Post Combustion Emission Controls

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Flue Gas Desulfurization (FGD) technology provides an example of both the scale and challenge of solving an environmental control comparable to carbon capture. Virtually all of the technologies for carbon capture will require conditioning the gas stream to eliminate the presence of sulfur (to prevent degradation of the solvents). FGDs were first installed in England to combat the environmental impacts associated with SO2 from coal burning. Using a process developed by ICI (Imperial Chemicals Industries), three large fossil steam plants were retrofitted early in the twentieth century, although work on FGDs came to a halt during the Second World War [2]. The first full-scale FGD in the US was installed in 1967 on a plant operated by Union Electric. Despite a several decade head start on the technology, many of the "first" adopters experienced significant challenges in plant reliability with what was perceived as a relatively simple emission control adaptation. A large number of fossil units constructed in the 1960s and 1970s, would later be retrofitted with FGDs.

Some of the early problems, which would take a decade or more to resolve include:

• Efficiency: FGDs inherently reduce the efficiency of a plant. Plant performance deteriorated by as much as 3-5% with the addition of the post combustion sulfur control. One impact is the need to reheat the flue gases to obtain buoyancy; and this usually meant extracting thermal energy from the base unit. At the time the retrofit program began, there was sufficient excess generator capacity that the loss was not a problem in terms of sufficiency of electricity supply.

• Corrosion: Early systems were plagued by corrosion on a large scale. Materials ranging from rubber to gunite pastes to stainless steel and titanium were tried over the years. Today, many "wet" designs have settled on using a tile and mortar coating that can stand up to the corrosive environment. In the early days, materials selection was more or less haphazard.

• Dewatering: Water is used to cycle sorbent and product through the system, and saturates the stack with water vapor in the process. This requires handling and disposition of the enormous quantities of water consumed in the process.

• Waste disposal: At the largest plants hundreds of tons of calcium sulfite and sulfate are generated each day. It was a major challenge to process these enormous quantities of waste. Entire valleys were filled with the sludge from FGDs before a substantial recycle route was used. Eventually the sulfate would become the primary product. Today, nearly 30% of all gypsum wallboard is recycled from sulfur captured at power plants.1

At some plants, those original FGDs are no longer in place, having been replaced by better designs that evolved out of this process. It would take until the late 1980s before most of the technical challenges (plugging, erosion, corrosion, water handling, additive usage, etc.) would be resolved, creating a jumping off point for the next phase that would become Title IV (the Acid Rain Program). This would allow for the technology expansion indicated in Fig. 10.4, although the expectations from the CAIR rule may come through a different regulatory pathway. The US EPA played a major role in deploying pilot scale demonstrations, catalyzing technology transfer via reports, papers, and symposia. This helped resolve many of the operability and environmental impacts that were revealed during these early years.

After the Clean Air Act of 1990, Title IV of that act initiated a large retrofit campaign to capture SO2 at the point of origin—the coal-fired power plant. Title IV was able to capitalize on the technical innovations and experience from nearly 20 years of trial and error. Even with that experience and knowledge, the graphic hints at the sheer length of time it can take to complete such a process—some 25 years. Earlier attempts to mitigate the impact of acid rain by treating rivers and lakes

1 The 2008 coal ash pond failure at the TVA plant in Kingston, Tennessee injected a new level of complexity in the concern over coal waste disposal.

had proved futile. Addressing the problem at the source achieved significant SO2 emissions reductions, as well as solving problems of acid deposition on streams and lakes, particulate haze, corrosion of infrastructure (including highways and bridges), and eventually resulting in significant health benefits. While these benefits were achieved with reduced plant output and degraded performance efficiency (and higher electricity rates), the benefits are widely acknowledged to have been worth the investment.

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