Gasification 101

By E. M. Morison
 

What is gasification?
Gasification is a thermochemical reaction that occurs when organic materials are heated to high temperatures without much oxygen. Think of it as slow, precisely-controlled combustion. This process generates a flammable gas known as producer gas, or synthesis gas, which is composed primarily of carbon monoxide and hydrogen. Synthesis gas, also called syngas, provides about one sixth the energy of an equal volume of natural gas.

What are the parts of a gasification system?
A gasification system consists of:
• a reactor, in which air, or air and steam, is forced through an incandescent fuel bed, producing gas,
• a scrubbing system to clean impurities from the manufactured gas, and

• a burner or engine and generator to convert the syngas into energy.
 

What is syngas used for?
Synthesis gas is used like natural gas to generate heat or electricity. It may be burned directly in a furnace or cleaned and used in more efficient power systems, such as gas turbines or internal combustion engines connected to electrical generators. Syngas is also processed into chemicals, fertilizers and liquid fuels.

In the future, experts say, renewable syngas will be used as a feedstock for manufacturing ethanol and biopolymers, and for making hydrogen for fuel cells.

What materials can be gasified?
Any carbon-based material can be gasified: coal, petroleum, oil refinery wastes, wood, mill and forest product residues, crops and crop residues, manure, agricultural processing coproducts, municipal solid waste, sewage sludge, tires, even plastics. A Georgia carpet mill is gasifying carpet scraps to run its plant.
 

Today, coal is the most common gasification feedstock, accounting for most of the syngas produced in the United States.

Why gasify biomass instead of burning it directly?
Most biomass power today is, in fact, produced by burning the fuel in a furnace to produce steam. But many types of biomass —distiller’s grains and rice hulls, for instance —are difficult to burn. Gasification can convert nearly any kind of biomass to syngas. Syngas can also be purified and filtered to remove tar, particulates and chemical contaminants. So it burns much more cleanly than the solid fuel it’s made from, reducing emissions.
 

Syngas can be used in more efficient integrated power systems called combined cycles, which couple combustion turbines and steam turbines to produce electricity. The efficiency of these systems can reach 60 percent — double the efficiency of direct-fired steam generators. Greater efficiency lowers
the energy unit cost.

Synthesis gas can also be transformed to methane, which can be piped to distant users and substituted for natural gas.

Does gasification have other benefits?

Promoters say biomass gasification could strengthen rural economies and cut reliance on foreign oil. Substituting renewable fuels for fossil fuels benefits the atmosphere. And the ash left after biomass gasification makes good fertilizer.
 

Is gasification a new technology?
Coal and wood gasification was first developed in the 1800s to manufacture “town gas” for lighting and cooking. In the 1940s, town gas was replaced by natural gas and centrally-generated electricity. After World War II, gasifiers were used to make chemicals and fertilizers from a variety of low-cost feedstocks, including coal and petroleum residues.
 

In the 1970s, gasification use began expanding in response to rising energy prices, and in the 1990s, world syngas capacity doubled. Now some 400 large-scale gasifiers worldwide are converting low-value coal and petroleumbased feedstocks into higher-value chemicals, fuels and electricity. The United States has about 20 large coal and petcoke gasification plants, which produce 15 percent of the world’s syngas.
 

Is biomass gasification used in the United States today?
Interest in biomass gasification first began in the 1970s, and a variety of biomass gasification methods have been developed. Although the technology is still emerging, dozens of small-scale biomass gasifiers are in use around the country, generating about two percent of the nation’s syngas supply — mainly from forest products, pulp industry residues and municipal solid waste.
 

What is the outlook for increasing U.S. biomass gasification?
Cheap fossil fuels have limited the economic competitiveness of biomass fuels, which are expensive to collect, transport and store. But that may be changing. Adoption of biomass gasification technology is being encouraged by higher oil and natural-gas prices, public demand for renewable energy, large supplies of biomass feedstocks and more stringent environmental regulations. Some recent
examples:

• A pilot wood gasification plant is supplying 12 megawatts of electricity to Burlington, Vt., augmenting the city’s existing power plant.
• In Salt Lake City, a pilot power plant is gasifying municipal solid waste, manure and agricultural residues to produce hydrogen for fuel cells.
• In Raleigh, N. C., clean wood residues are being gasified to produce fuel for a power utility.
 

What are the most promising types of biomass gasification?
Small biomass gasification systems hold the most commercial promise, because it is not cost effective to transport large amounts of bulky biomass over long distances to a central power plant. Systems are now being devised for manufacturing and processing plants, schools, farms and industrial parks.

Many experts say that agricultural processing plants will be the first to adopt gasification. These operations generate their own biomass coproducts, which are often low in value and could be profitably converted to energy.

Sources: United States Department of Energy, Energy Information Administration, University of North Dakota Energy and Environmental Research Center, Sebesta Blomberg & Associates, Inc.