Methane from non-biogenic (waste) streams? An exploration of potential, environmental effects, and policy

In the context of the upcoming obligation to blend green gas into natural gas and due to the sharply increased cost of natural gas as a result of the war in Ukraine, the question has arisen whether it might be worthwhile converting non-biogenic streams into methane gas.

There are a number of ongoing initiatives engaged in developing gasification plants that can convert biogenic waste in particular into biomethane, biohydrogen and biomethanol. These initiatives are also conducting experiments with non-biogenic waste. This usually involves a mix of biogenic waste and non-biogenic waste. Non-biogenic waste for gasification involves a variety of, mostly mixed, streams, where the energy value/non-biogenic carbon content consists largely of plastics. Examples include: mixed household waste, plastic and metal packaging and drink carton waste (PMG) and RDF, a paper and plastic fraction separated from waste. Currently, government support extends only to the biogenic part of the input from such a mixed waste stream.

Compared to other materials, such as glass, paper and metal, plastic is currently relatively under-recycled. Approximately 28% of all plastic waste in the Netherlands was recycled in 2020, which means about 12.5% of new plastic products are made with recyclate (the demand for recyclate exceeds the amount of plastic waste that recyclate can be made from). The 72% that is neither separated nor recycled is largely incinerated in waste incineration plants (AVIs);

According to the Dutch plastics transition agenda, new plastic should consist of 40% recyclate by 2030 (in total 2,400 kton), compared to 12.5% now (300 kton). Most of this increase (750 kton) would have to be met by more mechanical recycling, but also partly by chemical recycling (250 kton).

In addition to the recycling sector (both mechanical and chemical), plastic waste is being considered for use in aviation fuel (SAF), transport fuel (Recycled carbon fuels), cement kilns and the existing waste incineration plants. The total demand of all these sectors is about 3 times the supply.

Other studies show that gasification of biogenic streams (sewage sludge, etc.) provides a useful energy-rich supplement to fermentation and green gas and biogas. The following main conclusions emerge from this study into whether non-biotic waste is a useful supplement to this:

  • The demand for plastic waste for various applications (mechanical recycling, chemical recycling, cement kilns, transport fuel, aviation fuel and possibly methane for energy application) far exceeds the supply.
  • Both in terms of policy, CO2 reduction score and energy efficiency, recycling back to plastic is the preferred option.
  • To meet plastic recycling targets by 2030 (Plastics Transition Agenda), all plastic waste is needed for recycling.

While recycling is still in the development stage, it makes sense to use energy with the highest possible energy efficiency and CO2 score for the residual waste. In this regard, both the high-efficiency incinerator with CCS (part of a large heat grid) and the cement kiln (while still coal-fired) have the best scores. Gasification to methane has a slightly lower energy efficiency, comparable to the best three waste incineration plants currently in the Netherlands, but it also scores reasonably well in terms of CO2 due to CO2 capture.