Solid products

As can be inferred from the discussion so far on gaseous and liquid products, the nature and yield of solid products of HTC are strongly influenced by both process conditions and type of feedstock. In general, the solid product of HTC is a char that is elementally similar to lignite or sub-bituminous coal (Funke & Ziegler, 2010). In terms of its chemical characteristics, it is higher in carbon and relatively lower in both hydrogen and oxygen than the original feedstock, evidence of both dehydration and decarboxylation. As process severity increases, solid yields will decrease;

however, H/C and O/C ratios will also decrease, resulting in greater energy densification and higher heating values (Berge et al., 2011; Hoekman et al., 2011;

Sevilla & Fuertes, 2009). It has been noted, however, that slightly higher H/C ratios are associated with the HTC char of food waste and anaerobically digested sewage sludge (Berge et al., 2011). Slightly higher H/C and O/C ratios of HTC char compared to natural coal have been widely reported and are evidence of the presence of a higher number of functional groups in HTC char (Funke & Ziegler, 2010; Hu et al., 2010). This will be an important factor in later discussion. Typical values are shown in Figure 5.

Figure 5: Typical coalification diagram (Ramke et al., 2009)

Structural and chemical characteristics of HTC char have been of particular interest in recent years. It has been known for some time that HTC char obtained from non-structural carbohydrates are generally agglomerations of micrometre-sized carbon spheres that result in a sponge-like network of particles; although, feedstock and process conditions determine the exact nature of particle morphology (Titirici et al., 2007). Further, HTC particles exhibit different chemical properties in the core and on the shell of the particle. These differences are related to the fact that fairly stable oxygen bonds are established in the core, and less stable oxygen bonds are found on the shell (ibid.). Accordingly, the shells tend to be hydrophilic and the cores hydrophobic (Figure 6).

Figure 6: Formation of HTC char from cellulose (Sevilla & Fuertes, 2009)

However, longer carbonization times will result in a decrease in shell functional groups, rendering the particles more hydrophobic (He et al., 2013). This has been combined with observations that HTC char particles possess interesting carbon nanostructures (Titirici & Antonietti, 2010) that can be manipulated through the use of different templates or additives. For example, iron ions and iron oxide nanoparticles can both catalyse HTC reactions and influence the morphology of the resulting carbon nanomaterials. Further, the porosity of particles can be increased by performing HTC in the presence of nanostructure silica templates. In addition, the presence of Te nanowires during HTC can direct the formation of carbonaceous nanofibres. Next, hybrid materials can be produced such as carbon nanospheres and nanocables in the presence of noble metal nanoparticles and AgNO₃, respectively.

Finally, carbonaceous nanostructures can be doped with nitrogen to create a complex sponge-like mesoporous system by either adding nitrogen-containing substances to the HTC reaction or by using feedstock materials already high in nitrogen (ibid.).

Figure 7: a) Scanning Electron Microscope (SEM) image of monodispersed hard carbon spherules. b) Transmission Electron Microscope (TEM) image of carbon spheres. c) SEM images of carbonaceous materials. d) TEM image of hollow spheres (Hu et al, 2010)

Figure 8: a) SEM image of carbon nanofibres. b) TEM image of hallow carbon materials. c,d) SEM and TEM images of carbonaceous polymer nanotubes (Hu et al, 2010)

Figure 9: a,b) SEM and TEM images of nanocables with encapsulated, pentagonal-shaped silver nanowires (Hu et al, 2010)

For carbohydrates with structure, particularly those arising from biomass or waste, HTC char products can be quite different. The key determinant of the HTC solid product will be the nature of the crystalline cellulose structure. For so-called ‘soft’ or non-textured biomass, such as pine needles, that lacks an extended crystalline cellulose scaffold, a fairly unstructured collection of hydrophilic and water-dispersible spherical nanoparticles ranging from 20-200μm are observed. Particle size is a factor of process conditions. For ‘hard’ biomass made from crystalline cellulose, such as oak leaves, the original structure of the carbon material, for the most part, is maintained. However, this structure is penetrated by a continuous, sponge-like system of nanopores. This is often referred to as an ‘inverted’ structure of the ‘soft’ biomass (Hu et al., 2010; Libra et al., 2011; Titirici & Antonietti, 2010).

Figure 10: a) SEM image of the ‘soft’ biomass of pine needles before the HTC process; the inset shows an SEM image of after HTC process. b) SEM image of ‘hard’ biomass of oak leaf after the HTC process treatment. C) SEM image of the coexistence of carbon spheres and a microstructured biological tissue. D) SEM image of carbon scaffold replicating of the nonsoluble carbohydrates in rice (Hu et al, 2010)

The overall result of this discussion is that chemical properties, morphology and functionality of HTC solid products can be controlled, meaning carbon materials can be designed for a wide range of applications. One of the key observations that can be made at this time is that as chemical and morphological properties can be manipulated, so can the area and nature of the surface of the HTC char. Of equal importance is that not only can materials be designed for novel applications, but HTC can be viewed as a cheaper and more sustainable method of producing important carbon materials that have traditionally been manufactured by other means and from non-renewable or scarce resources (Titirici & Antonietti, 2010).