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Method for Calculating Calorific Value of Hydrogen-Blended Natural Gas using Speed of Sound and Light

Tomoo Ishiguro Riken Keiki 

There is much interest in power-to-gas (P2G), a technology where surplus power from solar power or wind generation is used to synthesize hydrogen, which can then be converted to fuel gas for storage or consumption. 

Furthermore, studies are ongoing for the feasibility of using natural gas pipelines for storage, transport, and delivery of hydrogen synthesized from P2G technology. This article introduces the RIKEN OPT-SONIC™ method that utilizes the speed of light and sound in gas mediums to resolve the calorific value of a new hydrogen-blended natural gas, heretofore never previously measured in the field.

 1. Emergence of Hydrogen-Blended Natural Gas
The adoption of clean energy, including solar power and wind generation, from renewable energy sources with zero CO2 emissions continues to grow.
However, the dependency of clean power generation on weather conditions presents a challenge to planned, demand-driven energy production.
P2G is an endeavor to solve the aforementioned challenge. When the supply of renewable energy exceeds demand, the surplus power is used to synthesize hydrogen by electrolyzing water. Essentially, the approach converts the difficult-to-store surplus electrical energy into fuel gas that can be stored or consumed.
Furthermore, feasibility studies are ongoing for the use of natural gas pipelines as transport and delivery systems for the energy in its converted fuel gas form.
The use of natural gas pipelines for storage also allows for CO2 methanation, a technique being researched in which the hydrogen generated by electrolysis is reacted with CO2 to create methane, the major component of natural gas, which is then injected into the pipeline. If there is not enough surplus energy available for methanation, plans are being explored for directly blending the renewable hydrogen into natural gas within permissible limits.
With the emergence of hydrogen-blended natural gas, natural gas calorimetry systems are now driven to adapt to hydrogen, which has never previously been measured in the field.

2. Calorimetry Methods and Challenges
There are several purposes for measuring the calorific value of natural gas. These include determining the transaction value of natural gas, quality control based on heating value standards for the injection of hydrogen, controlling plant combustion equipment for stable operation, and controlling air-fuel ratios for gas turbine generators that require precise combustion control.
High measurement accuracy, continuous measurement, and fast response, are requirements for calorific value measurement but the emphasis on each requirement will vary according to the purpose of the measurement.
Among the most trusted natural gas calorimeters providing high measurement accuracy are gas chromatograph (GC) calorimeters.
The principle in gas chromatography involves separation of the test gas into its components and quantifying the amount of each component. The caloric value is then calculated based on those amounts. The measurement accuracy of GC calorimeter is determined by the gas separation performance and quantification accuracy. However, the design of most natural gas GC calorimeters today has not anticipated hydrogen blending and therefore these instruments are incapable of separating and quantifying the hydrogen component and hence cannot measure the calorific value of hydrogen-blended natural gases.
Although there are GCs capable of separating the hydrogen component, the operational principle of gas chromatography (i.e., sampling the test gas, component separation, quantification and calorific value calculation) does not lend this method to control applications requiring fast response and continuous measurement.
Another widely adopted instrument is the combustion calorimeter which is known for its continuous measurement capability. The principle in combustion calorimetry involves the measurement of the heat generated or the concentration of the oxygen after it has been consumed in the combustion.
Even if hydrogen is blended into natural gas, this would not be a problem for measuring the calorific value as long as the air ratio can be adjusted to encourage combustion. However, because the detection principle is based on a combustion reaction, achieving high accuracy is a challenge.

3. The Principle of Calorimetry Utilizing the Speed of Sound and Speed of Light
There is a unique caloric measurement method called the RIKEN OPT-SONICTM Calculation Method (hereinafter, “Opt-Sonic Method”) that measures the speed of light in the fuel gas and the speed of sound to calculate the calorific value. This method is capable of measuring the calorific value with high accuracy, continuously, and fast response times.
The principle for the Opt-Sonic Method is described below.

Figure 1 shows the relationship between the refractive index of a medium and its calorific value for several gases. The refractive index is the ratio of the speed of light in vacuum to the speed of light in a specific medium.
The function Qopt, the straight line in Figure 1 formed by the square ( ■ ) data points, depicts the relationship between refractive indices and calorific values of gas mixtures comprising paraffinic hydrocarbon gases and hydrogen.
Figure 2 shows the relationship between the speed of sound in medium and its calorific value. The function Qsonic, the straight line in Figure 2, describes the relationship between the speeds of sound in hydrogen and paraffinic hydrocarbon gas mixtures and their calorific values.
If the fuel gas comprises only paraffinic hydrocarbons and hydrogen, the calorific value can be obtained from the functions Qopt or Qsonic. However, errors are introduced to the functions if the fuel gas (i.e., natural gas) includes miscellaneous gases such as N2, CO2, O2, and CO. These gases are indicated with solid triangles ( ▲ ) in Figures 1 and 2, and do not align with the linear functions Qopt and Qsonic.
The calorific value Q for gas mixtures containing the miscellaneous gases may be expressed by the following equations:

Q = Qopt – (kN2  XN2 + kO2 XO2 + kCO2 XCO2 )            (1)

Q = Qsonic – (k’N2 XN2 + k’O2 XO2 + k’CO2 XCO2)        (2)

Where Xi represents the volume fraction of gas component i, and k i and k’ i represent the error coefficients of gas component i. The latter error coefficients are shown as vertical dashed lines in Figures 1 and 2.
Although k i and k’ i have different values, the ratio between the two remains approximately constant as expressed in equation (3) regardless composition:

k N2 = α k’ N2,     k O2 ≈ α k’ O2,       k CO2 ≈ α k’ CO2    (3)

Using equation (3), equation (2) can be transformed into equation (4):

Q = Qsonic – α (k N2 X N2 + k O2 X O2 + k CO2 X CO2 )         (4)

Since the second term of equation (4) is α times the second term of equation (1), we get simultaneous equations (1) and (4) that can be solved to obtain equation (5), which can solve the calorific value from the speeds of sound and light:

Q ≈ Qopt – (Qopt – Qsonic) / (1- α)                  (5)

Figure 3 shows the relationship between the calculated results from equation (5) and the actual calorific value of each gas.
Every gas data point is on or near the straight line which has a slope of 1.
Figure 4 is data from a field test performed at a Japanese city gas company. The test was performed by intentionally varying the calorific value of gasified LNG by changing the amount of N2-blended BOG or gasified LPG being injected to the gasified LNG. The plot shows that the measurement results of the Opt-Sonic Method obtained from equation (5) perfectly matches the results of the GC measurements. Furthermore, the calorimeter implementing the Opt-Sonic Method can measure short-term calorific value changes that the GC, which measures in intervals, cannot pick up.
Note that the Opt-Sonic Method data has been shifted to make it easier to compare the results with the GC analyzer results. This was necessary because the T90 for the Opt-Sonic Method is fast at less than 5 seconds and was outputting results 30 minutes faster than the GC.

4. Calorimetry of Hydrogen-Blended Natural Gas
A test with simulated hydrogen-blended natural gas was performed with standard gas cylinders. Table 1 shows the evaluation results performed at a European gas company. Five standard cylinders with simulated hydrogen-blended natural gas were measured. The theoretical calorific values (calculated based on the composition) were compared with the measurement results from the Opt-Sonic method.
As shown in the table, the measurement accuracy is within ±0.5%, which is compliant to R140, CVDD Class A of the OIML (International Organization of Legal Metrology).
Internal testing at Riken Keiki has shown that accuracy within ±0.5% can be achieved for hydrogen concentrations of up to 10 vol%.

5. Summary
Adoption of environmentally sustainable energy sources including solar power and wind generation will continue to increase.
As a result, P2G technology will become increasingly indispensable as a solution for moderating the weather-induced fluctuations in energy production.
The calorimetry technology demonstrated by the Opt-Sonic Method is capable of measuring hydrogen-blended natural gas with high accuracy, continuously, and with fast response times, and is ideally positioned to provide a solution that will facilitate efficient use of new resources synthesized from renewable energy