Na/Ca Removal From Zhundong Coal By CO₂-water Leaching Method

Zhundong coalfield is the largest intact coalfield with forecast reserves of 390 billion tons.However,due to the high contents of AAEM?alkali and alkaline earth metals?,especially Na and Ca,Zhundong coal has caused serious fouling and slagging problems during combustion,which seriously affects its popularization and application.Removal of Na and Ca through washing is an effective way to solve this problem.We have proposed a CO₂-water leaching method in our previous study,which can remove AAEM from Zhundong coal effectively.However,the key influence factors and practical feasibility of CO₂-water leaching were not studied.Furthermore,the mechanism of the CO₂-water leaching was complex and not clear.Thus,an optimization method was put forward in this study,which called continuous CO₂-water leaching.The effect of the method on removing Na/Ca and practical application were investigated.The chemical mechanism of the method was further analyzed.The details are as follows:Firstly,the removal efficiency of Na/Ca and the influence time and temperature using continuous CO₂-water leaching were investigated.WCW coal was chose for continuous CO₂-water leaching and water-leaching experiments to explore its effect on Na/Ca removal.It was found that the removal of Na/Ca was improved obviously through comparing with previous study.The removal efficiency of Na increased by 33.6%.While the removal advantage of Ca was more obvious,and the removal efficiency increased from 10.7%to 57.4%.The effects of CO₂-water leaching were all better than that of water leaching under the same conditions.Because CO₂-water leaching can not only remove water-soluble alkali metals,but also remove a large number of organic alkali metals,which are extremely harmful.So that CO₂-water leaching can effectively inhibit the fouling and slagging problems of Zhundong coal.It was also found that the increase of temperature and time can promote the removal of Na,but have different influence on the removal of Ca.The removal efficiency of Ca showed fluctuating changes due to the dual effects of solubility and reaction rate.The increase in time was very favorable for the removal of Ca.The removal efficiency of Ca increased by 33.2%from 3 h to 12 h.Secondly,the practical application of CO₂-water leaching was preliminary explored in this study.In the case of having impurity ions in water,CO₂-water washing can still effectively remove Na/Ca from the coal,and the removal rate can reach 3.17 mg/g·coal and4.34 mg/g·coal respectively.In contrast,water leaching adsorbed impurity ions into the coal.CO₂-water leaching inhibitted the initial stage of gasification due to the removal of the catalytic minerals in the coal.But it changed the pore structure and removed ashes of coal,so that can accelerate gasification reaction at high temperature.In addition,simulated flue gas experiments show that the removal efficiencies of Na and Ca can reach 90%and 50%respectively when the CO₂ concentration is only 15%,which was much higher than that of water leaching.The above results proved that CO₂-water leaching has practical application prospects.And it is feasible to apply flue gas in coal-fired power plant to CO₂-water leaching.The removal mechanism of CO₂-water leaching was finally explored.It can be seen from the results of ion exchange and sequential leaching experiment that CO₂-water leaching can remove organic Na and Ca.These Na/Ca exists in the form of carboxylic and phenolic groups,the removal efficiencies can reach 65.1%and 57.3%,respectively.The removal mechanism includes two steps.The first step is the dissolution of CO₂ into water to form radical hydrogen?H⁺?.The second step is the removal of organic AAEM by H⁺through ion exchange.Continuous CO₂-water leaching can always provides H⁺to attack the electron cloud around carbon?C?,so that the cations linked to C is removed,which enhanced the removal efficiency of AAEM.Thus,this study proved that the CO₂-water leaching was an effective and promising method for AAEM removal from Zhundong coal.Which also has the foundation of practical application.The flue gas in coal-fired power plant can be used as cheap CO₂ source.