From most perspectives, an obvious place to consider capturing CO2 is at the source. This is the approach successfully used for SO2, NOx, and particulate emission controls, and one could extrapolate that such an approach is applicable to CO2. At the exhaust stack, the concentration of the CO2 is greatest, and it should be most amenable to recovery. There are several thousands of concentrated sources in contrast to hundreds of billions of CO2 receptors in the environment. CO2 presents its own unique chemical challenges. As already noted, CO2 is not as reactive as other pollutants (SO2 or SO3, for example). It's not easily transitioned to another chemical state. Some have suggested taking CO2 and converting it to carbon and oxygen, without realizing that energy accounting demands nearly as much energy released from the fuel is required to produce such a conversion. If one-tenth of 1% of CO2 was required to be converted in such a way, it might be viable but not on the scale of CO2 reduction being proposed. Some proposals call for 80-90% reduction in CO2 economy wide. Capturing this much CO2 from stationary and mobile sources would be as challenging as attempting to alter the global temperature directly. An alternative to capture, or an intermediate step, might be to reduce the CO2 emissions, or eliminate them entirely from the source. Various technical innovations to achieve this were extensively reviewed only a few years ago .
Fuel switching is perhaps the easiest choice for reducing CO2 emissions. Using a fuel with less carbon and more hydrogen in the fuel results in a reduce CO2 emission profile. In this respect, carbon is the mechanism of delivering hydrogen to the end-user. Fuel switching is not always as simple as it may appear. Fossil coal plants can easily substitute burning natural gas instead of coal or oil, but they do so at efficiencies far less than the most efficient combined cycles or even the simplest high compression diesel engines. Most combined cycles and peaking units are gas fired, providing limited options for fuel substitution. If fuel switching to natural gas is the option, that option should be used only in the most efficient systems available. That usually implies natural gas combined cycles, combined heat and power systems, or some comparable cycle. Because of its cost (and its high inherent fuel quality), the use a high quality fuel like natural gas in a low efficiency system is to be discouraged. Coal, on the other hand, is one of the least expensive fuels because it is so abundant. The reason for the low cost is that fuel is typically of poor quality (due to the presence of water and ash), as well as a high carbon content. The critical technology focus for coal utilization has been to find ways of extracting the thermal energy within the coal, while minimizing the release of pollutants to the environment, or damage to the hardware.
Another fuel switch option is the use of biomass (directly or blended with a fossil fuel). Biomass is derived from CO2 (and water vapor) absorbed from the atmosphere. Burning biomass may be one approach to reducing the CO2 emission factor from a conventional fueled power plant. In fact, there are quite few power plants around the world currently operating on biomass as a feedstock, although most of these are under 100 MWe in size (in the US, the average is closer to 50 MWe).
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