Dr. Allen Pfeffer, Alstom, USA
With coal expected to remain a major fuel source, the issue for the power generation industry is how this will square with tougher CO2 emission controls. Alstom is researching ways of capturing, storing and using the CO2 emitted from coal fired power plants.
Almost every projection shows a robust increase in the demand for electricity continuing for decades. The rate will be slower in the industrialized regions and faster in the underdeveloped world, with China and India leading. At the same time, the power industry is capital intensive and vast in size, which means that the actual operating part of the business is slow moving. Decades are required to develop and introduce new technologies, and useful plant lives of more than 30-40 years are the norm. It is probable that the new methods of power generation in the time period of 2015-2020 will be under development today, and demonstrated in the 2010-2015 period.
Figure 1. Alstom is developing an oxygen fired boiler as part of a US DOE greenhouse gas programme
Coal is abundant and is presently two to three times cheaper than gas. For countries with large domestic coal reserves – the USA, China, Germany, and India, for example – the combination of cost and security concerns will argue for coal’s continued widespread use into the future.
While the connection between man-made carbon dioxide (CO2) and global warming is far from clear, worldwide concern over greenhouse gases is driving research into reducing CO2 emissions from all sources. In the power industry, there are a wide range of fossil fuel technology options that are currently available or being developed to generate power in a world with carbon constraints. These CO2 mitigation technologies may be applied to both the existing fleet (retrofit technologies) and to new capacity. The technologies broadly fall into the three catagories of fuel switching, efficiency increase and CO2 capture.
Fuel switching technologies
A method of reducing CO2 emissions is to switch from coal to a less carbon intensive fuel, such as natural gas, renewable (biomass co-firing), or non-fossil power (ex. – nuclear energy). Fuel switching to natural gas can be achieved by repowering an existing coal fired plant with a topping natural gas combined cycle (NGCC). This would eliminate the use of the existing boiler and install new natural gas fired gas turbines with heat recovery steam generators to provide steam to the existing steam turbine. The application of this solution is limited by the volatility of natural gas prices.
Fuel switching an existing boiler to co-fire biomass with coal or building new plants designed for biomass is another viable means of reducing CO2 emissions. Biomass fuels are considered a CO2 neutral fuel because the CO2 released during combustion from a biomass derived fuel is recycled back into the next generation of crops from which they are derived, thereby creating a closed-loop CO2 recycle system. This option is particularly attractive in Europe where incentives offset the higher cost of the biomass fuel. However, the number of units that can be built is limited by the availability and transportation of the biomass fuel.
Improving the thermal efficiency of an existing plant will reduce CO2 emissions and conventional pollutants such as SO2, NOx, and particulates by an amount directly proportional to the efficiency improvement. Efficiency gains can be achieved for existing plants through relatively simple measures such as turbine, boiler, and other plant upgrades. More complex projects are also possible such as repowering to higher temperature and pressure steam conditions, or adding a topping gas turbine cycle and using the existing steam cycle as the bottoming cycle.
Repowering opportunities include replacing several smaller units with a single, larger, more efficient plant or even scrapping the older boiler and replacing it with a more advanced boiler, such as an oxygen fired circulating fluidized bed (CFB) boiler. All of these repowering opportunities involve extensive equipment modifications and replacements.
For new capacity, supercritical and ultra-supercritical pulverized coal (PC) and CFB plants will be the most cost-effective coal fired technologies for achieving limited CO2 reductions of up to 20 per cent. Larger CO2 reductions will require additional CO2 capture technologies. Efficiency improvements have been achieved by operation at higher temperature and pressure steam conditions, and employing improved materials and plant designs. Alstom is actively engaged in material technology advancement and steam plant design efforts to allow for coal power plants with greater than 50 per cent (LHV) net plant efficiency.
Efficiency gains can be achieved with technologies readily available today. Alstom is currently offering supercritical PC and CFB plants at steam conditions up to 300 bar/600ºC/620ºC. Development is ongoing through programmes such as EU Thermie and the USA Ultra-Supercritical Consortium to further increase steam conditions up to 350 bar/700ºC/720ºC.
There are few choices available today for the capture and sequestration of CO from fossil fuel power generation systems. However, many technologies are currently being developed and will be available as the demand for CO2 capture increases. Alstom is developing or closely monitoring some of these technologies. Many of these technologies can capture over 90 per cent of the CO2, but they are capital intensive and impose a large output reduction and energy penalty.
Conventional coal fired plants generate a dilute CO2 exhaust gas that is almost 80 per cent nitrogen from combustion in an air atmosphere. CO2 mitigation equipment is thus quite expensive and energy intensive because the entire gas volume must be processed to capture, concentrate, and compress the CO2 for sequestration.
Figure 2. A CO2 wheel uses a regenerative air-heater-like device with solid absorbent material for CO2 capture
Oxygen combustion avoids the problem of a dilute CO2 exhaust. By firing with nearly pure oxygen, nitrogen is not introduced into the products of combustion and a concentrated CO2 flue gas stream is produced. The CO2 gas stream can then be cleaned, purified, and compressed more cost effectively than a dilute CO2 stream from conventional combustion processes. The oxygen source could be from a cryogenic air separation unit (ASU), from oxygen transport membranes, or from a chemical looping (oxygen carrier) combustion system.
Oxygen fired plants can be very capital intensive and take a large energy penalty because of the power required by the cryogenic ASU needed to separate oxygen from the air. In the future, advanced oxygen fired plants using oxygen transport membrane (OTM) processes may significantly reduce these costs. Similarly, advanced oxygen fired plants using a chemical looping combustion process looks very promising with respect to both efficiency and economics.
Oxygen fired boiler
Alstom is actively developing an O2 fired CFB as part of a US Department of Energy greenhouse gas programme and in the framework of the ENCAP project funded by the European Union. An oxygen fired CFB is supplied oxygen from a cryogenic ASU. The boiler island provides a concentrated CO2 flue gas product stream to the gas processing system. There, the CO2 is captured, purified, and compressed for subsequent sequestration. This results in cost savings from a smaller boiler island, compared to an air fired CFB and cost savings on gas processing system equipment compared to amine-based CO2 scrubbing systems. It uses readily available commercial technologies, including oxygen production with an ASU and gas processing systems to produce enhanced oil recovery (EOR) quality CO2 product. A near term opportunity for O2-fired CFBs is for EOR applications, where the CO2 gas can be pumped directly underground. This opportunity is being developed by Alstom.
Phase I was completed last year, with preliminary techno-economic analysis of a 210 MW plant. Alstom is currently in the process of executing a pilot-scale testing to evaluate the concept at 9.9 MMBtu/h. Test will include two coals and one petcoke and combustion in O2/CO2 mixtures containing up to 70 per cent volume O2. The next steps after this programme will be to design a commercial plant for demonstration and then pursue the demonstration of oxygen fired CFB for EOR.
An oxygen transport membrane is a more effective method for oxygen production as compared to a cryogenic ASU, but requires high temperature air for the membranes to operate efficiently. The OTM can be integrated with a CFB or CMB process, which can supply a high temperature solids stream to preheat the air to the high temperatures necessary for the OTM.
Chemical looping combustion
Chemical looping combustion is indirect combustion of coal via chemical looping. An air fired boiler utilizes a continuously looping solid oxygen-carrier (e.g., CaSO4) which oxidizes the fuel into primarily H2O and CO2. Simple condensation of the H2O then yields a fairly pure CO2 product stream for compression and liquefaction.
A major advantage is that oxygen is supplied to the process without the large efficiency penalty associated with an ASU. The large investment cost associated with supplying oxygen is also avoided. Alstom is in the early states of development with this technology. The technology is being developed in separate DOE and EU funded projects, and is currently in proof of concept testing.
Tail-end CO2 capture
Tail-end CO2 capture includes a wide range of technologies that capture CO2 after it has already been generated in the boiler or combustion turbine. It includes technologies such as various amines for scrubbing, CO2 frosting, and CO2 wheel.
A monoethanolamine (MEA)-based absorption-stripping process can be used to capture CO2 from the flue gases leaving a NGCC or coal fired boiler, followed by a CO2 compression and liquefaction system. While this process is commercial, it is very energy intensive, consuming more than 30 percent of a power plant’s gross output.
The CO2 frosting process uses the principle of CO2 capture from flue gas by cryogenic refrigeration (or frosting) with liquid CO2 pumped to 2000 psig to capture CO2 from the exhaust gas from a boiler or NGCC. This process is being developed at the Ecole de Mines de Paris, France, with support from Alstom’s Environmental Control Services.
Figure 3. The carbonate flow chart process
A CO2 wheel uses a regenerative air-heater-like device with solid absorbent material for CO2 capture from the exhaust gas from a boiler or NGCC, followed by a CO2 compression and liquefaction system. This is a relatively low cost CO2 capture technology, although it is currently limited to capturing only about 60 per cent of the CO2 in the flue gas. This technology is being developed by Toshiba in Japan, with support from Alstom’s Air Preheater Business.
De-carbonization includes a range of technologies that remove or capture the CO2 prior to or during the combustion process. It includes technologies such as Integrated Gasification Combined cycle (IGCC), carbonate capture cycles and chemical looping gasification.
IGCC with CO2 capture involves a gasification process where coal or other carbonaceous feed stocks are exposed to steam and controlled amounts of air or oxygen at high temperature and pressure to form a fuel gas or syngas, which is comprised of primarily CO and H2. This gas can be further cleaned and then used in a variety of ways. The syngas can be used for power generation in an IGCC plant, or it can be shifted to increase the concentration of H2, where it has a wide range of uses including hydrogen powered automobiles and power generating fuel cells.
The syngas can be used as a chemical building block for a wide range of petrochemical products, such as ammonia, methanol, and fertilizers. It also can be used to make liquid fuels for transportation or as a substitute for natural gas that can be transported through pipelines. The carbon dioxide from an oxygen fired gasifier is also emitted as a concentrated gas stream that can be cleaned, captured, and compressed for sequestration at lower cost than from a more dilute gas stream.
Gasification is widely used today in the petrochemical industry. Further improvements are needed in reducing capital costs and improving availability to make IGCC plants an economical option for power generation.
A regenerative carbonate cycle uses a recirculating stream of lime to capture CO2 as calcium carbonate. Energy is then supplied to a calciner to recover the CO2 while regenerating the lime. This process is less capital and energy intensive than many other current solutions. Alstom is currently in the early stages of developing this technology.
Chemical looping gasification uses two primary chemical loops in the process to produce both a relatively pure CO2 stream and a medium Btu gas (more than 90 per cent hydrogen). The oxygen used in the gasification process is indirectly provided by a solid carrier. A continuously looping solid oxygen-carrier is first used to partially oxidize the fuel into primarily H2 and CO. Second, the CO is shifted to CO2 and a regenerative carbonate cycle is used for direct CO2 capture. Chemical looping is an advanced technology under early stages of development by Alstom that is specifically designed for CO2 capture. It represents an alternate technology path to IGCC. This very promising technology has the potential to have the lowest capital and operating costs of all advanced CO2 capture technologies considered and offers an attractive approach for reducing CO2 emissions.
IGCC processes produce a hydrogen-enriched synthesis gas that must be burned in a gas turbine to produce power. The gas fed to the gas turbine contains only 30-40 per cent hydrogen. In order to improve the CO2 capture characteristics, substantially higher hydrogen content will be required (as much as 70-100 per cent). An entirely new gas turbine technology must be developed to enable hydrogen-enriched combustion.
It is considered to be a huge scientific challenge to improve the combustion technology with hydrogen-enriched synthesis gas because ultra-low emissions must be compromised with stability and Alstom has joined efforts with other reasearch and development providers in this field in order to pursue this goal.
Fossil fuels such as coal will continue to be the fuel of necessity for the next few decades and the availability of coal will dictate its continued use. There is research underway that will allow coal to be a fuel of choice and still lead to a reduction in the amount of CO2 generated by power production. Alstom is part of a worldwide effort to ensure that the twin goals of satisfying the demand for electricity and a sustainable environment can be met.