Two-in-One Fuel Combining Sewage Sludge and Bioliquid

In this study, we first proposed hybrid sludge as a two-in-one fuel combining bioliquid (molasses and glycerol) with digested sewage sludge. The hybrid sludge is easily prepared by impregnation of sludge with bioliquid, and drying (or carbonization). After drying and carbonization treatment below 250 °C, the embedded bioliquid plays an important role as a artificial volatile matter or fixed carbon, which improves the calorific value of hybrid sludge. The hybrid sludge is denoted as hybrid sewage sludge by Korea Institute of Energy Research (HSK). Furthermore, the HSK is classified by the biomass types such as the glycerol-impregnated HSK (HSK-g) and molasses-impregnated HSK (HSK-m). Compared with digested sewage sludge, the net calorific value (NCV) of hybrid sludges increased by about 9–12 MJ/kg. The blended samples of dried sewage sludge and molasses (or glycerol) showed multistep combustion behavior, whereas hybrid sludge with molasses (or glycerol) confined inside sludge pores showed a single combustion pattern, which has a beneficial influence on an unburned carbon. In addition to such good combustion performance, the hybrid sludge provided a hydrophobic surface by dehydroxylation and decarboxylation reaction during thermal treatment. The hydrophobicity of HSK-m can prevent a falloff in the calorific value and a rise in transportation cost, which might be caused by readsorption of moisture.