Biochar and carbon sequestration. Is it really a solution to mitigate climate change?

For all of us it is no novelty to talk about climate change and global warming, well, I do not think that at this moment there is someone who has not heard about it, maybe some of us worry more than others, but I think that We are all aware of its effects and how we got to this point, that is, we are clear that in the name of the development that we enjoy so much we have released into the atmosphere a great demon that is wreaking havoc on the planet, CO₂.

Image source: pxhere.com, CC0

And although it seems that the discussion on the subject has focused more on who has greater responsibility, climate change is a fact that has altered our ecosystem, and the truth is that we have all done our part, we have become highly dependent on the processes that require energy derived from the burning of fuels, and there are so many interests involved, that many countries, starting with the more developed ones, refuse to face the issue. It is already a trite issue to remember how we got to this point, the truth is that we arrived, and the solutions should not take long to analyze, because this demon is getting stronger.

But just as in the movies, where a hero emerges with the firm intention of fighting the demon and sending it back to the underworld, where it is finally contained, a group of researchers use their knowledge in engineering to solve the problem, giving us the proposal to use biochar as the technology capable of doing so and mitigate climate change.

about a work on which I start to investigate (pyrolysis of biomass), we must be sure that we are not using the power of engineering with a simplistic view of the problem, but, as in the Marvel films, as scientists we run the risk of being the group of anti-heroes who do the wrong thing for the right reasons. Let's see then what I want to talk to you about.

Let's start by defining what is carbon sequestration

Just as the demon that is returned to the underworld to do no more damage, they will have already discovered that the purpose is to capture the CO₂ and place it underground. According to the FAO soil portal:

"Carbon dioxide levels can be reduced by reducing emissions or removing carbon dioxide from the atmosphere to store them in land reserves and aquatic ecosystems"

Apparently we want to try the second, as it is very difficult for us to do the first thing. I suppose that while other sources of energy are still not very accessible, we will remain married to the use of fossil fuels.

But going back to the subject, carbon sequestration is nothing more than capturing the carbon dioxide produced by anthropogenic activities and confining it safely under the ground or underwater.

Several ways of capturing CO₂ have been considered, it can be captured directly from the air, retained in the own source of emission of combustion gases or using biochar. In the former, it is necessary to develop filters that retain it from the gas flow that passes through them, which triggers the subsequent problem of giving adequate disposal to the filters saturated with CO₂, so perhaps the biochar has gained interest in the last years, since its disposition is made directly on the ground.

Biochar

This is produced by the thermochemical conversion of biomass in an atmosphere with little or no oxygen supply, a process known as pyrolysis, at temperatures not higher than 700 ºC. This is the main difference with regard to the production of common charcoal or activated charcoal, the other difference is that it has the prefix "bio" I do not know if it is because its most widespread use is as a soil improver or simply to make it look more natural In the following figure I show you the the pyrolysis of biomass with which we are working.

Laboratory scale biomass pyrolysis process. Source: @emiliomoron

I know you're still wondering, and how does this carbon sequestration work?

Well, as we know plants absorb CO₂ from the air as they grow, and release it again when they die, as a result of the decomposition of plant material or biomass. So, imagine that we can block this last part by burning dead or cut plants in a controlled way to produce the so-called biochar, so that dead biomass does not produce CO₂ but it is retained or "sequestered" in the coal. But there is also another situation, agriculture influences climate change as well as being affected by it, it is responsible for more 10% of the greenhouse gases produced by deforestation, desertification and changes in the surface of the earth.

Annual greenhouse gas emissions by sector, in 2010. "AFOLU" stands for "agriculture, forestry, and other land use". image source: wikipedia.com, CC0

And when biochar is added to soils, they release less methane and nitrous dioxide and make them more fertile.

Another benefit is that the pyrolysis of the biomass generated from agricultural activity is a way to obtain energy, because not only biochar is produced, but also vegetable oil and synthesis gas. So, CO₂ is removed from the atmosphere by photosynthesis to produce biomass, a part of that biomass produced as agricultural and forestry waste is converted by means of pyrolysis into biochar, biofuel and synthesis gas. Biofuel and gas are subsequently used to produce energy and heat, replacing the burning of fossil fuels and compensating for the CO₂ that these would provide; and the biocarbon would store carbon for a considerably longer time than would occur if only the decomposition of the biomass were allowed to follow its natural course.

Characteristics of biochar

Biochar, like any carbon, is a black amorphous, porous-looking solid. A preliminary elemental analysis that we have made of the material, starting from a mixture of waste from urban waste, composed of orange, potato and banana waste, has indicated that it is a compound rich in carbon but low in nutrients. But there is still a lot to review, since its composition varies according to the raw material of origin, in another publication it may only address the issue of biomass pyrolysis and its characterization.

biocarbon obtained by biomass pyrolysis.

Source: @emiliomoron

But then, is biochar a solution to mitigate climate change?

The main proponents of biochar claim that their production has a negative carbon balance, that is, the carbon emitted during the pyrolysis process (which consumes energy) is compensated by the carbon absorbed by the biomass during its growth. And not only that, they also affirm that in addition to retaining carbon, it improves the quality of the soils, making them more fertile, thus reducing the need for nitrogen fertilizers and with it the emissions of N₂O, another greenhouse gas. As an added effect, it is said that it purifies the water by avoiding its contamination with agrochemicals that are absorbed by it.

So in many places this technique is being publicized as a great solution, but the data seems to derive mainly from the observations of "Terra Preta" (or black land in Portuguese) an exceptionally fertile land achieved by the indigenous people of the central Amazon. A few thousand years, mixing charcoal with the soil of the tropical forest, usually lacking in nutrients. Although today there are only a few traces of the civilization that developed these soils for agriculture, they retain much of the original charcoal rich in carbon, which made them more fertile than the rest of the soil that surrounds them. So the idea arose that this technique could be used to increase the fertility of soils and at the same time retain carbon in the process.

Black land. Image source: pixabay.com, CC0

But can you reproduce your success? Is there enough evidence to recommend its use on a large scale? and what social impacts could it generate?

The evidence in Terra Preta seems circumstantial, the concrete technique used by the first farmers of these lands were lost along with their civilization, and what is known is that they mixed the earth with a variety of biomass that they later used to make charcoal , which can favor the creation of humus, which is very beneficial for the soil; but this is contrary to the proposal to use biochar, which by its method of production, involves mixing a portion of soil with a carbon that is biologically dead, in a study consulted suggests that biochar can retain nitrogen and phosphorus and free it from gradually, unlike inorganic fertilizers, which improved the growth of maize plants (height) but no significant differences were observed in the yield of the cobs, other articles make reference that even up to 100 years are required to achieve the interactions between microorganisms and charcoal such as those found in Terra Preta soil, or that may have negative long-term effects, especially due to the lack of information regarding the presence of contaminants such as aromatic hydrocarbons that may be present in certain types of carbon.

On the other hand, we must take into account the amount of biomass that would be needed to meet the demand for energy derived from fossil fuels, because although the proposal is to use waste materials, it is important that for its selection it does not compete with another use that can be given to these, or that does not compete with the production of food by the use of land to generate biomass.

Another important aspect is that its success in retaining carbon for a considerably long period distract us from the task of reducing greenhouse gas emissions, as there will be people who argued that this method created a kind of balance with current emissions, delaying thus any initiative to truly reduce emissions, because we must bear in mind that at any future time all this carbon will return to the atmosphere.

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Well friends, thanks for taking the time to read my post, I think this and other carbon sequestration technologies will only help us when we stop CO₂ emissions, but let me know what you think with your comments. Until next time!

References

If you wish, you can deepen the subject by consulting the following sources:

FAO. ¿Qué es el secuestro de carbono?
Wikipedia. Climate change and agriculture
Muerza, A. (2009). Biocarbón, ¿la solución para el cambio climático?
Shamim MIA (2015). Production of Biochar for Soil Application: A Comparative Study of Three Kiln Models
Lee, Yongwoon (2013). Production and characterization of biochar from various biomass materials by slow pyrolysis
Wikipedia. Terra preta