Research On Pyrolysis And Combustion Characteristics Of Zhundong Coal And Changji Oil Shale Mixtures And Their Kinetics

Alkali and alkaline earth metals（AAEMs）have an advanced effect on the thermal conversion of fossil fuels.The oil shale is rich in organic matter of kerogen,and its liquid product produced by pyrolysis is similar to natural oil,which is beneficial to the petroleum shortage.But Changji oil shale contains less yield of shale oil due to its high ash content,and Zhundong coal is rich in AAEMs content,therefore,it is of great research value to adopt AAEMs in Zhundong coal to promote the pyrolysis behavior of oil shale.In addition,when Zhundong coal is used for combustion power generation,AAEMs will volatilize to gas phase or cause melting and sintering.The Changji oil shale is rich in Si and Al elements and can be used as an additive for Zhundong coal combustion to solve the problems of fouling,slagging and corrosion caused by AAEMs.Furthermore,in Xinjiang province,Changji oil shale is used for power generation due to its higher transportation costs.However,the high ash content of oil shale hinders its combustion behavior.Thus,it is of practical significance to improve the combustion performance of Changji oil shale by blending with Zhundong coal during co-combustion.In summary,the inherent properties of Zhundong coal and Changji oil shale were considered,that is,Zhundong coal is rich in AAEMs,oil shale is rich in organic matter and Si and Al elements.And,these two fuels were made into mixtures to research their co-pyrolysis and co-combustion characteristics.By studying the co-pyrolysis behavior,the product distribution and product characteristics of pyrolysis were described,and the catalytic mechanism of AAEMs for the pyrolysis of Changji oil shale and the synergistic effect of the mixtures during co-pyrolysis were revealed,so as to achieve the effect of increasing oil and gas.By selecting oil shale/shale char as the additive of Zhundong coal to carry out alkali retention combustion,and the changes of alkali retention ratio,mineral composition,ash melting characteristics,ash particle morphology and particle size were studied.The migrations of AAEMs during co-combustion process of Zhundong coal and oil shale/shale char were found,in order to achieve the goal of inhibiting the volatilization of AAEMs and improving the ash melting point.In addition,the co-combustion behavior of Zhundong coal and Changji oil shale were investigated,the change of combustion characteristics was recognized,and the catalytic mechanism of AAEMs for Changji oil shale during combustion and the synergistic effect of co-combustion were proposed,so as to reduce the ignition point,burnout point and improve the combustion performance.The kinetics analysis was carried out and the catalytic mechanism functions of Changji oil shale during pyrolysis and combustion were established.These above provide a new way for the clean and efficient utilization of Zhundong coal and Changji oil shale.The main conclusions of this paper are as follows:（1）Alkali metals and alkaline earth metals catalyzed the pyrolysis of Changji oil shale.Sodium acetate,calcium acetate and magnesium oxide could promote the pyrolysis behavior of Changji oil shale.Among them,sodium acetate had the best catalytic effect,the shale oil yield increased by 6.7%and the pyrolysis gas yield increased by 5.6%.All three AAEMs promoted the release of H₂and CO₂,and inhibited the formation of C₂H₄and C₂H₆.AAEMs promoted the conversion of H and inhibited the conversion of C in the oil shale to shale oil during pyrolysis,improving the H/C ratio of shale oil.At the same time,AAEMs provided Lewis acid site,which promoted the cracking of long-chain aliphatic hydrocarbons and made shale oil lighter.The presence of sodium acetate and magnesium oxide reduced the activation energy of CJ oil shale pyrolysis,while calcium acetate exhibited the opposite trend.Na⁺and Mg²⁺could be embedded into the kerogen macromolecule structure as a polar center,which changed the covalent bond.The electron cloud was arranged,a dynamic induction effect occurred,and it gradually migrated to the C-C bond,which promoted the breaking of the C-C bond.The larger the radius of Ca²⁺,the more difficult it was to embed kerogen macromolecules.Ca²⁺provided active sites on the periphery of kerogen and gradually moved to the molecular center,which accelerated the pyrolysis reaction.With the increase of pyrolysis conversion rate,the activation energy of the CJ,CJ+Na,and CJ+Mg samples exhibited an upward trend,while CJ+Ca shown a fluctuation trend.And,Ca²⁺promoted the pyrolysis process of oil shale by increasing the active sites before the temperature of 500℃,and by reducing the activation energy after the temperature of 500℃.The method of loading AAEMs did not change the mechanism function model of Changji oil shale pyrolysis,that is,order models,but changed the n values.The pyrolysis mechanism functions of these samples were f_CJ（α）=（1-α）^2.1,f_CJ+Na（α）=（1-α）^3.1,f_CJ+Ca（α）=（1-α）^1.3,f_CJ+Mg（α）=（1-α）^2.4.（2）The synergistic effect occurred during co-pyrolysis of Zhundong Coal and Changji Oil Shale.In the pyrolysis process,Changji oil shale mainly produced oil,while Zhundong coal mainly formed pyrolysis gas.During the co-pyrolysis of Zhundong coal and Changji oil shale,the yields of the pyrolysis oil and gas were higher than the weighted values.When the proportion of Zhundong coal was 5%,the promoted effect of oil yield was the best,and the yield of pyrolysis oil was increased by 8.20%.The presence of Zhundong coal made the concentrations of H₂,CH₄and CO₂in the pyrolysis gas of the mixed fuels higher than the weighted values of the individual fuels,showing a promoting effect,while the concentrations of C₂H₄,C₂H₆and CO in the pyrolysis gas of the mixed fuels were lower than the weighted values of the individual fuels,showing an inhabited effect.The components of shale oil were mainly aliphatic hydrocarbons,while those of Zhundong coal tar were mainly aromatic hydrocarbons.The addition of Zhundong coal increased contents of short-chain aliphatic hydrocarbons in the pyrolysis oil of the mixed fuel,resulting in the pyrolysis oil lighter.The relationship between the oil yield of mixed fuels during co-pyrolysis and the H/C ratios of Zhundong coal and Changji oil shale is:η（wt.%,daf）=19.1935×ω×（H/C）_ZD+0.6314×（1-ω）×（H/C）_CJ.The synergistic effect of the co-pyrolysis process of oil shale and Zhundong coal occurred after 455℃,and when the proportion of Zhundong coal was 10%,the synergistic index was highest and the improvement of conversion rate was most obvious,thus,the synergistic effect was the most significant.The activation energy of the co-pyrolysis reaction was lower than the weighted value of the activation energy of the individual pyrolysis reactions,and produced a synergistic effect,which originated from the catalytic effect of AAEMs.At the same time,when the proportion of Zhundong coal was 10%,the pyrolysis activation energy of the mixed fuel MF2 was the lowest.Finally,the pyrolysis mechanism functions of these individual and mixed samples were f_CJ（α）=（1-α）^2.1,f_ZD（α）=（1-α）^3.1,f_MF1（α）=（1-α）^4.3,f_MF2（α）=（1-α）^3.4,f_MF3（α）=（1-α）^4.0and f_MF4（α）=（1-α）^4.5.（3）Changji oil shale/shale char effectively relieved the problems of fouling,slagging and corrosion caused by burning Zhundong Coal.The sodium in Zhundong coal was mainly water-soluble,and its content accounts for 74.36%in total sodium,while calcium was mainly in the form of HCl-soluble and insoluble,with the content of 85.61%in total calcium.During the high-temperature combustion of Zhundong coal,40%-50%of the sodium-containing minerals in the ash volatilized from the raw coal to the gas phase,while the calcium-containing minerals had lower volatility.When Zhundong coal was co-combusted with oil shale/shale char,they inhibited the volatilization of sodium/calcium in Zhundong coal,and the sodium retention rate of oil shale/shale char could reach 70%-88%,and the calcium retention rate could reach 88%-98%.The effect of oil shale and shale char on the sodium and calcium retention at low temperature was better than that of high temperature,and the effect of sodium and calcium retention of shale char was slightly better than that of oil shale.When the blending ratios of oil shale/shale char increased from 0%to 25%,kyanite and mullite with high-melting point were formed during high-temperature combustion,their softening temperatures ST increased by 79℃ and 91℃,respectively.Oil shale and shale char had a considerable effect on increasing the ash melting point of Zhundong coal.When the proportion of oil shale was 5%,the proportion of each component in the ash of MF1 was in the low-temperature eutectic zone,which made the four characteristic melting temperatures of MF1 decreased.（4）The migration paths of AAEMs in the co-combustion process of Zhundong coal and Changji oil shale/shale char were recognized.The primary minerals in Zhundong coal were mainly calcite,anhydrite and thenardite,which was easy to volatilize to gas phase or formed low temperature eutectic.After blending with oil shale or shale char,when the combustion temperature was 800℃-1000℃,albite,anorthite,gehlenite,and akermanite were formed in the ash of the mixture.When the combustion temperature further rise to 1000℃-1200℃,the nepheline and diopsode with high-melting point were further formed.Thus,the AAEMs-containing minerals gradually migrated to the aluminosilicate phase,and the AAEMs-containing minerals in Zhundong coal were deposited in the ash in solid form,which inhibited their volatilization and alleviated the problem of fouling and slagging caused by AAEMs in Zhundong coal.The primary minerals of Zhundong coal were mainly AAEMs with smaller grains,and the particles less than 10μm accounted for 84.31%.When the combustion temperature reaches 1150℃,the sintering and melting of the smaller-size fluxing minerals severely occurred,and the larger particles（>100μm）played a dominated role,forming a larger cluster-type ash.When Zhundong coal blended with oil shale or shale char during combustion,a large amount of minerals with large particle size and high melting point such as aluminosilicate and mullite generated.These refractory minerals minerals could maintain their shape at high temperature without melting and sintering.When the combustion temperature rise to 1150℃,the content of ash particles larger than 100μm in MC2 and MF2 decrease below5%,and the sintering and slagging problem were effectively suppressed and alleviated.（5）The AAEMs had improved the combustion performance of Changji oil shale and there is a synergistic effect in the co-combustion of Zhundong coal and Changji oil shale.Sodium acetate,calcium acetate and magnesium oxide had improved the combustion behavior of Changji oil shale and reduced its ignition temperature and burnout temperature.Among them,magnesium oxide exhibited the best effect.With the increase of oxygen concentration,the ignition temperature and burnout temperature of Changji oil shale decreased,the combustion performance improved,but the activation energy of combustion reaction increased.At the same oxygen concentration,the combustion performance of Changji oil shale in O₂/CO₂atmosphere was lower than that in O₂/N₂atmosphere.The synergistic effects occurred during the co-combustion process of Changji oil shale and Zhundong coal,which was from ignition temperature to 550℃.And the synergy effect increased with the increase of Zhundong coal proportion.The synergy effect of the co-combustion process of Changji oil shale and Zhundong coal originated from the catalysis of AAEMs.The AAEMs could promote the precipitation of volatiles in mixed fuels.At the same time,they could also act as oxygen carriers and form AAEM-O₂intermediates,which conveyed O₂to the surface of the particles to accelerate the combustion reaction.The presence of sodium acetate and calcium acetate reduced the activation energy of Changji oil shale combustion reaction,while magnesium oxide increased its activation energy.In addition,the combustion behavior was improved by promoting the cracking of kerogen and precipitation of volatiles when the sodium acetate was in the entire process and calcium acetate was before the peak temperature of weightlessness during Changji oil shale combustion.While,calcium acetate after the peak temperature of weightlessness and the magnesium oxide during the entire process of Changji oil shale combustion improved the combustion performance by the way of acting as an oxygen carrier.The method of loading AAEMs did not change the mechanism function model of Changji oil shale combustion,that is,order models,but changed the n values.The mechanism functions of combustion reaction of four oil shale samples were f_CJ（α）=（1-α）^2.4,f_CJ+Na（α）=（1-α）^3.1,f_CJ+Ca（α）=（1-α）^3.2,f_CJ+Mg（α）=（1-α）^3.5.