Deposition and Blade Fouling of Gas Turbines by Fuel Impurities and Additives

Abstract

CEGB gas turbines at present burn distillate fuel-oil and there would thus be an advantage in terms of fuel cost, if a less-refined fuel such as crude or residual fuel oil could be used. Compounds of sodium and vanadium which occur in such fuels, however, deposit on the blades and corrode them. This effect can be minimized by adding magnesium and silicon compounds to the fuel but these then become a major source of blade fouling the deposit is, however, less adherent and can be fairly readily removed. The present work was concerned with an experimental study of these effects.

The experiments were made in a high pressure combustor simulating a gas turbine environment. Sodium, vanadium, magnesium and silicon were normally added to the gas oil fuel as oil soluble compounds but in some tests magnesium was added as a suspension. The combustor itself was a modified form of the type used in the Rolls Royce Tyne engine. Deposition rates were measured on test surfaces simulating gas turbine blades at 1060 K and 1160 K (gas temperature 1080 K and 1180 K) at pressures and gas velocities typical of those used in current industrial gas turbines. They were found to compare quite closely with fouling rates reported for operational engines burning dirty fuels.

One conclusion drawn from the deposition data is that vanadium is deposited as particulate tetroxide, V₂O₄, which is then slowly oxidized to V₂O₅ which can then evaporate. The presence of sodium or magnesium in deposits, or a high carbon burden in the combustion gas, reduces the evaporation of deposited vanadium and, as a consequence, the amounts of vanadium in the deposit increases. On the other hand the apparent rate of deposition of sodium compounds was depressed by vanadium in the fuel. A possible explanation for this is offered in terms of the reduction of sodium sulphate within the deposit to the more volatile hydroxide by particles of depositing V₂0₄, i.e. Na₂SO₄ + V₂O₄ + H₂O → V₂O₅↑ + 2Na0H↑ + S0₂.

In other experiments when magnesium and vanadium were added to the fuel the total rate of deposition was constant and proportional to the impurity concentration in the fuel up to deposit weights of 60gm⁻². The overall deposition rate, when expressed as oxides in the deposit, approximated to 4×10⁻³gm⁻² per w.p.p.m. of the elements in the fuel per hour of exposure. This suggests that blade cleaning would be needed at intervals of10⁴ and 500h when burning crude and residual fuel oils, respectively, to maintain turbine efficiency within 90 to 95% of that for cleaned blades. The rate of fouling by Mg or Mg and V together was increased with sodium in the fuel but the addition of silicon to the fuel resulted in reduced fouling by Mg + V on Mg + V + Na.