Carbon dioxide mitigation using renewable power

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Energy is required to convert CO2 into useful products. Thermodynamic arguments can be made to show that, if the energy for the conversion of CO2 is provided by combustion of fossil feedstock, it is not possible to consume more CO2 than is produced. Renewable energy, specifically renewable power, is generally assumed to be the energy source for CO2 utilization. Various options are available for using renewable power for mitigation of CO2 emissions, including electroreduction of CO2 to hydrocarbons, electrolysis of water to hydrogen, and power for battery electric vehicles. Thermodynamic and practical arguments can be used to rank the effectiveness of renewable power for CO2 mitigation for these options, using power for light duty transportation as an example. Renewable power used to reduce water to hydrogen, which is then used to convert CO2 into liquid fuels, is only one-fourth as effective at mitigating CO2 emissions compared to use of the renewable power in a battery electric vehicle. The direct electroreduction of CO2 to liquid fuels, even if achieved with 100% thermal efficiency, is only one-third as effective at mitigating CO2 compared to the battery electric vehicle.

Section snippets

CO2 Utilization with fossil energy source

Stevenson examines the thermodynamics of making fuels or chemicals, comparing fossil-only feedstock routes with those that consume CO2 [2••]. He gives specific examples of methanol and acrylic acid synthesis. An abbreviated version of his argument is presented here, using his example of methanol synthesis from methane, versus synthesis from methane and CO2. For the methane-only case, the overall stoichiometry and enthalpy of the reaction are:4CH4+2O24CH3OHΔH=505kJ4molMeOH

It is assumed that

Alternate CO2 mitigation strategies using renewable power

Electroreduction of CO2 and/or water is one approach that uses renewable power to mitigate CO2, but other approaches are also available. These include:

  • 1

    Displacement of fossil fuels for light duty transportation in Internal Combustion Engine Vehicles (ICEV), via:

      • a

        Electroreduction of water to hydrogen using renewable power, with use of hydrogen in a Fuel Cell Vehicle (FCV).

      • b

        Renewable power to a Battery Electric Vehicle (BEV) or Plug-in Hybrid Electric Vehicle (PHEV). Note that a PHEV is equivalent

Discussion

Table 2 compiles the calculation results for the CO2 emissions mitigated from one mega-Joule of renewable power. It is probably no surprise that replacement of coal-fired power with renewable power has the greatest CO2 mitigation, at 291 g CO2 per MJ power. The next best scheme is to use renewable power for light duty transportation with battery electric vehicles, with 189 g CO2 mitigated per MJ. Next is replacement of natural gas-fired open cycle gas turbine power plants, with 141 g CO2

Conclusions

Thermodynamic arguments can be made to show that ‘utilization’ of CO2 requires energy, and if fossil feedstock is burned to provide the required energy, it is not possible to consume more CO2 than is produced. Rather, renewable energy must be used to provide the energy for conversion of CO2 into useful products, such that a net reduction in CO2 occurs. Renewable energy in the form of power can be used for the electroreduction of CO2/H2O to hydrocarbons, the electroreduction of H2O to hydrogen,

Conflict of interest statement

Nothing declared.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as

  • • of special interest

  • •• of outstanding interest

References (18)

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