Study on thermal properties of bio-char prepared by photo-thermal pyrolysis

Photo-thermal pyrolysis provides a new method for the preparation of bio-char through the energy absorption of the intermittent solar energy into the carbon materials to achieve the energy storage and negative carbon emissions. The evolution of thermal properties of different chars prepared by photo-thermal pyrolysis with different final-reaction temperatures and heating rates was studied. The study of specific heat capacity showed that the final-reaction temperature had a stronger impact than the heating rate. At a final-reaction temperature of 800 °C, the maximum specific heat capacity of rice straw and pine char reached 1595 J·kg−1·K−1 and 2070 J·kg−1·K−1 respectively. Measurements of thermal conductivity showed that the higher final-reaction temperature and the higher heating rate both increased the thermal conductivity of char. Rice straw char had a maximum value of 0.2203 W·m−1·K−1 at 800 °C and 500 °C/min, which was almost twice the thermal conductivity of char at 400 °C and 50 °C/min. Further correlation analysis showed the physical structure of the char, especially its density and pore size, had a greater impact on thermal conductivity. The multiple linear model of thermal conductivity was established, which not only had a good fitting effect but also well predicted the thermal conductivity of another kind of char, and the error was within 10%. This study demonstrates the potential of photo-thermal pyrolysis char as a heat carrier and light absorption medium during photo-thermal pyrolysis.