Analysis of two treatment technologies for coffee roasting matrixes: combustion and anaerobic digestion

Even if the demand for energy continues to increase, the European Union Directive 2009/28/EC has set a goal of obtaining 20% of all energy from renewable sources by 2020. In this context, this study investigated two different scenarios: direct combustion and anaerobic digestion. Direct combustion was evaluated using the mass and energy balance method in order to obtain indications of the energy revenue, the emissions, and consequently the necessary flue gas depuration equipment. Anaerobic digestion was evaluated by performing experimental tests in the laboratory, in order to obtain indications of the energy revenue. The results show that both the processes present some advantages: from the energy viewpoint the more suitable process is direct combustion (energy revenue equal to 1.16 MWh/tinput instead to the 0.48 MWh/tinput obtained for the anaerobic digestion), while from the viewpoint of greenhouse gas emissions (GHGs, in particular the CO2 emissions) the results are rather similar. (We evaluated that the emission is equal to 1.18 kg CO2/ kg input for the direct combustion and to 1.06 kg CO2/ kginput for the anaerobic digestion.) The anaerobic digestion presents more advantages from the viewpoint of features a simpler plant layout; the combustion scenario requires a more complex technology, in particular for dealing with NOx emissions. (The concentration coming from the process is equal to 900 mg/Nm3.) For this specific study, based on the obtained results, the anaerobic digestion proved to be the more suitable treatment process. Furthermore, the analysis considered here demonstrates a general methodology useful for energy production compatibility planning.