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What is Biochar
Pyrolysis offers a sustainable and effective means of producing energy from biomass and sequestering carbon from the atmosphere in a form called biochar.
However, there is a need for additional care to ensure the pretreatment, harvesting, transport, process of application, and even the post treatment are accounted for and to not create more carbon than it is captured by transforming the biomass into biochar.
Hence, this will create a need for innovation and a coherent strategy for low-carbon pyrolysis energy generation.
Biochar is called a "carbon negative" energy generator because it is capable of sequestering more carbon than is produced.
When compared to many other bio energy technologies, the vital advantage of biochar is that the wide variety of feedstock that it can be made from.
Additionally, biochar can be produced at a large scale range from a simple cook stove to mobile farm pyrolysis. Because of the transportation costs and carbon emissions, this is highly favorable for decentralized approaches and smaller scale ones.
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Note: For more information and a backgrounder about biochar, including notes about biochar production efficiencies and the cost and availability of commercially produced biochar (used in making biogas from various biomasses), please read "Biochar vs Charcoal - A Greener Alternative for the Soil and the Planet" also by this author and then return to this article.
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There are various systems for producing or making biochar. It is a complex subject and technical, too, but it is really reasonably easy to understand at its core.
Fundamentally, in the process of pyrolysis the biomass feedstock is heated in the range of 250 - 800 degree C in an oxygen free atmosphere.
Biomass is basically heated until the various cellulose and lignin products break down to generate a rich fuel of hydrogen that can be condensed and combusted for the generation of energy, and the high carbon-rich product that remains is biochar.
Biochar contains 17.60% ash, 18.70% volatile matter, and 63.70% fixed carbon. The heating value of biochar is 25.3 MJ/kg.
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How Biomass is turned into Biochar
The first pyrolysis phase is endothermic, in which it needs a higher level of energy than it generates. The last phase of pyrolysis is exothermic, in that it generates more heat than it needs to get started. 10% of the final energy it generates is making it effective at capturing valuable carbon.
Under the molecular pressure of heat, biomass is turned into charcoal. This process is thermal decomposition, where a stable, active flow of carbon monoxide (CO), hydrogen (H), and methane (CH4) gases which are known as syngas is created. The only part that remains, again, is charred materials of the carbon exoskeleton, and it is called biochar.
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Thermal Decomposition Process
The thermal decomposition process may include:
- slow pyrolysis
- intermediate pyrolysis
- fast pyrolysis
- microwave pyrolysis
- hydrothermal carbonization
- steam gasification
With this variety of systems, there is greater range of operating scales- from small pyrolysis units and gasification stoves that are small enough to be home based to large scale plants.
Thus both the small scale and larger scale plants will have the ability to deliver power and heat to houses, communities, and countries, and this will replace the demand of oil and coal in a carbon negative process of energy production.
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Slow pyrolysis is the most efficient method to turn biomass into biochar, and it is commonly cited in literature as the most promising technology to generate biochar.
Slow pyrolysis needs low to medium temperatures that range from 350 to 700 degree C at a very long residence time which takes a day and generates three yields: biochar of 30- 50% from the actual weight of biomass, water, and syngas.
The resulting syngas and biochar properties are greatly determined by temperature, feedstock, and residence time.
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Biochar generation is basically maximized with very slow pyrolysis, but fast pyrolysis offers the advantage of the production of bio-oil in addition to the biochar generation. This is called “flash carbonization," which occurs in a matter of 0.2 to 2 seconds with the modest temperature of 400 – 600 degree C. And this yields up 60% of bio fuels, 20% of biochar, and 20% of producer gases.
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Biochar Production at Small scale level
The eagerness for small-scale or home-based biochar units is at very high level now, and many people are attempting to do this process.
Biochar is made using simple techniques at home and at small-scale levels. Residential pyrolysis plants are an appropriate technology where biomass is easily available and cheaper than fossil fuels like natural gas, electric heat, or heating oil.
And some of methods that are frequently used are the simple two-barrel biochar retort, the experimental CarbonZero biochar kiln, and the simple two-barrel biochar retort with afterburner.
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Medium and large sized biochar pyrolysis units
Many companies are making biochar from biomass and are producing commercial scale systems to process paper by-products, agricultural waste, and even community waste.
There are three basic methods for deploying this pyrolysis system. The first is a centralized system- all the biomass in the region is collected and brought for the pyrolysis plant for processing.
The second one is a medium tech pyrolysis kiln, and the third is a mobile system in which a truck equipped with the paralyzer is driven to convert the biomass.
This could be powered with a syngas system, leaving the biochar to the environment. For biochar storage and transport, is important that everyone have care about storing, transporting, or combusting for this biochar is readily inflammable.
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Laird, D.A. (2008) “The Charcoal Vision: A Win-Win-Win Scenario for Simultaneously Producing Bioenergy, Permanently Sequestering Carbon, while Improving Soil and Water Quality", 100 Agronomy J. 178.
Brown, R. C. (2003) “Biorenewable Resources: Engineering New Products from Agriculture," Blackwell Press, Ames, IA.
Tilman, David; Hill, Jason; Lehman, Clarence. 2006. “Carbon Negative Biofuels from Low-Input-High-Diversity Grassland Biomass," Science, 314, 1598-1600.