Acid-based lignocellulosic biomass biorefinery for bioenergy production: Advantages, application constraints, and perspectives
Graphical abstract
Introduction
Most energy sources at present depend on fossil fuels, and this dependence causes the release of a considerable amount of greenhouse gases, which are dangerous toxic pollutants, to the environment (Sharma et al., 2020; Hoang, 2019). This release has resulted in serious environmental pollution and negatively affected local air quality (Atarod et al., 2020; Cao et al., 2020). Thus, searching for alternative fuels with low emission and renewable characteristics is crucial to mitigate environmental impacts and satisfy the ever-increasing energy demand (Chia et al., 2018). Lignocellulose biomass has valuable and sustainable energy sources for the generation of clean and environmentally friendly energy (Hoang et al. 2021a, 2021b). From this perspective, the use of biomass energy to replace conventional fossil fuels plays a key role in sustainable development strategies (Bhutto et al., 2016; Hoang and Pham, 2021). Lignocellulosic material, which consists mainly of cellulose, hemicellulose, and lignin, is a widely abundant and readily accessible source of organic matter on Earth (Mahmood et al., 2019; Parthasarathy and Narayanan, 2014). However, the major upfront capital investment and high expenditure associated with the production of lignocellulose-based liquid fuels, such as bio-oil and ethanol, present a major obstacle in scaling up current commercial operations (Lee et al., 2020). A fundamental understanding and a good grasp of various integrated processes are required in optimizing the cost competitiveness of lignocellulosic materials and related conversion technologies (Kumar et al., 2019a). Biomass pretreatment is an important process in addressing the inherent recalcitrance embedded in the structural components of lignocellulosic materials (Terlouw, 2013; Chatterjee et al., 2015; Gaspard and Chaker, 2014). Various findings have shown that pretreatment techniques allow polysaccharides to be increasingly susceptible to chemical and enzymatic reactions in the process of breaking down the main biomass components. Fig. 1 illustrates the basic differences between pretreated and unpretreated biomass and their impact on the production pathway of biofuels in terms of yield, productivity, and residue.
The pretreatment process is widely used in biofuel production and often accounts for the largest percentage of the overall project's expenditure due to the dominant role of pretreatment in deciding the characteristics of the final products (e.g., proportion, yield, and quality) and the other factors in the selection of the conversion steps and their respective performance. Even with high pretreatment costs, the potential to improve and enhance the efficiency and yield of the biofuel production process outweighs the cost (Bond et al., 2014; Behera et al., 2014). To successfully transform lignocellulosic materials into useful forms of energy, such as liquid fuels, the conversion process is often accompanied with a pretreatment phase in which each biomass component (cellulose, hemicellulose, and lignin) reacts differently during this process (Singh et al., 2015). Depending on the mechanisms, different approaches can be utilized in the pretreatment of lignocellulosic materials, which can be categorized based on physical reactions (e.g., comminution/breaking down), hydrothermolysis (microwave- and ultrasound-assisted methods) (Liu et al., 2014; Tan et al., 2019), chemical catalysts (e.g., acids, alkalis, ozone, and solvents) (Lee et al., 2019), physicochemical reactions (explosion treatment relying on hot steam or ammonia fiber) (Meenakshisundaram et al., 2021), and biological reactions (e.g., bacteria, microbes, and fungi) (Amini et al., 2017; Ong et al., 2020). Given the limitations associated with the stand-alone application of each individual approach, the integration of two or more processes helps overcome the various technical challenges associated with high energy consumption, large reaction volumes, and long residence times and can optimize the final yield and productivity of the intended products (Chen et al., 2017a). The development of pretreatment technologies has provided an impetus for advancing the production of bio-energy and valuable co-products and resolving the challenges that hinder the utilization of energy crops (Zheng et al., 2014). In the literature, there were a number of review papers related to biomass pretreatment by chemical methods. For example, Ho et al., (2019) have presented their assessment on the application of alkaline hydrogen peroxide in pretreating lignocellulosic biomass. In a recent publication, Xu et al., (2020) have reported the use of deep eutectic solvents for biomass pretreatment. In another case, Zhao et al., (2020) have published a review work on recent advances in biomass pretreatment by ammonia. The critical characteristics of each chemical method for the biomass pretreatment process were comprehensively discussed, indicating some disadvantages associated with the efficiency of biomass pretreatment for such chemical methods. On the way to seek the high-efficiency chemical methods for biomass pretreatment, there were a large number of publications on acid-based biomass pretreatment. Compared with other available technologies, acid pretreatment is generally preferred over other common methods due to its enhanced effectiveness in breaking down the structure of lignocellulosic materials and promoting crystalline–amorphous cellulose conversion (Brunecky et al., 2020). However, several shortcomings, such as the generation of inhibitory compounds and discharge of highly acidic effluent that is potentially harmful to the natural environment and human health, are associated with this technology (Young and Cabezas, 1999; Jönsson and Martín, 2016). A thorough analysis of acid pretreatment as an important step in the conversion of lignocellulosic biomass is necessary and needs to consider different factors, including research breakthroughs and advances in technology development, strengths and weaknesses in the application of a technology, and the potential effects on the economy and environment (Hosseini Koupaie et al., 2019; Ong et al., 2019). Thus, this study aims to provide insights into acid-based biomass pretreatment with the main purpose of improving the efficiency of biomass conversion to valuable chemicals and biofuels. To achieve this goal, this study focuses on the following: (i) presenting the main components of lignocellulosic biomass and the biomass potential for bioenergy synthesis, (ii) analyzing the main process variables for biomass pretreatment assisted by acid, (iii) comparing the efficiency of biomass pretreatment using various acid types, (iv) neutralization and detoxification after acid-based pretreatment, (v) economic aspect of acid-based pretreatment, and (vi) revealing the challenges in applying this acid-based biomass pretreatment in the future.
Section snippets
Components of lignocellulosic biomass
Biomass can be derived from a wide range of sources, including various portions (i.e., edible and non-edible) of food crops, perennial plants, and organic wastes produced from agricultural activities (Grass, 2004; Chen et al., 2021). Cellulosic wastes from the agricultural residues or non-edible portion of crops, such as leaves, stems, stalks, stover, trimmings, straw, and husk, make up a large portion of available biomass resources. Table 1 provides an overview of the current global annual
Characteristics of acid-based pretreatment
Inorganic compounds existing in the form of phosphate-, carbonate-, sulfate-, and chloride-containing minerals are commonly found in the biomass feedstock used in biofuel production (Chen et al., 2017b). Consequently, the presence of these minerals during biomass pyrolysis can exert a considerable impact on the reaction and final products. Moreover, the impurities left over as a result of the incomplete removal of these inorganic minerals adversely affect the physical characteristics of the
Acid-based lignocellulosic biomass pretreatment
Acid concentration is one of the most critical parameters influencing the efficiency of acid-based pretreatment of biomass. Therefore, various impacts on the structure of lignocellulosic biomass can be promoted by using concentrated or dilute acids. For example, hemicellulose, the first component in the lignocellulosic biomass structure, can be broken down during acid-based pretreatment. Hemicellulose can be easily hydrolyzed under the effects of dilute acids under conditions of concentration
Neutralization
In acid-based pretreatment for biomass, the concentration of the H+ ion exerts a noticeable impact on the hydrolysis rate of hemicellulose. Chemically, the neutralization capacity of a solution after acid-based pretreatment must be experimentally determined due to the difference in the mineral content of various biomass and the excess acid amount after pretreatment (Lloyd and Wyman, 2004). For industrial methods, the inorganic acids used for acid pretreatment are normally neutralized by
Economic assessment
The enhancement amount of obtained sugars is the main goal of acid-based pretreatment from lignocellulosic biomass. Therefore, acid-based pretreatment is suitable for producing bioenergy and value-added products. The yields and performance of sugar depend largely on operating conditions, characteristics of raw materials, and acid behaviors (e.g., acid type and acid concentration). Thus, an economic assessment of the biofuel production process involving acid pretreatment could be carried out as
Prospective and future challenges
Acid-based biomass treatment is effective in altering the molecular structure of biomass and providing added benefits in terms of the physicochemical characteristics of the final products. The method not only enables the reduction of inorganic compounds in the raw biomass materials but also enhances the solubilization of hemicellulose, thus making cellulose more accessible to subsequent enzymatic hydrolysis reactions. Consequently, the carbon and energy conversion efficiencies of biomass are
Conclusion
Acid-based pretreatment has an important role in the conversion of lignocellulosic biomass into sugar for further processing into biofuels and other industry-grade bio-products. The operating process parameters, such as acid types, acid concentration, and reaction temperature, for the biomass pretreatment process directly influence the yield of post-pretreated products. Given the advantages offered by various techniques, the potential for industrial application of acid-based pretreatment has
Author contribution
Anh Tuan Hoang: Conceptualization, writing-original draft preparation, Sandro Nizetic: Formal analysis, Reviewing and Editing. Hwai Chyuan Ong: Reviewing and Editing, Resources, supervision. Cheng Tung Chong: Reviewing and Editing. A.E. Atabani: Validation. Van Viet Pham: Visualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the strategic research fund of Ho Chi Minh city University of Technology (HUTECH), Vietnam, and the University of Technology Sydney, Australia.
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