Biochar is defined as a carbon-rich material produced during the pyrolysis process that is a thermochemical decomposition of biomass with a temperature below 700 degrees Celsius and done either in the absence of oxygen or with a limited supply of oxygen. Biochar has shown promise, and been suggested for mitigating climate change and improving soil quality while reducing waste and producing energy as a byproduct. Although biochar looks like charcoal, biochar is produced using a specific process to reduce contamination and safely store carbon.
Materials used in the production of biochar are organic materials such as wood chips, leaf litter, or dead plants, and often these materials are from wood cutting and agricultural wastes. In terms of physical attributes, biochar is black, highly porous, lightweight, fine-grained, and can have a large surface area. Around 70 percent of its composition is carbon and the remaining percentage consists of nitrogen, hydrogen, and oxygen among other elements. The chemical composition varies depending on the feedstock and the methods to make it and heat it.
A common pyrolysis system which generates biochar and biofuels.
The production of biochar through pyrolysis and the percentage of biochar produced depends on the biomass and the condition of the biomass. The percentage of biochar ranges from 10 to 35 percent of the initial biomass used. At low temperatures, around 450 to 500 degrees Celsius achieves a high quantity of biochar due to the low devolatilization rates and low carbon conversion. The quantity of biochar produced is reduced to 8 to 10 percent of biochar at moderate temperatures, around 550 to 650 degrees Celsius. While the yield is lower again at temperatures above 650 degrees Celsius.
Varying size and shape of biochar products.
Pyrolysis conditions such as reactor type and shape, biomass type, feedstock particle size, chemical activation, heating rate, residence time, and other related metrics will affect the physical characteristics of biochar. Higher heating rates, shorter residence times, and finer feedstock give finer biochar whereas slow pyrolysis with larger feedstock particle size gives a biochar with coarser particle size. In general, coarser biochar is produced from wood-based biomass while finer biochar is produced from crop residues and manures.
The main components of biochar are carbon, hydrogen, and various inorganic species in one of two structures. The first being a stacked crystalline graphene sheets. The second being randomly ordered amorphous aromatic structures.
Hydrothermal carbonization refers to the reaction of biomass in an underwater stagnant system for five minutes to sixteen hours and at pressures of 2 to 6 MPa in a relatively low temperature, usually less than to around 350 degrees Celsius. Because the hydrothermal carbonization process uses water as the reaction medium under high pressure and heating conditions, it is not easy to produce harmful substances, and biochar prepared by this method is more suitable for the absorption of water pollutants. However, this method is limited by preparation methods and requires expensive reactors.
Biochar, like charcoal, is a flammable solid and requires careful handling. According to the UN Hazardous Goods classification system, biochars are Class 4.2 Spontaneously Combustible Materials. This means they are capable of self-heating and even igniting when exposed to air. This classification stems from the testing of freshly pyrolyzed materials that have not yet surface oxidized. When such biochars are first exposed to oxygen in the air, the relatively fast surface oxidizations that occur release small amounts of heat that catalyze further oxidations and can cause ignition.
Biochar has been used in various processes, for example, it has been used in industrial processes as solid fuel in boilers, in the production of activated carbon, in the creation of nanotubes, and in the production of hydrogen rich gas. Biochar has been used for composting, as a raw wood charcoal for barbecuing, and fine particle biochar has been used to replaced the carbon used in, among other things, the manufacture of tires, rubber, and plastic components.
Biochar has been used for pollutant and waste water treatment, especially in wastewater that contains large amounts of organic matter, heavy metals, nitrogen, phosphorous, and other pollutants that can cause harm to the environment. And biochar has been promoted for being used to prepare soil for agriculture and for reforestation purposes, and to rehabilitate mining sites by retaining heavy metals.
One of the best understood and most used applications for biochar has been its use in wastewater treatment and pollutant treatment. This is in part because biochar, due to its structure, has significant absorption effects for organic pollutants such as antibiotics, phenols, and herbicides, which has further lead it to interest in using biochar for organic contaminants in livestock wastewater. This is in part because biocarbon can absorb antibiotic substances in water phase and its absorption mainly through electron donor/reception, hydrogen bonding, and cationic bridge.
For the absorption of heavy metals, which are toxic and cannot be biodegraded, biochar has been found to be useful, as it depends on the ion exchange on the surface of biochar, the chemical cross-linking between heavy metal ions, and its surface functional groups and the surface deposition between the ashes. Studies have found different absorption efficiencies dependent on the type of biochar, the processing of biochar, and the type of heavy metal.
For soil, especially in agriculture use cases, biochar has been suggested as a possible solution, especially in the case of degraded soils in order to enhance the overall soil quality. Some of the ways that biochar may help improve soil quality include:
- Enhancing soil structure
- Increasing water retention and aggregation
- Decreasing acidity
- Reducing nitrous oxide emissions
- Improving porosity
- Regulating nitrogen leaching
- Improving electrical conductivity
- Improving microbial properties
Some point to the use of compost, mulch, and smoldered plant matter to enrich soil in the Amazonian rainforests of Brazil as to the possible beginning of the idea of enriching soil with charcoal or biochar in the modern context, and that the modern day use of biochar for the purpose harkens back to these older practices. However, scientists disagree whether the soils in the Amazonian rainforests of Brazil were created on purpose, or if they were a refuse spot and an accidental byproduct of day-to-day village life.
There has been an increasing body of research on the role biochar can play in helping carbon-based practices go further and stabilizing the native carbon in the soil. This body of research has found that biochar has been shown to store roughly 50 percent of carbon in the biomass, which can last as a stable carbon source in the soil. A paper published in 2014 found that charcoal reduced carbon mineralization and off-gassing by between 64.9 to 68.8 percent, a phenomenon named the negative priming effect. This meant the 64.9 to 68.8 percent of native soil carbon, or the carbon already in the soil, was stabilized and kept in the soil. A meta-analysis paper confirmed these results, finding an average of 40 percent decrease in carbon off-gassing, known as respiration, while seeing an average of 18 percent microbial biomass.
However, despite the studies showing the potential for biochar to stabilize soil, it is otherwise difficult to predict biochar's influence on the soil. Different biochar made from different feedstock ends up with different physical and nutrient properties with different interactions in different soil types. As previous studies have found, the temperature and pH of the biomass has significant influences on the properties.
While studies have found uses for biochar in the stabilization of soil, there have been challenges to the idea that biochar has usefulness beyond the stabilization. It has been suggested that biochar has been useful for crop productivity and increasing soil quality. To understand if this was the case, the USDA National Laboratory for Agriculture and the Environment undertook multi-year field trials to assess the effects of biochar. The study amended 8 acres with biochar made from hardwood. Twelve plots received 4 tons per acre, and another 12 plots were treated with 8 tons per acre. The studies were conducted in field and laboratory in Idaho, Kentucky, Minnesota, Iowa, South Carolina, and Texas. These studies further confirmed that biochar can improve soil structure and increase sand's ability to retain water, but there was no significant difference found in the three-year average grain yield from either treatment, and overall soil fertility response was more variable.
Further, some studies have found that, despite biochar being billed as carbon negative, and useful for reducing emissions and the use of products such as fertilizers, the studies also found that the composition of biochar is increasingly important in the ability of biochar to achieve the results it has achieved. Furthermore, as biochar has been suggested as a substantial carbon sink, studies from Quebec to Georgia have found that the use of new biochar, while sinking carbon, also becomes a substantial oxygen sink, and in some cases, could deplete oxygen at a greater rate than it would deplete atmospheric carbon, which would result in a net effect of increasing atmospheric carbon. Further, these studies have found that biochar works best in humid environments.
The beginning of any supply chain is the natural materials for the end product. In the case of biochar, the sources of the natural materials is more diverse than in the case of other products. Most sources and most advocates for biochar emphasize that biochar sources come from used end-of-life biomass such as animal manure, forestry and agricultural residues, and sewage sludge. However, there have also been the use of purpose-grown energy crops such as switchgrass, Miscanthus, and corn. And, while the current prices mean that dedicated biomass plantations for producing biochar would not necessarily be seen unless the adoption of biochar was increased. Meanwhile, the possible production of biochar from indigenous forest clearing would threaten biodiversity conservation and, from a life cycle perspective, would not result in net emissions reduction.
Once the sources of biochar have been identified, the logistics of bringing the feedstock to conversion plants takes over. This includes activities such as harvesting, handling, collecting, processing, transporting, and storing of the biomass from the field or forest. Waste and byproduct biomass is typically concentrated at the site of processing for primary products, such as a processing plant, a concentrated animal feeding operation, or at a log landings and mills.
Most conversion systems require some biomass processing prior to pyrolysis. The purpose of processing in feedstock logistics is to make the feedstock more suitable for conversion and more homogenous, which improves mechanized handling and reduces variability in solid, liquid, and gaseous conversion outputs. While the need for processing also depends on the technical specifications necessary for the end biochar product, as reflected above, as the moisture, particle size, ash content, and other characteristics can change the resulting product and what use cases it is best suited for.
Diagram of the supply chain of biochar.
Storage as a component of logistics is also important because it can decouple conversion from feedstock production and delivery, allowing conversion to take place independent of feedstock production. This can be especially crucial in places where biomass is subject to season availability or disruptions in supply due to weather or market conditions, which is the case for many agricultural and forest biomass resources. For large biomass operations, it can be important to consider systems for managing feedstock degradation, fugitive dust emissions, and spontaneous combustion risk, which are hazards in biomass storage and handling.
The conversion part of the supply chain includes three categories of activities: the chemical and physical transformation of biomass feedstock into biochar, post-conversion treatments to enhance biochar effectiveness for specific end uses, and production of any co-products, which can include heat, power, energy, liquid fuels, and chemicals. More than any other component of the supply chain, conversion hinges on technology. With one of the most striking aspects of biomass conversion from a supply chain standpoint is the diversity of technologies and scales that can be used to transform biomass into biochar.
Example of an industrial scale biochar production facility.
On one end of the technical spectrum, small traditional charcoal kilns and more modern small batch systems can be employed by farmers, gardeners, and horticulturalists to process residue into biochar for smaller-scale, on-site applications. On the other end of the technical spectrum, biochar can be a co-product of biofuel production by large, integrated biorefineries deploying cutting-edge conversion technologies at large scales. With these contexts in mind, biochar supply chains are not simple and can have widely differing characteristics depending on the conversion technology employed, with biochar itself being variously a waste, byproduct, co-product, or sole primary product, depending on the operation.
Small-scale biochar production kilns.
Common co-products of pyrolysis include heat, bio-oil, and gas that can be used as fuel for combustion or as a raw material in the production of liquid fuels and chemicals through catalysis. But even through relatively comparable technologies, the supply chains can be variable, with different feedstock specifications and different outputs necessitating different downstream logistics. For example, past pyrolysis systems that produce bio-oil as a co-product must include systems for liquid fuel handling, storage, transportation and safety, and biochar production cannot be decoupled from bio-oil production, regardless of independent market demand for the two products.
Once produced, biochar can be used in its raw form to improve soils. However, in other cases, the performance as a soil amendment can be enhanced by post-conversion treatments. Such treatments include inoculation with desirable microbes, treatments to change pH or other chemical characteristics, granularization or pelletization to improve material handling and performance, composting or blending with chemical fertilizers and organics such as manure, and activation by chemical or physical means to increase surface area and promote ion exchange. This can also offer producers a chance to diversify their products to better meet the needs of end users. And these post-conversion treatments can increase the complexity of this stage in the supply chain.
This post-conversion part of the supply chain involves the activities to package, transport, and store biochar from the site of conversion to the site of end use. Depending on the feedstock and conversion method, biochar usually is either a fine powder, coarse charcoal, hydrophobic or hydrophilic, physically stable or friable, and homogenous or heterogenous in particle size and shape. Biochar may also be either dry or wet, depending on the cooling method used in production, and has various levels of performance in pneumatic and conveyor handling systems.
The different characteristics of the biochar have implications on the distribution of the product. This is as fine powders can be both difficult and dangerous to store and handle due to combustion risk, and pose health risks from breathing in the fine powder. And, while methods of pelletizing biochar can improve handling of the product, this process comes with added financial costs and energy requirements.
Example of biochar being distributed in metal drums.
For larger-scale applications and wholesale markets, biochar can be transported in bulk or rail or truck in specially designed rail cars and trailers. More commonly, raw biochar and biochar downstream products are packaged in those larger packages such as large polyethylene bulk bags, metal and plastic drums, and large multi-ply paper bags. Whereas for low volume or small scale consumer applications, often distribution is done through low-volume plastic bags and buckets.
From a logistics standpoint, bulk packaging can be efficient for producers, but may not also be able of meeting the needs of end users, especially if specialized equipment such as hydraulic lifts and rolling forklifts are needed for unloading. More broadly, distribution logistics must be well matched to both transportation modes and the capabilities of end users to handle and store the biochar before use.
As in most supply chains, the final stage is the end user. In biochar's case, this includes applications in agricultural and forest applications for the improvement of soil productivity, and represents a close to cyclical lifecycle in the supply chain. Other uses for biochar include use cases such as for mitigation and reclamation of mining sites, seed coating, potting media, storm water filtration, and restoration of soils on burned sites.
The end use segment includes not only the application of biochar to soils, but any blending or pre-application processing that may occur at the site of end use. In the case of any processing, biochar can be blended mechanically or by hand with soil and other soil additives, such as seeds, manure, compost or chemical fertilizers, and can include further grinding or screening, or additions of water or surfactants to improve handling during application. And, while application can be done by hand, it is more often performed by specialized agricultural and forestry equipment. Application generally relies on broadcasting by hand or application using planters, tillers, seeders, and spreaders at various scales and levels of mechanization.
Biochar can also be applied using hydroseeding systems that spread a pressurized aqueous slurry of biochar, mixed with additives, such as compost, mulch fertilizer, and tackifying agents to reduce loss of biochar in storm runoff. When biochar is used as soil amendment, biochar systems can meet a broad range of soil improvement, waste management, energy, and climate change mitigation needs. However, the same charcoal classified as biochar in soil applications has potential for use as a fuel and raw material in other applications. Alternative use include fuel pellets and briquettes, chemicals, feedstock for gasification, gunpowder, pigments and dyes, industrial sorbents, and a precursor in the manufacture of activated carbon.
In a project funded by Institute of Working Landscapes in the Oregon Forest Research Laboratory, an interdisciplinary research team that included Oregon State University, the U.S. Department of Agriculture's Agricultural Research Service, and USDA Forest Service studied whether large-scale biochar production for agricultural use is feasible, scalable, economically viable, and environmentally beneficial. Using the Upper Klamath Basin in southern Oregon and northern California as the model, the researchers considered the total costs of producing biochar, including transportation and the hypothetical construction, capitalization, and operation costs of biochar-producing facilities near the Oregon-California border. This study included an evaluation of the agricultural sector markets for biochar in the study of the region.
The biomass assessment undertaken with the study suggested that more than 5 million tons of biochar feedstock could be generated via forest restoration treatments on federal lands in the study area over a twenty-year period. Further, the analysis showed that high value crops, like potato and alfalfa, are the primary markets for biochar application in the study region, with organic potato production having the strongest potential for biochar. The team further concluded that biochar could create an effective link between forest restoration operations and commercial agriculture in the Upper Klamath Basin.
