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.

Many proposed clean coal technologies for power generation couple a gasification process with a gas turbine combined cycle unit. In the gasifier, the coat is converted into a syngas which is then cleaned and fired before entering the turbine. A problem is that coat-derived syngases may contain alkali metal impurities that combine with the sulfur and chlorine from the coal to form salts that deposit on the turbine blades, causing corrosion. This paper describes a new model, applicable to most types of coal, for predicting the dewpoint temperatures and deposition rates of these sodium and potassium salts. When chtorine is present the main atkati species in the mainstream gas flow are the chtorides; but when chlorine is absent, the Superoxides dominate. However, because the high-pressure turbine blades are film-cooled, they are at much lower temperatures than the mainstream gas flow and analysis then shows that the deposit is composed almost entirely of the sulfates in either liquid or solid form. This is true whether or not chlorine is present. Detailed calcutations using the new model to predict the alkali salt deposition rates on three stages of an example utility turbine are presented. The calculations show how the dewpoint temperatures and deposition rates vary with the gas-phase chlorine and sulfur levels as well as with the concentrations of sodium and potassium. It is shown that the locations where corrosion is to be expected vary considerably with the type of coal and the levets of impurities present.