Sorption-enhanced thermochemical conversion of sewage sludge to syngas with intensified carbon utilization

Highlights: • A novel two-stage sorption-enhanced thermochemical conversion process is proposed. • The CaO-based CO₂ carrying cycle promotes thermochemical syngas production. • A H₂- and a CO-rich gas stream are separately collected during syngas production. • Utilization of sewage sludge carbon is appreciably intensified via CO production. -- Abstract: Efficient transformation of sewage sludge into bioenergy is currently a promising option to combat the energy crisis and mitigate climate change. Most attention has been paid to thermochemical H₂ production, however, effective approaches to utilize the carbon in sludge are lacking. Here we propose a novel two-stage sorption-enhanced thermochemical conversion process, which relies on the integration of a CaO-based CO₂ carrying cycle, to intensify the utilization of sludge carbon. In the process, the CO₂ generated during sludge pyrolysis at the first stage is captured and stored in the form of CaCO₃, and is then released at higher temperatures (the second stage) to gasify the sludge char for CO production. Under the conditions investigated in this study, the proposed process could produce 284.7 NmL of syngas per gram of dry sludge with a gross CO/H₂ molar ratio of 2.3, via obtaining a H₂-rich gas stream at 550 °C and a CO-rich gas stream at 750 °C, respectively. We conclude that the proposed process offers an efficient option for the production of syngas from sewage sludge with significantly intensified carbon utilization.