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.
Optimisation study of a large waste-to-energy plant using computational modelling and experimental measurements
AbstractIn order to maximise the energy recovery efficiency of waste-to-energy plants, it is important to understand the physical processes that are occurring within the furnace. A mathematical model of the furnace section of a large waste-to-energy plant was constructed using FLIC to model combustion of the solid waste particles on the furnace grate and FLUENT to model the gas flow above the burning waste bed. The two models were coupled through their respective boundary conditions. A numerical simulation of the design-case setup of a large waste-to-energy plant was performed, which predicted the presence of a large flow recirculation zone in the radiation shaft. Further numerical simulations were performed using several different configurations of the secondary air jets, which revealed that the large flow recirculation in the radiation shaft could be avoided by changing the distribution of secondary air jets. Experimental measurements of the temperature profile within the burning waste bed of the plant ...
Computational model to investigate the effect of different airflows on the formation of pollutants during waste incineration
A numerical model is 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, previously reported in the literature, that computes 113 species involved in 892 reversible reactions. The calculations use the PSR model included in Chemkin II. The solid phase is not solved but instead the input species are generated from an experimental degradation study of the fuel. The computations are compared with experiments conducted in a laboratory-scale fixed-bed reactor. The reactor includes a primary air injection through the fuel and a secondary flow introduced downstream of the fuel bed. This geometry establishes primary and secondary zones of combustion. Atertiarynonreactive region follows after the secondary air-injection zone. Global measurements ofresidual oxygen, nitrogen oxide, carbon monoxide, and carbon dioxide are used to establish the validity of the model. The model predictions of residual oxygen as well as the carbon dioxide emissions show good agreement with the experiments. Nitrogen oxide is well predicted for a total air supply smaller than twice the stoichiometric requirement. For leaner operating conditions the stoichiometry resides outside the range for which the chemical model has been validated and significant discrepancies appear. The production of NO increases with the total airflow rate but is primarily affected by the stoichiometry in the primary zone of combustion. Two different regimes have been identified in the primary zone of combustion that are controlled by the airflow through the fuel. For primary airflows below the stoichiometric requirement, an oxygen-deficient combustion regime is established within the porous matrix. This regime is characterized by low reaction temperatures, favors endothermic pyrolysis, and greatly enhances the production of CO. Within this regime, an increase in primary airflow will result in a decrease in CO yield. For airflows above the stoichiometric requirement that the reaction establishes above the fuel, the temperature increases and the CO yield increases weakly with the total airflow rate. The model cannot describe this complex interaction between the fuel and the reacting flow. Thus, the predictions of CO differ greatly from the experimental measurements.