Advances in Low NOx Residual Oil Firing For Boilers

Residual oil continues to be the fuel of choice for many industrial steam users while environmental regulations allow its use. However, new emissions regulations on NOx and SOx are requiring substantial improvements in emissions performance or it will be regulated out of existence. This paper discusses the results and effectiveness of two applications of low NOx design modifications to residual oil firing industrial boilers. A comparison of the low NOx approaches required and results obtained for a package and field erected boiler are presented. Previous development in low NOx residual oil firing indicate that significant NOx reduction can be obtained through spray pattern modification to produce a staged spray flame with rich and lean zones. Industrial package boilers have specific performance criteria that impede the effectiveness of such low NOx techniques. Factors such as smaller furnaces, higher firing densities, lack of air preheat, widened operating envelope, simple controls, and variable fuel quality make the industrial package boiler a more challenging low NOx environment than a multiple burner field erected or utility boiler. The greatest NOx reductions are obtained in field erected boilers with low firing densities as a result of the ability to stage the combustion. The variability in fuel characteristics such as viscosity, distillation curve, carbon residue, and ash composition limits the potential emissions reduction and maintaining stable combustion.