Pyrolysis of dairy cattle manure: evolution of char characteristics

Livestock manure management constitutes a major challenge at this time. Traditionally, this waste has been used as fertilizer. Excessive application of this residual organic matter on agricultural soils can cause soil quality degradation due to heavy metals accumulation, migration of pathogens to water sources and food, and generation of greenhouse gases. As a promising alternative to land application, pyrolysis of livestock manure allows to obtain biochar, bio-oil and syngas. The goal of this work is to study slow pyrolysis of digested dairy cattle manure (DM) both through one-step and multi-step pyrolysis at increasing temperature in the range 250-600 degrees C. The non-condensable gases composition was continuously analyzed by gas chromatography. Char properties were characterized by ultimate analysis, heavy metals content, ash content, higher heating value (HHV), pH, electrical conductivity (EC), water holding capacity (WHC), cation exchange capacity (CEC), textural properties (specific surface area, pore volume and average pore width) and Fourier Transform Infrared (FTIR) spectroscopy. The experimental results showed that both the product distribution and the properties of char depended on pyrolysis temperature. Char obtained after the last step of multi-step pyrolysis had similar properties to that obtained in one-step pyrolysis. Thus, the cooling and re-heating of the solid between steps did not have a significant effect on the pyrolysis pathway. Pyrolysis at between 400-550 degrees C allowed to reach a compromise between char pH and electrical conductivity for its potential use as soil amendment.