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Show Table 3 Average Composition of Sea-water (pH - 8). Data from refs. 6 and 16. Species ClS042- HC03- Br Na+ Mg2+ Ca2+ K+ Fe V Cr Mn Co Cu Zn Hg Pb Concentration (g 1-1) 18.98 2.65 0.14 0.06 10.56 1.27 0.40 0.38 3 x 1Q-6 1.5 x 1Q-6 0.6 x 1Q-6 2 x 1Q-6 0.08 x 1Q-6 3 x 1Q-6 5 x 1Q-6 0.05 x 1Q-6 0.03 x 1Q-6 Concentrations of nitrite and nitrate in sea-water are lower than those of the major anions, being generally < 10-3 g 1-1. However, oxides of nitrogen are not readily soluble in sea-water as indicated by the low values of Henry's Law constant shown in Table 1 and in preliminary pilot plant measurements (13). INDUSTRIAL PLANTS As early as the late 1940's the Electrolytic Zinc Company in Tasmania built a plant to desulphurise smelter flue gas utilising tidal estuarine water (7). The gas stream was cooled from 180°C to 21°C in the scrubber and released to the atmosphere. The liquid effluent from the plant was returned to the estuary untreated. As much as 99% of the S02, present at a concentration of 30,000 ppmv in the untreated flue gas, was removed. In the 1960's sea-water was employed to desulphurise boiler flue gases at the Showa Denko company in Yokohama, Japan (8). In the 1970's three different flue gas desulphurisation systems based on sea-water scrubbing were patented in Japan (9,10,11). One of these, involving the electrolysis of sea-water, removed S02 with an efficiency of 99.9% (11). Another involved the spraying of the flue gas with sea-water containing CaC03, with or without CaS04 in order to obtain gypsum (10). Since then sea-water based desulphurisation plants have been installed in Norway, Sweden, U.S.A and India. The largest supplier of sea-water scrubbing technology is at present the Flakt group. Their Flakt-Hydro process involves counter-current absorption of flue gases into sea-water. The gases are first cooled in a quencher in order to enable the use of plastic lining in the scrubber and the cleaned gases are passed through a demister and reheater before release to the atmosphere. Reported flue gas desulphurisation efficiencies in seawater-based scrubbers are given in Table 4. 6 |
