11262 Process Modification

The primary process modification strategy to minimize CO2 emissions is reduced combustion of fossil fuels. Reduced fuel burning is advocated for many reasons, not just CO2 emission reductions. There are other pollutants like CO and NOx that are also reduced if less fossil fuel is consumed. Reserves of important fuels like natural gas and oil continue to dwindle, so reducing fuel consumption will help conserve these limited resources. The problem is that demand for energy continues to rise around the world. Therefore, fuel conservation would make other energy sources, particularly renewable sources like solar, wind, and hydroelectric energy, more attractive and cost competitive as usage increases. More research is likely to be funded for these alternative sources if fossil fuel use is restricted.

In some countries, particularly in Europe, CO2 emissions are being reduced by taxing fuels containing carbon. This is commonly referred to as a carbon tax. Another indirect method of reducing CO2 emissions is by imposing minimum efficiency regulations on equipment that directly or indirectly uses the energy produced by burning fossil fuels. This includes making electrical devices more efficient as most electricity in most countries is produced by burning fossil fuels. The more efficient the electrical devices, the less electricity is needed, which means less fuel is consumed at the power plant. Improving efficiency reduces fuel consumption, which then reduces CO2 emissions. This is always a preferred method to reduce emissions as it simultaneously conserves precious resources. Improving the thermal efficiency of an industrial combustion process reduces the amount of fossil fuel burned for a given production rate and therefore reduces CO2 emissions indirectly. Brune et al. [20] note that using heat recuperation is a method to increase system efficiency and reduce CO2 generation. A complication arises when dealing with existing low-efficiency equipment and how to impose and enforce standards in that case. Some recommend a combination of mandatory CO2 emissions reductions combined with a market-based emission trading system, where the emissions would be controlled in part based on permitting fuel sources [8].

Another suggested combustion modification strategy is to use fuels with less carbon than other fuels so less CO2 is generated per unit of energy produced. One recommendation is to replace some or all of the coal used in large power plants for producing electricity with natural gas [21]. The simplified global reaction for the combustion of methane, which is the primary component in most natural gases, can be written as

where the diluent nitrogen in the combustion air has been ignored as it carries through on both sides of the equation. A variant of this technique is to use pure O2 as the oxidizer instead of air, which makes it easier and simpler to recycle, capture, and/or separate CO2 from the exhaust gas stream [22]. Notice that the combustion products (excluding nitrogen) consist of one-third CO2 and two-thirds water on a volume basis. Compare this to the simplified global combustion reaction of coal:

where again the diluent nitrogen has been ignored. Notice that, excluding the nitrogen, all of the combustion products are CO2. An even better fuel for reducing CO2 emissions is hydrogen:

Figure 11.12 Process for production of CH4 from biomass. (From Ref. 23. Courtesy of CRC Press.)

where no CO2 is produced. However, it must be realized that some type of hydrocarbon fuel is normally burned to produce the hydrogen in the first place so a system analysis is required to determine how much CO2 is generated overall.

A variation of fuel switching is to use a biomass fuel, such as corn husks, or to use fuels derived from plants, such as methanol or methane (see Fig. 11.12) [23]. The idea here is that using biofuels or fuels derived from biomass balances CO2 generation with photosynthetic consumption so there is no net gain in CO2 production. At this time, the technology has not progressed sufficiently, nor are the economics favorable yet, to make this a viable option at this time for industrial combustion processes. However, this is likely to change in the future.

A more dramatic process modification strategy is to use completely different sources of energy such as nuclear, wind, or solar, which do not generate any CO2 emissions. However, in the case of nuclear power, there are other environmental issues such as the disposal of the used fuel rods that must be considered. At this time, "green" sources of energy like wind and solar have not been developed to the point to be economic or practical for the large quantities of energy that would be needed for large industrial users. Continued advances in technology should make these options more viable in the future.

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Guide to Alternative Fuels

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