Emissions and Soot in Partially Premixed Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 437

oxygen addition enhances the formation of soot by direct chemical interaction,
provided compensation is made for the dilution effects. This enhancement increases
with the increase in the oxygen concentration.
Yetter et al. ( 2000 ) experimented with asymmetric whirl combustion, in which the
fuel was injected off-axis or asymmetrically into the combustor, in an axial direction,
while the air is injected separately with purely tangential velocity. The combustor
exhibits unusual stability even for equivalence ratios ( ) less than 0.1. NO levels

U

increase with increasing , primarily because of the temperature rise. But at low NO

levels, CO concentration is higher due to lower temperatures at such lean conditions,
and so a trade-off exists. But all in all, the recorded CO and NO levels are extremely
low, CO being less than 25 ppm, and NO levels less than 15 ppm. It is observed that
between temperatures 1,100 and 1,700 K, CO can be reduced without increasing
NO x , as thermal NO x formation is insigni

U

cant at temperatures below 1,800 K.
Cheng et al. ( 2001 ) studied the effects of partial premixing on fl ame charac-

teristics and pollutant emissions in swirling methane fl ames. The fl flame length

decreases continuously with increasing partial premixing and the flame colour also

changes from yellow to blue. This indicates an improvement in combustion with
increased partial premixing, and the reduced

fl

flame lengths imply reduced residence
times. The EINO x and EICO decrease with increasing premixing, reach a minima at
U p ¼

fl

3, simultaneously, and then again start increasing as the premixing air is
further increased. Tae et al. ( 1996 ) measured NO emissions in laminar partially
premixed ethane-air

ow rate. NO pro-
duction has been found to occur mainly in the annular space, between the inner
premixed and the outer non-premixed

fl flames, as a function of the co- fl ow air fl

ame structure. The
inner premixed region is dominated by prompt NO mechanism, and the outer non-
premixed region by the thermal NO mechanism. A minimum EINO x is seen around
U p

fl flame fronts, in the dual fl

2, and it represents a compromise between thermal and prompt NO x mech-
anisms. The visible fl flame height also decreases with decreasing

U p. The

fl ame

resembles a premixed fl ame at

U p \

2 2 and a non-premixed fl ame at

U p [

2 2.

: :

The formation of soot was also initiated around

U p ¼

2 2, which could be related to

:

the minima in EINO x around this point. At lower

U p, the inner premixed zone

produces most of the NO x , while at higher

U p, most of the NO x

is produced by the

outer non-premixed zone.
The aim of this research was to obtain an optimum combustion process, which
might be a small effort in reaching towards a greener environment. A specially
designed burner was designed for this purpose which has been con

gured in such a

way, that the resulting flames. The objective

has been to optimize the level of partial premixing, so as to minimize the emissions
for a given fuel

fl flames are partially premixed, co- fl ow fl

flow rate. The idea is to keep the premixed mixture always rich,
such that the premixed equivalence ratio,

fl

1. Part of the fuel is combusted in
this premixing or primary air, and the remaining fuel is combusted in the presence
of secondary or diffusion air. The diffusion air helps make the combusting mixture
overall very lean.
Emissions like Carbon monoxide (CO), Carbon dioxide (CO 2 ), Unburnt
Hydrocarbons (UHC), Nitric Oxide (NO) and Soot; as well as temperature, have

U p [