Enhancement of hydrogen production in steam gasification of sewage sludge by reusing the calcium in lime-conditioned sludge

Gasification is a promising alternative process for sewage sludge energy utilization. CaO has been identified as an effective additive which can increase H2 content of syngas produced by coal, biomass, and sludge gasification. Considering that lime (CaO) is a widely applied conditioner for sewage sludge dewatering in filter press, this study investigated the enhanced efficiency of syngas, especially regarding H2 yield, in the catalytic steam gasification of dry dewatered sludge with physically mixed CaO and dry sludge dewatered with CaO as conditioner. The experiments were conducted in an electrically heated reactor at 873 K, 973 K and 1073 K, respectively. According to the results, conditioner CaO improved the H2 and syngas production more remarkably than additive CaO. It was identified by XRD and SEM-EDX that conditioner CaO was completely converted into Ca(OH)2 while additive CaO was still presented mainly as CaO. Furthermore, the Ca species of conditioner CaO was evenly distributed over the sludge matrix while Ca species of additive CaO maintained the original state with uneven distribution, both of which could increase the formation of H2 through interacting with produced gas and catalyzing thermal cracking of tar to some extent. In addition, the pore structure tests and XPS analyses revealed that, comparing to additive CaO, conditioner CaO was more favorable for the formation of pores, and it had a greater potential to encourage partial cleavages of C–C bonds and C–H bonds, resulting in the decomposition of organic macromolecules into relative small molecules, which might be more easily converted to the gaseous products. These indicate that it is valuable to reuse the Ca in lime-conditioned sludge during gasification process.