Abstract
The global energy demand is continuously rising due to industrial progress. Still, due to limited availability of fossil fuels, the acquisition of sustainable resources has become a major concern. Organic wastes such as food waste and sewage sludge are prominent examples that can be quickly distributed and utilized for commercial purposes in comparison to other renewable energy sources. The London Protocol banned the dumping of organic wastes, so noted to emphasize the importance of their efficient conversion into valuable resources. Food waste exhibits a high calorific value; however, it contains a significant amount of chlorine. On the other hand, sewage sludge possesses a low calorific value but a high ash content (Kim, 2012). To address these characteristics, this study aims to create bio-coal by blending food waste, sewage sludge, and paper sludge in specific proportions, which will be evaluated as a solid fuel. Consequently, the hydrothermal carbonization (HTC) process proves to be well-suited for handling food waste and sewage sludge, both of which exhibit a high moisture content of 80%. Moreover, the heavy metal levels within the bio-coal meet the requirements outlined in the Waste Management Act. Analyzing the bio-coal composition, it was observed that the fixed carbon content is 22.4% and the volatile matter accounts for 66%, which represents the highest values among the bio-coals derived solely from food waste. However, with an increased proportion of sewage sludge in the mix, there was a gradual decrease in both fixed carbon and volatile matter contents, accompanied by an increase in ash content. Regarding the calorific value measurement, the bio-coal produced from 100% food waste exhibited the highest value at 6,510 kcal/kg, while the bio-coal derived from 100% sewage sludge yielded the lowest value at 3,260 kcal/kg. With an increase in the proportion of sewage sludge, there was a decline in the calorific value and volatile content. Nonetheless, fuel ratio analysis indicates an enhancement in fuel quality. By utilizing the van Krevelen diagram analysis, the coal band was refined as the proportion of food waste in the mix increased. Consequently, the H/C ratio decreased, resulting in a fuel ratio that closely aligns with the C-H-O ratio of coal species. Industrial analysis of bio-coal derived solely from food waste revealed the highest volatile and fixed carbon contents. Regarding the ash composition, when bio-coal was produced using 100% sewage sludge and paper sludge, the ash content was determined to be 41.14% and 46.49%, respectively. With respect to the mixing ratio, an increase in the proportion of food waste led to higher volatile and fixed carbon contents, while the ash content decreased. The results of elemental analysis indicated that as the proportion of food waste increased, there was an increase in carbon and hydrogen content, accompanied by a decrease in oxygen content. Notably, higher mixing ratios of food waste yielded greater amounts of volatile and fixed carbon, leading to an overall increase in carbon content. This, in turn, is expected to enhance the fuel characteristics due to the decreased oxygen content and increased hydrogen content. The mixed bio-coal composed of sewage sludge and food waste, it was found that regardless of the mixing ratio, the calorific value was satisfied. Whereas, for mixed bio-coal consisting of paper sludge and food waste, it only met the solid fuel standard of 3,000 kcal/kg when the paper sludge content was below 70%. The thermogravimetric analysis (TGA) results revealed that with an increase in the proportion of food waste in the mix, the weight of bio-coal decreased within the temperature range of 180°C to 500°C. These findings indicate a decrease in ash content as the mixing ratio of food waste increased. View PDF for the rest of the abstract.
