Perspectives on Cleaner-Pulverized Coal Combustion: The Evolving Role of Combustion Modifiers and Biomass Co-Firing

Abstract

The article presents an extensive review of modern technological solutions for pulverized coal combustion, with emphasis on combustion modifiers and biomass co-firing. It highlights the role of coal in the national energy system and the need for its sustainable use in the context of energy transition. The pulverized coal combustion process is described, along with factors influencing its efficiency, and a classification of modifiers that improve combustion parameters. Both natural and synthetic modifiers are analyzed, including their mechanisms of action, application examples, and catalytic effects. Special attention is given to the synergy between transition metal compounds (Fe, Cu, Mn, Ce) and alkaline earth oxides (Ca, Mg), which enhances energy efficiency, flame stability, and reduces emissions of CO, SO₂, and NO_x. The article also examines biomass-coal co-firing as a technology supporting energy sector decarbonization. Co-firing reduces greenhouse gas emissions and increases the reactivity of fuel blends. The influence of biomass type, its share in the mixture, and processing methods on combustion parameters is discussed. Finally, the paper identifies directions for further technological development, including nanocomposite combustion modifiers and intelligent catalysts integrating sorption and redox functions. These innovations offer promising potential for improving energy efficiency and reducing the environmental impact of coal-fired power generation.

1. Introduction

The conscious and responsible use of coal, when using modern and appropriate technologies, will allow it to not only be treated as a foundation for development but also as a crucial element of industrial evolution—until more competitive energy sources are found or appropriately developed. The key direction remains the search for integrated solutions that combine economic needs with environmental protection [1]. In this study, Poland was used as an example of a representative coal-reliant country, in which coal still plays a crucial role in ensuring energy security and the stability of the power system. The aim was not to provide a detailed analysis of the energy policy of a single country, but rather to embed the technical issues discussed (pulverized coal combustion, combustion modifiers, and biomass co-firing) within a realistic system context that is characteristic of many coal-based economies worldwide. Beyond Poland, the most important country in which coal remains the primary energy source is China, where it accounts for a significant share of electricity generation and is fundamental to energy security and the operation of heavy industry. Other countries in which coal continues to play a major role include India, South Africa, Indonesia, Australia, and Kazakhstan, although their energy mix structures and the pace of energy transition differ considerably. Coal, which constitutes 64% of Poland’s domestic fossil fuel resources, plays a key role in ensuring energy security, and thus national security. It determines the country’s development, its sovereignty, and the quality of life of its citizens. According to statistics, at current mining levels, coal resources in Poland will suffice for 393 years of exploitation, making it a stable and long-term pillar of the energy sector. Currently, it accounts for 48% of primary energy, and as much as 80% of electricity production, which confirms its strategic importance [2]. However, energy security not only requires a reliance on coal, but also the diversification of raw material sources. It is also important to maintain the appropriate technical condition of infrastructure, and to invest in modern technological solutions. Energy security involves balancing primary energy demands with the processing possibilities of domestic energy capacities. Moreover, it also means that the sector’s reliability, which is understood as the ability to provide consumers with stable access to energy in the required quantity and quality, is ensured. At the same time, it is necessary to limit the negative impact on the natural environment by implementing technologies that support energy transformation and the gradual integration of more sustainable energy sources [3].

Pulverized coal plays a significant role in heavy industry and the energy industry due to its physicochemical properties and high calorific value [4]. Pulverizing the raw material allows for the efficient use of lower-quality coals, while simultaneously increasing boiler efficiency. Fine-grained coal particles have a significantly larger specific surface area than coarse coal, which in turn facilitates oxygen access and accelerates the combustion process. This technology ensures rapid and uniform fuel combustion and provides the foundation for modern power units [5]. The principle of pulverized coal combustion technology involves mixing finely ground coal with air in order to create a homogeneous mixture, which is then fed into the combustion chamber, where it burns quickly and efficiently [6]. The amount of primary air depends on the volatile matter content of the coal—for coals that are low in volatile matter, it is approximately 15% of the stoichiometric amount, while for coals that are rich in volatile matter, it can reach up to 25%. The remaining air, the so-called secondary air, is supplied directly to the burner. An increase in the primary air content hinders the ignition of the mixture in the furnace [5]. Lu et al. [7] studied the effect of pulverized coal fineness (50–200 μm) on, among other things, its flow characteristics in pipes. The results showed that the fineness of the ground coal had a significant impact on the flow pattern and its characteristics. Fine-grained pulverized coal tends to form a dense flow, while coarser-grained coal is more likely to form a dilute flow. The pulverization process, which is the stage of preparing pulverized coal for transport, is a complex issue. Its difficulty stems from both the need to consider the forces generated by the crushing equipment and the properties of the material itself. The heterogeneity of the raw material, the irregular grain shapes, the varying strength that results from varying degrees of cleavage, and the differences in the elasticity and the character of the surface (from smooth to rough) make this process non-uniform and difficult to predict [5,8]. A clear theory of material comminution has not yet been developed. Various approaches describe the process from slightly different perspectives. Rittinger’s theory assumes that the work required for comminution is proportional to the increase in the particle surface area. In turn, Kick’s theory focuses on work associated with deforming the entire material, while at the same time taking into account the volume and geometry of the grains. Bond’s theory is a combination of both approaches: initially—at the moment of crack formation—the work is proportional to the grain’s volume (as in Kick’s theory), and as comminution progresses, it is proportional to the surface area (as in Rittinger’s theory). Walker’s general formula integrates these approaches by taking into account both the effect of the particle’s diameter and volume on the workload in the crushing process [2,9,10]. Due to the fact that one of the basic goals of the energy industry is to ensure proper combustion quality, which in the case of pulverized coal furnaces is directly influenced by the degree of dust fineness, the selection of the grinding method and apparatus is an extremely important issue [2]. Among commercial solutions, three basic groups of mills can be distinguished: crushing mills (including ball-ring mills and roller-ring mills, both with a stationary and rolling roller), impact mills (in which the grinding process is mainly carried out by impacts), and impact-crushing mills—combining impacts and crushing [2,11]. Dust with a grain size of up to 200 µm is used in pulverized coal furnaces. The lower the volatile matter content in the coal, the finer the dust should be. This is because coal with lower reactivity requires greater fineness for efficient combustion. In practice, dust with a diameter of 88–200 µm is most commonly used, with the finest fraction being brown coal [5].

It is an undeniable fact that the combustion of pulverized coal contributes to serious environmental problems—polluting the air and negatively impacting human health and ecosystems [12,13]. It is necessary to seek solutions—technologies—that will reduce pollutant emissions and improve combustion dynamics [14,15]. One of the promising directions of development in the energy sector is the improvement of combustion processes through the use of various modifiers [16]. This includes the development of new compositions and formulation structures, as well as the search for effective methods for their implementation [17,18]. There are numerous literature reports confirming the great potential of hard coal, which can be understood as the ability to improve the efficiency of generated energy, and thus reduce greenhouse gas emissions [19,20,21,22].

Hard coal, like any other energy solution, has both advantages and limitations. Figure 1 presents a comparative analysis (conducted in paper [1]) of the suitability of the used fuels with regard to the adopted evaluation criteria. Among the greatest advantages of coal are its price and energy security. According to various forecasts and indicators, the price of coal can be expected to gradually increase. Despite this, this raw material will still remain one of the cheapest available energy sources [1]. Figure 2 presents a summary assessment of the compared energy sources.

Figure 1. The assessment of the future suitability of fuels for energy production in relation to selected criteria.

Figure 2. Assessment of the suitability of individual fuels in the energy sector.

The search for methods and technologies that will extend the presence of coal in the Polish energy sector is becoming a key challenge. Within the next 30 years, there is no realistic possibility of fully replacing this raw material with renewable energy sources. The alternative is an increased dependence on imports, which carries economic and geopolitical risks [1].

The main objective of the study is a comprehensive and comparative assessment of the effectiveness of various groups of modifiers used in the pulverized coal combustion process. This includes a quantitative evaluation of typical ranges of reduction in gaseous pollutant emissions, such as CO, NO_x, and SO_x, as well as an assessment of the impact of modifiers on combustion efficiency, fuel burnout degree, and the amount and properties of generated solid products, including ash. An important aspect of the study is also the identification of synergistic modifier systems that enable simultaneous influence on different aspects of the combustion process without deteriorating flame stability, increasing heat losses, or negatively affecting slagging, fouling, and corrosion phenomena in boiler components. In a broader perspective, the study aims to identify solutions that can be implemented in existing power generation facilities, enabling a reduction in the environmental impact of coal-based energy systems while maintaining high efficiency and safe operation.

2. The Theoretical Basis of Pulverized Coal Combustion

Pulverized coal combustion is a complex, step-by-step physicochemical process. The fact that coal is in a fine-grained form eliminates serious problems such as the inability to cope with load fluctuations due to its limited combustion efficiency, difficulties in removing large amounts of ash, and combustion disruptions caused by the resulting ash [23]. The first stage of the pulverized coal combustion process is the preparation of the coal fraction by grinding it in mills. The pulverized coal is then introduced into the combustion chamber by a hot stream of primary air using pneumatic pulverized coal ducts [24]. To ensure complete combustion, supplementary air (known as secondary air) is supplied separately to the combustion chamber. The combustion process itself can also be divided into specific stages, as schematically shown in Figure 3.

Figure 3. Stages of coal dust combustion [own elaboration].

The stream of the pulverized coal-air mixture (flowing from the pulverized coal burner) is introduced into the boiler. After mixing with the hot exhaust gases in the furnace, a flame is formed when the coal particles are ignited. It consists of three characteristic zones, as illustrated in Figure 4 [25].

Figure 4. Stages of coal dust combustion [25].

The most important factors influencing the process include dust particle size, ambient temperature, the oxygen concentration in the mixture, and the gas flow turbulence in the combustion chamber [26].

Combustion process control, understood as improving combustion dynamics or eliminating problematic issues related to pollutants and waste, can be achieved in many ways. The most important include process optimization, the modernization of existing systems, the development and implementation of combustion catalysts, as well as automation [27,28,29,30].

3. Combustion Modifiers—Division and Classification Criteria

There are many types of modifiers, and their use depends on the nature of the combustion process and the desired end results. Generally speaking, modifiers play a significant role in pulverized coal combustion, as they can improve the efficiency and purity of this process. However, selecting the appropriate modifier is a complex issue that requires special attention [31]. The incorrect selection of substances, or their excessive use, can lead to undesirable consequences, such as reduced combustion efficiency or increased emissions of harmful byproducts [32]. By actively influencing the chemical reactions occurring during combustion, modifiers can lower the process temperature, increase its efficiency, reduce pollutant emissions, and improve combustion stability and control. Therefore, they constitute a key element in the development of technologies that aim for a more efficient and environmentally friendly use of pulverized coal [33]. Pulverized coal combustion modifiers can be classified based on various criteria. A sample summary is presented in Figure 5.

Figure 5. Example division of combustion process modifiers.

The selection of chemical compounds comprising the pulverized coal combustion modifiers has been the subject of numerous studies, which in turn have resulted in numerous patent applications and implementations. The first of the analyzed division criteria allows for the separation of natural and synthetic modifiers. Among the natural modifiers, the following can be mentioned: clays, kaolins, dolomite, limestones, and silicon and aluminum compounds [34,35]. Their advantages include availability and low price, although they are characterized by variable effectiveness depending on chemical composition and combustion conditions [36]. Synthetic modifiers are intentionally produced in chemical or metallurgical processes. This group includes, among others, transition metal salts (e.g., iron, manganese, cobalt, nickel), metal oxides, and complex compounds [37,38]. Thanks to their controlled composition, they provide repeatable effects and often higher effectiveness when compared to natural modifiers [39].

Chyć [40] presented an extensive classification of modifiers based on their mechanism of action. He distinguished the following modifiers: oxidizing, oxidation catalysts, those capable of binding gaseous acidic oxides, anticorrosive, anticaking, flammable, those with sublimation properties, and fillers.

Xue et al. [41] investigated the feasibility and effectiveness of using kaolin as an in-furnace sorbent to reduce the emission of semi-volatile metallic elements and fine particles during coal and biomass combustion, as well as the co-combustion of both fuels.

Study [42] describes the investigation of the effect of the mineral siderite calcination products that are used as a catalyst on the combustion properties of bituminous coal and anthracite. It was shown that the combustion reactivity of pulverized coal can be improved by the addition of a catalyst, with the optimal amount being 1.5% for bituminous coal and 0.5% for anthracite, respectively. With the optimal addition, the combustion rate of bituminous coal and anthracite increased by 42.5% and 9.13%, respectively. The ignition temperature decreased by 7 °C for bituminous coal and by 3 °C for anthracite, and the complete combustion index increased by 49.2% and 14.88%, respectively, when compared to raw coal. The released heat for bituminous coal and anthracite increased to 4.7 kJ/mg and 4.66 kJ/mg after the addition of the catalyst.

In 1994, Borisovna patented an invention involving the periodic dosing of sodium chloride during hard coal combustion [43]. The optimal dose was considered to be 7–8 g of NaCl per m² of combustion area. This addition contributed to the increased thermal efficiency of the installation and reduced carbon moNO_x ide (CO) and nitrogen oxides (NO_x) emissions into the atmosphere. It was also found that it was possible to reduce the excess air coefficient in the flue gases and to reduce the heat losses from the flue gases by approximately 12%. Studies conducted in real heating installation conditions have shown that rock salt affects the kinetics of processes occurring in the combustion zone. Sodium chloride, commonly used as a component of fuel additives, is most often used in mixtures with other chemicals. It should be emphasized that the mechanism by which NaCl affects the obtained effects has not yet been fully and unambiguously explained [44].

Liquid and solid coal dust combustion modifiers differ primarily in their form, mode of action, and application. Liquid modifiers are solution- or oil-based mixtures consisting of a dispersant/wetting agent, a binding/adhesive component, and a catalyst [45]. They are applied to coal dust to improve fuel–air mixing, facilitate ignition, enhance complete combustion, and reduce emissions of NO_x and particulate matter; they require a carrier to ensure even distribution and controlled dosing. Solid modifiers, such as metal oxides [37,38], are used in powder or granule form and added directly to the coal dust or furnace. They act mainly as chemical catalysts that accelerate combustion and decrease harmful emissions, without the need for a liquid carrier, although their distribution may be less uniform than that of liquid modifiers.

Patent [46] concerns the development of a clean and effective coal combustion catalyst, which aims to increase the speed and efficiency of the combustion process while at the same time reducing pollutant emissions. The modifier consists of organic and inorganic compounds, including sodium dodecylbenzenesulfonate, polyethylene oxyethylated alkylphenol, dibutyl phthalate, trichloroethane, tributyl phosphate, dinitrotoluene (DNT), ferrocene, calcium stearate, and yellow soda. The entire composition is dissolved in diesel fuel (90–98% by volume). Various options for modifier dosing are assumed: during storage, transport and mixing, or directly by injection into the combustion chamber. The expected effects include carbon savings of 10–30%, SO₂ emission reduction by 50–80%, combustion efficiency improvement by 10–20%, no black smoke, reduced slagging and corrosion, low production costs, and ease of use.

Patent [47] proposes a high-energy catalytic additive to support coal combustion in the pulverized-fuel boilers of thermal power plants. The proposed modifier consists of a powder and a liquid. The powder is a mixture of inorganic compounds (rare-earth chlorides, metal oxides, hydroxides, lignosulfonates) that improve the fineness, reactivity, and combustion of pulverized coal. In turn, the liquid is a solution of strong oxidizers (e.g., perchlorates) and metal salts (cobalt, copper), which support combustion by generating additional combustible gases (H₂, CO, CH₄). The powder is added to the coal before grinding, improving the quality of the pulverized coal, increasing its reactivity, and facilitating complete combustion. The liquid is injected into the intense fire zone, where it rapidly decomposes, in turn creating a mixture of combustible gases that further enhance the combustion process. The liquid agent, manufactured according to a specific formula, is used in a ratio of 0.01% to fuel coal. The total dose of powder raw material and liquid raw material is 0.05% of the coal mass.

There are many scientific studies confirming that a material that is a production waste from one process can be successfully used as an additive that modifies the course of another operation [48]. In the case of coal dust combustion, the use of ash is often described due to its chemical composition—it is rich in iron and calcium oxides [49].

Wang et al. [50] studied the effect of a modifier (consisting of iron oxide and calcium oxide) on the combustion mechanism of coal dust. The authors determined the most favorable modifier composition based on the mutual ratio of oxides, identifying the 9Fe₁Ca system as the most effective and the 7Fe₃Ca system as the least effective. It was also shown, with an increase in the flux addition from 1% to 5% in the composite, that the values of the ignition index C and the characteristic combustion index S for the composite sample initially increased and then decreased. The most favorable range of composite flux addition was between 1% and 3%, while an excessive amount of it only had a limited effect on the efficiency of coal dust combustion. The oxides used in the study were the main components of a post-production by-product of the metallurgical and steel industry (BOF dust). However, it should be remembered that the ratio between the oxides may vary significantly [51].

Gao et al. [52] also explored the potential use of a composite composed of these two oxides. The study investigated the effect of the additives used separately, as well as their synergistic effect. Tests were conducted within a 1–5% additive concentration range, with the modifiers being the components of IBD dust. It was demonstrated that each of the process modifications reduced the activation energy and improved combustion efficiency.

Wang et al. [38] determined how fly ash from a steel mill and oil sludge from waste oil affect the combustion properties of coal dust. They demonstrated that incorporating these industrial wastes into the fuel mixture can improve combustion efficiency by facilitating ignition, increasing the reaction rate, and increasing heat release. Furthermore, no loss of catalytic efficiency was observed with moderate additive ratios, which means that these additives are possible to use. It is also worth noting that their use potentially reduces costs and energy consumption, while at the same time utilizing waste, which has ecological implications.

The effect of selected metal compounds and their oxides on the combustion process of various types of coal (lignite, hard coal, and anthracite) have been studied in Di et al. [53]. The effect of the additives was assessed based on thermogravimetric analysis. It was shown that the combustion effect was the result of both the used catalyst and the type of burned coal. All the used modifiers showed a positive supporting effect—they lowered the ignition and complete combustion temperature of the three types of coal, as well as increasing the combustion index (S) and the exothermic value (Q). The order of the catalytic effect was as follows: Na₂CO₃ > CaO > ZnO > CaCl₂. It was observed that metal oxide catalysts were most effective during the coke breeze combustion stage, while metal oxide catalysts mainly affected the thermal decomposition stage.

MnO₂, CaO, and Fe₂O₃ catalysts effects on combustion process of bituminous coal and anthracite have been investigated in Zou et al. [54]. They found that for bituminous coal, the relative order of catalyst activity in relation to the burnout rate was as follows: CaO > Fe₂O₃ > MnO₂, while for anthracite, the order was: Fe₂O₃ > CaO > MnO₂. They also observed that the catalysts enhanced chemical reactions that occur on the surface of coke breeze particles, and in its pores.

Guo et al. [55] developed a new catalytic combustion promoter that contained manganese dioxide along with other rare-earth and alkaline-earth metal oxides (or carbonates) in order to increase the combustion efficiency of pulverized coal. They found that with increasing promoter amounts, the ignition temperature decreased, with combustion efficiency significantly rising.

Wei et al. [56] demonstrated that CaO and MgO can improve the combustion characteristics of pulverized coal. The catalytic effect of CaO and MgO on pulverized coal combustion was found to be enhanced by increased volatile matter (VM) emissions from pulverized coal, which in turn led to a lower ignition temperature. MgO exhibited significantly higher catalytic activity than CaO in the combustion of solid coal. The mechanism of the catalytic combustion of pulverized coal in the presence of MgO differs from that of CaO.

The investigation in the research study performed by Salinas et al. [57] describe the effect of modifying the structure of a CeO₂-Al₂O₃ and La₂O₃-Al₂O₃ catalyst by incorporating potassium into its lattice. The presence of potassium oxides (K₂O) and oxygen associated with vacancies and/or lattice defects (O²⁻) was shown to correlate with high catalytic activity.

Reference [58] proposed using a safflower oil-based emulsion as a modifier for the pulverized coal combustion process. Experimental studies demonstrated a number of benefits of this solution, including: an increase in coal thermal efficiency by 10–15%, a decrease in harmful substance emissions by 15–25%, and a reduction in heavy metal content and slag capture by 18–23%, which in turn reduces the negative impact of the coal combustion process on the environment. The summary of the effects of selected modifiers is presented in Table 1.

Table 1. Summary of modifiers used in pulverized coal combustion and their effects.

Reference	Type of Modifier	Modifier Amount	Coal Type	Main Effects
[31,32,33]	General modifiers—the polymetallic modifier (modifiers containing salts of CuSO4 2H2O, NaCl, NH4Cl, MgSO4 7H2O, CaCl2 and urea)	the modifiers were used in the form of aqueous solutions; 2–3 dm3 per 1 Mg of coal	Pulverized coal	Improved combustion efficiency (boiler efficiency also increased from 65% in tests without modifier to 76% in tests with modifier), emission reduction (the CO, NOx and SO2 levels in the exhaust gas were reduced by approximately 9%, 12% and 10%, respectively, compared to the sample without modifiers), process stabilization
[34,35,36]	Natural: clays, kaolins, dolomite, limestone, Si and Al compounds		Pulverized coal	Low cost, easy availability; effectiveness varies depending on chemical composition and combustion conditions
[37,38,39]	Synthetic: transition metal salts, metal oxides, complex compounds		Pulverized coal	Repeatable effects, often higher effectiveness than natural modifiers
[40]	Mechanism-based: oxidizing, oxidation catalysts, gas-binding, anticorrosive, anticaking, flammable, sublimating, fillers	NaCl—7–8 g NaCl/m2 of combustion surface; potassium and sodium nitrates—100 g/Mg of fuel;	Pulverized coal	Different effects depending on mechanism; i.e.,: for NaCl—reduction in exhaust heat losses by approx. 12%, sodium nitrates—fuel savings of 5%;
[41]	Kaolin	1–5%	Pulverized coal/biomass	Reduced emissions of semi-volatile metals and fine particles; ash fusion temperature (AFT) increased by over 100–150 °C, which practically eliminated the problem of slagging when co-firing straw
[42]	Siderite calcination products		Bituminous coal, anthracite	Increased reactivity, lower ignition temperature, higher combustion index
[43,44]	Sodium chloride		Hard coal	Increased thermal efficiency, reduced CO and NOx emissions, lower heat losses
[37,45]	Liquid: dispersant/wetting agent + adhesive + catalyst; Solid: metal oxides		Pulverized coal	Liquid: improved mixing, easier ignition, NOx and particulate reduction; Solid: catalyzes combustion, reduces emissions
[38]	Steel Plant Flying Dust	Flying Dust: 5–15%; Waste Oil Sludge: 3–10%	Pulverized coal	Lowering the flash point by approximately 20 °C; Burnout Rate increased by 5% to 12%; reduction in combustion time by approx. 15–20%; the activation energy of the combustion process decreased by about 10–15 kJ/mol
[42]	Metal oxides (Fe2O3, CeO2, MnO2), alkali metal salts (Na2CO3, NaCl)	1.5% for bituminous coal, 0.5% for anthracite	bituminous coal, anthracite	lowering the flash point (a drop of about 7 °C—bituminous coal, of about 3 °C—anthracite); increase in burnout index (by 49.2% for bituminous coal, by 14.88% by anthracite); heat release increase (to a level of approximately 4.7 kJ/mg for bituminous coal and 4.66 kJ/mg for anthracite)
[46]	A catalyst with a complex composition, which is based on a combination of oxidants, metal salts and mineral carriers in diesel (Fe2O3, MgO, NaNO3, KMnO4, CaO)	0.05–0.2%	Pulverized coal	Coal savings 20–30%, SO2 reduction 55–70%, combustion efficiency +10–20%
[47]	High-energy catalytic additive (powder + liquid)	0.05%	Pulverized-fuel	Improved pulverized coal quality, increased reactivity, combustion support
[48,49,50,51]	Industrial waste: fly ash, BOF dust, oil sludge		Pulverized coal, lignite	Improved ignition, reaction rate, heat release (i.e., reducing the ignition temperature of lignite by approx. 112 °C); cost and energy savings
[52]	Fe2O3 and CaO	up to 5%	Pulverized coal	the comprehensive combustibility index (Sn) increases from 1.37 × 10−6 to 2.16 × 10−6%2 min−2 °C−3 as the Fe2O3 proportion increases from 0 to 5.0 wt% the activation energy (E) of PC combustion decreases after adding the above additives (the E decreases from 56.54 to 35.75 kJ/mol when the Fe2O3 proportion increases from 0 to 5.0 wt%), which supports the improved combustion performance
[53]	Na2CO3, CaO, ZnO, CaCl2	5%	Lignite, hard coal, anthracite	Lower ignition and complete combustion temperatures (all catalysts decreased the ignition temperature and burnout temperature of coals to the extent of 0.25–81.25 °C, 24.75–73.25 °C); higher combustion index (S) and exothermic value (Q)
[54]	MnO2, CaO, Fe2O3	no information	Bituminous coal, anthracite	Enhanced chemical reactions on coke surface and pores; different catalyst activity orders
[55]	MnO2 + rare-earth/alkaline-earth oxides or carbonates	0.4%	Pulverized coal	Lower ignition temperature with higher promoter amount, increased combustion efficiency, reduced coke consumption to 26.7 kg/t·Fe and decreased fly ash carbon content from 43.1% to 32.4%, yielding significant economic and environmental benefits
[56]	CaO, MgO	3 wt%	Pulverized coal	Improved combustion characteristics, lower ignition temperature; MgO higher catalytic activity than CaO
[57]	CeO2-Al2O3 and La2O3-Al2O3 modified with K	3–5%	Pulverized coal (soot)	Presence of K2O and O2– defects correlates with high catalytic activity
[58]	Safflower oil-based emulsion (concentration of safflower oil in the emulsion: 30%)	10–50% emulsion	Pulverized coal	Increased thermal efficiency 10–15%, reduced emissions 15–25%, reduced heavy metals and slag 18–23%, the melting point of coal can be lowered by 9.2%

A very important issue related to both coal combustion is the accompanying emission of harmful substances exceeding permissible regulatory limits. One method to counteract undesirable emissions through their reduction is the application of catalytic or sorption additives, as well as the optimization of process parameters. The authors of study [59] observed a simultaneous reduction in NO_x and SO_x emissions depending on the O₂ concentration at the boiler inlet, the mass flow rate of coal, and the air-to-fuel ratio. A significant improvement in SO₂ reduction was achieved when the O₂ concentration was 3.28 vol.% and the coal mass flow rate was 281 t/h. NO_x concentrations decreased markedly when the air-to-fuel ratio shifted from a lean condition (i.e., 7.14) to a richer condition (i.e., 7.29). In publication [60], a method for reducing SO₂ emissions through the injection of dry sorbents into the furnace of a coal-fired boiler was described. It was shown that the effectiveness of the process depends mainly on sorbent reactivity, flue gas temperature, and the quality of sorbent–flue gas mixing. Demonstration studies indicated that at a Ca/S molar ratio of 2.5, SO₂ reduction of approximately 40–55% could be achieved, while a maximum reduction of 72% was obtained at a Ca/S ratio of 3.8, confirming good agreement between experimental results and model predictions.

Among the additives limiting dioxin emissions from solid fuel–fired boilers, two categories can be distinguished: dioxin synthesis inhibitors (calcium oxide, ammonia, ammonium sulfate, sodium sulfate, sodium thiosulfate, sodium ammonium hydrogen phosphate(V), sulfur, and dolomite) and dioxin-reducing compounds (pyridine, quino-line, urea, ethylene glycol, amines, and EDTA) [34,61].

4. Co-Firing of Biomass with Coal Dust

Biomass is a key renewable fuel in the context of global decarbonization efforts and rising fossil fuel prices. Its use in energy production not only reduces greenhouse gas emissions, but also contributes to achieving carbon neutrality [62,63]. This is because the amount of carbon dioxide released during biomass combustion is equivalent to the amount that is absorbed by plants during photosynthesis, which makes this fuel carbon neutral [64,65]. Biomass includes a wide range of organic materials, such as agricultural and forestry waste (e.g., corn straw, wheat straw, pine sawdust, wood chips, and briquettes), as well as specially cultivated energy crops [66,67]. When compared to fossil fuels, biomass contains less sulfur and nitrogen, which in turn results in lower emissions of sulfur and nitrogen oxides during combustion [68,69]. A characteristic feature of biomass is its high content of volatile matter and oxygen, which allows it to ignite faster and be more reactive than coal [62,70,71]. It therefore aids the ignition process in fuel mixtures by accelerating degassing and improving overall combustion efficiency. Due to these properties, biomass is considered the fourth most important energy fuel after coal, crude oil, and natural gas. It stands out from other renewable energy sources not only for its wide availability and low cost, but also for its ability to produce stable energy regardless of the weather conditions [68,69,72].

Biomass co-firing is a technological process involving the simultaneous combustion of biomass with fossil fuels, most often coal, in the same energy device [73]. This method integrates the use of renewable energy with traditional combustion technologies, enables a reduction in coal consumption, limits pollutant emissions, and increases the share of renewable energy in the national energy mix [69,74]. Co-firing coal and biomass produces a synergistic effect, which is manifested by a reduction in the activation energy of the reaction and an increase in the reactivity of the mixtures [70,75]. This may result from the formation of a more reactive gas environment compared to the combustion of coal alone. At the same time, the lower average flame temperature and diluted oxygen limit the formation of NO_x and PM2.5.

Co-firing biomass with coal is considered one of the most effective and cost-effective uses of biomass. In many countries, it has been implemented in existing coal-fired power plants, and it requires only minor infrastructure modifications. This technology is distinguished by its short implementation time, low investment risk, and compatibility with existing power systems [76,77].

In addition to environmental benefits (reduction in CO₂, SO₂, NO_x, and CO), co-firing also contributes to increased energy security, the rational use of local resources, and improved power system stability [78]. Due to its adaptability to various boiler types and biomass types, this technology is a key element of the energy transition towards low-emission development [79].

In recent years, intensive research has been conducted on the co-firing of biomass with coal. It aimed to improve the energy efficiency of combustion processes and reduce gaseous pollutant emissions. Incorporating biomass into fuel blends is considered a key direction in the decarbonization of the energy sector, which will in turn enable the partial replacement of fossil fuels with renewable energy sources and reduce CO₂ emissions. An important issue in the context of coal and biomass co-combustion is the amount of ash, which can reduce combustion efficiency, promote the formation of deposits and slag in the furnace, and hinder heat transfer and flame stability.

Paper [62] describes research on the co-combustion of semi-coke from briquetted biomass and pulverized coal in order to improve energy efficiency and reduce CO₂ emissions. Four types of biomass were analyzed: sawdust, eucalyptus bark, corn straw, and rice husk. The biomass was gasified in a fluidized bed. Various proportions of biomass and pulverized coal were used. The analysis was conducted using thermogravimetry within the range of 25–900 °C. The best results were achieved for the sawdust and eucalyptus bark, where even an addition of 25% improved the combustion stability of pulverized coal and lowered the ignition temperature. Corn straw and rice husk showed negative effects at higher proportions (≥50%), mainly due to their high ash content, which hindered heat transfer. In turn, their addition reduced the combustion activation energy by up to 17 kJ/mol, which indicates that the reaction occurs more easily at lower temperatures.

The research presented in [80] focused on the combustion process of lignite, hard coal, and biomass (birch reed), as well as their mixtures. The biomass content in the composition of coal-based fuel mixtures was 10, 20, and 30 wt%. The studies were conducted under various conductive and convective heating conditions. The shortest ignition times were obtained for biomass due to its high porosity, low moisture content, and high volatile content. The longest ignition delay times were obtained for hard coal, as its reactivity was worse than that of biomass. Lignite ignited faster than hard coal, but slower than biomass. The addition of biomass shortened the ignition time and improved the degree of fuel combustion. It also reduced pollutant emissions. It was shown that the addition of 20 wt% of biomass to hard coal shortens the ignition delay time of particles during conductive heating by 53–70%, and during convective heating in a stream of heated air by 47–52%. Furthermore, the content of nitrogen oxides and sulfur oxides in the exhaust gases is reduced by 29% and 34%, respectively.

A significant effect of biomass type on gas emissions during co-combustion with coal was demonstrated in [68]. Four types of biomass waste were tested: poplar sawdust, rice husks, pine nut shells, and sunflower residues. Coal and biomass mixtures were prepared in the following proportions: 100:0, 90:10, 80:20, 70:30, 60:40, and 50:50 wt%. The best results were obtained for a mixture of 60% coal and 40% sunflower biomass, which demonstrated high efficiency and minimal gas emissions. In the case of the poplar sawdust, the best result was obtained with a 50:50 weight ratio. A 99% decrease in SO₂ emissions and a 38% decrease in CO emissions were observed. For the rice husks, a 72% reduction in CO emissions and a 90% reduction in SO₂ emissions were achieved when the carbon to biomass ratio was 70:30 wt%. In the case of the pine nut shells, an increase in the share of biomass in the mixture resulted in a reduction in harmful gas emissions.

In turn, the study presented in paper [71] analyzed the effect of the biomass (rice straw) content in pelletized mixtures with subbituminous coal (Zhundong) under rapid heating conditions. Pellets with a biomass content of 0%, 10%, 20%, 30%, and 40% by weight were prepared. The studies were conducted in a concentrating photothermal reactor. With an increasing biomass content, the degassing delay time decreased by approximately 70%, and the degassing duration increased by 22%. However, more intense slagging and the blistering phenomena occurred (related to the higher ash content in the biomass). Increasing the biomass content resulted in the appearance of additional CO emission peaks, which indicates the multistage nature of the reaction. CO₂ emissions decreased with an increasing biomass content, while nitrogen oxides initially increased with a higher biomass content and then decreased due to NO reduction by NH and CO radicals. CH₄ emissions were constant and independent of the mixture composition. Co-firing biomass with coal leads to significant synergistic effects—biomass accelerates degassing, lowers the activation energy of the reaction, and increases the reactivity of the mixtures. The greatest benefits were achieved for mixtures containing 20–30% biomass, which provided an optimal combination of low activation energy, high reactivity, and reduced emissions. Table 2 summarizes the key information presented in references [68,71].

Table 2. Comparison of co-combustion studies of coal and biomass.

Aspect	Reference [68]	Reference [71]
Coal type	Coal (not specified)	Zhundong subbituminous coal
Biomass type	Poplar sawdust, rice husks, pine nut shells, sunflower residues	Rice straw
Fuel form	Coal–biomass blends	Pelletized coal–biomass mixtures
Biomass share (wt%)	0, 10, 20, 30, 40, 50	0, 10, 20, 30, 40
Experimental conditions	Laboratory-scale co-combustion	Rapid heating, concentrating photothermal reactor
Optimal biomass content	40% (sunflower residues), 50% (poplar sawdust), 30% (rice husks)	20–30%
SO2 emissions	Up to 99% reduction (poplar sawdust, 50:50); 90% reduction (rice husks, 70:30)	Decreased with increasing biomass share
CO emissions	Up to 72% reduction (rice husks, 70:30); 38% reduction (poplar sawdust, 50:50)	Additional CO peaks observed (multistage reaction)
CO2 emissions	Reduced depending on biomass type	Decreased with increasing biomass content
NOx emissions	Not reported	Initial increase followed by decrease due to NO reduction by NH and CO radicals
CH4 emissions	Not reported	Constant and independent of mixture composition
Kinetic effects	Strong dependence on biomass type	Lower activation energy and increased reactivity
Negative effects	No significant issues reported	Enhanced slagging and blistering due to high ash content
General conclusions	Biomass type strongly affects emissions and combustion efficiency	Strong synergistic effects; optimal performance at 20–30% biomass

Paper [70] presents research on the co-combustion of coal and various types of biomass in an oxy-dioxide (O₂/CO₂) atmosphere. The authors aimed to better understand the mechanisms of the interactions between fuels, as well as analyze their impact on combustion efficiency and CO₂ emissions. The research material consisted of bituminous coal from the Shanxi Province and seven types of biomass: sawdust, rice husks, cotton stalks, rice straw, rapeseed straw, corn cobs, and cattle manure. Adding biomass significantly accelerated coal combustion and shortened the burnout time. Biomass with a high volatile matter content improved combustion efficiency. The co-combustion of coal and biomass demonstrated a clear synergistic effect—the mass of the mixture lost more than would be expected from the theoretical summation of the effects of burning the components separately. Interactions between the different types of biomass were weak or negligible. When comparing the TG curves obtained experimentally and theoretically, a longer combustion duration and a higher burnout rate were observed compared to the combustion of coal alone. This may be due to the irregular shape of biomass particles, which promotes better heat transfer and retention, leading to higher flame temperatures. The experimental and theoretical curves nearly coincide during the initial stage of combustion. However, after approximately 1380 s, the mass loss rate in the experiment exceeds the theoretical values, indicating a synergistic effect and accelerated combustion.

The importance of biomass treatment prior to co-combustion was emphasized in paper [77]. The effect of torrefaction of corn straw and rice husks, which were co-combusted with high-sulfur bituminous coal and low-sulfur subbituminous coal, were analyzed. Blends with 50:50 wt% were prepared. The coal particles were 75–90 µm in size, while the biomass particles were 90–150 µm in size. Emissions from fuel combustion in air were measured at the kiln outlet. The results showed that for both coal-raw biomass and torrefied biomass blends, SO₂ and NO_x emissions were lower than those predicted from the linear interpolation of the results obtained for the pure fuels. For coal-torrefied biomass blends, SO₂ emissions were lower than for coal-raw biomass blends. NO_x emission efficiency from pure torrefied biomass was slightly higher than from raw biomass because the raw biomass had a higher nitrogen content per unit mass. The emission of HCl from torrefied corn stover was lower than from its raw precursor because the former had a lower chlorine content. The emission of HCl from corn stover-coal mixtures was significantly higher than from pure coal combustion. The emission of HCl from high-sulfur coal-corn stover mixtures was higher than from mixtures of the same biomass with low-sulfur coal.

The beneficial effect of biomass on pollutant emissions is also confirmed by simulation studies presented in paper [81]. The CFD analysis included predictions of volatile matter release and char combustion from particles that were co-powdered with coal and biomass, as well as simulations of combustion chemistry occurring in the gas phase. The coal used was Canadian bituminous coal with a high sulfur content. The coal was blended with 5–20% of wheat straw. One significant result is the reduction in NO_x and CO₂ emissions due to co-firing. This reduction depends on the proportion of biomass (wheat straw) mixed with the coal. Increasing the mass fraction of wheat straw mixed with the coal results in a decrease in NO_x and CO₂ concentrations at the furnace outlet.

The aim of the study described in paper [82] was to analyze the effect of biomass in fuel blends on the combustion process and pollutant emission levels. Experiments were conducted with mixtures containing 0%, 25%, 50%, 75%, and 100% of biomass. It was demonstrated that an increase in the biomass content in fuel caused significant changes in the thermal parameters of the process and also affected combustion temperature, energy conversion efficiency, and the volume of gaseous combustion product emissions.

Kazagić et al. [83] conducted experimental studies of the co-combustion of low-calorific lignite with woody biomass (sawdust) and miscanthus, which aimed at assessing pollutant emissions and ash formation under conditions similar to those of actual pulverized coal boiler operation. Twelve fuel mixtures were tested at temperatures of 1250 °C and 1450 °C, and emissions of gases such as NO_x, SO₂, and CO were analyzed. The results showed that the co-combustion of coal with biomass (at a share of up to 30%) does not generate significant technological problems, and with appropriate process parameters allows for a reduction in NO_x and partially SO₂ emissions. Particularly favorable results were achieved when miscanthus was used, which demonstrated its better environmental properties when compared to sawdust. The combustion process efficiency remained satisfactory at biomass shares of up to 15%, while above this value, an increase in the content of unburned carbon in the slag was observed.

Wang et al. [84] analyzed the co-combustion of biomass, including straw and wood, with coal in a falling-tube furnace. The study focused on monitoring nitrogen oxides and carbon monoxide (CO) emissions at different furnace heights, as well as the assessment of the unburned carbon content in fly ash. The effects of the fuel type, process temperature, biomass content in the mixture, and air partition coefficient were taken into account. The results showed that the NO concentration increased in the initial combustion phase and then gradually decreased in the upper parts of the furnace, regardless of the fuel type. NO emissions from straw and wood combustion accounted for approximately one-third and one-half of the emissions generated by the coal, respectively. In the analyzed temperature range (900–1100 °C), it was observed that an increase in temperature led to an increase in NO emissions, while a reduction in the amount of unburned carbon in the ash had little effect on CO emissions. During biomass co-combustion, a significant decrease in NO, CO, and unburned carbon emissions was observed with an increase in the biomass content in the fuel. Furthermore, studies on the effect of air partitioning showed, in the case of coal combustion, that this treatment effectively reduced NO emissions. Moreover, it led to an increase in CO emissions and unburned carbon content. This effect was significantly weaker for biomass, which in turn confirmed its more favorable environmental properties during co-combustion.

The authors of study [85] investigated MILD combustion (flameless combustion at low temperature and under diluted oxygen conditions) as an alternative technology for large-scale, low-emission co-firing of biomass–coal blends. The results showed that this method not only reduces NO_x emissions but also effectively suppresses the formation of fine particulate matter. Compared with conventional swirl flame (SF) combustion, the application of MILD combustion to biomass fuel blends leads to a 45.5–49% reduction in heterogeneous fuel- NO emissions. For PM2.5 emissions, a reduction of 37–59% was observed relative to conventional low-NO swirl combustion, owing to the lower combustion temperature and oxygen concentration.

The aim of study [86] was to examine whether MILD combustion can be an effective method for the combustion and disposal of solid waste while simultaneously controlling emissions. The investigated material consisted of solid waste in the form of biomass (sawdust) combined with residual char (a by-product of thermal processes), i.e., a waste-derived fuel. In this case as well, a significant reduction in NO emissions (up to 54%) was demonstrated, achieving levels close to ultra-low NO emission standards, along with an approximately 50% reduction in PM2.5 emissions, which confirms the feasibility and relevance of implementing the discussed technology.

An important issue in the co-combustion of biomass and coal is the presence of alkali and alkaline earth metals (AAEM) and the lower ash melting temperature compared to coal [87,88]. It should be recognized that co-combustion can alter the mechanism of ash deposition on the heating surfaces of boilers, thereby directly affecting the efficiency of the process. The authors of the reference [89] demonstrated that deposits collected during the co-combustion of biomass and coal exhibit a three-layer structure, namely an outer sintered layer, an inner loose layer, and an initial adhesive layer. Using the conducted micromorphology analysis, it was shown that higher flue gas temperatures intensify the processes of agglomeration, sintering, and ash deposition, particularly in the outer layer. Additionally, the increase in flue gas temperature promoted the thermophoresis of low-melting fine particles rich in K, Ca, and Fe in the form of silicates and aluminosilicates. The condensation of alkali vapors and SO₂ was enhanced due to the elevated partial pressure in the high-temperature flue gas environment, facilitating the enrichment of de-posits with alkali metals and sulfur and increasing their adhesion to heating surfaces. Higher flue gas temperatures also promoted the formation reactions of low-melting silicates containing Ca and Fe.

It can be stated that the most advantageous solution from the perspective of simultaneous reduction in NO_x, SO₂, and CO₂ emissions while maintaining safe boiler operating conditions is co-firing biomass with pulverized coal at a biomass mass fraction of approximately 20–30%. Within this range, a clear combustion synergy effect is observed. Biomass, characterized by a high content of volatiles and oxygen and low sulfur and nitrogen content, lowers the ignition temperature of the fuel mixture and the activation energy of the reaction, improves flame stability, and accelerates carbon particle burnout. This results in a reduction in NO_x emissions by approximately 20–35%, SO₂ emissions by up to 30–40%, and a decrease in CO₂ emissions due both to partial substitution of fossil fuel with renewable fuel and to improved combustion efficiency. At the same time, the 20–30% biomass range is indicated as the upper limit where no significant increase in slagging, ash sintering, or high-temperature corrosion of heat exchange surfaces is observed. Exceeding this level leads to increased content of alkalis and chlorine in the ash, promoting the formation of low-melting mineral phases, deposits, and aggressive corrosive compounds, especially with straw-type biomass. For older boiler types or installations particularly sensitive to ash-related issues, a more conservative biomass share of 10–15% is recommended, which still provides a noticeable reduction in NO_x and SO₂ while minimizing the risk of process disturbances. The use of appropriate mineral and catalytic additives, such as CaO, MgO, kaolin, or halloysite, can further reduce slagging and corrosion by binding alkalis and increasing the ash melting temperature. The use of woody biomass allows for the most favorable results in terms of combustion behavior and emissions. It is characterized by relatively low ash content, low alkali and chlorine content, and a favorable mineral composition dominated by Ca and Mg. As a result, woody biomass exhibits good reactivity, promotes stable ignition, and reduces NO_x and SO₂ emissions, while minimizing the risk of slagging, ash sintering, and high-temperature corrosion. Woody fuels allowed for safe co-firing even at shares of 20–30%. Agricultural residues exhibit significantly worse ash properties, despite good reactivity and high volatile content. They are rich in potassium, sodium, and chlorine, leading to the formation of low-melting alkali chlorides and sulfates. These compounds intensify slagging, deposit formation, and corrosion of heat exchange surfaces. Consequently, although agricultural residues effectively reduce SO₂ and CO₂ emissions, their share in the fuel mixture must be limited (typically ≤20–30%) and in practice often requires the use of mineral additives (e.g., kaolin, halloysite, CaO), which bind alkali components in the ash and raise its melting temperature. Waste fuels are characterized by the greatest variability in composition. The text notes that they can improve combustion reactivity due to the presence of transition metal oxides, but they also carry the risk of increased pollutant emissions and ash-related problems if their mineral composition is not controlled. Safe use requires a low share in the fuel mixture and precise process control. Cleaner combustion is associated with a specific ash composition. The most favorable ash contains a low amount of alkalis and chlorine, is rich in calcium, magnesium, and aluminosilicates, and is characterized by a high melting temperature and low tendency to form low-melting phases [71,76,77,80,83,90].

5. Synergy Effect

There are many known combustion modifiers that have a varying effect on efficiency. For example, copper compounds contribute to soot reduction, improved surface cleanliness, and the increased efficiency and performance of boilers [91,92]. Nickel compounds intensify combustion, improve fuel burnout, and reduce CO emissions [93,94]. The use of modifiers containing sodium and potassium compounds results in better combustion and reductions in SO₂ and NO [94]. Titanium compounds also contribute to more efficient and stable combustion, as well as a reduction in the amounts of soot, NO_x, SO₂, and Hg [95,96,97]. Cerium compounds facilitate the post-combustion of CO and hydrocarbons, reduce NO_x, and improve combustion stability [98]. Iron compounds reduce the amount of unburned fuel, NO_x and dust, and limit agglomeration [99]. Manganese compounds help to reduce soot and improve fuel burnout [100]. Calcium compounds, among other things, improve the combustion process and reduce NO_x [101]. Magnesium compounds primarily reduce slagging in furnaces [102]. The synergy effect in boiler technology refers to the phenomenon of the mutual reinforcement of several chemical or mineral substances, the combined use of which produces better operational results than each of them used separately. This phenomenon includes chemical processes in the furnace, as well as physicochemical phenomena in ash, sediments, and on heat exchange surfaces. This leads to improved boiler efficiency, increased availability, and reduced emissions of harmful substances. The summary is presented in Table 3.

Table 3. Summarizes selected combustion modifiers, their primary effects, and reported synergistic interactions, as described in the literature.

Modifiers	Primary Effect	Synergistic Effects	References
Copper compounds	Soot reduction, improved heat exchange surface cleanliness, increased boiler efficiency	With Mn: reduced carbon activation energy, improved flame stability, reduced slagging	[91,92,103]
Nickel compounds	Enhanced combustion intensity, improved fuel burnout, CO reduction	—	[93,94]
Sodium and potassium compounds	Improved combustion efficiency, reduced SO2 and NO emissions	—	[94]
Titanium compounds (TiO2)	More stable combustion, reduced soot, NOx, SO2, and Hg emissions	With CaO or Zr: accelerated pyrolysis and oxidation, reduced NOx and SO2	[95,96,97]
Cerium compounds (CeO2)	Post-combustion of CO and hydrocarbons, NOx reduction, improved combustion stability	With Mn: enhanced CO oxidation and NOx reduction	[97,98]
Iron compounds (Fe2O3)	Reduced unburned fuel, NOx and particulate emissions, limited agglomeration	With Mn: enhanced redox activity, reduced unburned carbon	[37,99]
Manganese compounds	Reduced soot formation, improved fuel burnout	Synergy with Cu, Fe, and Ce improves combustion stability	[101,103]
Calcium compounds (Ca, CaO)	Improved combustion performance, NOx reduction	CaO2 provides oxygen and enables simultaneous SO2 capture	[102,104]
Magnesium compounds (MgO)	Reduced slagging tendency in furnaces	In PentoMag: ash stabilization and improved boiler availability	[105,106]
PentoMag (CuO∙CuCl2 + MgO)	Catalytic soot oxidation and ash stabilization	Improved boiler efficiency and availability	[106]
Thermact (Mn + Cu)	Reduced carbon activation energy	Improved flame stability and reduced slagging	[103]
Halloysite (aluminosilicate)	Combustion catalysis and mitigation of chlorine-induced corrosion	Higher ash melting temperature, reduced HCl emissions	[90,107]
Kaolin/Halloysite + KCl	—	Formation of stable potassium aluminosilicates; reduced slagging and corrosion	[90,107]
Fe2O3	High catalytic activity at 700–900 °C	With TiO2 and CeO2: uniform reaction conditions, reduced CO and NOx	[97,108]
TiO2 and CeO2	High catalytic activity above 900 °C	Synergistic temperature coverage with Fe2O3	[97,108]

Synergy between transition metals allows for the simultaneous enhancement of oxidation, reduction, and combustion stabilization effects over a wide temperature range. Cerium combined with manganese increases the efficiency of CO oxidation and NO_x reduction [97]. Combining iron compounds with manganese enhances redox activity, which in turn helps to reduce unburned ash [37]. Similar synergistic effects are observed with titanium and zirconium oxides. TiO₂ combined with CaO or Zr accelerates the pyrolysis and fuel oxidation processes, while at the same time reducing NO_x and SO₂ emissions [97]. Studies on nanoparticle TiO₂ have confirmed that this additive, in the presence of other metals (such as V, Mn, Ca, or Zr), exhibits greater catalytic activity than oxides that are used individually.

Many modern additives which are used in the energy industry are complex formulations containing mixtures of substances that have complementary effects. An example is PentoMag, which contains copper oxychloride (CuO∙CuCl₂) and magnesium oxide (MgO). As described in paper [106], CuO∙CuCl₂ acts as a catalytic agent and facilitates soot combustion, while MgO stabilizes ash and reduces slagging. The combined action of both components improves boiler efficiency and availability. Sadpal [106] also demonstrates synergistic effects—combining CuSO₄·5H₂O, NaCl, NH₄Cl, CaCO₃, and SiO₂ at high temperatures leads to the formation of copper oxychloride (CuO∙CuCl₂), which has a similar effect to PentoMag. This improves fuel combustion and reduces deposit formation. Another example is Thermact [103], which contains manganese compounds (Manganous Acetylacetonate) and copper compounds (CuSO₄ 5H₂O). Studies [103] have shown that the presence of both metals leads to a synergistic reduction in the activation energy of carbon, which in turn results in improved flame stability and reduced slagging.

A synergetic effect also occurs in the case of single compounds with multifunctional action. Calcium peroxide (CaO₂) (described in paper [104]) provides oxygen in order to support the combustion process. At the same time, upon decomposition, it forms CaO, which acts as a desulfurization sorbent. As a result, the same additive improves combustion efficiency and reduces SO₂ emissions. In turn, halloysite, a natural aluminosilicate described in [90] and [107], has a dual effect. It reduces chlorine corrosion by binding potassium in the form of aluminosilicates, and also catalyzes combustion processes due to the presence of iron oxides in its structure. Research conducted at the KIT Institute in Karlsruhe [104] found that adding halloysite to biomass fuel increases the ash melting temperature by 100–350 K and reduces HCl emissions from 54% to approximately 32% of the total fuel chlorine.

Reactions between fuel components and additives can also lead to synergistic effects. For example, studies [90] described the reactions of kaolin and halloysite with KCl, which lead to the formation of potassium aluminosilicates—kalsilite (KAlSiO₄) and leucite (KAlSi₂O₆)—with melting points above 1500 °C. The resulting mineral phases are stable and insoluble, and therefore reduce slagging and the corrosion of boiler surfaces. Studies have also shown that halloysite significantly reduces fly ash grain size, which in turn favors the formation of loose deposits instead of hard deposits [107]. This is due to a reduced tendency for particles to agglomerate, which improves heat transfer and facilitates the cleaning of heating surfaces.

Some additives exhibit their highest activity at different temperature ranges. Combining them allows for efficient combustion over a wider temperature range. As indicated in paper [108], iron oxides (Fe₂O₃) operate most effectively at temperatures of 700–900 °C, while titanium oxides (TiO₂) and cerium oxides (CeO₂) maintain high activity above 900 °C [97]. Their combined use allows for the maintaining of uniform reaction conditions throughout the combustion chamber by reducing CO and NO_x emissions and improving fuel burnout.

6. Summary and Prospects for the Development of Modifiers for Pulverized Coal Combustion

The synergy effect in the combustion of solid fuels is a key tool for improving the energy efficiency and environmental performance of boilers. Combining transition metals (e.g., Fe, Cu, Mn, Ce) with alkaline earth oxides (Ca, Mg) or aluminosilicates (SiO₂, Al₂O₃) simultaneously lowers the ignition and post-combustion temperature, improves the kinetics of the oxidation reaction, stabilizes the flame, reduces slagging and corrosion, and reduces CO, SO₂, and NO_x emissions. Modern combustion catalyst formulations, such as CuFeAl (KBM Affilips B.V., Oss, The Netherlands), PentoMag (Pentol, Cracow, Poland), Thermact (Enfit, Cracow, Poland) and Sadpal (SKWAT Sp. z o.o., Żółtki-Kolonia, Poland), utilize this synergy effect by combining catalytic, sorption, and stabilizing functions [109,110,111]. Selecting the appropriate proportions of the components and controlling their activation temperatures provide the basis for the further optimization of the combustion processes of pulverized coal and low-calorie fuels. In the context of biomass co-firing, the presence of volatile matter, alkali and alkaline earth metals, and low-melting-point components in biomass can alter the ignition behavior, flame temperature profile, and ash deposition characteristics in pulverized coal boilers. The review presented above shows that biomass co-firing can reduce peak flame temperatures, increase volatile release, and promote more uniform combustion, while also contributing to the formation of additional AAEM species that synergize with catalytic additives used in pulverized coal combustion. As an example, in biomass and carbon co-firing, magnesium oxide (MgO) acts as a catalyst/sorbent, enhancing CO₂ capture by forming carbonates (like MgCO₃) and improving the carbon matrix for adsorption, while reducing issues like slagging. The magnesium carboxylates/sulfonates are often intermediates or related to biomass pretreatment/fuel components, influencing the overall process by modifying biomass structure or capturing released acids, with MgO being key for carbon mineralization and creating high-surface-area adsorbents for cleaner energy application [112].

In the coming years, the development of combustion modifiers will focus on increasing the selectivity of redox reactions and integrating catalytic and sorption functions within a single material. A significant research direction is the development of multicomponent nanocomposites that combine the high chemical activity of transition metal oxides (Fe₂O₃, CuO, MnO₂) with the thermal and sorption resistance of mineral carriers such as Al₂O₃, SiO₂, or TiO₂. These materials allow for the precise control of the surface microstructure by increasing the number of active sites and enabling effective operation over a wide temperature range. Intelligent combustion additives that adapt their properties to changing operating conditions will become increasingly important in the context of industry. Examples include catalysts with controlled oxygen or hydrogen release, which automatically respond to an increased CO or unburned ash content. When combined with online monitoring technologies (exhaust gas composition and flame temperature sensors), it will enable the dynamic control of the combustion process, the minimization of emissions, and an increase in efficiency. Another development direction is the combining of catalytic additives with denitrification (DeNO_x) reagents such as urea or ammonia, which in turn will enable combustion and NO_x reduction processes to be integrated. Hybrid catalytic systems that combine SCR/SNCR properties with catalytic combustion are currently one of the most promising research directions in the power industry. Simultaneously, work is underway on sustainable, eco-friendly combustion modifiers, in which traditional metal salts are replaced with organic–mineral complexes or biocatalysts that reduce persistent pollutant emissions. Natural materials such as halloysite, kaolin, and dolomite, which can act as both catalysts and pollutant sorbents, are a source of inspiration here.

To sum up, the further development of pulverized coal combustion modifier technology is moving toward multifunctional systems that are capable of simultaneously activating oxidation and reduction reactions, stabilizing the thermal process, sorbing pollutants, and protecting heat exchange surfaces. The use of modern nanostructures, composites, and hybrid materials will significantly increase the energy efficiency of pulverized coal boilers in the future, while at the same time reduce pollutant emissions and operating costs.