Design and exergy analysis of highly efficient chemical looping reforming processes of coking dry gas in double and triple reactors

Abstract

Coking dry gas is a kind of refinery dry gas, which can be used as a feedstock for hydrogen production, but its steam reforming has the problems of high energy penalty and CO2 emissions. Therefore, novel processes of coking dry gas chemical looping reforming to produce CO and H2 using Fe-Cu as oxygen carrier were proposed in this study. Firstly, two processes of two-reactor for syngas production and three-reactor for syngas-high purity hydrogen production using external-heating were designed and the effects of the temperature difference between fuel and air reactor and the heat carrier addition were studied. When the temperature difference is increased to 150 °C, the exergy efficiencies of the two processes are 70.87 % and 77.13 % and the fuel reactors both produce 2.21 kmol syngas per kmol dry gas. While the inclusion of a steam reactor in the three-reactor system generates an additional 0.61 kmol high-purity hydrogen per kmol dry gas, thereby achieving higher efficiency compared to the two-reactor system. Heat carrier was used to fully transfer the heat from the air reactor to fuel reactor to obtain two auto-thermal systems. Their exergy efficiencies are as high as 84.39 % and 84.58 % and the fuel reactors both produce 3.02 kmol syngas/kmol dry gas, while the high-purity hydrogen production of three-reactor auto-thermal system is only 0.05 kmol/kmol dry gas. Obviously, the external-heating three-reactor process has notable advantages compared to the two-reactor. For auto-thermal systems, this work suggests developing high-temperature resistant Fe-Cu composite oxygen carriers and heat carriers with larger heat capacity to decrease the usage of heat carriers.