mixers. The addition of SCR NOx reduction technology is
a capital intensive investment. SCR technology is several
times more costly (capital) than SNCR.
While evaluating the total capital costs for the two offerings
on a “chute-to-stack” basis, it was discovered that the
additional capital cost associated with the SCR was more
than offset by the additional costs of the CFB boiler system.
Even though there were slight overall cost differences
between the CFB/SNCR and BFB/SCR systems, analysis
suggested a capital cost premium of 15% for the CFB design.
It should be recognized that there will be a point (or
points) where SNCR could be sufficient to meet the NOx
requirements. For CFBs this may be an evaluation based on
operating margin. Or the NOx requirement may show BFB/
SNCR being acceptable. Conversely, if NOx emissions are
ratcheted down to even tighter values SCR technology may
be required regardless of the combustion process.
Operating and maintenance costs For the case study
design, on a “chute-to-stack” basis, the CFB has higher
parasitic load as compared to comparable BFB boilers. For
some fuels, BFB technology may utilize flue gas recirculation
to control bed temperature. Should that be the case, the
BFB will require an additional fan in service thus increasing
parasitic load, and the power evaluations begin to approach
one another.
The BFB’s SCR has additional operating costs over an
SNCR system associated with a catalyst management plan
and increased gas-side system resistance. This is partially
offset by decreased reagent consumption.
With regard to maintenance, the CFB technology, with
its recirculating solids inventory, increases the potential
for abrasion and wear throughout the furnace cavity and
bed material separation equipment. This adds to the CFB
maintenance costs over the life of the unit.