The report gives results of pilot-scale experimental research that examined the physical and chemical phenomena associated with the NOx control technology of reburning applied to gas- and liquid-fired firetube package boilers. Reburning (staged fuel combustion) diverts some of the fuel and combustion air from the main burner(s) for injection into the post-flame gases, resulting in three distinct zones in the boiler radiant section: a fuel-lean primary zone, a fuel-rich reburning zone, and a fuel-lean burnout zone. NOx is reduced both by reduction of NOx formation and by destruction of primary flame NOx by secondary fuel radicals. The research showed that, with minimal facility modification, NOx emissions from a firetube package boiler can be reduced by 50% or more from an initial level of 200 ppm (measured dry, at zero % O2). Several hypotheses were evaluated in the study. Results indicate that the overall NOx reduction via reburning has a reaction rate order of about 1.5 with respect to primary NOx.