Abstract
In principle, coal-fired Combined Cycle power generation offers increased efficiency and reduced emissions compared to the systems currently in use. However, a serious problem facing the development of these schemes is the catastrophic corrosion of gas turbine components by alkali salts. Removal of 90 - 99% of the alkali is required if the flue gases from Pressurised Fluidised Bed Combustion or Gasification are to meet gas turbine manufacturers' limits. While filtration may be a suitable clean up technique for combustor gases, the removal of alkali salt vapours from a gasifier product stream represents a problem significantly more challenging, and was the focus of this thesis. Removing the corrosive alkali salt vapours by contacting them with a suitable alumino-silicate sorbent has been proposed as solution to this problem. However, several thermodynamic studies have suggested the high HCl concentrations commonly experienced upon gasification of British coal may substantially affect the performance of such sorbents. To study this, an experimental investigation was performed. First, a screening technique was developed that accurately and rapidly assesses a candidate material's potential as an alkali sorbent. Based on a standard thermal analysis technique it represents a significant improvement over previous screening methods. The results from the study of 26 candidate materials indicated that the sorbent's structure is a prime factor in determining its performance as an alkali "getter", with those possessing a layered silicate structure being identified as having the most potential. To test the selected sorbents further a high temperature fixed bed rig was designed and constructed. This allowed pelletised sorbent to be exposed to a gas flow of precise composition at temperature up to 927 +- 1. 5C, for up to 300 hours. Over 3000 hours of tests were performed using this apparatus, with gas compositions of 0 - 40 +- 0. 4 vpm NaCl, 0 - 160 +- 3 vpm HCl and 0 - 5 +/- 0. 17. H20 in nitrogen. These tests identified calcium montmorillonite, a 2:1 layer silicate, as a suitable sorbent. Long term tests showed it to have a saturation alkali capacity of 12. 6%. wt, suggesting an overall reaction such as: 2. 2 NaCl(g) + 1. 1 H2 O(g) + Ca 0. 2Al1. 6 Mg0. 4 Si4O10 (s) (Ca 0. 2 Al1. 6 Mg0. 4 Si4 O10 ). 1. 1 Na2 O(s) + 2. 2 HCl(g) The addition of HCl to the inlet gas was shown to alter drastically the overall rate of uptake by the solid; however it was not possible with this apparatus to determine the acid's effect on the effluent gas alkali concentration. To investigate further the mechanism and kinetics of alkali uptake, a theoretical description of the fixed bed alkali sorber was developed, based upon a "grain" model approximation for the gas-solid reaction occurring within each sorbent pellet. Four likely reaction mechanisms were tested against the experimental data to determine a suitable representation of reality. The most appropriate mechanism was the Two Reaction model, which visualises alkali uptake occurring by two separate, first order, reversible reactions. The rate of one reaction is assumed to be sensitive to HCl concentration whereas the rate of the other is virtually independent of it. This model contains three parameters which were recovered from the experimental data. Using this model the preliminary design of an Alkali Retention Unit for a 350 MW Pressurised Fluidised Bed Gasifier - Combined Cycle was performed. The unit consists of four, 4 meter diameter beds, filled to a depth of 5. 7 meters with 3 mm diameter sorbent pellets. Such a bed would last 4000 hours before breakthrough occurs, after which the sorbent would be discarded.