Integrated design, optimisation and evaluation of green hydrogen production by solid oxide electrolysis cells and pure-oxygen combustion for carbon neutrality

Abstract

To address the urgent issue of global warming, the chemical production requires decarbonization due to its significant greenhouse gas emission. Given existing technological constraints such as the limited temperature in electric heating and insufficient installation capacity of Carbon Capture Utilisation and Storage (CCUS), this dissertation introduces an innovative approach. An integrated system of Solid Oxide Electrolysis Cells (SOEC) and H2-O2 combustion is employed for low-carbon, high-temperature heat supply. This work demonstrates a promising solution through water electrolysis for green hydrogen production, whose electric efficiency is much higher than current commercialised technology such as alkaline electrolysis and proton exchange membrane electrolysis, and the utilisation of oxygen byproduct as a combustion agent. The impact of various operational parameters, such as SOEC temperature, current density, and combustion product temperature, on the system's energy efficiency is analysed. It identifies the optimal conditions for achieving thermoneutral or endothermic states, emphasizing the critical role of minimising ohmic and anode activation overpotentials. This analysis underscores the importance of converting electricity directly into the heat value of hydrogen rather than thermal energy, optimising the overall energy consumption and efficiency of the process. In addition, the practical challenges associated with implementation, especially the steam generation for hydrogen production and cooling water injection, are addressed. The energy demand during the separation and purification of hydrogen after electrolysis is analysed, which benefits the pressure increase for combustion. Additionally, this contribution explores the design of a water supply loop and the energy demands of evaporation, presenting a comprehensive case study that considers the heat pump system for waste heat recovery and steam generation, which saves 63% of previous thermal energy. The findings prove the feasibility of clean fuel replacement in industrial applications, keeping the production performance of cracking furnace stable. Additionally, life cycle assessments and economic evaluations reveal the lower global warming potential of the proposed system and its superior economic competitiveness, especially compared to existing natural gas with CCS and emerging electrochemical CO2 conversion in a large-scale industrial setting. The proposed system significantly reduces the global warming potential by 20.66% compared to CCS over a 25-year period. In addition, it demonstrates that the integration is not only environmentally friendly, including lower acidification, eutrophication, particulate matter formation, and photochemical oxidants creation potentials, but also more economically viable under various carbon and power price scenarios. For the target in the Paris Agreement that controlling greenhouse gas emission by 2050 for limited global temperature increase, the proposed technical pathway is more practical and easier to apply in large scale than electrochemical conversion. Finally, renewable energy storage solutions are explored to support the proposed decarbonization strategy. he electrification of certain devices is implemented to align with chemical production driven by renewable energy. A mixed-integer nonlinear programming model is introduced to assess the optimal balance between decarbonization effects and cost-efficiency. Physical hydrogen storage, including liquefied and compressed hydrogen, is compared with Compressed Air Energy Storage (CAES), and Photovoltaic (PV) power supply in Qinghai is used as a case study. The results indicate that CAES has the lowest SOEC cost, but when considering the total costs, including PV panels and energy storage units, CAES is the most expensive option, while compressed hydrogen is the cheapest. The decarbonization performance analysis reveals that even without any energy storage, clean fuel replacement and electrification can reduce CO2 emissions by 38.33%. However, as more energy storage units are applied, costs increase rapidly, making the goal of fully transitioning to clean fuel infeasible. In a mixed scenario where full hydrogen is supplied in summer, CO2 emissions are reduced by 62.85%. The study also emphasises that the type of renewable power storage in the electrical grid should be determined based on the specific energy demand of consumers, to avoid unnecessary energy loss during multiple stages of energy conversion.

Date of Award	9 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robin Smith (Co Supervisor) & Nan Zhang (Main Supervisor)