An experimental method for an on-line measurement of heat of combustion of a gaseous hydrocarbon fuel mixture of unknown composition is developed. It involves combustion of a test gas with a known quantity of air to achieve a predetermined oxygen concentration level in the combustion products. This is accomplished by a feedback controller which maintains the gas volumetric flow rate at a level consistent with the desired oxygen concentration in the products. The heat of combustion is determined from a known correlation with the gas volumetric flow rate. An on-line microcomputer accesses the gas volumetric flow data, and displays the heat of combustion values at desired time intervals

Chaturvedi, S. K.

Chegini, H.

English

NASA Technical Reports Server

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DEPARTMENT OF MECHANICAL ENGINEERING AND MECHANICS
COLLEGE OF ENGINEERING AND TECHNOLOGY
OLD DOMINION UNIVERSITY
NORFOLK, VIRGINIA 23529
ON-LINE MEASUREMENT OF HEAT OF COMBUSTION
By
S.K. Chaturvedi, Principal Investigator
and
H. Chegini, Research Assistant
Final Report
For the period ended April 30, 1988,
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Prepared for the
National Aeronautics and Space Administration
Langley Research Center
Hampton, Virginia 23665
Under
NASA Contract NAS1-17993
Task Authorization No.'s 60 and 77
Dr. Jag J. Singh, Technical Monitor
Instrument Research Division
*
Submitted by the
Old Dominion University Research Foundation
P.O. Box 6369
Norfolk, Virginia 23508
July 1988
https://ntrs.nasa.gov/search.jsp?R=19880016871 2020-03-20T06:47:54+00:00Z
ON-LINE MEASUREMENT OF HEAT OF COMBUSTION
By
••
Dr. S.K. Chaturvedi1 and H. Chegini2
ABSTRACT
An experimental method for an on-line measurement of heat of combustion
of a gaseous hydrocarbon fuel mixture of unknown composition is developed.
It involves combustion of a test gas with a known quantity of air to achieve
a predetermined oxygen concentration level in the combustion products. This
is accomplished by a feedback controller which maintains the gas volumetric
flow rate at a level consistent with the desired oxygen concentration in the
products. The heat of combustion is determined from a known correlation
with the gas volumetric flow rate. An on-line micro-computer accesses the
gas volumetric flow data, and displays the heat of combustion values at
desired time intervals.
1
 Associate Professor, Department of Mechanical Engineering & Mechanics, Old
Dominion University, Norfolk, Virginia 23529 /2
 Research Assistant, Department of Mechanical Engineering & Mechanics, Old
Dominion University, Norfolk, Virginia 23529
1. INTRODUCTION
Several well known techniques are currently available for the measure-
ment of heats of combustion of hydrocarbon fuels. For example, constant
volume bomb calorimeters are commonly used for determination of heats of
combustion of liquid and solid fuels. Similarly, constant pressure flame
calorimeters are employed for measuring the heat of combustion of gaseous
fuels (Ref. 1). Although these methods are reasonably accurate for most
applications, they are not suitable for applications which require an on-
line measurement of heat of combustion of fuels of unknown composition.
This is particularly the case in natural gas transmission industry where the
heat of combustion of natural gas of unknown composition may be required at
certain time intervals.
In the present study, we have developed a new experimental technique
for an on-line measurement of heat of combustion of a gaseous fuel of an
unknown composition. Preliminary results indicate that the technique has
the potential of measuring the heat of combustion values with a precision
approaching 0.30 percent. The proposed method is an extension of a method
developed by Singh and associates (Ref. 2). Their method involves combus-
tion of fuel with air of known oxygen concentration. The combustion bound
air is sufficiently enriched with oxygen so that the oxygen concentration in
the products equals that of the incoming air. It is further shown in Ref. 2
that the heat of combustion is correlated with the ratio of volumetric flow
rates of injected oxygen and the fuel. Although this study clearly
demonstrates the feasibility of the method, it has the drawback of oxygen
enrichment requirement which makes it unsuitable for an on-line field
measurement. A simplified theoretical method, eliminating the requirement
of oxygen enrichment, has also been proposed (Ref. 3). This procedure
involves combustion of a fuel with a fixed amount of air of a known oxygen
concentration. The fuel flow rate is adjusted until a desired oxygen
concentration is achieved in the combustion products. The heat of
combustion, for a given air flow rate, is shown to be correlated with only
one variable, namely the volumetric flow rate of the gaseous fuel. Two
correlations are presented for calculating the heat of combustion in the
high and low fuel volumetric flow rate regimes.
In the present study, following the ideas presented in Ref. 3, an on-
line gas calorimeter is developed for measuring the heat of combustion of
hydrocarbon gas fuel mixtures of unknown composition. The technique employs
a digital feedback controller to maintain the oxygen concentration in the
combustion products at a predetermined level dictated by the combustion
stability of the burner, and the oxygen sensor sensitivity. The volumetric
fuel flow rate is monitored and controlled at a level consistent with the
desired oxygen concentration in the products. The data from the experiment
is channeled to an on-line micro-computer which uses a software to display
the heat of combustion at desired time intervals. The details of the method
are described in the next section.
2. EXPERIMENTAL SET-UP
The prototype gas calorimeter is shown in Fig. 1. A photograph of the
set-up is included in Appendix A. The fuel gas and air of known oxygen
concentration was premixed and led into a specially designed burner. The
volume flow rate of the fuel was measured by a volumetric flowmeter with an
optical encoder mounted on its shaft. The volume flowrate (cc/min) was
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determined by measuring the counts (number/min) of the lines etched on the
disk of the encoder. The counts were recorded on a Hewlett Packard (HP)
data acquisition system and were also transmitted to a HP micro-computer and
a printer.
Initially, the burner presented severe combustion instability problems,
and the combustion could not be sustained with a porous plug type burner
over a wide range of operating conditions. Depending on the air and fuel
flow rates, either a blow-off or a flash-back of the flame resulted. These
problems of inflammability are common at low burning velocities (Ref. 4),
and special care was required in the burner design to sustain stable combus-
tion. Finally, a burner with internal honeycomb structure was employed to
damp out any large disturbances that might have resulted in combustion
instability. The use of this burner resulted in a stabilized flat flame
front over wide ranges of air and fuel flow rates. The burner was enclosed
in a glass container with a vented top. This eliminated any fluctuations of
oxygen concentration caused by diffusion of atmospheric oxygen toward the
combustion products.
The oxygen concentration in the products was measured by pumping the
products through a Y.O- - stabilized ZrO_ oxygen sensor. Since the sensor's
operational characteristics are similar to those described in Ref. 5, we
omit the details here. The mili-volt output of the oxygen sensor which
correlated uniquely with oxygen concentrations was linearized and amplified
during the experiment. The resultant output was directed to a MYCRO-352
single loop digital controller. A similar signal corresponding to 3.0
percent oxygen concentration, a value used in all experiments in the present
study, was also dialed into the controller. The controller using the
difference of the two signals, namely the one corresponding to the actual
oxygen concentration in products, and the one corresponding to the desired
3.0 percent value, manipulated a pneumatic control valve to vary, the fuel
flow rate, A higher than desired value of oxygen concentration prompted the
controller to open the valve wider and caused higher fuel flow rate through
the system. This led to faster depletion of oxygen due to enhanced
combustion, resulting in the return of oxygen concentration toward the
desired level.
Due to unsteadiness introduced by the volumetric flow meter and the
combustion process, the oxygen concentration in the products exhibited small
fluctuations around the mean value of three percent oxygen concentration.
To average the fluctuations in oxygen concentration, the mean and standard
deviation of a set of data points of heat of combustion at equal time
intervals was obtained. The experimental procedure and results are
described in the next two sections.
EXPERIMENTAL PROCEDURE
Prior to taking the data for heat of combustion, several preparatory
steps had to be taken to ready the set-up for experimentation. First, the
oxygen sensor was calibrated with air samples of known oxygen concen-
trations, so that a mV-oxygen concentration calibration curve could be
obtained. After this step, the system was flushed for several minutes with
air to scavenge any fuel left inside the system from the previous exper-
iment. The bottled air was used for combustion, and its oxygen concen-
tration was measured with the oxygen sensor before the start-up of the
experiment. The gas flowmeter was also calibrated so that the number of
counts per minute from the optical encoder can be readily converted into
flow rate in cubic centimeter per minute (ccm).
A fixed amount of air flow rate from a pressurized tank was initiated,
and the fuel flow rate was gradually increased to a level until a stable
flat flame front was established over the burner. The three percent oxygen
concentration was dialed as a set-point in the controller, and the
controller was activated to monitor the fuel flow rate. The fuel flow was
varied by controlling the valve opening until the set-point value was
achieved. As pointed out earlier, the oxygen concentration value in
combustion products could not be set exactly at three percent level due to
inherent unsteadiness of the combustion process and due to unsteadiness
introduced by the internal mechanism in the gas flowmeter.
The fuel flow rate data, in terms of counts per minute, was recorded at
three minute intervals. This value was then used in a correlation given in
Appendix B to obtain a value of heat of combustion at three minutes
interval. Several data points over a period of 15 to 30 minutes, depending
on the experiment, were recorded, and a mean value and the standard devi-
ation was computed for the period.
4. RESULTS
The present technique was applied to three gas samples, namely methane,
a mixture of methane and nitrogen, and ethane. The case of methane was used
to calibrate the procedure by using its known value of heat of combustion.
The remaining two cases served as examples for validating the procedure.
Table 1 shows the experimental data involving optical encoder counts, the
flow rate in ccm and the heat of combustion, calculated from the correlation
discussed in Appendix B. The oxygen concentration of the supply air was
20.93 percent and the air flow rate was 7000 ccm for this case. We note
TABLE 1
DATA FOR PURE METHANE
COUNTS/180 SEC. FUEL FLOWRATE HEAT OF COMBUSTION
(con) (Kcal/Kmol)
2125
2118
2108
2115
2116
2108
2110
2118
2112
590.3
588.3
585.6
587.5
587.8
585.6
586.1
588.3
589.4
211.7
212.5
213.5
212.8
212.7
213.6
213.4
212.5
212.1
Mean = 212.8 Kcal/Kmol
Standard Deviation = .62 Kcal/Kmol
that the standard deviation of the heat of combustion data points was 0.28
percent for this case.
Table 2 shows the data for the methane (92.5%) and nitrogen (7.5%)
mixture. The air flow rate and its oxygen concentration values remained at
the same levels. Based on a 7.5 percent non-combustible fraction in the
fuel, the heat of combustion of the sample was calculated to be 196.8
Kcal/Kmol. The mean value of nine data points, recorded at three minute
intervals, was 197.5, and the standard deviation of the data was 0.77
percent.
Table 3 shows the data for ethane. The air flow rate for this case was
7000 ccm, and the oxygen concentration measured prior to the experiment was
20.95 percent. The standard deviation of 16 data points showed a standard
deviation of 0.3%, while the mean value of the data was 376.7 Kcal/Kmol.
The corresponding heat of combustion reported in the literature is 372.8
Kcal/Kmol.
5. CONCLUSIONS
An experimental technique for measuring heat of combustion of gas
mixtures of unknown composition was developed and tested. The technique is
specially attractive for applications where an on-line measurement of heat
of combustion is required. The system worked reasonably well during the
course of this investigation, and the results validated the technique.
Although the data-base generated during the course of this study is somewhat
limited, the'results indicate that the technique is potentially capable of
measuring the heat of combustion of widely ranging hydrocarbon gas mixtures
of unknown composition with a precision approaching 0.30 percent.
TABLE 2
DATA FOR METHANE (92.5%) AND N2 (7.5%) MIXTURE
COUNTS/180 SEC. FUEL FLOW RATE HEAT OF COMBUSTION
(con) (Kcal/Kmol)
2297
2265
2284
2288
2260
2246
2262
2301
2280
671.7
662.3
667.9
669.0
660.8
656.7
661.4
672.8
666.7
195.6
198.4
196.8
196.4
198.9
200.1
198.7
195.3
197.1
Mean Value = 197.5 Kcal/Kmol
Standard Deviation = 1.5 Kcal/Kmol
TABLE 3
DATA FOR PURE ETHANE
COUNTS/300 SEC. FUEL FLOW RATE HEAT OF COMBUSTION
(con) (Kcal/Kmol)
2122
2119
2121
2122
2117
2112
2113
2113
2112
2107
2102
2108
2103
2103
2101
2104
353.7
353.2
353.5
353.7
352.8
352.0
352.2
352.2
352.0
351.2
350.3
351.3
350.5
350.5
350.2
350.7
374.8
375.4
375.0
374.8
375.7
376.6
376.4
376.4
376.6
377.5
378.4
377.3
378.2
378.2
378.6
378.1
Mean Value = 376.7 Kcal/Kmol
Standard Deviation = 1.3 Kcal/Kmol
10
REFERENCES
1. Sunner, S. and Mansson, M., "Experimental Chemical Thermodynamics,"
Vol.1 - Combustion Calorimetry, Pergamon Press, 1979.
2. Singh, J.J., Sprinkle, D.R., and Puster, R.L., "New Method for
Determining Heats of Combustion of Gaseous Hydrocarbons," NASA TP-2531,
December 1985.
3. Singh, J.J., Chegini, H., and Mall, G.H., "A Simplified Method for
Determining Heat of Combustion of Natural Gas," NASA TP-2682, April
1987.
4. Powling, J., "A New Burner Method for the Determination of Low Burning
Velocities and Limits of Inflammability," Fuel, Vol. XXVIII, No. 2,
1948.
5. Singh, J.J., Davis, W.T., and Puster, R.L., "Proposed Fast-Response
Oxygen Monitoring and Control System for the Langley 8-Foot High
Temperature Tunnel," NASA TP-2218, November 1983.
11
APPENDIX A
Photograph of the Set-Up of the Prototype Gas Calorimeter
ORIGINAL PAGE IS
QE £00* QUALITY A-2
APPENDIX B
Expression for Heat of Combustion of Gas Mixtures
EXPRESSION FOR HEAT OF COMBUSTION OF GAS MIXTURES
The heat of combustion, AH . , of a gas mixture of a pure hydrocarbon
III I /\
(C H ) and non-combustible inert chemical species is given by the
x y .
expression:
=
 f 1HCxHy
where f is the combustible fraction by volume of the mixture. If this gas
is mixed with air and burned, the amount of oxygen in the products of
combustion can be expressed as:
P = 4Q1 - nf(4x + y)
41 + 4n + nf(y-4)
where,
P = oxygen fraction of combustion products;
f = combutible fraction of test gas;
n = volumetric flow rate of test gas;
Q = oxygen fraction of air;
1 = volumetric flow rate of air;
x = carbon atoms in the fuel;
y = hydrogen atoms in the fuel;
B-2
Solving Eq. 2 for f, one gets:
f = 41(Q-P) - 4 Pn
n(4x + (1 + P)y-4P)
Substituting for f in Eq. 1, one gets:
AH .
 f = A H r , f41(Q-P)-4Pn ) (4)
mixture CxHy n(4x + (1 + p)y.4p) J
B-3


On-line measurement of heat of combustion

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