Page 216 - Synthetic Fuels Handbook

202 CHAPTER SEVEN

The reaction of coal or coal char with air or oxygen to produce heat and carbon dioxide
could be called gasification, but it is more properly classified as combustion. The principal
purposes of such conversion are the production of synthetic natural gas as a substitute gas-
eous fuel and synthesis gases for production of chemicals and plastics.
In all cases of commercial interest, gasification with steam, which is endothermic, is
an important chemical reaction. The necessary heat input is typically supplied to the gas-
ifier by combusting a portion of the coal with oxygen added along with the steam. From
the industrial viewpoint, the final product is synthesis gas, medium-Btu gas, or substitute
natural gas. Each of the gas types has potential industrial applications.
In the chemical industry, synthesis gas from coal is a potential alternative source of
hydrogen and carbon monoxide. This mixture is obtained primarily from the steam reform-
ing of natural gas, natural gas liquids, or other petroleum liquids. Fuel users in the industrial
sector have studied the feasibility of using medium-Btu gas instead of natural gas or oil
for fuel applications. Finally, the natural gas industry is interested in substitute natural gas,
which can be distributed in existing pipeline networks.
The conversion of the gaseous products of coal gasification processes to synthesis gas,
a mixture of hydrogen (H ) and carbon monoxide (CO), in a ratio appropriate to the appli-
2
cation, needs additional steps, after purification. The product gases—carbon monoxide,
carbon dioxide, hydrogen, methane, and nitrogen—can be used as fuels or as raw materials
for chemical or fertilizer manufacture.

7.1.3 Gasifiers
The focal point of any gasification-based system is the gasifier. A gasifier converts hydro-
carbon feedstock into gaseous components by applying heat under pressure in the presence
of steam.
A gasifier differs from a combustor in that the amount of air or oxygen available inside
the gasifier is carefully controlled so that only a relatively small portion of the fuel burns
completely. The partial oxidation process provides the heat and rather than combustion,
most of the carbon-containing feedstock is chemically broken apart by the heat and pres-
sure applied in the gasifier resulting in the chemical reactions that produce synthesis gas.
However, the composition of the synthesis gas will vary because of dependence upon the
conditions in the gasifier and the type of feedstock.
Minerals in the fuel (i.e., the rocks, dirt, and other impurities which do not gasify) sepa-
rate and leave the bottom of the gasifier either as an inert glass-like slag or other marketable
solid products.
Sulfur impurities in the feedstock are converted to hydrogen sulfide (H S) and carbonyl
2
sulfide (COS), from which sulfur can be extracted, typically as elemental sulfur. Nitrogen
oxides (NO ), other potential pollutants, are not formed in the oxygen-deficient (reducing)
x
environment of the gasifier. Instead, ammonia (NH ) is created by nitrogen-hydrogen reac-
3
tions and can be washed out of the gas stream.
In Integrated Gasification Combined Cycle (IGCC) systems, the synthesis gas is cleaned
of its hydrogen sulfide, ammonia, and particulate matter and is burned as fuel in a combus-
tion turbine (much like natural gas is burned in a turbine). The combustion turbine drives
an electric generator. And hot air from the combustion turbine can be channeled back to
the gasifier or the air separation unit, while exhaust heat from the combustion turbine is
recovered and used to boil water, creating steam for a steam turbine-generator.
The use of these two types of turbines—a combustion turbine and a steam turbine—in
combination, known as a combined cycle, is one reason why gasification-based power sys-
tems can achieve unprecedented power generation efficiencies. Currently, commercially
available gasification-based systems can operate at around 42 percent efficiencies; in the