Mathematical modeling of fluidized bed combustion. 4: N2O and NOx emissions from the combustion of char

Batch experiments were conducted to investigate the emissions of carbon oxi des and nitrogen oxides from the combustion of char prepared from a commerc ial coal in a bubbling fluidized bed. Combustion gases escaping from the su rface of the bed were sampled and analyzed for N2O, NO, NO2, CO and CO2 as a function of time by means of a Fourier Transform Infrared Spectrometer. T he experimental variables include char size and loading, inlet oxygen conce ntration, inlet CO concentration and bed temperature. A single particle reaction-diffusion model was developed first. The detaile d chemistry of NO and N2O formation destruction is complex. Homogeneous and heterogeneous reactions considered most pertinent were included in an exte nsion of the single particle char combustion model reported earlier. This s ingle porous char particle model was integrated into a three-phase hydrodyn amic description of the fluidized bed reactor. This system model for fluidi zed bed combustion was developed in response to the observation that most p revious system models, including those that incorporate details of NOx emis sions, are based on the two-phase theory of fluidization. However, two-phas e models are unable to predict the gas back-mixing and the recycle peak in solids-mixing. The non-isothermality of the bed resulting from the gas-phas e reactions was taken into account through inclusion of an energy balance f or the bubble phase. The effect of the variation in superficial gas velocit y on bubble properties and cross-flow was included through an overall mass balance. Calculations from the system model, including details of homogeneo us NOx reactions far from the char particles, compared well with data on th e emissions of CO, CO2, NO and N2O for various experimental conditions. The validated model was then used to investigate the influence of operating co nditions on the conversions of char-nitrogen to NO and N2O from the simulta neous combustion of char and propane in a fluidized bed with an air/propane mixture as the fluidizing medium. The most significant factors were found to be bed particle size and char diameter. (C) 2001 Elsevier Science Ltd. A ll rights reserved.