A part of the carbonization product may be recovered and returned to the microwave reactor, where it would be used as a microwave radiation absorber and it would make it possible to recuperate a part of the heat. A part of the gaseous product may be burnt to provide thermal energy for the feedstock pretreatment (e.g. drying). The process of pyrolysis with a reactor heated by microwave radiation has been schematically shown in Fig. 1.
The basic difference between the processes of conventional pyrolysis and microwave pyrolysis lies in the feedstock heating method. At the conventional heating, thermal energy is transmitted from the surface into the depth of the feedstock by convection, radiation, and conduction. Such a process is relatively slow and requires the feedstock material to be finely ground. Instead, the microwave heating is a process of conversion of electromagnetic energy into thermal energy. The microwave energy induces molecular motion by dipole rotation and ion migration .
The microwave heating is a contactless and fast process, which takes place throughout the entire material volume. Hence, the heat is generated throughout the entire volume of each material particle. The microwave heating is a selective process, i.e. the feedstock behaviour in a microwave field will not be identical for every feedstock material. The materials that are most susceptible to microwave radiation are dielectrics, such as e.g. water or methanol. Materials of this type are referred to as microwave radiation absorbers. The materials that have not dielectric properties will reflect or transmit microwave radiation without being heated. Therefore, the microwave pyrolysis may only be applied to the materials that fully or partly absorb microwave radiation, thanks to which the heating of such materials is possible.
An important process parameter is the residence time. The fast internal heating causes fast release of moisture from the feedstock and, in consequence, a growth in the volatile substances release area during the pyrolysis process. In a microwave reactor, volatile vapours (including small dipolar molecules) loaded with solid particles (chiefly fine particles of the carbonization product) can be easily heated by microwaves to a higher temperature. Therefore, the pyrolytic vapours must be carried away from the reactor very quickly for the secondary cracking of the vapours within the reactor space to be reduced and for water vapour to be removed. An important process parameter is the residence time. The fast internal heating causes fast release of moisture from the feedstock and, in consequence, a growth in the volatile substances release area during the pyrolysis process. In a microwave reactor, volatile vapours (including small dipolar molecules) loaded with solid particles (chiefly fine particles of the carbonization product) can be easily heated by microwaves to a higher temperature. Therefore, the pyrolytic vapours must be carried away from the reactor very quickly for the secondary cracking of the vapours within the reactor space to be reduced and for water vapour to be removed.
The efficient microwave energy transfer to the feedstock in the bed is another important issue related to the microwave reactor. The reactor vessel, made of materials transparent to microwaves (e.g. quartz), must be carefully and appropriately cleaned, especially at continuous operation mode. The presence of solid particles in the vapours and fine carbon particles on the vessel walls will cause significant difficulties in introducing microwaves into the reaction environment or even local burns of the reaction vessel. It is also important that the feedstock mixture should be homogenous for the microwave field action in the reactor to be uniform and for the forming of ignition points (“hotspots”) to be thus avoided.
The hotspots may easily cause the loss of control of local pyrolysis temperature. Thus, of the entire reaction process. When the pyrolysis reaction is completed, the solid residues, chiefly the carbonization product capable of catalysing the secondary cracking in the gaseous phase, must be efficiently separated. Even if the liquid reaction product has been cooled, the carbonization product conduces to the creation of stability problems because it speeds up the polymerization process and raises the viscosity of the liquid reaction product. For this reason, the carbonization product must be separated quickly and completely from the volatile vapours. This process may be carried out by means of solid substance separators such as those used for conventional pyrolysis. A liquid collector used at conventional pyrolysis may be used as well.
The collector is necessary to condense the vapours and 15 The use of microwave pyrolysis for biomass processing to collect the liquid phase. The liquid biofuel obtained from this process usually consists of two fractions, i.e. a water fraction and an insoluble oil fraction. The former one consists of water and water-soluble organic compounds, e.g. furfural, and the latter contains a mixture of the hydrocarbons that occur in oils and require further refining. Optionally, the pyrolysis process may include catalytic reforming, which is employed to improve the process selectivity in respect of the product type preferred (liquid or gas) . In this case, attention should also be paid to the vapour temperature and residence time in order to reduce the secondary vapour cracking.
 RUMIAN M.; CZEPIRSKI L.: Zastosowanie promieniowania mikrofalowego w technologii adsorpcyjnej (The use of microwave radiation in adsorption technology). Przemysł Chemiczny, 84/5 (2005), pp. 329–332.
 YIN C.: Microwave-assisted pyrolysis of biomass for liquid biofuels production. Bioresource Technology, 120 (2012), pp. 273–284.