Carbon burnout from the char of a single cylindrical pellet

Abstract

In countries with limited wood fuel resources, the use of straw for energy should be increased to comply with environmental commitments. Boiler houses burning whole straw bales have limited application because of unfavourable straw logistics and obstacles to their construction in most densely built-up settlements. Straw pellets are more convenient for transportation and safer for storage. Boilers with pellet burning can be operated in fully automatic mode. However, burning straw pellets instead of wood pellets is complicated with increased ash content and lower ash melting temperature. Approaches for low-temperature burning of straw pellets at temperatures below the initial deformation temperature of their ash are of practical interest. At low temperatures, burning is slowing and incomplete burnout with energy loss is possible. The rate of burnout also depends on the form and dimensions of the particle. It was common to accept the rate of carbon burnout as uniform on the reacting surface of the particle. This work aimed to study the low-temperature carbon burnout from the char of the cylindrical pellets of various lengths and to estimate the energy loss with unburned carbon. The aim was achieved by the mathematical description of pellet char burning, in which the actual cylindrical char was regarded as the infinite cylinder intersecting with the endless plate. The burnout of fixed carbon was accepted as layered, the carbon burning fronts as infinitely thin, and different rates of carbon burning in radial and axial directions were accepted. An experimental study of the low-temperature burning of fixed carbon from the char of wood and straw pellets was conducted at free air access at 700°C in the furnace, i.e., the temperature around a single pellet was certainly lower than the possible initial deformation temperature of the pellet ash. The duration of carbon burnout from the char of single cylindrical pellets depending on their lengths was studied. The research findings are as follows. Equations in a dimensionless form, describing changes in the remaining share of unburned fixed carbon in pellet char in time, were deduced analytically. At the experimental burning of single straw pellets, loose particles of ash with no signs of melting formed, but they contained unburned carbon. The share of unburned fixed carbon in ash was 0.016–0.020. The coefficient of determination of calculated and experimental duration of complete burnout of fixed carbon from wood pellet char was R2 = 0.96, and R2 = 0.87–0.91 at incomplete burnout from straw pellet char. The most significant scientific result is that for long pellets, the fixed carbon burnout is controlled mainly by its slower burnout rate in the radial direction, and for the shortest pellets by more intensive burnout rate in the axial direction. The practical value of the results obtained is that the use of shorter pellets, which are characterised by faster burnout, may become purposeful for intensive combustion. Conversely, for slow combustion, and especially with the aim of arranging low-temperature burning of straw pellets, it may be feasible to use longer pellets with extended burnout. In the given conditions of straw pellet burning, the unburned carbon presents in ash, but losses of the pellet energy with unburned carbon were estimated at 0.61–0.72%, which is acceptable for boiler burners.