Desenvolvimento e análise de uma fornalha para aquecimento direto e indireto de ar utilizando biomassa polidispersa / Development and analysis of a furnace for both direct- and indirect air heating by using polydispersive biomass

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

A heat-generation system with option to direct or indirect heating of the air, that could be used in drying agricultural products or heating zootechnical facilities was developed and evaluated. The developed furnace has a heat changer with concentric countercurrent-flux tube, and it was fed polydispersive biomass by pneumatic transport or manually with biomass by firewood. Some tests were accomplished to analyze the performance of the furnace, by using firewood as fuel (5, 10 and 15 kg h-1), coffee straw together with firewood, sawdust associated with firewood and sawdust only. When using only firewood as fuel, the developed furnace showed thermal efficiency varying from 68.9 to 74.2% for indirect heating of the air and 89.8% for the direct heating system. When using coffee straw associated to firewood and the sawdust associated to firewood, the efficiencies 58.6% and 62.9% were respectively obtained for the indirect air heating. In the system using only sawdust as fuel, an average efficiency of 73.3% was reached for the indirect air heating. The efficiencies showed to be excellent, compared to those indirect-fire furnaces mentioned in the available literature. The losses through exhaustion gases varied around 18% to 30%, as showing to be the highest ones. The furnace showed satisfactory thermal isolation, as the losses by cooling varied from 1% to 3%. The heat losses by the incomplete chemical combustion (CO) were lower than 4%, except for the tests using coffee straw, when they reached about 7%. When considering the initial gas temperatures as equal to the theoretical flame temperature, the values for effectiveness of the heat changer varied from 0.624 to 0.797, therefore close to that of the furnace. Both the pneumatic feeding system and the biomass distribution system showed to be efficient, easily operated and maintained. The sawdust combustion with flow above 10 kg h-1 and water content below 15% w.b. provided continuous flame, as dispensing the use of pilot flame with firewood. The total cost of the furnace to supply an energy demand around 140.0 MJ h-1 was R$ 3.615,60, a third from which is due to the pneumatic feeding system. The cost of the thermal-energy production is lower as higher is the energy release rate and the time of the annual furnace operation. The purchasing firewood costs (R$ ton-1) that are inferior to 27,00, 12,00 and 5,00 make less attractive the use of the coffee straw associated with firewood, the firewood-associated sawdust and the sawdust as fuel only, respectively than using only firewood. Thus, the developed furnace is an useful equipment for heat generation, and could be used in several agricultural and agroindustry sectors, since it provides clean heated air for several purposes at costs accessible to the agroindustries and rural producers, besides using fuels promoting the sustainable development.

ASSUNTO(S)

aquecimento de ar energia para secagem energia da biomassa energy for drying biomass energy air heating engenharia de processamento de produtos agricolas

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