Some Aspects of the Chemistry of M.H.D. Seed

The paper reports information assembled from the literature and from experiments about the physical chemistry of potassium sulphate at high temperatures and this enables a first assessment to be made of the difficulty of evaporating it quickly in an m.h.d. combustion chamber and recovering if after it leaves the generator duct. Measurements of the evaporation rate show that in oxidizing and neutral gases a 50 $\mu $m diameter particle, which could be produced by normal powdering methods, should be evaporated within 10 to 20 ms at 2800 degrees K and this agrees with calculations. The experiments suggest that evaporation should be much faster in reducing gases. Available thermodynamic data has been used to determine the amount of heat used in evaporation of the seed. It amounts to 6$\cdot $9% of the calorific value of oil fuel-allowing for seed impurities. A flue gas containing 0$\cdot $7 at.% of potassium should begin to condense at 1600 degrees K and the amount of liquid, solid, or smoke formed will depend on the rate of removal of heat from the flue gas and on the saturated vapour pressure of K$_{2}$SO$_{4}$. Accurate data for the latter determined by effusion and transpiration methods are reported. Potassium sulphate reaching metal tubes in the boiler and superheater may form liquids with excess of sulphur trioxide (including pyrosulphates, melting at about 670 degrees K) in the temperature range 670 to 970 degrees K. This could cause more severe corrosion than when potassium is absent. Measurements of the partial pressure of potassium sulphate in equilibrium with coal ash slags up to 2000 degrees K suggest that it should be possible to reject the slag without loss of potassium if it could be separated from the potassium containing gas at 2000 to 2100 degrees K according to the ash content of the coal.