Pollutants

Sulphur dioxide (SO₂) is an acid gas, also called sulphur oxide or sulphurous anhydride. It is non-flammable and non-explosive. This pollutant, which has a pungent odour, is toxic and a respiratory irritant. In plants that burn sulphur-containing fuel, SO₂ can, for example, be removed from the flue gas through dry sorbent injection, where SO₂ reacts with hydrated lime (Ca(OH)₂) to form calcium sulphite (1) or calcium sulphate (2): 

Ca(OH)₂ + SO₂ → CaSO₃ + H₂O (1)

Ca(OH)₂ + SO₂ + ½ O₂ → CaSO₄ + H₂O (2)

Hydrogen chloride (HC) is a colourless and odourless acid gas, also called hydrochloride or hydrochloric acid. 

Non-flammable and thermally stable, HCl is toxic and corrosive. HCl is very soluble in water. Dissolution generates large amounts of heat. 

Hydrogen chloride can be captured efficiently with calcium hydroxide (Ca(OH)₂) sorbents. Optimum capture performance depends on the flue gas composition, flue gas treatment system, and process temperature. The reaction mechanism is complex and takes place through the formation of calcium hydroxide-chloride (CaClOH) according to: 

Ca(OH)₂ + HCl → CaClOH + H₂O 

CaClOH + HCl → CaCl₂ + H₂O 

Hydrogen fluoride (HF) is a non-flammable and non-explosive gas, also called fluoric acid, hydrofluoride, hydrofluoric acid or fluorine monohydride. It has a pungent odour and is corrosive, irritating and toxic. Hydrogen fluoride in flue gas reacts readily with hydrated lime: 

Ca(OH)₂ + 2HF → CaF₂ + 2H₂O 

This compound is also called sulfuric anhydride or sulphur trioxide. SO₃ is a clear, oily liquid, often present as a gas. It should be handled with extreme care as it reacts violently with water to produce a highly corrosive sulfuric acid. SO₃ reacts with hydrated lime to form calcium sulphate: 

Ca(OH)₂ + SO₃ → CaSO₄ + H₂O 

Selenium dioxide (SeO₂), also called selenium oxide, is an acid gas. It is non-flammable and non-explosive. This pollutant is toxic and a respiratory irritant. In plants burning selenium-containing fuel, SeO₂ can be removed from the flue gas through dry sorbent injection, where SeO₂ reacts with hydrated lime (Ca(OH)₂) to form calcium selenite (1) or calcium selenate (2): 

Ca(OH)₂ + SeO₂ → CaSeO₃ + H₂O (1)

Ca(OH)₂ + SeO₂ + ½ O₂ → CaSeO₄ + H₂O (2)

Mercury (Hg⁰, Hg²⁺) is the only metal that is liquid at standard temperature and pressure conditions. It is highly toxic through ingestion or inhalation. 

This group of micropollutants (PCDD & PCDF) is classified as polychlorinated dibenzo dioxins/furans. They consist of chlorinated biphenyl molecules linked with different oxygen bridges and are highly toxic.