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M o r e i n f o r m
a t i o n :
w w w . g a s - f o r - e n e r g y . c o m
26 gas for energy Issue 3/2013
REPORTS Gas quality
Natural gas interchangeabilityin China: some experimental
researchby Yangjun Zhang and Chaokui Qin
Because of the difference between gas appliances in current China market and those research targets many
years ago, the well-established index- and diagram-based methods to predict interchangeability cannot be
taken as applicable to natural gases from different sources. 9 cookers and 6 water heaters were sampled and
tested to evaluate response to varying constituents. Some efficiency and CO emission changes were observed.
Results suggested further theoretical analysis and criterion be required to quantitatively define gas
interchangeability.
1. INTRODUCTION
Natural gas industry in China witnessed unprecedent-
edly rapid development in past 15 years. By the end of
2011 overall natural gas consumption increased to 108
billion cubic meters (BCM) annually from 24.5 BCM in
2000 [1]. The large-scale development of gas industry in
China dated back to late 1990s when the first transmis-
sion gas pipeline was put into operation in 1997, carry-
ing 3.6 BCM from Shanxi to Beijing each year. In 2004 a
3900km-long Western Gas (NO.1) pipeline was finished
and its annual transmission capacity was 12 BCM. The
second stage of Shanxi-Beijing pipeline was put into
operation with its capacity 12 BCM in 2005 [2]. Western
Gas (NO.2) pipeline construction was initiated in 2008
and finished in 2012, and its capacity was 30 BCM. Natu-
ral gas from Burma began to supply China in July, 2013.
It has been planned that natural gas from Russia can be
delivered to China by 2018. Table 1 and Figure 1 sum-
marized some distant pipelines in China.
Liquefied natural gas (LNG) began to play an impor-
tant role to satisfy rapidly-increasing demand in south-
B eg in ni ng Ye ar Fi ni sh Ye ar O ri gi n l oc at io n E nd l oc at io n L en gt h (k m) C ap ac it y
(BCM/year)
1 1992 1997 Shanxi Beijing 868 3.6
2 2000 2004 Xinjiang Shanghai 3900 12
3 2004 2005 Shanxi Beijing 918 12
4 2007 2009 Sichuan Shanghai 2206 12
52008 2012 Kazakhstan Shanghai/
Guangdong4895 30
6 2010 2013 Burma Yunnan/Guizhou 1727 12
7 2013 2018 Russia 38
Table 1. Some long-distance pipeline projects in China
Issue 3/2013 gas for energy 27
eastern China. In 2006 the first LNG terminal in Dapeng
(Guangdong) was put into operation, annual capacity
being 3.7 million tons (MMT). From then on several ter-
minals in Shanghai, Jiangsu, Dalian, Zhejiang were put
into operation (as shown in Figure 1), and the total
capacity increased to 24.2 MMT/a. It was expected that
10 terminals would be constructed along Chinese
coasts, and the total receiving capacity would climb up
to 50.9 MMT/a by 2017 [3].
The supply pattern of natural gas in China can be
summarized as “west-originated to east, north-originated
to south, sea-originated to land”. Accompanied with
gradual formation of long-distance pipelines and intro-
duction of LNG, more and more areas will be or have
been faced up with a fact that they are supplied with
gases from different sources. For example, there are 5 gas
sources in Shanghai and 6 gases in Guangdong. In Bei-
jing, there are also 4 sources.
The constituent differences of gases from different
sources may introduce an uncertainty related to perfor-
mance of appliances in end-users. There are as much as
80 gas sources in China (including some potential
sources), and their distribution in gas specification is illus-
trated in Figure 2 and Figure 3. The distribution of 80
gas sources is so wide, and LNG is generally richer thanpipeline natural gas (PNG) and offshore gas (OSG). CH4
vary from 72% to 100%, C2H6 and N2 can account up to
20%. In Chinese national standard GBT13611-2006 [4], it
was prescribed that Wobbe index of natural gas must fall
within 45.67 MJ/m3~54.78 MJ/m3. But no specific limit
with regard to constituent variation was strictly defined.
Figure 2. Properties of gas sources in China
Figure 3. The various gas constituents of gas sources in China
Figure 1. Distribution of gas pipeline in China
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Issue 3/2013 gas for energy 29
present theories so as to justify what can be accepted
when interchanged. In 2010 BP published “Gas inter-
changeability and quality control” to include some experi-
ments incorporating pipeline gas and LNG.
Previous research revealed that all the interchangea-
bility results are based upon experiments according to
realistic gas constituents and gas appliances. Both final
judgment criteria as “Yes” or “No” and testing method
vary from a country to another. Unfortunately, in this field
no systematic research had been carried out and no reli-
able conclusion can be referred in China. On the other
hand, the supply amount of natural gas and appliance
increased dramatically in past decade.
Some experiments were performed in Tongji Univer-
sity from 2011 to measure performance response of gas
cookers and water heaters to different gases. Despite no
well-established conclusion has been arrived, the work
turned out to reflect present status of potential influence
resulting from varying constituents.
3. EXPERIMENT PROCEDURES
3.1 Technical approaches
The gas interchangeability was literally defined as theability of one gas to substitute another one on some
appliances without materially changing the operation of
appliances, including performance and emission, etc. For
a specific appliance, the standards to which it is subjected
always prescribe performance (including efficiency, emis-
sion, safety issues, etc.) in terms of minimum require-
ments. The testing procedures involved are also included.
For example, a gas cooker must have efficiency higher
than 50% (or 55%) and its CO emission (air-free) must be
lower than 500ppm according to relevant Chinese stand-
ards. The performance data should be measured when
fuelled by 2kPa of 12T-0 (Chinese classification number,
equivalent to G20). For a “qualified” cooker which has
efficiency 52% and CO 300ppm when fuelled by gas “A”,
its efficiency drops to 48% while CO emission remains
350ppm when fuelled by gas “B”. It is quite difficult toconclude if gas “B” can substitute gas “A”. In other words,
both objective and subjective issues are involved in the
field of interchangeability research. As shown in Figure 4,
the detailed parameters used to describe performance
also change from one kind of appliance to another. For
atmospheric appliances in China, publicly accepted issues
include lift, flash-back, yellow-tip, and CO emission but
no consideration was taken to efficiency across the world.
In this paper the sampled appliances were domestic
cookers and water heater incorporating atmospheric
combustion. Related standards include “GB 16410-2007
Domestic gas stove” [10] and “GB6932-2001 Domestic gas
instantaneous water heater” [11].
3.2 TEST RIG
3.2.1 Gas blending system
The test gases were supplied by gas-blending system
through which CH4, C2H6, C3H10, C4H10 and N2, CO2, were
blended to ensure exactly the same constituents as test
gases. Also the same Wobbe indexes and CP wereachieved. Gas-blending system including a 5m3 storage
tank was shown in Figure 5. The purities of individual
components involved were as follows: CH4 99.9%, C2H6
99.5%, C3H10 99.95%, C4H10 99.95%, N2 99.999%, CO2
99.6%. After all the individual components were fed into
the storage tank, the mixture remained 3-5 hours while
propeller was working. Then gas was sampled and ana-
lyzed by gas chromatography. When the constituents of
blended gas fell within allowable limitations compared
with test gases (listed in Table 2) [12], and the Wobbe
indexes, heating values, of blended gases differed not
much from those of test gases, the blended gas could be
regarded as identical to test gases.
Figure 4. The relationship between gas appliance test standard and
performance
Figure 5. Gas blending system
PNG
C
3 H 8
C
H
4
N 2
Pressure regulator
Gas meter
Storage tank
C
2 H 6
to Burners C
O
2
C
4 H 1 0
28 gas for energy Issue 3/2013
REPORTS Gas quality
Therefore natural gas interchangeability problem arises in
recent years. Both appliance manufacturers and local
delivery companies show their concern with the problem
if serious combustion-related difficulties may result and if
it is necessary to strict ly limit individual constituents.
2. HISTORY OF GAS INTERCHANGE-
ABILITY RESEARCH: A REVIEW
Technically natural gas interchangeability can be treated
as some kind of extension of gas interchangeability. A
brief review of interchangeability research will be helpful
as how to configure out theoretical and experimental
approach, so as to answer following questions: will con-
stituent difference of gases from different sources lead to
difficulties of various burners? Can appliance manufactur-
ers have enough capacity to deal with such differences?
In 1915, a large-scale survey in US was performed,
finally leading to four kinds of instability phenomena, viz.
lift, flash-back, yellow-tip and incomplete combustion.
On a diagram with primary-air and port intensity as coor-
dinates, four curves represent these limits of instabilities.
It was put forward the relationship between operation
point of a specific appliance and four curves can deter-
mine a margin with which the appliance would operatestably. In 1926 an Italian engineer Wobbe established an
index to evaluate influence of gas properties upon heat
input rating of a burner. In 1927 American Gas Association
(AGA) initiated a 6-year research project, finally leading to
“C index” approach. Shortly soon it was suggested that
flame speed should be included as an influencing factor
in 1934. In 1941 Knoy derived a mathematical formula to
predict interchangeability of liquefied petroleum gas
(LPG). In 1946 AGA laboratory (AGAL) published “Research
Bulletin 36” which put forward three indexes to quantita-
tively describe the degree of unstable phenomena
related with atmospheric combustion. But incomplete
combustion was not considered due to some technical
reasons. The gas burners used by AGAL were made of
cast-iron, with round-ports and ribbon-ports. In 1951
Weaver published a 6-index method to predict inter-changeability, after analyzing relationship between flame
speed and experimental results of AGAL. Basically Weaver
indexes represent the relative tendency of unstable com-
bustion when interchanged.
A research project presided by Delbourg (Gaz de
France, GDF) was initiated in 1950 and a diagram-based
method was established to predict interchangeability of
manufactured gas, natural gas-air mixture, propane-air
mixture, etc, in 1956. Combustion Potential (CP) was put
forward to represent influence of inner cone height and
its impact upon lift, flash-back and CO emission. On a
diagram with corrected Wobbe index and CP as inde-
pendent coordinate, Delbourg triangle defines an area
within which a specific burner can operate satisfactorily.
Two additional indexes were combined to judge if soot-
ing and yellow-tip would be encountered.
Another diagram-based method was put forward in
1956 by Gilbert and Prigg to consider constituent
changes resulting from manufactured gas process and
raw materials. On G-P diagram flame speed was abscissa
and Wobbe index was y-axis, both of which can be cal-
culated directly from gas constituent. Four groups of
gases, G4, G5, G6, and G7 were experimentally tested on
typical burners to establish limits of various gases. In
1964 Harris and Lovelace put forward a modified dia-
gram to take into account introduction of natural gas
into England and future potential of substitute natural
gas (SNG). Later in 1978 their approach was refined by
Dutton and allowable areas for long-term and short-
term operation were experimentally determined. Dut-
ton method is still adopted to decide whether a gas can
be introduced into networks in England.
In 1980s Harsha, P. T. [5] pointed out that applicability
of multi-index methods remained to be further clarified
since most of these approaches were based upon experi-
mental results from appliances popular at the time when
experiments were carried out. In 1990s Liss, W.E. [6] sug-
gested that interchangeability research should beemphasized again to evaluate the diversity of natural
gases including LNG and coal-based natural gas. Ted A.
Williams [7] systematically summarized interchangeabil-
ity researches available and concluded that the main tar-
get of research should be transferred from natural
gases—manufactured gas to constituent differences
between natural gases from different sources, such as
pipeline gas and LNG.
In 2003 National Petroleum Committee (NPC) pub-
lished a report “Balancing Natural Gas Policy-Fueling the
Demands of a Growing Economy” to call for necessity to
refresh interchangeability method so as to introduce non-
conventional gas. In Europe EASEE-gas published EASEE-
gas CBP 2005-001/02 to put forward Common Business
Practice in regard to gas specifications. Halchuk-Harrington
et al [8] recommended that the concept of “interchangea-bility” should be extended to include ALL appliances,
including gas-turbine, gas-engines. Furthermore it was
pointed out that most researches available were based
upon fuel gas rather than natural gas, targeting at estab-
lishing standards for peak-shaving plants and blending
plants. In addition natural gas considered in previous
research was apparently different from current gases, and
only a small portion of gases had similar constituents with
today’s natural gas and LNG. In 2009 Ennis C.J. et al [9] com-
pared gas constituents and interchangeability methods in
US with those in Europe. Test gases were compared to help
understand interchangeability approaches and their appli-
cability. It was concluded that it essential to re-evaluate
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4. RESULTS AND ANALYSIS
4.1 Gas cooker
4.1.1 Heat input rating
Measured heat input rating of all sampled cookers under
different gases was shown in Figure 8(a). It can be foundthat heat input rating increases almost linearly with the
increasing Wobbe index for all cookers. This suggested
the heat input rating of cookers change consistently with
Wobbe index, and it also can be precisely predicted by
Wobbe index; on the other hand, it also means that
experiment results are quite accurate and reliable.
4.1.2 Thermal efficiency
Figure 8(b) shows the change of thermal efficiency
with varying gas constituents. Obviously the change of
thermal efficiency of gas cookers does not give any
regular pattern.
According to Ref [10], thermal efficiency of gas cooker
must be measured by use of both upper-limit pot and
lower-limit pot according to by heat input rating. Then the
thermal efficiency is calculated by interpolation of heat
intensity in terms of bottom area, viz. kJ/mm2. In fact such
procedure considers the flame and heat transfer in a more
objective way. However it tends to make the continuously
changing issue discrete. Secondly, the specific process ofcombustion and heat transfer were greatly influenced by
burner structure and those gas constituents which will
seriously affect flame characteristics, such as flame lumi-
nosity, height and speed. That is the reason why in most
published papers thermal efficiency was not taken as a
quantitative index to evaluate interchangeability in China.
It was prescribed in Ref [10] that thermal efficiency
must not be lower than 55% (for on-top cooker) or 50%
(for embedded cooker). From the experiment results it
can be concluded that with the change of gas constitu-
ents, thermal efficiency of all samples are not very satis-
factory. For all 9*10=90 operation points, only 75% (67
operation points) can be considered as “qualified” in
Figure 8. Measured performances of gas cookers when fuelled by different gases:
(a) heat input rating, (b)thermal efficiency, (c) CO emission, (d) NOx emission
30 gas for energy Issue 3/2013
REPORTS Gas quality
3.2.2 Test system
According to Ref[10] and Ref[11], a gas cooker test system
and a water heater test system, as shown in Figure 6 and
Figure 7 respectively, were set up to measure heat input
rating, thermal efficiency, flame shape and pollutant
emissions.
3.3 Test gas constituents and sampled gas
appliances
Three diffe rent types of natural gases, namely OSG,
PNG and LNG, will be introduced into Guangdong
networks successively from 2009 to 2020. Accord-
ingly 10 gases which have been presently available
and are planned were selected as test gases in this
paper. The detailed constituents of 10 test gases
were listed in Table 3, among which PNG1 has the
lowest Wobbe index and LNG5 has the highest
Wobbe index, and PNG3, LNG2 similar to 12T-0 (G20)
were in the middle of all gases.
9 sets of gas cookers covering 9 different brands
and three types of ports (namely, round, square, rib-
bon), and 6 sets of water heaters covering 3 differentbrands were selected as representatives of popular
burner structures in Guangdong. The popular struc-
ture of gas cookers in China markets includes injector
made of cast-iron, diffusion/distribution head made of
aluminum or cast-iron, together with burner head
made of casting copper alloys. Furthermore port inten-
sity of Chinese gas cooker was usually designed to be
7.0~9.0 W/mm2, much narrower than its US counter-
part in 1950s (4.5~16.8W/mm2) [5].
Component range
(mole %)
Reproducibility (%) Accuracy (%)
0~0.1 0.01 0.02
0.1~1.0 0.04 0.07
1.0~5.0 0.07 0.10
5.0~10 0.08 0.12
>10 0.20 0.30
Table 2. Accuracy and reproducibility of gas chromatography
Figure 6. Illustration of gas cooker test rig
Figure 7. Illustrative schematic of water heater testing rig
Gas storage tank gas meter
aluminum pot
flue gas measurement
thermometer
U-shape pressure gauge
gas cooker
thermometer
stirrer
sample ring
gas stoarge tank gas meter
water heater
flue gas measurementU-shape pressure gauge
Mole% CH4 C2H6 C3H8 C4H10 CO2 N2 HV (MJ/m3) WI (MJ/m3)
PNG1 96.00 0.70 0.20 0.10 2.30 0.70 37.04 48.3
PNG2 92.79 4.00 0.30 0.30 1.80 0.80 38.39 49.4
PNG3 98.90 0.20 0.22 0.20 0.00 0.46 37.70 50.7
OSG1 85.99 9.61 0.20 0.00 3.60 0.60 39.10 48.8
OSG2 84.40 4.70 2.40 5.20 0.90 2.40 43.84 52.4
LNG1 98.50 0.00 0.30 0.10 0.00 1.10 37.63 50.1
LNG2 97.00 1.90 0.30 0.10 0.00 0.70 38.30 50.6
LNG3 90.70 7.50 0.30 0.10 0.20 1.20 39.68 51.1
LNG4 89.40 6.00 3.20 1.10 0.00 0.30 42.12 52.9
LNG5 86.60 9.00 2.90 0.90 0.10 0.50 42.53 53.0
Table 3. Constituents and properties of natural gases for Guangdong test
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terms of efficiency, but there is no one gas cooker which
can give “qualified” thermal efficiency under all tested
gases. In other words, a lot of works remain to be finished
for the manufacturers before a cooker with enough flexi-
bility becomes available.
4.1.3 CO emission
From the very beginning of interchangeability CO emission
has been a significant issue to characterize appliance perfor-
mance. Usually there are clearly defined limits on CO emis-
sions for safety reasons. Figure 8(c)
illustrates CO emission of all samples
under different gases. The change of
CO emission with the gas constituent
doesn’t follow any regular pattern. A
slightly increasing trend of CO emission
with increasing Wobbe index was
observed, though the values of CO for
most samples fell below 500ppm. In Ref
[10] it is prescribed that CO emission (air-
free) must not be higher than 500ppm.
And it can be found that 6 sam-
ples can maintain lower CO emission
under all tested gases, accounting for
67%. For all 90 operation points, 78
operation points (about 87%) can be
considered as “qualified” in terms of
CO emission. So it can be concluded
that well-adjusted initial state of gas
cookers can be flexible enough not tomaterially increasing CO emission.
4.1.4 NOx emission
Figure 8(d) shows the NOx emission
of samples under different gases.
NOx emission of sampled cookers is
found to increase with increasing
Wobbe index, but NOx can be lower
than 90ppm under most test gases.
Anyway currently there is no manda-
tory requirement to specify the
allowable NOx emission.
4.1.5 Lift
For natural gas interchangeability
research, lift has always been a focus
issue due to the potential incomplete
combustion or even explosion haz-
ard. In Ref [10] lift is visually checked
immediately 15s after ignition. The
experiment results are shown in
Table 4. It can be found that lift tends
to happen to samples when fuelled
by lean gas such as PNG1, PNG2, OSG1
and LNG1. This can be attributed to
the smaller flame speed resulting
from higher inert contents.
C-A-1
PNG2 LNG1 LNG2
C-H-1
PNG1 PNG2 OSG1
C-D-1
PNG1 PNG2 PNG3
LNG1 LNG2 OSG1
C-I-1
PNG1 PNG2 PNG3
LNG1 OSG1
Table 4. Test results of lifting for gas cookers
Issue 3/2013 gas for energy 33
4.2 Water heater
As illustrated in Figure 9(a) heat input rating of all sam-
pled water heaters were found to increase linearly with
Wobbe index increment. Compared with gas cookers,
the increasing degree of input rating is somewhat differ-
ent, though Wobbe index can also be used to predict
changing trend of heat input rating. Thermal efficiency measurement results were
shown in Figure 9(b), it can be found that variations of
efficiency remain within 4~5 percentage points. It was
specified that efficiency of third-/second-/first- class
water heater should not be lower than 84%, 88% and
96%, respectively. Two condensing water heaters
(W-A-1 and W-C-2) gave efficiency higher than 96%
and can be labeled as “qualified” under all 10 gases.
Efficiency of W-C-1 dropped below 88% under PNG1
and LNG1. The remaining 3 water heaters can give effi-
ciency higher than 88%.
As shown in Figure 9(c), only 5 measured points were
observed to give CO emission above 600ppm (allowable
highest concentration) which is specified by Ref [11]. All
the other 4 water heaters can be termed as “qualified”
under 10 gases, accounting for 67%.
NOx emission did not give any regular pattern with
change of gas constituents. Except for W-B-1 gave a
90ppm+ under LNG4 and LNG5, all the other water heat-
ers can keep an emission lower than 90ppm. An apparent
trend can be found that NOx emission increase withincreasing Wobbe index of gases.
5. CONCLUSIONS
Most interchangeability research available was per-
formed by means of experiments on appliances popular
at the time when the research was done. Because the
structure, material, designing parameters of burners in
current Chinese market differ much from those gas appli-
ances almost 50 years ago, it is quite doubtful whether
the well-established prediction methods (index- or dia-
gram-based) can be applicable to present Chinese gases
from different sources.
Figure 9. Measured performances of water heaters when fuelled by different gases:
(a) heat input rating, (b)thermal efficiency, (c) CO emission, (d) NOx emission
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REPORTS Gas quality
9 gas cookers and 6 water heaters were experimen-
tally tested and heat input rating, thermal efficiency, CO
and NOx emission were measured when fuelled by 10
gases which have been/will be supplied in Guangdong.
The testing procedure and facilities involved strictly fol-
low related national standards. The conclusions can be
summarized as follows:
(1) The performance of domestic appliances (including
cookers and water heaters) are found to change with vari-
able gas constituents. None of the 9 cookers can give satis-
factory thermal efficiency under all 10 gases investigated.
1/3 of cookers were found to emit higher concentration of
CO and 44.4% of sampled cookers would have lift flame
under low Wobbe index gases. Only 3 cookers can keep
“qualified” in terms of both CO emission and lift, even if
thermal efficiency was not taken as a necessary measure-
ment index. 3 out of the 6 sampled water heaters can
maintain second-class efficiency under all 10 gases while
giving “qualified” CO emission. Half of sampled water heat-
ers would degrade with changing gas constituents.
(2) If a gas cooker was well-adjusted at the beginning
of measurement, CO and NOx emission would not change
materially. And most sampled appliances can tolerate the
variation range of constituents. Gas cookers tended to
decrease efficiency when gas constituents were changed,and more likely to become “unqualified”. For water heaters
tested, thermal efficiency could be maintained within a
range permitted by administrative requirement, and it
seemed no serious problem would occur.
The criterion as to decide whether a gas can be substi-
tuted by another one on a specific appliance depends on
both the standard to which the appliance is subjected and
the allowable margin, the latter of which is not fixed in terms
of quantitative indexes around the world. The work in this
paper can be considered as an initiating point to explore what
would happen to domestic appliances if the gas constituents
change to certain extent in China. Both theoretical analysis
and experiments are needed to further reveal applicable
index- or diagram-based method for Chinese gas industry.
REFERENCES
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[11] General Administration of Quality Supervision,
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AUTHORS
Yangjun Zhang, Ph.D. candidate
Gas Research Institute
China | Tongji University
Phone: +86 21 69583802
E-mail: [email protected]
Prof. Dr. Eng. Chaokui Qin
Gas Research Institute
China | Tongji University
Phone: +86 21 69583802
E-mail: [email protected]