An ignition delay time based criterion for the presence of envelope flame during black liquor droplet combustion

Predicting the occurrence of an envelope flame during black liquor combustion is crucial in order to determine which volatiles species reach the bulk gas from the droplet. The extent to which envelope flames occur in large scale black liquor combustion is currently not known. Experimental investigation in boilers is impractical and laboratory investigations can only cover parts of the conditions relevant for boilers. To obtain first estimates, this study proposes a new criterion for the prediction of envelope flame formation intended for boiler simulations. The criterion is based on the ratio of the residence time of the gases around the droplet and the ignition delay time of the black liquor volatiles. The criterion is verified with single droplet experiments conducted at two temperatures ,800 °C and 900 °C, with oxygen concentrations from 3% to 10% and gas slip velocities from 0.05 to 0.35 m/s. Ignition delay time is obtained from detailed chemistry simulations of black liquor volatile combustion. The criterion was compared to a previous developed criterion from literature based on CO oxidation Damköhler number. The performance in the experimental set was similar, with the ignition delay criterion performing better at the higher temperature. The criterion was then applied as a post processing step to the results a CFD simulation of a 5300tds/day-600MW _th recovery boiler to estimate the presence of the envelope flame. In the boiler conditions studied, 20% of the black liquor volatile release occurs with an envelope flame according to the ignition delay criterion.