The Biorefinery Paradigm: Technologies, Feedstocks, and Retrofitting for Future Sustainable Energy

Abstract

Biorefineries offer a sustainable approach to producing fuels, chemicals, food, and feed from biomass, presenting a viable strategy for mitigating greenhouse gas (GHG) emissions and reducing reliance on fossil fuels. This review provides a comprehensive overview of the biorefinery concept, with a particular focus on its integrated conversion processes, classification pathways, and the potential for retrofitting existing fossil fuel refineries. Emphasis is placed on the Gulf Cooperation Council (GCC) region, home to some of the world’s largest hydrocarbon processing infrastructures, as a strategic case study for deploying biorefinery technologies. This review presents the latest trends in integrated biorefinery configurations and the potential for upgrading to drop-in fuels. It examines conventional biorefineries in the GCC, outlines their processing capacities, and explores suitable biomass feedstocks that thrive under the region’s high-temperature and high-salinity conditions. By highlighting both technological advancements and regional opportunities, this study underscores the potential for leveraging existing infrastructure in oil-rich nations to facilitate the transition toward sustainable bioenergy systems.

1. Introduction

The world is currently experiencing a critical era in which society must make the difficult but necessary shift to a clean environment and sustainable development. The United Nations (UN) has identified several Sustainable Development Goals (SDGs), including affordable and clean energy, industry innovation and infrastructure, and climate action [1]. One option for achieving these goals is to replace crude oil with a clean energy resource while gradually promoting sustainability. Using the advantages of rapidly evolving biomass energy conversion technologies and well-established crude oil refining, the biorefinery concept has emerged as a viable strategy. Historically, the biorefinery concept first gained prominence in the late 1990s, driven by growing concerns about environmental degradation and the depletion of fossil fuel resources (coal, natural gas, and petroleum). The concept aimed to mimic the petroleum refinery model but using renewable resources, such as plant-based biomass or organic waste, instead of crude fossil fuel, while also considering socioeconomic and environmental factors, such as greenhouse gas mitigation [2]. When using organic waste, the biorefinery concept also serves as a sustainable waste management method, in addition to producing chemicals and refined fuels such as gasoline, diesel, jet fuel, and heating oil in an environmentally friendly and sustainable manner. The International Energy Agency (IEA) Bioenergy Task 42 [3,4] broadly defines a biorefinery as “a sustainable facility that converts biomass into a variety of marketable products and energy”. The National Renewable Energy Laboratory (NREL) and the United States Department of Energy (UoE) have been more specific by identifying biorefining as a facility that integrates biomass conversion processes and equipment to produce a spectrum of fuels and high-value chemicals [1,3,4]. Naira et al. [5] stated that the concept of a biorefinery is similar to that of a conventional crude oil refinery, except that, unlike fossil fuels, biomass must be processed to produce biofuels and platform chemicals. As per the primary report of Energy Transition Outlook 2023 [6], bioenergy is currently the most preferred renewable energy source and is predicted to play a crucial role in achieving renewable energy targets by 2050.

This review aims to illustrate the biorefinery concept and recent advancements in the sustainable production of energy carriers and co-products derived from biomass feedstocks. Covering 2004–2025, this review examines peer-reviewed papers and status reports to compare approaches and identify gaps. A transparent, reproducible workflow used Scopus, Web of Science, and Google Scholar, plus targeted searches of reputable agency reports; titles/abstracts and then full texts were screened with a standardized extraction template, and comparative tables harmonized terminology and metrics across studies. The review provides a timely synthesis and, to our knowledge, is among the first to examine the potential for biorefineries and retrofit strategies in Gulf Cooperation Council (GCC) countries. GCC countries (United Arab Emirates, Saudi Arabia, Oman, Qatar, Kuwait, and Bahrain) collectively produce about 23.2% of global crude oil, with about 17 million barrels per day [7] and operating approximately 21 refineries [8]. They are beginning to explore pathways for bioenergy deployment and integration with existing infrastructure. Despite growing interest, very limited publications have offered a comprehensive, GCC-focused assessment and retrofitting. This review also provides a brief overview of the different types of biorefineries, categorized based on key features such as feedstock, processing technologies, conversion platforms, and end products. It further explores the feasibility of retrofitting existing fossil fuel refineries into biorefineries, using the major oil-producing GCC countries as a case study, and highlights the associated challenges and prospects for biorefinery development in this region. In addition, it explores existing biorefineries in the GCC, detailing their processing capacities and assessing their potential for further conversion and integration.

2. Biorefinery Concept

The term “biorefinery” originates from the use of biological or organic feedstocks as raw materials, and from the refining processes employed to convert them into valuable products [9,10]. The biorefinery strategy comprises multistep technological processes that are applied subsequently or in parallel:

• Pretreatment (the treatment of the feedstock for further processing).
• Conversion of the biomass into useful forms of energy, fuels, chemicals, or other products through four main categories of processes: thermochemical, biochemical, mechanical/physical, and chemical processes.

An overview of the biorefinery concept is shown in Figure 1, illustrating the selection of raw materials (classified into four generations: edible crops, lignocellulosic biomass, algae, and non-edible oil crops), transformation technologies (thermochemical, biochemical, physical, and chemical), and the resulting value-added products. Biomass classification under various categories, such as first and second generation, including edible crops and agricultural waste, has been reported in several studies [11,12]. Most recently, there has been growing interest in synthetic biomass, such as genetically modified algae and engineered microorganisms, which are classified as fourth generation [13,14]. The system described in Figure 1 supports the production of bioenergy, biofuels, biomaterials, biochemicals, and food products through the sustainable utilization of biomass and optimized processing pathways.

A biorefinery can produce gaseous biofuels (biogas, syngas, hydrogen, methane), liquid biofuels for transportation (bioethanol, biodiesel, FT fuels, pyrolysis bio-oil), and solid biofuels (pellets, lignin, biochar). A biorefinery can also produce important chemical and material products such as (1) chemicals, including fine chemicals, building blocks, and bulk chemicals; (2) organic acids, such as succinic, lactic, and levulinic acid; (3) polymers and resins, such as starch-based plastics, phenol resins, and furan resins; (4) biomaterials, such as pulp, paper, and cellulose; (5) food and animal feed (protein-rich); (6) fertilizers; and (7) biopolymers, such as polylactic acid. Taking algal biomass as an example, this feedstock can be thermally broken down in a biorefinery to produce different kinds of biofuels through pyrolysis, gasification, combustion, or hydrothermal liquefaction processes. Recently, the emphasis on using microalgae to produce a single product has shifted to using a biorefinery technique for producing numerous products in addition to lipid-derived biofuels. As demonstrated in Figure 2, microalgae lipid can be converted into biodiesel through triglyceride and alcohol reactions in the presence of a catalyst [16]. To obtain the highest possible value of algal materials, the residue from the generation of biodiesel and high-value co-products can be fed into a thermochemical or biochemical process.

3. Biorefinery Classification

Studies differ in their classification approaches; some adopt the feature-based approach of IEA Task 42, which organizes systems by feedstock, platform, process, and products [3]. This approach is useful for comparing technology suites and policy planning, and it is widely adopted in status reviews. Other studies use the generational classification (first- to fourth-generation biofuels), which indicates feedstock sustainability and technology maturity. This approach is useful for policy framing, but less useful for process design comparisons [18]. Other studies classify biorefineries according to platform and value-chain focus for the technical pathway [19]. Most researchers integrate the above perspectives, where the generation, platform/process, and product portfolio (for economics) are used together to form a network and pathways. The next section will discuss classifications based on platform and process, while classification based on feedstock and products is discussed in a separate section on biorefinery pathways.

Classification of biorefineries based on the feedstock, platform, and products enables the comparison of different biorefinery systems, enhances our understanding of the global evolution of biorefineries, and helps us to identify technological gaps [20]. Feedstocks such as starch and sugar crops, lignocellulosic crops and their residues, aquatic biomass, and organic waste can be processed to create various “platforms” or biorefinery streams. Through a combination of chemical, biological, and thermal processes, these key platforms can subsequently convert intermediate products into a variety of final products that are profitable. This section describes classification based on platforms, processes, and pathways.

3.1. Platform Classification

Platforms are intermediates that link feedstocks and final products. Similarly to how crude oil is fractionated into many intermediates and products, the platform concept resembles that used in the petrochemical sector [20]. Since these platforms can be obtained through various conversion processes, they are recognized as the primary pillars of biorefinery classification. The most significant platforms in energy-driven biorefineries are summarized in Table 1 (in no order), along with brief comments on the technology employed in each. A brief discussion of each platform type is provided below.

In the biogas platform, high-moisture-content biomass, such as manure, food waste streams, or organic solids from municipal effluent treatment systems, can be processed in an anaerobic digester to produce a mixture of gas, primarily CH₄ and CO₂ [21]. Increasing the methane yield and the economic efficiency of the produced biogas can optimize this platform, in addition to the extraction of nutrients from the digestate streams for fertilizer. On the other hand, the syngas platform enables the production of a synthesis gas (syngas) mixture, mainly consisting of CO and H₂. Syngas, which is produced through the thermal conversion (gasification) of feedstock like organic residues and grasses, can be utilized in power generation or converted into lower alcohols, fuel (e.g., Fischer–Tropsch diesel), and chemical products [1,22]. Syngas can also be fermented to produce methanol, ethanol, ammonia (NH₃), and potentially other chemical building blocks [23]. In addition, through processes such as gasification coupled with an enhanced water–gas shift reaction, steam reforming, water electrolysis, and fermentation, the hydrogen platform enables high biomass-to-hydrogen conversion yields [4]. Among ecologically acceptable alternatives for many applications, H₂ produced from renewable sources is anticipated to be a powerful energy vector that can easily be stored, transported, and used [24]. Some of the available biomasses that can be used as feedstocks for hydrogen production are agricultural wastes, sawdust from forests, palm trees, municipal, and food wastes.

Sugar platforms implement processes to degrade sucrose into glucose or to hydrolyze starch or cellulose into glucose. In fermentation processes, glucose is used as a feedstock to provide many valuable chemicals. The C6 sugar platform utilizes six-carbon sugars such as glucose, fructose, and galactose (C₆H₁₂O₆), derived from sucrose or obtained through the hydrolysis of starch, cellulose, and hemicellulose. These sugars serve as feedstock for biological fermentation processes, including biofuel production and the synthesis of biochemicals and bioplastics [25]. Meanwhile, the mixed C5 and C6 sugars platform is based on mixed streams of six-carbon sugars (C₆H₁₂O₆) from the hydrolysis of hemicelluloses and side streams of five-carbon sugars (C5), such as xylose and arabinose (C₅H₁₀O₅) [26]. Various useful compounds can be produced by chemically modifying these streams. The mixed six- and five-carbon carbohydrates can undergo selective dehydration, hydrogenation, and oxidation reactions to produce wider products, such as furfural, levulinic acid, sorbitol, hydroxymethyl furfural (HMF), and glucaric acid.

The lignin platform presents a major chance to improve lignocellulosic biorefinery performance. As much as 30% of the weight and 40% of the energy content of lignocellulosic biomass comes from this incredibly abundant source material [27]. Due to its natural structure, lignin may be a key component in developing novel chemical feedstocks, particularly for synthesizing aromatic and supramolecular compounds. Lignin is a source of syngas products (methanol, di-methyl ether, ethanol, mixed alcohols, Fischer–Tropsch liquids, C1–C7 gases), hydrocarbons (like benzene, toluene, xylene, cyclohexane, styrene, and biphenyls), phenols (like substituted phenols, catechol, cresols, and coniferols), oxidized products like vanillin, and macromolecules like carbon fiber filters, polymer extenders, and pharmaceuticals [28].

The pyrolysis liquid platform, produced through thermal depolymerization of biomass, is a multi-component mixture of different-sized molecules, such as some petroleum-derived oils [29]. Therefore, the design of a biorefinery that uses pyrolysis oil is similar to a conventional refinery. The pyrolysis oil from different installations can be gathered at the biorefinery, where it is separated into different fractions. Various technologies can be applied to upgrade each fraction and ultimately obtain the best blend of high-value and low-value products from the pyrolysis oil. The main high-value compounds that are anticipated are levoglucosan, furfural, organic acids, and phenols [30,31,32]. In addition, upgrading pyrolysis oil through hydro-processing will produce sulfur-free biodiesel and lighter biofuels. For the plant-based oil platform, the main sources of oils (oleochemicals) with no-feed applications include palm, soy, coconut, algae, and waste oils from food-related activities. Oleochemicals are widely used in the production of biofuels, lubricants, coatings, adhesives, elastomers, sealants, household and industrial cleaning products, personal care items, pharmaceuticals, and nutraceuticals. The products obtained are cost- and performance-competitive compared to similar products from the petrochemical industry, while being non-toxic and environmentally friendly.

An algae-based platform enables easy conversion of algae in a biorefinery through thermochemical or biotechnological transformations, as the feedstock does not contain lignocellulose [33,34]. Algae can be produced on a large scale in all seasons with sunlight, CO₂, nutrients, and water, comprising a remarkable sector in the bioeconomy. Compared to palm oil, oil productivity per land area can be as much as ten times higher [28]. Additionally, microalgae can be cultivated in brackish water or saltwater on non-arable soil and do not compete with conventional agriculture for resources. Several possibilities exist for their utilization, including triglycerides for producing biodiesel, fertilizers, additives for the food industry, nutritional supplements, or important compounds such as chlorophyll-a, ß-carotene, phycocyanin, and γ-linolenic acid [1,35]. Pigments, antioxidants, fatty acids, vitamins, antiviral, antifungal, antimicrobial toxins, and sterols are other beneficial substances [36,37].

In the organic juice platform, wet biomass is first processed via dewatering and screw pressing, producing a nutrient-rich juice and a lignocellulosic cake. The juice, rich in proteins, amino acids, and carbohydrates, is refined using membrane technologies and used for fermentation, feed, or bioethanol and lactic acid production. Anaerobic digestion also generates energy. The fiber-rich cake contains cellulose, lignin, and pigments, and is utilized in feed, fiber products, or as feedstock for syngas and sugar platforms. The electricity and heat platform can be used internally to meet the energy needs of the biorefinery or sold to the grid. Effective use of solid residues (lignin and humins) through co-firing with coal or biomass can be integrated with a combined heat and power (CHP) plant or gasification plant, allowing for the economical use of excess steam and heat from the power plant.

Table 1. Biorefinery platforms for the production of biofuel and added-value products.

No	Platform Name	Technology Employed	Main Feedstock	Products	Utilization	Ref.
1	Biogas	Anaerobic digestion	Manure, food waste, and MSW biosolids	Mixture of gas (mainly CH4 and CO2)	Production of biomethane and hydrogen. Produces fertilizers as byproducts.	[21]
2	Syngas	Gasification	Organic residues and grasses	Synthesis gas, mainly CO and H2	Power generation. Production of alcohols, chemicals, Fischer–Tropsch diesel, methanol, ethanol, NH3, and building blocks.	[1,22,23]
3	Hydrogen	Water–gas shift reaction, steam reforming, water electrolysis, dark fermentation	Organic matters, lignocellulosic crops, and residues	H2 gas	Production of energy and chemicals.	[4,24]
4	C6 sugar	Hydrolysis process	Starch, cellulose, and hemicellulose	Six-carbon sugars such as glucose, and galactose (C6H12O6)	Production of ethanol. Synthesis of biochemicals and bioplastics	[25]
5	Mixed C6 and C5 sugars	Hydrolysis process	Starch, lignocellulosic crops, and residues	Mixed streams of six-carbon sugars (C6H12O6) and five-carbon sugars (C5H10O5)	Production of biofuel, furfural, levulinic acid, sorbitol, hydroxymethyl furfural (HMF), and glucaric acid.	[1,26,38]
6	Lignin	Pretreatment and separation of lignocellulosic crops and wastes	Lignocellulosic crops and residues	Lignin	Production of syngas. Produces liquid hydrocarbons, phenols, oxidized products, and macromolecules	[27,28]
7	Pyrolysis liquid	Thermal depolymerization (pyrolysis)	Oil-based residues, lignocellulosic crops, and residues	Pyrolysis liquid	Production of levoglucosan, furfural, organic acids, phenols, and polymers. Production of sulfur-free biofuels.	[30,31,32]
8	Plant-based oil	Pressing/disruption	Oil crops and oil-based residues	A mixture of methyl and/or ethyl esters of fatty acids	Triglycerides, fatty alcohol, epoxides, soap, and biodiesel.	[39]
9	Algae-based	Produced in all seasons with sunlight, CO2, nutrients, and water, then cultivated	Marine biomass	Microalgae can include significant amounts of protein, carbohydrates, and lipids	Produces biodiesel, fertilizers, additives for the food industry, nutritional supplements, pigments, fatty acids, vitamins, antimicrobial toxins, and sterols.	[1,35,36,37]
10	Organic juice	Dewatering and screw pressing	Wet biomass (starch crops, grain, grasses, and organic residues)	A nutrient-rich juice and a fiber-rich lignocellulosic cake	Production of energy. Production of proteins, amino acids, lactic acid, bioethanol, cake as feed pellets, fibers, lignin, syngas, and sugars.	[40]
11	Electricity and heat	Co-firing	Solid residues (lignin and humins)	Power and heat	Integrated with a combined heat and power (CHP) plant or gasification plant. Economical use of the excess steam and heat from the power plant.	[41]

Table 1. Biorefinery platforms for the production of biofuel and added-value products.

No	Platform Name	Technology Employed	Main Feedstock	Products	Utilization	Ref.
1	Biogas	Anaerobic digestion	Manure, food waste, and MSW biosolids	Mixture of gas (mainly CH4 and CO2)	Production of biomethane and hydrogen. Produces fertilizers as byproducts.	[21]
2	Syngas	Gasification	Organic residues and grasses	Synthesis gas, mainly CO and H2	Power generation. Production of alcohols, chemicals, Fischer–Tropsch diesel, methanol, ethanol, NH3, and building blocks.	[1,22,23]
3	Hydrogen	Water–gas shift reaction, steam reforming, water electrolysis, dark fermentation	Organic matters, lignocellulosic crops, and residues	H2 gas	Production of energy and chemicals.	[4,24]
4	C6 sugar	Hydrolysis process	Starch, cellulose, and hemicellulose	Six-carbon sugars such as glucose, and galactose (C6H12O6)	Production of ethanol. Synthesis of biochemicals and bioplastics	[25]
5	Mixed C6 and C5 sugars	Hydrolysis process	Starch, lignocellulosic crops, and residues	Mixed streams of six-carbon sugars (C6H12O6) and five-carbon sugars (C5H10O5)	Production of biofuel, furfural, levulinic acid, sorbitol, hydroxymethyl furfural (HMF), and glucaric acid.	[1,26,38]
6	Lignin	Pretreatment and separation of lignocellulosic crops and wastes	Lignocellulosic crops and residues	Lignin	Production of syngas. Produces liquid hydrocarbons, phenols, oxidized products, and macromolecules	[27,28]
7	Pyrolysis liquid	Thermal depolymerization (pyrolysis)	Oil-based residues, lignocellulosic crops, and residues	Pyrolysis liquid	Production of levoglucosan, furfural, organic acids, phenols, and polymers. Production of sulfur-free biofuels.	[30,31,32]
8	Plant-based oil	Pressing/disruption	Oil crops and oil-based residues	A mixture of methyl and/or ethyl esters of fatty acids	Triglycerides, fatty alcohol, epoxides, soap, and biodiesel.	[39]
9	Algae-based	Produced in all seasons with sunlight, CO2, nutrients, and water, then cultivated	Marine biomass	Microalgae can include significant amounts of protein, carbohydrates, and lipids	Produces biodiesel, fertilizers, additives for the food industry, nutritional supplements, pigments, fatty acids, vitamins, antimicrobial toxins, and sterols.	[1,35,36,37]
10	Organic juice	Dewatering and screw pressing	Wet biomass (starch crops, grain, grasses, and organic residues)	A nutrient-rich juice and a fiber-rich lignocellulosic cake	Production of energy. Production of proteins, amino acids, lactic acid, bioethanol, cake as feed pellets, fibers, lignin, syngas, and sugars.	[40]
11	Electricity and heat	Co-firing	Solid residues (lignin and humins)	Power and heat	Integrated with a combined heat and power (CHP) plant or gasification plant. Economical use of the excess steam and heat from the power plant.	[41]

3.2. Process Classification

The processes involved in the conversion of biomass feedstock into added-value products are classified in four main subgroups [20]:

• Mechanical/physical, such as cleaning, drying, and grinding.
• Biochemical processes such as anaerobic digestion, aerobic and anaerobic fermentation, and enzymatic conversion.
• Chemical processes such as hydrolysis, transesterification, hydrogenation, oxidation, and pulping.
• Thermochemical processes such as pyrolysis, gasification, hydrothermal upgrading, and combustion.

These processes require substantial energy and resources. The source of electricity and heat for the system’s energy needs is also identified by this process classification. At the beginning of any refining process, biomass may be initially separated by physical or chemical processes after or before mechanical pretreatment (by pressing, milling, etc.). One example of the separation process is reducing the biomass into its constituent components (such as wood biomass to cellulose, hemicelluloses, and lignin). Then, the cellulose can be converted to glucose via the saccharification process, and the glucose can be converted to ethanol and lactic acid through a fermentation process, while the lignin can be thermochemically degraded to produce phenolic products. Alternatively, biomass can be subjected to microbiological, chemical, or thermochemical processes to produce biofuels. In thermochemical processes, gasification and pyrolysis are the two most mature methods for converting biomass into chemical products and energy. In gasification, syngas is produced by heating the biomass to a very high temperature, more than 700 °C, with or without oxygen [42,43]. Fischer–Tropsch fuels, dimethyl ether, ethanol, isobutene, and other fuels and chemicals can also be produced using syngas as a chemical platform. Alternatively, syngas can be used directly as an energy source. Pyrolysis decomposes biomass at temperatures ranging from 300 to 600 °C and in the absence of oxygen [29]. In pyrolysis, the process produces solid biochar, light gases, and liquid oil (pyrolytic oil). The yields of these products can vary depending on the pyrolysis process conditions [44]. Nevertheless, bio-oil is the most desirable product of pyrolysis. The biofuel produced can furthermore be converted or entered into a conventional refinery. For example, pyrolytic oil can be subjected to depolymerization and deoxygenation to improve its quality. In particular, deoxygenation is crucial to producing transportation biofuels since oxygen tends to lower the heat content of molecules and increases their polarity, thus limiting their ability to be blended with fossil fuels [45,46]. In some cases, the presence of oxygen in chemical products (such as organic acids and polyols) gives the substance useful physical and chemical properties, depending on the intended application.

One of the most popular lower-temperature processes is hydrothermal treatment, which usually entails heating biomass in water at temperatures between 120 and 250 °C, high pressures up to 20 bar, and at residence times of 10 to 50 min [43]. Because it may not require size reduction, this technology is typically used on a large scale, which lowers operating and capital costs. On the other hand, biochemical processes operate at relatively lower temperatures (less than 80 °C). Enzymatic hydrolysis, anaerobic digestion, and fermentation are the three most common types of biochemical processes. In addition to the production of other chemical compounds, such as hydrogen, methanol, and succinic acid, ethanol is the most common fermentation product because of the high demand for liquid fuel for transportation and the early discovery of the sugarcane-to-ethanol conversion process (and, later, corn-to-ethanol conversion). Chemical processes, such as transesterification and enzymatic hydrolysis, produce biodiesel, which mainly consists of fatty acid methyl esters (FAMEs) or fatty acid ethyl esters (FAEEs). In this process, the biomass undergoes molecular-level chemical changes when it reacts with other substances, either with or without catalysts.

3.3. Biorefinery Pathways

The biorefinery pathway is a useful term to describe how the feedstocks are converted to products via platforms (intermediates) and conversion processes. IEA Bioenergy Task 42 [3] considers the platform characteristic to be the most important component in defining and differentiating biorefinery conversion pathways, as these intermediates serve as the foundation for a variety of end products and their combinations. Determining the pathways for biorefinery conversion has several advantages, including (i) making it easier to comprehend where various marketable bio-based products come from and how they are made, (ii) making it easier for users to recognize and differentiate between different kinds of biorefineries, (iii) giving various stakeholders more precise and identifiable information, and (iv) making it easier to study existing and emerging biorefinery systems [47,48].

According to the “EU Biorefinery Outlook to 2030” project [49], eleven biorefinery pathways were identified, as illustrated in Figure 3. Only the platforms resulting from primary refining (or primary conversion) were utilized for the definition of these pathways. Due to variations in the secondary refining process and the handling of co-products and residual materials, there are multiple process alternatives for every biorefinery pathway. The primary platform number and name, along with the feedstock names, are used to standardize the naming of the biorefinery pathways. The biorefinery pathways were split into two broad types according to how the biorefineries can be developed (see the red dotted circles in Figure 3). The bottom–up biorefinery pathways (A–D) are demonstrated at a commercial scale with a technology readiness level (TRL) of 9, where the TRL represents one of the main components of the biorefinery complexity index (BCI), such that a high TRL means a lower biorefinery economic and technical risk [50]. The bottom–up approach considers the extension and/or upgrading of the conventional biomass processing facilities currently in place (such as the pulp mills, sugar, starch, and oil). The top–down approach pathways (E–L) are mostly at a lesser degree of maturity (around TRL in the range of 4–8), at least for the production of chemicals and/or materials [50]. Before they can be used commercially, some of these pathways with low technology readiness levels (TRLs) need improvements to be made at many stages through research, piloting, or demonstration.

Recent studies have compared the main conversion under biochemical, thermochemical, and chemical pathways. Shahzad et al. [51] found that while thermochemical approaches are restricted by high processing costs and temperature requirements, biological processes are constrained by long processing times and low yields. This is in agreement with Osman et al. [52], who state that the strengths of biochemical pathways come from the well-developed process and the high selectivity to specific molecules (fermentation products). Their weaknesses comprise their sensitivity to lignin and ash, which require intensive pretreatment and enzymes (higher cost) [52]. Meanwhile, thermochemical pathways are characterized by their robustness to feedstock variability and ash content, where gasification and pyrolysis can handle woody wastes, MSW, and high-ash feedstocks better. Also, HTL handles wet biomass without drying. Its strengths come from the flexibility of producing fuel products. In addition, it represents better carbon utilization in some scenarios. Its weaknesses include being capital-intensive, sensitive to tar/catalyst issues, and requiring costly upgrades of bio-oil [53]. Chemical pathways include transesterification of oils to biodiesel, catalytic upgrading, and hydrogenation. These are used when lipid and oil feedstocks (vegetable oil, waste cooking oil, algal oil) are available. These pathways are suitable for biodiesel production, although feedstock availability and fuel versus food concerns are important [54].

The hybrid and integrated pathways include thermochemical to biochemical combinations, such as syngas fermentation and pyrolysis substrate fermentation. There is an active research area that focuses on obtaining the best of both examples (gasification to syngas, then microbial fermentation to ethanol/ethanol equivalents; and pyrolysis-derived substrates fermented to products) [55]. Studies highlight promising yields and better resource recovery, but also show technological complexity and integration challenges. The relative advantage of biochemical vs. thermochemical depends on feedstock type, allocation rules for coproducts, local electricity grid carbon intensity, and scale [56]. Several comparative studies show comparable economies under different assumptions. According to other reviews, thermochemical pathways are better suited to handle diverse and wet waste feedstocks like MSW, sewage sludge, and energy crops with a high ash content, while biochemical pathways work best with feeds that are low in lignin and rich in sugar.

3.4. Biorefinery System Networks and Types

Three examples of the biorefinery network classification are illustrated in Figure 4. These are classified based on the number of platforms utilized. The pathways in each platform (feedstock, process, platform, and products) are represented by different shapes and colors. Figure 4a, which represents a single platform, shows a C6 sugar biorefinery that uses starch crops and produces ethanol and animal feed through a fermentation process [20], while Figure 4b, which represents multiple platforms (C5 and C6 sugars, electricity and heat, lignin) and processes (pyrolysis, combustion, and fermentation), shows a single biorefinery system that produces multiple products consisting of bioethanol, electricity, heat, and phenols [57]. A more detailed and comprehensive overview of the biorefinery system network encompassing major types of feedstock and various options of platforms, processes, and products is shown in Figure 4c. This network’s top level displays the different feedstock types that are converted into platforms and/or products through the combination of different processes. Some platforms and conversion processes are also connected, merging two or more separate biorefinery systems, because some processes are appropriate for many platforms. Therefore, the number of platforms involved is a measure of the complexity of the biorefinery system.

Recently, with the advancement of the biorefinery concept, new classifications have been made based on the raw material input, type of technology, status of technology, and the main or intermediate products, as presented in

Table 2. A summary of the most recent classifications and studies of biorefineries.

Type of Biorefinery	Processing Techniques	Feedstock	Products	References
Type 1: Green biorefineries	Pretreatment, Gasification, Compression, Methanation, Digestion, Pyrolysis, Adsorption, Distillation	Organic waste fractions and agricultural waste	Syngas, methanol, Fischer–Tropsch fuels, pyrolytic oil, biochar, and hydrogen	[59,60]
		Grass and silage	Lactic acid, proteins, and methane	[61,62]
		Red clover, clover grass, and oil seed radish	Methane, byproducts (press cake and brown juice)	[63]
		Mango waste	Pectin, oil seeds, polyphenols, and cattle feed	[64,65]
Type 2: Blended types of waste biorefineries	Pretreatment, Gasification, Compression, Methanation, Anaerobic Digestion, Distillation	Sludge and manure	Biogas, syngas, methanol, Fischer–Tropsch fuels	[66,67]
	Pretreatment, Gasification, Compression, Methanation, Adsorption, Distillation, Size Reduction, Heat Steam Recovery, Pyrolysis, Separation, Anaerobic Digestion	Municipal solid waste	Syngas, methanol, Fischer–Tropsch fuels, hydrogen, biogas, biochar, bio-oil, pyrolysis gas, olefins, aromatics, and energy	[67,68]
	Hydrolysis, Size Reduction, Pyrolysis, Adsorption, Saccharification, Fermentation, Separation, Anaerobic Digestion	Food waste	Biogas, biochar, bio-oil, pyrolysis gas, hydrogen, and lactic acid	[69,70]
	Hydrolysis, Fermentation, Separation, Anaerobic Digestion	Slaughter waste	Polyhydroxyalkanoate (PHA)	[71,72]
	Pretreatment, Transesterification, Separation	Waste oil	Fatty acid methyl esters (FAMEs), and glycerol	[73,74]
	Grinding, pyrolysis, Extraction, Transesterification	Date palm pits	Pyro-gas, pyrolytic oil, biochar, and biodiesel	[75,76]
Type 3: Whole-crop biorefineries	Pretreatment, Hydrolysis, Size Reduction, Enzymatic Hydrolysis, Dark Fermentation, Saccharification, Separation, Distillation	Wheat straw	Butanol, bioethanol, hydrogen, and biogas, butyric acid, ethanol, biomethane, electricity, and phenols	[77,78]
	Pretreatment, Drying, Hydrolysis, Size Reduction, Gasification, Fermentation, Dehydration, Distillation	Sugarcane	Ethanol, electricity, gypsum, fertilizers, animal feed, sugar, power, n-butanol, acetone, and butanol	[79]
Type 4: Lignocellulose-based biorefineries	Dilute Acid Pretreatment, Hydrolysis, Size Reduction, Enzymatic Hydrolysis, Pyrolysis, Combustion, Saccharification, Fermentation, Dehydration, Heat Recovery, Evaporation, Separation, Distillation	Corn stover	Ethanol, succinic and acid electricity, biomethane, phenols, gypsum, methane, phthalic anhydride, naphtha, diesel range fuels, and bioethanol	[80,81]
	Dilute Acid Pretreatment, Simultaneous Saccharification, Fermentation, Distillation	Corn	Ethanol	[82]
	Drying, Aerobic Digestion	Yellow onion	Quercetin and biogas	[83]
	Incineration, Extraction	Birch forest	Electricity and betulin	[83]
	Pretreatment, Gasification, Compression, Methanation, Adsorption, Heat Steam Recovery, Separation, Distillation	Wood and forest waste	Gas, methanol, dimethyl ether, Fischer–Tropsch fuel, ammonia, hydrogen	[84,85]
	Pretreatment, Hydrolysis, Size Reduction, Gasification, Pyrolysis, Compression, Adsorption, Fermentation, Heat Steam Recovery Separation, Distillation	Lignocellulose	Gasoline, diesel, Fischer–Tropsch fuel, biochar, methanol intermediates, bio-oil, ethylene, propylene, acetone, butanol, ethanol, biogas, and hydrogen	[86,87,88,89]
	Pretreatment, Drying, Dilute Acid, Washing, Hydrolysis, Enzymatic Hydrolysis, Simultaneous Saccharification and Fermentation, Pyrolysis, Aerobic Digestion, Distillation	Salicornia bigelovii	Ethanol, biogas, biochar, and bio-oil	[90,91]
	Size Reduction, Pyrolysis, Dehydration, Separation, Distillation	Jatropha curcas	Light gases, naphtha, jet fuel, and diesel	[92]
	Pretreatment, Hydrolysis, Combustion, Fermentation, Aerobic Digestion	Switch grass	Bioethanol, biomethane, electricity, and phenols	[93]
	Pretreatment, Size Reduction, Saccharification, and Distillation	Palm oil tree	Bioplastic and poly(3-hydroxybutyrate)	[94,95]
Type 5: Marine biorefineries	Drying, Harvesting, Transesterification, Extraction, Anaerobic Digestion, Pyrolysis, Distillation	Microalgae	High-value products, power, biodiesel, bioethanol, biogas, glycerol, and bio-oil	[96,97,98]
	Pretreatment, Washing, Fermentation, Separation, Anaerobic Digestion, Distillation	Microalgae (Chlorella strain)	Naphtha, biogas, renewable diesel blendstock, and anaerobic digestate	[99,100]
	Dilute Acid Pretreatment, Dark Fermentation, Anaerobic Digestion, Pyrolysis	Brown macroalgae	Hydrogen, pyrolytic oil, biochar, and methane	[98,101]

. Green biorefineries (Type 1), where green biomass is separated into a fiber-rich press cake containing cellulose, starch, dyes, and protein-rich juice composed of proteins and free amino acids, are becoming well known and popular. In Type 2 biorefineries, which are more relevant to regions of limited plant-based biomass, several types of waste streams can be blended in waste biorefineries; however, it is necessary to make sure that these input streams are comparable in structure and composition so that the processing steps can tolerate a certain amount of feedstock heterogeneity. Whole-crop biorefineries (Type 3) regard whole crops as the first-generation feedstock. These include sugar crops like sugarcane, sugar beet, and sorghum, as well as a range of cereal crops such as wheat, corn, potato, and cassava [58]. Due to the ongoing disputes around food versus fuel and the developments in lignocellulose processing, agricultural residue is now the preferred feedstock for biorefineries, and this is identified under Type 4 (lignocellulose-based biorefineries). For a long time, macroalgae and seaweeds have generated considerable interest in bioenergy applications due to their high potential to produce bio-products and biofuels. This is identified under Type 5 biorefineries, where microalgae grown in saline and brackish water are used as feedstock. Type 5 is particularly relevant to coastal arid and semi-arid regions, where marine microalgae can be cultivated for large-scale biorefinery operations.

4. Retrofitting for Biomass-Based Processing

By adapting existing infrastructure which was originally designed to process crude oil or natural gas, fossil fuel refineries can be retrofitted to handle biomass-based feedstocks. The utilization of existing assets allows for a reduction in the cost and time required to build completely new facilities. Also, retrofitting processes support energy transition, providing a practical pathway for traditional fossil fuel industries to transition toward cleaner and more sustainable energy systems. Another benefit of retrofitting processes is that they can help retain jobs in traditional refining sectors. Retrofitting may involve all or some of the following:

• Feedstock conversion by using biomass sources in place of fossil fuel feedstocks, which may involve preprocessing steps like pyrolysis, grinding, or drying.
• Process modification and technological integration, which may involve modifying essential components (e.g., by reforming, hydrotreating, fermentation, gasification, etc.) to manage the various chemical compositions and contaminants present in the biomass.
• Material upgrades to make sure that the materials used in equipment can withstand the more caustic nature of intermediates made from biomass.

4.1. Technical and Economic Feasibility of Retrofitting

The integration of a new biorefinery into an existing fossil fuel refinery is both technically feasible and economically beneficial. Although the catalytic processes involved are similar, it is technically possible to upgrade biorefinery intermediates, such as stable pyrolysis oil, using the conventional refinery units like hydrocrackers and fluid catalytic cracking (FCC) units with minimal retrofitting. Effective co-processing without significant technological changes is made possible by this compatibility. In a study by Mahmoud and Shuhaimi [102], the proposed integrated plant network shown in Figure 5 was found to significantly improve profitability. The existing petroleum refinery units, shown in blue, consist of the standard four sections: fractionation, hydrotreating, upgrading, and blending to process single or mixed crude oils. The proposed standalone biorefinery is shown in green. Part of the stable pyrolysis oil after passing through the debutanizer unit (De-C4) can be mixed with the treated gas oil in the refinery at an optimum S1/T4 ratio equal to 20/80 wt% [103] before entering the fluid cracking unit. The recovered light naphtha (LTN) from the naphtha splitter can be upgraded into the catalytic reformer of the existing petroleum refinery. The heavy stabilized oil stream can be a feedstock to the existing hydrocracker in the petroleum refinery. The study demonstrated that the integrated enterprise plant design increased overall profit by 2.82% (USD 21.5 million per year) compared to operating the facilities separately. Furthermore, integration shortened the biorefinery’s investment payback period from 6.9 to 4.7 years (lowered by 32%), enhancing financial viability. The shared infrastructure, hydrogen supply, and blending pools contribute to lower capital and operational costs while optimizing product yields such as gasoline and diesel. The integration approach not only supports the sustainable production of transportation fuels but also strategically positions refineries to meet future low-carbon fuel standards more cost-effectively.

A study by Tanzil et al. [104] showed that co-processing biorefinery intermediates within existing refinery units reduces the minimum fuel selling price (MFSP) by 10–19% compared to standalone biorefineries, while fully repurposing shutdown refinery assets can lower MFSP by up to 34%. Technically, this integration is enhanced by the similarity of downstream operations between biomass upgrading and conventional petroleum refining, including hydrotreatment, isomerization, and product distribution. The integration scenarios, including five lignocellulosic SAF technologies, were formulated based on the existing facilities’ capital structure [104], as shown in Figure 6.

On the other hand, an experimental study by Eschenbacher et al. [105] demonstrated that co-processing bio-oil with conventional fossil fuels in a refinery fluid catalytic cracking (FCC) unit requires careful consideration to avoid issues related to metals, oxygen, and sulfur-containing compounds, aromatics, and basic nitrogen present in the bio-oil. When upgraded bio-oils—obtained from the catalytic treatment of wheat straw pyrolysis vapors—were blended with atmospheric residue for co-processing, the result was an increase in yields of dry gas, CO, CO₂, and coke, accompanied by a 2.8% decrease in naphtha yield and an overall conversion drop of approximately 2.5%. This is because of the high aromaticity and basic nitrogen content in bio-oils. Hence, co-processing bio-oils, especially from hydrothermal liquefaction into jet-range fuels, is technically possible in refineries that already process vacuum gas oils (VGOs) or other middle distillates. The advantage of this approach lies in avoiding the duplication of infrastructure, where bio-oils can be introduced at various points in the refining process, thereby reducing the need for new, capital-intensive equipment for bio-oil treatment. However, there remains the challenge of refinery catalysts being poisoned by metals and chemicals (such as oxygen, sulfur, and nitrogen) found in bio-oils [106].

4.2. Retrofitting Configuration

In a biorefinery, retrofitting may include integrating new processing units, redirecting existing material flows, altering operational configurations, or even repurposing idle units to support a different type of feedstock or product output. It is considered the most effective and affordable step to close the gap between the petroleum-based economy of today and the bio-based economy of tomorrow. The point where biocrude is introduced in a refinery may vary depending on the biocrude characteristics and the final product needed. The National Advanced Biofuels Consortium (NABC) [107] proposed three insertion points for processing biofuels in a petroleum refinery, as shown in Figure 7. Insertion point 1 is at the crude distilling unit (CDU) inlet that blend the raw biocrude produced from biomass with the crude oil; insertion point 2 is considered in certain processing units after the biocrude has been slightly upgraded; and insertion point 3 is at the blender pools with the finished fuel after biocrude upgrading.

The lowest risk to the refinery is when the upgraded biocrude is inserted alongside petroleum fuels as finished fuels, whereas the biggest risk is when it is inserted before the CDU stage [108]. The introduction of biocrude in some processing units after minor upgrading yields the highest advantage for the refinery. The fluid catalytic cracker, hydrotreater, or hydrocracker can potentially be used for co-processing since these processing units must be able to crack as well as remove oxygen from the biocrude [109,110]. Zhang et al. [111] used mixed-integer linear programming (MILP) models to optimize retrofitting of a conventional fossil-based refinery into a hybrid biomass-based refinery. Figure 8 illustrates the primary processes and material flow of six biomass-based technologies considered in the study. The results offered flexible design possibilities, for example, with the FCC being fully retrofitted with the biomass fast pyrolysis (BFP) technology, in which the FCC unit no longer imports crude products from a conventional refinery. The study concluded that FP and Virent’s BioForming (VB) technologies are more competitive than other alternatives and have greater retrofit compatibility, which makes them cost-effective and thus preferred in most planning scenarios.

5. GCC Countries Case Study

The term “GCC countries”, as used throughout this paper, refers to the six Middle Eastern nations: Saudi Arabia, the United Arab Emirates, Qatar, Kuwait, Bahrain, and Oman. In some energy-related studies, these countries are also referred to as the Gulf Countries. An arid climate, scarce freshwater resources, and sandy soils characterize GCC countries. Consequently, large-scale production of plant-based biomass for biorefineries or general bioenergy applications in this region FACES significant challenges. However, alternative feedstocks may be available from unconventional organic matter sources, which can be classified into four groups:

• Group 1: Halophyte crops and agricultural residues (e.g., date palm waste, Salicornia, and vegetable crop residues).
• Group 2: Animal and organic waste (e.g., manure, general household waste, food waste).
• Group 3: Industrial waste (e.g., paper waste, plastic waste, wood construction waste).
• Group 4: Aquaculture (e.g., algae, seaweed, fish waste).

5.1. Feedstocks

The data in Figure 9 represent the most up-to-date estimates of current biomass and organic waste in GCC countries, including the main feedstocks within the group classifications described above. Municipal solid waste (MSW) and animal waste are both highly promising feedstocks in the Gulf region, with consistently comparable quantities across all countries. Saudi Arabia has the highest potential for the supply of both feedstocks because it has the highest population and the largest land area. There are fewer opportunities for sewage waste and, to some extent, agricultural residue; therefore, the initial development of biorefineries in the region may rely heavily on MSW and animal waste rather than conventional plant-based biomass. The United Arab Emirates and the rest of the Gulf countries also generate a significant volume of solid waste from livestock activities, emphasizing the need to research applications for poultry industry waste and manure from camels, sheep, goats, horses, and cattle. These various biomass resources offer significant prospects for sustainable supply to biorefineries [112,113].

Table 3. Types of organic waste with high potential as feedstock for biorefineries in some GCC countries [117].

Country	Food Waste		Date Palm Waste		S. bigelovii 1
	Tons/Year	Kg/Capita/Year	Tons/Year	Kg/Capita/Year	Tons/Year	Kg/Capita/Year
Saudi Arabia	3,594,080	94.6	1,539,756	40.5	7,695,890	198.1
United Arab Emirates	923,675	81.9	323,478	28.7	299,288	25.9
Oman	470,322	62.6	372,572	49.6	1,108,010	144.1
Qatar	267,739	74.5	39,651	11.0	41,460	11.3
Bahrain 2	230,000	140.0	NA 3	NA	2792	1.7
Kuwait 2	400,000	91.2	118,953	27.1	63,788	14.2
Total	5,885,816		2,394,410		9,211,229

¹ Assuming 10% of each country’s arable land is used to grow Salicornia (full plant + seed). ² Food waste in Bahrain and Kuwait taken from [118,119]. ³ NA: not available.

presents data on specific high-potential feedstocks classified under Groups 1 and 2, namely date palm, food waste, and Salicornia. Food waste, a component of municipal solid waste (MSW), is generated in large quantities and ranks among the highest per capita in the world. Bahrain and Saudi Arabia have yearly food waste amounts of 140 and 94.6 kg per capita, respectively. The United Arab Emirates comes fourth, with food waste mostly from the hospitality sector, especially during the high tourism season. On the other hand, although there is currently a lack of thorough, reliable statistics, all of the Gulf countries deal with the same problems in managing food waste. Date palm waste, which is a major crop residue produced in the Gulf countries throughout the year, especially during the date fruit cultivation season, is significantly high in Saudi Arabia. However, Oman has the highest waste production per capita (49.6 kg/capita/year). S. bigelovii, a plant that can withstand salt, grows in coastal areas and also produces oil-rich seeds that can be used to produce biofuels [114,115]. Reliable statistics on the availability of this feedstock are not available; however, it is possible to predict the future supply based on recent studies on the potential cultivation scale. As shown in Table 3, it is estimated that this plant can produce a significant amount of biomass, nearly double that of food waste.

Apart from the challenge associated with biomass availability in GCC countries, there is the challenge of the significant difference in feedstock characteristics and degree of maturity in bioenergy applications. For example, organic waste, agricultural residues, and aquatic biomass differ in characteristics and processing technology, making the option of co-feeding more complex. To provide a simple guide, Figure 10 presents a chart that visualizes the relationship between potential feedstock availability and the potential for bioenergy applications using various biomass and waste types in the GCC region. Municipal solid waste (MSW) is the most attractive biomass source for large-scale, short-term bioenergy projects in the Gulf countries due to its abundance, supportive policies, and its well-established global use in bioenergy applications. Waste-to-energy initiatives are being implemented by countries such as Saudi Arabia and the United Arab Emirates as part of their circular economy strategies [120].

Halophytes such as S. bigelovii and aquatic organisms such as microalgae are predicted to play a role in bioenergy, particularly in the production of aviation fuel; however, this remains at the experimental stage [121,122]. Therefore, despite their anticipated moderate-to-high availability, it is more appropriate to view them as long-term research and development prospects rather than immediate additions to current bioenergy portfolios. Jatropha, similar to Salicornia, is an example of a well-suited halophyte for arid climates and salty soil conditions. Both crops can be grown on marginal lands that are unsuitable for traditional agriculture [114,123]. Jatropha has been used for biodiesel production in China and India, and, along with Salicornia, is now being explored as a potential biofuel feedstock in parts of the Middle East, Africa, and GCC countries, as discussed further below. Microalgae and seaweed have the advantage of containing significant glucan levels [124,125], indicating that these types of biomass contain fermentable sugar which can be converted into a wide range of bio-based products in a biorefinery through microbial fermentation. Furthermore, algae can grow in various water types and use wastewater or CO₂ to enhance their growth; therefore, they offer [126] a method for directly capturing and storing CO₂, either for the short term in the form of carbon sequestrated in biomass or longer term (100–1000 years) when applying biochar in soil following a pyrolysis process [127]. Also, other algae-derived pathways (with chemical and thermochemical processes) in biorefineries are feasible for biofuel production due to algae’s moderate-to-high availability. This makes algae very suitable due to its high potential for use on an industrial scale.

Animal manure and food waste have great potential for distributed or medium-sized projects, especially in rural areas or with hotel clusters. Significant quantities of animal waste are accessible and used in the production of biogas. For example, the GCC produces large quantities of waste from chicken farming and its associated industry, especially in Saudi Arabia. The emirates of Abu Dhabi and Dubai in the United Arab Emirates also produce plenty of organic waste as a result of their extensive livestock sector (including poultry, dairy, sheep, goats, and camels). On the other hand, food waste, despite its high per capita generation and rich lipid content, is available only in relatively limited quantities. However, efforts have been made to reduce food industry waste, such as the project led by Lootah Biofuels and municipality collection pilots in Dubai, where used cooking oil (UCO) is used as a biodiesel at a scale of about 500,000 L/month and about 770 tons of biodiesel are produced per year [128].

The major source of agricultural residues in GCC countries comes from the date palm tree. Although date palm waste is an abundant byproduct, it is quantitatively lower than other organic waste streams in the region. While industrial-scale utilization is technically feasible, it requires pretreatment and logistical development. The resulting biochar has potential as a carbon sink and soil amendment for desert agriculture. A case study in Saudi Arabia has shown that date palm fiber can be used for saccharification to bioethanol [129]. Sewage sludge produced at wastewater treatment plants is a sustainable source of organic matter. With increasing urbanization in GCC countries, it is expected to contribute to the supply of organic feedstock for bioenergy applications. However, its use is technically challenging due to its high ash content, and its quantity remains relatively low, particularly in countries with smaller populations.

Taking the United Arab Emirates as an example, the two feedstocks that have potential and are currently explored for bioenergy are aquatic biomass, such as microalgae, and drought-tolerant plants, such as S. bigelovii. This is because the United Arab Emirates is situated along a coastline that extends more than 2000 km and is close to vast empty sandy deserts [124]. S. bigelovii has been experimentally grown in several sites in the United Arab Emirates and is currently being explored to produce sustainable fuels. The Sustainable Bioenergy Research Consortium (SBRC) in the United Arab Emirates explored the potential of Salicornia and mangrove silviculture with additional co-benefits, using a system called the Seawater Energy and Agriculture System (SEAS) [130] to produce sustainable aviation fuel (SAF) and seafood in the same environment as the crop. The SEAS platform, shown in Figure 11, is designed to address the food–water–energy nexus from a multidisciplinary and comprehensive perspective [131]. Such a concept may benefit from large-scale implementation if developed and integrated with a biorefinery. On the other hand, seaweed screened across the island of Abu Dhabi, the United Arab Emirates capital, has demonstrated good growth in water with salinity between 50 and 70 ppt and with temperatures as high as 34 °C in the summer [124]. This could significantly enhance the commercial viability of biorefinery operations in GCC countries. Figure 12 shows a typical algal biorefinery with upstream and downstream processes [132]. This feedstock provides multiple products for various applications, including nanotechnology applications, which adds an attractive feature to the biorefinery and makes it economically attractive due to opening new market opportunities. Another example of a strategic industry move towards SAF is the recent partnerships between Saudi Arabia, TotalEnergies, and the Saudi Investment Recycling Company (SIRC). The project looks into the development of SAF and other low-carbon fuels, including animal fats, tallow, and electro-fuel pilots, with a view to establishing an SAF plant in the Kingdom [133].

5.2. Current Refineries and Transition to Biorefinery

In recent years, strategies for expanding biomass-based renewable energy have focused on utilizing existing infrastructure for energy production and petroleum refineries. Major American energy and logistics companies, such as World Energy, ExxonMobil, and Phillips 66, are actively evaluating proposals to convert their operating refineries to produce renewable fuels [102,134,135]. Similarly, green hydrogen, green ammonia, biofuels, renewable diesel, and sustainable aviation fuel (SAF) projects have been a key focus of strategic energy planning in the GCC [130,136]. This shift is driven by the need to diversify away from fossil fuel dependence, and several green energy facilities are expected to come online [137]. However, these projects have not yet been developed commercially in the region, likely due to several factors, including the region’s abundant crude oil reserves, the absence of laws and legislation requiring the use of biofuels, the lack of feedstock for biofuel production, and limited research-based knowledge in the field. Nevertheless, recent advances have been made in lignocellulosic biorefineries at the demonstration or pilot phase [104,138]. Globally, in Portugal, the Galp Energia, in partnership with Japan’s Mitsui, is constructing a hydrogenated vegetable oil (HVO) plant at the Sines refinery, aiming to produce 270,000 metric tons per year of biodiesel and SAF from waste materials like used cooking oils [139]. The facility will utilize green hydrogen generated by wind or solar-powered electrolysis and is expected to commence operations in 2026. As in the United Arab Emirates, an agreement has been signed between Masdar and TotalEnergies in Abu Dhabi to develop a commercial project that will convert green hydrogen to methanol and then to SAF [140]. This project aims to decarbonize industries, including aviation and maritime, by 2031 using green hydrogen and CO₂ from industrial sources.

GCC countries currently account for around 35% of the active refineries in the Middle East [137,141]. The crude distillation unit (CDU) capacity of these refineries is expected to grow by 10.7% by 2027 [142]. By country, Saudi Arabia is expected to have the highest CDU capacity in 2027 [143]. A list of operating, recently upgraded, and upcoming refineries in GCC countries with their processing capacities is presented in

Table 4. Major upgrades, new, and upcoming fossil fuel refineries in GCC countries.

Name/Location	Capacity	Country	Owner
Upgraded refineries [145]       (b/d)
Ruwais	837,000	United Arab Emirates	ADNOC
Sitra	267,000	Barain	Bapco
SASREF	305,000	Saudi Arabia	Aramco
Petro Rabigh	400,000	Saudi Arabia	Rabigh
Mina al-Ahmadi	466,000	Kuwait	KNPC
Mina Abdullah	270,000	Kuwait	KNPC
Riyadh	140,000	Saudi Arabia	Saudi Aramco
Jebel Ali	140,000	United Arab Emirates	ENOC
New refineries [145]          (b/d)
Al-Zour	615,000	Kuwait	KPC
Duqm	230,000	Oman	Joint
Lizan/Jazan	400,000	Saudi Arabia	Saudi Aramco
Kuwait	NA	Kuwait	KNPC
Brooge	180,000	United Arab Emirates	BPGIC
Upcoming refinery [137]     (kTPA)
Amiral complex	1650 (ethylene and propylene)	Saudi Arabia	Aramco and Total
Ras Laffan Petrochemical	2080 (ethylene)	Qatar	Chevron Philips and Qatar Energy
Borouge 4	6400 (polyolefins)	United Arab Emirates	ADNOC and Borealis
Al Zour Petrochemicals	2340 (ethylene)	Kuwait	KIPIC
Duqm Petrochemical	1600 (ethylene)	Oman	Joint

NA: not available, kTPA: kilo tonnes per annum, b/d: barrel per dat.

. These significantly high processing capacities offer the opportunity for retrofitting or complete conversion to sustainable biorefineries. According to Eni’s Oil World Review 2020 [144], the average Nelson Complexity Index (NCI), which is indicative of different cracking refineries, is approximately 7 for the Middle East. This is lower than the average complexity of refineries in the US and Europe. However, the GCC is home to several complex refineries, such as the Ruwais refinery in Abu Dhabi, which are undergoing expansions and will eventually create a more sophisticated plant with a 14 NCI.

5.3. Upgrade to Drop-In Biofuels

One of the objectives of biorefineries and their retrofitting with existing fossil fuel refineries is to produce drop-in biofuels. These biofuels are fully compatible with the current petroleum infrastructure and are functionally equivalent to petroleum-based transportation fuels [146]. Traditional drop-in biofuels are primarily based on the upgrading of lipids and oleochemicals. However, future advancements in GCC countries are likely to come from drop-in biofuels derived from organic waste and, at a later stage, from alternative feedstocks such as halophytes and aquatic biomass. Bio-oils can be produced via thermochemical processes, such as gasification, pyrolysis, and hydrothermal liquefaction. On the other hand, co-processing drop-in biofuels with conventional petroleum refining could significantly reduce capital costs.

Most current biofuels are oxygen-containing compounds such as bioethanol and fatty acid methyl ester (FAME) biodiesel, which are not typically classified as drop-in biofuels. In contrast, renewable diesel, also known as green diesel, hydrotreated/hydrogenated vegetable oil (HVO), hydrotreated esters and fatty acids (HEFAs), hydrotreated biodiesel, or hydrogenation-derived renewable diesel (HDRD), is considered a drop-in biofuel [147]. It is gaining increasing interest for use in sectors such as marine and rail, and also serves as a viable route for bio-jet fuel production. Removing oxygen is one of the major challenges in upgrading lipids or biomass into drop-in biofuels [146]. Oxygen lowers a fuel’s energy density and reduces its stability; therefore, biofuels containing oxygen can only be blended in low volumes. Additionally, the presence of oxygen-containing functional groups increases the risk of corrosion in pipelines and storage tanks, as they tend to attract water. The most common process for upgrading bio-oil in refineries is hydrotreating [148], a catalytic process in which bio-oil reacts with hydrogen to remove oxygen, yielding alkanes (e.g., paraffins, which are components of renewable diesel). The catalyst and conditions used in the upgrading process are very similar to those used in the petroleum refining industry, either nickel-molybdenum or cobalt-molybdenum [149,150].

Previous research used an effective H/C ratio to rank the different potential lipid/biomass feedstocks for drop-in fuel, as shown in Figure 13a. This provides an indication of the level of upgrading that will normally be necessary to produce a deoxygenated drop-in biofuel. Additionally, it demonstrated how upgrading will raise the need for hydrogen as biomass’s oxygen concentration rises. Research indicates that feedstocks with a higher H/C ratio are more easily upgraded than those with a lower ratio, and that low H/C feedstocks are frequently associated with other characteristics, such as increased coking [151]. In contrast, a feedstock with a higher O/C ratio is more difficult to upgrade and needs an external source of hydrogen supply. According to this analysis, the level of upgrading required for feedstocks of potential use in GCC countries, ranked based on their H/C and O/C ratios, is illustrated in a Van Krevelen diagram shown in Figure 13b. Sewage sludge, located in the lower right corner of Figure 13b (with the highest H/C and moderate O/C ratios), may produce biofuels that require the least upgrading in biorefinery processing. In contrast, biofuels derived from animal waste, having the lowest H/C and highest O/C ratios, may prove to be the least favorable due to the extensive upgrading required.

5.4. Challenges and Future Perspectives

Retrofitting conventional petroleum refineries into biomass-based biorefineries in GCC countries faces various technical and commercial challenges. These challenges are influenced by the region’s unique characteristics, including its infrastructure, feedstock availability, and market dynamics. Junghare et al. [152] identified five challenges in biorefineries: (i) the variability in feedstocks, (ii) the collection and coordination for transporting biomass, (iii) seasonal shifts, particularly for agricultural biomass, (iv) competition with land and the food supply chain, and (v) market viability and financial sustainability. This section discusses the key challenges and future perspectives related to the biorefinery concept in GCC countries.

5.4.1. Technical Challenges

Unlike crude oil, lignocellulosic biomass feedstocks vary widely in chemical composition, moisture content, and ash content. This challenge is even greater for feedstocks available in GCC countries, as they originate from diverse and unconventional sources such as food waste, microalgae, agricultural residues, municipal waste, and halophytes. These differences significantly influence processing behavior, yields, and product quality [153]. General agricultural waste or crop residues, such as date palm waste and Salicornia, are composed of cellulose, hemicellulose, and lignin, components that behave very differently from sewage sludge and animal waste. For example, sewage sludge has a high moisture content and ash content, which reduces its energy efficiency. Similarly, food waste and microalgae typically have a high moisture content at the time of collection (~80%), requiring energy-intensive drying steps before further processing. In addition, biomass with a high ash content is likely to contain significant amounts of inorganic elements (e.g., potassium, chlorine, sulfur), which contribute to slagging, fouling, and corrosion in high-temperature equipment. Another problem is the incompatibility of the oxygenated bio-oils with the existing catalysts and processes in conventional refineries. These oxygenates can cause excessive coke formation, leading to faster catalyst deactivation [154]. This increases the demand for hydrogen to treat it, hence impacting the hydrogen supply infrastructure. Thus, catalyst redesign, unit modification, and the addition of hydrogen infrastructure for deoxygenation are often necessary for efficient biomass conversion. Other infrastructure modifications are required for retrofitting, as existing hydrotreatment units may need adjustments to process biomass-derived feedstocks. The variability in biomass feedstock quality necessitates dynamic process control systems to maintain consistent product quality and process efficiency, making process optimization essential. Moreover, biomass processing generates different waste streams compared to fossil fuels, requiring the development of new waste treatment and disposal strategies.

5.4.2. Commercial Challenges

GCC countries have limited biomass resources, at least in the short term, until a sustainable local supply is developed. There is also the problem of seasonal variability that significantly affects feedstock availability and cost. For example, date palm waste peaks during trimming and cultivation seasons, while food waste production increases during Ramadan and the peak tourism season in major cities such as Dubai. These issues create a potential reliance on imported feedstocks, introducing supply chain complexities and cost uncertainties. Additionally, some feedstocks with potential use in the region, such as food waste and sewage sludge, are more susceptible to degradation because of high temperature (reaching up to 50 °C during the summer), therefore requiring specialized and costly storage and handling infrastructure.

The economic viability of retrofitting existing refineries may benefit from technological advancement; however, some old refineries may require significant capital investment in new equipment, infrastructure modifications, and technology integration. It is also recognized that biofuels and bio-products must compete with established fossil fuel-based products, which are often more cost-competitive due to existing subsidies and market structures. Other challenges may also arise from the absence of robust policy incentives and regulatory frameworks for biofuels in the Gulf countries because of the use of unconventional stocks. Nevertheless, the United Arab Emirates has published a national policy on biofuels [155] and is actively implementing used cooking oil recycling and other related programs, which strongly signals the move towards local feedstock mobilization and domestic biofuel industry support. To assess the current practice in the GCC [156],

Table 5. Comparison of GCC practice against policy elements used internationally to accelerate retrofits and sustainable biorefinery deployment.

No.	Policy Elements	The Best Global Practice	GCC Practice	Gaps/Strengths
1	Demand certainty	Binding blending mandates and clear SAF/renewable fuel trajectories as the best practice creates large, predictable markets that justify refinery retrofits [157].	Limited mandatory blending quotas or tradable obligations at scale in general, and limited binding, long-term offtake mandates, or regional targets for the United Arab Emirates.	Gap
2	Carbon/credit markets	Provide ongoing revenue streams for low-CI fuels and let markets set prices [158].	No widely adopted LCFS equivalent at the national level. The GCC’s carbon markets are still in their early stages, and investment depends on export markets or direct funding.	Strength/gap
3	Feedstock sustainability and traceability	Using strict sustainability criteria such as LCA rules ensures positive climate outcomes.	As a partial strength, United Arab Emirates policy emphasizes recycling UCO and infrastructure, but region-wide robust sustainability standards and enforcement mechanisms for a diverse set of feedstocks are still developing.	Partial strength/gap
4	Capital and operating incentives	Direct grants, state aid packages, concessional finance, tax credits, and loans reduce CAPEX barriers and accelerate retrofits [159].	Although there is a considerable sovereign capacity to provide incentives, there have not been many targeted national programs for refinery rehabilitation up to this point.	Opportunity
5	Technical enabling measures	Funding for pilots, streamlined permitting, and technical standards help de-risk retrofits. Internationally, these have proven effective to scale integration trials [160].	The GCC has a strong advantage due to its major refineries with technological capabilities, existing hydrogen and utility infrastructure, and the ability to fund pilot plants quickly [133]. However, there is currently no regional standard for regulatory testing of environments or systematic co-processing of trial frameworks.	Strength
6	Feedstock mobilization	Municipal collection mandates, UCO bans/controls, feedstock pooling, and preprocessing hubs are used globally to aggregate feedstock at a commercial scale [161,162].	UCO collection pilots and export restrictions are positive steps for the GCC in the United Arab Emirates. But for larger streams of MSW organics and agricultural residues, collection infrastructure and preprocessing hubs need scaling and policy support.	Partial strength

shows a comparative evaluation of the framework and gaps in the region against global policies and practices in the EU, US, and the specific California Low Carbon Fuel Standard (LCFS) [157,158,159]. Accordingly, the strengths of GCC policy to retrofit refineries are the availability of refinery hydrogen, utilities, and funding to accelerate pilots. Its weaknesses lie in the limited mature demand-side mandates or tradable credit systems, inconsistent regional policy harmonization, and still-developing feedstock collection infrastructures.

5.5. Future Perspectives

Globally, the number of new biorefinery facilities is rapidly growing. However, enhancing the sustainability of biorefineries and retrofitting involves several key actions, some of which are directly relevant to arid and semi-arid regions in general, and to GCC countries in particular. Extensive research and development (R&D) is needed to develop innovative methods for upgrading biomass and biofuels derived from unconventional feedstocks and waste streams (e.g., food waste, date palm waste), ideally with minimal environmental impact. R&D is also necessary to enable the use of multiple feedstocks (co-feeding), both to mitigate the limited supply of any single feedstock and to facilitate the production of diverse products without compromising process consistency or product quality. Recent research on co-feeding date palm waste and S. bigelovii [122] has demonstrated the feasibility of this biomass mixture for producing high-quality pyrolysis bio-oil, supporting the technical viability of bio-oil integration. Such findings are likely to encourage the adoption of retrofitting strategies in existing fossil fuel refineries across the GCC region. As with most industrial systems, future research should also focus on minimizing waste byproducts and greenhouse gas (GHG) emissions generated during biomass pretreatment and conversion within biorefineries. For example, fly ash produced as a byproduct of crude oil refining, which contains heavy metals [163], could be utilized as a catalyst for biomass conversion or combined with biochar for use as a soil amendment.

Technically, there is significant potential for utilizing carbon dioxide in biorefineries to produce various bio-based materials and chemicals. Research has demonstrated promising applications of subcritical and supercritical CO₂ as versatile tools in biomass hydrolysis processing [164,165]. Within the biorefinery framework, high-pressure CO₂ is especially interesting as a solvent that can be employed in a wide range of reactions [166,167]. These include direct biomass processing, catalytic dehydration of carbohydrate-derived intermediates, and transformations of compounds derived from biomass. Therefore, the use of CO₂ in biomass processing should be further explored, especially in contexts where CO₂ emissions, such as those from fossil fuel refining, are already being captured. Although very few studies addressed combining retrofitting refineries and CCUS, some studies indicated the potential for Biomass Energy with Carbon Capture and Storage (BECCS) and the co-location of capture facilities with storage sites [168]. This is an interesting area for future research, especially by utilizing data-driven decision-making to reduce emissions and meet climate goals. Digital tools such as artificial intelligence (AI), the internet of things (IoT), and advanced modeling are important for enhancing the performance of existing biofuel production processes, though the integration of permanent CCUS systems is still developing.

Another area that requires attention is life cycle assessment (LCA) and techno-economic analysis (TEA). Several articles specifically highlight this as a research gap [169]. Besides developing value-chain pathways and standards to monetize lignin and co-product markets, logistics research for cost reductions in feedstock densification (torrefaction, pellets) and decentralized preprocessing is needed to improve supply chain economics. The perspective of biorefinery in GCC countries also relies on drawing a practical roadmap. Firstly, this can start with quantifying recoverable feedstock in a decentralized survey and using conservative collection efficiencies; some studies suggest around 30–60% recoverable mass [128]. Secondly, as reported in some studies [170], it is important to run small FCC and hydrotreater trials with local bio-oils to determine allowable blend ratios and upgrade needs. Thirdly, it is recommended to build decentralized preprocessing hubs (torrefaction, pelletizing, bio-oil stabilization) near the main feedstock sources to reduce freight and improve feedstock quality. Lastly, future perspective depends on setting policy incentives and offtakes for SAF and biofuels to de-risk CAPEX and crowd in refinery integration projects.

6. Conclusions

This review explores the biorefinery concept and its classification in recent studies, aiming to provide a promising platform for advancing a sustainable, biomass-based industry that produces biofuels and other bio-based products. Biorefinery is poised to play a central role in the transition of the energy sector from petroleum to biomass sources. The integrated pathways involve the most recent classification scheme used in the literature, in which feedstock generation, platform, process, and product are used to classify biorefinery types. This review concludes that thermochemical conversion pathways of biomass are more established and better understood than biochemical pathways. This indicates that the gaps in research for biochemical pathways need to be filled, especially when combined with thermochemical and chemical pathways to incorporate the benefits of all pathways. Studies on retrofitting refineries into biorefineries, by adapting existing operational units to work with biomass-based technologies, show that fast pyrolysis is the most competitive option, especially when lignocellulosic feedstocks are used. This is primarily because existing refinery units, such as hydrotreaters and hydrocrackers, can be repurposed, reducing capital investment. As a case study, the major oil-producing GCC countries are tracked in the literature to study the availability of feedstocks and the potential for bioenergy applications. It is found that, in the Gulf countries, municipal solid waste (MSW) is the most desirable biomass source for large-scale biorefineries because of its abundance and its proven worldwide applications in bioenergy. It is also shown that biofuels produced from sewage sludge require the least upgrading in biorefinery processing because they have the highest H/C and moderate O/C ratios. Other feedstocks, such as halophytes like Salicornia Bigolovii, may contribute if arid land is prepared and utilized for large-scale plantations. Halophytes can be cultivated as bioenergy feedstocks and as sources of food or livestock feed. Finally, it is important to conclude that the rapid production of SAF and drop-in fuels is attractive for refineries seeking low-CAPEX pathways, although pilot plants are required to optimize the limits for bio-oil blend ratios and upgrades are needed in co-processing.