Sludge Pretreatment Combined With Catalytic Supercritical Water Gasification Of Sewage Sludge For Hydrogen Production

The municipal sewage sludge,the end product of biological treatment in wastewater treatment plant.It is of great significance to carry out effective treatment and resource utilization.At present,supercritical water gasification technology has become a new technology of sludge treatment,because it is not limited by water content and can convert organic matter in sludge into combustible gas containing hydrogen.But there is a large amount of inorganic ash in sludge,which not only increases the energy consumption of heating,reduces the gasification efficiency of organic matter for H₂production,but also easily causes problems such as reactor blockage.In view of these problems,this paper put forward the technology for H₂production that sludge pretreatment combined with catalytic supercritical water gasification.The organic matter in sludge was transferred to liquid phase by pretreatment,and only the liquid products that riched in organic matter were catalytic gasified to produce H₂,thus eliminated the influence of inorganic matter.The effects of different pretreatment methods（hydrothermal liquefaction,thermal-alkaline hydrolysis）on the transformation of organic matter in sludge were explored in this paper,and the hydrogen production characteristics of combined treatment technology for H₂production were investigated.From the perspective of sludge organic matter composition,the mechanisms of combined treatment technology for promoting H₂production were clarified and verified based on the sludge model biomass experiments.The major conclusions of the present study can be summarized as follows:（1）The combined treatment technology of hydrothermal liquefaction pretreatment combined with catalytic supercritical water gasification was put forward,and the influence of hydrothermal liquefaction pretreatment on the transformation of organic matter in sludge was determined.The organic matter in sludge was continuously transferred with the increased of liquefaction temperature and liquefaction time.The optimal hydrothermal liquefaction pretreatment condition of 250°C,1 h.Under this condition,the TOC concentration in liquid products reached 15,255 mg/L,about 87%of carbohydrates,62%of humus and 49%of proteins in sludge were transferred.Compared with the direct catalytic gasification of sludge,this combined treatment process that hydrothermal liquefaction combined with catalytic supercritical water gasification greatly improved the H₂yield and H₂selectivity with using using Al Cl₃,KOH,K₂CO₃and Ca O as catalysts.The yield of H₂increased by 17.1%-120.8%and the H₂selectivity increased by 2.04-13.45 times with different catalysts（except Ca O）.（2）A combined treatment technology of thermal-alkaline hydrolysis combined with Catalytic supercritical water gasification was put forward,and the influence of thermal-alkaline hydrolysis pretreatment on the transformation of organic matter in sludge was investigated.The organic matter in sludge was continuously transferred with the increased of hydrolysis temperature,time and alkali addition.The optimal thermal-alkaline hydrolysis pretreatment condition of 180°C,1h,m _{Na OH}:m _OM=25%.Under this condition,the TOC concentration in liquid products reached about 16000mg/L,and the concentrations of soluble protein and polysaccharide were 15000mg/L and 2400mg/L,respectively.At the same time,about 70%of proteins,50%of humus,70%of carbohydrates and 35%of lipids in sludge were transferred.Compared with the direct catalytic gasification of sludge,this combined treatment process that thermal-alkaline hydrolysis combined with catalytic supercritical water gasification greatly improved the H₂yield and H₂selectivity with using using Al Cl₃,KOH,K₂CO₃and Ca O as catalysts.The yield of H₂increased by 172.46-661.11%and the H₂selectivity increased by 4.94-51.92 times with different catalysts.（3）To explore the mechanisms of promoting hydrogen production by the two combined treatment technologies,oleic acid,starch,bovine serum albumin,fulvic acid and dealkalized lignin were used as sludge model biomass for direct gasification and combined gasification respectively.Fulvic acid,starch and bovine serum albumin were easily gasified,while lignin and oleic acid were gasified difficultly.The two combined treatment technologies both improved H₂yield and H₂selectivity to a certain extent,and the H₂yield could be increased by 114.11%at the highest.The essence of sludge pretreatment is the screening process of organic matter in sludge.A part of easily gasified organic matter（such as proteins,humus and carbohydrates）were transferred to the liquid phase after pretreatment,while another part of hard gasified organic matter（such as lipid and lignin）still remained in the solid phase.In addition,the pretreatment process also promoted the degradation of complex macromolecular organic matter in sludge into small molecular organic matter,which were easy to produce H₂during the subsequent supercritical water gasification process.So the combined treatment technology significantly improved the H₂ yield.