Experimental Investigation Of Ash Deposits Characteristics Of Co-combustion Of Coal And Rice Hull

Co-combustion of coal with biomass does not only reduce the use of fossil fuels, but also provide an effective means to reduce the net emission of atmospheric pollutants and greenhouse gases. It is widely applied in the industrial production. However, because of a lack of understanding of ash deposition mechanism during co-firing coal with biomass, co-combustion of coal with biomass cause severe ash deposition, fouling, and slagging problems.In this paper, a designed oil-cooled probe which can simulate the water wall was used to collect the ash deposits. The ash deposit growth and shedding characteristics were investigated a digital imaging technique when co-firing Da Tong (DA) coal with different proportions of rice hull (0%,5%,10%, and20%, based on weight) in a pilot scale furnace. The CCD camera record the deposit growth and shedding process to analyze the mechanism of deposit growth and shedding. The deposit heat flux which represented the heat transfer efficiency was calculated. The microstructure, semi-quantitative chemical composition, and mineralogy of the ash deposits were evaluated by scanning electron microscopy with energy dispersive X-ray (SEM-EDX) and X-ray diffraction (XRD).The addition of rice hull increased the porosity of deposits and the deposits showed loose, easy removable. The deposit growth process consisted of four stages: the initial stage, the rapid growth stage, the slow growth stage, the stable stage which corresponded to the heat flux demonstrated three-stage mode, as follows:the initial stage, the slow decreasing stage, the stable stage. The stable thicknesses of deposits increased exponentially with the increase of rice hull, thus causing the reduction of heat flux, affecting the heat transfer efficiency. The selective deposition of elements resulted in the layered structure due to the different elemental composition. The key problem of abrasion wear resulting from the high SiO2content of deposits during the co-firing of coal with rice hull should be paid attention to.The shedding deposits with similar morphology showed no obvious layering. The main shedding mechanism of ductile deposits was the falling of molten ash deposits as a result to gravity while it was erosion and thermal shock for brittle deposits. The successive deposit shedding events resulted in fluctuation in the thermal load which seriously affects the safe and economic operation of the unit. The microstructure, chemical composition, and mineralogy of shedding deposits were similar to the sintered layer of deposit on probe. The shedding deposits were mainly from the sintered layer. The deposit shedding was liable to occur between the initial layer and the sintered layer.