KINETICS OF THERMOLYSIS AND BURNING OF SOLID BIOFUEL PARTICLES. PART 4. INFLUENCE OF PELLETS ANISOTROPY ON OXIDATION RATE OF CARBON FROM COKEASH RESIDUE IN AXIAL AND RADIAL DIRECTIONS

• Дата: 2017
• Автор: Zhovmir, M.
• Формат: Стаття
• Мова: Ukrainian
• Опубліковано: Institute of Renewable Energy National Academy of Sciences of Ukraine 2017
• Теми: solid biofuels; pellets; cokeash residue; carbon; burning out; rate; anisotropy; твердое биотопливо; гранулы; коксозольный остаток; углерод; выгорание; скорость; анизотропия; тверде біопаливо; гранули; коксозольний залишок; вуглець; вигорання; швидкість; анізотропія
• Онлайн доступ: https://ve.org.ua/index.php/journal/article/view/29
• Назва журналу: Vidnovluvana energetika

In the previous part of paper research on rate of carbon oxidation from cokeash residue of long wood and straw pellets with insulated ends were presented. Assuming the rate of carbon oxidation is the same across surface of cokeash particle durations of carbon full burnout were calculated for residue of different length pellets. It was a satisfactory matching of the calculated and experimental data for long pellets (above 20-25 mm), but for a shorter ones calculations gave data significantly exceeding experimental. Observations on progress of carbon burnout from pellet’s cokeash residue allowed assume anisotropy on carbon oxidation rate in axial and radial directions. Based on analysis of pellet production features in presses with ring or flat shaped die with rolls it can be supposed more significant biomass compacting in axial (longitudinal) direction       of pellet than that in transversal (radial) direction. It is known that the thermal conductivity of porous solids grows with increase of their density. Due to predominant biomass compressing in axial direction, one can assume that the density and thermal conductivity of wood pellets in the axial direction should be higher than in the radial, i.e. biofuel pellet and cokeash residue formed out of it are anisotropic. Intensification of the transfer phenomena in direction of greater material density can stipulate the increase of carbon oxidization rate in axial (longitudinal) direction of pellets. This paper provides results of experimental and analytical–experimental research on carbon burning out rate from cokeash residue of wood and straw pellets in axial (longitudinal) direction. Mathematical model of carbon burning out from cokeash residue formed from thin round plate cut from cylindrical pellet is proposed. Dependence for determining rate of carbon burning out in axial direction wca is based on experimental data about dimensions of round plate, its mass and duration of complete burnout: here:               ρc – average carbon density in particle of cokeash residue; δ0 – semithickness of round plate cut from pellet; tb – duration of carbon complete burnout from round plate of cokeash residue. Experiments were carried out at temperature in muffle furnace 700oC and free air access with same lots of pellets as in previous paper. It was found carbon burning out rate from cokeash residue of 6 mm wood pellets in axial direction is wca = 3.2 g/(m2s) and is 1.7…2.0 times higher than that in radial direction, and for cokeash residue of 6mm straw pellets wca = 2.3…2.5 g/(m2s) is 1.4…1.5 times higher than that in radial direction. That should to be taken into account at calculation of pellets burning duration.