M.A. Savitenko – ANO “Center for Research and Development in the Field of Energy” Hydrogen Technological Solutions ”
B.A. Rybakov, Ph.D. – SK-Engineering LLC
There are many talks about decarbonization these days, with an emphasis on carbon emissions. But few people talk about water vapor, which has even more disastrous effect. Have you forgotten or is it not time yet?
Decarbonization: are we going the right way?
The transition to hydrogen energy is justified by the need to reduce carbon dioxide emissions into the atmosphere. This process is called decarbonization.
It is known that an increase in the concentration of carbon dioxide in the Earth’s atmosphere leads to an increase in the average air temperature, that is, “global warming”.
To stimulate the decarbonization process, the European Union plans to adopt a so-called “carbon tax” on all imported goods.
In July last year, KPMG analysts calculated the likely losses of Russian exporters in the event of the introduction of the fee. In the worst case scenario, it will appear in 2022 and will affect both direct and indirect emissions. Then suppliers will pay 50.6 billion euros until 2030. Under the baseline scenario, the tax will be introduced in 2025 and will be extended only to direct emissions, which will cost Russian exporters 33.3 billion euros. The most positive scenario assumes the appearance of a tax in 2028, in which case manufacturers will pay 6 billion dollars.
With the development of hydrogen energy, carbon dioxide emissions into the atmosphere will decrease and water vapor emissions will increase, since energy and water vapor are released during the reaction of hydrogen and oxygen.
It is believed that water vapor is harmless to humans and the environment.
Is there a connection between water vapor and the greenhouse effect?
Water vapor and the greenhouse effectWater in the Earth's atmosphere contains:
Water in the Earth’s atmosphere contains:
– in the form of steam (water gas formed during the evaporation of water)
– in a liquid state (cloud elements in the form of water droplets, raindrops),
– in a frozen state (cloud elements in the form of ice crystals, snowflakes, hailstones).
A distinctive feature of water from other constituent elements of the Earth’s atmosphere is that its content in the atmosphere is constantly changing.
Decarbonization: are we going the right way?The water content in the Earth's atmosphere depends on:
– air temperature,
– the state of the evaporating surface.
In the Arctic countries, the air temperature is very low, so the atmosphere contains a very small, hardly measurable amount of water – i.e., the air is dry. In hot countries, where the evaporation process is very intense, on the contrary, the amount of water in the Earth’s atmosphere can reach 4% and the air is very humid.
The higher the air temperature, the more water vapor can be contained in the air.
Does the water vapor dissolved in the air affect the climate, and if it does, can its effect be compared with the influence of carbon dioxide?
Below are excerpts from the publications of foreign researchers of the influence of water vapor on the climate of our planet:
“The importance of water vapor in climate regulation is undeniable. It is the dominant greenhouse gas that retains the Earth’s heat more strongly than other substances.”
“Water vapor is the most important greenhouse gas. Carbon dioxide is the second most important greenhouse gas.”
“The authors found that in the case of clear sky, the contribution of water vapor in the reflection of the long-wave radiation is 75 W/m2, while carbon dioxide – 32 W/m2.”
“Water vapor is the dominant greenhouse gas, the most important source of infrared opacity in the atmosphere”.
“The dominant role of water vapor as a greenhouse gas seen for a long time”.
“Generally speaking, water vapor is the only atmospheric absorber of infrared radiation.”
“In society, only carbon dioxide is known as a greenhouse gas. In reality, water vapor makes a more significant contribution to the increase in atmospheric temperature.”
Natural gas combustion
Natural gas consists of a mixture of marginal hydrocarbons, such as methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), pentane (C5H12) and hexane (C6H14), as well as a small amount of inert gases.
The characteristics of natural gas, such as density and heat of combustion, can be determined with great accuracy using the characteristics of the first four homologues. The general formula for the reaction of marginal hydrocarbons with oxygen has the following form:
СnH(2n+2) + 0,5 (3n+1)O2→ nCO2 + (n+1) H2O,
If n is the number of carbon molecules and the ordinal number of the hydrocarbon homologue.
Let us consider combustion reaction equations of the first four homologues of hydrocarbons C1, C2, C3 and C4, which are in a gaseous state under atmospheric conditions.
If n=1
CH4 + 2O2 → CO2 + 2 H2O
(i.e., 1 mole of methane, combining with 2 moles of oxygen, forms 1 mole of carbon dioxide and 2 moles of water vapor).
When burning one kilogram of methane (CH4), 50 MJ of thermal energy is released, as well as 2.75 kg of carbon dioxide (CO2) and 2.25 kg of water vapor (H2O), that is, water vapor emissions into the atmosphere are slightly lower than carbon dioxide emissions.
If n=2
C2H6 + 3,5O2 → 2CO2 + 3 H2O.
When burning one kilogram of ethane (C2H6), 47.8 MJ of thermal energy is released, as well as 2.93 kg of carbon dioxide (CO2) and 1.8 kg of water vapor (H2O), that is, the mass fraction of water vapor in ethane combustion products is less than when burning methane.
If n=3
C3H8 + 5O2 → 3CO2 + 4 H2O.
If n=4
C4H10 + 6,5O2 → 4CO2 + 5 H2O.
From these expressions it can be seen that with an increase in the number of the homologue of a hydrocarbon, a larger volume of oxygen is required for its complete oxidation, while a larger volume of carbon dioxide and water vapor is released than when methane is burned. The volume of CO2 released during the combustion of hydrocarbons is proportional to the ordinal number of the homologue, and the water vapor is n+1. When methane is burned, twice as much water vapor is released as carbon dioxide, and with an increase in the proportion of “heavy” hydrocarbons in the burned gas, this proportion decreases.
Combustion of hydrogen and methane
Let’s compare the amount of greenhouse gas emissions into the atmosphere from the combustion of hydrogen and methane, which is the main component of natural gas.
When two hydrogen molecules combine with one oxygen molecule, two water molecules arise. The reaction of hydrogen and oxygen compounds is accompanied by the release of energy.
2*Н2 + О2 → 2*Н2О + energy (1)
When burning one kilogram of hydrogen (H2), 120 MJ of thermal energy and 9 kg of water vapor (H2O) are released.
The addition of one molecule of methane with two molecules of oxygen creates two molecules of water and one molecule of carbon dioxide. The reaction of methane and oxygen compounds is accompanied by the release of thermal energy.
СН4 + 2*О2 → 2*Н2О + СО2 + thermal energy (2)
When burning one kilogram of methane (CH4), 50 MJ of thermal energy is released, as well as 2.75 kg of carbon dioxide (CO2) and 2.25 kg of water vapor (H2O), that is, 5 kg of greenhouse gases.
To obtain 120 MJ of thermal energy, it will be necessary to burn 2.4 kg of methane. At the same time, 6.6 kg of carbon dioxide and 5.4 kg of water vapor will enter the atmosphere, that is, 12 kg of greenhouse gases.
The data of the above calculations are summarized in Table 2.
Table 2.
| The mass of the burned gas | Heat energy release,
MJ |
Mass of water vapor | Carbon dioxide mass | Total mass of greenhouse gases |
| 1 kg of hydrogen (H2O) | 120 | 9 kg | – | 9 kg |
| 2.4 kg of methane (CH4) | 120 | 5,4 kg | 6,6 kg | 12 kg |
From this calculation, it can be seen that when the same amount of energy is obtained, the total greenhouse gas emissions from methane combustion are 30% higher than when hydrogen is burned.
At the same time, water vapor emissions from natural gas combustion are 40% lower than from hydrogen combustion.
It can be seen from the publication /3/ that the effect of water vapor on the greenhouse effect is 2.3 times higher than the effect of carbon dioxide.
If we take this fact into account, the greenhouse effect of burning 1 kg of hydrogen will be commensurate with the burning of 2.4 kg of methane. That is, when the same amount of energy is released, the effect of hydrogen and methane on the greenhouse effect is commensurate.
At the same time, according to the publications of foreign mass media (mass media), it can be concluded that only carbon dioxide affects the increase in the greenhouse effect!
In /7/ it is reported that studies conducted by scientists from the University of Miami Rosenstiel School of Marine and Atmospheric Science have confirmed that water vapor in the troposphere – the layer of the atmosphere located between the Earth’s surface and extending to a height of 5-20 km – will play an increasing role in climate change in the future.
Researchers from Florida reported that the increasing amount of water vapor in the atmosphere is caused by human activity.
If this is the case, then along with reducing carbon dioxide emissions into the atmosphere, it is necessary to control water vapor emissions.
Sources of water vapor emissions
As noted above, when using hydrocarbon gases such as methane, ethane, propane and butane, water vapor is formed along with carbon dioxide emissions.
In thermal and nuclear power engineering, the working fluid involved in the generation of electrical and thermal energy is water vapor, for the condensation of which various types of cooling towers are used, as well as direct-flow cooling with water from rivers, lakes and reservoirs.
In 2008, direct-flow cooling was banned in the design and construction of new power plants in Russia.
The so-called “wet” cooling towers are sources of water vapor emissions into the atmosphere.
To reduce water vapor emissions in the energy sector, “dry” cooling towers, as well as air condensing units (VCS) allow.
Hydrogen-the fuel of the future
As noted above, the substitution of natural gas with hydrogen as a fuel does not lead to a decrease in the greenhouse effect if the steam formed as a result of the combustion of hydrogen-containing gas is not condensed.
This also applies to fuel cells in which electrical energy is generated by electrochemical means, since water vapor is formed along with electrical energy at the outlet of the fuel cell.
That is, during the transition to hydrogen energy, it is necessary to stimulate not only technologies that reduce carbon dioxide emissions into the atmosphere, but also water vapor.
References
- “Climate Modeling through Radiative-Convective Models”, Ramanathan & Coakley, Reviews of Geophysics and Space Physics (1978).
- “Atmospheric Radiation: Theoretical Basis”, Goody & Yung, Oxford University Press (1989, 2nd edition).
- “The Importance and Nature of the Water Vapor Budget in Nature and Models”, Lindzen, Climate Sensitivity to Radiative Perturbations: Physical Mechanisms and Their Validation (1996).
- “Earth’s Annual Global Mean Energy Budget”, Kiehl & Trenberth, Bulletin of the American Meteorological Society (1997).
- “The Radiative Signature of Upper Tropospheric Moistening”, Soden, Jackson, Ramaswamy, Schwarzkopf & Huang, Science (2005).
- “Radiation and Climate”, Vardavas & Taylor, Oxford University Press (2007).
- «Rising Levels of Human-Caused Water Vapor in Troposphere will Intensify Climate Change Projections”, Rick Panpaleo, (2014)