Interactions between Kaolinite, Organic Matter, and Potassium Compounds at Elevated Temperatures during Pyrolysis of Caking Coal and Its Density-Separated Fractions

Large coal agglomerates (cakes), which form in slow heating fixed-bed commercial gasifiers, result in low carbon efficiency and carbon loss in the ash assemblage. In an earlier study, laboratory-scale pyrolysis (600, 720, or 920 degrees C for 15 min) of a bituminous coal and its beneficiated fractions (>75 mu m <1000 mu m) was conducted to determine chemical reactions, which may inhibit caking propensity. It was found that the <1.5 g /cm(3) float achieved 70% caking compared to feed coal (18%) and its >1.9 g/cm(3) sink (0%). In this study, blends of the same <1.5 g/cm3 float and either 2.7 M (Molarity) KOH or 2 M KCl or 0.9 M K2SO4 or 1.5 M KNO3 or 1.1 M K2CO3 or 1.5 M KCH3CO2 were pyrolyzed to gain valuable insights into the reactions inhibiting caking propensity. Kaolinite, organic matter, and potassium compound interaction effects in the blends, KOH concentrations, and temperature effects on the caking propensity were therefore investigated. X-ray diffraction (XRD) results indicated that the potassium compounds added to coal resulted in the formation of noncaking oxygen-containing potassium minerals (illite, sanidine, nepheline, microcline, and muscovite) at elevated temperatures due to the reactions of Al2O3.2SiO(2) and K+ in the chars. The addition of high proportions of potassium compounds to coal may result in gasifier blockage/corrosion/erosion and carbon loss due to clinker/slag formation at elevated temperatures and should be considered with caution. Nuclear magnetic resonance results further show that polycyclic aromatic hydrocarbons and carboxylic acids present in coal and KOH blends reacted with KOH to form oxygenates (23%) in the noncaking chars. Large cakes derived from the <1.5 g/cm(3) float contained 11% oxygenates. Also, Fourier transform infrared (FTIR) results for the 2.7 M KOH blend char confirmed higher intensities of peaks of oxygen-containing compounds, which are associated with caking propensity inhibition during pyrolysis. These chemical reactions in the presence of organic matter, kaolinite, and KOH to form oxygenates and potassium feldspars in the chars significantly reduced the highest coal particles caking (70%) to <2 or 0%. Based on these findings, the <0.5 M KOH blend strategy should be considered for use in commercial gasifiers to inhibit severe coal particle caking.

Automated summary

Potassium compounds added to coal can lead to gasifier blockage/corrosion/erosion and carbon loss due to clinker/slag formation at elevated temperatures. During pyrolysis, organic matter, kaolinite, and potassium compounds form oxygenates and potassium feldspars in the chars, significantly reducing coal particle caking.