Avaliação da eficiência térmica de caldeiras em regime de carga parcial e plena por métodos direto e indireto via modelagem computacional

Abstract

This work aims to analyze, through computational modeling, the influence of boiler operating load on the thermal efficiency of a biomass-fired boiler, evaluating this behavior using both the direct and indirect methods of efficiency determination. To this end, a model was developed in EES (Engineering Equation Solver) software, based on energy balances, a stoichiometric combustion balance derived from the fuel's elemental analysis, and the formulations established in the literature for the direct and indirect methods of boiler efficiency calculation. Four load levels were simulated (100%, 60%, 30%, and 10% of nominal capacity), with operating parameters, excess air, flue gas temperature, and CO contente, obtained from published experimental data for a biomass grate boiler, ensuring greater adherence of the model to real operating conditions. The results showed that thermal efficiency undergoes significant degradation under low-load conditions, particularly below 30% of nominal capacity, and that this relationship is not strictly monotonic, exhibiting non-linear behavior at intermediate loads due to the interaction between increasing excess air and varying flue gas temperature. The comparison between methods showed good numerical agreement between them, while also reinforcing that the indirect method is more suitable for solid fuels such as biomass, since it does not require direct measurement of fuel flow rate and allows individual identification of which loss mechanism is responsible for efficiency degradation under each load condition, with radiation and convection losses identified as the mechanism most sensitive to load reduction. Finally, a gap was identified in the literature regarding the quantitative variation of operational combustion parameters as a function of load for biomass boilers, suggesting as a proposal for future work the conduction of an experimental measurement campaign on a real boiler, in order to validate and refine the relationships adopted in this study.