Table 1.

Flue gas desulfurization processes.

Non-regenerable processes: wet methods
Flue gas desulfurization process	Sorbent/additives	SO2 removal efficiency	Final product/s	Remarks	Reference
Limestone forced oxidation (LSFO)	Limestone	Up to 99%	CaSO4.2H2O	Prevent scaling. Complete oxidation of CaSO3	Srivastava et al., 2001, Srivastava, 2000
Limestone inhibited oxidation (LSIO)	Limestone, sodium thiosulfate	~95%	CaSO3	Difficult to dewater the waste. Form large crystals of CaSO3	Srivastava et al., 2001, Srivastava, 2000
Jet bubbling reactor	Limestone	90%	CaSO4.2H2O	Provide large liquid/gas interface for SO2 sorption. Lower slurry pH (3.5–4.5), hence 100% of limestone utilization	Zheng et al. (2003)
Lime and Mg enhanced lime	CaO, Mg salts	99%	CaSO4.2H2O lighter in color (higher value), if desired Mg(OH)2	More expensive than limestone processes, but produces high quality gypsum	Srivastava (2000)
Dual alkali process	Sodium solution (Na2SO3) for scrubbing, lime	90%	CaSO4/CaSO3 sludge	By-product contains Na salts, hence needs to be treated before disposal	Mo et al. (2006)
Sea water process	Natural sea water	>96%	SO2 dissolved in sea water	Discharged into oceans	Abrams et al. (1988)
NH3-based wet desulfurization process	(NH4)2SO3	50–60% of SO3 absorption	NH4HSO3	Higher SO3 absorption	Huang et al. (2016)
Non-regenerable processes: dry methods
Flue gas desulfurization process	Sorbent/additives	SO2 removal efficiency	Final product/s	Remarks	Reference
Lime spray drying	Lime slurry	Up to 98%	CaSO3	SO2 absorption efficiency strongly depends on the ratio of the water evaporation rate to the absorption rate	Hill and Zank, 2000, Babcock and Wilcox, 2009
In-furnace sorbent injection	Hydrated lime	27–72%	CaSO4	The furnace temperature and the residence time are important for proper SO2 sorption	Shemwell et al. (2000)
Limestone injection into furnace and activation of unreacted calcium (LIFAC)	Finely pulverized limestone	>80%	CaSO4	Flue gas temperature, residence time in the furnace, droplet size of water, and residence time in the reactor affect SO2 removal efficiency	Srivastava and Jozewicz (2001)
Economizer sorbent injection	Lime	80%	CaSO3	Porosity of calcitic hydrate (Ca(OH)2) is important for an effective SO2 absorption	Muzio and Often (1987)
Duct sorbent injection	Finely dispersed hydrated lime, NaHCO3	50–60% with lime 80% with NaHCO3 85% with lime and NaOH as additive	CaSO4	Surface moisture content of lime is important for efficient SO2 removal	Stouffer et al. (1989)
Duct spray drying	Slaked lime slurry	>50%	CaSO3 and CaSO4	Adequate mixing of reactant and resident time are significant for maximum SO2 removal	Murphy et al. (1986)
Circulating fluidized bed	Hydrated lime	–	CaSO3	Semi-dry FGD process	Neathery (1996)
Hybrid pollution abatement system (HYPAS) sorbent injection	Dry mixture of lime and recycled solids	Near commercial, but not reported	CaSO4	Byproducts and remaining fly ash collected in a pulse jet fabric filter	Srivastava and Jozewicz (2001)