Biochar slow pyrolysis

Slow pyrolysis of biomass operates at relatively low heating rates (0. l-2°C/s) and longer solid and vapor residence time (2-30 min) to favor biochar yield (Nanda et al., 2014b). Slow pyrolysis operates at temperature lower than that of fast pyrolysis, t q)ically 400 10°C and has a gas residence time usually > 5 s. Slow pyrolysis is similar to carbonization (for low temperatures and long residence times). During conventional pyrolysis, biomass is slowly devolatilized facilitating the formation of chars and some tars as the main products. This process yields different range of products with their properties dependent on temperature, inert gas flow rate and residence time. 

Brown, T.R., Wright, M.M. and Brown, R.C. (2011) Estimating profitability of two biochar production scenarios slow pyrolysis vs fast pyrolysis. Biofuels, Bioproducts and Biorefining, vol 5, pp. 54—68. 

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. 

Besides stability, reflecting chemical composition, the physical properties of fast pyrolysis biochar also differ from those of slow pyrolysis biochar. Research by Bruun et al. (2012) showed somewhat higher surface area of fast pyrolysis biochar, and its... 

Due to the wide range of processing conditions (temperature and residence time) available and feedstock options (woody biomass, agricultural biomass, and diverse organic residues) that can be processed in slow pyrolysis units, the yield and properties of biochar can vary widely. This provides an opportunity to optimize the production to yield biochar with properties matching its application. In their research Ronsse et al. (2013) and Zhao et al. (2013) showed that certain biochar properties are primarily affected by processing conditions (eg, surface area, pH, carbon sequestration potential), while others are mainly feedstock-dependent (eg, content of total organic carbon. 

Ronsse, F., van Hecke, S., Dickinson, D., Prins, W., 2013. Production and characterization of slow pyrolysis biochar influence of feedstock type and pyrolysis conditions. GCB Bioenergy 5 (2), 104—115. http //dx.doi.org/10.llll/gcbb.12018. 

Biochar from dairy manure also has pofenfial for environmenfal remediation or for creating slow release phosphorus fertilizers (Cau and Harris, 2010). Dairy manure was converted by heating at temperatures below pyrolysis temperatures (< 500 °C) and in the presence of air. The potential benefit for lowering GHGs was nof defermined but the products have the potential of creating new markets for manure. 

Bmun, E.W., Ambus, P., Egsgaard, H., Hauggaard-Nielsen, H., 2012. Effects of slow and fast pyrolysis biochar on soil C and N turnover dynamics. Soil Biology and Biochemistry 46, 73—79. http //dx.doi.0rg/lO.lOl6/j.soilbio.2Oll.ll.Oi9. 

Kloss, S., Zehetner, F., Dellantonio, A., Hamid, R., Ottner, F., Liedtke, V., Soja, G., 2012. Characterization of slow pjTolysis biochars effects of feedstocks and pyrolysis temperature on biochar properties. Journal of Environment Quality, http //dx.doi.org/10.2134/ Jeq2011.0070. 

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