Machinery | Heat Waves

Variable natural gas quality presents problems in glass fabrication and manufacturing
Brian Hammarsten
April 16, 2019
COMMERCIAL, RETAIL, FABRICATION : INSIGHTS

Mixer and flow control system

Mixer and flow control systems are one solution to problems with fluctuating natural gas quality. The equipment adjusts the heating value of the incoming natural gas supply, eliminating the need for manual adjustments to gas fired equipment within the facility.

The bottom line

The abundance of natural gas due to the shale gas revolution in the United States is a positive event for the energy-intensive glass industry. However, variations in gas energy content can pose problems in production. Natural gas conditioning can help.

Modern glass production must control a precise blend of many elements. One critical element is heat, and a lot of it. Glass manufacturing depends heavily on natural gas as an energy source to generate the high temperatures required. 

With the advent of shale gas, natural gas is abundant. However, the quality—specifically its heating value—has become more variable than in the days of pre-shale gas. 

This variability can often be measured in hours, not days. Some pipelines have tariffs that allow for a range between 950 and 1,250 Btu per cubic foot. These fluctuations may be problematic and may have an adverse effect on the burners, the throughput of glass fabrication and manufacturing, as well as the quality of the end-product. This creates a concern that glass manufacturers may need to face.

Why are there greater fluctuations in natural gas heating value?

The natural gas in some of these newly developed fields is a “wet” gas, meaning that the natural gas stream contains a large amount of NGLs or natural gas liquids. These NGLs (ethane, propane, butane, etc.) can be stripped from the natural gas stream and supplied to their own marketplace. However, stripping these NGLs is a relatively expensive process and areas with processing limitations and low demand for NGLs such as ethane may choose to leave them in the natural gas stream. This is known as ethane rejection. Consequently, low NGL commodity prices in the marketplace can result in a higher heat content in the natural gas stream delivered to pipelines. 

For example, if we look at ethane, we see the gross heating value is approximately 1,760 Btu/cf. Natural gas is typically in the 1,000 Btu/cf range. It doesn’t require much ethane to drive up the overall gross heating value of the natural gas stream.

Precision gas fired glass making/tempering equipment may not be able to accommodate a variable natural gas heating value supply. While the equipment can typically be adjusted, the natural gas heating value changes again and additional equipment adjustments must be made. As a result, product quality may suffer or become unusable before manufacturing equipment adjustments can be made. 

Adjust the fuel, not the process equipment

An efficient way to address the natural gas quality issue is to “fix” the gas heating valve. The use of a calorimeter is one way to view the natural gas component stream and heating value. Based upon this information, equipment adjustments can be made. However, the calorimeter is not an instantaneous output device, so approximately five minutes is lost between sample and information output. Again, output is only indicating the gas is changing, and manual burner adjustments are still required. The problem is not truly solved.

In order to solve the problem, an instrument utilizing an optical tunable filter spectrometer is employed. This unit gives a relatively instantaneous reading of the natural gas quality. This information can then be used to feed a gas mixer based upon mass flow. Air or inert gases can be added to the natural gas stream to limit the gross heating value of the gas to a level the combustion equipment finds acceptable. This way the equipment is not adjusted—the gas quality is.

This system would also be employed should a situation arise where the natural gas stream suffers from diminishing energy content (for example, delivered natural gas of 950 Btu/cf). Through basically the same process, the equipment can add an enrichment gas (propane, ethane, butane, etc.) to increase the natural gas gross heating value to within an acceptable range. 

The abundance of natural gas due to the shale gas revolution in the United States is a positive event for the energy-intensive glass industry. However, pipeline tariffs typically allow for a wide variation in gas energy content, which may be problematic. Natural gas conditioning is one way to take full advantage of the shale revolution and continue manufacturing high-quality glass. 

Brian Hammarsten is account manager for Standby Systems Inc., which provides packaged fuel system solutions and custom gas blending for industrial, commercial and utility customers. He can be reached at bdhammarsten@standby.com or 612/721-4473.