Figures 1 and 2 illustrate optimized processes for high-calorific <30 mm and low-calorific <80 mm refuse-derived fuel (RDF) and solid recovered fuel (SRF) production.
Stepping up alternative fuels production: 2 optimized production lines
The 80 mm RDF line illustrated in Figure 1 is equipped with a minimum range of components for making 80–100 mm preheater fuel. There is no eddy current or optical sorting and therefore, the content of non-magnetic metals and chloride is not under control. The application features only one shredder, a pre-shredder without a screen that will reduce the material down to <150 mm of which 80% will be <80 mm.
The remaining oversized particles will be recirculated until the required grain size is achieved. This simple and straightforward solution will be easy to extend with more classification equipment, as all basic components are identical to the ones that are usually integrated in the more advanced plants that produce high-calorific value AF.
Figure 2 represents a production line for high-calorific value <30 mm RDF/SRF for the main burner. In this case, we have integrated a pre-shredder and a fine-shredder. The pre-shredder will ensure that the input material is reduced to < 300 mm, and all the waste bags are opened to allow screening, magnets, the eddy current, optic sorting and the wind shifter to work properly to secure a “clean” input material for the fine shredder, where stones and metals would cause production interruptions and very often major damage. Wear costs on fine-shredders are much higher than on pre-shredders and therefore, the abrasives need to be removed before they reach the fine-shredder.
Some RDF/SRF producers who process 100 per cent clean material streams will of course be able to benefit from a one-step solution. If they produce a <80 mm product, they may be able to achieve a reasonable throughput in a single stream process with a fast-running fine shredder with sharp tools. However, they will always have to take into account that the productivity of a fine-shredder increases considerably if fed with pre-shredded material. In other words – it is easier to achieve good and constant feeding of a fine-shredder with a conveyor feeding <300 mm material than feeding bulk material into a fine-shredder with a crane.
Selecting the right shredder
When selecting a pre-shredder for an RDF/SRF production line, the equipment should have the following features:
- A robust machine with a high run rate that will not suffer from unshreddables in the waste stream.
- Ability to shred everything and not let unsortable items pass and cause production stoppages
- Ability to open all waste bags and generate as little fines as possible. The option to screen out all fines results in a higher calorific value and less wear in the process equipment.
- Ability to generate a homogeneous output with a constant flow and a uniform layer on the extraction conveyor to enable magnets, screens and the other sorting equipment to work properly.
A fine-shredder for an SRF production line should
- Produce homogeneous output material
- Generate as little oversize material as possible to ensure smooth feeding
- Feature easy service access for fast maintenance to give a high run factor while keeping service and maintenance cost as low as possible.
Producing high quality RDF/SRF for the cement industry, for example, has become easier with the sophisticated technology used today. The optimal line design depends on the required final fuel quality with finer grain sizes requiring additional process steps to remove unwanted elements and ensure a homogenous end product. Good equipment design also takes easy maintenance and high uptime into account.