Characteristics of In-Bed Sulphur Removal and Staging of Combustion Gas in Fluidized-Bed Coal Combustion by a Mixture of Oxygen and Recycled Flue Gas
1999 Volume 25 Issue 5 Pages 735-741
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1999 Volume 25 Issue 5 Pages 735-741
The influences of limestone addition and staging of combustion gas on the emission reductions of SO2, NO and N2O are experimentally examined for coal combustion in a bubbling fluidized-bed where CO2/O2 combustion system is applied. An Australian bituminous coal was burnt by a mixture of a recycled flue gas and oxygen in a 0.158 m i. d. and 3 m high combustor. Tsukumi limestone crushed to sizes 1 to 3 mm was used for sulphur removal. The results obtained in CO2/O2 combustion are compared with those in conventional once-through air combustion.
Sulphur removal efficiencies in CO2/O2 combustion at a bed temperature ranging from 1090 to 1150 K are much higher than those in air combustion. It is confirmed from a model analysis based on mass balance that such higher SO2 removal efficiency in CO2/O2 combustion is mainly produced by extension of mean contacting/reaction time between limestone particles and SO2 due to flue gas recycling into the combustor. Two-stage combustion depressed NOx and N2O emissions in CO2/O2 combustion, as well as in air combustion. Compared with single-stage combustion, emissions are reduced to less than 2/3 when the ratio of primary/total oxygen flow rate is approximately 0.8. Thus, we can say that CO2/O2 combustion is an excellent system not only for CO2 recovery, but also for emission reductions of SO2, NOx and N2O.
