From plants to biofuel: An explanation of biomass pre-treatment
Bioethanol plant at Seal Sands, Teeside, UK [Image source: Nick Bramhall, Flickr]
Renewable energy from biomass is now a key component of renewable energy development worldwide. The fuel used for this process, known as feedstocks, consist of lignocellulosic biomass, that is to say plants with a complex structure containing polymers of cellulose, hemicellulose and lignin. Typically, these are materials such as straw, corn stover, switchgrass or wood waste, mostly used for the production of biofuels, such as bioethanol, which must be subjected to a process of fermentation in which the sugars turn to alcohol (ethanol being a form of alcohol). Most vehicle warranties accept a maximum of 5 percent bioethanol/95 percent petrol blend. Stronger mixes are possible but they usually require modification of the vehicle in order to be successful.
Prior to the fermentation process, the sugars must be released from the lignin via a number of pre-treatment processes. These processes can also produce various by-products through integrated biorefinery processes.
The carbohydrate polymers cellulose and hemicellulose and the aromatic polymer lignin contain carbon sugars which are tightly bound to the lignin. That is to say they are trapped within the lignocellulose. This means that they must first be disconnected from the lignin and then hydrolyzed using acid or enzymes, in order to break them down into sugars (simple monosaccharides) so that they can be used for biofuel.
The initial step in this procedure is a mechanical one. The plants must be shredded and ground in order to reduce their size, decrease crystallinity, reduce polymerization and increase the effect of acid or enzyme hydrolysis. This also improves the energy density of the biomass so that it can be more easily transported from field to point to use. Typically, the biomass is converted into pellets, cubes or pucks (similar in size and shape to an ice hockey puck). They can also be turned into ‘bio-coal’ or ‘bio-oil’ through heat and pressure treatment.
Fractionation is the process in which the biomass is converted into lignin, cellulose and hemicellulose, which can be more easily processed in a biorefinery.
The next step is steam explosion, in which the fibrous structure of the biomass is broken down with high pressure steam and then rapidly depressurized. This destroys the fibrous thereby enabling subsequent pre-treatment processes. Other methods of achieving the same result include ammonia fiber explosion, in which the biomass is treated with liquid ammonia at high temperature and pressure, and super-critical carbon dioxide explosion in which the biomass is treated with carbon dioxide.
US Department of Agriculture (USDA) Agricultural Research Service scientists add a new yeast strain to a corncob mix to test its effectiveness in fermenting ethanol from plant sugars [Image source: US Department of Agriculture, Flickr]
Alkaline hydrolysis involves the treatment of the biomass with a high concentration of alkaline at low temperature for a long time period. Substances used for this purpose include sodium hydroxide, calcium hydroxide or ammonia. The advantages of this process include lower temperatures and pressures, less degradation of the sugars and the ability to recover many of the caustic salts. However, the lengthy time period required and the high concentration of alkaline are the main disadvantages.
Low Temperature Steep Delignification (LTSD) is a process developed by Bio-Process Innovation Inc that uses small amounts of non-toxic chemicals. A one ton pilot plant was constructed by the company in Indiana, USA, but the process is now widely commercially available for use in biorefineries elsewhere.
Co-solvent Enhanced Lignocellulosic Fractionation (CELF) uses an organic compound called tetrahydrofuran (THF) in combination with dilute sulfuric acid for fractionation. It can produce a high sugar for fermentation. It can also produce a number of useful organic compounds including furfural (which can be used as an aid to the delivery of agricultural herbicides and as a chemical solvent), 5-hydroxymethylfurfural, and levulinic acid which can be catalytically converted into chemicals or fuel. This process was developed by the University of California and licensed by CogniTek. A company called MG Fuels has been established to commercialize the process.
Organosolv uses organic solvents such as ethanol, methanol, butanol and acetic acid to make lignin and hemicellulose soluble. A patented organosolv process has been developed and patented by American, Science and Technology AST although it is presently only at pilot scale. The process converts lignocellulosic biomass into sugars, pure lignin, pulp and biochemical and involves fractionation and hydrolysis to produce more than 95 percent yield of sugars.
Ozonolysis is the treatment of biomass with ozone prior to enzyme hydrolysis.
Pyrolysis is one of the most well-known processes, involving chemical decomposition by heating. Flash pyrolysis achieves this within 1-2 seconds using temperatures of up to 500°C. Mobile pyrolysis units are currently in use among a number of organisations worldwide, as well as being deployed at local level in developing countries to achieve community scale production of biofuels. Torrefaction is a milder form of pyrolysis, in which the thermochemical process is conducted at 200-350 °C in the absence of oxygen which produces torrefied biomass or char, more commonly known as ‘bio coal’.
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