coal selection criteria for industrial pfbc firing project 3.2 - CCSD

“Coal Selection Criteria for Industrial PFBC Firing”
depended on CO emissions (i.e. carbon conversion). Higher oxygen partial pressure
resulted in more complete combustion and hence lower CO emissions. The overall NOx
emissions were lower in PFBC than in AFBC (Nagel, Spliethoff et al. 1999).
Abe et al. measured emissions from the Wakamatsu demonstration plant and concluded
that the cyclone gas temperature (Tc) controlled the emissions of CO, N2O, NOx and SO2
under stationary conditions (Abe, Sasatsu et al. 1999). Analyses found that N2O and SO2
emissions were more dependent on gas temperature (α Tc) compared with CO and NOx
emissions (α Tc 1/2 ). Bed temperature also had some role in explaining the spikes of SO2
and N2O emissions during partial load (Abe, Sasatsu et al. 1999). Sudden changes in bed
temperature due to changes in combustion (e.g. increase in coal load) may decrease the
oxygen concentration in the burner zone thus increasing NO2 and SO2 concentrations.
The following series of ASH TR equations could be used to estimate the concentrations of
exhaust gases under PFBC operations (Abe, Sasatsu et al. 1999):
PCO = PO2 1/2 x exp (13.431 x 10 3 /Tc – 21.562) (R 2 = 0.9794) (Eq. 4)
Calculated NOx conversion = ([O2]/3.5) 1/2 x {[F1/(1+F1) – F2/(1+F2)]} (Eq. 5)
F1 = PO2 1/2 x 5.00 x 10 -4 x exp (0.21 x 10 3 /Tc) (Eq. 6)
F2 = PO2 1/2 x 4.57 x 10 -7 x exp (12.1 x 10 -3 /Tc) (Eq. 7)
NOx = NOx conversion x input [N] x 22.4/gas flow rate x 10 6 (ppm) (Eq. 8)
It was found that maximum reduction of NOx emissions (up to 70%) could be achieved
when the bed was operated at the stoichiometric air ratio (Hippinen, Lu et al. 1993). Air
staging was only useful in reducing the emissions if it changed the temperature
distribution of the reactor, as NOx is highly dependent on reactor temperature. Sulfur
retention efficiency decreased when operating the bed with primary air ratio below 1
(Hippinen, Lu et al. 1993). Air staging could also cause increased emissions of CO and
unburnt carbon in the fly ash, thus reducing the combustion efficiency although fly ash
recycling had been implemented. This problem could be prevented by operating at higher
temperature or by using secondary air pre-heating which facilitated the production of
NOx (Hippinen, Lu et al. 1993).
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