Ammonia and methane emissions during drying of dewatered biogas digestate in a two-belt conveyor dryer

- During drying of dewatered digestate NH3 is released to the environment.
- During digestate drying CO2 is released but no CH4 above background level was found.
- NH3 emissions can be reduced up to 94% by using ammonia scrubbers.
- The release of NH3 and CO2 does not depend on the drying temperature.
- During drying 80% of NH4-N was released, P and K remain in the dried product.

Aim of the present study was to identify type and amount of emissions during the drying of biogas digestate in a two-belt conveyor dryer at different temperature settings and to investigate the effect on its nutrient content. Furthermore, the possibility of recovering nitrogen from the exhaust air was investigated. Emissions of CH4, CO2 and NH3 were measured by Fourier transform infrared spectroscopy. Biogas is mainly composed of CH4 and CO2, hence gas release from the digestate during drying was expected to increase the concentration of these components. Although CO2 concentration was elevated above the background concentration, CH4 did not exceed the background concentration. Maximum NH3 concentration of 183.3 mg·m−3 was detected during drying. A NH3 concentration of 10.8 mg·m−3 was measured in the exhaust air of the ammonia scrubber, which is equal to a NH3 reduction rate of 94%.