I.3. The ways to reduce CO2 emission

There is a six main ways to reduce carbon dioxide emission:

• the substitution of fuels with a low value of H:C ratio (such as coal) by fuels with a high value of this ratio (such as natural gas),
• the economical use of energy (especially implementation of low energy technology in the industry ),
• the application of the combined production of electricity and heat energy in the form of more efficient processes (the technological progress in the power sector, implementation of gas-fired combined-cycle natural gas blocks etc.),
• the use of renewable energy on a larger scale (water, sun, heat pumps, wind energy),
• the development of nuclear energy [7].

Carbon dioxide has attracted a lot of attention over last 20 years, as scientific research has established a direct link between the increases in CO₂ emission with the rise in global temperatures, which has led the government and industrial around the world actively working on solutions to reduce CO₂ emissions. Many different technologies have been proposed to reduce this pollution from environment with economically energy use.

Current approaches within the present fossil-fuel energy scenario for the reduction of CO₂ emission from large-scale fossil energy facilities (e.g. power stations and cement works) are primarily focusing on carbon capture and storage (CCS, with three generic option being proposed: post-combustion capture, pre-combustion capture and oxy fuel combustion). All three approaches are based on different physical and chemical processes involving absorption adsorption and cryogenic capture of CO₂ [8].

CO₂ could be stored in natural gas reservoirs. The general idea of CO₂ disposal in gas reservoirs is that the recoverable hydrocarbons stored underground (gas and condensate) are replaced by CO₂.

The mechanism of underground carbon dioxide storage in gas reservoirs can be considered as simply replacing the volume of gases and fluids produced from the reservoir by supercritical ‘liquid’ carbon dioxide. According to this view, the volume of the net underground withdrawal is a measure of the capacity for CO₂ storage [11]. However, it is important to note that there are unknown ecological and environmental risk associated with this technology.

Due to CO₂ capture and sequestration, large amount of CO₂ will be available.

The chemical and catalytic conversion of CO₂ into useful substances such as transport fuels or another chemicals forms, is one of the most important technologies to be developed. A successful conversion of CO₂ into fuels can lead to the closure of the carbon cycle, by recycling carbon taken originally from fossil fuels. However, CO₂ conversion into hydrocarbon fuels require reduction, which is thermodynamically not a favorable process. The high amount of energy required for conversion should, therefore, be taken from sources which do not produce CO₂ or produce less CO₂ than the amount chemically transformed. As such energy sources, among other, nuclear, geothermal, solar, and wind energies are proposed [11].

Catalysts plays an important role in CO₂ chemical transformation by lowering and optimizing the process.

Today there is an increasing interest focused on the concept of recovered CO₂ being used to synthesis fuels through Fisher-Tropsch chemistry. The broad range of FTS products can be processed to the desired hydrocarbons fractions. The Fischer-Tropsch reaction is highly exothermic and requires the use of heterogeneous catalysts in a temperature range between 200 and 350˚C under elevated pressure.

On the other hand, other valuable chemical products may also be obtained from CO₂:

• methane,
• methanol,
• formic acid,
• gasoline,
• other hydrocarbons.

Methanol and methane is especially desirable where fuels are concerned. These products may be directly used as a raw material for other synthesis or the source of energy for other processes.

Commercially, methanol is produced from syngas obtained from natural gas or coal. The reaction feed contains CO and H­₂ and a small amount of CO₂. This reaction is usually catalysed by Cu/ZnO-based catalysts which have high reactivity and selectivity.

On the other hand, methanol could be produced from natural (biological) materials, in the following scheme:

Biomass is converted to biogas (mainly methane) using anaerobic digestion. Biogas can then be converted into a mixture of CO and H₂ (synthesis gas) using catalytic processes. Synthesis gas is then converted to liquid fuel and gaseous hydrocarbons using catalysts such as Co, Rh, or Fe in Fischer-Tropsch process [12].

Catalytic hydrogenation of CO₂ is one of the most effective methods to convert CO₂.