Effects of pyrolyses temperature on some of physical and chemical characteristics of biochars

In this study, various physical and chemical properties of biochars produced by slow pyrolysis at 3 different temperatures (300, 400 and 600 ºC) from seven different feedstocks were determined and compared. Tomato wastes (stem, roots and leaves), poplar sawdust, wheat straw, corncob, bean plant wastes, rice husk and cattle manure were used as raw materials. Yield, specific surface area (SSA), field capacity (FC), wilting point (WP) and available water contents (AWC), pH, electrical conductivity (EC), cation exchange capacity (CEC), total carbon (TC) and nitrogen (TN), calcium (Ca), magnesium (Mg), potassium (K), zinc (Zn), iron (Fe), copper (Cu) and manganese (Mn) concentrations of biochars produced were determined. Increasing the pyrolysis temperature from 300 ºC to 600 ºC resulted in a decrease in yield and SSA of all biochar types. The most significant decrease in SSA was observed in sawdust in which the surface area at 300 ºC was reduced from 758.3 m2 g-1 to 250.8 m2 g-1 at 600 ºC. The increase in pyrolysis temperature although not statistically significant caused an increase in FC, WP and AWC. Similar to the moisture contents, the increase in the pyrolysis temperature caused a statistically significant increase in the pH values of all biochar types. However, EC values were not significantly affected by temperature rise. The CEC values significantly varied depending on the biochar types. The CEC value at 300 ºC pyrolysis temperature was between 33.47 and 88.16 cmolc kg-1, while it was between 41.87 and 78.68 cmolc kg-1 at 400 ºC, and between 23.27 and 68.03 cmolc kg-1 at 600 ºC. The TC content of the biochar produced from bovine manure decreased with the increase in temperature, no change was observed in the bean biochar, but the TC content of the other five biochars increased. The increase in temperature led to a significant decrease in the TN content, especially after 400 ºC. Increasing the pyrolysis temperature from 300 to 600 ºC in all biochar types led to an increase in P and K contents. The results indicated that the yields and properties of biochars strongly depended on the pyrolysis temperature. High temperature (600 ºC) produced biochar having high pH and EC values and micro (Cu, Fe, Zn and Mn) and macro (P, K, Ca and Mg) concentrations.