Characterizing moisture occurrence state in coal gasification fine slag filter cake using low field nuclear magnetic resonance technology

Gasification fine slag, a carbon-rich solid waste produced during coal gasification, is currently disposed of through conventional landfilling methods. Its high-water content hampers resource utilization by reducing its calorific value. This study employs low field nuclear magnetic resonance (LF-NMR) analysis to investigate the water occurrence states in a filter cake with 70.7% moisture content derived from fine gasification slag of the Ningxia coal water slurry furnace. The analysis reveals that free water constitutes a major proportion (68.28%), while combined and capillary water accounts for 2.62% and 2.33%, respectively. The removal of moisture from the coal gasification fine slag filter cake is significantly affected by the drying temperature. The drying rate initially increases and then decreases with different temperatures. For drying temperatures of 60, 70, 80, 90, and 100°C, the maximum drying rates are achieved at 1.09, 1.23, 2.18, 3.86, and 4.01%/min, respectively, with corresponding durations of 40, 40, 25, 30, and 35 min. The T₂₂ relaxation peak area continually decreases and shifts leftward on the coordinate axis. The behavior of the T₂₃ and T₂₂ peaks follows a similar pattern, indicating that water with higher mobility is easily eliminated during drying, while the T₂₁ peak exhibits a robust signal strength throughout the drying process. At a drying temperature of 100°C, the proportion of free water (A₂₂) decreases from 87.3% to 0, the A₂₃ fluctuates between 0 and 5.3%, and the bound water peak area (A₂₁) increases from 10.4% to 94.7% as drying time prolongs. A relationship model is developed between the moisture content of the coal gasification fine slag filter cake and the total integral area of the LF-NMR test, enabling rapid detection and analysis of the total moisture content and distinct occurrence states in the filter cake.