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The combination of temperature gradient and volatile release has been identified as the main cause for primary fragmentation (breakage of fuel particles during devolatilization). A mathematical model of the primary fragmentation in a fluidized bed has been developed, incorporating both causes. It takes into account the type of the coal, size of the coal particles, and the fluidized bed temperature. The model simulates fragmentation of a batch of coal particles. For each particle in the batch, the model follows propagation and merging of cracks, starting from randomly distributed pre-existing pores, leading to possible breakage of the particle. The model calculates volume of the fragmented particles and volume diameters, classifying them into size classes. For each size class, the number of particles is counted, and the mass fraction is calculated. The results are the distribution of mass and number of char particles after the devolatilization and the primary fragmentation parameters.

There is a growing need for the use of alternative and renewable energy sources, in terms of sustainable energy development. One of the potentially biggest renewable energy sources in Serbia is agricultural biomass, quite available and cheap at the moment. For many years, the Vinca Institute of Nuclear Sciences, Laboratory for Thermal Engineering and Energy, has been working on the development of clean technologies for efficient utilization of biomass, and as a result utilities and equipment (boilers and furnaces) with wide range of use have been developed. These devices have complete combustion control and primarily burn baled biomass with no adequate utilization. The results of the development and of the tests of an experimental demonstrativel boiler burning small bales of soya straw have been presented in this paper. In the boiler, the combustion has been organized on the 'cigarette burning' principle, and fuel feeding has been done by gravity. The technical scheme of the boiler and the results of preliminary tests carried out in real operation conditions have been given.

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Harmonization of environmental protection and the growing energy needs of modern society promote the biomass application as a replacement for fossil fuels and a viable option to mitigate the greenhouse gas emissions. For domestic conditions this is particularly important as more than 60% of renewables belongs to biomass. Beside numerous benefits of using biomass for energy purposes, there are certain drawbacks, one of which is a possible high emission of NO x during the combustion of these fuels. The paper presents the results of the experiments with multiple biomass types (soybean straw, cornstalk, grain biomass, sunflower oil, glycerin and paper sludge), using different combustion technologies (fluidized bed and cigarette combustion), with emphasis on the emission of NO x in the exhaust gas. A presentation of the experimental installations is given, as well as an evaluation of the effects of the fuel composition, combustion regimes and technology on the NO x emissions. As the biomass combustion took place at temperatures low enough that thermal and prompt NO x can be neglected, the conclusion is the emissions of nitrogen oxides primarily depend on the biomass composition -it is increasing with the increase of the nitrogen content, and decreases with the increase of the char content which provides catalytic surface for NO x reduction by CO.