Study on MSW catalytic combustion by TGA

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Abstract

By thermogravimetric analysis (TGA), the influences of different catalysts on the ignition and combustion of municipal solid waste (MSW) have been investigated in this paper. MSW combustion can be divided into two steps. One is the emission and combustion of volatiles, and the second is the combustion of char. The existence of catalysts in the first step enhances the emission of volatiles from the MSW. When the concentration of volatiles around the MSW is high enough, the MSW ignites. The action of catalysts in the second step of MSW combustion may be as a carrier of oxygen to the MSW char. Two indices have been used to compare the characters of ignition and combustion of MSW under different catalysts. One is the temperature when the rate of conversion of the MSW is at 5%, while the other is the combustion when the temperature is at 850 °C in the TGA system. By comparing the different values of the two indices, the different influences of the catalysts on the ignition and combustion of MSW have been studied.

Introduction

The technology of municipal solid waste (MSW) combustion has been proven as an attractive method of waste disposal for many years due to the primary advantages of hygienic control, volume reduction and energy recovery. The combustion of MSW also brings about air pollution such as the emission of dioxins [1], PAHs and PCB [2], [3] in MSW incinerators.

MSW is extremely heterogeneous. Its moisture content and heating value may vary over wide ranges, and its “particle” size is always very big. The direct combustion of MSW is sometimes very difficult. After being crushed, dried and solidified, parts of MSW are changed into refuse derived fuel (RDF). RDF is easy to handle for transportation and storage, and its heating value is higher than that of the original MSW. When calcium based sorbent is added to the RDF, the emission of sulfur dioxide, dioxin and PAHs will be reduced during RDF combustion [4], [5]. The efficiency of the furnace is reported to reduce the heat efficiency from 82.38% to 81.61% when RDF is added to co-combust with coal. This may be due to low mass transfer between the RDF and the oxygen in the furnace [6].

If the RDF is produced with the addition of catalysts similar to the calcium based sorbent, the combustion of RDF will be enhanced, and the air pollution will be reduced at the same time. It is valuable to study the catalytic combustion of MSW. There are very few papers about the catalytic combustion of MSW. It is reported that alkali and alkaline earth metal salts can greatly accelerate the ignition of coal [7]. Some IA, IIA and B area metal elements in the periodic system of elements are found to be catalysts for soot and carbon combustion. These metal salts include KVO3–RbVO3, RbVO3–CsVO3, KVO3–CsVO3, CuCl2–KCl–NH4VO3, Fe2O3 and V2O5 [8], [9], [10]. MSW always contains more volatiles than coal and carbon. What kinds of catalysts will be suitable for MSW combustion? In this paper, the topic will be studied in detail by thermogravimetric analysis (TGA).

Section snippets

The experimental conditions

MSW is obtained from a resident area in Nankai University. Metal, glass and battery are removed from the MSW. The remaining parts of the MSW, including plastic, biomass, paper, cloth, remaining food and slag, are dried, broken, grinded and screened. The sizes of all particles of the material from the MSW are less than 1 mm. After treatment, they are mixed together according to their original percentages. The proximate analysis of the MSW is 2.1% water, 62.5% volatiles, 21.4% carbon and 14.0%

The experimental results and analysis

The data from the NETZSCH TG 209 analyzer are divided into four groups: temperature (°C), time (s), weight loss (%) (described as w/w0) and first derivative of weight loss with respect to time. In order to compare the different influences of the different catalysts on the MSW combustion, the data of the DTG and TG are obtained from the original data according to the formulae:DTG=dαdt,TG=α=w-w0w-wwhere w is the weight of sample in the TGA experiment at time t. w0 is the initial weight of the

Conclusions

By TGA study, the combustion of MSW can be divided into two steps. One is volatiles emission and combustion, the second is the remaining volatiles and char combustion. For the first step of MSW combustion, the existence of catalysts enhances the emission of volatiles, which makes the ignition of MSW happen at lower temperatures than that without catalysts. The temperature when the rate of conversion of MSW is at 5% is used to describe the ignition index. According to the value of the ignition

Acknowledgement

The authors gratefully acknowledge the financial support from the National Nature Science Fundation of China (No. 20347002).

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