## 1 Introduction
However, the production of this variety of valuable products, besides the main equipment (reactor), requires additional equipment and processes for further separation and/or purification, e.g., distillation, in order to obtain a fuel that could be used in automobile applications [2]. In addition, processes such as pyrolysis are endothermic and require a countable amount of energy. Thus, an interesting alternative is the combustion of plastic waste which is an exothermic process; that is, it produces energy and also requires much less equipment for separations, etc. Of course, energy is recovered by using pyrolysis products as fuels. However, similar effects take place during the combustion of polymers. In general, thermal oxidation (combustion, presence of oxygen) and thermal degradation (pyrolysis, absence of oxygen) are distinct processes. However, for polymers, these processes are strongly related. There is a variety of complex chemical reactions during the thermal oxidation of polymers, which also include pyrolysis reactions [11]. These pyrolysis products are combusted and/or react with each other. In other words, during the combustion of polymers, to some extent, similar products as for the case of pyrolysis are produced; simply, these products are directly combusted instead of being separated, purified, and then used as fuels. The complexity of thermal oxidation and degradation of polymers is also related to the basic disadvantage of the combustion of plastics, that is, the formation and emission of various toxic substances, such as polycyclic aromatic compounds, polychlorinated biphenyl, and others [11]. Reactions among decomposition products lead to the formation of other products and hinder complete combustion. Appropriate selection of parameters such as high gas temperature and high residence time can contribute to the minimization of such emissions [11]. In addition, excess oxygen could assist in this direction. Also, there are various available anti-pollution technologies that could be used in order to capture and recover pollutants and/or non-combusted substances. Thus, the combustion behavior of plastic waste is a topic that attracts continuous research attention [11; 12; 13; 14]. Also, the co-combustion of fuels may be related to certain advantages such as increased energy production, lower emissions, and better combustion behavior, e.g., due to lower activation energy arising from synergistic effects. In this direction, behavior and kinetic analysis of the co-combustion of plastic waste or plastic-derived fuel with biomass, e.g., wood, is attracting continuous attention [15; 16; 17]. Various studies have been reported on the advantages of co-combustion of biomass waste with lignite [18; 19; 20; 21]. Plastic waste has been explored for its potential to substitute coal or to be co-fired with coal in cement kilns [14]. To the best of our knowledge, the co-combustion of plastic waste with lignite rarely has been reported. In addition, due to the complexity of polymer degradation, it is common to study the combustion of each polymer separately. Of course, very useful insights are extracted from such studies; however, as mentioned above, there are cases where the separation of the plastic waste is almost impossible, e.g., for plastic waste that has been abandoned for years on the ground's surface. For such plastic waste with random/unknown composition (from municipal solid wastes), combustion may be a viable management solution. Thus, the scope of this work is to study the combustion and co-combustion behavior (including main gas emissions and ash quality) of plastic waste of the MSW (random composition) as well as its blends with lignite.