Study On The Interaction Mechanism Between Different Components Of Volatiles From Medium And Low-temperature Pyrolysis Of Low-rank Coal And Their Reaction Behaviors Under The Strengthening Effects Of Carbon-based Medium

With the rapid development of economy,the demand for energy has been increasing.Based on the fossil energy reserve situation of“rich coal,poor oil and little gas”in China,the role of coal as energy supply and ballast stone is more prominent.It is an important way to construct clean and efficient coal utilization system for achieving the goal of"carbon peaking and carbon neutrality".Low-rank coal reserve is relatively abundant in China which is mainly used for direct combustion to generate power at present.Low-rank coal is characterized by high volatile content,high moisture content and low calorific value.In the process of direct combustion of low-rank coal,there are some problems such as high energy consumption and low power efficiency,which aggravates the carbon emission.The grading and qualitative conversion of low-rank coal through pyrolysis technology as the core is of great significance to realize clean and efficient utilization of low-rank coal.However,the large-scale development and commercial application of low-rank pulverized coal pyrolysis technology are hindered due to the system blockage resulting from the volatiles’reaction in the thermal environment and the difficulty in the separation of oil and dust.Granular bed dust removal technology can achieve dust removal by deep filtration.In addition,it is expected that the surface characteristics of the filter medium can act a role in changing the reaction path of volatiles,so as to achieve the upgrading of pyrolysis tar and gas.Therefore,this paper focuses on exploring the interaction mechanism of different components in coal pyrolysis volatiles in the thermal environment,as well as the in-situ reaction mechanism of volatiles under the strengthening effects of carbon-based medium,which can provide theoretical guidance for the large-scale industrial development of low-rank coal pyrolysis technology.In this paper,the medium and low-temperature coal tar first was distillated at atmospheric pressure to obtain six different distillates（light oil,phenol oil,naphthalene oil,washing oil,anthracene oil and pitch）.The reaction behavior of each distillate at different temperatures and the interaction between the distillates were investigated in order to reveal the influencing mechanism of the reactivity and interaction of different components in the volatiles in the thermal environment on coke formation.Further,commercial activated carbon was used as a carbon-based medium model to reveal the influencing mechanism of coal pyrolysis volatiles over activated carbon on coke formation.On the basis of the above studies,porous carbon-based medium with high abrasive resistance was prepared by means of the difference in thermoplasticity between biochar and caking coal.The carbon-based medium was further modified by loading nickel.The in-situ reaction behavior of coal pyrolysis volatiles under the strengthening effects of carbon-based medium was revealed in a downer-bed continuous pyrolysis reactor.The main results are as follows:（1）The reaction behavior of each distillate and the interaction between distillates at different temperatures were investigated.The difference of thermal reaction behavior of each distillate and the influencing mechanism of the interaction between distillates on coke formation were analyzed.Results show that temperature is a key factor affecting the distillates’reaction to form coke.The coke yields increase with temperature during thermal reaction for all six distillates.Pitch,as a heavy component,has the highest coke yield at different temperatures compared to other distillates.Pitch is an important precursor of forming coke in the thermal environment.The hydrogen-rich free radicals generated from the cracking of light oil at high temperatures bond with large molecular free radicals in pitch,which results in the fact that the interaction between the two distillates compresses coke formation.By contrast,the interaction between other distillates（phenolic oil,naphthalic oil,wash oil and anthracene oil）and pitch promotes coke formation.The highest extent of interaction occurs between phenolic oil and pitch,which is related to the high content of oxygenated compounds in phenolic oil and the high electron density of macromolecular aromatic compounds in pitch.（2）The effects of activated carbon on the coal pyrolysis product distribution were analyzed.The tar’s composition and coke yield at different feed times were investigated in a downer-bed continuous pyrolysis reactor.The coke formation process of coal pyrolysis volatiles’reaction over activated carbon was analyzed.Results show that a great amount of coke is generated on the activated carbon and the pore structure of activated carbon is not conducive to the diffusion of macromolecular compounds.With the increase of feed time,the amount of coke on activated carbon increases,while the rate of coke formation decreases,thus the coke yield reduces.The increase of coke amount on activated carbon results in the blocking of pore structure of activated carbon.The specific surface area of the activated carbon and its cracking activity on tar and pitch in the tar decreas.Hence,the yields of tar and pitch increase.The tar’s composition analysis shows that the content of oxygenated compounds in the tar increases with the increase of feed time,which indicates that the coke formation of volatiles’reaction over activated carbon originates from the adsorption and cracking of the oxygen bond.With the increase of coke amount on the activated carbon,its cracking activity on oxygenated compounds decreases,which also results in a decrease in the rate of coke formation.（3）The carbon-based medium was prepared by co-carbonization of biochar and caking coal and the carbon-based medium was furtherly activated under steam（VICM and VACM represent the inactive and activated carbon-based medium,respectively.）.The volatiles generated from coal pyrolysis reacted over VICM and VACM,and the quartz beads were used as the control group.The effect of the physicochemical structure of VICM and VACM on the volatiles’reaction was investigated by analyzing the yield and composition of the pyrolysis products.Results show that the abrasive resistance of VICM and VACM is high which is suitable to be applied in the granular bed duster.The micropores,mesopores and macropores all exist in VICM and VACM.VICM and VACM improve the tar’s quality.The light oil content increases,meanwhile,the number-average molecular weight and weight-average molecular weight of tar decrease over VICM and VACM.The contents of 1-4 ring aromatics,1-2 ring phenols and C9-C28 aliphatic hydrocarbons in tar increase,while the contents of oxygenated compounds and heterocyclic compounds containing N and S decrease over VICM and VACM.Besides,the yield of light tar（<360 ℃）increases over VICM,which indicates that small molecular hydrogen-rich free radicals are effectively bonded with the large molecular free radicals.VACM promotes adsorption and cracking of tar due to more pores,oxygen-containing functional groups and carbon defects,resulting in the excessive cracking of both light and heavy tar,thus their yields decrease.The mesopores and macropores of VICM and VACM provide a mass transfer channel for the diffusion of macromolecular compounds produced by the volatiles’cracking and the coke yield on VICM and VACM is significantly lower than that on activated carbon.Characterization of coke suspended in tar with a particle size less than 0.45μm（Coke-S）shows that its condensation degree decreases over VICM and VACM.The free radical formed by the breaking of the oxygen-containing weak bond binds with the Coke-S precursor free radical,and the oxygen in the tar has been transferred to the Coke-S.（4）Nickel was introduced into the biomass by ion exchange,and then the volatiles of the biomass were removed to obtain biochar containing nickel.The blend of biochar containing nickel and caking coal was carbonized and activated to prepare the nickel-loaded carbon-based medium（Ni/ICM and Ni/ACM represent the inactive and activated nickel-loaded carbon-based medium,respectively.）.The coal pyrolysis volatiles’reaction over Ni/ICM and Ni/ACM was investigated.The effects of physicochemical structures and metal sites of Ni/ICM and Ni/ACM on the distribution and composition of pyrolysis products were analyzed.Results show that the nickel content of Ni/ICM and Ni/ACM reduces with the increase in the solution’s pH value during the nickel ion exchange procedure.The average crystallite size of Ni and NiO in Ni/ICM is 5-7 nm,and that is slightly larger in Ni/ACM.Ni/ICM further improves the tar quality,and the content and yield of light oil increase compared to ICM.The light oil yield improves by 37.5%over Ni_1.9/ICM(Ni_1.9/ICM represents the Ni/ICM with the nickel content of 1.9 wt.%.).The light tar yield is higher and the coke yield is lower over Ni/ICM than over Ni/ACM,which may result from better matching between small molecular hydrogen-rich free radicals and large molecular free radicals over Ni/ICM with higher nickel dispersion.The acid sites of Ni/ICM are conducive to the decomposition of macromolecular aromatics to produce 1-4 ring aromatics,while the excessive acid sites of Ni/ACM cause the polycondensation of aromatics to form heavier components or coke.The coke in tar is composed of large aromatic clusters crosslinked by oxygen and fine ash particles.The large aromatic clusters,as heavy components in tar,are found that its condensation degree and aromaticity decrease over Ni/ICM and Ni/ACM in comparison with ICM and ACM,respectively,which also means that the quality of tar is improved.Combined with the above research,the high reactivity of the components in volatiles is a key factor leading to the volatiles’reaction in the thermal environment to form coke.The highly active oxygen-containing compounds are the initiators of the volatiles’reaction,and pitch is the main contributor to coke formation.The coke on carbon-based medium from volatiles’reaction affects its activity.The hierarchical pore structure of carbon-based medium can effectively reduce the coke on the carbon-based medium.It is necessary to balance the carbon-based medium’s ability of generating small molecular hydrogen-rich free radical and large molecular free radical and strengthen the matching hydrogen supply of small molecular hydrogen-rich free radical to large molecular free radical in the absence of external hydrogen sources for achieving the upgrading of pyrolysis tar and gas.