The integration of heat pumps with biomass boiler flue gas for generating hot water

Abstract

Achieving the goal of net zero greenhouse gases by 2050 requires reducing demand and switching from fossil fuels to renewables. Process heat in New Zealand is a significant source of greenhouse gas emissions. Transitioning to biomass boilers could lower emissions and offer opportunities for waste heat recovery by integrating heat pumps with the flue gas. This thesis evaluates heat pump integration for hot water production, supplying low temperature process heat. Using an industrial case study, a boiler model is developed and confirmed against industrial measurements. Flue gas composition is modelled to determine temperature-enthalpy profiles, and therefore the heat available for recovery and upgrade. Combustion is modelled for different biomass fuels and varying flue gas oxygen levels. The higher moisture fuels showed increased availability of heat for utilisation, which decreased as flue gas oxygen levels increased. Three heat pump cycles are explored to maximise efficiency. Refrigerant selection is considered with respect to environmental factors and thermodynamic suitability. Using a basic cycle heat pump the three most promising refrigerants, ammonia, propane and cyclopropane, were identified and considered in more detail for the industrial case. Subcooling improved efficiency and should be maximised. An internal heat exchanger also improves efficiency and reliability and should be considered. Two zeotropic mixtures were investigated to take advantage of the temperature glide. The propane-pentane (50wt%-50wt%) blend matched the temperature profile more closely and provided increases in COP. To make the most of the latent heat in the flue gas, the evaporator temperature must be much lower than the dew point. However, absorbing heat from low source temperatures results in lower COPs, but higher volumes of hot water generated.