Biochar benefits composting in 10+ ways
What is biochar?
Pyrolysis process summarised in the burning of a match metaphor1
Biochar is made during a process called pyrolysis where the volatiles are combusted and the remaining material left behind is high in carbon. Biochar can be made from any biomass and it’s preferable if this is a waste of some sort. The resulting porous matrix of solid carbon2 is stable and durable at time scales exceeding hundreds to thousands of years3. It has multiple benefits as a soil amendment and so is fundamentally a permanent improvement in human scale timeframes.
Pyrolysis Methods:
This can be as simple as a pit or trench in the ground to sophisticated and often automated machines.
This kind of pit can be dug with a spade in a location near the biomass4
In between there are various form factors of kilns and retorts. The kontiki kiln is well documented and makes around 1000 litres in an afternoon5.
When the top layer of biomass begins to grey with ash, it is the time to add the next fuel layer. This blocks the air to the lower layers and allows them to char completely6
Kontiki kiln at lighting (done on top to minimise smoke) on left and burning with shield added for safety and efficiency7 on the right.
The Ring of Fire kiln is another option:
This kiln is made from sheets of metal bolted together (fitting into a station wagon) and is large enough (diameter of 2.3m) to be loaded by a machine.
Drawing showing the pre-heated air that enters in such a way to ensure a clean burn8
Or a mobile retort for a batch process - fill the vessels with biomass as well as around in the outside chamber, light, and then wait until it completes (flames finish):
Portability and high quality clean biochar results from a unit such as this9
Residence time and highest treatment temperature (HTT) are critically important characteristics of pyrolysis. In general, HTT of 450° C is the minimum required to achieve the formation of linked ring structures (aromaticity) at the molecular level, making it recalcitrant, and very long lived in the soil. HTT of 600° C is a good benchmark for greatest potential aromaticity and porosity. Temperatures in excess of 750° C start to produce diminishing returns, as the graphene structures begin to fuse into plates and collapse the internal surface area10.
Feedstock form factor is another consideration that will be at least partially dictated by the type of pyrolysis employed. Most continuous process equipment requires incoming material to be chipped or shredded to achieve an optimum average particle size and density factor. Batch retorts can be more tolerant of greater length and diameter pieces, but will need longer HTT treatment time at highest temperature.
Feedstock preparation
For general biochar creation, the most favourable feedstocks are those woody materials that are high in lignin and can include things like forestry slash and debris, sawdust, bark, offcuts and residuals from timber milling, prunings and cullings from horticulture and viticulture, and woody branches and brushy material in the municipal solid waste stream (green organics). These produce a biochar with high surface area and high carbon levels and these attributes make it highly prized for soil amendment, environmental management, and water quality11.
Non-woody components of waste streams are best left for input into the composting process as their resultant biochar properties are lower in stable carbon and higher in mineral (ash) content12.
Quality Control
Cross contamination of feedstock has its risks. Treated lumber and painted wood contain heavy metals such as cadmium and lead and these pose the highest risk to human health and to environment harm. Burning these can lead to the offgassing and escape to the atmosphere of volatile compounds including arsenic, and the resulting biochar could have unacceptably high levels of toxic products bound up in its structure. Biomass contamination with chemical compounds containing chlorine or fluorine can also lead to the formation of toxicants. It is interesting to note that small amounts of non-chlorinated hydrocarbons such as oil or plastics at levels less than 2% by dry weight do not pose a hazard, as the high heat of pyrolysis causes complete thermal decomposition of the polymers13.
For commercial biochar production regular sampling and testing to ascertain the quality of biochar being produced is important. These tests verify that it is within acceptable limits for substances of concern. The International Biochar Initiative publishes assessment categories and protocols for biochar producers14.
10 Beneficial attributes of biochar for composting
Biochar is an ideal input into composting systems because of many benefits to the system itself. An outline of a number of positive benefits that enhance the efficiency and product yield, as well as mitigating some of the negative environmental effects that arise in commercial composting operations follows shortly. The addition of biochar at rates from 2%-10% w-w can lead to improvements in the following areas:
1. Water retention Biochar’s sponge-like structure can hold considerable quantities of water. Typical ranges for softwood biochar range from 3-4 times by weight to achieve saturation. This characteristic not only helps to maintain moisture levels in the compost, but can also help buffer excess water infiltration during rainfall events and prevent leaching and runoff and is also a moderating agent helping maintain optimum moisture levels for microbial activity15.
2. Aeration Biochar does not decay under microbial activity and the porosity and high internal surface area (as much as one hectare per 25 g biochar) provides air exchange at the microscale16. This quality helps maintain better aerobic conditions during the composting process, potentially reducing the number of passes required to aerate windrows.
3. Nutrient adsorption The cation exchange capacity of biochar means that nutrient ions in solution are attracted and held by it. This means losing substantially less gaseous nitrogen during composting, thus improving the C:N ratio of the finished product17.
4. pH regulation Biochar is typically alkaline (often around 9) and so its addition reduces the acidity in composting processes. The presence of calcium, potassium and other metallic elements makes biochar an effective liming agent18.
5. Microbial habitat The combination of better aeration, nutrient adsorption, water retention, and pH moderation leads to an environment that is highly beneficial to the microbiology required to create an effective composting process. The carbon matrix of biochar in compost is rapidly colonised by bacteria and fungi and this home and reservoir keep the decomposition process going better than if they were not there and for the whole compost process19.
6. Minimise leaching The nutrient holding ability and water holding functionality of biochar mean that nutrient leaching from the compost pile (or windrow) is reduced significantly20 21 . However, for times of high rainfall, the deployment of biochar in sediment traps as a filter will catch nutrients from the runoff that can then be returned to another compost batch once the sorption limit is reached22.
7. GHG emissions reduction Nitrous oxide and methane have heating potential of about 300 times and 30 times that of carbon dioxide, respectively, and under certain conditions both are formed during composting. When biochar has been added, it also favours facultative bacteria that prevent or reduce the formation of anerobic pockets within a compost pile/windrow and thus the formation of methane and nitrous oxide23.
8. Carbon credits & carbon sequestration Producers of biochar are able to claim carbon credits on the international voluntary markets because biochar is recognised as an effective means of storing carbon safely for long time scales (100+ or 1000+ years). These markets offer accreditation based on criteria that include feedstock sourcing, life cycle assessment of the production and application methods, and quality assurance testing of the biochar itself. The income from these carbon offset and removal credits improve the business case for making biochar as a compost input. The long term non-decay of carbon in biochar has also led to its designation by the IPCC as one of only a handful of viable and scalable carbon drawdown and removal (CDR) technologies. Estimates of its potential to reduce atmospheric carbon are by as much as 15% percent of annual emissions worldwide24. Every application of biochar containing compost to the soil thus benefits the climate in taking carbon out of the carbon cycle.
9. Market differentiation Biochar enhanced compost sets it apart from competitors and can be promoted on the basis of its increased benefits to soil and plant life as well as the environmental benefits accrued during the compost making process itself.
10. Air Quality Strong smelling ammonia and hydrogen sulfide are significantly reduced when biochar is an input to the composting process25. These objectionable odours and potential health hazards are often an issue for neighbouring land use and make gaining or keeping consent more difficult.
The Future?
The benefits of biochar as a co-composting addition to compost inputs are numerous and compelling. Imagine a future when a pyrolysis kiln or machine is a standard part of any commercial composting operation. Compost creation is more environmentally friendly with the carbon sequestered in the end users soil application leaving it permanently improved for future generations as well as being a positive action mitigating climate change.
COMBI (co-composting biochar) flowchart highlighting the benefits26
References
1 Match image from https://biochar-international.org/about-biochar/how-to-make-biochar/biochar-production-technologies
2 International Biochar Initiative Biochar Standards version 2.1, Section 1 https://biochar-international.org/biochar-standards
3 Rodionov, A., W. Amelung, N. Peinemann, L. Haumaier, X. Zhang, M. Kleber, B. Glaser, I. Urusevskaya, and W. Zech (2010), Black carbon in grassland ecosystems of the world, Global Biogeochem. Cycles, 24, GB3013, https://doi.org/10.1029/2009GB003669
4 https://www.reddit.com/r/Permaculture/comments/ewnxg7/biochar_batch_via_trench_pit_method_ready_to
5 Schmidt, Hans-Peter & Taylor, Paul. (2014). Kon-Tiki flame cap pyrolysis for the democratization of biochar production. Ithaka Journal for biochar materials, ecosystems & agriculture. 1. 338-348.
6 https://www.ithaka-journal.net/en/ct/151-kon-tiki-the-democratization-of-biochar-production
7 https://www.thebiocharrevolution.com/biochar-production-in-kon-tiki-australia
9 https://biocharkilns.com/biochar
10 Lehmann, J., Joseph, S. (2015), Biochar for Environmental Management 2nd Edition, ISBN: 978-0-415-70415-1 p 111-137
11 Lehmann, J., Joseph, S. (2015), Biochar for Environmental Management 2nd Edition, ISBN: 978-0-415-70415-1, p 719
12 Ibid p 719
13 IBI Biochar Standards version 2.1, Section 3.1 (https://biochar-international.org/biochar-standards)
14 IBI Biochar Standards version 2.1, Section 4
15 Sanchez-Monedero, M.A., Cayuela, M.L., Roig, A., Jindo, K., Mondini, C., Bolan, N., Role of biochar as an additive in organic waste composting, Bioresource Technology (2017), https://doi.org/10.1016/j.biortech.2017.09.193
16 Ibid
17 Ibid
18 Lehmann and Joseph, p 148-150
19 Sanchez-Monedero et al
20 Kammann, C. I.; Schmidt, H-P.; Messerschmidt, N.; Linsel, S.; Steffens, D.; Müller, C.; Koyro, H.-W.; Conte, P.; Joseph, S., Plant growth improvement mediated by nitrate capture in co-composted biochar. Scientific. Reports, 2015. https://doi.org/10.1038/srep11080
21 El Hanandeh, A., Bhuvaneswaran, A., de Rozari, P. (2017), Removal of nitrate, ammonia and phosphate from aqueous solutions in packed bed filter using biochar augmented sand media, MATEC Web Conf. Volume 120, 2017 International Conference on Advances in Sustainable Construction Materials & Civil Engineering Systems (ASCMCES-17), https://doi.org/10.1051/matecconf/201712005004
22 Kuoppamäki, K., Hagner, M., Valtanen, M., & Setälä, H. (2019). Using biochar to purify runoff in road verges of urbanised watersheds: A large-scale field lysimeter study. Watershed Ecology and the Environment, 1, 15–25. https://doi.org/10.1016/j.wsee.2019.05.001
23 Yin, Y., Yang, C., Li, M., Zheng, Y., Ge, C., Gu, J., Li, H., Duan, M., Wang, X., & Chen, R. (2021). Research progress and prospects for using biochar to mitigate greenhouse gas emissions during composting: A review. Science of The Total Environment, 798, 149294. https://doi.org/10.1016/j.scitotenv.2021.149294
24 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories Appendix 4: Method for Estimating the Change in Mineral Soil Organic Carbon Stocks from Biochar Amendments: Basis for Future Methodological Development https://www.ipcc-nggip.iges.or.jp/public/2019rf/pdf/4_Volume4/19R_V4_Ch02_Ap4_Biochar.pdf
25 Sanchez-Monedero et al
26 Antonangelo, J. A., Sun, X., & Zhang, H. (2021). The roles of co-composted biochar (COMBI) in improving soil quality, crop productivity, and toxic metal amelioration. Journal of Environmental Management, 277, 111443. https://doi.org/10.1016/j.jenvman.2020.111443
Further Information
Here are some great supporting resources we have found. Let us know if you have further ones to add:
USBI brochure on composting with biochar (pdf)
Video featuring biochar made at a composting facility in Spain: https://youtu.be/oHbdtJxb6ZE?si=ckop5L8FBJg0OURr (4.5 mins)
Thorough and referenced blog post: https://www.compostmagazine.com/biochar-in-compost
Extremely well referenced white paper by Pacific Biochar: https://pacificbiochar.com/wp-content/uploads/Pacific-Biochar_Biochar-Compost_white-paper.pdf
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