Study On Slagging And Fouling Characteristics Of High-Alkali Low-rank Coal During Thermochemical Conversion Process

Zhongdong(ZD)district in Xinjiang has the largest integrated coalfield in China with the estimated reserve of about 390 Gt.Due to the low exploitation cost,high reaction activity and easy burn-out,ZD coal is considered as a good choice for power and gasification.However,the high contents of alkali and alkaline earth metal(AAEM)elements,especially for Na2O in coal ash(usually above 2%),are responsible for severe slagging and fouling characteristics during its utilization.At early stage,severe ash-related problems occurred in pulverized-coal-fired boilers because of the poor understanding of this coal and lack of effective solutions,which greatly restricted its popularization and application.Based on the difficult utilization of ZD coal,the application of ZD coal was attempted in the bench-scale circulating fluidized bed(CFB)test rig,as well as lab-scale experimental facilities and thermodynamic calculation software Factsage 6.1.Through these works,the possible ash-related problems,their corresponding mechanisms and potential solutions during the ZD coal utilization in CFB were determined,explored and proposed,respectively.Finally,a new predicting method of slagging and fouling was provided to confirm the suitable operating temperature of the special coals.The main work and conclusions are listed below:1)The influence of Na-based materials on slagging characteristics of ZD coal was investigated.The results show a vital catalytic role was played by Na-based materials in slagging and various mechanisms corresponded to various Na-based species.The formation of low-melting-point sodium silicates via reactions between Na-based species and SiO2 was the main slagging mechanism of ZD coal.The self-melting of Na2SO4 was the mian reason for enhancing slagging.Besides,slagging was also induced under the interaction of low temperature eutectics formed via reactions between SiO2 and Na2O/Na2CO3 released from organic Na.Overall,the promotion of NaAc and Na2SO4 on slagging was higher compared to NaCl.2)Experiments about ZD coal were conducted under different air equivalence ratio(ER)conditions in 0.4 T/D CFB test rig.The results show that at reducing atmosphere,silica-based bed material would react with Na-based compounds in ZD coal forming low-melting-point sodium silicates(Na2O·nSiO2),and induced slagging in riser;at oxidizing atmosphere,massive AAEM-based species would be released from the raw coal and then deposited on low-temperature tailing surfaces via diffusion,condensation,aggregation and thermophoresis,resulting in obvious layered deposits at 607?735 ?.3)The feasibility of minerals as gasified bed material of ZD coal during CFB gasification was studied.It is found that silica-,aluminum-and phosphorus-based bed materials had a strong capturing capacity of Na.The former one usually reduced the ash fusibility of gasified ash increasing the risk of slagging/agglomeration in gasifier,while the cases for the latter two were reversed.The results of experiment and calculation reveal that CaO in the industrial CFB boiler ash had the reaction priority with minerals(SiO2 and Al2O3)in coal ash over Na-based species,which restrained the retention of Na and then increased the ash fusibility of gasified ash.Hence,the industrial CFB boiler ash is an effective and low-cost potential bed material for ZD coal to avoid the slagging in CFB gasifier.4)The influence of wall temperature on slagging during ZD coal gasification in CFB was studied.When wall temperature was above 882 ?,slagging would occur on the probe,while fine ash particles would deposit on the wall as the temperature gradient between gas phase and wall gradually increased.The selective deposition of mineral elements was confirmed under the action of wall temperature:The enrichment of Ca,Si,Na,Fe and S was the main cause of slagging,and the condensation of volatile species such as NaCl was responsible for the deposition of fine ash particles.The deposits on low-temperature walls could be easily removed and not affect the normal operation of gasifier because of the poor viscosity during gasification.5)The feasibility of controlling wall temperature to relieve the ash deposition on tail heating surfaces during ZD coal combustion was studied.Experimental results illustrate the decreasing wall temperature enhanced the ash deposition due to the constant condensation of NaCl,leading to the deterioration in the heat transfer characteristics of the probe.On the contrary,the increasing wall temperature matched the decreasing deposition amount.However,the higher wall temperature enhanced the corrosive reactions between the initially deposited NaCl and alloys,causing serious damage to heating surfaces.6)The corrosion characteristics of ZD coal during CFB combustion was investigated.Two corrosion phenomena including high temperature gas corrosion and tail deposited ash corrosion were found during Shaerhu coal(SEHc)combustion in CFB.Thermodynamic calculation results indicate that the priorities of protective oxide formation and corrosion reactions among different alloy elements were basically opposite.The ordinary alloy material sprayed with the anticorrosion Al2O3 coating seems the optimal case.7)The measured compositions of ZD coal ash prepared at 815 ? couldn't reflect its real ones,which was the main reason for the inaccuracy of slagging prediction based on the Chinese Standard ashing temperature.The liquid phase ratio-temperature curve was obtained using Factsage 6.1 with the ash compositions at 500 ? as input data,the safe operating temperature corresponding to 25%liquid phase ratio was confirmed.The fouling characteristics of ZD coal resulted from a combination of ash compositions and temperature.Based on the gas-solid distribution of coal ash as a function of temperature,the contributions of residual and volatile phase to fouling were determined,and one new fouling prediction method was proposed.