Pre-heating of green coffee
In times of high raw material and energy costs, the coffee roasters must not only care about the quality of their roasting processes, but also have to optimise them especially in terms of energy savings and cost reduction. Usually, only a combination of different measures entails a significant reduction of the energy costs. An efficient way to save energy is to pre-heat the green coffee.
Roasting of coffee requires a thermal energy of 700 to 750 kJ/kg of beans by physical laws. This heat transfer into the bean takes place by different thermodynamic processes:
- Conductive (heat transfer through contact with heated surface)
- Convective (heat transfer by hot air)
- Radiant heat
The methods depend on the type of roasting machine and the selected roasting process. The different roasting machines and their heat transfer cause exhaust gas temperatures of about 250 to 450 °C, depending on the roasting process and the exhaust gas cleaning method. This thermal energy of the roasting exhaust air can be recovered by preheating of the green coffee prior to roasting. The principle of green coffee preheating is as follows: Before the actual roasting process, the green coffee batch is preheated in a preheating chamber to a temperature slightly below or at 100 °C before it is fed into the roasting chamber. The thermal energy stored in the beans cannot only shorten the subsequent roasting process, but the energy quantity expended during roasting decreases, which in turn leads to a reduction of CO2 emissions.
The preheating systems can both reduce the energy costs by up to 25 % and the CO2 emissions by about a quarter. By the preheating measure, not only costs and emissions are reduced, but the capacity of the roasting machines can also be increased by up to 20 %.
The cost savings effects depend on the one hand on the roasting time. The longer the roasting time, the more heat can be introduced for preheating. On the other hand, the temperature of the exhaust gases plays an important role.
However, the preheating technology is only sensible and efficient from a roasting time of more than four minutes so that the positive effects become noticeable. From a roasting capacity of 500 kg per hour, the acquisition of or retrofitting with a green coffee preheating system will be worthwhile.
So far, only a few roasters use this opportunity to save energy. The manufacturer of roasting machines and plants Neuhaus Neotec, however, assumes that the preheating of green coffee will soon belong to the standard equipment of new roasting plants.
The saving effect depends on:
1. Roasting time (the longer, the more energy can be recovered)
2. The exhaust air temperature
Neuhaus Neotec offers two variants:
Clean air preheating (ambient air is heated via a heat exchanger)
Exhaust air preheating (the exhaust air is in direct contact with the green coffee beans, resulting in a higher efficiency)