Article pubs.acs.org/EF
High-Temperature Conversion of SO2 to SO3: Homogeneous Experiments and Catalytic Effect of Fly Ash from Air and Oxy-fuel Firing Lawrence P. Belo,† Liza K. Elliott,† Rohan J. Stanger,† Reinhold Spörl,‡ Kalpit V. Shah,† Jörg Maier,‡ and Terry F. Wall*,† †
Chemical Engineering, University of Newcastle, Callaghan, New South Wales 2308, Australia Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, 70569 Stuttgart, Germany
‡
ABSTRACT: The reaction of SO2 with fly ash in the presence of O2 and H2O involves a series of reactions that lead to the formation of SO3 and eventually H2SO4. Homogeneous experiments were conducted to evaluate the effects of the procedural variables, i.e., temperature, gas concentrations, and residence time, on the post-combustion conversion of SO2 to SO3. The results were compared to existing global kinetics and found to be dependent upon SO2, O2, residence time, and temperature and independent of H2O content. For a residence time of 1 s, temperatures of about 900 °C are needed to have an observable conversion of SO2 to SO3. Literature suggested that the conversion of SO2 to SO3 is dependent upon the iron oxide content of the fly ash. Experiments using three different fly ash samples from Australian sub-bituminous coals were used to investigate the catalytic effects of fly ash on SO2 conversion to SO3 at a temperature range of 400−1000 °C. It was observed that fly ash acts as a catalyst in the formation of SO3, with the largest conversion occurring at 700 °C. A homogeneous reaction at 700 °C, without fly ash present, converted 0.10% of the available SO2 to SO3. When fly ash was present, the conversion increased to 1.78%. The catalytic effect accounts for roughly 95% of the total conversion. Average SO3/SO2 conversion values between fly ash derived from air and oxy-fuel firing and under different flue gas environments were found to be similar.
1. INTRODUCTION
in power plants because of the innately low sulfur content of Australian coals. The current paper focuses on understanding the mechanisms of the conversion of SO2 to SO3 under post-flame conditions in air and oxy-fuel flue gas environments. Belo et al.8 discussed in a previous paper the reaction routes of sulfur in a combustion system. SO2 is formed from the decomposition and oxidation of organic and inorganic sulfur associated in the coal matrix. SO2 is then converted to SO3 via either homogeneous gas-phase reactions or heterogeneous catalytic reactions.8,11 Conversion of SO2 to SO3 may be influenced by the following factors, namely, SO2 partial pressure,12−15 O2 partial pressure,13−15 presence or absence of moisture,12−16 presence of catalytically active components in the fly ash (e.g., Fe2O3),12,13,15,16 and temperature-residence time profile of the plant.13 The principal mechanisms for SO3 production in combustion systems are via
For centuries, fossil fuels (coal, oil, and natural gas) have been the primary source of energy for power generation, and this is expected to continue in the distant future. Coal, the cheapest fossil fuel, consists mainly of carbon and results in emissions of carbon dioxide when combusted. A total of 41% of the approximately 30 gigatonnes of CO2 emissions in 2010 was contributed by energy generation alone.1 Apart from being the largest source of greenhouse gases (chiefly CO2), coal-fired power generation also contributes significant amounts of other pollutants to the environment. Among the major pollutants are SO2 and SO3, collectively known as SOx, which apart from being a health concern2 plays an important role in the formation of photochemical smog and acid rain.3,4 CO2 capture and storage (CCS) technologies have been developed to address this growing concern of greenhouse gas and pollutant emissions. One promising CCS solution is oxy-fuel combustion, where coal is burned in a mixture of oxygen and recycled flue gas (RFG) instead of air. The resulting flue gas, rich in CO2, is cleaned and sequestered. Studies have shown that using recycled flue gas in oxy-fuel results in not only higher CO2 but also SO2 in the combustion zone, which could change the sulfur partitioning in the flue gas, resulting in higher SO3 concentrations.5,6 This is of particular importance when no SOx cleaning of the recycle stream is in place, and SO2 concentrations can be as high as 4 times that of air combustion.1,7−10 The increased SO2 concentration resulting from recycling of the flue gas is challenging in the Australian scenario, where no flue gas desulfurization is applied © 2014 American Chemical Society
SO2 + O2 ⇄ SO3 + O
(1)
SO2 + O ( +M) ⇄ SO3 ( +M)
(2)
with eq 2 occurring at flame temperatures. Burdett et al.17 stated that, unless there is a high concentration of O atoms present, the concentration is not high enough to facilitate SO3 production via eq 2. Fleig et al.,11 Jorgensen et al.,13 and Cullis and Mulcahy18 stressed that, even though photochemical oxidation of SO2 had Received: September 11, 2014 Revised: October 23, 2014 Published: October 23, 2014 7243
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Figure 1. Experimental setup for homogeneous and heterogeneous SO2 to SO3 conversion experiments. atm. A water saturator maintained at a controlled temperature was used to introduce water vapor into a N2 or CO2 carrier gas. The amount of water present was measured by a Testo 350XL relative humidity probe. Reactant gases were heated with heating tapes to approximately 80 °C before entering the reactor. The experiment consisted of two parts: homogeneous conversion reaction with gases only and heterogeneous conversion, with the reaction of gases in the presence of fly ash. During the homogeneous experiments, the quartz tube was empty, except for the feed gas flowing through it, but during the heterogeneous experiments, the quartz tube held a packed bed of fly ash (∼0.5 g) supported by quartz wool placed in the center of the isothermal region. The calculated residence time in the isothermal zone and the calculated contact time with a ∼1 cm thick bed of fly ash are presented in Table 1.
been studied in the past, there is limited convincing evidence of an uncatalyzed homogeneous reaction at temperatures below 900 °C. However, according to Cullis and Mulcahy18 and Rees et al.,19 oxidation of SO2 to SO3 is rather difficult but can be achieved catalytically. Cullis and Mulcahy,18 Marier and Dibbs,16 Spörl et al.,20 and Jorgensen et al.13 have shown that fly ash could act as a catalytic oxidizer to SO2 given the presence of metal oxides in its matrix even at temperatures as low as 400 °C. One of the best known catalysts is Fe2O3. The reaction is as follows:2,16,19 SO2 +
Fe2O3 1 O2 ⎯⎯⎯⎯⎯⎯→ SO3 catalyst 2
(3)
During oxy-fuel combustion, the increased amount of O2 and SO2 has the potential of affecting the degree of oxidation of SO2 to SO3, which has been reported in several studies.1,6,10 Limited literature exists on the conversion of SO2 in the absence of combustibles. Spörl et al.20 stressed that the ratios between heterogeneous and homogeneous SO3 formation are still vague. The presence of SO3 is significant in power plants because increasing amounts of SO3 in the flue gas increase the acid dew point (ADP). At temperatures below ∼400 °C,11 SO3 is highly reactive with water vapor to form H2SO4, reaching complete transformation at ∼200 °C via the reaction8,20 SO3(g) + H 2O(g) ⇄ H 2SO4 (g)
Table 1. Calculated Residence Time in the Reactor and Contact Time with Fly Ash with an Input Gas Flow of 0.5 L/ min at 298 K and 1 atm
(4)
Upon cooling to temperatures below the ADP, H 2 SO4 condenses, which is highly corrosive to boiler components (e.g., air heaters).1,13,21−23 To avoid corrosion, it is imperative that all of the components of the boiler be kept and operated at temperatures greater than the ADP, which will, in turn, increase the overall plant efficiency losses associated with heat recovery.1 The purpose of this work is to investigate the SO2 to SO3 conversion as a function of SO2, O2, and H2O in post-flame conditions in the absence of combustibles. The extent of catalytic conversion because of fly ash was also investigated.
temperature (°C)
residence time at temperature (s)
contact time with fly ash (ms)
400 500 700 900 1000
1.50 1.30 1.04 0.86 0.79
60.1 52.3 41.6 34.5 31.8
To determine the conversion of SO2 to SO3, a modified condenser based on the principles of the controlled condensation method25−28 (CCM) was employed. CCM assumes that, if an excess partial pressure of water vapor is maintained in the reaction zone,16 all SO3 is converted to H2SO4, which can then be separated from the gas stream by condensation. Passing the gas stream through a condenser at temperatures below the sulfuric acid dew point (90 °C)16,25,27−29 and above the water dew point30 (generally below 60 °C25,29) allowed for collection of H2SO4. Quartz wool was placed inside the quartz condenser to provide more surface area for condensation. Conversion of SO2 to SO3 in the gas mixture was determined by the amount of H2SO4 produced during each experiment, completed after 60 min. A known quantity of distilled deionized (DDI) water was used to flush out the condenser to collect H2SO4. The H2SO4 concentration was then determined using a Dionex Dx-100 ion chromatogram (IC). To determine the amount of quartz wool needed to fully capture the acid condensate, different amounts (0.25, 0.50, 0.75, 1.0, and 1.5 g) of quartz wool were placed in the condenser to collect prepared concentrations of H2SO4 vaporized within the furnace. The concentration of H2SO4 was approximately a magnitude higher than the equivalent amount of SO3 generated in power plants. Figure 2 shows the graph relating the mass
2. EXPERIMENTAL SECTION The experimental rig used in this study on the post-flame conversion of SO2 to SO3 is shown in Figure 1. The setup consisted of a 12 mm inner diameter (S/V ratio = 3.33 cm−1) quartz tube flow reactor placed in an electrically heated horizontal furnace with an isothermal length of 25 cm. Quartz was chosen as the reactor material and packing material because previous researchers13,24 verified the inert properties of quartz to SO2 oxidation. Reactant gases (SO2, O2, and N2/CO2) were supplied by gas cylinders and controlled by Brooks mass flow controllers with a combined flow rate of 0.5 L/min at 298 K and 1 7244
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Figure 2. Quartz wool capture test with 1.0 and 0.1 M H2SO4. Figure 3. ADP estimations relating SO3 and H2O vapor concentrations simulating conditions in the flue gas using ZareNezhad’s31 ADP correlation.
of the quartz wool and the recovery (%) of H2SO4 used. From the test, it was observed that an average 92.16−99.98% capture was achieved with greater than 1.0 g of quartz wool for both high and low H2SO4 concentrations. Hence, this amount of quartz wool was used throughout the experiments. The sulfuric acid dew point (ADP) temperature is one of the conventional ways of estimating the amount of SO3 and H2O present in the flue gas in power plants.28 ZareNezhad’s correlation31 was used for estimating the ADP, as shown in eq 5
Homogeneous conversions investigated the effects of gas concentrations of O2, SO2 and H2O. Heterogeneous conversions were carried out in the presence of fly ash heated to the reaction temperature of, i.e., 400, 500, 700, 900, and 1000 °C. Fly ash samples used in the heterogeneous conversions were fly ash obtained from a joint Australian−German study evaluating coal behavior in oxy-fuel. Fly ash samples A, B, and C derived from three Australian sub-bituminous coals (A, B, and C) were obtained from the 20 kWth combustion rig located at the Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Stuttgart, Germany.9 The rig was operated at 1350 °C, capable of combustion investigations with 0.5−3 kg of pulverized fuel/h and having a constant product rate of 11.5 m3 [standard temperature and pressure (STP)]/h to maintain comparable residence times for the different modes of firing, i.e., air, oxy-fuel with partial cleaning, and oxy-fuel full recycling without cleaning. The fly ash was obtained from the bag filter of the rig maintained at an inlet temperature of 225 ± 30 °C and outlet temperature of 195 ± 15 °C. Details of the conditions and the rig are discussed in previous literature.8−10 The X-ray fluorescence (XRF) data of the fly ash used in the experiments is presented in Table 4. It can be noted that the Fe2O3 data in the table are bold to emphasize differing values, with sample B being the lowest and sample C being the highest.
Tdew = 150 + 11.664 ln(pSO ) + 8.1328 ln(pH O ) − 0.383226 3
2
ln(pSO )ln(pH O ) 3
(5)
2
where Tdew is the sulfuric acid dew point temperature (°C), pH2O is the partial pressure of H2O in the flue gas (mmHg), and pSO3 is the partial pressure of SO3 in the flue gas (mmHg). Selection of the inlet gas concentrations (SO2 and H2O) were completed using values measured in pulverized fuel (PF) combustion and predicted for oxy-fuel combustion6,9,32 as a guide (Table 2).
Table 2. Parameters Used in the Estimation of the Sulfuric Acid Dew Point Temperature mode
[SO2] (ppm)
[SO3] (ppm)
[H2O] (vol %)
air oxy partial cleaned recycle oxy uncleaned recycle
500 1000 2000
0−100 0−100 0−100
3 10 30
3. RESULTS AND DISCUSSION Experiments were performed to quantify the conversion of SO2 to SO3 under post-flame conditions using simulated air and oxy-fuel flue gas at relatively low temperatures of 400−1000 °C. 3.1. Homogeneous SO2 to SO3 Conversion. 3.1.1. Effect of the Residence Time. Experiments with varying gas velocities were performed to obtain insight into the effect of the residence time on the system. Gas velocities of 0.5−1.5 L/min (dry, STP) were used to obtain residence times of 0.3−0.9 s. Figure 4 presents the effect of the residence time on SO2 conversion to SO3 compared to a previous study completed by Flint and Lindsay,24 who used a minimum residence time of 1 s to generate about 0.4% SO3/SO2 conversion. The experimental results are also compared to a kinetic model produced by Burdett et al.,17 as given by eq 6
Experiments labeled as “air” represent PF combustion, in which cleaned air is used as the oxidant. Experimental concentrations of 1000 ppm of SO2, 5 vol % O2, and 3 vol % H2O reflect air combustion flue gas levels using medium−high sulfur coals. Practical oxy-fuel combustion with partial flue gas cleaning (with simulated removal of nominally 20 vol % H2O, 20 vol % SO2, and 50% Hgtot from flue gas based on theoretical maximum conversion9 before being recycled to the furnace) is simulated in the experiments termed here as “oxy partial cleaned recycle”. Oxy-fuel combustion with flue gas recycling without cleaning is termed in this study as “oxy uncleaned recycle”. Assuming 2% conversion of SO2 to SO3 and the input gas concentrations shown in Table 2, the ADP estimations show that, by changing from air combustion to oxy-fuel combustion, the ADP increases from 120 to 140 °C, presented in Figure 3, while if the flue gas was fully recycled without any cleaning, the ADP could potentially increase by 40 °C, from 120 to 160 °C. ZareNezhad’s correlation was also used to estimate the operating temperature of the controlled condensation experiment. It estimated that, with even a low SO3 concentration of 0.5 ppm and water concentrations of about 3 vol %, the ADP sits at a temperature of 93 °C, still higher than the condenser temperature range of 60−90 °C (75 °C was chosen for this study). To investigate the individual kinetic effects of SO2, O2, and H2O in the post-flame conversion of SO2 to SO3 at temperatures of 400−1000 °C, the following experimental design was used in this study (Table 3).
d[SO3] k A[SO2 ][O2 ] (−B / T ) e = 1 [SO2 ][O2 ] = dt RT RT −1
(6) 3 −1
where k1 = A exp(−B/T), A = 2.6 (±1.3) × 10 mol cm s , B = 23 000 ± 1200 K (leading to B/R = 190 ± 10 kJ mol−1), and [SO2], [O2], and [SO3] are partial pressures.17 Increasing SO3/SO2 conversions were observed as the residence time in the reactor was increased. It could be noted 12
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Table 3. Effect of Varying Gas Concentrations on the Uncatalyzed, Post-flame Conversion of SO2 to SO3 at 900 °C experiment
control
variable
(1) SO2 effect
[O2] = 5 vol % [H2O] = 3 vol % [SO2] = 1000 ppm [H2O] = 3 vol % [SO2] = 1000 ppm [O2] = 5 vol %
[SO2] (ppm) = 500, 1000, 1500, and 2000
(2) O2 effect (3) H2O effect
[O2] (vol %) = 3, 5, and 10 [H2O] (vol %) = 3, 4, 7, and 9
Table 4. XRF Data of the Fly Ash Samples Used in the Study fly ash sample A
a
B
C
(%, dry basis)
air
oxy
air
oxy
air
oxy
SiO2 Al2O3 Fe2O3a CaO MgO Na2O K2O TiO2 MnO2 P2O5 SO3 BaO SrO total percent carbon (unburned)
55.2 33.3 6.55 0.95 0.741 0.144 0.512 2.06 0.097 0.178 0.132 0.042 0.049 100
