Strategies for producing biochars with minimum PAH contamination

Abstract

With the aim to develop initial recommendations for production of biochars with minimal contamination with polycyclic aromatic hydrocarbons (PAHs), we analysed a systematic set of 46 biochars produced under highly controlled pyrolysis conditions. The effects of the highest treatment temperature (HTT), residence time, carrier gas flow and typical feedstocks (wheat / oilseed rape straw pellets (WSP), softwood pellets (SWP)) on 16 US EPA PAH concentration in biochar were investigated. Overall, the PAH concentrations ranged between 1.2 and 100 mg kg -1 . On average, straw-derived biochar contained 5.8 times higher PAH concentrations than softwood-derived biochar. In a batch pyrolysis reactor, increasing carrier gas flow significantly decreased PAH concentrations in biochar; in case of straw, the concentrations dropped from 43.1 mg kg -1 in the absence of carrier gas to 3.5 mg kg -1 with a carrier gas flow of 0.67 L min -1 ; for woody biomass PAHs concentrations declined from 7.4 mg kg -1 to 1.5 mg kg -1 with the same change of carrier gas flow. In the temperature range of 350-650°C the HTT did not have any significant effect on PAH content in biochars, irrespective of feedstock type, however, in biochars produced at 750°C the PAH concentrations were significantly higher. After detailed investigation it was deduced that this intensification in PAH contamination at high temperatures was most likely down to the specifics of the unit design of the continuous pyrolysis reactor used. Overall, it was concluded that besides PAH formation, vaporisation is determining the PAH concentration in biochar. The fact that both of these mechanisms intensify with pyrolysis temperature (one increasing and the other one decreasing the PAH concentration in biochar) could explain why no consistent trend in PAH content in biochar with temperature has been found in the literature. AbbreviationsHTT, highest treatment temperature; I.D., inner diameter

Effects of Pyrolysis Conditions on Physicochemical Properties of Oat Hull Derived Biochar

“…Also, it was found that the conversion technology is a factor to be considered. Regarding the temperature, Buss et al (2016), after analyzing the samples, were unable to recommend a particular pyrolysis temperature for the production of biochar with low PAH content. González et al (2017).…”

“…Moreover, Devi and Saroha (2015) and Alburquerque et al (2015) reported a decreasing PAHs concentration when the temperature increased, similar to our results. Buss et al (2016) collected and analyzed 46 biochar produced under different operational conditions and different configuration of reactors and concluded that feedstock characteristics highly influenced on PAH concentrations. Also, it was found that the conversion technology is a factor to be considered.…”

“…A higher nitrogen flux reduced the chance for aromatic radicals to undergo condensation, which promoted the PAHs precursors to be conducted in the gas flux, and therefore prevented the formation of PAHs (Buss et al 2016).…”

“…However, these parameters have been studied separately or not systematically. Only Buss et al (2016) reported the effect of the residence time, highest treatment temperature (HTT), and carrier gas flow rate on the total concentrations of 16 US EPA PAHs in biochar. However, there is no known correlation between the biochar physicochemical properties, PAHs content, and pyrolysis conditions.…”

Torrefaction of Agricultural Residues: Effect of Temperature and Residence Time on the Process Products Properties

“…Additionally, biochar adsorbs hydrophobic organic pollutants in the soil [12,13]. However, the process of biochar production (torrefaction) is accompanied by the formation of polycyclic aromatic hydrocarbons (PAHs) as a result of Diel-Alder reaction occurring during the process [14]. These hydrocarbons are considered to be toxic, and for this reason, their emissions to the environment are legally limited and strictly monitored.…”