Study On Hydrogen-Rich Fuel Gas Production From Pyrolysis Of Deep-dewatered Sewage Sludge Cakes After Conditioned With Fenton's Reagent And Red Mud

Sewage sludge is a by-product of municipal wastewater treatment, which has a complicated composition, the high water content, and is difficult to be effectively dewatered. It has become a challenge research topic to reduce the sludge water content and realize the reasonable utilization of sludge cake.This study investigated the production of hydrogen-rich gas from the pyrolysis of the deep-dewatered sewage sludge cakes with the water content around 60 wt% after conditioned with the Fenton's reagent and red mud. The paper studied the effect of water content on the hydrogen-rich gas production and the catalytic effect of red mud in organic matters decomposition during the pyrolytic process. The main contents are as follows:1. Pyrolysis characteristics analysis of deep-dewatered sludge cakesThe pyrolysis characteristics of dried raw sludge and deep-dewatered sludge cake were analyzed by thermogravimetric analysis. According to TG and DTG curves of sludge samples, the decomposition of organic matters in raw sludge was occurred within the range of 200-600 oC with a weight loss of 49.15%, while the decomposition of organic matters in deep-dewatered sludge cake was occurred in three ranges of 100-200 oC, 200-600 oC and 600-900 oC with a total weight loss of 47.31%. The composite conditioners can reduce the pyrolytic temperature of biodegradable organic matter, humus and non-biodegradable organic matters, decrease the pyrolytic weight loss rate, thus improving the gas yield.2. Effect of water content on the yield of fuel gas from pyrolysis of deep-dewatered sldudge cakesPyrolytic experiments were conducted under the temperatures of 700, 800, 900 oC on the deep-watered sludge cakes with water contents of 0%, 14.2%, 40.9%, 52.2% and 65.2% and PAM conditioned sludge cakes with water contents of 14.6%, 40.4%, 52.8%, 65.1% and 85.2%, respectively. It was found that when pyrolytic temperature reached 900 oC, the gas production of deep-dewatered sludge cakes reached the maximum as the water content was 65.2%, and the H2 yield reached the maximum as the water content was 52.2%. Under 900 oC, as the water content of PAM conditioned sludge cakes was 65.1%, both the gas production and H2 yield reached the maximum. The maximum gas output of deep-dewatered sludge cake was 36.0 % higher than PAM conditioned sludge cake, and H2 yield was elevated by 89.1%. According to results of GC-MS, the main contents of tar are aromatics, aromatic nitriles, substituted aromatics, oxy-compounds and aliphatic hydrocarbons. Among them, the aromatics, especially polycyclic aromatic hydrocarbons(PAHs) accounts for the main component. As water content of deep-dewatered sludge cake increased, oxycompounds and aliphatic hydrocarbons gradually disappeared, the relative content of aromatic nitriles and substituted aromatics raised gradually. Moreover, the relative content of poisonous and harmful aromatics, especially the content of five and six ring PAHs decreased. According to FTIR results of solid char, when the temperature reached 700 oC, most of the organic matters in sludge has been degraded completely. As the water content of deepdewatered sludge cake increased, the ratio of the water participated in pyrolytic reactions increased and the weight of solid char decreased gradually. As the water content reached 65.2 %, water provided the steaming atmosphere of pyrolysis process, and improved the water vapor concentration involving in secondary cracking reforming of tar and char gasification reactions, which therefore increased the yield of fuel gas, especially the H2 production.3. Catalytic study of red mud on the gas production from pyrolysis of deepdewatered sludge cakePyrolytic experiments were conducted on the deep-dewatered sludge cakes conditioned with Fenton's reagent and different kinds of red mud at the temperature of 700, 800 and 900 oC. It was found that the type of red mud has great influence on the gas output of deepdewatered sludge cakes conditioned with Fenton's reagent and red mud, especially the H2 yield. The deep-dewatered sludge cake conditioned with the type of red mud with the highest Fe2O3 content produced the highest gas and H2 yield, especially at the temperature of 700 oC, this type of red mud has the best enhancement on the cracking of organic matters in sludge to produce fuel gas. Further study found that the higher Fe2O3 content was in red mud, the higher relative content of PAHs produced in tar, and the highest content reached 88.54%. Moreover, according to FTIR results, the stretching vibrations of C-Har in solid char of deepdewatered cakes conditioned with red mud were weaker and decreased faster with increasing pyrolytic temperature. It implies that red mud could enhance aromatics cracking and gasification reactions, which improve the relative content of PAHs in tar. According to DielsAlder reaction model, red mud could accelerate dehydrogenation, cyclization and aromatization reactions to promote the production of PAHs, and the higher Fe2O3 content was in red mud, the better enhancement red mud performed, which greatly increased the gas output, especially the H2 yield.Moreover, the effect of mixing method of red mud and sludge on the gas production was studied. It was found that the red mud as a component of composite conditioner, not only can has a deep-dewatered effect, but also has the catalytic effect on cracking of organic matters in sludge cake, secondary cracking reforming of tar and solid char gasification reactions in the pyrolytic process.This study provides a novel route for the deep-dewatering of sewage sludge and the subsequent recycling of dewatered sludge cakes.