Municipal solid waste has been gaining interest as a potential feedstock for biofuels development as it is highly organic in nature and it is a waste product requiring very little processing to become a suitable feedstock. The main focus of this research project was to evaluate whether municipal solid waste (MSW) is a good source for bioenergy development, in particular, as a feedstock for conversion to biofuels. And if densification of MSW is a feasible process to integrate into waste disposal systems in Canada. These topics were addressed through a comprehensive review of classification of MSW in Canada with focus on suitability for biofuels development and a subset of experiments that produced information on the characteristics of MSW refuse-derived fuel (RDF) and the parameters required to produce a quality, densified fuel product.
A review of existing systems in Canada was conducted to establish how different regions currently classify waste; then, a classification framework produced specifically for energy recovery from MSW was used to analyze the strengths and gaps in those existing systems. Finally, a discussion regarding the suitability for biofuels development in each region was made based on the analysis. The City of Edmonton was used as the reference jurisdiction due to their established waste-to-biofuels project, and a geographic distribution of regions that were reviewed included Vancouver, Saskatoon, Toronto, and Halifax. The review determined that most jurisdiction classify MSW by material or product, with the former method being more suitable for investigating alternative utilization methods. Each region has potential for pursuing biofuels development, however, the greatest barrier appears to be whether there is a driving socio-political reason for doing so in the area.
Characterization of MSW-RDF fluff sample received from Edmonton showed that the composition of the material was approximately 35% paper, 22% plastics, 14% fabrics, 6% organics/wood, and 23% fines by mass. The RDF was densified, as well as the biodegradable (paper and wood) fraction of the RDF stream to compare quality of pellets for the two material compositions. A characterization of the thermochemical and biochemical properties of MSW RDF-fluff was conducted to evaluate the suitability of MSW RDF-fluff for biofuels application. The ash content of RDF material was 19-39% while that of the biodegradable material samples was 20-23%. Proximate analysis resulted in a CHNS ratio of 33-41% carbon, 5-6% hydrogen, 0.6-0.8% nitrogen, and 0.2-0.5% sulfur for all samples. From the results of the proximate analysis, the higher heating value (HHV) for MSW RDF-fluff was calculated to be 14-16 MJ/kg. Fibre analysis of the biodegradable fraction determined that it contained 28% insoluble lignin, 1 % soluble lignin, 22% glucose, and 0% xylose.
A single pelleting trial was conducted to examine the compaction parameters that would produce high quality pellets: grind size, moisture content, pelleting pressure, and pelleting temperature. It was determined that quality pellets, for both materials, were formed at a grind size of 6.35 mm at 16% moisture under pelleting conditions of 90°C and 4000 N applied load. The compact density of pellets produced from RDF ranged from 880-1020 kg/m3; the compact density of the biodegradable pellets ranged from 1120-1290 kg/m3. Fitting of the Walker and Jones models to the experimental data both indicated that the biodegradable material fraction has a higher compressibility than the RDF material, where neither moisture content nor grind size at all levels had a significant effect on the compressibility of either material. The Kawakita-Lüdde model estimated the porosity of the pelleted samples, while the Cooper-Eaton model indicated that the primary mechanism of densification was particle rearrangement. Application of the Peleg and Moreyra model for analysis of relaxation properties of the compressed materials determined the asymptotic modulus of the residual stress to be between 89 and 117 MPa for all experimental parameters; however, the RDF material produced more rigid pellets than the biodegradable material.
Pilot-scale pelleting was then completed to emulate industrial pelleting process utilizing the parameters from the single pelleting operation that were deemed to produce quality pellets. All six of the sample treatments produced durable pellets (88-94%), with the ash content around 20% for all samples. A techno-economic feasibility study determined that 6.35 mm diameter pellets could be produced for an average cost of $38/Mg and includes both size reduction and densification procedures, although the aggressive process of the size reduction required indicates that it may not be a technically feasible option
