Effect Of CO₂ On The Transformation Behavior Of Mineral Matters And The Ash Fusion Temperatures During Coal Gasification

The ash fusion temperatures?AFTs?are important parameters affecting the normal operation for the pressurized fixed-bed dry bottom gasifier.In order to ensure the ash discharge in solid condition,excessive steam is injected into the gasifier to control the temperature,thus inevitably leading to the emissions of phenolic wastewater.Utilization of CO₂ instead of partial steam as partial gasification agent in the gasifier has been proposed to save excess steam,reduce phenolic wastewater emission and realize CO₂ resource use.Because the difference on physical and chemical properties of CO₂ and H₂O,and the reactivity of char gasification with CO₂ is lower than that with H₂O.The diffusion rate of O₂ under CO₂ atmosphere is relatively slower than that of O₂under steam.The introduced CO₂ as the gasification agent to the gasifier will affect thermal chemical process of beds.Therefore,it is necessary to investigate the influence of CO₂ on structure and gasification reactivity of chars,reveal the transformation regulation of mineral during coal pressurized gasification,and explore fusing characteristics of coal ash under CO₂ atmosphere.For the study to set up relevant theoretical support has important theoretical and practical value.Based on the above background,in this study,the pressurized fixed-bed dry bottom gasifier using coal as raw material.A series of coal char and coal ash were obtained under different conditions via a fixed-bed reactor and a horizontal tube furnace.The effect of CO₂ on structure and gasification reactivity of char and transformation behavior of mineral matters during gasification and the ash fusion temperatures was conducted by a series of characterization techniques,such as ICP and SEM-EDS.Additionally,the effect of CO₂ on coal ash related properties was revealed by selection of ash sample for CO₂ before and after back to gasifier.The discussion results are shown as follows:?1?The influence of CO₂ on structure and gasification reactivity of chars were investigated,and the results suggest that the chars obtained under CO₂atmosphere display lower char yields,lower ratios of small to large aromatic rings,and larger specific surface area compared with that of char under N₂atmosphere.The chars with higher AAEMs contents have better gasification reactivity.The gasification reactivity of char-CO₂ gradually increased with the extension of constant time,whereas the demineralized char-CO₂ shows reverse trend.The demineralized char prepared under CO₂ delivers lower gasification reactivity than that under N₂ atmosphere due to the catalytic effect on gasification caused by AAEMs in the coal chars.And the influence of the specific surface area on gasification reactivity can be ignored compared to the ratios of small to large rings in chars after the exclusion of mineral catalysis.?2?The effect of CO₂ on transformation behavior of mineral during coal pressurized gasification was studied.Intertestingly,the different metal elements of Yining raw coal exist in a different modes.Na and K mainly exists in the form of water-soluble,whereas Ca,Fe and Mg mainly exists in the form of hydrochloric acid soluble.With the increase of temperature,the specific surface area of char increased and release of Ca,Fe,Mg,K and Na enhance gradually.The specific surface area of chars obtained under H₂Oatmosphere at 1.0 MPa were largest.Similar phenomenon can be observed by the release of Ca,Fe,Mg,Na and K.The specific surface area of chars were obtained under H₂O atmosphere were higher than that under CO₂ atmosphere.Compared to the H₂O atmosphere,CO₂ atmosphere inhibit the release of Na,K and Fe.However,CO₂atmosphere promote the release of Ca and Mg of the pressure at 0.1 MPa,and inhibit the release of Ca and Mg at 1.0 and 2.0 MPa.?3?The effect of CO₂ on transformation behavior of ashes were analysed.For Yining ash,atmosphere mainly affects the existence of Ca form and content.The content of Ca in ash obtained CO₂ atmosphere is lower compared with that of Ca under H₂O atmosphere.The content of CaO in ashes increases with the enhanced temperature.Ca main existence form is CaCO₃ under CO₂ atmosphere at 750 and 850°C,while CaS is manily generated under CO₂ atmosphere at950°C.In addition,Ca main existence form is CaO under H₂O atmosphere,and Ca mainly exists in the form of CaSO₄ under CO₂/O₂ atmosphere.In terms of Zhundong ash,the atmosphere has little effect on the components of Na and Fe minerals,but shows obvious influence on Ca.In detail,Na and Fe is generated in the form of NaAlSiO₄ and Fe₂O₃,respectively.The calcium minerals in the coal ash under CO₂/O₂ atmosphere are easy to carbonate,forming more carbonic acid at a lower temperature.?4?The influence of CO₂ atmosphere on ash fusion temperatures of ashes were studied.The ash fusion temperatures of ash obtained under different atmosphere is increased with the increase of temperature,which is mainly due to the content and existing form of Ca in ash.The ash fusion temperatures of ash obtained under CO₂ atmosphere is lower than that under H₂O atmosphere at 750and 850°C,but higher than that H₂O atmosphere at 950°C.The ash fusion temperatures of ash obtained under CO₂/O₂ atmosphere is higher than that under H₂O/O₂ atmosphere.For Zhundong ash with low ash fusion temperatures,and is easy to slag coal due to the coal ash in the generation of nepheline.The change of atmospheres have little impact on the ash fusion temperatures of Zhundong ash.?5?The related properties of different size ash for CO₂ before and after back to gasifier is studied.CO₂ back to the gasifier affect the particle size distribution and carbon residue content of ash.The residual carbon content of ash decrease with the rising of the particle size.Compared with the CO₂ before back to the gasifier,the residual carbon content of different particle size ash enhances.After CO₂ back to the gasifier,the influence of the composition of coal ash is not obvious,but had an influence on the distribution of elements in ash content,and ash fusion temperatures enhanced,which can improve the substitute steam ratio.