Process-Level Investigation of the Flue Gas Latent Heat Recovery Using a Condensing Heat Exchanger in a Biomass-Fired Boiler

Abrahams, Zaina

Process-Level Investigation of the Flue Gas Latent Heat Recovery Using a Condensing Heat Exchanger in a Biomass-Fired Boiler

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Mechanical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

Biomass-fired boilers offer an opportunity to reduce reliance on fossil fuels in South Africa but are often hindered by efficiency losses. This is particularly relevant for high-moisture fuels where a considerable amount of the combustion energy is consumed in evaporating the fuel moisture. Water vapour exits with the flue gas, carrying the energy used for evaporation as latent heat. Unrecovered latent heat ultimately decreases the overall boiler efficiency. One way to recover a portion of this latent heat is to cool the flue gas below its dew point temperature using a condensing heat exchanger (CHE). This study develops a process-level, steady-state, one-dimensional numerical model of a shell-andtube CHE for flue gas latent heat recovery in a biomass-fired boiler. The heat exchanger model is divided into an Economiser section, where sensible heat is recovered, and a Condenser section, where simultaneous heat and mass transfer occurs once the flue gas temperature drops below the dew point. Flue gas condensation is modelled using the Colburn-Hougen method, with the Ackermann correction factor included to account for potential convective mass transfer induced by vapour condensation. The CHE geometry is discretised into control volumes, and local heat and mass transfer rates are solved simultaneously. The model is validated against experimental data from the literature, with good agreement observed between predicted and measured outlet temperatures and overall heat transfer performance. This confirms that the numerical model can adequately capture the underlying condensation physics to perform engineering feasibility studies and early-stage CHE design. The CHE model is applied to a 4 t/h biomass-fired boiler firing bagasse to evaluate process-level effects. Various heat sink configurations are considered, including make-up water preheating, feedwater preheating, reduced condenser pressure operation, and an independent cooling water stream. The results illustrate a quantified operating envelope in which meaningful latent heat recovery can be achieved for the case-study boiler. When limited heat sinks are available, efficiency improvements from a CHE are marginal. Meaningful efficiency gains can be achieved by significantly reducing condenser pressure or by using an independent cooling water stream. The model highlights the impact of the CHE on key process parameters, which provides an engineering tool for future prototype development and cost-benefit analysis.