The dew points of acidic vapours in simulated power station flue gases

During combustion, sulphur and chlorine, which are present
as the major impurities in fuel oil and coal, are partially
converted into sulphuric acid and hydrochloric acid
respectively. Condensation of such corrosive products is clearly highly undesirable and regions of plant have been
identified where acid dewpoint corrosion impinges upon the efficiency and maintenance cost of oil and coal fired
power stations.
It is further recognised that prevention of acid deposition
can be achieved if the flue gas temperature is maintained
above the acid dewpoint. However, expulsion of flue gas at temperatures higher than the acid dew point decreases the energy yield from the fuel and is therefore wasteful and uneconomic. Clearly, precise determination of the acid dewpoint is vital.
A literature survey showed that dewpoint data for the
H₂SO₄/H₂O system have considerable scatter within a range ± 10K. Recently, Halstead and Talbot (CERLLeatherhead) have developed an equilibrium gas saturation technique which gives an uncertainty of only ±2K. There remains however some doubt about the estimate of the enthalpy of vaporisation of sulphuric acid. Furthermore little is known about the potentially more interesting three component system H₂SO₄/HCI/H₂O.
This work has sought to tackle these deficiencies by applying
an improved version of Halstead and Talbot's transpiration
apparatus to dewpoint investigations in the
H₂SO₄/H₂O system and to the H₂SO₄/HCI/H₂O system.
The results obtained essentially confirm Halstead and Talbot's
findings in the H₂SO₄/H₂O system and enabled a more precise determination of the vaporisation enthalpy of sulphuric acid. Very importantly, it has been established that the dewpoint is independent of the presence of HCl.