Characterization of oxy-fuel flames - their composition, temperature and radiation

Doctoral thesis, 2007

Oxy-fuel combustion is receiving growing attention as one of the promising CO2 capture technologies in connection to power production. Yet, significant efforts are required in research and development of this combustion process in order to be able to design and optimize a full scale power plant. In oxy-fuel combustion, the fuel is burnt with oxygen of high purity instead of air. Typically, a large amount of flue gas is recycled back to the combustion chamber to adjust the flame temperature and heat transfer conditions. As a consequence, the system is in principle free from air borne N2, whereas the combustion products, such as CO2, H2O, NOx and SOx, are accumulated to different degree in the system, which has implications on both combustion and heat transfer related processes. This work focuses on the flame and radiation characteristics and the emissions formed during oxy-fuel combustion of both gas and coal. The work is based on experiments carried out in a 100 kW oxy-fuel test facility, which was designed and constructed as a part of this work. In the present work dry recycling conditions were applied to study the effect of CO2 on the properties mentioned. The experiments show that the temperature, radiation, composition and ignition of the oxy-fuel flames are effectively controlled by the flue gas recycle rate. The gas and particle radiation was studied during combustion of both propane and lignite. The total radiation intensity was measured and compared against model results of the gas radiation intensity. In the propane oxy-fuel flame it is seen that the spectrally continuous radiation from soot is significantly influenced by the amount of CO2 recycled. Thus, in addition to effects on flame temperature and gas band radiation, the amount of CO2 recycled may have an important impact on the particle radiation. The same methodology was applied to study these relationships also in the lignite flames. The share of gas radiation in high temperature regions of the flame increases from about 30% during air-firing up to 45% in the oxy-fuel flames. However, the particle radiation seems to be little influenced under oxy-fuel conditions. Thus, during oxy-fuel combustion of lignite and with the recycling conditions applied (i.e. with an O2 fraction between 25 and 29 vol.% in the RFG) the particle radiation still dominates the measured total radiation intensity signal in important parts of the flame, which reduces the influence of an increased gas radiation in the lignite-fired flames. The NO formation was measured and modeled during combustion of lignite. The experimental results show that the NO emission [mgMJ-1] is reduced to 25-30% of the emission typical for air-firing. The modeling gives that the lower emission from oxy-fuel combustion is explained a by faster rate of reduction of formed and recycled NO in the system. Furthermore, the complete gas phase nitrogen chemistry was modeled to evaluate the potential for primary NOx reduction measures by means of gas phase reduction paths. The results demonstrate two different strategies possible (which could be combined): reduction via reburning at low temperatures (around 1000ºC) or reverse Zeldovich at high temperatures (above 1500ºC). Besides being sensitive to temperature, reburning is closely related to the stoichiometric conditions whereas the reverse Zeldovich path requires absence (or near) of free N2 and a relatively long residence time in order to be effective.