Waste-to-energy conversion of dried sewage sludge using sorption-enhanced thermochemical technology

Thermochemical conversion of sewage sludge (SS) into bioenergy is a promising and sustainable approach to combat the energy crisis and mitigate the climate change. Among all the products from the thermochemical conversion, syngas, a mixture of hydrogen (H₂) and carbon monoxide (CO), has attracted increasing attention since it is a versatile and flexible platform feedstock for the production of value-added chemicals and fuels via Fischer-Tropsch synthesis (FTS) process. However, conventional thermochemical conversion of biomass mainly focuses on H₂ production, and effective approaches for CO production are still lacking. In this thesis we proposed a novel two-stage sorption-enhanced (TSSE) 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 from SS at the first stage (a lower temperature around 500-600 °C) is captured and stored in the form of CaCO₃ to enhance H₂ production and is then released at the second stage (a higher temperature around 700-800 °C) to gasify the sludge char for CO production. Thus, the syngas production from the TSSE thermochemical conversion of SS is significantly improved in the following two aspects: (1) producing syngas with separated H₂- and CO-rich streams at the first stage and the second stage, respectively, and (2) improving the utilization efficiency of carbon for CO production. The TSSE pyrolysis of SS sample with a CaO/SS mass ratio of 1:1 (Ca/SS-1:1) could produce 284.7 NmL/g_{dry ss} of syngas with the gross H₂/CO molar ratio of 0.4, obtaining 62.4 vol% of H2-rich gas stream at 550 °C and 72.5 vol% of CO-rich gas stream at 750 °C, respectively. The carbon utilization in the SS could reach as high as 20.4% using the proposed TSSE pyrolysis process, and the yield of CO is remarkably higher than that using other conventional sorption-enhanced thermochemical conversion processes.

The maximum conversion of syngas into downstream synthetic products via the FTS process requires a controllable H₂/CO ratio in the syngas to match the usage ratio of the FTS reactors, while the lack of tunable H₂/CO ratio in the syngas limits direct industrial application of SSderived syngas. By the introduction of steam into the first stage of the TSSE steam gasification of SS, the H₂ production at the first stage is significantly enhanced, and the H₂/CO ratio of produced syngas is tunable from 0.9 to 4.7 by controlling the CaO and steam contents. The SS sample with a CaO/SS mass ratio of 3:7 (Ca/SS-3:7) produces the maximum syngas production reaching 323.8 NmL/gdry SS with an H₂-rich gas stream (72.2 vol% purity) at the first stage and a CO-rich gas stream (60.5 vol% purity) at the subsequent second stage. The performance characterization of the TSSE steam gasification process shows a high yield of tar, indicating that the proposed TSSE steam gasification process still has a great potential to promote the decomposition of tar for enhanced syngas production.

The tar in syngas would block and corrode the downstream equipment, restricting the industrial and practical application of syngas. To address the issue of tar removal in the syngas and further enhance the syngas production, the Ni-CaO/Al₂O₃ catalyst and biochar were introduced into the TSSE steam gasification of SS. A synergistic effect of CaO and Ni in the Ni-CaO/Al₂O₃ catalyst was observed to enhance the H₂ production at the first stage, while CaO is a factor of vital importance for CO production at the second stage. The presence of steam and Ni₅Ca₄₀/Al catalyst shows unprecedented performance in enhancing the H₂ production at the first stage due to the promotion of tar cracking/reforming and water-gas shift reaction. Biochar supplements carbon source for CO production, remarkably promoting the CO production at the second stage. Upon introducing steam, Ni₅Ca₄₀/Al catalyst and biochar, a 9.3 times higher yield (396.9 NmL/g_{dry SS}) of H₂ at the first stage was obtained compared to the TSSE pyrolysis of Ca/SS-3:7, and the yield of CO (208.4 NmL/gdry SS) at the second stage triples that without biochar (66.6 NmL/g_{dry SS}). Therefore, there is a complementary effect of steam, Ni-CaO/Al₂O₃ catalyst, and biochar on the enhancement in H₂ production at the first stage and CO production at the second stage. With the introduction of steam, Ni-CaO/Al₂O₃ catalyst, and biochar, the yield of syngas further increases to 645.5 NmL/g_{dry SS} with an 88.2 vol% of H₂ at the first stage and a 55.6 vol% of CO at the second stage, and a H₂/CO ratio of 2 is achieved, which is desirable for the downstream synthesis of value-added chemicals and fuels via FTS process. And the percentage of tar is eliminated by 22.0% and the total gas increases by 42.7% compared to those from the TSSE steam gasification process.

This work develops a new TSSE thermochemical conversion process that has been demonstrated to effectively utilize the carbon in SS for high-purity CO production in addition to the sorption-enhanced production of high-purity H₂, and to achieve the production of syngas with tunable H₂-to-CO molar ratios through the inherent separation of H₂ and CO generation. This technology makes it possible to achieve the waste-to-energy conversion by direct integration of the TSSE thermochemical conversion with the syngas application via FTS process where H₂ and CO could be mixed in desirable ratios for the downstream synthesis of value-added chemicals and fuels.