Study On Pyrolysis Characteristics Of Sludge And Leaching Behavior Of Trace Elements In Its Char

Sewage sludge, which is one of the main by-products generated from municipal sewage treatment plant, is in an ever-increasing amount due to the continuous development of industry and agriculture as well as the improvement of living standards. However, how to dispose of tremendous sewage sludge to eliminate the potential hazardous effect on environment has become a serious challenge with which people have to face recently. As a new type of sludge disposal technology, sludge pyrolysis technology is receiving more and more attention due to it has many advantages. In recent years, domestic and foreign research institutes and enterprises have done a lot of work on sludge pyrolysis technology. The high-speed carbide sludge technology, which is developed by Tomoe Engineering Co., Ltd, is one of the best representatives. This research is supported by Hubei Both Environment&Energy Co., Ltd which will introduce the technology. In view of this, a continuous high-speed carbide sludge system has been studied, to discover the release characteristic of volatile during pyrolysis of sludge. Toxicity leaching experiments as well as batch leaching experiments have been carried to investigate the leachability of trace elements in char under different conditions.First of all, three kinds of sludge samples have been performed in a horizontal quartz reactor, to discover the characteristic of volatile during fast pyrolysis of sludge. In this experiment, CH4, CO, NO, SO₂, NH₃, HCN and NO₂ have been investigated. In addition, the experiment have also detected a small amount of HCl, HF, COS and CS₂. The results show that the pyrolysis temperature has an important impact on fast pyrolysis of sludge. With the increase of final pyrolysis temperature, the content of CH4 in pyrolysis gas increases, CO level increases slightly, SO₂, NO₂ and HCl slightly decreases at high final pyrolysis temperature. During the pyrolysis time of 15min, total release of nitrogen compounds in pyrolysis gas has the trend of NH₃>NO₂>HCN>NO. Then, a thermogravimetric analyzer with a Fourier transform infrared spectrometer has been used to investigate the release characteristic of volatile during temperature-programmed pyrolysis of sludge. The results show that the main volatile gases are CO₂, CH4, H2O, and some other organic gases.In order to assess the potential harm of disposal and use of sludge char to the environment, a leaching toxicity experiment has been performed. The experimental results have showed that the leaching of trace elements (Cr, Cr (VI), Ni, Pb, Cu, Zn and Hg) is far below the standard limit concentration, which indicate that the char from sludge pyrolysis does not have leaching toxicity. Batch leaching experiments have been carried to investigate the factors for trace elements leaching. It is shown that, both leaching time and pH have important influence on the leaching of trace elements in sludge pyrolysis char. The results for each element are not consistent, which may due to different binding forms of trace elements in char products. Compared with the results of sludge (ad), the leaching level of char is much lower except Hg, which shows that pyrolysis of sludge can effectively restrain the leaching of trace elements.