Investigation on the effects of different forms of sodium, chlorine and sulphur and various pretreatment methods on the deposition characteristics of Na species during pyrolysis of a Na-rich coal

In this work the effects of different forms of sodium, chlorine and sulphur and various pretreatment methods on the evolutionary behaviors and deposition characteristics of Na species were systematically investigated. The pyrolysis experiments of Na-loaded coal samples (in which Na was loaded by NaOH and CH3COONa) blended with various Cl- and S-bearing additives were conducted in a lab-scale pressurized fixed bed reactor at 1000 °C, and the transformation of Na was subsequently analyzed. Finally, the mechanism on the evolutionary behaviors of Na was proposed. Experimental results show that acid elution treatment could significantly elute corrosive elements from the coal and the absence of anions in gas phase could significantly reduce Na deposition even adequate Na was loaded into the acid washed coal. However, when Cl- and S-bearing substances were added to the Na-loaded coal, abundant NaCl and Na-S-O crystals were deposited on the probes and Cl is more competitive in enhancing Na-bearing compounds deposition, compared with S. During Zhundong coal pyrolysis, the crystalline process of Na species, morphologies of the deposits and Na retention in char are significantly affected by the interactions between Na-bearing compounds and coal matrix or other inorganic species. More importantly, the deposition of crystal NaCl was decreased with rising pretreatment temperature. While the addition of CaO in coal or char was effective in capturing volatile Cl atoms and/or HCl to generate CaCl2, resulting in the absence of NaCl in the deposits during coal or char pyrolysis. Furthermore, the addition of CaO in char could also enhance the interactions between active Na species and coal matrix or minerals, resulting in the increased Na content in CaO-loaded char.