## 1 Introduction  
Currently, in many countries, the production of energy from biomass constitutes the largest share in the structure of the use of renewable energy, and biomass is believed to be most likely to increase the use of this fuel as a source of renewable energy [1, 2, 3, 4]. Statistically, biomass is one of the three most important sources of fuel used for generating electricity and thermal applications [2]. Nowadays, many modern technologies are used on a global scale to effectively convert biomass into heat, steam energy, as well as liquid and gaseous biofuels [5, 6, 7, 8]. Due to the high potential of wood to be used for commercial biofel production, it is expected to continuously increase wood resources, especially so-called energy plantations. However, from an economic point of view, these production systems are not financially advantageous, and the clones of trees such as willows and poplars produced by cultivation programs do not grow fast enough to obtain profitable biofuel production [9]. The net amount of biomass in wood depends on the tree species but also on the available amount of solar energy, the amount of energy captured by the plant, the efficiency of photosynthesis conversion or the loss of biomass as a result of plant life processes [10]. The most important parameters characterizing wood and other thermally transformed fuels in terms of their energy use are combustion heat and calorific value [11; 12]. These parameters may differ significantly even within the same fuel, which is affected by many factors, and so, for example, pinewood in air-dry conditions may reach values from 18.6 MJ\(\cdot\)kg\({}^{-1}\)[13] to 20.54 MJ\(\cdot\)kg\({}^{-1}\)[14]. The heat of fuel combustion decreases with increasing water content, which in the case of wood is stored in the cell walls and in the spaces between dead cells [15; 16]. When determining the heat of combustion with the use of a calorimetric bomb, the time and temperature of the medium receiving heat from the bomb body are automatically recorded [17]. As a result of this registration, a time-temperature characteristic is created. Measurement of the temperature of individual periods of the combustion process enables to determine the so-called corrected temperature rise needed to estimate the calorific value. In turn, the duration of individual periods is used to determine the so-called amount of heat exchange in the calorimeter, which allows defining the correction for heat exchange between the calorimeter and the thermostat [18].  
It should, however, be noted that there are indications suggesting that the characteristics of the heat of combustion can be modified by proper conditioning of the biological material [19; 20; 21; 22; 23]. The most popular methods of pretreatment of organic substances include thermal treatments, which, while improving the conditions of mass transfer, simultaneously cause several unfavorable changes in the product, e.g., irreversible changes in color or degradation of biologically active compounds [24; 25; 26]. One of the most promising nonthermal pretreatment techniques prior to drying is the use of high voltage pulses that generate a time-varying electric field--pulsed electric field (PEF) [27; 26; 28]. The interaction with a pulsed electric field causes changes in the structure of the cell membrane called electroporation, which is a process of creating new or enlarging already existing membrane pores. The perforation of the natural structure of the cell membrane facilitates the diffusion of water and components dissolved in it. This reduces the resistance to mass movement (accelerates many processes, e.g., drying or osmotic dehydration) and improves the extraction conditions [29; 30; 31].  
The aim of the presented research was to determine the impact of the pulsed electric field on the characteristics of the combustion heat of the selected biological material and further to determine the combustion curve for each combination of the experiment.