Thursday, December 5, 2013

Biochar manufacturing process conditions.

Temperature, heating rate and heating time
     For the same feedstock, biochar yield is highly dependent upon the conditions under which pyrolysis is carried out; namely, temperature, heating rate, heating time and particle size (Shafizadeh, 1982;Williams and Besler, 1996; Demirbas and Arin, 2002; Uzun et al., 2006; Tsai et al., 2006).While it is well documented that biochar yield decreases with increasing temperature and that the yield temperature relationships are different with different feedstocks (Horne and Williams, 1996; Williams and Besler, 1996; Tsai et al., 2006).


    Depending upon the operating conditions, the complex and varying changes of biomass during pyrolysis affect both the composition and chemical structure of the resulting biochar, with significant implications for nutrient contents and, especially, nutrient availability to plants. Changes in the composition of biochars during pyrolysis of organic matter using molecular techniques indicate a gradual decrease in the amounts of OH and CH3 and an increase in C=C with increasing temperature (150°C to 550°C), suggesting a change from aliphatic to aromatic C structure of the biochar.

      In contrast, biochars formed at lower temperatures (300°C to 400°C) are only partially carbonized, with high H/C ratios and O contents, and have a lower surface area. Consequently, low temperature biochars are found to have higher amounts of acid–basic surface functional groups. Therefore, increasing temperature during pyrolysis results in changes in the molecular composition, as well as changes in biochar charge properties.

     Biochars containing large proportions of mineral matter (ash) produced at low temperatures also have a much greater concentration of sub-grain boundaries and defects on the surface than the same biochars produced at high temperatures. Mineral matter in low temperature biochar is more likely to dissolve since these defects are centers for reactions with liquids and gases. These changes should have effects on the total nutrient content as well as their availability.


    Porosity of biochar significantly increases between 400°C and 600°C, and may be attributed to increases in water molecules released by dehydroxylation acting as pore-former and activation agent, thus creating very small (nanometer-size) pores in biochar (Bagreev et al., 2001).

    Biochar production cannot be properly discussed without first distinguishing it from char and charcoal. All three forms of carbonaceous material are produced from pyrolysis; the process of heating carbon (C) - bearing solid material under oxygen (O2)-starved conditions. Char is defined here as any carbonaceous residue from pyrolysis, including natural fires. Thus, char is the most general term to employ in scientific descriptions of the products of pyrolysis and fires, whether from biomass or other materials. Charcoal is char produced from pyrolysis of animal or vegetable matter in kilns for use in cooking or heating.

     Biochar is carbonaceous material produced specifically for application to soil as part of agronomic or environmental management. No standard currently prescribes the composition or preparation of biochar to distinguish it from charcoal produced as fuel. However, understanding of what makes for ‘good’ charcoal in agronomic and environmental management applications will inevitably encourage separate designations for charcoal and biochar. Although C is the major constituent of charcoal, its exact composition and physical properties depend upon the starting material and the conditions under which it is produced.

     Charcoal contains 65 to 90 per cent C with the balance being volatile matter and mineral matter (ash) (Antal and Grønli, 2003). Superficially, charcoal resembles coal, which is also derived from vegetable matter; indeed, the word charcoal may have originally meant ‘the making of coal’ (Encyclopedia Britannica, 1911).


    However, the geological processes from which coal is derived are quite different from charcoal-making, resulting in important differences in chemical composition, porosity and reactivity. Charcoal is readily generated in open fires, whether forest fires or campfires. Thus, it was available to early humankind whose first apparent use of it was in the creation of spectacular cave paintings during the last Ice Age (Bard, 2002).

Lascaux Cave Paintings - Horse: Drawn 10,000-15,000 B.C.

Chauvet Cave in the valley of the Ardèche River in France is filled with paintings, engravings and drawings created more than 30 000 years ago, of cave lions, mammoths, rhinos, bison, cave bears and horses. It contains the earliest known cave paintings, as well as other evidence of Upper Paleolithic life. It is situated on a limestone cliff above the former bed of the Ardèche River. The cave was first explored on December 18, 1994. As well as the paintings they discovered fossilised remains, prints, and markings from a variety of animals, some of which are now extinct.  Source:

    Charcoal eventually found application in other fields, including agronomy, medicine, metallurgy, pyrotechnics and chemical manufacture. However, its largest application has always been in the preparation of smokeless fuel for cooking, residential heating, smelting and steel making. The process of charcoal making removes most of the volatile matter responsible for smoke during burning. Charcoal is a relatively clean burning fuel that represented an important innovation in the controlled use of fire. Biochar as a C sequestration agent and soil amendment, on the other hand, is still poorly understood.

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