Investigation of selective catalytic reduction for control of nitrogen oxides in full-scale dairy energy production

Selective catalytic reduction (SCR) was used to reduce exhaust gas nitrogen oxides (NOx) from the emissions of a 710kW combined heat and power system fueled by dairy biogas. Exhaust gas NOx was reduced from 63.1±31.9 to 14.2±17.5ppmvd@15% O2 such that emissions were 0.33±0.40gkW−1h−1, based on data averaged over 15min intervals. Online exhaust gas sensors with integrated process control algorithms were effective in improving NOx removal by automated control of urea, the ammonia source used for catalysis of NOx reduction reactions. Pre-SCR NOx was most strongly correlated with equivalence ratio (R2=0.39), indicative of the air–fuel ratio. A concave relationship between NOx production and thermal conversion efficiency was not observed since lean-burn operation of the engine was consistent and only altered under low engine load. Following installation of pre- and post-SCR NOx sensors, average daily exhaust gas NOx reduction in the SCR was 82.6±8.5%. Post-SCR NOx emissions were typically impacted by pre-SCR NOx (R2=0.36), suggesting that altered operation of the anaerobic digesters or modifications to the engine would be effective in reducing NOx emissions as well as urea demand. After nearly three years of operation, the SCR catalyst remains in service without requiring replacement. Average daily urea demand was 31.8±16.3Ld−1 for the system that produced 369±136kW of electricity. During the second year of observation the regulatory limit of 0.804gkW−1h−1 was met 94% of the time while the regulatory target of 0.201gkW−1h−1 was only met 45% of the time, based on data averaged over 15min intervals. These results provide guidance for dairy energy projects in locations with stringent NOx emissions standards.