## 1 Introduction  
The current EU policy and legislative framework, including the energy market, is increasingly moving towards sustainable development, energy self-sufficiency, zero emission and circular production. These objectives are being followed up by the policies of individual institutions and countries by adopting the necessary measures, including subsidies or regulations, which affect all areas of the economy [1; 2; 3; 4]. Biomass is an energy source composed of organic materials and natural resources [5]. After traditional fossil fuels such as oil (32%), coal (27%) and natural gas (22%), biomass (9.8%) is the next most important energy source [6; 7]. Compared to traditional fossil fuels, biomass has a number of beneficial characteristics that predispose it to wider use. It is a hydrocarbon fuel with a high oxygen content that has a zero carbon balance. This means that the carbon consumed in the energy generating process was generated entirely during relatively recent short period of time [8]. Utilization of biofuel energy is relatively weather-independent compared to solar, wind and hydro power, and can be produced and consumed locally and adequately sized or managed. However, it is still seasonal and dependent on local agricultural activity. The boiler combustion technology is relatively simple, easily adapting to greater variability in feedstock and use conditions. For these reasons, the combustion of biofuels, and especially those using waste feedstock, is increasingly in demand within the energy mix. A further benefit is that the possibility of using biomass has relatively low cost associated with its conversion into biofuel form [9]. The ongoing structural changes of the energy systems require better use of regional energy resources [10]. For these reasons, it is necessary to carry out a detailed analysis of all regional raw materials that may represent a promising source of energy [11]. Increasingly, therefore, attention is being paid to the use of various types of biomass that, until now, have been considered less suitable for energy use because of their characteristics or lack of availability [12]. There is a growing global trend towards more efficient use of agricultural and food by-products [13]. However, it should be true that only biomass otherwise unsuitable for further efficient use is burned and that it is indeed a true waste material [14]. This article focuses on waste from the food processing industry processing of fruit, grapes and nuts, which are relatively biologically active. This waste cannot be stored for long periods of time or transported over long distances for further use [15]. Improper collection and disposal of this waste, in particular that from the fruit processing industry, can cause serious environmental and ecological problems as well as significant loss of biomass [16]. EU Member States produce approximately 100 million tonnes of biowaste and by-products per year. Of this amount, fruit processing accounts for 14.8% and the distilling industry for approximately 4% [17]. A closer examination of the different types of fruit processed reveals significant species variation in terms of the amount of waste produced. Moreover, waste production for each species is influenced by seasonality, the concentration of cultivation areas in a given area and climatic factors. Depending on the commodity, the type of fruit and its processed parts (seed, pericarp, skin, etc.) and the processing technology chosen, the amount of waste materials can vary significantly. The largest proportion of waste products is generated during the processing of stone fruit (35%), followed by grapes (30%), nuts (30%), pome fruit (12%) and small fruit (3%) [18; 19]. The waste products produced may vary in their characteristics, such as moisture, consistency or even chemical composition, which may have an impact on their overall energy potential [20].