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Stewart Wyatt
Homework 5
MANE 6960
AWPPCE
14 October 2013
Q1 Thermal and fuel NOx control during combustion
Thermal and fuel NOx are formed during the combustion process. Modifications to combustion devices
to minimize the amount of NOx generated are investigated. These methods are called primary methods
of NOx control [1].
The formation mechanisms of oxides of nitrogen (NOx) considered are [1]:
o
o
o
Thermal NO route, also known as the Zel’dovich NO mechanism
Prompt NO route, also known as the Fenimore NO mechanism
Fuel bound nitrogen (FN) route
Thermal and prompt NO are formed by the combustion of clean fuels (which do not contain nitrogen
compounds) with air (which contains atmospheric N2). NO is formed mainly by the Zel’dovich
mechanism [1]. The Zel’dovich mechanism requires breaking of the strong triple bond in the N2
moelcule, hence this mechanism is considered unimportant at temperatures below 1800 K [1].
For processes dominated by Thermal NO formation, time spent at the peak temperature, temperature
magnitude and the concentrations of N2 and O are the primary variables controlling NOx generation [2].
Reducing peak temperatures and the time the combustion products spend at the peak temperature
suggest methods used to significantly reduce NOx emissions.
Staged combustion is a NOx control strategy that uses a rich-lean or lean-rich combustion sequence. For
example, a fuel rich combustion process has good stability and low NOx generation due to its nonstoichiometric flame temperature, followed by a lean stage to complete the combustion of unburned CO
and hydrocarbons, results in lower thermal NOx than would be achieved with an unstaged near
stoichiometric flame. For staging to be effective, rapid mixing of the rich products and air must be very
rapid [2]. With any real process, the mixing is not instantaneous so additional NOx is generated when
passing through the stoichiometric region between the fuel rich and fuel lean stages.
Thermal NOx may also be controlled by combustion with oxygen only, instead of air, as it removes the
source of nitrogen. However, this is rarely seen in practice due to the air separation requirements.
Thermal NOx reduction strategies may inadvertently increase emissions of CO and unburned
hydrocarbons as these are more readily formed at lower combustion temperatures.
Fenimore [3] noted that some NOx formation preceded the Zel’dovich NO and called it prompt NO.
Prompt NO was not found in nonhydrocarbon CO-air and H2-air flames, suggesting a reaction between a
hydrocarbon species and atmospheric nitrogen. Hence a control mechanism for removing prompt NO is
the use of non-hydrocarbon fuels if available.
Fuel bound nitrogen has been shown to be an important source of NO emissions [4]. The NO appears to
be only slightly dependent on temperature, which is in contrast to the strong temperature dependence
of Thermal NO. The fuel bound nitrogen route is not important for fuels with little or no bound nitrogen
Stewart Wyatt
Homework 5
MANE 6960
AWPPCE
14 October 2013
such as natural gas and gasolines, however, pulverized coals and heavy distillate fuels can contain
significant quantities of fuel nitrogen [2].
Strategies employed for the reduction of NOx from industrial combustion equipment includes [2]:
o
o
o
o
Staged combustion – Typically a rich-lean staging of combustion is used. The NOx reductions
range from 10 to 40 %.
Temperature reduction – Reducing the amount of air preheat and/or injecting water into the
flame reduce the combustion temperature, and, consequently NOx formation. Similarly, water
injection and flue gas recirculation (FGR) act as diluents to reduce the flame temperature. The
flue gas recirculation may occur within the furnace or externally. NOx reductions of 50 to 85 %
may be achieved with FGR in gas fired industrial boilers.
Oxy/Gas combustion – The concentration of nitrogen in the combustion system is reduced by
supplying additional oxygen. Enough oxygen is required to offset the increased combustion
temperature with the reduced nitrogen concentration. If oxygen is used instead of air and no
nitrogen is present in the fuel, NOx production may be eliminated.
Reburn – Nominally 15 % of the total fuel is introduced downstream of the main fuel lean
combustion zone. NO is reduced in the reburn zone via reactions with hydrocarbons. Additional
air is then supplied for the completion of the combustion process. NOx reductions of 60 % have
been achieved with reburn.
Post combustion devices for all NOx formation mechanisms may also be employed. These include
Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR).
Combustion control mechanisms for NOx have been shown to be reliant upon the reduction of
combustion peak temperatures, reduction of the time at peak temperatures, reduction of N2
concentration and/or the use of a fuel with reduced bound nitrogen.
References
1. Kuo K. K., 2005, Principles of Combustion, John Wiley & Sons, Hoboken.
2. Turns S. R., 2012, An Introduction to Combustion: Concepts and Application, 3rd edition, McGraw
Hill, New York.
3. Fenimore C. P., 1971, “Formation of Nitric Oxide in Premixed Hydrocarbon Flames,” Symposium
(International) on Combustion, Volume 13, Issue 1, pages 373-380.
4. Glassman I., 1987, Combustion, 2nd edition, Academic Press, Orlando.
Stewart Wyatt
Homework 5
MANE 6960
AWPPCE
14 October 2013
Q2 SO2 stripping from air by water
The equilibrium data for SO2 in air and water is given in Table 1 and Figure 1.
Table 1 Equilibrium data for SO2 in air and water
Figure 1Equilibrium data for SO2 in air and water
The operating line for a certain absorption tower used for SO2 removal is:
58.1395 ∗
0.005
1
1
This is solved to give y as a function of x:
1.162695 10
1.142695 2010
The equilibrium and operating lines are plotted in Figure 2.
Stewart Wyatt
Homework 5
MANE 6960
AWPPCE
14 October 2013
Figure 2 Operating line of SO2 tower and SO2 equilibrium
The operating line is modified by trial and error adjustment of the gradient until it approaches the
equilibrium line as shown in Figure 3.
Figure 3 Modified operating line
# MANE6960 Homework 5 Q 2
# Stewart Wyatt
restart :
ye d 37.836$x K0.0045;
# gradient of molar concentration of SO2 in air (y) to molar concentration of SO2 in water
(x) at equilibrium
ye := 37.836 x K0.0045
y
x
> e1 d
= 58.1395$
C0.005; # given operating line of SO2 absorption tower
1 Ky
1 Kx
y
58.1395 x
e1 :=
=
C0.005
1 Ky
1 Kx
> yo d solve e1, y ; # given operating line of SO2 absorption tower
1.16269 105 x C10.
yo :=
1.14269 105 x C2010.
>
>
>
>
>
>
>
>
F d plot ye, x = 0 ..0.003, color = blue :
G d plot yo, x = 0 ..0.003, color = red :
with plots :
display F, G ;# equilibrium ye and operating line of tower yo
(1)
(2)
(3)
0.15
0.10
0.05
0
0.001
0.002
0.003
x
Operating line of SO2 tower
SO2 equilibrium curve
> # rotate the operating line yo by adjusting the gradient
> f d 0.68 : # by trial and error adjust f to make yo1 (black) approach ye (blue)
1.16269e5$f$x C10
> yo1 d
;
1.14269e5$f$x C2010
79062.92 x C10
yo1 :=
77702.92 x C2010
> H d plot yo1, x = 0 ..0.003, color = black :
> display F, G, H ;
(4)
0.15
0.10
0.05
0
0.001
0.002
x
Modified operating line yo
Equilibrium line ye
>
Operating line yo
0.003
Stewart Wyatt
Homework 5
MANE 6960
AWPPCE
14 October 2013
Q3 Formaldehyde removal by a platinum catalyst
Formaldehyde (HCHO) is a volatile organic compound (VOC) that may be emitted as a product of
combustion and indoor sources. Catalysts may be used as a post combustion means of oxidizing
formaldehyde to CO2 and H20. This process is investigated.
When all participating species in a chemical reaction (reactants and products) exist in a gaseous phase,
the reaction is termed homogeneous [1]. A heterogeneous reaction consists of participating substances
that do not exist in a single phase [1]. Gas-solid heterogeneous reactions are used for post combustion
oxidation of formaldehyde with a platinum catalyst.
Characteristics of catalysts in general, but also the platinum catalyst used for the oxidation of
formaldehyde, include [1]:
o
o
o
The catalyst increases the speed of the reaction without undergoing change or being consumed
itself.
The reaction (oxidation of formaldehyde) when supported by a noble metal catalyst (such as
platinum) may proceed at a temperature too low for significant gas phase oxidation to occur.
The presence of the catalyst does not alter the equilibrium composition of the mixture, but it
can be used to take a slowly reacting, non-equilibrium system to it equilibrium state.
Peng and Wang [2] examined the oxidation of formaldehyde with consideration of the active metal,
dispersion of the active metal within the catalyst support material, active metal loading, temperature,
formaldeyde concentration and space velocity.
Peng and Wang [2] compared catalyst metals with a TiO2 substrate for the oxidation of formaldehyde.
Pt showed the highest activity with a conversion of 40.1 % and 99.6 % at room temperature and 60 C
respectively. The activity of the catalyst metals in decreasing order is Pt, Pd, Rh, Mn, Cu as shown in
Figure 1.
Peng and Wang [2] compared catalyst support materials with Pt as the active metal. The support
material role in the catalyst is to promote a very high dispersion of the active metal and high specific
surface areas which maximizes the available active sites. The optimum support materials in decreasing
order are TiO2, SiO2, Ce0.8Zr0.2O2, Ce0.2Zr0.8O2 as shown in Figure 2.
The adsorption process of using a catalyst to oxidize formaldehyde to carbon dioxide and water is a
means of reducing post combustion pollution. However, the optimum catalyst is a function of different
parameters including active metal, support structure, and operating temperature. Additional items to
be considered for a practical application may include the ability of the catalyst to oxidize other
pollutants found in the gas stream, susceptibility to poisoning by other material in the gas stream and
pressure drop through the catalyst.
Stewart Wyatt
Homework 5
Figure 1 Formaldehyde oxidation using different catalyst metals [2]
Figure 2 Formaldehyde oxidation using Pt with different catalyst supports
MANE 6960
AWPPCE
14 October 2013
Stewart Wyatt
Homework 5
MANE 6960
AWPPCE
14 October 2013
References
1. Turns S. R., 2012, An Introduction to Combustion: Concepts and Application, 3rd edition, McGraw
Hill, New York.
2. Peng J., Wang S., 2007, “Performance and characterization of supported metal catalysts for
complete oxidation of formaldehyde at low temperature,” Applied Catalysis B: Environmental,
73, pages 282-291.