Microwave heating processes involving carbon materials

TL;DR: In this paper, the microwave-assisted processes in which carbon materials are produced, transformed or used in thermal treatments (generally, as microwave absorbers and catalysts) are reviewed and compared with those obtained by means of conventional (non-microwave-assisted) methods in similar conditions.

About: This article is published in Fuel Processing Technology.The article was published on 2010-01-01 and is currently open access. It has received 866 citations till now. The article focuses on the topics: Microwave.

Summary

1. Introduction to the microwave heating of carbons

• Microwaves lie between infrared radiation and radiowaves in the region of the electromagnetic spectrum.
• The interaction of charged particles in some materials with the electric field component of electromagnetic radiation causes these materials to heat up.
• Due to these advantages, microwaves are used in various technological and scientific fields in order to heat different kinds of materials [2-4].
• Among these materials, carbons are, in general, very good microwave absorbers, so they can be easily produced or transformed by microwave heating.
• Few research groups have determined the dielectric loss tangents of carbons and the data that can be found are scattered throughout bibliography.

2. Synthesis of carbon materials

• Microwave plasma-enhanced chemical vapor deposition (CVD) has been widely used for growing carbon nanotubes [19, 20] or diamond deposition [21].
• The synthesis of nanocarbons by the direct microwave irradiation of catalyst particles on a solid support is another possibility for producing carbon nanofilaments by CVD [22, 23].
• A microwave-based device specially designed for the production of expanded graphite has been patented [34].
• Thus, microwave heating can be used to facilitate the polymerization or curing of polymers by eliminating solvents.
• By using microwave drying, the process for obtaining carbon gels has been greatly simplified.

3. Production, modification and regeneration of activated carbons

• Activated carbons are, in general, produced from different organic precursors, such as biomass, coal, polymers, natural or synthetic fibres, etc, which are subjected to carbonization and activation processes.
• On the other hand, chemical activation consists in the simultaneous carbonization of the precursor with an activating agent, such as ZnO, H3PO4, KOH, etc, at temperatures ranging from 673 to 1073 K. Both microwave-assisted activation processes have been recently reviewed by Yuen and Hameed [46].
• This effect has been used to produce carbon molecular sieves (CMS) by subjecting acrylic textile fibres to microwave action [56].
• In particular, CMS produced by this method showed a very high selectivity for CO2/CH4 and O2/N2 gas separations [57].
• It was found that the rapid heating of the exhausted activated carbons by microwave energy leads to a quick regeneration.

4. Metallurgy and mineral processing

• Microwave heating has been investigated for use in various metallurgical processes, including pyrometallurgy, hydrometallurgy [63] and mineral processing [3].
• Yet in the particular case of processes involving carbons, the microwave-assisted reduction of metal oxides with different carbon materials has been extensively investigated.
• Microwave heating is also employed in the gold mining industry to recover gold from the activated carbon used in the so called carbon-in-pulp process (CIP).
• Then, after the carbon is subjected to an acid wash to remove inorganic compounds, it is regenerated at 923-1123 K in a steam atmosphere to remove other foulants such as flotation reagents, lubricating oils and humic acids which would foul the carbon and undermine its performance.
• Coals are, in general, very poor microwave absorbers (see Table 1), since they do not possess graphene lattices of a size large enough to allow delocalized π-electrons to move in order to couple with the electromagnetic field of the microwaves (i.e., heating by interfacial polarization).

5. Thermal valorisation of biomass and biosolids

• So far, none of the methods used, from land reclamation, such as landfill or organic fertilisers, to incineration, is exempt from drawbacks, like collateral pollution or high costs of treatment.
• Drying, pyrolysis and gasification of the sewage sludge take place at the same time, giving rise to a larger gas fraction with a high syngas (CO+H2) content [79, 84, 85] and to an oil fraction with a low polycyclic aromatic hydrocarbons (PAHs) content [82, 83].
• Unlike other conventional pyrolysis methods and due to the high temperatures that are reached during the process, a partially vitrified solid residue can be obtained by microwave-assisted pyrolysis [80].
• Given that microwave-assisted pyrolysis maximizes the gas fraction obtained (oils are produced but in very small amounts) and the fraction of the carbonaceous residue can be used as microwave receptor and consumed by auto-gasification with the CO2 obtained in the process [72], this method can be used for the thermal valorisation of biomass and biosolids, by producing mainly syngas-rich gases.

6. Microwave enhancement of carbon catalyzed reactions

• Owing to their particularly strong interaction with microwave radiation and high thermal conductivity, graphite and certain other carbons are efficient sensitizers.
• Carbon particles are used to selectively heat the catalyst and substrate without bulk heating the solution.
• A variety of carbon materials, such as activated carbons, metallurgical cokes, chars or anthracite, were used.

7. Conclusions

• Carbon materials are, in general, very good microwave absorbers.
• This explains the increasing interest over the last decade in using them in a wide variety of microwave-assisted thermal processes.
• These processes include the synthesis of a wide variety of carbon materials (i.e., nanostructures, graphite, active carbons, polymers, etc.), the purification or even chemical and/or physical modification of carbons in a quick and controlled way, the enhancement of different processes involving coal, chars or even biomass/biosolids, and the clear improvement in the efficiency of some carbon-catalyzed reactions.
• These processes are attracting considerable attention given their possible use in commercial applications and some of them have already been demonstrated at pilot or even industrial scale.

Figures

Figure 1. Evolution of the number of research papers published on microwave-assisted processes involving carbon materials (Source: Scopus®).

Table 2. Examples of microwave-enhanced heterogeneous catalytic reactions, in which carbons are used as catalysts and microwave receptors

Figure 2. Carbon nanofilaments grown on a carbon fiber. (Reprinted from Ref. [23] with permission from Elsevier).

Figure 3. Comparison of the phenol adsorptive capacities of an activated carbon after various cycles of regeneration: (N850EF) electric furnace at 1123 K in a N2 atmosphere; (N850MW) microwave furnace at 1123 K in a N2 atmosphere; (C850EF) electric furnace at 1123 K in a CO2 atmosphere; (C850MW) microwave furnace at 1123 K in a CO2 atmosphere. Adapted from reference [60].

Figure 5. Variation, with temperature, in the gas composition obtained from the pyrolysis of a biomass residue using conventional (closed symbols) and microwave-assisted (opened symbols) pyrolysis. Adapted from reference [72].

Table 1. Dielectric loss tangents for different carbon materials at a frequency of 2.45 GHz and room temperature, ca., 298 K

Figure 4. Representative coal molecular structure and char after devolatilization. Coal doesn’t possess many delocalized π-electrons. After devolatilization aromatic rings fuse given rise to small graphitic planes with π-electrons free to move.

Frequently asked questions

Q1. What are the contributions in this paper?

In this paper some of the microwave-assisted processes in which carbon materials are produced, transformed or used in thermal treatments ( generally, as microwave absorbers and catalysts ) are reviewed and the main achievements of this technique are compared with those obtained by means of conventional ( non microwave-assisted ) methods in similar conditions.