1235

8.8 Calorimeters

P. FOUNDOS

(1982)

G. LIPTÁK

(1995)

D. LEWKO

(2003)

Type of Designs:

A. Direct measurement by burning of fuel gasB. Inferential by calculation from composition and physical analysis, including chro-

matography (Section 8.12), mass spectrometry (Section 8.29), etc.C. Special designs such as reaction calorimeters of Mettler-Toledo, designs for the

measurement of partial molar heat capacities of biopolymers by CSC, and totalabsorption calorimeters made by Opal

Applications:

1. Custody transfer2. Process monitoring and control3. Blending and mixing of fuel gases4. Gas and liquefied natural gas (LNG) processing5. Compliance recording

Operation:

a. Continuousb. Cyclicc. Portable

Performance:

(1) Controlled environment(2) Varying ambient(3) High speed of response(4) Inaccuracy

±

0.5% of full scale or better(5) Inaccuracy

±

1.0% of full scale or better(6) Inaccuracy

±

2.0% of full scale or better

Area Classification:

(a) General purpose(b) Explosion-proof

Cost:

Under $10,000 [A, 2/3, a, (3)/(5)/(6)]$10,000 to $15,000 [A, 2, a, (3)/(5)/(6), (b)]$15,000 to $30,000 [A/B, 2/3, a/b, (1)/(2)/(4), (a)/(b)]

Partial List of Suppliers:

Ametek Process & Analytical Instruments (www.thermox.com)Cosa Instrument Corp. (www.cosa-instrument.com)Daniel Measurement and Control (www.danielind.com)Delta Instrument LLC (www.deltainstrument.com)EG & G Chandler Engineering/Ranarex (www.chandlerengineering.com)Galvanic Applied Sciences (www.galvanic.ab.ca)

INTRODUCTION

This section does not discuss the special calorimeter designssuch as reaction calorimeters (Mettler-Toledo), designs for themeasurement of partial molar heat capacities of biopolymers

(CSC), or total absorption calorimeters (Opal), but only directsthe reader to the web pages of their respective suppliers.

Inferential calorimeters, which determine the heat contentby calculation from composition and physical analysis, includ-ing chromatography (Section 8.12) and mass spectrometry

ATI

HeatValue

Flow Sheet Symbol

AIE

© 2003 by Béla Lipták

1236

Analytical Instrumentation

(Section 8.29), are not covered in detail either, because theyare discussed in the above-noted sections.

Calorimeters are analyzers that measure the heat value orenergy content of gaseous fuels. There are two broad cate-gories of this type of instrument: those that can be consideredtrue calorimeters, because they are actually burning the gasand directly measuring its heating value, and inferential cal-orimeters, which analyze the composition of the gas or mea-sure a physical parameter to determine the heating value.

TERMINOLOGY

Basic terms and definitions used in gas calorimetry are givenhere and may also be found in the references listed at theend of this section. A summary of the measurement calibra-tion techniques is outlined in Table 8.8a.

British Thermal Unit

A British thermal unit (BTU) is the amount of heat requiredto raise the temperature of 1 lb of water by 1°F at or near 60°F.

BTU Dry

This is the heating value that is expressed on a dry basis. Thecommon assumption is that pipeline gas contains 7 lb (orless) of water vapor per million standard cubic feet (SCF).

BTU Saturated

This is the heating value that is expressed on the basis thatthe gas is saturated with water vapors. This state is definedas the condition when the gas contains the maximum amountof water vapors without condensation, when it is at basepressure and 60°F.

Combustion Air Requirement Index

The combustion air requirement index (CARI), a dimension-less number, indicates the amount of air required (stoichio-metrically) to support the combustion of a fuel gas. Mathe-matically, the CARI is defined by Equation 8.8(1):

8.8(1)

Gross Calorific Value

This is the heat value of energy per unit volume at standardconditions, expressed in terms of BTU per SCF, kilocalorieper cubic Newton meters (Kcal/N.m

3

), or other equivalentunits.

Net Calorific Value

This the measurement of the actual available energy per unitvolume at standard conditions, which is always less than thegross calorific value by an amount equal to the latent heat ofvaporization of the water formed during combustion.

Wobbe Index

American Gas Association (AGA) 4A defines the WobbeIndex as a numerical value that is calculated by dividing thesquare root of the relative density (a key flow orifice param-eter) into the heat content (or BTU/SCF) of the gas. Mathe-matically, the Wobbe Index is defined by Equation 8.8(2):

8.8(2)

The Wobbe Index accounts for composition variations interms of their effect on the heat value and specific gravity, whichaffect the flow rate through an orifice. In essence, the WobbeIndex is a measurement of the available potential heat, and itcan be used in conjunction with the gas flow measurement toproduce a measurement of heat flow rate (see Section 2.5).

In the following paragraphs, some basic terms and gascalorimeter design variations are described. The AmericanSociety for Testing Materials (ASTM) Standards listed in thereferences serve to compliment this information.

UNITS, ACCURACY, AND OUTPUT SIGNALS

When calorimeters are used for custody transfer of naturalgas, the unit of measurement is often the BTU. The NaturalGas Policy Act of 1978 (15USC 3311, Supplement 1981)established the BTU as the basic measurement of natural gasfor pricing purposes, supplanting the traditional volume basesmeasurement. As a result, the market demand for custodytransfer type calorimeters increased markedly. In Europe andin other countries where the metric system is used, naturalgas calorimeters are calibrated in mega-Joule units.

The response time can be a critical consideration inselecting the right analyzer for closed-loop control applica-tions. On the other hand, for custody transfer applications,one should maximize accuracy even at the expense ofresponse time, while improved response time even at theexpense of accuracy is justified in some critical process con-trol applications.

The analog output of the analyzer may represent the grosscalorific value (sometimes referred to as upper heating valueor gross heating value), the net calorific value (sometimesreferred to as lower heating value or net heating value), orthe Wobbe Index.

CARIAir/Fuel Ratio

s.g.=

Wobbe indexcalorific valuespecific gravity

=

© 2003 by Béla Lipták

8.8C

alorimeters

1237

TABLE 8.8a

Summary of Calorimeter Features and Specifications

Type

Type Application

a

Operation Performance

Area Class

1 2 3 4 5 Con

tinu

ousl

y

Cyc

lic

Stan

dard

Sam

ple

Em

piri

cal

Cal

ibra

tion

Am

bien

tL

imit

s

°

F (

°

C)

Loc

al R

eado

ut

Rem

ote

Tran

smit

ters

Ran

ge i

n B

tuF

ull

Scal

e

Acc

urac

y ±

%of

Ful

l Sc

ale

Spee

d of

Res

pons

e (9

0%)

Dir

ectt

Infe

rent

ial

Gen

eral

Pur

pose

Ex-

Pro

of

GROSS CALORIFIC VALUE

Water

∆

T

� � � � � �

72–77(22–25)

� �

130–3300 0.5 3 min

Air

∆

T

� � � � � �

72–77(22–25)

� �

120–3600 0.5 15 min

Gas Chromatograph

� � � � � � � � �

0–100(–18–38)

�

Any 0.5 10 min

Adiabatic Flame Temperature

� � � � � � �

N/A

� �

N/A 0.5 N/A

NET CALORIFIC VALUE

Airflow Calorimeter � � � � � � � � �

50–90(10–32)

� �

130–3300 1.0 8 sec

Gas Chromatograph � � � � �

0–128( − 18–53)

�

Any 0.5 10 min

Expansion Tube Calorimeter � � � � � �

N/A

� �

120–3300 1.0 3.5 min

Specific Gravity � � � � � �

0–128(–18–53)

� �

Varies 2.0 N/A

Process Chromatograph � � � � �

60–90(16–32)

� �

150–3600 2.0 4.5 min

Thermopile Calorimeter

� � � � � � � N/A

�

150–3300 2.0 55 sec

WOBBE INDEX

Airflow Calorimeter

� � � � � � � �

50–110(10–43)

� � 130–3300 0.75 8 sec

Gas Chromatograph

� � � � � � � � �

0–120(

−

18–49)

� Any 0.5 10 min

Expansion Tube Calorimeter

� � � � � �

N/A

� � 120–3300 1.0 3.5 min

Thermopile Calorimeter

� � � � � � �

N/A

� 150–3300 2.0 55 sec

a

See feature summary at begining of section.

© 2003 by Béla Lipták

1238

Analytical Instrumentation

DESIGN VARIATIONS

In the following paragraphs, a brief description is given ofthe various gas calorimeter designs that are used to detectthe heating value of gaseous fuels and waste gases.

Water-Temperature-Rise Calorimeter

One variation of this design is illustrated in Figure 8.8b. Herea constant (capillary-controlled) flow rate of the gas is mixedwith a constant flow rate of oxygen (or air) and is burned.The resulting heat of combustion is removed by a constantflow rate of water. Both the flow rates and the temperaturesof the entering streams are controlled at a constant value.

Thermistors measure the temperature of the water enteringand leaving. The temperature difference or temperature rise istherefore a direct measure of the heating value of the burnedgas. This calorimeter can be compensated for the variationsin barometric pressure and can therefore measure the grosscalorific value of fuel or waste gases.

Figure 8.8b also shows a calibration heater. During thecalibration cycle this electrical resistance heater is turnedon while the gas flow is off. Thereby a known accuratelydetermined amount of heat is introduced and the correspond-ing temperature rise on the water side is measured. When thetemperature rise is the same as it was when gas was being

burned, the amount of electric heat introduced matches theheating value of the gas. This type of calibration can be madeat any time and for any reading of the calorimeter.

Air-Temperature-Rise Calorimeter

The measurement is accomplished by continuously transfer-ring all the combustion heat of a metered quantity of gas toa metered quantity of air (see Figure 8.8c). The temperaturerise of the air is measured and is related directly to grosscalorific value of the gas. The unit can be modified so thatthe heat-absorbing air is not separated from the products ofcombustion, thus resulting in a more accurate measurementof the net calorific value.

Airflow Calorimeter

In this design the variations in the heat released by the con-tinuous burning of the fuel gas are offset by a continuous,varying airflow that maintains the temperature of the productsof combustion constant (see Figure 8.8d). Thus, the airflowis correlated to the heat value of gas or to the Wobbe Index.With the addition of a constant-volume gas-metering pump,and compensating for specific gravity variations, the instru-ment can be calibrated for the net calorific value.

FIG. 8.8b

Water-temperature-rise-type calorimeter provided with electric heater for direct calibration.

1

Water PressureRegulator

Water Inlet

CalibrationHeater

WaterCapillary

SolV

SolV

SolV

SolV

SolV

Water InletThermistor

Water HeaterGas Capillary

Oxygen

Oxygen Pressure Regulator

Oxygen Heater

Water Overflow Non-Condensible Gas Vent

Water Outlet Thermistor

Combustion Chamber Shell

Ignition Electrode

Insulation Jacket

CalibrationHeater

Controller

Gas Pressure Regulator

Gas Inlet

Gas Capillary

Gas Heater

MixedGas Tube Sol

V

© 2003 by Béla Lipták

8.8 Calorimeters

1239

Residual Oxygen Calorimeter

In this design, a continuous gas sample is mixed with dry airat a precisely maintained ratio. The ratio is dependent on theBTU range of the gas to be measured. The fuel–air mixture

is oxidized in a combustion furnace, and the oxygen concen-tration in the spent sample is measured using a zirconia oxidecell. With the addition of a precision specific gravity cell, theanalyzer can be calibrated to provide a measurement corre-sponding to the calorific value of the gas.

FIG. 8.8c

Air

∆

T calorimeter.

FIG. 8.8d

Airflow calorimeter.

Gas Vent toOutside

ThermocoupleTube

PressureRegulator

GaugeLocation

Inlet OrificeSight Plug

InletOrifice

Bleeder BurnerFlame

SafetyShut-OffValve

BleederGas

MeteredGas

Piping

PrimaryArt

OrificeCap

Secondary AirOrifice Cap

Combustion AirMeter Orifice

(9)Outgoing Air Thermometer

Combustion AirMeterCombustion

AirOverflow Weir

GasMeter

Overflow WeirDrainpipe

DrainManifolds

Gas andPrimary Air

Secondary Air

Heat Exchanger

Main Burner Flame

(A)Incoming Air Thermometer

Heat Absorbing Air Outlet Connector

Heat Absorbing AirTank Water Level

Heat Absorbing Air Meter

Productsof

Combustion

CondensatePan

Burner Water Seal WaterPump

AuxiliaryTank

Supply

Flapper/Nozzle

Thermal Expansion Element

Products of Combustion

Heat Exchange Air

AirOrifice

Air ControlValve

Burner

GasOrifice

CombustionAir

Adjustment

D/PTransmitter

Safety ShutdownSolenoid

PrecisionGas

Regulator

Gas

M

Air Supply

Calorific Value-CV

Gas

MixingBaffle

Ambient Compensating Tubes

Wobre Index = cv2g√

© 2003 by Béla Lipták

1240

Analytical Instrumentation

Chromatographic Calorimeter

A conventional chromatograph can also be used to analyzegas composition (see Section 8.12), and a microprocessorcan be used to calculate the heating value and specific gravityof the gas from empirical data held in memory by the micro-processor. This information can be used to calibrate for thegross or net calorific value of the Wobbe Index.

Expansion Tube Calorimeter

In this design, the gas sample is delivered through a precisionregulator system (Figure 8.8e), which is independent of spe-cific gravity and atmospheric pressure. The gas is burned atthe base of a differential expansion tube unit that respondsto the temperature of the products of combustion and excessair. The differential signal is calibrated as the net calorificvalue. Modification of the regulator to respond to specificgravity changes allows calibration to the Wobbe Index.

Adiabatic Flame Temperature

The gross calorific value of fuel gas is proportionate to theratio of air to a fuel that maximizes the adiabatic flame tem-perature of the mixture. Therefore, in this design, flows arecontrolled and two burners are used to obtain the mathematical

derivative of the flame temperature composition that allowscalibration for gross calorific value.

Thermopile Calorimeter

The thermopile calorimeter measures the temperature of thehot products of combustion mixed with a constant volume ofair supplied by a fan. The sample to the burner is providedwith an orifice bypass, which is needed for specific gravitycompensation; thus, the resulting measurement is in terms ofnet calorific value. To measure the Wobbe Index, the bleedis blocked and the sample goes through the burner.

APPLICATIONS

There are five general areas of application:

1. Custody transfer: Sale or purchase of fuel gas withaccuracy being the primary consideration.

2. Process monitoring and control: To effect on-linemanual or automatic control of process or efficientburning of the gas by using heat value measurementas one of the measured variables. Such applicationsinclude feed-forward control of fuel gas-fired heatersor boilers, stove-firing control, vaporizer control, andsynthetic natural gas (SNG) reactor control. In theseapplications, measurement of heat value is importantas a process measurement for efficient control of energyconsumption, or to limit the flow of heat to an energy-sensitive process. This application requires high speedof responses as well as reliability to achieve effectivecontrol.

3. Blending and mixing of fuel gas: To obtain a uniformquality gas or to utilize waste or by-product gases byblending them into the main fuel gas. Blending is oftenused to achieve a desired ratio of streams such aspropane/air or blast furnace gas/coke oven gas. How-ever, when by-product or waste gases are to be injectedinto a stream and they vary significantly in quantity,monitoring of the mixture is necessary to make properuse of the fuel. Speed of response and reliability areessential.

4. Processing of gas and liquefied natural gas (LNG)operations: LNG must be vaporized and conditionedfor efficient consumption. Coke oven and blast furnacegases are the main fuels used by the steel industry, andrefinery gas is a major by-product that is used in petro-leum refining. All these fuel gases are produced inspecific processing operations that require monitoringand conditioning for efficient use.

5. Compliance recording: For government-regulatedenergy transfer by pipeline and utility distributors tothe consumers. Accuracy and traceability are essentialcriteria.

FIG. 8.8ePrecision regulator used for expansion tube-type calorimeter.(Courtesy of Cosa Instrument Corp.)

Air Flow

First Regulator

Pressure Set at1 Inch (25 mm) H2O

SampleInlet

Air Breather

Pressure Approx.Atmospheric

Sample Flowingto Expansion TubeCalorimeter Burner

Air Breather

PrecisionOil-SealedBell-Regulator

© 2003 by Béla Lipták

8.8 Calorimeters 1241

SAMPLE CONDITIONING

The requirements for the sample conditioning system aredependent on the limitations of the selected analyzer and theminimum, normal, and maximum limits of the process streambeing monitored. In many industrial applications, fuel gasesare generated as by-products of other processes, and these“off-gases” can be extremely dirty and require sample con-ditioning. In some cases, special consideration must be givento the dew point of the off-gas when designing the sampleconditioning system. Refer to Sections 8.2 and 8.3 for samplesystem options and design features.

CONCLUSION

Gaseous fuel energy is a costly commodity that is beingconsumed with much more care and efficiency than in thepast. The key to its efficient consumption is measuring theavailable heat of the fuel gas. There are several calorimetersavailable for reliable on-line measurement of gas heatingvalues. For closed-loop control applications, the designs thatprovide a speed of response of less than a minute (Table 8.8a)are preferred.

In addition, there are several manufacturers that offercalorimeters specifically for custody transfer applications.These analyzers offer improved accuracy at the expense ofresponse time (Table 8.8a).

Reference

1. Christopher, D. E., “Direct Energy Measurement,” Measurements andControl, December 1991.

Bibliography

AGA, Report 3.AGA, Report 8.AGA Gas Measurement Manual, Section 11A.2, “Determination of Heating

Value of Gas,” p. 11A2.1.Armstrong, G. T., “Standard Combustion Data for the Fuel Gas Industry,”

AGA, 1972.

ASTM Standard D 900-55(70), Calorific Value of Gaseous Fuels by theWater Flow Calorimeter, Philadelphia: American Society for Testingand Materials, 1970.

ASTM Standard D 1826-77, Test for Calorific Value of Gases in Natural GasRange by Continuous Recording Calorimeter, Philadelphia: AmericanSociety for Testing and Materials, 1977.

ASTM D 1945, “Analysis of Natural Gas by Gas Chromatography,”Philadelphia: American Society for Testing and Materials.

Bowles, E. B., “Small Errors in BTU Measurement Can Add Up to LargeLosses in Revenue,” Pipe Line & Gas Industry, November 2000.

Broadwater, S. R., “Columbia Gas Moves toward Heating-Value Measure-ment,” Oil and Gas Journal, August 25, 1980.

Christopher, D. E., “Direct Energy Measurement,” Measurement and Con-trol, December 1991.

Distribution Conference 72-D-76, American Gas Association, Arlington, VA,1972.

Foundos, A. P., “Measuring Heat Release Rate from Fuel Gases,” Instru-mentation Technology, Instrument Society of America, 1977.

GPA, “Calculations of Gross Heating Value, Relative Density and Com-pressability Factor for Natural Gas Mistures from CompositionalAnalysis.”

GPA, “Standard 2145-Table of Physical Constants for Hydrocarbons andOther Compounds of Interest to the Natural Gas Industry.”

GPA Reference Bulletin, “Heating Value as a Basis for Custody Transfer ofNatural Gas,” 1984.

Green, D. and Perry, R. H., Perry’s Chemical Engineer’s Handbook, 6thed., New York: McGraw-Hill, 1984.

Hawkins, J. and McGowan, A., “Theoretical Introduction to the Use of aResidual Oxygen Measurement Method for the Analysis of Combus-tion Air Index (CARI) and the Wobbe Index of Fuels,” ISA Chicago,October 2002.

Kizer, P., “Natural Gas Energy Determination Review,” ISA Proceedings,1991.

Kizer, P., “Energy Measurement Using On-Line Chromatography,” in 71stInternational School of Hydrocarbon Measurement,” 1996.

Kizer, P., “Operation of On-Line Gas Chromatographs,” in American Schoolof Gas Measurement, 1998.

Lange, N. A. and Forker, G. M., Handbook of Chemistry, 10th ed., NewYork: McGraw-Hill, 1967, pp. 842–843, and Columbia Gas SystemData.

Larson, B., “Heating Value Technologies for 2001,” in Winnipeg CGA GasMeasurement School, June 5, 2001.

Lide, D. R., Handbook of Chemistry and Physics, 71st ed., 1990–1991.McCoy, R., “BTU Determination by Process Chromatograph,” Applied

Automation, Inc., a subsidiary of Phillips Petroleum.Melrose, D. C., “Comparison of Calculated and Measured Heating Value of

Natural Gas,” presented at AGA Distribution Conference 72-D-2,American Gas Association, Arlington, VA, 1972.

Pannill, W. and Sharples, R. J., “Calculation of Gas Heating Value Is Com-plicated by the Courts,” Oil & Gas Journal, July 2, 1984.

© 2003 by Béla Lipták