Computational model to investigate the effect of different airflows on the formation of pollutants during waste incineration

Abstract

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.