The present work studies the possibility of energy recovery by thermal conversion of combustible residual materials, namely tires and rubber-plastic, plastic waste from outdoor luminaires. The waste has great potential for energy recovery (HHV: 38.6 MJ/kg for tires and 31.6 MJ/kg for plastic). Considering the thermal conversion difficulties of these residues, four co-combustion tests with mixtures of tires/plastics + pelletized Miscanthus, and an additional test with 100% Miscanthus were performed. The temperature was increased to the maximum allowed by the equipment, about 500 °C. The water temperature at the boiler outlet and the water flow were controlled (60 °C and 11 L/min). Different mixtures of residues (0–60% tires/plastics) were tested and compared in terms of power and gaseous emissions. Results indicate that energy production increased with the increase of tire residue in the mixture, reaching a maximum of 157 kW for 40% of miscanthus and 60% of tires. However, the automatic feeding difficulties of the boiler also increased, requiring constant operator intervention. As for plastic and rubber waste, fuel consumption generally decreased with increasing percentages of these materials in the blend, with temperatures ranging from 383 °C to 411 °C. Power also decreased by including such wastes (66–100 kW) due to feeding difficulties and cinder-fusing problems related to ash melting. From the study, it can be concluded that co-combustion is a suitable technology for the recovery of waste tires, but operational problems arise with high levels of residues in the mixture. Increasing pollutant emissions and the need for pre-treatments are other limiting factors. In this sense, the thermal gasification process was tested with the same residues and the same percentages of mixtures used in the co-combustion tests. The gasification tests were performed in a downdraft reactor at temperatures above 800 °C. Each test started with 100% acacia chip for reference (like the previous miscanthus), and then with mixtures of 0–60% of tires and blends of plastics and rubbers. Results obtained for the two residues demonstrated the viability of the technology, however, with mixtures higher than 40% it was very difficult to develop a process under stable conditions. The optimum condition for producing a synthesis gas with a substantial heating value occurred with mixtures of 20% of polymeric wastes, which resulted in gases with a calorific value of 3.64 MJ/Nm3 for tires and 3.09 MJ/Nm3 for plastics and rubbers.
The present study evaluates the digestion of cork boiling wastewater (CBW) through a biochemical methane potential (BMP) test. BMP assays were carried out with a working volume of 600 mL at a constant mesophilic temperature (35 °C). The experiment bottles contained CBW and inoculum (digested sludge from a wastewater treatment plant (WWTP)), with a ratio of inoculum/substrate (Ino/CBW) of 1:1 and 2:1 on the basis of volatile solids (VSs); the codigestion with food waste (FW) had a ratio of 2/0.7:0.3 (Ino/CBW:FW) and the codigestion with cow manure (CM) had a ratio of 2/0.5:0.5 (Ino/CBW:CM). Biogas and methane production was proportional to the inoculum substrate ratio (ISR) used. BMP tests have proved to be valuable for inferring the adequacy of anaerobic digestion to treat wastewater from the cork industry. The results indicate that the biomethane potential of CBWs for Ino/CBW ratios 1:1 and 2:1 is very low compared to other organic substrates. For the codigestion tests, the test with the Ino/CBW:CM ratio of 2/0.7:0.3 showed better biomethane yields, being in the expected values. This demonstrated that it is possible to perform the anaerobic digestion (AD) of CBW using a cosubstrate to increase biogas production and biomethane and to improve the quality of the final digestate.
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