Distribution, occurrence and leaching dynamic behavior of sodium in Zhundong coal

- The sodium content in Zhundong coal decreases with the increasing depth to roof.
- Part of sodium would release during high temperature ashing process.
- H2O-Na predominated in Zhundong coal and is affected by migration and evaporating of coalbed water.
- Shrinking core model was employed to describe the leaching process of sodium in a seam coal and parting.
- 88.63 and 96.62 wt% of H2O-Na is leached out from the seam coal and parting, respectively.

The utilization of rich-reserve Zhundong coal is limited due to the high sodium content, which can cause severe fouling and slagging troubles in a traditional boiler. The collected drill data of sodium, chlorine and ash in Zhundong coal, as well as the vertical samples including roof, up seam (US) coal, parting, low seam (LS) coal and floor were characterized and investigated. It is found that the sodium content in Zhundong coal is a magnitude higher than chlorine content and decreased with the increasing depth to roof of the coal seam, which agrees well with the changing trend in vertical samples: roof > US coal > LS coal > parting > floor. The determined Na2O content in ash was lower than the converted content indicated part of sodium volatilized during ashing process. Water soluble sodium (H2O-Na) predominated and accounted for 65.50, 82.49 and 64.35 wt% in US coal, parting and LS coal, respectively. H2O-Na content is affected by migration and evaporating of coalbed water. SEM-EDX and XRD results indicated that Na, S, O and Ca are the main elements in sodium-rich minerals of roof. With higher H2O-Na and less specific surface area and microporous volumes, parting has an obviously higher initial leaching rate and leachability. The shrinking core model shows that the diffusion rate constant (Kp) is 0.002 and 0.005 h−1 for LS coal and parting, respectively. 88.63 wt% of H2O-Na in LS coal was leached out, and final Na2O content is markedly lower than the threshold value of coal-fired boiler.