CARE: Biochar Production

Biochar - The Processes and Benefits

Conversion And Resource Evaluation Ltd

Category: Environmental Improvement Solutions | 19/11/2009 - 04:05:09

The International Biochar Initiative defines biochar as “a fine-grained, highly porous charcoal that helps soils to retain nutrients and water” [i]. Biochar differs from charcoal and similar materials by the fact that it is produced for agricultural and not energy purposes. Nevertheless, it may be alternatively used as an energy carrier.

Biochar is produced from the thermal decomposition of natural organic materials such as crop waste, wood chip, municipal waste, or manure in a limited oxygen atmosphere by a process called pyrolysis [ii]. The application of heat breaks down the organic matter into condensable vapours, non condensable gases (syngas) and a solid, carbon-rich residue (biochar). The condensable vapours yield a liquid known as pyrolysis liquids or "bio-oil", which contains a wide range of oxygenated chemicals. Syngas consists of carbon monoxide, carbon dioxide, hydrogen, methane and higher hydrocarbons.  Bio-oil and syngas can be burnt to produce heat, power or combined heat and power.

Biochar Production

At present, there are very technologies specifically for biochar production for agricultural purposes. As a result there are various technologies currently in use as presented in Table 1. Biochar is usually produced in higher yields if slow pyrolysis is used as shown.

Table 1 Pyrolysis processes [and gasification for comparison]

Process Liquid
(bio-oil)
Solid (biochar) Gas (syngas)
Slow pyrolysis
  • Long residence times
  • Low-moderate reactor temperature
30%
(70% water)
35% 35%
Intermediate pyrolysis
  • Low-moderate reactor temperature
  • Moderate hot vapour residence time
50%
(50% water)
25% 25%
Fast pyrolysis
  • Moderate reactor temperature (~500°C)
  • Short hot vapour residence time (<2 s)
75%
(25% water)
12% 13%
Gasification [for comparison]
  • High reactor temperature (>800°C)
  • Long vapour residence time
5% tar
(contains 5% water)
5%
(not used as biochar)
90%

The use of biochar for agricultural purposes has been triggered by investigations in the Amazon basin that have attributed the high fertility of the dark earths (terra preta) to their high biochar content and revealed the unique properties of biochar as a soil conditioner. Biochar can facilitate [2, 3, 4]:

  • Increased nutrient retention and cation exchange capacity
  • Increased crop production
  • Decreased soil acidity
  • Decreased uptake of soil toxins
  • Improved soil structure
  • Improved conditions for earthworm populations
  • Increased water-holding capacity of the soil
  • Decreased release of non-CO2 greenhouse gases (CH4, N2O)
  • Improved fertiliser use efficiency by preventing fertiliser runoff and leeching

In addition to its agricultural benefits biochar can help combat climate change via carbon sequestration.  Biomass fuels are carbon neutral; the carbon captured in the biomass by photosynthesis returns to the atmosphere during combustion.  Biochar systems can be carbon negative because they retain a substantial portion of the carbon captured by photosynthesis.  The result is a net reduction of carbon dioxide in the atmosphere, as shown in Figure 2 [1].  Biochar can sequester carbon in the soil for hundreds and even thousands of years.  By improving soil fertility and stimulating plant growth, it also leads to the consumption of more carbon dioxide in a feedback effect.  Furthermore the bio-energy generated as part of biochar production (bio-oil and syngas) can replace carbon positive energy from fossil fuels [2].

However it should be noted that the benefits of biochar as a soil conditioner are not a certainty. They depend on the quality of the biochar and the quality of the soil. Some biochars may have adverse effects on plant growth. The quality of the biochar is greatly affected by the pyrolysis process parameters, principally temperature and solids residence time, and the type of the feedstock.  With regards to the quality of the soil, highly degraded and nutrient-poor soils usually show increased crop production after biochar application, whereas fertile and healthy soils do not always yield a positive change.  Research on matching the unique properties of biochars to different applications is ongoing.

In order to boost biochar production and application the uncertainty around investment in the sector and the market for carbon offsets need to be addressed through sound economic and full life-cycle analysis.

Conversion and Resource Evaluation Ltd. (CARE Ltd) provides specialist technical services in bio-energy and waste-to-energy projects for heat, power, combined heat and power and renewable products such as biochar. We can offer the following services:

  • Source biochar for soil applications
  • Arrange for the testing biochar, e.g. pot trials
  • Design a process for biochar production from research to commercial scale.
  • Perform techno-economic evaluations of biochar systems

Conversion And Resource Evaluation Ltd. is working with companies and research institutions to further the development of standards and processes for optimal biochar production and has also worked on the determination of standards and GHG assessment of pyrolysis processes [5, 6].

Biochar Production Diagram

Figure 1 Biochar production diagram by Johannes Lehmann [1]

Biochar Production is a Carbon Negative Process

Figure 2 Biochar production is a carbon negative process by Johannes Lehman [7]

Mangium Seedlings

Figure 3 Charcoal additions to A. mangium Seedlings showing height and diameter significantly increased at age of 6 months in comparison to a control [8]

References

1.International Biochar Initiative (IBI) http://www.biochar-international.org/

2.Dr Evelyn Krull, CSIRO Land and Water, Biochar Factsheet http://www.csiro.au/resources/Biochar-Factsheet.html

3.Biochar Info http://www.biochar.info/biochar.biochar-overview.cfml

4.Australia and New Zealand Biochar Researchers Network http://www.anzbiochar.org/biocharbasics.html#xone

5.S. Joseph, C. Peacocke, J. Lehmann and P. Munroe, "Developing a Biochar classification and test methods'',Biochar for Environmental Management: Science and Technology, Johannes Lehmann and Stephen Joseph (eds.), 2009, Earthscan, London, p. 107-126.

6.B.A. McCarl, C. Peacocke, C.-C. Kung, G. Cornforth, R. D. Sands and R. Chrisman, ''Economics of Biochar Production, Utilisation and Greenhouse Gas Offsets", Biochar for Environmental Management: Science and Technology, Johannes Lehmann and Stephen Joseph (eds.), 2009, Earthscan, London, p. 341-357.

7.Lehmann J., 2007, A handful of carbon, Nature 447: 143-144 doi: 10.1038/447143a

8.Siregar, (2004 Indonesia), Forest and Nature Conservation Research and Development Center