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In order to obtain and document knowledge about conditions when firing grain in smaller furnaces, an investigation of a newly erected plant has been carried out during the winter 1993-94. The investigation includes measurements and calculations of emissions and efficiency as well as experiences of the operation of the furnace. The combustion plant consists of a 40 kW solid fuel furnace. The furnace is fed through an auger from behind with fuel from a silo of 5 m(sup 3) standing outside and delivers its heat to an accumulator tank of 5 m(sup 3). The construction is fully automated except for the ash handling. During the ordinary operation of the furnace there were test periods of 14 days length for the different fuels. Winter wheat, spring barley and fresh wood chips were fired during the test periods. The output had mean values between 12 and 15 kW. The efficiency of the furnace was rather similar for wheat and barley, 73 and 70 % respectively. The smoke gas emissions of carbon monoxide were during the periods extremely low for wheat and barley, 55 and 27 ppm respectively, while firing with wood chips it was as high as 3300 ppm. For the discharge of nitrogen oxides, NO(sub x), the conditions were reversed. Mean values of NO(sub x) during the periods were for wood chips, wheat and barley 220, 453 and 383 mg/MJ respectively. Ashes from the different fuels varied to a large extent. Ashes from barley and wood chips were very voluminous, which disturbed the combustion and demanded frequent ash cleaning. Wood chips needed ash cleaning every second day and barley every day. The demand of fuel handling also showed great differences. For wheat and barley it was necessary to fill the silo about once a week but for wood chips it had to be filled almost every second day. 7 refs, 24 figs, 11 tabs