Technology About Gas

Towards greener natural gas

 By Robin Wylie
About gas

Evolving technologies could allow natural gas to play an even greater role in the transition to a low-carbon future…

(Cover pic by Duke Energy, CC BY-NC-ND 2.0)

The increasing use of natural gas for electricity generation is one of the most significant energy trends of the past 30 years. In 1987, natural gas accounted for 8 percent of global electricity production. Now that figure is 22 percent, and rising steadily. (By 2040, around 30 percent of total worldwide energy generation is expected to come from natural gas).
As our consumption of natural gas continues to rise, however, so too will the need for greener sources of power, as humanity comes to grips with the realities of climate change. Natural gas has been widely praised for its relatively low emissions compared to other hydrocarbons such as coal. But technology is continuously pushing the boundaries of how green natural gas can be.

A new generation

One way to make natural gas greener is to increase the thermal efficiency of the power plants that use it – more efficient plants need to burn less fuel to produce a given amount of energy, thereby reducing carbon emissions; a one percent rise in efficiency reduces greenhouse gas emissions by between 1 and 3 percent.
Today, the most efficient way to generate electricity on a wide scale using natural gas is using “combined-cycle” power plants. These operate using a double step generation process, first using combusted natural gas to power one turbine, before using the exhaust gases to heat water, producing steam, which is then used to drive a second turbine.
Combined-cycle natural gas plants are significantly more efficient than traditional “simple cycle” power plants, which have no capacity for waste heat recovery. Simple-cycle plants typically achieve efficiencies of 32 to 38 percent, while combined-cycle plants can now reach efficiencies of over 60 percent. In 2016, General Electric inaugurated a combined-cycle natural gas plant with a record-breaking 62.2 percent efficiency.
Currently, simple-cycle power plants are the most widely used in natural gas power generation, but the use of combined cycle technology is on the rise. Combined-cycle plants accounted for roughly three-quarters of global additions in 2015, and now provide more than 80 percent of natural gas-fired generation in the United States.

Caledonia combined-cycle plant (tva.gov)

Using CO to cut CO

Recycling the waste heat from power generation is one way to cut down on carbon emissions from natural gas. But another way to achieve this could be to use the carbon emissions themselves.
In the past few years there has been increasing interest in the possibility of using the waste carbon dioxide from power plants to produce additional power, by subjecting the gas to high pressures and temperatures, and using it to drive a separate turbine.
When CO is simultaneously heated to above 88°F and pressurized to above 73 times atmospheric pressure, it enters what is called a “supercritical” state, where its density increases significantly while still moving like a gas. And this combination of properties makes it an ideal substance for driving a turbine.
By converting waste CO into a supercritical state, various kinds of thermal power plant — including natural gas plants — can potentially increase their generation capacity without burning any extra fuel, thereby raising their efficiency, and cutting their net carbon emissions. The potential of supercritical CO turbines to modern power generation is so significant that the technology was recently the subject of an article in the prestigious journal Science, with the title: “Turbines can use CO to cut CO.”
There are various engineering challenges associated with using supercritical CO to drive turbines. For one, the turbine blades need to be able to withstand higher mechanical stresses, and their design also need to be optimized to work efficiently with the consistency of supercritical CO, which is somewhere between a liquid and a gas.

Liquid CO₂ is heated in a pressure cell until it reaches the critical point were it changes into a supercritical fluid

But that isn’t stopping developers from taking advantage of the opportunity offered by this new technology. Dresser-Rand, a turbine supplier to the oil and gas industry, has already commercialized a functioning supercritical CO turbine, which has a maximum output of 8 MW. And a partnership between NET Power, CB&I, Toshiba, and Exelon is currently developing a 50 MW demonstration plant which incorporates a supercritical CO turbine, in Texas. It is expected to be completed later this year.
And aside from these private companies, the United States Department of Energy announced this year that it is building a prototype power plant that uses supercritical CO turbines. The project, which has a budget of $80 million, is expected to go online in 2023.

Spotting the leaks

Improving turbine technology is one way towards greener natural gas. But there is another significant opportunity to curb carbon emissions, at an earlier stage in the supply chain.
There is wide concern about the environmental impact of fugitive gas emissions — accidental leaks of methane and other greenhouse gases that can occur during the production of hydrocarbons. Fugitive gas emissions can result from the production of oil and coal, but the greatest potential is from natural gas wells.
The scientific consensus is that fugitive emissions are unlikely to offset the environmental benefits of natural gas compared to other fossil fuels. But nevertheless, reducing such emissions is a key area where the carbon footprint of natural gas could be significantly reduced.
And that’s where new technologies can help. Fugitive emissions are notoriously difficult to identify, given that natural gas is invisible to the human eye. But electronic eyes are a different story.

One of the most promising techniques for identifying fugitive gas emissions is drone-mounted methane sensors, which allow gas producers to spot gas leaks from the air. Previously, the best option for detecting fugitive emissions was to use handheld infrared cameras; switching to automated, airborne monitoring using drones could therefore allow much more complete measurements to be made with less manpower.
Methane-detecting drones have already been developed by numerous organizations. One of the first was developed by NASA. The drone was initially intended for use on Mars (where methane could be a fingerprint of life), but the technology has since been used to hunt for fugitive emissions in natural gas fields in Canada.
And last year, General Electric presented “Raven”, a methane-detecting helicopter drone developed by its R&D department. On a test run, the drone successfully found gas leaking from a two oil wells in Arkansas.
These machines, and the various other methane-sniffing drones, which are operating or in development, could enable the natural gas industry to detect, quantify — and most importantly, plug — their elusive fugitive emissions.
Whether in power plants or during gas production, new technological advances are making greener natural gas a possibility. With sufficient commitment to implementing them in practice, there is every reason to think that natural gas can maintain — and even improve upon — its position as a leading low-carbon fuel source for the future.

READ MORE: Hybrid power plants by Nicholas Newman

about the author
Robin Wylie
Freelance earth/space science journalist. Currently finishing off a PhD in volcanology at University College London.