Digestate as bio-fuel in domestic furnaces

• The digestate from a biogas power plant was investigated as fuel in a common air furnace.
• The digestate drying process was optimized in order to achieve the maximum higher heating value of the fuel.
• The digestate was pelletized and tested in an air furnace.
• Ash sintering issues were found using pure digestate; mixed digestate–wood pellets were created to overcome this problem.
• The mixed pellets were suitable for combustion in the air furnace.

This study investigates the use of the biogas power plants byproduct (digestate) as biofuel in an ordinary domestic air furnace. The digestate, disposed by a 1 MW biogas plant located in Italy, was dried out and pelletized in order to be used as fuel in a wood pellet furnace with 29 kWth of nominal power, commonly installed in industrial HVAC systems. The first test was carried out starting from a heavily dried pellet called “digestate 0” characterized chemically and physically in order to obtain its composition, while its ashes were tested using an optical thermal dilatometer for the softening point evaluation. This first test outlined that the “digestate 0” pellets were not suitable for combustion applications even when mixed with an equal part of pure wood pellets. The research then focused on the raw digestate drying process through a set of physical and chemical tests. It was found that a temperature of 150 °C maximizes the higher heating value of the new “digestate 1” at 16.6 MJ/kg. However, to further avoid the ash sintering, “ultimate digestate” pellets were prepared mixing 50% of “digestate 1” and 50% of wood. The digestate obtained in such a way was experimentally tested through several runs of the air furnace. In these tests, the overall efficiency as well as the furnace emissions was measured.