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Show Upper Fuel Gas Over-Fire Air Zone In the upper part of the boiler furnace, a portion of the fuel gas will be burned by over-firing the fuel gas with air. For reburning techniques this zone is often called the oxidation zone. This will bring the fuel utilization efficiency above the 99.5% level. Final NOx production from the ASC process is expected to be up to 50 ppm. For some existing boilers, the lower furnace combustion zone could operate with a reducing atmosphere, and the existing burners will be modified for staging combustion with over-fired air. The NOx level will be expected to be higher, but the simpler furnace modification and operation could become an important issue. ADVANTAGES OF ASC IN BOILER RETROFIT Repowering the retiring boiler with ASC offers the following advantages. o The entire process can be installed in place of the existing fuel-feed system. This allows coal-fired repowering of the unit without the necessity of obtaining any additional space. o The other advantage is the possibility of converting pulverized-coal units to a clean coal technology at an overall cost lower than any of the available or emerging alternatives. Preliminary estimates indicate that coal-fired repowering could be achieved at a cost of $1 50 to $ 200/kW without lifeextension costs. Depending on the amount of life extension required, this cost can rise to between $300 to' $350/kW. Comparable coal-fired repowering options range from $500 to $1400/kW (Table 1). o The ASC process provides advantages in particulate control over other available and emerging coal-fired repowering options. Existing particulatecontrol systems suffer severe impacts from most coal-fired repowering options. Using an AFBC, micronized coal, pulverized coal (LIMB), or using a two-stage pulverized coal-slagging combustor requires modifying or upgrading the existing particulate-control system. This is necessary because of the increased ash loading of the system and the increased ash resistivity. o The high sulfur-capture rate, low NOx production, and low particulate emissions of ASC coal-fired repowering without the use of post-combustion technologies are far superior to any of the available or emerging coal-fired repowering options. 13 |
