Effect of molding and burning temperature on ash physicochemical characteristics of refuse derived fuel

Abstract: Waste to energy (WTE) is one of the most reasonable managements for solving impacts of rubbish on our environment. Methods of WTE are major in incineration and gasification, by which to release thermal energy from organic substances. WTE plants have some more serious problems to solve because of the complexity of raw refuse. Refuse derived fuel (RDF) with specific pretreatment is confirmed as an optimum feedstock with higher efficient and more stable performance for incineration and gasification than the raw refuse. However, the RDF ash is still about 20% of original weight after burning. It needs to be considered how to dispose RDF ash in the right way. It is worth mentioned that RDF ash has specific property, such as low toxicity, well-distributed and inorganic nature. Therefore, some experts indicated that RDF ash could be used as source materials of building wall or road basement. This study focused on the characteristics of RDF ash such as weight, particle size, chemical composition, and discussed the mechanism of reaction change during the combustion. In this experiment, refuse from a WTE plant in Guangdong Foshan was made into RDF by screening and smashing process. Pelleted RDF made by additional extrusion and forming process. Ash samples were obtained from both pelleted and non-pelleted RDF burned at temperature of 600, 700, 800, 900, 1 000 and 1 100 ℃ in muffle furnace, respectively. The weight, particle size and chemical composition of these samples were analyzed. The results showed that chemical compounds of the RDF ash have decomposition reaction mainly below 1 000℃, such as decomposition of organic residue, chloride, sulfate, thus particle size decreased. While the burning temperature was rising over 1 000 ℃, combination reaction turned to be dominant during the whole reaction, the higher temperature is, the more serious reaction happened, which resulted in the products with composite silicates. Meanwhile, particle size increased and even formed into massive structure because more and more ash substances melted and bonded at or over their melting points. As a result, lump amounts increased after temperature returned back. Some compounds containing non-metallic elements such as C, Cl, S decomposed and turned into gaseous compounds, which escaped from the base-ash, so the ash weight reduced. High temperature burning reduced the weight of RDF ash. Compared with non-pelleted RDF ash, ash of pelleted RDF has more active at the same temperature, because of its higher density which means that there is shorter distance among reaction substances causing lower resisting force to overcome. Pelleted RDF ash contained more crystal substance quantities than the non-pelleted one, so it has bigger particle size, but the weight of the pelleted RDF ash is less than that of non-pelleted RDF ash.