NOx formation in a bed of fuel particles is studied by modeling and by experiments with a continuously weighted pot furnace. The bed was ignited at the top and ignition front propagated down against the airflow. A sharp decrease in NO was usually detected just after the ignition wave reached the bottom of the bed. There was a peak at the end, when practically all organic mass had been combusted. A numerical model for the combustion and gasification in the bed including NO formation was developed. The measured NO emissions were interpreted by model calculations. Different factors such as air rate, fuel moisture, and particle size affecting NO formation is studied, and ways to decrease NO formation are discussed.