Measurements of the NO_x precursors and major species concentrations above the grate at a waste-to-energy plant

Satisfactory results from modelling of NO_x formation in CFD during combustion of Municipal Solid Waste (MSW) in grate-fired waste-to-energy plants is highly depended on accurate descriptions of the temperature field in the free-board above the waste bed. Accurate modelling of the temperature field relies on an accurate determination of the boundary conditions; the species concentration, gas temperature and gas velocity from the waste bed into the computational domain. In this study the gas temperature and the gas concentration profiles of O₂, H₂O, CO, CO₂, CH₄, C₂H₂, C₂H₄, NH₃ and HCN along the waste bed were measured. The measurements were performed on a 9 ton/h grate-fired waste-to-energy plant, Affald+ unit 4 in Denmark. The species concentration profiles were determined by gas extraction using a 6 m water cooled probe and FTIR spectroscopy, while the gas temperatures were determined by suction pyrometer. The major combustible gasses were determined to be CO, CH₄ and C₂H₄, which are only found above the first half of the grate. Furthermore, it was determined that during grate-fired MSW combustion the majority of the NO_x precursors are released as NH₃; more than 80%. The influence of NO_x precursor composition on the NO_x formation was examined through CHEMKIN simulations. It was shown that for reliable modelling of the NO formation at high excess air ratios, λ > 1.1, it is important to determine the correct ratio between NH₃ and HCN. The importance of an accurate precursor determination increases considerably with decreasing temperatures.