"Clean Coal" technology – an imperfect option

Over the years, pollution control laws have prompted the development of so-called "clean coal technologies" that can reduce the sulfur dioxide, nitrogen oxides, and soot emanating from coal-fired power plants. In Indiana, "clean coal technology" is defined as a technology that directly or indirectly reduces airborne emissions of sulfur or nitrogen-based pollutants associated with the combustion or use of coal. Clean coal technologies generally fall into four main categories:

1. Coal washing involves grinding the coal into smaller pieces and passing it through a gravity separation process designed to lower the level of sulphur and minerals in the coal.

2. Pollution control devices are designed to reduce emissions of particulate matter (i.e., devices such as fabric filters, and electrostatic precipitators or ESPs, where flue gases are passed between collecting plates, attracting particles using an electric charge), nitrogen oxides (i.e., low NOx burners designed to reduce the formation of NOx by controlling the flame temperature and chemical environment in which the coal combusts), sulphur dioxide (i.e., flue gas desulphurization or FGD, also known as wet scrubbing, using a sulphur absorbing chemical such as lime to absorb SO2), and trace elements emissions such as mercury, cadmium and arsenic, which can be reduced by particulate controls, fluidized bed combustion and FGD equipment.

3. Efficient combustion technologies, which include:

  • supercritical pulverized coal combustion (SPCC), which can increase the thermal efficiency of a power plant from 35% to 45%, thereby reducing emissions due to less coal being used;

  • fluidized bed coal combustion (FBC), which allows coal combustion at relatively low temperatures to reduce NOx formation and uses a sorbent to absorb sulphur; and

  • coal gasification, in which coal is reacted with steam and air or oxygen under high temperatures and pressure to form synthetic gas, or "syngas" (mostly carbon monoxide and hydrogen), which can be burned to produce electricity or processed to produce fuels such as diesel oil. Coal gasification technologies include:
    • Integrated Coal Gasification Combined Cycle (IGCC) - a coal gasification technology in which coal is not combusted directly, but reacts with oxygen and steam to form a “syngas” (predominantly hydrogen and carbon monoxide), which is burned in gas turbines to produce electricity, and where exhaust heat from the turbine is used to produce steam to power a steam turbine (a second generation cycle) and provide steam to the gasification process.

    • Integrated Gasification Fuel Cells (IGFC) is another coal gasification technology that uses hydrogen from coal gasification in a solid fuel cell to produce electricity.
    In a general sense, coal gasification is similar to refining oil. Coal gasification takes a dirty coal package and refines it into cleaner hydrocarbons. "When coal gasification technology is used to produce SNG [substitute natural gas – pipeline quality gas that can serve end use consumers or can be used as fuel to produce electric power to supply electric utility service to end use consumers], emissions of regulated pollutants are very low because there is only limited combustion of already cleaned syngas." IGCC technology has the potential to take pollution control one step further, capturing CO2 before it escapes into the atmosphere.

4. Carbon capture and sequestration (CCS) involves separating out or "capturing" the carbon dioxide, and storing it deep underground, in theory to prevent the greenhouse gas from entering the atmosphere. Storage methods being considered include pumping CO2 into disused coal fields to displace methane which can be used as fuel, pumping CO2 into saline aquifers deep underground for long-term storage, and pumping CO2 into oil fields to help maintain pressure and make oil extraction easier.

"The few IGCC plants in existence emit large amounts of CO2, but this CO2 could be separated prior to combustion and then potentially captured and stored underground. It is much more cost-effective to incorporate carbon capture technology into and IGCC plant than it is to retrofit conventional coal plants with this technology." "Unfortunately, though nearly 20 new IGCC coal plants have been proposed, only one – a long-term Department of Energy demonstration project dubbed FutureGen – would be equipped with the technology needed to capture the millions of tons of CO2 these plants would produce every year. There is also no infrastructure in place for transporting the CO2 and storing it in a permanent location, and researchers have yet to determine whether large amounts of CO2 can be safely and reliably stored underground (in former oil and natural gas wells or deep saline aquifers) for long periods of time. Early studies have been encouraging, particularly when compared with the known risks of releasing CO2 into the atmosphere."

"There are no coal fired power stations in commercial production which capture all carbon dioxide emissions, so the process is theoretical and experimental and thus a subject of feasibility or pilot studies. It has been estimated that it will be 2020 to 2025 before any commercial scale clean coal power stations (coal burning power stations with carbon capture and sequestration) become commercially viable and widely adopted. This time frame is of concern because there is an urgent need to mitigate greenhouse gas emissions and climate change to protect the world economy according to the Stern report. Even when CO2 emissions can be caught, there is considerable debate over the necessary carbon capture and storage that must follow."


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