Environmental Engineering Reference
In-Depth Information
mechanisms. McLean et al. (
1994
) and Vagelopoulos and Egolfopoulos (
1998
)
reported premixed
fl
flame speeds at pressures from atmospheric to a few atmo-
spheres. K
é
romn
è
s et al. (
2013
) reported a more extensive study on high-pressure
premixed
fl
flames. Additional data on syngas
fl
flame speeds can be found in the
references cited above. In addition to unstretched
ame response to
stretch and the resulting cellular instabilities are important considerations. These
phenomena are of fundamental relevance for
fl
ame speed, the
fl
fl
flame extinction, turbulent
fl
ame
propagation,
ame
response to stretch is characterized in terms of the Markstein length or non-
dimensional parameter, Markstein number. Spherically expanding
fl
flame stabilization, blowout, and transition to detonation. The
fl
ames have been
commonly used to determine this parameter, and details can be found in Aggarwal
(
2013
) and Kishore et al. (
2011
).
Since syngas generally contains other species in addition to CO
2
and H
2
O, it is
important to examine the effects of various diluents on syngas combustion and
emissions. Moreover, dilution is often used to lower the
fl
ame temperature and
thereby limit NO
x
emissions. Several researchers have examined the effects of
diluents on laminar
fl
flame speed, stability, and emissions (McLean et al.
1994
;
Natarajan et al.
2009
; Kishore et al.
2011
; Burke et al.
2007
; Burbano et al.
2011
;
Das et al.
2011
). It is important to note that only NO
x
emission is relevant in syngas
fl
fl
ames.
NO
x
in hydrocarbon flames can be formed due to four mechanisms, namely thermal
(Zeldovich), prompt (Fenimore), N
2
O, and NNH mechanisms (Briones et al.
2007
;
Guo and Smallwood
2007
; Fu et al.
2012
). Thermal NO involves reactions:
O+N
2
⇒
flames, while both soot and NO
x
emissions are important for hydrocarbon
fl
N + NO, and N + O
2
+ NO + O, and N + OH
NO + H, while prompt
⇒
NO formation is initiated through the reaction CH + N
2
⇒
NCN (or HCN) + H (or
N). Thus, the prompt mechanism is absent in syngas
flames, since it is linked to
hydrocarbon combustion chemistry, which produces CH radicals. The prompt NO,
however, may be important for syngas mixtures containing CH
4
. The N
2
O-inter-
mediate mechanism involves N
2
+O+M
fl
N
2
O + M as the initiating reaction and
is important for lean mixtures and high pressures. Finally,
⇒
the NO formation
through NNH route involves reactions: N
2
+H
NNH and NNH + O
NO + NH
⇒
⇒
(Bozzeli and Deam
1995
). Ding et al. (
2011
) investigated NO
x
formation in lean
premixed syngas counter
flames and observed that the NO was formed pre-
dominantly through the NNH and N
2
O intermediate routes. The contribution of
thermal NO was small due to the low-
fl
ow
fl
ame temperatures. In addition, increasing
the CO fraction in syngas was found to increase the amount of NO formed.
Combustion in many practical devices involves non-premixed (diffusion) and
partially premixed
fl
flames (PPFs) (Giles et al.
2006
; Som et al.
2008
). While
numerous studies exist of such
fl
flames with hydrocarbon fuels, relatively few
investigations have focused on syngas fuel. Studies on non-premixed syngas
fl
ames
have been reported by Giles et al. (
2006
), Hui et al. (
2007
), and Park et al. (
2004
),
while those on PPFs have been reported by Som et al. (
2008
) and Ouimette and
Seers (
2009
). Giles et al. (
2006
) also examined the effects of diluents (N
2
,H
2
O, and
CO
2
) on NO formation in non-premixed
fl
flames. Som et al. (
2008
) examined the
effects of strain rate, equivalence ratio, and syngas composition on the detailed
fl
Search WWH ::
Custom Search