Thermo-Energetic and Environmental Assessment of Alternative Fuels in Cement Clinker Production: A Review

Abstract

Cement clinker production is a thermal- and emissions-intensive process requiring high-temperature heat for drying, calcination, and sintering. This review provides a process-based assessment of refuse-derived fuel (RDF), solid recovered fuel (SRF), tire-derived fuel (TDF), and biomass as partial substitutes for coal and petcoke in modern dry-process cement kilns. The study synthesized the evidence from plant-scale trials, pilot and laboratory experiments, process modeling, computational fluid dynamics, emissions studies, life-cycle assessment (LCA), techno-economic analysis (TEA), and regional case studies to evaluate alternative fuels across fuel properties, kiln-zone suitability, process stability, clinker quality, emissions performance, and environmental outcomes. The review shows that stable co-processing generally requires fuels with net calorific values above 14 MJ kg⁻¹ and moisture contents below 15%, although TDF can provide 26–33 MJ kg⁻¹ and sustain high-energy kiln duty when sulfur, zinc, and steel residues are controlled. RDF, SRF, and biomass require pre-processing, homogenization, calibrated dosing, and continuous fuel-quality monitoring to limit incomplete burnout, deposit formation, volatile circulation, and clinker-quality variation. LCA studies show that 20% RDF thermal substitution can reduce global warming potential by about 3.3–4.2%, increasing to approximately 6.7% when avoided landfill methane credits are included. Modern abatement systems can maintain particulate matter at about 10–30 mg Nm⁻³ and PCDD/F below 0.1 ng TEQ Nm⁻³ under stable operation. The review concludes that alternative fuels are quality-dependent co-processing options whose mitigation role is complementary to clinker-factor reduction, energy-efficiency improvement, low-clinker binders, electrified heating, oxy-fuel calcination, and carbon capture.

1. Introduction

1.1. Background and Alternative-Fuel Context

Cement production remains one of the most energy-intensive and carbon-intensive industrial processes because the clinker intermediate product is produced through high-temperature pyroprocessing and carbonate decomposition. The sector contributes about 7–8% of anthropogenic carbon dioxide (CO₂) emissions [1,2,3,4], arising mainly from limestone calcination and the combustion of fossil fuels such as coal and petcoke used to supply kiln heat [5,6,7]. Calcination refers to the thermal decomposition of CaCO₃ → CaO + CO₂ during clinker production. The thermal requirement for clinker production is primarily concentrated in the kiln system, and thermal energy consumption accounts for approximately 80% of total energy use, with the remaining 20% consumed by electrical energy [8]. Modern dry-process preheater–precalciner kilns are substantially more efficient than wet or long dry kilns, yet they still require approximately 3.0–4.0 GJ of thermal energy per ton of clinker [4,6,9], depending on kiln design, raw-meal properties, fuel quality, and operating conditions [8,10].

In efficient dry-process systems, direct emissions commonly range from 0.8 to 1.0 t CO₂ per ton of cement [11,12]. These values, however, are not uniform across the global cement fleet, since older wet-process and less efficient kiln systems generally operate at substantially higher specific energy use and CO₂ intensity. This high emission intensity has made cement production a major focus of industrial decarbonization research, particularly because global cement demand remains strongly linked to infrastructure expansion, urbanization, and construction-sector growth [6,9]. Under increasingly strict climate constraints, this high thermal demand has intensified interest in alternative fuels as a practical strategy to reduce fossil-fuel dependence and improve the environmental performance of clinker production.

Alternative fuels are generally defined as selected wastes or by-products with recoverable energy that can partially displace the fossil-fuel demand of clinker production [8,10,13,14]. Refuse-derived fuel (RDF), tire-derived fuel (TDF), biomass, sewage sludge, waste oils, solvents, and other recovered fuels can provide usable thermal energy while reducing dependence on conventional fossil fuels in kilns [1,15,16,17,18,19,20]. Among these alternative fuels, RDF, TDF, and biomass are the most prominent substitutes used in cement kilns [5,20,21,22]. Their use also supports waste valorization through co-processing, in which selected waste streams are thermally utilized, and their mineral fractions are incorporated into clinker under controlled kiln conditions [20,21]. Previous studies indicate that alternative fuels can reduce fossil-fuel consumption, lower net greenhouse gas (GHG) emissions, and support waste-management objectives when fuel quality, feeding strategy, and emission-control systems are properly managed [22,23,24]. In 2023, the Global Cement and Concrete Association (GCCA) reported 26.85 Mt of alternative fuels used in cement kilns [25]. Figure 1 presents the global distribution of alternative-fuel and biomass-waste categories used in cement production, highlighting the dominance of RDF (including plastics), TDF, mixed industrial wastes, solvents, impregnated sawdust, waste oil, and agricultural or organic biomass fractions.

These fuels provide dual benefits by partially substituting fossil fuels and supporting waste valorization through co-processing, i.e., the simultaneous thermal destruction of waste and recovery of useful energy and mineral content in cement kilns. Recent industry data show that the use of alternative fuels in cement production has advanced substantially [4,6,24,26,27]. In the European Union (EU), the sector substituted an average of 56% of its fossil-fuel consumption with waste-derived alternative fuels in 2023, confirming that co-processing is already a mature decarbonization and circular-economy strategy in many European plants [28]. GCCA policy reporting similarly indicates that Europe has achieved average substitution rates above 50%, with some plants operating at considerably higher thermal substitution rates (TSR) [29]. Nevertheless, adoption remains uneven across countries and regions.

Although European case studies provide strong evidence of mature co-processing systems and high TSR, the global significance of alternative-fuel deployment depends strongly on developments in Asia. Figure 2 presents cement-production trends in China, India, and the world between 2022 and 2024. Cement-production statistics show that global cement output was approximately 4.1 billion tons in 2022 and 2023 before declining slightly to 4.0 billion tons in 2024 [30,31]. China remained the dominant producer, declining from about 2.1 billion tons in 2022 to 2.0 billion tons in 2023 and 1.9 billion tons in 2024, corresponding to approximately 51.2%, 48.8%, and 47.5% of global production, respectively, while India’s production increased from about 380 million tons in 2022 to 420 million tons in 2023 and 450 million tons in 2024, increasing its global share from approximately 9.3% to 10.2% and 11.3% over the same period. The data illustrate the continued global importance of these two countries for cement-sector decarbonization and alternative-fuel deployment.

Consequently, while EU experience provides important evidence of technical feasibility, the largest absolute decarbonization gains from alternative-fuel deployment are likely to depend on broader adoption in China and India, where co-processing remains more heterogeneous and less mature at scale. Within Europe, substitution rates also vary considerably by country. Figure 3 compares the 2022 alternative-fuel shares across selected EU countries, with the EU average shown for reference. In that year, Austria reported the highest TSR at 82%, followed by Poland (78%), the Czech Republic (77%), and Germany (76%), whereas Spain reached 44% and Italy 24%, indicating uneven diffusion driven by policy support, waste-supply chains, and plant readiness [32].

Life-cycle assessment (LCA) studies further indicate that increasing solid recovered fuel (SRF) use from 0 to about 50% of kiln fuel energy can reduce net GHG emissions by about 20%, with reductions approaching 30% at 80% substitution under favorable conditions [4]. Co-processing can also reduce landfill demand and associated methane burdens, particularly in regions where waste would otherwise be landfilled or openly burned without energy recovery. Adoption is therefore globally relevant but unevenly implemented. In China, interest in RDF and biomass is increasing, yet by 2020, only about 17% of production lines had co-processing capability [33]. In India, policy targets and pilot programs are accelerating the use of RDF and agricultural residues [34]. In South Africa, waste tire management initiatives have enabled partial replacement of coal with TDF in selected plants [17]. Despite this progress, the technical, energetic, environmental, and operational implications of alternative fuels remain heterogeneous across fuel types, kiln configurations, and regional waste-management systems.

1.2. Thermo-Energetic Basis for Alternative-Fuel Evaluation

Energy inputs occur across four main stages: (i) raw material preparation (crushing, drying, blending), (ii) pyro-processing to clinker, (iii) cement grinding/blending, and (iv) storage/dispatch [35]. The pyro-processing stage (also called clinker production) uses a rotary-kiln system with a preheater and often a precalciner. Here, the raw meal is heated to remove moisture, decomposed (calcined), and sintered to form clinker; pyro-processing consumes around 88% of the total fuel and 91% of the total energy used in all cement-making procedures, and accounts for a similar share of CO₂ emissions [36]. Modern preheater/precalciner kilns perform 60–65% of fuel combustion and more than 90% of calcination in the precalciner vessel, allowing the kiln to complete sintering [35]. After clinker is produced, finish grinding/blending converts clinker and mineral additives into cement powder; this stage is also electricity-intensive. The storage and dispatch stage involves silos and bagging/packaging equipment. Decarbonization strategies, therefore, distinguish between fuel-derived emissions and process emissions. A systems-dynamics review reports that roughly half of cement-plant CO₂ emissions are from the calcination process (chemical decomposition of CaCO₃), while the remainder comes from fuel combustion and electricity use [37].

Because calcination emissions are inherent to limestone decomposition, they are largely unaffected by the choice of fuel, whereas fuels directly determine fuel-CO₂ and combustion-related pollutants. Consequently, evaluations of alternative fuels must specify which thermal duty they address: fuels used in the precalciner provide heat for calcination and account for most of the CO₂ released [33,35], whereas fuels used at the main kiln burner supply the high-temperature sintering heat. Alternative-fuel substitution in cement production cannot be assessed as a generic replacement for fossil fuels because clinker production requires heat delivered across distinct process zones with different temperatures, residence times, and combustion requirements. In modern dry-process systems, raw meal is progressively dried and preheated in the cyclone preheater before entering the precalciner. The precalciner serves as the principal decarbonation reactor in dry-process kiln systems.

Multiple studies identify the main calcination range as 850–900 °C under controlled mixing and oxygen availability, whereas more recent dynamic modeling indicates that industrial calciners may operate more broadly within 900–1100 °C, depending on local process conditions [38,39,40]. The partially calcined material then enters the rotary kiln, where temperatures continue to rise until the burning zone reaches conditions required for clinker mineral formation, with material temperatures of about 1400–1450 °C needed to sustain liquid-phase formation and clinker mineralogy development [41]. Clinker mineralogy refers to the phase composition of clinker, especially alite (C₃S), belite (C₂S), aluminate (C₃A), and ferrite (C₄AF), which governs cement reactivity and strength development. The clinker is subsequently cooled rapidly to stabilize mineral phases and recover sensible heat for reuse as secondary and tertiary combustion air. The literature consistently reports that about 85–95% of limestone calcination is completed before the meal enters the rotary kiln, and recent models estimate a typical value of about 90% at the calciner exit [38,39]. Kiln gases arising from waste co-incineration must be raised in a controlled and homogeneous manner to at least 850 °C for 2 s, even under unfavorable operating conditions; where hazardous waste contains more than 1% halogenated organic substances expressed as chlorine, the required temperature increases to 1100 °C [42].

Table 1. Major stages of the dry-process cement kiln.

Process Stage	Temperature Range	Thermal Function	Specific Heat Demand	Fuel and a Typical Feeding Point	Reference
Raw milling and drying	Preheater exit gas typically 300–400 °C; conditioned mill-entry gas often 180–260 °C	Moisture removal, raw meal drying, size reduction, homogenization	Uses recovered exhaust heat rather than the plant’s highest-grade heat	Limited direct relevance for AF choice; affected indirectly by kiln/preheater gas temperature and false-air control	[43,44,45]
Cyclone preheater	Upper stages are broadly several hundred °C; calcination onset can begin around 600 °C; lower-stage meal approaches the calciner window.	Sensible heating and initial decarbonation	A major part of total kiln efficiency comes from heat recuperation here; poor recuperation increases kiln fuel demand.	Usually, there is no dedicated direct firing in dry systems; strongly influenced by gas chemistry, deposits, and volatile circulation.	[38,43,46,47]
Precalciner	Common calcination window about 850–900 °C; industrial operation often described as roughly 900–1100 °C	Main decarbonation reactor; completion of ~85–95% of calcination before kiln entry	Calcination of CaCO3 requires about 1.7 MJ kg−1, validated modern-clinker systems can operate near 3.3 GJ t−1 clinker total thermal intensity	Principal AF feeding point in dry kilns; fuel is introduced with tertiary air; medium-LHV or coarser solid fuels are often more suitable here than at the main burner	[38,39,40,48]
Rotary kiln transition and burning zone	Feed rises rapidly through the transition zone; solids in burning zone about 1450 °C; gas/flame broadly 1450–2000 °C	Final decarbonation, solid-state reactions, liquid-phase formation, and clinker mineral development	This stage has the highest thermal requirement; practical kiln-system demand remains well above theoretical reaction heat due to sensible heating and losses.	Main-burner duty favors premium, low-moisture, fast-burning fuels; unsuitable fuels lengthen the ignition distance and weaken flame intensity.	[40,43,46,48,49]
Clinker cooler	Entering clinker broadly 1200–1400 °C; discharged clinker is often targeted near 100–150 °C or below about 100–200 °C, depending on plant practice	Rapid quench to preserve clinker mineralogy and recover sensible heat	Secondary and tertiary air can account for about 50% of total cooler heat output, discharged air about 40%, and hot clinker about 9% in measured balances	No primary fuel input; cooler performance controls secondary-air and tertiary-air temperature, which in turn affects both main-burner and calciner combustion	[50,51]

summarizes the principal stages of the dry-process cement kiln, listing their indicative temperature windows, dominant thermal roles, specific heat-demand considerations and the relevance of each zone to alternative-fuel selection.

These thermo-energetic requirements define the practical operating envelope for alternative fuels. Within cement production, fuel suitability is determined not solely by heating value, but by the combined effects of moisture, particle-size distribution, volatile release and burnout kinetics, ash quantity and chemistry, chlorine and sulfur contents, alkali input, and trace element loading, all of which influence combustion efficiency, recirculation phenomena, build-up formation, atmospheric emissions, and clinker mineralogy [52]. These properties affect ignition distance, flame temperature, heat-release location, gas volume, oxygen demand, internal circulation of volatile species, and the risk of deposits or reducing conditions in the kiln system. The technical feasibility of RDF, TDF, and biomass depends on whether their combustion characteristics can satisfy the thermal and chemical requirements of the preheater, precalciner, rotary kiln, and clinker cooler without compromising process stability or clinker quality. RDF and many biomass streams commonly exhibit lower heating values (LHV) and Higher-moisture contents than coal or petcoke, which can increase the fuel mass flow required to deliver the same heat duty and may alter flame shape, burnout behavior, and flue-gas volume [20,21]. TDF has a higher calorific value and can provide stable thermal input, but it may introduce sulfur and zinc, which require careful control to protect kiln chemistry and clinker mineralogy [21,53,54]. Therefore, the evaluation of alternative fuels must extend beyond simple fossil-fuel displacement to include feeding-point suitability, combustion stability, thermal efficiency, volatile-species management, clinker-phase development, and emissions performance.

1.3. Research Gap and Contribution of This Review

Previous studies often assess RDF, TDF, or biomass in isolation, and differences in methodology, system boundaries, trial duration, kiln configuration, and performance metrics limit direct comparison across the literature [19,20,22,23,24]. In addition, many existing discussions emphasize emissions, waste utilization, or substitution rates without systematically linking fuel properties to kiln-zone thermal requirements, combustion dynamics, clinker chemistry, and cement performance. This limits the transferability of published findings, especially at high TSR or under variable industrial operating conditions.

This review addresses that limitation by providing a process-based comparative evaluation of RDF, TDF, and biomass in cement production. Specifically, it examines the thermo-energetic behavior of these fuels, their interaction with kiln operation and clinker formation, their emissions and environmental implications, and the methodological approaches used to assess their performance. Beyond synthesis, the review identifies key research gaps and methodological constraints that limit reliable scale-up, including the need for long-duration high-TSR datasets, a clearer understanding of multi-fuel interactions under kiln conditions, stronger links between trace chemistry and cement performance, harmonized LCA assumptions, and improved digital control and real-time fuel management. Therefore, this study reframes the assessment of alternative fuels as a process-based suitability problem rather than a simple fossil-fuel replacement strategy, thereby providing a clearer technical basis for comparing alternative fuels in sustainable cement production.

Because alternative fuels primarily reduce fuel-derived emissions, this review views RDF, TDF, and biomass as complementary mitigation options that must be integrated with kiln-efficiency improvements, clinker-factor reduction, supplementary cementitious materials (SCM) and carbon capture for deeper decarbonization.

Although this work does not present new experimental measurements, its contribution lies in the critical integration of process-engineering evidence, fuel-property constraints, kiln-zone thermal requirements, emissions behavior, and LCA boundary assumptions into a unified framework for evaluating alternative fuels in cement clinker production.

Compared with previous studies, this review provides a more integrated process-based interpretation of alternative-fuel co-processing. Earlier work has often focused on individual fuel types, specific national case studies, emissions performance, or LCA-based environmental outcomes. These studies consistently show that RDF, SRF, TDF, and biomass can reduce fossil-fuel demand and, under controlled conditions, lower selected environmental burdens. However, the present review extends this evidence by comparing alternative fuels through a unified framework that links fuel properties, kiln-zone thermal requirements, feeding-point suitability, combustion stability, clinker-quality effects, direct emissions, LCA boundary assumptions, techno-economic constraints, and regional transferability. In this way, the review reframes the evaluation of alternative fuels from a simple substitution problem to a process-specific suitability assessment.

2. Technical Performance and Process Considerations

The technical feasibility of replacing coal or petroleum coke with alternative fuels in cement production depends not only on their net calorific value but also on how their moisture content, ash chemistry, and combustion behavior interact with the thermal requirements of the clinker-production train. Stable sintering and complete burnout require pre-processing, appropriate injection systems, and robust control strategies to maintain both energy efficiency and product quality.

2.1. Fuel Properties and Combustion Behavior

The suitability of a fuel for cement-kiln co-firing depends on its net calorific value, moisture content, ash chemistry, particle-size distribution, and concentration of problematic constituents. Stable RDF use generally requires a net calorific value of about 15–20 MJ/kg, a moisture content near or below 15%, chlorine below approximately 0.8%, sulfur below about 2.5%, and low heavy-metal concentrations. TDF generally exhibits higher calorific values from 28 to 32 MJ/kg and generally has a low moisture content of 10% and chlorine content of 0.5%, but it introduces sulfur and zinc from the rubber vulcanization process, while biomass shows wider variability of 8–18 MJ/kg depending on feedstock and moisture content of 15–20% and chlorine content of 0.2–0.3% [14,21,55,56].

The high variability of alternative fuels remains a major technical constraint in cement-kiln co-processing. RDF, SRF, and biomass can vary substantially in calorific value, moisture content, particle-size distribution, ash chemistry, chlorine, sulfur, alkalis, and trace-metal content, depending on waste origin, collection system, sorting efficiency, seasonal conditions, and pre-processing quality [57]. This variability affects heat-input stability, ignition delay, burnout completeness, gas volume, volatile-species circulation, deposit formation, and clinker chemistry [58,59]. Therefore, alternative fuels require stricter quality-control procedures than conventional coal or petcoke. Practical control measures include source exclusion, removal of PVC plastics and metals, shredding, screening, drying or biodrying, homogenization, blending, covered storage, routine proximate and ultimate analysis, calorific-value testing, halogen and heavy-metal monitoring, and controlled dosing through calibrated feeding systems. Where fuel variability remains high, feed-forward control based on fuel-quality data and real-time adjustment of fuel feed rate, oxygen supply, and calciner or burner settings are required to maintain stable kiln operation and emissions compliance [60,61].

These variability-related control requirements are summarized in

Table 2. BAT acceptance limits and alternative-fuel characteristics in cement kilns [21,55,62,63,64].

Parameter	BAT Guidance	RDF	TDF	Biomass	Kiln Operational Relevance	Pre-Treatment and Controls
Net calorific value (MJ kg)	14	15–20	28–32	8–18	Stable flame and heat input; clinker burnability control	Blending, size reduction, routine sampling and calorific-value testing
Moisture (wt. %)	15	15	10	15–20	High moisture lowers the flame temperature, increases the gas/solids flow rate, and causes instability.	Drying/biodrying; covered storage; drainage or dewatering
Chlorine (wt. %)	0.5–0.8	0.6–0.8	0.5	0.2–0.3	Volatile cycling to ring/coating formation; bypass requirement; corrosion risk	Remove polyvinyl chloride plastics; improve sorting; implement input control; kiln bypass if required.
Sulfur (wt. %)	2.5	2.5	2.0–2.5	0.5–1.0	Sulfate/alkali balance affects build-ups and emissions	Raw-mix balancing; bypass control; sulfur-to-alkali ratio ratio management
Ash (wt. %)	Site-specific	Low–moderate	Low	Low–moderate	Alters raw-mix chemistry and clinker mineralogy; affects free lime	Ash accounting in mix design, feeder calibration, supplier quality assurance

, which compares BAT guidance values, representative fuel characteristics, kiln operational relevance, and pre-treatment measures for RDF, TDF, and biomass.

For co-processing, the EU Best Available Techniques (BAT) framework explicitly links the thermal operating objective to waste destruction efficiency. From Table 2, BAT guidance values indicate that stable operation usually requires RDF to have a net calorific value above about 14 MJ/kg, a moisture content below 15%, chlorine below 0.8%, sulfur below 2.5%, and low heavy-metal concentrations. The BAT conclusions further require continuous and constant waste feeding, indicating that alternative-fuel quality, moisture content, calorific value, particle-size distribution, and feeding stability are not only operational variables but also compliance-critical parameters [42]. Consequently, co-processing assessments should evaluate whether the fuel can maintain stable heat release at the intended feeding point while limiting emissions of nitrogen oxides (NO_x), sulfur dioxide (SO₂), hydrogen chloride (HCl), heavy metals, and organic pollutants [65,66,67]. Due to the batch variability that persists despite contractual specifications, pre-treatment, such as shredding, drying, or biodrying, removal of PVC plastics and metals, blending, and quality assurance, is necessary to maintain a homogeneous heating value of roughly 15–18 MJ/kg for RDF [1,48,68].

2.1.1. Kiln Feeding and Combustion Dynamics

In modern dry-process preheater–precalciner plants, raw meal is heated progressively in multistage cyclones before entering the precalciner, where rapid pre-calcination occurs under controlled combustion conditions at roughly 890–900 °C [69,70]. The rotary-kiln burning zone must then sustain material temperatures near 1450 °C to complete clinker mineral formation and limit free lime [41]. The hot clinker is subsequently quenched in the cooler, where sensible heat is recovered by generating secondary and tertiary air, which are returned to the kiln and precalciner, respectively, thereby improving overall thermal efficiency [71]. Computational fluid dynamics (CFD) studies describe how the kiln feed is heated in the preheater and then sent to the precalciner, where typically 85–95% of the CaCO₃ decomposition occurs [39]. Figure 4 presents a simplified schematic of a modern preheater–precalciner cement kiln.

The schematic shows the principal feeding points for RDF, biomass, and TDF, including calciner injection, main-burner injection, kiln-inlet feeding, and mid-kiln tire feeding. The figure highlights that fuel suitability depends on particle size, moisture content, residence time, combustion characteristics and the thermal duty of each kiln zone. Injection location and burner configuration are selected, along with fuel properties and acceptance limits, to ensure process stability and complete combustion. Pre-processed RDF or biomass, such as pulverized, pelletized, or chipped, is typically introduced through the precalciner and/or the main burner, whereas whole or large tires are fed at the kiln inlet or the back end to take advantage of longer residence times [72].

Table 3. Kiln integration effects of RDF, biomass and TDF [21,61,73,74].

Integration Aspect	Kiln Injection Point (s)	Basis for Selection	Combustion Behavior	Thermal Profile Effect	Operational Adjustment
RDF/biomass (processed)	Precalciner/main burner	Better dispersion and burnout under controlled particle size and moisture	Slower burnout than coal; distributed heat release.	Heat release may shift downstream, producing a longer, cooler flame	Burner settings, flame momentum, and secondary-air distribution/temperature
TDF (whole/chipped)	Mid-kiln/kiln inlet/back end	Longer gas residence time supports complete breakdown	Stable energy release, but compositional concerns sulfur and metal (Zn)	Comparable flame-shaped effects depending on feed form and dosing	Controlled feed rate, handling and metering
Process implication	Residence time matched to burnout.		longer flame and lower peak temperature at the main burner	Hot-zone displacement risk, requiring air and burner correction	Maintain sintering temperature range (1400–1450 °C) and clinker quality

summarizes how RDF/biomass and TDF are integrated at different kiln locations, along with the corresponding combustion behavior, thermal profile impacts, and control responses. Alternative fuels often burn more slowly than coal and release heat more evenly; when fired in the main burner, they can lower peak flame temperature, lengthen the flame, and shift the clinker-forming zone [21].

Pre-processed RDF/biomass is typically fed to the calciner and/or main burner, while whole or large tires are introduced at the kiln inlet/back end to take advantage of a longer residence time. For example, stable operation at approximately 30% RDF substitution was achieved by increasing the preheater secondary-air temperature and enhancing flame momentum, while maintaining clinker quality [62,73]. These results underscore the need to align fuel characteristics with appropriate injection locations and to implement adaptive process-control strategies that respond dynamically to variations in heat-release rates and flame geometry.

2.1.2. Kiln Stability and Control

Co-firing RDF, biomass and TDF introduces additional variability in heat input and gas composition, sensitive to fluctuations in fuel density, particle size and energy content, requiring improved process control.

Table 4. Key kiln stability challenges and mitigations under alternative-fuel co-firing [21,55,76].

Challenges	Cause Factor	Mitigation
Heat-input variability	Variation in calorific value, moisture and particle size	Install accurate metering and dosing; employ online calorimetry; implement fuel blending
Incomplete combustion/CO surges	Large particle size, high moisture, poor dispersion	Ensure good dispersion; maintain adequate oxygen; temporarily suspend alternative-fuel feeding during start-up/shut-down or low O2 conditions
Deposit formation, clogging and ring build-up	Volatile Cl, S and alkalis condense in cooler zones	Limit Cl and S input; install bypass for high Cl; adjust raw-mix chemistry; manage temperature profile
High flue-gas volume/dust carryover	Higher mass flow for low CV fuels; increased primary air for conveying	Optimize preheater and cooler settings; adjust fan capacity; improve dust collection
Increased process variability	Rapid changes in fuel quality or feed rate	Use advanced process control (model predictive control); implement feed-forward control based on fuel properties

summarizes key challenges and mitigations [62]. To address this, plants employ precise dosing and metering, sometimes supported by online calorimetry or thermogravimetric analysis, to adjust fuel feed rates in real time. Industrial evidence shows that, with appropriate technical and operational modifications, cement plants can co-fire substantial shares of alternative fuels without compromising process stability or clinker quality. For example, co-firing a rice-husk/RDF blend at the Kattameya plant in Egypt reduced coal use by 17% and electrical power demand by 13%, while maintaining clinker quality and lowering NO_x and SO₂ emissions [75].

Similarly, at Arabian Cement in Egypt, a thermal substitution rate of 13.8%, comprising 10.6% RDF, 1.3% tires, 0.4% sludge, and 1.5% agricultural residues, reduced coal and natural gas consumption and avoided approximately 108 kt CO₂ [77]. Another trial in Egypt also shows that introducing up to 15% hazardous-waste RDF did not alter clinker mineralogy or reduce cement strength [78], and German plants routinely achieve more than 50% thermal substitution when fuel-feeding systems and process control are robust [1]. However, substitution rates above 60–70% TSR require capital investment in multi-fuel burners and improved pre-processing to maintain stability [55].

The strong chemical and energy heterogeneity of RDF, biomass, and TDF also limits the direct reproducibility of industrial co-processing results. Reported TSR, emissions responses, and clinker-quality outcomes are strongly influenced by local fuel composition, pre-processing quality, kiln configuration, feeding point, residence time, oxygen availability, and process-control capability [21,79,80]. Therefore, results obtained in one plant cannot be transferred directly to another plant without considering differences in waste origin, fuel preparation, raw-meal chemistry, burner design, calciner configuration, bypass operation, and emission-control systems. To improve reproducibility, industrial studies should report standardized fuel-characterization data, including calorific value, moisture, ash chemistry, particle-size distribution, chlorine, sulfur, alkalis, and trace metals, together with kiln operating conditions and substitution rate. Such reporting would support stronger cross-study comparison and more reliable scale-up of RDF, biomass, and TDF co-processing.

2.2. Clinker Quality and Cement Properties

Co-firing alternative fuels affects clinker quality through two pathways: ash incorporation into the raw mix and changes in thermal profile. Ash from RDF, biomass and TDF becomes part of the clinker feed, effectively acting as a supplementary raw material.

Table 5. Major ash constituents and clinker-quality implications of alternative fuels.

Fuel Type	Main Ash/Minor-Element Inputs	Main Mechanisms Affecting Clinker Formation	Clinker/Cement Impact	Practical Control Limits	Monitoring and Control Strategy	Reference
Biomass	Alkalis (K2O, Na2O), SiO2, CaO, MgO, P2O5; composition strongly feedstock-dependent	Phosphorus can enter silicate phases and stabilize belite (C2S) at the expense of C3S; alkalis intensify internal circulation and may alter melt chemistry and burnability	Higher belite fraction; Increased free lime and reduced C3S formation at elevated P2O5 input; possible reduction in early-age strength; altered clinker reactivity; greater process instability where alkali input is high and kiln-cycle stability depending on ash composition	Control total P2O5 input; avoid excessive alkali loading; avoid approximately 0.5–1.0 wt. % where belite/free-lime effects become significant; site-specific thresholds should be set through raw-mix and clinker chemistry control	XRF/XRD monitoring, raw-mix correction, biomass selection/blending, ash accounting in kiln feed, tighter alkali–sulfur balance control	[21,81,82]
TDF	Zn, Fe, S, Ca; minor metals from steel and additives in tire structure	Zinc can partition into clinker phases and promote Zn-bearing spinel formation (e.g., franklinite); excessive Zn can suppress C3S formation; sulfur contributes to sulfate balance and internal circulation	Potential inhibition of C3S formation, higher belite fraction, possible adverse effect on early strength at excessive ZnO input; sulfur-related deposit/build-up risk if sulfate balance is disturbed	Control total ZnO and SO3/S-alkali balance in clinker/system input; maintain site-specific metal and sulfur acceptance limits; avoid excessive TDF share without chemistry compensation	Continuous fuel-quality control; metal-content surveillance; sulfur balance control through raw mix and bypass where required; correlate clinker-phase composition with TDF input	[21,83,84]
RDF	Heterogeneous ash containing SiO2, Al2O3, CaO, alkalis, Cl, S, trace metals; composition depends on waste source and pre-processing quality	Highly heterogeneous ash affects raw-mix chemistry; chlorine, alkalis, and sulfur promote volatilization-condensation cycling; trace metals may accumulate in dust/clinker; indirect effects arise through process instability and altered thermal profile.	Greater variability in clinker chemistry; risk of kiln instability, ring formation, coating build-up, and indirect effects on clinker mineralogy and cement consistency; product quality risk increases with poor fuel specification control	Control total Cl input, sulfur input, ash variability, and trace-metal content; site-specific acceptance criteria required for stable long-duration operation	Continuous fuel QC; improved sorting and PVC removal; blending and pre-processing; online or routine monitoring of ash chemistry, halogens, sulfur, and heavy metals; integrate ash into kiln-feed calculations	[55,74,85,86]

lists the major ash-derived constituents introduced by alternative fuels and their potential effects on clinker mineralogy, along with recommended limits to maintain cement performance. Biomass ash often contains silica, alkalis (K₂O, Na₂O) and phosphorus; high phosphorus levels (0.5–1.0 wt. % P₂O₅) can stabilize belite (C₂S) and increase free lime, thereby suppressing C₃S formation. RDF ash can include a heterogeneous mix of SiO₂, CaO, Fe₂O₃, Cl, and heavy metals; elevated chlorine and heavy-metal content can lead to refractory corrosion, ash deposits, and modifications to clinker mineralogy [72]. TDF ash introduces iron and zinc; excess zinc forms spinel phases (e.g., franklinite) that inhibit C₃S formation and enlarge clinker crystal size [64]. Controlling P₂O₅ (0.5 wt. %), ZnO (0.2 wt. %) and Cl/S input helps prevent these adverse effects.

Cement kilns also immobilize many heavy metals within clinker and hydrated phases, reducing environmental release when process control is effective [87,88]. Thermal-profile changes can affect the clinker microstructure and cement properties. Alternative fuels often generate longer, cooler flames than coal, which can promote the growth of larger C₃S crystals and increase the proportion of intergranular melt. At very high substitution rates, these mineralogical changes may reduce clinker reactivity and compromise early-age strength development [72]. Largely, the literature indicates that RDF, biomass, and TDF can be co-processed at substantial replacement levels provided that fuel specification control, fit-for-purpose feeding systems, and chemistry/thermal monitoring are implemented [48,89,90], with stable operation reported across multiple countries at TSR spanning approximately 15–90% for RDF under varying degrees of process adjustment [73,78,91,92,93].

3. Environmental Performance and Life-Cycle Perspective

In Section 2, the study established the thermo-chemical bases for alternative-fuel substitution in cement production; the decision to displace fossil fuels must also consider environmental performance across both direct process emissions and full life-cycle impacts. This section critically reviews available evidence on pollutant emissions, global warming potential (GWP), and other environmental impact categories associated with RDF, TDF, and biomass utilization. Furthermore, key research gaps hindering comprehensive sustainability assessments are identified to inform future investigation and policy development.

Fossil-based RDF from plastics offers little climate benefit. Moreover, the variability in waste can pose a pollution risk to kiln systems via heavy metals and chlorine. To mitigate these risks and satisfy environmental standards, rigorous pollution control strategies and real-time emissions monitoring protocols must be enforced [94]. LCA is a standardized methodology for quantifying environmental impacts across all stages of a product system, from raw material extraction to production, use, disposal, and recycling. In cement production, LCA enables systematic evaluation of emissions, energy use and resource consumption across key process stages, including raw material preparation, clinkerization, fuel combustion, and waste management. This approach allows consistent comparison of environmental burdens and benefits associated with different technological and operational strategies. Globally, numerous LCA studies have assessed mitigation options in the cement sector, including alternative-fuel use, clinker substitution, and carbon-capture technologies [94,95,96,97,98,99].

3.1. Direct Emissions and Air-Quality Implications

Co-firing of alternative fuels in cement kilns modifies the combustion atmosphere, potentially altering emissions of particulate matter (PM), NO_x, SO₂ and other pollutants. A multi-year modeling study conducted at an Egyptian cement plant evaluated emission scenarios as the fuel mix shifted from natural gas to coal blended with RDF, TDF and dried sewage sludge [100]. Results indicated that maximum ground-level concentrations of total suspended particles (TSP), nitrogen dioxide (NO₂), and SO₂ varied significantly with fuel composition. TSP concentrations showed positive correlations with coal, RDF, and dried sewage sludge, but negative correlations with natural gas, diesel, and TDF. NO₂ levels showed a positive correlation with DSS and a negative correlation with TDF, displaying intermediate behavior for diesel, coal, and RDF. SO₂ concentrations were positively correlated with natural gas and DSS, but negatively correlated with RDF, TDF, and coal. The direct-emission response to alternative-fuel co-processing is therefore fuel- and kiln-specific. Emission performance depends on combustion temperature, oxygen availability, fuel nitrogen and sulfur contents, chlorine input, raw-meal chemistry, and the effectiveness of particulate and acid–gas control systems. This explains why RDF, TDF, biomass, and sewage sludge can produce different emission responses under similar substitution rates, and why well-controlled co-processing may remain within regulatory limits even when individual pollutants shift slightly relative to coal or petcoke firing [21,42,100,101]. SO₂ emissions are governed mainly by fuel, raw sulfur input and the sulfur-retention capacity of the kiln system. Typical SO₂ stack levels in modern kilns are 50–400 mg/Nm³, with most below 3 kg SO₂/t, largely controlled by raw-mix optimization using free lime to absorb sulfur. As a result, increased sulfur input does not necessarily translate into proportional increases in stack SO₂ [21,101]. Co-processing low-sulfur wastes generally reduces SO₂ emissions. Nielsen et al. [101] showed that sulfates remain relatively stable under oxidizing conditions but can decompose under local reducing conditions, especially near the kiln inlet, releasing gas-phase SO₂.

Ige et al. [102] integrated LCA with systems-dynamics modeling to project long-term environmental impacts of South African cement production. The study assessed CO₂, NO_x, and SO₂ emissions and their effects on GWP and human health. Policy recommendations included eco-blended cement production and carbon taxes to mitigate emissions. This innovative approach aims to guide sustainable industry development. Industrial evidence further suggests that higher shares of RDF or TDF do not systematically increase SO₂ emissions when sulfur balance and kiln atmosphere are properly controlled [21,76,100]. Available plant and review studies indicate that regulated emissions often remain comparable to traditional coal or petcoke firing when co-processing is well managed [21,100,103].

3.2. Life-Cycle Assessment Results

LCA provides a systematic basis for evaluating the environmental impacts of alternative-fuel substitution by accounting for upstream fuel preparation, transport, kiln operation, and, where applicable, avoided-waste-management burdens. However, LCA results for RDF, SRF, TDF, and biomass should not be interpreted only as a function of thermal substitution rate. Reported environmental benefits also depend on system boundary definition, baseline fuel, allocation procedure, waste-treatment assumptions, fuel-processing energy, transport distance, and treatment of biogenic and fossil carbon fractions.

Across RDF/SRF studies, technical performance and life-cycle outcomes are governed by both fuel quality and boundary assumptions. Stable co-processing depends on calorific value, moisture content, particle-size distribution, chlorine input, sulfur content, ash chemistry, and the selected feeding point, because poorly homogenized RDF/SRF can increase heat-input variability, incomplete burnout, volatile-species circulation, CO excursions, and deposit-formation risk [58,61]. Therefore, RDF/SRF should not be treated as a uniform fuel category with fixed environmental benefits, but as a quality-dependent co-processing option whose performance is controlled by fuel specification, kiln configuration, feeding strategy, and LCA boundary assumptions [104,105].

Regional LCA studies show that RDF/SRF substitution generally reduces fossil-fuel demand and GWP, although the magnitude varies considerably. Ige and Kabeya [105] reported that 20% RDF thermal substitution in South African and Ethiopian cement production reduced GWP by 3.3–4.2% per kg cement, with the reduction increasing to 6.7% when avoided landfill methane emissions were included; fossil resource depletion declined by approximately 10%, while other midpoint categories changed only marginally. This result indicates that moderate RDF substitution provides measurable but limited direct climate benefits under a cradle-to-gate cement-production boundary, while broader waste-management credits can substantially increase the estimated benefit. Figure 5 summarizes representative GWP reductions reported in selected LCA studies of alternative-fuel substitution. The figure compares the spread of reported outcomes across RDF, RDF with avoided landfill methane credit, RDF/biological sludge, and biomass-substitution scenarios.

The reviewed LCA studies report a wide range of GWP reductions, from modest improvements under direct cement-production boundaries to larger reductions where higher substitution rates, different baseline fuels, or avoided-waste-management credits are included, as shown in Figure 5. This variation confirms that alternative-fuel co-processing can provide measurable environmental benefits, but the magnitude of reduction remains strongly scenario-specific.

Other studies report larger reductions where higher substitution rates, different baseline fuels, or expanded waste-management assumptions are applied. Georgiopoulou and Lyberatos [20] found that replacing 30–40% of conventional fuel with RDF and biological sludge reduced GWP by approximately 22–30% relative to petroleum coke in European cement kilns. Similarly, Salaripoor et al. [104] reported that RDF blends, particularly those containing organic waste, reduced GWP and overall environmental impacts compared with fuel oil or coal in Tehran cement plants during natural gas shortages. Çankaya [106] reported that climate-change impacts decreased by 12% when 15% of fossil fuel was replaced with RDF and by 27% when RDF substitution reached 30%, whereas dried sludge at 15% substitution slightly increased climate-change impact relative to the baseline. These results show that RDF generally performs better than dried sludge for climate-change mitigation, but the outcome depends on fuel composition, moisture content, the substituted fossil fuel, and LCA boundary assumptions.

Biomass-based substitution shows greater potential to reduce fossil CO₂ emissions, particularly when locally available residues displace coal or petcoke with minimal additional processing. Beressa and Vijaya Saradhi [95] reported that replacing 50% of imported coal with coffee husks at the Mugher Cement Factory reduced GHG emissions by 14%, improved thermal efficiency by 1.2%, and reduced coal cost by 36%. However, biomass benefits remain sensitive to assumptions regarding collection, drying, storage, transport, land-use effects, and biogenic carbon neutrality. High moisture and volatile matter can increase the fuel mass required per ton of clinker and may reduce the net environmental advantage if drying energy and long-distance transport are included [65].

TDF has comparatively limited LCA coverage relative to RDF and biomass. Its environmental benefits are mainly linked to its high-energy density and the displacement of coal or petcoke, but its sulfur, zinc, and metal contents require strict kiln chemistry and emission-control management [64]. Çankaya and Pekey [107] reported that alternative fuels, including TDF and dried sludge, reduced GHG emissions by 16% compared with conventional fossil fuels. However, because TDF contains a large fossil-derived fraction, its climate benefit is generally lower than that of sustainably sourced biomass and more dependent on avoided tire-disposal assumptions.

LCA studies on clinker substitution, although not strictly equivalent to fuel substitution, provide a useful context, showing that fuel switching alone cannot deliver the full mitigation required in cement production. Stafford et al. [108] and Moretti and Caro [109] showed that replacing clinker with industrial residues or SCMs can substantially reduce CO₂ emissions, often by 25–40%, depending on the replacement level and material type. These findings indicate that alternative fuels should be viewed as one component of a broader decarbonization strategy that also includes clinker-factor reduction, energy-efficiency improvements, and carbon capture.

In general, the reviewed LCA evidence indicates that RDF, SRF, TDF, and biomass can reduce fossil-fuel demand and lower GWP when fuel quality, kiln operation, and boundary assumptions are properly controlled. However, reported benefits are not directly transferable across regions or plants. Transparent reporting of system boundaries, avoided-waste credits, fuel-processing energy, transport distance, substitution rate, carbon-allocation assumptions, and uncertainty ranges is necessary for meaningful comparison across studies.

3.3. Research Needs and Limitations

Although modern cement kilns can achieve very low stack emissions when fitted with fabric filters or high-efficiency electrostatic precipitators, with particulate matter commonly reported at 10–30 mg Nm⁻³, several knowledge gaps remain [42,56]. These gaps relate not only to pollutant concentrations but also to pollutant fate, health relevance, long-term operational reliability, high-TSR validation, regional transferability, regulatory comparability, and integration with emerging low-carbon kiln technologies.

3.3.1. Emissions Monitoring, Trace Elements, and Health-Risk Assessment

Direct emissions of dust and heavy metals are more dependent on kiln stability and abatement performance than on the mere presence of alternative fuels. While total heavy-metal emissions from properly controlled kilns generally remain below about 0.5 mg Nm⁻³, volatile elements such as mercury, cadmium, and thallium may escape capture if fuel quality, raw-meal chemistry, or volatile circulation is poorly controlled [56]. Research should therefore prioritize continuous monitoring and modeling of volatile-metal transport and partitioning, particularly under high TSR and multi-fuel mixtures.

Most LCAs treat dioxin/furan (PCDD/F) formation as negligible because measured concentrations at European kilns are typically below 0.1 ng TEQ Nm⁻³, with many plants achieving concentrations below 0.01 ng TEQ Nm⁻³. Nevertheless, the dependence of PCDD/F formation on chlorine input, sulfur chemistry, oxygen availability, temperature history, and abatement conditions warrants further research, especially when waste-derived fuels introduce higher chlorine or metal loadings. Long-term datasets at high substitution rates remain limited; therefore, future studies should assess how increasing RDF, SRF, biomass, or multi-fuel shares affect PCDD/F formation, NO_x-control strategies such as SNCR/SCR, and the interaction between combustion control and organic-pollutant suppression.

Trace element emissions and their impacts on clinker quality are also often insufficiently represented in LCA inventories, even though metals such as Zn and Pb can influence both environmental releases and cement performance [64]. Incorporating heavy-metal fate, immobilization, and potential release pathways into LCAs would improve assessment robustness. While emission factors suggest that most heavy metals are retained in clinker or fly ash and that volatile fractions of Cd and Pb may account for less than 0.006% of total input [110], isolated studies report that sewage-sludge co-processing can increase particle-phase Cd and Pb emissions [111]. This underscores the need for systematic studies on the fate of trace elements across different waste types and fuel blends, including the effectiveness of bypass systems, raw-mix adjustment, and fuel-quality limits in mitigating releases.

Health-impact assessment remains underdeveloped in studies of alternative-fuel co-processing. Although modern abatement systems can maintain low stack concentrations of particulate matter, heavy metals, and PCDD/F, these pollutants remain health-relevant because their impacts depend on particle-size distribution, metal speciation, atmospheric dispersion, population exposure, deposition, and potential food-chain transfer. Fine particulate matter can contribute to respiratory and cardiovascular risks, while volatile metals such as Hg, Cd, and Tl require attention because their mobility and toxicity differ from those of less volatile metals, which are retained mainly in clinker or kiln dust. Similarly, PCDD/F emissions are typically low in well-controlled kilns, but their persistence and bioaccumulation warrant continued surveillance when chlorine-rich waste-derived fuels are used. Most available studies focus on stack-emission compliance rather than exposure-based health-risk assessment. Future research should therefore combine continuous emissions monitoring with dispersion modeling, intake-pathway analysis, and human-health-risk characterization to determine whether measured emissions translate into material exposure risks under different fuel blends, TSR levels, and local population settings [55,58,112,113].

3.3.2. Operational Reliability and High-TSR Validation

Long-term operational risk remains insufficiently quantified in the available literature. Although many studies report short-term process stability during RDF, SRF, TDF, biomass, or sewage-sludge co-processing, fewer studies provide multi-year industrial evidence on deposit formation, coating instability, corrosion, refractory wear, bypass-dust generation, thermal instability, and maintenance frequency under sustained high-TSR operation. These risks are strongly influenced by chlorine, sulfur, alkali and trace-metal inputs, fuel moisture, particle-size variability, local reducing conditions, and kiln/calciner temperature profiles. Therefore, short-duration trial results should be interpreted cautiously when assessing long-term plant reliability. Future industrial studies should report longitudinal indicators such as kiln availability, unplanned shutdown frequency, refractory lifetime, bypass-dust production, coating stability, chlorine–sulfur–alkali balance, fuel-quality variability, maintenance demand, and emissions compliance over extended operating periods [21,55,58,68].

Validation at very high TSR remains another important limitation. Although industrial and pilot studies demonstrate that moderate TSR can be achieved without major deterioration in clinker quality or emissions performance, evidence becomes more limited above approximately 50–70% TSR [21,55,58]. At these higher substitution levels, fuel heterogeneity, moisture content, particle-size variability, chlorine–sulfur–alkali circulation, ash chemistry, oxygen demand, and heat-release distribution become more difficult to control. Therefore, high-TSR claims should be supported by long-duration plant-scale data rather than short-term trials alone. Future studies should evaluate sustained operation at TSR levels of 50–70% or higher using standardized indicators such as kiln availability, specific heat consumption, burnout completeness, CO excursions, bypass-dust generation, coating stability, clinker-phase composition, cement strength, regulated emissions, and maintenance frequency.

3.3.3. Regional, Regulatory, and Methodological Transferability

A further limitation is the regional concentration of available evidence. Many plant trials, LCA studies, and policy-oriented assessments are derived from Europe and selected national contexts such as Egypt, Turkey, India, China, Ethiopia, and South Africa [20,24,95,100,105,114]. These studies provide useful evidence, but their results are not directly transferable across all cement-producing regions because alternative-fuel performance depends on local waste composition, fuel-preparation infrastructure, kiln technology, fuel prices, transport distances, landfill practices, regulatory limits, carbon-pricing mechanisms, and emissions-control requirements. Consequently, reported TSR values, GWP reductions, pollutant responses, and techno-economic outcomes should be interpreted as context-dependent rather than universal. Future research should expand the geographical coverage of plant-scale datasets and apply harmonized reporting protocols for fuel properties, kiln operating conditions, emissions, LCA boundaries, and economic assumptions to improve cross-regional comparability.

Regulatory comparability is another important limitation. The European BAT framework provides a useful reference for emission control, waste co-processing, thermal treatment requirements, and monitoring practice; however, it should not be interpreted as a universal regulatory condition for all cement-producing regions. In developing and emerging markets, alternative-fuel deployment is shaped by national emission limits, waste-classification rules, permitting systems, landfill policy, continuous-emissions-monitoring capacity, and enforcement strength. Differences in waste segregation, RDF/SRF certification, fuel-quality assurance, and institutional monitoring capacity can strongly influence whether the same alternative fuel can be used safely and consistently across countries. Therefore, future studies should report the regulatory context under which co-processing data are generated, including applicable emission limits, monitoring requirements, waste-acceptance criteria, and enforcement mechanisms. Such reporting would improve the transferability of results beyond European BAT-based systems and support more robust comparisons across industrial and regulatory contexts.

3.3.4. Integration with Emerging Low-Carbon Kiln Technologies

Finally, as cement plants increasingly adopt carbon-capture technologies, oxy-fuel calcination, electrified heating, raw-mix modification, and low-clinker binders, future assessments should evaluate how these strategies interact with alternative-fuel co-processing. This is particularly important under high-TSR operation, where fuel chemistry, kiln atmosphere, chlorine–sulfur–alkali circulation, and flue-gas composition may influence capture efficiency, clinker quality, emissions control, and overall LCA performance.

4. Energy Characteristics and Efficiency

Efficient cement manufacturing requires that alternative fuels deliver sufficient thermal energy while maintaining a stable kiln profile. Modern dry-process kilns consume, on average, about 3.6 GJ of thermal energy per ton of clinker and 100 kWh of electricity per ton of cement [4,6,9]. In 2022, fossil fuels still provided roughly 90% of the sector’s thermal energy, and the average emissions intensity of cement remained around 0.58 t CO₂ per ton of product [30]. The IEA net-zero scenario demands reducing these to below 3.4 GJ/t clinker and 90 kWh/t cement by 2030, underscoring that improvements in kiln efficiency and fuel substitution are both necessary for deep decarbonization [30]. A recent study estimated that replacing 1 kg of coal (approximately 26 MJ/kg) needs about 1.5 kg of RDF (approximately 17 MJ/kg) to supply the same energy [21,55].

4.1. Fuel Calorific Value and Mass Flow

Fuel energy density and moisture content determine the mass of fuel required to meet a given heat duty. RDF typically exhibits a net calorific value of 8–15 MJ/kg, a moisture content of 25–40%, and a particle-size distribution up to 400 mm [63]. SRF produced by tighter sorting and drying achieves 15 MJ/kg and 15% moisture. In contrast, many biomass residues, such as wood chips and agricultural waste, have calorific values of 12–20 MJ/kg and moisture contents of 10–60%, whereas [55,115], TDF is highly energy-dense at 26–33 MJ/kg [21,55,64]. Lower-energy fuels require proportionally higher mass flow to supply the same heat, increasing flue-gas volume and potentially altering flame shape and residence time. Higher-moisture fuels further reduce effective heating value because part of the energy is consumed in vaporizing water [63]. TDF provides a high calorific value comparable to traditional coal, enabling substantial substitution in clinker production. Still, its elevated sulfur and zinc contents require careful control of kiln chemistry and emissions management to maintain clinker quality and environmental performance [64,116].

4.2. Thermal Substitution Rate and Co-Processing Maturity

The TSR measures the share of kiln thermal energy provided by alternative fuels. However, TSR differs markedly across regions because it is shaped not only by fuel availability but also by kiln configuration, waste-management infrastructure, fuel preparation, policy support, and process-control maturity.

Table 6. Comparative alternative-fuel deployment and substitution potential in selected cement markets.

Region	Cement-Production Relevance	TSR (%)	Potential Rate (%)	Long-Term Rate (%)	Key Interpretation	Reference
EU	A minor share of global cement output, but the most mature co-processing region	56	65–80	80–90	Most mature co-processing benchmark, supported by established waste-supply chains and strong regulatory frameworks	[27,28,117,118]
US	Major industrial producer with advanced decarbonization policy activity	16	-	-	Demonstrates growing but still moderate substitution relative to leading EU practice	[30]
China	Largest global cement producer	<2	-	-	Highest potential global decarbonization leverage, but routine co-processing remains limited.	[30,33]
India	Second major global anchor	2.5	19	25	Large future upside for emissions reduction if co-processing expands at scale	[112,113,119]
Egypt	Useful non-EU comparison with published sector-level estimates for current and future substitution	6.4	20	30	Illustrates practical adoption outside Europe under developing-market conditions	[100,120]

presents comparative data for plants in selected countries and regions, showing actual substitution rates of around 56% in the EU, 16% in the US, less than 2% in China, 2.5% in India, and 6.4% in Egypt. The EU remains the most mature benchmark for co-processing, with current TSR around 56% and reported long-term levels in advanced systems reaching 65–90% [27,28,117,118].

This performance reflects established waste-supply chains, strong regulatory support, and widespread operational experience with multi-fuel firing. The US represents an intermediate case (16%), indicating meaningful progress but still substantially lower substitution than in Europe [30]. Outside Europe, substitution rates remain modest: the cement sector in Canada uses a mix of scrap tires (37%), used oil (15%), waste solvents (16%), and other wastes, amounting to only a small share of total thermal input [121]. India and China, which account for more than half of global cement production, have TSR values below 10% [63], although India has published sectoral estimates indicating potential growth of about 19% in the medium term and 25% in the longer term [33,112]. Therefore, scaling up TSR in high-output regions remains a critical challenge.

Beyond thermo-energetic and environmental performance, alternative fuels can also improve fuel-supply resilience by reducing dependence on imported coal and petcoke, particularly where locally available biomass, RDF, or TDF streams are well developed. However, this advantage remains conditional on the availability of fuel-preparation infrastructure, quality control, and plant-specific capacity to accommodate variable waste-derived fuels. Accordingly, the energy value of alternative fuels depends on their ability to provide stable heat release within kiln operating limits, rather than solely on the thermal substitution rate.

4.3. Techno-Economic Considerations of Alternative-Fuel Co-Processing

The techno-economic viability of alternative-fuel co-processing depends on the balance between avoided fossil-fuel costs and the additional costs of preparing, transporting, storing, feeding, and controlling heterogeneous fuels [58,122]. Recent studies emphasize that RDF, SRF, TDF, and biomass can reduce dependence on and expenditures for coal and petroleum coke when locally available waste-derived fuels are supplied at competitive costs, or when gate fees and waste-diversion incentives improve the economic case and when fuel quality is sufficiently controlled to ensure stable kiln operation [14,48,60,91,123]. However, these economic benefits are conditional on fuel quality and plant readiness rather than automatic. Waste-derived fuels often require sorting, shredding, drying, blending, quality assurance, covered storage, fire-safety systems, dedicated conveyors, dosing equipment, and modifications to burner or calciner feed systems before they can be used reliably in cement kilns [12]. At higher TSR, additional capital investment may also be required for multi-fuel burners, improved metering systems, chlorine bypass capacity, enhanced emissions monitoring, oxygen enrichment, and advanced process-control systems.

Operating and maintenance costs are also fuel-specific. High-moisture RDF or biomass can increase fuel mass flow, fan load, drying demand, and heat losses, whereas chlorine, sulfur, alkalis, and trace metals can intensify deposit formation, coating instability, corrosion risk, bypass-dust generation, and maintenance frequency [57,124,125]. TDF has a high calorific value and can provide stable thermal input, but sulfur, zinc, and steel residues require careful control to protect clinker chemistry, equipment reliability, and emissions performance [126]. Recent optimization-based work further shows that alternative-fuel blending should be evaluated alongside raw-mix chemistry and operational constraints, as cost reduction and CO₂ mitigation depend simultaneously on fuel price, heat value, clinker-quality constraints, and emissions limits [13]. Similarly, thermally dried sewage sludge can partially substitute for fossil fuels, but its economic and operational benefits depend on drying energy, sludge composition, raw-meal compatibility, and the ability to maintain clinker quality during co-processing [127]. Therefore, the economic performance of alternative fuels is governed not only by purchase price but also by net usable energy, pre-processing intensity, transport distance, kiln compatibility, process stability, maintenance demand, and quality-control requirements.

At the plant level, profitability improves when alternative fuels displace expensive fossil fuels without reducing kiln availability, clinker output, or cement quality. Case-study evidence on RDF production and utilization shows that economic benefits are strongest when cement-kiln co-processing is integrated with municipal waste-management systems, because landfill diversion, waste-to-energy recovery, and fossil-fuel displacement can be considered together [123]. Similarly, industrial co-combustion evidence also indicates that RDF–biomass mixtures can support stable clinker production when fuel blending, combustion control, and process monitoring are properly managed [114]. Previous studies on cement-sector energy efficiency show that fuel and process-energy savings are economically meaningful only when operational reliability is maintained and when process modifications do not introduce excessive maintenance or control costs [9,10]. Nevertheless, economic benefits may be reduced where fuel supply is inconsistent, transport distances are long, moisture content is high, or additional abatement systems are required. Consequently, techno-economic assessment should be integrated with thermo-energetic and environmental evaluation when comparing RDF, SRF, TDF, and biomass for cement clinker production.

5. Methodological Approaches Used in the Literature

Evaluating alternative fuels in cement production requires a combination of experimental observation and modeling to capture both combustion dynamics and system-level impacts. Four primary methodological streams are evident in the research literature: plant-scale trial runs, pilot- and laboratory-scale experiments, process simulation and CFD, and life-cycle assessment (LCA). Each of these approaches provides complementary information on fuel performance, energy efficiency, pollutant formation and environmental trade-offs.

5.1. Plant-Scale Trials and Industrial Case Studies

The most direct evidence on fuel substitution comes from industrial demonstrations in operating kilns. Full-scale kiln trials are widely used to evaluate alternative fuels by progressively increasing substitution rates while monitoring operating performance. They provide the most direct approach for confirming that a new fuel can be co-fired safely without compromising process stability, clinker quality, or overall product performance. Industrial demonstrations typically begin with a low share of alternative fuel, e.g., 5% of thermal input and increase the substitution level in stages while monitoring kiln stability, fuel consumption, emissions, and clinker mineralogy [78].

Rivera Sasso et al. [78] reported that industrial case studies typically employ a step-by-step substitution methodology, introducing alternative-fuel adoption at approximately 5% RDF content and progressively increasing to 15–20% or higher, with systematic monitoring of kiln stability, emission profiles, specific heat consumption, and clinker quality at each operational stage. Some plants conduct A/B comparisons under the same raw mix and similar conditions by operating a baseline period on 100% coal and a comparable period under coal–alternative-fuel co-firing. Typical measurements include fuel feed rate, flame temperature monitored with optical/infrared pyrometry, kiln shell temperature profiles via infrared scanning, fan power demand, and clinker chemistry and strength testing to detect changes in mineralogy and cement performance [21,76]. Hashem et al. [128] documented industrial trials conducted in Egypt using rubber and plastic waste as alternative fuels, demonstrating CO₂ emission reductions while maintaining clinker quality and process stability, facilitated by optimized fuel-injection configurations. Similarly, Kukreja et al. [76] reported that the National Council for Cement and Building Materials in India conducted pilot-scale trials of co-firing biomass fuels, including rice husk and bamboo chips, at TSR of 10–15%, accompanied by comprehensive process and emission monitoring. These studies are resource-intensive and site-specific, so extrapolation requires caution.

Reza et al. [123] evaluated the utilization of RDF in cement kilns through a case study integrated with the Metro Vancouver municipal solid waste (MSW) management system. Their analysis demonstrated that RDF derived from MSW could successfully displace a portion of fossil-fuel demand in cement kilns while delivering concurrent environmental and economic benefits, including reduced landfill utilization and enhanced waste-to-energy recovery efficiency. The authors reported that the tested RDF exhibited an LHV of 17.8 MJ/kg, indicating its viability as a supplementary energy source. Sai Kishan et al. [129] conducted industrial-scale trials at a Greek cement plant employing TDF as a partial substitute for coal. Their results demonstrated that TDF supports stable combustion and high thermal efficiency due to its high calorific value and favorable combustion properties. They reported that no statistically significant changes in clinker mineralogy or final cement quality occurred when substitution levels remained within established operational parameters, thereby confirming the technical suitability of waste tires as a supplemental fuel in rotary-kiln systems.

Kara [91] performed industrial-scale experiments evaluating RDF co-firing with petroleum coke at substitution rates of 8%, 12%, and 15%. The study found that emissions of NO_x, SO₂, CO, and PM remained below regulatory thresholds prescribed by environmental directives and within industry-accepted benchmarks. These results indicate that controlled RDF co-firing can be implemented without compromising environmental compliance or process stability, provided that fuel quality and injection parameters are rigorously managed. Kahawalage et al. [58] conducted a comprehensive review of plant experience with SRF co-processing in cement kilns, concluding that fuel characteristics, pre-processing methodologies, injection location, and kiln operating conditions collectively govern process performance and emissions. The study emphasized that achieving stable TSR while maintaining environmental compliance requires rigorous control over combustion behavior, ash chemistry, and holistic process integration.

Nakomcic-Smaragdakis et al. [126] investigated the trial implementation of TDF as an alternative fuel in an industrial cement plant. The authors reported plant-scale measurements across a range of 0–15% tire-derived energy contribution to total heat production. Their results indicated that clinker mineralogical and physical characteristics remained statistically unchanged relative to baseline operations, while NO_x and SO₂ emissions exhibited modest reductions under TDF co-firing conditions. These results confirm the technical feasibility of controlled TDF co-processing under representative industrial operating conditions, provided that fuel quality and injection protocols are appropriately managed. Plant-scale trials are expensive and site-specific, but they provide real-world data on ignition delay, flame shape, heat release, pollutant formation, and process control, and they often reveal practical issues such as feed-system blockages or deposition tendencies that are not apparent in small-scale studies.

5.2. Pilot- and Laboratory-Scale Combustion Experiments

Pilot- and laboratory-scale combustion studies complement full-scale kiln trials by enabling the systematic evaluation of alternative fuels under controlled, reproducible conditions. Laboratory techniques, including thermogravimetric analysis (TGA) and drop-tube furnace testing, are particularly useful for quantifying devolatilization behavior, ignition delay, burnout characteristics, and temperature-dependent kinetic parameters under high-temperature, short-residence-time conditions relevant to cement processing [78]. Pilot rotary kilns, typically several meters in length, provide an intermediate platform between bench-scale experiments and industrial application by allowing controlled adjustment of fuel feed rate, temperature profile, and solids’ residence time, while also permitting direct sampling of reacting materials, which is generally not possible in commercial kilns.

The interpretation of pilot- and laboratory-scale combustion results requires careful specification of operating conditions. For example, the observation that conduction from the meal bed can dominate heat transfer is most applicable to coarse solid alternative-fuel particles introduced near the kiln inlet or into the material bed, where the fuel is in direct contact with hot calcined meal or clinker-forming solids. Under these conditions, particle heating, drying, and devolatilization are governed by bed temperature, particle size, local solids mixing, and residence time. However, this mechanism should not be generalized to all fuel-feeding configurations. In calciner firing or main-burner injection, convective heating, turbulent gas–solid mixing, radiative heat transfer, oxygen availability, and flame aerodynamics become more important. Therefore, conclusions from pilot- and laboratory-scale combustion studies should not be generalized directly to industrial preheater–precalciner kilns. Their transferability depends on feeding location, temperature field, gas-flow regime, particle-size distribution, residence time, thermal substitution rate, and kiln configuration [21].

Husillos Rodríguez et al. [127] investigated thermally dried sewage sludge and concluded that it was technically viable and could replace up to 14% of raw materials without altering raw-meal moduli, thereby reducing fossil-fuel consumption by approximately 70%. The sludge exhibited a calorific value of 8293 J/g (1990 kcal/kg), and the experimental design preserved key raw-meal moduli during clinkering.

Wang et al. [130] found that 10–20 wt. % biomass substitution improved combustion rates by 0.52–2.28% and reduced ignition temperature by up to 56 °C. However, under strongly alkaline ash-simulated conditions, clinker mineral formation shifted unfavorably, including the suppression of C₃S formation and an increase in free CaO. These findings indicate that ash chemistry remains a critical control variable, particularly at higher substitution rates. Nakhaei et al. [131] investigated petcoke and SRF co-firing in a full-scale cement calciner using a computational particle-fluid dynamics (CPFD) model, validated against plant measurements of gas temperature and O₂/CO₂ concentration at multiple sampling locations and across the calciner cross-section. Their analysis demonstrated that calciner performance was strongly governed by fuel distribution, gas–solid mixing, and flow development, with direct implications for fuel burnout and calcination efficiency.

Abu-Elyazeed et al. [114] reported an Egyptian clinker-production case study in which an RDF–biomass mixture was co-combusted with bituminous coal. Their results showed that partial substitution with alternative fuels could be achieved under industrial operating conditions while maintaining stable clinker production, provided that fuel blending, combustion control, and process monitoring are properly managed. Wojtacha-Rychter and Smoliński [73] conducted a multi-case assessment of cement-kiln operating conditions. They showed that substituting coal with RDF and sewage sludge could reduce both fossil CO₂ emissions and operating costs. Their results indicated that replacing up to 90% of coal with RDF could save as much as 28.6 Mg/h of coal, while increasing the share of sewage sludge in total heat consumption by 6% reduced emissions by 17 kg CO₂ per Mg of clinker. Hercog et al. [132] conducted co-firing experiments in a 1 MW pilot facility using petcoke in combination with RDF, biomass, and hydrogen. Their results showed that high proportions of alternative fuels could be combusted without adverse effects on overall combustion performance. They further concluded that, under the investigated burner conditions, the conventional base fuel could be almost entirely replaced by RDF or biomass.

In general, pilot- and laboratory-scale combustion experiments provide critical mechanistic understanding of ignition, burnout, ash transformation, volatile release, pollutant formation, and clinker-phase development. These studies are indispensable for screening candidate fuels, identifying process constraints, and defining suitable operating windows before industrial implementation. Nevertheless, laboratory and pilot experiments are invaluable for identifying governing mechanisms, such as heat-transfer mode, devolatilization kinetics, and ash-related mineral transformations, and for screening fuels or fuel blends before committing to plant-scale tests. Their results also inform process-simulation models by providing kinetic parameters and baseline data for validation.

5.3. Process Simulation and Computational Fluid Dynamics

Process simulation and CFD have become essential tools for evaluating cement-kiln performance under alternative-fuel firing, complementing laboratory, pilot-scale, and full-scale investigations. These approaches range from high-fidelity CFD models to simplified heat- and mass-balance frameworks capable of representing combustion, gas–solid flow, and heat transfer in burner, calciner, and rotary-kiln systems [133,134,135,136]. CFD resolves burner combustion, particle trajectories, and radiative heat transfer, clarifying impacts on kiln aerodynamics and thermal fields [137,138]. Integrated process models utilize platforms such as ASPEN Plus, ChemApp, and in-house codes [40,79,139,140,141]. Rahman et al. [142] developed a simulation incorporating fuel kinetics and thermodynamics to predict flame temperature and clinker-phase development, quantifying effects such as reduced peak flame temperature from moist RDF.

Consequently, process simulation is now routinely applied before major fuel substitutions to anticipate operational risks and to design targeted mitigation measures. Alcaide-Moreno and Castán-Lascorz [135] developed a hybrid CFD–one-dimensional framework for a 64 m kiln to solve coupled gas and clinker phases, balancing computational efficiency with physical robustness to improve representation of fuel-particle behavior. This hybrid formulation improved the representation of fuel-particle behavior and internal kiln dynamics relative to conventional one-dimensional models, while remaining computationally efficient. Advanced modeling facilitates complex scenario analysis where physical measurements are difficult. CFD evaluates calciner efficiency and pollutant emissions [143], while full-plant models integrate carbon capture and oxy-combustion to assess the impacts of flue-gas recirculation and power-to-gas conversion [141]. Specific applications include regression-based surrogates for rapid prediction in 4200 t/day plants [40] and coupled shrink-core calcination frameworks for precalciner residence time [137].

Sensitivity analyses quantify risks, such as the intensification of chloride cycling from high-chlorine RDF, and identify kiln bypass thresholds. Energy-balance models estimate changes in specific energy consumption due to fuel moisture and hydrogen content. Since predictions depend on input data, models must be calibrated against plant measurements to ensure accuracy. Ultimately, process simulation and CFD are standard pre-implementation tools that enable engineers to anticipate operational challenges, optimize kiln settings, and mitigate risks during large-scale adoption of alternative fuels.

5.4. Life-Cycle Assessment

Life-cycle assessment provides a system-wide evaluation of the environmental implications of alternative fuels, considering upstream fuel production, transportation, kiln operation, and downstream waste management. Cradle-to-gate LCAs often compare a coal-only baseline with scenarios in which RDF, TDF, or biomass provides a fraction of thermal input. Recent LCAs have compared baseline coal or petroleum coke operations with scenarios incorporating RDF, TDF, or biomass [4,20,94,123,144]. Panahandeh et al. [94] conducted an LCA of clinker production in Tehran using natural gas, natural gas–mazut, and natural gas–RDF blends; co-firing RDF (5–30% heat input) reduced acidification by 2.14–11.5% and GWP by 0–1.3% relative to natural gas alone, with greater reductions versus the mazut scenario. Khan et al. [4] found that increasing SRF substitution reduced GWP from 1036 to 832 kg CO₂-eq per functional unit, with a projected 725 kg CO₂-eq at 80% substitution, demonstrating emission reductions across both cement production and waste treatment.

LCA requires detailed inventory data across multiple stages, including emissions from waste collection, RDF pre-processing (shredding, sorting), and transport. By aggregating these contributions, LCA provides a system-wide assessment that may reveal net environmental credits. For example, co-processing waste tires in cement kilns can avoid emissions associated with dedicated tire incineration, with these avoided burdens credited to the cement system. Across the literature, most LCAs conclude that alternative fuels provide net environmental benefits, particularly for climate-change mitigation and mitigating fossil resource depletion [4,20,105]. However, some studies report impact shifting: while GHG emissions decline, categories such as human toxicity may show limited improvement or slight increases due to emissions from fuel pre-processing or trace-metal handling in kiln dust [94,145]. To enhance comparability and robustness, recent research increasingly follows standardized sectoral guidance from the IPCC and the World Business Council for Sustainable Development [146,147], which provides harmonized frameworks for cement-sector LCAs.

5.5. Limitations and Cross-Study Comparability

Each methodological approach has limitations. Plant trials provide invaluable real-world evidence but are expensive, site-specific and often not publicly documented. Pilot and laboratory experiments provide high-resolution kinetic and thermochemical data, but may not capture the complex interplay among heat transfer, volatile circulation, and clinker mineral formation observed in industrial kilns. Simulation models can evaluate hypothetical scenarios and inform burner design, but their accuracy depends on the fidelity of input parameters and on their validation against measured data. LCA results depend strongly on the selected system boundary, inventory data and allocation method; differing assumptions about waste processing and biogenic carbon can lead to divergent conclusions across studies. Consequently, building a transferable evidence base on alternative-fuel performance will require coordinated research that integrates plant trials, pilot studies, validated simulations and harmonized life-cycle models.

6. Comparative Synthesis with Existing Literature

Figure 6 summarizes the process-based framework used in this review to compare alternative fuels across fuel properties, kiln operation, clinker quality, emissions, and life-cycle or techno-economic dimensions.

The framework links alternative-fuel properties to kiln-zone suitability, process stability, clinker quality, emissions performance, and LCA/TEA outcomes. It summarizes the main contribution of this review by showing that RDF, SRF, TDF, and biomass should be evaluated as process-specific and quality-dependent co-processing options rather than as uniform substitutes for coal or petcoke. As shown in Figure 5, the reviewed evidence indicates that alternative-fuel performance depends on the sequential interaction between fuel properties, kiln-zone suitability, process stability, clinker quality, emissions control, and life-cycle or techno-economic outcomes. This framework provides the basis for comparing the findings of this review with existing process-focused, LCA-focused, and regional case-study literature.

The results of this review are consistent with the existing literature, which shows that RDF, SRF, TDF, and biomass can reduce fossil-fuel demand and improve selected environmental indicators when fuel quality, kiln stability, and emissions control are properly managed [12,20,21,58,107,148]. Earlier studies have demonstrated that alternative fuels can be co-processed without substantial deterioration in clinker quality or regulated emissions under controlled operating conditions [21]. The present review confirms these conclusions but also shows that reported benefits are strongly conditional on fuel composition, moisture content, particle-size distribution, feeding point, kiln configuration, thermal substitution rate, and abatement performance [58,122]. Compared with LCA-focused studies, this review confirms that reported reductions in GWP are not directly transferable across plants or regions. Georgiopoulou and Lyberatos [20], Çankaya and Pekey [24], and Ige and Kabeya [105] showed that RDF/SRF substitution can reduce fossil-fuel demand and climate-change impacts, but the magnitude of reduction depends on system boundary assumptions, avoided landfill or incineration credits, baseline fuel, fuel-preparation energy, transport distance, and treatment of biogenic and fossil carbon fractions [149]. Therefore, the same substitution rate can produce different environmental outcomes depending on whether the assessment applies a narrow cradle-to-gate cement-production boundary or a broader waste-management boundary.

Compared with process-focused literature, the present study further emphasizes that alternative-fuel evaluation is a kiln-zone-specific suitability problem rather than a direct calorific replacement of coal or petcoke [49]. Fuels suitable for calciner firing may not necessarily meet main-burner requirements, as main-burner combustion requires rapid ignition, high flame intensity, and stable heat release for clinker mineral formation. This distinction is particularly important for low-calorific or high-moisture fuels, where pre-processing, homogenization, and feeding-point selection determine practical performance [48,62].

Generally, this review extends the existing literature by integrating fuel-property variability, thermo-energetic kiln-zone requirements, clinker-quality implications, emissions behavior, LCA boundary assumptions, techno-economic considerations, and regional transferability into a single process-based framework. This synthesis indicates that RDF, SRF, TDF, and biomass should be assessed as quality-dependent co-processing options rather than uniform substitutes for fossil fuels.

7. Key Research Gaps and Strategic Future Directions

Despite significant progress in both research and industrial practice, several knowledge gaps and uncertainties remain in the deployment of alternative fuels in cement production. Addressing these gaps is essential to further optimize the use of RDF, TDF, and biomass, ensuring their long-term sustainability and technical, environmental, and social acceptance.

7.1. Long-Term and High-Substitution Effects

Many studies examine alternative fuels at moderate substitution TSRs of 10–40% and over short trial periods, but evidence is limited for sustained operation at very high TSRs of 80% over multiple years. Key gaps include the long-term effects on kiln lining longevity, maintenance cycles, and gradual system contamination. Although available reports often indicate stable high-TSR operation, longitudinal datasets remain scarce. Addressing this requires plant-level studies at facilities with sustained high alternative-fuel use, integrating operational, chemical, and maintenance records. Refractory fatigue under modified flame conditions, elongated flames, and localized hotspots from heterogeneous combustion also warrant investigation. Accelerated laboratory exposure of refractory samples to alternative-fuel ash can simulate multi-year conditions and clarify degradation and corrosion mechanisms, which remain understudied for waste-derived fuels.

7.2. Fuel Quality and Pre-Processing Optimization

Another gap lies in improving the quality and consistency of RDF and biomass fuels supplied to cement plants. Priority measures include advanced sorting to remove chlorine-bearing plastics and metals, optimized shredding to narrow particle-size distributions for stable pneumatic conveying, and densification routes that convert heterogeneous RDF into specification-grade SRF pellets. These upgrades would improve combustion predictability and reduce operational disturbances. Although AI-enabled automated sorting using vision systems is emerging in industry [150,151], further research is required to optimize performance, quantify removal efficiency, and evaluate impacts on combustion stability, kiln operation, and overall fuel performance.

Integrating real-time fuel data into kiln control would allow dynamic adjustment of fuel rates and combustion settings, mitigating short-term variability and improving thermo-energetic performance. Feed-forward control strategies based on real-time fuel measurements have been proposed [152], but plant implementations and empirical evidence remain limited. A broader objective is to make RDF comparable to coal in handling and consistency, including via pelletization into uniform SRF. Further work should quantify how pelletization, including the use of binders or densification additives, alters combustion kinetics, burnout, ash chemistry, and emissions formation to support standardized, high-quality SRF products.

7.3. Co-Processing of Emerging Waste Types and Multi-Fuel Blends

While RDF, TDF, and biomass remain the primary focus of current research, a growing range of emerging waste-derived fuels, such as industrial sludges, meat and bone meal (MBM), refined waste oils, and various solvent mixtures, are increasingly being introduced or proposed for use in cement kilns [60]. In practice, these fuels are often co-fired rather than in isolation, e.g., combining RDF with sewage sludge and biomass, yet the scientific literature broadly evaluates each fuel type in isolation. This creates a notable research gap regarding the synergistic or antagonistic effects of co-firing multi-fuel blends under kiln conditions. Most existing literature assesses alternative fuels individually, whereas in industrial reality, a kiln could be feeding a fuel blend, for example, firing of RDF 15%, biomass 10% and waste oil 5%, which may raise important questions about how such mixtures influence flame structure, combustion stability and emissions. It is plausible that specific interactions may be beneficial; for instance, the high hydrogen content of biomass could enhance ignition and combustion of plastic-rich RDF fractions, while other blends might increase the risk of trace emissions through additive or catalytic effects of chlorine, alkalis, sulfur, and metals.

7.4. Integration with Carbon Capture and Emerging Clinker Technologies

As the cement sector advances toward net-zero targets, alternative fuels will likely be used alongside carbon capture and storage (CCS) technologies or novel low-carbon clinker chemistries. However, a substantial research gap remains regarding how alternative fuels influence these emerging process configurations and whether they introduce new constraints or opportunities. The combustion environment, heat-transfer dynamics, and emissions chemistry in CCS-enabled kilns may differ significantly from those in conventional systems. For example, oxy-fuel combustion, one of the leading CCS technologies, in which the kiln is fired with nearly pure oxygen to generate a CO₂-rich exhaust suitable for capture, can alter ignition behavior, burnout rates, and heat-release profiles compared with air-firing [108]. Alternative fuels could alter ignition behavior, burnout rates, and heat-release profiles compared with air-firing [153]. Early indications suggest that higher oxy-fuel conditions may produce higher flame temperatures, potentially improving the burnout of difficult-to-combust fractions such as plastics or high-ash biomass. At the same time, the shift to some oxygen-rich atmosphere may modify reaction pathways for pollutants and other trace species, and partitioning may also change, potentially affecting NO_x chemistry, sulfur and chlorine cycling and the distribution of metals between clinker and dust. Systematic experimental and modeling studies are therefore needed to define operating envelopes, emission outcomes, and material-quality impacts when alternative fuels are combined with CCS-enabled firing modes and novel clinker systems. Similarly, the adoption of calcium looping carbon-capture systems introduces additional uncertainties regarding the interaction between waste-derived fuels and sorbent behavior. Elements commonly present in alternative fuels, such as alkalis, chlorine, sulfur, and heavy metals, can influence sorbent reactivity, alter sintering behavior, or alter carbonation–calcination kinetics.

Ammonia (NH₃) is emerging as a potential long-term fuel option for cement decarbonization strategies. As a carbon-free fuel at the point of combustion and a hydrogen carrier, NH₃ is attracting increasing interest as a means of reducing fossil thermal energy demand in hard-to-abate industries. In cement production, its relevance is likely to be greatest in integrated decarbonization pathways pursued alongside carbon capture, oxy-fuel combustion, and emerging low-clinker technologies. Cement-specific evidence indicates that NH₃ co-firing in rotary kilns is technically feasible. However, kiln response, clinker quality, and NO_x emissions remain highly sensitive to co-firing conditions, with substitution above about 30% not recommended under the investigated conditions [154]. More broadly, practical deployment is constrained by low flame speed, narrow flammability limits, difficulty in ignition, flame-stability concerns, and a strong tendency to form NO_x, indicating that staged combustion, blending, burner redesign, or partial cracking may be required [155,156,157]. Cross-sector studies further support NH₃ strategic relevance as a zero-carbon energy vector for high-temperature systems, although implementation remains dependent on combustion control and heat-integration design [158].

8. Conclusions

This review assessed the thermo-energetic, operational, environmental, life-cycle, and techno-economic implications of using RDF, SRF, TDF, and biomass as alternative fuels in cement clinker production. The review synthesized evidence from plant-scale trials, pilot- and laboratory-scale combustion experiments, process-simulation and CFD studies, emissions assessments, LCA studies, techno-economic analyses, and regional case studies. This methodological basis allowed the review to compare alternative fuels not only by calorific value, but also by kiln-zone suitability, combustion behavior, clinker-quality effects, emissions performance, and life-cycle outcomes.

The results show that the interaction between fuel properties and kiln-process requirements governs the suitability of alternative fuels. RDF, SRF, TDF, and biomass differ substantially in lower heating value, moisture content, particle-size distribution, volatile release, ash chemistry, chlorine, sulfur, alkalis, and trace-metal content. Fuels with low-calorific value, high-moisture content, or coarse particle size require pre-processing, homogenization, controlled dosing, and careful selection of the injection point to avoid incomplete burnout, unstable heat release, CO excursions, deposit formation, and clinker-quality variation. Calciner firing is generally more tolerant of medium-LHV and coarser fuels, whereas main-burner firing requires faster ignition, higher flame intensity, and stable heat release to sustain clinker mineral formation.

Process evidence indicates that alternative fuels can be co-processed without significant deterioration in clinker quality when fuel specifications, ash chemistry, feeding systems, and kiln control are properly managed. However, the review also shows that waste-derived fuels should not be treated as uniform substitutes for coal or petcoke. Their variable composition can affect raw-mix chemistry, volatile-species circulation, coating stability, refractory wear, clinker mineralogy, and cement strength development. TDF provides high-energy density, but sulfur, zinc, and steel residues require careful control. RDF and SRF offer useful waste-recovery and fossil-fuel-displacement benefits, but their performance depends heavily on sorting, drying, blending, chlorine control, and stable feeding. Biomass can reduce fossil CO₂ emissions, but its benefit depends on moisture content, feedstock type, drying energy, and supply-chain conditions.

The environmental evidence confirms that regulated emissions can remain within accepted limits under stable kiln operation and effective abatement. Modern dust-control systems can maintain particulate matter at approximately 10–30 mg Nm⁻³, while PCDD/F emissions in well-controlled kilns are generally below 0.1 ng TEQ Nm⁻³. Nevertheless, volatile metals, chlorine–sulfur–alkali circulation, PCDD/F formation, and trace element partitioning require continued monitoring, particularly under high-TSR and multi-fuel operation. The review also identifies health-impact assessment as an underdeveloped area because many studies report stack-emission compliance rather than exposure-based health-risk indicators.

The LCA synthesis shows that alternative-fuel substitution can reduce fossil-fuel demand and GWP, but the magnitude of the reductions depends strongly on methodological assumptions. For example, 20% RDF thermal substitution in South African and Ethiopian cement production reduced GWP by approximately 3.3–4.2%, while the reduction increased to about 6.7% when avoided landfill methane emissions were included. Other studies reported larger reductions at higher substitution rates, with different baseline fuels, or with broader waste-management boundaries. These findings demonstrate that LCA outcomes depend on system boundary, avoided-waste credits, fuel-processing energy, transport distance, substitution rate, biogenic and fossil carbon allocation, and regional waste-management conditions.

The techno-economic synthesis indicates that alternative-fuel co-processing can reduce fossil-fuel expenditure and improve fuel-supply resilience when locally available waste-derived fuels are supplied at competitive cost or supported by gate fees and waste-diversion incentives. However, economic feasibility depends on fuel quality, pre-processing requirements, transport distance, storage systems, dosing equipment, burner or calciner-feeding modifications, emissions monitoring, maintenance requirements, chlorine bypass needs, kiln availability, and clinker-quality preservation. Therefore, TEA should be integrated with thermo-energetic and LCA when comparing RDF, SRF, TDF, and biomass for cement production.

Several research gaps remain. Long-duration plant-scale datasets are still limited, especially above 50–70% TSR. Future studies should quantify kiln availability, refractory lifetime, coating stability, bypass-dust generation, chlorine–sulfur–alkali balance, burnout completeness, clinker-phase composition, cement strength, regulated emissions, and maintenance frequency under sustained high-TSR operation. Further work is also required on trace element fate, high-frequency monitoring of mercury, cadmium, thallium, and PCDD/F, exposure-based health-risk assessment, regulatory comparability outside Europe, and regional transferability of LCA and TEA results. Harmonized reporting of fuel properties, kiln operating conditions, emissions data, LCA boundaries, and economic assumptions is necessary for meaningful comparison across studies.

Future assessments should evaluate how alternative-fuel co-processing interacts with emerging low-carbon kiln technologies, including oxy-fuel calcination, electrified heating, raw-mix modification, low-clinker binders, ammonia co-firing, hydrogen, and carbon capture. This is particularly important under high-TSR operation, where fuel chemistry, kiln atmosphere, flue-gas composition, and volatile-species circulation may affect capture efficiency, clinker quality, emissions control, and overall environmental performance. In general, this review confirms that RDF, SRF, TDF, and biomass can provide technical and environmental benefits within defined kiln-operating limits; however, their role in emissions mitigation is complementary to clinker substitution, energy-efficiency improvement, and carbon capture rather than sufficient as an isolated decarbonization pathway. This process-based interpretation provides a stronger basis for evaluating alternative fuels in sustainable cement clinker production.