COMBUSTION EXPERIMENT
Performance of a Flat Flame Burner,
Combustion Generated Pollutants, CO and NOx,
and Catalytic Oxidation of CO
Chemical Engineering
Laboratories I, II
Revised September 2000
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TABLE OF CONTENTS
OBJECTIVE
SESSION I
Preliminary-Overview/Objectives
A. Combustor Assembly/Analytical Equipment
B. Dimensions - Combustor, Catalyst
C. Operations/Calculations
SESSION II - Laboratory
SESSION III - Report
Calculations
a)
b)
c)
d)
Burner Performance/Pollutant Generation
Catalyst Performance
Questions to be addressed
Oral Presentation
SESSION IV - Executive Summary
REFERENCES
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COMBUSTION EXPERIMENT
Combustion is a combined Unit Operation and Unit Process of unsurpassed importance to
modern civilization and technology. The linkages between practical devices, combustion
phenomena, and the underlying scientific and engineering disciplines are illustrated in Figure 1.
Combustion is the rapid conversion of fuel and oxygen to carbon dioxide and water in the presence
of a flame. It can also be a major source of several pollutants.
OBJECTIVES
In this experiment propane will be burnt in a flat flame burner to evaluate burner performance
and pollutant emissions, such as carbon monoxide and nitric oxide. A second objective is the
performance of a fixed bed catalyst for carbon monoxide conversion (to CO2).
Session I Preliminary - Overview/Objectives
This session is to acquaint you with the detailed objectives of the experiment, the equipment,
and the data to be taken. You should become familiar with certain definitions and combustion
terminology such as: stoichiometry, equivalence ratio, adiabatic flame temperature, volumetric and
surface heat release rates, flame velocity, stack draft, fuel types and composition, etc. Consult the
reference list at the end of this handout for background and some theory. In addition to these books
there are two premier journals in the combustion field: Combustion and Flame; Combustion Science
and Technology.
A.
Combustion Assembly/Analytical Equipment
A flat flame burner (Figure 2) is used to combust a light hydrocarbon gas, propane, C3H8.
The flame is essentially very thin, i.e., quasi-onedimensional. The combustor assembly is shown in
Figure 3. It consists of a platform upon which the burner is mounted. The burner is topped by a
short quartz glass chimney (27 cm high) with a transition to a galvanized steel ducting section (70
cm). This section includes a cooling coil (water) which may be used to regulate catalyst inlet and
outlet temperatures (T3 & T4), as well as an air inlet. This is followed by a flanged stainless steel
section containing the catalyst (cf. catalyst description below) and from there to galvanized exhaust
ducting. To the right of the burner table are three flow meters and associated valves to control
cooling water, and reactant (propane and air) flowrates, respectively. In lieu of one or more of these
flow meters, mass flow controllers may be used. THE TA WILL INSTRUCT IN THEIR USE
AND SETTINGS. The burner surface temperature is controlled by cooling water flowing through
the base of the burner. Thermocouples T1 and T2 measure the water T in and out, respectively.
Propane and air are controlled separately and are mixed in the base of the burner, prior to ignition
with an electric spark.
Downstream from the flame are several ports which are used for temperature measurements
(via thermocouples) and gas sampling. The principal fixed sampling probes are located upstream
and downstream of the catalyst, together with thermocouples T3 and T4, respectively (Figure 3).
These gas sampling lines connect to a pair of valves (located on the left front side of the support
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frame), which can be set to the desired sampling location (pre-or postcatalyst). For this experiment,
the second value should be set to "COMBUSTION".
From here, all samples taken go through a sample conditioning train (located below the
burner table). The principle function of this train is the removal of water prior to entry of the
noncondensable gas stream containing CO2, CO, and NO (in addition to N2 and any O2 not
consumed) into the gas analyzer modules.
To the right of the burner table a movable console is located which holds three gas analyzers
for carbon dioxide, carbon monoxide, and nitric oxide measurements (Figure 4). These
measurements are displayed continuously on each gas analyzer's screen and the computer monitor.
Part of your task will be the measurement of these pollutants under a variety of burner operating
conditions. This monitor is mounted on another console and - in addition to the above gases displays temperature (from all active thermocouple locations) in real time.
Under appropriate conditions, such as  < 1 (= equivalence ratio) the catalyst - converts
carbon monoxide to carbon dioxide. As part of this experiment you will evaluate the catalyst
performance as a function of inlet (T3) and outlet (T4) temperatures, inlet CO-concentration and space
velocity, at several equivalence ratios.
B.
Dimensions
- Burner
Base - 120 mm
Inside diameter - 62.5 mm
Sampling probes are located
below the catalyst (precatalyst, T3)
and after the catalyst (post catalyst, T4)
close to the respective thermocouples.
Glass chimney - 125 mm (i.d.)
- Honeycomb Catalyst (Pt/alumina)
Bed Length
Bed Diameter
Individual Channel area (open squares)
% open
C.
160 mm
70 mm
1.75 x 1.75 mm2
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Operations/Calculations
Operation of all equipment and instrumentation should be reviewed during Session I. The TA will
demonstrate the operation of the burner, thermocouples, gas analyzers, suitable operating ranges, and
the sampling train. Assignments should be made during this preliminary session of tasks to be
performed by each group member during the lab session. These assignments and their execution are
critical to the success of Session II.
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Using typical assigned flow meter or mass flow controller (propane and air ranges) settings (cf.
below), calculate flowrates for a series of equivalence ratios and the expected CO2-concentrations
(assuming complete conversion of propane to CO2). The actual settings which may require pressure/
temperature corrections, are read directly in lpm (liters per minute at standard conditions from the
rotameters). Calculate equivalence ratios PRIOR TO THE LABORATORY SESSION. Note the
location of the propane supply (tank to left rear).
Session II - Laboratory
You must have the calculations described below completed before entering the
laboratory to run the experiment!
Before the beginning of the laboratory session, the teaching assistants will have lit the burner
to equilibrate the system with the surroundings. Exhaust fan must be on.
Record the ambient temperature, pressure, and water flow rate (this value should be  2 lpm).
Safety is very important for this experiment. One group member should always be
watching the flame in case it goes out. If so, that person should immediately turn off the propane
supply using the valve on the top of the propane tank. Note: turning off the propane supply is the
only time a student may be behind the combustion platform. The teaching assistant then should be
notified to relight the burner.
Experimental Variables:
1)
Fp = propane flow; Fa = air flow (lpm). With constant propane flowrate of Fp = 0.6-0.7 lpm,
choose 3-4 values of the equivalence ratio in the range 0.5    1.1. Calculate the
corresponding air flow rate, Fa. Set the flow rates and allow the system to come to steady
state (steady flame and constant CO2). Record (on the computer and in your laboratory
notebook) all system temperatures, precatalyst gas concentrations (CO, CO2, NOx). A
sample data sheet is shown in Figure 5.
Record flame characteristics such as thickness, distance from burner, coloration, and
geometry (flat, bumpy, wavelets, etc.). Make estimates only from in front of the combustion
platform!
2)
With a constant air flow rate (Fa near the lower bound given below) choose 3-4 values of the
equivalence ratio in the range 0.5    1.2. Calculate the corresponding propane flow rate,
Fp, which must be in the range 0.6  Fp  1.l lpm. Record all of the system temperatures and
all of the precatalyst and postcatalyst gas concentrations.
3)
With a constant  = 1.0, choose 3-4 values of the propane flow rate in the range 0.6  Fp 
1.0 lpm. Calculate the corresponding Fa. Record data as in step 2.
4)
With a constant  at some value less than 0.9, choose 2 values of Fa which differ by 5-10
lpm. Calculate the corresponding Fp. Record data as in step 2.
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5)
For a low-mid range Fa-value (14-20) gradually increase Fp so that  > 1.2. Observe the
color and behavior of the flame. The reported sooting limit for propane is  = 1.56. (Sooting
is the incipient formation of carbon particles.) See if you can approach it. In this portion of
the experiment the flame must be observed very closely; it may go out (note instructions
above if that occurs). Try again, beginning at   1.2. If unsuccessful a second time,
abandon this portion.
Even if unsuccessful, this portion must be followed by a run at  < 1.0 for 15 minutes.
Otherwise, soot may accumulate in the catalyst bed.
6)
In addition to the data to be collected as outlined in steps 1-4 above, some of those conditions
should have the precatalyst gas concentrations measured twice, i.e. measure pre, post, and
then precatalyst gas concentrations. Also, periodically throughout the experiment, you
should return to a single base set of conditions (Fp and Fa) to check for reproducibility and
any changes in the experimental system.
Do not exceed the bounds listed under 7 (with the exception of 5., above).
7)
For a variety of reasons (can you think of what they might be) T3  T4 during your
experimental measurements. For some settings the difference may approach a T of 200o,
but for most settings it should be well below 150o. In calculations, use a mean temperature.
0.5    1.2 (cf. 5, above, however)
0.6  Fp  1.1
14  Fa  35
Suggested Operating Ranges:
N.B. The temperature at T3 should never exceed 400oC. If it does shut off propane
flow and restart (only with a lower Fp and Fa) when the temperature has dropped below
400oC (this temperature may be controlled by the water flow through this section).
(fuel/air)actual
Equivalence ratio,  = (fuel/air)
stoichiometric
(Fuel/air)stoichiometric assumes complete conversion of propane to CO2 and H2O (for  > 1,
that means unconverted propane must appear on the product side).
Combustion Stoichiometry
C3H8 + 5(O2 + 3.78 N2)  3CO2 + 4H2O + 18.9 N2
 CO2 
 100
Combustion efficiency, CE, % = 
 CO  CO2 
So, at a propane flowrate of 1 lpm, an equivalence ratio,  = 1 requires 23.8 lpm of air. If air
flow is lowered (or if propane flow is increased),  > 1, and not all of the fuel is converted.
N.B. On the right rear corner of the burner platform a rotameter is located which
serves to supply air needed to oxidize CO to CO2 when the combustion itself is
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conducted substiochiometric (not shown in Figure 3). This flow may enter either at the
igniter inlet or just below the catalyst bed (cf. Figure 3). The setting at all times should
be at least 130 (SS-float).
Session III - Report
Prepare for the interim report by following IIIa and IIIb below, and address the questions in
IIIc. .
Calculations
IIIa.
Burner Performance/Pollutant Generation
Calculate heat loss through the burner base (BB) using T1 and T2 and Fw and the ratio of heat
lossBB to total heat release; equivalence ratios; combustion efficiency; adiabatic flame T (assuming no
heat loss) and pseudo adiabatic flame T (based on: Hc - heat loss through burner base; where Hc =
heat of combustion of propane at 298 K = 530,600 cal/g mole); residence time from flame surface to
T3.
Calculate  for the observed sooting limit.
Plot r (inverse equivalence ratio) calculated from your data against nitrogen mass fraction (cf.
reference 1, Figure 3) as shown in Figure 6. N.B. The definition of  as given in reference l is not
the same as given above.
Give a reasonable estimate of an energy balance (heat release vs. all possible heat losses from
the total system).
Your measured CO- And NO-concentrations are to be compared to calculations based on
equilibrium and kinetic rate expressions from the literature, as given in references 2 and 3. Use
measured and calculated temperatures in both equilibrium and rate equations. Use pseudo adiabatic
flame T for NO-calculations (cf. also reference 5, under General References to Combustion, chapter
4, p. 119, and p. 538, for NO-calculations; copies of this material may be obtained from Prof.
Altwicker).
Estimate your flat flame (in the absence of cones, bumps, etc.) thickness from
 =
l
C po v
where

=
~
=
thermal conductivity of unburnt mixture
thermal conductivity of air
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Cp
o
v
=
=
=
heat capacity
gas density
gas approach velocity (through burner base)
Then calculate a characteristic , flame residence time (you will need this value for
calculations of NO-formation).
IIIb. Catalyst Performance
Evaluate the performance of the catalyst bed for CO-oxidation in terms of
equivalence ratios, space velocities, inlet and outlet CO-concentrations (i.e.,
conversion), inlet and outlet temperatures
cf. reference 2, p. 359-364 and
reference 6, Chapters 4 and 6.
IIIc.
Address the following questions:
1.
By what mechanism is CO(g) mass transported to the channel wall, where chemical
consumption (to produce CO2) occurs? What is the relevant transport coefficient, and to
what energy-transfer process and transport property coefficient is this "analogous"?
2.
Estimate the Schmidt number Sc  /DCO-mix for CO Fickian diffusion through the
prevailing combustion gas mixture, using:
1.73
0.216
cm2
 T 
DCO-mix  DCO/N2 =~ p
• 300
, s
 
where p is the prevailing pressure (expressed in atmospheres) and T the mixture temperature
(expressed in kelvins), and  = kinematic viscosity.
3.
Under typical flow rate, temperature, and pressure conditions found and using the mass
transfer equation given below, estimate the catalytic duct length, z, required to consume 95%
of the inlet CO concentration, and the mixing cup (bulk) stream temperature at this length.
CCO z
= exp
CCO IN
[
4z
- Re Sc d
eff
]
= 0.05
where deff = 1.75 mm; Re = Reynolds # in catalyst channel. How does this result compare to
your experimental results?
4..
If the catalyst were "poisoned" (e.g., by lead compounds), what would happen to the CO exit
concentration? Which of the assumptions used in predicting the required converter length, z,
would be violated?
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IIId.
This report will be an ORAL PRESENTATION WITH VIEWGRAPHS in which all
members of the group are to participate. It is useful if you provide a handout of your overheads.
This presentation typically last 50-60 minutes and should include Introduction and Background,
Experimental and Analytical System (including positions for measurements), Results, Discussion,
Conclusions, References used. Grading emphasis will be placed on the organization and clarity of
the presentation. Your grade will be based on this presentation, plus the Executive Summary (cf.
below).
Questions to Consider:
1.
Summarize the purpose of this experiment
2.
Be able to explain the general theory of combustion in relation to the
experimental setup (what are the advantages of a flat flame burner?) and the
parameters measured, including definitions of, for example
a.
b.
c.
d.
Stoichiometry of combustion reactions
Burning velocity
Diffusion flame vs. premixed flame
The three T's of combustion (time, temperature and turbulence)
e.
f.
g.
h.
i.
Equivalence ratio and its significance to pollutant formation
Adiabatic flame temperature
Combustion products
Heat transfer mechanisms
Equilibrium vs. kinetics as related to NO- and CO-formation
Discuss the performance of the flat flame burner as shown in Figure 6 and compare to your
experimental results. How does flame color relate to equivalence ratio?
3.
Catalyst performance and your results.
THESE ORAL PRESENTATION TIMES SHOULD BE SCHEDULED SEPARATELY WITH
PROF. ALTWICKER.
Session IV - Executive Summary
Provide a two page Executive Summary of the experiment and your results; you may add one or two
figures and/or tables.
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REFERENCES
1.
Vantelon, J.P., P.J. Pagni, and C.M. Dunsky, Cellular Flame Structures on a Cooled Porous
Burner. This reference is attached.
2.
Cooper, C.D. and F.C. Alley, Air Pollution Control, 2nd Ed., Waveland Press, 1994,
Chapters 11, 337-364; Chapter 15, 485-499.
3.
Flagan, R.C. and J.H. Seinfeld, Fundamentals of Air Pollution Engineering, Prentice Hall,
1988, Chapters 2 and 3.
4.
Prucker, S., W. Meier, and W. Stricker, Rev. Sci. Instrum., 65, 2908-2911 (1994).
5.
Senser, D.W., J.S. Moore, and V.A. Cundy, ibid., 56, 1299-1284 (1985).
6.
Fogler, H.S., Elements of Chemical Reaction Engineering, Prentice Hall, 1992, Chapters 4 &
6.
7.
Holman, J.P., Heat Transfer, McGraw-Hill Inc., 1997.
General References to Combustion
1.
Glassman, I., Combustion, 3rd Ed., Academic Press, 1996.
2.
Chigier, N., Ed., Energy and Combustion Science, Student Edition 1, Pergamon Press, 1974.
3.
Borman, G.L.. and K.W. Ragland, Combustion Engineering, McGraw Hill, 1998.
4.
Puri, I.K., Ed., Environmental Implications of Combustion Processes, CRC Press, 1993.
5.
Turns, S.R., An Introduction to Combustion, McGraw Hill, 1996. This reference has a
sample NO-calculation.
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Figure 1
Combustion Linkages
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Figure 2
Flat Flame Burner
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Figure 3
Combustion Assembly
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Figure 4
Combustion Sampling & Analysis Layout
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Figure 5
Date
psi, Air Pressure (line)
psi, Propane Pressure (at tank)
°C Ambient Temperature
Group Number
15
1)Rotameter or
Mass Flow Controller Setting
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Figure 6 (cf. Reference 1)
Flame Stability map for N2-O2-C3H8 mixtures at different STP total flow rates: a) 200 cm3s-1, b)
395 cm3s-1, and c) 590 cm3s-1. No change in the map is observed for flows in excess of 500 cm3s-1.
The hatched tail in zone 1 indicates a yellow flame with cells. The hatched lobe has the cellular
structure of primary interest here. The dotted zone has a conical blue flame with an inner flat green
flame. Zone 2 has a conical blue flame with an inner flat blue flame, while zones 3 and 4 are similar
to zones 3 - 5 (in Fig. 2 of reference 1)
inverse equivalence ratio, r (cf. reference 1)
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