NET Power's CO2 cycle: the breakthrough that CCS needs

10 July 2013



NET Power's proposed oxyfuel power plant technology, with supercritical CO2 as the working fluid in a radically new cycle, makes carbon capture an inherent feature of the process, not an add-on with very large parasitic loads, as with "conventional" CCS approaches. The result: 100% carbon capture with extraordinarily high target efficiencies, 58.9% LHV for gas, 51.44% LHV for coal.


By Rodney J Allam

Using a radically new, proprietary power generation cycle, NET Power's technology promises electricity generation from fossil fuels that is not only cost competitive with the current lowest-cost, pollutant-emitting, technologies, but which can also eliminate all emissions to air from the power generation process without degrading the plant's overall economic performance.
NET Power's cycle, which can be seen as a breakthrough in power generation technology, addresses the primary shortcoming of "conventional" carbon capture concepts: they are intrinsically additive (or parasitic) processes that increase capital costs and reduce overall plant efficiencies. Despite efforts to reduce the cost of these technologies, by their very nature they will always result in an increase in the cost of electricity when applied to traditional power systems.
The NET Power approach, in contrast, was designed from the ground up to overcome these limitations by using a high pressure, highly recuperative, oxyfuel, supercritical CO2 cycle that makes carbon capture part of the core power generation process, rather than an add-on. The result is high efficiency power generation that inherently produces a pipeline-quality CO2 byproduct at no cost to the system's performance.
NET Power is partnering with CB&I, Toshiba Corporation, and Exelon Corporation to demonstrate this new system in a 50 MWt natural gas power plant.


CO2 as a working fluid
The NET Power cycle relies on several key principles to achieve high efficiencies, low capital and O&M costs, and no atmospheric emissions. Most important, NET Power turns the CO2 problem into the solution by exploiting the special thermodynamic properties of carbon dioxide as a working fluid. This avoids the energy losses that steam-based cycles encounter as a result of heat loss inherent in the unavoidable vaporisation and condensation of water. In the process, NET Power generates - at no additional cost - a high-pressure, high-quality CO2 byproduct that is ready for pipeline removal.
At the core of the cycle is a supercritical CO2 loop, where high-pressure CO2 passes through a turbine, is cooled to remove water and impurities, and then is re-pressurised and reheated in a heat exchanger against the hot turbine exhaust stream and returned to the combustor.
The cycle (Figure 1) begins at the top end by burning natural gas (or a gasified solid fuel) with nominally pure oxygen in a high-pressure, oxy-fuel combustor. This hot, high-pressure feed drives a single turbine that operates with an inlet pressure in the range of 200 to 400 bar and a pressure ratio of 6 to 12.
After exiting the turbine, the high-temperature exhaust flow enters an economiser heat exchanger, where its heat is transferred to the high pressure CO2 flow that is heading back into the combustor. After exiting the economiser heat exchanger, the turbine exhaust flow is cooled to near atmospheric temperature, and any water in the flow is condensed and separated. The remaining working fluid, which is predominantly CO2, is compressed, reheated in the economiser heat exchanger, and then sent again into the combustor, where it dilutes the combustion products. This process has the double effect of retaining heat in the system and lowering the turbine inlet temperature to an acceptable level.
As fuel and oxygen are continuously added to the system for combustion, an excess of CO2 byproduct is created (alongside H2O, which is continuously removed). This excess CO2 is removed from the high pressure recycle flow at a high purity and pressure, enabling delivery to a CO2 pipeline without negatively impacting the performance of the overall system.
The cycle's high efficiencies are driven in part by maintaining temperatures and pressure above the critical point of carbon dioxide. The compression system for the CO2 recycle flow at the lower end of the cycle (after the H2O is removed and before it is reheated for return to the combustor) raises the working fluid's pressure to approximately 80 bar and then cools it to near ambient temperature. With a density above 700 kg/m3 at these conditions, the recycle CO2 flow can then be pumped (rather than compressed) to higher pressures with a multi-stage centrifugal pump. Taking advantage of this unique aspect of supercritical CO2 saves considerable energy as the recycle stream is brought back to the pressures required for reintroduction into the combustor.
Another important driver of NET Power's high power generation efficiency is its ability to achieve a close temperature approach at the hot end of the heat exchanger. In the pressure-enthalpy diagram for CO2 (Figure 2), the imbalance between the heat liberated by the low-pressure turbine exhaust and the heat required to raise the temperature of the high-pressure recycle flow from its lowest temperature point is apparent. This imbalance is caused by a very large increase in the specific heat of CO2 in the high-pressure recycle flow at the low-temperature end of the economiser heat exchanger.
The NET Power cycle addresses this issue at the low-temperature end of the heat exchanger by incorporating waste heat (from an external source) into the CO2 flow. One NET Power configuration accomplishes this by utilising waste heat generated by the air compressors of the cryogenic air separation plant associated with the oxy-combustion system. This integration also enables the system to dramatically mitigate the large parasitic impact of oxygen separation that is typically experienced by oxy-fuel systems.


A technology platform
This basic natural gas cycle provides NET Power with a core technology platform from which many other derivative systems can be developed. One important variation is the NET Power coal cycle (Figure 3), which integrates the base cycle with a commercially available coal gasifier.
In this system, the low grade heat from the gasifier is recovered to the low temperature region of the high pressure CO2 recycle, where it is used to heat a side stream from the economiser heat exchanger (thus serving a similar purpose to the air separation unit in the natural gas configuration). The result of this close coupling is near 100% thermal efficiency of the gasifier, thereby driving efficiencies significantly higher than any other coal-based generation system.
The coal-derived fuel gas is combusted, and the predominant impurities in the low-pressure turbine exhaust stream are SO2 and NO/NO2. These, in turn, are converted to H2SO4 and HNO3, which occurs mostly within the cold-end passages of the heat exchanger in the presence of condensed liquid water and excess oxygen. The pressure of the turbine exhaust flow, in the range of 16 bar to 66 bar, ensures that the reaction kinetics are fast. This is particularly marked for the NO oxidation reaction, which is accelerated by the relatively high partial pressures of the NO and the excess O2 present after combustion.
This process step has been demonstrated in several locations, including Vattenfall's Schwarze Pumpe pilot plant in Germany.
Further, the nitric acid present will largely remove mercury contaminant, and the H2SO4 can be converted directly to CaSO4 by reaction with a limestone slurry in a simple stirred tank reactor. The Ca(NO3)2 is highly soluble in water and can be separately recovered if desired.
Other applications where the NET Power cycle offers large cost, efficiency, and environmental gains include:
integration with liquefied natural gas regasification facilities (increasing efficiencies into the high 60s);
solar-natural-gas hybrid plants that integrate heat from concentrated solar power (increasing efficiencies into the mid 70s);
direct integration with enhanced oil recovery facilities, using any sour gas as a combustion fuel; and
integration with existing steam power cycles, where steam in existing power stations is superheated by the NET Power cycle, increasing the total cycle efficiency.


Gas and coal development
Together with CB&I, Toshiba Corporation, and Exelon Corporation, NET Power is developing a 50 MWt natural gas power plant (Figure 4) that will generate electricity and capture all produced emissions. The plant is a scaled-down version of a larger commercial plant, currently designed at 250 MWe, such that the operability and performance demonstrated by the testing and commissioning programme will be directly applicable to the future scale-up of the system.
To that end, the plant's components have been initially designed at the 250 MWe level, and CB&I is conducting a 250 MWe pre-FEED study concurrently with the pre-FEED and FEED studies for the 50 MWt plant. The demonstration plant is anticipated to come online within two years.
NET Power is in addition currently undertaking the early development stages of the first large-scale gas-fuelled commercial facility.
Work is also proceeding on development of a coal system. Given the heavy reliance on coal in countries with very large anticipated power growth, including in particular China and India, NET Power views this as a very important technology, especially in the light of the projected performance and environmental gains over conventional coal systems. In 2012, the United Kingdom Department of Energy and Climate Change awarded NET Power a £5 million grant as part of the CCS Innovation Competition. A portion of that funding is supporting further design and analysis of NET Power's coal cycle, which is being undertaken in the UK.
NET Power anticipates the coal development programme to continue in parallel to the natural gas demonstration programme so that, upon commissioning of the natural gas demonstration plant, the coal system can be commercialised soon thereafter.


Toshiba turbine and combustor
Toshiba has undertaken the development of the new combustor and turbine that will be required due to the pressures, temperatures, and working fluid of the NET Power cycle.
Toshiba's approach to the turbine involves combining well-known gas and steam turbine technologies, as the turbine inlet temperature is well within the operational parameters of current gas turbine technologies, and the turbine pressure is not beyond that of advanced steam turbine technologies.
Toshiba is well-positioned to develop this turbo-machinery, as it has a strong track record in both of these fields, including in the areas of high pressure and high temperature ultrasupercritical and advanced ultrasupercritical turbines.
Toshiba's design approach has focused on utilising proven technologies whenever possible and developing a scaled-down version of a full commercial turbine for use in the 50 MWt NET Power demonstration plant.
The turbine design employs a double-shell structure to allow for the system's high pressure. A CO2 cooling flow will fill the space between the outer and inner casing, enabling the outer and first inner casings to utilise CrMoV casting. The smaller inner casing, where temperatures exceed 700°C and moderate cooling is applied, will require Ni-based materials. The central portion of the rotor will also use Ni-based forgings; however, the ends of the rotor are able to employ CrMoV forgings, thereby limiting the use of more expensive materials.
Toshiba has developed its own Ni-based materials, "TOS1X" and "TOS3X," which are used for forging and casting respectively (the product names mentioned being trademarks of Toshiba). Both of these materials have higher creep rupture strength than commercial Ni-based materials and are ideal for this turbine.
The cooling approach being employed calls for the utilisation of a CO2 cooling flow supplied through the centre bore of the rotor. Radial holes in the rotor enable the flow to be distributed throughout the various stages of the turbine. Detailed analysis has shown that film cooling typical of high temperature gas turbines will not be needed.
Toshiba is developing a combustor design capable of handling the high-pressure combustion required by NET Power. The combustor's design benefits from being able to use more stable, simpler diffusion combustion; NOx is not a concern since NET Power is an oxyfuel process. Additionally, the combustor's temperature profile enables the use of proven cooling techniques, such as back-side convection cooling.
A scaled down version of the combustor is being used for testing (Figure 5).


Approaches to heat exchange and oxygen supply
The economiser heat exchanger required by the cycle needs a very large specific surface area to allow for the high flows of CO2 and the large temperature range, between 50°C and 750°C. In order to handle the very high pressures, two different configurations are available.
The first uses passages fabricated from flat metal sheets that are shadow masked with a pattern of the required passages on one side of the metal surface, chemically etched to form the passages, stacked to form a heat exchanger block and diffusion bonded in a heated chamber. The resulting complete heat exchanger block is essentially one continuous metal element with no boundaries present between the passages. Heat exchangers of this variety have been used extensively in the oil industry and recently in experiments on CO2 heat transfer for nuclear reactors.
The second configuration consists of separately fabricated sections of corrugated sheet metal that form a finned surface when used in heat exchanger passages. The low pressure passages can be fabricated from formed fins that are sandwiched between the high pressure passages. As an alternative to chemical milling to form the high pressure passages, fins held between plain parting sheets can be used. This method of fabrication has been used for gas turbine economiser heat exchangers fabricated from stainless steel.
NET Power's oxygen requirements can be met by pumped LOX cycle cryogenic air separation units. An O2 compressor will deliver the oxidant mixture to the combustor at the required pressure. A commercial 250 MWe natural gas system would require about 3000 tonne/day at 99.5% purity.


High efficiency, low cost, valuable byproducts
Both the natural gas and coal NET Power cycles have high target efficiencies that have been modelled extensively with all parasitic loads included. With a target net efficiency of 58.9% LHV, the NET Power natural gas system is on a par with current natural gas combined cycle systems; the NET Power system, however, includes 100% carbon capture, while the natural gas combined cycle does not. For coal fuels, NET Power's target net efficiency of 51.44% LHV, again with 100% carbon capture, is far superior to that of conventional coal systems that do not include any capture systems whatsoever.
Both cycles also benefit from very high target gross efficiencies, which have been modeled at 82.7% LHV for natural gas and 74.9% LHV for coal. This enables the cycles to absorb the high parasitic power requirements of the air separation unit and the CO2 compressors while maintaining high net efficiencies. Additionally, such high gross efficiencies indicate that there is significant room to increase the cycles' net efficiencies with reductions in power requirements for auxiliary equipment, especially air separation systems and CO2 compressors.
The target efficiencies (%) for the NET Power technology are summarised in Table 1, with full CO2 capture at 300 bar:
There are several features of the NET Power cycle that will enable it to maintain low capital costs. Having eliminated the entire steam section of a combined cycle plant, including the HRSG, main steam piping, reheat steam piping, steam headers, and the entire steam turbine block, the NET Power cycle is much smaller than a typical CCGT plant.
Extensive layout designs indicate that NET Power's gas cycle will have a footprint of one-third to one-half the size of a traditional combined cycle plant of similar output. This small footprint is also a result of the high pressure nature of the cycle, which leads to smaller components. Moreover, the elimination of the large, expensive emissions control systems and chimney stacks that conventional plants require also provides cost savings and footprint advantages while increasing potential siting opportunities.
The NET Power cycle does require the addition of two significant components, though: the air separation unit and the heat exchanger block, tempering some of the cost reductions.
But one of the most important aspects of the NET Power system from a cost perspective is the ability to inherently capture all produced CO2 without degrading plant performance. While this capability has clear environmental benefits, it also greatly enhances a NET Power plant's overall economic performance by providing a low-cost, pipeline-ready byproduct that can be used for CO2-based enhanced oil recovery or industrial purposes.
Finally, 8 Rivers Capital, the original inventor and developer of NET Power, continues to develop new technology around power generation, and patents for improvements have been applied for that will result in a further cost reduction of approximately 30%.


A significant breakthrough
NET Power's new cycle presents an important opportunity for the power generation industry. In the face of increasing atmospheric CO2 levels, cleanly utilising abundant fossil fuels without raising the cost of electricity is a critical challenge that must be addressed.
By offering a breakthrough system that makes clean, fossil fuel based power an economically prudent choice over conventional, emitting technologies, NET Power offers the solution.

Figure 1. NET Power cycle
Figure 2. Log pressure vs specific enthalpy
Figure 3. NET Power coal cycle
Figure 4. Layout of Power plant
Figure 5. New combustor
Table 1. Target efficiencies


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