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.