Page 256 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

232 8 Pre-combustion Air Emission Control

water and the toxicity of H 2 S. Removal of CO 2 also improves the heating value of
the natural gas because CO 2 is not combustible and it simply lowers the combustion
efficiency.
The great partial pressure of H 2 S and CO 2 allows them to be removed by
physical adsorption and membrane separation from the natural gas outside of the
well. Physical solvents like Rectisol, Purisol, and Selexol are preferred over
chemical solvents for high pressure gas purification processes. However, the most
popular technology is amine-based absorption. The separated H 2 S can be converted
into elemental sulfur as explained above. The treatment of separated CO 2 will be
elaborated in Sect. 12.6.

8.2 Fuel Substitution

As introduced previously the air emissions are dependent on the fuels. Relatively
speaking, natural gas is cleaner than gasoline, and coal is the dirtiest fossil fuel.
Solid fossil fuels, biomass, and waste-derived fuels generate a wide spectrum of air
pollutants in addition to solid waste. From this point of view gaseous or liquid fuels
are considered cleaner because they hardly contain ash-forming elements, which
make them suitable for application in internal combustion engines and gas turbines.
As a result, the emissions per unit heat or power generated by liquid/gaseous fuels
are less than solid fuels.
It is technically feasible to use fuel substitution such as co-combustion of oil and
coal, in order to reduce the air emissions. In reality, other factors, likely economical
consideration, also contribute to the final decision of which fuel is used for certain
process. So far, coal is still the cheapest and most polluted fuel in environment. In
the United States, coal is mainly used as an electric utility fuel. Although oil is more
expensive than coal to recover, it can be easily transported by pipelines. Like oil,
natural gas is more expensive to recover from wells, but contrary to oil, it cannot be
easily stored or shipped. Nonetheless natural gas is widely used because of its ease,
efficiency, and cleanliness of combustion.
It is obvious that replacing a high-sulfur fuel with a low-sulfur fuel will reduce
the amount of SOx in the flue gas. Replacing solid fuel with oil or gas can also
reduce the ash formation. In some regions, local authorities may enforce fuel
substitutions to reduce the local air pollution. For example, low-sulfur coals must be
burned in the plants within the capital of China.
The success of fuel substitution depends on the fuel-flexibility of the burner and
the economics of operation with another fuel. It is relatively easy to retrofit a coal-
fired boiler with natural gas; however, it may not be the same using oil. Further-
more, we have to consider the operation of downstream air cleaning units. For
example, sulfur content in the fuel affects the dust separation performance of an
electrostatic precipitator (ESP). In-depth analysis will be introduced Chap. 10 for
post-combustion air emission control approaches.