Coal, which is the most abundant and economically stable
fossil fuel in the United States, continues to grow in use while under pressure
to meet the most stringent federal and local emissions requirements.
This trend has added to the cost and complexity of coal
combustion technologies. Emission-control methods that facilitate the use of
coal in power plants can be classified as
Precombustion processes
In situ combustion processes
Postcombustion processes
Precombustion processes include methods to clean the coal of
sulfur-bearing compounds by wet separation, coal gasification, and coal
liquefaction techniques. Coal gasification involves the partial oxidation of
coal to produce a clean gas or by production of a “clean fuel” through coal
liquefaction.
Sulfur and ash are removed in these processes. The use of
coal to produce a gas is not a new idea; it has been used to produce “town gas”
for over 200 years. But its use in the United States had almost disappeared by
1930, because natural gas was abundant and low in cost.
Concerns about the availability and economic stability of
gas supplies, along with environmental trends, have renewed interest in coal
gasification to produce substitute natural gas (SNG) and low- and medium-heat
content (LBTU and MBTU) gas for chemical feedstock or power plant fuel.
Coal gasification in the combined-cycle mode has been well
established as a viable technology for producing power with very low emissions
both in the United States and Europe. New plants are using technologies such as
high-temperature gas turbines, hot-gas cleanup to remove 99% of the sulfur
(H2S), and higher-pressure combined steam cycles to achieve overall
efficiencies of greater than 40%.
New integrated gasification combined-cycle (IGCC) plants of
as much as 250 MWe are available. IGCC technology produces very low emissions
per kilowatt of power and is therefore very attractive for the production of
power.
Likewise, coal liquefaction is not a new technology, but is
only in limited commercial use in the United States. South Africa is the
largest producer of synthetic liquid fuels from coal. Large-scale production of
synthetic liquid fuels from coal began in 1910 in Germany with the
Fischer-Tropsch process, which is used to produce a variety of fuels.
In fluidized-bed combustion, an in situ
combustion-emission-control process, 90% to 95% of the SO2 is captured during
combustion by a sorbent (limestone). In this process, the NOx production is low
because of the low temperature at which the combustion reaction takes place.
NOx levels well fired below 0.25 lb/MBtu have been achieved with certain coals.
Fluidized-bed combustion was developed in the 1950s and is now available for
electric power plants of up to 300-MWe size.
The technology hasthree distinct types of units: bubbling
bed, hybrid velocity, and circulating fluidized bed (CFB). CFB technology is
the most popular fluidized-bed process and has evolved as a low-emission
technology with excellent fuel flexibility for the production of power.
Bubbling and hybrid-velocity fluidized-bed technologies have demonstrated low
emissions while burning low-rank coals, waste fuels such as petroleum coke, and
renewable fuel such as wood and peat.
Hybrid-velocity fluidized-bed combustion can be readily
retrofit to many older boilers that need pollution-control technology.
Pressurized fluidized-bed combustion is used to achieve low sulfur and NOx
emissions of fluidized-bed combustion integrated with a gas turbine to achieve
high cycle efficiency, and therefore make more efficient use of coal.
Postcombustion control processes are widely used for the
capture of sulfur and particulate. Processes and equipment for removal of NOx
from flue gases leaving boilers have been widely used in Europe and are being
applied in the United States. In situ control of NOx by modifications to firing
technology and over-fire air can reduce NOx as much as 50%. Selective
noncatalytic control (SNCR) involves ammonia or urea sprayed in the proper
place in the boiler to reduce NOx.
More NOx reduction can be achieved by selective catalytic
reduction (SCR), which uses ammonia in a postcombustion control system. SCR can
reduce NOx levels well below those from a conventional pulverized-coal boiler.
Coal gasification is an efficient way to produce electric
power while minimizing the emissions from the combustion of coal. Coal
gasification can achieve cycle efficiencies above 40% when the gas turbine
cycle is completely integrated with the steam cycle.
This is referred to as the integrated gasification combined
cycle. In an IGCC plant, the gas from the gasification process is burned in a
boiler or gas turbine for the generation of electric power. The process also
uses the heat from the gas turbine exhaust to produce electric power from a
steam cycle.
In the gasification process, coal is partially reacted with
a deficiency of air to produce low heatingvalue fuel gas. The gas is cleaned of
particulate and then sulfur compounds in a hot-gas cleanup system. Elemental
sulfur is disposed of or sold.
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