3.1. Validation of Dutton’s ICF Index
When gas constituents change, the operation of gas cookers, namely the primary air coefficient α′ and heat input Q, will be changed from (α′,Q)1 to (α′,Q)2. When investigating if a three-component mixture can reproduce the same amount of CO as that fueled with original gas containing CH4, C2H6, C3H8, C4H10, C5H12, CO2, and N2, the “equivalence” should be evaluated for all possible primary air coefficients and heat inputs corresponding to gas constituents’ variations. Hence, the relationship between CO emission and heat input, together with the primary air coefficient of a gas, was studied first.
For different gas constituents, CO emission was found to change in an exponential pattern.
Figure 3 shows the relationship between ln(
CO) emissions from E13/ET13 and reference gas CH
4 under different primary air coefficients as an example. The “equivalence” of some groups of E13/ET13 test points is not satisfactory. Inevitably, there are some errors existing in all the measured data. To minimize the influence of these errors, it is more reasonable to build up a function and check the trend of CO changing with
α′ and
Q than to compare measured COs directly. When comparing CO curves of E13/ET13, it can be found that the two functions tend to overlap each other. Both E13/ET13 have similar emissions under the same working conditions (
α′,
Q). Intuitively, the distance between the E13/ET13 function and CH
4 function increases gradually with the increasing primary air coefficient. Due to the difference in the combustion equipment investigated, Dutton did not need to consider the effect of the primary air coefficient. For the water heaters in Dutton’s research, a unique
ICF number could be calculated for each three-component mixture and used in the gas quality management. However, for the Chinese cooker, a different
ICF number can be calculated under a different primary air coefficient.
Table 4 shows the comparison of Dutton’s
ICF index with the calculated
ICF based on experimental data. Hence, the
ICF index cannot be applied to the Chinese gas cooker discussed herein.
3.2. Validation of Three-Component Mixture
Within the operation range (heat input
Q and primary air coefficient
α′) investigated, the CO emission increases exponentially with heat input
Q and decreases with the primary air coefficient
α′. Three-component mixtures tend to give similar CO emissions. Furthermore, comparison between CO fueled by the other five sets of original gases and their three-component mixtures can lead to similar conclusions. To quantitatively evaluate the difference between CO emission fueled by an original gas and that by a three-component mixture under different heat input and primary air, a characteristic curved surface can be configured as shown in Equation (6). Given a test gas, such a surface can be fitted by 30 measured data. For different gas constituents, the coefficient matrix
A is different.
Figure 4 is a three-dimensional view of CO under the three-component mixture ET8 and CO under the original gas E8. It can be found that the maximum CO appears at highest heat input and minimum primary air, while the minimum CO appears at the lowest heat input and maximum primary air. Meanwhile, it can be found that the CO surface for original gas and its corresponding three-component mixture are comparatively close to each other.
where
CO is the CO emission of the gas cooker, ppm;
Q is the heat input of the cooker, kW; and
α′ is the primary air coefficient.
To quantitatively assess how “equivalently" CO emission from an original gas can be reproduced by a three-component mixture, a parameter
I can be defined as the mathematical expectation of the absolute value of difference between two functions, as shown in Equation (7). Within the range of (
α′,Q) measured experimentally, namely
α′ = 0.4–0.6,
Q = 3.42–4.18, several points of (
α′,Q) are selected. The value of |
COO-
COT| on each point of (
α′,Q) are calculated. The parameter
I equals the arithmetic mean value of |
COO-
COT| on each point. Apparently, its physical meaning is the average distance between two surfaces.
where
COO is the CO emission function of original gas, ppm; and
COT is the CO emission function of the three-component gas, ppm.
If
I < σ + σT, it can be concluded that a three-component mixture can give “equivalent” CO as that from original gas.
Table 5 lists the calculated values of
I,
σ, and
σT for six sets of original gases and their three-component mixtures. For all sets of gases, the calculated
I’s are small enough. It can be concluded that the selected three-component mixtures can equivalently represent their original gases, viz., if a certain gas is input into a gas cooker, the CO emission would be the same as that from its three-component mixture operating at the same heat input
Q and primary air coefficient
α′.
3.3. Influence of Gas Constituents on CO Emission
All gases falling into the category 12T in Chinese standard GB 13611-2007 could be reduced to a three-component mixture. It was found that all natural gas fell within an area:
W = 45–55 and
PN = 0–15. Compared to the range within which Dutton established his prediction method (
W = 45–55,
PN = 0–100) [
21], the exploration range in this paper was narrowed down a lot.
In order to input 12T-0 into the left-side burner of tested cooker, first, we adjusted the primary air shutter until a satisfactory flame appeared.
Figure 5 shows flame shapes under different primary air coefficients.
Figure 5a is a result of smaller primary air, which is usually called a “soft” flame, and would lead to excessive CO emission.
Figure 5d corresponds to a higher primary air coefficient. The flame is “hard” and tends to lift when substituted by other gas rather than adjustment gas.
Figure 5b,c show a satisfactory flame. The primary air coefficient is properly moderate and results in a quite acceptable flexibility to changing gas constituents. The adjustment of the flame above was set according to the report published by the Gas Research Institute (GRI) [
27]. Unstable combustion phenomena, such as lift and excessive CO emission, will not occur when gas constituents fluctuate. The underlying principle for initial adjustment when fueled with 12T-0 is that the satisfactory flame shape in
Figure 5b,c appears to ensure maximum flexibility. The flame shape gave an intuitionistic determination of the initial primary air coefficient. However, it could not be quantitatively analyzed. In practice, the gas–air mixture was extracted from the gas separator of the cooker by an injector and sent into gas chromatography to analyze the molar volume fraction of various components. Then, the primary air coefficient could be calculated. Therefore, it was possible to make the same condition for combustion under different gases. All the combustion cases were measured under the same initial air coefficient.
After the primary air shutter was fixed, we let the burner that injected primary air operate at a nominal heat input of 3.8 kW. Then, the burner remained unchanged, and different three-component mixtures were input into the burner to record CO emissions. To systematically investigate the influence of
W and
PN upon CO emission, the input mixture was deliberately designed to remain constant
W or constant
PN.
Figure 6 shows the measurement result, in which all blue figures donate CO emission in ppm.
Figure 7 shows the CO emission changing with
PN for a constant
W index. When the Wobbe index was kept unchanged, the observed increase of CO emission could be attributed to an increasing tendency for incomplete combustion resulting from increasing propane, and to an increase of contact time between the flame and cold bottom being heated because of increasing N
2. Meanwhile, when the
PN index increased, the port intensity (kW per square meter of burner port area) decreased because the heating value decreased, leading to a decreasing of secondary air. All these factors cause the CO emission to increase with increasing
PN, in a linear manner.
To obtain the relationship between CO emission and gas quality parameter, it is necessary to investigate the influence of the Wobbe index on the formation of CO.
Figure 8 shows the CO emission changing with
W under a constant
PN number. Due to the change of the Wobbe index, the operation point of the cooker changed accordingly. Given a fixed
PN number, increasing the
W index means a higher heat input and lower primary air. Both factors enhanced the CO formation in an exponentially increasing pattern. Therefore, CO tends to increase exponentially with W. The change of the Wobbe index affects the formation of CO in two aspects: the gas properties and the operation condition of the burner.
CO emission was found to be increasing monotonically with both
W and
PN. Equation (8) can describe the relationship between CO and
W,
PN quite accurately.
where
CO is the CO emission of the gas cooker, ppm;
w1,
w2, and
w3 are the coefficients;
W is the Wobbe index, MJ/m
3; and
PN is the volumetric percentage of C
3H
8 plus N
2 of the three-component mixture.
Let CO be equal to a constant value, e.g., 500; then, an equation depicting CO = 500 can be derived, as shown in Equation (9). In similar manner, a set of equal-CO lines for the tested cooker corresponding to initial primary air can be derived, as shown in
Figure 9.
To examine the precision of Equation (8), some supplemental measurements were made to compare the predicted CO emission and measured data. Several deliberately designed three-component mixtures, as listed in
Table 6, were input into the burner. Measured CO emission were given as well (in carmine square), as shown in
Figure 9. From the difference between the predicted equal-CO lines and measured values, the derived equation can be precise enough. In addition, for a gas cooker that was initially adjusted under 12T-0, its CO emission can be predicted by the formula
CO = (
w1 ×
PN + w2)e
w3×W.
It is stipulated in Chinese National Standard GB16410-2007 [
24] that the CO emission from a cooker under test gas should not exceed 500 ppm.
Figure 9 shows that the CO emission of the tested cooker will not exceed 500 ppm when fueled with three-component mixtures that fall below a 500 ppm equal-CO line (thickening line). It can be found that the range of gas to ensure a qualified emission is not large, and the choice of gases are quite limited. However, the national standard does not specify what the CO emission from a gas cooker should be when the gas constituents change. In fact, the Gas Safety (Management) Regulations (GS(M)R) of the UK stipulated that the CO emission from combustion equipment should not exceed the national standard of 2
0.48 = 1.4 times [
20] under the varying gas composition. Therefore, the highest CO emission
COmax can be set after a safety survey to expand the range of selectable gas sources.
The gas cooker geometry affects the CO emission significantly [
28,
29,
30]. As for a cooker with different geometry, its CO emission can be predicted by the approach proposed above. To adopt the above conclusion to manage gas quality in China, all gas cookers in the market should be classified according to their structure, including the shape, angle of the ports, clearance from the surface being heated, etc. For each type of cooker, the following procedures can be applied step by step. (1) Adjust primary air with 12T-0 so that a nominal heat input and satisfactory flame shape can be achieved. (2) Measure CO emission under different three-component mixtures while maintaining the burner as unchanged, so that a series of equal-CO lines can be obtained. (3) After a safety investigation, the maximum CO emission of a gas cooker under various gas constituents is adopted. For each type of gas cooker, there would be a line corresponding to
CO =
COmax. Apparently, the public area enveloped by different types of gas cookers’
CO =
COmax would define the gas quality, which can ensure that the CO emission will not exceed the National Standard. For gas quality outside this public area, some additional measures such as blending must be so as to ensure that the natural gas entering the network can meet the safety standards.