Comparison of Ashes Produced in a Biomass Moving Grate Boiler by Wood Chips and Sewage Sludge

One option to fight global warming is to convert our use of fossil energy into renewables such as biomass energy. However, the forest preservation and the quality of the ambient air are also two major issues. Therefore, the use of biomass waste without any supplementary emissions could represent a part of the solution. In this study, two fuels were considered for a 200 kW moving grate boiler. A multicyclone and a bag filter were fitted on the boiler. The first fuel consisted of classical wood chips whereas the second was a mixture of wood chips with sewage sludge. This second fuel presented a high ashes mass ratio compared to wood chips. The aim was to verify the possibility to burn this kind of fuel without any modification of the installation. The first relevant result is that the conventional pollutants, i.e., CO and NOx, remained under the emissions limits even with the sewage sludge combustion. The Total Suspended Particles emissions at the exhaust were always under 5.4 mg.Nm(-3) dry based corrected at 6% of O-2, which is low with respect to the standard limitation. The majority of the ashes remained on the combustion room. However, with both fuels, about 5% of ashes mass remained in the heat exchanger. Nevertheless, the heat exchanger was more clogged with the second fuel, which produced five time more ashes. This may lead to a yield loss. Thus, sewage sludge can be used in a wood boiler without any issue if an automatic exchanger sweep is fitted on the installation.