Intermittent waste heat recovery: Investment profitability of ORC cogeneration for batch, gas-fired coffee roasting

Coffee roasting is a highly energy intensive process with much of the energy being lost in intermittent cycles as discharged heat from the stack. CHP systems have been investigated to provide heat to the roasting process by a micro gas turbine (MGT). However, much of the heat released in a coffee roaster is from the afterburner that heats up the flue gas to higher temperatures to remove volatile organic compounds and other pollutants. In this paper, a solution to utilising waste heat is assessed through energy and material balances of a rotating drum coffee roasting with partial hot gas recycling. A cost assessment methodology is adopted to compare the profitability of three systems configurations integrated into the process. The case study of a major coffee torrefaction firm with 500 kg/hr production capacity is assumed to carry out the thermo-economic assessment, under the Italian energy framework. The CHP options under investigation are: (i) regenerative topping micro gas turbine (MGT) coupled to the existing modulating gas burner to generate hot air for the roasting process; (ii) intermittent waste heat recovery from the hot flue gas through an organic Rankine cycle (ORC) coupled to a thermal storage buffer. The results show that the profitability of these investments is highly influenced by the natural gas/electricity cost ratio, by the coffee torrefaction production capacity and intermittency level of discharged heat. MGT seems to be more profitable than waste heat recovery via ORC due to the intermittency of the heat source and the relatively high electricity/heat cost ratio.