Biochar
Biochar is a charcoal-looking, carbon-rich substance that is produced when organic materials are subjected to a process called pyrolysis. The carbon can then be safely stored for hundreds to thousands of years in this form and delivers more ecosystem benefits when applied to soil.
Introduction
Biochar is a powerful environmental and climate ally due to its permanence and the environmental benefits it delivers. This scalable solution is both cost-efficient and effective at storing carbon for hundreds to thousands of years when added to soils or buried at depth.
Biochar is a great example of human engineering and nature working together to store carbon. It is a stable solution that can be circular. While accessibility of biomass is a limiting factor, current agricultural systems produce many excess products, making biochar highly scalable.
The Steps
The three general steps in biochar production and storage are outlined below. Biochar is a prime example of a hybrid carbon removal method as both natural and engineered processes are involved.
Biomass
CO₂ is trapped by plants and converted to carbon-rich biomass in a process called photosynthesis.
Pyrolysis
Biomass is heated to between 300 and 800°C in full or partial absence of oxygen to produce the biochar and other by-products, some of which are suitable as input for renewable energy generation.
Use
Biochar is primarily used as a soil amendment for the improvement of agricultural and forest soils, but it can also be used in the construction industry.
How it works
Carbon Sequestration
The sequestration step in carbon dioxide removal via biochar production is facilitated by photosynthesis. Plants, such as agricultural crops and trees in forests, take up water from the soil and absorb CO₂ from the atmosphere.
Using energy from the sun, a green pigment called chlorophyll converts the water and CO₂ to oxygen and glucose. The oxygen is released into the atmosphere and the glucose goes on to nourish the growing plant and create biomass. Large quantities of waste biomass is produced in agriculture, forestry, food processing and manufacturing, and in municipalities, and this biomass is suitable for biochar manufacturing.
Pyrolysis
In the absence of oxygen, the pre-treated waste biomass that otherwise would have rotted and reemitted CO₂ to the atmosphere, is subjected to thermal decomposition through high pressure and temperature in a reactor. This is pyrolysis.
This process generates three products: synthetic gas, bio-oil, and biochar. The biochar is separated from the other by-products in a cyclone, while the other products can be used for energy recovery.
Among the different types of pyrolysis, slow pyrolysis yields the highest proportion of biochar due to the lower temperatures (300-500°C), slower heating rates, and longer residence time of the biomass in the reactor.
Carbon Storage & Use
The biochar produced from pyrolysis is generally very stable and contains >65% carbon. This biochar could just sit in storage anywhere for centuries but there are opportunities to optimise its use.
For example, it can be used as a soil amendment in agriculture and forestry, where it confers many co-benefits while safely and permanently storing most of the biochar-carbon. Biochar can alternatively be incorporated into long-lasting building materials, such as plaster, concrete, bricks, and insulation.
An accessible, scalable, circular solution
Stabilizing carbon in biochar is a perfect example of how human engineering and nature can work together. If we can produce and protect this biochar-carbon at scale and in a responsible manner, significant positive climate, social, and economic impacts are possible.
The technology and knowledge are in place to make this work, but significant financing is required to achieve the level of production necessary to realize these impacts.
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The majority of biochar’s climate change mitigation potential can be attributed to its slower composition compared to the raw material from which it is produced.
Biochar could remove and securely store CO₂ on the gigatonne scale in the near future (estimates from leading researchers are between ~2-3.7 Gt CO₂ e per year), but only if barriers, such as financing, can be overcome. Aside from this, biochar has a positive climate impact via some secondary mechanisms.
N20 and CH4 emissions are reduced, plant growth is promoted, leading to a positive feedback of increased CO₂ removal from the atmosphere, while reducing emissions from fertilizer use.
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When used as a soil amendment, biochar can improve fertilizer use efficiency and soil fertility by reducing nutrient leaching and mitigating gaseous nitrogen losses.
Other benefits include enhanced rates of pesticide and chemical degradation, and improved soil structure and water holding capacity. Biochar can also contribute to the greening and improvement of the construction industry. It can increase the strength of cement and it is an excellent insulating material for buildings as it maintains stable humidity levels.
Lastly, one of biochar’s greatest attributes is that it can be produced anywhere with the right investment, and so, could provide a meaningful form of income for many rural communities around the world.
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