Deputy Editor Tim Probert reports from the University of Leeds, which in conjunction with the Chinese Academy of Sciences, is developing an integrated system for thermal power generation, electrical energy storage and carbon capture. The new system combines an open nitrogen cycle with a closed, gas fired Brayton cycle, with carbon dioxide captured in the form of dry ice. Unsurprisingly, the project has attracted interest from utilities and manufacturers from around the world.
Tim Probert, Deputy Editor
Who says British engineering is dead? Not at the University of Leeds in northern England, the brains behind a novel concept to develop an integrated system that appears to combine the ‘Holy Trinity’ of power engineering at a relatively low cost: efficient gas fired power generation, electrical energy storage and easy-to-store carbon capture.
The system is designed to be used in conjunction with a combined-cycle gas fired turbine power plant, either by retrofitting an existing power plant or by installing the system at a new facility. The University of Leeds says the integrated system could be also used for coal fired generation, but the coal would first have to be gasified.
The integrated power generation project utilizes a novel cryogenic energy storage system and CO2 capture. The new system combines a direct open nitrogen (cryogen) expansion cycle with a natural gas fired closed Brayton cycle, and the CO2 produced is captured in the form of dry ice.
Excess electricity from the baseload units of a large scale combined-cycle gas turbine power plant produces cryogen, e.g. liquid nitrogen or liquid air. During periods of peak demand, ambient heat boils the cryogen, which expands in an engine or a turbine to generate power (see box on page 32 for full technical explanation).
Joint UK/China project funding
The project, funded by the UK’s EPSRC (Engineering and Physical Sciences Council) and Beijing’s Chinese Academy of Sciences, is barely at the prototype stage, but Professor Yulong Ding, of the University of Leeds’ Institute of Particle Engineering believes his system has the potential to be in commercial deployment within five years.
“The problem with existing CCS technology is the high cost and that at present there is not yet any storage space for the CO2,” he says. “This concept aims to solve these two issues. A further problem is the increasing need for peaking power. This system can increase the capacity of power plants by as much as 50 per cent.”
Ding says the cost of the system, which requires the cryogenic system, an air separation unit and an extra gas turbine, would not be prohibitive (see box on page 34 for economic analysis). “Air Liquide can supply off-the-shelf air separation units (ASU) – this is a relatively commonplace, mature technology,” he says. The ASUs are powered by off-peak electricity, with the oxygen fed to the combustor to burn with natural gas. The high temperature gas is then fed to the gas turbine. The liquid nitrogen produced by the ASU is used in an open cycle to provide additional power. The cold energy produced in the process is also used. The high grade cold is recovered through power generation and low grade cold is used after power generation to capture CO2 in the form of dry ice.
The waste cold is -43 oC, sufficient to capture CO2. The dry ice can be stored and used subsequently to generate another revenue stream. The dry ice, effectively pure CO2, can also be used to produce fuels.
Novel cryogenic system
Ding says the advantages of the cryogenic energy storage technology, which is at the early stages of development, include low capital costs per unit energy, relatively high energy density and long storage duration. It is very environmentally friendly. However, cryogenic energy storage has a relatively low round trip efficiency of around 40 per cent due to an inefficient liquefaction process.
|A computer program based on Matlab 7.0 was written for simulating the new cycle. The whole system was assumed to be in the steady state. The letters and numbers in brackets after entries in the first column relate to the flow sheet on page 32|
Ding concedes that the efficiency of the ASU is not spectacular, but combined with the ability to store energy and capture carbon, the project is an appealing prospect.
“At off-peak time, a lot of electricity is wasted. By running the air separation plant during off-peak to produce nitrogen and oxygen, this is a form of energy storage that can be used for peaking power,” he says.
The total power output of the system is 46.12 MW, approximately twice that of an oxy-fuel combined-cycle
Professor Ding adds the inefficiency of the cryogenic system is mitigated by the use of low-grade heat from the power plant to superheat the working fluid. The efficiency of the cryogenic energy storage can be increased substantially if waste heat is considered as a free energy source. In addition, there is plenty of room for improving the efficiency of the liquefaction process.
The University of Leeds research laboratory prototype uses a Honda 5.5kW engine
Ding says the cycle’s exergy efficiency is as high as 64 per cent under the baseline conditions, whereas the corresponding electricity storage efficiency is about 54 per cent.
Sensitivity analyses indicate that the above baseline performance can be enhanced by increasing the gas turbine inlet temperature, decreasing the approach temperature of the heat exchange process, operating the combustor at an optimal pressure of 7 bar and operating the cryogen topping pressure at 90 bar.
Professor Ding says further enhancement could be achieved by increasing the isentropic efficiency of the gas turbine and the liquefaction processes. He suggests the power capacity installation of peak-load units and fuel consumption could be halved by using the newly proposed system. However, Ding concedes that further work is needed to study the economic aspects associated with the process, particularly on the cryogen storage and helium make-up due to possible leakage in the closed cycle.
At present, the project is still very much at the early stages of development. Professor Ding’s team has built a prototype in a research laboratory at the University of Leeds which has an air separation unit and a 5.5 kW Honda engine, which generates enough electricity to be fed into the National Grid via an ABB inverter (see photograph on page 28).
|The project prototype features a rudimentary ASU and a 5.5 kW Honda engine which generates power fed into the grid|
Interest from the private sector
The next step for the project is to build a pilot-scale demonstration plant, which would require private investment. Ding has had enquiries from around the world about the project. As well as attracting interest from British and Chinese utilities in building a demonstration unit, the University of Leeds project has also caught the eye of academic institutes in the USA.
“We have also had enquiries from a plastics manufacturer that uses a great deal of CO2,” says Ding. “Such a company could either retrofit an existing plant to produce CO2 for the manufacture of plastics, or it could transport CO2 from power plants in insulated trucks. Either way, the carbon footprint of such an integrated power plant would be greatly reduced, while bringing the benefits of additional revenue streams from the sale of dry ice, as well as lowering carbon permit bills.”
|The process flow sheet for the newly proposed cycle Source: Institute of Particle Science and Engineering, University of Leeds & Institute of Process Engineering, Chinese Academy of Sciences|
Ding says his project is unique and seems slightly surprised such a system has not hitherto been developed. “People ask me why no one has attempted this before. I tell them ‘I don’t know!’ But if we have investment from a utility, I believe we could see commercial-scale deployment of this project within five to eight years.”
1. Yongliang Li, Haisheng Chen, Yulong Ding, Institute of Particle Science and Engineering, University of Leeds, UK; Y. Jin, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China; Chunqing Tan, Institute of Engineering Thermoplastics, Chinese Academy of Sciences. An integrated system for thermal power generation, electrical energy storage and CO2 capture, 2010.
2. Bubbico R, Di Cave S, Mazzarotta B. ‘Preliminary risk analysis for LNG tankers approaching a maritime terminal’ in the Journal of Loss Prevention in the Process Industries, 2009.
3. Cornot-Gandolphe S, Appert O, Dickel R, Chabrelie M-F, Rojey A. ‘The challenges of further cost reductions for new supply options’ at the 22nd World Gas Conference, Tokyo, Japan, 2003.
HOW THE SYSTEM WORKS1
At off-peak hours, excessive electricity generated by the baseload units is used to power the air separation and liquefaction (ASU) plant to produce oxygen and liquid nitrogen while the rest of the system is powered off.
The produced oxygen and liquid nitrogen are stored in a pressurized vessel and a cryogenic tank, respectively, for generating power via the high-pressure turbine (HT) and low-pressure turbine (LT), and assisting combustion in the combustor (B) at peak hours.
At peak hours, natural gas is compressed in the compressor C1 to the working pressure. The working fluid then mixes with oxygen in the combustor (B) where combustion takes place to give high-temperature and high-pressure flue gas consisting of CO2 and H2O.
Combustion of natural gas in an oxygen environment can produce a temperature that is too high for the gas turbine. To control such a temperature, an appropriate amount of helium gas is mixed with the flue gas before entering the gas turbine for power generation through a generator (G). The helium gas is not consumed but circulates in the system.
The flue gas containing helium from the gas turbine then goes through a series of heat exchange processes via heat exchangers 1 (HE1), 2 (HE2) and 3 (HE3) to recover the waste heat by passing the heat to a nitrogen stream from the ASU. During the heat recovery processes, steam in the flue gas is removed via a condenser (WS), whereas CO2 is removed in the form of dry ice through a solidification process in CS (the triple point of CO2 is 5.718 bar and 56.61 oC).
As a result, the flue gas stream after CO2 removal contains only helium.
The helium stream is then cooled down further in HE3 and compressed in compressor C2 to the working pressure, and finally goes through further heat exchange in HE2 and HE1 before flowing back to the combustor. Note that there may be a very small amount of CO2 in the separated water stream (WS), but for simplification of the calculations, it is assumed that water, CO2 and helium are fully separable.
The nitrogen stream starts from the cryogenic storage tank where liquid nitrogen is pumped to the working pressure by a cryogenic pump (P). The high-pressure nitrogen is then heated in heat exchangers (HE3, HE2 and HE1 in series) and expands in two stages via respectively a HT and a LT to generate electricity.
HE1 serves as an inter-heater between the two-stage expansion. After expansion, the pure nitrogen can be used to purge the sorbent bed of the ASU air dryer. From the above, one can see that the newly proposed cycle consists of a closed-loop topping Brayton cycle with He/CO2/H2O as the working fluid and a open-loop bottoming nitrogen direct expansion cycle. The topping Brayton cycle can be identified as 4-5-6-8-9-11-12-13-14-15-16-4, whereas the bottoming cycle is 18-20-21-22-23-24-25-26.
It is the combination of the two cycles that produce electricity at peak hours. The Brayton cycle uses a small amount of natural gas, which is burned in the pure oxygen produced by the ASU during off-peak hours. Helium is only used to control the turbine inlet temperature and is recirculated. The working fluid of the open cycle, nitrogen, is the actual energy carrier of the off-peak electricity. As CO2 is captured, only water and nitrogen are emitted from the process.
ECONOMIC ASPECTS of the integrated system1
The proposed new system is for large-scale EES. It is recognized that economic analyses are complicated and are beyond the scope of this work. However, we are able to discuss the following factors that are related to the economic aspects.
First is the storage tank volume of oxygen and nitrogen and the costs for making the tanks. Considering the example system runs continuously for eight hours, the storage volume for the required liquid nitrogen is about 1131 m3. Such a volume should not present an issue in terms of tank manufacturing because insulated cryogenic tanks with a capacity that is greater than 50 000 m3 have been widely used for liquefied natural gas (LNG) storage and transportation .
Although no exact cost can be given at the moment for making a cryogenic tank with 1131 m3 volume, it is expected to be at least several millions of US dollars based on the costs of making LNG tanks (e.g. about $170 million are needed to make an LNG tank with a maximum capacity of 135 000–138 000 m3 ).
Oxygen can also be stored in the liquid state and, as a consequence, the total volume should be smaller than that required for liquid nitrogen. The costs for making the tank are expected to be at a similar order of magnitude as those for the liquid nitrogen tank.
Second is the utilization of helium as the working fluid of the gas turbine cycle. Helium is selected not only to avoid NOx production in combustion but also for its better compression and expansion behaviour. If compressed isentropically from the ambient pressure to the topping pressure, the eventual temperature of helium is much higher than that of nitrogen.
This implies that helium could recover the high-grade cold more efficiently. On the other hand, if expanding isentropically from the topping pressure to the ambient pressure, the eventual temperature of helium is much lower than that of nitrogen as more thermal energy converts to power through the gas turbine.
These two merits make helium more attractive to be the working fluid. It is recognized that there is a costing element here, as helium is more expensive. Although helium recirculates in the closed-loop cycle, leakage is unavoidable in a realistic system and as a result helium makeup will be needed. Currently, the amount of helium makeup cannot be estimated accurately due to the separation processes of steam and dry ice in the helium cycle. Further experimental work will be needed in this respect.
Third is the cost of a gas turbine for a working fluid consisting of a mixture of helium, CO2 and steam. This is believed to be less challenging. The inlet pressure of the turbine is about half that of the reference gas turbine. This makes it relatively easy to design and manufacture.
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