Abstract
In the present study reduction of nitrogen oxides using reburning technology, during combustion of sewage sludge (fuel I) and the mixture of sewage sludge, wasted bleaching earth and CaO (fuel II), was carried out. The experimental works were conducted in a laboratory-scale fluidized bed reactor (power up to 10 kW) with application of two types of beds: chemically inert bed (sand) and chemically active bed (CaO). The second combustion (reburning) zone in the reactor was formed by dosing into an area above the bed, additional gaseous fuel (propane). Obtained reduction in emissions of nitrogen oxides in both types of beds was at a level 70% - 79%. Additionally bed of CaO has the desulfurizing effect and also reduces the CO concentration in the exhaust fumes. A significant drawback of active bed is the adverse effect on increase of the primary NO which enters the second combustion zone. The result of this fact is higher NOx emission during combustion of the same fuel in bed of CaO in comparison to the combustion of this fuel in the sand bed, when the same maximum degree of reduction of NOx will be obtained for both types of beds.
Highlights
Application of combustion processes to thermal utilization of waste materials with a positive net calorific value is considered as the best method of processing the high amount of wastes produced by human, as well as providing alternative, to fossil fuels, energy sources
Fuel II was composed of municipal sewage sludge (30%mass), wasted bleaching earth (62%mass) and CaCO3 (8%mass), the intended particle size and dosage form suitable for fluidized-bed reactor have been obtained in several subsequent operations
For optimal effect of reburning the combustion process should be conducted in the way to obtain air excess coefficient close to 1.0 in the second combustion zone
Summary
Application of combustion processes to thermal utilization of waste materials with a positive net calorific value is considered as the best method of processing the high amount of wastes produced by human, as well as providing alternative, to fossil fuels, energy sources. There are conducted worldwide extensive researches into different methods of thermal utilization of sewage sludge [5,6,7,8,9,10,11,12,13,14,15] Some of these tests is carried out using a fluidized bed reactor, because of the advantages resulting from the specific hydrodynamic conditions of the bed. The serious problem of sewage sludge combustion processes is, as mentioned earlier, the high content of nitrogen and sulfur in the processed material [26] The result of this situation is that in the case of thermal processing of the sludge it is necessary to reduce the content of SO2 and NOx in the exhaust fumes. The aim of latter studies was to determine the range of air excess coefficient in reburning zone— r, by which sufficiently low concentration of NOx in flue gases, while reduction of N2O and absorption of SO2 in the bed is achieved
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