Torrefaction and pyrolysis of metal-enriched poplars from phytotechnologies: Effect of temperature and biomass chlorine content on metal distribution in end-products and valorization options

Torrefaction (290 °C) and pyrolysis (450 and 800 °C) trials were performed at pilot scale with a patented reactor on metal-enriched poplars from a contaminated soil managed by phytotechnologies and poplars cultivated on uncontaminated soil, for comparison. This study emphasized the influence of temperature on end-product yield, metal distribution in end-products and metal behavior. Results showed that the evolution of the end-product yield, i.e. biochar, bio-oil and gas fractions, was depending on temperature rather than other parameters such as the origin or metal content of the tested poplars. Torrefaction decreased the processed poplars weight, leading to metal-enriched torrefied biomass as a major end-product. At 450 and 800 °C, metal behavior was governed by temperature. Nevertheless, the different chlorine content of initial biomass was hypothesized to be an important factor that could modify the metal behavior by increasing metal volatilization at process temperatures. Amongst the studied metals, Zn and Pb were hypothesized to form volatile metal chlorides, therefore increased metal volatilization from Control poplar biomass could be explained by increased chlorine content of such biomass. At 450 °C, metals were recovered in biochar, then in bio-oil and finally in gases. At 800 °C, similar results were obtained for Cu, confirming that Cu was not easily volatilized. On the contrary, Zn, Pb and Cd were mostly recovered in gases, which became the second metal recovery compartment. According to metal concentrations, valorization options were discussed for end-products.