The report describes the design and development of a versatile conbustion-test rig for investigating combustion of residual fuel oils at conditions simulating either the residence time or turbulence level of combustion in industrial boilers. The rig was designed to permit adjustment of the following factors: excess air, firing rate, combustion-air swirl, atomization-nozzle type, atomization temperature, fuel, and wall temperatures (with cooling surface added). Successful operation of the rig was achieved at firing rates of about 1.8 and 7.0 gph with No. 6 residual fuel oil and a 20 percent water-in-residual fuel oil emulsion. Smoke numbers (Bacharach) are reported for a number of combustion trials at 2 to 30 percent excess air. Preliminary measurements are given for NO(x) concentrations.