(218c) Enhanced CO2 Sequestration From a Combined Cycle Power Plant

At present 80 % [1] of world energy is being generated by the combustion of fossil fuels because of their easy availability and technical maturity in transforming raw material to final useful products. The products of the burning of the fossil fuel carbondioxide (CO₂), which is a greenhouse gas, and nitrogen and sulphur oxides, which have long been recognized as pollutants. In order to avoid continuous accumulation of CO₂ in the atmosphere, efforts on several fronts are being carried out worldwide to tackle CO₂ build-up in the atmosphere. CO₂ capture and storage (CCS) is one of the most promising of such measures, which can reduce emissions from fossil fuel-fired power stations by 80?90% [2]; here, the CO₂ released from the combustion of hydrocarbon fossil fuels is captured at the source itself, for example, in a natural gas fired combined cycle power plant, and the flue gas is prevented from being released into the atmosphere, by storing it in liquid form in an underground geological formation. However, the difficulty with these conventional plants is the very low CO₂ concentration (~3.5% by volume) in the flue gas. Hence, the capture process is difficult, and powerful solvents are required to remove CO₂ in a conventional air-fired power plants. In addition, the presence of NOx and other pollutants also is a problem in the monoethanolamine (MEA)-based CO₂ absorption process [3] leading to a low CO₂ removal efficiency. These problems can be overcome if the flue gas is produced under oxy-fuel combustion mode. Here, combustion occurs in a CO₂/ recycled flue gas environment rather than in nitrogen environment (as in conventional combustion) and a CO₂-rich flue gas (up to 85% by volume) can be achieved. After the removal of water vapour, the remaining gases, mostly CO₂, will be sent for direct compression and then for storage.

In the present paper, a variation to the oxy-fuel combustion, namely, enriched CO₂ flue gas recycling is proposed [4] which is based on the concept of intermediate-temperature diluted oxygen carbon sequestration (INDOCS) [5] method in which preheated oxidant with diluted oxygen content is used to moderate the flame temperature. The oxidant is made up of recirculated exhaust gas enriched in CO₂ (obtained by partial removal of water vapour) and nearly pure oxygen from an air separation unit. It is shown here that this mode of operation is particularly suitable for a combined cycle, gas-fired power plant.

In a normal air combustion plant, air is drawn from the atmosphere, and is sent for compression. The compressed air at high velocity passes through to the combustion chamber where it mixes with fuel in desired proportion to generate sustained flames continuously. Usually, gas turbine combustion process occurs in the pressure range of 12-15 bar and gas turbine inlet temperature is in the range of 1200-1400 ^o C. The exhaust gas temperature from gas turbine is at 700-500 ^oC depending upon the downstream applications, the energy in the flue gas from gas turbine can be recovered using water/steam. This steam is sent for additional power generation in high pressure steam turbine and low pressure steam turbine. After recovering the energy, the gases will be sent through the stack to the atmosphere. The flue gases at the exit are in the temperature range of 125 to 175 ^oC. In the new variant proposed here, the flue gases at the exit from the gas turbine and steam generation are sent through the secondary energy recovery system to be brought down to a temperature in the range of 150 ^oC, and are then sent through a flue gas conditioner. Here a part of the flue gas is condensed and is drained out. The resulting mixture has now less moisture and is thus CO₂-enriched; it is preheated to a temperature in such way that, after compression, enriched CO₂ flue gas is in the range of 327 ^oC to 577 ^oC and is mixed with oxygen in such a proportion as to have a decreased O₂ concentration in the oxidizer of 16% by mass [5]. This will increase the CO₂ concentration in the exhaust gas which enhances the CO₂ sequestration efficiency. The advantages of the proposed CO₂-enriched flue gas recirculation over the conventional flue gas recycling for oxy-fuel are (i) reduction in the overall volumetric flow rate of the gases through the gas turbine resulting in decreased size of the turbine, and (ii) decreased amount of non-condensable gases in the exhaust leading to a more efficient sequestration of CO₂.

References

[1] IEA, Key World energy statistics, International Energy Agency, Paris, 2008

[2] Holloway S. Carbon dioxide capture and geological storage, Phil. Trans. R. Soc. A 2007;(365):1095?1107, (doi:10.1098/rsta.2006.1953)

[3] Yang H, Xu Z, Fa, M, Gupt, R, Sliman, RB, Bland AE, Wright I, 2008 Progress in carbon dioxide separation and capture: A review, Journal of Environmental Sciences;2008 (20): 14-27.

[4] Seepana S. and Jayanti S. Optimized enriched CO₂ recycle oxy-fuel combustion for high ash coals, Fuel 2009 (Accepted for publication).

[5] Seepana S. and Jayanti S., A new stable operating regime for oxy-fuel combustion, 2008, ASME Conference, Boston. (#IMECE2008-67091)