Effect of Coffee Grounds and Waste Engine Oil as Additives in Food-Processing Biomass Pellet Combustion

The purpose of the present research is to quantitatively assess the combustion characteristics of three types of food-processing biomass – pumpkin shells (PS), bean pods (BP), and cherry stalks (CS) – analyzed both as individual feedstocks and in composite mixtures containing 10% spent coffee grounds (SCG) namely: M1SCG (PS10%SCG); M2SCG(BP10%SCG); M3SCG(CS10%SCG) and 10% waste engine oil (WEO): M1WEO (PS10%WEO); M2WEO (BP10%WEO); M3WEO (CS10%WEO). Additionally, a goal of this study was to evaluate the suitability of two ternary blends M4SCG (30%PS+30%BP+30%CS+10%SCG) and M4WEO (30%PS+30%BP+30%CS+10%WEO) for pellet production according to European solid biofuel standards. Calorific values were measured with a model 6200 combustion calorimeter in accordance with ASTM D5865. Nitrogen content was evaluated through the formation of nitric acid, while sulphur content was examined by converting it to sulphates and determining BaSO4 gravimetrically. Ash and moisture content, bulk density (CEN/TS 15103), porosity index, volatile matter (EN ISO 18123:2015), and fixed carbon (ASTM D3172–13) were determined using standardized procedures. Energy density, fuel value index, and combustion efficiency were computed following the established methods reported in the literature.The combustion and physicochemical characteristics of all individual biomasses and biomass-additive mixtures were found to be in accordance with the European standards set. The two composite mixtures M4SCG and M4WEO demonstrated improved calorific performance along with advantageous density-related attributes. Both M4SCG and M4WEO fulfilled essential quality standards, which suggests their technical feasibility as pelletizable biofuels and their potential as sustainable alternatives to fossil fuels. This study provides the first systematic characterization of pumpkin shells, bean pods, and cherry stalks as solid biofuel feedstocks. The first comparative evaluation of SCG and WEO as fixed 10% additives under identical experimental conditions is performed. By integrating underutilized food-processing residues with waste-derived additives, the results extend current knowledge on biomass pelletization, additive-assisted combustion enhancement, and circular waste-to-energy strategies.