Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 86

incoming combustion air. It is worth comparing these features with those in a stirred
reactor from an engineering point of view.
The WSR is an idealized reactor that may be constructed using our concepts. It
is a combustion vessel having inlet and outlet ducts, in which rapid combustion
reactions proceed in the homogeneous mixture, in the molecular sense, of incoming
reactants and hot burned products. The combustion efficiency of the reactor depends
on its volume and the flowrate, that is, the residence time, and it can be related to
chemical processes. Therefore, the temperature and composition in the reactor are
theoretically uniform until its blowout limit. The actual characteristics of a real
reactor, however, often vary with the nature of the flow inside and are not those of
the well-stirred condition except in a small laboratory reactor. In that sense, the
practical reactor should be called a jet-stirred reactor. Combustion states in a jet-
stirred condition can be categorized with two parameters: i.e., a turbulent time scale,
τ m , and a chemical time scale, τ ch . The ratio of those parameters is known as
Damköhler number, Da = τ m /τ ch , by which we can classify ordinary burner combus-
tion and combustion in a jet-stirred reactor. In general combustion processes the
chemical time scale, τ ch , can be overwhelmingly shorter than the turbulent time scale
τ m , which could be expressed as combustion with a large Damköhler number, that
is, a flamelet regime. Accordingly, the idealized combustion, where rapid reactions
take place in the homogeneous mixture even on the molecular level can be regarded
as the low Damköhler number combustion sufficiently less than 1.0. It has been held
that combustion in a jet-stirred reactor is characterized by low Damköhler number
combustion, where uniformly distributed reactions, hence a uniform temperature
distribution, prevail the flow in homogeneous mixture, in the scale of turbulence
sense. There seem to be similar features observed in the combustion in a furnace
operated with high temperature air. However, in a practical furnace, rapid mixing
for producing uniform mixtures, which is caused by high momentum jet flows, shows
a gap between idealized and actual combustion. The latter experiments actually
revealed that decomposition and dissociation of fuel were proceeding simultaneously
with combustion reactions at around the outer edge of fuel jet issued into the furnace,
yielding nonuniform weak reaction zones. The high temperature air combustion
seems to realize a smaller Damköhler number by decreasing the chemical time scale,
τ ch

, with the use of low oxygen concentration air, although a turbulent time scale,
τ m , is mostly the same as usual combustion. Further, the flow inside the furnace
would not be the same in a jet-stirred reactor because the furnace is far too large
for the incoming jet momentum to stir uniformly. However, even though high
temperature air combustion is not the same as in a jet-stirred reactor, knowledge
about the similarity and difference between the two would still be useful for further
improvement of high temperature air combustion.

2.3.4 P OLLUTANT F ORMATION

2.3.4.1 Nitric Oxides

Oxides of nitrogen, referred to as NO x , are emitted from combustion processes,
mainly as one of the pollutant species in burned products. The two major oxides of