Water vapour


Molar mass 18.01528 g/mol

Water vapour is the most important greenhouse gas in the atmosphere, needed for life. Water is constantly cycling through the atmosphere. Its concentration depends on temperature and weather patterns, and varies a lot across the globe and through the year.

Carbon dioxide


Molar mass 44.01 g/mol
Lifetime in atmosphere No single lifetime can be given
Global Warming Potential over 100 years 1
Estimated emissions in 2008 51,762,916 Gg
Atmospheric concentration in September 2013 393,510,000 ppt

Nearly all of the carbon content in incinerated waste is emitted to the atmosphere as carbon dioxide. Municipal solid waste contains approximately the same mass fraction of carbon as does carbon dioxide itself (27%), so incineration of 1 tonne of waste is estimated to produce approximately 1 tonne of carbon dioxide.

Carbon dioxide emitted by human activity is of course the main cause of global warming leading to climate change. Under the Climate Change Act 2008, the UK is committed to reduce greenhouse gas emissions from 1990 levels by at least 80% by 2050. In 1990, emissions from energy consumption were 10.5 tonnes of carbon dioxide per capita, so the target is slightly over 2 tonnes per capita.

In 2011, UK emissions from energy consumption were 8 tonnes per capita, the reduction from 1990 largely due to the replacement of coal by gas in electricity generation. The Exeter Incinerator is designed to accept up to 60,000 tonnes per year of waste, from Exeter and the immediate surrounding area in Devon. With a population of about 120,000, that means 0.5 tonnes of carbon dioxide added to every person’s carbon budget. But suppose the Incinerator replaced some carbon emissions from other energy plant……

Incinerators have electricity generation efficiencies of 14-28%. The waste heat can be used in a district heating network, giving efficiencies higher than 80%. The Exeter Incinerator will initially provide electricity to the national grid, and has the potential to export heat but only if a district heating network is established on the Marsh Barton estate.

So the Incinerator will produce electricity at a substantially lower efficiency than the rest of the national grid, and displace lower carbon alternatives.



Polychlorinated dibenzo-p-dioxins (PCDDs; known colloquially and inaccurately as dioxins) are subject to the European Waste Incineration Directive, which puts strict limits on emissions to air. Incineration is controlled to minimise their production, and the flue gas is treated post-combustion. The resulting toxic fly ash must be handled as hazardous waste.

Emissions of dioxins and furans from an incinerator typical of those currently operating in the UK (230,000 tonnes per year) are approximately equivalent to emissions from accidental fires in a town the size of Milton Keynes (population 230,000). That is, emissions from the Exeter Incinerator will be equivalent to half the emissions from accidental fires in Exeter.

The structure of dioxins comprises two benzene rings (six carbon atoms) joined by two oxygen atoms. Chlorine atoms may be attached to this structure at any of positions 1–4 and 6–9 in the above picture, which gives 75 flavours. Hydrogen atoms are attached to the remaining positions.

Dioxins are commonly regarded as highly toxic compounds that are environmental pollutants and persistent organic pollutants. Of the 75 flavours, the seven below are considered toxic by the World Health Organization (WHO). 2,3,7,8-Tetrachlorodibenzo-p-dioxin* became known as a contaminant in Agent Orange, and is the most toxic of all. It is therefore designated the reference molecule for rating toxicity.

Flavour (DD stands for
dibenzo dioxin)


WHO Toxicity
Equivalency Factor






















For more information, see the Wikipedia articles about polychlorinated dibenzodioxins and specifically 2,3,7,8-Tetrachlorodibenzo-p-dioxin.

*The ‘p’ stands for ‘para’, indicating the oxygen atoms are opposite each other. The oxygen atoms could be next to each other, which would be indicated by ‘o’ for ‘ortho’, but this molecular configuration is unstable.