Envelope Flame and Thermal NO in Black Liquor Combustion: Experimental and Modelling Studies

The work presented in this thesis consists of experimental and modelling studies of the role of the envelope flame and thermal NO in black liquor combustion in recovery boilers. The envelope flame refers to the flame that can form around a black liquor droplet during pyrolysis as the pyrolysis gases react with oxygen present in the surrounding gas.
Experimental work in pyrolysis of black liquor was done to quantify the initial split of nitrogen between volatiles and char. Altogether, 12 black liquors were analysed, including 4 softwood and 3 hardwood ones corresponding to different levels of lignin reduction. A large difference was seen in the split among liquors, indicating that the split of nitrogen between volatiles and char is liquor specific. Lignin removal did not affect the split for the softwood liquor, while the nitrogen remaining in the char increased for the hardwood liquor.
The amounts of nitrogen that remained in the char and of nitrogen that formed NO were studied using interrupted laboratory-scale combustion experiments. Complementary modelling work was done to determine which species were unaccounted for in experiments. Modelling of the experiments showed that only NO and N2 were present as post flame species, regardless of the primary nitrogen species.
Experiments were carried out with either Ar or N2 as background gases to determine if thermal NO was formed in the envelope flame. Thermal NO was formed in temperatures above 900°C with 15% O2 and 1000°C with 10% O2. A recovery boiler CFD model was used to investigate how much volatiles were released under conditions where thermal NO was observed in experiments. According to the recovery boiler CFD model, no volatiles are released in the boiler under these conditions.
CFD simulation of the recovery boiler including thermal NO formation was carried out to also investigate formation of thermal NO in the bulk furnace gas. Outlet NO increased 18% when thermal NO was considered in the model. Thermal NO was not generally formed in the furnace gas. Instead, thermal NO formed at the bottom of the boiler were the primary air encountered the hot char bed.
To determine the presence of the envelope flame in the boiler, a new criterion for the presence of the envelope flame was developed. The criterion is based on the ignition delay time of the volatiles leaving the droplet. Experiments to build a flame map were conducted to validate the criterion. The criterion accurately predicted the presence of the envelope flame at 900°C. At 800°C, it showed inconsistencies, most likely due to the volatile composition used for the ignition delay calculations.
A fully coupled CFD simulation with coupled envelope flame prediction using the developed envelope flame criterion was conducted. Source terms were adjusted to account for the presence of the envelope flame. In the model, close to 20% of the volatile release occurred with envelope flame. The presence of the envelope flame did not significantly impact the temperature and oxygen concentrations of the boiler. Nitrogen chemistry was affected, with 20% less NO being found at the outlet. This NO reduction was found to be due to the reduction to N2 in the envelope flame as well as a more active NO reburning chemistry.