Biochar

Biochar is charcoal used as a soil amendment. Biochar is a stable solid, rich in carbon, and can endure in soil for thousands of years. Like most charcoal, biochar is made from biomass via pyrolysis. Biochar is under investigation as an approach to carbon sequestration, as it has the potential to help mitigate climate change. It results from processes related to pyrogenic carbon capture and storage (PyCCS).Independently, biochar can increase soil fertility of acidic soils (low pH soils), increase agricultural productivity, and provide protection against some foliar and soil-borne diseases. Regarding the definition from the production part, biochar is defined by the International Biochar Initiative as 'The solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment'. Biochar is charcoal used as a soil amendment. Biochar is a stable solid, rich in carbon, and can endure in soil for thousands of years. Like most charcoal, biochar is made from biomass via pyrolysis. Biochar is under investigation as an approach to carbon sequestration, as it has the potential to help mitigate climate change. It results from processes related to pyrogenic carbon capture and storage (PyCCS).Independently, biochar can increase soil fertility of acidic soils (low pH soils), increase agricultural productivity, and provide protection against some foliar and soil-borne diseases. Regarding the definition from the production part, biochar is defined by the International Biochar Initiative as 'The solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment'. The word 'biochar' is an English neologism derived from the Greek word βίος, bios, 'life' and 'char' (product of carbonisation of biomass, as charcoal). Pre-Columbian Amazonians are believed to have used biochar to enhance soil productivity. They seem to have produced it by smoldering agricultural waste (i.e., covering burning biomass with soil) in pits or trenches. European settlers called it terra preta de Indio.Following observations and experiments, a research team working in French Guiana hypothesized that the Amazonian earthworm Pontoscolex corethrurus was the main agent of fine powdering and incorporation of charcoal debris in the mineral soil. Biochar is a high-carbon, fine-grained residue that today is produced through modern pyrolysis processes; it is the direct thermal decomposition of biomass in the absence of oxygen (preventing combustion), which produces a mixture of solids (the biochar proper), liquid (bio-oil), and gas (syngas) products. The specific yield from the pyrolysis is dependent on process condition, such as temperature, residence time and heating rate. These parameters can be optimized to produce either energy or biochar. Temperatures of 400–500 °C (673–773 K) produce more char, while temperatures above 700 °C (973 K) favor the yield of liquid and gas fuel components. Pyrolysis occurs more quickly at the higher temperatures, typically requiring seconds instead of hours. The increasing heating rate will also lead to a decrease of pyrolysis biochar yield, while the temperature is in the range of 350–600 °C (623–873 K). Typical yields are 60% bio-oil, 20% biochar, and 20% syngas. By comparison, slow pyrolysis can produce substantially more char (≈35%); it is this which contributes to the observed soil fertility of terra preta. Once initialized, both processes produce net energy. For typical inputs, the energy required to run a “fast” pyrolyzer is approximately 15% of the energy that it outputs. Modern pyrolysis plants can use the syngas created by the pyrolysis process and output 3–9 times the amount of energy required to run. Besides pyrolysis, torrefaction and hydrothermal carbonization process can also thermally decompose biomass to the solid material. However, these products cannot be strictly defined as biochar. The carbon product from the torrefaction process still contains some volatile organic components, thus its properties are between that of biomass feedstock and biochar. Furthermore, even the hydrothermal carbonization could produce a carbon-rich solid product, the hydrothermal carbonization is evidently different from the conventional thermal conversion process. Therefore, the solid product from hydrothermal carbonization is defined as 'hydrochar' rather than 'biochar'. The Amazonian pit/trench method harvests neither bio-oil nor syngas, and releases a large amount of CO2, black carbon, and other greenhouse gases (GHGs) (and potentially, toxins) into the air, though less greenhouse gasses than captured during the growth of the biomass. Commercial-scale systems process agricultural waste, paper byproducts, and even municipal waste and typically eliminate these side effects by capturing and using the liquid and gas products. The production of biochar as an output is not a priority in most cases. In a centralized system, all biomass in a region is brought to a central plant (i.e. biomass-fueled thermal power station) for processing into biochar. Alternatively, each farmer or group of farmers can operate a lower-tech kiln. Finally, a truck equipped with a pyrolyzer can move from place to place to pyrolyze biomass. Vehicle power comes from the syngas stream, while the biochar remains on the farm. The biofuel is sent to a refinery or storage site. Factors that influence the choice of system type include the cost of transportation of the liquid and solid byproducts, the amount of material to be processed, and the ability to feed directly into the power grid. The most common crops used for making biochar include various tree species, as well as various energy crops. Some of these energy crops (ie Napier grass) can also store much more carbon on a shorter timespan than trees do. For crops that are not exclusively for biochar production, the Residue-to-Product Ratio (RPR) and the collection factor (CF) the percent of the residue not used for other things, measure the approximate amount of feedstock that can be obtained for pyrolysis after harvesting the primary product. For instance, Brazil harvests approximately 460 million tons (MT) of sugarcane annually, with an RPR of 0.30, and a CF of 0.70 for the sugarcane tops, which normally are burned in the field. This translates into approximately 100 MT of residue annually, which could be pyrolyzed to create energy and soil additives. Adding in the bagasse (sugarcane waste) (RPR=0.29 CF=1.0), which is otherwise burned (inefficiently) in boilers, raises the total to 230 MT of pyrolysis feedstock. Some plant residue, however, must remain on the soil to avoid increased costs and emissions from nitrogen fertilizers.