Effect of stripper off-gas burning on NOx emissions

We conducted tests to: (1) determine the ammonia content of kraft pulp mill condensates sent to steam strippers; (2) quantify the efficiency of steam strippers for stripping ammonia from condensates; (3) determine the amount of NH3 in stripper off-gases (SOGs) oxidized to NOx in lime kilns, boilers and thermal oxidizers; and (4) correlate any NOx emission increases to operating parameters such as combustion temperature and oxygen level in the combustion zone. The ammonia content of the condensates in the study ranged from 94 mg/L to 508 mg/L. The strippers removed essentially all of the ammonia from the condensates. The amount of ammonia in the SOGs that was oxidized to NOx ranged from 6.3% to 34.4%, with an average of 19.7%. Apparently, the amount of ammonia converted to NOx was not related to the type of combustion device. The increase in NOx emissions due to SOG burning ranged from 0.25 to 2.76 pounds/thousand gallons of condensate stripped (KGCS) and averaged 1.13 lb/KGCS. At the mills tested, this corresponds to NOx emissions increases ranging from 4.7 lb/hr to 49 lb/hr. Tests at a limited number of facilities showed that the fraction of ammonia that was not converted to NOx did not leave the stack as ammonia, which suggests that the ammonia not oxidized to NOx was reduced to molecular nitrogen.