Energy And Polymer Recycling

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Increasingly, there is a demand, as with other materials, that plastics should be recycled. For most thermoplastics this is a possible route for postconsumer material, and this is dealt with in more detail in other chapters. However, there is some confusion over the energy savings that can be achieved by plastics recycling, and this can lead to conflicting decisions.

The problem is best illustrated by considering first the recycling of metals. Primary aluminum, for example, is produced by the electrolysis of fused alumina and is an energy-intensive process with a gross energy of 150 MJ/kg aluminum. Suppose that 100% of all aluminum can be recovered, that there are no losses during remelting, and that remelting uses an energy of 20 MJ/kg, then for the average energy required to use 2 kg of aluminum is 150 + 20 = 170 MJ or the equivalent of 85 MJ/kg. In other words the primary production energy is spread over more than one use. Repeated recovery and remelting would, of course, reduce the average energy per kilogram even further. In practice the average saving will be lower because it is not possible to recover 100% and some melt losses do occur. The example does, however, illustrate the critical point that repeated recycling can lead to energy savings because the initial production energy is spread over more than one use.

For plastics the situation is somewhat different. The total gross cumulative energy required to produce virgin polymer is the sum of the polymer production energy and the feedstock energy. The polymer production energy is the equivalent of the production energy for aluminum, but the feedstock energy has no corresponding component in the aluminum example. Thus when considering polymer recycling, it is essential to keep separate the production energy and the feedstock energy, and different techniques are needed to affect these components.

There are essentially two techniques applied in polymer recycling: mechanical recycling and energy recovery. In mechanical recycling, the postconsumer material is recovered, cleaned, and flaked so that it can be used again to fabricate further products. There may be a deterioration in the properties of the recycled polymer, which means that it is used in products with a less demanding specification; but, nevertheless such recycled material replaces polymer that would otherwise have to be produced from virgin sources. This process is identical to the recovery and reuse of aluminum because the production energy is spread over more than one use. The feedstock energy is unaffected by mechanical recycling. If 1 kg of polyethylene has a feedstock energy of 45 MJ/kg, then this will be present in the virgin polymer and also in the recycled polymer; burning recycled polymer yields the same energy as burning virgin polymer. Feedstock energy can therefore be regarded as a circulating load that remains as long as the polymer exists.

In energy recovery, the postconsumer material is recovered and burned as a fuel. This process essentially recovers that proportion of the feedstock energy that remains in the polymer. In practice, the whole of the feedstock energy is seldom available for recovery. Feedstock energy is the calorific value of the hydrocarbon feed to the polymer production system. However, chemical changes and losses during production mean that the feedstock energy will be different from the calorific value of the final polymer. Despite this, it is clear that energy recovery acts on the feedstock energy and has nothing to do with the production energy.

This distinction between production energy and feedstock energy is very important in recycling for two main reasons:

1. Because the two recovery methods act on different components of the gross energy, they are not mutually exclusive. Mechanical recycling acts on production energy by spreading it over multiple uses. In contrast, energy recovery acts on feedstock energy by attempting to recover it. These two recovery techniques are often presented as if they were mutually exclusive so that a choice must be made between mechanical recycling or energy recovery. In fact this choice need never be made. If the ultimate goal is energy saving, then mechanical recycling should always be followed by energy recovery.

2. To achieve energy savings in mechanical recycling, the recovery and reprocessing of postconsumer waste must use less energy than is needed to produce virgin polymer. To assess whether or not a recycling process is energy saving, it is necessary to identify the window of opportunity for recovery. For HDPE, the gross energy to produce virgin polymer (i.e., production energy plus feedstock energy) is 80 MJ/kg. Of this 48 MJ/kg is feedstock and 32 MJ/kg is production energy. The recovery process therefore must not use more than 32 MJ/kg.

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