Impacting factors on NOX emissions and reduction through air staging during pulverised biomass combustion

Controlling NO X emissions from solid fuel combustion remains a challenge. This study presents an experimental investigation into the factors influencing NO X formation during biomass combustion in an entrained flow reactor. Eight different biogenic fuels, including pre-treated samples, were tested alongside two additional fuels doped with KCl and coal fly ash. The experiments examined the combined effects of temperature, overall stoichiometric ratio, fuel-bound nitrogen content, and additive presence on NO X emissions under both non-staged and air-staged combustion. Results show that increasing temperature, stoichiometric ratio, and fuel-N content led to higher NO X formation, whereas the addition of specific inorganic additives significantly reduced emissions, with KCl lowering NO X by up to 49.4%. Compared with the used fuels, bark pre-treated through steam explosion displayed an enhanced NO X formation. Air-staging experiments demonstrated that temperature, stoichiometry, and residence time in the reduction zone strongly affected NO X levels, with the lowest emissions achieved at stoichiometric ratios between 0.7 and 0.9. Extended residence times further decreased emissions and shifted the optimal λ towards 1, while double air staging yielded an additional 43.8% reduction compared with single-stage operation. The conversion of fuel-N to NO decreased with increasing nitrogen content for both staged and non-staged conditions. Overall, the results highlight that both combustion conditions and fuel characteristics impact NO X emissions and that optimised air staging, in conjunction with suitable fuel selection and additive use, enables substantial primary reduction of NO X emissions from pulverised biomass combustion.