Transformation Of Sodium During The Pyrolysis And Combustion Of CO₂-water Washed Zhundong Coal

The high content of alkali and alkaline earth metals?especially Na and Ca? in Zhundong coal will volatilize and react with SO3?fly ash, forming complex sulfate during coal combustion, these sulfates will lead to series of issues, such as severe slagging and fouling during coal combustion. To solve this problem, an effective method is to remove Na and Ca from Zhundong coal before coal combustion, such as coal washing, washing solution like water and acid are the traditional coal washing solution, Na associated with oxygen can not be removed with water washing, while acid solution can remove most organically bound sodium, but acid washing is still in the laboratory research scale, so an efficient and economical method for Na removal is required. In this paper, the transformation of Na during the pyrolysis of Zhundong coal are investigated, based on the above research, CO_2-water washing was used to remove Na from Zhundong coal, then the effect of CO_2-water washing on the transformation of Na during the pyrolysis of Zhundong coal and the influences of CO_2-water washing on combustion characteristics of Zhundong coal are investigated.First of all, the occurrence modes of Na and Ca in Zhundong coal are examined by sequential extraction with deionized water ?0.1mol/L HCl. Experimental results show that 66.2% of Na in Zhundong coal is water soluble, while the proportion of acid soluble Na is 28.3%?in which the majority is organically bounded?, most of Ca is water insoluble but acid soluble. Volatility of Na increases as the pyrolysis temperature rising, the volatility of Na mainly come from the decomposition of organically bound Na, while the pyrolysis temperature is below 700?. Water soluble Na?like NaCl?Na2SO4? start to volatile as pyrolysis temperature rising, part of water soluble Na and acid soluble Na transform into acid insoluble Na.Then CO_2-water washing was used for Na especially organically bound Na removal, CO_2-water washing can remove more than 80% of Na and part of Ca from Zhundong coal, removal rates of Na and Ca increase with the increasing extraction time and temperature. CO_2-water washing has little effect on coal quality, but can remove 40% of ash from coal. The volatility of Na decreases to 3% during the pyrolysis of CO_2-water washed coal at 500?, and the volatility of Na and Ca decrease at 900?, too.At last, the combustion characteristics of CO_2-water washed coal are studied, it is found that ignition temperature and burnout temperature of CO_2-water washed coal are lower than raw coal, the volatility of Na decreases after CO_2-water washing, alkali-acid ratio?B/A? of ash decreases from 2.50?raw coal? to 1.03(CO_2-water washed coal). CO_2-water washing change the slagging properties of Zhundong coal.CO_2-water washing is an effective method for Na removal from Zhundong coal.