In an oxyfuel plant, the input fuel is burned in an oxygen-rich environment, generally containing 95% to 99% O2 on a molar basis. When combined with flue gas recirculation to keep combustion temperatures reasonable, the flue gas of an oxyfuel combustor is comprised of almost entirely CO2 and H2O, assuming a low sulfur, low ash fuel. Carbon dioxide can easily be separated from water by condensing the water out of the flue gas stream, and the lack of other significant constituents, such as nitrogen, means that this single, simple step can provide high purity CO2for sequestration or other uses. The ability to forgo an energy-intensive CO2 separation process offsets the power consumption of the Air Separation Unit that produces the concentrated O2 stream, resulting in a more efficient system overall when compared to a standard air-fuel combustion plant.
The pressurized oxyfuel cycle shown in the first figure below is a further refinement of the oxyfuel concept. Carbon dioxide is generally compressed for delivery, and this compression process generally consumes a significant amount of power. For example, in the atmospheric pressure oxyfuel cycle shown in the second figure below, the intercooled compression of the CO2 to a pressure of 2200 psia (152 bar) comprises around 30% of the plant’s entire auxiliary load. Increasing the pressure of the entire cycle decreases the compressor’s power load while at the same time eliminating the losses associated with throttling the output of the ASU to atmospheric pressure, resulting in a more efficient cycle.
Pressurized Oxyfuel Cycle Utilizing a CFB
In the model above, an ASU provides a flow of 99% oxygen by volume at a pressure of 435 psia (30 bar). This gas is diluted through the recirculation of approximately 70% of the flue gas by mass, then supplied to the pressurized circulating fluidized bed. Flue gas treatment includes an ESP for particulate removal, flue gas cooling to condense its water, and then compression of the final flue gas stream to deliver 97% CO2 by volume at 2200 psia (152 bar).
A single reheat supercritical steam cycle with nine feedwater heaters is used in this plant. It has high pressure conditions of 4050 psia/1080°F (279 bar/582°C), reheat conditions of 1000 psia/1110°F (69 bar/520°C), and a condenser pressure of 0.7 psia (48 mbar). With these conditions, plant gross output is 500 MWe, net output is 370 MWe, and net LHV efficiency is 33.6%.
For comparison, a similar conventional power plant without CO2 capture and a gross output of 500 MWe would have a net output of 475 MWe and net LHV efficiency of 41.4%. When equipped with post-combustion CO2 capture using proven amine-based technology, such a plant would have net LHV efficiency of 30.5. A similar plant utilizing oxyfuel combustion at atmospheric pressure with a gross output 500 MWe would have a net output of 338 MWe and a net LHV efficiency of 30.7%.
Oxyfuel Cycle at Atmospheric Pressure with Conventional Furnace
Note that the ESP and CFB are modeled after equipment built to function at atmospheric pressure. The effects of a pressurized cycle on equipment size and cost are not known at this time.