The British Coal Topping Cycle is a proposed advanced coal-fired power generation system in which gases derived from the coal and containing low levels of alkali vapours are passed to a gas turbine. The deposition of these vapours onto the turbine blades needs to be assessed because deposited alkali salts may accelerate corrosion and hence reduce blade lifetimes.
A model for the thermochemical behaviour was derived by assuming frozen chemistry in the external and boundary layer flows, but equilibrium at the surface and within the deposit, including an allowance for non-ideal behaviour in the deposit. The model is currently formulated for sodium and potassium chlorides and sulphates in the deposit and can deal with deposition to either a solid or liquid deposit. The boundary layer mass transfer has been calculated by both a simple heat transfer analogy model and a numerical solution of the two-dimensional diffusion equation within the boundary layer.
Some examples of general results obtained with the model are described, including a comparison of the effects of different deposit phase, non-ideal behaviour and film cooling. Some predictions are compared with measurements of alkali salt deposition obtained in experiments simulating the conditions expected in a Topping Cycle system.