The double-alkali method is a flue gas purification technology that uses sodium-based desulfurizers for in-tower desulfurization. Due to the strong alkalinity of sodium-based desulfurizers and the high solubility of reaction products after absorbing sulfur dioxide, its process involves countercurrent spraying of flue gas with circulating alkali liquor in the absorption tower to wash SO₂ in the flue gas. The desulfurization products after washing are discharged into a regeneration tank, where calcium oxide is used for reduction and regeneration. The regenerated sodium-based desulfurizer is then pumped back to the desulfurization tower for cyclic use. The double-alkali desulfurization process has low investment and operating costs, and is less prone to problems such as supersaturation crystallization and scaling/clogging, making it suitable for desulfurization retrofitting of small and medium-sized boilers.

The double-alkali flue gas desulfurization technology uses sodium hydroxide solution as the initial desulfurizer. The prepared sodium hydroxide solution is directly injected into the desulfurization tower to wash SO₂ in the flue gas, thereby achieving flue gas desulfurization. The desulfurization process system mainly consists of 4 parts: (1) Preparation and supplement of absorbent; (2) Spraying of absorbent slurry; (3) Reduction of sodium-based alkali in the regeneration tank slurry; (4) Gypsum dewatering treatment.