Characterization of high heating-rate chars from alternative fuels using an electrodynamic balance

Important advantages in the use of alternative and renewable fuels (CO₂ reduction in the atmosphere, recovery of energy from wastes, limited SO_x, NO_x and heavy metal emissions) can be obtained only by solving technological and economical problems that make direct combustion of such fuels impractical. This is possible after a detailed investigation to determine the most important features of these materials in all steps of the thermal process. At present, few data can actually be found for the char properties of these fuels. Nevertheless, the knowledge of properties of chars (especially after severe devolatilization) is crucial for both modeling purposes (reactivity, kinetics of combustion and gasification, morphology variations, composition, and fate of pollutant precursors) and practical applications (boiler efficiency, ash deposition, and condensation causing fouling and slagging problems).This work deals with the characterization of chars from different classes of materials (biomasses, waste, and low and high volatile matter (VM) coals) obtained after a devolatilization performed in severe thermal conditions, i.e., high temperature and high heating rate. A methodological approach is developed, applied, and discussed, using an electrodynamic balance that is a versatile analyzer for the study of properties of single levitated particles. The specific heat, size, and shape distribution, and density variation between the char and the parent material are evaluated for all materials. Scanning electron microscopy (SEM) analysis is also carried out to investigate morphological variations and support the major results obtained with the electrodynamic analyzer.     

Effect of steam on the single particle ignition of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions

This article investigates the effect of steam on the ignition of single particles of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions. Three solid fuels, all in the size range 125–150 µm, were used in this study; specifically, a low rank sub-bituminous Colombian coal, a low-rank/high-ash sub-bituminous Brazilian coal and a charcoal residue from black acacia. For each solid fuel, particles were burned at a constant drop tube furnace wall temperature of 1475?K, in six different mixtures of O₂/N₂/CO₂/H₂O, which allowed simulating dry and wet conventional and oxy-fuel combustion conditions. A high-speed camera was used to record the ignition process and the collected images were treated to characterize the ignition mode (either gas-phase or surface mode) and to calculate the ignition delay times. The Colombian coal particles ignite predominately in the gas-phase for all test conditions, but under simulated oxy-fuel conditions there is a decrease in the occurrence of this ignition mode; the charcoal particles experience surface ignition regardless of the test condition; and the Brazilian coal particles ignite predominately in the gas-phase when combustion occurs in mixtures of O₂/N₂/H₂O, but under simulated oxy-fuel conditions the ignition occurs predominantly on the surface. The ignition delay times for particles that ignited in the gas-phase are smaller than those that ignited on the surface, and generally the simulated oxy-fuel conditions retard the onset of both gas-phase and surface ignition. The addition of steam decreases the gas-phase and surface ignition delay times of the particles of both coals under simulated oxy-fuel conditions, but has a small impact on the gas-phase ignition delay times when the combustion occurs in mixtures of O₂/N₂/H₂O. The steam gasification reaction is likely to be responsible for the steam effect on the ignition delay times through the production of highly flammable species that promote the onset of ignition.     

Direct observations of single particle fragmentation in the early stages of combustion under dry and wet conventional and oxy-fuel conditions

This article investigates the single particle fragmentation of three solid fuels in the early stages of combustion under dry and wet conventional and oxy-fuel conditions. The three solid fuels studied were a low rank sub-bituminous Colombian coal, a low-rank/high-ash sub-bituminous Brazilian coal, and a charcoal residue from black acacia. Particles, with size in the range 125–150 µm, were burned in a drop tube furnace with a constant wall temperature of 1475?K, under six different mixtures of O₂/N₂/CO₂/H₂O, which allowed simulating dry and wet conventional and oxy-fuel combustion conditions. A high-speed camera was used to record the fragmentation process during the early stages of combustion and the collected images were treated to characterize the fragmentation mode, probability and time. The observed fragmentation modes are characterized by the occurrence of exfoliation, radial fragmentation or a combination of both. The results disclose that the fragmentation mode is strongly affected by the fuel type, but less affected by the atmosphere; the fragmentation probability is strongly affected by both the fuel type and the atmosphere; and, finally, fragmentation in air occurs significantly dispersed after ignition, but it tends to cluster closer to the ignition under simulated oxy-fuel conditions.