Options for reducing a coal-fired plant’s carbon footprint, Part II

Dr. Justin Zachary, Bechtel Power Corp.

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A conventional coal plant’s CO₂ emissions can be reduced either after combustion (see Part I of this article in POWER, June 2008) or before. In the latter case, typified by integrated gasification combined-cycle (IGCC) plants, the fuel used is synthesis gas (syngas), which contains mostly hydrogen (H₂) and CO. A water-shift reactor converts the CO to CO₂, which is then removed by physical absorption. The H₂ in the syngas is used to fuel one or more gas turbines.

Because this approach is substantially different (it requires replacing the plant’s coal-fired boiler with combustion turbines), we will not discuss it further here. Instead, we will concentrate on other possible methods of CO₂reduction.

Oxyfuel combustion

Among those methods is oxyfuel combustion. Burning coal in the presence of oxygen, rather than air, produces a highly pure CO₂ exhaust that can be captured and sequestered at relatively low cost. Often, the oxygen is mixed with the exhaust to regulate combustion and to raise the level of CO₂ in the flue gas.

In the Rankine steam cycle with oxyfuel combustion, the volume of flue gas leaving the boiler is considerably smaller than the volume of air entering it. Why? The large amount of N₂ in combustion air (78%) does not become part of the flue gas. As a result, the volume of flue gas from an oxyfueled plant is about 75% lower than that from a plant using air for combustion. The flue gas from an oxyfueled plant consists primarily of CO₂. Figure 1 shows a block diagram of a typical oxyfuel process, and Table 1 highlights its advantages and limitations.

1. A typical oxyfuel combustion block diagram. Source: Bechtel Power Corp.

Table 1. Oxyfuel combustion’s pluses and minuses. Source: Bechtel Power Corp.

Theoretical and experimental research into this technology has intensified over the past two years. Several pilot plants are now operating in the U.S., Germany, and Japan.

At the heart of the oxyfuel process is an air separation unit (ASU), a large vessel that consumes much electricity. In an effort to reduce its load and penalty on plant output, new, more energy-efficient oxygen separation technologies are in development. They include ion transport and oxygen transport membranes and BOC Group’s ceramic autothermal recovery process for oxygen production.

Air-fired combustion has better thermal performance than oxyfueled combustion, as Table 2 shows. Although the level of unburned carbon in both processes’ flyash is similar, more coal needs to be burned in oxyfuel combustion to achieve the same net output.