Mathematical modeling of a bubbling fluidized bed combustor co-fired with cotton residue and lignite

Yaşar, Mehmet Soner
A comprehensive system model, previously developed for prediction of combustion behaviors of Turkish lignites, lignite/hazelnut shell and lignite/olive residue blends in fluidized bed combustors, is extended for modeling of co-combustion of Turkish lignite and cotton residue in a bubbling fluidized bed combustor. Cotton residue has high nitrogen content (~ 4.1 wt. % a.r.) unlike lignite (~ 0.9 wt. % a.r.), olive residue (~ 1.6 wt. % a.r.) and hazelnut shell (~ 0.5 wt. % a.r.), which leads to relatively high emissions of nitric oxides (NOx) and nitrous oxide (N2O). For accurate prediction of emissions of NO and N2O, a sufficiently detailed NO and N2O formation and reduction reaction scheme is incorporated into the existing model. The assessment of the accuracy of the model is tested by comparing its predictions against the experimental data obtained in METU 0.3 MWt atmospheric bubbling fluidized bed combustor (ABFBC) where a typical Turkish lignite is co-fired with limestone and cotton residue. Reasonable comparisons are obtained between the predicted and measured O2, CO, CO2, SO2, NO and N2O concentrations and temperature profiles. The results obtained in this study show that the addition of cotton residue increases emission of total nitrogenous species due to its high nitrogen content, but the main effect is observed in the increase of N2O formation. Furthermore, determination of the fuel nitrogen partitioning into char and volatiles and distribution of volatile nitrogen species are found to be the most important parameters for modeling of NO and N2O emissions in bubbling fluidized bed combustor.