Use of biogas digestate as solid fuel

A common biogas plant with a power of 500 kW emits more than 10,000 t of digestate per year with a dry matter of about 10%. Up to now, such digestates were used as fertilizer. Yet, using the dried digestate as solid fuel seems to be a promising alternative. In this study, two digestates were pressed into pellets and burned in a biomass heating unit. The composition of the flue gas (O-2, CO2, NOx, and CO) and dust were measured, as well as fuel properties of digestate. The fusibility and the basic composition of the combustion chamber ash was determined according DIN 51730 and by using X-ray fluorescence (XRF). Chemical composition and physical properties of digestate fuel pellets depended on the mixture of substrates used as feedstock for biogas production. The digestates investigated in this study can be recommended as a fuel for combustion. The calorific value, the ash properties and the emissions allow their use in the investigated solid biomass combustion unit.