Biochar for soil amendment
Patrick BrassardStéphane GodboutVicky LévesqueJoahnn H. PalaciosVijaya RaghavanAhmed AhmedRichard HogueThomas JeanneMausam Verma
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Biochar production cannot be properly discussed without fi rst distinguishing it from
char and charcoal. The literature contains
many examples where these three terms for
carbonaceous materials are freely interchanged, which causes unnecessary confusion. All three forms of carbonaceous material
are produced from pyrolysis, the process of
heating carbon (C)-bearing solid material
under oxygen-starved conditions. Pyrogenic
carbonaceous materials (PCM) are defi ned
here as any carbonaceous residue from pyrolysis (Chapter 1). Thus, PCM is the most general term to employ in scientifi c descriptions
of the products of pyrolysis and fi res whether
from biomass or other materials. Char is the
PCM residue from natural fi res. Charcoal is
PCM produced from pyrolysis of animal or
vegetable matter in kilns for use in cooking or
heating, including industrial applications such
as smelting. Biochar is carbonaceous material
produced specifi cally for application to soil
for agronomic or environmental management. In 2012 the International BiocharInitiative (IBI) released the fi rst Guidelines
for ‘Biochar that is used in Soil’ to formally
defi ne this carbonaceous product and describe
its desired characteristics (www.biocharinternational.org/characterizationstandard).
However, continuing research is required to
understand what constitutes ‘good’ biochar in
agronomic and environmental management
applications.
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This study shows the significant differences in the fuel quality and ash properties of biochars produced from the slow pyrolysis of various biomass components (leaf, wood, and bark). The objective is to identify which component is likely to cause problems in subsequent utilization processes if biochar produced from various components of mallee trees is used as a fuel. It is found that the pyrolysis of different biomass components produced biochars with distinct characteristics, largely because of the differences in the biological structure of these components. Leaf biochar showed the poorest grindability, possibly because of the presence of abundant tough oil glands in leaf. Even for the biochar prepared from the pyrolysis of leaf at 800 °C, the oil gland enclosures remained largely intact after grinding. Biochars produced from leaf, bark, and wood components also have significant differences in ash properties. Even with low ash content, wood biochars have low Si/K and Ca/K ratios, suggesting that these biochars may have a high slagging propensity, in comparison to bark and leaf biochars. It appears that, in the utilization of biochar prepared from mallee biomass, the grindability is likely to be limited by the leaf fraction while ash-related problems could be due to the wood and bark components.
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Using biochar as a soil amendment agent can recycle a majority of inherent inorganic nutrients in biomass to the soil, largely enhancing the overall sustainability of pyrolysis technology. This work investigates the effects of pyrolysis conditions and biomass properties on the leachability and recyclability of nutrients in mallee biochars. Understanding the relationships between biochar preparation conditions and biochar nutrients recyclability will aid the optimisation of suitable conditions to produce biochar with excellent leachability.
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Biochar is a carbon-rich and porous material that finds application in different sectors and can be extremely useful in agriculture as soil improver. This paper provides a comparison between biochars produced with different slow pyrolysis processes and biochar produced in a downdraft gasifier. A blend of residual lignocellulosic biomasses (hemp hurd and fir sawdust) was pelletized and used as starting feedstock for the tests. The biochars produced were analyzed and compared. Temperature proved to be the main driver in conditioning the chemical-physical characteristics of the biochars more than residence time or the configuration of the pyrolysis process. The higher the temperature, the higher the carbon and ash content and the biochar pH and the lower the hydrogen content and the char yield. The most noticeable differences between pyrolysis and gasification biochars were the pH and the surface area (considerably higher for gasification char) and the low content of hydrogen in the gasification biochar. Two germinability tests were carried out to assess the possible application of the various biochars as soil amendment. In the first germinability test, watercress seeds were placed in direct contact with the biochar, while in the second they were placed on a blend of soil (90%v/v) and biochar (10%v/v). The biochars with the best performance were those produced at higher temperatures using a purging gas and the gasification biochar (especially mixed with soil).
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Biomass as a fuel suffers from its bulky, fibrous, high moisture content and low-energy-density nature, leading to key issues including high transport cost and poor biomass grindability. This study investigates the possibility to pretreat biomass to produce biochar as a solid biofuel to address these issues. Biochars were produced from the pyrolysis of centimeter-sized particles of Western Australia (WA) mallee wood in a fixed-bed reactor at 300 to 500 °C and a heating rate of 10 °C/min. The data show that, at pyrolysis temperatures ≥320 °C, biochar as a fuel has similar fuel H/C and O/C ratios compared to Collie coal that is the only coal being mined in WA. Converting biomass to biochar leads to a substantial increase in fuel mass energy density from ∼10 GJ/ton of green biomass to ∼28 GJ/ton of biochars prepared from pyrolysis at 320 °C, in comparison to 26 GJ/ton for Collie coal. However, there is little improvement in fuel volumetric energy density, which is around 7−9 GJ/m3 in comparison to 17 GJ/m3 of Collie coal. Biochars are still bulky and grinding is required for volumetric energy densification. Biochar grindability experiments show that the fuel grindability increases drastically even at pyrolysis temperature as low as 300 °C. Further increase in pyrolysis temperature to 500 °C leads to only a small increase in biochar grindability. Under the grinding conditions, a significant size reduction (34−66% cumulative volumetric size below 75 μm) for biochars can be achieved after 4 minutes grinding (in comparison to only 19% for biomass after 15 minutes grinding), leading to a significant increase in volumetric energy density (e.g., from ∼8 to 19 GJ/m3 for biochar prepared from pyrolysis at 400 °C). Whereas grinding raw biomass typically results in large and fibrous particles, grinding biochars produces short and round particles. The results in this article indicate that biochar has desired fuel properties and potentially a good solution to address the key issues including high transport cost and poor grindability associated with the direct use of biomass as a fuel.
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