Waste to Energy – Energy Recovery of Green Bin Waste: Incineration/Biogas Comparison

Abstract

This study presents how to determine marginal incinerator energy efficiencies. This concept should be applied in assessments of the treatment of specific waste streams to create a level playing field when comparing different technologies, for instance in life-cycle assessments. The marginal efficiencies depend on the technical level, the surrounding energy system, and the waste type/heating value. The concept of marginal efficiency is in this article applied to the treatment of green bin waste. The thermal treatment options considered are incineration and anaerobic digestion with biogas combustion in a reciprocating gas engine. The comparison is made only with regard to energy yield. The comparison is carried out for three different types of energy systems: power producing, combined heat and power, and combined heat and power with flue gas condensation. For a power-only energy system, the electrical efficiency of anaerobic digestion is found to be comparable to incineration. However, for an energy system with district heating as an option, the energy recovery of incineration is much higher than biogas. Furthermore, if the waste heat recovery technology flue gas condensation is used, the total efficiency of incineration is almost twice as high as the biogas technology, with the same boundary conditions applied. Although this study produces specific figures of energy recovery yields for a number of different scenarios for incineration and biogas production, the aim of the study is not a technology comparison as such. A proper technology comparison, including life-cycle assessments, should deal with many other issues than just energy recovery, for instance, other environmental factors, initial cost, operating and maintenance costs, and commercial aspects. However, this study does show that when undertaking such technology assessments, it is crucial to the results that: • Proper and relevant boundary conditions are applied: if district heating is possible, this changes results dramatically. Optimally, a specific site should form the basis for a comparison. • Energy recovery efficiencies must be linked to the actual waste steam considered. For green bin waste, this study has shown that margin efficiencies of up to 125 % can be achieved. This is much higher than efficiencies reported in the literature (95 %) [1•, 2•]. Therefore, one main conclusion is that efficiencies used for comparisons of technology must be waste-type (and calorific-value) specific to obtain reliable results in the comparisons.