CO_2 CAPTURE FOR PC-BOILERS USING OXY-FUELS - A TRANSITION STRATEGY

摘要

Retrofitting pulverized coal (PC) boilers with flue gas recirculation to recover CO_2 was conceived and tested in the mid-1980's by Argonne National Laboratory as a low-cost CO_2 source for enhanced oil recovery (EOR). The fundamental concept involves replacing combustion air with oxygen diluted by recirculated CO_2 from the flue gas. This eliminates N_2-CO_2 separation, permitting more economical CO_2 recovery than competing amine systems. A CO_2/O_2 molar ratio of ～3 is necessary to preserve the heat-transfer performance and gas-path temperatures. In a site-specific study retrofitting a low-sulfur coal-fed (0.2% S) boiler, CO_2 recycle shows competitive, operating, and environmental release advantages when compared to amine scrubbing systems. Anticipated breakthroughs in O_2 production will further improve the advantages. Oxy-fuel costs at $42/tonne CO_2 compare favorably to $50/tonne CO_2 with monoethanolamine (MEA) scrubbing. Higher sulfur contents degrades solvents faster, and this puts amines at an additional disadvantage since the oxy-fuels require no solvents or solvent disposal. At present, power station derating is similar in both approaches. Both have the same internal power demand of 94 MW_(internal) for 300 MW_(gross) station. For oxy-fuels high-sulfur operation using flue gas desulfurization (FGD) should be practical, and in a related experimental study, we found that high CO_2 partial pressures from oxy-fuels have a negligible impact on SO_2-reagent use although FGD solids handling will require redesign. One of the special opportunities afforded by CO_2/O_2 recycle is that it should prove very attractive at sites considering repowering and converting to an integrated-gasification-combined-cycle (IGCC) to produce H_2 and electricity. Early efforts to construct a CO_2 pipeline and build an oxygen plant would permit companies to approach CO_2 sequestration in a phased manner, particularly if construction schedules for CO_2 pipelines need long lead-times.