Fluidised incineration of moist paper pellets using straw as a secondary fuel

In this study, the characteristics of the drying, devolatilisation, and char burn-out processes of moist paper pellets, and the effect of the addition of straw on the combustion of moist paper pellets were investigated. Paper pellets, 10 mm in diameter, 8-10 mm long, 0.5-0.7 g in weight, and with 20-50 wt% moisture content, were either pyrolysed (i.e., in the absence of oxygen) or burned in the muffle furnace, and were combusted batch-wise in a 100 mm diameter, 1250 mm long, laboratory-scale, fluidised bed incinerator, at 750, 825, or 900 °C. Occasionally, straw in the amount of 15 or 30 wt% was added to pyrolyse or burn with the paper pellets. The 50 wt% moisture content paper pellets were also burned continuously at 825 °C in the fluidised-bed incinerator either with or without the addition of straw (30 wt%). The results from the muffle furnace showed that drying, devolatilisation, and char burnout time decreased with an increase in temperature. The effect of temperature was found to be less when the samples were burned in the fluidised-bed. It was found that water in the samples, when the moisture content was 20 wt% evaporated almost immediately after they were put into the furnace. At higher moisture contents (i.e., at 35 and 50 wt%), it took 2-4 minutes for the water to evaporate. It was also found that the dried samples became more porous when the moisture content in the original samples increased. This resulted in a decrease of the char burn-out time of the samples. Straw assisted the evaporation of moist paper pellets and, thus, led to a decrease of the drying time and an increase in the stability of the combustion of paper pellet-straw mixtures, even when moisture contents in paper pellets were high. The percentage of carbon burned during devolatilisation was found to be 50-76% of the total carbon, despite the fact that the devolatilisation time was 2.5-4 times lower than that of the char burn-out. The average rate of the devolatilisation was found to be 5-6 times that of the char burn-out. A spike of CO was observed during devolatilisation at 750 and 825 °C when straw was added; however, the spike was not observed when only paper pellets were burned. The percentage of CO emitted during devolatilisation increased from 30 - 40%, when only paper pellets were burned, to 80-90%, when straw was added. However, at 900 °C, the percentage of CO emitted during devolatilisation of paper pellets was found to be similar to that of paper pellet-straw mixtures (i.e., 80-90%), and the spike of CO was also observed when only paper pellets were burned. The results from the continuous fluidised-bed test runs showed that the combustion the mixture of paper pellets and straw could achieve a high combustion efficiency, while emitting low amount of CO.