Effects of pyrolysis conditions on the thermal transformation of chicken manure into char

Biochar is a fine-grained and highly porous carbonaceous substance, arising from the pyrolitic decomposition of natural or synthetic organic materials. It is lately applied to soils to favorably affect soil physico-chemical properties, such as water and nutrient retention and cation exchange capacity (CEC). The extent of the effect of biochar on crop productivity is very variable due to the different biophysical interactions and processes that occur when it is applied to soils. Char properties are greatly influenced by both natures of feedstock and process conditions. An accurate characterization of biochar is crucial to evaluate the possibility to amend soil with such material by avoiding environmental damages. The aim of this work was the chemical-physical characterization of biochar from chicken manure in order to investigate its potentiality either as fertilizer or as metal bio-sorbent in soil remediation procedures. Biochar was produced at different temperatures and residence times into a heating muffle. Then, evaluation of carbon, hydrogen and nitrogen contents, transversal relaxation time (T2) by NMR as well as metal content such as Na, K, Fe, Cu, Mn, Al, Ca, Mg, and Pb has been achieved. Environmental Scanning Electron Microscopy (ESEM), Differential Scanning Calorimetry (DSC) and Thermogravimetry (TG) studies have been also carried out. Elemental Analysis revealed a decrease in the C, H, N content with increasing Pyrolysis Temperature (PT). These changes appeared not to be dependent upon residence time (RT) in the furnace. The ash content increased from 46.09% in 300°C-obtained biochar to 66.54% in 600°C-obtained biochar. TG profiles from biochars pyrolized at the same PT but at different RT were similar, while there were statistically significant differences among biochars obtained at increasing temperatures. From TG and DSC curves, the presence of one or two main different processes, each corresponding to a mass-loss step, was recognised. Biochar samples can be divided into different groups depending on their distinctive thermal behavior. For samples obtained below 450°C, the first mass loss occurred between 180°C and 350°C. It can be attributed to the destruction of alkyl systems, CH and amino groups. The second mass loss is comprised in the range 350-600°C and corresponds to the thermo-oxidation of aromatic, mono or polycyclic, rings as well as decarboxilation. Samples produced above 500°C revealed only one single process supporting the evidence that biochar produced at higher temperatures contain mainly aromatic compounds. 450°C-produced biochars showed intermediate characteristics. T2-NMR results revealed an increase in all relaxation times with PT. The shift of T2 towards higher values is an indication of the presence of water into larger pores and suggests that char treated at higher temperatures may contain larger-sized pores. According to our results, the chemical composition and the physical structure of chicken manure biochar vary mainly depending on process temperature. During pyrolysis, biochar loses volatile compounds gaining more stability. Biochar formation can be described as an aromatic growth and polymerization process, resulting in carbon enrichment. In addition, porosity development is achieved as volatile matter is removed. The high ash content of this kind of char has both positive and negative implications. The ash contains important plant nutrients (K, Ca, Mg) and a very basic pH, which are generally positive traits. However, they can be detrimental if biochar is applied at high concentrations or on alkaline/calcareous soils.