Session: 33-02 Deposition and erosion in hot engine components

Paper Number: 151130

Investigation of Ash Deposition Behaviour From Biogenic Solid Fuel Under Gas Turbine Conditions With Film Cooling 

The aim of the global energy supply is to achieve a sustainable character in the coming decades. Gas turbines can make a significant contribution realising this goal. This can be achieved by replacing the current use of fossil fuels with fuels from renewable and regenerative resources. With fuel from a sustainable source, a gas turbine can then be used to stabilise the electricity grid or to provide thermal energy for accompanying processes or for district heat. One possible source of substitute fuel is biogenic residual and waste materials. However, these materials are predominantly available in a solid form with high contamination potential.

Several attempts have already been made in the past to enable the operation of a solid fuel with a gas turbine. One of these approaches was the development of the IGCC (Integrated Gasification Combined Cycle) process. Other approaches have pursued either the direct utilisation of flue gas from solid fuel combustion or indirect utilisation by heating a gas through a heat exchanger. The main reasons why these processes have not become widely accepted is the trade-off between costly fuel gas cleaning and allowing deposits to build up in the turbine, as gas turbines generally have high maintenance costs. In order to solve the problem, a combustion system consisting of a fluidised bed gasifier, hot gas cyclone and staged combustion chamber was created, which allows a low ash content in the flue gas. As residual particles still remain in the flue gas, a small gas turbine was also developed, which allows economical operation from solid fuel utilisation. In order to further increase the service life, the next step is to integrate a film cooling system on the blades of the turbine to reduce deposition.

Aim of the work carried out here is to demonstrate the development process of film cooling and physical considerations in order to reduce the deposits on the turbine blades.

First, different turbine blade profiles with film cooling were created, which correspond to the blade design of a gas turbine. CFD simulations were performed for the turbine blade profiles to verify the cooling effect in advance and to sort out inefficient versions for the reduction of experimental design. The most promising configurations were then manufactured and analysed in a newly developed test rig for deposition tests. For this purpose, individual blades were exposed to synthetic test ashes under defined operating conditions. Test ashes were created on the basis of calculations and analyses from a real operation, which were used for subsequent deposition tests. The ash composition consists of typical fly ash components such as gypsum, sand, lime, salts, sulphides and other oxides. The evaluation of the film cooling with regard to the reduction of ash deposits and the cooling effect was based on the gravimetric determination of the deposited ash quantity and the chemical and structural composition of the test ashes using REM-EDX. In addition, deposition sites were compared with flow pattern from the CFD simulations.

As known from the literature, the blade configurations with fanned cooling holes showed the best cooling effect with the best cooling air distribution. Overall, however, all configurations showed good protection against the feedstock in areas of the critical cross-section of a turbine blade where deposits have the most dominant influence. There was no significant difference in deposition tendencies between aligned holes and fan shaped holes. The influence of temperature on the formation of deposits was also negligible above a critical range. However, a significant influence of the angular difference between the blade inlet and outlet was found. An average of Δβ=40° seems to cause the lowest deposit build up.

Overall, the integration of film cooling appears to be a promising application for increasing service life. The next step is to integrate the film cooling on the turbine blades of the developed small gas turbine in order to carry out tests with real fuels.

Presenting Author: Wunder Luis Technsiche Universität Dresden

Presenting Author Biography: Luis Wunder studied mechanical engineering at Coburg University of Applied Sciences, Bavaria, with a bachelor's and master's degree in thermodynamics and turbomachinery. After graduating, he began his doctorate at the Technical University of Dresden, where he now works as a research associate.

Authors:

Wunder Luis Technsiche Universität Dresden
Daniel Bernhardt Technische Universität Dresden
Michael Beckmann Technische Universität Dresden