Coal Pyrolysis For Oil And Gas With Infrared Quick Heating And Staged Reaction Control

The characteristics of "rich in coal,poor in oil,less in gas" for China’s energy resources determines the long-term dominant position of coal in the country’s energy structure.The overall quality of coal is poor in China,and more than half of its total reserves are low metamorphic coal(low rank coal).Pyrolysis technology presents a way to utilize coal with added value and for hydrogen-rich gas as the alternative energy resources.Rapid heating ensures high tar yield for coal pyrolysis.However,literature studies using gram-scale reactors usually suffer serious secondary reactions to volatile matter,while other milligram reactors cannot calculate the yield of pyrolysis products and take product samples for analysis.The real characteristics of rapid pyrolysis products have not been revealed by far.A new type of infrared rapid heating reactor is taken in this paper to treat grams of coal as sample and suppress-minimizes secondary reactions of volatile.The pyrolysis characteristics of low-rank coal at was tested at wide-varying temperatures(500-1000 ℃)and heating rates(1-1000 ℃/min)in the infrared heating reactor.The reaction control method for maximizing the initial pyrolysis products(tar and gas)was obtained.Considering the practical possibility of rapid heating,a staged reaction control strategy was proposed and also verified experimentally to obtain high yield of volatile through rapid pyrolysis,high gas yield via deep condensation of char at high temperature and high quality of tar via selective adsorption on low-temperature particles and in turn cracking in reheating.The main findings are as follows.1.Structure variation of low-rank coal in slow-heating treatment.The evolution of micro/meso pores and chemical composition of a kind of low-rank coal during mild thermal treatment was studied based on the observation of coal structure changes with the gradual detachment of organic matters from the coal.Aromaticity and CH2/CH3 ratio of coal organics are correlative with the volumes of super-micropores and mesopores,respectively.The super-micropores are found to consist of inter-layer distance between stacks of aromatic rings,and mesopores are the space between macromolecular aromatic rings which are inter-connected via aliphatic chains.2.Infrared fixed-bed pyrolysis with different heating rates.The pyrolysis process of low-rank coal has the characteristics of complexity,nonuniformity and continuity.The entire process of reaction was described using the distributed activation energy model(DAEM),and the change of pyrolysis kinetic parameters with conversion was analyzed.The activation energy distribution curve was in accordance with the Gauss distribution and mainly distributed in the range of 200～240 kJ/mol.The maximum value of pyrolysis f(E)appeared at the activation energy of 220 kJ/mol.The frequency factor first increased with the increase of activation energy to show the compensation effect.When the activation energy was more than 220 kJ/mol,the frequency factor tended to become stable.The influence of heating rate(6-667 ℃/min)on pyrolysis of 3 g low-rank coal was studied at a specific temperature(700 ℃)in the infrared quick-heating reactor.It was found that the higher heating rate promoted the breakage of covalent bonds in macromolecular networks to generate more volatile components.The internal and external pressure difference of the coal particles is increased to improve the mass transfer efficiency.The residence time pyrolysis tar precursors inside particles and also in the bed is reduced.These consequently,decrease the yields of pyrolysis char and gas but increase the yield of tar and also the total volatile yield.With the increase of heating rate,more organic compounds escape from the tested coal sample,and the residence time of pyrolysis tar precursors decrease both inside coal particles and in the coal bed.The yields of tar and total volatile increase,by accompanying the decrease in pyrolysis gas and char.The higher heating rate promotes the breakage of covalent bonds in macromolecular networks and generate more heavy components to result in higher yield and heavier composition for tar.The heavy oil content in tar increases from 19.4 wt.%to 48.5 wt.%.The total yield of tar is above that of the Gray-King(G-K)assay at the heating rates above 18 ℃/min,and reaches its peak at 667 ℃/min(134%of the G-K tar yield).The contents of alkanes and aromatic compounds decrease,while the contents of alkenes and heteroatomic compounds increase.The tar produced at the tested highest heating rate may well reflect the structure and composition of the primary tar.It has the characteristics of high boiling point,low alkane/alkene ratio,low aromaticity and high heteroatom composition.With the increase in heating rate,the smoothness of char surface decreases,and the oxidation activity of char slightly decreases.3.Infrared fixed-bed pyrolysis at different pyrolysis temperatures.For the adopted infrared-heating reactor,the maximal heating rates corresponding to the finally reached temperatures of 500,600,700,800,900 and 1000 ℃ are 300,314,667,978,1335 and 1723 ℃/min,respectively.With increasing the pyrolysis temperature from 500 ℃ to 1000 ℃,the pyrolysis gas yield increases,pyrolysis char yield decreases,tar yield first increases and then decreases to have its peak at 700 ℃.The tar yield is always higher than that of the G-K assay,while the total volatile matter is higher than that of the G-K assay at temperatures above 700 ℃ and the gas yield becomes higher than that of the G-K assay above 800 ℃.At 1000℃,the yields of tar,pyrolysis gas and total volatile matter reach 125%,129%and 128%of the corresponding yields of the G-K assay respectively.The total gas production increases from 62.67 L/kg to 295.04 L/kg to proportionally correlate with the extraction rate of hydrogen from coal.With the increase of temperature,the content of light tar increases from 39.33 wt.%to 55 wt.%.The alkane content in tar decreases,while alkene content and tar saturation both increases.The surface porosity of char increases,and the oxidation activity of char obviously decreases.When the pyrolysis temperature is above 900 ℃,the final carbon conversion of char in its reactivity test sharply decreases.4.Staged reaction control for pyrolysis.Based on the proceeding tests an idea of staged reaction control was proposed and further experimentally validated It is a first step of low-temperature but rapid pyrolysis and a second step of high-temperature char polycondensation incorporating with the selective adsorption and cracking of heavy components.For the first stage at medium temperatures it can suppress the secondary reactions to the produced volatile matters that may be maximized via quick heating of particles.The heavy tar obtained in the rapid-heating pyrolysis is in turn internationally adsorbed onto ash,char or/and raw coal.Reheating the tar-adsorbed particles leads to upgrading of heavy tar,while high yield of gases ensured by polycondensation of char at rather increased temperatures.Testing different adsorbents found that the realized upgrading effect from large to small follows an order of ash,char and raw coal.Via such a staged reaction control strategy,it well ensures the production of high-yield tar with high content of light components and high-yield gas rich in hydrogen.This reaction control mechanism rightly reflects the principle of reaction control for the pyrolysis in the so-called fixed/moving bed with internals.