According to the waste management hierarchy published by the U.S. EPA, waste reduction and reuse are the most preferred modes of waste management, followed by recycling, energy recovery and lastly disposal. As many communities in the U.S. work towards sustainable waste management practices, recycling tends to be a cost-effective and common solution for handling municipal solid waste. With the introduction of single-stream recycling and automated materials recovery facilities (MRFs), where commingled recyclables are sorted into various commodity streams for sale to recycling facilities, recycling rates have steadily climbed in recent years. Despite increasing total recycling rates, contamination and diminishing returns for higher recovery ratios causes MRFs to landfill 5–25% of the incoming recycling stream as residue. This residue stream is composed primarily of plastics and fiber, both of which have high energy content that could be recovered instead of buried in a landfill. Plastics in particular are reported to have heat contents similar to fossil fuels, making energy recovery a viable end-of-life pathway. Sorting, shredding and densifying the residue stream to form solid recovered fuel (SRF) pellets for use as an alternative fuel yields energy recovery, displaced fossil fuels and landfill avoidance, moving more disposed refuse up the waste management hierarchy. Previous studies have shown that plastic, paper, and plastic-paper mixes are well suited for conversion to SRF and combustion for energy production. However, these studies focused on relatively homogenous and predictable material streams. MRF residue is not homogenous and has only a moderate degree of predictability, and thus poses several technical challenges for conversion to SRF and for straightforward energy and emissions analysis.
This research seeks to understand the energetic and environmental tradeoffs associated with converting MRF residue into SRF for co-firing in pulverized coal power plants. A technical analysis is presented that compares a residue-to-SRF scenario to a residue-to-landfill scenario to estimate non-obvious energy and emissions tradeoffs associated with this alternative end-of-life scenario for MRF residue. Sensitivity to key assumptions was analyzed by considering facility proximity, landfill gas capture efficiency, conversion ratio of residue to SRF and the mass of residue used.
The results of this study indicate that the use of MRF residue derived SRF in coal fired steam-electricity power plants realizes meaningful reductions of emissions, primary energy consumption, coal use and landfill deposition.