A novel cascade absorption heat transformer process using low grade waste heat and its application to coal to synthetic natural gas

Energy challenge is becoming more and more serious. Improving energy utilization efficiency seems to be a feasible way to tackle energy problems. A large amount of waste heat is discharged in chemical process industry. It is reported that in a coal chemical process waste heat accounts for 17-67% fuel consumption, of which 60% can be recyclable in theory. Waste heat recovery is a reliable and reasonable way for efficiency improvement. However, the low grade waste heat is difficult to be utilized. Thermodynamic cycles such as organic Rankine cycles (ORC), absorption chillers, absorption heat pumps, absorption heat transformers, and mechanical heat pumps are able to utilize wasted thermal energy for the generation of electrical power, chilling and heat at a higher temperature. In this paper, absorption heat transformer (AHT) is studied and novel cascade absorption transformer (NCAHT) is proposed to recover low grade waste heat. The NCAHT is testified in a coal to synthetic natural gas plant. Results show that the total capital investment of a 12 MW NCAHT is 924,000 USD. Financial Internal Rate of Return (FIRR) is used as an economic evaluation index. The FIRR is 62.16% and the payback time is 0.77 years. When the steam price reduced to 9 USD or the cooling water price higher than 0.96, the FIRR will be less than 30%.