An experimental feasibility study on fast pyrolysis of MSW-derived trommel fines for energy recovery and waste management

Abstract

Trommel fines are solid wastes with particle sizes of <25 mm, which are obtained from the MSW recycling facilities. They represent a class of wastes with significantly high proportions of both inorganic and organic (biodegradable and non-biodegradable) materials, making their disposal problematic. In this research, the technical and economic feasibility of fast pyrolysis of trommel fines has been experimentally investigated for energy recovery, thereby eliminating the organic load and leaving an inert solid residue (glass, stones, ceramics etc.) for final disposal or other uses. A batch of trommel fines, obtained from a UK Waste Management company, was fully characterized and prepared into suitable feedstocks for fast pyrolysis using an existing bubbling fluidised bed reactor system. A combination of physical processes including drying grinding, sieving and manual separation was used to obtain a dry pre-treated feedstock (PT) of suitable size range for fast pyrolysis. In addition, the PT feedstock was applied in an agitated wet treatment procedure to obtain two new feedstocks for fast pyrolysis by washing with water (AW) and with a 1.00 vol. % Decon Neutracon surfactant aqueous solution (AWS). Characterization results showed that different size fractions of the ‘as received’ trommel fines have differences in properties. Ash contents were high (>30 wt.%) and along with the volatile matter and higher heating values (HHV), varied in relation to particle sizes. By careful pre-treatment process design, the size fraction 0.5 mm – 2 mm, which was suitable for fast pyrolysis, had an experimental energy content of 13.8 MJ kg -1. The energy contents of the AW and AWS feedstocks increased with a reduction in ash contents after the respective washing procedures. A 300 g h-1 bubbling fluidised bed fast pyrolysis rig was used to investigate the effect of temperature and moisture content on product yields and process conversion efficiency of dry physically pre-treated trommel fines (PT) to determine their optimum processing conditions. Investigations were also undertaken to study the effect of feedstock pre-treatment method; dry (PT) and wet (AW and AWS) on both the pyrolysis products and process conversion efficiency. Using PT feedstock, the highest organic liquid yield and highest conversion efficiency was obtained between 500 °C and 550 ºC with <3 wt% feedstock moisture content. The organic liquid yield and the process conversion efficiencies increased with AW and AWS feedstocks, with AW feedstock giving the best results. The HHV of primary condensate from all feedstocks was greater than 30 MJ kg-1 and the washing procedure was found to reduce the nitrogen contents of the liquid products especially in the secondary condensate liquids.The fast pyrolysis results were used to determine the economic feasibility of the fast pyrolysis technology at PT optimum processing conditions for energy recovery and management of trommel fines at different processing capacities. The PT fast pyrolysis was found to be economically feasible from 2000 kg h-1 processing capacity, with a capital investment payback period of 8.6 years at 20% interest rate. The net present value (NPV) increased with the AW and AWS feedstocks and further analysis showed that processing these feedstocks could still be economically feasible at capacities of approximately 1000 kg h-1. 3 Overall, the results of this study suggest that the laboratory-scale fast pyrolysis rig used in this study and the developed economic model can form the basis for future research and process development for treatment of MSW.