Before and after each test, samples were weighed on a balance (section 3.1.3);
the data were gathered and are shown below as columnar graphs (Figure 4.1 - Figure 4.8). The graphs represent either the difference in salt weights before and after the exposure or a percentage change of their mass.
Test 1 and Test 2 – 22 different salt mixtures; 7% O2 + 100 ppm SO2 + 350 ppm HCl + bal. N2; 600 and 550°C respectively
Figure 4.1 shows the comparison of the mass change data for the salts exposed in Test 1 at 600ºC (blue colour) and in Test 2 at 550ºC (red colour).
The mass change was calculated as a difference between the mass of the salt after and before the test. The salts can be identified as five groups: pure chlorides, pure sulphates, mixtures of two chlorides, mixtures of one chloride and one sulphate and mixtures of two chlorides and two sulphates.
Among 22 salt mixtures in Test 1, only one showed mass gain of 0.09 mg (mixture 12, initially 100% of Na2SO4), the highest mass loss of 40.22 mg was observed for mixture 11 (initially 100% K2SO4), the lowest mass loss of 0.61 mg was noticed for mixture 6 (initially 100% NaCl) and slightly higher but still very low mass loss (1.15 mg) for mixture 1 (initially 100% KCl).
It can be noticed that pure chlorides and the mixtures of two different chlorides showed the lowest mass loss, whereas pure sulphates or the mixtures of sulphates and chlorides performed the highest mass loss (for some cases 35 times higher than for chlorides).
Figure 4.1 Mass change data for the same 22 salt mixtures after 50 hours of exposure in Test 1 at 600°C and Test 2 at 550ºC
As seen in Figure 4.1, all salts exposed in Test 2 showed mass loss with the highest for mixture 13 - initially 25% KCl + 40% NaCl + 25% Na2SO4 + 10%
K2SO4 (17.62 mg). Four salts characterised similar mass loss of 1.05 – 1.15 mg and they were initially 100% KCl, 100% NaCl, 80% KCl + 20% K2SO4 and 20%
KCl + 80% K2SO4. Mixture 12 (initially 100% Na2SO4)showed a bit higher, but still low mass loss of 1.39 mg. Generally, it can be seen that the mixtures of two chlorides and two sulphates presented higher mass loss in comparison to other salt mixtures.
It can be noticed that for 85% of the salts exposed in this test, the mass loss was lower (or almost the same in two cases) than for their equivalents exposed in Test 1 at 600ºC.
Since the amount of the salt applied on each sapphire disc varied, the percentage mass change was calculated as a difference between the mass of the salt remained after the test and the amount of the salt before the test, all divided by the mass of the salt at the start. Figure 4.2 depicts the comparison between the percentage change in mass for the same salts exposed in Test 1 and Test 2. Blue columns represent the data gathered after Test 1, red columns – after Test 2.
Figure 4.2 Comparison in percentage mass change for 22 salt mixtures after 50 hours of exposure in Test 1 (600ºC) and Test 2 (550ºC)
It can be seen that for 54.5% of the salts exposed at 550ºC (Test 2) their % change of mass was lower in comparison to their equivalents at 600ºC (Test 1).
This was mainly for mixtures of different molar ratios of KCl : NaCl : K2SO4 : Na2SO4.
A trend could be observed between the pure chlorides and mixtures of KCl:NaCl. For all of those, the % change in mass was lower at 600ºC than at 550ºC. At 550°C it was increasing with the increasing amount of NaCl in the mixture, with the exception of mixture 5 (initially 20% KCl + 80% NaCl). When
K2SO4 was introduced (KCl:K2SO4 combinations), the % change started to be lower at 550 than at 600ºC, with the exception of mixture 8 (originally 60% KCl + 40% K2SO4). For most of the mixtures containing four different salts the % change was higher at 600ºC with the exception of mixtures 14 and 15 (originally 25% KCl + 30% NaCl + 25% Na2SO4 + 20% K2SO4 and 25% KCl + 20% NaCl + 25% Na2SO4 + 30% K2SO4 respectively). For the initially pure K2SO4 the % change was higher at 600ºC, whereas Na2SO4 displayed a negligible mass gain in Test 1 (0.41%) which could be considered as no mass change at all – it showed mass loss of about 10% in Test 2 (550ºC).
Test 3 – 12 mixtures of salts; 350 ppm HCl + bal. N2; 550ºC
In this test, 2 sets of the same 12 salt mixtures (pure chlorides, pure sulphates, mixtures of two chlorides and mixtures of one chloride and one sulphate) were investigated. Figure 4.3 presents the mass change data obtained after 50 hours of Test 3. Blue columns show the first set of 12 salt mixtures; red columns show the second set of the same mixtures exposed at the same time.
Figure 4.3 Mass change data obtained after 50 hours of exposure in Test 3 at 550ºC
The lowest mass loss was observed for mixture 7 (initially 80% KCl + 20%
K2SO4) and it was 0.45 and 0.18 mg. The highest mass lost showed mixture 11 (initially 100% K2SO4) and it was 6.56 mg in the first set and 12.5 mg in the second set. The only mass gain (0.06 mg) was displayed for mixture 1 (originally 100% KCl) which also showed a slight mass loss (0.80 mg) in the second set.
Similarly to Tests 1 and 2, the amount of the salt applied was not equal for all the sapphire discs, therefore, the percentage change in mass was calculated and it is presented in Figure 4.4. It can be noticed that for 50% of the salt mixtures, their % change was very similar in two sets, whereas for the other half of the salts, their % change varied quite noticeably.
Figure 4.4 Percentage change in mass after 50 hours of exposure in Test 3 at 550ºC
Figure 4.5 shows the comparison between the 12 salt mixtures (pure chlorides, pure sulphates, KCl:NaCl and KCl:K2SO4 combinations) which were exposed in Tests 1 – 3.
Figure 4.5 Percentage change in mass after 50 hours of exposure in Test 3 at 550ºC
Between the same 12 salt mixtures (pure chlorides, pure sulphates, KCl:NaCl and KCl:K2SO4 mixtures) exposed in Tests 1 – 3, the highest mass loss was observed for originally 100% K2SO4 (mixture 11) and the lowest for 100% KCl (mixture 1). Mixture 7 (originally 80% KCl + 20% K2SO4) followed the trend and displayed lower % change over the decreasing temperature and excluding SO2
from the gas mixture. However, 100% K2SO4 (mixture 11) behaved similarly (the highest mass change in Tests 1, 2 and 3), with the exception of its higher mass change in set 2 (Test 3). Mixture 12 (initially 100% Na2SO4) showed quite stable behaviour and similar mass changes at the same temperature (550ºC);
only at 600ºC a small mass gain (0.41%) could be observed. In the case of KCl:K2SO4 mixtures in set 1 (Test 3) their mass % change increased with the increasing molar ratio of K2SO4 in the mixture. The same situation was observed for set 2 with the exception of mixture 10 (originally 20% KCl + 80%
K2SO4).
Test 4 – 6 mixtures of chlorides; 350 ppm HCl + bal. N2; 550ºC
In this test two sets of 6 chloride mixtures were investigated (two pure chlorides and 4 mixtures of KCl and NaCl). They all showed mass loss with the lowest one for initially 100% KCl (0.2 and 0.05 mg) and the highest for two salts:
originally 20% KCl + 80% NaCl (1.86 and 0.44 mg) and 100% NaCl (0.71 and 1.54 mg). Figure 4.6 presents the mass change data gathered after Test 4. Blue columns show the first set of six salt mixtures; red columns show the second set of these mixtures exposed at the same time.
Similarly to Tests 1 – 3, the amount of the salt applied was not the same for all the sapphire discs therefore the percentage change in mass was calculated for the two sets and is shown in Figure 4.7. It can be seen, that the % change varied for the same salts exposed in two sets at the same time.
Figure 4.6 Mass change data obtained after 50 hours of exposure in Test 4 at 550ºC
Figure 4.7 Percentage change in mass after 50 hours of exposure in Test 4 at 550ºC
Six the same mixtures of salts (pure chlorides and chloride-based mixtures) were exposed in Tests 1 – 4 and the comparison between their % changes in mass is shown in Figure 4.8. The highest mass losses were observed for them in Tests 1 and 2 (with SO2). The exception was pure NaCl which displayed the highest mass loss in Test 3 (at 550ºC, without SO2, where KCl:K2SO4 were present in the furnace). In all cases, the lowest mass loss was displayed in Test 4, where S was excluded from the test atmosphere completely. Comparing these four tests the lowest mass change was observed for KCl. One negligible mass gain (0.6%) was observed for KCl (Test 3, set 2) which could possibly be considered as no mass change or the experimental error.
Figure 4.8 Comparison between % change in mass of 6 the same salt mixtures (DO1 – DO6) exposed in Tests 1 – 4