As countries around the globe seek to mitigate the dangers of disruptive climate change, alternatives to fossil fuels are being urgently sought. One of these is hydrogen, which has many of the advantages of both fossil fuels and renewable sources such as wind and solar.

It can be produced with low or zero-emissions and can be stored and transported. It is also clean burning, producing only water as a by-product.

Despite its evident advantages, hydrogen also has its drawbacks. It can lead to pipelines that carry it becoming brittle and it is also three times less energy dense than methane, the major component of natural gas.1

To overcome these, hydrogen blending has been proposed as a solution. This is where hydrogen is mixed into the natural gas grid, reducing the total amount of natural gas needed. Blending hydrogen at 20% of the natural gas volume is considered feasible, a rate which avoids the need to make changes to consumers’ cookers, boilers or heating systems. The UK has set itself a target for Britain’s gas network to be ready to deliver hydrogen at a 20% level to homes and businesses around the country   from 2023.2

Because of the limit on blend ratios, it is important to measure the calorific value of the resultant mix. Continuous gas analysers can perform this measurement in seconds, speeding up production and minimising downtime. ABB’s infrared analyser technology measures most of the stream composition in the natural gas. This is then used to define the background gas for a thermal conductivity analyser that measures the hydrogen concentration.

Measuring the CV of natural gas

The calorific value (CV) of natural gas is its most important parameter. This figure defines the quantity of energy per unit of mass or volume released when the gas undergoes oxidation during the combustion process.

The units of measurement most commonly used in the industry to express this parameter are KJ/Kg, KJ/m3, MJ/m3, Kcal/Kg and Kcal/m3. For example, the CV of gas in the UK’s national grid ranges from 37.5MJ/m3 to 43.0MJ/m3(3).

The calorific value of natural gas is measured according to the requirements of ISO 6976. This standard also stipulates measurement requirements for the density, relative density and Wobbe index from composition. This measurement is most often performed by gas chromatography, as this is the most precise and widespread industrial standard technology. Examples of instruments using the technique include ABB’s NGC8206 Natural Gas Chromatograph.

In some circumstances, a reliable on-line continuous, rapid reading method is required, which can be achieved through measuring the density of the gas or recording the speed of sound in the gas. 

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Source: ABB

Infrared methods

An alternative measurement method uses a non-dispersive, infrared photometric gas analyser, or NDIR. This is widely used in industry to measure gases that are active in the infrared spectrum. These gases include CO (carbon monoxide), CO2 (carbon dioxide), NO (nitric oxide), SO2 (sulfur dioxide), CH4 (methane) and other hydrocarbons.

Applications of this technique include controlling impurities in air separation units (ASUs) continuous emission monitoring, assessing quality in cement and lime manufacture, and the analysis of hydrocarbons in refineries.

ABB offers an NDIR-based continuous gas analyser in the form of its Uras26 model, which can calculate calorific value, as well as other associated parameters. As such, Uras26 is an advanced system that eliminates much of the guesswork associated with using conventional instruments. 

The Uras26 can quantify very low concentrations of gas and can simultaneously monitor four different types of gases. The three main components of natural gas, methane (CH4), ethane (C2H6) and propane (C3H8) are measured simultaneously, along with carbon dioxide (CO2). Since the hydrocarbons are active in the infrared spectrum, this method can be used as an alternative to traditional methods, applying the calculation equations from the ISO 6976 standard. 

On the basis of these calculations, the calorific value, relative density, Wobbe index, and percentage of nitrogen (%N2) are all calculated using the advanced algorithms installed on its digital signal processor.

The main drawback of NDIR for this application is that, unlike a chromatograph, it does not measure the complete range of hydrocarbons present in natural gas. However, a high level of accuracy can still be achieved because the three main components of methane, ethane and propane make up the vast majority of the calorific value of the gas. 

Real-world tests on a variety of natural gases from a range of sources have shown that the calculation accuracy of the ABB Uras26 is comparable with the chromatographic method, showing an error of between 0.5% and 1%.

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Adding hydrogen to the grid 

Based on the infrared technology used by the Uras26, it can clearly be utilised as an alternative for the rapid measurement of the CV of natural gas. However, it is a different story when we start to blend hydrogen. This is because hydrogen does not react with infrared radiation and also does not absorb it. This would make the alternative method of CV determination by Uras26 impossible.

However, hydrogen has a high thermal conductivity of 0.18 W/mK, the highest conductivity of any gas. This means we can accurately measure the concentration of hydrogen in a mix with another gas by measuring the thermal conductivity of the complete gas stream. 

Using the Uras26 module, we can determine the concentration of the four most important constituents of the hydrogen-enriched natural gas - CH4, C2H6, C3H8 and CO2. This concentration data can then be used by the thermal conductivity analyser to calculate the theoretical thermal conductivity resulting from these components. 

To measure thermal conductivity, ABB offers its Caldos25 thermal conductivity analyser. Able to measure even in corrosive atmospheres, thanks to the unique design of its measuring cell, the Caldos25 has a rapid response to even the slightest change in stream composition. All the modules are controlled by analyser platforms from ABB’s AO2000 or EL3060 series. This guarantees correct calculation of the infrared-related stream composition, allowing the hydrogen concentration to be determined from the calculated and measured thermal conductivity.

Testing the AO2000-Uras26 technique

Tests of measurement techniques for calorific value were conducted at a burner test bed of a German institute. The primary objective of the tests was to evaluate the effect of fluctuating natural gas compositions on the burner characteristics and subsequently on industrial processes. A secondary goal of the test programme was to investigate the accuracy of available technology for CV measurement, such as gas chromatography, speed of sound and IR absorption spectroscopy.

A measurement set-up consisting of a AO2000-Uras26 and Caldos25 was one of the techniques investigated. This test rig was installed at the gas inlet of the burner, which was supplied with natural gas from the local gas grid. To simulate the expected fluctuations in gas quality, the natural gas was diluted with hydrogen. The measurement results of the AO2000-Uras26 and Caldos25, and the individual components and the CV parameters, were compared with the results of a gas chromatograph. The results are shown in Figures 2 and 3.

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Source: ABB

Conclusion

Tests conducted both in the laboratory and in the field on the measurement of calorific value using NDIR show that it can offer a viable alternative to techniques already used in natural gas production and transport. The accuracy of NDIR-based measurement is comparable to that achieved using the chromatographic method, and there are also competitive advantages in maintenance, operation and response time.

The technology can be used not only in the production of natural gas, but also in large-scale gas consuming industries to control the calorific value of the gas supplied to the network in a rapid and secure manner, saving time and money in process operations.

Due to the fast response time of the solution by adding Caldos25 for hydrogen measurement, it can also be used to monitor the blending of hydrogen into the natural gas grid.

As such, the total solution with Uras26 and Caldos27 offers support to gas producers and users as they transition to more sustainable energy sources.