| Summary: | A form and dimensions of fuel particles influence the intensity of their burning and approaches to the mathematic description of the process. Known methods do not allow correctly measuring all pellets’ lengths and describing pellets’ lengths distribution. The purpose of the study is to substantiate method for determining the individual pellet length and to specify statistical characteristics of pellets’ lengths distribution. The purpose was achieved by applying the proposed method of indirect determination of the length of each pellet by weighing it, followed by calculation of the equivalent length and modal cluster analysis of the distribution of pellets by length, based on the probability density distribution. The most noteworthy results are that the experimental calculation of the equivalent length gives results that coincide with direct measurements for pellets of the correct shape, but in contrast to direct measurements can also be used to determine the equivalent lengths of irregularly shaped pellets and their fragments. Clustering allowed grouping pellets around objectively existing local maxima in the probability density distribution, which can be identified at intervals of pellet lengths not exceeding 2 mm. The importance of the obtained results is that the indirect method of determining the length of pellets allows replacing the measurement of pellet lengths by their weighing, which eliminates subjective factors when measuring the length of irregularly shaped pellets and their fragments. Clustering characterised the granulometric composition of pellets with histograms of probability, mass fraction, and average length by clusters. Upon using proposed approaches, granulometric composition of industrially produced pellets was specified and increased probabilities were noted for 8 mm pellets in clusters of smaller lengths, compared to 6 mm pellets; while straw pellets are characterised by a higher probability in clusters with shorter lengths compared to wood pellets
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