Circulating Fluidized Bed Boiler Model for Predicting Emission Levels of Tire Fuel using Base Case of Lignite Fuel

Abstract

Government regulations on emission standards and constantly fluctuating fuel availability and prices have led to the need of more readily usable and available models for power generating combustors. A common, versatile, power generating combustor is the circulating fluidized bed (CFB) boiler. Waste tire is viewed as a potential fuel in a CFB boiler. Extensive literature review is used to create a model of a CFB boiler.

The model is programmed using Microsoft Excel software. The program consists of four major tabs being the ‘Input’, ‘Bed’, ‘Riser’, and ‘Output’ tabs. The ‘Input’ tab consists of appropriate and minimal input parameters to allow for enough variability while not over burdening the user with acquiring large amounts of data. The ‘Bed’ tab describes the instantaneous chemical reactions assumed to take place in the dense bed region of the combustor. The ‘Riser’ tab tracks the time dependent chemical kinetics through a predetermined number of time steps divided evenly throughout the lean riser region of the combustor. The ‘Output’ tab calculates the concentration at the exit of the combustor riser for ten different species important to both combustion and government regulations being CO, H2O, O2, N2, NH3, HCN, SO2, CO2, H2, and NO. With 5,000 chemical kinetics calculation rows, the computation time for varying an input is approximately ten seconds.

The model has been validated against an anonymous industrial CFB boiler firing lignite coal fuel resulting in emission concentrations being 22% different or less between the model predictions and the actual emissions when kinetics are slightly modified. The model was also validated against a Babcock and Wilcox pilot/laboratory scale bubbling fluidized bed (BFB) boiler firing subbituminous coal fuel resulting in emission concentrations being 10% different at most between the model predictions and the actual emissions without kinetics modification except for the O2 concentration being off by about 74% due to differences with CFB.

The anonymous CFB boiler had fuel switched to waste tire fuel, and 24 inputs were varied as part of a parametric analysis of the fuel to discover emission trends.