Municipal solid waste is a poor-quality fuel and its pre-processing is necessary to convert it into fuel pellets to improve its consistency, storage properties, handling characteristics, combustion behaviour and calorific value. Technological improvements are taking place in the realms of advanced source separation of MSW, resource recovery and production of MSW fuel pellets in both existing and new plants for this purpose.
There has been an increase in global interest in the preparation of MSW fuel pellets or Refuse Derived Fuel (or RDF) so let us take a close look at this interesting alternative fuel.
Pelletization of Muncipal Solid Waste
Pelletization of municipal solid waste involves the processes of segregating, crushing, mixing high and low heat value organic waste material and solidifying it to produce fuel pellets or briquettes, also referred to as Refuse Derived Fuel (RDF) or Process Engineered Fuel (PEF) or Solid Recovered Fuel (SRF).
The process is essentially a method that condenses the waste or changes its physical form and enriches its organic content through removal of inorganic materials and moisture. The calorific value of RDF pellets can be around 4000 kcal/ kg depending upon the percentage of combustible matter in the waste stream, in addition to additives and binder materials used in the process.
The calorific value of raw MSW is around 1000 kcal/kg while that of MSW-based fuel pellets is 4000 kcal/kg. On an average, about 15–20 tons of fuel pellets can be produced after treatment of 100 tons of raw garbage. Since pelletization enriches the organic content of the waste through removal of inorganic materials and moisture, it can be very effective method for preparing an enriched fuel feed for other thermo-chemical processes like pyrolysis/ gasification, apart from incineration.
Pellets can be used for heating plant boilers and for the generation of electricity. They can also act as a good substitute for coal and wood for domestic and industrial purposes. The important applications of RDF in the Middle East are found in the following spheres:
- Cement kilns
- Waste-to-energy plants
- Coal-fired power plants
- Industrial steam/heat boilers
- Pellet stoves
The conversion of solid waste into fuel briquettes provides an alternative means for environmentally safe disposal of municipal solid waste which is currently disposed off in non-sanitary landfills and waste dumps.
In addition, the MSW pelletization technology provides yet another source of renewable energy, similar to that of biomass, wind, solar and geothermal energy. The emission characteristics of RDF are superior compared to that of coal with fewer emissions of pollutants like NOx, SOx, CO and CO2.
RDF production line consists of several unit operations in series in order to separate unwanted components and condition the combustible matter to obtain the required characteristics. The main unit operations are screening, shredding, size reduction, classification, separation either metal, glass or wet organic materials, drying and densification. These unit operations can be arranged in different sequences depending on raw MSW composition and the required RDF quality.
Various qualities of fuel pellets can be produced, depending on the needs of the user or market. A high quality of RDF would possess a higher value for the heating value, and lower values for moisture and ash contents. The quality of RDF is sufficient to warrant its consideration as a preferred type of fuel when solid waste is being considered for co-firing with coal or for firing alone in a boiler designed originally for firing coal.
There are several issues that have to be kept in mind:
Processing MSW to RDF is a relatively expensive undertaking. Without a tipping fee, this is not finance-able. In Europe, where up to €200 tipping fees are paid, they can get excellent results. The plants are fully automated and have an excellent energy balance. However, they have still to pay tipping fees to the cement kilns, because RDF is considered “waste.” Therefore, a process that provides most “bang for the buck” is needed to make ends meet.
Burning the RDF is of course a solution, but not very cost efficient, profitable and environmentally friendly. Also there are major issues with the combination of the RDF – if there is more than 3% PVC in the plastic content, you have to expect that hydrochloric acid evolves that eats into the machinery. Japanese cement kilns and power plants refuse taking any RDF with more than 3% PVC content. There are huge mountains of unused RDF of this kind.
What most people do not recognize, well processes RDF is an excellent feedstock for diesel or jet fuel production. One ton of bone-dry RDF yields about 400kg or about 450 liters of diesel fuel. While this is still not enough to completely finance both, the pre-process from MSW to RDF and the conversion to renewable fuel, it makes a huge difference in the total calculation.
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