1. The Necessity of Pelletizing Municipal Solid Waste into Refuse-derived Fuel Pellets
2. Processings of Turning Trash into Treasure
3. Achievements having been Made in Solid Waste Management during the Past Decades in the World
The Necessity of Pelletizing Municipal Solid Waste into Refuse-derived Fuel Pellets
Solid wastes are any discarded materials. They can be solid, liquid, semi-solid or containerized gaseous material. In the light of a survey conducted by World Watch Institute, the total volume of municipal solid waste (MSW) is about 1.3 billion tons per year in the world (MSW is measured before disposal, so data on it include material that is later diverted for recycling). Moreover, it is predicted that the number will be doubled annually by 2025.
The disposal of MSW is a matter of increasing concern for governments all around the world. Previous managements are still adopted by municipalities and goverments. Innovative ideas are on the way.
Pyrolysis is a process of chemically decomposition of organic materials by heat in the absence of oxygen which produces various hydrocarbon gases. During pyrolysis, the molecules of object are subjected to very high temperatures leading to very high vibrations. Therefore every molecule in the object is stretched and shaken to an extent that molecules starts breaking down. The rate of pyrolysis increases with temperature. Fast pyrolysis produces liquid fuel known as bio oil. Slow pyrolysis produces gases and solid charcoal. Solid products of this process contain metals, glass, sand and pyrolysis coke which cannot be converted to gas in the process. Pyrolysis is extensively used in the petrochemical industry and can be applied to municipal waste treatment where organic waste is transformed into combustible gas and residues.
Incineration technology is the controlled combustion of waste with the recovery of heat to produce steam that in turn produces power through steam turbines. The incineration process produces two types of ash. Bottom ash comes from the furnace and is mixed with slag, while fly ash comes from the stack and contains components that are more hazardous. Such systems rely on minimum guaranteed waste flows. It indirectly promotes continued waste generation while hindering waste prevention, reuse, composting, recycling, and recycling-based community economic development. It costs cities and municipalities more, provides fewer jobs than comprehensive recycling and composting, and also hinders the development of local recycling-based businesses.
Landfills may cause numbers of problems. Damage can include infrastructure disruption, such as damage to access roads by heavy vehicles. Pollution of the local environment may occur as well, such as contamination of groundwater or aquifers by leakage or sinkholes or soil contamination.
As existing landfillls become filled to capacity and it is more costly to site new landfills, the development of alternative disposal methods is becoming essential. In addition, the wastes being buried contains considerable quantities of energy that can replace conventiaonal fossile fuels.
Producing energy from trash is known as a “waste-to-energy” option. Several such options have existed for many years and are in extensive use throughout Europe. One of the more exciting options that has been proposed within the last decade is to convert waste into solid recovered fuels (SRF). SRFs are engineered blends of nonrecycled combustible waste (general combustible components of MSW is paper, cardboard, plastics, textiles, rubber, leather, wood) condensed into fuel pellets or briquettes. Taking advantage of those fuel pellets or briquettes can be recovered to provide environmental, economic and resource conservation benefits.
Advantages of Pelletizing MSW into Fuel Pellets
The calorific value of raw MSW is around 1000 kcal/kg while that of 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. Recycling rates have increased over the past few decades, such as in American, 1.5 pounds wastes are recycled and another 0.5 pounds are incinerated-out of the 4.4 pounds of trash being prodeced every day, but there are still about 50% of the waste ending up buried in landfills. Now, with the use of pelletisation technique, we could reduce the amount that is sent to landfills by about 10 percent and produce a fuel that is relatively clean and more energy dense than coal. This technology is especially suitable for plastics that are difficult to recycle, or that decompose slowly in landfills—like baby diapers.
There are significant reductions of SOx, NOx and CO2, and the trapping of the chlorine combustion products such as HCI that are formed from the remaining plastics in the trash, which makes the pelletized binder-enhanced SRFs a part of the environmental solution, rather than a part of the problem.
MSW fuel pellets (suitable to consume blending with coal) by the electric utility industry represents a very large potential market for the lime industry. Even with the conservative assumptions used in conducting the market analysis, an annual lime usage of over 1,410,000 tons per year was indicated. Other potential markets, including the cement industry, paper industry, agricultural processing, and military installations.
Creating fuel pellets out of trash could significantly reduce the amount of trash that goes into landfills and could offset fossil fuel use.
Processings of Turning Trash into Treasure
As mentioned above, material suitable for pelletization is positively selected, with a small amount of residue set aside for disposal. The materials suited for fuel pellet production are shredded, fiberized and stored in storage silos.
In primary shredding, the materials are delumped into 25-40 size to enable easy drying and separation. Delumped materials are dried from 50% moisture to 25% moisture, either on a paved sun drying yard or in a mechanical dryer.
Dried waste materials are passed through a rotary sieve for separation of fine dirt and sand; fine materials can be sent as soil conditioner for further processing.
Screened waster materials is passed through density separation phase in air density separator. Heavy particles are rejected and sent for dumping.
Light fractio is passed through a cage mill for further size reduction with hot air for faster drying and misture is reduced from 25%-15%. Dried combustible material having 25-40mm size is refuse-derived fuel and its calorific value is about 3000 kCal/kg.
Refuse-derived fuel can be ground further in a secondary shredder for making it suitable for pelletisation, then it can be later combined with high-BTU admixture materials such as carpet waste, poly film or other acceptable plastic derivatives. They are transferred through pellet mills to produce the final fuel pellet with different diameters i.e. 10mm to 25mm, suitable for different uses.
These fuel pellets are hard and odorless, can be stored for up to three years without sigificant biological or chemical degradation, and due to their increased bulk density, are more durable and can be more easily transported. High bulk density and regular size makes its transport, storage, conveying and combustion easier as compared to other fuels.
|Parameters of refuse-derived fuel pellets are as follows|
Before briquetting, municipal waste has to be processed for size reduction, adding binder agents and reducing the moisture content.
In general, the moisture content, fraction size, pressing temperature, and compacting pressure of compressed waste are the most important parameters to manufacture briquettes with acceptable quality. The pressing temperature and compacting pressure depend on the type of briquetting machine used. Fraction size has great influence on the briquetting process. The coarser the fraction is, the higher compacting power is needed for briquetting. Briquette has lower homogeneity and stability. By increasing the fraction size, the binding forces inside the material decrease which effects on faster decay by burning.
Post time: Jun-05-2020