Biomass Gasification
Biomass gasification is a thermal process that converts plant matter into mixtures of combustible gases (hydrogen, carbon monoxide, methane, ethylene, etc). It is accomplished through direct or indirect heating of the biomass in the presence of limited oxygen, causing partial oxidation of the biomass polymers. The basic process has been known since 1792, when a Scottish engineer first gasified coal to light his home. A similar gas, also known as “town gas,” was distributed throughout Europe and the U.S. beginning in the latter half of the 18th century for street lighting and home heating.
Gasification can convert 90% of the chemical energy contained in dry plant material into chemical fuel gas plus recoverable waste heat. Direct gasification can be accomplished with air or steam and oxygen. When using air, it is called "air blown" gasification and generates “producer gas” with an energy density of about 150 Btu/scf. The nitrogen in the air is not utilized in the reaction and dilutes the fuel gas. When using steam and oxygen instead of air, "synthesis gas” is produced. Synthesis gas or “syngas” is a richer fuel gas with an energy content of 300 Btu/scf.
Gasification differs from combustion in several ways. When oxygen is limited in the gasification reaction, combustible products like hydrogen, carbon monoxide, and methane are produced. If sufficient oxygen is provided, as it is in combustion, the products fully oxidize to water vapor and carbon dioxide. While combustion is useful for providing immediate heat, the produced gases have little chemical energy remaining.
The gasification process has several advantages over conventional combustion:
- Biomass gasification results in a combustible gas, called “producer gas” that can be conditioned to burn cleanly, like natural gas. This means a customer can install a gasifier system in an existing facility and reuse much of the same boiler, dryer, and ducting equipment. Frontline BioEnergy utilizes a flexible-fuel burner that allows instantaneous natural gas backup.
- True gasification separates production of the gas from combustion of the gas so that the gas can be conditioned before it is burned to produce heat. This provides a unique opportunity to remove particulates and other contaminants in the gas stream before entering a boiler or other gas appliance. Processes utilized by competitors often require post combustion gas cleaning, which requires a much larger system to handle the higher volume of gas.
- Gases produced through biomass gasification are capable of being further processed into chemicals, liquid fuels, and gases (eg. hydrogen, methanol, ethanol, and ammonia). This provides a path for “biomass-to-ethanol” which is not limited to the cellulose portion of the biomass. Converting the entire biomass material to gas has the potential to produce over 100 gallons of ethanol per ton of biomass, much higher than enzymatic processes.