The report gives results of field tests of a subscale rotary cement kiln to evaluate the effect of various combustion modifications on gaseous emissions, using a research kiln donated by a cement company. The test unit was 8.2 m (27 ft) long and 0.38 m (15 in.) inside diameter and will produce 68 kg (150 lb) of clinker per hour. A coal burner was designed and built for the test program, and the kiln was reworked to incorporate heated secondary air and flue gas recirculation (FGR). The effects on NO emissions of FGR, diluted primary air, primary/secondary air ratio, burner tip velocity, and secondary air temperature were evaluated. The most effective combustion modification for coal firing was determined to be dilution of primary air with inert gas, which reduced NO by nearly 40 percent when the primary-air oxygen concentration was reduced to 12 percent.