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
In situ combustion 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.