By Ligang Zheng
Research Scientist, Natural Resources Canada
Research Scientist, Natural Resources Canada
The urgency of developing, demonstrating, and deploying CCS technologies is supported by the recently released Intergovernmental Panel on Climate Change report, “Climate Change 2013: The Physical Science Basis”.1 Coal is the dominant fuel for electricity production and is responsible for generating about 40% of electricity in the world. Also, out of the cumulative CO2 emissions from fuel combustion, coal was responsible for 43%. Unlike CO2 emissions in the transportation sector, due to its large quantity and concentrated nature, coal for electricity generation is at the center of the global effort to fight climate change.
An example of regulations to curb CO2 emissions is one made by the U.S. Environmental Protection Agency. The rules propose that the limit for carbon emissions from new coal-fired power plants would be 499 kg CO2/MWh on average over a 12-month period. Upon the release of the proposed CO2 emissions rules for new coal-fired power plants, the U.S. EPA further indicated that proposed regulations for existing coal-fired power plants are to be released in June 2014.
As shown in Table 1, even for the best coal-based generation technology, the ultra-supercritical steam cycle, a reduction of more than 30% of the emitted CO2 must be achieved to meet the proposed rules. Other countries also have either already enacted similar regulations or are in the process of discussions. For example, almost exactly one year ago, Canada announced that new and end-of-life coal-fired power plants will have to meet a performance standard of CO2 emission rate at 420 kg/MWh by 1 July 2015.
Table 2 shows the average CO2 emissions rate for electricity generation in select countries. For those countries heavily dependent on coal generation, such as India and China, the emission rates are very high. Clearly, major efforts are needed to develop technologies to address emissions of those existing plants in order to decrease their CO2 emissions.
Reducing CO2 Emissions with Oxy-Fuel Technology
To reduce CO2 emissions, many innovative ideas have been suggested and a number of technologies have been developed. The best strategy is to employ high-efficiency generation technology whenever possible, since it alone could reduce CO2 emissions by up to 20%, as shown in Table 1. However, to meet the newly proposed CO2 rules for new power plants and to reduce CO2 emissions by existing plants, some of the generated CO2 emissions from coal-fired power plants, even those from ultra-supercritical sources, must be captured and sequestered.
Currently, post-combustion capture (PCC), oxy-fuel combustion, and integrated gasification and combined cycle (IGCC) are the front runners to capture CO2 , and major demonstration projects for these three technologies are underway.3 Both PCC and oxy-fuel combustion are based on pulverized coal combustion and yet the two approaches are very different. PPC uses chemical solvents to capture CO2, which is accomplished by adding equipment and processes to the existing power plants. Although PCC imposes very stringent standards on SOx and ash emissions, it does not have a significant impact on operations of the power plant itself, though operating the PCC train adds considerably more complexity. Due to the nature of the technology, it is also the best option for partial CO2 capture.3
Oxy-fuel combustion, on the other hand, significantly changes how the combustion is conducted. It uses oxygen instead of air, thus eliminating nitrogen from the oxidant gas stream and producing a CO2-enriched flue gas. This flue gas is ready for sequestration after water has been condensed and other impurities have been separated out. A simplified schematic of oxy-fuel combustion is presented in Figure 1.
Oxy-fuel combustion for CO2 capture consists of three main components: the air separation unit (ASU) that provides oxygen for combustion, the furnace and heat exchangers where combustion and heat exchange take place, and the CO2 capture and compression unit. Due to the large quantity of high-purity oxygen typically required in oxy-fuel combustion, cryogenic air separation is currently the technology of choice for oxygen production. A large portion of the flue gas must be recycled back to the furnace for combustion temperature moderation and gas volume reconstitution to ensure proper heat transfer.
It is interesting to note that the concept of large-scale oxy-fuel combustion was first suggested in 1982—before climate change became a global concern—as a means to obtain CO2 for enhanced oil recovery (EOR). Oxy-fuel combustion has also been employed for productivity enhancement, fuel reduction, and NOx emissions reduction in the glass, aluminum, cement, steel, and incineration industrial sectors. These industrial applications are much smaller in scale compared with power generation and traditionally there was no intention to capture CO2.
Oxy-fuel technology has been developing rapidly since the late 1990s. The successes of in-depth theoretical examination of oxy-fuel combustion and accumulated bench and pilot-scale tests have led to several industrial-scale demonstrations since 2008. This progress is mainly due to the perceived superiority of the technology: It is viewed as a simple, but effective. Unlike post-combustion capture, there is no need to add a complicated chemical process to capture CO2. There is also no need for the power generation industry to adopt a completely new process (such as IGCC). The major components of oxy-fuel combustion, that is, coal combustion and air separation, are mature technologies that have been extensively employed, so that the retraining requirements for personnel are minimal.
One of the most noticeable advantages of oxy-fuel combustion is the low NOx emission, thanks both to the use of oxygen for combustion which eliminates nitrogen from air and to the NOx re-burning mechanism with flue gas recycling. More interestingly, recent research has shown that integrated emissions control of SOx, NOx, and mercury (Hg) may be possible as part of the oxy-fuel flue gas CO2 capture process.4 This alone could significantly reduce the cost of oxy-fuel combustion technology. Oxy-fuel combustion is also being mentioned as an excellent option for retrofitting the existing fleet of modern pulverized coal-fired power plants for CO2 reduction.5
Pilot-scale oxy-fuel demonstrations have so far confirmed that plant operations can be effectively switched from air-firing to oxy-fuel firing, air infiltration can be effectively limited, a highly enriched CO2 flue gas can be produced for transportation and storage, and significant NOx emissions reduction can be achieved. Based on these successful demonstrations, it would appear that there are no major technical obstacles in implementing oxy-fuel combustion for CO2 capture.
Challenges Facing GHG Control Technologies
Both PCC and oxy-fuel combustion technologies significantly reduce power plant net efficiency by up to 15%, according to the literature, while simultaneously raising capital costs, thus increasing the cost of electricity (CoE) for end users by as much as 100%. For example, in a recent paper6 examining the economic performance of both PCC and oxy-fuel combustion technologies, the authors found that the net efficiency of the PCC power plant decreased from 45% of the reference plant (supercritical 1200 MWe gross) to 30%, while that of the oxy-fuel power plant decreased to 35%. The same paper also showed that the CoE for a PCC plant increased by 65% while that of an oxy-fuel plant increased by 48%. Another paper7 carried out an extensive review of literature on this topic, the results of which are summarized in Figure 2, where PCC and oxy-fuel combustion are compared. It is important to point out considerable uncertainties are naturally associated with these types of studies. This is mainly due to the complexity of the processes and the fact that no commercial operating plants exist. Based on cost reductions achieved during the commercialization (i.e., learning by doing) of other low-emissions technologies, there is certainly reason to hope that CCS costs can be reduced and net efficiency can be improved, but large-scale demonstrations will be instrumental.
Whereas Kanniche and colleagues6 showed that PCC had significant efficiency loss compared to oxy-fuel, Rubin and colleagues7 showed that the efficiency losses for PCC and oxy-fuel are relatively close with a slight advantage for the oxy-fuel case. However, efficiency or even CoE should not be the only factor when considering which technology is more appropriate.
One significant advantage of the PCC process is that it can produce very high-purity CO2 ready to be compressed and transported, which is not the case for the oxy-fuel process. The main penalties of PCC are due to the requirement for solvent regeneration and solvent loss. Many research activities are currently addressing these issues.7 PCC is sometimes considered a “messy” technology because of its use of large amounts of chemical solvent and the size of the equipment. The use of chemical solvent also gives PCC an edge in retrofitting existing power plants and in building the so-called “CO2 capture ready” power plants. One perceived weakness of PCC technology is that it requires very clean flue gas to minimize solvent loss due to impurity contamination. (This requirement also has ramifications when retrofitting existing power plants with PCC as most of the flue gas cleaning equipment will likely have to be upgraded as well.) Again, this disadvantage can be turned into an advantage because the PCC train can be easily turned off during periods when CO2 capture is not necessary (for example, when the power plant has reached its annual CO2 capture goal) while meeting emissions requirements for other air pollutants. Currently, SaskPower of Canada is retrofitting a 150-MWe unit at its Boundary Dam location resulting in a 110-MWe PCC power plant. Note also that there are several other large-scale PCC-based power plants either under construction or being planned around the world.3
Making Oxy-Fuel Plants More Attractive
The main drawbacks for oxy-fuel combustion are associated with the air separation unit (ASU) and the CO2 purification unit (CPU). Recently, significant progress has been made in developing an efficient CPU to process oxy-fuel flue gas. By its nature, the flue gas coming out of the oxy-fuel process will have some impurities in it, such as H2O, SOx, NOx, O2, and N2. These impurities must be removed or their concentrations reduced before the flue gas can be sent to pipelines to ensure safe transportation of CO2 . Recent CPU development has demonstrated that the usual air pollutants such as SOx, NOx, and Hg can be completely removed from the flue gas stream so that the CPU acts not only as a CO2 purification unit but also as an emissions control unit. As a result, an oxy-fuel power plant can do away with equipment such as the flue gas desulfurization (FGD), selective catalytic reducer (SCR), and Hg control devices like activated carbon injection. This can lead to significant savings on capital investment and improved efficiency of the plant. Compared to PCC, the current state of oxy-fuel technology does not offer as much flexibility. Once a plant is built and optimized for oxy-fuel operation, it is difficult, sometimes even impossible, to revert back to a sustained air-firing mode, especially if all the emissions control units have been removed. An oxy-fuel power plant is also not amenable for partial CO2 capture. As such, it requires long-term commitment and insurance that a viable CO2 market will exist during the entire life of the plant.
For a typical oxy-fuel power plant, the ASU accounts for almost two-thirds of the loss in efficiency. The ASU also is a major capital investment. As oxy-fuel technology was developed, it became clear that the cost of oxygen production must be improved to make oxy-fuel technology a viable contender as a GHG control technology. Although innovative technologies based on membranes have been in development in the past decade, notably the ion transport membrane and oxygen transport membrane, they are still far from being able to reliably produce the large amounts of oxygen required for a commercial-scale oxy-fuel plant. It is also not clear how membrane-based oxygen production can be effectively used in coal combustion applications.8 It seems that, for the near future, traditional cryogenic air separation will be predominant. This process has long been used in a wide variety of industries, and is highly optimized and reliable. New opportunities for further optimization arise when the ASU can be integrated in the thermal cycle of the power plant. According to Air Liquide,9 its new cryogenic low-energy oxygen production technology (called Oxy LE) is able to reduce oxygen production costs from the current $200 kWh/t O2 to about $185 kWh/t O2, and with thermal integration (Oxy XLE) the cost can be further reduced to $165 kWh/t O2. Air Liquide estimates that, by 2017, it can reduce the oxygen cost further to $150 kWh/t O2.
Another innovative approach to reduce oxygen cost involves oxygen storage. The basic idea is to operate the ASU during the evening when electricity demand is low and store the produced O2 to be used during the day when ASU usage is kept to a minimum. By combining various innovative ideas, it is possible to considerably reduce the efficiency loss due to O2 production, probably reducing the ASU’s share of efficiency loss from two-thirds to less than one-half, resulting in a lower net plant efficiency loss from the current 8–10 percentage points to roughly 6–7 percentage points.
Other areas for further improvement to oxy-fuel combustion consist of minimizing air ingress, which would lead to increased costs for CPU operation, and developing new high-temperature materials to allow the oxy-fuel plant to operate with higher O2 concentrations, thus reducing the energy requirement for flue gas recycle.
The Current State of Oxy-Fuel
Globally, three pilot-scale oxy-fuel plants are in operation3; interestingly, all three are quite distinct, thus offering valuable experience in building various oxy-fuel plants. Vattenfall GmBH has had a 30-MWth newly built PC-fired oxy-fuel plant in operation since 2008 in Schwarze Pumpe, Germany. In Biloela, Queensland, Australia, Callide Power Station has retrofitted a 30-MWth existing PC-fired power plant to operate in oxy-fuel mode. And, in Ciuden, Spain, Endesa has a newly built 30-MWth oxy-fuel plant using circulating fluidized bed combustion (CFBC) technology. Despite these different approaches, all three projects are successful in that they can all produce high CO2 concentration flue gases that can be purified for pipeline transportation. Even though most R&D work has been focused on PC-fired oxy-fuel plants, Endesa successfully showed that a CFBC is just as capable. The Australian project showed that it is possible to retrofit an existing and rather old power plant for oxy-fuel operation.
Based on the operating experience of these pilot projects, several boiler manufacturers are confident that they can build a reliable, commercial-scale oxy-fuel power plant right now. As a FutureGen 2.0 project, Babcock & Wilcox has started the front-end engineering and design work for a 168-MWe oxy-fuel plant in Illinois, U.S., with the goal of capturing approximately 1.1 million tonnes of CO2 per year, which represents more than 90% of the power plant’s carbon emissions. Foster Wheeler is planning a 323-MWe oxy-fuel CFBC power plant in Compostilla, Spain, and Alstom is planning a 426-MWe oxy-fuel plant in White Rose, UK, as well as a 350-MWe oxy-fuel plant in Daqing, China.10
Another area that is getting more attention recently is the required quality of the CO2 for pipeline transportation. Unlike PCC, which essentially produces a stream of nearly pure CO2, an oxy-fuel plant may produce a less concentrated CO2 due to non-condensable gases. While technologies exist to further purify the CO2, additional costs and energy penalties are incurred. It is important to know what concentration of CO2 and individual impurities limits are needed for safe pipeline transportation. On the system side, operating the boiler at elevated pressures has received considerable attention due to the increased efficiency it provides.11
In conclusion, oxy-fuel combustion technology is at a point where it is considered near commercial from a technological point of view. What is needed now is a successful large-scale demonstration plant. This step is now being undertaken in several countries such as U.S., UK, and China, albeit cautiously. By further improving the economics of the oxy-fuel combustion, for example, by reducing the cost of O2 production and reducing the energy penalty due to both the ASU and CPU, it is likely that oxy-fuel combustion will overcome the last hurdles and reach full commercialization.
- IPCC (Intergovernmental Panel on Climate Change), Climate Change 2013: The Physical Science Basis, 2013, www.ipcc.ch/report/ar5/wg1/#.UqDC6MRDvvo
- International Energy Agency, IEA Statistics: CO2 Emissions from Fuel Combustion, 2012 Edition.
- Global CCS Institute, The Global Status of CCS: 2013, 2013, www.globalccsinstitute.com/publications/global-status-ccs-2013
- V. White, K. Fogash, Purification of Oxyfuel-Derived CO2 : Current Developments and Future Plans, 1st IEA Oxy-fuel Combustion Conference, Cottbus, Germany, 7–10 September 2009.
- T. Wall, R. Stanger, Industrial Scale Oxy-fuel Technology Demonstration, “Oxy-fuel Combustion for Power Generation and Carbon Dioxide (CO2) Capture”, Zheng (editing), Woodhead Publishing, U.K., 2011.
- M. Kanniche, R. Gros-Bonnivard, P. Jaud, J. Valle-Marcos, J.-M. Amann, C. Bouallou, Pre-combustion, Post-combustion and Oxy-combustion in Thermal Power Plants for CO2 Capture, Applied Thermal Engineering, 2010, 30, 53–62.
- E. Rubin, H. Mantripragada, A. Marks, P. Versteeg, J. Kitchin, The Outlook for Improved Carbon Capture Technology, Prog. Energy Combust. Sci., 2012, 38, 630–671.
- M. Prosser, M. Shah, Current and Future Oxygen Supply Technologies for Oxy-fuel Combustion, “Oxy-fuel Combustion for Power Generation and Carbon Dioxide (CO2) Capture”, Zheng (editing), Woodhead Publishing, UK, 2011.
- P. Terrien, R. Dubettier, M. Leclerc, V. Meunnier, Engineering of Air Separation and Cryocap™ Units for Large Size Plants, Third IEA GHG Oxy-fuel Combustion Conference, 2013, Ponferrada, Spain.
- D.K. McDonald, FutureGen 2.0: Power Block Design and Integration, IEAGHG OCC3 Conference, Ponferrada, Spain, 11 September 2013.
- B. Clements, L. Zheng, R. Pomalis, T. Herage, High Pressure Oxy-fuel (HiPrOx) Combustion System, “Oxy-fuel Combustion for Power Generation and Carbon Dioxide (CO2) Capture”, Zheng (editing), Woodhead Publishing, U.K., 2011.