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Hydrogen and Oxygen
Carbon Capture Using the Oxygen Produced by Electrolysis
Bruce Barbour - July 2020 - Updated September 2021
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 (H2O) 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 - in fact exactly half the volume of the hydrogen 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, we have to also start removing existing carbon dioxide from the atmosphere to bring the carbon dioxide concentration down to acceptable levels somewhat closer to pre-industrial concentrations.
| Carbon
Capture and Storage (CCS) CCS is extremely controversial in Australia and around the world. The main reason is that CCS has been championed by the fossil fuel industry as a way to prolong their existence. The fossil fuel industry use it in two ways. Firstly to provide a veneer of cleanliness over a dirty industry - e.g. the whole "clean coal" crap. The second is as a delaying tactic - "We're working on this technology to clean up our product - just give us a bit more time and we will get there, we will be clean. And give us a few more million dollars of taxpayer money." It is solely a PR tool for them. In the meantime they continue business as usual - raking in the money and polluting the atmosphere. The trouble is despite all of their claims of being able to implement CCS and despite millions of dollars of Government subsidies they have been singularly unsuccessful in implementing it on any meaningful scale - in Australia and around the world. Not one cent of Government money should go to the fossil fuel industry - for CCS research or any other subsidy. In fact they should be paying us for the damage they have caused. CCS should not be used by the fossil fuel industry to prolong its life. However given that the World is going to overshoot its so called carbon budget humanity is going to have to draw down the existing carbon from the atmosphere. This could be done by a number of technologies - biochar, deep ocean storage, rewilding/tree planting. In conjunction with these direct air capture with CCS and also OFBECCS as discussed on this page, may be part of the long term solution. |
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, more likely, oxygen mixed with exhaust gases, primarily CO2, for better controlled combustion. Effectively the nitrogen from air has been removed from the combustion chamber and therefore does not have to be removed from the exhaust gases for CCS. For example a mix of, say, 30% oxygen and 70% exhaust flue gases / carbon dioxide could be used. (Note: these percentages are a complete guess. Actual percentages would be whatever mix is found to work the best.) The burning of the fuel is going to consume most, if not all, of the oxygen in the CO2 / oxygen mix. As a consequence of this the exhaust gases will be primarily carbon dioxide. The flue gases would be captured. A proportion of the gases would be sent back to the combustion chamber after having been mixed with the appropriate concentration (30% in the case of the example) of oxygen to continue the process. The remainder would be pumped into subterranean storage with minimal further concentration or refinement. For example it may be decided that the removal of moisture generated from the burning process - moisture that came from the fuel being not perfectly dry prior to burning - was warranted as the process of removing most of the moisture is relatively easy. For other gases - probably some nitrogen and toxic gases from the combustion of some of the waste products - it may be decided not to remove them due to the cost.
Burning of fuel in an oxygen rich environment is called Oxy Fuel Combustion. When it is combined with burning of natural organic material using carbon capture and storage it could be referred to as Oxy Fuel combustion for Bio Energy with Carbon Capture and Storage. (OFBECCS). References: (1) Wikipedia, (2) Science Direct, (3) Science Direct 2).
So what fuel would be used in this process? While any carbon based fuel could be used the preferred fuel would be an organic waste product. Waste is ideal as a fuel as it doesn't require further energy input for its production. Just some additional energy input for the collection, transport and some pre-treatment (e.g. cutting and drying) of the waste.
The advantage of using organic waste product is that the process will result in the removal of carbon from the atmosphere. In the photosynthesis and growth process organic food and crops take in carbon dioxide from the atmosphere. Normally an organic waste product would be allowed to rot in the field or in landfill or in a composting area. In the process of rotting it would release the carbon dioxide back into the atmosphere. This is not generally thought of as harmful as it is part of the normal carbon cycle - except if methane is produced in the process which it can be, especially in landfill. The OFBECCS process would divert the carbon dioxide from returning to the atmosphere instead it would go into 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% renewable as it removes the extra carbon from the atmosphere through the capture and storage process.
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.
What sources of organic waste products could be used?
Domestic household organic waste from food waste and also cuttings from residential gardens is one source. Victoria is introducing a 4 bin waste/recyclables collection system which will have a separate bin for the collection of food scrap waste and garden cuttings. While it was planned that the organic waste would be composted it may be determined that a better use would be this OFBECCS process. (It also may not be the best use of organic waste. The world is going to have to decrease its use of artificial fertiliser. Compost material may be valuable as a replacement fertiliser.) The moisture content of household domestic waste is an issue if the waste is to be burnt, meaning that the waste would have to dried (in the sun) prior to burning. While paper and cardboard could also be combusted it may have a better carbon balance to recycle it. A proportion of domestic hard waste is comprised of timber/wood (e.g. from furniture). This would also be suitable as a fuel.
Victoria is planning energy from waste plants. Energy from waste plants are also very common overseas. The waste will probably be partially organic and partially from fossil fuel based materials (plastics). If these plants in Victoria were to go ahead they should involve carbon capture and storage technology as well as toxic gases capture and storage.
If a crop waste was to used, the normal food crop would be harvested and sent to market. The waste stems and leaves and other plant product would also be collected and sent to be used as the fuel. Waste from the plantation timber industry could also be used.
Another option for a waste fuel source is dried sewage farm sludge.
A crop could be grown specifically as a fuel source, i.e. no other crop is harvested. This, of course, is not waste product. The crop could be some kind of fast growing plant - perhaps a bamboo?? However there is an additional energy input and financial cost associated with that. And it could displace other food production crops which may become more of an issue as the impacts of climate change are felt more heavily. This might still be considered if it was determined that this OFBECCS process was a good means of capture and storage of carbon.
Plastic waste, for example supermarket food and drink containers, while it would work as a fuel, would not result in carbon removal from the atmosphere. It would effectively be fossil fuel generated electricity with carbon capture. The oxy fuel combustion process should not be used to prolong the fossil fuel electricity generating industry.
The OFBECCS process would generate heat from the burning of the waste, which would be used to boil water for generating electricity in the usual manner.
As this would be a new process for Australia the first step would be to set up a trial site to determine the technical difficulties prior to wider adoption.
The ideal trial site would be somewhere adjacent to a significant natural gas distribution pipeline, say coming from the Bass Strait gas fields off the coast of Gippsland (in Victoria, Australia). It would also have to be adjacent to potential carbon storage sites. The site should also have a source of renewable electricity although the renewable electricity can always come in through a sufficiently large grid connection and the process could use some of its own generated electricity. The site would also have to have a significant source of organic waste, so maybe adjacent to a largish rural city in Gippsland, where the domestic organic waste collection could be used as the primary fuel source, and sewage sludge, crop waste and a dedicated crop could be trialed.
For the trial, a plant to produce hydrogen and oxygen from water electrolysis would be built. The hydrogen could be pumped into the natural gas line, if there is no other better use for it. Hydrogen can be mixed with natural gas at a concentration of up to 10% without requiring any modification to the pipelines or the gas appliances. Another option would be to use the hydrogen to generate more electricity by running an internal combustion engine powered generator, a fuel cell or using it as an additional heat source for the main boiler and generator. The hydrogen could be stored and the electricity produced from the hydrogen generated at peak times for a better financial return. The oxygen produced would be used in the OFBECCS process.
When fully up and running the OFBECCS process would make the production of hydrogen by electrolysis more economical and therefore a more attractive proposition. The oxygen generated would value add 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 of the electricity generated the real benefit is that OFBECCS 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.
Going into the future we are going to have to remove large amounts of carbon from the atmosphere. OFBECCS may be part of this approach however it could not work as a sole solution. If it was it would require a significant proportion of the arable land to be devoted to growing crops for OFBECCS, displacing food production in a time when it is likely that the food productivity of the land will be declining. It would have to work in conjunction with other methods of atmospheric carbon removal such as (deep) ocean and soil sequestration (e.g. biochar) and direct air carbon capture and storage (DACCS). However the most important requirement, without which all other action is futile, is to stop putting carbon into the atmosphere.
