## 1 Introduction  
Among the multitude of energy sources used to generate both electricity and heat, biomass is considered the most promising fuel in many countries [1; 2; 3; 4]. This opinion is confirmed by the fact that statistically, biomass is one of the most-used energy sources in the world [2]. An even greater increase in the share of energy derived from the combustion of wood biomass was seen in energy plants such as willlow (_Salix stimuli_) or poplar (_Populus tremula_). However, in retrospect, these systems turned out to be economically unprofitable. The reason for this is that growth is not fast enough to achieve profitable biofuel production [5]. Therefore, various, often highly advanced technologies began to be used to transform the energy contained in the wood of trees [6; 7; 8; 9]. From the point of view of energy use of fuel (including wood), some of the most important physicochemical parameters are the heat of combustion and the calorific value [10; 11]. The aforementioned parameters may have different--often very different--values, even for the same fuel. An example is birch wood, which in an air-dry state can reach a calorific value from 17.90 MJ\(\cdot\)kg\({}^{-1}\)[12] to 18.72 MJ\(\cdot\)kg\({}^{-1}\)[13]. The heat of combustion is influenced by many factors, including the content of nonflammable parts (often due to contamination of the wood) and the water content. Water in wood may be present in the form of transient moisture, which is the result of the material and the surrounding air reaching the moisture balance, as well as permanent moisture, i.e., that stored in the cell walls and in the spaces between dead cells [14; 15]. Usually, the determination of the heat of combustion is carried out with the use of a calorimeter. The combustion process in the calorimetric bomb is automatically registered. Due to this, it is possible to determine the duration of individual combustion stages and the temperature increase, which is an indirect measurement of the temperature of the medium receiving heat from the bomb body [16]. As a result of this registration, a time-temperature characteristic is created. To obtain reliable data, it is also necessary to designate the so-called corrected temperature-rise and corrections for heat exchange between the calorimeter and the thermostat [17].  
Some researchers suggest that using appropriate conditioning of biological material, it is possible to modify the heat of combustion of this material [18; 19; 20; 21; 22]. Thermal methods constitute a large group among the methods of pretreatment of biological material. The action of the temperature factor contributes to the improvement of the mass exchange conditions, but at the same time, irreversibly changes some properties of the conditioned products, such as their color or the amount of biologically active compounds [23; 24; 25]. However, from the perspective of wood used for combustion in energy-generating installations, these negative effects are of minor importance. Among the techniques of nonthermal processing, the use of high voltage pulses, which generate a time-varying electric field--PEF (Pulsed Electric Field), is becoming interesting and gaining popularity [26; 27; 28]. PEF causes the effect of electroporation in biological material, i.e., changes in the structure of the cell membrane by enlarging the existing or creating new pores of the membranes. The resulting perforation of the natural structure of the cell membrane causes the diffusion of water. Thus, the resistance to mass movement is reduced, which contributes to osmotic dehydration, acceleration of the drying process, and improvement of the extraction conditions [29; 30].  
The aim of the research presented in this study was to determine the effect of pulsed electric field on the time-temperature characteristics of the heat of combustion of selected broadleaved wood species by determining the combustion curve for each of the analyzed variants.