Shredding waste before transportation and disposal can save on disposal costs, increase the lifespan of landfills and decrease the need for capital investments
By Gary Dickie
Project Execution Manager, Ketek Group
Shredding homogenizes and reduces the volume of Municipal Solid Waste (MSW), improves the operating characteristics of incinerators and decreases the need for capital investments. It also increases the lifespan of landfill sites, reduces disposal costs and potentially produces revenue.
Reduce disposal costs
Increase lifespan of landfills
Decrease capital investment
Homogenize and reduce volume
Improve operating characteristics of incinerators
Shredding of MSW for Incineration
Chemical rate and heat transfer theories indicate that the productivity of a batch incinerator should be enhanced by pre-shredding the MSW, reducing the average particle size, homogenizing the feed, and increasing its bulk density by an estimated 30 per cent. Smaller particle sizes enhance reaction kinetics and flame propagation speed due to the higher surface-to-volume ratio and lowers the amount of combustion air needed to meet the required combustion rates. Minimizing the primary combustion air supply rate lowers the total amount of flue gases and can result in decreased costs of the Air Pollution Control system.
Smaller and more homogeneous particles increase bed mixing coefficients and reduce retention time required for complete combustion. The benefits realized through the pre-processing of MSW by means of modern shredding equipment were evaluated quantitatively for the new generation of Low Speed, High Torque (LSHT) shear shredders. The shearing mechanism used in these low-rpm devices produces a more uniform particle distribution at a lower energy cost per ton of MSW processed than incinerating raw MSW.
There are three major benefits of size reduction
First, shredding breaks the raw MSW into its basic components so material recovery and separation will be more effective.
Secondly, shredding reduces particles to a more manageable size that can be better handled by any subsequent processing equipment or personnel.
Lastly, and most importantly for ferrous metal recovery, overband magnets on the shredder discharge allow for automated ferrous metal separation from the waste going into the incinerator. Many metals can be recovered for revenue.
Shredding also allows for other material separation operations such as air classifiers, screens and optical sorters.
Moisture content in MSW can vary widely, from as little as 10 per cent up to 60 per cent in some food waste. This moisture content can have a large effect on the power consumption of an incinerator. Pre-shredding helps reduce the moisture content in MSW.
In an incinerator, the burn is controlled by particle size, heating value, air supply velocity, and the heat-transfer environment. Decreasing the particle size improves the reaction kinetics as a result of a larger overall surface area. Smaller particles can achieve higher burn rates at lower air supply rates. Lowering the air supply rate lowers the convective heat transfer by lowering Reynolds number and the convective heat transfer coefficient between the air and the particle.
These facts make apparent the difficulties that can be faced in designing a system meant to combust MSW particles of highly heterogeneous nature in heating value and/or particle shape and size. Processing the MSW into a more homogenous stream allows the designer of the system to choose an air supply rate that reaches maximum flame propagation speeds with minimal excess supply air, without quenching the flames of the smaller particles. Low excess air results in higher overall thermal efficiency. Uniform temperature distribution will maximize heat transfer in the incinerator.
The overall effect of shredding tends to reduce particle size between three and four times, with an average size of less than 100 mm, depending on feed composition, rotor speed and sizing bars. Of course, decreasing the particle size of combustible materials increases the surface-to-volume ratio, allowing for quicker heat and mass transfer and combustion rates.
Shredding of MSW in Landfills/Transfer Sites
The MSW capacity of a sanitary landfill is governed by the available airspace determined by zoning restrictions and the in-place density of the refuse. It is common practice in landfilling operations to use compactors to increase the density and stability of the refuse face. Several landfill operators have extended the operating life of their landfills using shredders.
It has been proven to be economically feasible and profitable to operate with a shredder on site. Say a landfill receives monthly revenues of $1,000,000 from tipping fees. A volume reduction of 30 per cent in the landfill density can extend the expected life of MSW management by one month for every three months of operating with the shredder, easily generating enough revenue to overcome shedding costs. An additional benefit of increased MSW density is that a greater tonnage can be deposited each day between the required daily applications of Daily Cover (e.g. 15 cm of soil is required by EPA).
In-place density is defined as the relationship between the solid waste tonnages to the airspace volume used for a specific time period. An investigation has shown that shredding MSW can lead to an increase of nearly 30 per cent in the in-place density, with an average improvement of 20 per cent.
The benefits of shredding are not limited to volume reduction. The decomposition rate of waste in landfills is increased with shredded material. The increased rate of decomposition generates larger quantities of methane on an annual basis. The net production of landfill gas will remain the same; however, the time frame for collection is decreased significantly due to decreased particle size. Landfill gas collection systems must be employed to both recover energy from the waste and to mitigate greenhouse gas emission (GHG). The landfill gas production rate also benefits from the more uniform flow of leachate throughout the refuse; the more evenly packed waste eliminates bridging that causes leachate to flow through channels. More densely compacted MSW can achieve the necessary saturation to enter the anaerobic zone more readily with less of a need for leachate recirculation. This leads not only to more rapid decomposition but more uniform decomposition lending to an landfill gas collection system with more simple controls and regulation.
Landfill gas is a combination of methane and carbon dioxide, in many cases a landfill will collect this gas and simply flare the combustible mixture to avoid added GHG emissions. The problem with landfill gas is that methane is a much stronger greenhouse gas than carbon dioxide and thus it is required that landfills be cognizant of this and either convert the methane to CO2 by flaring or running a landfill gas turbine to extract useful energy from the gas. Shredded refuse has an increased production rate and higher quality of landfill gas, allowing for easier collection. It means the landfill can be capped with fewer concerns about bio-degradation once the majority of methane has been produced.
The benefits of increased density due to shredding refuse go beyond improved storage capacity. A higher MSW density can save money in the transportation aspect of MSW management. Collection and transportation represent as much as 70 per cent of the cost of managing MSW. When it is necessary to transport waste over long distances to landfills, small collection trucks (three-four tonnes) usually deliver to a Waste Transfer Station, where long-distance trucks carrying 20 tonnes are loaded to transport the waste to its final destination. This allows for fewer trips, smaller crews and decreased operating costs. This allows for:
Transfer stations are generally equipped with one or more waste compacting devices, which increases the density of the waste by two-to-three times. It is clear that compacted MSW is higher density than non-compacted shredded MSW. But shredded MSW can compress further than unprocessed waste due to the increased packing efficiency that is possible with smaller particle sizes. The more uniform shredded MSW also results in less wear and tear on the compactor. In the event that a landfill decides it will benefit from shredding MSW, it could be beneficial to do this at the transfer station and thus capitalize twice on the increased density of shredded MSW.
Shredding of Construction and Demolition Waste on site
Recycling construction and demolition waste is a profitable and environmentally friendly way to produce aggregates and reuse valuable materials that would otherwise be disposed of. Shredding and processing the waste near the worksite also reduces the need for truck transportation, resulting in lower logistics costs.
Construction and demolition (C&D) debris often contains bulky heavy materials, including wood, roofing shingles, gypsum, metals and plastics, as well as salvaged doors, windows and plumbing fixtures. Low speed, high torque industrial shredders can grind and process this material to increase density for transportation and facilitate downline separation and recycling.
C&D wastes are made up of many economically valuable materials such as reusable aggregates, bitumen, brick, cardboard, concrete, metals, mineral wool and wood, many of which can be sold directly or used in new products, construction materials or energy production.
In an optimal case, this waste is processed near the demolition site, making the discarded matter a continuous stream of raw materials for new roads, buildings, bridges and urban landscape
Recovering as much material as possible also has another benefit. Often, heavy and bulky waste is expensive to dump in landfills or store in stockpiles. The larger the proportion of materials reused, the greater are the savings in waste management costs.
Many of the challenges in processing C&D waste are caused by the variable properties of the materials. By nature, the waste generated at demolition sites is bulky and often heavy, which increases transportation costs.
Reducing the waste to smaller and uniform grain size by means of shredding makes it easier to handle. Furthermore, the different materials can be separated from each other earlier in the recycling process, resulting in increased efficiency and productivity.
The closer to the worksite this shredding and processing is done, the less there is need for expensive and time-consuming loading, unloading, and truck traffic to and from the site, which not only costs but also generates unnecessary emissions and safety risks for the workers.