Study Of Western Coal Chemical Looping Hydrogen Generation With Iron-based Oxygen Carrier

The continued consumption of fossil fuels has caused increasing pollution.It is the focus of today's society,so the concept of"Hydrogen Economy"is proposed.The technology of hydrogen production,to a large extent,affects the sustainable development of the"Hydrogen Economy".In China,coal reserves are abundant and there are many ways to use coal.The technology for clean and efficient use of coal has attracted wide attention.Based on the chemical looping technology?CLC?,the coal chemical looping hydrogen production?CLHG?process was developed.It has the characteristics of high conversion efficiency,low cost and little impact on the environment.The current research on CLHG mainly focuses on three aspects:the feasibility of different types of fuels,the design and preparation of oxygen carriers and the optimization of the reaction process.In this paper,CLHG experiments of coal with low-rank lignite as the raw material were performed.An iron-based oxygen carrier loaded with different metals was designed and prepared,and NaAlO₂ modified iron-aluminum oxygen carrier has the best reaction performance.In a fixed-bed reactor,the feasibility of a two-reactor double-cycle coal chemical looping hydrogen production process was studied.These works are of great significance for the practical application of the coal chemical looping hydrogen production process.The thesis mainly includes the following three parts:?1?The Fe₂O₃/Al₂O₃ composite metal oxygen carrier was prepared using impregnation method.Zhaotong lignite was used as raw material,and the chemical looping hydrogen production of coal were carried out in a fluidized bed.The effects of operating temperature,mass ratio,steam flow rate and coal water content on the hydrogen production process were investigated.The optimal operating conditions for low-rank lignite coal direct chemical looping hydrogen production were as follows:reduction period temperature was 900°C,temperature of steam oxidation period was 900°C,the COAL/OC mass ratio was 3g/12g,and the steam flow rate was 0.5 g·min^-1. Under these optimal conditions,the carbon conversion achieved 91.6%,the H₂ yield was 0.87 m³·kg^-1?daf?,and the H₂ concentration was 84.9%.After ten cycles,the oxygen carrier still has good reaction performance.?2?A series of experiments with respect to the CLHG process using sodium modified Fe₂O₃/Al₂O₃ was conducted in a batch fluidized bed reactor.When the Coal/oxygen carrier?OC?mass ratio was 0.05,the carbon conversion and hydrogen yield were the highest,and the OC was primarily reduced to Fe²⁺.In order to improve the reaction performance of oxygen carriers,Mo-Fe-Al,Cu-Fe-Al,Ca-Fe-Al,Na-Fe-Al and Ca-Fe oxygen carriers were prepared by impregnation method and co-precipitation method.Here,Na0.5Fe4Al6 presented the best reducibility for hydrogen generation when compared with the different addition amounts.This is attributed to the addition of NaAlO₂,which has a high melting point and can generate NaFeO₂,as seen in the XRD analysis results.Moreover,its unique layer structure can loosen the structure between Fe₂O₃ and Al₂O₃.Thus,the surface structure and pore structure were improved,which are responsible for the enhancement in reactivity rather than the catalytic effects of the alkali metal.During multi-redox tests,the cycle performance of Na0.5Fe4Al6 remained stable after eight-redox cycles,and no serious sintering phenomena was observed;in addition,the carbon conversion and hydrogen yield remained above 86%and 1.40 L?g^-1,respectively.?3?The dual-cycle coal chemical looping hydrogen production?Dual-CLHG?process uses only a fuel reactor and a steam reactor to complete the circulation of the oxygen carrier.Compared with the CLHG process of the three reactors,the air reactor is omitted.Experiments showed that Dual-CLHG with Fe₃O₄ as an oxygen carrier can achieve high carbon conversion and produce high-purity hydrogen.The hydrogen yeild was 1.29 L·g^-1,which is 19.4%higher than the CLHG process of the three reactors.During the cycle,carbon deposited on the surface of the oxygen carrier blocked the pore structure.Meanwhile,the high temperature leaded to sintering and agglomeration of the oxygen carrier.These factors caused the decline of the oxygen carrier cycle performance.