(419e) Composition and Chemistry of Oxy-Coal Flames: a Discussion On Corrosion-Related Aspects

The present work addresses recent findings on the oxy-fuel combustion chemistry and the related composition of oxy-fuel flames. The two main topics are the sulphur chemistry occurring in coal combustion including SO₂, SO₃ and H₂S formation and the formation and oxidation of CO in various coal flames. The implications on corrosion issues in oxy-fuel boiler design are discussed, as well as future research needs. The results presented include experimental work from the literature and results by the oxy-fuel group at Chalmers. The experiments at Chalmers are performed in a 100 kWth oxy-fuel test facility, which has been used extensively since its commissioning in 2003. In addition, modeling of the gas phase chemistry is discussed and related to the experimental work. All results presented for oxy-fuel combustion are related and compared against air-fired conditions.

The sulphur chemistry in oxy-fuel combustion differ from that under air-fired conditions; the main areas discussed in the paper are 1) conversion of fuel-S to SO₂, 2) H₂S formation in the flame (at local fuel rich conditions) and 3) SO₃ formation. The SO₂ concentration increases drastically in oxy-fuel compared to air-fuel conditions. Both experiments and modeling show that the formation of H₂S is promoted by the high SO₂ concentration in the oxy-fuel environment, given sub-stoichiometric conditions. The SO₃ concentration is also expected to increase, due to the elevated SO₂ concentration. Increased concentrations of SO₃ in combination with high moisture content in the flue gases (when recycling wet flue gases) will lead to a higher acid due point temperature with corrosion implications at low temperature parts of the boiler.

Both experimental and modeling work indicates that the CO formation is influenced by both homogeneous and heterogeneous reactions occurring in oxy-fuel combustion. Modeling of the homogenous chemistry in methane flames has shown that the main gas-phase reaction influencing the formation is

CO₂ + H ↔ OH + CO (1)

Reaction 1 gives, besides a direct effect on CO formation, also an effect on the radical pool composition by promoting the formation of OH, which influences the SO₃ formation as is discussed further in the paper. High levels of CO are also found in experimental work on oxy-coal flames where gasification reactions are suggested to contribute to the CO formation, in addition to the homogenous reactions. The main reaction considered is the Boudouard reaction

C(s) + CO₂ ↔ 2CO (2)

However, there are different opinions on the influence of gasification reactions on the CO formation in oxy-coal combustion, as will be discussed further in the paper. To summarize, high levels of CO and H₂S are observed in the flame and near burner zone as well as higher SO₂ concentrations in the combustion zone in general. As mentioned, the SO₃ levels are expected to increase in oxy-fuel compared to air-fired conditions, but the formation of SO₃ needs to be studied in more detail under oxy-fuel conditions. For corrosion phenomena occurring at combustion temperatures, design principles otherwise employed during reburning and air/fuel staging in air-fired boilers may be adopted in oxy-fuel firing. For coals with high sulphur content and ashes with low sulphur retention capabilities, the temperature window of low temperature corrosion may be drastically influenced. Thus, oxy-fuel combustion is expected to cause a more corrosive environment than air-firing, due to the elevated concentration of combustion products.