There is a growing interest in using municipal solid waste and industrial waste as an alternative fuel. Several methods have been studied to show the best approach to convert waste to clean and economical energy. Because most of polymer wastes such as plastics, tire, rubber and leather have higher calorific values, pyrolysis, thermal gasification and incineration/combustion of these wastes have been frequently considered to meet the renewal energy needs. However, polymer wastes have difficult burning problems due to the massive air pollutants emission and material corrosion of incinerator.
The objectives of this study are to develop the reaction kinetic model involved in the pyrolysis of plastic waste considering various input mixing ratio, pyrolysis temperature and pyrolysis time as the major operation parameters affecting the pyrolysis model. It was attempted to measure and assess experimentally the characteristics of gasified products, inorganic pollutants and residues generated from a full-scale gasification incinerator of waste tire. Also, in order to investigate the influence of the design and operation parameters on the rotary kiln for mixed PE waste, air velocity, temperature and pollutant distribution were measured and analyzed. Simplified radiation heat transfer model considering flow and heat release pattern was developed to obtain the temperature profile with varying amount of air supply, air injection pattern, waste charge and preheated combustion air. The chemical equilibrium program(CEC 71) was applied to analyze the effect of operation parameters on air pollutants emission.
Reaction kinetics at various temperatures for the pyrolysis of mixed plastic waste including polyethylene(PE) and polystyrene(PS) were modeled in terms of five types of overall pyrolysis reaction. The model development was based on the assumption that as plastic wastes were heated in a non-reactive environment, they were decomposed homogeneously to various products of gas, oil and char by a first-order rate, irreversible reaction, and isothermal condition. Among the five models, the type II model in which the activated polymer exists as an intermediate product was the most accurate one in predicting the pyrolysis products of pure PE and pure PS. And also, for mixtures of plastics both type II and IV models can be used to explain the composition of pyrolysis and the products. Furthermore, from the analysis of variance(ANOVA), the mixing ratio and the temperature were shown to be the parameters that have the greatest effect on the pyrolysis reaction of plastic waste mixture.
The overall chemical reaction in the gasification reactor injected with air for partial oxidation took place in two steps; the first being true pyrolysis and the second being the gasification step. Greater portion of oil(200 ~ 300 mg oil/N㎥) was produced at the beginning of the gasification process and was controlled by the initial pyrolysis reaction. More noncondensed fuel was produced in the middle of gasification process. The oil products mainly included aromatic compounds such as naphthalene, l-limonene, methylbenzene, dimethyl- quinone, vinylidene, benzothiazole, diphenylhydrazine, and etc. Zinc content in ash was the highest with the exception of iron. Si, Al, CA, Cu, S, and Cd were also observed. Polynuclear aromatic hydrocarbons were obtained at an amount of 150 μg/g of bottom ash and at an amount of 294-420 μg/g of fly ash. And also, other hydrocarbons such as combustion byproducts was observed.
From the trial burn test and model analysis of rotary kiln, operation conditions for the reduction of air pollutants emission were achieved at the 70 ~ 100 % excess air supply, with the higher rotation speed(2.0 rpm), and also observed at 55 ~ 65% of upper air supply. $NO_x$ seemed to be the main thermal $NO_x$ in the flame area and $SO_2$ was generated in the post combustion zone. Total hydrocarbons (THC) emission was linearly correlated to CO emission. From two dimensional radiation heat transfer model combined flow pattern, it was possible to estimate optimal air supply and the air distribution pattern in rotary kiln. In the preliminary design procedure, the optimal treatment capacity according to dimension of rotary kiln can be suggested by comparison of the predicted temperature profile and design temperature. And also, the calibrated chemical equilibrium model can predicted pollutants profile in rotary kiln and air pollutants emission with varying the temperature and the equivalent ratio.
