Carbon capture

Carbon capture and storage (also known as carbon sequestration) is a method of mitigating the CO₂ emitted from the combustion of fossil fuels. It is a way of storing the gas before it is allowed to enter the atmosphere and more recently it also refers to scrubbing CO₂ from ambient air, for example using carbon scrubbers or converting it into a useful product via artificial photosynthesis.

Carbon storage

Various methods have been suggested for storing CO₂, which include storing it in existing geographical formations, deep in the oceans or in the form of mineral carbonates. These will be discussed in more detail below.

Geological storage

This method involves injecting CO₂ into underground geological formations including old oil fields, gas fields, saline formations and old coal mines. By injecting CO₂ into declining oil fields, this can force more oil out of the well and the cost of storing the gas can be partially offset by the sale of the extra oil recovered. However they do have a limited capacity and locations are also finite. In the case of mines, CO₂ attaches itself to the surface of the coal which in turn releases methane which can be sold to offset the cost of storing the gas in the first place. However like pushing out and using the extra oil, burning the methane will produce more CO₂. It also is important that the coal field is not over permeable otherwise the gas could leak. However good locations are available in all the geographical locations where the gas could be stored for millions of years.

Oceanic storage

There are two major methods of ocean storage; the first is pumping the gas down to depths of 1-3km and letting the gas rise, where it will dissolve into the seawater, this will slowly come out of the water back into the atmosphere. The other method is pumping it down further than 3km, and at this depth the pressure will cause the CO₂ to liquefy, and as this liquid is denser than the sea water it will fall to the bottom of the sea and form non reactive pools of liquid CO₂.  Both these methods have potential but the effect of additional quantities of the gas may well upset ecosystems, so more research needs to be completed in this field.

Mineral storage

This method involved reacting the CO₂ with metal oxides, forming metal carbonates. This reaction requires heat, however the metal oxides needed for the reaction are plentiful (such as Magnesium) and the produced carbonates are very stable so they will hold on to the gas for millennia. The major drawback is the heat required for the reaction, so research is ongoing to make this a more economically viable option.

What are Carbon Dioxide Scrubbers?

In other sections of this site we have covered the concept of carbon capture and storage, which is the process of removing carbon dioxide from entering the atmosphere and storing it. Most of the technologies we have looked at focus on removing the gas where it is developed in high quantities, for example in the exhaust gases produced by coal power stations.

Carbon dioxide is found in the air everywhere though; so why just concentrate on where the gas is emitted in higher quantities?

It is possible to scrub CO₂ from the air anywhere; the technology has been around for decades and used on Submarines and Spacecraft, to name but a couple of examples. So the potential is there to research and build on this technology, scaling it up so it can be positioned to effectively scrub the air in any location.

How does CO₂ Scrubbing work?

Despite several different designs currently being in development, they all are based on a common chemical reaction. Air is sucked into the machinery where it is bought into contact with a sorbent material which chemically binds with the Carbon Dioxide. A sorbent material is one that simply absorbs a gas or liquid (e.g. sponge is sorbent as it absorbs many times its own weight in water).

The greater the surface area of the sorbent, the more efficiently it will absorb the gas or liquid, therefore different mechanisms have been suggested to maximise exposure of the sorbent to the carbon dioxide, thereby maximising it’s scrubbing ability.

The Palo Alto Research Centre has proposed to draw the CO₂ through a fine mist of liquid sorbent. Housing this technology in towers that are several metres high, the mist would react with the gas and be collected in a chamber where they would once again be separated. The pure CO₂ could be compressed into liquid form and removed, while the sorbent would be recycled and used again to collect more of the gas.

Klaus Lackner, has created another proposal to maximise the surface area of the sorbent, and this to apply solid sorbent to thin sheets and allow the Carbon dioxide to react with it. Once the initial reaction has taken place, liquid chemicals are washed over the sheets that create a stronger bond with the CO₂ than the sorbent. The liquid can then be collected (as it now is bound to the CO₂), and this can be heated which will allow the CO₂ to be stripped from the liquid, and so once again the pure CO₂ could be compressed into liquid form and removed, while the liquid can be recycled and used again to wash future CO₂ from the sheets.

Issues facing the technology

The air-capture machines are electrically powered, and most electricity produced (via non-recyclable methods) has carbon dioxide emissions associated with it. So an important question is whether the carbon dioxide stripped from the air is in excess of the carbon dioxide ‘produced’ to drive the machine. In fact, Klaus Lackner’s prototype uses 100kwh of electricity to remove 1 tonne of CO₂ from the air, and this power required equates to 35kg of CO₂ being produced as emissions, so the ratio of gas removed far outweighs the amount produced. In fact, if the energy used to drive it is derived from renewable energy forms then the figures become even more attractive.

Another issue facing the technology is that the sorbent material cannot be recycled for ever, and has a finite lifecycle, after which it has to be replaced so this makes sorbent supply high (and expensive), in addition there will be maintenance costs associated with swapping the sorbent material over. The cost currently associated with removing 1 tonne of CO₂ from the atmosphere is about £150 per tonne when using these carbon scrubber methods, while the cost of trading a tonne of carbon is about £6-£13 (see carbon trading). Only when the cost of removing a tonne is lower than the trading cost will this become truly commercially viable. Dr Lackner has suggested that he feels that with technical improvements and economies of scale achievable if the products become commercially successful, then the cost will come down to approximately £30 per tonne. In addition trading carbon prices will rise in the future, as countries looking to fulfil their green promises endeavour to make the cost of emitting CO₂ unattractive.

Finally, where do we position the carbon scrubbers? In the grand scheme of things, CO₂ present in the atmosphere is found in very low percentages, roughly 0.04% (with 99% made up of Nitrogen and oxygen), so actually removing it out of the atmosphere is difficult. Therefore when we produce the compressed gas as a result of the carbon scrubbing technologies, it is important that we have a use for it. CO₂ is a useful gas in it’s own right – it can be used to pump into commercial greenhouses to increase plant growth, it can be used to inject into natural gas reserve beds to drive more of the gas out; It can even be transformed into fuel for transport. So we should erect the carbon scrubbers where the CO₂ produced can be then used to perform a useful function. In the future, if the technology takes hold and is profitable (the stored gas can be sold for more than it costs to remove it), then we could see the widespread launch of carbon scrubber ‘orchids’, simply acting to remove the gas from the atmosphere, but until that time it is important that we spend time considering the best places and set ups for these technologies.

What is biochar?

Biochar, also known as charcoal or Terra Preta, is a carbon-rich product created from the pyrolysis of biomass. Pyrolysis simply refers to the thermochemical decomposition of plant derived organic matter in a low or zero oxygen environment. During this process the biomass is converted to biochar and syngas (which can be captured and used to make heat and power). The lack of oxygen prevents combustion and the hotter the temperature at which the reaction takes place, the quicker pyrolysis takes pace.

The biochar produced varies depending on both the biomass source used and the temperature that pyrolysis takes place. At temperatures of approx 400°C, it will produce more char (the solid part of biochar) while pyrolysis at higher temperatures yields more syngas and a more porous and absorptive biochar, which has greater potential to adsorb toxic substances.

The two main methods of pyrolysis are “fast” pyrolysis and “slow” pyrolysis. Fast pyrolysis yields 60% bio-oil, 20% biochar, and 20% syngas, and can be done in seconds, whereas slow pyrolysis can be optimised to produce substantially more char (~50%), but takes on the order of hours to complete.

Why is biochar such a big deal?

The biochar process makes impressive reading when you see how much and how quickly carbon dioxide is actually released into the atmosphere by plants, animals and humans. Carbon dioxide released by human activities makes up approximately 70% of greenhouse gas emissions. Aside from lowering individual energy use and producing the electricity from greener or renewable sources as mentioned elsewhere on the site, in an effort to actually decrease the amount of Carbon Dioxide in the atmosphere we need to ‘trap’ it. See also Carbon Capture & Storage.

In Photosynthesis, plants and trees use carbon dioxide combined with water and sunlight to produce sugars, thereby locking the carbon within their very matter. Until a plant or tree dies, it will continue to use and store the carbon dioxide, using it as fuel for growth. Upon death, the plant or tree will start to decompose in the air as microbes and fungi begin to break it down. This process releases the carbon dioxide back into the atmosphere within one or two years so it only acts as a carbon sink for a relatively short time.

During pyrolysis, 50% of the carbon locked in the biomass is converted into biochar and the rest is converted to syngas which can be captured and used to produce heat and power. The biochar produced is chemically and biologically more stable than the original carbon form it comes from, and can remain stable in soil for hundreds to thousands of years. Therefore biochar has the potential to play an important role as a long term carbon sink, sequestering the carbon from the atmosphere and partially offsetting greenhouse gas emissions produced by burning fossil fuels.

Biochar use in agriculture

The addition of biochar to agricultural soils can improve crop yield, aiding in retaining nutrients and water, decreasing soil acidity and decreasing the release of non- carbon dioxide green house gases such as methane and nitrous oxide. As mentioned previously, the physical properties of biochar are dependant on the temperature at which it is produced, therefore it can be designed with specific qualities to target the distinct properties of soils. For example, if a more porous biochar is produced and applied to the soil, it will adsorb and lock potential contaminants within its structure; therefore the crops grown will be more containment free.

Is biochar economically viable?

The economic viability of biochar is dependant on a few factors; firstly the cost of the feedstock. If feedstock is available close to the pyrolysis equipment then this makes the process more attractive as the associated transport costs are minimal. In addition, if the feedstock would otherwise require a waste disposal fee, then cost of production of biochar would be further reduced. Another key driver of the economic viability of biochar is related to the returns achievable through the sale of the syngas (resulting from the Pyrolysis process) to energy producers. Biochar can also be used in agriculture, reducing the requirement of fertilizer and also increasing yield. Futhermore the biochar producer or user may benefit from some sort of carbon credit under an emissions trading scheme.

The future of biochar

Biochar, or charcoal, has been produced for hundreds of years and is made regularly on a small scale allowing subsistence farmers to produce small quantities of biochar for their farms of gardens. However, industrial scale production is still in its infancy, with research currently ongoing within the scientific and technological communities focusing on the most effective method of producing it on a large scale.

Biochar offers a very viable solution of carbon sequestration, however there is a long way to go in terms of research and building enough pyrolysis plants until this technology will make a large contribution to mitigating climate change. The UK biochar Research Centre, with its headquarters at the University of Edinburgh, is making good progress in developing this research in this areas and promoting the benefits of biochar.

Benefits

• Biochar is very stable carbon sink, and therefore can become a viable carbon sequestration method.
• As part of the pyrolysis process, syngas is produced as a waste product that can be used to produce power.
• The biochar can be produced to target specific deficiencies in soils to aid agriculture.
• Biochar is easy to produce on a small scale.

Limitations

• Producing biochar on an industrial scale is still at the research level, with the best techniques yet to be established.
• Biochar requires a source of feedstock, which unless harvest locally can be expensive to transport.
• The systems used to create biochar are based on the feedstock type – it is ambitious to expect a ‘One size fits all’ standard system.

What is CCS algal synthesis?

Bio CCS algal synthesis is a new process in which the carbon cycle that normally takes millions of years can be reproduced on an Algal synthesiser in 24 hours. It is the latest in a long line of potential Carbon Capture and Storage technologies, although it is more focused on the capture of carbon dioxide and turning it into a useful product rather than simply CO₂ storage.

MBD Energy Limited and an algal research team from James Cook University have developed a 5000m² test facility designed to produce 14,000 litres of oil and 25,000kg of algal meal from every 100 tonnes of CO₂ consumed.

How does this form of carbon capture work?

The process of algal synthesis involves injecting captured flue gases (the exhaust gased from burning fossil fuels) into a waste water growth medium infused with locally selected strains of micro algae, contained in an enclosed membrane system. The algae photosynthesise with sunlight, using the CO₂ as fuel for growth, doubling its mass every 24 hours causing the waste water to thicken as the growth takes place. The algae is harvested daily and crushed to produce algae oil, algae meal and clean water (35% oil and 65% algae meal). In this manner, the algae capture CO₂ that would otherwise find its way directly into the atmosphere, thereby offsetting one of the major greenhouse gases.

What can we use the resulting by-products for?

Biofuels – The algae oil produced as a by-product of carbon CCS method is ideally suited to biofuel production. In addition, the glycerine, which is a secondary by-product of this biofuel production process can be used in such areas as pharmaceuticals, cosmetics and food production. The algae oil also can be used to make plastics.

Feed for livestock – The algae meal that results from CCS algal synthesiser can be used as feed for livestock, The algae meal has an advantage over conventional livestock feed in that there is a lot less cellulose in it, this is because algae is supported in water, where as plants need to support themselves in air so need the cellulose for rigidity. The breakdown of cellulose by the ruminant digestive system causes the release of methane, so low cellulose feed has the benefit of not producing as much methane as a by-product of this reaction.

Biomass – the algae meal can also be used as biomass for fertilizer, bio plastic production and energy production.

MBD Ltd are currently in the process of moving from the test facility to full scale display plants, at a number of Australia’s major coal burning power stations. These ‘proof of concept’ projects that commenced last year take the greenhouse gases from the power stations flue chimneys and produce the oil and the algae meal out of this process.

Benefits

• Compelling sustainable solution to 3 significant world issues: Oil, food and CO2.
• Bio CCS removes CO2 from the air, therefore mitigating one of the gases that is thought to be responsible to Global Warming.
• The Carbon Cycle process, that normally takes millions of years, can take 24 hours.

Limitations

• Commercially harvesting the technological process is still in its infancy – more trials are needed to prove the technology has a worthwhile payback period.

What is photosynthesis?

Photosynthesis is the process by which green plants use sunlight to synthesise foods from carbon dioxide and water. The process combines 6 molecules of carbon dioxide and 6 molecules of water to produce one molecule of glucose and 6 oxygen molecules. The glucose is stored in the plant as starch and cellulose which are simply long chain glucose molecules (known as polysaccharides) as a source of food for the plant to survive and grow. The oxygen that is produced as a byproduct of photosynthesis is what most animals rely on to breath, so the process plants and trees fulfil on our behalf is critical to our survival.

We already can harness light energy from the sun to produce electricity via solar photovoltaic cells, however there is a fundamental issue with electricity that we are currently facing and that is that we have no suitable way of storing the electricity produced (batteries are limited), so we have to use the electricity as it is produced otherwise we essentially lose it.

The beauty of photosynthesis is that it locks energy from the sun within the chemical bonds in the glucose molecule. Therefore plants are not only producing energy, but they also have the ability to store it.

An introduction to renewables

Read more

Artificial photosynthesis

If we could somehow artificially replicate the photosynthetic process completed by plants, we would be able to lower carbon dioxide concentrations in the atmosphere, while also producing sugar that we could use for food and energy production. The ultimate goal though is to take the natural process of photosynthesis and improve it, making it more efficient, absorbing more light, at a wider range of wavelengths, potentially even in the dark to produce more energy.

There are three major scientific challenges in artificial photosynthesis that we need to find answers to before we can create fuel directly from sunlight on an economical scale:

1. Light capture and moving the electrons to the reaction centres

2. Splitting water into Hydrogen and Oxygen

3. Reducing Carbon Dioxide

Overcoming the challenges in artificial photosynthesis

In plants, light capture is handled by leaves which contain the green pigment chlorophyll. This pigment (along with accessory chlorophyll pigments) absorbs all photons with a wavelength of ~430-700nm, and through a complex process first splits water into its constituent parts, then combines hydrogen with carbon dioxide to make the sugars. In artificial photosynthesis we are proposing to use nanoparticles to not only replicate this process but improve its efficiency.

If we use light capturing titanium dioxide nanoparticles on any surface it dramatically increases the surface area and therefore the light capturing potential of the surface. If this titanium dioxide is coupled with a dye and then immersed in an electrolyte solution with a platinum cathode, electrons are excited to the extent they are displaced and produce a current.

This current can then be used to split the water into its molecular components, thereby storing the solar energy in chemical bonds, particularly in the reduced form of hydrogen, again in the presence on nanoparticles, more specifically iridium oxide nanoparticles.

In the final part of natural photosynthesis (known to biologists as the Dark Reactions), carbon dioxide is captured by the chemical ribulose biphosphate, before undergoing the Calvin Cycle, eventually producing one molecule of glucose.

Scientists are currently trying to establish the most efficient form of naturally occurring ribulose biphosphate, with a view to making a wholly artificial nanotechnology-based version that is more efficient than it’s naturally occurring relative.

Artificial photosynthesis research today

There are many barriers to actually recreating the natural process of photosynthesis that we need to overcome, so we are still very much in the early research and development phase on making artificial photosynthesis as a viable energy source.

Research is  going on all over the world currently trying to crack what could one day become the renewable fuel of the future, not only using the sun’s energy more effectively than Solar PV currently does (and leaves themselves), but also helping to remove some of the carbon dioxide that humans have added to the atmosphere through burning fossil fuels.

Thomas Fuance, a professor at the Australian National University, feels that artificial photosynthesis could one day be the game changer, providing cheap fuel for everyone in the world, regardless of location, and is pushing for a worldwide collaborative approach to research this area, similar to ITER, or the Human Genome project, sharing data and thereby concentrating on the most promising emerging technological solutions.

In August 2011 he coordinated the first international conference dedicated to the created of  Global Artificial Photosynthesis Project at Lord Howe island.

Other approaches being used to mimic photosynthesis

Nanotechnology is not the only avenue we are using trying to mimic photosynthesis. Another method involves using giant parabolic mirrors direct and concentrate sunlight onto two chambers separated by a ring of cerium oxide. The energy from the sun heats this cerium oxide up to 1500⁰c, which in turn releases an oxygen atom into one of the chambers and is pumped away. The deoxidised cerium is then moved into the other chamber, where carbon dioxide is pumped in, and the deoxidised cerium steals one of the oxygen molecules, creating carbon monoxide, and the more stable cerium oxide, which can be reused in the reaction. A similar reaction is used to separate water into its native elements, a hydrogen molecule and oxygen.

Finally, a process first carried out in the 1920s, named after its inventors Franz Fischer and Hans Tropsch, can be completed. The Fischer-Tropsch process involves reacting the hydrogen molecule with the carbon monoxide using a transition metal catalyst such as cobalt to produce a hydrocarbon that can then be used as a fuel.

Research experiments for the process outlined above have been created in the lab, however these reactions are currently unsustainable.

Artificial photosynthesis conclusion

As mentioned elsewhere on TheGreenAge, photovoltaic panels and solar heat collectors are two of our most popular mechanisms for taking advantage of solar energy, however these are currently both inefficient. As humans, we have really struggled to replicate nature’s photosynthetic process, where a plant transfers simple molecules into others with richer energy content, which is probably the most effective way to storing solar energy. Therefore, if either of the techniques above can be mastered, then we are going some way to replicate one of natures best kept secrets.

Benefits

• We are going someway to mimic natures most effective process for creating energy rich products from simple input materials.
• Producing a new fuel that can power vehicles from naturally occurring input materials, CO2, water and Sunlight.
• It makes Carbon storage more economically viable as the CO2 can be used to create a saleable product.
• If we are able to tap into existing big producers of CO2, such as power station exhaust we are able to use the CO2 twice before it enters the atmosphere.

Limitations

• So far the reactions are inefficient / unsustainable.
• The reaction needs the heat generated from the sunlight to power it, so is not suitable for operation worldwide.