Note: Descriptions are shown in the official language in which they were submitted.
3~ 3~
1 Constant temoerature heatin value measurement
, _ ~ . 9
apparatus
The present invention relates in general to the field
of devices for measuring the heating value (e.g.,
BTU's) of a fuel gas, and in particular to devices
in which changes in the energy supplied to a heating
source within a fuel gas reaction chamber provide an
indication of the heating value.
Because of the ever-increasing costs of energy,
utilization of the energy contained in the wild gas
streams generated by many refining, metallurgical and
chemical processes has become economically important.
Uncontrolled fluctuations in the heating value of the
material being combusted lead to gross inefficiencies
in the combustion process, and may, in extreme cir-
cumstances, produce hazardcus conditions or unaccept-
able variations in the characteristics of the
products being manufactured.
To minimize these inefficiencies and waste, various
20 control schemes have been introduced which attempt
to monitor the fluctuations in the heating value of
the fuel gases being introduced into a combustion
chamber, and, via feedback mechanisms, to adjust the
rate at which the fuel is introduced, so as to main-
25 tain delivery of a uniform heating value per unittime to the chamber. This is particularly important
in the case of furnaces or boilers which combust a
variety of fuel gases simultaneously, since at
different times, the mix of fuel gases within a given
30 volume may vary widely. Representative of such prior
3~
1 art control schemes is the apparatus shown in u.S.
Patent No. 4,329,873. This patent discloses an
apparatus in which a sample gas and combustion gases
are combined in a reaction chamber, and they are
oxidized by the action of a catalytic heating ele-
ment. This heating element is monitored for changes
in its resistance caused by the heating of the com-
busted gases, and a feedback signal based on this
resistance change controls the flow rate of the
sample gas into the chamber. A similar system dis-
closed in U.S. Patent No. 4,329,874 monitors the
temperature difference between one end of a cataly-
tically-coated heating element located within the
reaction chamber and the opposite end located outside
the chamber. The end of the heating element within
the reaction chamber is heated by the oxidation of
the fuel gas, however, a controller varies the
current to the element to maintain a predetermined
temperature differential between its two ends. The
measure of the difference in the electric current
supplied to the element can be correlated to the
calorific heating content of the fuel gas.
In U.S. Patent No. 4,170,455, there .is shown a system
in which the temperature of an incoming fuel gas is
measured. The gas is passed through a perforate
metal heat shield into contact with a bed of particu-
late catalyst, thereby causing combustion. The
temperature of the combusted gas is mea~ured, with
the temperature difference being correlatable to the
concentration of the gas of interest. The assembly
formed of the particulate catalyst and the surround-
ing metal mesh screen is configured to maximize the
39
1 bed volume-to-surface area ratio, so as to maximi2e
the flow capacity-to-heat loss ratio of the system.
A major drawback of many such prior art systems is
that, since they are based on a catalytic reaction
5 and localized temperature measurement, the response
of the system is inherently dependent on the composi-
tion of the fuel gases. Catalysts are generally not
of universal applicability, and the rate of catalytic
reactions varies greatly for different reacting sub-
10 stances. Therefore if a combustion process utilizesfuel gases of a widely varying nature, separate
measurement systems using separate catalysts and/or
calibrations would be required. When the composition
of the fuel is unknown, the design of the measur-
15 ing unit must take into account the highly variablerates of oxidation of the component gases, which may
range from hydrogen (very reactive) to methane and
other alkanes. Any heat value measurement derived
from the heat released by combustion of only part of
20 the sample is probably composition dependent, as the
fraction which will react depends on the chemical
species present and the stoichiometry of the mixture.
It should be pointed out that catalysts are suscep-
tible to "poisoning" when contaminated by foreign
25 substances, which often are an unavoidable part of
the incoming fuel stream. Also, catalysts tend to
age, and their performance characteristics change
over time, thereby adversely affecting the accuracy
and precision of the control process.
30 Therefore, there is still a need within the industry
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1 to provide a more universal measuring apparatus for
measuring the BTU9 or heating value or calorific
content, of a fuel gas, which is capable of measuring
a wide variety of sample gases, either individually
or in mixtures, and without recallbration.
It is a further object of the present invention to
provide such a system which is not susceptible to
poisoning or degradation by foreign fuel gases, and
is tolerant of wide fluctuations in the type of fuel
gas being supplied.
It is a further object of the present invention to
provide an apparatus to perform the above functions
in a manner that is adaptable to existing furnaces,
boilers and other similar combustion apparatus, and
15 which performs its functions in an economical manner.
A fuel gas heating value measuring apparatus in
accor~ance with the present invention comprises a
combustion unit, including an outer shell having a
high coefficient of thermal conductivity, forming an
2û inner combustion chamber. Fuel gas and oxidizing gas
inlets communicate with the interior of this combus-
tion chamber. A heating element positioned within
the combustion chamber maintains the chamber at a
temperature above the combustion temperature of any
of the constituents of the fuel gas, with a source
of energy provided for the heating element. The ap-
paratus includes a mechanism For achieving complete
combustion of the fuel gas within the combustion
chamber by the heating element, while also transmit-
ting the heating effects of the combustion uniformly
~2~ 39
1 to the outer shell. A temperature sensor applied tothe outer shell produces an output signal correspond-
ing to the sensed temperature, and provides this
signal to a mechanism for varying the amount of
energy supplied by the energy source to the heating
element in response to the output signal, so as to
maintain the temperature of the outer shell constant.
Finally, a mechanism is provided for measuring the
variation in energy supplied by the energy source and
correlating this variation to the heating value of
the fuel which has been combusted.
In a particular embodiment of the present invention,
the mechanism for achieving the complete combustion
of the fuel gas is a tightly compacted bed or aggre-
gate of particles~ having a high thermal conductivi-
ty 7 such as alumina, beryllia or silver. These par-
ticles are densely packed within a combustion chamber
having a relatively small internal volume. The fuel
gas is injected into one end of the chamber, upstream
of the oxidizing gas. This initially produces a
stratified, i.e., non-homogeneous, mixing of the fuel
and the oxidizing gas (a "rich" mixture) ~hich facil-
itates the ignition of the fuel gas. Subsequent
diffusion of the fuel and oxidizing gases through the
bed of particles results in complete mixing of the
gases and effects total combustion of the fuel gas,
i.e., a release of the total chemical energy content
of the fuel gas. Because of the intimate contact
among the particles of the bed and their high thermal
conductivity, the totality of the heating effects
caused by combustion at any point within the chamber
is immediately transmitted to the outer highly con-
39
1 ductive thermal shell, and is distributed evenlythroughout the surface of the shell. In effect, the
bed of particles surrounding the heating element acts
as an extension of the heating element, producing,
as it were, a widely distributed heating element.
Because of the near instantaneous transfer of the
total heating effect to the outer shell, and the
uniform temperature distribution across the shell
surface, ~very point on the surface is theoretically
an indicator of the thermal effect due to the com-
bustion of the fuel gas. Therefore the temperature
at any arbitrary point on the surface can be corre-
lated to the heat content of the gas which has
effected the sensed change in temperature.
The numerous operating features and advantages of the
present invention will be made clear by the following
detailed description, in conjunction with the accom-
panying drawings in which:
FIG. 1 is a schematic, in block diagram form, depict-
ing a measuring apparatus built in accordance withthe present invention;
FIG. 2 is an elevation view, in section9 of the fuel
gas combustion unit shown in FIG. l; and
FIG. 3 is a graph showing the correlation of fuel
power versus electrical power supplied to the heater,
for a variety of fuel gases.
Referring now to FIG. 1, a fuel gas heating value
Z~3~
1 measuring apparatus in accordance with the present
invention, depicted generally by reference numeral
11, includes a fuel gas combustion unit 13, to be
described in further detail hereinafter. The func-
5 tion of the combustion unit is to combine the samplegas containing combustible constituents (e.g.~ hydro-
gen, methane, carbon monoxide) and an oxidizing gas
(e.g., air or oxygen) to achieve total combustion of
the fuel gas. A heater 15, located within the com-
10 bustion unit 9 and heated to a temperature above thenormal combustion point of the fuel gas in question,
initiates the oxidizing reaction. The total heating
effects achieved by the combustion as well as the
output of the heater are transferred to the outer
15 surface of the combustion unit, the temperature of
which surface is to be maintained at a constant,
p.edetermined level.
This surface temperature is a function not only of
the amount of heat generated by the heater 15 itself,
20 but also of the heat generated by the combustion of
the fuel gas and the oxygen. Naturally, as the
heating value or calorific content of the fuel gas
varies, the amount of heating effect produced by
combustion of this fuel gas changes accordingly.
25 Therefore, in order to maintain the surface tempera-
ture constant, the amount of heat provided by the
heater must change inversely with changes in the
heating value of the gas, i.e., more heat being re-
quired from the heater when the heating value of the
30 gas decreases, and less heat required from the heater
when the heating value increases.
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1 The operation of the heater 1~ is controlled by the
amount of energy supplied to the heater from an
external energy source 19, with more or less energy
being supplied to the heater as appropriate. The
5 temperature of the outer surface is monitored by a
conventional temperature sensor 21, such as a thermo-
couple or resistance-temperature device (RTD),
securely fastened thereto. The output signal from
this temperature sensor, directed along a line 23,
10 is fed into a conventional feedback controller 25.
Representative of such a conventional controller is
the SPEC 200 electronic analog controller rnanu-
factured by The Foxboro Company, Foxboro, Massachu-
setts, assignee of the present application. As the
15 actual temperature sensed by the temperature sensor
deviates from a temperature set point programmed into
the controller, the controller, in a well-known
conventional manner, generates an error signal. This
signal is fed back along a line 27 to the heater
20 energy source 19, to appropriately modify the amount
of energy supplied to the heater 15 and counteract
the increase or decrease in surface temperature
produced by the internal combustion of the fuel
gas. In the depicted embodiment, the heater is a
25 resistance wire (see also FIG. 2), and the energy
being supplied thereto is in the form of an electri-
cal current. A typical such resistance wire is one
formed of a 60 percent nickel, 24 percent iron and
16 percent chromium alloy, sold under the trademark
30 Nichrome. However, the present invention can operate
effectively with a wide variety of conventionally
known heaters, powered by electrical or non-electri-
cal sources. A monitoring device 29, interposed be-
~'
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1 tween the combustion unit heater and the heaterenergy source measures the variation in the energy
supplied to the heater. The difference between the
electric power required with and without fuel gas
flow equals the heating value supplied by the fuel.
An indicator 31, whose input is derived from the
monitoring device, provides a direct indication o~
the heating value of the fuel gas in question, its
scale having been appropriately calibrated to the
10 proper units of measurement (e.g., BTU per unit
volume).
Referring now to FIG. 2, the specific combustion unit
13 fùr achieving efficient combustion of the fuel gas
is shown in greater detail. The entire combustion
15 unit is enclosed within a hermetically sealed metal
jacket 33, which is evacuated so as to thermally
insulate the combustion unit from its surroundings
in order to limit the total amount of power required
to maintain it at the desired operating temperature.
20 Such insulation also may be achieved, e.g., by en-
closing the combustion unit in a guard heater held
at some elevated temperature below the desired oper
ating ternperature. The outer shell 35 of the combus-
tion unit, which defines an interior combustion or
reaction chamber, is made of a material having a
high thermal conductivity, for example, silver.
shiny platinum ~oil 37 (which does not tarnish at
elevated temperatures) is tightly wrapped about the
outer shell to reduce radiative heat loss. This type
3û of construction produces an overall device that is
isothermal and which has a constant and controllable
rate of heat loss to its surroundings.
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1 The outer shell 35 is penetrated by two inlet lines,
38, 39, the first of which delivers the fuel gas
sample to be measured, while the other delivers the
oxygen~ The interface between these inlet lines and
the outer shell are securely brazed, welded or simi-
larly bonded to insure a gas-tight construction.
There is also an exhaust line 41 similarly attached
to the outer shell and communicating with the com-
bustion chamber for exhausting the combustion
products. An inlet conduit 43 provides access for
the heating element 15, comprising an alumina
(A1203) support member 45 with a resistance wire
47 wrapped around the exterior of the alumina member
at its upper end. A pair of electrical leads 49
connected to the resistance wire passes through the
interior of the alumina support and out through its
bottom end via a gas seal such as epoxy or a ceramic
material which prevents the gaseous content of the
chamber from exiting the chamber except through
exhaust line ~1.
Filling the entire remaining volume of the combustion
chamber is a compacted bed of particles 53 having a
high coefficient of thermal conductivity. These
particles surround the heater 15 and are in intimate
contact with the heater and each other. Typical
particle materials usable in the present invention
are beads of silver, alumina, or beryllia
(~e203). In the specific embodiment shown, these
particles are of alumina, having a size distribution
of 8-2û mesh. The intimate contact of the particles
with both the heater 15 and the outer shell 35, as
well as with each other, insures that localized
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1 heating effects occurring anywhere within the com-
bustion chamber are almost instantaneously trans-
ferred to all parts of the combustion unit, including
the outer surface of the silver shell. This rapid
5 transfer is aided by the relatively small internal
volume of the combustion unit, typically 2.5 cm3.
In essence, then, the highly thermally conductive
configuration of the combustion unit achieves an
integrating effect, in that the temperature of the
10 outer shell is a function of the sum total of the
heating effects throughout the combustion chamber.
The temperature sensor is securely bonded to the
silver shell through a hole in the foil 37. Place-
ment of the sensor is not critical, because the
15 temperature is uniform across the entire outer sur-
face 35, a direct result of the optimum thermal con-
ductivity of the combustion unit as a whole.
In operation the fuel gas is introduced at the lower
end of the combustion unit 13, with the oxidizer
20 being introduced at a higher point, downstream of the
fuel gas. This condition makes the initially formed
gas mixture fuel-rich and, as is well known, enhances
the ability of the fuel gas to be ignited. The
passage of the two gases through the many circuitous
25 paths within the bed of alumina particles 53 produces
a turbulent interaction, to achieve a thorough mixing
of the fuel gas and the oxygen, which produces com-
plete combustion of the fuel gas. Also, the bed of
highly thermally conductive particles effectively
30 operates as an extension of the resistance wire
heater, by achieving a more widely distributed
heating surface, and further facilitating complete
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1 combustion. Many prior art devices, which depend on
combustion of only a portion of the total fuel gas-
oxidizer sample mixture, require a separate calibra-
tion for different sample compositions because the
5 temperature profile generated by heat of combustion
will depend on the reactivity of the fuel gas
species. However, such recalibration is not required
within the present invention, because of the previ-
ously mentioned integrating effect. Rather, the
10 total combustion, and the high thermal conductivity
of the overall construction combine to achieve an
accurate characterization of the gas sample's heating
value, regardless of the reactivity of the gas.
It should be pointed out that although the terms
15 ~complete combustion~ and ~total combustion" are used
herein to distinguish the mode of operation of the
present invention from prior art devices which use
only sampling techniques, this is not to say that the
present invention will not function with anything
20 less ~han 100 percent combustion of the fuel gas.
The consequence of less than lûO percent combustion
is a corresponding loss in accuracy. For example,
if only 99 percent of the total fuel gas sample
combusts, the accuracy of the heating value measure-
25 ment will not be better than one percent.
The actual combustion of the fuel gas and the oxygenis initiated by the heated resistance wire ~l7.
Unlike pr.ior art catalytic combustion devices as
discussed above, which operate at a temperature
30 below the normal combustion point of the sample
gases, the resistance wire is heated to raise the
~ ~2~L~3~
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1 temperature of the chamber above the combustion point
of any of the constituent elements within the fuel
gas. As mentioned above, the electric current sup-
plied to the resistance wire varies, depending on the
heating value of the fuel gas being combusted.
~ssuming there is a known, constant rate of fuel gas
flow for the duration of the measurement, the magni-
tude of the change in the number of watts of electri-
cal power consumed is equivalent to the heating power
(e.g., in BTU's) of the fuel gas sample. This change
in electrical power is detected by using a conven-
tional wattmeter as the monitoring device 29.
Experiments with devices such as the above described
embodiment, using pure fuel gases such as H2, C0
and CH4 have yielded efficiencies of 97 to 100 per-
cent, with a response time of about twenty seconds.
The graph of FIG. 3 clearly depicts the wide applica-
bility of the present invention, by showing the
equivalence of fuel power to changes in electrical
20 power for these three gases. These actual measure-
ments were made without the need for any recalibra-
tion after changes in the species being measured.
Instead of the measured heating value of the fuel gas
being merely displayed on the indicating device 31,
25 the heating value information can be fed back to a
supply valve (not shown) governing the rate of flow
of the fuel gas to a furnace or similar combustion
apparatus, to continuously deliver a constant heating
value per unit time thereto. By maintaining a con-
30 stant heating value input, the furnace output simi-
larly can be maintained constant.
~L~24~3139
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l Similar results are achieved by directly varying the
amount of fuel gas supplied to the combustion unit
13 to maintain the constant temperature at the outer
shell 35, rather than by varying the electric current
to the heater wire 47. In this situation, the same
signal which controls the gas supply to the combus-
tion unit is used to control the gas supply to the
main burner or furnace, so that the present invention
functions as a set point controller, rather than a
mere heating value indicator.
Although the particles 53 are chosen primarily for
their high thermal conductivity, it is recognized
that, in the case of certain fuel gases, there may be
an attendant amount of catalytic action. However, as
discussed above, the present invention is not intend-
ed to require a catalytic reaction. The fact that
the particle material is not a catalyst allows the
present invention to be applied to a wide variety
of fuel gases, while avoiding the common problem of
poisoning catalysts by sulfides, lead compounds, etc.
Although the above embodiment has been described in
quite specific terms, it is understood that certain
modifications may become apparent to those skilled
in the art. For example, materials other than silver
may be usable for the outer shell of the combustion
unit, such as for example palladium which has high
thermal conductivity but a far higher melting point
than silver. Also 9 at higher operating temperatures
it may be unnecessary to use oxygen as the oxidizing
agent, and normal atmospheric air may be sufficient.
Nevertheless, it is intended that these and other
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l similar modifications be included within the scope
of the following appended claims.
