The role of the air supply on NOx generation in PK-35 steam boiler

Detelin Markov1, Peter Stankov1, Flemming Andersen2
1 Technical University of Sofia, Department of Hydroaerodynamics and Hydraulic
Machines,
8 Kliment Ochridsky boulevard, 1000 Sofia, Bulgaria
2Aalborg University, Department of Thermodynamics and Mechanical Energy
Systems,
101 Pontppidanstraede, DK-9220 Aalborg, Denmark

ABSTRACT

The effect of air supply quality on the NO generation in a PK-35 natural gas
fired steam boiler
furnace was investigated.
The PK-35 steam boiler is equipped with two jet type burners located
symmetrically on the front
wall at elevation of 3.15 m above the furnace bottom. The air is supplied to
the
burners by one
centrifugal fan trough a pipe line so that a misbalance of intake air supply to
the burners is
possible and hence the combustion process within the furnace volume could be
asymmetric.
The principal goal of the paper is to study the effect of the intake air supply
quality on the
NO generation within the furnace volume. The intake air supply quality is
characterized by flow
rate uncertainty and the degree of misbalance of intake air supply to the
burners. The intake air
flow rate uncertainty is presumed to be equal to 0%, +-2.5%, +-5% and +-7.5%.
The degree of
misbalance of air supply to the burners is presumed to be equal to 0%, 1.25%,
2.5%, 5% and 10%
and  the total number of cases studied is 35. The excess air at burners outputs
varies within the
range [0.9805;1.1395] and hence within the furnace volume there are regions
where the combustion
goes at understoichiometric, stoichiometric and overstoichiometric conditions.
The thermal NO formation is evaluated using the extended Zeldovich mechanism.
The prompt NO
formation is evaluated on the basis of the mechanism proposed first by
Fenimore.
The reactive
nitrogen species are usually present in trace amounts in gas flames and have
only a small impact
on the heat release process. Therefore the NO generation within the furnace
volume is evaluated
at the post-processing step using the Eddy-Dissipation Concept combined with
global formation rate
expression for both thermal and prompt NO.