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Application of vermiculite and limestone to desulphurization and to the removal of dust during briquette combustion

¹ Department of Geology, Peking University, Beijing 100871, P. R. China
² Electron Microscopy Center and Department of Geological Sciences, University of South Carolina, 701 Sumter St., Columbia, SC 29208, USA
³ School of Materials Science and Technology, China University of Geosciences, Beijing 100083, P R China

^* E-mail: ahlu{at}pku.edu.cn

Small domestic cooking furnaces are widely used in China. These cooking furnaces release SO₂ gas and dust into the atmosphere and cause serious air pollution. Experiments were conducted to investigate the effects of vermiculite, limestone or CaCO₃, and combustion temperature and time on desulphurization and dust removal during briquette combustion in small domestic cooking furnaces. Additives used in the coal are vermiculite, CaCO₃ and bentonite. Vermiculite is used for its expansion property to improve the contact between CaCO₃ and SO₂ and to convey O₂ into the interior of briquette; CaCO₃ is used as a chemical reactant to react with SO₂ to form CaSO₄; and bentonite is used to develop briquette strength. Expansion of vermiculite develops loose interior structures, such as pores or cracks, inside the briquette, and thus brings enough oxygen for combustion and sulphation reaction. Effective combustion of the original carbon reduces amounts of dust in the fly ash. X-ray diffraction, optical microscopy, and scanning electron microscopy with energy dispersive X-ray analysis show that S exists in the ash only as anhydrite CaSO₄, a product of SO₂ reacting with CaCO₃ and O₂. The formation of CaSO₄ effectively reduces or eliminates SO₂ emission from coal combustion. The major factors controlling S retention are vermiculite, CaCO₃ and combustion temperature. The S retention ratio increases with increasing vermiculite amount at 950°C. The S retention ratio also increases with increasing Ca/S molar ratio, and the best Ca/S ratio is 2–3 for most combustion. With 12 g of the original coal, 1 to 2 g of vermiculite, a molar Ca/S ratio of 2.55 by adding CaCO₃, and some bentonite, a S retention ratio >65% can be readily achieved. The highest S retention ratio of 97.9% is achieved at 950°C with addition of 2 g of vermiculite, a Ca/S ratio of 2.55 and bentonite.

KEYWORDS: coal, desulphurization, dust removal, air pollution, vermiculite, CaCO₃, briquette, ash, China

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