Thermochemical Conversion of Biomass to Biofuels via Gasification


The thermochemical conversion process uses heat to break down biomass into into intermediates, such as gas or bio-oil that can be upgraded into fuel and other products. One type of thermochemical conversion process is gasification, a method that uses heat to turn biomass into a hydrogen- and carbon monoxide-rich gas called synthesis gas, or syngas. This in turn is then converted into liquid transportation fuels or other products. Gasification occurs at relatively high temperatures. Here’s one example of a gasification conversion process. Wood material such as forest residue is a common feedstock for the gasification process. For best results, feedstock wood chips are no more than two inches in size and have 20%-30% moisture content by weight, depending on the gasifier. In the gasifier, the biomass is broken down into vapors in a carefully controlled environment that typically contains oxygen and/or steam. The syngas produced, made up of carbon monoxide and hydrogen. Char, a solid byproduct of gasification, is typically removed using a cyclone. Cyclonic separation allows removal of particulates from a gas stream, without a a filter, by using rotational effects in combination with gravity to collect the particles. Tars and methane are also produced. Their quantity and composition depend on the gasification environment. Tars are high molecular weight organic compounds. If they are not removed, these tars will contaminate downstream equipment and inhibit fuel synthesis. One way to remove methane and tars is by using special catalysts to convert the compounds into additional syngas, thus eliminating the methane and tar problem while also improving the efficiency of the process. Additional cleaning and conditioning steps may be required to remove other contaminants such as ammonia, sulfur, and carbon dioxide. The syngas is then conditioned to produce the desired ratio of carbon monoxide and hydrogen to achieve optimal chemical reactions in subsequent steps. The pressurized, hot syngas is then passed over a catalyst to form a liquid. The catalyst is contained in a reactor, and the syngas is passed through the reactor, where the carbon monoxide and hydrogen molecules combine to form larger molecules. These molecules are subsequently cooled, condensed and refined into clean, renewable transportation fuels and other products. The Department of Energy has supported development of innovative technologies that result in higher yields and superior quality of biomass syngas. These efforts have also resulted in a much more thorough understanding of the gasification process. Enabled by these advancements, industry is deploying gasification technologies that will bring clean, renewable transportation fuels fuels and other biomass-based products to the marketplace.

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