Fundamental Study Of Integated Coal Pyrolysis System Coupled With CO₂ Capture And Conversion Technology

The carbon dioxide produced in the fossil fuel utilization industry now seriously thereatens this planet with the greenhouse effect.Considering the chanllenge of energy conservation and carbon dioxide emission reduction,propelling the coal-to-chemicals technology without carbon emission is one of the most important pathway to deal with the environmental issues.In this research,an innovative integrated system named integrated coal pyrolysis system coupled with CO2 capture and conversion technology is proposed,in which the methane chemical looping combustion technology（CLC,unit for heat）,CH₄-CO₂ reforming technology（CCR,unit for syngas production）,coal fast pyrolysis under syngas atmosphere（unit for products）and hydrogenation of CO₂ for CH₄（CO₂ conversion unit）are all coupled and integrated.The CLC is the source of heat in the system to combust the coal pyrolysis gas and methane,producing highly pure CO₂ The CO₂ from the unit 1 is hydrogenated in the unit 2 for CH₄ production and then in unit 3,reformed with CH₄ to produce syngas.In unit 4,the syngas from the upper stream provides hydrogen source for the coal fast pyrolysis to produce high value-added chemicals.In this research,the fundamental studies for the former three units were performed,to provide the manufacturability support for the proposed system.The target of the CLC in unit 1 is to provide heat while capture the CO₂ for the system.CLC is an innovative CO₂ capture technology with nearly zero energy penalty,the active oxygen contained in the crystal of oxygen carrier（OC）reacts with the methane to generate the CO₂ and H₂O.In this unit,the CuO based OCs were prepared with three different methods including mechanical mixture（MM）,impregnation（IM）and co-precipitation（CP）,and evaluated on thermal gravimetric analyzer（TGA）,and fixed bed rector.The fresh and spent OCs were characterized with different methods including surface area and pore analysis,X-ray diffraction（XRD）,H₂ temperature programming reduction（TPR）,scanning electron microscope（SEM）.The result indicated that the carbon deposition was proved the major reason for the low combustion efficiency and the deactivation of OCs in redox cycles,causing 27%,25% and 44% decreases in CO₂ selectivity for three kinds of OCs after 10 cycles CLC in fixed bed reactor;meanwhile,40%,47% and 31% decreases in BJH pore volume were observed for these three OCs.With the introduction of steam,the CO₂ selectivity was improved to nearly 100% with the gasification reaction between the steam and the carbon.In addition,the pore volume of the OCs was well preserved.Furthermore,the structure analysis of OCs before and after cycles illustrated that the steam would greatly intensify the redox degree of OCs at 800 ℃,strengthen the redistribution of Cu on the surface of Al₂O₃,and thus contribute to the fine and uniform distribution of CuO.Therefore,considering the high cost of the OC used in the CLC technology,the steam could act as a cheap OC protector which has a promising future to be extensively used to realize the low-cost running of CLC.The fly ash（FA）from the coal fired power plant was tested as the support for Cu-based OCs,which performed well with outstanding anti-carbon deposition capability compared with Al₂O₃ support.Especially the FA IM OC prepared with impregnation method,achieved 94¹00% methane conversion and 90⁹4% CO₂ selectivity,as well as the excellent stability with minor deactivation in ten redox cycles.Thus,as the waste of the power plant,the fly ash is a promising material to be utilized as the support of OC in CLC.Highly effective and stable catalyst is the key for CCR technology in unit 2 to convert the CO₂ and CH₄ from the upstream and produce the syngas for the coal fast pyrolysis in the downstream.The nano-structured Ni based catalysts supported on spinel（MgAl₂O₄）was developed.The evaluation results indicated that,the best performed 20%Ni catalyst realized conversions of CO₂ and CH₄ as high as 95.19% and 98.50% at 900 ℃ respectively,with gas hourly space velocity（GHSV）as high as 912000ml·gCat-1·h-1.Moreover,the nano-structured Ni based catalysts was stable in 10 hours without any deactivation.When the GHSV went up to 1368000 ml?gCat-1?h-1,the conversion of the CO₂ and CH₄ were still around 80%,indicating the outstanding reactivity and processing capacity of the Ni catalyst developed in this study,which was performing better than those catalysts reported in the literatures.The goal of the coal fast pyrolysis in unit 4 was to improve the yield and quality of the chemicals contained in the coal tar,in the atmosphere of syngas produced in the upstream.The coal was pyrolyzed at 700 ℃ under atmospheres including N₂,H₂,CO,H₂-CO and syngas produced form the last unit,also the double layer reforming-pyrolysis was performed with that catalyst on the upper layer and the coal placed at the lower layer.The characterization of the tars indicated the improvement of quality and quantity with the reducing atmosphere compared with the inert,and the hydrogen partial pressure was found the most critical parameter for that improvement.In the atmosphere for H₂,the tar was increased by 31.3% and the contained BTX（Benzene,toluene and xylene）and naphthalene was increased by 27.1% and 133.4%.The produced syngas also performed outstandingly,with 25.4% increment of tar yield and 25.0% and 79.4% for the BTX and naphthalene.The double layers reforming-pyrolysis was not efficient enough due to the low temperature in which the catalyst was not fully activated.In addition,the energy and material calculations were performed for the proposed system,the result indicated that the heat self-sufficiency could be realized with the combustion of the pyrolysis gas.Meanwhile,highly pure CO₂ was captured in the system companied with the production of high-quality coal tar,coal char and the depleted air.Therefore,the prefixed targets of the system design were accomplished,and the proposed concept was proved scientifically feasible.Furthermore,the basic study of the three units involved in this study were progressive in their areas,which were of great importance to propel the developments of the carbon capture and storage technology and coal conversion technology.