High-Temperature Conversion of SO₂ to SO₃: Homogeneous Experiments and Catalytic Effect of Fly Ash from Air and Oxy-fuel Firing

Abstract

The reaction of SO 2 with fly ash in the presence of O 2 and H 2 O involves a series of reactions that lead to the formation of SO 3 and eventually H 2 SO 4 . 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 SO 2 to SO 3 . The results were compared to existing global kinetics and found to be dependent upon SO 2 , O 2 , residence time, and temperature and independent of H 2 O content. For a residence time of 1 s, temperatures of about 900°C are needed to have an observable conversion of SO 2 to SO 3 . Literature suggested that the conversion of SO 2 to SO 3 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 SO 2 conversion to SO 3 at a temperature range of 400−1000°C. It was observed that fly ash acts as a catalyst in the formation of SO 3 , with the largest conversion occurring at 700°C. A homogeneous reaction at 700°C, without fly ash present, converted 0.10% of the available SO 2 to SO 3 . When fly ash was present, the conversion increased to 1.78%. The catalytic effect accounts for roughly 95% of the total conversion. Average SO 3 /SO 2 conversion values between fly ash derived from air and oxy-fuel firing and under different flue gas environments were found to be similar.

Oxyfuel combustion for CO2 capture in power plants

“…Catalytic SO 3 formation is highly dependent on temperature, with a maximum of the SO 2 /SO 3 conversion at about 700 • C (see Fig. 14) (Belo et al, 2014;Marier and Dibbs, 1974). The difference between air and oxyfuel gas atmospheres was reported to be low in lab scale testing on the catalytic SO 3 formation on coal fly ashes (Belo et al, 2014).…”

“…convective section of the boiler) is also influenced by the presence and amount of catalytically active compounds in the ash (e.g. Fe 2 O 3 ), with higher contents leading to higher SO 3 formation (Belo et al, 2014;Walsh et al, 2006). Catalytic SO 3 formation is highly dependent on temperature, with a maximum of the SO 2 /SO 3 conversion at about 700 • C (see Fig.…”

“…14) (Belo et al, 2014;Marier and Dibbs, 1974). The difference between air and oxyfuel gas atmospheres was reported to be low in lab scale testing on the catalytic SO 3 formation on coal fly ashes (Belo et al, 2014). Ellison and Weiler (2005a,b) reported that SO 3 could also be entrained in the fly ash particles, as well as on ash deposits.…”

“…The homogeneous SO 3 formation, like its catalytic counterpart, clearly shows a temperature-dependent behaviour (Belo et al, 2014;Fleig et al, 2011b). Both reaction mechanisms are relatively slow (Belo et al, 2014;Walsh et al, 2006), which is the reason why in a practical firing systems, no equilibrium concentrations of SO 3 can be observed.…”

The Effect of Different SO2/SO3 Catalytic Media on High-Temperature Corrosion Processes (Hot Corrosion, Fireside Corrosion, Sulfidation–Oxidation)

“…Marier et al [11] and Belo et al [12] showed that the presence of fly-ash has a catalytic effect on the conversion of SO 2 to SO 3 due to the presence of iron oxide (redox cycle reaction, Fe 2? $ Fe 3?…”