In-Furnace Sulfur Capture by Co-Firing Coal With Alkali-Based Sorbents

Over the last quarter of a century, since the 1990 US Clean Air Act Amendments were enacted the gaseous sulfur emission, in the form of sulfur dioxide, have been reduced [1] by a factor of 4, by switching to lower sulfur content coals, installation of flue gas desulfurization (FGD) sorbents or switching altogether to natural gas as a fuel. Penetration of alternative energy generation also has had a positive impact. However, current emissions of sulfur dioxide are still voluminous, amounting to 3,242,000 short tons annually in the USA [2]. As wet flue gas desulfurization is both real-estate- and capital-intensive, infurnace dry sorbent injection has been considered over the years to be a viable alternative. However there is still uncertainty on the best selection of the sorbents for particular coals and furnace operating conditions. This is particularly the case when it is economically attractive for the power-plant operator to burn locally-sourced high-sulfur coal, such as the case of Illinois bituminous coals. This manuscript presents experimental results on the reduction of sulfur oxide emissions from combustion of a high-sulfur content pulverized bituminous coal (Illinois #6 Macoupin). The coal particles were in the size range of 90–125 μm and were blended with dry calcium-, sodium-, potassium-, and magnesium-containing powdered sorbents at different proportions. The alkali/S molar ratios were chosen to be at stoichiometric proportions (Ca/S = 1, Mg/S = 1, Na₂/S = 2, and K₂/S = 2) and the effectiveness of each alkali or alkali earth based sorbent was evaluated separately. Combustion of coal took place in a drop-tube furnace, electrically-heated to 1400 K under fuel-lean conditions. The evolution of combustion effluent gases, such as NO_x, SO₂ and CO₂ were monitored and compared among the different sorbent cases. The use of these sorbents helps to resolve the potential of different alkali metals for effective in-furnace sulfur oxide capture and possible NO_x reduction. It also assesses the effectiveness of various chemical compounds of the alkalis, such as oxides, carbonates, peroxides and acetates.