Ignition Risks of Biomass Dust on Hot Surfaces

Combustible biomass dusts are formed at various handling stages, and accumulations of these dusts can occur on hot surfaces of electrical and mechanical devices and can pose fire risks. This study evaluates the ignition characteristics of dust from two types of biomass commonly used in the U.K. power stations: herbaceous miscanthus and woody pine. The ignition risks of the individual biomass and their blends in two different weight ratios, 90 wt % pine to 10 wt % miscanthus and 50 wt % pine to 50 wt % miscanthus, were investigated. Biomass–biomass blends represent the power plant scenario where a number of biomass are fired under daily operation, and thus, dust sedimentation could consist of material blends. The influence of washing pretreatment (particularly to remove catalytic potassium) on the ignition behavior of these dusts was investigated. Fuel characterization via proximate and ultimate analyses was performed on all fuels and combustion characteristics via thermogravimetric analysis (TGA). The risk of self-ignition propensity of both untreated and washed biomass was ranked graphically using the activation energy (Ea) for combustion and the temperature of maximum weight loss (TMWL) determined from the derivative TGA (DTG) curve. It was found that the TMWL and Ea of washed biomass were higher than those of the untreated biomass, implying a lower self-ignition risk. Similar analyses were performed on untreated and washed blends, and comparable results were observed. The ignition characteristics were studied following the British Standard test methods for determining the minimum ignition temperature of a 5 mm dust layer on a heated surface. It was found that the washed individual biomass and their blends revealed slightly higher dust ignition temperatures than their respective untreated counterparts, a 20 and 10 °C difference for individual biomass and blends, respectively. The effect of washing on the ignition delay time was more obvious for pine than for miscanthus, but the time difference between the untreated and washed biomass never exceeded 4 min for all biomass and blends. The biomass pretreatment method of washing did change the combustion and self-ignition characteristics of biomass dust, and there was evidence of potassium being leached from the fuels upon washing (particularly miscanthus). This is considered the main reason for the increase in the minimum ignition temperature. While the washed biomass is found to have a lower ignition risk, it should be noted that the result (validated for up to 5 mm thickness) is not significant enough to influence plant operations for the ignition risk from thin dust layers according to the National Fire Protection Association (NFPA) standards