Towards a Cleaner Use of Coal:
Technology is Reducing Emissions

The term “clean coal technologies” is used to describe a wide range of processes that reduce or eliminate emissions produced from coal-fired electricity generation. Coal is typically used to produce electricity by pulverizing it into a fine dust, which is then blown into a combustion chamber that heats a boiler.

The boiler produces steam that propels a turbine, which, in turn, generates electricity. Emissions found in the flue stack gases of these plants, which are emitted to our atmosphere, include

Sulphur dioxide (SO2) – creates acid rain when combined with water vapour

Carbon dioxide (CO2) – the largest greenhouse gas by volume emitted by Canada

Nitrous oxides (NOx) and particulate matter – creates smog and contributes to the production of acid rain, and

Mercury – a neural toxin.

Some clean coal technologies include

Supercritical boilers that employ higher temperatures and pressures to increase combustion efficiency, which reduces the amount of all emissions released per unit of electricity produced.

Scrubbers/solvent capture that removes SO2 and NOx from the flue stack gases of a typical coal plant before the gas is run through a solvent that captures CO2.

Oxygen/CO2 recycle combustion or oxyfuel that separates the nitrogen and oxygen in the air, feeding only the oxygen into the combustion process of a conventional pulverized coal operation. Since coal won’t burn using oxygen alone, flue gas containing CO2 is recycled back to the burner and used to dilute the oxygen, resulting in high concentration of CO2 in flue gas and making it more economic to capture. This process reduces CO2 and other emissions.

Integrated Gas Combined Cycle (IGCC) technology offers an entirely different approach by turning coal into a gas. Gasification technologies differ widely but share certain general production characteristics.

Coal is fed into the gasifier in either dry or slurried form. In the gasifier, the coal is subjected to high temperatures and pressure, and low levels of oxygen to create synthesis gas, or syngas, without burning the coal. Syngas is a mixture of hydrogen and carbon monoxide that has about one-quarter of the energy value of natural gas. The gas is then treated or cleaned to remove trace elements or impurities such as sulphur that are either recirculated to the gasifier or recovered and marketed.

The syngas is then burned in an advanced gas turbine (the first cycle). Waste heat from the gas turbine is captured instead of being released to the atmosphere, and used to create steam. The steam drives a steam turbine that generates more electricity.

Since one fossil fuel source provides the energy to drive two turbines in this combined cycle process, it is more efficient than pulverized coal plants and fewer emissions are produced.

One of the biggest benefits of the Integrated Gas Combined Cycle process is that it creates a pure CO2 stream that can be more easily captured and geologically stored or used in other industrial processes.

Coal isn’t the only fuel source for IGCC technology. Any number of carbon-based feedstocks can be used such as petroleum coke, the carbon residue from upgrading oilsands bitumen to synthetic crude, and the high-sulphur bottoms produced by refinery operations. Typically these products would be disposed of as waste.

Some clean coal technologies are used in commercial applications throughout the world. IGCC is gaining in popularity in the United States and Asia for electricity generation but it is still in the preliminary research and development stage here. EPCOR’s Genesee Phase 3 plant addition, southwest of Edmonton, will be the first operation in Canada to use the supercritical process in conjunction with a number of other emission reducing technologies.

-reprinted with permission from C3 Views, the monthly newsletter of Climate Change Central.

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September 2003 PEGG


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