First fire supercritical CO2 power plant
What if power stations would reuse CO2 and produce water instead of respectively producing and using these resources? The American company NET Power has achieved first fire of the combustor at a demonstration plant. The heart of the so-called supercritical CO2 plant has been designed using the so-called Allam cycle, effectively producing water and (partly) reusing CO2.
Header Photo: A bird’s eye view of the plant in La Porte. The facility is located in one of the largest industrial zones in the world, along the Houston Ship Channel. (Photo courtesy of NET Power)
Article by Lucien Joppen
The end of May 2018, NET Power published a press release, proudly announcing first fire at the 50MW demonstration plant in La Porte, Texas. This small city, located in the Sugarland/ Bay Area of Houston, can pride itself not only 0n being a tourist destination, due to its proximity to the bay, but it also harbours the world’s first and only supercritical CO2 power plant and CO2 cycle test facility. NET Power effectively is a start-up, based in Durham, North Carolina. The company has developed a power system that is based upon the Allam Cycle. The company has the support of Exelon Generation, a company specialized in green energy production, McDermott (technology, engineering and production) and 8 Rivers Capital, a future technology development and financing company. One of the lead inventors of the Allam cycle, Rodney Allam himself, is partner at 8 Rivers.
The Allam cycle is a process for converting fossil fuels into mechanical power and as such resembles conventional gas fired power stations. But there the analogy stops. While traditional power plants burn gas or gasified coals with air, the crucial part of the Allam unit - the combustor - is operating on a mixture of oxygen, natural gas and CO2, the mass of the latter being around 94 percent of the total. This is achieved by using pure oxygen instead of air to burn the fuel.
After driving the turbine, the exhaust gas is cooled in a heat exchanger, the minor amount of water in the process stream is condensed and separated from the flow, becoming a potential source of fresh water. The carbon dioxide is compressed and pumped mechanically, and a small amount, matching the amount continuously added through combustion, is captured at high pressure, ready for pipeline transmission. The rest of the carbon dioxide is reheated in the heat exchanger (to 720 degrees Celsius) and recycled into the combustion unit, where it continues to form the vast majority of the working fluid.
The Allam principle, as demonstrated in La Porte, Texas, required a new design of the commercial-scale combustor and the turbine. These components have been designed by Toshiba Energy Systems & Solutions. In the 4th quarter of 2016, the Japanese company shipped the custom-built equipment off to the United States.
Commenting on the shipment, Takao Konishi, Vice President of the Thermal & Hydro Power Systems & Services Div. of Toshiba’s Energy Systems & Solutions Company said, “I am delighted to reach this milestone of successful shipment of the turbine (...). By combining casing technology for the ultra-super critical steam turbine with material technology and cooling technology for the high temperature gas turbine, we have achieved a highly reliable turbine. With our advanced technologies in thermal power generation, we are committed to producing electricity efficiently.”
Enhanced oil recovery
According to Toshiba, this efficiency translated into the ability to capture and re-use CO2. Compared to a combined cycle plant, which is capable of capturing ‘only’ 90 per cent of the CO2, the Allam/Toshiba plant is able to capture nearly 100 per cent CO2. As mentioned before not all CO2 generated will be used in the energy production cycle. Once the cycle is full of the CO2 working fluid, all of the CO2 created from combustion will be removed from the plant at high purity and high pressure, where it can be used for various purposes, such as food & beverage (carbonated drinks) or building materials. Another option is sequestration via so-called enhanced oil recovery (EOR). This would mean that for every atom CO2 ‘harvested’ from the ground, two atoms of CO2 would be sequestered, according to the CEO of NET Power, Bill Brown, in an interview with Forbes. He also mentioned sequestration of CO2 into products, a route that is being explored in various parts in the world.
Viable with no price on CO2
Not only would the Allam-power plant be superior in terms of CO2-emissions compared to conventional facilities, it would also be cost-competitive, NET Power claims. As is the case with most technology development, competing on cost with established technologies is always difficult. Being costcompetitive in an early stage is therefore quite remarkable.
NET Power stated that its technology will be price competitive on electricity sales alone, through economy-of-scale, after 30 plants (300 MW each) have been built. This excludes possible revenues from CO2. The company will use other revenues from the plant, including selling the CO2 for EOR or industrial purposes, generating tax credits for CO2 capture, and selling other industrial gases the plant produces, to provide compelling economics even with the first plants.
As for sequestration, one can imagine that the route via products would be more beneficial to the business model as it generates value whereas sequestration underground only involves cost, except when used for EOR. Whatever scenario is employed, it will be very difficult to predict a future CO2- market which is dependent on political measures to reduce CO2-emissions (see the Paris agreement, ed.), and NET Power is not expecting to be reliant on these.
Meanwhile various experts have been asked about their opinions regarding the technology and its viability in the real world. Daniel Cohan, Professor at Rice University, stated that it is the first concept that could be a triple win for climate, environment and cost. “Other carbon-capture concepts use huge amounts of energy so they require more coal mining or natural gas drilling and the environmental damage that comes with it.” For the time being, NET Power will be testing its technology in Texas. According to Brown, speaking to Forbes.com, the company is “flipping various switches” at the moment. As for a commercial-scale plant, NET Power is targeting deployment of that plant in around 2021.
Valve World reached out to NET Power and received feedback regarding the use and testing of critical valves in the process.
Which (types) of critical valves are used in the process?
“We have predominately air operated ball and EHC valves in the critical portion of the plant that are used for control purposes. There are also stop, check, and relief valves in critical parts of the plant designed for pressures up to and above 4500 psig. Critical valve bodies are casted 316SS in high temperature locations. Oxygen valves are specialty monel valves.”
In terms of heat and/or pressure, what are the consequences for these components?
“We have had to be careful on selection of soft parts in the valves given the unique conditions in the process, including heat, pressure, and the unique process chemistry. Failure modes were discussed and deliberated in detail in multiple HAZOP meetings.”
Which testing procedures are employed for critical valves?
“We conducted CFD on critical valves to validate max Cv and then conducted flow tests in the factory prior to shipping to validate Cv. During commissioning, all valves were stroked during cold flow with CO2 or their primary working fluid. During testing, instrumentation on certain valves was added to enable validation of Cv curves, flow rates and dp across the valve.”
Diagram of the Allam Cycle. Note the small amount of water used, which illustrates that it is not a steam cycle. (Photo courtesy of NET Power)