The report gives results of Phase I of an investigation of combustion modification techniques for controlling NOx emissions from pulverized-coal-fired utility boilers. The techniques--studied on a 5-million Btu/hr single-burner pilot unit--included: excess air; air preheat; rating; flue gas recirculation; staged combustion; quench; and swirl. The study showed that NOx reductions of up to 50% are possible either using staged combustion or by lowering excess air levels from 30 to zero %. Flue gas recirculation yielded only moderate NOx reductions. Fuel-bound nitrogen conversion increased with increasing excess air level and decreasing temperature. At substoichiometric conditions, the final precursors for NO formation from either fuel-bound nitrogen or thermal atmospheric fixation appeared identical. For existing units, control of excess air promises to be the best method for NOx reduction; however, for new units, staging (with physical separation of the two stages) appears to be the most promising. (Author)