Biochar gasification

Brewer, C.E., et al., 2009. Characterization of biochar from fast pyrolysis and gasification systems. Environmental Progress Sustainable Energy 28 (3), 386—396. 

Most thermochemical conversion technologies used for production of biofuels and biochemicals yield solid residues as a byproduct. Depending on the technology, this residue contains more or less carbon, and can therefore be considered biochar in its own right, or a biochar precursor. The key relevant technologies, fast pyrolysis, gasification, and hydrothermal carbonization are discussed individually in the following sections. 

Given the specifics of the fast pyrolysis process in terms of feedstock requirements and process conditions, ie, fast heating and short residence time in reactor, it can be expected to yield biochar with a different set of properties compared to other conversion processes, such as slow pyrolysis or gasification. The short residence time can lead to incomplete charring of the biomass particle, as observed by Bruun et al. (2011,2012). This in turn leads to lower environmental stability of biochar, and therefore lower carbon sequestration potential. This is the case even when the biomass conversion during pyrolysis is apparently complete, as reported in Brewer et al. (2009). These authors observed lower stability of fast pyrolysis biochar, assessed based on fixed carbon content and aromaticity, compared to slow pyrolysis and gasification biochar produced from the same feedstock. 

In gasification units with high carbon conversion efficiency the solid residue, ie, gasification ash/char, contains only a small amount of carbon, and is therefore less suitable for use as biochar compared to biochar from pyrolysis, at least from the carbon sequestration point of view (Leiva et al., 2007). One possible exception could be fly-ash from fluidized bed gasification, as its carbon content can be up to 70% (Pels et al., 2005). However, in this case, close attention would need to be paid to contaminant content in this material, as harmful organic and inorganic contaminants can be present in high concentrations. 

In addition, in some systems, water is used to cool down biochar discharged from the gasification chamber, and this water is often recycled and used in tar scrubbers, thus contaminating biochar (Shackley et al., 2012b). Therefore, if gasification biochar is to be used for soil application, the production system needs to be set up to minimize the risk of contamination. 

Hansen, V., Miiller-Stover, D., Ahrenfeldt, J., Holm, J.K., Henriksen, U.B., Hauggaard-Nielsen, H., 2015. Gasification biochar as a valuable by-product for carbon sequestration and soil amendment. Biomass and Bioenergy 72 (1), 300—308. http //dx.doi.org/10.1016/ j. biombioe.2014.10.013. 

Muter, O., Lebedeva, G., Telysheva, G., 2014. Evaluation of the changes induced by gasification biochar in a peat-sand substrate. International Agrophysics 28 (4), 471—478. http // dx.doi.org/10.2478/intag-2014-0037. 

Shackley, S., Carter, S., Knowles, T., Middelink, E., Haefele, S., Haszeldine, S., 2012a. Sustainable gasification-biochar systems A case-study of rice-husk gasification in Cambodia, part II field trial results, carbon abatement, economic assessment and conclusions. Energy Policy 41, 618—623. http //dx.doi.Org/10.1016/j.enpol.2011.ll.023. 

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