A sustainable syngas cryogenic separation process combined with ammonia absorption refrigeration pre-cooling cycle

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With the goal of effectively utilising low-grade waste compression heat by investigating potential improvements in energy efficiency, this study proposed a novel syngas liquefaction process combined with an ammonia absorption refrigeration cycle driven by available low-grade compression heat. The systems were evaluated from thermodynamics, as well as techno-economic aspects, to reduce the energy consumption and greenhouse gas emission. This study attempted to tackle two vital issues: how to effectively convert low-grade compression heat to cooling capacity to improve energy efficiency and design refrigeration cycles (both ammonia absorption refrigeration cycle and main refrigeration cycle) tailored with Heat Integration. The process was modelled, simulated, and optimised, aiming to reduce refrigerant and energy. The case study results demonstrated that the syngas liquefaction process with ammonia absorption cycle dramatically reduced the refrigerant consumption by 15.3%, contributing to a 7 MW reduction in electricity consumption. The specific energy consumption was decreased from 3.2 to 2.36. The coefficient of performance increased from 0.11 to 0.14. As for the environmental effect, the total greenhouse gas emissions were decreased from 0.12 Mt CO2-eq to 0.09 Mt CO2-eq. The exergy analysis showed that the new process decreased the total exergy destruction by 6.7 MW, leading to a higher exergy efficiency of 54.5%. In view of the economic performance, the total capital investment and total product cost were lowered by 10.8% and 20.1%. The proposed syngas liquefaction process exhibited the significant potential to improve environmental, thermodynamic, and techno-economic performances. The sensitivity analysis showed that the product cost of the new process was less sensitive to the variation of the electricity price.