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) standard

Gas phase potassium release from a single particle of biomass during high temperature combustion

A notable characteristic of solid biomass fuels as compared to coal is their significantly higher potassium content. Potassium influences ash deposition and corrosion mechanisms in furnaces and boilers, the effects of which may differ depending on phase transformations of potassium species in the gas phase and condensed phase. An understanding of how potassium is released from biomass fuels during the combustion process is therefore useful for plant designers and operators assessing means of avoiding or mitigating these potential problems. An experimental method is used to measure release patterns from single particles of biomass fuels using flame emission spectroscopy and a single-particle combustion rig. The experimental arrangement also allowed simultaneous thermal imaging of the combusting particle in order to determine the surface temperature. A model of the single particle combustion is presented. Using experimental data on devolatilisation and burnout times for different sized particles and the measured surface temperature profiles, the thermal and kinetic sub-models are verified. A model for potassium release is described and this is integrated to the single particle combustion model to allow prediction of the temporal patterns of release of gas-phase potassium. The modelled release patterns were compared with those observed. Good agreement between modelled and measured potassium release patterns was attained confirming that the proposed mechanisms affecting potassium release are valid

Observations on the release of gas-phase potassium during the combustion of single particles of biomass

One of the more significant characteristics of solid biomass fuels as compared to coal is the quantity of potassium that they contain. Potassium content influences ash deposition and corrosion mechanisms in furnaces, the effects of which may differ depending on phase transformations of potassium species in the gas phase and condensed phase. The fate of potassium from the fuel during the combustion process is therefore an important concern. To investigate this, an experimental method is presented in which the release patterns from single particles of various biomass fuels are measured by use of flame emission spectroscopy implemented using a custom-built photo-detector device. Single particles of fuel are combusted in a methane flame with a gas temperature of ∼1800 K. The observed potassium release patterns for thirteen solid biomass materials are presented. The data are analyzed to examine the relationships between: the level of potassium in the fuel particle; the fraction of potassium released at each stage of combustion and the peak rate of release of potassium to gas-phase during combustion. Correlations between these quantities are identified with key trends, patterns and differences highlighted. The analyses provide useful information for the development and validation of modelling of potassium release during combustion of biomass

Fundamentals and environmental aspects in the thermochemical conversion of biomass

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Entrained Metal Aerosol Emissions from Air-Fired Biomass and Coal Combustion for Carbon Capture Applications

Biomass energy with CO2 capture could achieve net negative emissions, vital for meeting carbon budgets and emission targets. However, biomass often has significant quantities of light metals/inorganics that cause issues for boiler operation and downstream processes; including deposition, corrosion, and solvent degradation. This study investigated the pilot-scale combustion of a typical biomass used for power generation (white wood) and assessed the variations in metal aerosol release compared to bituminous coal. Using inductively coupled plasma optical emission spectrometry, it was found that K aerosol levels were significantly greater for biomass than coal, on average 6.5 times, with peaks up to 10 times higher; deposition could thus be more problematic, although Na emissions were only 20% of those for coal. Transition metals were notably less prevalent in the biomass flue gas; with Fe and V release in particular much lower (3–4% of those for coal). Solvent degradation may therefore be less severe for biomass-generated flue gases. Furthermore, aerosol emissions of toxic/heavy metals (As/Cd/Hg) were absent from biomass combustion, with As/Cd also not detected in the coal flue gas. Negligible Cr aerosol concentrations were found for both. Overall, except for K, metal aerosol release from biomass combustion was considerably reduced compared to coal