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.
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