Abstract
A comprehensive study of the energy yield from slow pyrolysis of the organic fraction of municipal solid waste (OFMSW) and energy recovery from the aqueous liquid product by anaerobic digestion has been carried out. In this paper, the results of the liquid pyrolysis product characterisation are presented, with toxicity and methane potential assessments of the aqueous liquid product. The OFMSW feedstock was obtained from a UK waste treatment plant. Shredded samples dried to different moisture contents (12.7–45.8%) were processed in a 300 g per hour auger screw pyrolysis reactor at temperatures from 450 to 850 °C. Sixteen pyrolysis runs were performed, with process mass balance closures above 90% obtained (wet feed basis). Pyrolysis liquids showed clear phase separation under gravity. With increasing processing temperature, the liquid yield (both organic and aqueous fraction) reduced but the gas yield increased. An investigation into the product energy distribution indicated that processing temperature had a strong effect on the product energy distribution, while the effect of feedstock moisture was relatively small. Batch anaerobic testing of the aqueous fraction showed that toxicity increased with pyrolysis processing temperature and decreased with feedstock moisture content. Statistical analysis confirmed that the pyrolysis processing temperature was the dominant factor affecting the toxicity of the aqueous product. Careful acclimatisation of the microbial consortium to the applied substrate and loading is likely to be necessary for improved digestion of the aqueous fraction.
Highlights
Over the past 20 years, the focus of waste management in EU countries has increasingly moved from disposal to prevention, reuse or recycling, or recovery
For this reactor with a certain raw material and a fixed feed rate, the vapour residence time is a function of the vapour production rate, which is dependent on the thermal processing temperature: the higher the reaction temperature, the higher the vapour production rate, and the shorter vapour residence time, which is calculated for each run
Screening tests were carried out based on an extended form of the assay in BS ISO 13641-1:2003, in which specific methane production was estimated by determination of gas composition at the end of the incubation period
Summary
Over the past 20 years, the focus of waste management in EU countries has increasingly moved from disposal to prevention, reuse or recycling, or recovery. Pyrolysis is a thermochemical conversion process that has been widely used for centuries in the manufacture of charcoal It has been the subject of extensive research as a means of processing organic waste for energy recovery, and in particular for converting solid biomass into valuable liquid and gaseous biofuels as well as charcoal [5,6,7]. Anaerobic digestion (AD) is a biological process that is suitable for wet wastes and produces biogas (a mixture of CO2 and methane) with a semi-solid residue digestate It has seen increasingly widespread adoption in recent years for the treatment of source segregated organic wastes due to the recovery of a valuable fuel gas and the environmental benefits associated with the process [9], including use of the digestate which may have value as a source of plant nutrients [10]. Statistical analysis was employed to assess the effect of processing condition on the process mass balance, product energy distribution and the results of AD screening
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