Research On The Characteristics Of Sewage Sludge Fast Pyrolysis And Its Economic Accessment

The pyrolysis technology is one of promising methods to deal with sewage sludge with clean and efficient characteristics. However, the commercial application of sewage sludge pyrolysis technology is facing several difficulties including environmental issues in pyrolysis process and the economy assess of large-scale industrial process. The control methods of heavy metals are directly determined by their releasing characteristics during sewage sludge pyrolysis process. The economic evaluation of the sewage sludge pyrolysis will provide necessary fundamental data for commercial application in future.At first, the distribution characteristics of sewage sludge in China's large and medium-sized cities were statistically analyzed and compared with those of foreign countries in this dissertation. The results suggested that the level of heavy metals in sewage sludge was relatively lower in our country except for the arsenic element. After clustering analysis of heavy metals content in most of Chinese provinces, it was found that heavy metal pollution in Jiangsu, Zhejiang and Shanghai were the most serious ones, and the level of heavy metals in sludge in most other regions was relatively lower. Taking the thermal stability, eco-toxicity and other factors into account, this study mainly focused on the following heavy metals: Cr, As, Cd and Pb.Secondly, the typical sludge in Wuhan city was taken as the representative of the fast pyrolysis experiment under different temperatures and different residence times to study the release characters of volatiles and heavy metals in pyrolysis process. The results suggested that the release proportion of the volatiles matter was more than 80% at 450?. For another, the release rate of volatile matter and combustible gases increased at higher temperature. The inductively coupled plasma mass spectrometry(ICP-MS) was employed to investigate the release characteristics of Cr, As, Cd and Pb. The study showed that most of the Cr retained in the sludge char, but the release rate of Cr at high temperatures would increase significantly. Most of the Arsenic element released at a low temperature of 450? with the maximum release rate of 60.4% and nearly all of the Cd and Pb released at 850?. Obviously, emission of these heavy metals would cause secondary pollution during sewage sludge fast pyrolysis. Combined with the release rate of volatile matter and the heavy metals release properties, it was found that the release rate of Cr and As increased almost linearly with the emission of volatiles. But unlike Cr and As, most of the Cd and Pb escaped at the final stage of pyrolysis. Therefore, pyrolysis conditions didn't have much effect on the control of pollution for Cr and As. However, the release of Cd and Pb could be suppressed effectively by reducing the residence time or reaction temperature.At last, considering the distribution of the products at the pyrolysis experiment, the commercial pyrolysis process of sewage sludge was constructed using Aspen Plus software. The investment and operation cost of commercial pyrolysis technology were calculated based on the model and the influence of different factors on the pyrolysis technology economy was considerd. At the same time, the economic assessment was used to compare coal/sludge co-combustion technology and sewage sludge pyrolysis technology. The results showed that both of the economy of pyrolysis and co-combustion technology were not so satisfied because of low prices of fossil fuels. In addition, the annual investment return rate related with fossil energy price, transport cost, carbon tax and government subsidies. Comparing to co-combustion technology, the economy of sewage sludge pyrolysis technology was relatively well. When the prices of crude oil and coal reach 52 US dollars per barrel and 418 CNY per ton respectively, the two technologies mentioned above will make profit. The co-combustion technology is suitable for small and medium sized cities when the transport distance is less than 15 km. By contrast, sludge pyrolysis technology is more economical in large and medium cities with a longer transportation distance. The annual rate of return on investment of the pyrolysis and co-combustion technology would be 2.4% and 7.3% respectively if our country imposed carbon tax. In case of the government subsidies for these two technologies is up to 95 CNY per ton and 24 CNY per ton respectively, an internal rate of return of the two technologies can reach to 30%.