Fuel entrained oxygen compensation for calorific content analyzer

A calorific content analyzer for determining the calorific content of a combustible gas while compensating the analysis of the calorific content for fuel entrained oxygen. The gas is combusted to achieve substantially stoichiometric combustion by varying the air/fuel ratio supplied to a combustion chamber by a rotary valve which is speed controlled in response to a sensing of oxygen in the combustion products. A first measurement of the calorific content of the fuel gas is made using a normal operation of the analyzer and a second measurement is made using a known amount of air added by the rotary valve to the combustible gas stream. The true air/fuel ratio and the calorific content of the fuel gas is then computed using a predetermined relationship among the system variables.

CROSS-REFERENCE TO CO-PENDING APPLICATION 
Subject matter disclosed but not claimed herein is shown and claimed in 
U.S. Pat. No. 4,386,858 issued on June 7, 1983 and assigned to the same 
assignee as the present application. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to gas analyzers. More specifically, the 
present invention is directed to analyzers for determining the calorific 
content of a combustible gas. 
2. Description of the Prior Art 
The measurement of calorific content, e.g., BTU content, of a combustible 
gas such as that supplied for home heating by a public utility, etc., 
provides a measure of the quality of the gas being supplied and, hence, 
the appropriate rate or cost for the gas be billed by the public utility 
to a customer who formerly was charged a rate based simply on a cubic 
volume of gas consumption. Conventional gas analyzers for determining the 
composition of an unknown gas have usually involved a variety of time 
consuming methods. The basic prior art analyzer is known as the Orsat type 
and is used to absorb the constituent gases one at a time from a gas 
mixture and to determine the constituent quantities from a resulting 
decrease in the gas pressure exhibited by the mixture. The resulting gas 
analysis could be used as a basis for consumer billing. In another 
apparatus, chromatographic analysis of the gas constituents has been used 
to compute the actual heating value or calorific content of a combustible 
gas from the percentage composition of the combustible constituents of the 
mixture. In still another device, the heat content of the gas has been 
determined by measuring the amount of heat liberated in burning one cubic 
foot of the gas in a closed volume at standard conditions of temperature 
and pressure. The heat so liberated is absorbed by a known quantity of 
surrounding water, and the subsequent temperature rise of the water is 
used to calculate the heating value. However, all such prior art devices 
are wholly impractical for mass installation on-line in gas consumer 
locations particularly home consumers, since such methods involve 
expensive instrumentation and considerable labor to perform the 
measurements and calculations while introducing substantial time delays. 
Accordingly, it is desirable to have a so-called on-line system which can 
measure the calorific content of the combustible gas in an unattended 
location and which is suitable for mass installations. Known gas analyzers 
of this latter type include ones based on the use of the thermal 
conductivity of the known gas which gas is analyzed by comparing its rate 
of thermal conductivity with that of a standard reference gas. Another 
prior art gas analyzing device uses a catalyzing wire which has its 
temperature affected by a gas being burned adjacent to the wire to produce 
an output signal which is used to ascertain the percentage of combustible 
gas in the gas being tested. An additional group of gas analyzers are 
based on an optical analysis of the color, etc., of a gas flame to provide 
a measure of combustible gas content. However, all of these prior art 
devices have serious shortcomings in providing a rapid and accurate 
measurement of the calorific content of the combustible gas while 
utilizing a compact and simple structure suitable for mass production, 
on-line installation and capable of being used over extended periods of 
time without significant maintenance. 
SUMMARY OF THE INVENTION 
To overcome the aforesaid defficiencies of the known methods of calculating 
the heat content of a combustible gas, a system has been developed using 
an electrochemical cell as an oxygen or combustible product sensor to 
sense the residual products of combustion based on ceramic compunds such 
as ZrO.sub.2. Under normal conditions, the combustible gas is burned with 
excessive oxygen to assure complete combustion and an absence of carbon 
monoxide in the products of combustion. This leads to the presence of an 
amount of excessive oxygen after combustion which indicates that such an 
oxygen sensor could provide a relatively inexpensive and rapid solution 
into the problem of determining the heating value for the combustible gas 
mixture based on the air-fuel ratio. Thus, such a system uses a ceramic 
based electrochemical sensor which is known to exhibit a Nernstein voltage 
output when exposed to different partial pressures of oxygen on each side 
of the ceramic material. Such a sensor can be used to sense the amount of 
oxygen present in the products of combustion. That system proposes to 
utilize a known volume ratio between the fuel and oxygen supplied to a 
burner in conjunction with a measurement of excess oxygen after combustion 
using the aforesaid Nernstein relationship to provide a basis for deriving 
the heat content of the fuel. That system eliminates both the need for 
precise temperature control of the sensor and the errors introduced by 
inert constituents of the combustible gas mixture. 
The heat content measuring system includes a volumetric measuring system to 
accurately proportion the fuel which is mixed with the oxygen from a 
suitable source, e.g., air. A single burner in combination with the oxygen 
sensing system provides the necessary air-fuel information necessary to 
effect a determination of the heat content measurement of the fuel gas. 
The system is designed to control combustion substantially at the 
stoichiometric point wherein the electrochemical sensor exhibits a 
step-change function to produce increased accuracy. At this point, the 
precise volumeric ratio of fuel to air is accurately known from the 
fuel-air ratio controlling system, and the heat content of the fuel can be 
accurately determined from that ratio in a manner which is simplified by 
the elimination of the effects of several undesirable variables. Thus, 
this heat content measuring system includes a precise and adjustable 
metering system which accurately proportions the amount of fuel gas or 
calorific gas to be tested with a known amount of air such that any given 
time, the volumetric ratio of air to fuel is precisely known. 
The mixture is fed to a combustion system in which the fuel is combusted in 
the presence of a solid state ceramic electrochemical cell which provides 
a step-change in its output voltage as the amount of residual oxygen in 
the products of combustion approaches zero, i.e., as the combustion 
approaches the point of stoichiometry. An electrical output signal from 
the electrochemical cell is utilized with a programmable electronic 
processing system to adjust the fuel-air mixture in accordance with the 
output of the electrochemical cell to achieve the stoichiometric air-fuel 
ratio is signalled by the rapid change in electrical output of the cell at 
that point. The air-fuel ratio at that point is known from the measuring 
system and, therefore, the heat content of the fuel can be readily 
determined therefrom. However, in the blending of a combustible gas by a 
gas supplier, e.g., a gas utility, it is common to add air to the blend to 
lower the BTU value of the gas supplied to the consumer. The added oxygen 
contained in the mixture produces in a calorific content analyzer using 
the aforesaid method an error in its determination of the calorific 
content of the fuel in proportion to the amount of oxygen added. This 
occurs as a result of the oxygen in the fuel which is not accounted for by 
the means for providing the variable ratios of the fuel and oxygen whereby 
the determination of the air to fuel ratio is erroneous. Accordingly, it 
is desirable to provide a compensation for the fuel entrained oxygen in 
the determination of the calorific content of the combustible gas. 
An object of the present invention is to provide an improved combustible 
gas calorific content analyzer having fuel entrained oxygen compensation. 
In accomplishing this and other objects, there has been provided, in 
accordance with the present invention a combustible gas calorific content 
analyzer having a fuel entrained oxygen compensation for a system 
delivering to a combustion means a variable ratio of gas and air to 
produce substantially stoichiometric combustion by sensing combustion 
products in a first mode of operation. 
The analyzer comprises a first connection means for a source of a fuel gas, 
second connection means for a source of combustion air, ratio control 
means for producing a mixture of fuel gas and air in a selectively 
variable ratio, said ratio control means having a first inlet, a second 
inlet, a third inlet and an outlet arranged to receive a flow mixture 
obtained from said first, second, and third inlets, means connecting said 
first connection means for a fuel gas source to said first inlet of said 
ratio control means, means connecting said second connection means for a 
source of air to said second inlet of said ratio control means, 
selectively controlled valve means for providing a connection between said 
third inlet and said first connection means for a source of fuel gas in a 
first mode of operation and between said third inlet and said connection 
means for a source of air in a second mode of operation, combustion means 
connected to said outlet from said ratio control means for producing a 
combustion of said mixture of fuel gas and air, sensor means for sensing 
combustion products from said combustion means to produce an output signal 
representative of the combustion state of said combustion means and 
controller means arranged to respond to an output from said sensor means 
for controlling said ratio control means and for switching said valve 
means between said first and second modes of operation to produce 
substantially stoichiometric combustion of said mixture of fuel gas and 
air in said first and second modes of operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Detailed Description 
Referring to FIG. 1 in more detail, there is shown a pictorial diagram of a 
calorific content measuring system embodying an example of the present 
invention. An fuel gas inlet pipeline 2 is arranged to supply a fuel gas 
to a pressure regulator 4. The regulated flow output of the pressure 
regulator 4 is applied to a solenoid controlled flow selection valve 6 and 
a rotary mixing valve 8. An air inlet pipeline 10 is arranged to supply 
combustion oxygen, e.g., air to a second pressure regulator 12. The 
regulated flow output of the second pressure regulator 12 is applied by a 
pipeline 13 to a first input of the selection valve 6 and through a 
temperature measuring device 14 to a first input of a rotary mixing valve 
8. The output of the selection valve 6 is supplied through pipeline 15 to 
a second input of the rotary mixing valve 8. A first output from the 
rotary mixing valve 8 is supplied through pipeline 16 to a temperature 
measuring device 17. The output of the temperature measuring device 17 is 
supplied through pipeline 18 to a burner 20 located in a primary or 
measuring combustion chamber 22. 
An electrochemical combustion product sensor 24 is located in the 
combustion chamber 22 for sensing the combustion products from the burner 
20. An output signal from the sensor 24 is connected to a first input of a 
system control apparatus 26. The system control 26 may be any suitable 
device for producing an output in response to a predetermined relationship 
of input signals applied thereto, e.g., a microprocessor operating under 
control of a fixed stored program, such devices being well-known in the 
art. A first ignition and flame safeguard device 28 is also located in the 
combustion chamber 22 and is controlled over signal line 30 by the system 
control 26 to effect ignition of the burner 20 and to sense the presence 
of a burner flame. A second output from the rotary mixing valve 8 is 
connected through pipeline 32 to a second burner 34 located in a second or 
flare combustion chamber 36. A second ignition and flame safeguard device 
38 is located in the second combustion chamber 34 and is controlled by the 
system control 26 over signal carrying line 40. A motor 42 is arranged to 
drive the rotary mixing valve 8 by a motor shaft 44 connected 
therebetween. The motor 42 is energized by the system control 26 over a 
signal carrying line 46 connected to a first output of system control 26 
and may be any suitable drive device capable of being precisely controlled 
for rotational speed, e.g., a stepping motor. 
The first temperature measuring device 14 is connected by a signal carrying 
line 48 to a second input of the system control 26 while the second 
temperature measuring device 17 is connected by a signal carrying line 50 
to a third input of the system control 26. A second output from the system 
control 26 to the electrically control selection valve 6 is applied by a 
signal carrying line 52. A third output from the system control 26 is 
connected by a signal carrying line 54 to a display 56 for displaying the 
calorific content of the combustible fuel gas being tested, e.g., BTU 
content. A keyboard 58 connected to the system control 26 by a signal 
carrying line 60 is provided to supply control signals to the system 
control 26 which is previously mentioned may include a microprocessor 
having a memory for storing control signals as digital words therein. 
MODE OF OPERATION 
In the embodiment of the invention shown in FIG. 1, fuel and air from 
pressure regulators 4, 12, respectively, are fed through a rotary mixing 
valve 8 in which the proportion of air to fuel gas transferred 
therethrough to the primary or measurement combustion chamber 22 depends 
on the speed of rotation of the rotary valve 8. The rotary mixing valve 8 
contains hollow chambers or transfer buckets in a rotor driven by the 
motor 42 which are alternately filled with fuel and purged with air as 
they are rotated past a fixed plate having first slots therein supplied 
with the fuel gas and air for filling the buckets, and second slots for 
receiving the fuel gas and air from the buckets and which are connected to 
the primary and secondary combustion chambers 22, 36, respectively, as 
described more fully hereinafter with respect fo FIGS. 2 and 3. The fixed 
volume slots and buckets are arranged to interact in a manner whereby the 
air flow remains constant and the fuel gas introduced varies with the 
angular speed of the rotor whereby the air-fuel ratio of the mixture 
supplied to the measurement combustion chamber 22 is controlled by the 
rotational speed of the rotor. 
The motor 42 driving the rotor is speed controlled by the system control 26 
to allow for flexibility in adjusting the rotational speed of the rotor 
and, thus, the air-fuel ratio to achieve substantially stoichiometric 
combustion. Specifically, the oxygen sensor 24 detects the excess oxygen 
in the combustion products from the burner 20 and produces a step change 
in its output signal at substantially stoichiometric combustion. The 
output signal from the sensor 24 representative of the detected oxygen 
level is applied via a connecting wire to the system control 26. The 
system control 26 is arranged to respond to the output signal from the 
sensor 24 to produce a first controller output signal on line 46 for 
controlling the speed of the motor 42, and, consequently, the rotor within 
the valve 8. The system control 26 is also arranged to provide a second 
output signal on line 54 representative of the speed of the motor 42 for 
application to a display device 56 for providing display of the calorific 
content of the fuel gas, e.g., the display device 56 may be a digital 
display for displaying the calorific content in BTU's. 
Inasmuch as fuel and air are fed to the measuring or primary burner 20 at 
the same regulated pressure and, if packaged together in a measuring 
instrument, are substantially at the same temperature, this system 
eliminates additional variables relating to differences in temperature and 
pressure which would otherwise have to be compensated for by measuring the 
temperature and pressure in adjusting the calculation. Temperature and 
pressure compensation, of course, may be provided e.g., temperature 
sensors 14, 17 in the fuel and air supply, for applications where the 
maintenance of equal pressure and temperature is not feasible. Because 
precise volumetric measuring by the rotary valve 8 is used, the need for 
compensating for changes in molecular weights or combustibles or for the 
addition of inert components is also eliminated. The calculation of the 
heat content of the combustible gas is made by utilizing a microprocessor 
system operating in accordance with a fixed stored program to solve a 
predetermined equation from a simplified known constant relationship 
between the air-fuel ratio at substantially the stoichiometric point which 
is known from the speed of the sampling or mixing system and the 
corresponding fuel heat content. The results are subsequently recorded or 
displayed in a suitable manner. The aforesaid system, however, has a 
disadvantage in that the combustible fuel gas often contains entrained 
oxygen which provides oxygen for combustion with the combustible gas 
constituents at the burner in addition to the oxygen provided by the air 
supply. Thus, by utilizing the straightforward application of the air-fuel 
ratio, the deviation produced by the entrained oxygen in the combustible 
gas is not accounted for which produces an error in the calorific content 
measurement since the rotary valve speed is faster than that which would 
be required for the same fuel gas without entrained oxygen inasmuch as the 
entrained oxygen displaces fuel gas. 
This error can be corrected by making two measurements of the unknown 
combustible gas. The first measurement is taken in the normal way as 
described above using the calorific content analyzer, and the speed of the 
rotary valve providing the variable air-fuel ratio for substantially 
stiochiometric combustion is controlled and stored in the system control 
26. The second measurement is made when substantially stoichiometric 
combustion is attained with a known amount of air added to the unknown 
combustible gas stream. The true air/fuel ratio for substantially 
stoichiometric combustion can then be computed by the system control 26 
using the following equation. 
##EQU1## 
Where: C.sub.of2 =known concentration of air added to fuel 
f.sub.1 =measured result without C.sub.of2 
f.sub.2 =measured result with C.sub.of2 
T.sub.F =temperature fuel 
T.sub.A =temperature air 
C.sub.o =concentration of oxygen in moist air 
K.sub.r =rotary valve gain constant 
K=air/fuel ratio 
Q.sub.a =air volumetric flow rate 
This equation is derived from an analyzer system model analysis with oxygen 
in fuel and without oxygen in fuel with a final subtraction of the two 
descriptive equations. The problem accordingly reduces to one of 
introducing an accurately measured oxygen concentration into the gas 
stream and producing a substantially stoichiometric combustion of the 
resulting mixture. This problem is solved by adding an additional set of 
transfer buckets to the rotor of the rotary valve 8. 
In FIG. 2, there is shown an exploded pictorial illustration of a rotary 
mixing valve for use with the present invention. The rotary mixing valve 
includes a motor 42 having an output shaft 44 connected to a rotor 62. The 
rotor 62 has surface depressions forming chambers of fluid carrying 
transfer buckets 64 located in its surface with a first group of chambers 
or buckets 64 being located at a first radial distance from the center of 
the rotor 62 and the second group of buckets 66 being located at a second 
radial distance from the center of the rotor 62. A fixed plate or stator 
68, described more fully hereinafter is connected to the motor 42 and is 
used to provide the connections for the pipelines 5, 13, 15, 16 and 32 
shown in FIG. 1. A cover 70 is arranged to cover the rotor 62 and is 
attached to the stator 68 by any suitable means such as machine screws 72. 
Finally, the layered package of the rotor 62, the stator 68 and the cover 
70 is attached to the motor 42 by any suitable means such as machine 
screws 74 to form a layered structure. 
In FIG. 3, there is shown a front view of the face of the stator 68 
adjacent to the face of the rotor 62 having the transfer buckets 64, 66 
therein. As shown in this view, the face of the stator 68 has three groups 
of coaxial slots or grooves therein for supplying and receiving gases. 
Thus, some of the slots receive air or fuel gas supplied to the rotary 
valve 8 for application to the buckets 64, 66 while other ones of the 
slots supply air and gas to the main burner 82 and the flare burner 80 
after receiving the air and gas from the buckets 64, 66. A first group of 
slots 76, 78 are arranged at an outer radius from the center of the stator 
68 while a second group of slots 80 and 82 are arranged at an intermediate 
radius from the center of the stator 68. A third group of slots 84, 86 are 
arranged at an inner radius from the center of the stator 68. Of the 
longer slots 78, 80 and 86 in the face of the stator 68, slots 78 and 86 
encompass approximately 153.degree. while the intermediate slot 80 covers 
approximately 208.degree.. Of the shorter slots 76, 82, 84 in the face of 
the plate 68, slots 76 and 84 cover approximately 49.degree. while 
intermediate slot 82 covers approximately 70.degree.. Thus, the air 
receiving slots 76 and 84 are shorter than either the fuel gas receiving 
slot 86 or the selective air-gas receiving slot 78. Similarly, the main 
burner supply slot 82 is shorter than the flare or secondary burner supply 
slot 80. Internal drillings (not shown) in the stator 68 connect the slots 
76, 78, 80, 82, 84 and 86 to respective ones of the pipelines 5, 13, 15, 
16 and 32. The relationship of the slots 76, 78, 80, 82, 84 and 86 in the 
face of the stator 68 and the groups of transfer buckets 64 and 66 in the 
rotor 62 are shown in FIG. 3 by a phantom representation of the groups of 
buckets 64 and 66. The rotor 62 is urged against the stator 68 to provide 
a substantially fluid-tight seal between the contacting faces thereof 
having the buckets 64, 66 and the slots 76, 78, 80, 82, 84 and 86 therein. 
This seal may be enhanced by coating the contacting faces of the rotor 62 
and the stator 68 with a low friction material, e.g., 
polytetrafluorethylene. 
To make the initial measurement, the selection valve 6 is set to enable 
fuel gas purging of the outer buckets. The total volume of gas transferred 
by each bucket pair is: 
EQU V.sub.T =V.sub.1 +V.sub.2 
Where 
V.sub.1 =buckets 64 volume 
V.sub.2 =buckets 66 volume 
The normal operation of the calorific content analyzer then results in the 
determination of f.sub.1 based on the required rotary valve motor speed 
needed to achieve substantially stoichiometric combustion. After f.sub.1 
has been determined, the selection valve 6 is switched to enable air 
purging of the outer transfer buckets 66. On the other hand, fuel gas 
purging is maintained on the inner buckets 64. The concentration of air 
added to the fuel gas when the rotary valve dumps the transfer buckets 64 
and 66 into the primary or measurement burner 20 is proportional to the 
ratio of V.sub.2 / V.sub.1 +V.sub.2. The concentration of the air added to 
the gas is fixed by the size of the transfer buckets 64, 66 and therefore 
is known by: 
EQU C.sub.OF2 =V.sub.2 /(V.sub.1 +V.sub.2) 
The true calorific content value of the fuel gas can then be determined by 
solving the first equation shown above where f.sub.2 is found by 
determining the rotary valve motor speed required for substantially 
stoichiometric combustion with the added air. In the actual operation of 
the analyzer, the value of C.sub.OF2 would be determined by means of a 
calibration gas of known calorific content. This calibration gas would be 
measured during substantially stoichiometric combustion with and without 
the added air to indirectly determine C.sub.OF2. 
Accordingly, it may be seen that there has been provided, in accordance 
with the present invention a calorific content analyzer having 
compensation means for fuel entrained oxygen.