Status of Flue Gas Desulphurisation (FGD) systems from coal-fired power plants: Overview of the physic-chemical control processes of wet limestone FGDs

This paper presents a general review of the Flue Gas Desulphurisation (FGD) technologies used to abate sulphur emissions from coal-fired power plants, and exposes the major physic-chemical processes occurring during wet limestone FGD. The abatement capacity of major, minor, and trace elements and the fate of trace pollutants during wet limestone FGD, as well as the features of wet limestone FGD by-products are discussed. It can be stated that wet limestone FGD system is the FGD process most widely used because of its high desulphurisation performance and low operating cost. Among control parameters evaluated in this review, pH range > limestone reactivity and SO₂ concentration > efficiency of particulate control devices > water re-circulation to the scrubber > entrainment of particles by the OUT-FGD gas, are the key factors of wet limestone FGDs. It is noted that, the enrichment of inorganic trace pollutants in FGD waters because of the re-circulation of water to the scrubber from gypsum slurry filtration, and the entrainment of accumulated fly ash particles in gypsum sludge, those of unreacted limestone, and the particles and droplets from gypsum slurry by the outgoing FGD (OUT-FGD) gas reduce considerably the desulphurisation efficiency and the abatement capacity of trace pollutants by wet limestone FGDs. The paper concludes with a number of issues to be investigated in depth in view of the worldwide FGD market growth and the forthcoming implementation of one of the most promising technologies to reduce CO₂ emissions, oxy-fuel combustion, in Pulverised Coal Combustion (PCC)-FGD processes at an industrial scale.