Methodology for Comparing End-of-Life Pathways for Non-Recycled Materials

Recycling plastics is widely accepted as the most beneficial end use of plastic products. Consequently, many cities are turning towards single-stream recycling to make it easier for consumers to recycle and to increase the total amount of municipal solid waste (in particular, energy-dense plastic waste) that is diverted to recycling facilities. However, single-stream recycling Materials Recovery Facilities (MRFs) are now faced with sorting more diverse material flows with increased contamination from the mixing of recyclable and non-recyclable materials, leading to roughly 5–10% of the incoming material being sent to landfills. Converting the energy dense MRF waste material into solid recovery fuel (SRF) pellets creates an additional use for the products, diverts the material from the landfill, and displaces some fossil fuel use. However, there are some non-obvious energetic and environmental tradeoffs that require analysis to quantify. That is the intent of the research presented here. To analyze the potential of SRFs as viable alternative fuel sources, a first-order thermodynamic materials and energy balance was constructed using cement kilns as a test-bed. The proposed methodology allows for a range of traditional fuels to be compared with and without supplemental SRF. The SRF case can be benchmarked against the reference case, or conventional plastic end-of-life pathway, landfilling of the non-recycled plastic. The comparison includes transportation and processing steps required for each pathway, including any additional sorting needed for creating the SRF as well as the pelletization process itself. A robust methodology was created that allows for the MRF residue to be adjusted on a compositional basis because residue composition varies by season and location, which affects the analysis. Additionally, proximity to SRF conversion facilities and cement kilns will vary for each MRF and can impact the analysis so the methodology allows these factors to be adjusted. A test case was studied to compare the landfilling or combustion of MRF residue in a cement kiln at a rate of 0.9 metric tons per hour (7884 metric tons for a one year period). The analysis details the total energy consumed, landfill avoidance, amount of fuel displaced, and the total equivalent CO₂ emissions of each scenario. The methodology successfully models the reference and SRF case and is robust enough to be used with a wide variety of potential SRF scenarios. A few parametric studies were performed on the transportation and landfill variables to determine their relative effect on results. It was found that additional transportation would have minimal effect of total energy consumption. When using SRF as a supplementary cement kiln fuel, the equivalent CO₂ reductions are higher in scenarios with low methane capture efficiency at the landfill. Overall, it was found that using SRF as a supplementary fuel at cement kilns reduces the total fossil energy consumption and total equivalent CO₂ reductions by 6% and 76%, respectively.