Abstract:

A new kind of coal gasification based on Zn/ZnO redox pair via a two-step cycle is proposed.Firstly，coal is mainly gasified to CO by reacting with ZnO in a highly endothermic step at about 1200℃.Secondly，in an exothermic step at a lower temperature of about 500℃，hydrogen gas and solid ZnO are produced via the hydrolysis of liquefied zinc produced from the first step.The produced ZnO can be then recycled to the first step.In this paper，both the auto-thermal reactor with partial oxidation of coal and solar reactor were considered as heat supply in the first step.The theoretical energy analysis of the new system based on the 1st and the 2nd law of thermodynamics was performed.The result showed that the system could reach an ideal thermal efficiency as high as 89% when equipped with the auto-thermal reactor，and 67% when solar reactor was used considering the current thermal efficiency of solar collector.The relative exergy efficiencies were 80% and 66% respectively.The major sources of exergy losses came from the irreversibility of reaction in auto-thermal reactor，and heat losses from solar reactor.A simple environmental evaluation for the system was also performed based on the ratio of produced CO to H₂（CHR）in the system.Apparently，the solar heated system was more environmentally friendly than the auto-heated system，the CHR of the former was smaller than 1/6 of the latter.