Pathways of lignocellulosic biomass deconstruction for biofuel and value-added products production

Highlights

• Physical and chemical pretreatment show great effectiveness but need huge energy.

• Biological pretreatment can address most challenges but with long incubation times.

• Hybrid technologies gain popularity and improve the chemical yield.

• Pretreatment of lignocellulosic biomass account for 40% of total production costs.

• Optimization of process parameters need for techno-economically feasible.

Abstract

As the world attempts to transition from fossil fuels, lignocellulosic biomass (LCB) serves as a promising alternative due to its high abundance. Hydrolysing LCB can generate various bioproducts, such as biofuels and value-added chemicals. However, the presence of lignin inhibits the solubilization of LCBs, presenting a major techno-economic challenge in the biorefinery concept. Therefore, this paper addresses the gaps left by most of the recent review works that fail to comprehensively review different pretreatment methods and the full scope of applications of LCBs, and do not incorporate techno-economic considerations of the technologies, the latter being the greatest bottleneck in the commercialization of the processes. The literature review revealed that while many of the physical and chemical pretreatment methods exhibit great effectiveness, they have a huge dependence on energy, chemicals, water, and/or specialized equipment, and produce harmful waste and inhibitory compounds. The pretreatment of lignocellulosic biomass can account for 40% of total production costs. Biological pretreatment can address these challenges but is limited by long incubation times. For instance, the bacterial pretreatment can noticeably reduce sawdust cellulose, hemicelluloses, and lignin contents by 35.8%, 37.1%, and 46.2%, respectively. Recently, integrated/coupling (hybrid) methods, such as chemical-assisted liquid hot water/steam and microwave or ultrasound-assisted alkaline pretreatment, have been gaining popularity due to their potential to improve chemical yield, but at the expense of the high cost of operation. To make pretreatment processes more techno-economically feasible, there is a need for process integration and the standardization and optimization of process parameters.

Introduction

The world population is expected to grow to 9.7 billion by 2050. Even though the COVID-19 pandemic has brought huge uncertainty in terms of global energy needs [43], energy demand was projected to grow by 12% between 2019 and 2030 as a result of the growing population before the pandemic, and this is still likely to manifest [72]. Therefore, the growing human population and the consequent environmental pollution [8], biodiversity loss, natural resource depletion, and climate change [44] are increasingly bringing sustainable energy policies and the need to lower the effects of greenhouse gas (GHG) emissions to the attention of policymakers and researchers [18]. Moreover, the rapid exhaustion of fossil fuel reserves is directing the world towards a devastating energy security crisis [107]. Currently, fossil fuels meet more than 80% of the global primary energy use, and are thus contributing to large amounts of GHG, particularly carbon dioxide (CO₂) emissions [73]. Therefore, the need to phase out fossil fuels is driven by many reasons, such as the increasing energy demand and subsequent fossil fuel depletion, the consequent accumulation of atmospheric CO₂ that is causing climate change, and finally, threats to national energy security and rural economic development [108], [47]. Shifting away from fossil fuels towards renewable energy sources provides a unique opportunity with multiple benefits to address many of the issues in the contemporary world, where humanity is attempting to transition from unsustainable concepts of production, consumption, and disposal, to a more sustainable way of life that also simultaneously results in socio-economic development [67].

Contributions to the global energy supply system can come from renewable sources [4], such as geothermal, solar, wind, hydroelectric, and tidal power. It can also originate from biomass, which currently serves as the fourth largest energy source after the dominant fossil-based sources [23]. The total primary energy supply from biomass resources constituted 70% of the total share among all the renewable energy sources in 2016, providing 56.5 exajoules of energy [63]. Currently, lignocellulosic biomass (LCB) is produced at a rate of approximately 2 × 10¹¹ tons per year [40], [47]. The forecast return is expected to be so large that both natural and social scientists, as well as policymakers, have coined the term biobased economy to refer to a kind of economy that is driven by biobased goods, which may also be identified as a circular bioeconomy due to the abundance and renewable nature of biomass [145]. Biomass has advantages over several other energy sources due to its economic feasibility and environmental friendliness [27]. LCB represents one of the most economical, sustainable, and largest feedstocks for biobased energy production. LCB denotes the dry plant matter, primarily made of cellulose, lignin, and hemicellulose, which is present in complex associations in the plant cell wall [17]. A small portion of ash, proteins, pectin, extractives, and inorganic compounds are also present in LCB [98]. This matter is primarily supplied from the agricultural, forest, and industrial sectors where forest residues and agricultural waste are one of the most abundant and cheap feedstocks [36]. To transform LCB into biofuel, it must undergo numerous pretreatment and conversion processes [156].

Pretreatment, also known as fractionation, is a critical step in the bioconversion of LCB for application in biorefineries [110]. Lignin protects the cellulose and hemicellulose from degradation by making the cell wall impermeable to the action of enzymes [17]. The diversity in the composition of LCB poses a great technological and operational challenge in making this biomass available for conversion to biofuel [53], [113]. Therefore, pretreatment processes, such as physical, chemical, biological, and hybrid methods, are used to tackle the recalcitrance of LCB by improving its biodegradability [1], [84], [87]. Pretreatment methods are employed to modify the structures of the LCB to make the biomass matter available for conversion into bioenergy [117] by disrupting and depolymerizing the cell wall and preventing further cellulose crystallinity [83]. These processes produce efficient second generation sugars that drive biorefineries and act as natural intermediates in chemical and biological transformations [78], [133]. Even though the hemicellulose and lignin are removed after pretreatment, the cellulose is preserved as much as possible to ease downstream steps [71]. The resulting physicochemical properties of the biomass determine their engineering application [36], [80]. Many methods have been introduced for the pretreatment of LCB, each offering different kinds of advantages. Appropriate techniques are selected based on the extent of energy and chemical inputs, the generation of toxic and hazardous by-products, and the ease of producing lignin as a high-value commodity [38].

The basic understanding of the different phases of the various processes and their respective techno-economic feasibility plays a crucial role in the commercialization of biomass for the production of biofuel [16]. As a result, pretreatment methods have been the subject of numerous comprehensive reviews works in the past few years. Many papers have focused on extolling the environmental sustainability of biorefineries using LCB and delve into discussions of the challenges of commercialising biorefineries, such as lack of investment, immature technologies, and other issues with scaling up [20], [38], [47]. Other kinds of works that this table does not contain are the review papers that focus on specific pretreatment methods or specific solvents. Moreover, many of the recent review works that are focused on pretreatment technologies overall do not provide comprehensive insights into the biological techniques, potentially because the techniques are too vast to be covered along with the other kinds of methods. Consequently, advances in physical and chemical pretreatment methods are discussed in much detail in these papers. There has also been considerable progress in the development of hybrid pretreatment methods to improve the effectiveness of treatment processes and tackle the limitations of single pretreatment methods. Nevertheless, most papers have primarily discussed physicochemical methods, thereby failing to provide insights into the technologies that use ultrasound, microwave, and irradiation-assisted methods.

The implications of the converted biomass also range far and wide. However, biofuel, namely bioethanol, has been more widely studied even though the products of the pretreatment techniques can serve as precursors for other kinds of important biofuels, such as methane [62], [101] and hydrogen [6], as well as numerous value-added biochemicals [26], [47], [77] and biochars [118]. Biofuel is a type of fuel that is produced from lignocellulosic biomass. Ethanol is one of the most abundant biofuels produced from lignocellulosic biomass. Hot water treatment is a common treatment strategy among the many processes suitable for the production of ethanol from sugar cane, for example. However, it does not apply to lignocellulosic materials. This process is highly specific for water-soluble products and as one of these ethanol is one of the major by-products of the process [68]. A combination of other treatment mechanisms along with hot water treatment helps to enhance the yield. High ethanol yield is also possible using microwave-assisted chemical and ultrasound-assisted alkali pretreatment mechanisms [46]. Several lignocellulosic residues, e.g., wheat straw, rice straw, sugarcane tops, cotton stalk, and corn stalk, are resources high in ethanol content that are commonly treated with dilute acids to produce the ethanol [31], [136]. Hydrogen is another major type of biofuel produced from dilute acid-assisted processes of lignocellulosic biomass pretreatment. Other treatment strategies, e.g., catalytic cracking and steam treatment, have also been found to be useful hydrogen-producing pretreatment processes [52].

Lignocellulosic biomass produces a wide range of value-added goods that include bio-oil, biogas, additives, reagents, and solvents. Bio-oil has been produced as a value-added product from multiple lignocellulosic pretreatment processes, such as microwave-assisted processes, pyrolysis, and fluidized bed reactors [52]. Bio-oil has a high market value that encourages treatment industries to set up bio-oil producing facilities [26]. Biogas is another value-added product [7] that can be commonly extracted using biomass pretreatment processes. However, capturing the biogas produced from such pretreatment is difficult and requires a sophisticated setup [65]. Additionally, other reagents, solvents, and additives can also be produced as value-added goods. Biochar is another product commonly extracted using lignocellulosic biomass treatment. Biochar refers to a carbon-rich residue that is commonly used as a low-grade fuel or as a soil additive. Biochar increases soil fertility by increasing its water-containing potential [52]. Biochar is mostly produced from different pyrolysis processes adopted for biomass pretreatment [89]. Biochar can also be the end product of other pretreatment technologies.

Many studies exclude considerations of the techno-economic feasibility of pretreatment methods, which omits a crucial understanding of whether LCB can be applied at a commercial scale. Even though the low cost and high production rate of LCB can make it a promising source for energy and value-added products [47], the operating and capital cost can exceed the total processing cost by 40% [19], [31], denoting the huge need for pretreatment methods to be economically feasible. The large-scale implementation of conventional treatment methods is hindered by the extreme environment (such as high pressure and temperature) and the intensive resource, chemical, and energy requirements of the processes [22]. Such knowledge gaps hinder the advancement of research and development of improved and/or novel pretreatment methods, augmented by the lack of future perspectives in recent scholarly work. Therefore, to address these gaps, this paper comprehensively reviews the different kinds of traditional and emerging pretreatment technologies that have been introduced, explores the variety of applications of the products of LCB pretreatment, and evaluates the techno-economic feasibility and viability of launching the technologies on an industrial scale. As a constantly evolving field of research, this paper will provide crucial information regarding the major bottlenecks for the commercialization of LCB due to the challenges of the existing pretreatment technologies, and nudge those working in relevant fields towards potential avenues for future research.