Design and performance analysis of a small-scale prototype water condensing system for atmospheric emissions abatement

This article presents the design and performance analysis of a flue gas condensation system coupled to a biomass plant, that is an objective of the national supported PRIN2017 BIOCHEAPER project. The analysed system is composed of a biomass combustion plant fed by wood chips producing flue gases that are condensed by a double heat exchanger system capable of extracting water and heat from the flue gases and consequently reducing the concentrations of CO, CO2, NOx, and PM, in addition to releasing the gases at a temperature close to that of the external environment. The biomass plant has a capacity of 100 kW and operates at a steady state regime with a fuel consumption rate of 50 kg of wood chips per hour. The fuel material used is composed of chips of ligno-cellulosic material with a lower heating value of 18.98 MJ/kg. After going through the process of heat exchange and cleaning by the cyclone, the combustion process produced gases in output at 430°C. The central condensing works in two stages of cooling. After passing through the cyclone, the gases produced are sent to a first cooling stage consisting of a high-temperature tube heat exchanger (combustion gas/air heat exchanger). Gases enter at a rate of approximately 584 m3/h at 430°C. In this phase, sensible heat is recovered and transferred to the heat carrier (air), and the gases enter the second exchanger at a temperature of 65°C. In the second stage, in the second heat exchanger (flue gas/water heat exchanger) the sensible and latent thermal energy is recovered in the heat carrier (water), producing water condensation. The flue gas is released into the atmosphere at about 33°C. During the operation, all the conditions were measured, including temperatures of flue gases, the mass flow of flue gases, the rate of fuel consumption, the temperature of heat carriers, and the rate of water condensation. According to the tests, the condensing system operating under the described conditions was able to condense water from the flue gas at a rate of about 75 g/min at a temperature of 22°C. In addition, it was observed that the condensation system reduced the emission of some pollutants. A reduction in the order of 20% of CO2, 19% of CO, 30% of NO and 26% of NOx emerged comparing the gas before and after treatment by the condensation system.