Investigation of carbonation of wheat stems from central Europe during slow pyrolysis at different temperatures
Abstract
Slow pyrolysis of wheat stems from south -west Thuringia in Central Germany favours the interaction of lignin, cellulose and hemicellulose. As a result, after low pyrolysis temperatures of 600 °C, 800 °C or 1000 ° C crystallinity of bio - carbon is pronounced. As shown by Raman spectroscopy, with increasing pyrolysis temperature, the intensity ratio I D/IG of D band to G band increases. This gives evidence of a growing amount of aromatic graphitic rings with a lot of disorder in the carbonaceous biogenetic material. With increasing pyrolysis temperature more 6 -fold, still defective carbon rings form out of the amorphous sp2-C-C matrix. High resolution transmission electron microscopy identifies parallel layers of crystalline carbon that are oriented in bands after pyrolysis at 800 °C. This degree of order after a relatively low pyrolysis temperature is excellent. For many applications, regulated and oriented crystallinity is essential. Steering the orientation and fraction of crystalline bio -carbon could be favourable for producing carbon fibres of higher quality from suitable biomass. By regulating the composition of lignin to cellulose, hemicelluose or other substances, the bio -carbon crystallinity could be adjusted .
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Introduction
Pyrolysis products obtained from natural sources provide bioenergy and carbons with advantageous structural configurations. Such carbons are mainly composed of aromatic carbon with several degrees of aromaticity (Mohanty et al., 2013). There are plenty of highly sophisticated applications for carbon materials from biological sources. The pyrolysis conditions as well as the selected biomass type and composition influence the suitable applications strongly.
Pyrolysis of biomass for biochar as a soil remediation usually demands lower pyrolysis temperatures around 300 °C until 600 °C or slowly higher. Such biochars significantly increases seed emergence, soil and crop productivity (Anawar et al., 2015; Mohanty et al. 2013). Properties like high surface area, high porosity, variable charge, and presence of special functional groups increase soil water-holding capacity, pH, cation exchange capacity (CEC), surface sorption capacity, base saturation, and crop resistance to disease, which varies with pyrolysis conditions or biomass type (Singh et al., 2010). Biochar strongly sorbs salts (Thomas et al., 2013) and ameliorates salt stress effects on plants in contaminated soils or agricultural areas recovered from marine water in cost region. It was shown, that the addition of biochar to soil increases the availability of P and Zn, and total N concentration, and hereby, it increases crop yields (Blackwell et al., 2010).
Carbonisation at higher temperatures effects high performance functional and/or extraordinary mechanical properties. In example, multiwalled carbon nanotubes (MWCNTs) synthesis is promoted by selected bio-carbon, like bamboo charcoals, which contained Mg SiO calcium silicate, responsible for the nucleation and growth of MCNTs at 1200-1400 °C (Zhu et al., 2 4 2012).
In most cases, an additional chemical treatment or thermo-mechanical pre-treatment before or during the pyrolysis is essential. Wu et al. (Wu et al., 2015) suggest an one-step carbonisation of alkali-treated wheat for the synthesis of threedimensionally (3D) interconnected honeycomb-like porous carbon foam (HPC). The HPC electrode exhibits a high specific capacitance, outstanding electrochemical stability, and delivers an ultrahigh energy density much higher than most carbonbased supercapacitors. While wheat grains are produced for human feeding mostly, wheat straw is annually generated in abundance worldwide 529 million tons/year (Buranov et al., 2008). North America is the largest producer with 15 % of global wheat production after Asia with 43 % and Europe with 32 % (Kim and Dale, 2004).
The present study focuses on investigation of pyrolytic carbon of wheat straw from Germany. It clarifies microstructure by SEM and Raman spectroscopy after pyrolysis at different temperatures. Due to the relatively high pyrolysis temperatures, the intended applications of the produced biogenic cellular wheat straw carbon is envisaged in several areas including structural (lightweight design) and functional advantages (adsorption of gases or catalytic effects).
Conclusion
Wheat stems contain cellulose, hemicellulose, lignin, and maybe residues from the soil. In the present study, wheat stems of south -west Thuringia in Central Germany were characterized and pyrolysed at 600 °C, 800 °C and 1000 °C top temperatures. Raman spectroscopy confirm a good crystallinity of carbon already after heating up to 600 °C. In addition, most of the shrinkage, namely 73 %, is reached at 600 °C already. Furthermore, HR-TEM images give evidence of crystalline carbon fractions in an amorphous matrix. Parallel layers of crystalline carbon are oriented in bands after pyrolysis. There is a goo d orientation of the graphites basal planes. This result is surprising, because lignin is known to retard crystallisation and tends to convert into amorphous carbon. Apparently, the co -pyrolysis or interaction of lignin with cellulose and/or hemicellulose seems to be favourable for improving carbon fraction in crystalline structures. I n Steering the orientation and fraction of crystalline bio -carbon could be favourable for producing carbon fibres of higher quality from suitable biomass.
