Introduction to Gasification
Gasification is a technological process that uses heat, pressure, and steam to convert any carbonaceous (carbon-based) raw material into synthesis gas. Syngas, for short, is so called because of its history as an intermediate in the production of synthetic natural gas. Composed primarily of the colorless, odorless, highly flammable gases carbon monoxide (CO) and hydrogen (H2), syngas has a variety of uses. For example, syngas can be used to create electricity, refined into pure hydrogen, or transformed into liquid transportation fuels . Gasification systems are also increasingly being used to turn feedstocks like coal into useful chemical products like ammonia.
Gasification offers an alternative to more established ways of converting feedstocks like coal, biomass, and some waste streams into useful products like electricity or fuels. The advantages of gasification in specific applications and conditions, particularly in the generation of electricity from coal, may make it an increasingly important part of the world's energy and industrial markets.
As with any technology, widespread market penetration of gasification relies on economic conditions and enabling infrastructures that allow it to be competitive with other alternatives. For gasification, the largest potential market is electricity generation from coal, where integrated gasification combined cycle (IGCC) plants are being proposed with increasing frequency. The graphic below is a simple representation of a gasification process for coal.
Gasification vs. Combustion
Gasification differs from more traditional energy-generating schemes in that it is not a combustion process but rather a conversion process. Rather than a fuel being burned to create usable heat or expansion, the fuel is combined with steam and oxygen in a heated, pressurized vessel. The atmosphere inside the vessel is starved of oxygen to prevent or limit combustion, and the result is partial oxidation of the fuel to produce a syngas.
Feedstocks and Products
An advantage of gasification is its applicability to a variety of feedstocks. Almost any carbonaceous substance can be gasified: fossil fuels such as coal, oil, or natural gas, biomass (which is any kind of agricultural waste such as corn stover or various crops), asphalt, or even sewage, plastics, and municipal solid waste. Gasification can be used to produce a variety of products, including electricity, natural gas, liquid fuels (such as gasoline, diesel, and jet fuel), hydrogen, and various chemicals.
Typical Syngas Composition
The syngas produced from gasification can vary depending on the feedstock and the gasification process involved. However, typical syngas produced from coal in an oxygen-blown gasifier is by volume about 30 to 60% CO , 25 to 30% H2 , 0 to 5% methane (CH4), 5 to 15% carbon dioxide (CO2), and some water.
Governing Chemical Equations
A number of chemical reactions take place in the gasification process depending on the particular application. In general, the carbon in the fuel reacts with oxygen, CO2, steam, or H2 in the following reactions:
||C + ½O2 ↔ CO
C + CO2 ↔ 2CO
C + H2O ↔ CO + H2
C + 2H2 ↔ CH4