Sulphur
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SOx emissions and control - 2.5 Predicting sulphur emiss...
Page 1 of 2
This effect of the ash constituents has been known for many years. For example, Raask (1982) reported studies on sulphate capture in ash and boiler deposits in relation to SO2 emissions from firing British bituminous coals in pulverised-fuel boilers. Species that can be sulphated include sodium chloride (NaCl), and magnesium and calcium carbonates (MgCO3 and CaCO3). Raask (1982) concluded that, in coal-fired boilers burning British bituminous coal, an average 15% of the total sulphur in the coal was retained in the ash and boiler deposits in the form of sulphates. However, later work carried out by the UK’s Central Electricity Generating Board indicated that this estimate was too high (Gibb, 2000). For low-rank coals, the fixation of sulphur in the ash may be more effective than for higher-rank coals. At the 300 MWe Unit 3 of the Ag. Dimitrios power plant, firing Greek lignite with a sulphur content of 0.42–0.6%, ash sulphation reduced SO2 emissions from the expected 230 ppm to 120–170 ppm (Kakaras and others, 1991;1995). The lignite yielded around 18% ash, the ash composition being about one-third CaO, with an MgO content of more than 4%. Elsewhere, Pleasance and Johnson (1995) reported that for brown coals from Victoria (Australia), the degree of sulphur fixation ranges from 10 to 70%. Western US subbituminous coals, particularly those from the Powder River Basin (PRB), are low in sulphur and high in calcium. Kang and others (1994) give sulphur contents of 0.38% and 0.73% for Eagle Butte and Wyodak subbituminous coals, with corresponding calcium oxide in ash values of 33.76% and 24.80%. In comparison, a Kentucky No.9 bituminous coal contained 3.67% sulphur and 6.01% CaO in the ash. Thus blending these coals should not only reduce SO2 emissions simply by dilution, but also possibly capture some sulphur in the higher-calcium ash. The calcium in the PRB coals is usually atomically dispersed but during char combustion, partially molten aluminosilicate ash particles on the char surface scavenge the calcium. Computer-controlled scanning electron microscopy revealed that most of the final ash particles were calcium-rich aluminosilicates. Results of calculations suggested no thermodynamic barriers to their sulphation in the temperature range 25–1200ºC, but entrained flow reactor (EFR) experiments were carried out in order to demonstrate the feasibility of the capture of sulphur in coal by calcium from the fly ash in a temperature environment similar to that found in utility boilers. Experiments with ash generated from subbituminous coal in the EFR showed that sulphur capture with the ash derived from a high-calcium coal was possible, while testwork using a blend of Kentucky No.9 coal and Eagle Butte coal showed that the amount of SO2 reduced decreased with increasing amounts of Kentucky No.9 in the blend. Kang and others (1995) also carried out EFR experiments using coal ash collected from the ESP hopper of Detroit Edison’s Belle River Unit 2 (St. Clair, MI, USA). The unit burns a Decker subbituminous coal from the Powder River Basin. When this as received ash was injected into a flue-gas stream in the reactor at a gas temperature of 1300ºC, the SO2 concentration increased from 390 ppm to 520 ppm. This was because the Belle River ash was already sulphated, primarily as calcium sulphate (CaSO4). Above 1200ºC, the CaSO4 decomposed, releasing SO2 back into the gas stream. Subsequent experiments were carried out with treated ash at a gas temperature of 1050ºC. As soon as the ash feeder was turned on, the SO2 concentration decreased rapidly from 348 ppm to about 322 ppm and thereafter kept decreasing at a slower rate. The rapid decrease in SO2 was attributed mainly to in-flight sulphur capture by ash. The slower decrease reflected the reaction with the deposited ash on the EFR walls. This represented a significant amount of SO2 capture, implying that, in practical coal- combustion systems, potentially significant reduction by ash deposits is possible. A full-scale test was performed at the Belle River Unit 1. Temperatures around 1000ºC correspond to the secondary superheater and first reheater region. The test verified the experimental findings and showed that about 10% of the SO2 emission was reduced by ash sulphation. Uzun and others (1995) defined ‘combustible sulphur’ as the difference between the total sulphur in the coal and that captured by the ash. Their first attempts to obtain correlations based on total sulphur and proximate analysis were unsuccessful. However, Uzun and Özdogan (1997) were able to find correlations when they were based on the pyritic, organic, and total sulphate sulphur forms. Inclusion of the CaO content in the correlations also affected the results favourably. Further work also incorporated the MgO content (Uzun and Özdogan, 1998). These experiments were not, however, carried out under combustion conditions. The coal ash samples were prepared according to ASTM standard D3174-89 at a temperature of 750ºC, far lower than the temperatures normally encountered in pulverised-coal combustion. Increasing the temperature to 950ºC resulted in an increase in the combustible sulphur content, confirming that there are indeed temperature effects. This also implies that the sulphur capture by ash in pulverised-coal combustion cannot be predicted by equations obtained at lower temperatures. Uzun and Özdogan (1998) pointed out that new formulae have to be developed for combustion conditions that differ from those of their study.
2.5
Predicting sulphur emissions Despite the evidence for sulphur capture by ash, Okamoto (1998) pointed out that the amount is so small that it can be neglected for practical purposes. Accordingly, he calculated the SOx emissions as f ll
http://www.coal.org.uk/catalogues/coalonline/81432/6041/html/6041_30.html
10/9/2551
SOx emissions and control - Page 31
Page 1 of 2
Amount of SOx per tonne of coal (m3) = 1000 kg × (S wt%/100) × (22.4/32) 1 gram-molecule of gas occupies a volume of 22.4 litres at 0ºC and at 1 atmosphere, and the atomic mass of sulphur is 32. The value obtained can be divided by the emission gas volume to obtain the concentration in ppm. Other sulphur-emission calculations take into account sulphur capture by ash and boiler deposits. For example, the European Environment Agency (EEA) published a method of determining SO2 emission factors (McInnes, 1996). The calculation procedure is performed in three steps: 1
The fuel sulphur reacts stoichiometrically with oxygen (O2) to sulphur dioxide (SO2). The result is the maximum attainable amount of sulphur dioxide (CSO2.max) given by: CSO2.max = 2 CSfuel where: sulphur content of fuel (in mass element/mass fuel [kg/kg]) CSfuel maximum attainable amount of sulphur dioxide (in mass pollutant/mass fuel CSO2.max [kg/kg])
Default values for the sulphur content (CSfuel) in hard and brown coal were provided, based on the geographical origin of the coal. For example, a sulphur content of 0.9 wt% was given for German Ruhr bituminous coal, and 0.8 wt% S for German Rhenish brown coal. The EEA document quotes the sulphur contents on a ‘maf’ basis (moisture and ash-free), equivalent to ‘daf’ (dry, ash free). 2
The maximum attainable amount of sulphur dioxide CSO2.max is corrected by the sulphur retention in ash s. As a result, the real boiler emission of sulphur dioxide C SO2.boiler fuel is obtained: C SO2.boiler fuel = CSO2.max(1-s) where: C SO2.boiler fuel is the real boiler emission of sulphur dioxide (in mass pollutant/mass fuel [kg/kg]) is the maximum attainable amount of sulphur dioxide (in mass pollutant/mass fuel CSO2.max [kg/kg]) is the sulphur retention in ash s
The sulphur retention in ash depends on, for example, the fuel characteristics and the temperature inside the boiler. If there no data for s are available, default values for various fuels are given:
Dry-bottom boiler Wet-bottom boiler
Hard coal
Brown coal
0.05 0.01
0.3
Therefore, for bituminous coal in a dry-bottom boiler, 5% of the SO2 is captured by the ash, while 1% is captured in a wet- bottom boiler. The brown coal value was an average; in practice a range of 0.05 to 0.60 can occur, depending on the origin of the brown coal. 3
The boiler emission of sulphur dioxide can be further corrected by the reduction efficiency and availability of the secondary flue-gas desulphurisation system installed. The EEA compared the calculated SO2 emission data with measured values from mainly German power stations. These are shown in Figure 4.
Good correlations between measured and calculated values were claimed for calculations that were only based on plant- specific data provided by power-plant operators. However, for most of the calculations a mixture of plant-specific data and default values for missing parameters was used, leading to deviations from the middle axis. The EEA document claims that, ‘in particular, strong differences occur for SO2 emissions, which show a tendency to be overestimated’. This is attributed to assumptions with regard to the default values; for example, the sulphur retention in ash varies greatly depending on the data availability. However, examination of Figure 4 reveals that there is only one significant overestimate and that the tendency is to underestimate the SO2 emissions. The data on which Figure 4 is based confirm that the figure’s axes are correct. The underestimation may be caused by low default values for the sulphur contents of German coals, or because the default value of 5% SO2 retention in the ash may be too high. This latter explanation is less likely given that the s value at the Münster power station was given as 0.15, and this station was one for which the calculated value (1310–1650 mg/m3), although underestimated, was reasonably close to the measured value (1644–1891 mg/m3). For a station at Karlsruhe, the s value was quoted as 0.4, which seems high, and may account for the underestimate of 1310–1650 mg/m3 calculated compared with 1600–2000 mg/m3 measured. It is also possible that the default values for the efficiency and availability of the flue-gas desulphurisation equipment were too high.
http://www.coal.org.uk/catalogues/coalonline/81432/6041/html/6041_31.html
10/9/2551
SOx emissions and control - Figure 04 Comparison of meas...
2900
Page 1 of 2
are used, but attempts have been made to estimate the extent of sulphur capture. For example, Raask (1982) suggested that the fraction of the total sulphur required to convert the chloride sodium and carbonate calcium to sulphates can be expressed in terms of the chloride and carbonate contents of the coal, assuming no ‘loss’ of sodium or calcium to silicates:
2400 1900
1400
SE = (0.27 Cl + 0.58 CO2)/S where SE is the fraction of total sulphur converted to sulphate and S, Cl, and CO2 are the sulphur, chlorine, and carbonate contents on a weight per cent basis.
900
400 Raask (1982) allowed for the excess of chlorine over leachable sodium and potassium, and for the fact that Measured values, mg/m3 iron and manganese carbonates in the coal ash would not form stable sulphates. He also noted that, in practice, Comparison of measured flue gas the quantities of sulphates formed are significantly less concentrations of SO2 with calculated flue as a result of the capture of sodium and calcium by the gas concentrations downstream of the fused silica particles in the coal flame. At boiler flame boiler (McInnes, 1996) temperatures of between 1300 and 1500ºC, alkali metal and alkaline earth metal silicates are more stable than the corresponding sulphates. In fact, as Mraw and others (1983) pointed out, at furnace flame temperatures, no SO2 absorption by calcium can take place since CaSO4 is thermodynamically unstable. It becomes stable slightly below 1200ºC as the flue gas enters the convection passes of the boiler. However, the CaO in silicate melts has an extremely low activity that reduces the temperature still further before CaSO4 becomes stable at the SO2 pressures involved.
400
Figure 04
900
1400
1900
2400
2900
An ‘alkali loss’ term was introduced by Raask (1982): 1–kW , where W is the total mass of ash and the constant k was set at 0.09 based on analytical data for the sulphate content in ash, boiler deposits, hopper ash and so on. The quantity of SO2 emitted in kg per tonne of coal burned was calculated: 2/3
SO2em = 20 [S – (0.27 Cl + 0.58 CO2)(1 – 0.09 W )] 2/3
Raask (1982) pointed out that the carbonate content of coal is not determined by routine analysis and that analytical data were scarce. An alternative equation uses the acid-soluble alkali and alkaline earth metal content of the coal to calculate the SO2 emitted: SO2em = 20 [S – (0.70 Na +0.41 K + 0.80 Ca + 1.32 Mg)(1 – 0.09 W )] 2/3
where Na, K, Ca, and Mg are the weight percentages of the acid-soluble sodium, potassium, calcium and magnesium in the coal. Pohl and others (1997) reported that work at ACIRL in Australia produced the following proportional absorption of sulphur dioxide by calcium, sodium and potassium in the ash: % S capture = 90 (Ca/S) + 30 (Na/S) + 70 (K/S) where the sulphur capture and the elemental ratios are molar ratios. Uzun and Özdogan (1997) obtained several correlations for combustible sulphur with sulphur forms and the CaO content. The best correlation (with a correlation coefficient of 0.99) was: SC = -0.130477 CaO + 1.449782 SP + 0.935385 SS + 0.748601 SO where: Sc is combustible sulphur, wt%, db CaO is the calcium oxide content, wt%, db SP is the pyritic sulphur content, wt%, db SS is the sulphate sulphur content, wt%, db SO is the organic sulphur content, wt%, db.
http://www.coal.org.uk/catalogues/coalonline/81432/6041/html/6041_32.html
10/9/2551
SOx emissions and control - Page 34
Page 1 of 2
The presence of calcium chloride however, is not all beneficial, and can potentially cause operational problems. Blythe and Rhudy (1991) reported experience at two EPRI test facilities in the USA – the Arapahoe 2.5 MW pilot unit in Denver, CO, and the High Sulfur Test Center (HSTC) at Somerset, NY. At both plants there were repeated failures of 300-series stainless steel clamps used to attach the bags in the fabric filters. Failures of stainless steel bag caps and anti-collapse rings were also noted. These failures were attributed to stress corrosion cracking caused by exposure to chloride. At the Arapahoe facility, the failures were noted at chloride levels equivalent to 0.2 wt% chlorine in the coal. At the HSTC, all testing was conducted with a coal chlorine content of only 0.1 wt%.
http://www.coal.org.uk/catalogues/coalonline/81432/6041/html/6041_34.html
10/9/2551
Page 1 of 2
This effect of the ash constituents has been known for many years. For example, Raask (1982) reported studies on sulphate capture in ash and boiler deposits in relation to SO2 emissions from firing British bituminous coals in pulverised-fuel boilers. Species that can be sulphated include sodium chloride (NaCl), and magnesium and calcium carbonates (MgCO3 and CaCO3). Raask (1982) concluded that, in coal-fired boilers burning British bituminous coal, an average 15% of the total sulphur in the coal was retained in the ash and boiler deposits in the form of sulphates. However, later work carried out by the UK’s Central Electricity Generating Board indicated that this estimate was too high (Gibb, 2000). For low-rank coals, the fixation of sulphur in the ash may be more effective than for higher-rank coals. At the 300 MWe Unit 3 of the Ag. Dimitrios power plant, firing Greek lignite with a sulphur content of 0.42–0.6%, ash sulphation reduced SO2 emissions from the expected 230 ppm to 120–170 ppm (Kakaras and others, 1991;1995). The lignite yielded around 18% ash, the ash composition being about one-third CaO, with an MgO content of more than 4%. Elsewhere, Pleasance and Johnson (1995) reported that for brown coals from Victoria (Australia), the degree of sulphur fixation ranges from 10 to 70%. Western US subbituminous coals, particularly those from the Powder River Basin (PRB), are low in sulphur and high in calcium. Kang and others (1994) give sulphur contents of 0.38% and 0.73% for Eagle Butte and Wyodak subbituminous coals, with corresponding calcium oxide in ash values of 33.76% and 24.80%. In comparison, a Kentucky No.9 bituminous coal contained 3.67% sulphur and 6.01% CaO in the ash. Thus blending these coals should not only reduce SO2 emissions simply by dilution, but also possibly capture some sulphur in the higher-calcium ash. The calcium in the PRB coals is usually atomically dispersed but during char combustion, partially molten aluminosilicate ash particles on the char surface scavenge the calcium. Computer-controlled scanning electron microscopy revealed that most of the final ash particles were calcium-rich aluminosilicates. Results of calculations suggested no thermodynamic barriers to their sulphation in the temperature range 25–1200ºC, but entrained flow reactor (EFR) experiments were carried out in order to demonstrate the feasibility of the capture of sulphur in coal by calcium from the fly ash in a temperature environment similar to that found in utility boilers. Experiments with ash generated from subbituminous coal in the EFR showed that sulphur capture with the ash derived from a high-calcium coal was possible, while testwork using a blend of Kentucky No.9 coal and Eagle Butte coal showed that the amount of SO2 reduced decreased with increasing amounts of Kentucky No.9 in the blend. Kang and others (1995) also carried out EFR experiments using coal ash collected from the ESP hopper of Detroit Edison’s Belle River Unit 2 (St. Clair, MI, USA). The unit burns a Decker subbituminous coal from the Powder River Basin. When this as received ash was injected into a flue-gas stream in the reactor at a gas temperature of 1300ºC, the SO2 concentration increased from 390 ppm to 520 ppm. This was because the Belle River ash was already sulphated, primarily as calcium sulphate (CaSO4). Above 1200ºC, the CaSO4 decomposed, releasing SO2 back into the gas stream. Subsequent experiments were carried out with treated ash at a gas temperature of 1050ºC. As soon as the ash feeder was turned on, the SO2 concentration decreased rapidly from 348 ppm to about 322 ppm and thereafter kept decreasing at a slower rate. The rapid decrease in SO2 was attributed mainly to in-flight sulphur capture by ash. The slower decrease reflected the reaction with the deposited ash on the EFR walls. This represented a significant amount of SO2 capture, implying that, in practical coal- combustion systems, potentially significant reduction by ash deposits is possible. A full-scale test was performed at the Belle River Unit 1. Temperatures around 1000ºC correspond to the secondary superheater and first reheater region. The test verified the experimental findings and showed that about 10% of the SO2 emission was reduced by ash sulphation. Uzun and others (1995) defined ‘combustible sulphur’ as the difference between the total sulphur in the coal and that captured by the ash. Their first attempts to obtain correlations based on total sulphur and proximate analysis were unsuccessful. However, Uzun and Özdogan (1997) were able to find correlations when they were based on the pyritic, organic, and total sulphate sulphur forms. Inclusion of the CaO content in the correlations also affected the results favourably. Further work also incorporated the MgO content (Uzun and Özdogan, 1998). These experiments were not, however, carried out under combustion conditions. The coal ash samples were prepared according to ASTM standard D3174-89 at a temperature of 750ºC, far lower than the temperatures normally encountered in pulverised-coal combustion. Increasing the temperature to 950ºC resulted in an increase in the combustible sulphur content, confirming that there are indeed temperature effects. This also implies that the sulphur capture by ash in pulverised-coal combustion cannot be predicted by equations obtained at lower temperatures. Uzun and Özdogan (1998) pointed out that new formulae have to be developed for combustion conditions that differ from those of their study.
2.5
Predicting sulphur emissions Despite the evidence for sulphur capture by ash, Okamoto (1998) pointed out that the amount is so small that it can be neglected for practical purposes. Accordingly, he calculated the SOx emissions as f ll
http://www.coal.org.uk/catalogues/coalonline/81432/6041/html/6041_30.html
10/9/2551
SOx emissions and control - Page 31
Page 1 of 2
Amount of SOx per tonne of coal (m3) = 1000 kg × (S wt%/100) × (22.4/32) 1 gram-molecule of gas occupies a volume of 22.4 litres at 0ºC and at 1 atmosphere, and the atomic mass of sulphur is 32. The value obtained can be divided by the emission gas volume to obtain the concentration in ppm. Other sulphur-emission calculations take into account sulphur capture by ash and boiler deposits. For example, the European Environment Agency (EEA) published a method of determining SO2 emission factors (McInnes, 1996). The calculation procedure is performed in three steps: 1
The fuel sulphur reacts stoichiometrically with oxygen (O2) to sulphur dioxide (SO2). The result is the maximum attainable amount of sulphur dioxide (CSO2.max) given by: CSO2.max = 2 CSfuel where: sulphur content of fuel (in mass element/mass fuel [kg/kg]) CSfuel maximum attainable amount of sulphur dioxide (in mass pollutant/mass fuel CSO2.max [kg/kg])
Default values for the sulphur content (CSfuel) in hard and brown coal were provided, based on the geographical origin of the coal. For example, a sulphur content of 0.9 wt% was given for German Ruhr bituminous coal, and 0.8 wt% S for German Rhenish brown coal. The EEA document quotes the sulphur contents on a ‘maf’ basis (moisture and ash-free), equivalent to ‘daf’ (dry, ash free). 2
The maximum attainable amount of sulphur dioxide CSO2.max is corrected by the sulphur retention in ash s. As a result, the real boiler emission of sulphur dioxide C SO2.boiler fuel is obtained: C SO2.boiler fuel = CSO2.max(1-s) where: C SO2.boiler fuel is the real boiler emission of sulphur dioxide (in mass pollutant/mass fuel [kg/kg]) is the maximum attainable amount of sulphur dioxide (in mass pollutant/mass fuel CSO2.max [kg/kg]) is the sulphur retention in ash s
The sulphur retention in ash depends on, for example, the fuel characteristics and the temperature inside the boiler. If there no data for s are available, default values for various fuels are given:
Dry-bottom boiler Wet-bottom boiler
Hard coal
Brown coal
0.05 0.01
0.3
Therefore, for bituminous coal in a dry-bottom boiler, 5% of the SO2 is captured by the ash, while 1% is captured in a wet- bottom boiler. The brown coal value was an average; in practice a range of 0.05 to 0.60 can occur, depending on the origin of the brown coal. 3
The boiler emission of sulphur dioxide can be further corrected by the reduction efficiency and availability of the secondary flue-gas desulphurisation system installed. The EEA compared the calculated SO2 emission data with measured values from mainly German power stations. These are shown in Figure 4.
Good correlations between measured and calculated values were claimed for calculations that were only based on plant- specific data provided by power-plant operators. However, for most of the calculations a mixture of plant-specific data and default values for missing parameters was used, leading to deviations from the middle axis. The EEA document claims that, ‘in particular, strong differences occur for SO2 emissions, which show a tendency to be overestimated’. This is attributed to assumptions with regard to the default values; for example, the sulphur retention in ash varies greatly depending on the data availability. However, examination of Figure 4 reveals that there is only one significant overestimate and that the tendency is to underestimate the SO2 emissions. The data on which Figure 4 is based confirm that the figure’s axes are correct. The underestimation may be caused by low default values for the sulphur contents of German coals, or because the default value of 5% SO2 retention in the ash may be too high. This latter explanation is less likely given that the s value at the Münster power station was given as 0.15, and this station was one for which the calculated value (1310–1650 mg/m3), although underestimated, was reasonably close to the measured value (1644–1891 mg/m3). For a station at Karlsruhe, the s value was quoted as 0.4, which seems high, and may account for the underestimate of 1310–1650 mg/m3 calculated compared with 1600–2000 mg/m3 measured. It is also possible that the default values for the efficiency and availability of the flue-gas desulphurisation equipment were too high.
http://www.coal.org.uk/catalogues/coalonline/81432/6041/html/6041_31.html
10/9/2551
SOx emissions and control - Figure 04 Comparison of meas...
2900
Page 1 of 2
are used, but attempts have been made to estimate the extent of sulphur capture. For example, Raask (1982) suggested that the fraction of the total sulphur required to convert the chloride sodium and carbonate calcium to sulphates can be expressed in terms of the chloride and carbonate contents of the coal, assuming no ‘loss’ of sodium or calcium to silicates:
2400 1900
1400
SE = (0.27 Cl + 0.58 CO2)/S where SE is the fraction of total sulphur converted to sulphate and S, Cl, and CO2 are the sulphur, chlorine, and carbonate contents on a weight per cent basis.
900
400 Raask (1982) allowed for the excess of chlorine over leachable sodium and potassium, and for the fact that Measured values, mg/m3 iron and manganese carbonates in the coal ash would not form stable sulphates. He also noted that, in practice, Comparison of measured flue gas the quantities of sulphates formed are significantly less concentrations of SO2 with calculated flue as a result of the capture of sodium and calcium by the gas concentrations downstream of the fused silica particles in the coal flame. At boiler flame boiler (McInnes, 1996) temperatures of between 1300 and 1500ºC, alkali metal and alkaline earth metal silicates are more stable than the corresponding sulphates. In fact, as Mraw and others (1983) pointed out, at furnace flame temperatures, no SO2 absorption by calcium can take place since CaSO4 is thermodynamically unstable. It becomes stable slightly below 1200ºC as the flue gas enters the convection passes of the boiler. However, the CaO in silicate melts has an extremely low activity that reduces the temperature still further before CaSO4 becomes stable at the SO2 pressures involved.
400
Figure 04
900
1400
1900
2400
2900
An ‘alkali loss’ term was introduced by Raask (1982): 1–kW , where W is the total mass of ash and the constant k was set at 0.09 based on analytical data for the sulphate content in ash, boiler deposits, hopper ash and so on. The quantity of SO2 emitted in kg per tonne of coal burned was calculated: 2/3
SO2em = 20 [S – (0.27 Cl + 0.58 CO2)(1 – 0.09 W )] 2/3
Raask (1982) pointed out that the carbonate content of coal is not determined by routine analysis and that analytical data were scarce. An alternative equation uses the acid-soluble alkali and alkaline earth metal content of the coal to calculate the SO2 emitted: SO2em = 20 [S – (0.70 Na +0.41 K + 0.80 Ca + 1.32 Mg)(1 – 0.09 W )] 2/3
where Na, K, Ca, and Mg are the weight percentages of the acid-soluble sodium, potassium, calcium and magnesium in the coal. Pohl and others (1997) reported that work at ACIRL in Australia produced the following proportional absorption of sulphur dioxide by calcium, sodium and potassium in the ash: % S capture = 90 (Ca/S) + 30 (Na/S) + 70 (K/S) where the sulphur capture and the elemental ratios are molar ratios. Uzun and Özdogan (1997) obtained several correlations for combustible sulphur with sulphur forms and the CaO content. The best correlation (with a correlation coefficient of 0.99) was: SC = -0.130477 CaO + 1.449782 SP + 0.935385 SS + 0.748601 SO where: Sc is combustible sulphur, wt%, db CaO is the calcium oxide content, wt%, db SP is the pyritic sulphur content, wt%, db SS is the sulphate sulphur content, wt%, db SO is the organic sulphur content, wt%, db.
http://www.coal.org.uk/catalogues/coalonline/81432/6041/html/6041_32.html
10/9/2551
SOx emissions and control - Page 34
Page 1 of 2
The presence of calcium chloride however, is not all beneficial, and can potentially cause operational problems. Blythe and Rhudy (1991) reported experience at two EPRI test facilities in the USA – the Arapahoe 2.5 MW pilot unit in Denver, CO, and the High Sulfur Test Center (HSTC) at Somerset, NY. At both plants there were repeated failures of 300-series stainless steel clamps used to attach the bags in the fabric filters. Failures of stainless steel bag caps and anti-collapse rings were also noted. These failures were attributed to stress corrosion cracking caused by exposure to chloride. At the Arapahoe facility, the failures were noted at chloride levels equivalent to 0.2 wt% chlorine in the coal. At the HSTC, all testing was conducted with a coal chlorine content of only 0.1 wt%.
http://www.coal.org.uk/catalogues/coalonline/81432/6041/html/6041_34.html
10/9/2551
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