Meet the Methane Economy
Much of our economy and many of our homes use natural gas. This resource, like oil, is the result of biological action many millions of years ago and is finite. One day it will run out taking with it the transport industry, as it is currently totally fossil fuel based, the plastics and fertilizer industries that require fossil fuels as feedstock and the power generation system that uses natural gas. What is required is a long term supply of natural gas that will not run out and can supply all the needs of the electricity generation industries, chemical industries and home consumers.
A recent advance by a team of researchers at the University of NSW has pointed the way forward. Their work combined with two other technologies can lead to a method of producing methane, methanol or diesel fuel from sunlight, water and air.
The Process is an Integrated System
Firstly sunlight is used to split water into hydrogen and oxygen in the Solar Hydrogen Plant using new materials from the UNSW research. CO2 is captured from the air using a modified Solar Chimney. The power chimney technology for power generation is proven and a working model is due to be built in NSW. The resultant H2 and CO2 are reacted in the Sabatier/RWGS reactor with a catalyst to produce methane, CH4. With modification of the reactant ratios a gas mixture of CO and H2 can be produced which can be reacted in another converter to form methanol or ethylene.
Hydrogen from Water
A team from the University of NSW has recently identified the major variables needed to optimise materials for anodes and cathodes in water splitting cells. With this research there holds the promise of developing cells to separate hydrogen and oxygen from water using sunlight. The original scientific work by Fujishima and Honda in 1972 has been built on by this team and it is now considered the most advanced to produce an efficient cell within the next 5 to 7 years.
In a practical cell the electrodes are constructed from semiconducting oxides, the exact composition of which is the subject of intense research at the UNSW and other institutions to find the best combination of photo-sensitivity, corrosion resistance and low band gap. The leading contenders at the moment are TiO2 , CaTiO3 and SrTiO3.
In operation the plant would consist of large numbers of cells exposed to the sun. These cells would produce hydrogen and oxygen from water with sunlight providing the energy. The resultant gases would collect at the anodes and cathodes which would be collected by network of pipes and pumps. There is no requirement for the cells to produce electricity and then breakdown water to hydrogen and oxygen. This is a one step process with the cell emitting hydrogen and oxygen on exposure to sunlight. The oxygen can be liquefied and sold and the hydrogen also can be sold as gaseous or liquid hydrogen. However, as detailed later in the article, there are important reasons why the hydrogen should be converted to methane in a Sabatier/RWGS reactor and not marketed on a widespread basis as hydrogen.
CO2 From the Air
To make the whole process CO2 neutral the carbon dioxide must be obtained from the air. A paper by Klaus Lackner et al called “Extracting Carbon Dioxide From the Air” describes a convection tower to extract CO2 and provide electricity. The authors meant this as a method of offsetting CO2 emissions from fossil fuel plants by taking the CO2 out of the air and sequestering it. However this method is ideal for providing CO2 for the process of conversion of hydrogen to methane. It is even more ideal when you realise that the convection tower that Klaus describes is already being developed and built in Australia as the Solar Tower by Enviromission and is itself and exiting new method of generating renewable power. The concept of the Solar Tower is exactly the convection tower the authors of the paper envisaged.
In the Solar tower the sun’s heat is used to heat a large body of air which is then forced to move as a hot wind up a large tower through large turbines by natural convection. The Solar Tower required for the CO2 collecting would be much smaller than the pure power generating towers.
In a CO2 collecting tower an amine solution would be pumped up to the top of the tower and allowed to fall from the top to be collected at the bottom. As the rising air stream passes over the falling amine stream, CO2 is absorbed from the air by the solution which is then collected at the bottom, heated to release the CO2 and then pumped up to the top of tower again. As this process occurs after the air passes through the wind turbines the Solar Tower would generate most of the electricity needed for the integrated plant. As it does not interfere with the power generation capabilities of the tower it is possible that CO2 absorbers could be retro-fitted to existing Power Towers. This could reduce the cost of implementing Solar Methane by using existing structures.
Currently there are plans to capture CO2 from fossil fuel power station and sequester it underground. If this CO2 is shipped to the Solar Methane plant instead this can supplement the CO2 stream from the Solar Tower. This means that the CO2 from the fossil fuel plant is at least used twice thereby decreasing the net impact of the CO2 released
Converting Hydrogen to Methane
I first became of aware of the Sabatier reaction form reading a book “The Case For Mars” by Robert Zubrin. He envisioned that a mission to Mars would make it’s return fuel from the Martian atmosphere, which consists of 99% CO2, and hydrogen carried aboard. This way a small amount of hydrogen could be multiplied to become enough methane and oxygen, from reacting with CO2, to make such a mission viable. The Sabatier reaction is a well known reaction where H2 and CO2 react in the presence of a catalyst to form CH4 and H2O. This is the reaction
While this reaction will work there is another called the Reverse Water Gas Shift (RWGS) reaction. Its main product is CO and H2 in this reaction
The product of this reaction is called a synthesis gas and can be fed to a reactor to produce methanol. This would give a liquid that can be used directly as a transport fuel or reacted further to produce bio diesel.
The combined reaction that can happen is with a change in parameters is:
This means that the RWGS reactor, just by changing reactor pressure and temperature, can produce methane and a synthesis gas and can adapt to market requirements. This reaction was chosen by the Mars Direct team as the best and lightest system to produce the most methane and oxygen in the right proportions for rocket fuel. With this is mind there is great room for improvement as a ground system is not constrained by weight and size like a system designed for space.
Why Convert Hydrogen to Methane?
Why do this? Why go to all the trouble of converting hydrogen to methane? Why not just use the hydrogen? Why even bother producing hydrogen at all – after all can’t we just generate electricity from sunlight? Well there are a number of very compelling reasons for producing hydrogen and converting it, which I will detail.
- No Infrastructure Changes Required At the moment our industry uses natural gas in enormous amounts. It is not only a fuel for electricity production but as a feedstock for many industrial processes. It is also widely used in homes for heating and cooking. This represents a massive installed base of machines designed to run on methane. You might think that it would be easy for these to switch to hydrogen however this is not the case. Some seals that are used for natural gas are not compatible with hydrogen. The orifice size for burners is different as is the feeder pipe size. Hydrogen, being less dense than natural gas, needs higher pressures to deliver the same amount of gas. The present pipes might need replacing as they probably cannot handle the higher pressures. All this does not sound too bad but when you consider the sheer amount of industrial ovens, domestic ovens, domestic and industrial water heaters, domestic and industrial air heaters and the millions of kilometres of piping and the billions of pumps and seals involved then you begin to get an idea of the scale of the switch to hydrogen. Converting the hydrogen to methane at the point of production totally eliminates this conversion pain and can save industry and domestic consumers billions of dollars in conversion costs. It also makes hydrogen easier to sell as it is in a form that can be used by industry with no changes.
Solar Methane Overcomes Grid Instability Problems While wind energy is a great idea and will form an important part of the future energy picture there are some problems with it. Wind power connected to a grid can and does produce some grid instability. This has been noted in Denmark where wind power accounts for about 30% of Denmark’s power. This is not an easy problem to solve as to overcome it a ‘spinning reserve’ must be maintained to overcome the periods when renewables are not available. This actually is the main objection conventional power companies have against renewable power. To them they must maintain several power stations, usually coal fired, at low or zero power output but still ready to instantly ramp up to high power levels to maintain supply to the power grid as wind generators come on and off stream. They actually see little benefit from wind power because when the large power stations are idle they are still consuming large amounts of fuel and still produce large amounts of CO2 as they are running at very low efficiencies. This results in the CO2 savings of renewables to be less than the face value of the renewable power generated. When the power system is less centralised and homes have more of their own power supplies this will be less of a problem. Battery Electric Cars and Pluggable Hybrid Electric cars if they are configured properly can be the missing storage in the renewable power grid. Even with all this there is a need for base load generation - the 24 by 7 power that industry needs rain, hail, or shine.
Producing Natural Gas from sunlight completely eliminates these problems and provides a method of producing a 24 X 7 base load. Solar Methane production is CO2 neutral and does not contribute to Global Climate Change. The normal power stations running off solar methane become renewable power plants. As methane is readily storable, enough can be set aside to fuel the power stations at night and on cloudy days. Power companies are not under threat as their infrastructure remains in place so their livelihood is secure and they will therefore feel much more comfortable with this form of renewable power. There is also no need to modify the current natural gas co-generation natural gas plants as the power companies will be buying the same gas from the solar plants that they are at present getting from offshore wells. It is possible that even coal plants could be converted to natural gas which could extend their lives until they wear out.
Property Hydrogen Methane Gaseous Density ratio (air = 1) at STP 0.07 .55 Boiling Point (degrees C) -253 -161 Liquid density at boiling point (kg/m3) 70.973 422.62
Natural Gas is Easier to Handle and Store Hydrogen is a very light gas. It is only 0.07 as dense as air at standard temperature and pressure. This is in contrast to methane which is 0.55 as dense as air. What this means is that hydrogen needs very large tanks or small tanks compressed to extremely high pressures to be stored as a compressed gas. Alternatively it can be stored as a liquid however, as it boils at -253 degrees, it has to be kept extremely cold to remain as a liquid. This vastly increases the cost of the storage as the tankage needs to be very heavily insulated and the boil off is harder to manage.
Methane on the other hand boils at -161 degrees almost twice that of hydrogen. As a liquid it has a density of 422.2 Kg/cubic meter. This means that more gas can be stored in smaller tanks that do not have to be insulated nearly as well as hydrogen. Additionally LNG is a standard commodity and is transported today by ship and road and is very well understood. Liquid hydrogen is not nearly as common as it is really only used in the space industry as a component of rocket fuel. It is certainly not as widespread as liquid natural gas.
In summary methane is easier to store, is better understood and accepted by more people, and can store larger amounts in smaller cheaper tanks.
Many Other Hydrocarbons can be Produced With the conversion of hydrogen to methane comes the prospect of other alternative and useful products. If you use the RWGS reaction the synthesis gas can be fed to a reactor to produce methanol. Methanol can be used directly in cars as a fuel or reacted with vegetable oil to make bio diesel. Methanol being a liquid at room temperature is even easier to store and handle then methane. The same H2 and CO2 can be fed to a separate reactor where in the presence of a iron Fischer Tropsch catalyst they can be reacted in accordance with:
2CO + 4H2 = C2H4 + 2H2O
C2H4 is ethelyne which is the base feedstock for polyethelyne or polythene. This means that our plastics industry does not have to come to a screaming halt when the oil becomes scarce. This is a bigger issue than it seems on the surface. Without oil there is no plastic. Can you imagine a world without plastic – no drink bottles, no bags, no Tupperware, and the list goes on. It would hard to live without plastic however this is what we have to do if oil runs out or becomes scarce.
Producing hydrocarbons from air water and sunlight gives us the long term energy supplies that our civilization needs without releasing CO2 to trigger global climate change. After all plants do this and they are the basis of all human life on the planet and we think nothing of it. We call it photosynthesis and plants have been doing it for billions of years. The oil and natural gas we use now are the products of solar energy millions of years ago.
It is quite certain that the basic outline that I have presented will be changed. For instance the outer canopy area of the Power Tower could be made from the hydrogen producing cells. The water splitting process only uses the higher energy photons leaving the infra red part of the spectrum untouched. If the water splitting cells can be made transparent then they can form the canopy of the solar tower so that they do double duty. As the hydrogen modules contain an aqueous salt solution, having the electrolyte as part of the canopy will result in the solution storing heat. This will allow the power tower to operate into the night.
Possibly the greatest impact of solar methane is the total elimination of the need for nuclear power. Nuclear power is being increasingly touted as the saviour of the planet due to the fact that it does not emit CO2. This completely ignores the fact that nuclear power generates radioactive waste, can never be 100% safe and is a magnet for terrorist groups seeking targets and materials for attacks. A solar tower cannot have an accident that spreads radioactive material over a whole region. With solar methane the base load that the nuclear advocates say that only they can generate can be maintained with conventional gas turbine power plants running on easily stored solar methane. Countries that do not have the right conditions for Solar Towers can simply buy gas transported by conventional tankers as they do now. However there are very few countries that cannot setup Power Towers of their own.
Solar hydrogen is on a replacement for other renewables. The future energy supply of this country must be a diverse mix of technologies. With the decentralization of power generation and the widespread conservation of energy use we will need less base load generators to supply demand. Promotion of electric cars and pluggable hybrids will allow the power companies to end the wasteful practice of keeping generators running on idle as spinning reserve. The hopefully millions of battery cars plugged into the grid can supply this surge power leaving only the minimum of base load generation running at full capacity and full efficiency. They certainly can give faster reacting gas turbines time to start from cold. Wind, electricity producing Power Towers, and other renewable energy sources will always be a part of a sustainable decentralized energy solution that can maintain this fossil fuel addicted civilization of ours into the foreseeable future. We also might reverse some of the effects of two hundred years of environmental neglect that has left us on the brink of a possibly catastrophic global climate change. Without a solution to Peak Oil and Global Climate Change our society as we know could well collapse and lead to the deaths of billions due to starvation and disease. We need to act now to secure our future.
H2 and CH4 data http://www.airliquide.com/en/business/products/gases/gasdata/index.asp?GasID=41
Tom Meyer - RWGS/Ethylene System for Mars http://spot.colorado.edu/~meyertr/rwgs/rwgs.html
T.Bak, J Nowoty, M. Rekas, C.C. Sorrell - Photo-electrical hydrogen generation from water using solar energy. Material-related aspects International Journal of Hydrogen Energy 27 (2002 991-1022)
J. Nowotny, C.C Sorrell, L.R. Sheppard - Solar-hydrogen: Environmentally safe fuel for the future International Journal of Hydrogen Energy
Klaus S. Lackner, Patrick Grimes, Hans-J.Ziock – Capturing Carbon Dioxide From the Air First National Conference on Carbon Sequestration, National Energy Technology Laboratory http://www.netl.doe.gov/publications/proceedings/01/carbon_seq/7b1.pdf
Y.Soong, M.L. Gray, R.V. Siriwardine, K. L. Champagne - Novel Amine Enriched Solid Sorbents for Carbon Dioxide Capture First National Conference on Carbon Sequestration, National Energy Technology Laboratory
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