Landfill or incineration: what is the best disposal route for our waste plastic materials?
Large numbers of energy from waste (EfW) plants exist right across Europe and many of the UK’s municipal solid waste (MSW) and mixed recyclables ‘processing’ facilities are simply exporting a high proportion of mixed material refuse derived fuel (RDF) bales to utilise spare European capacity.
Further EfW plants are being planned and built in the UK, while landfill is frowned upon as the accepted ‘worst option’ for disposal. Revisiting the decision-making metrics that have led to this ‘accepted waste hierarchy’ might point to a stark choice between the two waste disposal options.
There are valid arguments for the ‘pros’ and ‘cons’ of both waste disposal methods. But what if the true environmental cost of CO2 emissions was also factored into deciding the ‘best option’?
Good or bad waste material?
When the full carbon-cost of the disposal method is expressed in ‘pound notes’ - reflecting the impact that large-scale CO2 release has upon the earth - then this metric could really change decisions about what is ‘good’ waste material to burn as a fuel and what is ‘bad’.
Existing designs of EfW incinerators are a compromise engineering solution. The non-homogeneous nature of the waste fuels requires robust moving-grate burners to move the combustible materials through the unit. Water-filled side boilers must be used for heat transfer to capture the heat-energy produced.
Even the most modern burner designs are relatively inefficient at energy recovery, generating lower amounts of electrical power per tonne of fuel burned when compared to high efficiency, combined cycle gas turbine systems (CCGT). Both power generating units are ultimately doing the same task: converting carbon-rich fuels into electricity (and occasionally combined with ‘useful-heat’ production), while sending atmospheric-polluting carbon emissions up the exhaust stack as a major environmental cost associated with the beneficial electrical power supplied into the local grid.
So what are the solutions? High-efficiency gas turbines are a much more efficient way to generate each kilowatt of power, plus some heat, from fossil fuel sources if measured in terms of the mass of CO2 released per unit of power output.
However, large scale waste-burners consume huge tonnages of waste materials that would otherwise have been landfilled. Siting an EfW plant close to large urban areas can sometimes deliver useful heating into local industry and households. However, nearly a third of this input waste volume ends up as residual incinerator bottom ash (IBA). This begs the question: where does all that IBA tonnage end up?
If the major components in the infeed waste fuel mix to a modern EfW unit are renewable carbon, such as wood, papers, cardboard or organic matters, then the ‘short-life’ carbon atoms released back into the atmosphere via the exhaust stack are ecologically balanced with their earlier carbon-capture in a tree, plant or living organism. So this fraction of the waste ‘fuel’ shows a carbon-neutral effect.
As for the plastic content in residual household or commercial waste, the carbon-rich molecules that create the long-chain polymers (e.g. ethane, propane, styrene, etc.) are derived from crude oil refineries and are then polymerised to make plastics. Burning these is essentially the same as driving a petrol car or taking a flight – power created at the ‘expense’ of long-life carbon release.
But what if the waste plastic could be separated from the ‘organic or renewable carbon’ wastes and the ‘inert’ carbon-rich, stabilised plastic stored either in the ground or in a covered storage system. That would represent a long-term carbon-sink and remove those fossil-based materials from the EfW infeed mix. Clearly it would be better to recycle these materials if a technically and economically viable process was available to do that.
A typical EfW plant has efficiencies of up to 30% for converting feed material into electricity; in contrast, a combined cycle gas turbine’s efficiency is typically about 50%. As shown in below, this disparity in efficiencies means that producing 1 MWh of electricity from a CCGT produces just 40% of the CO2 emissions for the same energy from plastic incinerated at an EfW plant.
It is true that a best-in-class EfW plant with integrated heat recovery can recover a further 35%; however, this heat could instead be generated by a gas boiler that has an efficiency of at least 90%. Even taking this additional efficiency into account, a combination of CCGT and boiler still emits about 65% of the CO2 of the leading EfW plants.
Using the CO2 metric alone suggests that it makes more sense to bury large amounts of plastic in a long-term ‘carbon sink’ in the ground and efficiently combust natural gas to satisfy our immediate power needs. However, until world leaders are prepared to transform the taxation on fossil fuel use in a way that truly reflects the high cost of ‘free carbon release’, then this numeric analysis remains an esoteric academic study.
Perhaps we should start calling this form of end-of-life waste treatment ‘sky-fill’ to compare it with the alternative ‘land-fill’ disposal route for carbon mass?
The Paris Agreement commits countries to taking action to hold temperature rises to well below 2C above pre-industrial levels - and to try to stabilise emissions at a level which would see a temperature rise of no more than 1.5C.
Following the agreement’s signing by the largest CO2 producers and as the COP22 meeting in Marrakesh draws to a close, some world-leading countries may start to introduce the taxation of fossil-fuel carbon release as a means to get the world’s atmosphere back under control and remain within the stated, and agreed, 1.5C global warming limit. However the major contributors to global carbon emissions, USA and China, appear to remain heavily dependent upon coal-fired power plants and oil-based fuel systems in their economic activity.
Eminent scientists worldwide have calculated that a very large proportion of the known (and often privately-owned) reserves of oil, gas and coal already available for extraction and combustion will have to stay in the ground as part of tackling climate change and staying within agreed limits.
The huge shift in corporate and national energy-habits required to leave fossil fuels in the ground will only happen with a carbon tax; particularly on the creation of electrical power and directly linked to the tonnes of CO2 released into the atmosphere per unit of fossil carbon consumed.
If that happens, it might be the time to return to that ‘mine’ of carefully stowed thousands of tonnes of good plastic and look at the economics of turning it into new polymer. With a huge carbon tax slapped on burning it, then the economics would probably work. So these plastics may not have to stay in the ground for too long.
Looking at the bigger picture, we should all be concerned about the wholesale damage of completely uncontrolled burning of fossil fuel. That’s what we’re doing when we’re burning plastic that’s encapsulated amongst the mixed MSW we put in our black bin bags.
The short-term political and economic viewpoint is that ‘we’re getting some electrical power from it so it must be a good thing to do’. But this I think reflects the market failure created by very high landfill taxes that are not balanced by an equivalent taxation method to discourage ‘sky-fill’.
It’s a complex and challenging issue that reaches out over the next 20 years; a critical period in our history. Until we get a carbon tax that puts some seriously big pound notes on the cost of throwing carbon into the atmosphere, I don’t see there being any real change. After all, the Earth doesn’t have a bank account – it’s us humans who operate under that monetary metric.
Axion Polymers is part of the Axion Group that develops and operates innovative resource recovery and processing solutions for recycling waste materials. The Group works with a wide range of clients within the recycling and process industries on the practical development of new processing and collection methods.