Char particle reaction and attrition in fluidized bed combustors: Modelling and measurement.

Abstract

Two slightly different models for predicting combustion efficiency for burning coal chars in bubbling fluidized bed combustors are developed. The models proposed assume that char particles are consumed simultaneously by combustion reaction and attrition with the rates of both processes following the shrinking particle model. To solve the gas-solid reaction problem, an approach quite different from the traditional method has been attempted. Characteristic times are introduced based on mean residence time concepts to characterize the reaction and attrition kinetics and particle size of different groups of char particles (ie original, attriated, elutriated, and withdrawal particles). Based on fundamental material balances and the proposed reaction-attrition mechanism, the models link combustion efficiency to some measurable parameters, including a few of the introduced characteristic times. Experimental techniques employing two types of batch experiments to determine the model parameters were developed. The first type experiments involve burning char particles to completion, while the second type experiments involve freezing the char combustion reaction, separating attrited particles from the original mother chars, and subsequently burning off attrited fines. With experimental records of exit gas concentrations of carbon oxides vs time for first type experiments as input, the models can be used to evaluate the parameters which characterize the reaction of burning char particles. When similar records for second type experiments are used, other parameters characterizing the attrited and elutriated particles can be obtained. The method also allows carbon losses from both types of experiments to be estimated. The new developments give a way to estimate the attrition rate and the elutriation fraction of post-attrition combustion char fines in fluidized beds, problems not previously solved. Experiments were performed with a bench-scale fluidized bed combustor to demonstrate the model parameter measuring techniques and to test the model features. The results show that the measured parameters are within reasonable ranges and generally agree well with the model assumptions. On the other hand, probably due to secondary fragmentation effects and the difficulty of precise burnout time measurement, most results evaluated for the power of dependence of fractional conversion on time are lower than expected, and discrepancies exist between the results of carbon loss estimated from the two types of experiments. The influences of coal type and operating conditions on the model parameters are also investigated. The results shown generally agree with past experience and are interpreted with the proposed model of the reaction-attrition mechanism.