Abstract–A well-stirred combustor (WSC) was constructed capable of burning natural gas, and residual fuel oil at equivalence ratios of 0.4 to 1.8 and residence times of 50 or 175 ms. gold-flow tests verified the well-stirred condition. NOy emissions produced by burning these fuels in the WSC were higher than previous WSCs with 1-10 ms residence time, but generally lower than industrial boilers.