Abstract
Fuelwood is the primary source of heat energy for tea processing, but its availability is declining due to population growth and logging restrictions. This study aimed to optimize the economics of biomass fuel mixtures for tea processing boilers by integrating macadamia nutshells as a supplementary fuelwood. The objective was to develop a cost-effective fuel mix strategy using Response Surface Methodology (RSM) and MATLAB simulations. The methodology involved proximate and ultimate analyses to assess the energy potential of fuelwood and macadamia nutshells. Various scenarios of moisture content and wood availability were simulated to determine the necessary quantities of macadamia nutshells to address fuelwood shortages. RSM was then applied to optimize the fuel mix by minimizing costs while maximizing energy efficiency. Key findings revealed that macadamia nutshells have a higher bulk density (680-745 kg/m³) and lower moisture content (7.86-10 %) than eucalyptus wood (322-358 kg/m³, 15-50 % moisture content). Additionally, macadamia nutshells have a superior calorific value (21,296.56 kJ/kg) compared to eucalyptus (18,765.24 kJ/kg), though they are more expensive (USD 0.10/m³ vs. USD 0.04/m³). The regression analysis showed that moisture content significantly increased fuel costs (18 % per unit increase), while wood availability reduced costs by 17 % per unit increase. The quadratic model (R(2) = 0.9995) confirmed these interactions. The study supports the use of macadamia nutshells as a viable alternative or supplementary fuel source, enhancing the sustainability of tea processing operations.