| Author: | Shi, Tao |
| Title: | Sustainable waste valorisation: design, simulation, optimization and decision-making |
| Advisors: | Ren, Jingzheng (ISE) Lee, Carman (ISE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Sewage sludge Sewage disposal Sewage sludge -- Recycling Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Industrial and Systems Engineering |
| Pages: | xxi, 246 pages : color illustrations |
| Language: | English |
| Abstract: | As the rapid growing population associated with increasing quantity of municipal solid waste, it has become increasingly urgent to effectively address the waste-derived problems to promote the sustainable development of cities. In Hong Kong alone, approximately 1,200 tons of sewage sludge are generated daily from the 12 major sewage treatment plants, while in mainland China, the quantity reaches around 3.89 million tons. It is concerning that a majority of the sewage sludge is currently managed through landfilling and incineration methods, which give rise to environmental and social issues, including extensive land occupation, significant carbon emissions, and the potential permeation of waste inorganics. Therefore, the waste sewage sludge that produced during the sewage treatment facilities has been the research focus to answer the questions of what the sewage sludge treatment status is, how to treat the waste more environmentally friendly, and which treatment schemes may be the best choice for the residents and the government future implementations. To better indicate the utilization of waste sludge, a preliminary step is to get insights about the waste sludge production, current sewage sludge utilization, some potential waste-to-energy technologies and the associated challenges. It has been found that in the context of Hong Kong, incineration for power generation plays an important role in the reduction of sewage sludge volume, landfilling replacement and environmental protection as the launch of fluidized-bed incinerator in Tuen Mun district. However, the carbon emissions after the post-combustion of waste sludge have to be further considered for achieving the carbon neutrality at source and manufacturing sessions. In addition, there is a lack of works that comparatively studied the conventional incineration-based process with chemical looping combustion (CLC) technology and the new supercritical water gasification (SCWG) process for the sludge-to-power processes. Take the fluidized-bed incineration with carbon emission design as a benchmark, a comparative energy-economic-environmental analysis of different sludge-to-power processes are investigated firstly to start up the development of waste sludge-to-energy. Results show that the CLC-based scheme performs the best from the perspective of economic criterion as well as the environmental indicator but not good in the energy efficiency. CLC has the superiority in both carbon separation and fuel combustion in different reactors while other power generation scheme requires individual chemical solvent absorption which is highly energy intensive. Nevertheless, the separated reactor design means a low energy efficiency due to additional exergy destruction. Implications may be obtained that constructing a new waste combustion plant by CLC technology could be a good choice for carbon capture and carbon emissions reduction. Promoting the sustainable waste sludge treatment does not only be limited to the conventional waste sludge incineration scheme, some innovative sludge valorisation for value-added chemicals production should be also studied. Among these chemicals, methanol as a versatile chemical with high energy density and good transportation feature which can be potentially used to replace the auto fuels has attracted worldwide attentions. The conventional chemical production pathway of methanol is relatively mature and reliable nonetheless it greatly depends on the nonrenewable fossil fuels and causes large quantity of carbon emissions. Based on the abovementioned issues, a complete sludge-to-methanol process is also studied to get a deep understanding of the energy-economic-environmental performance. The whole process is constructed based on the robust thermodynamical-based process simulation with gasification model validation. The economic estimation revealed the methanol production cost is around 579.62$/ton. Compared to the current fossil fuel-to-methanol process (100-250 $/ton), the production cost under nowadays technical status is not so competitive and attractive as external subsidy is required if the local government want to support the process implementation. On the other side, a demonstration plant of sewage sludge-to-methanol would be a good choice for developed cities with huge quantity of sewage sludge resources because of the long-term research and development. The current work can serve as a reference for the future sludge treatment planning. One of the big challenges during the emerging waste valorisation design is the determination of suitable operational parameters. In terms of the sludge-to-power, the operational parameters can be obtained from previous publications and the chemical engineering experiences which may not be always effective. As involving more objectives during the process optimization, the problem solving would be more difficult due to the computation burden and simulation complexity. Therefore, it is necessary to address the process optimization problem for such complex waste valorisation schemes by introducing a composite sustainability index. The sustainability index objective can include multiple assessment aspects (i.e., energy, economic, environmental, and safety) thus the process optimization can achieve the sustainability-oriented process design. The methodology framework is verified in the sludge-to-hydrogen process design because the hydrogen fuel is environmentally friendly. Detailed energy-economic-environmental-safety assessment shows that the process is not competitive regarding the hydrogen production cost of 6.66$/ton. To some extent, proposed waste sludge-to-hydrogen scheme needs to be further improved. Sensitivity analysis implies that subsidy fees, H2 market price, and total equipment cost are significant factors in the financial profit. Subsequent study is a make-up for the process optimization framework with the aim of improving the optimization efficiency. By incorporating the machine learning methods, robust mechanism-based models can be replaced by artificial neuro network (ANN) models. ANN regression model in the process optimization has shown its superiority according to the solving time and operational parameters comparison. Eventually, we have comprehensively studied various ways including the sludge-to-power, sludge-to-methanol, and sludge-to-hydrogen shown in the previous chapters. However, for promoting the sustainable valorisation of sewage sludge, there is a final knowledge gap in the waste valorisation domain that which way is most recommended for practical implementations among the comprehensive options considering both soft and hard criteria. As such, the last work of thesis aims to conduct a decision-making analysis by considering more sustainability criteria including technical, economic, environmental, and social criteria. The quantification of hard criteria are obtained from the process simulation and evaluations while the soft criteria are took into account by collecting the opinions from some experts in this area. The final results reveal the advancement of the emerging CLC technology for power generation meanwhile we do not recommend the conventional incineration strategy based on the established criteria system. Throughout the study, we follow the system thinking and achieve the comprehensive study on sustainable waste valorisation that starting from the primary thermochemical conversion, complete waste-to-energy process development and optimization, to final multicriteria decision-making analysis. The detailed energy-economic-environmental-safety data of developed sludge processes can provide a guideline for future planning of waste treatment facility construction. In addition, the last decision-making assessment find out the great potential of CLC technology and the significance of value-added methanol production in waste utilization. |
| Rights: | All rights reserved |
| Access: | open access |
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