Condensing Economizer for Biomass Combustion Boilers: Technical Design, Performance Analysis and Experimental Validation

This study presents the design, thermodynamic analysis, and experimental validation of a condensing economizer dedicated to biomass-fired heating boilers. The proposed device incorporates a centrally arranged flue-gas distribution chamber and a water-cooled tube bundle that enables recovery of both sensible heat and the latent heat released during water-vapor condensation. The economizer, protected under Patent No. PL247342B1 was evaluated through analytical modelling and laboratory testing performed on pellet boilers with rated outputs of 25–100 kW. The heat-transfer model, which couples convective and condensation mechanisms, demonstrated that the presence of water vapor in the flue gas significantly increases the effective heat transfer coefficient, thereby enhancing the overall energy recovery potential. Prototype units with heat-exchange surfaces ranging from 0.63 to 2.90 m2 achieved thermal outputs between 1.8 and 7.8 kW, with stable condensation occurring when the boiler’s return-supply temperature difference was maintained at ΔT ≥ 25 °C. Experimental results confirmed a substantial reduction in flue-gas temperature (down to 59 °C at nominal load and 34 °C at reduced load), which translated into an increase in gross boiler efficiency to 107% and 119%, respectively. All emission indicators complied with the Ecodesign Directive (EU) 2015/1189, while seasonal efficiency analysis yielded an Energy Efficiency Index of 167.8 (A+++). The findings demonstrate that the integrated counterflow–crossflow economizer significantly improves waste-heat recovery, reduces fuel consumption, and lowers particulate and gaseous emissions. Owing to its modular construction and corrosion-resistant design, the system is suitable for practical implementation in modern biomass heating installations.