Techno-economic assessment of a waste hydrogen recirculation system

Today, 97 Mton of hydrogen are consumed globally in the industry sector. The International Energy Agency has estimated that 6% of the global energy consumption of the industrial sector is used to produce H2 causing the emission of more than 900 Mton of CO2 in 2021. Thus, decarbonizing the industry sector requires the development of innovative solutions for the sustainable H2 production and reuse. This study focuses on a real industrial application: a semiconductor production plant using H2 as a gas carrier is analysed. The plant requires around 155 Sm3/h of H2 (5.0 purity) and it operates 8712 hours per year. Currently, H2 is produced from fossil fuels and it is supplied regularly by trucks. After the production processes, the H2 is contaminated with some ppm of HCl and SIHCl3 and some ppb of doping agents. At present, H2 is treated to reduce the pollutant concentration and then it is emitted into the atmosphere. In order to evaluate the feasibility of reusing the waste H2 into the production cycle, a techno-economic analysis of a recirculation system capable of collecting and purifying the contaminated H2 is carried out. Firstly, the different technologies for the H2 purification available on the market (Palladium Membranes, ElectroChemical H2 Compressor (ECHC) and Pressure Swing Adsorption) are reviewed and analysed. Then, the Pd Membranes are identified as the most promising solution and the purification plant is designed. The proposed system consists of Pd membranes, compressors and heat exchangers. These components are required to correctly operate the purification system at 350 °C and supply H2 at 20 barg to the industrial process. The system is able to purify H2 up to 5.0 grade but it has an efficiency of 95%, thus it cannot fully recover the waste gas and new H2 must be added regularly. The economic feasibility of the proposed solution is then assessed by performing a discounted cash flow analysis and evaluating the Net Present Value (NPV) and the PayBack Time (PBT). The investment costs are determined basing on the actual quotation from the manufacturers and adopting the NETL capital cost estimation methodology. The operating costs account for the natural gas for heat production, the electricity consumption for the compressor and the replacement of the membranes occurring every 5 years. The revenues are generated by the reduction in the quantity of 5.0 purity H2 that is purchased yearly. The outcomes are encouraging, with positive NPV values after 20 years of plant life and reasonable PBT. Then, a sensitivity analysis is conducted to understand the profitability of the plant as function of the H2 cost. The results show that the higher the price of H2, the higher the savings and thus the shorter the PBT. Finally, the CO2 savings generated by recirculating the waste H2 are estimated. The H2 used in the plant is produced by steam methane reforming with carbon intensity of 9.3 kgCO2eq/kgH2. The net CO2 reduction is calculated by considering the savings associated to the production and the new emissions from natural gas and electricity consumptions. For completeness, a similar approach is adopted to determine the CO2 saved if H2 were produced by coal gasification. The results show that it is possible to reduce the CO2 impact of the plant by using this recirculation system. The same methodology is applied to an EHCP system to compare the performances of the two systems. The results confirm that EHCP is not yet a mature technology due to higher operating costs and lower efficiency.