Applicability of biogas digestate as solid fuel

Biogas digestate is a byproduct in biogas plants. Using the dried digestate as solid fuel seems to be a promising alternative. Objectives were to verify whether digestate from biogas plants is suitable as a solid biomass fuel and to classify the digestate according to current regulations for biofuels. Combustion experiments in a biomass combustion facility were carried out to ascertain both, emissions and combustion behavior.Two different digestates were used as test fuel and pressed into pellets. Net calorific value of digestate pellets were between 15.8 MJ/kg and 15.0 MJ/kg with water content of 9.2% and 9.9%. Ash content was between 14.6% and 18.3%, with softening temperature between 1090 °C and 1110 °C. Major compounds of ash were calcium 13.6-17.0%, phosphorous 20.4-26.7%, silicon 18.0-30.4% and potassium with 8.5-15.5%. The average concentration of carbon monoxide was between 104 mg/m³ and 275 mg/m³ and 334-398 mg/m³ of nitrogen oxides. Average dust concentration of 100-106 mg/m³ has been detected, which was reduced to 40-43 mg/m³ by using an electric filter.Chemical composition and physical properties of digestate fuel pellets depend on the blend 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. Further investigations are required to cover a broader range of digestates and combustion techniques.