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

Ratale H Matjie; Jeanett M Lesufi; John R Bunt; Christien A Strydom; Harold H Schobert; Romanus Uwaoma

doi:10.1021/acsomega.8b01359

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

ACS Omega. 2018 Oct 26;3(10):14201-14212. doi: 10.1021/acsomega.8b01359. eCollection 2018 Oct 31.

Authors

Ratale H Matjie^{1

2}, Jeanett M Lesufi¹, John R Bunt¹, Christien A Strydom¹, Harold H Schobert³, Romanus Uwaoma¹

Affiliations

¹ Centre of Excellence in Carbon-Based Fuels, School of Chemical and Minerals Engineering and Centre of Excellence in Carbon-Based Fuels, School of Physical and Chemical Sciences, North-West University, Potchefstroom 2520, South Africa.
² Chemical Engineering - Faculty of Engineering & Technology, Vaal University of Technology, Vanderbijlpark Campus, Vanderbijlpark 1911, South Africa.
³ The EMS Energy Institute, Penn State University, University Park, 16802 Pennsylvania, United States.

Abstract

The objective of this study, the first of its kind on these specific South African low-sulfur coals, was to capture H₂S and SO₂ 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 H₂S and SO₂ 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, FeS₂ produced Fe₂O₃, FeO, and SO₂. It transformed to FeS and released H₂S 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 SO₂ and O₂ 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 H₂S to SO₂ 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 K₂CO₃ 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.