Hydrogen as a fuel

If we are to have a sustainable transportation infrastructure that includes hydrogen fuel cell vehicles, we must produce the energy carrier-hydrogen- in large quantities from clean, non-fossil energy sources, and that means from renewables.

So exactly what options do we have to put us onto the clean hydrogen path and what challenges need to be overcome along the way?

As we plan for the clean, non-petroleum-fuelled automobile and truck fleet of the future, we envision a propulsion technology portfolio that includes biofuel powered, electric drive, and hydrogen fuel cell vehicles (FCV).

The last of these is perhaps the most technically challenging, but also the most attractive technology in terms of its ability to dramatically decrease oil consumption, CO2 greenhouse gas emissions, and tail pipe pollution.

However, hydrogen is not an energy source – it is an energy carrier. To fully realise its benefits, we must produce it not from fossil sources, but from renewable energy.

The world produces huge quantities of hydrogen today for industrial and commercial purposes, probably in excess of 50 million tonnes/year. Most of that production is fossil-energy based, either from reforming natural gas, or electrolysis using electricity produced 

from coal, natural gas, petroleum, or nuclear.

Renewables on the other hand are a desired energy source for hydrogen production due to their diversity, regionality, abundance, and potential for sustainability. That being asserted, there are many challenges to producing Hydrogen from renewables – and perhaps the major one is reducing the cost to be competitive with gasoline and diesel.

Electrolysis – splitting water into hydrogen and oxygen using electricity from one of the many renewable sources;

Biomass conversion – via either thermochemical or biochemical conversion to intermediate products that can then be separated or reformed to hydrogen; or fermentation techniques that produce hydrogen directly;

Solar conversion – by either thermolysis, using solar-generated heat for high temperature chemical cycle hydrogen production or photolysis, in which solar photons are used in biological or electrochemical systems to produce hydrogen directly.

The order above is, in general, also representative of the technological maturity of these pathways, and thus roughly the chronological order in which we might expect to see them commercially available.

There is a substantial worldwide business in producing electrolysers, and building electrolysis facilities for hydrogen production. The challenges for transportation-ready renewable hydrogen are both in cost, and in understanding the logistics and economics of large central production plants versus smaller distributed facilities located nearer the vehicle users.

The worldwide electricity production potential from renewables is staggering. If addressed and utilised aggressively, there is sufficient resource to support not only large inputs to the electrical grids across the planet, but also significant hydrogen production.

Because biomass is our only renewable source of hydrocarbons, conversion of a small portion of the planet’s huge biomass resource to fuels is an important option for our transportation needs. Hydrogen can be produced from this renewable feedstock..

Biomass-to-hydrogen is complex, not only because of the technical details of the conversion processes themselves, but also because of the many process types that could be employed.

Gasification – whether steam, air/oxygen, catalytic, or indirect – involves subjecting the biomass to elevated temperatures and pressures in order to reduce the organic materials to hydrogen and carbon monoxide/dioxide gases (along with varying quantities of undesirable solid and gaseous byproducts). From there, the hydrogen can be separated out by membrane, chemical, or catalytic steps.

Ethanol produced from lignocellulosic materials could be further reformed to hydrogen, as could other biofuels or intermediate products of various biochemical routes Certain regional implications, feedstock types, or end-use requirements might make this a viable, if not a widespread, option.

More interesting perhaps is dark fermentation, a process that uses anaerobic microorganisms to produce hydrogen directly, much in the way that bacteria or yeast can produce ethanol via fermentation. Such organisms might be enhanced to better perform the hydrogen production task. They typically need to start with glucose, so the cellulosic ethanol pretreatment and hydrolysis techniques that are being developed now to break down cellulose into glucose would also be required for the dark fermentation pathway.

Perhaps the most intriguing options, with huge potential but requiring more development time, are solar conversion techniques. These are thermolysis and photolysis, respectively.

Thermolysis involves using the heat produced from concentrated solar power (CSP) to drive one of many thermochemical reactions (hundreds of which are known) that can produce hydrogen, or to drive electrolysis at very high temperatures for more efficient water decomposition.

Photolysis may be the ultimate “holy grail” for hydrogen production, using solar photons to produce hydrogen directly via biological or electrochemical systems.

Reference Source; www.renewableenergyfocus.com

Provided by Robin Burn.

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