Feedstocks

Biochar can and should be made from biomass waste materials. Making biochar from biomass waste materials should create no competition for land with any other land use option—such as food production or leaving the land in its pristine state.

Biomass waste materials appropriate for biochar production include crop residues (both field residues and processing residues such as nut shells, fruit pits, bagasse, etc), as well as yard, food and forestry wastes, and animal manures.

Large amounts of agricultural, municipal, and forestry biomass are currently burned or left to decompose and release CO₂ and methane back into the atmosphere. They also can pollute local ground and surface waters—a large issue for livestock wastes. Using these materials to make biochar not only removes them from a pollution cycle, but biochar can be obtained as a by-product of producing energy from this biomass. Feedstocks must not contain unacceptable levels of toxins such as heavy metals which can be found in sewage sludge and industrial or landfill waste.

It is important to study all the aspects of feedstock supply on a local community/business before setting up a biochar operation. Feedstock availability can vary year to year and within years. In developing country areas where feedstocks are often used as heating and/or cooking fuel, biochar production can be an important side benefit if done sustainably. The biochar can then be used on fields to potentially increase food production by improving soil fertility (See Chapter 9 Biochar Systems in Biochar for Environmental Management for a series of case studies on biochar production and sustainable feedstock models in developing countries).

Feedstock effects on Nutrient Properties of Biochar

The composition of biochar (the amount of carbon, nitrogen, potassium, calcium, etc) depends on the feedstock used and the duration and temperature of pyrolysis. As an example, biochar produced from feedstocks which have greater contents of potassium (such as animal litters) often have higher potassium contents than biochar made entirely from wood (which often have higher carbon contents). However, pyrolysis conditions greatly affect nutrient properties contents and so biochar should be tested on a batch by batch basis to determine specific properties.

Economic Considerations

The choice of feedstock will be affected by the biomass resources in the immediate area and availability. Due to collection, and transport and storage costs, it often makes the most economic sense to use local feedstocks (if they are also an environmentally sustainable option). The most basic costs and benefits to consider with feedstock choices are highlighted below:

• Feedstock production and collection: If the feedstock is a residue, such as municipal biomass waste, logging or cropping residues, or a by-product such as bagasse, then production is less an economic issue than if the feedstock is purposely grown for the production of biochar—such as switchgrass—which would include the costs and inputs needed for the growing and harvesting of the crop.  Revenue in the form of tipping fees may be obtained from certain waste feedstocks.
• Use Tradeoff: This would include the potential nutrient value lost from using feedstock for biochar production rather than as a direct fertilizer on the field. How this trade off works will vary on the area and on the feedstock. For example, chicken litter may be valuable in some areas as a direct fertilizer while in other areas it may be treated as a waste and represent a disposal cost.
• Feedstock transport: When waste biomass is found far from the place where it will be used, transportation costs can be very high. In some situations it may make sense to densify the biomass by chipping or pelletizing before transport.
• Feedstock storage and pre-processing: Many feedstocks will need to be dried before pyrolysis. Depending on the feedstock choice, the drying process could occur passively through careful storage or may need more intervention—such as using a drier (thus requiring energy and labor). Energy for drying could in some cases be obtained from the pyrolysis of previous batches of feedstock.

See Chapters in Biochar for Environmental Management: Chapter 1: Biochar for Environmental Management; Chapter 5: Nutrient Properties and their Enhancement; Chapter 9 Biochar Systems; and Chapter 19: Economics of Biochar Production, Utilization, and Greenhouse Gas Offsets.