Abstract
In late 2013, an open call for charcoal and biochar samples was distributed in an effort to compare a wide range of char samples by Raman spectroscopy. The samples contributed to this survey included: laboratory produced biochars, recent biochars produced in field conditions, and ancient char samples previously analysed by carbon dating. By using selected Raman measurements, the char samples could be ranked in terms of the degree of thermochemical alteration or extent of carbon nanostructural development. The Raman results for recently produced biomass chars were generally consistent with the conversion of amorphous carbon formed at lower temperatures into condensed, polyaromatic, and graphene-like carbon formed at higher temperatures. A number of parameters calculated from the Raman spectra could be used to estimate the effective heat treatment temperatures in the recently produced biochars. Other samples such as anthracite coal, tire pyrolysis carbon, and ancient chars departed from the trends observed in the recently produced biomass chars using this approach. In total, 45 samples were analysed by Raman spectroscopy for this survey. Ancient and buried char samples displayed higher intensities for features in the Raman spectra associated with amorphous carbon.
Highlights
Raman spectroscopy measurements have previously been used for evaluating the heat treatment temperature (HTT) of chars produced from Japanese cedar and proposed as a means of quality control for charcoals [9]
Based on correlations found between the HTTs and Raman results in a previous study of low-ash chars [6], ‘effective HTT values’ could be calculated for each sample in the survey
A wide range of chars and other carbonaceous materials were provided for this Raman survey and these samples represented a diverse selection of precursors and different production methods
Summary
Even within a single processing technology, the production conditions can often be varied by altering factors such as the heat treatment temperatures and the amount of time the material spends at elevated temperatures. These precursor and processing variables mean that chars (or biochars) represent a wide range of different materials with a range of morphologies, porosities, nanostructure, chemistry, and properties. Production temperature is widely regarded to be one of the most important factors for the development of the carbon chemistry/nanostructure within the chars and the highest heat treatment temperature (HTT) is used as a common signpost for charring intensity or the degree of thermochemical alteration/transformation [2,3].
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