|
Hydrogen has been touted by many as the fuel of the future and while it is true that at the point of use the major byproduct of its use is water it is not as simple as just converting everything to run on hydrogen rather than fossil fuels.
Fossil fuels are hydrocarbons, which means they are made up of compounds that consist mainly of hydrogen and carbon but they can also contain other elements like sulphur. When fossil fuels are mixed with air and burnt the hydrogen combines with oxygen to release energy and produce water H2O. However the carbon and other compounds also burn producing energy and substances like carbon dioxide CO2 and sulphur dioxide SO2.
There is another problem with thermal engines which is associated with the nitrogen that makes up around 79% of the atmosphere. To increase the efficiency of engines the combustion of fuel is carried out at high pressures and temperatures and under these conditions the nitrogen in the atmosphere can form nitrous oxides NOX and other nitrogen compounds that are highly toxic and react with sunlight to produce photochemical smog.
While all the combustion byproducts of fossil fuels have an effect on the atmosphere and environment it is the CO2, SO2 and NOX compounds that cause the majority of the problems.
In theory, hydrogen is an ideal fuel as when it is burnt in air the major byproduct is water, H2O, and while it is not totally without environmental effects they are minimal when compared to the byproducts of burning many fossil fuels. However like anything it's never that simple and there are some serious drawbacks:
- Like fossil fuels if H2 is burnt at high temperatures and pressures it can also produce nitrogen compounds?
- While H2 has a good energy per mass ration when compared with fossil fuels its density means that at normal atmospheric temperatures and pressures the volume makes storing it as a gas impractical. There are however ways to store H2 and currently the most promising are:
- Liquid H2. Hydrogen liquefies at -252° C so you can store it as a liquid but you must keep it a cryogenic temperatures. This uses up energy in the chilling process and temperatures as low as this are difficult to maintain. Temperatures this low can also change the properties of the material used to construct storage vessels and if liquid H2 ever comes in direct contact with living tissue it freezes and severely damages both the contact area and surrounding tissue..
- High pressure H2. Hydrogen is often stored under pressure but to gain a relatively useful energy to volume ratio you need pressures of around 70 Mpa (10,300 PSI). Constructing pressure vessels that can withstand pressures of this magnitude is no simple feat. There are also secondary problems of cooling a condensation as the fuel is used. Compressing the H2 to these pressures is also not easy and consumes a not insignificant amount of energy which has a detrimental effect on the overall efficiency.
- Hydrides Rather than storing hydrogen in its elemental form it can be stored compounded with other chemicals in a form that will release the hydrogen relatively easily.
- Absorption Media Similar in concept to storing the H2 as a hydride but rather than having the hydrogen chemically bonded it is stored in a manner that is not too dissimilar to the way a sponge soaks up water.
- Another problem with storing H2 is the size of the molecule. H2 is an extremely small molecule and is impossible to fully contain. No matter how good the container you build it will leak and H2 leaks can be extremely dangerous.
- Hydrogen also burns with a flame that is virtually invisible to the naked eye, so detecting leaks and burning H2 is difficult without specialized equipment.
Before we go any further let's look at how much H2 would be needed to replace current annual consumption of fossil fuels. In 2004 the world consumed about 407 EJ (407 x 1016 Joules) of energy from fossil fuels. Given hydrogen releases around 286 KJ mol-1 when combined with oxygen and H2 has a molecular mass of 2 x 1.00794 we can now calculate roughly how much H2 would need to be produced to replace the fossil fuels as follows:
Currently the global production of H2 is around 50 million metric tons but this would need to increase to at least 3,500 million metric tons per year. In reality it is more complex than this but it gives us a starting point that we can compare with existing technology and what would need to be done to replace the fossil fuel economy with a hydrogen economy.
If we are to move to a hydrogen economy it is going to involve a whole raft of different technologies so over the next few weeks we will look at some of the technologies that have the potential of making the hydrogen economy a reality rather than an empty pipe dream.
The first obvious source of H2 on earth is the water that makes up the world's oceans as water is made up of 2 atoms of hydrogen plus one atom of oxygen. The simplest way to separate the water forming H2 and O2 is to use electrolysis. However if we are to reduce the damage we are doing to the environment we need to find the energy needed to split the hydrogen from the oxygen from a renewable non polluting source. If we simply use existing fossil fuels like coal to generate the electricity you would end up producing more pollution than we currently are and we would end up being worse off than just using the fossil fuels directly.
So can we do it? Can we possibly ever produce anything like the 3.5 billion metric tons of H2 needed each year from the world's oceans and if not, how much can we realistically expect to be able to produce? What about the energy needed, where can we get 407 EJ plus the production, distribution and storage losses, of energy without creating even more pollution than we currently are? Do we really need to produce that much H2 or are there other alternatives?
| 
Re: Future Energy Sources 3.1.1 Water & the Hydrogen Economy
