Characterizing Impacts of Operating Parameter Variability on the Performance and Reliability of a Double-Stage Trommel

Abstract

A principal equipment used in waste pre-processing systems is a rotary screen (trommel). Despite the application of trommel screens in waste processing, the full-scale performance of this method has been shown inconsistencies with theoretical models and has not been thoroughly detailed. There were two high-level sets of research goals defined in this work, which studied screening performance and operation performance of a full-scale trommel. The first set of research objectives was to quantify and assess the impact of feed rate variation and seasonal variation in waste characteristics, in terms of particle size distribution and composition, on trommel’s screening performance during full-scale operation throughout the year. Also investigated was the impact of clogging of screen apertures on screening of material. The second set of objectives were defined to characterize the operation performance of the waste processing system, with a primary focus on the trommel, using system analysis methods including system availability, maintainability and throughput. A two-stage trommel, respectively, with 5 cm and 23 cm screens was evaluated in this study. The trommel design capacity was 55 tonnes per hour (t/h) and it was operated at a municipal solid waste processing facility located in a cold region (Edmonton, Canada), where weather and temperature variation is extreme. The facility is currently at maximum capacity and is fed with co-mingled refuse with the inorganic recyclable material removed. The variation in size separation efficiency and recovery was monitored with respect to the total feed rate, the overs loading feed rate, the season, operation time, and waste characteristics. The characteristics of the feedstock and separated waste streams were determined by sieve and compositional analyses. Separation efficiency and recovery results verified that the performance of the first stage varied seasonally, primarily due to changes in the particle size distribution of the feedstock; secondly, because of a greater feed rate. The seasonal variation in the compostable fraction of the waste streams was found to be the primary reason for changes in the trommel’s performance. On the contrary, the particle size distribution of the inorganic fraction of feedstock suitable for refuse-derived fuel production remained similar throughout the year and had steeper sigmoid curves. This indicated that the refuse-derived fuel material was more uniformly distributed, making it more sensitive to sieve size, which should be taken advantage of by selecting a smaller cut-off size for separating compost and refuse-derived fuel material from each other. A strong linear correlation was found between the recovery results and the corresponding overs loading rate. This correlation not only varied between seasons, but also varied within the operation cycle during the winter tests due to the clogging of the screens. A non-linear equation was fitted to quantify clogging formation according to the net operation time and feed rate. It showed that the 5 cm screens in the trommel’s first stage were completely clogged after 10 hours at feed rate of 40 t/h and after 1.8 hours at feed rate of 60 t/h. The system analysis results indicated that the majority of downtimes (by total number and duration) originated from the first-hand-sort room followed by the second-hand-sort room, which were located before and after the trommel. Other types of downtimes (e.g., jammed disc screen and conveyors), mostly occurred when the waste pre-processing system was overloaded, especially during the peak (summer) season. Overall, availability of the system decreased non-linearly in relation to the increased feed rate. The most probable downtime (probability >50%) measured 47.6 ± 1.1 sec when feeding was <50 t/h, which on average was 25 seconds longer than the most probable downtime (probability >35%) when the feeding was >65 t/h, as indicated by the Lognormal probability density function, fitted to the mean time to repair results. Accordingly, the Weibull cumulative distribution functions fitted to the mean time between failures results showed that the probability of operating for longer periods was higher when the feeding was managed at lower rates. This research quantified full trommel operations in different seasons and during the operation cycle. The aforementioned valuable findings of this research can be utilized in the development of a simulation model using discrete event simulation that can be used as a tool to assess the behavior of the pre-processing system under different operating conditions.