Numerical Modeling of Gas-Phase Waste in Incinerator: Focus on Emissions and Energy Recovery under Air-Fuel Ratio and Air Volume Control

Abstract

Traditional incinerators achieve the thermal requirements through heat transfer and heat radiation. However, the early recovery of flue gas preheats the air and yields nitrogen oxide (NOx) to rise in the combustion of overoxygen. The operation of an incinerator inevitably implies the release of greenhouse gases and emissions of NOx harmful to the human health. The administrator of one laboratory incinerator in Taiwan sought to optimize the operation conditions such as temperature or oxygen level of the combustion products within the combustion chamber to minimize the release of pollutants and maximize the efficiency of combustion. In this phase, air-fuel ratio control and air volume control are regarded as the first priority. A numerical model of the laboratory-scale plant in southern Taiwan is established by using enhanced wall treatment and coupling the thermochemical conversion of volatile waste to the gaseous combustion of the released syngas. The model allows users to characterize the temperature and retention time of the combustion products for the verification of the fulfillment of the existing regulation for NOx and oxygen level in incineration plants. It shows trade-off relationship between combustion efficiency of fuel and emissions (NOx and CO) in surveying cases of air-fuel ratio (AFR) ranges from 1.5∶1 to 14.4∶1 according to numerical results. Increasing the air volume enhances this trend. In this study, it shows the lowest emissions of NOx in case of AFR=1.5∶1, but worse combustion efficiency. Meanwhile, to increase the air volume by 1.15 times suppress most CO and about 28% NOx, but increases by 6% the residual fuel. The averaged distribution of retention time of particles in this study ranged from 30 to 50 s, and is provided for further improvement of geometry in the next phase.