The collection of waste and production of RDF may be complex and usually includes several steps. A goal is to produce homogenous material without substances or other categories of waste unsuitable for combustion. The first step is household waste collection. The requirements of regulations in Poland assume containers, bins, or containers should be placed in front of the property to allow a collection company to conveniently pick up the waste. Depending on housing type and access roads a collection company must use suitable vehicles. In narrow streets or residential access in a block of flats area, large trucks have difficulties in moving, therefore smaller vehicle types are necessary. After collection, a vehicle must unload the waste. After unloading a waste handling machine upload the waste onto the processing line for removal of unnecessary objects, other waste categories unsuitable for combustion, and hazardous materials. The remaining material for the production of RDF is shredded into small particles and is loaded onto a heavy goods vehicle (HGV) for shipping to the WtE plant. Material and energy flow analyses are useful to identify a volume or mass of collected waste and power requirements – as energy consumption for each process (Rotter et al. 2004). The main concept of our study is presented in Figure 2.
The calorific value of produced RDF is the reference value of energy potential. The tests of calorific values complied with PN-EN 15403:2011 standard – (Solid recovered fuels — Determination of ash content. The calculated calorific value).
The case study is for the rural and urban communities in the South of Poland (Figure 3). The process starts with waste collection from households. The collection is for urban and rural communities. Various types of vehicles for the collection and shipping RDF to cement plant. The collection depends on the schedule and assigned plan of routing therefore the distances for a scheduled collection vary.
The collection vehicles’ routes are selected to access each household in the area. Two categories of vehicles have been compared in this study – a specially adapted LCV – Figure 4a, and a GT – Figure 4b, with a waste compacting capability. After a vehicle’s loading capacity is reached the vehicle has to unload in a pre-treatment facility. For the shipment of RDF from the pre-treatment facility two types of vehicles were selected for comparison in this research – Tipper Semi-Trailer (TST) - Figure 4d, and Walking Floor Trailer (WFT) Figure 4c. The vehicles for the collection and RDF shipment are shown in the Figure 4 including the handling machine for loading.
The separation of waste is a helpful method for selecting households' various waste categories depending on the main material. It helps to classify materials of categories having similar calorific values. In this process, output materials are fractions of combustible materials like plastics, paper, and cardboard. Other materials like glass or metal can be recycled. Another category of waste from households containing materials of high calorific value is textiles, wood, and furniture belonging to another waste stream – bulky waste (Fig. 5).
This study focuses on the estimation of energy consumption in the entire reverse supply chain of the bulky and MSW. The primary source of waste in households and the final destination is a WtE plant – the cement plant. This study estimates the energy consumption for waste collection, transportation, handling, processing, compacting, and shipping to a WtE plant - cement plant. The case study will focus on the separately collected waste stream – bulky waste and mixed MSW from municipalities in the Silesian region in the south of Poland.
The case study includes three categories of waste for collection, processing, and production of RDF (Fig. 6). For the collection of the bulky waste and separately collected waste specially adapted LCV with a cage can be used. Another vehicle is a GT with a waste compacting capability. The GT is also used in the collection of MSW. The calorific value of diesel is 37 MJ/l (National Statistics UK 2021).
After traveling to a waste processing company the vehicles unload collected waste ant it is transferred into a shredder. The processing of bulky waste is simple. Bulky waste comprises furniture, large garden equipment, upholstery, carpets, and various kinds of wood including chipboards and fiberboards. In a processing facility, a diesel engine mobile shredder with a magnetic separator is used for processing the waste. Approximately 90 percent of the mass of bulky waste is suitable for the production of RDF.
Calorific value RDF produced that way is higher when the fraction of polymers is higher than a wood-related fraction. MSW has a lower calorific value and includes many fractions and impurities that require to be removed from the waste stream (Zhou et al. 2014; Dianda and Munawar 2017). The second scenario is for the production of RDF from mixed MSW. GT with waste compacting capability is used for routing in communities for the collection of waste. The fuel consumption for GT is 30 liters per 100 km. The processing is much more complex compared to bulky waste. In this scenario after unloading the waste-collecting truck – the hydraulic grip waste loading machine fills the bunker with the MSW. At the first stage after passing the bag opener, the Trommel screen removes fine fractions from the waste stream. Then a belt conveyor transfers the waste onto two-stage shredding machines including preliminary and secondary shredding. A magnetic separator is installed for removing ferromagnetic parts other than the RDF fraction. Finally, a wind shifter is used for the efficient removal of plastics, film and foils, cardboard, and paper from screened overflow particles. The percentage of fraction
In the third scenario separately collected waste enters the processing facility. The separation of waste is at the first stage focus on recycling materials, a lower quality fraction of polymers and other mixed materials from the separately collected waste goes through sorting on a conveyor. Optical, magnetic, and pneumatic sorting methods are applied in the facility. Approximately 60 percent of the waste stream is processed as RDF.
The final transportation of RDF to the WtE plant is by using large trucks with a trailer. Commonly used vehicles are TST and WFT. Processed RDF is loaded and shipped to WtE – the cement plant located 100 km from the RDF pre-treatment facility.
Depending on the category of waste several operations are required to achieve purity of the material and fraction size. In this case, the power supply and several processing machines and conveyors have the main impact on energy consumption per weight of the processed waste. Finally, loading the RDF and shipping to the WtE plant are the last energy-consuming processes evaluated in this study. The waste transportation company can select different types of vehicles. In Formula (1) energy required for the collection, handling, processing, and transportation of waste is expressed as Ec in MJ per reference mass unit (RMU). Reference mass unit is calculated as the mass of RDF per one shipment to WtE multiplied by the calorific value of RDF. The calorific value depends on the category of waste from three scenarios. The coefficient of energy consumed for the production of RDF concerning energy for recovery in one total load of RDF can be expressed as Eloss in formula 1.
Energy consumption of fuel and electricity in transportation and processing of waste can be calculated from the formula:
Eloss = EC/ERMU [%] (1)
where: Emru – is the calorific value of reference mass of RDF and EC is the energy required for transportation, handling, and processing waste to produce RDF. EC is energy from fuel combustion in vehicles and electric energy required in the processing (conveyors, shredders, separators, etc.). It is expressed as MJ per reference mass unit (mass of RDF in one shipment).
EC = Ecol + Eproc + Eh + Et [MJ/rmu] (2)
Ecol – the calorific value of fuel in waste collection and transportation,
Eproc – the calorific value of fuel or electric energy consumption in processing of waste
Eh – the calorific value of fuel in handling and loading machines in waste processing plant
Et – the calorific value of fuel in RDF transportation to the WtE plant
All values are converted to MJ/rmu (reference mass unit) – one shipment to the WtE plant.
The study evaluates also the environmental burden of emissions in the transportation phase of the waste collection. For the comparison of emission for three scenarios, the research includes NOx, PM, CO, and CO2 and data from HBEFA emissions database (Keller and Wüthrich 2014).