Additional information about the equation described in the methodology to calculateenergy efficiency of burners
Generally energy efficiency can be defined as the ratio between quantity of input
energy and utilized energy.
In the same manner, energy efficiency of burners in the proposed methodology canalso be calculated by “the quantity of energy which the burners actually utilize forgenerating heat, divided by the quantity of energy input into the burners”.
-“quantity of energy input to the burners” is derived from “total calorific value ofnatural gas input to the burners”-“quantity of energy utilized by the burners for combustion” is derived bysubtracting “the calorific value contained in the exhaust gas” and “the calorificvalue contained in excess air” from “the total calorific value of natural gas
input into the burners”

Therefore, the following equation is applied to calculate energy efficiency of burners
in the proposed methodology.
Calorific value contained in
the excess air

Calorific value contained in theexhaust gas

η=NCVNG - [GwNG * c1 * (T1 - T2) + A0,NG * (m - 1) * c2 * (T1 - T2)]
NCVNG
Total calorific value of natural gas
input to the burners

In case of regenerative burners, energy efficiency of burners approaches to 100%(h=1) if the temperature of exhaust gas and excess air becomes closer to that of theambient temperature by heat exchange between exhaust gas/air and inlet gas/air.
Default values for each parameters of the equation are set by selecting conservativevalues and referring to the values published by the IPCC and the Japan IndustrialStandards.
[Key assumptions for the establishment of default efficiency]
Typical composition of natural gas, according to the published IPCC’s document
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“Properties of CO2 and carbon-based fuels”, contains 98.4% of hydrocarbon (CnHm)
as shown in the following table.
Table Typical composition of natural gas
Ratio

Component

Mol% (dry)Carbon dioxide CO2

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Nitrogen N2

1.1

Methane CH4

94.4

Ethane C2H6

3.1

Propane C3H80.5

Isobutane C4H10

0.1

N-butane C4H10

0.1

Pentanes+ (C5+)

0.2

Hydrogen sulphide (H2S)
Helium (He)0.00040Source: IPCC “Properties of CO2 and carbon-based fuels”
When natural gas is combusted with burners, components of natural gas and air
react and are released as exhaust gas, where hydrocarbons (and other substances)in natural gas are oxidized. Typically, excess amount of air is pumped into thefurnace than the amount which is needed to fully oxidize the natural gas (by a fewpercentage points) in order to avoid incomplete combustion. Therefore, energy istransported out of the furnace in the form of heat contained in exhaust gas andexcess air.
[Calculation of quantity of calorific value contained in the exhaust gas]
Four parameters are required to calculate the calorific value contained in theexhaust gas;
1. Gw: Theoretical volume of wet exhaust gas generated from combustion ofnatural gas [Nm3/Nm3]
2. c1: Average specific heat at constant pressure of wet exhaust gas generated
form combustion of natural gas, at specific temperature of exhaust gas [kJ/ Nm3･
degree Celsius]
3. T1: Temperature of exhaust gas [degree Celsius]
4. T2: Ambient temperature [degree Celsius]
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1. Gw: Theoretical volume of wet exhaust gas from combustion of natural gas
・ Combustion equation (CmHn + (m+n/4)O2 → mCO2 + (n/2)H2O) provides
the theoretical amount of air, dry exhaust gas and wet exhaust gas.
i

Theoretical amount of air = (m+n/4)/0.21
Theoretical amount of air is the minimum volume of air required forcomplete combustion of fuel. Since oxygen is required for combustion,
theoretical amount of air can be derived from dividing the total volumeof oxygen by the ratio of oxygen to air (21% in volume, 23.2% in mass).

ii

Volume of dry exhaust gas = Theoretical amount of air*(1-0.21)+m
Dry exhaust gas is a gas resulting from complete combustion of fuel.
Volume of dry exhaust gas is summation of gases other than oxygen
consumed at combustion and carbon dioxide generated by combustion.

iii

Volume of wet exhaust gas = Volume of dry exhaust gas+(n/2)
Volume of dry exhaust gas is summation of dry exhaust gas and vapor
generated at combustion.

・ Since natural gas usually contains four types of hydrocarbon (CH4, C2H6,C3H8, C4H10), theoretical amount of air, volume of dry exhaust gas andvolume of wet exhaust gas can be calculated from the corresponding
volumes of each hydrocarbon combusted and their composition ratio.
・E.g.
a. Theoretical amount of air = 9.69 m3
b. Volume of dry exhaust gas = 8.68 m3
c. Volume of wet exhaust gas = 10.69 m3
d. Total volume of exhaust gas = Volume of wet exhaust gas + Volume ofgases other than hydrocarbon = 10.71 m3

2. c1: Average specific heat at constant pressure of wet exhaust gas generated form
combustion of natural gas, at specific temperature of exhaust gas
・ Average specific heat at constant pressure of wet exhaust gas generated
form combustion of natural gas can be derived from Appendix Table 2“linear prediction is used for the estimation” of “JIS G 0702:1995 Method ofheat balance for continuous reheating furnace for steel”.
・ Average specific heat of natural gas can theoretically be derived from thespecific heat of four types of hydrocarbon (CH4, C2H6, C3H8, C4H10) andtheir composition ratio.
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・ Although the temperature of exhaust gas temperature of reference burner
is set as 750 degree Celsius in the methodology, the Appendix Table 2 onlyshows data at every 100 degree Celsius. Therefore, theoretical value(1.481kJ/Nm3) is derived from averaging two specific heat values of 700
degree Celsius and 800 degree Celsius.
3. T1: Temperature of exhaust gas
・ Temperature of exhaust gas in the methodology is set in a conservativemanner(pleaserefer“ADDITIONALINFOFORREFERENCEEMISSIONS”)
4. T2: Ambient temperature
・ Annual average ambient temperature in Jakarta is adopted in themethodology.
[Calculation of quantity of calorific value contained in the excess air]Five parameters are required to calculate the quantity of heat contained in exhaustgas;
1. A0: Theoretical amount of air of natural gas [Nm3/Nm3]
2. m: Air ratio for burner
3. c2: Average specific heat at constant pressure of air, at the specific temperatureof exhaust gas [kJ/Nm3･degree Celsius]
4. T1: Temperature of exhaust gas [degree Celsius]
5. T2: Ambient temperature [degree Celsius]
1.

A0: Theoretical amount of air of natural gas
・ Theoretical amount of air of natural gas can be derived with the same
method as the calorific value contained in exhaust gas, which is 9.69m3.

2.

m: Air ratio for burner
・ As a practical matter, air more than theoretical amount is required forstable combustion. The recommended operational value in the manual of
the actual burner or the default value set as 1.05 in case of the reference
burner can be selected in the methodology in a conservative manner.

3.

c2: Average specific heat at constant pressure of air, at the specific temperature
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of exhaust gas
・ Average specific heat at constant pressure of air can be derived from
Appendix Table 2 “linear prediction is used for the estimation” of “JIS G
0702:1995 Method of heat balance for continuous reheating furnace forsteel”.
・ E.g. Average specific heat of air at 750 degree Celsius is almost equal to1.380kJ/Nm3
Rationale for deriving parameters of 4. and 5. is the same as those mentioned above forthe exhaust gas.

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