Effect of Additives to Improve Calcium-Based Sorbents in Semi-Dry Flue Gas Desulphurization Ítalo W. L. França 1 & Samuel J. M. Cartaxo 1 & Moises Bastos-Neto 1 & Luciana R. B. Gonçalves 1 & Fabiano A. N. Fernandes 1 Received: 15 April 2019 /Revised: 15 October 2019 /Accepted: 28 January 2020 # Springer Nature Switzerland AG 2020 Abstract Semi-dry flue gas desulfurization (FGD) technology is used by some coal-based power plants to remove sulfur dioxide from flue gas. In semi-dry FGD systems, SO 2 is removed by calcium-based sorbents in a reactive spray dryer, but the level of desulfurization is lower than in wet FGD systems. This work evaluates the effects of seven additives in the desulfurization of the flue gas: magnesium hydroxide, ammonium nitrate, ammonium acetate, ammonium phosphate, sodium hydroxide, citric acid, and urea. Slurries with calcium hydroxide concentrations of 10 and 20 wt% containing 2 and 4 wt% of an additive were applied in the removal of SO 2 . Additives containing ammonium intensified the removal of SO 2 and increased the yield of the reaction. The SO 2 content in the flue gas reduced by 73.8% when applying a slurry containing 4 wt% of ammonium nitrate and 10 wt% of Ca(OH) 2 . The use of this slurry increased the removal of SO 2 by 13.5% when compared to the best result obtained using calcium hydroxide slurry without any additive. Keywords Calcium hydroxide . Spray dryer . Sulfur dioxide . Semi-dry flue gas desulfurization 1 Introduction Desulphurization is an important step in coal-fired power plants that aims pollution control. Coal contains sulfur at weight percentages that typically ranges from 0.6 to 3.0% [1]. Coal-fired power plants produce sulfur dioxide during combustion of coals containing sulfur. This sulfur dioxide needs to be partially removed from the flue gas to comply with anti-pollution laws. Semi-dry desulphurization processes consist of a reactive spray dryer that removes SO 2 from the furnace flue gas. The SO 2 -rich gas stream enters the spray dryer and gets in contact with a fine spray of calcium hydroxide slurry. The product exiting the spray dryer consists of a fine powder of calcium sulfite, calcium sulfate, and unreacted calcium hydroxide. High SO 2 removal levels from flue gas are difficult to achieve in semi-dry FGD, and plants run at efficiency levels (% of SO 2 removal) ranging from 40 to 60% and hardly exceed 70% [2, 3]. Higher efficiencies would hypothetically require an excess of Ca(OH) 2 , but since the reaction occurs primarily on the external layer of the slurry droplets, the excess of Ca(OH) 2 generates larger droplets and reduces the total surface area available for the chemical reaction between Ca(OH) 2 and SO 2 [2]. Under excess of Ca(OH) 2 , the efficiency of semi- dry FGD plants can drop to as low as 30%. Studies on desulphurization by spray drying absorption [4–6] concluded that sulfur removal is incomplete due to the formation of an insoluble product layer at the surface of the particle, leaving an unreacted core inside the particle. The re- moval of sulfur in semi-dry flue gas desulphurization can be improved by forming smaller slurry droplets, increasing the solubility of the reaction products in the droplet, and creating a more favorable reaction medium for SO 2 removal. Additives could be mixed with calcium hydroxide to enhance one or more of the conditions that would improve the desulphurization pro- cess. Several researchers [7, 8] studied the role of additives on wet FGD systems, but little is known on the effects of additives on semi-dry FGD systems. In this work, we have studied the effects of seven different additives (magnesium hydroxide, ammonium nitrate, ammo- nium acetate, ammonium phosphate, sodium hydroxide, citric * Fabiano A. N. Fernandes fabiano@ufc.br 1 Departamento de Engenharia QuímicaCampus do Pici, Universidade Federal do Ceará, Bloco 709, Fortaleza, CE 60115-222, Brazil https://doi.org/10.1007/s40825-020-00156-0 Emission Control Science and Technology (2020) 6:105–112 /Published online: 4 2020 February