Solar assisted pyrolysis for biochar production

Pyrolysis represents an environmentally sustainable strategy for the exploitation of biomasses grown on contaminated or physically degraded lands. It combines the recovery of a valuable material, the char, to the production of a combustible vapor phase. The energy efficiency of pyrolysis is significantly reduced by the need to provide the extra energy input required by the heating of the biomass up to the pyrolysis temperature and by the endothermicity of the pyrolysis reactions as a whole. Pyrolysis becomes more appealing from the energetic point of view if the heat requirements of the process are provided by concentrated solar energy. Nevertheless, solar pyrolysis reactors operate in more severe thermal conditions than the ones typically allowed in traditional pyrolysis reactors. Consequently, the dependence of char properties and gas composition on the thermal conditions of the solar assisted pyrolysis process has to be assessed in order to optimize the operating conditions for the valorization of char and the exploitation of the energy content of the vapor phase. Solar assisted pyrolysis tests on Populus nigra L. have been conducted using a solar reactor (volume 6 L) heated by a tunable solar incident radiation [1, 2]. A wide range of heating rate (5-150°C/s) and final pyrolysis temperature (800-1400°C) have been explored with the aim to find the optimal conditions for gas production and to evaluate the effects of temperature and heating rate on char properties. Preliminary results obtained for poplar char (see Figure 1) show that heating rate has a mild effect on char yield: a slight decrease of char yield is observed between 10 and 50 °C/s at the expense of gas yield, whereas the variations observed for the liquid yield are inside the experimental error. Higher heating rates do not produce remarkable variations on the product yields. The same trend is observed for gas composition. An increase of heating rate from 10 to 50 °C/s determines higher concentrations of H2 and CO in the gaseous products at the expense of CO2, thus producing an increase of the calorific value of the gaseous product. Temperature has a stronger effect on the pyrolysis process. Indeed, when pyrolysis is conducted at increasing temperature from 800 to 1400 °C the yield of char decrease as well as the liquid yield. On the contrary, gas yield increases enriching in CO and H2. The effect of thermal conditions on char chemico-physical properties has been studied by performing elemental analysis, porosimetry tests and inorganics analysis on the collected char samples. Both temperature and heating rate has a strong effect on the structural characteristics of the solid residue in terms of BET surface and pore volume.