Torrefaction of Simulated Separated Municipal Solid Waste

Torrefaction is low temperature pyrolysis that can serve as thermal pretreatment for biomass prior to energy conversion. Torrefaction increases the specific energy density of biomass by removing moisture and low molecular weight volatiles to create a more homogeneous and hydrophobic solid feedstock. This treated feedstock is reported to maintain quality better in storage and may have superior mechanical properties for size reduction and other processing compared to the original feedstock. For these reasons, the solid char product of torrefaction is of most interest as feedstock for entrained-flow and other types of gasifiers. The aim of this study was to characterize the products, mainly char, from torrefaction of simulated post-Material Recovery Facility (MRF) municipal solid waste (MSW). The effects of residence time and torrefaction temperature on the physical and chemical properties were investigated for torrefied products or chars produced in a laboratory scale moving-bed pyrolyzer. Methods for producing the synthetic refuse-derived feedstock were described along with those used for measuring tar compounds in the condensation products of the torrefaction gas. The pyrolysis mechanisms of the synthetic MSW components and composite mixture were also studied through thermogravimetric (TG) and differential thermal (DT) analysis techniques. Characterization values for the synthetic post-MRF MSW components and the composite mixture were determined in this study and compared with accepted literature values originating from as early as the 1960s. For most of the MSW materials, ash contents and heating values were found to be larger than the values reported in the literature. These differences in characteristic values emphasized the need for updated MSW composition data. Toluene, ethylbenzene and para-xylene concentrations in the tar samples were determined through GC/MS tar analysis. The concentration results had relatively high standard deviations and did not show clear trends that might correlate treatment temperature and residence time to the concentration of a particular tar compound. Generally, toluene, ethylbenzene and para-xylene concentrations increased with increasing treatment temperature. Statistical analysis indicated that the toluene, ethylbenzene and para-xylene concentrations in tar samples collected from experiments conducted at a temperature of 350°C were significantly different (at α=0.05) from those found for tar samples generated at temperatures 300 and 250°C. For the torrefaction experiments, the mildest thermal treatment conducted in the moving-bed pyrolyzer (temperature of 250°C and a residence time of 5 minutes) resulted in a nearly 10% increase in higher heating value (HHV) for the torrefied synthetic MSW from the original feedstock whereas the most severe thermal treatment (temperature of 350°C and a residence time of 9 minutes) resulted in a large HHV increase of 43% on a dry basis. Increases in fixed carbon contents from the original feedstock for the torrefied synthetic MSW char products were measured to be in the range 12.4% to 26.9% on a dry basis. This increase in the fixed carbon contents of the torrefied synthetic MSW products correlated well with the increase in HHV observed. The average mass yields for the torrefaction experiments were 86.8%, 78.7% and 67.6% (on a dry basis) for treatment temperatures 250, 300 and 350°C respectively. The energy yield was greater than the mass yield for all solid char products produced in the torrefaction experiments. The difference between energy and mass yields increased as torrefaction treatment temperature increased. Specifically, each 50°C increment in torrefaction temperature corresponded to around a 10% increase in the difference between energy and mass yields. For the three torrefaction temperatures studied, energy yields for the torrefied MSW produced in the moving-bed pyrolyzer remained high around 95% indicating that the overall energy content of the feedstock was conserved during torrefaction.