Ethanol from Lignocellulosic Biomass

Cellulosic ethanol technology is one of the most commonly discussed second-generation biofuel technologies worldwide. Cellulosic biofuels are derived from the cellulose in plants, some of which are being developed specifically as “energy” crops rather than for food production. These include perennial grasses and trees, such as switchgrass and Miscanthus. Crop residues, in the form of stems and leaves, represent another substantial source of cellulosic biomass.

The largest potential feedstock for ethanol is lignocellulosic biomass, which includes materials such as agricultural residues (corn stover, crop straws, husks and bagasse), herbaceous crops (alfalfa, switchgrass), short rotation woody crops, forestry residues, waste paper and other wastes (municipal and industrial). Bioethanol production from these feedstocks could be an attractive alternative for disposal of these residues. Lignocellulosic feedstocks do not interfere with food security and are important for both rural and urban areas in terms of energy security reason, environmental concern, employment opportunities, agricultural development, foreign exchange saving, socioeconomic issues etc.

Production of Ethanol

The production of ethanol from lignocellulosic biomass can be achieved through two different processing routes. They are:

  • Biochemical – in which enzymes and other micro-organisms are used to convert cellulose and hemicellulose components of the feedstocks to sugars prior to their fermentation to produce ethanol;
  • Thermochemical – where pyrolysis/gasification technologies produce a synthesis gas (CO + H2) from which a wide range of long carbon chain biofuels, such as synthetic diesel or aviation fuel, can be reformed.

Lignocellulosic biomass consists mainly of lignin and the polysaccharides cellulose and hemicellulose. Compared with the production of ethanol from first-generation feedstocks, the use of lignocellulosic biomass is more complicated because the polysaccharides are more stable and the pentose sugars are not readily fermentable by Saccharomyces cerevisiae. In order to convert lignocellulosic biomass to biofuels the polysaccharides must first be hydrolysed, or broken down, into simple sugars using either acid or enzymes. Several biotechnology-based approaches are being used to overcome such problems, including the development of strains of Saccharomyces cerevisiae that can ferment pentose sugars, the use of alternative yeast species that naturally ferment pentose sugars, and the engineering of enzymes that are able to break down cellulose and hemicellulose into simple sugars.

Ethanol from lignocellulosic biomass is produced mainly via biochemical routes. The three major steps involved are pretreatment, enzymatic hydrolysis, and fermentation. Biomass is pretreated to improve the accessibility of enzymes. After pretreatment, biomass undergoes enzymatic hydrolysis for conversion of polysaccharides into monomer sugars, such as glucose and xylose. Subsequently, sugars are fermented to ethanol by the use of different microorganisms.

Pretreated biomass can directly be converted to ethanol by using the process called simultaneous saccharification and cofermentation (SSCF).  Pretreatment is a critical step which enhances the enzymatic hydrolysis of biomass. Basically, it alters the physical and chemical properties of biomass and improves the enzyme access and effectiveness which may also lead to a change in crystallinity and degree of polymerization of cellulose. The internal surface area and pore volume of pretreated biomass are increased which facilitates substantial improvement in accessibility of enzymes. The process also helps in enhancing the rate and yield of monomeric sugars during enzymatic hydrolysis steps.

Pretreatment of Lignocellulosic Biomass

Pretreatment methods can be broadly classified into four groups – physical, chemical, physio-chemical and biological. Physical pretreatment processes employ the mechanical comminution or irradiation processes to change only the physical characteristics of biomass. The physio-chemical process utilizes steam or steam and gases, like SO2 and CO2. The chemical processes employs acids (H2SO4, HCl, organic acids etc) or alkalis (NaOH, Na2CO3, Ca(OH)2, NH3 etc). The acid treatment typically shows the selectivity towards hydrolyzing the hemicelluloses components, whereas alkalis have better selectivity for the lignin. The fractionation of biomass components after such processes help in improving the enzymes accessibility which is also important to the efficient utilization of enzymes.

Conclusions

The major cost components in bioethanol production from lignocellulosic biomass are the pretreatment and the enzymatic hydrolysis steps. In fact, these two process are someway interrelated too where an efficient pretreatment strategy can save substantial enzyme consumption. Pretreatment step can also affect the cost of other operations such as size reduction prior to pretreatment. Therefore, optimization of these two important steps, which collectively contributes about 70% of the total processing cost, are the major challenges in the commercialization of bioethanol from 2nd generation feedstock.

About Salman Zafar

Salman Zafar is a renowned expert in waste management, biomass energy, waste-to-energy and sustainable development. He is proactively engaged in creating mass awareness on renewable energy, waste management and environment. Salman is the CEO of BioEnergy Consult, apart from being the Founder of Cleantech Solutions and EcoMENA. Salman has successfully accomplished a wide range of projects in the areas of biogas technology, biomass energy, waste-to-energy and solid waste management. He has participated in numerous national and international conferences and has authored many articles in reputed journals and magazines. Salman can be reached at salman@bioenergyconsult.com or salman@cleantechloops.com.
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2 Responses to Ethanol from Lignocellulosic Biomass

  1. We are currently researching second generation biofuels (Stellenbosch Unversity) and one of the major obstacles that still needs to be overcome is the inability of current industrial yeast strains to ferment both xylose and glucose simultaneously efficiently. Much work has being going in to modifying yeast strains such as saccharomyces cerevisiae to be able to do this as well as hardening the strains to be resistant to byproducts that are formed during pretreatment steps. The progress that we are seeing is positive and we are confident about the future of second generation bioethanol produced from waste lignocellulosic materials such as sugarcane bagasse and the like.

  2. Paul Hu Lim says:

    Dear Mr S Zafar and the reporter Mr P McIntosh:
    Indeed you are in part correct but not any more. Try Applied Biofuels Malta Limited through the contacts Hurrellconsult@aol.com, or Josephmicallef@yahoo.co.uk) then you will get more information.
    These Companies are spear-heading the procedures in Malta Israel Italy the UK and Holland (through Dr Roelof Niezen at Niezengr@yahoo.com) and will assist you.

    Making Ethanol from Biomass is the current way forward but the issue is the Carbon Dioxide Residues. These company are using this to make Methanol as well and thus improve the performace.

    A typical Biomass to ethanol Facility working on converting 300,000 tonnes of biomass (assumed dry matter) will generate over 85 Million litres of Ethanol per year more as the process gets up to steam. And if you consider that to build this in the EU will be less than €90 Million then with an Internal Rate of Return approaching 55% after year 5 it has to be a good investment. Maybe at Stellenbosch University you might be interested.

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