Tag Archives: thermochemical

Overview of Biomass Conversion Technologies

  By Salman Zafar | April 15, 2012 - 2:58 am | Biomass Energy, Cogeneration, Thermochemical
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A wide range of technologies are available for realizing the energy potential of biomass wastes, ranging from very simple systems for disposing of dry waste to more complex technologies capable of dealing with large amounts of industrial waste. Conversion routes for wastes are generally thermo-chemical or bio-chemical, but may also include chemical and physical.

Thermal Technologies

The three principal methods of thermo-chemical conversion corresponding to each of these energy carriers are combustion in excess air, gasification in reduced air, and pyrolysis in the absence of air. Direct combustion is the best established and most commonly used technology for converting wastes to heat. During combustion, biomass is burnt in excess air to produce heat. The first stage of combustion involves the evolution of combustible vapours from wastes, which burn as flames. Steam is expanded through a conventional turbo-alternator to produce electricity. The residual material, in the form of charcoal, is burnt in a forced air supply to give more heat.

Co-firing or co-combustion of biomass wastes with coal and other fossil fuels can provide a short-term, low-risk, low-cost option for producing renewable energy while simultaneously reducing the use of fossil fuels. Co-firing involves utilizing existing power generating plants that are fired with fossil fuel (generally coal), and displacing a small proportion of the fossil fuel with renewable biomass fuels. Co-firing has the major advantage of avoiding the construction of new, dedicated, waste-to-energy power plant. An existing power station is modified to accept the waste resource and utilize it to produce a minor proportion of its electricity.

Gasification systems operate by heating biomass wastes in an environment where the solid waste breaks down to form a flammable gas. The gasification of biomass takes place in a restricted supply of air or oxygen at temperatures up to 1200–1300°C. The gas produced—synthesis gas, or syngas—can be cleaned, filtered, and then burned in a gas turbine in simple or combined-cycle mode, comparable to LFG or biogas produced from an anaerobic digester. The final fuel gas consists principally of carbon monoxide, hydrogen and methane with small amounts of higher hydrocarbons. This fuel gas may be burnt to generate heat; alternatively it may be processed and then used as fuel for gas-fired engines or gas turbines to drive generators. In smaller systems, the syngas can be fired in reciprocating engines, micro-turbines, Stirling engines, or fuel cells.

Pyrolysis is thermal decomposition occurring in the absence of oxygen. During the pyrolysis process, biomass waste is heated either in the absence of air (i.e. indirectly), or by the partial combustion of some of the waste in a restricted air or oxygen supply. This results in the thermal decomposition of the waste to form a combination of a solid char, gas, and liquid bio-oil, which can be used as a liquid fuel or upgraded and further processed to value-added products.

Biochemical Technologies

Biochemical processes, like anaerobic digestion, can also produce clean energy in the form of biogas which can be converted to power and heat using a gas engine. Anaerobic digestion is a series of chemical reactions during which organic material is decomposed through the metabolic pathways of naturally occurring microorganisms in an oxygen depleted environment. In addition, wastes can also yield liquid fuels, such as cellulosic ethanol and biodiesel, which can be used to replace petroleum-based fuels.

Anaerobic digestion is the natural biological process which stabilizes organic waste in the absence of air and transforms it into biogas and biofertilizer. Almost any organic material can be processed with anaerobic digestion. This includes biodegradable waste materials such as municipal solid waste, animal manure, poultry litter, food wastes, sewage and industrial wastes. An anaerobic digestion plant produces two outputs, biogas and digestate, both can be further processed or utilized to produce secondary outputs. Biogas can be used for producing electricity and heat, as a natural gas substitute and also a transportation fuel. Digestate can be further processed to produce liquor and a fibrous material. The fiber, which can be processed into compost, is a bulky material with low levels of nutrients and can be used as a soil conditioner or a low level fertilizer.

A variety of fuels can be produced from biomass wastes including liquid fuels, such as ethanol, methanol, biodiesel, Fischer-Tropsch diesel, and gaseous fuels, such as hydrogen and methane. The resource base for biofuel production is composed of a wide variety of forestry and agricultural resources, industrial processing residues, and municipal solid and urban wood residues. The largest potential feedstock for ethanol is lignocellulosic biomass wastes, which includes materials such as agricultural residues (corn stover, crop straws and bagasse), herbaceous crops (alfalfa, switchgrass), short rotation woody crops, forestry residues, waste paper and other wastes (municipal and industrial). The three major steps involved in cellulosic ethanol production 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. Bioethanol production from these feedstocks could be an attractive alternative for disposal of these residues. Importantly, lignocellulosic feedstocks do not interfere with food security.

Production of Cellulosic Ethanol

  By Salman Zafar | September 10, 2011 - 7:51 am | Agricultural Residues, Bioethanol, Biofuels, Biomass Energy, Energy-from-Waste, Thermochemical Conversion, Waste-to-energy
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The production of biofuels from lignocellulosic feedstocks can be achieved through two very 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;
  • Thermo-chemical – 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.

Lignocellulosic processing pilot plants have been established in the EU, in Denmark, Spain and Sweden. The world’s largest demonstration facility of lignocellulose ethanol (from wheat, barley straw and corn stover), with a capacity of 2.5 Ml, was first established by Iogen Corporation in Ottawa, Canada. Many other processing facilities are now in operation or planning throughout the world.

Ethanol from lignocellulosic biomass is produced mainly via biochemical routes. The three major steps involved are pretreatment, enzymatic hydrolysis, and fermentation as shown in Figure. 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 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.

Presently, a ton of dry biomass typically yields 60-70 gallons of bioethanol. 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.