CARBONIZATION OF CELLULOSE TO VALUE-ADDED CARBON USING SUPERCRITICAL CARBON DIOXIDE (sCO2)

Carbonization of biomass in the presence of supercritical-CO₂ provided carbon solids with enhanced properties and potential to provide sustainable pathway for high-value solid products which are currently resourced from expensive and carbon driven fossil-fuel routes. Carbonization of cellulose using supercritical CO₂ at temperatures of 523 K and 623 K at ∼ 100 bar pressure in a stirred reactor for 1 to 8 hours of residence times was characterized for morphology using scanning electron microscopy (SEM), surface graphitization using Raman spectroscopy, thermal stability using thermogravimetric analysis, and crystallinity using powder-XRD. The solid chars were found to be dominated by clusters of micro-spheres (< 5 μm), especially at temperatures of 623 K. Raman spectroscopy revealed the formation of graphitic crystallite units connected aliphatic (sp3) carbons, suggesting significant graphitization.

Other high-pressure pyrolytic techniques investigated includes solvothermal process such as hydrothermal carbonization (HTC) and other supercritical solvent-based carbonization techniques (Dimitriadis and Bezergianni, 2017; Elliott et al., 2015). HTC involves feedstock carbonization in the presence of water at temperatures of 400–750 K and autogenic pressures greater than 100 bar to reach near-critical water conditions at residence times ranging from 1 to 24 hours. CO₂ is a relatively inert and non-toxic, non-flammable feedstock which is readily available as flue gas at low cost while its low-critical point (304.1 K and 7.38 MPa) makes its production significantly easier compared to other supercritical fluids such as water and alcohols. Low and gas-like viscosity, with high density, zero surface tension and high diffusion capabilities of supercritical CO₂ along with its quadrupole moment, and polar C=O bonds makes it an effective non-polar solvent while also dissolving functional groups such as –OH, –F, and –CO. Supercritical CO₂ for solvothermal carbonization from its non-polar solvent capabilities can improve condensation reactions while the non-polar tar-products can be easily separated out and dissolved in supercritical CO₂, which makes it a better pathway than HTC. In addition, the use of CO₂ results in more carbon utilization, which then also lowers carbon emission into the atmosphere.

The X-Ray diffraction analysis (XRD) patterns of high residence time supercritical CO₂ carbonization (SCC) of char samples obtained at 523 K and 623 K at 5 hours of residence time, see Fig. 1. The results reveal that while the char samples are mostly amorphous, increase in peak value at 23° and ∼ 46° was observed at high temperature which correspond to (002) and (100) crystal planes of graphitic layers in amorphous carbon in the char samples. They can be mildly seen at 523 K with a small increase at 20°. This suggests the growth of graphitic components while the broadband behavior of these peaks suggests the char to be mostly amorphous.

The morphology of filtered and dried char samples using SCC of cellulose using SEM provided the type of carbon structures and porosity attained. Figure 2 shows the morphology of chars obtained at 523 K at residence times if 2 and 5 hours. One can clearly see the increase in porosity and separation of the particles with increase in time from 2 to 5 hrs. The results reveal the presence of linked chains of carbon microspheres as seen in the magnified images. Other disordered particles were also formed while the surface was found to be of low porosity, similar to that from hydrochars. Our results showed the increase in porosity and separation of the particles with increase in time from 2 to 5 hrs. Figure 2(c) shows the partially separate cluster of carbon microspheres along with pores of size less than 1 μm on the connecting surface. This suggests that long residence time of 5 hours at 623 K is favorable to obtain improved graphitic content along with morphology containing carbon microspheres and surface with higher porosity. Increase in SCC temperature provided chars with more porosity and improved microsphere content. These results establish that the chars obtained from SCC of cellulose to be of comparable quality to high-ranking coals. They offer additional advantages over HTC while being sustainable and carbon neutral compared to fossil-fuels. This makes SCC of biomass and biowastes as a novel alternative carbonization pathway which not only converts low-grade carbon neutral feedstocks into valuable carbon but also provides a utility for supercritical CO₂ with lowered carbon emissions compared to any other carbonization pathway.