Emissions and Soot in Partially Premixed Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 436

premixed combustor, than a diffusion flame combustor. With a decrease in the

equivalence ratio, CO emissions increase and the NO x emissions decrease, as the
combustor is cooled with an excess of air. So, the lean premixed combustor is
shown to be highly ef

fl

cient when it comes to NO x reduction.
Rink and Lefebvre ( 1987 ) studied the formation of different pollutants for liquid
fuel combustion with changes in spray characteristics, like the mean spray diameter,
equivalence ratio. The air supplied was atomized with airblast atomizers, and the
fuel used for this study was light diesel oil (DF-2). The UHCs decrease as

U
increases, reaches a minima at around stoichiometric, then increases again. The
initial high levels of UHCs at low

imply incomplete combustion of the fuel, due to
low reaction temperatures, while higher levels of UHCs at higher values of

U

1
are due to incomplete combustion of the richer mixture, due to a paucity of O 2 .
Hence, CO 2 concentration showed a trend opposite to that of UHCs. NO increases as
U

U[

increases, reaches a maximum slightly on the lean side of the stoichiometric
conditions, and then reduce. The initial rise in NO is due to an improvement in
combustion, leading to higher temperatures, while the subsequent reduction in NO
can be due to reduced flame height and lowered residence times. As the atomization

quality improves and the mean drop size decreases, the combustion quality also
improves, because smaller fuel particles can vaporize much faster and hence com-
bust more quickly. Hence, though the concentration of UHCs is reduced, that of CO 2
and NO increases due to increased residence times, as the combustion now occurs
sooner. The particulate emissions show an increase with an improvement in the
atomization quality, as the effective fuel

fl

air ratio, i.e. the ratio of the mass of fuel
vapours to the mass of air, increases. For lean zones, reduced drop size leads to
reduced particulate emissions, as it helped enhance the evaporation rates, and so,
most of the fuel could burn in the premixed mode. For richer zones, as the atom-
ization improves, the enhanced evaporation leads to higher effective equivalence
ratios, which increases soot formation.
G

-

lder ( 1995 ) studied the effects of oxygen addition on the soot formation in
overventilated, co-

ü

flames of methane, pro-
pane and n-butane. The addition of oxygen on the fuel side had three main effects:
one was the dilution effect (changing carbon content per unit mass of the fuel gas
mixture), another was the thermal effect, occurring due to a change in the

fl ow, axisymmetric, laminar diffusion fl

fl ame

temperature field upon diluent addition, and lastly, a direct chemical interaction

(excluding the changes in the rates of chemical reactions, due to changing tem-
peratures), due to changes in the species concentration. Whenever oxygen is added
to the fuel side, the soot formation is chemically suppressed. The addition of
oxygen on the fuel side promotes the pyrolysis of fuel, and hence the production of
hydrocarbon radicals and H atoms which enhance soot formation. Simultaneously
aromatic radicals and some important aliphatic radicals like C 2 H 3 and C 4 H 3 are
removed by reactions with molecular oxygen and oxygen atoms. These two
opposing effects result in a net suppression in soot formation. Acetylene, formed
from vinyl radicals and consumed by the OH, H and O atoms, is known to be a soot
precursor. And the higher concentration of H atoms explains the reduced concen-
tration of acetylene and hence of soot. For higher alkanes like propane, n-butane,