Análise técnico-econômica do aproveitamento dos gases residuais da carbonização contínua de madeira para a geração de eletricidade

Abstract

Brazil is the only country that produces charcoal on a large scale. However, traditional carbonization processes waste large amounts of energy through the residual carbonization gases that are released into the atmosphere, which cause air pollution and environmental degradation. This fact is a consequence of the technology currently used to produce charcoal, the large rectangular masonry kilns, which make the recovery and use of gases quite difficult. The carbonization gases contain approximately 40% of the energy content of wood, and the large production of charcoal in the Minas Gerais state can represent a considerable energy potential. The objective of this work is to assess the possibility of recovering the energy of the gases produced during wood carbonization in a "Lambiotte" type reactor (retort). Since these reactors operate continuously, they allow better use of wood energy through a gas recovering system that uses part of the gas energy in the charcoal production process itself and part for electricity generation. The carbonization reactor considered can produce 20 tons of charcoal per day, besides making available up to 0.84 of thermal power in the residual gases. For converting the gases’ energy into electricity, two technologies were analyzed: a conventional steam Rankine cycle and an Organic Rankine Cycle (ORC), which presented efficiencies of 9.5% and 17.7%, respectively. The economic analysis indicated the steam Rankine Cycle as the most viable technology for the case under study, where it is estimated a specific generation of 130 kWh per ton of charcoal, considering a gravimetric yield of 33%. The analysis also considered a charcoal gravimetric yield of 42%, which resulted in a specific electricity production of 62 kWh per ton of charcoal. Continuous carbonization technology proves to be quite effective for large-scale charcoal production. In addition to producing better quality and more homogeneous charcoal, it manages to provide the full use of the energy contained in the wood with the generation of electric energy through the surplus gases of the process. The emission of highly polluting gases, such as CH4 and CO, is eradicated with the burning of the residual gases, thus avoiding one of the greatest problems of this industrial sector, the emissions of greenhouse gases.