Repurposing Natural Gas Lines:
The CO2 Opportunity
Authors: Caroline Kenton and Ben Silton
1.6 million miles. That’s long enough to go to the moon, circle it, come back to the Earth, and circle that too… three times! (With 100,000 to spare, too.)
It’s also the number of gas pipeline miles we have in the U.S. alone. Over the last decade alone, we’ve added about 100,000 miles of gas distribution infrastructure (while transmission growth has remained largely flat) as fracking has pushed down the cost of natural gas. In tandem, natural gas has fueled our transition away from coal generation, as gas turbines have the same valuable load-following and inertial support capabilities that are highly synergistic with intermittent renewables – yet burn much cleaner than coal.
What was prized as cleaner than coal has now become our grid’s new dirty fuel: between methane leaks and relatively high carbon emissions as renewables penetrate further and further, America’s “bridge fuel” to a cleaner energy future is now increasingly vilified by environmental advocates. Indeed, new generation capacity is increasingly renewables-heavy: just 16% of new generation capacity is expected to come from natural gas in 2021.
Meanwhile, Carbon Capture Utilization and Storage (CCUS) is increasingly called upon, as shown by the $6.2B of funding in the December stimulus package, to get the world to Net Zero by balancing out emissions from hard-to-abate industries such as cement and steel production. We’re moving to reduce carbon on multiple fronts – generation, demand, and everything in between.
The transition away from natural gas could precipitate a financial crisis from all the stranded natural gas transmission and distribution assets; after all, the U.S. has more than ten times the gas line-miles than any other country – including Russia – and it’s a hard pill to swallow to leave all these relatively young lines just sitting there, idle and empty. Instead, those lines may ultimately prove valuable in a new energy economy with strong roles for CCUS and hydrogen (H2).
In this two-part blog series, we will look at two opportunities to repurpose natural gas lines: CO2 and H2. Both have their distinct challenges, but it will be critical to find ways to utilize the infrastructure we currently have to accelerate the transition towards a cleaner energy future while avoiding the financial and environmental burdens of millions of miles of stranded lines.
A CO2 Pipeline? Really?
The ACT Acorn Project, established by the European Commission under the Horizon 2020 Programme for Research and Innovation, estimates that the costs to repurpose a hydrocarbon pipeline for CO2 transportation will be just 1-10% of the cost of building new pipelines. While this high-level estimate may be economically favorable, there are many integrity and stability issues that need to be addressed prior to conversion.
Although CO2 takes center stage in emissions debates, there are currently 5,200 miles of CO2 pipelines in the U.S, which pales in comparison to current natural gas infrastructure. In the United States, the majority of the active CO2 pipelines are located in West Texas for Enhanced Oil Recovery (EOR) purposes. EOR refers to changing the properties of the hydrocarbons in fields that have heavy oil and poor permeability, in order to increase oil recovery.
There is a tertiary method of recovery that involves injecting gases such as CO2 into oil reservoirs to decrease viscosity of oil and subsequently increase flow. Traditionally, CO2 EOR has been focused on minimizing the amount of CO2 injection required because it can be expensive; however, as carbon sequestration technologies (and tax credits) develop, it may become easier and more favorable to inject larger and larger volumes of CO2.
While EOR is the most cost-effective use of captured carbon today, it unfortunately does still support more extraction of fossil fuels. Good news: EOR is certainly not the only use case for carbon utilization. Carbon capture opportunities abound in more effective sequestration in cement production and – with technical development to increase scale – agriculture enrichment.
Or, if the demand side of the equation doesn’t materialize, the sequestration of CO2 in geological features may be necessary for the world to subsidize. Already, between the 45Q tax credit, the Low Carbon Fuel Standard, corporate incentives from Shopify, Microsoft, and Stripe, and other carrots such as Elon Musk’s X-Prize, the incentives to capture carbon are converging with the cost of carbon capture, likely in the range of $300/ton and falling through the efforts of firms such as Carbon Engineering.
One major challenge for CCUS is that the sequestration of carbon – particularly from point source emissions but also from dilute streams – is rarely co-located with the end use or sequestration site. Efficient transportation from the point of capture to the point of utilization/storage will be critical.
Similar to the struggle we have with transporting renewable energy to areas with intermittent or non-existent supply, transporting carbon across the country will be a major hurdle to overcome for effective CCUS. As leading innovators focus on how to best capture CO2 or additional applications for CO2, we can not ignore the obvious — captured CO2 is a wasted resource if we can’t deliver it to a storage facility or end use site.
While the repurposing of natural gas pipeline infrastructure to transport CO2 appears to be a viable way to avoid stranded assets, it is important to first understand the technical hurdles along this path. While natural gas and CO2 are both gases at standard temperature and pressure, they have different requirements for safe transport in pipelines.
Supercritical fluid is usually the most economical means of transporting CO2 because it has the high density of liquid CO2 but the low viscosity and flow of its gaseous form. Natural gas is generally transported in pipelines in gaseous form and at pressures between 800-1,160 psi. Unfortunately, CO2’s critical point is at a temperature of 30.9°C and pressure of 1,070 psi. Therefore, the pressure for CO2 transportation must be at least 1,200 psi — to avoid phase changes from temperature fluctuations — which is much higher than the standard operating parameters for existing natural gas pipelines. In other words, if CO2 were to be transferred at the same pressure as natural gas, it would change phases and cause significant frictional losses that would limit transportation. (So, it should come as no surprise that all major CO2 pipelines today transport at pressures above 1,900 psi!)
Limited Case Studies
Published projects on the conversion of pipelines in the United States are few. Overseas, the Organic Carbon Dioxide for Assimilation of Plants (OCAP) pipeline in the Netherlands is a repurposed oil pipeline that has been pushing CO2 since 2004. The pipeline had previously been out of commission for nearly 25 years, but the 26 inch, 51-mile pipeline now transports CO2 as a gas at 101-304 psi and supplies 300 Mt per year of CO2 captured from hydrogen production to local greenhouses. (Side note: look out for a future blog post on opportunities for reusing fossil fuel CO2 for agriculture. Email us if you want to collaborate on this!)
The only example today of a natural gas to CO2 conversion in the U.S. is the West Gwinville Pipeline, a 16-inch line spanning just 50 miles operated by Denbury Resources in Mississippi. No doubt, there are sure to be more as the CCUS industry takes off: these types of projects are promising and certainly two-birds-with-one-stone-y, but they do require major technical changes.
To convert a hydrocarbon pipeline into a CO2 pipeline, a few modifications must be made. For starters, a dehydration system is required to minimize water content, since wet CO2 offers a high risk of corrosion. Additionally, high-pressure CO2 pipelines require crack arrestors to prevent catastrophic failure in the event of corrosion. Further, there must also be modifications to the gaskets and non-ferrous materials of the original pipeline so they are resistant to deterioration in the presence of concentrated CO2.
Because pipelines carry rather volatile substances, there are many physical conditions to be considered in their design and operation. The following chart summarizes physical properties and the effects they have on pipeline operations.
|Considerations for Pipelines|
|Pipe Diameter||The larger the internal diameter of the pipeline, the more fluid movement capacity.|
|Pipe Length||The greater the length of a pipeline, the greater the total pressure drop.|
|Density||Different fluids have different weights per unit volume.|
|Specific Gravity||Specific gravity is the density of a fluid divided by the density of water or air, depending on whether the fluid is a liquid or a gas.|
|Compressibility||Compressibility represents the deviation of real gas from the ideal gas model. It represents the change in relative volume as a result of pressure increases.|
|Temperature||Temperature may affect the capacity in pipelines.|
|Viscosity||Viscosity is the property that resists flow between adjacent fluid parts, which affects line size and pump power requirements in pipelines.|
|Reynolds Number||The Reynolds number describes the type of flow which affects the pressure drop in a pipeline.|
|Friction Factor||The friction factor relates to the roughness of the inside pipe wall.|
This table shows how much of pipeline design and operation is driven by the material in transit and its properties. Considering that many of the above properties are heavily inter-dependent, converting pipelines to carry new materials is anything but a trivial challenge.
You probably weren’t surprised to learn that switching from natural gas to CO2 requires arduous technical planning; we know different substances have different physical properties which affect their ability to flow. But the key consideration here is whether CO2 pipelines would be more economical than decommissioning pipelines altogether.
Unfortunately, this question has no easy answer, but innovation in the EOR and CCUS industries, as well as other CO2 applications, could not only alleviate our greenhouse gas emission issues but also create a critical mass of pull forces for a pipeline repurposing opportunity.
ADL will be launching a Gas Line Innovation Challenge to solicit novel solutions from the entrepreneurial ecosystem for repurposing natural gas pipes to transport CO2, H2, or other materials. Request more information on the challenge by clicking the “Schedule a Conversation” button below.