Lignocellulosic feedstock materials are the most abundant renewable bioresource material available on earth. It is primarily composed of cellulose, hemicellulose, and lignin, which are strongly associated with each other. Pretreatment processes are mainly involved in effective separation of these complex interlinked fractions and increase the accessibility of each individual component, thereby becoming an essential step in a broad range of applications particularly for biomass valorization. However, a major hurdle is the removal of sturdy and rugged lignin component which is highly resistant to solubilization and is also a major inhibitor for hydrolysis of cellulose and hemicellulose. Moreover, other factors such as lignin content, crystalline, and rigid nature of cellulose, production of post-pretreatment inhibitory products and size of feed stock particle limit the digestibility of lignocellulosic biomass. This has led to extensive research in the development of various pretreatment processes. The major pretreatment methods include physical, chemical, and biological approaches. The selection of pretreatment process depends exclusively on the application. As compared to the conventional single pretreatment process, integrated processes combining two or more pretreatment techniques is beneficial in reducing the number of process operational steps besides minimizing the production of undesirable inhibitors. However, an extensive research is still required for the development of new and more efficient pretreatment processes for lignocellulosic feedstocks yielding promising results.

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Recent updates on different methods of pretreatment of lignocellulosic feedstocks: a review

The hunt for alternative sources of energy generation that are inexpensive, ecofriendly, renewable and can replace fossil fuels is on, owing to the increasing demands of energy. One approach in this direction is the conversion of plant residues into biofuels wherein lignocellulose, which forms the structural framework of plants consisting of cellulose, hemicellulose and lignin, is first broken down and hydrolyzed into simple fermentable sugars, which upon fermentation form biofuels such as ethanol. A major bottleneck is to disarray lignin which is present as a protective covering and makes cellulose and hemicellulose recalcitrant to enzymatic hydrolysis. A number of biomass deconstruction or pretreatment processes (physical, chemical and biological) have been used to break the structural framework of plants and depolymerize lignin. This review surveys and discusses some major pretreatment processes pertaining to the pretreatment of plant biomass, which are used for the production of biofuels and other value added products. The emphasis is given on processes that provide maximum amount of sugars, which are subsequently used for the production of biofuels.

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An overview of key pretreatment processes employed for bioconversion of lignocellulosic biomass into biofuels and value added products

Second-generation bioethanol can be produced from various lignocellulosic biomasses such as wood, agricultural or forest residues. Lignocellulosic biomass is inexpensive, renewable and abundant source for bioethanol production. The conversion of lignocellulosic biomass to bioethanol could be a promising technology though the process has several challenges and limitations such as biomass transport and handling, and efficient pretreatment methods for total delignification of lignocellulosics. Proper pretreatment methods can increase concentrations of fermentable sugars after enzymatic saccharification, thereby improving the efficiency of the whole process. Conversion of glucose as well as xylose to bioethanol needs some new fermentation technologies to make the whole process inexpensive. The main goal of pretreatment is to increase the digestibility of maximum available sugars. Each pretreatment process has a specific effect on the cellulose, hemicellulose and lignin fraction; thus, different pretreatment methods and conditions should be chosen according to the process configuration selected for the subsequent hydrolysis and fermentation steps. The cost of ethanol production from lignocellulosic biomass in current technologies is relatively high. Additionally, low yield still remains as one of the main challenges. This paper reviews the various technologies for maximum conversion of cellulose and hemicelluloses fraction to ethanol, and it point outs several key properties that should be targeted for low cost and maximum yield.

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An overview of key pretreatment processes for biological conversion of lignocellulosic biomass to bioethanol

Bioenergy, derived from biomass and/or biological (or biomass-derived) waste residues, has been acknowledged as a sustainable and clean burning source of renewable energy with the potential to reduce our reliance on fossil fuels (such as oil and natural gas). However, many bioenergy processes require some form of pre-treatment and/or upgrading procedure for biomass to generate a modified residue with more suitable properties and render it more compatible with the specific energy conversion route chosen. Many of these pre-treatments (or upgrading procedures) involve some form of substantive heating of the biomass to achieve this modification. Microwave (MW) heating has attracted much attention in recent years due to the advantages associated with dielectric heating effects. These advantages include rapid and efficient heating in a controlled environment, increasing processing rates and substantially shortening reaction times by up to 80%. However, despite this interest, the growth of industrial MW heating applications for bioenergy production has been hindered by a lack of understanding of the fundamentals of the MW heating mechanism when applied to biomass and waste residues. This article presents a review of the current scientific literature associated with the application of microwave heating for both the pre-treatment and upgrading of various biomass feedstocks across different bioenergy conversion pathways including thermal and biochemical processes. The fundamentals behind microwave heating will be explained, as well as discussion of the imperative areas which require further research and development to bridge the gap between fundamental science in the laboratory and the successful application of this technology at a commercial scale.

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The application of microwave heating in bioenergy: A review on the microwave pre-treatment and upgrading technologies for biomass

Lignocellulosic and waste materials represent a considerable potential for biofuel production. Currently, they are not fully utilised due to low biofuel yields from biological processes while thermo-chemical conversion technologies and transesterification processes suffer from certain drawbacks of conventional heating. Consequently, several technologies have been studied to improve biofuel production from lignocellulosic and waste materials, with microwave (MW) irradiation gaining increased interests over the years. This study reviewed the use of MW irradiation for assisting biofuel production from biological and thermo-chemical conversion processes as well as biodiesel production from the transesterification process. The principles underlying MW irradiation were initially described followed by the several benefits of MW over conventional heating and the different effects of MW irradiation on pre-treatment of lignocellulosic biomass and waste materials such as defragmentation of lignocellulosics, organic matter solubilisation and enhanced hydrolysis. Although MW irradiation generally enhanced biofuel production from biological processes, the extra biofuel (bio-ethanol, bio-methane or bio-hydrogen) produced could not compensate for the energy input due to MW irradiation, resulting in negative efficiencies for the pre-treatment technique. Regarding MW-assisted thermo-chemical conversion of biomass and waste materials to biofuels, it can be deduced that MW-heating is much more beneficial as opposed to conventional heating based on the products quality and yields obtained but the energy efficiency aspects of MW-heating are contradictory among studies. As for MW-assisted biodiesel production from the transesterification process, studies are almost unanimous as to the benefits of MW-heating over conventional heating such as reduced processing times or increased biodiesel yields. Finally, this article discussed some of the challenges of MW irradiation such as formation of inhibitors on biological processes, energy efficiency of the technology, technical aspects viz. poor dielectric properties of some substrates as well as difficulties for large-scale implementation of the technology before concluding on the future directions of MW-irradiation.

P Boonmanumsin

Papers

Release of monomeric sugars from Miscanthus sinensis by microwave-assisted ammonia and phosphoric acid treatments