The conventional wisdom of the day is that we have to replace our coal-fired power plants with gas-fired ones because they produce too much carbon dioxide (CO2). The presumption is that the emitted CO2 is contributing to global warming. Natural Gas fired plants still emit CO2, but less than coal-fired ones. The thought is that the benefits from replacing all these plants are worth the substantial costs. The U.S. Energy Information Administration (EIA) has produced several reports analyzing this topic (see for example http://www.eia.gov/forecasts/aeo/electricity_generation.cfm ). They have consistently added very high costs to the evaluation of coal plants in an attempt to account for a need to remove CO2 from the flue gas from the coal-fired plant. These studies produced several somewhat surprising results including:
- Gas-fired plants appeared to be cost effective when compared to coal-fired plants and
- Several alternative technologies appeared to be cost effective when compared to coal.
I suspect both sets of conclusions are somewhat disingenuous. How soon we forget the Natural Gas shortages of the 1980’s and price spikes of just a decade ago. I’m not at all sure that we can depend on low Natural Gas prices well into the future – especially if our manufacturing and chemical industries recover. This is particularly strange when the US has often been called the Saudi Arabia of coal.
The second of these can be immediately dismissed just by looking more carefully at the clarifying statements in the EIA reports. The alternative technologies cited were Wind, Solar (Photovoltaic and Thermal) and Hydro. Solar Thermal was off the chart high in cost and is not a viable consideration. Solar Photovoltaic (Solar PV) was somewhat handicapped by assuming it had to be converted to AC and put on the existing electrical grid. Since Solar PV is a DC source, this conversion process for the purpose of transmission results in high costs and high conversion losses. A different use of Solar PV might make better sense. This could be could local use by consumers of electricity (e.g. solar panels on houses), or centralized use that required no conversion or transmission (more on that later).
Wind power is listed among the “Non-Dispatchable” forms of power in the EIA reports. This is a rather cryptic way of saying that it cannot be adjusted to demand. The wind blows more or less randomly and not in concert with electrical needs. These forms of power are not very useful for powering a grid. Hence, none of these “Non-Dispatchable” power technologies is really practical for supplying power to the grid. This is often overlooked when the Department of Energy or the Administration is promoting “Green Energy.” The truth of the matter is that neither Wind, Solar or Hydro are very practical for powering the electric grid beyond minor assistance to more robust systems such as Coal, Natural Gas, Nuclear and the forever underdevelopment Biomass technologies. Of these Coal remains the most reliable and abundant in the US.
So what can be done to improve the CO2 performance of Coal power? One approach that has been getting a lot of attention is Carbon Capture and Sequestration. The basic idea is to develop cost effective ways to capture the CO2 in the flue gas coming from the coal-fired plant and then do something with it. This sounds easy, but it is not. The concentration of the CO2 in the flue gas is only a few percent (3% to 13%). This makes it difficult to remove it from the flue gas. Higher concentrations of CO2 in the flue gas can be created if Oxygen (O2) instead of air is used for combustion. Unfortunately O2 is expensive to produce. An additional problem is what to do with all the CO2 that is produced. Even a medium-sized coal-fired power plant produces a train load of CO2 per day. One approach is to use it for oil and gas production. It can be used for fracturing formations and flushing them to recover additional oil. Where a power plant is near oil and gas formations this may make some sense, but that is not always practical.
Another approach would be to use locally generated, low voltage Solar Photovoltaic energy to reduce CO2 to a usable product and generate O2 at the same time in electrochemical cells. One of the more promising reactions produces methanol (MeOH) and O2:
2CO2 + 4H2O -> 2CH3OH + O2
This is composed of the two half reactions:
CO2 + 6H+ + 6e- -> CH3OH 0.046V ( versus SHE)
2H2O -> 4H+ + 4e- + O2 1.229V (versus SHE)
If these reactions can be done with low overvoltage it only requires 1.277 volts. If good current efficiency can be achieved, a metric ton (very close to a US ton) of both methanol and oxygen can be produced using approximately 6,400 kW-Hrs of electricity. That is approximately $320 of electrical power cost at current industrial rates. The current price for a metric ton of methanol is $516 (see: http://www.methanex.com/products/documents/MxAvgPrice_Feb282013.pdf ).
Methanol can be used as a motor fuel (alone or blended), a turbine fuel (especially for backup to Natural Gas), a precursor for a diesel fuel replacement (DME) and a precursor for making plastics. If used to make plastics, the CO2 is virtually sequestered permanently.
The price of oxygen is a little more difficult to determine since it is often produced and consumed on-site. It is usually generated by separating it from the nitrogen in air. This can be done by mechanical refrigeration processes, but this is pretty costly. If O2 is generated at a power plant site, it would almost certainly used in combustion rather than compress or liquefy the O2 for sale. It would probably be used on-site to replace or enhance air in the combustion of the coal. This would increase the concentration of CO2 in the flue gas and make CO2 removal much more efficient. When solar power was available (during daylight), the plant could run on the oxygen being produced and CO2 capture would be improved.
The above electrochemical reactions are under intense investigation by several groups – including Stites & Associates, LLC. There have been several important improvements made in the past few years. Efficiencies of nearly 100% and over-potentials of less than 200mV have been reported for short periods of time with specially treated metal electrodes1. Similar results have been obtained with organic bases added as a catalyst – especially when used in conjunction with a light activated, semi-conductor electrode2. Stites & Associates, LLC, will be trying to reproduce some of these more promising developments in the Brighton lab in the next few weeks. We will also be testing a couple of new ideas that we have developed.
These technical problems do not seem to be insurmountable. Furthermore, there appears to be a reasonable chance that direct photochemical conversion of CO2 to methanol or other useful products may be practical3. Hence, in the not too distant future we may see a hybrid Coal/Solar PV power plant. Such a plant could combine the robustness of a coal-fired power plant with the capture of CO2 and conversion of it to methanol and oxygen using clean, locally generated Solar PV power.
1Summers, David; Leach, Steven; Frese, Karl; “The Electrochemical Reduction of Aqueous Carbon Dioxide to Methanol at Molybdenum Electrodes with low Overpotentials,” J. Electroanal. Chem, 205 (1986), 219-232.
2Barton Cole, Emily; Lakkaraju, Prasad; Rampulla, David; Morris, Amanda; Abelev, Esta; Bocarsly, Andrew; “Using a One-Electron Shuttle for the Multi-electron Reduction of CO2 to Methanol,” Journal of the American Chemical Society, 132, 11539-11551.
3Barton Cole, Emily; Rampulla, David; Bocarsly, Andrew; “Photo-induced CO2 Reduction with GaN Electrode in Aqueous System,” Journal of the American Chemical Society, 2008, 130, 6342-6344.