Reburning of Nitric Oxide in Oxy-Fuel Firing—The Influence of Combustion Conditions

Abstract

Experiments were carried out to obtain data on the efficiency of nitric oxide (NO) reduction by reburning under oxy-fuel conditions. The work was carried out in a 100 kW test facility fired with propane. The data were analyzed by means of a combustion model, which included a detailed description of the reburning chemistry. The importance of several combustion parameters on the reburning efficiency was studied: inlet oxygen concentration, flue gas recycle ratio, temperature, and stoichiometric ratio. The inlet oxygen concentration was kept between 25 and 37 vol % and the burner stoichiometric ratio between 0.7 and 1.15. NO was injected in the oxidizer. As expected, similar to air-firing, reburning in oxy-fuel is favored by substoichiometric conditions. A decrease in combustion temperature, caused by a lowered stoichiometric ratio, is shown to be advantageous for reduction of NO under oxy-fuel conditions. The effect of inlet oxygen concentration on reburning is not significant as long as the combustion conditions are fuel-lean. However, the amount of recycled flue gas, which increases with decreasing oxygen content, significantly improves the total reduction. Instead, when the stoichiometric ratio is decreased, the recycle flow of the flue gas is reduced, which, to some extent, counteracts the otherwise positive effect of fuel-rich conditions in the flame zone in oxy-fuel conditions. Thus, during oxygen-rich combustion, the total reburning efficiency in oxy-fuel combustion is superior to once-through reburning in air-firing, but during substoichiometric conditions, the reduction in air and oxy-fuel combustion is comparable.

NO reburning in oxy-fuel combustion: A comparison between solid and gaseous fuels

“…These data originate from published studies Kühnemuth et al, 2011), and only a brief summary of the experimental unit, measurement equipment, operating conditions, and models used for evaluation is given here. The furnace is top-fired and operates with air or a mixture of oxygen and recycled flue gas (dry or wet).…”

“…2. This mixing rate has been shown to give a fair representation of the furnace concentrations (Kühnemuth et al, 2011). The combustion chemistry is described by a detailed reaction mechanism, which is based on the work of Mendiara and Glarborg (2009) on C1, C2 and nitrogen chemistry under oxy-fuel conditions updated with a C3 subset from Frassoldati et al (2003).…”

“…Furthermore, the reduction of NO by char is enhanced by the presence of CO, and the reduction could, thus, be of special interest in oxy-fuel combustion where the CO concentration is locally increased. The authors of this work have previously performed a series of investigations on the emission of nitrogen oxides from oxy-fuel combustion Kühnemuth et al, 2011;Normann et al, 2008Normann et al, , 2009Normann et al, , 2010. From the results it was concluded that the emission of nitrogen oxides, related to the amount of fuel supplied, is significantly reduced in oxy-fuel combustion compared to air-firing, because of increased reduction of formed and recycled NO x .…”

“…In a previous work (Kühnemuth et al, 2011), the sensitivity of the reburning reduction to the combustion conditions was quantified by propane experiments with NO injection in the oxidizer. The influence of temperature variations in these experiments was separated from the combustion parameters investigated by means of modelling.…”

“…The effect of temperature is indicated by the shaded area of the bars. Based on measurements by Kühnemuth et al (2011). under air-firing. This is mainly because of an increased sensitivity to combustion temperature under oxy-fuel conditions, which may be explained by the changes in radical chemistry, as discussed above.…”