Biomass Explosion Residue Analysis

On account of its greenhouse gas advantages there is increasing use of pulverized biomass in power generation. However, there is little information on the combustion properties of pulverized biomass and on the explosion hazards they create in the mills, dust conveyor systems and biomass storage silos. This work uses the ISO 1 m3 dust explosion equipment to study the explosion properties and combustion characteristics of pulverized biomass dust clouds. An unreported feature of this apparatus is that in rich concentrations only about half the dust injected is burned in the explosion. This work was undertaken to try to understand, through measuring the mass and composition of the debris at the end of the explosion, why all the pulverized biomass injected did not burn and the consequences for the measured parameters of flame speed, Pmax and Kst. One possible explanation of the results is that the residue material was formed from biomass dust blown ahead of the flame by the explosion induced wind and deposited on the vessel wall, where it was compressed as the pressure increased in the vessel. The flame side underwent flame impingement pyrolysis and the metal side was heated and compressed in the explosion but not burned. This was supported by photographic and pressure decay data that indicated the presence of a “cake” of dust being formed on the wall of the vessel. The results also show that the overpressures remain high for very rich equivalence ratios of up to 6. The reactivity of biomass was higher than coal for the two types of biomass investigated. No rich combustion limit was found. This challenges the general industry assumption that operating in very rich conditions in mills is safe. An explanation is proposed for the high peak pressures under rich conditions.