IMPROVING THE PROPERTIES OF CO-FUEL PELLETS FROM SEWAGE SLUDGE AND RICE STRAW

Abstract

This study investigates an approach to improving the properties of fuel pellets derived from wastewater sludge, which typically exhibit low heating values and poor mechanical strength due to the absence of natural binding components within their structure. Rice straw was incorporated as a co-feedstock in the production of mixed sludge-based fuel pellets. Five mixing ratios were formulated and pelletized, after which their physical and energy-related properties were analyzed. In addition, an economic feasibility assessment was conducted at both laboratory and industrial scales using NPV, IRR, B/C ratio, and payback period as key performance indicators. The results indicate that the incorporation of rice straw enhances the heating value of the mixed pellets, with the value increasing proportionally to the rice straw content HHV 12.56–17.19 MJ/kg. Higher proportions of rice straw also significantly improved pellet durability due to the presence of lignin, which acts as a natural binder that strengthens the structural cohesion of the pellets, thereby reducing the likelihood of breakage. From the economic evaluation, it was found that mixed fuel pellets at laboratory scale begin to show favorable economic returns from the SS50:RS50 ratio onward, yielding an NPV of 6,060.29–48,328.03 THB, an IRR of 4.97–31.19%, a B/C ratio of 105.17–144.64%, and a payback period of 3.00–7.94 years. At the industrial scale, positive economic feasibility was observed starting from the SS65:RS35 ratio, with an NPV of 14,393,556.80–43,271,672.47 THB, an IRR of 8.75–23.64%, a B/C ratio of 111.11–135.17%, and a payback period of 2.85–3.96 years.