水溶Na对高碱煤热解及氮热变迁的影响

Influence of water-soluble sodium on the pyrolysis and thermal nitrogen transformation of high alkali coal

新疆准东煤田煤炭资源丰富,部分新疆煤含有超过0.4%的高碱金属含量而被称为高碱煤。在热利用过程中碱金属Na作为催化剂可改变煤的反应活性,进而影响其热反应过程及氮氧化物的排放。为研究水溶Na对高碱煤热解特性及氮热变迁规律的影响,选取准东煤作为典型新疆高碱煤,并采用盐酸对其进行脱灰预处理,在脱灰煤中添加不同含量的NaCl代表高碱煤中水溶钠,利用热重-差示扫描量热、红外、质谱(TG-DSC-FTIR-MS)联用仪开展热解实验,运用Coats-Redfern积分法对煤热解反应进行动力学分析。结果表明,少量Na促进低温段(40~295℃)的干燥脱气反应,大量Na促进高温段(>800℃)二次脱气反应,Na对主热解阶段(中温段)影响较小。动力学分析表明Na含量≤0.6%时,煤热解表观活化能降低,Na对煤热解具有催化作用。通过红外与质谱的综合分析,发现在低加热速率条件下,对煤燃烧过程NOx气态前驱物以HCN为主,其生成量远大于NH3。Na的添加可明显促进煤热解NH3生成,0.7%Na的含量使NH3的析出量增加61%。较少的Na可促进HCN的生成,而当Na含量≥0.6%时,Na对HCN的生成起抑制作用。总体上Na含量超过0.6%时,可减少NOx前驱物的生成。研究认为0.6%的Na含量有利于高碱煤的热解利用:促进高碱煤热解,减少NOx前驱物的生成。

Zhundong coalfield has a huge coal resource,some of which is called high alkali coal because the alkali metal content exceeds 0.4%.In the process of thermal utilization,Na acts as a catalyst to affect the thermal reaction process and NOx emissions by changing the activity of the coal reaction process.In order to study the effect of water-soluble Na on the pyrolysis and nitrogen transformation of high alkali coal,Zhundong coal was selected as a typical high alkali coal.Different content of NaCl was added into Zhundong coal after washing by hydrochloric acid solution to represent water-soluble sodium.Pyrolysis experiments were carried out by thermogravimetry and differential scanning calorimetry,Fourier transform infrared spectroscopy,and mass spectrometry(TG-DSC-FTIR-MS).The Coats-Redfern integration method was used to analyze the kinetics of coal pyrolysis reaction.The results show that a small amount of sodium promotes the degassing reaction at low temperature of 40-295℃,excessive sodium exhibits a promoting effect at high temperature of greater than 800℃,and the effect of sodium on the main pyrolysis stage is not obvious.The kinetic analysis shows that the apparent activation energy decreases when the amount of added Na is less than 0.6%,indicating that Na has a catalytic effect on the pyrolysis characteristics of high alkali coal.The comprehensive analysis of infrared and mass spectrometry found that HCN is the main gaseous precursor of NOx during coal combustion process under slow heating conditions,and its production is much larger than NH3.The addition of Na can significantly promote the formation of NH3 during coal pyrolysis,which can be increased by 61%at the Na content of 0.7%.The amount of HCN produced increases when the amount of Na is less than 0.6%,but the addition of Na inhibits the formation of HCN when its content is more than 0.6%.The generation of NOx precursor is reduced at the Na content above 0.6%.Therefore,the pyrolysis utilization of high alkali coal is beneficial at the Na content of 0.6%,which promotes coal pyrolysis and reduces the formation of NOx precursors.