With the Bosch reaction carbon dioxide can be burnt with hydrogen into solid carbon and water. I don’t know the conditions required for this reaction, such as the efficiency, catalysts required or whether carbon needs to be regularly scraped off the catalysts, the speed of the reaction and whether it can be done for large amounts of CO2, and so on The temperature seems high but the process releases some heat energy so it might pay part of its way in terms of energy. http://en.wikipedia.org/wiki/Bosch_reaction
One issue has to do with the supply of hydrogen gas. Apparently common industrial processes for creating hydrogen from hydrocarbons result with carbon monoxide and by a further reaction carbon dioxide as by-products. With the steam reformation of natural gas and through the water gas shift reactions, from the wikipage http://en.wikipedia.org/wiki/Hydrogen_production
For the proposed hydrogen fuel energy systems to be carbon neutral you would obviously have to make sure that CO2 isn’t a by-product for the production of large amounts of hydrogen, and high temperature electrolysis is proposed as a method to do that, I suppose.
One point that might be worth considering might be a strategy for eventually reducing CO2 emissions from power stations or places where CO2 is produced industrially. It takes energy to collect and process the wastes including CO2. If that energy is taken from burning more carbon fuels it would lessen the efficiency of the power plant for producing power, and it would dilute the overall reductions in CO2 emissions if a percentage reduction is aimed for, since more fuel needs to be burnt. One strategy that would keep the efficiency of the power station as it is, and keep the certainty of the carbon power station supply that is required as it is, would be to try to source the energy needs for capturing and processing the wastes of carbon power stations from renewable energy sources. The wastes would most likely need to be stored for some short time until the renewable energy is available to process the wastes, but that shouldn’t be much of a problem.
There are a number of advantages to this approach, even if it may seem expensive to start with and even if it might seem like a wasteful way to use renewable energy to start with. So it might sound counter-intuitive on a purely technical evaluation.
Reductions to CO2 emissions are likely to be staged over the next few decades. By using renewable energy sources to power the removal and processing of CO2 from the wastes, the percentage of CO2 emissions that could be removed from the wastes of any station might more easily become a function of the amount of renewable energy sources installed around the power station. This would have no negative effect on the amount of electricity produced by the carbon power station, even as the percentage of CO2 emission reductions is increased. As a direct function that results in reducing CO2 emission levels it would be easier to justify major investments in renewable energy around existing carbon power stations, and having those investments paid for at least in part through carbon credits.
Another point has to do with the need for baseload power. The requirement for baseload that carbon power stations deliver is partly a consequence of electricity networks not having much of a storage capacity in them. If it was possible to store over a day’s worth of electricity locally in an electricity grid then baseload power would not be essential. The power stations could power the grid during the day as well as charge up storage banks to cover the energy needs overnight. The power station would only need to provide energy during the day and during the night the energy needs are taken from that stored energy. Managing such an energy regime would be quite complicated, but the point is that once electricity storage technology is developed and installed the need for baseload power may diminish. In that case there might be a switchover point when it may be cheaper and more efficient to shut down a coal power station and feed the renewable energy sources surrounding that power station, that were previously used to power the reduction in CO2 emissions, straight into the grid. There would be other ways to efficiently manage renewable energy sources near carbon power stations, for instance reducing the output of the carbon power station and having the shortfall filled through renewables, if and when appropriate. This might work well in parts of Australia but maybe not everywhere or at all times of the year.
This could be one part of a strategy to change over the energy systems to more renewable and CO2 neutral sources while maintaining the certainty of electricity supply that carbon power stations provide at the moment. It may seem an expensive way to reduce CO2 emission initially and it may seem to be a wasteful use of renewable energy sources to start off with, but it ensures certainty of supply while ensuring that CO2 emission reduction targets can be met without undue reductions in power station efficiency or through increasing use of carbon fuels, it provides a clear justification for the investment in renewables, and it provides a pathway to sustainable CO2 neutral energy systems from what we have now. The carbon power stations may also serve as backup generators, or as seasonal power sources over winter, for example.
Geosequestration of CO2 may also be a temporary technique for storing away wastes during winter, for example, that can be neutralised during more favourable months. I don’t think, however, that geosequestration will be some kind of cure-all for reducing CO2 emissions. It simply is not a sustainable strategy for dealing with CO2 over the long term, and it would be difficult to argue that it could ever be. Incidentally, one meaning for the word ‘sequestrate’ is ‘to make bankrupt’.
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