Gearing up for a new supercritical CO2 power cycle system

Toshiba has almost completed detailed design in preparation for a turbine that will use carbon dioxide as the working fluid.

25MWe Demo Plant. Design features a single can-type combustor and double-shell turbine structure, scaled-down model of a 250-300MWe turbine design for a commercial plant.

The case for building new coal fired plants or back-fitting existing coal plants with carbon capture technology is economically unattractive. The case for equipping gas-fired plants with carbon capture and storage (CCS) is even more difficult to justify.

 

High capital cost, combined with the penalty in efficiency that the capture process places on the power plant, has so far proved to be a major stumbling block in the commercial deployment of power plants with CCS.

 

A solution that has been under serious development for the last 5 or 6 years, however, has now reached the stage where the key component – a new type of turbine and combustor – is close to the start of manufacturing.

 

The turbine and combustor, being designed and built by Toshiba, essentially combine gas turbine and steam turbine technologies, with the potential to deliver a power plant with:

 

● Efficiency of about 59% (LHV) when running on natural gas

● Efficiency of 51-52% (LHV) when running on gasified coal (syngas)

● Full 100% carbon capture at 300 bar without any efficiency penalty.

 

Since 2012, Toshiba has been developing a new turbine and combustor for the new CO2 power cycle together with NET Power, CB&I, Exelon and 8 Rivers Capital. The five companies have now completed major agreements to build a 25MW gross electric (50MWt) demonstration plant in Texas.

 

Through the successful completion of operating tests, the demonstration plant is intended to provide the basis for construction of the first 295MWe full-scale commercial plant.

 

CO2 power cycle

NET Power LLC was formed almost six years ago by 8 Rivers Capital, a technology commercialization firm based in North Carolina (and inventor of the supercritical CO2 power cycle) with a clear and different approach to tackling the problem of burning fossil fuels more cleanly.

 

Instead of trying to “fix” supercritical coal, IGGC (integrated gasification combined cycle) or natural gas combined cycle power plants, 8 Rivers looked at designing a fossil based system from scratch that achieves the desired end result. The cycle produces pipeline-ready CO2 and no air emissions without reducing plant efficiency or increasing costs.

 

In 2009, Rodney Allam, a former head of technology development at Air Products, joined the company to work on a new thermodynamic cycle so all the emissions are controlled from the outset.

 

Process description

The cycle is not a combined cycle. Instead, it exploits the special thermodynamic properties of carbon dioxide as a working fluid by eliminating the energy losses that steam-based cycles encounter due to the heat of vaporization and condensation.

 

According to the company, the Allam Cycle (named after its lead inventor) removes the steam Rankine Cycle from the process and improves upon the simpler, more efficient Brayton Cycle. The Allam Cycle combusts natural gas or synthetic gas (derived from a coal gasification system) with pure oxygen (oxyfiring), as opposed to burning gas with air.

 

Following a Brayton Cycle-like expansion across its turbine, CO2 is recirculated back to the beginning of the cycle in a highly recuperative process. The system eliminates the expensive steam cycle components and avoids the inefficiencies of traditional Rankine cycles.

 

This process generates a relatively pure stream of high pressure carbon dioxide and some water while significantly reducing or eliminating other pollutants such as NOx. At very high pressures, CO2 exhibits a greater energy density and work output, enabling the cycle to reach extremely high efficiencies.

 

 

CO2 Power Plant Project Partners

 

Toshiba, NET Power, CB&I, Exelon and 8 Rivers Capital are working together to develop and commercialize the application of supercritical carbon dioxide power cycle technology for efficient emissions-free electric power generation.

 

They have completed major agreements to build a 25MWe gross electric (50MWt) demonstration plant in the U.S. for test and evaluation that will provide the basis for the design and construction of a full-scale 295MWe commercial plant.

 

● Toshiba is to provide a first-of-a-kind turbine that will utilize supercritical CO2 as a working fluid to produce low-cost electricity while eliminating NOx, CO2 and other pollutants.

 

● CB&I to provide engineering, procurement and construction services including pre-FEED (front-end engineering design) and FEED studies for the demo plant and pre-FEED study of a commercial 500MWt power plant.

 

● Exelon, one of the leading competitive energy providers in the US, will be responsible for siting, permitting and commissioning the demo plant facility.

 

● NET Power will be responsible for project management, overall system engineering and integration, coordination between the partners.

 

● 8 Rivers Capital , inventor of the supercritical CO2 cycle, will provide ongoing engineering and technology development services.

 

Timetable calls for Toshiba to begin delivery of key equipment to the demo plant site in August 2016. The completed plant is expected to enter the commissioning stage before the end of 2016.

 

The working fluid is expanded through a turbine that has an inlet pressure in the range of 200 bar to 400 bar and a pressure ratio between 6 and 12. It is then cooled through a heat exchanger, and H2O is separated from it to create a CO2 stream. The CO2 stream is pressurized and a major part of this flow is fed back to the combustor to begin the cycle anew.

 

This novel cycle separates almost all of the CO2 from the other combustion products, producing a sequestration-ready CO2 byproduct that is at pipeline quality and pressure. The need for a separate CO2-capture system is thus eliminated. There is therefore no efficiency penalty of adding a capture process, which can typically result in a loss of around 10 per cent in overall electrical efficiency.

 

An important factor in achieving high net cycle efficiency is to use a high turbine inlet temperature. This temperature, however, is limited by the maximum allowable temperature of turbine exhaust that flows directly into the heat exchanger.

 

The operating temperature at the hot end of the heat exchanger is thus in the range of 700°C to 750°C. This leads to a typical turbine inlet temperature constraint in the range of 1100°C to 1200°C.

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