Wireless Meter of Methane Number and Mass Flow of Natural Gas

Tu Xiang Zheng

  

Natural gas is used in an amazing number of ways. Although it is widely seen as a cooking and heating fuel in most households, natural gas has many other energy and raw material uses that are a surprise to most people who learn about them. In 2012 about 30% of the energy consumed across the United States was obtained from natural gas. It was used to generate electricity, heat buildings, fuel vehicles, heat water, bake foods, power industrial furnaces, and even run air conditioners.

Natural gas is a gaseous mixture chemically composed by methane, smaller fractions of higher molecular weight hydrocarbons and inert gases (mainly N₂ and CO₂). The different components ratio in the gas mixture determines its physical and chemical properties and consequently, its quality. Concretely, composition fluctuations affect to properties such as methane number.

The methane number is the parameter used to quantify the quality of the natural gas.  A 100 methane composition is given 100 methane number and as the higher hydrocarbons and inert gases percentage increases the methane number decreases. It is assigned that a 100 methane composition is used as the knock resistant reference fuel. Every natural gas engine has a higher than a 65 methane number to prevent engine knocking.

For methane number measurement many different sensor techniques are available in a variety of classes. Sensor principles include electrical techniques, like electrochemical detection, or electrical detection of adsorption by induced capacitance changes, optical techniques; for instance infrared (IR) adsorption or Raman spectroscopy, chromatography, calorimetry and acoustic analyses. What most sensor techniques have in common is that their applicability into real time monitoring systems is limited, either because sensors are hard to integrate based on practical considerations like, size, cost or response time, or because the sensors rely on principles that generally do not apply to all gasses.

The present paper proposes a new method to determine on line the methane number of natural gas. The method is based on the measurement of the gas density and the correlation between the density and the methane number of natural gas. The gas density can be measured with a thermopile flow sensor combining a differential pressure sensor. These two sensors are installed in an orifice plate. When natural gas flows through the orifice plate the mass flow rate and the pressure drop of the gas flow can be measured simultaneously. Then the density of the natural gas can be calculated based on the Bernoulli equation which states that there is a relationship between the pressure drop and velocity of the natural gas flow.

The correlation between the specific gravity and the methane number of natural gases is shown as the following table. The table gives methane number: 48.1, 66.2, 76.4, 80.8, 91.4 and 100 and the volume percent of their corresponding compositions. Using the specific gravity of each composition the specific gravity of each methane number can be calculated which is also given in the table. The specific gravy of each composition of the natural gas is shown in another table. It can be seen that the methane number increases and the specific gravity of the different methane number gases decreases. It is not surprising because the specific gravity of methane is lower than all other composition of the natural gas. 

A proposed wireless natural gas meter is shown in the above figure. The meter can measure both the methane number and wirelessly send the data to a smart phone for the user to monitor the consumption and the quality of the natural gas precisely. In order to do so a key component of the meter is a thermopile flow sensor developed based on a mix of integrated circuit manufacturing and micro-machining process. Some of the advantages of the thermopile flow sensors can be listed as

• Direct mass flow sensing;
• Large dynamic range;
• Fast response;
• Excellent low flow sensitivity;
• Low power consumption;
• Small size, mass, volume;
• low cost; and
• Easy to integrate in gas or fluid transport networks.