When hydrocarbons are burned they produce CO2 and water. The great challenge is to reverse this process and to produce - efficiently and economically - hydrocarbon fuels and materials from CO2 and water. Of course Nature, in its process of photosynthesis, captures CO2 by green plants from the air and converts it with water, using the Sun's energy and chlorophyll as the catalyst, into new plant life. Thus, plant life replenishes itself by recycling atmospheric CO2. The difficulty is that the conversion of plant life into fossil fuels is a very long process, taking many million years. As we cannot wait that long, we must develop our own chemical recycling processes to achieve it within the required very short time scale. The most promising approach is to convert CO2 chemically by catalytic or electrochemical hydrogenation to methanol, and subsequently to hydrocarbons. Chemists have known for more than 80 years how to produce methanol from CO2 and H2. In fact, some of the earliest methanol plants operating in the 1920 to 1930s were commonly using CO2 and hydrogen, generally obtained as byproducts of others processes, for methanol production. Efficient catalysts based on metals and their oxides (notably copper and zinc) have been developed for this reaction. These catalysts are very similar to those used presently in the industry for methanol production via syn-gas. In view of the understanding of the mechanism for methanol synthesis from syn-gas, this is not really unexpected. It is now well established, that methanol is most probably formed almost exclusively by hydrogenation of CO2 contained in syn-gas on the catalyst s surface. In order to be converted to methanol, the CO present in the syn-gas must first undergo a water gas shift reaction to form CO2 and H2. The formed CO2 then reacts with hydrogen to yield methanol .
Although further improvements are needed, many methods of producing methanol from CO2 and H2 are already known [220, 221, 221a, 221b], and there should be no major technological barrier for their application on a large commercial scale. The limiting factor for the large-scale use of such a process is the availability of the feedstocks, namely CO2 and H2. CO2 can be obtained relatively easily in large amounts from various exhaust sources such as those of fossil fuel-burning power plants and various industrial plants. Eventually, even the CO2 contained in air can be separated and chemically recycled to methanol and desired synthetic hydrocarbons and their products. Hydrogen is presently mainly produced from non-renewable fossil fuel-based syn-gas, and thus its availability is limited. In view of our diminishing fossil fuel resources, this is not the way of the future. In addition, the generation of syn-gas from fossil fuels releases CO2, which contributes further to the greenhouse effect.
To address this problem, while fossil fuels are still available improved routes to produce hydrogen from them have been investigated. In a process called "Car-nol", which was developed at the Brookhaven National Laboratory, hydrogen and solid carbon are produced by the thermal decomposition of methane . The generated hydrogen is then reacted with CO2 recovered from fossil fuel-burning power plants, industrial flue gases or the atmosphere to produce methanol. Overall, the net emission of CO2 from this process is close to zero, because CO2 released when methanol is used as a fuel was recycled form existing emission sources. The solid carbon formed as a stable byproduct can be handled and stored much more easily than gaseous CO2, and be disposed of or used as a commodity material, for example in soil conditioning or as a filler for road construction. Clearly, the process depends on the availability of methane:
Methane decomposition: CH4
Thermal decomposition of methane occurs when methane is heated at high temperature in the absence of air. To obtain reasonable conversion rates under industrial conditions, temperatures above 800 °C are required. This process has been used for many years, not for the production of hydrogen but of carbon black used in tires and as a pigment for inks and paints. For the primary generation of hydrogen, different reactor designs have been proposed. Attention has recently been focused on reactors operating with a molten metal bath, such as molten tin or copper heated to about 900 °C, into which methane gas is introduced.
Being an endothermic reaction, thermal decomposition of methane requires about 18 kcal mol-1 to produce 2 moles of hydrogen from methane, or 9 kcal of energy for the production of 1 mol of hydrogen. As a comparison, this is less than the highly endothermic methane steam reforming reaction, which requires about 49.2 kcal mol-1 to produce 4 moles of hydrogen, or 12.3 kcal mol-1 hydrogen. It should be clear that, whereas methane steam reforming produces four molecules of hydrogen for every molecule of methane used, methane decomposition, like partial oxidation of methane, yields only two. On the other hand, methane's thermal decomposition byproduct (i.e., carbon) can be easily handled, stored and used without much further treatment. The suppression of CO2 emissions generated by methane steam reforming or partial oxidation is more difficult and energy-intensive. The CO2 must first be collected, concentrated and finally transported to a suitable sequestration site; this may be distant from the production site and require an extensive pipeline network. Methane thermal decomposition process is still in its infancy, and will require considerable further investigation to mold it into a mature and efficient technology for commercial application. In a carbon-constrained world which is aiming to reduce greenhouse gas emissions, methane thermal decomposition could be a useful alternative for the generation of hydrogen and methanol, avoiding the problems associated with CO2 emission and sequestration. It would allow mankind to use extensively the world's remaining natural gas resources, including unconventional sources such as the huge methane hydrates deposits under the ocean floors and arctic tundra (given that an effective means of collecting them is found) with limited effects on the Earth's atmosphere.
Ultimately, however, all of these resources are finite and non-renewable. Over time, they will become depleted or economically too prohibitive to exploit. In the long term, therefore, the large-scale, cost-effective production of hydrogen by electrolysis of water is the key to the development of CO2 to methanol processes, and thus to the methanol economy. The electrolysis of water to produce hydrogen is a well-developed and straightforward process, and is achieved by applying an electric current between electrodes inserted into water with some elec trolytes present. Hydrogen evolves at the cathode, and oxygen at the anode. The electricity needed for the process can be provided by any form of energy. At present, a large part of the electricity produced is derived from fossil fuel, but in the future, in order to be sustainable and environmentally sound, the electric power required for large-scale electrolysis of water should be obtained from atomic energy (fission and later fusion, if proven technically and commercially feasible) and any renewable energy source, including hydro, solar, wind, geothermal, wave and tides. Such production methods of hydrogen, avoiding the emission of CO2 to the atmosphere, are discussed in Chapter 9.
Although the catalytic reduction of CO2 with hydrogen is feasible, one drawback is that the process requires 3 moles of hydrogen for every mole of CO2. As the generation of the required hydrogen is highly energy-consuming, the production of methanol from CO2 was often considered economically less attractive than its formation from carbon monoxide and hydrogen.
Olah and Prakash have recently found ways of overcoming some of the difficulties of converting CO2 into methanol. Using electrochemical or photochemical reduction of CO2, besides methanol, formaldehyde and formic acid can be obtained in high selectivity and good conversions. The formic acid and formaldehyde in a subsequent step, not unlike the previously discussed oxidative conversion of methane, can then be converted to methanol, with formic acid providing the required hydrogen [223, 223a, 223b, 223c].
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