Experimental Study Of Zhundong Coal Chemical Looping Combustion Using Hematite And Transfer Characteristic Analysis Of Sodium And Chlorine

Zhundong coalfield is the biggest integrated coalfield in China even the world.But the issues of fouling and slagging often take place on the internal furnace heating surface during conventional combustion process because of the high sodium concentration in Zhundong coal.Although adding the solid additions including Al2O3 and SiO2 could relieve the fouling and slagging,the operation is not solved radically.Therefore,seeking for a new combustion process maybe is a better solution instead of focusing on conventional combustion process.Chemical looping combustion(CLC)is an innovative combustion method,which utilizes the lattice oxygen from oxygen carrier to deliver oxygen.And the technology of coal chemical looping combustion using hematite as oxygen carrier is proven to be one of the promising and effective coal combustion technologies.A certain amount of Al2O3 and SiO2 in hematite could also have the same benefits to relieving the fouling and slagging as the solid additions.In the meantime,the slow coal gasification rate is the biggest problem in the coal CLC process.Alkali and Alkaline Earth Metal(AAEM)is effective catalyst for coal gasification progress and the high sodium concentration in Zhundong coal could solve the problem.Besides,the oxygen carrying capacity of hematite is relatively poor,but relatively high oxygen concentration in Zhundong coal could load off the pressure of transferring lattice oxygen.At last,compared to the oxidized atmosphere of high temperature in conventional combustion process,CLC has a reduced atmosphere of low temperature,which must affect the release of sodium and chlorine in the combustion process.Therefore,this paper comprehensively investigates the Zhundong coal chemical looping combustion using hematite as oxygen carrier.At first,the feasibility study of Zhundong coal chemical looping combustion using hematite as oxygen carrier is conducted in the batch fluidized bed and 1kWth reactor of interconnected fluidized beds,respectively.The results based on the batch fluidized bed show that:(1)high content of sodium in Zhundong coal gives rise to an increase of carbon conversion and carbon conversion rate and accelerates the carbon conversion process;(2)the absorption reaction between hematite and sodium is influenced by the gasification agent and steam agent is more effective compared to N2 or CO2,where more sodium exists in the formation of higher melting aluminosilicates.Na2O·Al2O3·6SiO2;(3)the reactivity of hematite is increased with cycle number and a certain amount of sodium transfers from Zhundong coal to hematite forming Na3FeO2,which further improves porous properties of hematite.The results based on the 1kWth reactor of interconnected fluidized beds show that:(1)in the reactor which is more close to the engineering operation condition,Zhundong coal chemical looping combustion using hematite as oxygen carrier exhibits a better combustion performance and sodium in the fly ash exists in the formation of high melting point sodium aluminosilicate,Na2O·Al2O3·6SiO2 instead of low melting point sodium silicates;(2)NOx release is relatively less in the operation process;(3)defluidization is not detected during the 10 h continuous operation and the porous properties of hematite are enhanced due to the sodium in Zhundong coal.The separation of coal ash from oxygen carrier particles is still not perfectly realized in the recent coal CLC technology,so the existence of coal ash would affect the oxygen carrier property in the operation process.This paper investigates the influences of Zhundong coal ash with a thermogravimetric analyzer in terms of ash formation characteristics,oxygen carrying capacity and effects of other solid additions.The results indicate that:(1)the Zhundong coal ash formation rate is increased with the temperature and steam agent compared to CO2 agent is more conducive to ash formation process;(2)sodium content fixed in ash in CLC process is lower than that in conventional combustion process,especially for the CLC with steam gasification agent;(3)Zhundong ash could release oxygen and regenerate in the CLC process,which enhances the reactions in fuel reactor(FR);(4)pure SiO2 addition is the least effective for avoiding fouling and slagging problem of Zhundong coal CLC,and effects of Kaolin and bauxite are similar,while pure Al2O3 is the most effective.Next,the influences of reaction order and agent to the reactions between NaCl,Al2O3 and SiO2 are investigated.The results indicate that the order of that NaCl first reacts with SiO2 and then reacts with Al2O3 is more conducive to the sodium absorption.And higher gasification temperature results in more sodium absorbed in the formation of sodium aluminosilicate.In addition,when NaCl first reacts with SiOand then reacts with Al2O3,the sodium absorption efficiency is based on the production of Na2O·3SiO2 from the reaction between NaCl and SiO2.The sodium absorption efficiency at different agents is in the sequence of O2>H2O>CO2>N2.When NaCl first reacts with Al2O3 and then reacts with SiO2,the sodium absorption efficiency is based on the molten NaCl in the products from the reaction between NaCl and Al2O3.The sodium absorption efficiency is in the sequence of N2>CO2>H2O>O2.At last,the Zhundong coal CLC reaction in FR is split into three steps:pyrolysis,gasification and reaction with hematite.In each step,the transfer characteristics of chlorine and sodium are investigated and the transfer sketches are given.Chlorine and sodium exhibit different transfer characteristics at different reaction temperature.However,the final formation of chlorine always includes most gaseous HCl and less gaseous NaCl,and the final formation of sodium always includes less gaseous NaCl and most solid Na2O·3SiO2 and Na2O·nAl2O3·mSiO2.