Fossil in production of greenhouse gases, particularly, carbon

Fossil fuels play a major role as
a leading source of energy for a foreseeable future and their persistent use
inherently result in production of greenhouse gases, particularly, carbon
dioxide. Therefore, carbon capture, carbon sequestration and other carbon
dioxide abatement techniques have gained major focus in the research and
development sections of many industries and universities. At the same time,
national and international governments of many countries especially in Europe
have put forward strict regulations on greenhouse gas emissions and in
improving renewable energy sources.  There
are many methods currently in research for carbon capture and one of them is the
electrochemical reduction of carbon dioxide.

Electrochemical methods for
biofuel/high value chemical production has attracted many researchers with a
bulk of research articles published on Bioelectrochemical CO2
reduction in the last 5 years (Aryal et al.,
2017; Bajracharya et al., 2016; Cai et al., 2016; Guo et al., 2013, 2017;
Januszewska et al., 2014; van Eerten-Jansen et al., 2015; Zhao et al., 2016). A large part of the research focuses on Microbial
Electrochemical Cells (MEC), where the microorganisms act as catalysts for the electrochemical
reactions at the cathode. The Microbial Electrochemical Cell (MEC) is a
modified version of Microbial Fuel Cell (MFC), where an applied potential
difference results in overcoming non-spontaneous reaction such as hydrogen
formation through electrolysis of water (Logan et al.,
2008). 

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The microorganisms are able to
generate an anode voltage of about -0.300 V by feeding on the acetate present
in the electrochemical cell. However, this voltage is not sufficient to
generate hydrogen at cathode, which requires a cell voltage about -0.414 V.
Therefore, with the help of the extra potential supplied through a potentiostat
or main electric supply, hydrogen evolution is achieved at the cathode (Liu et al., 2005a). An extension of this reaction with help of
suitable microorganisms and optimised experimental conditions would lead to
efficient carbon dioxide reduction (CO2) with the formation of
methane (CH4) (Call and Logan,
2008). This
concept was developed as a cheaper option with the use of bio-cathodes and
easier storage options compared to hydrogen production and storage (van Eerten-Jansen et al., 2015). Additionally, the volumetric energy density is very
low (11 MJ/m³), which is much lower compared to methane (36 MJ/m³) making it a
very impractical fuel to serve as an energy source. Therefore, methane becomes
an ideal fuel source that can be stored and transported inexpensively and used widely,
especially for public transport (Balat et al., 2008).

There are two pathways to achieve electrochemical
carbon dioxide reduction:

The
theoretical potential for the reaction (1) that reduces CO2 to CH4
with a transfer of 8 electrons is below the theoretical potential for the
electrolysis reaction of H2 (reaction 2a). Therefore, it is
understood that microorganisms involved in direct electron transfer would have
a greater energy gain than organisms that use indirect electron transfer where
hydrogen molecule acts as a shuttle for electron transfer. The electrochemical
principle states that a lower potential for the transfer of particular quantity
of electrons is efficient against a use of higher potential for the transfer of
same quantity of electrons. Therefore, for the same reaction, CO2 to
CH4, using a lower potential is preferable and cultivating direct
electron transfer is desired (Mueller, 2012).

Figure 1: Graphical
representation of CO2 reduction for biomethane production (Mohseni et al., 2017)

Latest
trends of energy statistics in some of the European nations such as Germany and
Norway (Fig. 2) show that renewable electricity could be one of the major
sources of energy. Therefore, there is a need to create demand for this
electricity and the best way for that is to find energy sectors that are able
to replace fossil energy sources. The chief customer officer of the company
SolarFuel in Germany, Stephan Rieke has reported that the excess renewable
energy in Germany grew from 150 gigawatt-hours per year to 1,000 gigawatt-hours
per year in two years. The amount is expected to continue to grow as Germany
pursues ambitious goals to cut greenhouse-gas emissions 80 percent by 2050
using largely renewable energy (Gotz et al., 2011; Götz et al., 2011; Graf et al., 2011; Hoekman et al.,
2010; Specht et al., 2010; Sterner, 2009). A similar situation is to arise in the Scandinavian
nations and especially in Norway where already 96% of the electricity is
sourced through hydroelectric power stations (SSB, 2017a).

Figure 2: Electric
Power production and consumption trends in Norway for a period of 10 years
between 2006 and 2015 (SSB, 2017a).

Figure 2
depicts the electric power production and consumption trends in Norway where
different power generation sources such as Hydro, Thermal and Wind have been
included. It can be observed from the figure that gross power consumption
(black) in Norway has always been lesser than power production (green) every
year except for 2010. For example in the year 2015 the power production was
around 145000 GWh whereas the gross power consumption was only 130000 GWh
leading to almost 15000 GWh of excess energy production. In the same year, the
contribution from hydroelectric power (blue) to the total electricity
production was 139000 GWh, which in itself is more than the gross consumption.
This leads to an excess electricity production of about 2500 GWh through wind
power (orange) and 3500 GWh through thermal power (not shown).

Also, in
the 5 year period of 2011 to 2015 it can be observed that the wind power
(orange) production has been more than doubled from 1200 GWh in 2011 to 2500
GWh in 2015 which indicate that Norwegian policies for renewable energy power
are very encouraging (SSB, 2017a). At present the excess
electricity is being exported to the neighbouring countries and therefore
Norway is rewarded financially. The capacity of these renewable sources for
electricity would only increase in the coming years and an alternative to
exporting the excess electricity, is for it to replace fossil energy sources especially
in the transport sector. This approach of carbon capture addresses the specific
issue excess renewable energy that is going to be available in the coming
years. This technology is otherwise coined as ‘Power to Gas’ technology (Mohseni et al., 2017).

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