Anaerobic treatment of palm oil mill effluents: potential contribution to net energy yield and reduction of greenhouse gas emissions from biodiesel production

Abstract

The effluents from palm oil mills for biodiesel production are generally treated in open ponds, causing large amounts of greenhouse gas (GHG) emissions. This study assesses the use of palm oil mill effluents (POME) as feedstock to produce biogas via anaerobic digestion. Biogas from POME can be converted into electricity and heat to eventually reduce the greenhouse gas (GHG) emissions of biodiesel production from palm oil. This study is using two system boundaries, firstly, system a “gate-to-gate” concerning the POME treatments, and secondly a “cradle-to-gate/total combustion” when we assess the impact of varying POME treatments within the biodiesel chain. The research draws on field and experimental data from palm oil and biogas production in Sumatra, Indonesia. The findings show that the energy output from the conversion of POME to methane via anaerobic digestion and the subsequent combustion of the methane in a combined heat-power plant exceeds the energy consumption of the palmoil milling process. Treating POME in an anaerobic digester and using the biogas to generate electricity and heat has the potential to significantly reduce the GHGemissions of biodiesel production from palm oil. In the studied case, the energy output from the conversion of POME to electricity and heat is 0.44 MJ kg 1 biodiesel and the net energy yield is 0.42 MJ kg 1 biodiesel. The ratio of energy output to energy input of the conversion process is about 23.1. The potential reduction of GHG emissions is 658 g CO 2-eq kg 1 biodiesel or 15.96 g CO 2-eq MJ 1 . This is equivalent to about 33% of the total GHG emissions of biodiesel production from palm oil. Against this background we recommend to further develop and implement the treatment of POME in anaerobic digestion combined with the purposeful use of the methane, electricity and heat produced from the POME. This can make a significant contribution toward meeting international targets of emissions reduction for biodiesel production.