METHOD FOR CALCULATING THE CALORIFIC VALUE OF A HYDROGEN-BLENDED NATURAL GAS UTILIZING THE SPEED OF SOUND AND LIGHT
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 calorifi c value of a new hydrogen-blended natural gas, heretofore never previously measured in the fi eld.
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 diffi cult-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 fi eld.
2. Calorimetry Methods and Challenges
There are several purposes for measuring the calorifi c 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 calorifi c 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 quantifi cation 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 calorifi c 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, quantifi cation and calorifi c 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 calorifi c 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.
IET September / October 2019
www.envirotech-online.com
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 calorifi c value. This method is capable of measuring the calorifi c 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 calorifi c value for several gases. The refractive index is the ratio of the speed of light in vacuum to the speed of light in a specifi c medium.
The function Qopt, the straight line in Figure 1 formed by the square ( ■ ) data points, depicts the relationship between refractive indices and calorifi c values of gas mixtures comprising paraffi nic hydrocarbon gases and hydrogen.
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