Making the most of hydrogen

As hydrogen refueling stations begin to appear, including one from Shell opening near London, Toyota makes its Mirai hydrogen fuel cell vehicle more widely available and iron ore smelters even look to hydrogen as an alternative reducing agent (rather than coal) that doesn’t involve carbon, it would appear that this important energy carrier is beginning to show its potential.

The final energy (i.e. energy we use) mix in use today is less than 20% electricity, in large part dictated by the need to transport, store and use energy well away and disconnected from the point of production. Vehicles are a common example of this, with the requirement to power them on the move. While electricity storage is coming of age with batteries, hydrogen offers an alternative route forward that provides much of the flexibility we have come to rely upon with liquid fuels.

Hydrogen also offers the possibility of providing intense heat very quickly in a confined area, such as required by various industrial processes. For these reasons, and as society looks to shift away from direct use of fossil fuels for the purpose of carbon dioxide mitigation or simply needs to augment the energy system with other carriers, hydrogen shouldn’t be ignored.

Hydrogen production comes via three routes. The first is as a by-product from some industrial processes (e.g. chlorine manufacture). When I first started work in a Shell refinery I was assigned to follow the operation of the platformer, a unit which increases the octane number of naphtha by creating cyclic ring compounds (aromatics). These have a lower hydrogen content than straight chain molecules, so hydrogen is a byproduct. For the most part this source of hydrogen is in full use, either within other processes in refineries or in neighbouring facilities.

The second is steam reforming of natural gas, which produces hydrogen and carbon dioxide. Most industrial hydrogen in the world today is produced via this route. The carbon dioxide is relatively pure and can be used or captured and stored, making the process very carbon efficient. In the Shell refinery in the Netherlands some carbon dioxide from its hydrogen manufacturing unit is sold to local greenhouses to provide the enhanced growing conditions that they need. In Alberta, Canada, one million tonnes per annum of carbon dioxide from the Shell Scotford Refinery hydrogen unit is captured and geologically stored.

Finally, hydrogen can be produced by electrolysis of water, an experiment almost everybody has done in chemistry class at school. This route has renewable energy enthusiasts very excited as it can provide a ready use of electricity when wind and solar production exceeds demand, a situation many grids may face as renewable energy penetration reaches very high levels. But this process suffers from a relatively low efficiency, with only a few percent of global hydrogen being produced via this route. Generating hydrogen by electrolysis is only (optimally) about 60% efficient and the use of this hydrogen in a car is (optimally) also only about 60% efficient, so two thirds of the energy input is lost. This can be improved to some extent if heat recovery methods are employed.

The economics of the electrolysis route can be difficult, particularly if it relies on renewable energy surpluses, which won’t always be available. That would leave the electrolyzer idle or having to use purpose generated electricity. Further, most current electrolyzers vent the oxygen that is made, which is effectively waste energy and explains why the process can only be 60% efficient if hydrogen is the only product being made.

The global oxygen industry is very large; today it is around 500+ million tonnes per annum, with the metallurgical industry being the largest consumer. Oxygen is typically manufactured by cryogenic distillation of air. In the context of a hydrogen economy based on electrolysis rather than steam reforming of natural gas, oxygen production would be significant. According to Toyota, the Mirai consumes 0.76 kg H2 per 100 km. Using these numbers as a basis, a 200 million vehicle hydrogen fuel cell fleet might consume 23 million tonnes of hydrogen per annum, which when produced by electrolysis would also generate around 180 million tonnes of oxygen, or around one third of the current demand. This of course presumes it can be efficiently captured and transported.

Significant production of oxygen may also benefit another facet of an energy system striving for net-zero emissions. Biomass could be used for power generation, but in an oxygen combustion system (i.e. combusting the biomass with pure oxygen rather than with air which is 79% nitrogen). This means that the waste gas would be mainly carbon dioxide, which in turn would lead to easier capture and storage. This could then be an important negative emission technology in the net-zero sum for the economy as a whole.

The hydrogen economy offers many new opportunities and it is good to see that it is starting to move forward. The Shell hydrogen refueling station that opened in Cobham, uses an on-site electrolyzer to produce its hydrogen, with the electricity coming from a certified green source. It is the first of three hydrogen stations Shell plans to open in the UK in 2017.