Modelling for combustion and sulphur retention efficiencies of fluidized bed combustors

Abstract

Studies on the combustion of coal and sulphur capture by limestone/lime particles are carried out analytically and verified with the experimental data reported elsewhere. Extensive experimental data are available for small scale FBC. However, the usefulness of these data to large scale FBC is rather limited since the geometry and operating conditions are widely different from those of small scale FBC. The present work concerns with the correlation of combustion efficiency, η and sulphur retention efficiency, S[subscript ret] with a dimensionless group called Damköhler number. The results are expected to be useful in utilizing the results of small scale FBC to large scale FBC. Damköhler number is defined as a ratio of characteristic residence time to the reaction time. Even though Damköhler number has been used in the conventional combustion literature, it has not been introduced to FBC in correlating η and S[subscript ret]. The present work provides such a correlation with Damköhler numbers. Two kinds of Damköhler numbers are introduced: (1) D[subscript FBC,C]; Damköhler number for correlating η of FBC; (2) D[subscript FBC,S]; Damköhler number for correlating S[ret] of FBC. The D[subscript FBC,C] is a ratio of characteristic residence time to reaction time of solid fuel particles while D[subscript FBC,S] is a ratio of characteristic residence time to reaction time of lime particles. Essentially population balance equations and two phase reactor theory are used in developing the model for combustion and sulphur retention efficiencies of FBC. Simplifying assumptions are made for obtaining the results in terms of Damköhler numbers. The mass balance equations of solid fuel particles, CaO particles, oxygen, and sulphur dioxide yield the solutions to (i) oxygen and sulphur dioxide concentrations in the emulsion phase, (ii) dimensionless weight of solid fuel particles and CaO particles in the bed, (iii) burnt char fraction, (iv) burnt volatiles fraction, (v) CaO conversion efficiency to CaSO₄, (vi) combustion efficiency, and (vii) sulphur retention efficiency. Generalized plots are given for the weight of char, bed material, burnt char fraction, CaO conversion efficiency, and sulphur retention efficiency vs. Damköhler numbers. These correlations are then verified with the experimental data for combustion and sulphur retention efficiencies reported in the open literature for FBCs operated under various conditions and widely varying geometrical parameters. The effects of attrition, fragmentation, swelling, and overflow are investigated. It is found from the correlations that the FBC must be operated such that D[subscript FBC,C] is greater than 5-10 for acceptable combustion efficiency and D[subscript FBC,S] is greater than 10-20 for desired sulphur retention efficiency.