Effect of calcium dispersion and graphitization during high-temperature pyrolysis of beech wood char on the gasification rate with CO2

Abstract This paper presents thermal deactivation of beech wood chars during secondary pyrolysis in a drop-tube reactor. Pyrolysis temperature was varied between 1000 °C and 1600 °C at a constant residence time of 200 ms. The effect of pyrolysis conditions on initial conversion rate R0 during gasification, graphitization of the carbon matrix and ash morphology was investigated. Gasification experiments for the determination of R0 were conducted in a thermogravimetric analyzer using pure CO2 at 750 °C and isothermal conditions. A linear decrease in initial conversion rate R0 was observed between 1000 °C and 1400 °C. However, a strong increase of R0 at 1600 °C was encountered. Micropore surface area of the secondary chars showed no correlation with the initial conversion rate R0 during gasification with CO2. Graphitization of the carbon matrix was determined using X-ray diffraction and Raman spectroscopy suggesting the growth of aromatic clusters and graphite-like structures for increasing pyrolysis temperatures up to 1600 °C. Furthermore, CaO dispersion was analyzed quantitatively and qualitatively using temperature-programmed reaction at 300 °C as well as SEM/TEM. CaO dispersion DCaO decreases steadily between 1000 °C and 1400 °C whereas a strong increase can be observed at 1600 °C, which is in good accordance with the development of the initial conversion rate R0 as a function of pyrolysis temperature. SEM/TEM images indicate the formation of a thin CaO layer at 1600 °C that is presumably responsible for the strong increase in initial conversion rate R0 at this temperature. When excluding the catalytic activity of CaO via formation of the ratio R0 DCaO−1, increasing graphitization degree has a linear negative influence on char reactivity at pyrolysis temperatures between 1000 °C and 1400 °C.