Near Infrared Spectroscopic Evaluation of Biochars Made from Wheat Straw: A Second Look at the near Infrared for the Study of Chars with Implications for Coal and other High Carbon Materials

Abstract

Biochars result from the pyrolytic processing of organic materials. There is an increasing interest in the production and use of biochars from agricultural wastes, to sequester C in the soil and improve soil quality. Near-infrared spectroscopy, which has been used for decades to determine the composition of the varying agricultural materials used in making biochars would thus appear to be an obvious method for analysing these materials. However, previous work on charred cellulose, lignin, pine bark and wood showed that, while near-infrared spectroscopy using a Fourier transform spectrometer could be used for quantitative analysis, spectral interpretation beyond the bands found in the 4000 cm−1 to 5000 cm−1 (2500–2000 nm) region was difficult, to near impossible, due to the nature of the char spectra produced, the high degree of baseline curvature and the tendency to be very noisy. The objective of this work was to re-examine the question of near infrared (NIR) spectroscopy and chars using biochars made from wheat straw charred at various temperatures for 3 h. Results indicated that using spectrometer settings which work well for other materials such as soils or forages, especially with a Fourier Transform spectrometer, are not satisfactory for working with biochars or similar materials such as coals. Results obtained with a scanning monochromator, which also scanned a larger sample area (25× to 100× as large), were far superior to those obtained with a Fourier spectrometer with a praying mantis-style diffuse reflectance device and similar scanning conditions. Although previous work has shown that quantitative analysis can be carried out using NIR spectra collected on the same Fourier transform instrument, under the same conditions, these results and those show that qualitative analysis or spectral interpretation in the NIR require a different setup/procedure to be used. Collecting more scans, and at a lower resolution, greatly improved the spectra, although heating the sample could be a problem. Using a different DRIFTS device, which allowed a significantly larger sample area to be scanned, could also be useful, but was not tested.