Combining thermal–alkaline hydrolysis pretreatment with catalytic supercritical water gasification for hydrogen production from sewage sludge

Abstract

An integrated process consisting of a thermal–alkaline hydrolysis pretreatment (TAHP) combined with catalytic supercritical water gasification (CSCWG) was developed to treat sewage sludge to produce H2. TAHP facilitated the transfer of organics from sludge into liquid products for subsequent CSCWG to produce H2. Upon increasing the hydrolysis temperature, time, and alkali dosage, the continuous transfer of organics to liquid products was observed, wherein the hydrolysis temperature had the greatest effect. Under the optimal pretreatment conditions (180 °C, 1 h, and mNaOH/mOM = 25 %), approximately 70 % of proteins, carbohydrates, and 50 % of humus in sludge, i.e., easily gasified organic species, were transferred. Using this process, the TOC concentration in the liquid products reached about 17,000 mg/L, and the polysaccharide and soluble protein concentrations reached 2400 and 15,000 mg/L, respectively. With this integrated process, the H2 yield and selectivity increased by 3.7–7.6 times and 5.94–52.92 times, respectively, compared with the directly catalyzed gasification of sludge. This improvement occurred because TAHP promoted the transfer of easily gasified organic species into the liquid products, while the difficult-to-gasify organics and inorganics in ash remained in the solid phase, which prevented them from inhibiting the catalyst.