U.S. Department of Energy - Energy Efficiency and Renewable Energy
Bioenergy Technologies Office
Thermochemical Conversion Processes
In gasification conversion, lignocellulosic feedstocks such as wood and forest products are broken down to synthesis gas, primarily carbon monoxide and hydrogen, using heat. The feedstock is then partially oxidized, or reformed with a gasifying agent (air, oxygen, or steam), which produces synthesis gas (syngas). The makeup of syngas will vary due to the different types of feedstocks, their moisture content, the type of gasifier used, the gasification agent, and the temperature and pressure in the gasifier.
Gas Cleanup and Conditioning
The syngas produced undergoes clean-up and conditioning to create a contaminant-free gas having the appropriate hydrogen-carbon monoxide ratio prior to the catalytic conversion step.
Among the contaminants removed during clean-up are tars, acid gas, ammonia, alkali metals, and other particulates.
Syngas is then conditioned: hydrogen sulfide levels are reduced by sulfur polishing, and hydrogen-carbon monoxide ratio is adjusted using water-gas shift.
In pyrolysis processing, biomass feedstocks are broken down using heat in the absence of oxygen, producing a biooil that can be further refined to a hydrocarbon product. The decomposition occurs at lower temperatures than gasification processes, and produces liquid oil instead of a synthesis gas. Oil produced varies in oxygen content or viscosity according to the feedstock used.
Oil produced in pyrolysis processing must have particulates and ash removed in filtration to create a homogenous product. The oil is then upgraded to hydrocarbon fuels via hydrotreating and hydrocracking processing, which reduces its total oxygen content.