## 4 Discussion and Conclusions
Thus, it becomes possible to accelerate certain technological processes based on the production of energy from fuel combustion, or, on the contrary, to extend the time of energy release, which is important, for example, from the point of view of central heating installations in buildings. Therefore, we propose introducing the term of rate of heat of combustion, which will cover both the amount of heat produced and the time of its release. Its basic unit should be the expression kW\(\cdot\)kg\({}^{-1}\), which is also used in some fields to express the so-called specific power. The presented results indicate that birch and ash react to PEF conditioning in a similar manner. The second group, due to the similarity of the obtained results, comprises oak and linden. Within both groups, the trees were compared in terms of their botany, especially the content of elementary chemical elements, the composition and structure of cell membranes, physical properties (weight density, porosity, contractility), as well as anatomical features (size and arrangement of vessels in the growth ring). It would seem that the main factor of changes should be porosity, which affects electroporation, and this in turn is the basis for modifications caused in wood during treatment with a pulsed electric field. However, the only demonstrated correlation between the obtained results and the biological, physical, and chemical parameters of the tested samples concerned the average content of pentosan-free cellulose in wood. Its values are 45.50% for birch, 45.50% for ash, 42.80% for oak, and 43.16% for linden, respectively [58]. Therefore, it seems that it is the cellulose content (in addition to the electric field strength shown in previous studies) that will be the factor determining the effect caused by PEF in wood. Analyzing the obtained data in terms of their utilitarian use, an important parameter is the energy efficiency of the described process. The highest recorded consumption of electricity needed to generate an electric impulse in the used station is equal to 71.3 mWh, as described in the introduction [31]. Taking into account the amount of material that is introduced into the cell and the number of pulses used in each variant, the energy needed to stimulate 1 kg of sawdust is about 17.1 MJ. Referring again to the introduction to this paper--where, as an example, the amount of energy obtained from the combustion of 1 kg of birch sawdust, which is in the range from 17.90 to 18.72 MJ\(\cdot\)kg\({}^{-1}\)[12; 13], is possible without any analysis--this process is unprofitable from an energy point of view. It should be emphasized, however, that the devices used by the authors were made in 2017, and its subsequent modifications concerned the control elements, and not the components responsible for generating the pulse. It seems that the use of the most modern methods would be able to reduce the amount of energy needed during stimulation, but research is still required to reduce the energy consumption of this process. Likewise, further experimentation is needed to cover a wider range of plants and other electric field parameters, leading to the parameterization of the described phenomenon. Conceptualization, P.K. and E.P. ; methodology, P.K. ; software, E.P. ; validation, P.K. and E.P. ; formal analysis, E.P. ; investigation, E.P. ; resources, P.K. ; data curation, P.K. ; writing--original draft preparation, E.P. ; writing--review and editing, P.K. ; visualization, E.P. ; supervision, P.K. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The data presented in this study are available upon request of the respective author. The data are not publicly available due to the possibility of their commercial use by the unit in which the authors are employed and the use of source data for further research and development works of an accredited laboratory (Polish accreditation number AB 1698), whose staff are the authors of this publication.