Hybrid Energy System Created by Conversion of CO2 to Power or Fuel

Exxon-Mobil- Fuel Cell Energy Partnership

The greatest opportunity for future large-scale deployment of carbon capture and sequestration (CCS) may be in the natural gas power generation sector, since capturing CO, from coal-fired generation is roughly twice as expensive. In 2016, ExxonMobil announced a partnership with FuelCell Energy, Inc. to advance new technology that may substantially improve CCS efficiency, effectiveness, and affordability for large natural gas-fired power plants. The research indicates that by applying this new technology, more than 90% of a natural gas pow'er plant’s carbon dioxide emissions could be captured [33-36].

Scientists at ExxonMobil and FuelCell Energy, Inc. are jointly pursuing new' technology that could reduce the costs associated with current CCS processes, by increasing the amount of electricity a power plant produces while simultaneously delivering significant reductions in carbon dioxide emissions. At the center of these efforts is a carbonate fuel cell. Laboratory tests have indicated that applying carbonate fuel cells to natural gas pow'er generation could capture carbon dioxide more efficiently than current, conventional CCS technology.

During conventional carbon capture processes, a chemical reacts w'ith the carbon dioxide to extract it from power plant exhaust. Steam is then required to release the carbon dioxide from the chemical—steam that w'ould otherwise be used to move a turbine. The effect is to decrease the amount of electric power the turbine can generate. Using fuel cells to capture carbon dioxide from power plants can result in a more efficient separation of carbon dioxide from power plant exhaust with an increased output of electricity. Power plant exhaust is directed to the fuel cell, replacing air that is normally used in combination with natural gas during the fuel cell power generation process. As the fuel cell generates power, the carbon dioxide becomes more concentrated, allowing it to be more easily and affordably captured from the cell’s exhaust and stored.

ExxonMobil’s research indicates that a typical 500 MW power plant using a carbonate fuel cell may be able to generate an additional 120 MW of power, while current CCS technology actually consumes about 50 MW of power. ExxonMobil has been assessing a number of carbon capture technologies for many years and believes that carbonate fuel cell technology offers a great potential. The technology’s capability has been tested in the laboratory, and data from those simulations is currently under analysis. Further development will involve a more detailed examination of each component of the system and optimization of the system as a whole [33-36].

In theory, carbon capture is simple. The carbon dioxide produced when fossil fuels are burned to produce electricity is captured and then stored deep underground instead of being released into the atmosphere, where they become heat-trapping greenhouse gases. Because large amounts of energy are required to concentrate carbon dioxide molecules together so that they can be caught, current carbon capture technology is expensive. It’s just one of several technical and economic hurdles facing large-scale use of carbon capture. The fuel cell could be a fundamental shift in carbon capture because it can trap the gas while also generating electricity. This is important in power generation, where every percentage increase in efficiency matters. Fuel cell will also considerably reduce emission of C02 into the atmosphere.

When natural gas is burned in a gas turbine, the exhaust produced is only about 4% carbon dioxide. Carbonate fuel cells can grab that carbon dioxide and concentrate it into a stream that is around 70-80% carbon dioxide while creating more electricity at the same time. Further processing increases the carbon dioxide concentration to over 95%. While that’s a promising start, ExxonMobil and FuelCell Energy are planning to test and improve the technology to further increase its efficiency and demonstrate it at larger scale. The goal is to minimize emissions while maximizing power output. Such a treatment of CO, will also make power plants hybrid energy systems.

The unique feature of these fuel cells is that they act as a CO, pump while using natural gas to generate power. As C02 stream is passing through the fuel cell membrane, it is getting concentrated so that it can then go into storage, while generating additional electricity. FuelCell Energy’s scalable and affordable carbon capture solution captures carbon emissions from existing coal- or gas-fired power plants, while simultaneously producing power. The solution is scalable so the amount of carbon capture can be increased over time. The fuel cells also destroy approximately 70% of the plant’s smog-producing pollutants. The capability to efficiently and affordably capture carbon while simultaneously producing power enables emission compliance in a manner that provides a revenue stream and returns on capital from the sale of power produced by the fuel cells. This process is called “Sure Source.”

Benefits of Sure Source carbon capture include [33-36]:

  • • Scalable and affordable: Fuel cell plants can be added incrementally in a cost-effective manner; starting with as little as 5% capture with no appreciable change in the cost of power and with minimum capital outlay. To achieve 90% capture, the cost of power for a coal-fired plant is increased by only $0.02/kWh, while conventional carbon capture technologies for coal- fired power plants almost double the cost of power.
  • • Produces, rather than consumes, additional power: The process generates additional power—cleanly and efficiently—during the carbon capture process, contributing to the existing coal plant’s total output. In comparison, conventional carbon capture technologies consume about 20% of the plant’s overall power output.
  • • Destruction of pollutants: Captures and separates CO, from the flue gas of coal- and gas-fired power plants while simultaneously destroying approximately 70% of the smog-producing nitrogen oxide (NOx).
  • • Return on investment: Generates a return on capital rather than an increase in operating expense, extends the life of existing coal-fired power plants, and enables low-carbon utilization of domestic coal and gas resources.
  • • Proven solution: Millions of megawatt hours of ultra-clean power are generated by Sure Source power plants globally.

Since Sure Source power plants produce power efficiently and with virtually zero emissions, the net result is a compelling carbon capture solution for preventing the release of greenhouse gases by coal- or gas-fired power plants, while simultaneously increasing overall net efficiency and clean power output in an affordable manner for ratepayers.

CCS has long been an ultimate goal for many energy companies, for both money saving and environmental reasons. It is the process by which the carbon dioxide, which would otherwise be released as waste from power plants into the atmosphere, is captured, compressed, and injected underground for permanent storage. Fuel cells have been a rapidly expanding option for mini-grids and distributed generation because they produce electricity directly from a chemical reaction, and devices are generally fairly small and easy to install and transport. In this case, the fuel cells would run directly off the power plant emissions, removing them from the air and, in turn, producing additional electricity to feed it back into the system or indeed sell off.

Many facilities have been able to capture C02 emissions since the 1970s, but in very energetically exhaustive ways. Existing CCS technology actually consumes up to 25% of electricity at a power plant, equating to a large amount of money. With power plants already facing financial challenges from the growing interest in renewable energy, falling oil prices, and an increasing unpopularity of coal, a 25% jump in power generation isn’t usually feasible. For example, Southern Company built a coal-fired power plant in Kemper, Mississippi, which was supposed to incorporate CCS. The project ended up costing $7billion in total, three times the original estimate. In 2015, ExxonMobil claimed that it captured 6.9 million metric tons of CO, using the CCS process, which is the equivalent amount of fumes from over a million cars. Researchers think that pursuing new CCS technology could actually help to reduce costs. While current CCS processes are associated with added expenditures, by combining with fuel cell, the new method could increase the amount of electricity a power plant produces. In addition to the cost saving and environmental benefits, carbonate fuel cells also produce a chemical feedstock called “syngas,” which is primarily made of hydrogen and can be used as a fuel for internal combustion engines.

In September 2015, FuelCell Energy announced a $23.7 million cost-shared project with the Department of Energy to demonstrate that its technology could capture 90% of the CO, from a small stream of coal exhaust and concentrate it to 95% purity. In the first phase of the project, a modified version of its commercially available 2.8 MW SureSource 3000 fuel cell system will capture 54 metric tons of carbon dioxide per day at the Barry plant. That’s just a small fraction of the CO, emitted by the facility. To put things in perspective, a typical 500-MW coal plant emits about 3.3 million metric tons of carbon dioxide per year—which works out to about 9,000 metric tons per day—and so capturing 90% of those emissions would require about 400 MW of fuel cell capacity [33-36].

This leads to one potential setback for the technology in that the amount of CO, that can be captured depends heavily on how many fuel cells are in operation. For example, a 500 MW combined cycle plant would require at least 120 MW of fuel cells to achieve a level of 90% carbon capture. An equivalent coal plant might need as much as 400 MW of fuel cells because coal plants are generally less efficient and emit higher levels of CO,. That’s a lot of fuel cells. In terms of capital investment, fuel cell power is three to four times as expensive as conventional coal power. With a carbonate fuel cell, the cogenerated heat and electricity could boost the host plant’s power output by 80%. Advocates point out that when full life cycle costs are taken into account, along with the difficult-to-quantify environmental benefits, the balance begins to favor such a combined cycle. Either way, FuelCell Energy and Exxon scientists will focus on increasing the efficiency in separating the CO, from gas turbine exhausts and are likely to learn a lot in the process. They are working to better understand the chemical processes and working out how they respond to different compositions and concentrations of the flue gas. If successful, the next steps will be to launch a pilot project for more testing and then integrate it to a larger scale pilot facility after. Eventually, the goal is to grid-tied build a 2 MW-3 MW demonstration plant that would run alongside a coal-powered plant. Table 2.3 compares conventional CCS technology with CCS based on CO, fuel cell technology [33-36].

Table 2.3 Comparison of performance of coal power plant with various options for carbon capture [33-36].

500 MW coal power plant with no carbon capture: Emits 3.6 million tons of CO, per year. This is equivalent to more than 685,000 cars annually. Cost of electricity 6 /kWr.

TABLE 2.3

Comparison of the Three Scenarios Analyzed [37]

Scenario

Thermal Product

Storage

#1—Electric boiler

Variable

None

#2—Electric thermal storage

Constant

Thermal storage from electricity

#3—Electric boiler thermal storage

Constant

Thermal storage from electricity and thermal sources

  • 500 MW coal power plant with conventional carbon capture: Carbon capture by conventional absorption technology consumes significant amount of power plant’s output (20% or 100 MW) to capture 90% of CO,. This causes cost of electricity to increase by 80% and increase in pollutants by 25% (lbs/MWh). The cost of electricity would be 110/kWh.
  • 500 MW coal power plant with fuel cell C02 capture: In this process, flue gas from the coal plant is routed into the fuel cells, which then concentrate and capture CO, as a side reaction during power generation. The coal plant remains at full power while the fuel cells affordably capture CO, and destroy approximately 70% of the coal plant NOx emissions. This will increase total power generation by 80% to 900 MW, increase cost of electricity by 33%, and decrease pollutants by 78% (lbs/MWh). The cost of electricity would be 80/kWh.
 
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