Abstract
This paper reports the measurements of SO3 emissions with and without limestone under unstaged and staged fluidized-bed combustion, carried out on a m2 and 2 m high stainless-steel combustor at atmospheric pressure. The secondary air was injected 100 cm above the distributor. SO3 emissions were monitored for staging levels of 85 : 15, 70 : 30, and 60 : 40, equivalent to a primary air/coal ratio (PACR) of ~0.86, 0.75, and 0.67. Experiments were carried out at 0%–60% excess air level, 1-2 m/s fluidizing velocity, 800–850°C bed temperature, and 20–30 cm bed height. During unstaged combustion runs, SO3 emissions were monitored for a wide range of Ca/S ratios from 0.5 to 13. However, for the staged combustion runs, the Ca/S ratio was fixed at 3. SO3 was retained to a lesser extent than SO2, suggesting that SO2 reacts preferentially with CaO and that SO3 is involved in the sulphation process to a lesser degree. The SO3 emissions were found to be affected by excess air, whereas the fluidizing velocity and bed temperature had little effect. SO3 was depressed on the addition of limestone during both the staged and unstaged operations, and the extent of the reduction was higher under staged combustion.
1. Introduction
The presence of SO3 in flue gas corrodes the equipment and ducts of combustion system and therefore needs to be removed [1]. In order to control emissions of SO3, more studies on its formation and dissociation are required under air-fired and oxy-fired combustion conditions. The simulation study of Zheng and Furimsky [2] shows that SO3 emissions would be unaffected during oxy-fuel combustion, being governed only by oxygen concentration. The kinetics of reactions occurring in the combustor were studied by Burdett et al. [3] using a TGA microbalance. They proposed the following mechanisms for the formation of SO3.
1.1. SO2/SO3 Homogeneous Gas Phase Reaction
SO2 may be oxidized to SO3 by two reactions: where M is a chaperon third body molecule. The large temperature dependence of reactions (1) and (2) ensures that the rate of production falls rapidly with decreasing gas temperature, and, in fact, 90%–95% of SO3 is formed in the bed and freeboard and the remaining 5%–10% in the region between the freeboard and sampling point. SO3 increases sharply with temperature, but the homogeneous reaction cannot account for all the SO3 produced.
1.2. Heterogeneous Catalysis of SO2 on Bed Particles and Heat Transfer Surfaces
In a coal burning combustor, a more effective catalytic material, iron oxide, is present in fly ash. While the SO3 formation in this process is important, the experimental data is insufficient to quantify the SO3 formation.
Dennis and Hayhurst [4] used an 80 mm diameter fluidized-bed combustor and mass spectrometer for measuring the concentration of SO3. They confirmed the amount of SO3 formed at atmospheric pressure to be very low and much less than the equilibrium concentration. The rate measured was 100 times faster than expected for oxidation in the gas phase.
Willium and Gibbs [5] measured SO2 and SO3 emissions from a 0.3 m2 fluidized-bed combustor. They reported that the higher the excess air level, the lower the SO2 emissions (on removal of the dilution effect, SO2 increased with an increase in excess air, reaching a limiting value of 1300 ppm above 30% excess air), and that SO3 emissions increased slightly with the increase in excess air. Barnes [6] also observed the similar effect of excess air on SO2 and SO3 emissions. Willium and Gibbs [5] and Barnes [6] found that the higher the sulphur content of fuel, the higher the SO2 emissions, while the SO3 was unaffected by the fuel sulphur content [5]. Coal fed to the bed caused SO2 and SO3 emissions to increase than when fed to the surface [5]; SO3 shows a weak dependence on bed temperature [5, 6].
Barnes [6] studied the effect of sand particle size, fluidizing velocity, and bed depth on SO2 and SO3 emissions. Barnes’ findings indicate that fine sand (0.300 mm) produces high SO2 and SO3 emissions. Increasing fluidizing velocity from 1 to 2 m/s caused reduced formation of SO2 in the bed and freeboard. An increase in bed depth increased SO3 emission; a deep bed (30 cm) and fine sand resulted in a slight decrease in SO2 emission.
Oxygen availability and fluidizing characteristics within the bed also affect SO3 formation. Ahn et al. [7] found that for pulverized coal, concentrations of SO3 and SO2 were significantly higher for oxy-fired conditions as compared to air-fired conditions. In circulating fluidized bed, SO3 concentrations were notably higher for oxy-fired conditions too. For higher sulfur coal, SO3 concentrations were 4–6 times greater on average. Their findings contradict the finding of Barnes [6].
Hindiyarti et al. [8] investigated the reaction of SO3 with H, O, and OH radical. The revised rate constant calculated by them suggests that SO3 and O reaction is found to be insignificant during most conditions. According to them, is the major consumption reaction for SO3.
Stanger and Wall [9] reviewed published work on SO3 concentrations and emissions under oxy-fuel firing. Their conclusion is that the conversion of SO2 to SO3 is considerably variable.
Willium and Gibbs [5] found that coal char had a very significant removal effect on SO3 emissions because SO3 above the bed was 50% greater than in the exit. Barnes [6] said that unburnt char does not have a major effect on SO3 as carbon carryover increases under the given operating conditions. Her results showed that the quantity of inert particles (30 cm deep bed) resulted in an increase in heterogeneous catalytic reaction of SO2 to form SO3.
Burdett et al. [3] carried out experiments in a microbalance to study the effect of limestone on SO3 emissions. According to Burdett et al. [3], the reaction of CaO, O2, and SO2, in order to yield CaSO4 (reaction (3)), must occur in two separate steps. Two possibilities exist, either SO2 reacts with CaO and CaSO3 is formed, which is then oxidized, or else the formation of SO3 in gas phase or on a stone surface is followed by an attack on the CaO. Consider the following: Route 1 Route
It is not possible, due to the lack of available experimental data, to say which of these mechanisms is operative under given conditions, although both may be important. A most interesting comparison between the level detected with and without limestone (proposed by Burdett et al. [10]) is the SO2/SO3 ratio. Without limestone, the ratio they found was 2200/33 (or 67 : 1), and with limestone it increased to 350/1.5 (or 230 : 1). It is clear that SO3 is depressed to a greater extent than SO2 on the addition of limestone, and they attribute this to the higher reactivity of SO3 compared with that of SO2. Burdett [11] and Burdett et al. [10] have assumed that SO2 oxidizes to SO3 in the particles at a rate dependent on local SO2 and O2 concentrations, with SO3 diffusing through the CaSO4 shell and reacting with CaO. The rate of formation of CaSO4 may be linked to different rates of production of SO3 at different locations within the stone. At a high oxygen level, oxidation increases preferentially at the edge of a particle. High utilization is achieved when SO2 diffusion in the interior of the stone is maximized, and this, in turn, implies a low SO3 formation rate. Barnes [6] reported a decrease in SO2 conversion to SO3 with oxygen concentration and an increase with SO2 concentration.
Fieldes et al. [12] reported the achievement of a high fractional sulphation (0.36) when coal is burnt in the bed. It appears that the fraction of the sulphur gas phase, which is SO3, has a substantial effect on the fractional sulphation of the limestone. Ash also appears to remove SO3 selectively. The mechanism of this hypothesis supposes that the direct reaction of CaO with SO3 is faster than a reaction via the CaSO3 intermediate. This paper examines the factors responsible for formation and reduction of SO2 and SO3 from a coal-fired fluidized bed under varying operating conditions.
2. Apparatus and Procedure
The main features of the fluidized-bed combustor and ancillaries are presented in Figure 1. The bed consisted of silica sand of mean size 0.700 mm. Fluidizing air was supplied by a fan and metered and introduced through a distributor plate. For staged combustion, the secondary air was introduced into the combustor through a stainless-steel pipe 100 cm above the bed surface. In staged combustion mode, the total combustion air is separated into a primary air stream supplied to fluidize the bed and a secondary air stream injected above the bed to complete the combustion. For example, in 70 : 30 staging, 30% of the total air is injected as secondary air.
The bed was preheated by a propane burner that was fixed above the bed, and the fluidizing airflow rate was adjusted to the lowest level to minimize heating time. Coal was fed in the combustor when the bed temperature reached 550°C. When the bed temperature reached 800°C, the desired coal feed rate was adjusted to a constant value, the propane burner was switched off, and the fluidizing air was adjusted to the required level. The bed temperature was maintained constant by using an adjustable cooling coil with circulating water. Concentrations of O2, CO, CO2, and SO2 were recorded continuously by an ADC-RF infrared gas analyzer.
The experiments were carried out at bed temperatures of 800–850°C, fluidizing velocities of 1-2 m/s, and excess air levels of 0%–60%. Static bed height was 20–30 cm. Two types of coal bituminous Linby and Daw Mill of 3–16 mm (large) diameter in size and two types of limestone, Ballidone and Penrith of <3 mm mean diameter in size, were used. In both cases, the coal was premixed with the limestone and fed overbed at 42 cm above the distributor (see Table 1 for proximate and ultimate analyses of the coal and Table 2 for chemical composition of the limestone). The Ca/S ratio was 3 : 1 mole per mole or otherwise as indicated. Three levels of staging (15%–40% secondary air) were used to investigate the effect of fluidizing velocity, bed temperature, and excess air on SO3 reduction during air staging.
3. Sampling of SO3
An SSL/MEL SO3 analyzer, developed by Severn Science Labs/Marchwood Eng. Labs, was used for continuous monitoring of the SO3. A detailed account of the principles and operating procedures of an SSL/MEL analyzer can be found in Jackson et al. [13] and Hotchkiss et al. [14].
The representative sample of flue gas was extracted from the sample point located near the exit of flue gas to the cyclone (200 cm above the distributor), where the gas temperature was around 550°C. Under these conditions, the use of a quartz sampling probe was found to be adequate. This enabled the extraction of the acid-containing gas directly into the filter-contactor of the SO3 analyzer. The temperature of the sampled gas was maintained (by keeping the length of the tube as short as possible) at a value in excess of the acid dewpoint and below the temperature at which significant dissociation to SO3 occurred; accurate determination of the acid gas content in the gas could then be made precisely over a significant period of time. This procedure effectively eliminates any interaction between the SO3 in the gas sample and other species within the sampling probe itself.
4. Results and Discussion
4.1. SO3 Emissions without Limestone under Unstaged Combustion
The effect of some operating variables on the SO3 emissions under unstaged combustion is presented in Figure 2. The results are corrected to 5% oxygen in the flue equivalent. The flue gas SO3 emissions ranged from 5 to 10 for the Linby coal. SO3 emissions decrease with excess air when corrected for dilution, increase slightly with excess air, reach limiting values, and then gradually decrease. The fluidizing velocity also affects emissions to some extent. The effect of bed height on SO3 emissions was associated with the size of sand particles. As the fine sand produces more reducing environment, the oxygen stoichiometry influences the rates of oxidation of SO2, and as a result, the generation of SO3 is reduced. The SO3 emissions were also less sensitive to change in bed temperature. The rise in emissions was typically 0.5/10°C.
In another set of experiments, the Daw Mill Coal was tested for SO3 emissions. The flue gas SO3 emission ranged from 4 to 19.5 ppm for Daw mill coal. Changing the operating parameters resulted in a maximum change in SO3 emission of 13 ppm. Increasing the bed depth resulted in higher SO3 emission, the fluidizing velocity also affected emission depending on the size of bed material used. The SO3 emission increased as the bed size was varied from coarse to fine with the bed depth. SO3 emissions were weakly dependent on temperature typically rising 0.7 ppm/10°C.
4.2. Comparison to Reported Work
Dennis and Hayhurst [4] have reported that the SO3 formation under atmospheric pressure was very low (e.g., mole fraction of SO3 in off gas = 7 × for mole fraction of SO2 gas entering a bed = 2.3 × at 875°C). The oxidation rate in the gas phase was 100 times faster than expected. An Eley-Rideal mechanism was proposed in which O2 and SO2 competitively chemisorb on the surface, and the rate of reaction is controlled by gas-phase molecule of SO2 reacting with adsorbed O atom.
Willium and Gibbs [5] have tested many coal types for SO3 concentration without limestone in 750–900°C temperature range. Their findings suggest that ash (having traces of Ca, Mg, Na, K, etc.) is the principle removing species of SO3. In another experiment, when pure SO2 was introduced, the SO3 reacted with added char at 850°C in the absence of oxygen to give SO3 of 7 vpm in the outlet, which suggests that char is important in the removal of SO3. He also observed a 50% reduction in SO3 in the freeboard. According to Willium, the reduction was due to the reaction of SO3 with unburnt char.
SO3 emissions are dependent on the oxygen and sulfur dioxide concentrations and were found to follow a similar trend. Willium and Gibbs [5] found that in contrast to the effect on SO2 emissions, fine coal produced lower SO3 emissions. In this study, SO3 emissions were slightly higher when fine sand was used and tended to increase with bed height. This suggests that unburnt char does not have a significant effect on SO3 emissions. The results of this study indicate that the amount of particles in the bed could have a significant effect on SO3 emissions, resulting in an increase in the heterogeneous catalytic reaction of SO2 to form SO3 as the quantity of bed particles increases. Higher bed height, therefore, will also result in high SO3 emissions. The oxygen concentration and fluidizing velocity will also affect SO3 formation.
4.3. SO3 Emissions with Limestone under Unstaged Combustion
SO3 emissions decrease in the presence of limestone and the reduction is temperature sensitive. The SO3 reductions were less sensitive than the reductions achieved for SO2 at similar conditions [15, 16]. At a temperature around 850°C, the SO3 reductions were only 28% of the SO2 reductions, but at 800°C, the reductions reached 70% of the SO2 reduction level. The results corresponding to the operating conditions are shown graphically in Figures 3 and 4. It should be noted that the SO3 reduction shown in Figure 3 was obtained when limestone was injected 12 cm above the distributor, and Figure 4 represents the results when limestone was injected 42 cm above the distributor.
At a temperature of 850°C, some of the SO2 will always be converted to SO3 via reaction (6). The conversion decreases with oxygen concentration and increases with sulfur dioxide concentration. An increase in temperature enhances the rate of SO3 formation. SO3 can react with CaO to form CaSO4 via reaction (7). The rate of this reaction is temperature dependent. Yilmaz et al. [17] studied the thermal dissociation of SO3 in the range of 800–1200°C under atmospheric pressure. At the location in the flame where the net SO3 formation rate is zero, he determined a rate constant of 6.9 × 1010 cm3 mol−1 s−1 for SO3 + N2 → SO3 + O + N2; that was consistent with other flame results. A high temperature lowers the reaction rate. Therefore, at a high temperature, more SO3 is produced but less will be consumed in sulphation. As a result, a larger decrease in SO3 emissions is observed at lower temperature.
The effect of excess air can also be seen in these graphs. SO3 emissions have been found to increase with excess air, but upon removing the dilution effect, the increase is within a narrow range, indicating that there could be an optimum reduction at a particular excess air beyond which the SO3 reduction decreases. An increase in the fluidizing velocity has little effect on the overall reduction of SO3 emissions.
During another set of experiments, Penrith limestone was added to the Daw Mill coal. It was observed that SO3 emissions were decreased in the presence of limestone, and the reduction was temperature dependent. At the higher temperature of 850°C, the SO3 reductions were 18%–20% of the SO2 reduction, but at 800°C the SO3 reductions reached 55% of the SO2 reduction level. Figure 5 shows the results of this set.
4.4. Comparison to Reported Work (Conducted on Microbalance or Small Bed of 36–78 mm ID)
Burdett et al. [3] have reported that the reaction between limestone and sulfur oxides is highly sensitive to changes in O2, SO, and SO3 concentrations. Absorption of SO3 by the coal ash cannot be quantified on the microbalance, and the microbalance results are not applicable to fluidized combustor.
Fieldes et al. [12] have reported that extent of SO2 oxidation to SO3 varied with SO2 and O2 concentration. The coal combustion test showed that the lower SO3 concentrations are due to its selective removal by ash. They had tested a variety of limestone, and in all the cases the mole fraction of CaO converted to CaSO4 was affected by inlet oxygen in the same way as Penrith limestone.
Thibault et al. [18] have conducted experiments on a small (6 mm) fixed bed packed with CaO particle. They have tested two grain size of the sorbent and reported that for efficient capture of SO3 a small grain size and open macropore structure are essential.
4.5. Comparison to Reported Work (Conducted on Pilot Scale)
Burdett et al. [19] have reported fractional conversion of SO2 to SO3 decreased from about 1.5% in the limestone-free case to around 0.35% when the limestone and alkaline ash were present, which was due to the greater reaction of SO3 with limestone compared with ash, the absorption occurring both in the bed itself and in the freeboard.
Burdett et al. [10] have reported that combustion of a 3% sulfur coal in a bed burning at 900°C generated 33 vpm of SO3 and proposed that the effect of O2 on sulphation capacity results from the formation of SO3 within the pores of the stone.
4.6. SO3 Emissions without Limestone under Staged Combustion
Merryman and Levy [20] have conducted staged experiments on a quartz tube methane burner producing stable methane-H2S flame within desired fuel-air ratio without a sorbent presence under staged combustion conditions. They have reported that when the remaining excess air was injected into these gases, the maximum amount of SO3 formed was greater than formed when this additional air was included with the initial combustion air, the overall excess of air being the same in both cases. The experimental conditions of Merryman and Levy do not match with our fluidized bed; therefore, their results are not comparable with this study.
During this study, the SO3 emissions under staged combustion without limestone could not be monitored extensively due to malfunctioning of SO3 analyzer.
4.7. SO3 Emissions with Limestone under Staged Combustion
The concentration of SO3 emissions at 1.5 m/s and 20% excess air was 17.0 ppm, which decreased to 5.5 ppm in the presence of limestone. The SO3 emissions at 70/30 staged (1.5 m/s, 850°C) combustion (without limestone) were similar to those of unstaged combustion (without limestone). However, it was observed that in the presence of limestone, staged combustion results in a higher reduction of SO3 than unstaged. Figure 6 gives the SO3 emissions as a function of PACR. The emissions at 15% secondary air were 1.5 ppm and increased to 7.2 ppm at 45% secondary air. It is clear that SO3 is depressed on the addition of limestone during both the unstaged and staged operations, and the extent of reduction was higher under staged combustion.
Figure 6 shows that the maximum removal of SO3 occurs at a lower staging levels of 85/15, and, as the bed becomes more substoichiometric, the rate of SO3 removal decreases. This trend indicates the formation of SO3 in the freeboard which bypasses the limestone and appears in the flue. This increase in SO3 reduction with the in-bed air ratio is in agreement with Barnes [6] findings.
The results of the staged combustion test with Daw Mill coal in the presence of Penrith limestone indicate that SO3 emissions varied little with changes in excess air. However, if excess air is coupled with fluidizing velocity, then it had some effects on the emissions. At higher velocity of 2 m/s, the change was up to 4 ppm. The concentration of SO3 emissions at 1.5 m/s and 30% excess air was 20 ppm which decreased to 8 ppm under staged combustion in the presence of limestone. The results of Daw Mill coal test are shown in Figure 7.
It should be noted that there is no published work on SO3 emissions under staged combustion conditions with or without limestone on any scale. Therefore, the results of this study could not be compared.
5. Conclusion
The experimental data shows that during unstaged combustion without limestone, SO3 emissions are dependent on oxygen and SO2 concentration. SO3 emissions increase slightly with excess air, reaching a limiting value, and then slowly decrease. SO3 emissions are less sensitive to change in bed temperature. However, the fluidizing velocity and bed height affect the emissions.
In the presence of limestone, SO3 emissions are reduced during both staged and unstaged operations, and the reduction is temperature sensitive. However, during staged combustion, the reduction is enhanced. As staged fluidized-bed combustion is a proven technique to reduce and SO2 emissions, therefore, it should be possible to operate a fluidized-bed combustor under a staged mode with limestone to keep SO2, SO3, and emissions to a minimum.