Breaking ground for a groundbreaker: the first Allam Cycle power plant

The 50 MWt demonstration plant is being built in La Porte, Texas.

“NET Power is the first technology that allows policy and economics to work together, instead of against each other, to ensure the world meets our climate targets," said NET Power's CEO, Bill Brown. The groundbreaking "marks a significant step for our world-class team, including Exelon, CB&I, 8 Rivers and Toshiba, towards delivering a technology that will be the cornerstone of a modern global energy infrastructure that is clean, affordable and flexible."

Executives from each of the companies gathered on the site to mark the start of construction of the demonstration plant. The $140 million programme - which not only includes demonstration plant design and construction, but also ongoing technology advancement, a full testing and operations programme, and commercial product development - is funded by a combination of cash and in-kind contributions from Exelon and CB&I. Toshiba has developed and is now manufacturing a new supercritical carbon dioxide turbine and combustor fortheproject.CB&Iisperformingthe engineering, procurement, and construction of the plant. Exelon is providing operations, maintenance, and development services. 8 Rivers, in which Rodney Allam is a partner, owns the technology and continues to develop it. Hideo Nomoto, formerly with Toshiba and largely responsible for the Japanese company's involvement, saying he became "quickly fascinated" by the Allam Cycle when it was first introduced to him, has also recently joined 8 Rivers.

The Allam Cycle based plant produces only electricity, liquid water and pipeline- ready carbon dioxide, while at the same time operating as efficiently as the best natural gas power plants available today, says NET Power, capable of a net efficiency of 58.9%
(LHVbasis)ongas(withfullcarboncapture), 51.4%(LHVbasis)oncoal(employingan integrated gasifier), again with full carbon capture.

In addition, for a small reduction in efficiency, the technology "can operate without water, actually becoming a net water producer."
NET Power's 50MWt demo plant will be a "fully operational unit that will generate power to the grid while demonstrating all key aspects of the Allam Cycle", with commissioning expected to begin in late 2016 and be completed in 2017.

The facility will also provide the validation to begin constructing the first 295 MWe, commercial-scale plants says NET Power, which "is already engaged with customers across several industries on the design and development of these projects", including in the USA and UK.

As well as the new turbine, combining elements of both gas and steam technology, and the new combustor (which has been successfully tested on a test rig in California), key to implementation of the Allam Cycle is the recuperative heat exchange system downstream of the turbine (see item 7 of the flow diagram, right), which must be highly efficient while coping with onerous conditions, including high temperatures and pressures. The outlet temperature of the carbon dioxide stream leaving the turbine is about 750°C. This is cooled to around 50°C and the heat transferred to the 300 bar carbon dioxide stream entering the combustor, which is heated to about 720°C prior to entering the combustor.

Heatric has been chosen to supply four "printed circuit" heat exchangers (PCHEs) for this part of the 50 MWt demonstration plant. Heatric's printed circuit heat exchangers are well established in the demanding conditions of offshore gas production, with some 2700 units in operation around the world. The Heatric units are manufactured using a diffusion bonding "solid state" process, originated at the University of Sydney. This "creates a much stronger, more efficient and compact exchanger, almost impervious to the harsh offshore operating environment", says Heatric. "The process creates an exchanger core with no joints, welds or points of failure, resulting in a unit combining exceptional strength and integrity with high efficiency and performance in a unit up to 85% smaller and lighter than traditional technologies such as shell and tube exchangers." Heatric was initially established in Australia in 1985 and acquired by Meggitt in 1990, subsequently moving to the UK in 1990.

The flow plates in a Heatric exchanger 'stack' are diffusion bonded using a high temperatures and pressures with no melting or deformation of channels, ensuring flow integrity and complete bonding of all plates throughout the stack. No brazing flux or filler is used in the diffusion bonding manufacturing process, which acts by generating metal grain growth between the upper and lower surfaces of each plate. The result of this is process is a very high integrity solid block of the parent metal with heat exchange flow channels running throughout its core.

The Toshiba developed carbon dioxide turbine combines elements of both advanced steam (including the ability to operate at a pressure of 300 bar, with pressure ratio about 10) and the high temperature capabilities of gas turbines. The inlet temperature of the turbine is 1150°C, which is nevertheless "not that high" compared with modern GT practice, Hideo Nomoto pointed out at a recent IMechE conference in Manchester, UK (Steam turbine and generator user group, 16-17 March 2016) where he, together with Rodney Allam, presented an update on the project.

The novel turbine, for example, which is currently being manufactured by Toshiba in the UK, with first firing expected at the plant site in March 2017, employs thermal barrier coatings and the kind of intricate cooling channel arrangements familiar from advanced gas turbine practice.

Materials employed in the turbine include the following: Ni based forging and CrMoV forging welded together for the rotor (to minimise requirement for Ni based material); CrMoV casting for outer casing; Ni base casting and CrMoV for inner casings; Ni base casting for blades. Goodwin Steel Castings of the UK has been working with Toshiba on development and manufacture.

The required Ni based materials had already been developed by Toshiba for gas turbines and advanced ultrasupercritical coal plant applications (eg, TOS1X and TOS3X) and were "ready for use", so "time consuming materials R&D was not necessary", according to Hideo Nomoto, although the welding has proved challenging (as it did in the EU AD700 R&D programme).

Rig testing on the Toshiba developed combustor has been successfully completed at a test rig in California. This is a first of a kind in view of the high pressure, and working fluid, but because there are no NOx emissions from atmospheric nitrogen, a stable diffusion flame can be used.

A key feature of the Allam Cycle is that heat goes in at two temperature levels, high, via fuel combustion, and low, less than 400°C. The low temperature heat is vital in attaining high efficiency and can be derived from, eg waste heat from the air separation unit, waste heat from a gasifier quench, LNG regasification, solar power, existing power station. This means the cycle can be seen as "a platform with diverse applications", its developers say.