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

Abstract

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.

Catalytic Effect of Reactive Extraneous Mineral Composites on Char and Tar Distribution during Pyrolysis of Highveld Partially Oxidized Fine-Coal Reject and Its Beneficiated Residues

“…The ashes of these samples comprised mainly SiO 2 , Al 2 O 3 , CaO, SO 3 , Fe 2 O 3 , and MgO with minor percentages of TiO 2 , K 2 O, Na 2 O, and P 2 O 5 which are consistent with the ash composition of SA feed-coals to commercial gasifiers. 26 , 27 Mphahlele et al 28 found that higher concentrations of trace elements (Co, Zn, As, Rb, Sr, Zr, Ag, Cd, Ba, In, and Sn) and rare earth element (Y) are concentrated in the density-separated fractions of the FCR samples compared to the typical concentrations for SA Highveld ROM coals, as reported by Wagner and Hlatshwayo. 29 This could be associated with health and environmental issues during the storage of FCR samples in stockpiles and slime dams.…”

Production of sodium-based zeolites and a potassium-containing leach liquor by alkaline leaching of South African coal fines ash

“…Other changes observed in the mineral composition include a decrease in diopside concentration from 4.4% to zero and an increase in illite from zero to 4.6%. Matjie et al (2021), Rautenbach et al (2020) and Rautenbach et al (2021) state that Al2O3•2SiO2 reacts with K + from the added potassium species to form K-feldspar in the chars and coal ashes upon pyrolysis and combustion of South African feed-coals. A lower pseudomullite proportion was also noted in the SA2 blend ash sample.…”

Production of Al(HI)-K(I)-Ti(IV)-sulphate-containing leach liquor from metakaolinite-containing ash derived from South African coal fines

“…An increase in the proportion of insoluble microcline in the SA2 blend ash samples prepared at 1050°C can be attributed to the reaction between potassium ions from the K₂CO₃ melt or K₂O reacting with metakaolinite, at elevated temperatures (Tables III and IV) (Rautenbach et al, 2021, Matjie et al, 2018. Matjie et al (2021) studied the reaction between metakaolinite and potassium cations to form synthetic K-feldspars by heating a blend of 10% K₂CO₃ and coal at 700°C and 1050°C in air. In addition, Collins et al (2018) used FACTSAGE™ modelling to predict microcline and kalsilite crystallization at temperatures between 1050°C and 1250°C from the melt of the SA2 blend.…”

“…The synthetic potassium feldspar minerals, including microcline, form when potassium species react with metakaolinite at 600-700°C (Heller-Kallai, 1978;Heller-Kallai andLapides, 2003, Matjie, et al, 2021). Other potassium feldspars present in the coals begin to transform at temperatures of >1050°C (Matjie et al, 2021).…”