CFD analysis of municipal solid waste combustion using detailed chemical kinetic modelling
Nitrogen oxides (NO x ) emissions from the combustion of municipal solid waste (MSW) in waste-to-energy (WtE) facilities are receiving renewed attention to reduce their output further. While NO x emissions are currently 60% below allowed limits, further reductions will decrease the air pollution control (APC) system burden and reduce consumption of NH3. This work combines the incorporation of the GRI 3.0 mechanism as a detailed chemical kinetic model (DCKM) into a custom three-dimensional (3D) computational fluid dynamics (CFD) model fully to understand the NO x chemistry in the above-bed burnout zones. Specifically, thermal, prompt and fuel NO formation mechanisms were evaluated for the system and a parametric study was utilized to determine the effect of varying fuel nitrogen conversion intermediates between HCN, NH3 and NO directly. Simulation results indicate that the fuel nitrogen mechanism accounts for 92% of the total NO produced in the system with thermal and prompt mechanisms accounting for the remaining 8%. Results also show a 5% variation in final NO concentration between HCN and NH3 inlet conditions, demonstrating that the fuel nitrogen intermediate assumed is not significant. Furthermore, the conversion ratio of fuel nitrogen to NO was 0.33, revealing that the majority of fuel nitrogen forms N2.
Measurements of the NOx precursors and major species concentrations above the grate at a waste-to-energy plant
Abstract Satisfactory results from modelling of NOx 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 NOx precursors are released as NH3; more than 80%. The influence of NOx precursor composition on the NOx 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.
The role of early lithification in development of chalky porosity in calcitic micrites: Upper Cretaceous chalks, Egypt
Abstract Petrological and geochemical examination of Upper Cretaceous chalks from the Abu Roash area and Bahariya Oasis (Egypt) provide insights into factors which control porosity development in fine-grained calcitic carbonates. Petrological studies indicate that primary skeletal fabrics of coccoliths and foraminifera in micrites of Abu Roash are well preserved. In contrast, primary skeletal fabrics of the Bahariya Oasis chalks are pervasively altered. Geochemical analysis of these chalks reveals a striking contrast in composition. The high porosity chalks at Bahariya Oasis have depleted isotopic values ( δ 13 C = −5.0‰, δ 18 O = −8.9‰ PDB ) and low concentrations of trace elements (Sr and Na). In contrast, the low porosity chalks at Abu Roash are less depleted in terms of their isotopic values ( δ 13 C = 1.0‰, δ 18 O = −4.0‰ ) and possess elevated concentrations of Sr, Na, Fe and Zn. Importantly, fracture and collapse veins, present only at Abu Roash, are filled with calcite whose δ18O value is coincident with that of altered micrites at Bahariya Oasis. The lithic fragments cemented by this calcite, however, retain compositions identical to other lithified limestones at Abu Roash. Differences in fabric and chemistry of the pelagic marine micrites observed between Abu Roash and Bahariya Oasis reflect the degree of early lithification and compaction prior to exposure and meteoric diagenetic alteration associated with regional Late Cretaceous/pre-Eocene subaerial unconformity.