Gasification is a thermo-chemical process that converts solid fuel materials into combustible gases that can then be used for heat and power.
Gasification itself is a century-old technology that flourished before and during the Second World War. The technology disappeared soon after, as liquid fuel again became readily available. Today, as fossil fuel use is becoming less and less desirable, there is renewed interest in gasification technology.
When biomass is heated with no oxygen, or a fraction of what is needed for efficient combustion, it gasifies into a mixture of carbon monoxide and hydrogen — synthesis gas or syngas. Combustion occurs as a result of mixing oxygen with hydrocarbon fuel. Because gaseous fuels mix with oxygen more easily than liquid fuels, which in turn mix more easily than solid fuels, syngas inherently burns cleaner and more efficiently than the solid biomass from which it was made. It also results in the production of a low carbon, renewable energy.
One advantage of gasification technology is that it is a decentralized energy conversion system that operates economically even when used in small-scale applications. Although the technology is currently not commercially available in the United States, it has proven to be economical in many locations and its use can lead to greater self-reliance in a fuel crisis.
Theoretically, almost all kinds of biomass with a moisture content of 5-30 percent can be gasified. All gasifiers have fairly strict requirements for the shape and size of fuel as well as moisture, volatile matter, carbon, and ash content.
The complete gasification system consists of the gasifier, a cooling and cleaning system, and an energy converter — burners or an internal combustion engine. Mixed with air, the syngas can be used in a gasoline or diesel engine with little modification. As a diesel engine cannot be operated on syngas alone, it needs to be operated on a dual-fuel mode or converted completely in a sparkignition engine.
Technically attractive gasification systems are still in the early phases of development and demonstration. Assessments on the cost effectiveness of energy-generating systems with integrated biomass gasification have to be proven by practical experience in regular operation. Nevertheless, there are indications that technical advances in developing reliable systems for biomass gasification and efficient gas utilization can lead to economic advantages over combustion.
Most small- and medium-sized combustion power plants have overall efficiencies in the range of 15-20 percent, whereas their gasification counterparts can achieve overall efficiencies of approximately 35 percent, with 45-50 percent as a near-term possibility.