Effects of pyrolysis temperature on carbon retention and stability of biochar

Abstract: Turning biomass wastes into biochar under the conditions of low temperature and limited oxygen has recently proven to be a promising approach for long-term carbon sequestration. However, the ultimate carbon sequestration efficiency of biochar depends not only on the feedstock type and production condition, but also on the environmental conditions of soil. In order to reveal the effects of pyrolysis temperature which is main parameter of biochar production condition on carbon retention and biochar stability, and provide more information for further improvement of carbon sequestration potential by turning biomass into biochar, the characteristics of biochar derived from walnut shell under lab condition were analyzed. During a typical slow pyrolysis process, the biochar was heated at a speed of approximately 20℃/min in a Muffle Furnace under limited oxygen and held at 200-700℃ for 2 h; then, biochar yield, elemental composition, functional groups distribution using Fourier transform infrared spectroscopy (FTIR) analysis and its chemical stability determined by potassium dichromate (K2Cr2O7) and potassium permanganate (KMnO4) oxidation methods, were all analyzed. The results showed that the carbon (C) content of biochar increased with the rise of the pyrolysis temperature from 54.0% at 200℃ to 83.7% at 700℃, on the contrary, the hydrogen (H) and oxygen (O) contents decreased with the rise of the temperature from 6.31% to 2.22% for H content and 45.1% to 10.6% for O content, respectively. Moreover, the H/C and O/C, usually used as two indexes to estimate carbon aromaticity for biochar, also decreased with the rise of the pyrolysis temperature from 1.25 to 0.32 for H/C and 0.54 to 0.09 for O/C, respectively, which indicated the carbon aromaticity of biochar was strengthened with the rise of the pyrolysis temperature, which is beneficial for the improvement of biochar stability. After the pyrolysis of walnut shell, especially when pyrolysis temperature was above 200℃, compared to the feedstock, the -C-O and O-CH3 groups did not exist and the aliphatic-CH gradually disappeared while more aromatization were shown by new aromatic compounds with the increasing of pyrolysis temperature based on FTIR analysis. Biochar yield decreased with the rise of the pyrolysis temperature from 79.1% at 200℃ to 19.4% at 700℃. The carbon retention, defined as the proportion of the original carbon, which was from plant photosynthesis and sequestrated the CO2 in feedstock from atmosphere, was retained in the biochar after the pyrolysis, and also decreased with the rise of the pyrolysis temperature from 69.4% to 11.0%, all of which indicated that high pyrolysis temperature aggravated carbon loss. However, high pyrolysis temperature strengthened the carbon stability, and especially for biochar derived from 500℃, the carbon loss of biochar oxidated by K2Cr2O7 and KMnO4 were 10.4% and 1.66%, respectively, which were smaller than the other biochars derived from other pyrolysis temperatures. There was significant negative correlation between pyrolysis temperature and the yield and carbon retention of biochar, while there was significant positive correlation between pyrolysis temperature and biochar stability. The stability of biochar in real soil and the optimal temperature of biochar for maximizing its carbon sequestration capacity need to be researched in the future.