Experiments and modeling on oxy-fuel combustion chemistry during lignite-firing

Other conference contribution, 2007

This paper presents experimental and modeling work on the combustion chemistry of the oxy-fuel (O2/CO2 recycle) process with focus on the difference in NO formation between oxy-fuel and air-fired conditions. Measurements have been carried out in a 100 kW test unit, which facilitates oxy-fuel combustion with real flue gas recycle. These measurements include in-furnace gas concentrations and temperature profiles from lignite-fired tests. The tests comprise a reference test in air and three oxy- fuel test cases with different oxygen fractions in the recycled feed gas. Additional oxy-fuel experiments were performed in order to study the sensitivity of the NO formation to both stoichiometry and air ingress. The results show that for the burner settings used in this work, lignite oxy-combustion with a global oxygen fraction of 25 vol % in the feed gas results in flame temperature levels close to those during air-firing. Similar to previous work, it is seen that the NO emission levels in [mg/MJ] during oxy-fuel operation are reduced to less than 30 % of the emission level during air-fired conditions. The results from the modeling shows that the reduction of NO emissions during oxy-fuel combustion is caused by an increased destruction of formed and recycled NO. Further experimental tests on the OF 27 condition show that an increased stoichiometric ratio (from l = 1.18 to 1.41) as well as an increased N2 content in the feed gas (from about 1% to 15%) only has a small effect on the NO formation during oxy-combustion.