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

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posted on 2023-02-24, 01:33 authored by Xiaoxia Yang

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/gdry 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/gdry 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/gdry 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/gdry 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.


Table of Contents

Chapter 1. Introduction -- Chapter 2. Literature review -- Chapter 3. Sorption-enhanced thermochemical conversion of sewage sludge to syngas with intensified carbon utilization -- Chapter 4. Tunable syngas production from two-stage sorption-enhanced steam gasification of sewage sludge -- Chapter 5 Syngas production from co-gasification of sewage sludge and biochar using two-stage sorption-enhanced catalytic thermochemical conversion process -- Chapter 6. Conclusions and future work

Awarding Institution

Macquarie University

Degree Type

Thesis PhD

Department, Centre or School

School of Engineering

Year of Award


Principal Supervisor

Yijiao Jiang


Copyright: Xiaoxia Yang Copyright disclaimer:




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