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Assessment of Methods to Recover Energy from Waste Plastic

Summary

Purpose

The purpose of the study was to describe and assess various methods for recycling/energy recovery of the plastic fraction in mixed waste. The potential amounts were calculated to be 2,900,000 tons of mixed waste that is currently incinerated in waste incineration plants. 8-10% of this is plastic waste. Plastics already being collected for recycling purposes are not considered in the investigation.

The starting point for the investigation is the total amount of mixed waste in Denmark. The plastic fraction of this can be treated in different ways, and the methods assessed in the current project are:

• Waste incineration (of the total amount collected)
• Separation of the waste in an energy-rich fraction and in a residual fraction. The energy-rich fraction can be produced in two qualities (both termed “sub-coal”), one with a calorific value of about 20 GJ/ton (PPF) and one with a calorific value of about 30 GJ/ton (MPF).
  • Use of PPF and MPF as a substitute for coal in combined heat and power production
  • Use of PPF as a substitute for coal in cement kilns
• Recycling to basic chemicals by degassing/synthesis

Assessment method

The different technologies for treatment of mixed waste are described and subsequently compared by:

• Establishing energy balances
• Calculation of selected emissions related to the treatment methods
• Calculation of the contribution to selected environmental impacts
  • Global warming
  • Photochemical ozone formation
  • Acidification

Treatment methods

Several methods for sorting the plastics out from mixed waste have been developed. In the current project the focus is on a Dutch method for mechanical separation of a paper/plastics fraction (PPF) and further refinement to a mixed plastics fraction (MPF).

The first step – the mechanical separation producing a paper/plastics fraction – can be found in industrial scale in the Netherlands. The second step – a wet separation process yielding a mixed plastic fraction – has been tested in pilot scale, also in the Netherlands. The resulting product is a fuel that can be further processed and used in production of combined heat and power or in cement kilns as a substitute for coal.

In the Netherlands it has been concluded that the second step is economically viable only if the out-sorted paper fraction can be exploited commercially, e.g. by recycling into paper. Collection of waste paper for recycling is very common in Denmark, and utilisation of a “new” fraction of waste paper of a significantly lower quality than usually found is possible, but not very realistic. The assessment of the use of sub-coal is therefore focused on the paper/plastics fraction.

PPF and MPF are assumed to be produced from Danish waste. The potential amounts were estimated at 333,500 ton PPF and 130,500 ton MPF. The estimated amounts are relatively uncertain, being estimated from total Danish amounts assuming that the composition of Danish waste is comparable to that of the Dutch waste. Using the Dutch experience, 9-11 mechanical separation plants are needed in connection with existing incineration plants.

PPF and MPF can be used in Danish combined heat and power plants. Enstedværket was chosen as the model plant, and based on the composition of PPF and MPF as well as incineration parameters, the emissions were estimated. In a possible future practice, the use of PPF and MPF can be assumed to take place on plants in Jutland and Zealand. Sub-coal can also be used in cement kilns, and the Aalborg Portland plant was used as a model. Based on information about selected quality parameters, PPF and MPF are judged to be suitable as a fuel. As an example, the content of heavy metals is assessed not to have any influence on the quality of the final cement product.

An alternative to energy utilisation of the plastics fraction is chemical degradation of the polymers. Degradation and degassing requires a complex plant that, with additional input of oxygen, can produce methanol, electricity and heat. Degassing can be performed on the plastics fraction alone, or on mixed waste. The SVZ plant in Germany was chosen as a model. The processes at this plant were developed to treat a combination of different types of waste: household waste, contaminated wood, plastics, sludge, tar, etc. The degassing option can potentially be used on mixed waste in Denmark or – following a separation step – on the plastics fraction alone, either in Denmark or Germany.

Results

The assessment of alternative methods for treatment of mixed or processed waste comprises the following scenarios:

• Base case scenario: Incineration of mixed waste in waste incinerators with energy recovery
• Scenario I: Incineration of mixed paper/plastics fraction (PPF) in combined heat and power plant; residual waste is incinerated in waste incinerators with energy recovery
• Scenario II: Incineration of mixed plastics fraction (MPF) in combined heat and power plant; residual waste is incinerated in waste incinerators with energy recovery
• Scenario III: Incineration of mixed paper/plastics fraction (PPF) in cement kilns; residual waste is incinerated in waste incinerators with energy recovery
• Scenario V: Treatment of mixed plastics fraction by degassing, residual waste is incinerated in waste incinerators with energy recovery

By using the model developed by the Danish Energy Agency for allocation of CO₂ emissions between heat and power production, the combined yield of energy in the form of heat (for district heating and processes), electricity, and methanol (as fuel) can be established:

With the selected allocation method the greatest yield is observed in Scenario I, in which PPF substitutes coal in a combined heat and power plant. An almost similar yield is observed in Scenario III, in which PPF substitutes coal or petrol coke in cement kilns. Refinement of the paper/plastics fraction to a mixed plastics fraction with a higher calorific value does not increase the overall yield of energy. It is mentioned that economic calculations using the prices for delivered energy (heat and power) also show that the Scenario I is the most favourable option. The calculations does not take the cost of building and maintenance of the separation plant into account and should only be seen as a very rough indication.

A sensitivity analysis examining a 10% increase of the calorific value of the PPF and MPF fractions does not change the results significantly and does not change the relative ranking of the scenarios.

The contributions to potential environmental impacts are shown in the Table below.

It appears from the Table that the highest contribution to global warming is found in Scenario V, which on the other hand also shows the largest potential for reduction of the contribution to photochemical ozone formation and acidification.

Conclusions

The assessment indicates that it is possible to achieve a higher yield of energy by separation of mixed waste into a fraction with a higher calorific value and a residual fraction. Scenario I (PPF in combined heat and power plants) and Scenario III (PPF in cement kilns) give the best utilisation of the energy content of the waste, either in the form of increased production of electricity or in the form of process energy in the cement kiln. The contribution to the selected potential environmental impacts shows a heterogeneous picture, where Scenario V has the highest contribution to global warming, but at the same time also provides the largest reduction of the contribution to photochemical ozone formation.

It is, however, stressed that the choice of system boundaries – primarily with respect to which technologies are affected by changed handling of mixed waste – is of great importance for the results. This, especially when combined with the rather inhomogeneous data base that was available for the assessment, means that the above conclusions should be regarded with great caution.

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Version 1.0 Januar 2004, © Miljøstyrelsen.