Effects of Feedstock Size and Shape on Combustion in Biomass Boilers, 2016 Pulping, Engineering, Environmental, Recycling, Sustainability (PEERS) Conference Proceedings

The feedstock for biomass (hog fuel) boilers in kraft pulp mills consists mostly of bark, rejected wood chips and sawmill residue, which come in different sizes and shapes. This high non-uniformity of the feedstock is believed to have a great effect on combustion in biomass boilers, particularly those with stoker-grate beds. A laboratory study was conducted to systematically investigate the effects of feedstock size and shape on combustion using a thermogravimetric combustor under a variety of conditions. Biomass pieces made of pine (softwood) and maple (hardwood) at different thicknesses (3 to 20 mm) and lengths (10 to 50 mm), and of different shapes (cylindrical, rectangular and triangular prisms) were used as model feedstocks. The results show that in all cases, the combustion occurs through three distinct stages categorized as time-to-ignition, volatile burning and char burning. The volatile burning is greatly affected by the sample thickness, while the time-to-ignition and char burning are not. The overall combustion rate is governed mostly by the sample thickness, not length, and it follows a simple hyperbolic relationship: Rate=Constant/Thickness.

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