|At a research-scale combustion unit at Ohio State University, engineers are testing a clean coal technology that harnesses the energy of coal chemically, without burning it. Here, doctoral student Elena Chung (left) and master’s student Samuel Bayham (right) display chunks of coal along with pulverized coal (bottle, center) and the iron oxide beads (bottle, right) that enable the chemical reaction. Photo by Jo McCulty, courtesy of Ohio State University.|
By Liang-Shih Fan and Elena Chung, Ohio State University
Coal plays a big role in America’s energy present and future. According to the U.S. Energy Information Administration (EIA), coal comprises 20 percent of the primary U.S. energy fuel consumption and provides 42 percent of our country’s electricity generation.1 The EIA projects that coal’s share of U.S. electricity generation will fall by 2040, but still remain the leader at 35% percent.2 Thus, coal will continue to be important in satisfying growing U.S. energy demands, but how exactly that future plays out will depend a great deal on the development of technologies that eliminate coal emissions.
Here in Ohio, coal is used to generate about 78 percent of electricity, according to the Ohio Public Utilities Commission.3 With such a high dependence on coal, Ohio has a long history of pushing innovative technologies that use domestic energy sources while protecting health and the environment.
Our lab at Ohio State University is led by Liang-Shih Fan, professor of chemical and biomolecular engineering, who has committed his research career to developing cleaner and more environmentally-friendly fossil fuel conversion solutions. In particular, we focus on applying chemical engineering fundamentals in particle technology, gas-solid fluidization and reactor design to develop new chemical process technologies that can address energy and environmental issues.
Throughout his career, Fan has continuously researched engineering technologies for pollution control of fossil fuel combustion. Specifically, our group has studied methods for sulfur dioxide (SO2), nitric oxides (NOx) and toxic metal (arsenic, selenium and mercury) removal from coal-fired power plants. In the early 2000s, we successfully demonstrated clean coal technologies such as the Ohio State Carbonation Ash Reactivation (OSCAR) process for flue gas desulfurization and toxic heavy metal removal and the CARBONOX process for NOx removal from the flue gas using activated coal char.
With increasing environmental concern about greenhouse gas emissions from coal-fired power plants, CO2 capture research was a natural evolution for our research, starting in 1998. We began developing technologies for carbon capture, utilization and sequestration (CCUS). This first resulted in the development of the carbonation-calcination reaction (CCR) process for carbon dioxide removal.
More recently, our chemical looping processes were developed out of Fan’s lifelong passion of cleaning the environment, protecting public health and providing efficient, affordable energy. We want to continue to push the forefront of clean coal innovations with research in order to secure reliable, domestic energy that has a reduced environmental impact.
Chemical Looping is an innovative chemical process that converts carbon-based fuels such as coal, biomass, syngas and natural gas to electricity, liquid fuels and/or hydrogen with low to negative net carbon emissions. The process is a series of reduction-oxidation reactions where initially a carbon-based fuel is reacted with metal oxide at high temperatures. The carbon reacts with the oxygen from the metal oxides to form carbon dioxide and steam. By producing only carbon dioxide and steam gases, the carbon dioxide can be easily separated and captured by condensing the steam. Then using air, the reduced metal particles are re-oxidized back to metal oxides that can be circulated and used again in the chemical looping process.
Though today’s application of chemical looping is novel, the fundamental concept of the technology started with the steam-iron process in which iron was used to produce hydrogen in the 1900s. Then a similar circulating reaction scheme was used in the 1950s to produce pure carbon dioxide for the beverage industry. The environmental applications of such chemical looping technology were first reported by the Tokyo Institute of Technology in the 1980s.
Chemical looping is a type of pre-combustion and oxy-combustion carbon capture technology where the oxygen source comes from a metal oxide particle rather than air. Among possible carbon capture technologies listed on the U.S. Department of Energy (DOE)’s Carbon Emission Control Technology Roadmap, DOE projects chemical looping to be one of the most economical.4
One crucial advantage of chemical looping is the flexibility in terms of fuel source and products. Chemical looping systems have been demonstrated around the world with both gaseous fuels such as synthesis gas (syngas) and natural gas, or solid fuels such as coal, biomass and waste. The heat generated from such systems can be used for electricity production. This technological process can also be designed to produce hydrogen, syngas, chemicals and liquid fuels. Chemical looping reactors can be redesigned to handle many different types of fuel and to produce a variety of products.
In Ohio State’s Clean Energy Research Laboratory, we feel that the simplicity of the chemical looping design is critical; The beauty of the process is the streamlined and flexible design. In place of a conventional pulverized coal-fired boiler, our chemical looping combustion process uses separate reactors to avoid mixing air and fuel, which eliminates the need for costly, energy-intensive gas separation systems. In comparison to other carbon capture technologies, the chemical looping reaction lends itself to higher efficiencies in capturing carbon dioxide.
Our patented chemical looping technique is the result of more than 10 years of extensive research from Fan and more than 60 undergraduate students, graduate students and post-doctorates. The lab explores both calcium- and iron-based chemical looping, where the looping media used is calcium oxide and iron oxide, respectively. Two of these processes—the Syngas Chemical Looping (SCL) system and the Coal-Direct Chemical Looping (CDCL) system—have been progressing closer to commercial scale.
Syngas Chemical Looping (SCL)
The SCL process utilizes the gaseous fuel-based chemical looping process that converts syngas to hydrogen and heat. As compared to other chemical looping technologies, the SCL system is unique in that it uses three reactors that can allow for co-production of electricity and pure hydrogen. This hydrogen can be utilized economically by the petrochemical and chemical industries. Unlike other chemical looping gasification technologies, our chemical looping systems use a unique counter-current moving bed reducer reactor and fluidized bed combustor reactor.
We’ve laboratory tested the SCL process for over 100 hours, exhibiting a product stream of pure hydrogen with complete CO2 capture at bench scale (around 2.5-kWth). With this success, we pressed on with a larger scale 25 kWth sub-pilot unit in Columbus, OH. Over 300 demonstration hours, the SCL sub-pilot unit exhibited above 99 percent purity for CO2 with over 99 percent syngas conversion and 93-99 percent hydrogen purity.
With the success at smaller scales, we are now preparing to scale up the technology to a 250 kWth pilot plant in collaboration with Babcock and Wilcox Power Generation Group (B&W PGG), CONSOL Energy, Inc., Particulate Solid Research, Inc. (PSRI), Clear Skies Consulting LLC (Clear Skies) and Air Products and Chemicals, Inc. (Air Products). This SCL pilot scale is sponsored by the Ohio Department of Development (ODOD) and the U.S. DOE’s Advanced Research Programs Agency-Energy (ARPA-E)—the agency that specifically helps advance high risk, potentially breakthrough technologies to commercial scale.
At the U.S. DOE’s National Carbon Capture Center (NCCC), operated by Southern Co. in Wilsonville, Alabama, Ohio State’s SCL pilot demonstration project will be the largest pressurized scale-up of chemical looping technology for hydrogen generation from coal and biomass. Using a slipstream of syngas from Southern’s transport gasifier, the fully integrated 250 kWth pressurized unit will begin commissioning in the third quarter of 2013. This demonstration unit will be used to verify the operability and feasibility of advanced chemical looping technologies.
Based on independent economic analyses, both the commercial-scales of the SCL and CDCL technologies are projected to meet the U.S. DOE’s goal of less than 35 percent increase in cost of electricity for the production of a new power plant. Further, we anticipate that the commercial scale chemical looping processes can be used for repowering existing coal power plants or for integrating into newly constructed coal power plants.
Coal-Direct Chemical Looping (CDCL)
|The Coal-Direct Chemical Looping sub-pilot demonstration unit at Ohio State University in Columbus, Ohio.|
Following a similar advancement pathway of the SCL unit, the CDCL sister project—the solid fuel-based chemical looping combustion system—was funded by the U.S. DOE and ODOD. In the CDCL process, finely powdered coal directly reacts with iron oxide beads. Similar to the SCL process, in the first reactor, coal reacts with the oxygen from the iron oxides to form carbon dioxide and steam, which are removed from the system. In the CDCL process, the solid iron and coal ash are left behind. One inherent benefit of the CDCL design is that the iron beads can be easily separated from the coal ash because of the size difference. The coal ash is easily removed from the entire system with a cyclone and without the need for any additional fine removal device. Then in the second reactor, the iron beads are re-oxidized to be recycled and reacted with fresh coal powder in the first reactor.
Recently, in Columbus and in collaboration with B&W PGG, Clear Skies, and Air Products, we demonstrated the fully integrated chemical looping combustion 25 kWth unit. From more than 600 hours of testing, Ohio State has operated the CDCL sub-pilot system with nearly full conversions of different types of coals while producing over 99 percent pure carbon dioxide. Most recently, the CDCL system completed a milestone by successfully testing a continuous 200 hours test with sub-bituminous and lignite coals and metallurgical coke.
During the CDCL sub-pilot demonstrations, the system produced other pollutants that have been demonstrated to be easily manageable. For example, the process produced NOx and SO2 concentrations that are comparable to quantities produced from a conventional pulverized coal combustion boiler equipped with a low NOx burner. The NOx concentrations are also typically lower because of the lower reaction temperatures. These NOx and SO2 pollutants can be removed using traditional selective catalytic reduction units and flue gas desulfurization units, respectively.
With these successful CDCL demonstrations, Ohio State plans to further scale up with a CDCL pilot scale demonstration unit in the near future with B&W PGG.
Future Plans and Applications
With the upcoming demonstration at NCCC of the SCL pilot unit, we hope to lead the commercialization of chemical looping technologies. Future testing would require integrating the carbon capture technology with partners for carbon utilization or sequestration.
On a commercial scale, the SCL and the CDCL technologies can be implemented as a part of a greenfield plant or as a repowering of the aging fleet of coal-fired boilers in a traditional pulverized coal power plant.
With the fuel source and product flexibilities of chemical looping technologies, the possibilities for chemical looping are abundant. Currently, we are also exploring a concentrated solar-power-related thermal energy generation technology. As a part of DOE’s National Renewable Energy Laboratory’s SunShot initiative, Utah State University, B&W PGG and Ohio State are developing a solid-particle solar receiver that can be integrated into a solar chemical looping process.
The recent shale gas boom has also pushed efforts for natural gas conversion technologies. We have demonstrated the viability of using natural gas for carbon capture and chemical synthesis. Ohio State has tested methane at both the bench scale and sub-pilot scale for nearly full natural gas conversion with 100 percent carbon capture. With the fuel flexibility of the SCL pilot unit at NCCC, we plan to test methane during its operation at in early 2014. Additionally, the chemical looping process can be used to produce various chemicals and liquid fuels. At Ohio State, bench scale tests have also validated the conversion of methane to syngas using the chemical looping technology. We are also researching the application of coal or natural gas chemical looping for gas-to-liquids, coal-to-liquids and chemical production. The potentials of chemical looping technologies are extensive, as such conversion processes could be used to generate electricity, to fuel vehicles and to produce useful chemicals with minimal greenhouse gas emissions. We believe that, ultimately, chemical looping is a potentially game-changing technology that can truly make a difference in our energy landscape.
1. U.S. Energy Information Administration, What is the Role of Coal in the United States?: http://www.eia.gov/energy_in_brief/article/role_coal_us.cfm
2. U.S. Energy Information Administration, Annual Energy Outlook, DOE/EIA-0383(2013), May 2013.
3. Public Utilities Commission of Ohio, Where Does Ohio’s Electricity Come From?: http://www.puco.ohio.gov/puco/index.cfm/consumer-information/consumer-topics/where-does-ohioe28099s-electricity-come-from/
4. Figueroa, J.D., Fout, T., Plasynski, S., McIlvried, H., Srivastava, R.D. “Advances in CO2 Capture Technology—The U.S. Department Energy’s Carbon Sequestration Program”, International Journal of Greenhouse Gas Control 2008, 2, 9-20.More Power Engineering Issue Articles
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