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We investigated guide vane Optimization of the Wood pellet boiler using Numerical Analysis. Wood combustion is a complex process. It involves many physical and chemical processes. Burning phases of wood pellet can be classified into three modes, 1) gasification mode, 2) flame burning mode and 3) charcoal burning mode. The test equipment used moving grate combustor. Primary air is supplied from the bottom of the grate and the flame is formed above moving grate. The flame is formed longer than gas or oil flame. Therefore Flames burning very close to a cold wall may be partially quenched by heat conductivity to the wall. Considering the length of the flame, the two chambers are developed. The flame in the 1st combustion chamber is moved to the second combustion chamber along the guide vane. The result of guide vane influence on the internal flow structure is analyzed from the velocity profiles and the vector plot.
We investigated a combustion characteristics of wood pellet in a combustion test equipment that can burn a single wood pellet. The test equipment can supply flue gas with constant temperature up to 850 ℃ and can control O2 concentration. The higher heating value of the wood pellet (dry base) is 19.1 MJ/㎏ (4 550 ㎉/㎏) and water content as received is 8.28 %. The results show that there are 3 modes of wood pellet combustion, 1) Gasification (including pyrolysis), 2) Flame burning and 3) Charcoal burning. We measured the time duration of each mode. In order to shorten the ignition time, the duration time of gasification (mode 1) should be reduced. The minimum temperature for ignition of a wood pellet is 500 ℃. The mass reduction rate was measured. During gasification mode (mode 1), the mass reduction is about 20 ~ 30 %. The mass reduction during flame combustion is about 55 ~ 70 %.
The developments of clean and renewable energy sources are needed in order to reduce dependency of fossil fuel and global warming. Wood pellet is one of clean renewable energy sources and has been considered as one of the substitutes for fossil fuels. Wood combustion is a complex process. It involves many physical and chemical processes. Burning phases of wood pellet can be classified into three modes, 1) gasification mode, 2) flame burning mode and 3) charcoal burning mode. In this study, the characteristics of NOx(nitrogen oxides) reduction of wood pellet boiler was investigated by using flue gas recirculation, that is FGR.
Recently domestic wood pellet boilers are installed in rural and forestry houses. The fuel price per lower heating value of wood pellet is about 20 % lower than that of heating oil on July 2010. In spite of lower price of wood pellet, a few user of wood pellet boiler complain expensive fuel cost. One of this reason is inaccurate or improper air-fuel ratio setting of wood pellet boiler. O2 concentration of flue gas of domestic wood pellet boiler is about 9.7 % and there are few domestic wood pellet boiler which can control air-fuel ratio automatically. We tested a domestic wood pellet boiler in changing boiler thermal output. The nominal boiler thermal output is 25 ㎾ (21 500 ㎉/h). We measured thermal efficiency at 100 % boiler thrmal load and in the same air supply condition of 100 % boiler load we measured thermal efficiency at 40, 60, 80 % boiler thermal load. The results show that at boiler load 60 % the thermal efficiency is about 4 % point lower than that of boiler load 100%.