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No Smoke Just Steam 10 December 2004 The visibility of the plume of water vapour from the chimney of the Energy-from-Waste (EfW)
facility has been the subject of recent newspaper articles. These have
raised a number of questions about the plume - what it is and what can be done
to reduce it. The Department of Local Government
and Environment believes it is important these questions should not be left
unanswered and that the public should be informed.
Is it smoke?
We tend to associate chimneys with smoke. Smoke is caused when there is a fire
but not all the fuel is burnt. Small pieces of un-burnt material rise together
in the hot air and we see them as smoke, and the more pieces there are the
blacker the smoke. For example, paper burns more quickly than plastic. If we
have a fire fuelled by paper only, the smoke almost looks white. If we add
plastics the smoke becomes black as plastics are difficult to burn.
When we see a plume coming from the EfW chimney it would be natural to think
this is smoke from the incineration process and that something is wrong with the
EfW when the plume appears. Under normal operating conditions, however, the EfW
does not produce smoke because
- combustion temperatures are so high that very little material remains
un-burnt, even plastics;
- the chimney is fitted with specially designed equipment to ensure any
un-burnt material and waste gases from the flue are not released to the
atmosphere
These un-burnt particles and gases are collected separately and known as "Air
Pollution Control Residues" (APCRs), which are disposed of under "licence" in
the UK. If anything ever went wrong with the incineration process, the
monitoring equipment would immediately alert the EfW Control Room. If the
pollutants exceed permitted levels, except briefly, the EfW will be shut down.
This ensures that the public and the environment are not exposed to potentially
harmful levels of pollution.
If it's not smoke then what is it?
The fact of the matter is that the plume you see is simply water vapour i.e.
water in the form of steam. The water comes from the waste itself and how much
vapour we see depends on how wet the waste is, and what the weather conditions
are. If you imagine the air around the chimney as a big sponge, when it is dry
it can soak up the water coming from the chimney. However, in damp or rainy
conditions the air is like a wet sponge, which means the water coming out of the
chimney has no place to go and when it cools down it turns into the steam we see
(it condenses). Cold weather also causes this effect in the same way we can see
our breath on cold winter days
So why does the plume look white some days and grey on others?
The colour of the plume never actually changes it just looks different in
different light and weather conditions. This happens in the same way the sea
appears to change colour; some days the sea looks grey, and on others it looks
blue.
What can be done to make the plume invisible? If the steam coming out of the
chimney is reheated the water vapour will spread out well away from the chimney
before it turns to droplets and we will not be able to see them. However, to do
this we would need to burn oil to heat the steam. This would mean burning fossil
fuels, which are not renewable and can cause air pollution.
The conditions imposed by the planning consent on the operation of the EfW do
allow a visible plume in certain weather conditions, as described above. What
that consent certainly does not allow is smoke to be produced during normal
operating conditions. Consequently, the emissions from the EfW are tightly
controlled in order to protect our health and the environment. However, the
Department does not believe that it makes sense to burn fossil fuels simply to
reduce the visibility of a plume of harmless steam, which is why a plume is
sometimes visible coming out of the EfW chimney.
What other Checks are there?
The Department has established the Richmond Hill Consultative Committee (RHCC)
to oversee, independently, the operation and, more importantly, the regulation
of the EfW. The Minister for Local Government and
the Environment chairs the RHCC, with the other representatives being elected
members from Douglas Corporation, Braddan Parish
Commissioners, Onchan District Commissioners and the
Isle of Man Municipal Association. The RHCC meets at least four times a year
but, as the EfW is new, it has decided to meet every six weeks initially. The
January 2004 Agenda will include an item on the issues surrounding the visible
plume from the EfW. More stories >>>
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