ARTICLE pubs.acs.org/IECR
SO3 Formation under Oxyfuel Combustion Conditions Daniel Fleig,* Klas Andersson, Fredrik Normann, and Filip Johnsson Department of Energy and Environment, Division of Energy Technology, Chalmers University of Technology, SE-412 96 G€oteborg, Sweden ABSTRACT: The sulfur chemistry in oxyfuel combustion systems has received growing attention lately. The formation of SO3 is of special concern, because of the elevated SO2 concentrations found in oxyfuel, compared to air-fuel conditions. The present study focuses on the gas-phase chemistry and examines the impact of different combustion parameters and atmospheres on the formation of SO3 in oxyfuel and air-fuel flames, using a detailed gas-phase model. The work also includes a summary of the presently available SOx data from experiments in laboratory and pilot-scale combustors. The reviewed experimental data, as well as the modeling results, show significantly increased SO3 concentrations in oxyfuel, compared to air-fuel conditions. The modeling results reveal a complex behavior of the SO3 formation, which is influenced by direct and indirect effects of the SO2, O2, NOx, and CO content in the flue gas. One of the main contributors to the increased SO3 concentration in oxyfuel, compared to air-fuel conditions, is the high concentration of SO2 in oxyfuel combustion. The modeling also shows that the stoichiometry, residence time, and flue-gas cooling rate are critical to the SO3 formation. Thus, in addition to the stoichiometry of the flame, the flue-gas recycling conditions are likely to influence the formation of SO3 in oxyfuel combustion.
1. INTRODUCTION Sulfur trioxide (SO3) is formed during the combustion of sulfur-containing fuels; however, the amount of SO3 in the flue gas is orders of magnitude less than the amount of sulfur dioxide (SO2). A SO3/SOx formation ratio of 0.1%1% is often observed for air-fired pulverized-coal combustion. The formation of SO3 during combustion is undesired, because of the involvement of SO3 in high- and low-temperature corrosion. The altered combustion conditions in oxyfuel atmospheres will affect the sulfur chemistry and SO3 formation; consequently, the sulfur chemistry in oxyfuel combustion must be explored. Here, the gasphase SO3 formation under oxyfuel and air conditions is investigated by a detailed kinetic model of the sulfur chemistry involved in hydrocarbon combustion. Equilibrium calculations for typical flame conditions show that, under oxygen-rich conditions, the only noteworthy sulfurous species are SO2 and small amounts of SO3 and sulfur oxide (SO), whereas under fuel-rich conditions, the amount of SO3 is insignificant.1,2 SO3 formation in a coal-fired boiler occurs during the cooling of the flue gas in the post-flame region where oxygen (O2) is in excess, mainly via the oxidation of SO2, SO2 þ O ð þ MÞ h SO3 ð þ MÞ
ð1Þ
and secondary formation via HOSO2, SO2 þ OH ð þ MÞ h HOSO2 ð þ MÞ
ð2Þ
HOSO2 þ O2 h SO3 þ HO2
ð3Þ
3
Glarborg et al. found that the presence of SO2 inhibits carbon monoxide (CO) oxidation by radical consumption, mostly by reaction 1. Since reactions 13 are relatively slow and temperature-dependent, the cooling rate of the flue gas governs the formation of SO3.4 Furthermore, the formation of SO3 is strongly dependent on the concentration of SO2 and O2.5 Therefore, the r 2011 American Chemical Society
excess O2 concentration was commonly kept low in oil-fired boilers, to minimize SO3 formation (0.3%0.5% O2 in excess).5,6Again, the SO2 concentration will be dependent on the sulfur content of the fuel and its conversion to SO2 (CSO2), which relate to coal type79 and combustion conditions.10 A low conversion of fuel-bound sulfur (coal-S) to SO2 means lower SO2 concentrations and also indicates a high capture potential of SO3 by basic oxides in the ash. In oxyfuel combustion, the SO2 concentration is significantly increased,1118 compared to air firing, because of the removal of airborne nitrogen (N2). Measurements performed under oxyfuel conditions also show an increase in SO3 concentration.1521 In this paper, observed conversions of coal-S to SO2, as well as the SO3 concentrations under oxyfuel conditions from literature, will be discussed in a separate section. Besides increased SO2 and carbon dioxide (CO2) concentrations, also other combustion products are enriched under oxyfuel conditions. For oxyfuel combustion with wet flue-gas recycle, high concentrations of water (H2O) vapor can be expected. As discussed in this work, the different combustion conditions will have an effect on the formation of SO3, because of the influence of CO2 and H2O on the O/H radical pool.22 The modeling of SO3 formation in this study considers only gas-phase reactions. However, SO3 may also participate in heterogeneous reactions.4,23 For example, the formation of SO3 is favored by fly ash that contains iron oxide (Fe2O3)2326 or vanadium pentoxide (V2O5),25,27 whereas if the alkalinity of the ash is high, it might capture SO3 instead.5 Since heterogeneous reactions are strongly dependent on the fuel used, it is, as a first approach, reasonable to investigate the SO3 formation only in the gas phase, to show general trends. Received: March 16, 2011 Accepted: June 1, 2011 Revised: May 24, 2011 Published: June 01, 2011 8505
dx.doi.org/10.1021/ie2005274 | Ind. Eng. Chem. Res. 2011, 50, 8505–8514
Industrial & Engineering Chemistry Research
ARTICLE
The formation of SO3 during combustion is important for the boiler operation. The involvement of SO3 in low-temperature corrosion on air-heater and economizer surfaces is welldocumented.2831 During cooling of the flue gas, SO3 starts to combine with H2O at temperatures below 500 °C to form gaseous sulfuric acid (H2SO4):32,33 SO3 ðgÞ þ H2 OðgÞ f H2 SO4 ðgÞ
ð4Þ
At temperatures of 1470 K) is similar in all cases, probably because the concentration is near the equilibrium concentration. Figure 5 summarizes the main formation routes for SO3: primary formation of SO3 with reactions between O-radicals and SO2 (reaction 1) and secondary formation of SO3 via HOSO2 (reactions 2 and 3). In the high-temperature region, SO3 is mainly produced via primary formation (reaction 1) while it is consumed, to some extent, by reaction with H-radicals: SO3 þ H h SO2 þ OH
For temperatures of
