Global Carbon Dioxide Recycling

Coal, petroleum and natural gas were formed by fossilization of bodies of organisms spending hundreds of millions of years. They are the gift of our planet. After industrial revolution the industrial and economic growth is always supported by an increase in their combustion, leading to an increase in atmospheric carbon dioxide concentration. In the 1970s we came to be afraid the complete exhaustion of fossil fuel.

In the early 1970s we were thinking to supply hydrogen to the whole world producing hydrogen by electrolysis of seawater using electricity generated by solar cell installed on rafts floating on sea. At the same time we felt the difficulty to use hydrogen as the major fuel. We have no widespread technologies for storage, transportation and combustion of hydrogen. Facing rapid consumption of large amounts of resources by high industrial activities in the 1970s after high economic growth we begun to worry the complete consumption of resources and the damage of the natural environment by waste emissions.

The solar energy absorbed by lands, oceans and the atmosphere is released in the form of infrared thermal radiation into space. Greenhouse gases absorb the infrared radiation and maintain stable climate. Among greenhouse gases carbon dioxide had been kept almost constant at about 280 ppm by a balance in biogeochemical carbon cycle before industrial revolution. After industrial revolution it became higher than 290 ppm for about 100 years. During subsequent 100 years from the 1870s the industrial development of the world led to its constant increase at about 0.28 ppm every year. After 1970, the carbon dioxide emissions were too much to be treated on our planet. Carbon dioxide was accumulating in the atmosphere with an increase in rate and its atmospheric concentration exceeded 400 ppm. It has been said in 2007 that the atmospheric carbon dioxide concentration reached the level in 3.5 million years ago, in spite of the fact that our Homo Sapiens appeared only 200 thousand years ago. It has been known that in 3.5 million years ago the atmospheric carbon dioxide concentration was between 360 to 400 ppm and that the mean global temperature and sea level were 2–3 °C and 15–25 m higher than the pre-industrial levels, respectively. Our planet spent 2.5 million years to decrease it to the preindustrial level by forming carbonate solids due to chemical weathering of the Himalayas. It is clear how the current level is hazardous, and we need to avoid the carbon dioxide emissions higher than the pre-industrial level.

From the beginning of the 20th century, the world average temperature rose more than 1 °C. In the northern hemisphere, after the World War II to the middle of the 1970s the cooling trend appeared because human activities emitted ever more dust and smog particles that blocked sunlight. After the middle of the 1970s the air pollution was almost settled in developed countries and the greenhouse effect was enhanced by a rapid increase in the atmospheric carbon dioxide concentration. The temperature rise became sharper with time and was about 0.26 °C for 10 years from 2007. The abnormal weather induced by global warming resulted in the disaster to cause a large number of dead people in many areas in the world. Implementation of no fossil fuel combustion must be done urgently by the cooperation of the whole world.

Both primary energy consumption and carbon dioxide emissions in the world increased continuously showing the same trends because nearly 90% of primary energy consumption was fossil fuel combustion. Only the economic depression of the world led to suppression of increases in primary energy consumption and carbon dioxide emissions. Primary energy consumption and carbon dioxide emissions of developed countries kept high levels and those of developing countries increased with higher rates after 2000. Primary energy consumption and carbon dioxide emissions of developing countries exceeded those of OECD countries in 2010, but their populations were 78.0 and 18.1% of world population in 2015, respectively. If all people in the whole world are allowed the world average primary energy consumption and carbon dioxide emissions per person in the world, developing country people are allowed almost 1.5 times as high as energy consumption and carbon dioxide emissions per person in 2015, while OECD country people must decrease 60% of the values in 2015. Furthermore, the world population is increasing at a constant rate of 83.1 million every year. Because the increase in the primary energy consumption is necessary for industrial and economic development, only the solution for prevention of further increase in atmospheric carbon dioxide concentration is to use renewable energy without fossil fuel combustion.

In the world primary energy consumption in 2015, fossil fuel, nuclear power and renewable energy were 85.6, 4.7 and 9.7%, respectively. The world primary energy consumption increased by a factor of 1.01861 every year from 1980 to 2015. If the world primary energy consumption increases at this rate all world reserves of petroleum, natural gas, uranium and coal will be completed exhausted until the middle of this century inducing intolerable global warming. Only the solution is to establish and spread the technologies to use renewable energy by which all people in the whole world can keep the sustainable development.

Nuclear power generation started in 1951 in the USA. After passing more than 60 years, the share of nuclear power in the world primary energy consumption in 2015 was only 4.7% in spite of the fact that nuclear power generation was carried out as the national project in all countries. Nevertheless, uranium resources itself is limited. The risk cannot be deduced, because the consequences of the worst-case nuclear incident are unknown. Once accident occurs hundreds of thousands of people must evacuate from their hometowns for several tens of years or more, in addition to inevitable cancer cases of childhood and workers. Developed countries that induced global warming are responsible for showing the technologies by which the whole world can survive only using renewable energy, without taking selfish attitude clinging to nuclear power generation that cannot spread to the world.

For the prevention of further progress of global warming and complete exhaustion of fossil fuel, carbon dioxide emissions in the whole world must be limited to the preindustrial level by the use of only renewable energy without fossil fuel combustion. We need to establish and spread the technologies by which the whole world can keep sustainable development by the use of only renewable energy. On our planet there are superabundant renewable energy souses. For survival of the whole world renewable energy must be converted to currently used fuel for which infrastructures of storage, transportation and combustion exist in the whole world.

The fuel synthesis from renewable energy must be performed by simple technologies without requiring the sophisticated system, because the fuel synthesis must be done in the whole world. We succeeded to create effective catalysts on which carbon dioxide was rapidly converted to synthesized natural gas, methane by the reaction with hydrogen at ambient pressure with almost 100% methane selectivity. On the basis of finding of the catalyst for carbon dioxide methanation we made a proposal of global carbon dioxide recycling, consisting of hydrogen production by water electrolysis using electricity generated from renewable energy, carbon dioxide methanation by the reaction with hydrogen, methane combustion and capture of carbon dioxide which will be sent back to the carbon dioxide methanation plant. Realization of global carbon dioxide recycling will lead the world to use renewable energy forever without emitting carbon dioxide into atmosphere. We started the study of key materials for global carbon dioxide recycling about 30 years ago.

Key materials are the cathode and anode for hydrogen and oxygen production by water electrolysis and the catalyst for carbon dioxide methanation by the reaction with hydrogen. We succeeded to create active Ni-Fe-C and Co-Ni-Fe-C alloy cathodes by electrodeposition. Their activity for hydrogen generation is mechanistically highest. Alloy formation results in charge transfer from nickel atom to iron atom in the alloys, which accelerates the electron transfer from the cathode to hydrogen ion to form hydrogen atom. We could create oxygen production anodes without forming chlorine in direct seawater electrolysis. The effective electrocatalysts for oxygen formation in seawater electrolysis were MnO₂-type oxide containing Mo, W, Fe and/or Sn, on which 99.9% oxygen evolution efficiency was kept for more than 4200 h in 0.5 M NaCl at the current density of 1000 Am⁻². Because of urgent requirement of electrolyzers for hydrogen generation, we currently produce industrial electrolyzers using hot alkaline water electrolysis, creating active anodes and cathodes. We created Ni supported ZrO₂-type oxide catalysts for very rapid carbon dioxide methanation at ambient pressure with almost 100% methane selectivity. The ZrO₂-type oxide should be tetragonal containing oxygen vacancies, which is effective for adsorption of bidentate carbonate. It was clarified that hydrogenation of carbon dioxide to form methane proceeds from the bidentate carbonate adsorption on the catalyst.

On the basis of creation of effective key materials, we could construct the world’ first power to gas plant of the methane production at the rate of 0.1 Nm³ per hour in 1996 at Tohoku University. The plant consisted of power generation by solar cell, hydrogen production by seawater electrolysis, carbon dioxide methanation and methane combustion. The methanation system and the methane combustion furnace were connected by double piping and after combustion of methane with oxygen diluted by carbon dioxide, carbon dioxide was spontaneously sent back to the methanation system. In 2003, industrial scale pilot plant was constructed at Tohoku Institute of Technology. This plant consisted of seawater electrolyzer and carbon dioxide methantion system, which produces methane at the rate of 1 Nm³ per hour. After 2011, the plant construction and industrialization in progress by international and domestic collaboration of companies.

Europeans have been making the effort to use renewable energy for prevention of global warming from the early 1980s. Germany initiated the first green electricity feed-in tariff scheme in the world by Electricity Feed-in Act in 1991. Germany is now performing “Energiewende” from 2010. Germany will accomplish power generation only from renewable energy until 2050, by which 80% carbon dioxide emissions will be decreased. When power is generated only from intermittent and fluctuating renewable energy, supplying of a deficiency and leveling by stable stored power are necessary. For long term storage, our technology of conversion of surplus power to the synthesized natural gas, methane, and electricity regeneration from methane by natural gas power plant are efficient and convenient. In conventional power generation by fossil fuel and nuclear power, more than 60% of energy is exhausted in the form of warm waste water. There is no energy conversion loss in power generation from renewable energy. In the transportation sector the change will be done from gasoline and diesel cars of about 15% energy efficiency into electric and plug-in-hybrid cars. The energy efficiency of electric cars using electricity generated from renewable energy is about 70% for driving and charging, without emitting carbon dioxide. About 50% energy consumption decrease in business and civic life sector will be done by nearly energy-zero new buildings and renovation. The success of “Energiewende” of Germany will lead the whole world to follow their success, ending the dependency on fossil fuels and nuclear power.

Hydrogen is attractive because of formation of only water by combustion. However, no widespread technologies for storage, transportation and combustion of hydrogen. Hydrogen fuel cell car was the potential application of hydrogen fuel. However, the resources are limited, because the electrocatalyst for hydrogen fuel cell car is platinum. The world and car industries advance to electric cars using the secondary energy, electricity without emitting carbon dioxide. The energy efficiency of the electric car is far higher than the hydrogen fuel cell car in which hydrogen is the tertiary energy. Unless effective application is found, hydrogen cannot be the major fuel for direct combustion.

Establishment of local power supply system is important and effective. The system consists of direct use of power generated from renewable energy, storage of surplus power in the form of synthesized natural gas, methane, regeneration of power by natural gas power plant using methane and the use of warm water formed. The surplus power will be used to water electrolysis to form hydrogen and oxygen. Hydrogen will be used for methanation of carbon dioxide captured from the power plant. Methane will be used for regeneration of stable power. Oxygen generated by water electrolysis will be used for combustion of methane at the power plant after dilution with carbon dioxide captured at the power plant. Warm water discharged from the power plant will be used in the local area. In this system, carbon will be recycled between the methanation plant and the power plant. The exhaust gas does not contain nitrogen because oxygen diluted with carbon dioxide is used for methane combustion instead of air, and carbon dioxide capture from the exhaust gas is easily carried out. Captured carbon dioxide is used for methane formation and dilution of oxygen. The excess power supplied to the outside will support industry, transportation, etc.

Current increasing rates of atmospheric carbon dioxide concentration and global temperature are in the critical situation.