Applications Of Coal Char To Hydrogen Production By Methane Decomposition

Catalytic methane decomposition?CMD?is a clean process for hydrogen production.The two products with gas and solid phases could be easily separated and thus the process is simple.The solid product carbon has the potential economic value,which would greatly reduce the cost of hydrogen production by methane cracking.Both transition metals and carbon materials can be used for the process of methane decomposition,but compared with the metal catalysts,carbon material catalyst has great advantages.As a special carbon material,the coal char?CC?sample?known as blue coke?is overcapacity especially in northern of Shaanxi.In this paper,methane decomposition was conducted by using K₂CO₃/CC hybrids as the catalysts.On the one hand,porous carbon can be obtained from CC by K₂CO₃ activation,and on the other hand,co-production of hydrogen-rich gas and carbon fibers can be obtained.Firstly,effects of the reaction temperature and space velocity on methane decomposition were investigated.The results showed that the appropriate reaction temperature is 850?and the methane space velocity is 900 mL/(h·g_CC).Under the optimized conditions,hydrogen-rich gas and fibrous carbons can be simultaneously obtained.During the reaction process,K₂CO₃could play a positive role on the formation and growth of fibrous carbons by enhancing the oxygen transfer at the higher temperature.The increased feed can lead to the formation of stubby fibrous carbons.In addition,K₂CO₃ plays a positive role on methane activation and conversion and even on the activation of the carbons?including the initial CC sample and the produced carbons?.However,the process also brings some K₂CO₃ consumption and loss.And thus,less stable methane conversion was found along with the increasing loss of the introduced K₂CO₃.In order to reduce the influence caused by the loss of K₂CO₃ during the reaction,the CMD was proposed with addition of some CO₂ as a co-feed.The results show that the introduced amount of CO₂ has great impact on the methane conversion,H₂/CO molar ratio and the produced carbon morphology.But the CO₂ conversion under the given condition keeps as high as 98%.This work takes the CH₄/CO₂ feed ratio of 75:15 as an example to carry out the qualitative and quantitative analysis and found that the process is different from the traditional methane reforming with CO₂.Similarly,the process is neither the sum of methane decomposition and CO₂ gasification.The introduced CO₂ as co-feed could enhance the oxygen transfer cycle during the reaction,reducing the loss of K₂CO₃,thus enabling the catalyst to show higher activity and stability.In addition,comparing to the catalytic activity of K₂CO₃ and CC themselves under the experimental conditions,it is found that there is synergy between K₂CO₃ and CC.Comparing with inert supports,such as MgO,CaO and Al₂O₃,CC can be used as an effective support of Ni-based catalysts,but the catalyst is easily to become deactivation because of the produced carbons.Based on the good activation of K₂CO₃ on the carbons mentioned above,several Ni-K based catalysts were also prepared and used for CMD.It was found that the stability of the catalyst can be improved and the formation of fibrous carbons can still be obtained on the catalyst surface.