U.S. Department of Energy - Energy Efficiency and Renewable Energy

Bioenergy Technologies Office

Thermochemical Conversion Processes

Gasification

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.

Figure illustrating Thermochemical Platform Integration for Gasification Conversion: Inputs from Feedstock Logistics and/or Biochemical Conversion  (lignin residue) stages undergo two types of processing. Processing and handling, combustion for heat and power recycling output. Processing and handling, gasification, gas cleanup, gas conditioning, fuel synthesis, and output to biofuels distribution.

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.

Pyrolysis

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.

Figure illustrating Thermochemical Platform Integration for Pyrolysis Conversion: Inputs from Feedstock Logistics stage undergoes feed processing and handling, pyrolysis, bio-oil stabilization, fuel processing, and is output to biofuels distribution.

Bio-Oil Cleanup

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.