In Situ Capturing and Absorption of Sulfur Gases Formed during Thermal Treatment of South African Coals

Abstract

The objective of this study, the first of its kind on these specific South African low-sulfur coals, was to capture H 2 S and SO 2 produced under inert and oxidizing conditions from sulfur compounds present in the coals. The capturing agents were calcium and magnesium oxides formed during the transformation of calcite and dolomite. The effectiveness of two different scrubbing solutions (0.15 M cadmium acetate and 1.1 M potassium hydroxide) for absorption of volatilized H 2 S and SO 2 was also investigated. The bituminous coal (coal A) contained dolomite, calcite, pyrite, and organic sulfur. Lignite (coal B) had a high organic sulfur content and contained gypsum, no or low dolomite and pyrite contents, and no calcite. A third sample (coal C) was prepared by adding 5 wt % potassium carbonate to coal A. Under oxidizing conditions and at elevated temperatures, FeS 2 produced Fe 2 O 3 , FeO, and SO 2 . It transformed to FeS and released H 2 S under inert conditions. Organic sulfur interacted with organically bound calcium and magnesium at 400 °C in an inert atmosphere to form calcium sulfate and oldhamite ((Ca,Mg)S). CaO, produced from calcite or dolomite, reacted with SO 2 and O 2 at 950 °C to form calcium sulfate. Treatment of lignite at 400–950 °C resulted in 96–98% evolution of sulfur as gases. Hydrogen sulfide formation increased with the increasing thermal treatment temperature under inert conditions for the three coals. Under oxidizing conditions, sulfur dioxide formation decreased with the increasing temperature when heating coals B and C. The lowest ratio (0.01) of H 2 S to SO 2 was achieved during thermal treatment of the blend of coal and potassium carbonate (coal C), implying that almost all of sulfur was retained in the coal C ash/char samples. In situ capturing of sulfur gases by CaO and MgO and by the added K 2 CO 3 in coal C to form calcium/magnesium/potassium sulfates and potassium/calcium/magnesium aluminosilicate glasses during utilization of these and similar coals could reduce the percentage of sulfur volatilized from the coals by 54–100%, thereby potentially decreasing their impact on the environment.

Mineralogy and Petrology of Chars Produced by South African Caking Coals and Density-Separated Fractions during Pyrolysis and Their Effects on Caking Propensity

“…As expected, pyrite in the South African coals reacts under reducing/inert conditions to form pyrrhotite. 33,34,47 The heated Sink1.9 samples that are rock fragments contained a high proportion of excluded FeS 2 (14 and 17%). As can be visually observed in Figures 3, 8, and 9, the presence of these noncaking or nonswelling minerals (rock fragments of extremely high ash carbominerite particles) in the sink fraction can reduce the caking of coal particles or inhibit caking during pyrolysis of the sink particles.…”

“…Following the literature review stating that the maximum swelling propensity of the lump Waterberg coal is 550 °C, , this temperature was also selected for the pyrolysis experiments. Also, it is expected that the organically associated inorganic elements in the macerals react with each other at 400–450 °C and 0.87 and 30 bar could contribute positively to the caking propensity of the caking feed coal constant heating rate of 6–7 °C/min of the oven employed during experimentations. The residence time of approximately 15 min at the final pyrolysis temperature was used during the thermal treatment of the sample under inert atmosphere using nitrogen gas with an ultrahigh purity of 99.99%, as supplied by African Oxygen (Afrox) Ltd, Industrial Gas Company, South Africa.…”

“…It has been reported that highly aliphatic liptinite and vitrinite macerals in the caking coal contribute significantly to the formation of metaplast (liquid-molten vitrinite), and in the plastic temperature range (350–550 °C) to the development of fluidity and swelling, hence caking. The included kaolinite, submicron minerals associated within macerals (for example, pyrite (FeS 2 ), siderite (FeCO 3 ), calcite (CaCO 3 ), and dolomite (CaMg­(CO 3 ) 2 )) and organically associated inorganic elements in the coal react with each other to form artifact minerals at temperatures less than 500 °C. , The polluting gases, i.e., H 2 S, COS, and SO 2 , are released from the sulfur-bearing compounds (organic sulfur and sulfur-bearing minerals) during the pyrolysis of coal under inert atmospheres . Pyrolysis controls the char quality and the pyrolysis mechanisms depending on a number of coal properties that have been studied extensively. − The coal properties of important gasification and combustion technologies include coal rank, coal petrographic, and mineralogical composition; caking propensity, and operating parameters (particle size, temperature, heating rate, pressure, and operating atmosphere). − In addition, during pyrolysis, the plasticity of the caking coal mass is mostly seen at temperatures ranging between 400 and 500 °C. , The caking propensity of caking coal with high proportions of vitrinite- and liptinite-containing functional groups could initiate caking, which is an inevitable technical problem that occurs in fixed-bed coal gasification. − Caking coal particles can soften, swell, and coalesce (forming large cakes) in the fixed-bed gasifier during the gasification process. − This subsequently reduces gas flow and causes channeling and an unstable operating pressure leading to degraded syngas quality.…”

“…4,5 The polluting gases, i.e., H 2 S, COS, and SO 2 , are released from the sulfur-bearing compounds (organic sulfur and sulfur-bearing minerals) during the pyrolysis of coal under inert atmospheres. 47 Pyrolysis controls the char quality and the pyrolysis mechanisms depending on a number of coal properties that have been studied extensively. 5−8 The coal properties of important gasification and combustion technologies include coal rank, coal petrographic, and mineralogical composition; caking propensity, and operating parameters (particle size, temperature, heating rate, pressure, and operating atmosphere).…”

Effect of the V5+ to V4+ Molar Ratio on H2S Absorption and Conversion to Elemental Sulfur

“…Elemental Sulfur Mass Balance. The unreacted H 2 S contained in the off-gas from the experiment was absorbed by the cadmium acetate solution, and the concentration of the unreacted H 2 S was determined by the analytical method which is described in Matjie et al 30 This was done for mass balance purposes because the amount of the incoming sulfur as H 2 S and elemental sulfur formed were known, but the unreacted H 2 S leaving the apparatus was not known.…”