Coffee roasting has a high demand for thermal energy. Roasters typically operate with a hot air temperature stream between 300°C and 540°C and the beans are roasted for a period of time ranging from a few minutes to about 20 minutes.
Roasting machines are usually horizontal rotating drums, centrifugal bowls, fluidised beds or tangential bin roasters where the green coffee beans are tossed around in a flow of hot combustion gases. The roasters operate in either batch or continuous modes and can be indirectly or directly fired.
In the roasting operation the heat is transferred to the beans from hot air. Hot air is drawn through the drum by a fan. The gaseous emissions resulting from roasting operations are typically ducted to a treatment system to reduce VOCs (alcohols, aldehydes or organic acids) and particulate matter (roasters are followed by a cyclone that removes the chaff released by the beans). The energy from these air treatment systems is frequently directly exhausted to the atmosphere.
This best practice focusses on the preheating of the coffee beans immediately before the roasting operation by means of the heat available in cleaned exhaust gases. This energy-saving technique can be combined with other energy-efficient techniques, such as the partial reuse of the roast gases in the same roasting system either directly (roasters with recirculation) or by means of a heat exchanger, or to use the roast gases to produce warm water or to heat buildings. Each of these techniques allows significant energy savings in the roasting operation; from less than 10% in the case of heat exchangers installed in the exhaust gas ducting to approx.30% in the case of roast gas recirculation machines.