| China is the largest producer and consumer of coal all over the world.The low rank coal,accounting for 57.4% of coal resources in China,is an important source for the energy supply in future.Low rank coal has a low degree of coalification and a high content of alkali and alkaline earth metal(AAEM).AAEM will have significant impacts on thermal utilization process of coal.On the one hand,AAEM will catalyze the pyrolysis process of low-rank coal.On the other hand,residual AAEM in char will affect the reactivity of char.Therefore,the pyrolysis characteristics of low rank coal represented by Zhundong coal were investigated,especially the catalytic and migration characteristics of AAEM in the process of pressurized pyrolysis of coal.This paper provides theoretical basis for the wide,efficient and clean utilization of low rank coal.On the basis of adequately referring to the research on the wire mesh reactor home and abroad,the pressurized wire mesh reactor(PWMR)experimental system is designed and built.The basic structure,gas pipeline system and temperature control system of PWMR are described in detail,and the debugging and operation process are introduced in this paper.The output characteristics of high frequency pulse wide modulation(PWM)switching power supply can increase the requirement of limit heating rate of PWMR to 3300 °C/s.The temperature regulation algorithm designed on this basis can accurately control the heating rate within 1000 °C/s.The construction of PWMR system provides the experimental basis for this research.The relationship between the structure and reactivity of coal is an important research content in the field of coal chemistry,it is also the basis for the development and progress of many coal processing and conversion technologies.In order to clearly characterize the coal structure at the molecular scale and effectively predict the coal reactivity,a series of infrared spectrum structural parameters that could reveal the structural changes of different components in coal at the molecular scale are constructed in this paper.The relationship between the infrared spectrum structural parameters and pyrolysis reactivity is also established.The results show that with the increase of the degree of coalification,the total content of oxygen-containing functional groups in coal gradually decreases,the content of C-H functional groups first increases and then decreases,and the length of aliphatic side chain and bridge bond undergoes a process of stability-increase-decrease.These structural changes can be effectively characterized by the defined infrared spectrum structural parameters.Based on the above infrared spectrum structural parameters,a comprehensive infrared spectrum structural parameter S is built,and a linear relationship between S and the pyrolysis reactivity is established,so as to achieve the purpose of predicting the pyrolysis reactivity of coal.It provides a theoretical basis for the selection of subsequent research methods in this paper.In this paper,chars pyrolyzed under different conditions are prepared by pressurized thermogravimetric analyzer(PTGA)and PWMR,and the influence of AAEM with different occurrence forms on the structure of Zhundong coal char is studied.The increase of pyrolysis temperature lead to the release of functional groups in char,and the release of functional groups increases at the low heating rate.At different pyrolysis temperatures,the influence of pyrolysis pressure on the transformation of functional groups in char is different.When the pyrolysis pressure is below 1MPa,the pressure has a significant impact on the carbon skeleton structure of char,which is mainly manifested as the promotion of graphitization of char samples.However,the phenolic hydroxyl group,ether and aliphatic-aromatic ratio in char are not significantly affected by the pyrolysis pressure.When the pyrolysis pressure is over 1MPa,the water-soluble AAEM inhibites the decomposition of phenolic hydroxyl groups,and the ion-exchangeable AAEM inhibites the decomposition of carboxyl groups and catalyzes the cracking of long aliphatic chains.AAEM in coal will migrate during pyrolysis process.In this paper,the volatilization of AAEM with different occurrence forms in Zhundong coal is investigated.The AAEM in Zhundong coal is mainly composed of Na,Mg and Ca.The proportion of insoluble AAEM in Zhundong coal is very low,and its thermal stability is very high.Therefore,there is no obvious volatilization behavior in the whole pyrolysis process.The ion-exchangeable AAEM is mainly composed of Mg and Ca,which is the main form of AAEM in Zhundong coal,and the water-soluble AAEM is mainly Na.When pyrolysis temperature increases from 600 to 800 °C,the volatilization of ion-exchangeable and water soluble AAEM increases.The increase of pyrolysis pressure reduces the volatilization of Mg and Ca,and made a complex influence rule on the volatilization of the water-soluble state AAEM.The structure of the char and the occurrence state of AAEM will affect the reactivity of the char.The effects of chemical structure of the char and AAEM on the reactivity of char during the pyrolysis process in PTGA and PWMR are studied in this paper.Demineralization reduces the reactivity of char obviously.The insoluble AAEM in Zhundong coal has little effect on the reactivity of char.Ion-exchangeable AAEM improves the reactivity of char.Water-soluble AAEM inhibites the reactivity of char.In the process of pyrolysis,the secondary reaction results in the reduction of functional group structure in the char.In the process of PTGA pyrolysis,when the pyrolysis temperature is 600 °C,the occurrence state of AAEM in char dominates the reactivity of the char,and the influence of pyrolysis pressure on the reactivity of char is relatively complex.When the pyrolysis temperature is 700 °C and the pyrolysis pressure is 0.1 MPa,the enrichment of AAEM and char structure dominates the reactivity of char jointly.When the pyrolysis temperature or pressure further increases,the char structure dominates the reactivity of char. |
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