Browsing by Author "Nel, Joshua Lourens"

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    Technical and economic comparison of pyrolysis and pelleting for valorisation of pulper rejects from a secondary tissue mill, as alternative to disposal
    (Stellenbosch : Stellenbosch University, 2021-03) Nel, Joshua Lourens; Gorgens, Johann F.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: South African paper mills are researching solutions to divert commercial and industrial waste (C&IW) from landfills. This problem is driven by increasing landfill fees, and pressure from the Department of Environmental Affairs and corporate policy. This study evaluates two recycling methods for one C&IW from secondary tissue mills, known as pulper rejects. Pulper rejects are rich in plastics and contain some fibres. The methods of recycling included secondary (pelleting) and tertiary recycling (pyrolysis) to convert rejects into marketable solid recovered fuel (SRF) and condensable product (fuel oil), respectively. Rejects were pretreated through drying, milling, extracting ferrous metal, and pelleting. The Ø6 mm pellets underwent analysis to test the technical, economic, and environmental factors according to two British SRF standards. The results from analyses proved the pellets to be feasible for cofiring as SRF in the primary burner of cement kilns. This was possible due to the high lower-heating value (LHV of 29.7 MJ/kg), low ash content (8.65 wt.%), and specific concentration of mercury, which was lower than the maximum permissible concentration according to the Air Quality Act 39 of 2004 (0.0574 mg/MJ). Comparatively, the pellets were subject to pyrolysis for conversion to condensable product, char, and gas. The highest condensable product yield of 62.4 wt.% was achieved at 500 °C on bench-scale and the condensable product was 70% wax and the rest, oil. Consequently, temperatures from 450 to 550 °C were tested on pilot-scale, with a condensable product yield of 51.9 wt.% being achieved at 500 °C. This product contained wax, organic phase oil and aqueous phase oil being 47.6, 20.6 and 31.8 wt.% of the total, respectively. The wax and organic phase oil had a higher heating value (HHV) of 38.9 and 43.6 MJ/kg, respectively and formed the fuel oil product. The gross-energy conversion (GEC) represented the energy transfer from feedstock to products. At 500 °C, the GEC to char and condensable product from bench-scale and pilot-scale was 86.5% and 74.1%, respectively with the difference attributed to the aqueous phase oil reported for pilot-scale. However, the separation of aqueous phase resulted in improved oil HHV from 20.6 MJ/kg (bench-scale) to 43.6 MJ/kg (pilot-scale). Techno-economic models were developed for the pelleting and pyrolysis processes to compare profitability at mill capacity. The pelleting process was developed according to the recovery factor transform function method to evaluate multiple pelleting lines with the SRF selling price based off its LHV. The pyrolysis process was developed on Aspen Plus, using pilot-scale data, and the price of char and fuel oil determined according to their HHV. Neither process was profitable with the economic settings. The minimum fuel selling price (MFSP) of SRF calculated at 25% discount rate was R6 269/ton and more than 5.16 times the current price (R1 214/ton). Similarly, the MFSP for fuel oil was R29 137/ton and 5.85 times the current price (R4 978/ton). Hence, unless waste disposal fee is drastically increased or feedstock capacity increased, both processes will remain unprofitable.