Research status and future development of biomass liquid fuels

Zhang, J., Yang, J., Zhang, H., Zhang, Z., and Zhang, Y. (2021). "Research status and future development of biomass liquid fuels," BioResources 16(2), 4523-4543.

Abstract

Due to the combined pressures of energy shortage and environmental degradation, bio-liquid fuels have been widely studied as a green, environmentally friendly, renewable petroleum alternative. This article summarizes the various technologies of three generations of biomass feedstocks (especially the second-generation, biomass lignin, and the third-generation, algae raw materials) used to convert liquid fuels (bioethanol, biodiesel, and bio-jet fuel) and analyzes their advantages and disadvantages. In addition, this article details the latest research progress in biomass liquid fuel production, summarizes the list of raw materials, products and conversion processes, and provides personal opinions on its future development. The aim is to provide a theoretical basis and reference for the optimization of existing technology and future research and development of biomass liquid fuels.

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Research Status and Future Development of Biomass Liquid Fuels

Ji Zhang,^a,b Junling Yang,^a Huafu Zhang,^a Zhentao Zhang,^a^,* and Yu Zhang ^a

Keywords: Biomass liquid fuel; Bioethanol; Biodiesel; Biomass jet fuel

Contact information: a: Key Laboratory of Energy Saving Technology of Food and Pharmaceutical Storage and Processing and Transportation Equipment in Good Quality (China National Light Industry), The Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences, Beijing 100190, China; b: College of Forestry, Jiangxi Agricultural University, National Forestry and Grassland Bureau Woody Spice (East China) Engineering Technology Research Center, Nanchang 330045, China; Tianjin 300222, China; *Corresponding author: zzt@mail.ipc.ac.cn

INTRODUCTION

The development of modern industry provides prosperity but leads to the wasting of biomass resources. In recent years, the transformation and utilization of these green, inexpensive, and readily available waste biomass resources have become urgent and important issues in the fields of environmental protection and chemical industry. Coupled with the dual pressures of energy shortage and environmental deterioration, countries around the world have begun to compete to develop safe, environmentally friendly, and renewable biomass energy. The development and application of biomass liquid fuels have attracted unique attention and extensive research in many countries. Biomass liquid fuels generally refer to liquid fuels, such as bioethanol, biodiesel, and bio-jet fuel, which are converted from biomass using various technologies. Biomass liquid fuel technology has developed rapidly in the past three decades. Biomass liquid fuel has become the most promising class of alternative fuel, and biodiesel and bioethanol have achieved particularly large-scale development (Dabros et al. 2018; Efeovbokhan et al. 2019).

As shown in Table 1, current biomass liquid fuels are mainly developed from three generations of raw materials. The first generation of biofuels mainly uses food crops and animal fats as raw materials. Bioethanol and biodiesel are produced via fermentation and transesterification reactions. The relevant production technology has a long history of use, but due to food scarcity concerns, it has become a less favorable option. The second-generation biofuels mainly use non-grain crops, such as lignocellulosic biomass as raw materials, which include straw, hay, bagasse, rice husks, wood chips, and others. The use of second-generation biofuels causes many technical problems, and there are major problems with conversion rates and production costs.

In the past few decades, in-depth research on second-generation biomass materials had focused on the enhancement of biomass pretreatment, the production of cellulolytic enzymes, and the improvement of strains and processes. Such efforts have eliminated some major technical bottlenecks. However, there is a need to further reduce processing costs to obtain sufficient market competitiveness. In particular, it is important to reduce the energy requirements in the pretreatment process, increasing sugar concentration, improving enzyme activity and strain recycling, and increasing the utilization of by-products (Guerrero et al. 2018). The third-generation biofuels mainly use algae as a raw material to extract oil and fat. Algae has wide distribution, high oil content, strong environmental adaptability, a short growth cycle, and high yield. Its growth does not occupy the two major resources of land and fresh water, but the production process is still in the laboratory stage. To achieve commercial large-scale production, many technical problems need to be solved (Kabir et al. 2019; Vintila et al. 2019)

RESEARCH STATUS OF BIOETHANOL TECHNOLOGY

Bioethanol refers to the conversion of various biomass into fuel alcohol via the fermentation of microorganisms. In contrast with first-generation biomass ethanol, which uses sugar- and starch-based crops as raw materials, second-generation biomass ethanol is mainly produced from lignocellulosic biomass, which can be divided into agricultural, forestry residues, energy crops (such as bagasse, rice husk, and straw). Second-generation bioethanol is produced through pretreatment, hydrolysis, and fermentation (Ayodele et al. 2019). Table 2 shows current research of bioethanol produced from second-generation biomass using different pretreatment methods, microorganisms, and fermentation conditions. The purpose of biomass pretreatment is to reduce the crystallinity of cellulose, increase the specific surface area of the substrate, break the barrier effect of hemicellulose and lignin, and facilitate the contact and reaction of cellulose with hydrolytic enzymes. The pretreatment of lignocellulose can adopt physical (such as mechanical communication and irradiation), chemistry (such as acid, alkali, ionic liquid, organic solvent, and ozone pretreatment), physical chemistry (such as steam explosion, ammonia fiber, and CO₂ explosion) and biological (such as microorganism) pretreatment methods. Among them, steam explosion is a particularly promising pretreatment method for large-scale production of bioethanol. Guerrero et al. (2018) used steam explosion to pretreat banana waste and found that it has a high hemicellulose recovery rate and cellulose enzymatic hydrolysis efficiency, low energy consumption, and a lack of recycling issues. The hydrolysis process is the degradation of the hemicellulose and cellulose polymeric sugars in the raw materials into fermentable monosaccharides. The fermentation reaction uses the hydrolyzed monosaccharide as a raw material, and it is transformed into bioethanol by the metabolism of yeast or bacteria.

The common processes after pretreatment mainly include separate consolidated bioprocessing, separate hydrolysis and fermentation, and simultaneous hydrolysis and fermentation (SHF). Among them, the SHF method after steam pretreatment of the second-generation biomass feedstock is widely regarded as one of the most promising methods for producing bioethanol. To maximize ethanol production in bioethanol production, it is necessary to consider the use of xylose in the hemicellulose portion of lignocellulose while minimizing the inhibitory effect on the fermentation process. As Saccharomyces cerevisiae cannot effectively ferment xylose, Subsamran et al. (2019) studied the engineered strain of Saccharomyces cerevisiae and found that it can effectively co-ferment glucose and xylose, thereby greatly improving the ethanol yield. The final yield of bioethanol mainly depends on the pretreatment technology, hydrolysis process, and fermentation efficiency.

The second-generation bioethanol mainly uses lignocellulosic materials as raw materials. The main limitation is that it contains a large amount of lignin and requires a large amount of arable land or forest land for cultivation. The use of algae raw materials, such as microalgae and macroalgae, to produce ethanol is called third-generation bioethanol. Macroalgae is a particularly rich source of carbohydrates in bioethanol production. Due to the high carbon and low lignin content of algae raw materials, the third-generation bioethanol, which uses algae as raw materials, has higher octane number, higher heat of vaporization, and less greenhouse gas emissions (Yahmed et al. 2018). The production of bioethanol from macroalgae consists of mechanical pre-processing, pretreatment, and microbial fermentation. The most important step is pretreatment, which destroys or changes the cell wall through physical, chemical, and biological means and releases biomolecules, such as cellulose and sugar polymers, thereby improving the saccharification and bioethanol production efficiency. The efficiency of the pretreatment process is mainly determined by cellulose crystallinity, hemicellulose fraction, and the accessible surface area for enzymatic hydrolysis. In addition, an effective pretreatment method should retain some portion (such as pentose) of hemicellulose and reduce the formation of fermentation inhibitors (Dave et al. 2019). Table 3 summarizes recent examples of the production of bioethanol using macroalgae as a raw material and using different pretreatment methods, such as mechanical, chemical, and enzymatic pretreatment. In most cases, enzymatic hydrolysis after dilute acid treatment or alkali treatment is an economically viable pretreatment method for the production of bioethanol. Saravanan et al. (2018) found that, prior to saccharification, mechanical pretreatment of the macroalgal biomass, such as washing, drying, and grinding, coupled with acid and enzymatic hydrolysis, could improve saccharification efficiency. Chemical methods using weak base treatment or dilute acid treatment have also been widely used. The disadvantage of alkaline pretreatment is that it requires a large amount of water for desalination, which increases production costs. In acid pretreatment, dilute sulfuric acid can be used for almost every kind of macroalgae. The factors that ultimately affect the pretreatment process include pH, temperature, processing time, substrate concentration, and the reagents used for pretreatment.

RESEARCH STATUS OF BIODIESEL TECHNOLOGY

Biodiesel mainly refers to long chain fatty acid alkyl esters, which are formed by transesterification and esterification of alkyl alcohol with lipid biomass raw materials, such as animal oils, vegetable oils, and marine microalgae. The common biodiesel production technologies and their advantages and disadvantages are shown in Table 4. The production of biodiesel through the catalytic distillation method is a green process that could reduce costs and improve economy. Because of the high cost of traditional precious metal catalysts, low-cost new catalysts that could be recycled and high-activity and high-selectivity enzymatic catalysis had been extensively studied (Singh et al. 2019a). The reaction conditions of bio-enzyme-catalyzed synthesis of biodiesel are relatively mild, and the reaction process is more environmentally friendly, but the activity and stability of lipase need to be further improved (Wong et al. 2019). Dilution was a method of reducing the amount of solute in a solution by increasing the amount of solvent. Bioethanol and biodiesel could be used as solvents for diluting oil, but the result of this process was that the density and viscosity of the oil were reduced (Mahlia et al. 2020). Fazal et al. (2019) found that the use of micro-emulsification technology could increase the viscosity of biodiesel. As the temperature increased, the viscosity of the micro-emulsified oil gradually decreased. The processing of microemulsions was easier. But, less volatility, less stability, and high viscosity were still some issues with micro-emulsification. Gude et al. (2013) used microwave technology to produce biodiesel and found that it had the following advantages: low energy consumption, greatly reduced reaction time and solvent requirements, improved selectivity, improved conversion rate, and less by-product formation. However, this method had higher requirements for catalyst, solvent and temperature control. Pyrolysis is a method of preparing biodiesel at high temperature in isolation of air or in an inert atmosphere.

Koh and Ghazi (2011) used Jatropha curcas seed as a raw material to successfully produce biodiesel through pyrolysis and passed the ASTM 7554-10 test, but the production cost was relatively high. Joda et al. (2019) found that biodiesel that was produced by reactive distillation technology had higher reaction conversion efficiency and yield. Compared with the traditional method, its production cost decreased by 15.54%, and the power consumption increased by 2.28%. The supercritical catalytic method for preparing biodiesel has a short reaction time and simple pre-reaction and post-reaction treatments, but extreme process conditions and high equipment costs restrict its further development (Chua et al. 2020).

Among these conversion technologies, transesterification is the most economical and common method for preparing biodiesel, and the biodiesel produced has properties comparable to diesel. The key to the preparation of biodiesel by transesterification is the catalyst. According to the different catalytic methods, transesterification can be divided into homogeneous acid-base catalysis, heterogeneous acid-base catalysis, biological enzyme catalysis, and supercritical catalysis (Quah et al. 2019). Heterogeneous alkali catalysts have the advantages of easy recovery, low corrosion, and environmental friendliness, but the active center of the catalyst is easily lost, the stability is poor, and the preparation steps are complicated. Heterogeneous acid catalysts have high stability and low corrosivity, and they are easily recovered, regenerated, and separated from products. However, the catalytic activity is not high, and the reaction needs to be performed under high pressure and high temperature over a long duration (Yesilyurt et al. 2019).