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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 °C for 15 min) of a bituminous coal and its beneficiated fractions (>75 μm <1000 μm) 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/cm 3 float and either 2.7 M (Molarity) KOH or 2 M KCl or 0.9 M K 2 SO 4 or 1.5 M KNO 3 or 1.1 M K 2 CO 3 or 1.5 M KCH 3 CO 2 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 Al 2 O 3 •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.

This study assessed V5+ concentration and V5+ to V4+ ratio effects on the H2S absorption and oxidation to elemental sulfur from H2S gas streams in simulated Stretford solutions. Laboratory-derived sulfur products were characterized by inductively coupled plasma optical emission spectrometry, X-ray diffraction (XRD), particle size distribution (PSD), and differential scanning calorimeter (DSC) analyses for probing the chemical, mineralogical, and physical properties of the sulfur products. H2S absorption and sulfur production efficiencies increase with an increase in the V5+ concentration. However, the sulfur’s PSD results are finer and the purity decreases as the V5+ concentration increases because of K/Al/Si/Mg/Fe impurities in the added technical grade sodium ammonium vanadate (SAV) to the Stretford solution. XRD and DSC results of the orthorhombic sulfur (α-S8) products indicate that these products have similar mineralogical properties and melting points compared to those of the commercial α-S8. V5+ to V4+ ratios decreased with a decrease in α-S8 production efficiency, and a total vanadium loss of 89% was achieved because of the V4+ precipitation from the Stretford solution without disodium salt of 2,6 and 2,7 isomers of anthraquinone-disulphonic acid (Na2[ADA]). Low V5+ to V4+ ratios were found to be responsible for the sulfur sol formation and subsequent dark-colloidal V4+ and fine α-S8 precipitation in the Stretford units. The Stretford process must be operated with Na2[ADA] for the V4+ oxidation in the solutions. Research outcomes as well as the total replacement for SAV with a pure V5+ salt can assist the Stretford operators to further improve α-S8 quality and its production efficiency.

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