Análise exergética de caldeira de biomassa em processo KRAFT de obtenção de celulose

Abstract

A detailed model was proposed in this work for exergy analysis of a real biomass boiler in a kraft pulp industry. The model was proposed based on an equilibrium model which uses Gibbs Energy minimization and energy balance to calculate the flue gas composition, the temperature of the furnace and the temperature of the flue gases along the boiler. The exergetic efficiency was calculated by the indirect method: exergy destruction was evaluated in each of the major components of the boiler, and was calculated separately on the material streams (water/steam and flue gases). The calculation of the exergy destruction was made both by exergy balance in the control volumes and by the entropy generation term, obtained by entropic balance. The Gibbs minimization model predicted successfully the complete combustion of the biomass, and can be adapted to another thermochemical processes. Results showed that the furnace and the water walls have the higher exergy destruction, accounting for 47% and 30% of the total exergy destruction, respectively. The higher exergy destruction can be explained due to the irreversibilities of the combustion process. In the other parts of the boiler, the exergy destruction was higher in the water and steam flows than in the flue gases, and the results indicate that exergy destruction was higher in streams at lower temperatures. The global exergetic efficiency was 42.47%.