Study on Drying and Pyrolysis of Municipal Sludge Treated by Microwave Inducement
|School||Huazhong University of Science and Technology|
|Course||Energy and Environmental Engineering|
|Keywords||municipal sludge microwave inducement thin-layer drying pyrolysis fixedbed rheological properties diffusion numerical simulation|
With the acceleration of the construction of wastewater treatment plant, the scale of municipal sludge has been increasing year after year. Reduction and resource treatment technologies are need. In view of the deficiency of the current sludge treatment technology on drying efficiency and pyrolysis energy consumption, this paper focused on the drying and low-temperature pyrolysis technology based on microwave inducement. Experimental research and numerical simulation are carried out on the key technology for sludge treatment.Through the municipal sludge drying experiments, the effects of temperature and microwave inducement on the drying characteristics of the sludge are considered. The results show that the higher the drying temperature, the faster the rate of sludge drying. When the drying temperature increases from 70℃to 160℃, the corresponding maximum drying rate changes from 0.005 g·(g·min)-1 to 0.060 g·(g·min)-1. Under the experimental conditions, the time of microwave-induced sludge drying can be reduced by nearly 30% compared with original sludge. The microwave-induced sludge drying is relatively easier than primitive sludge drying. Microwave radiation can significantly improve the diffusion coefficient of municipal sludge. The diffusion coefficient increases by 228%,295%,388% than the original sample under irradiation time 60s,90s,180s with 15 w/g radiation dose; and increases by 293% under irradiation time 60s with 20 w/g radiation dose. Calculated by Fick’s law under the same temperature, the diffusion coefficient of the municipal sludge is three orders of magnitude bigger than that of paper mill sludge. Processing the experimental data with the Arrhenius equations, the average activation energy of the municipal sludge is 37 kJ/mol. There is a big difference of the composition and structure between the municipal sludge and paper mill sludge. Under the same thermal conditions, the drying process of municipal sludge absorbs more heat and consumes more energy than that of paper mill sludge. Through the best fit selection, it is found that the WeiBull model is the best to simulate the sludge drying process compared to other dynamic models.A TG-DSC experiment was designed to observe the pyrolysis characteristics of sludge from two wastewater treatment plant. Coats-Redfern method was used to carry out dynamics calculation concerning pyrolysis of the sludge. When the heating rate is 20K/min, the mean active energy were 45kJ/(mol-k) and 41kJ/(mol·k). Malek optimization method was used to find the most generously function in the mechanism function, and proof of sludge pyrolysis is mainly in accordance with three-dimensional diffusion reaction.In order to understand the effect of low dose microwave radiation on traditional pyrolysis to improve productivity and energy saving effect, TG-DSC experiments were designed to observe the characteristic of sludge treated with various doses of microwave radiation, and compared with the mixture of sludge and biomass pyrolysis characteristics. Coats-Redfern integral method was used to calculate the apparent activation energy, the reaction order and frequency factor of the sludge thermal decomposition reaction. The experiments showed that the reaction depth was improved by the microwave radiation. Microwave-induced sludge samples were treated with the dense of 10 (w/g),20 (w/g) and 25 (w/g) in 10K/min heating rate conditions. And the reductions of mean active energy were 19.2 kJ/(mol·K),2.6 kJ/(mol·K) and 12.7 kJ/(mol·K), an average scale of 24%. On the other hand, the mass-loss rates were increased by a range of 4.6%,5.7% and 11.6%. In lOK/min heating rate conditions, the microwave radiation had slight impact on the reaction rank in the experiments. It was improved that adding 5% of wheat stalk or wood chips could give a boost to the pyrolysis.During the experiments of fixed-bed pyrolysis of sludge, the effect of microwave cracking to the pyrolysis of sludge was analyzed qualitatively through spectrometry and gas chromatography-mass spectrometry analysis and so on. Through comparing the productivity of the tri-state product from the fixed-bed pyrolysis experiments and microwave cracking of the sludge under different ultimate temperature, it was found that the pyrolysis in microwave cracking was easier to proceed and the reaction intensity was strengthened. GC-MS analysis indicated that the bio-oil derived from sludge pyrolysis was dominated by single-ring aromatic compounds and nitrogen-containing aromatic compounds. By contrast, the content of nitrogen-containing aromatic compounds increased in the product of the sludge by microwave cracking. For example, the content of amine and ketone had increased by 152 and 118% respectively. It was also found that the yield of gas was higher under lower final temperature in the pyrolysis of sludge by microwave cracking, but the maximum productivity could not be promoted. The contrast analysis of FTIR spectra indicated that the microwave reduced the contents of organic compounds such as alkanes, olefins, alcohols and ketones in the pyrolysis residue.By measuring the viscosity of municipal sludge, the relationship data of the shear rate and shear stress of sludge were obtained, and the shear-thinning phenomenon was observed. The initial yield stress and critical shear rate were simultaneously solved by piecewise fitting method, and then a full range of sludge flow curve was gained. The results, which numerical simulation of pipe flow sludge through establishing a simplified model, were close to the empirical formula and experimental results of references. And it is found that diameter, flow rate and flow index have significant effects on the pipeline flow. When other conditions remain unchanged, reducing the flow diameter of 33% can lead driving pressure to increase 152.1%, and increasing the flow rate 10 times can lead driving pressure to increase 11 times, as well as increasing the flow index 10 times can lead driving pressure to decrease 20.2%.The Aermod model, K-εmodel and K-ω-sst model were used to simulation and prediction of harmful gas diffusion and pollution. The result of Aermod model showed the most far beyond 600 meters factory boundary concentration exceeded the sewage plant emission standards at the national level, but to meet two criteria. Contrast research discovery, the same source of pollution discharge conditions, calculation using the Gaussian diffusion model as the core of the AERMOD model results than the K-εmodel calculates the value is too large; K-ω-sst model calculation results with the K-εmodel, the former dispersion of pollutants faster.