## 1 Introduction  
The concept of 'waste to carbon' is the conversion of organic waste into valuable carbon materials, including fuels with a higher concentration of carbon [1]. For that purpose, the torrefaction process has been adapted to different types of organic waste [2; 3; 4; 5]. Tests were carried out indicate that it is possible to convert municipal solid waste (MSW) into carbonized refuse-derived fuel (CRDF) with a calorific value between 24 and 26 MJ/kg, i.e., comparable to that of bitumen coal [6]. In another study [7], lower calorific value of refuse-derived fuel (RDF) increased due to torrefaction from 21.0 MJ-kg\({}^{-1}\) to 25.9 MJ\(\cdot\)kg\({}^{-1}\). As well as this, [PERSON] et al. [8] indicated the increase of RDF lower heating value from 19.7 MJ\(\cdot\)kg\({}^{-1}\) to 21.2 MJ\(\cdot\)kg\({}^{-1}\) due to 90 min torrefaction at 220 \({}^{\circ}\)C. CRDF addresses the problem of MSW management, in particular for MSW of organic origin. The waste can be transformed into CRDF, and thus it is not landfilled and could be used as fuel. The MSW torrefaction is one of the developing technologies, creating new, potential solutions for the 'waste to energy' concept [9]. In Poland, for example, the total annual production of MSW is \(\sim\)12 min Mg [10] of which \(\sim\)2.6 min Mg of RDF was produced in 2016 [11], while only \(\sim\)1.2 min Mg was thermally utilized with energy recovery [11] in cement kilns. The remainder of RDF (\(\sim\)1.4 min Mg) is thermally treated in incineration plants or stored up for future processing. One of the undesired consequences of that is that the number of fires of unprocessed RDF almost doubled from 75 to 132 between 2012 and 2017 [10]. Such a situation enables research in waste (biomass)-derived carbon materials, including the development of novel technology of RDF torrefaction and production of CRDF with improved calorific value fuel properties. One of the requirements for torrefaction technology adoption is that it must be based on the sustainable approach, while the produced CRDF should be safe in handling and use. One of the identified challenges related to the development of MSW torrefaction technology is the potential impact of biochar (including CRDF) on the environment, through emissions of volatile organic compounds (VOCs) [12; 13] from carbon materials. Similar observations of VOCs emission occur during wood pellets storage, causing concerns of indoor air quality [14; 15; 16; 17]. Moreover, the storage, transport, and use of combustible dust poses severe safety issues, as dust may explode when accidentally dispersed in air and ignited [18]. Thus, pelletization of CRDF is also useful to reduce the explosion risk [1]. Additionally, the emitted VOCs may be a source of self-ignition and pose a risk of fire during CRDF storage. However, [PERSON] et al. [19] showed that torrefaction does cause depletion of volatiles components, which increases available surface active sites for oxidation (self-heating). Therefore, the self-heating propensity increases with fixed carbon content in biochar. The development of the torrefaction--a new technology of waste treatment, shifting towards a recycling-based sustainable society, without evaluation of safety aspects, and embarking too readily on the practical application of newly developed CRDF, may trigger similar (fire) disasters, such as that which occurred at the refuse-derived fuel power station in Mie Prefecture in 2003 [20; 21]. Similarly, the development of MSW torrefaction technology focused also on solving the problem of storage and transportation of CRDF, a bulky, powder-like material. Our recent work has shown that pelletization of CRDF can reduce its volume, thus improving the logistics of storage, transport, and use as fuel [1]. Pellets have the additional feature of being a standardized fuel that is easy to use in the fully automatic operation, simplifying the construction and operation of burners [22].