Global biochar deployment holds the potential to reduce emissions in agriculture and other industries. New research published in October 2023 in the Biochar journal, commissioned by the International Biochar Initiative (IBI), quantifies biochar’s carbon removal potential across 155 countries and confirms once again its impact on removing CO2 while benefiting the environment. The research is called Biomass residue to carbon dioxide removal: quantifying the global impact of biochar.
We interviewed Dr. Thomas Trabold, one of the paper’s co-authors and a research professor at the Rochester Institute of Technology’s Golisano Institute for Sustainability. He talked about his work related to biochar production and shared some of the insights from the report, best practices in biochar handling that increase its carbon removal capacities, and more.
Dr Thomas, why did you decide to take part in the research the International Biochar Initiative recently published?
The reason I was very excited about joining this project is that my research here at the Rochester Institute of Technology is focused on the valorization of biomass residues.
About 10 years ago, I was concentrated on food waste, in particular, as a feedstock for value-added processes such as anaerobic digestion, fermentation, etc. More recently, I’ve transitioned toward biochar because I think the potential is much greater. And the technology is suited for a much wider array of feedstocks. This is really where I’m focusing all of my research efforts now.
Would you please summarize for our readers the main findings from the research?
The main finding is that readily available biomass residues from existing agricultural, forestry and wastewater systems, can be converted into carbon sequestering biochar, and on a global basis, offset over 6% of greenhouse gas emissions. At an individual country level in some cases, that impact can be even higher than 20% and greater than 10% in 28 countries.
How is biochar produced? What are the systems for its production and do they emit directly greenhouse gas emissions into the atmosphere?
Biochar is produced by a process called pyrolysis, which is essentially high-temperature treatment in an oxygen-reduced environment. By treating the biomass, we can effectively convert much of the starting carbon into a stable form of carbon that we call biochar.
By doing this, we can essentially lock away carbon that would otherwise be converted back into carbon dioxide by the normal natural process of biomass growing through photosynthesis, dying, falling back to earth and through the process of degrading, emitting CO2 into the atmosphere as part of the natural carbon cycle.
By using pyrolysis, we’re able to transfer some of that carbon to a stable solid form, instead of all CO2 going back to the atmosphere.
The pyrolysis process needs a small amount of electricity just to drive the mechanical systems that move the biomass into it, and then transport the biochar out of it. It also needs a little bit of thermal energy to get the process up to a temperature. In most cases, there is enough energy in the material itself to drive the whole reaction.
We typically run the system without the need to add additional energy. At our university, we do this fairly frequently and we run a large number of different types of biomass – from woody waste, food waste to animal manure and other things. In that sense, it can be more or less self-sustaining. There is always a little bit of electricity needed to run the controls, augers and air blowers and things of that nature, but overall, it’s a pretty self-sustaining system.
There are also byproducts from the process like syngas or oil, is that correct?
That is correct. We usually think of pyrolysis systems having three co-products – biochar, bio-oil, and hydrogen-rich syngas. Depending on how you run the process, you can favor one of those co-products over the other. We are mostly interested of course in biochar, so we favor the production of biochar.
We typically run a so-called slow pyrolysis process, meaning that the residence time in the pyrolysis reactor is relatively long and that favors biochar production. The fast pyrolysis system will favor the formation of bio-oil.
In your research, you state that the greenhouse gas emissions associated with biochar production are not included in the analysis. Is that because the pyrolysis process is self-sustained or is there another reason for that?
In the research, we did account for the emissions associated with the feedstock collection transport, the biochar production itself, as well as the transport of the final biochar product. We outlined in the paper, the methodology for doing that and we subtract the emissions associated with biochar production. The final numbers that we reported are actually net carbon sequestration from the biochar.
For quantifying the biomass residue for biochar production, you consider four different sources – crop residues, animal manure, forestry, water residues and wastewater biosolids. Do the net emission savings of the produced biochar differ among the different types of residue sources used to make it?
Yes, they certainly will vary depending on the nature of the starting biomass. That was all accounted for in the analysis. Different starting feedstocks produce biochar with different levels of organic carbon and different levels of biochar permanence. In fact, one of the important factors determining the quality of the final biochar is the nature of the startup feedstock.
Things like woody biomass typically produce high-quality biochar, that sequesters relatively high levels of carbon, whereas other materials like biosolids from wastewater treatment typically would not sequester as much carbon on the same mass basis. That would be considered biochar that is not as high quality. In short, the starting feedstock has a significant impact on the amount of carbon that can be sequestered.
That also means biochar made from manure, for example, would ultimately result in less net carbon removal benefits.
That is correct. I would just add to that, the vast majority of resources available, based on our analysis, are agricultural residues. Typically those materials will produce relatively high-quality biochar that can sequester significant amounts of carbon.
In the research, you also say you exclude some biochar benefits to achieve a more generally conservative approach. Would you please explain with some examples what are the benefits that are excluded?
We intentionally conducted a conservative analysis, meaning that we only accounted for the carbon sequestration benefit within the biochar itself. Additional benefits that you would expect to have from using biochar like, for example, increasing crop yields, or reducing nitrous oxide emissions from soil, are not included in our analysis.
We made reference to a number of earlier studies that had looked at the total global impact of biochar, not at the level of individual countries as we did. Their estimates were much higher, instead of 6% of global greenhouse emissions being offset, they were pointing out to as high as 12% and 15%. That is because they added those additional benefits that we expect from biochar, mostly related to the reduction of synthetic fertilizer use and nitrous oxide.
Do you account for collected biomass that has been purposely grown for biochar production or only biomass that would have been left on the field to decay?
There is no consideration of purpose-grown biomass at all in our analysis. We only account for all residues from agriculture, woody residues from forestry operations, animal livestock manure and biosolids from wastewater treatment.
Do you know what usually happens to the residues that are not collected for another use? How do farmers typically dispose of them?
In many cases, they are just left on the field or tilled back into the soil. In some cases, there is actually open field burning. For example, I have a former student from Ivory Coast where there is a huge cacao industry. You may know that cocoa beans are contained within a very thick pod, and that pod material is considered waste.
There are mounds of these cocoa pods in Ivory Coast that are just burned because there are so many of them that farmers don’t have anything to do with. In the case of other biomass feedstocks like biosolids from wastewater treatment, they are often landfilled. I know this is a common practice here in the US. Obviously, landfills have a lot of very negative environmental impacts.
We can take the material that would have been otherwise landfilled and convert it to biochar which obviously has significantly positive environmental benefits.
Is there a difference in terms of improving soil health, or other benefits, when comparing producing and applying biochar to soil and just leaving those residues to decay in soil, apart from the slower release of CO2 of course?
I think you touch on the actual benefit – if we leave those residues in the field, they will certainly contribute with carbon and other nutrients to the soil. But they will break down relatively quickly, in a year or less. Whereas if we take some of those residues and convert them to biochar, the carbon will be much more stable and will be retained in that form for hundreds of thousands of years, instead of immediately.
I would note that in the paper, we assumed that 30% of agricultural residues are retained in the field. We understand you can’t just take all the residue to make biochar you need to keep some in the field to retain soil health. Our analysis assumes that beyond 30%, that material can be converted to biochar to create a stable form of carbon emergency.
A lot of the nutrients that are in the biomass, like nitrogen, phosphorus, potassium, will also be released over time. I must say, I’m not an expert in what is the timescale. We do know that those nutrients in organics like phosphorus, potassium, calcium, and so forth, are usually contained in the biochar through the pyrolysis process. If you are starting with biomass rich in phosphorus as an example, your biochar will also be rich in phosphorus.
What is the durability of biochar in terms of years when applied to soil? We know there are studies with conflicting results, so what are the factors that influence its long-term durability?
We talked about this at length in the paper. The main metric or determinant of permanence is the biochar production temperature. Higher temperatures will get you a larger permanence factor. The permanence factor is defined by the amount of carbon retained after 100 years divided by the original amount of carbon.
We found that with biochar produced at so-called high temperatures, greater than 600 degrees centigrade, the permanence factor will vary anywhere from 0.75 to 0.95. That range of variation is due to the soil temperature. The permanence will depend upon the production temperature of the biochar, as well as the soil temperature.
Warmer soils, like in the tropical regions will give you lower permanence, whereas colder soils in the northern climates will give you a higher permanence. That was based on prior research from Woolf et al. published in 2021.
What would you say are the best practices in biochar collection, production and application to ensure larger carbon removal benefits?
I would say producing it again at a high temperature greater than 600° C. But also even beyond the production of biochar itself, other factors like collection and pretreatment of the material are very important. Then even going beyond the technical round, looking at things such as defining your markets before producing biochar is extremely important.
In the light of the fact that most of our residues are agricultural, it is very important to educate and coordinate with the farming community. Whatever country we’re talking about, educating farmers in the agricultural industry about the benefits of biochar, and coordinating that activity with other stakeholders, state government agencies, investors and so forth is really critical and should be part of the best practices.
What are the estimates of how much carbon dioxide is removed for one tonne of biochar produced and applied to soil?
Based on our analysis, in aggregate, looking at all of the global biomass resources identified, we computed a factor of 0.36 tonnes of CO2 equivalent per tonne of dry biomass feedstock. We estimated the total amount of biomass globally at 7.4 billion tonnes per year, which would result in 2.6 billion tonnes of CO2 equivalent impact.