Latest Buzz...


Monday, July 9, 2018

Keeping waste plastic out of landfill

The March 30, 2018 report by the Australian Packaging Covenant Organisation describes the fall in the price of waste plastic following Chinese import restrictions for packaging.

There is a relatively small drop in the price for Plastic - HDPE ($575/tonne during 2015-2017 to $500/tonne in 2018), a larger drop in price for Plastic - PET ($575/tonne during 2015-2017 to $375/tonne in 2018) and a substantial fall in price for Plastic - mixed ($325/tonne during 2015-2017 to $75/tonne in 2018).

One avenue that raises the value of waste plastic is to convert it to methane and use it with natural gas.

A number of projects are underway to convert renewable energy to hydrogen and inject that hydrogen into natural gas distribution pipelines.

The wholesale price of energy delivered via that method is about $10/GJ. The retail price is about $35/GJ.

One tonne of methane can be made from about 875 kilograms of plastic waste and 125 kilograms of hydrogen. The energy content of this methane is about 55 gigajoules. At $10/GJ its value is $550.
Steam Hydrogasification in a hydrogen environment

The 125 kilograms of hydrogen made with renewable energy has an energy content of about 15 gigajoules. It contributes about $150 to the $550 value of the tonne of methane.

Put another way, 875 kilograms of plastic waste adds $400 to the value of the hydrogen that is to be injected into natural gas pipelines. This lifts the value of plastic waste to about $455/tonne: more than the $325/tonne price for Plastic - mixed during 2015-2017 and substantially more than the $75/tonne in 2018.

Using hydrogen to produce of methane is the subject of a number of research papers and patents.

The availability of hydrogen that is intended to be injected into natural gas pipelines means that this established body of knowledge is increasingly likely to find commercially viable applications.

Some References

  1. Hydrogasifcation of biomass to produce high yields of methane, U.S. Patent 4,822,935 April 18, 1989
  2. Production of Substitute Natural Gas by Biomass Hydrogasification, M. Mozaffarian, R.W.R. Zwart, Netherlands Energy Research Foundation, ECN, April 7, 2008
  3. The steam hydrogasification reaction, which researchers at the University of California, Riverside’s Center for Environmental Research and Technology engineers began developing in 2005, has been found to be 12% more efficient, with 18% lower capital costs, compared to other mainstream gasification technologies, September 15, 2011.
A different process is helpful if, for instance, you want to upgrade biogas - a mixture of carbon dioxide and methane - to be suitable for injection into a natural gas pipeline. One method is to separate the carbon dioxide from the biogas.

Another method available if you have a quantity of hydrogen that is to be injected into the same natural gas pipeline is to convert the carbon dioxide that it is in the biogas into methane with the hydrogen. This avoids both the overhead and cost of separating the carbon dioxide from the biogas. It also dodges the limitation that the quantity of hydrogen that can be safely mixed with natural gas should be no more than 10 percent: the hydrogen gets converted into methane.