Reduction of the N2O Emission from Fluidized Bed Combustion by Afterburning

Doctoral thesis, 1995

A method for reducing N2O emissions from fluidized bed combustion is presented and investigated. In contrast to flame combustion, fluidized bed combustion generates significant amounts of nitrous oxide (N2O) which are emitted to the atmosphere. The method investigated, called afterburning, means that a secondary fuel is introduced downstream the combustor of a circulating fluidized bed boiler. Combustion of the secondary fuel raises the temperature of the gases, which is accompanied by a steep decrease in N2O emissions. The injection fuel ratio, i.e. energy in secondary fuel divided by energy in primary fuel, is in the range 10-20%. Afterburning has been investigated both by chemical modelling and by full-scale experiments in a 12 MW CFB boiler. In the experiments, Liquefied petroleum gas (LPG), fuel oil, pulverized coal, pulverized wood and sawdust were used as afterburning fuels. The primary cyclone of the fluidized bed boiler was used as an afterburning chamber. The results showed that the N2O emission can be reduced by 90% or more by this method. The resulting N2O emission was principally a function of the gas temperature achieved in the afterburning fuel. The excess-air ratio in the gases from the primary combustion also influence the results, in the way that a low excess-air ratio decreased the N2O emission. No negative effects on NO or CO emissions or on sulphur capture were recorded. The amount of afterburning fuel needed to achieve a given N2O reduction was influenced by various losses, e.g. cyclone cooling, heating of particles and unburned afterburning fuel. In a plant designed and optimized for afterburning, these losses could be reduced considerably, and it is shown that in such a plant an injection fuel ratio of 10% should be sufficient to remove practically all N2O. Chemical modelling was performed in order to study the governing processes in afterburning. A kinetic scheme describing hydrocarbon (C1-C3) oxidation, nitrogen chemistry as well as interactions between hydrocarbon and nitrogen-containing species was used together with a Perfectly stirred reactor model. The results showed that N2O was destroyed by both radical reactions, primarily with hydrogen radicals, and by thermal decomposition under the conditions prevailing in the experiments. Heterogeneous reactions such as quenching of radicals and heterogeneous decomposition of N2O were shown to have small influence on N2O emissions from afterburning. A comparison between modelling of full-scale experiments and corresponding experimental data shows good correlation at higher temperatures, but the difference grows larger at lower temperatures.