This is a final report for a test program to evaluate the long-term effectiveness of combustion modifications on industrial boilers. Previous short-term tests had been performed on industrial boilers to determine the effect of combustion modifications on air pollutant emissions such as NOx, SOx, CO, HC, and particulate. The objective of this program was to determine if the combustion modification techniques which were effective for the short-term tests are feasible for longer periods. The report gives results of a 30-day field test of a 29.3 MW output (100,000 lb steam/hr) coal-fired spreader stoker. Low excess air was used to control NOx emissions. Results indicate that low excess air firing is an effective long-term NOx control for spreader stokers. The as-found NOx concentration was 243.9 ng/J (400 ppm at 3% O2, dry) with the boiler load at 80% of design capacity. Firing in the low excess air mode reduced the as-found condition by about 20%. Low excess air firing also increased efficiency by about 3%.