Study On Influence Mechanism Of Sodium On Zhundong Coal Pyrolysis And Gasification

Zhundong coalfield is the largest integrated coalfield in China and in the world,very suitable for mining and utilization.As a unique coal species,Zhundong coal has high sodium content and which is easy to cause fouling and slagging problems in utilization.In order to use the catalytic property of inherent sodium on pyrolysis and gasification process of coal and solve the high cost problem caused by catalyst deactivation and depletion,the experimental and theoretical research on inherent water-soluble sodium which is the the main sodium form in coal was conducted in this essay.It is a new method that the supramolecular compound of crown ether is employed in the research for the study of the effect of inherent water-soluble sodium on the pyrolysis and gasification process of Zhundong coal.The mechanism of inherent water-soluble sodium on pyrolysis and gasification were revealed and the kinetics model of catalytic gasification reaction was established,which provide theoretical basis and basic data support for the large-scale and efficient use of Zhundong coal.The main contents and results were summarized as follows:The mineral composition of Zhundong coal was studied using an inductively coupled plasma mass spectrometry,ion chromatography and X ray diffraction?XRD?.The results showed that the content of sodium in Zhundong coal is 3.82 mg/g,being about 72% of the total sodium.Water-soluble anions are Cl^-,SO₄^2-,HCO₃^-,CO₃^2-and et al.However,the sodium elements existing in sodium salt crystals form was not found through XRD analysis of coal.The influence of step coal washing on surface area and average pore size was studied by physical adsorption instrument.It was found that the hole channel can be dredged by the coal washing and the surface porous structure of coal sample increased significantly,especially when the mineral matter combined with organic matter was washed away by ammonium acetate solution and hydrochloric acid solution.The influence of coal washing on the pyrolysis and gasification reaction was studied by thermogravimetric analyzer?TGA?and XRD.It was found that the coal washing process promotes the evolution of the coal char microcrystalline structure to the graphitization,which reduces the gasification reactivity of coal samples.When the mineral content is low in coal,the gasification reactivity is mainly determined by the pore structure.To research the effect of Na ion on pyrolysis/gasification process of Zhundong coal and eliminate the interference of other metal ions,the non-Na ions in washing water were extracted by supramolecular compounds of crown ether.The influence of types of crown ether,organic phase diluent,extraction time,concentration of crown ether,extraction temperature on metal ions extraction from in multiple ions mixed system was studied.The results showed that the more polarity of diluent,the better the crown ether extraction.The coordination ability of crown ether with the same pore size to same kind of ion is different due to the difference of the substituents on the crown ether.The extraction rate can be improved by increasing the concentration of organic extractant,while the separation coefficient doesn't increase with the increase of extractant concentration.Increasing temperature is not conducive to the extraction reaction as the extraction reaction is exothermic.The non-Na ions can be completely extracted from Zhundong coal washing and no extra pollution was produced to the solution when 20 mmol/L and 10 mmol/L dicyclohexyl-18-crown-6 was used in sequential twice extraction.The Zhundong coal sample contained inherent water-soluble sodium was prepared with back feeding method.The effect of inherent and additional water-soluble sodium on pyrolysis characteristics of Zhundong coal samples was investigated in TGA.The micro-structures of coal and char were analyzed by scanning electron microscopy,energy dispersive spectroscopy and XRD.The results show that the presence of sodium can promote the weight loss of coal samples in the initial pyrolysis stage and the pyrocondensation stage.The sodium will also hinder the volatilization of volatile components in the main pyrolysis stage and inhibit the evolution of microcrystal structure to the graphitization of the coal char.When the load of Sodium was constant,the inherent water-soluble sodium is easier to react with coal particle and promote pyrolysis as it mainly distributes on surface of coal particles.Increasing heating rate can improve the reactivity of coal pyrolysis and increase the total loss weight and maximum weight loss rate of coal samples.The calculating the kinetic parameters of the pyrolysis process shows that the presence of sodium can significantly reduce the activation energy of pyrolysis.The effect of inherent water-soluble sodium and additional water-soluble sodium salt on characteristics of Zhundong coal gasification was studied by TGA.The results show that inherent water-soluble sodium has the strongest effect on the coal catalytic gasification reaction at the same sodium load,which reduced the reaction temperature of gasification from 816.3 to 657.8? in the temperature programmed gasification process and shortened gasification time 67.8% as the carbon conversion rate reaching 90% in isothermal gasification process.CO₂ strong chemical adsorption ability of coal samples can reflect the number of active sites on coal sample surface in gasification reaction,which explains the differences of coal gasification reactivity after loading inherent water-soluble sodium and other water-soluble sodium salt.The sodium has an effect on the pore structure and crystallite structure of char during isothermal gasification process,which also affect the following gasification reaction.A kinetic model for coal catalysis gasification was established,and the kinetic parameters of the model were verified by the non-conversion method.It shows that the model fitting results are in good agreement with the corresponding experimental data and can describe gasification reaction process of Zhundong coal loaded sodium element.The parameter calculation shows that the apparent activation energy was reduced by about 46%.