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.
COMPUTATIONAL MODEL TO INVESTIGATE THE MECHANISMS OF NOx FORMATION DURING WASTE INCINERATION
A numerical model has been developed to study the incineration of solid waste within a fixed-bed reactor. The model simplifies the flow to a one-dimensional succession of perfectly stirred reactors (PSRs) and includes a detailed chemical model. The calculations use the PSR model included in CHEMKIN II. The model predictions or residual nitrogen oxide are in good agreement with the experiments and have permitted validation of the model. The model has been used to investigate the mechanisms of NO formation during the combustion process of solid waste and to determine the principal reactive zones into the setup. NO is formed rapidly as gaseous fuel enters the reactor and occurs in three different steps. The principal intermediates are NCO, NH, and HNO. The rate of formation is dependant on the local oxygen content. A second region of NO production can be identified later in the trajectory of the fuel. Here the NO already formed will react with reducing species such as NH2, NH, NCO, CH3, and HCCO to produce N2.
Premixed Methane-Air Flame Spectra Measurements Using UV Raman Scattering
Abstract The underfed combustion of dried bagasse in the forms of loose fibre, dense fibre, small pellets and large pellets has been investigated. The large pellets ignited rapidly on the surface and each pellet burnt slowly as an individual fuel lump, so that a bed depth greater than ∼700 mm would be necessary to reduce primary air to the stoichiometric rate. The other three forms exhibited the characteristic ignition and burn-out stages at low air rates. Their maximum combustion rates were ignition limited and decreased as the bulk density increased. Since the corresponding primary air flow rates were less than or equal to stoichiometric, their combustion at the maximum rate would be practicable. The ash from the small pellets fused into clinker, so they should only be fired, in existing bagasse boilers, as a supplement to wet bagasse at temperatures below the ash fusion temperature.