Riding Bikes on Paved Trail in Houston Park (view of river and skyline of downtown Houston) - Houston, Texas, USA

A zero carbon natural gas

For the last 25 years or so, natural gas has played an increasingly important role in global electricity generation.

by Robin Wylie
11 February 2020
5 min read
by Robin Wylie
11 February 2020
5 min read

One of the key factors driving the growth of gas-fired power is the substantial environmental benefit it offers over other fossil fuels: when used to generate electricity, natural gas can emit as much as 60 percent less carbon dioxide than coal, while natural gas power plants are also better suited to provide the essential back up power required by renewable energy technologies such as wind and solar.
As it is currently used, natural gas has the potential to act as a “bridge” fuel, helping the world shift its energy infrastructure towards renewable alternatives. However, as a significant source of CO2, the days of gas power might seem to be numbered in the long-term. But an experimental power plant currently being tested in Texas could herald a new future for natural gas.

The new face of natural gas?

Located just outside Houston, the Net Power pilot plant is probably the most talked-about power plant in the world right now. The MIT Technology Review named it on their list of 10 breakthrough technologies to watch in 2018, while the plant has also been covered by various major magazines, including Forbesand Slate.
The hype is well deserved. That’s because Net Power claims that it is on the verge of achieving something quite remarkable in Texas: producing electricity from natural gas with zero carbon footprint.
The plant’s developers aim to achieve this formidable goal by utilizing a novel thermodynamic cycle called the Allam Cycle, which uses captured waste CO2from natural gas combustion to power a separate turbine. It achieves this by subjecting the CO2 to pressures greater than 72.9 atmospheres (1071 psi), so that the gas enters what is known as a “supercritical” state, which makes it a more efficient working fluid than regular CO2.
As well as being a highly efficient means of producing electricity, using the waste CO2 as an integral part of power generation means that the gas can be easily retained, rather than being vented to the atmosphere as occurs in most thermal power generation. By storing or re-using this captured CO2, the Net Power plant should be able to produce cheap electricity with near-zero net carbon emissions.

As well as the colossal advantage of preventing direct CO2 emissions, using the Allam Cycle to produce power from natural gas will allow the Net Power plant to maintain heat within the generation system, thus requiring less fuel to reach operating temperature. What’s more, by using pure oxygen for gas combustionrather than air (which contains 78 percent nitrogen), practically all of the NOx emissions of traditional natural gas plants will be eliminated.

Net Power, a collaboration between technology development firm 8 Rivers Capital, Exelon Generation, and energy construction firm CB&I, began construction on the Texas pilot plant in 2016. As of January 2018, the company was still in the process testing the plant’s individual components. However, it is expected to begin supplying electricity to the grid before the end of the year. The plant will have an initial capacity of 50 MW, enough to power around 40,000 local homes. But the impact of the project could reach far beyond Texas. While it won’t be the first natural gas power plant to attempt carbon capture and storage, the fact that it could do so at no extra cost relative to standard natural gas power generation could quite revolutionize the use of natural gas worldwide.

Capturing the waste CO2 from thermal power generation has traditionally proven tricky, mostly due to the difficulty in separating low-concentration carbon dioxide from the mixture of exhaust gases. The Allam Cycle, however, circumvents this issue — by using this CO2 as a working fluid, the gas has already been captured once generation is complete, so no extra steps — or funds — are required.

If Net Power can demonstrate the viability of their Allam Cycle plant, it would offer the first economically viable way to produce electricity from natural gas (or any other fossil fuel for that matter), with effectively zero carbon emissions.

Of course, even if the pilot plant succeeds, there remains the question of what to do with the recovered CO2. One potential use could be in the manufacturing of commodities such as cement, plastics and other carbon-based materials. In the short term, however, waste CO2 is most likely to be used in Enhanced Oil Recovery (EOR), a technique in which gas is injected into an oil will in order to increase output.

Using captured CO2 to effectively release more CO2 seems counter-productive. However, according to Net Power CEO Bill Brown, the process could potentially lead to a net reduction in CO2 emissions, since for every carbon atom extracted from the ground during EOR, two will be stored underground as compressed CO2.

Finding an appropriate way to utilize captured CO2 will be essential if natural gas power production is ever to be a truly zero-carbon process. Nevertheless, finding an economically viable way to capture the carbon in the first place is a vital first step towards this goal, depending on how Net Power’s ambitious pilot plant turns out.