Study On Anti-Slagging Mechanism Of High-Alkali Low-Rank Coal During Gasification Process

High-alkali low-rank coal has the characteristics of high reaction activity and easy burn-out.However,severe slagging ease to occur during the process of thermochemical conversion,even in a circulating fluidized bed(CFB)with a relatively low operating temperature,due to the high content of alkali and alkaline earth metals in high-alkali coal.Therefore,its popularization and utilization of coal-based chemicals,synthetic ammonia,natural gas,power,and other industries based on CFB coal gasification technology were restricted.Aiming at the slagging problem of high-alkali coal gasification,the slagging characteristics were analyzed in this study firstly.The thermodynamic calculation software FactSage 7.2 was used to obtain the generation curve of liquid slag with temperature and provide support for the selection of coal types and suitable operating temperatures.Secondly,the influences of gasification temperatures and residence times on slagging characteristics and the migration and transformation rules of alkali metals were discussed.Besides,according to the slagging condition of CFB gasifier in different parts,the influences of the reaction atmospheres and other components on slagging were further analyzed.Finally,the effective anti-slagging control schemes for coal ash components were explored by combining experiments and thermodynamic calculations.In addition,the anti-slagging method that suitable for engineering application was verified by the stable operation tests carried on the CFB gasifier,which provide support for the safe application of high-alkali coal in CFB industrial gasifier.The main work and conclusions are detailed as follows.(1)The gasification experiments of different coal qualities were carried out,and the generation curve of liquid slag with temperature was obtained was established through thermodynamic calculations.The differences in slagging characteristics of six high-alkali coals were discussed and the reasonable operating temperature ranges were optimized.As indicated by the results,the low retention rate of Na(RNa)lead to high ash fusion temperatures(AFTs).However,when the content of Ca in ash was high,the AFTs would be affected by the existential forms of Ca.Reducing RNa,promoting the gas phase release of alkali metals,and inhibiting the formation of Ca silicate and aluminosilicate can effectively prevent slagging.Based on slagging prediction model of the high-alkali coal,by compared the temperature corresponding to the 25%liquids line with the initial temperature of the rapid growth of liquid slag,the operating temperature of the six high-alkali coals(in order of Mulei coal,Tietou coal,Qiyal coal,Qiya2 coal,Shaerhu coal,Yihua coal)should be controlled below:1150?,950?,1125?,1125?,1100?,and 1000?,respectively.(2)The high sodium coal gasification experiments under different temperatures and residence times were carried out,and the slagging characters and the migration and transformation rules of alkali metals were discussed in combination with thermodynamic calculations.The results showed that,the higher the gasification temperature,the easier it was for the inorganic matter in the char particles to penetrate to the surface of the particles.Then the dense layer was formed under the capture effect of SiO2 and Al2O3,and lead to the higher viscosity of the particles.With the increase of residence time,these particles would stick to each other,agglomerate and grow,and eventually caused slagging.In YNc,sodium existed mainly in H2O-soluble and NH4Ac-soluble forms,the proportion of which tends to decrease during the gasification process.Ultimately,Na existed in the gasification residue as the insoluble and HCl-soluble forms.As for NH4Ac-soluble potassium,it could be transformed into H2O-soluble and HCl-soluble forms during the gasification process.Ultimately,it existed mainly in an insoluble form.Eventually,the Na which was retained in the gasification ash was as high as 70%,and K only accounted for 40%.The slagging problem caused by the alkali metal Na was more serious.(3)The gasification experiments of high-alkali metal and alkaline earth metal(AAEM)coal under different atmospheres were carried out.The growth rules of the liquid phase were analyzed through the phase equilibrium calculation,and the influences of the composition change on ash fusion characters were discussed through the phase diagram calculation.The results showed that in the typical interval of CFB gasification operating temperature(950?1050 ?),high-alkali coal displayed the highest proportion of liquid phase in reducing atmosphere and the lowest in water vapor atmosphere.The ash concentration,CO2 concentration,and reaction temperature at the bottom of the furnace were strong.Once the particles agglomerated,the rapid growth of liquid slag would occur,therefore the risk of slagging was the highest.The fluidization speed of particles in the return feeder was low and the material concentration was high.The introduction of water vapor could reduce the temperature,promote the gas phase release of Na,reduce the viscosity and agglomeration between the particles,and effectively alleviate the slagging.Although the upper part of the furnace was in a strongly reducing atmosphere,the particle speed was fast and the material concentration was low,therefore the risk of slagging is low.As for high-AAEM coal with similar ash composition,the higher the RS/A,the lower the softening temperature(ST)of the coal ash,and the higher the risk of slagging.The ash fusibility could be effectively controlled by adjusting the relative content of SiO2,Al2O3,Na2O,and CaO in the coal ash.(4)The coal blending gasification tests of YNc and the additives gasification tests of YHc were carried out,and thermodynamic calculations were combined to explore the effective anti-slagging control scheme.For YNc,finding suitable blended coal in the project site(Guangxi)could reduce the cost and promote the utilization of local inferior coal.The phase diagram was used to guide the selection of blended coal types and proportions,the results gained from combining the experiments showed that:adding 25%of Baikuang coal could reduce the retention rate of alkali metals(RR),reduce the formation of low-temperature eutectic induced by alkali metals in ash,and effectively preventing slagging.For YHc used in industrial gasification in Xinjiang,adding additives was a more suitable way.According to the measured AFTs and the calculated coal ash melting characteristic curve,the likelihood of slagging(from high to low)for coal ash after adding additives was:ripe vermiculate,raw vermiculate,silicate,YHc only,kaolin,boehmite,and corundum,and the most suitable addition ratio was 6%.(5)The gasification tests based on CFB gasifier were carried out,the effects of different steam-to-coal ratios and oxygen-to-coal ratios on slagging characteristics of high-alkali coal gasification were explored,and the stable operation tests verification were completed.According to the test results,the lower the oxygen-to-coal ratio,the stronger the reducing atmosphere in the riser,the more obvious the material crystal phase represented by nepheline(NaAlSiO4),sodium(potassium)feldspar((K,Na)AlSi3O8),and Na2SiO3 in the gasification ash and the higher the risk of slagging.Increasing the steam-to-coal ratio could effectively reduce the temperature at the bottom of the riser and promote the release of alkali metals in the gas phase,so that the risk of slagging could be mitigated.The bottom of the riser and the return feeder were at the highest risk of slagging inside the CFB gasifier.The use of corundum as the bed material or the kaolin as the additive could be effective in reducing local enrichment of AAEM elements,so as to suppress slagging.Considering the stability of the long-term operation of the system,the addition of 6%kaolin was confirmed as the most effective scheme for anti-slagging.