With fossil fuels still being the dominant energy source for power generation, and projected to remain so for decades, there is a need to control CO2 output through the use of carbon capture.
There have been a number of reports on achieving global climate goals that conclude that carbon capture is an essential technology for eliminating CO2 emissions from the power and industrial sectors, and eventually for removing CO2 directly from the atmosphere.
Carbon capture technology is not strictly an emissions control technology bolted on to the back-end of conventional power plants to separate CO2 out of their flue gas, with the CO2 then being compressed and piped for sequestration. Carbon capture can also involve a broader range of technologies.
NET Power LLC was formed by 8 Rivers Capital, a technology commercialization company based in North Carolina, USA, which then received investment from McDermott, Exelon, and Oxy Low Carbon Ventures. NET Power has a clear approach to burning fossil fuels more cleanly. Instead of trying to ‘fix’ supercritical coal use, integrated gasification combined cycle (IGCC) or natural gas combined cycle power plants, 8 Rivers designed a fossil-based system from scratch to achieve the desired end result.
As a result of this, NET Power developed a power generation technology that has been designed to inherently produce its carbon emissions as a nearly pure, high pressure stream that can be sent directly into a pipeline without requiring specialised carbon capture equipment.
In May 2018, NET Power announced that it had successfully achieved first fire of its supercritical CO2 demonstration power plant in La Porte, Texas, USA. This included firing of the 50 MW Toshiba commercial scale combustor, which involved integrated operation of the full NET Power process. The 50 MWth, 25 MWe plant was constructed over a two-year period, and it is said to be the world’s only industrial scale supercritical CO2-based power plant and CO2 cycle test facility. The plant is designed to demonstrate NET Power’s Allam Cycle technology, which uses a new turbine and combustor developed specifically for the process by Toshiba. The development includes combining gas turbine and steam turbine technologies in the production of the supercritical CO2 turbine.
The turbine inlet temperature of the cycle is not high for gas turbines, but it is very high for steam turbines. Similarly, the pressure of the cycle is high for gas turbines, but it does not exceed typical levels for advanced steam turbines. The combustor has been designed to cope with a gas pressure of 300 bar, more than 10 times the gas pressure typically used in conventional gas turbines.
Toshiba’s technology has the following potential capabilities:
- Efficiency of about 55-59 per cent when running on natural gas
- Efficiency of about 51-52 per cent when running on gasified coal
- 100 per cent carbon capture with no efficiency penalty.
The combustor process in the cycle is critically important, because the working fluid and pressure are different from typical heavy-duty gas turbines. The major characteristics of this combustor are:
- Zero NOx emissions because of the use of oxygen as opposed to air
- The temperature is not as high as existing combustors for heavy-duty gas turbines
- The pressure is much higher than is the case in existing combustors.
According to a spokesperson for NET Power, the potential market for carbon capture is enormous. “Fossil fuels provide a low-cost, reliable dispatchable fuel source. In the developing world in particular, countries are focused on using these resources. If we can affordably eliminate air emissions from the processes that use these fuels through carbon capture technologies, it will play a major role in the global energy mix going forward.”
Fossil fuels are still by far the most dominant energy source worldwide, and they will remain so for decades to come, according to most projections.
NET Power carbon capture
NET Power’s Supercritical CO2 technology is different than conventional carbon capture technologies, as it is fundamentally a new power cycle, called the Allam Cycle. This is more fully described in Gas Turbine World, September-October 2016, but in basic outline, it produces its own carbon dioxide as a pipeline-ready by-product. The Allam Cycle does this by combining two processes. Firstly, it uses oxy-combustion, in which fuel is burned in pure oxygen. The combustion products from burning methane in oxygen rather than air are just CO2 and water. If this stream is cooled, the water drops out as a liquid, and the stream consists of pure CO2.
Oxy-combustion is not a new concept. However, in order to provide pure oxygen, an air separation unit (ASU) is required. This is a standard piece of equipment commonly used in the industrial gas industry. However, the extra capital cost and power required for the ASU hurts the economics of oxy-combustion when applied to conventional power cycles, in the same way that carbon capture does, as it is essentially pushing the carbon capture cost to the front-end of the cycle rather than the back-end.
However, the Allam Cycle does not use a conventional power process. Instead of using steam or ambient air to drive its turbine, it uses CO2 above its critical state – at a temperature and pressure above its critical point where liquid and gas phases are indistinguishable. Supercritical CO2 acts as a very efficient working fluid, enabling the core power process to have a higher internal efficiency than conventional power systems. This higher performance can be used to supply the requirements of the ASU, while still remaining competitive with a conventional power cycle.
The overall process is as efficient as a conventional natural gas combined cycle, between 55.1 and 58.9 per cent. Because it is a simple process that only requires a single turbine, and has no need of steam equipment, it is low cost. This enables the unit to produce low-cost power and pipeline-ready CO2, with no air emissions. In addition, the ASU also produces saleable argon and nitrogen, which can further improve the economics of the plant.
Burning methane in oxygen
One advantage of oxy-combustion is that because combustion of an alkane in pure oxygen generates only CO2 and water, there is no nitrogen present to create NOx emissions. Any dirt or water in the methane needs to be removed before combustion, as is typically already the case with pipeline grade natural gas.
The combustion system can be adapted to use any alkane gas, and it can burn sour gas, syngas, and biogas.
La Porte pilot facility
The Supercritical CO2 pilot plant on the La Porte site is a 50 MWth NET Power plant, using a natural gas-fueled Allam Cycle. The design has been scaled down 10x from NET Power’s commercial product, which is a 300 MWe natural gas power plant. The combustor, however, is at full commercial scale, and the turbine at 200 MWth is a 2.5x scale down. For the purpose of the demonstration plant, piped oxygen is being used rather than constructing an on-site ASU. It was felt that as the ASU is already a highly proven piece of equipment; it was not necessary to demonstrate it in the pilot plant.
The plant is connected to the grid and will begin selling power soon. It is operated as a real power plant, in addition to being used as a test site.
NET Power’s investors funded the development of the plant, and they include: 8 Rivers Capital, the technology developer; Exelon Generation; McDermott; and Oxy Low Carbon Ventures, a subsidiary of Occidental Petroleum. Toshiba developed the turbine and combustor used in the Allam Cycle.
So far, the project has demonstrated and proven the most novel piece of equipment used by the Allam Cycle, which is the combustor. NET Power has also worked out the overall integrated operability of the process.
NET Power broke ground on the project in March 2016, with commissioning starting in late 2017. First fire was announced in May 2018, and the primary test program is scheduled to be completed in early 2019.
NET Power said that until the primary test program was complete, it would be premature to release detailed results of the testing. However, It was able to say that the most sensitive testing, that of the supercritical CO2 oxycombustion system combustors, had demonstrated very positive outcomes to date. It also said that the testing to date had confirmed their expectations.
The next phase of the project will see construction of a full-scale 300 MWe plant.
A spokesman said that there were several projects currently in the planning and development stages. There was a good synergy working in the oil production industry in particular. Oil companies often want a supply of CO2 for enhanced oil recovery, while power plants wish to dispose of CO2 without releasing it into the atmosphere.
Interest in the system has been expressed from several places, including the USA, notably Texas, the UK, New Zealand, and the Middle East. NET Power says that it expects that one or more projects would move into the detailed planning phase in 2019.
It has been noted that it is possible sell CO2 to oil production facilities, to provide an additional revenue stream. As well as this, the ASU separates out nitrogen and argon, which can then also be sold to provide other lines of income.
NET Power said that these additional income streams would make the first unit commercially competitive with standard gas turbine power plants. After a number of units have been produced, the costs should fall, and the company expects that it will not be long before the units are cost-competitive on electricity sales alone.
According to NET Power, the units are expected to be capable of more flexible operation than a combined cycle plant with regards to ramp rate. It also pointed out that because operating the ASU involves a relatively large parasitic load, a temporary increase in output of around 30 MWe can be achieved by turning the ASU down, and using on-site stored oxygen to run the plant. Typically, a unit might have storage sufficient for 4-5 hours, but this is entirely dependent on the size of the storage tank, which can be adjusted based on customer needs. The unit can be on the grid straight away, and it is very effective at low loads.
NET Power plants are projected to be economic without requiring a tax on carbon, and they currently can claim the 45Q carbon capture tax credit. However, such incentives are important to deploying such technologies, as these help customers overcome the increased risk of building a new technology, and they help the technology come down the cost curve. As more copies are built, the cost will fall.
With an efficiency of 55.1-58.9 per cent, NET Power concludes that the system is comparable in efficiency with combined cycle gas turbine power plants, and expects it will be cost-competitive with combined cycle plants very quickly because of the additional revenues from CO2 and Argon.
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