Activated carbon production from sewage sludge employing low-oxygen flue gas

Abstract: Sludge treatment and disposal have been a significant environmental problem because of the huge yield due to the high-speed urbanization and industrialization in China. Recently, in virtue of potential energy contained in organic matters of sewage sludge, thermal conversion technologies, such as pyrolysis, gasification, combustion and some high value-added products production, have been recognized as promising methods to deal with sludge aiming at energy recovering. Among them, sludge-derived activated carbon had been developed and worked as one of the most effective methods to realize the harmless sludge treatment and energy recycling simultaneously. However, the cost of the sludge-based activated carbon production was still high due to the huge amount of heat and gas required during the carbonization and activation. Therefore, this study proposed to produce the sludge-derived activated carbon with flue gases. The feasibility of exploiting low oxygen flue gas to prepare activated carbon from the sludge was investigated by focusing on the operating conditions, such as the activation method (physical activation and ZnCl2 activation), activation temperature, activation time, steam content, the ratio of sludge to ZnCl2 and oxygen content. The Brunauer-Emmett-Teller (BET) surface area and the adsorption value of the methylene blue and methyl orange of the derived activated carbon were taken as the indexes to evaluate its performance.The results showed that the optimal operating parameters of activation temperature, activation time and steam content were 800°C, 90 min and 34.8% respectively for the physical activation. However, the products were not so good comparing with the commercial products. The BET surface area, the yield of activated carbon, and the adsorption value of methylene blue and methyl orange were only 246.3 m2/g, 46%, 18.1 mg/g and 14.8 mg/g respectively. It was indicated that the single use of flue gas was not sufficient for physical activation of sludge to produce activated carbon due to its low vapor content and high operation cost to support the energy demands of high activation temperature. Some other activation method should be incorporated or modified to improve the thermal efficiency of this whole process as well the performance of sludge-based activated carbon.In respect to the ZnCl2 activation, the existence of oxygen can promote formation and development of the microspores of activated carbon. The BET surface area of the products produced under an oxygen content of 4%, was improved 6.82% comparing with what was prepared without oxygen. Correspondingly, the adsorption properties were improved with an increment of 2.75 times in the adsorption value of methylene blue. The optimal oxygen content was about 2%-4% for ZnCl2 activation. Under the optimal condition of activation temperature 550°C, activation time 90 min and oxygen content 2%, the yield of sludge-derived activated carbon as 59%; its BET surface area and methylene blue adsorption value were the highest of 516.1 m2/g and 129.8 mg/g, respectively; the pore volume, microspore volume and average pore size were 0.29 cm3/g, 0.16 cm3/g and 3.95 nm, respectively. All the data demonstrated that combination with chemical activation, the flue gas with high temperature and low oxygen content could be used as a heat resource, shielding gas and activation agent to produce activated carbon from dewatered sludge with moisture content higher than 80% directly. This study could provide a theoretical basis to lower the cost of sludge-derived activated carbon production from dewatered sewage sludge. It also provided a viable option to realize the harmless sludge treatment and cost-effective energy recycling simultaneously.