Investigation On Pyrite Transformation And Its Effects On Slagging During O₂/CO₂ Combustion

Oxyfuel?O₂/CO₂?combustion is one of the most promising technologies to reduce CO₂emission from coal combustion power plants.With the development of oxyfuel combustion technology,slagging in the furnace is still a problem should be solved.Pyrite has been identified as a major contributor to ash slagging.Switching from air combustion to oxyfuel combustion may change the transformation behavior of pyrite,which will affect ash slagging properties.Therefore,investigation on pyrite transformation behavior and the effect of which on slagging properties during O₂/CO₂ combustion are of great help to better understanding ash slagging behavior in oxy-coal combustion.However,so far only limited relevant investigations have been reported.In this research,pure mineral,synthetic char and pulverized coal were used as experiment samples.Experiments were carried out in a fixed bed reactor and a drop tube furnace?DTF?to investigate pyrite transformation behavior and its impact on ash slagging properties during O₂/CO₂ combustion.The details of this research are summarized as follows:Firstly,effect of O₂/CO₂ combustion on pyrite transformation and the products slagging properties was investigated in a high temperature DTF.Poisson distribution model was used to estimate pyrite fragmentation in different combustion atmospheres.Pyrite transformation model in air combustion was modified to apply in O₂/CO₂ combustion case.To exclude the influence of other mineral,natural pyrite was selected as experiment sample.Results show that under low O₂ level conditions,major iron species in solid products are pyrrhotite and Fe₃O₄.Chemical property of CO₂ plays a dominant role on pyrite transformation.Comparing with O₂/N₂ combustion,O₂/CO₂ combustion promotes pyrite fragmentation and one pyrite particle averagely fragments to two particles.The amount of Fe₃O₄ in products increases from 34%to about 64%in O₂/CO₂ combustion.Higher content of Fe₃O₄ in solid products will decrease ash slagging propensity.Under high O₂ level conditions,major iron species in solid products are Fe₃O₄ and Fe₂O₃.Physical property of CO₂ plays a dominant role on pyrite transformation.Comparing with O₂/N₂ combustion,pyrite combustion temperature decreases in O₂/CO₂ combustion because of lower O₂ diffusion rate in CO₂.The amount of Fe₂O₃ in the solid product increases from 38.9%to 46.8%in O₂/CO₂ combustion case.With O₂ increasing from 21%to 32%,the amount of Fe₂O₃ in the solid product increases from 46.8%to 67.7%.Under high O₂ level conditions,combustion atmospheres have insignificant effect on ash slagging propensity.Secondly,pure mineral reaction experiment in fixed bed reactor and symthertic char combustion experiment in DTF was carried out to investigate the effect of O₂/CO₂combustion on pyrite and kaolinite interaction and its impact on ash slagging.Results show that comparing with O₂/N₂ combustion;O₂/CO₂ combustion promotes pyrite and kaolinite interaction.The amount of iron glass in solid products increases from 40.2%to 58.2 and ash particles size distribution increases in O₂/CO₂ case.With the O₂ concentration in combustion atmospheres increasing from 21%to 32%,interaction between and kaolinite is suppressed.The amount of iron glass in solid products decreases from 58.2%to 43.2%and ash particles size distrbituion decreases.Char combustion temperature has dominant effect on included pyrite and kaolinite interaction and their ash particle size distribution in different combustion atmospheres.Except for char combustion temperature,chemically participation of CO₂ in pyrite transformation also promotes the interaction between included pyrite and kaolinite in some degree.In different combustion atmospheres,ash slaggging properties of synthetic char are mainly influenced by particle size distribution of bulk ash.Coarser particle size distribution will increase ash impaction efficiency and promote ash slagging.In order to investigate the effect of O₂/CO₂ combustion on ash properties and pyrite transformation,SH coal riched in excluded pyrite and HM coal riched in included pyrite was subject to combust in the DTF,respectively.Results show that comparing with O₂/N₂combustion with the same O₂,O₂/CO₂ combustion promotes mineral interaction.The PSDs and sintering strength of bulk ash increase,while the ash melting temperature decreases in O₂/CO₂ combustion.With O₂ in combustion atmospheres increasing,mineral interactions are suppressed.The PSDs and sintering strength of bulk ash decrease,while the ash melting temperature increases.Though combustion atmospheres have similar effect on the interactions of pyrite and calcite with aluminosilicate,their effect on the interactions of pyrite with aluminosilicate is more evident.Combustion atmospheres have similar effect on the interactions of included and excluded pyrite with other mineral.However,they have more evident effect on the interactions of included pyrite with other mineral than that of excluded pyrite.In order to investigate the impacts of ash properties variation of pyrite-rich coal on ash slagging during O₂/CO₂ combustion,slagging experiments of SH and HM coal were carried out in DTF.Results show that combustion atmospheres have similar effect on the slagging propensity and chemical composition of SH and HM coal.Comparing with O₂/N₂combustion with similar O₂,O₂/CO₂ combustion increases ash slagging propensity.The relative contents of SiO₂ and Al₂O₃ in the slag increase,while that of Fe₂O₃ decreases.With O₂ increasing,the ash slagging propensity decreases.The relative contents of SiO₂ and Al₂O₃ in the slag decrease,while that of Fe₂O₃ increases.However,combustion atmospheres have insignificant effect on slagging morphology.For both coals investigated,changing of ash slagging propensity in different combustion atmospheres has closely correlation with interaction behavior of pyrite and aluminosilicate.On one hand,interaction of pyrite and aluminosilicates will result in the formation of low melting point iron glass.It will increase capture efficiency of ash particle;on the other hand,interaction of pyrite and aluminosilicates will increase particle size distribution of ash.It will increase capture efficiency of ash particle.