Techno-economic optimisation and life cycle assessment of alternative technologies for converting municipal solid waste into chemicals and electricity in the UK

This research aims to develop a systematic life cycle assessment approach for alternative technologies for converting UK’s municipal solid waste (MSW) to energy and chemicals. The study focuses on seven scenarios, which are combustion, gasification, methanol synthesis, indirect dimethyl ether (DME) synthesis, direct DME synthesis, solid oxide fuel cell (SOFC) and methanation. The LCA model is developed based on one ton of MSW where three waste compositions are considered. The environmental impacts are evaluated by SimaPro software using IMPACT 2002+ method and Ecoinvent 3 as database. This work does not consider on the MSW collection and transportation, the plant construction and uncertainty of the MSW. The syngas composition employed in this study is predicted from a non-stoichiometric equilibrium gasification model using Lagrange multiplier and Gibb’s free energy minimisation approach, which is simulated in MATLAB software. Moreover, a dynamic kinetic model based on the reactions in the gasification is also developed and simulated. The model output is compared to the experimental results and the parameters enhanced using two estimability analysis approaches. The environmental impact results show that the SOFC process has the least impact on most end-point categories for the base case of the waste composition (MSW 1) because it can generate the highest amount of electricity. The other two waste compositions are also compared and found that in case of the second MSW composition (MSW 2), SOFC is in the most environmentally friendly based on the end-point impact categories. For the third composition (MSW 3), the direct DME synthesis becomes the top technology in the human health, whereas SOFC is still the top in the climate change and resource categories. The methanation always has the lowest ecosystem quality impact. A sensitivity analysis is performed by increasing the total yields for methanol, indirect and direct DME synthesis. It appears that when the total yield increases from 0.4 to be 0.6, all three processes have lower global warming (GW) impact, because there are more carbon atoms converted to methanol and DME in the process, but still more than SOFC process. A techno-economic analysis is also carried out for each scenario. It is found that the net present value (NPV) from SOFC process are always higher than other process when the waste capacity is more than 160 kilotons per annum for MSW 1 and MSW 2. However, the direct DME synthesis has higher NPV than the SOFC in case of MSW 3. Lastly, a multi-objective optimisation is performed to help achieve a more holistic decision making on these scenarios by using ϵ-constraint method that consists in

maximising the NPV and minimising the global warming potential (GW). The results confirm that the SOFC is the optimal technology for both MSW 1 and MSW 2 (in the capacity ranges of 84-481 kton MSW 1 per year and 92-602 kton MSW 2 per year). For MSW 3, SOFC is the optimal process in the range between 36-103 kton CO2 eq. per year (capacity 59-168 kton MSW 3 per year) and the direct DME synthesis is the optimal technology in the range between 214-678 kton CO2 eq. per year (capacity 218-691 kton MSW per year).