Flue gas desulphurisation under South African conditions

Abstract

Coal is the primary energy source in South Africa and accounts for about 75% of the total energy consumption in the country and 90 per cent of the total electricity capacity of 43 142 MW is coal-derived. Upon combustion, the sulphur contained in the fuel is oxidized to sulphur dioxide (SO 2 ). Emissions of SO 2 lead to the formation of acid rain which is harmful to the environment. Besides this environmental problem, SO 2 is a health hazard causing problems to the respiratory and cardio-vascular system. The objective of this investigation was to study and/or rank the performance of locally available materials (i.e. limestone, dolomite, or calcrete) as sorbents in the capture of SO 2 emissions from coal-fired power plants. Two experimental procedures were adopted in this work: the pH-stat method was used to simulate conditions encountered in wet flue gas desulphurisation (WFGD); and the fixed-bed reactor was used to simulate conditions encountered in the dry in-duct flue gas desulphurisation (DFGD) process. The dissolution rate in the pH-stat apparatus was observed to be a strong function of the particle size; the finer the particles the higher the conversion within the same time period. The dissolution rate decreased with an increase in the pH for the range tested (4 ≤ pH ≤ 6). This result indicated that the dissolution of calcium – based materials is controlled by the mass transfer of the hydrogen ions accompanied by chemical reactions in the mass transfer boundary layer. The origin of the sorbent material and its chemical composition had an influence on the dissolution process; calcitic limestones generally displayed a higher rate than their dolomitic counterparts. The reaction rate was observed to increase with an increase in the reaction temperature for the range tested (30 ≤ T ≤ 70oC).Kinetic analysis of the results indicated that the dissolution process takes place according to the shrinking core model with surface control. The reaction rate constant obtained varied from low values for dolomite to high values for the limestone samples (0.00415 ≤ k ≤ 0.0190 min - 1 ). Thus the performance of the limestone samples is expected to be better than that of the dolomite samples in full-scale operation. The activation energy also showed a similar trend with that of limestone samples being higher than that for the dolomite samples (14.7 ≤ E a ≤ 27.5 kJ/mol). Thus a ranking of the sorbents can be obtained either by using the reaction rate constant or the amount of acid consumed over a specified period during the pH-stat reactivity experiments. Experiments of the desulphurisation activity in the fixed-bed reactor showed that the capture capacity depends on the sorbent type as well as its chemical composition. It was observed that an approximation of the reactivity of the sorbent can be obtained by observing the relative rise in temperature (ΔT) during the hydration of the calcined sorbent to produce the relevant slaked lime. Sorbents with a higher reactivity were exhibited a higher temperature rise than those with a lower capture capacity. This is thought to be due to a more porous structure that results with a higher temperature rise. Variation of relative humidity (0 ≤ RH ≤ 40%) indicated that the relative humidity is the single most influential parameter on the capture capacity in conditions similar to those encountered in dry FGD. An increase in the relative humidity resulted in an increase in the capture capacity. The influence of the SO 2 concentration (1000 ≤ C 0 ≤ 2500 ppm) was studied at a reactor temperature of 80°C and 0% RH. Although the initial sorbent activity increases as the concentration of SO 2 increases, the maximum conversion of the sorbent is independent of the SO 2 concentration. The reaction temperature on the other hand was seen to have a minimal effect on the sorbent activity. The desulphurisation activity in the fixed-bed can be satisfactorily modeled using the surface coverage model. The hypothesis of this model is that the sorbent is made up of plate grains and that the reaction is controlled by chemical reaction on the surface of a grain and the reacting surface area of the grain decreases with the deposition of solid product. A comparison of the model and experimental results showed a close agreement between the two. A comparison between the pH-stat and the fixed bed results showed that the pH-stat process can be used to reliably rank the performance of the different sorbent materials as sorbents in the flue gas desulphurisation process. The materials that had a higher dissolution rate also showed a higher sulphur capture capacity in the fixed-bed reactor.