This paper compares the results of an advanced CFD calculation with measurements of a heavy fuel oil fired low NOx boiler. First, a state of the art boiler was investigated and the impact of boiler load and excess air on the NOx emissions was measured. In a second test run a staged combustion technology was integrated using the over fire air concept. The over fire technology is well known and well tested in coal fired boilers. In this pilot boiler it was shown that the over fire air technology could be used for oil fired boilers, too — leading to an enormous NOx reduction without any increase in CO and soot emissions. It was shown that the influence of boiler load, excess air and over fire air on the NOx and CO emissions can be predicted very well in the CFD calculation. Detailed numerical investigations showed that two-phase effects, a good turbulence model, gas and soot radiation and a detailed chemical kinetics mechanism are a must when modeling (staged) heavy oil combustion. The results of the CFD calculation showed an excellent agreement with the measurements over a very wide range of boiler settings and load factors although NOx is extremely difficult to predict.

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