Pulverised Biomass Flame Propgation

A resource analysis for Pakistan has demonstrated that abundant crop residues offer a viable, and environmental-friendly alternative to currently inadequate, oil-based power generation. Similarly, in many countries there is legislative pressure and incentives to replace coal with biomass, in electricity production. Efficient and safe exploitation of such biofuels requires data on flame propagation rates and explosibility characteristics. Crop residues (bagasse, rice husk, wheat-straw, corn-cob and peanut-shell) and different raw and thermally treated woods were tested using the modified Hartmann tube and the modified 1 m3 explosion vessel. The modified Hartmann tube was operated for varying ignition delays using a digital timer. A hemispherical disperser with drilled pipe was calibrated for the testing of fibrous and coarse size biomass mixtures. Thermogravimetric analysis data from these materials enabled the application of two different models for the determination of volatile release kinetics. Biomass samples were found to have lower activation energies with higher volatile release rates at low temperatures, compared to coals. Despite their higher ash content, pulverised crop-residues showed leaner minimum explosible concentrations (0.2-0.5 equivalence-ratio) than woods (0.3-0.7) - depending on particle size. Biomass samples were more sensitive to explosion than coal, resulting in flame propagation in coarse-size-range fractions (300-500-μm) that was not experienced with coals. Maximum explosion pressures of near 9 barg were measured for the fine size fraction (less than 63-μm) samples, with no less than 7 barg for coarse size fraction (less than 1 mm). Milling of thermally treated biomass samples resulted in higher fines fraction than untreated biomass, for the same sieve size and this was considered as one of the reasons of the higher reactivity (higher flame speed and higher deflagration index of these samples). The detailed data from this work are usable in the design of safe and efficient combustion systems for power generation from crop residues and other biomass fuels.