Waste to Energy: Solid Fuel Production from Biogas Plant Digestate and Sewage Sludge by Torrefaction-Process Kinetics, Fuel Properties, and Energy Balance

Abstract

Sustainable solutions are needed to manage increased energy demand and waste generation. Renewable energy production from abundant sewage sludge (SS) and digestate (D) from biogas is feasible. Concerns about feedstock contamination (heavy metals, pharmaceuticals, antibiotics, and antibiotic-resistant bacteria) in SS and D limits the use (e.g., agricultural) of these carbon-rich resources. Low temperature thermal conversion that results in carbonized solid fuel (CSF) has been proposed as sustainable waste utilization. The aim of the research was to investigate the feasibility of CSF production from SS and D via torrefaction. The CSF was produced at 200~300 °C (interval of 20 °C) for 20~60 min (interval 20 min). The torrefaction kinetics and CSF fuel properties were determined. Next, the differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of SS and D torrefaction were used to build models of energy demand for torrefaction. Finally, the evaluation of the energy balance of CSF production from SS and D was completed. The results showed that torrefaction improved the D-derived CSF’s higher heating value (HHV) up to 11% (p < 0.05), whereas no significant HHV changes for SS were observed. The torrefied D had the highest HHV of 20 MJ∙kg−1 under 300 °C and 30 min, (the curve fitted value from the measured time periods) compared to HHV = 18 MJ∙kg−1 for unprocessed D. The torrefied SS had the highest HHV = 14.8 MJ∙kg−1 under 200 °C and 20 min, compared to HHV 14.6 MJ∙kg−1 for raw SS. An unwanted result of the torrefaction was an increase in ash content in CSF, up to 40% and 22% for SS and D, respectively. The developed model showed that the torrefaction of dry SS and D could be energetically self-sufficient. Generating CSF with the highest HHV requires raw feedstock containing ~15.4 and 45.9 MJ∙kg−1 for SS and D, respectively (assuming that part of feedstock is a source of energy for the process). The results suggest that there is a potential to convert biogas D to CSF to provide renewable fuel for, e.g., plants currently fed/co-fed with municipal solid waste.

Medical Peat Waste Upcycling to Carbonized Solid Fuel in the Torrefaction Process

“…The experimental mass loss in specific temperatures can be seen in Figure A2a, while mass loss predicted using calculated k values in Figure A2b. In the previous work [39], we summarized and recalculated serval waste material torrefaction kinetics by the same method. The graphic of k values at specific temperatures for wood refuse-derived fuel (RDF), sewage sludge, and digestate is shown in Figure A3.…”

“…Experiment results showed material decomposition in the function of time and allowed for the determination of decomposition kinetics. Moreover, TG-DSC results were used for the calculation of the theoretical mass and energy balance of the torrefaction process (Section 2.2.5) according to previous work [39]. The decomposition kinetics were determined using the well-known Coats-Redfern method (CR) [40].…”

“…Figure A3. The constant reaction rates (k) in function of temperature for different materials at specific temperatures[39].…”

Scenario Analysis for Selecting Sewage Sludge-to-Energy/Matter Recovery Processes

“…The rapid increase depends on two main factors. The first one is strictly related to the increase in world population; the second one is the indirect consequence of the forced implementation of the European Council Wastewater Treatment Directive 97/271/EC, which requires a higher quality of the effluent treated with the unavoidable increase of the SS produced as a byproduct of wastewater treatments [9].…”

Pine Wood and Sewage Sludge Torrefaction Process for Production Renewable Solid Biofuels and Biochar as Carbon Carrier for Fertilizers

“…Due to the carbonization of biomass taking place during torrefaction, the biomass obtains properties that make it similar to coal [31][32][33][34][35]. Torrefied biomass is used not only as a renewable fuel in the energy sector [36][37][38][39][40][41][42][43][44]. Due to its developed specific surface and microporosity, it can also be used in environmental protection to remove pollutants from water and sewage [45,46].…”