A model of chunkwood combustion

The burning rate of yellow-poplar and black spruce wood chunks (15 cm diameter by 5, 7.5, and 10 cm thick) was measured by inserting and removing individual chunks in a spreader stoker boiler fired with coal (50 Mg/hr. of steam). The chunk shrinking rate was approximately constant at 1.8 mm per minute. The char thickness varied from 2 to 15 mm, depending on conditions. Temperature profiles were calculated from a transient heat conduction model with implicit boundary conditions. The calculated temperature profiles agreed with char thickness observations. A burning rate model was developed assuming external oxygen diffusion control. The total burn time depends on the initial particle radius, wood char density, oxygen concentration, and an oxygen diffusion coefficient. Because of turbulence and irregular shaped particles, the effective diffusion co-efficient was determined from the best fit of the data. The burning rate model accurately predicts the chunk-wood mass versus time data. The wood chunks were predicted to burn out in 17 to 32 minutes depending on the size, but independent of the initial moisture content. The model provides a basis for analyzing the heat release rate and beat release distribution in large boilers operating with chunkwood or whole-tree sized fuel. Comparisons between boilers operating with chunk-wood and coal are discussed. Chunkwood is an attractive boiler fuel.

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