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Hydrogen and Oxygen

Carbon Capture Using the Oxygen Produced by Electrolysis

Bruce Barbour - July 2020

At some stage in the future the much talked about hydrogen production will get off the ground. Hydrogen will have many uses on the way to and in a post carbon society:- fuel for automobiles and trucks, a store of energy for later conversion into electricity, a heat source for industry and an export commodity for all those purposes.

Hydrogen can be produced in a number of ways. The two main ways are by: (1) splitting hydrocarbons / fossil fuels, and (2) electrolysis of water, which uses electrical energy to split water into hydrogen and oxygen. The first method produces carbon dioxide as a byproduct and is not sustainable/renewable. The second method, electrolysis, can either be unsustainable if fossil fuel generated electricity is used in the process or can be fully sustainable if the electricity used is generated from renewable sources, such as solar or wind. The production of hydrogen by electrolysis using renewable energy is the preferred method, and hopefully this method will be utilised as more renewable energy is generated into the future.

Production of hydrogen by electrolysis means that a lot of pure oxygen (O2) will also be produced. While it could be released into the atmosphere without any impact that would be a waste. Instead this oxygen produced as a byproduct of hydrogen generation should be looked on as a great opportunity in a number of areas.

A lot of the commentary that I read on tackling climate change indicate that it is quite likely that we, that is humanity, are going to overshoot the desirable maximum target of carbon dioxide concentration in the atmosphere. This means that not only do we have cut our net carbon dioxide emissions to zero, after that goal is achieved (hopefully) we have to start removing existing carbon dioxide from the atmosphere to bring the carbon dioxide concentration down to acceptable levels, whatever that may be.

One way to do this is via carbon capture and storage. Carbon capture and storage up until now is usually discussed in relation to the burning of fossil fuels, capturing the exhaust gases and then usually extracting the carbon dioxide from the exhaust gases and then pumping the concentrated carbon dioxide into underground storage caverns where it will hopefully be stored for a very long time. The big issue with this process has been the difficulty of extracting the carbon dioxide from the exhaust gases for storage. While all the exhaust gases could be put underground this would waste limited storage space. Most of the exhaust gases would be the inert gas nitrogen, as nitrogen constitutes approximately 78% of air.

This is where the use of the generated pure oxygen comes in.

Instead of combusting the fuel to generate electricity with atmospheric air it could be combusted with pure oxygen (or pure oxygen mixed with exhaust gases, in this case CO2, if that is better for the controlled combustion. References: (1) Wikipedia, (2) Science Direct, (3) Science Direct 2) As a consequence of this the exhaust gases are going be primarily carbon dioxide which could be pumped into subterranean storage without further concentration/refinement.

So what fuel would be used in this process? The preferred fuel would be crop waste. A normal food crop would be harvested and sent to market. The waste stems and leaves and other plant product would also be collected and then sent to be burnt in a special furnace designed to operate with pure oxygen. The furnace would boil water for generating electricity in the usual manner. The exhaust gases from the combustion would be collected and stored underground. As part of the photosynthesis and growth process the crop takes in carbon dioxide from the atmosphere. Normally crop waste would be left in the field and would rot and release the carbon dioxide back into the atmosphere. The process outlined at the start of the paragraph would divert that carbon from returning to the atmosphere to underground storage decreasing the amount of carbon in the atmosphere. And of course the electricity generated would be completely renewable, in fact better than 100% as it removes the extra carbon from the atmosphere through the capture and storage process.

There are a couple of other options for fuel for the furnace. The crop could be grown specifically as a fuel source, i.e. no other crop is harvested. This could be some kind of fast growing plant - perhaps a bamboo?? This might be considered if it was determined that this process was a good means of capture and storage of carbon.

The ash generated from crop waste and crop burning also contains carbon, as well as other chemicals. The ash could also be buried as carbon storage or there could be other uses for it.

The ideal set up would be for the furnace / boiler / generator to be built in conjunction with a solar / wind farm. The solar / wind farm could either be dedicated for the production of hydrogen and oxygen or could sell part of the solar / wind generated electricity directly into the grid if the demand is there - though I think it would probably be a dedicated plant. The ideal location would be an area surrounded by crop farming as a source for the crop waste to the boiler. It should also have access to an underground storage site for the carbon dioxide, preferably close by, and of course access to the electricity grid with suitable capacity. And a good supply of clean water for the electrolysis and the boiler.

This arrangement would make the production of hydrogen by electrolysis more economical and therefore a more attractive proposition. The oxygen generated value adds to the whole process, instead of being wasted or a produced as a low value byproduct. Is it actually competitive with other electrical generation? I don't know. However it is one of these processes that would greatly benefit if Australia had a system of carbon taxes or carbon credits.

Regardless of the economics the real benefit is that burning crop waste in this manner has negative carbon generation. It takes carbon out of the atmosphere - which is desperately required at this time and this by itself may be sufficient to recommend this process. The generation of clean electricity from the process may come to be considered as the very useful byproduct.

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