Characterization of biogas digestate for solid biofuel production

Abstract

In this study, suitability of biogas digestate which is normally used as a fertilizer but wasted during the dry season when farm activities are at standstill, was investigated as a solid fuel for thermal applications. The digestate was obtained at different retention times from laboratory scale and household digesters and characterized. Carbonized briquettes were produced from the digestate followed by physico-chemical characterization and assessment of their combustion and mechanical properties. Results of the proximate analysis of the digestate were: moisture content (6.1 to 18.3%), volatile matter (27.9 to 47.7%), ash (15.0 to 48.9%), and fixed carbon (9.1 to 17.1%). The ultimate analysis results for the digestate were: Carbon (19.5%), Hydrogen (3.3%), Oxygen (20.8%), and Nitrogen (7.0%). The developed briquettes showed a moisture content, volatile matter, ash, and fixed carbon in the range of 3.7 to 8.9%, 9.9 to 21.5%, 45.6 to 76.4%, and 8.2 to 22.8%, respectively. Their ultimate analysis results were: Carbon (21.1%), Hydrogen (1.3%), Oxygen (1.8%), and Nitrogen (1.9%). The briquette combustion properties revealed an ignition time, burning rate, and water boiling time of 5.35 seconds, 0.16 g/min, 31.1 minutes, respectively with higher and lower heating values of 14.87 and 7.88 MJ/kg, respectively. The briquette ash compounds were Sodium 1718.5 ppm, Potassium 20017.8 ppm, Copper 6.12 ppm, Cadmium 1.22 ppm, and Lead 25.6 ppm. TGA/ DTG analysis indicated high mass loss rates at 105 °C and maximum energy release between 600 and 900 °C. The mechanical compressive strength was between 19 and 50 MPa, with bulk density between 1.82 and 2.02 g/cm3. The higher calorific value of 14.87 MJ/kg therefore provides enough heat. Thus, the briquettes from the biogas digestate demonstrate potential for domestic thermal applications in Uganda.