Enhanced ignition of biomass in presence of NO_x

Accumulation of combustible biomass residues on hot surfaces of processing machineries can pose fire hazards. In addition, the presence of nitrogen oxides (NO_x) from plant equipment alters the local conditions, aggravating the propensity for low temperature ignition risks. This study presents an experimental study on a relative effect of NO_x on ignition temperature of morpholine, an important surrogate of biomass, to reveal the sensitising role of NO_x in ignition of biomass fuels and to gain mechanistic insights into the chemical aspect of this behaviour in fire. The experiments employed a flow-through tubular reactor, operated at constant pressure and residence time of 1.01 bar and 1.0 s, respectively, and coupled with a Fourier-transform infrared spectroscope. For a representative fuel-rich condition (Φ=1.25), the concentration of NO_x as small as 0.06% lowers the ignition temperature of morpholine by 150 °C, i.e., from approximately 500 °C to 350 °C. The density functional theory (DFT) calculations performed with the CBS-QB3 composite method, that comprises a complete basis set, characterised the dynamics and energies of the elementary nitration reactions. We related the observed reduction in ignition temperature to the formation of unstable nitrite and nitrate adducts, as the result of addition of NO_x species to morphyl and peroxyl radicals. Furthermore, the reaction of NO_x with low-temperature hydroperoxyl radical leads to the formation of active OH species that also propagate the ignition process. The present findings quantify the ignition behaviour of biomass under NO_x–doped atmospheres. The result is of great importance in practical applications, indicating that safe operation of wood-working plants requires avoiding trace concentration of NO_x within the vicinity of biomass residues. This can be facilitated by proper (and separate) venting of engine exhausts.