As testing begins in Texas, Allam shares his thoughts on building emissions-free natural gas plants with new carbon capture technology
THE Allam cycle is set to revolutionise carbon capture technology by using the captured carbon dioxide to power turbines in natural gas plants.
The technology has been developed by NET Power, which is a company co-owned by technology company 8 Rivers, electricity provider Exelon, and energy services company McDermott International. A demonstration plant in Texas was successfully fired up on 30 May and is currently undergoing testing. The Chemical Engineer caught up with Rodney Allam, Fellow of IChemE and partner at 8 Rivers, to discuss the NET Power demonstration plant and the motivation behind the zero-emissions project.
“I started working on these [carbon capture] systems when I was chief engineer of Air Products back in about 1978 and we looked at removing CO2 from conventional power systems. That got me thinking about the whole problem, and then of course concern about atmospheric pollution and in particular global warming being apparent in the early 1980s. We did studies at Air Products for the UK government and for the EU, and then I worked with the UN Intergovernmental Panel on Climate Change (IPCC). We accurately determined, based on all the published data, exactly what these designs looked like and what the costs were. A number of demonstration plants were built in the world and some of them still operate, but you can’t get away from this very high cost and it’s a very unsatisfactory situation.”
“You have to do something about [climate change]. You can’t just wring your hands and say well these costs are very high but people will have to bear them – because people won’t bear them. They’ll rebel against the high costs, and climate change and global warming will just get worse and worse. Renewables are an incredibly valuable method of reaching this carbon free future, but they aren’t capable of satisfying world demand for power. The only solution I could see was to make the cost basically zero, by making a system that could use fossil fuels and could get rid of the CO2 without increasing the cost of power generation.”
Conventional natural gas plants work by combusting natural gas with air to heat water into steam which is then used to drive a turbine to generate power. Steam is an inefficient working fluid as 30-40% of the original energy is lost through cooling towers. The process emits CO2 into the atmosphere along with polluting NOx, a by-product of burning the nitrogen in the air.
NET Power’s demonstration plant uses the Allam cycle, so that instead of burning the natural gas with air, the fuel is burned with pure oxygen, which eliminates the need for any NOx reduction equipment. Also present in the combustor is preheated CO2 from a previous cycle which is at a pressure 300 times that of the atmosphere. The CO2 is in a supercritical state, which means that it can expand to fill a container like a gas while still having the density of a liquid. When the CO2 is heated further in the combustor, it becomes a working fluid that can drive the turbine to produce electricity. The outlet pressure from the turbine is around 30 times atmospheric pressure, and the only products from the combustion are CO2 and water. Some of the CO2 is pressurised back up to 300 bar and sent back to the combustor, and the rest can be sequestered.
“What you have is a huge recycling system which goes round in a circle and you add a little bit of fuel and oxygen at the top end, and you take pure CO2 and water out at the bottom end,” said Allam.
As pipeline-quality CO2 is produced as a by-product, it can be used in industrial processes such as enhanced oil recovery where CO2 is used to extract more oil from mature oilfields, while storing the CO2 underground.
“CO2 is actually a very valuable commodity in areas of high oil demand. People say that the oil that you’re producing will produce pollution, but it doesn’t have to produce pollution because it can be recycled into a carbon-free system such as this one. Effectively, it vastly extends our ability to handle the insatiable demands of energy of mankind and we can do it in a clean manner and extend our fossil fuels reserves.”
No energy is lost through cooling towers as the Allam cycle keeps heat within the system and less fuel is needed for the turbine to reach operating temperature. The technology can also operate without water, using air for cooling instead.
The only drawback is that the system can’t be integrated into conventional plants, as Allam explains.
“You can’t really take an existing gas turbine system and suddenly retrofit it to make it work in this manner. It’s necessary to wait until the gas turbine system becomes obsolete and instead of replacing it with polluting technology you replace it with sustainable technology.”
“The challenge of course is that in many countries much investment has taken place in conventional coal-fired systems and these plants are not going to be suddenly shut down and replaced. The trajectory of introducing new technology like this extends over the life of the current conventional systems which will be in operation for 30 or 40 years. You can’t imagine a sudden step change where it all suddenly becomes very easy to produce power with no emissions. You have to look at a progression in which existing systems are replaced in a progressive manner.”
If testing at the 25 MW(t) (50 MW(e)) demonstration plant is successful, it could begin generating electricity later this year.
“We’re very pleased with the results of testing so far and the demonstration plant and operations teams have done an excellent job of designing and methodically commissioning the plant,” said Allam. “The commissioning is using a team of commissioning engineers and operators from Exelon, and there’s a team of engineers from Toshiba who are responsible for the testing of the turbine and combustor, and there a team of engineers from 8 Rivers who look after the process and the control system.”
NET Power plans to construct 300 MW(e) commercial scale plants – enough to power 200,000 homes – which could begin operations as early as 2021. A full-size plant would cost around US$300m to build, generate 800,000 t/y of CO2, and produce electricity for the same cost as a conventional natural gas plant.
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