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1 December 2008
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Introduction
Have you ever wondered what happens to the plastic
rubbish bag that is collected from in front of your house? If it weighs 10
kg, on an average it has the same energy in it as 2½ kilograms or 3 litres
of fuel oil. Nevertheless, the chances are that it is taken to a
rat-infested, smelly, landfill, where it will slowly decompose, releasing
methane, a bad greenhouse gas, and the rest will remain there for ever. That
waste is a waste of money, because it has a value, if used sensibly. This
essay shows how the inhabitants of Lausanne, Switzerland, and further afield
exploit the value, both energetically and environmentally.
The main purpose of publishing this page is to
disseminate the fact that it is better to extract value out of waste rather
than simply allowing it to pollute the environment in senseless landfills. A
separate page describes how a similar type of installation could enhance
the Cypriot economy and environment, although the information here could be
a good introduction to the notion of enhancing the value of waste in any
country. More information can be found in the Tridel website at
www.tridel.ch (in French), including a
video describing the plant.
This enhancement of the value of household rubbish, as
well as organic industrial and building waste, is achieved with the help of
Tridel SA, a corporation funded by investments, bank credits and subsidies
from the Swiss Confederation, the Canton of Vaud and the City of Lausanne.
The latter pioneered the technique some 40-odd years ago, but the existing
installations no longer had the necessary capacity, required too much
maintenance and did not conform to the newer strict Federal or EU emissions
standards (OPAir). In 2001, a referendum was held and the people
democratically decided to have a new state-of-the-art generation plant,
which entered into operation in 2006.
Before describing our visit, the background of the old
plant needs to be mentioned. The steam generated was sent to a power station
at Pierre-de-Plan, a few hundred metres distance, where it was used to
generate electricity and to distribute steam for space and water heating to
the Cantonal University Hospital complex (CHUV) and to a number of apartment
blocks. The old and new plants are situated in the steep-sided valley of the
Flon, about 200 metres above Lake Geneva, Tridel being the best part of a
kilometre higher up the valley. Both plants are almost hidden from view
within the valley. As we shall see later, this geographical layout is
important.
Catchment area
The main catchment area for trash is Lausanne city and
a large part of the Canton of Vaud, some 145 communes with a total
population of about 380,000. Apart from Lausanne, other large towns include
Morges, Yverdon-les-Bains, Orbe and Sainte Croix. These communes produce
about 137,500 tonnes of non-recyclable rubbish/year. However, it should not
be forgotten that the Swiss citizen is disciplined and separates the rubbish
into all kinds of recycling bins, so the per capita quantity of
non-recyclable waste is low, which is actually a disadvantage for Tridel. In
places where recycling of paper and plastics is minimal, the quantity of
available fuel would be considerably higher, for the same population.
Other than locally produced rubbish, some places
outside the Canton of Vaud also send waste, even over considerable
distances, such as the Canton of Ticino in the south-east of the country.
About 15 per cent of the waste comes from outside Switzerland, notably from
Germany. Also, if some other incineration plant or landfill cannot be used
for any reason, the excess can be sent to Tridel (or vice versa!).
Arrival of waste
There are two ways the rubbish can arrive,
either by truck or by rail. The latter is innovative (and costly!). Because
of the altitude of Tridel, it was thought that trucks coming from the bottom
half of Lausanne would be a nuisance and cause too much noise and pollution,
climbing up the hill. It was therefore decided that it would be logical to
discharge them at Sebeillon station, compact it into special containers and
bring it up by rail. This meant piercing a 4 km long S-shaped tunnel 50 m
under Lausanne. This has another advantage; the containers could be loaded
anywhere, even hundreds of km away, and brought to Sebeillon by ordinary
goods trains, where they can be shunted directly up to the plant.
The local trucks are mainly those that service the
higher parts of the city and the surrounding communes on the plateau.
However, some distant places, such as Freiburg-im-Bresgau in Germany
(400 km), send waste here by truck, with it still being economical.
The trucks and containers go up a spiral ramp to
discharge (1) the contents through one of eight automatic doors into a 30 m
deep storage silo (2) which can contain 10 000 m3 of waste for up
to two weeks of continuous production. The air in the storage silo may be
dusty and contain combustible or even explosive gases if they are allowed to
accumulate. For this reason, it is continuously drawn off at a high volume
(4) and fed into the incinerator to remove any dangerous emanations or
emission of greenhouse gases. Accidental fires in the silo are avoided by
continuous monitoring of the infra-red radiation and other detection
equipment. Water cannons can be directed to the heart of any fire that may
break out. From the silo, an enormous grab (3) can lift 2.4 tonnes and
transfer it into a hopper equipped with a screen and a mill to break up
large items, such as planks of wood. The grab can be seen at the bottom
right of the right-hand photograph above. There is no other sorting at this
stage.
Incineration and flue gas treatment
There are two incinerators into which the
rubbish is pushed from the hopper onto an inclined oscillating firebed (5),
which has a forced air feed from underneath to assist the combustion which
occurs at 1 000 - 1 200°C. The clinker falls through the firebed and to the
bottom with a particle size of about 5 -15 mm, for further operations after
cooling.
The steam is generated in a boiler and superheater (6)
through 40 kilometres of pipework around which the flue gases circulate. At
full capacity, the steam production from both 40 MW incinerators has a
recoverable energy equivalent of 60 MW. The gases then go through an
electrostatic precipitator (7) to eliminate most of the fly ash. They are
then subjected to a series of treatments, starting with a quench (8) and
other treatment (9), described in more detail later, and through to the
chimney (10).
Solids and water are treated to render them inoffensive
to the environment in a sophisticated series of operations (11), as
described in detail later in this essay.
Steam usage
The steam generated in the incinerator
boilers is piped to a steam turbine at 400°C and 40 bars, driving an
alternator, similar to that in any other thermal or nuclear power station,
although smaller than those in large power plants. This can generate up to
20 MW of electricity, enough for the average consumption of a town of about
23 000 inhabitants. 3 MW is consumed internally to run the Tridel plant and
the rest is fed into the grid. The steam is still hot, coming from the
turbine, and is fed into a heat exchanger, which heats pressurised water to
175°C. This is sent, in a closed circuit, through insulated pipes in a
tunnel, to the old Pierre-de-Plan station to heat water for the CHUV and
neighbouring apartment blocks, in summer.
Winter requires heat for space heating in the same
buildings and there is not sufficient energy left in the steam to heat both
the hot water and the space. The turbine is equipped with two stages and by
using just a small part of the turbine, enough electricity can be generated
to run the plant and the rest of the steam can be used for adequate heating
in the coldest weather, down to less than ‑15°C. Obviously, the number of
joules required for space and water heating varies with the weather and all
the excess energy left over from these requirements is used to generate
electricity which is sold to the grid. This means that, to obtain the best
rates, it is necessary to sell it as a future, a day or two in advance, so
the evolution of weather patterns is studied, to forecast the probable
available capacity.
One can speculate that this balance of heat and
electricity production is more than favourable. As the temperature drops in
the evening, so the heat requirements will increase and electricity
production will decrease. As maximum electricity demand is during the
daytime, for industry and commerce, this can produce a more profitable
equilibrium in the “between seasons”: maximum electricity production by day
and maximum heat by night.
If the plant is running at full electrical output and
the waste heat is used only for water heating, there is no miracle; the
overall thermal efficiency is the same as for any other thermal power
station, between 30 and 35 per cent. On the other hand, the thermal
efficiency rises in winter, because most of the energy is used to heat the
buildings. It can reach over 80 per cent. The overall average year-round
thermal efficiency is just over 50 per cent.
Environment
One thing of note that, even after nearly two years of
exploitation, nowhere in the plant is there any trace of smell or undue
dust. It is one of the cleanest factories I have visited.
The first thing that comes to mind is the quality of
what comes out of the chimney. As mentioned, this is largely nitrogen, of
which air is composed of 80 per cent. The
tables on the Tridel website show that the toxic elements of most of the
substances resulting from the combustion are reduced to better than 10 per
cent of the values allowed under the very strict Swiss legislation (Opair)
and under EU legislation, with the sole exception of nitrogen dioxide, which
is roughly down to 50 per cent of the permitted values according to the
Swiss legislation or 25 per cent according to the EU. This can only be
described as extremely satisfactory and public health is in no way
endangered. It should also be mentioned that these values are automatically
monitored in permanence and any major deviation from the guaranteed values
will cause a shutdown of the line, no matter the cause.
Referring to the schematic, the incoming flue gases,
after the precipitator, are cooled through a quench column (1) and then goes
through a water scrubber (2) which eliminates any residual fly ash,
dissolves acid gases and heavy metal salts, including mercury compounds. The
temperature at this stage is quite low, so it goes through a heat
exchanger (3) to heat it up again to between 250°and 300°C. At the same
time, the incoming gases are pre-cooled, prior to the quench. The re-heated
gas then goes through a catalytic converter (4) to reduce nitrogen oxides
and any residual insoluble organic gases. A ventilator (5) ensures there is
no reflux of the gases into the plant, after which they are passed through
the silencers (6) to the chimney (7). The raw, untreated, waste water is
collected in a tank (8).
As for waste water, the schematic shows a complex
treatment. The fly ash from the electrostatic precipitator is mixed with the
sludge from the scrubbers. A solid mass is obtained by mixing with limewater
and this is mixed with the cinders to be sent by rail to a non-hazardous
landfill. The volume thus landfilled is about 10 per cent of the volume of
the original compacted rubbish or 20 per cent, by weight. Interestingly,
there remains a small proportion of carbon in the landfill which remains
somewhat bioactive, albeit sterile. All the waste waters are then treated
with sodium hydroxide (caustic soda) to precipitate heavy metals in the form
of hydroxides, which are filtered out. The precipitate is bagged and sent to
Le Havre by train for recuperation of the metals. After neutralisation, the
waste waters, conforming to well under the limits set by the Federal
Ordinance on the treatment of waste water (Opeau) are sent to the public
sewage system.
As a matter of interest, to
reduce the consumption of potable quality water, the rain water collected
from the roofs and the grounds of Tridel are collected in a cistern and are
used in the scrubbers and for other technical purposes.
The clinker contains
non-combustible material, such as glass and aluminium which have melted
during incineration, and ferrous and non-ferrous metals which have not
melted. The waste which has melted has a certain nuisance value because it
forms lumps. The ferrous metals are separated by a powerful electromagnet
and recovered as scrap iron which has a small market value. Non-ferrous
metals are not separated for the moment but an induction separator is
planned for the near future.
Environmental balance sheet
So, what does the holistic
environmental balance sheet look like?
On the positive side, the fuel
to convert to heat and electricity is organic and renewable. It will contain
a small proportion of fossil-fuel derived waste products, such as plastics,
but it can be said that these have already been consumed in their primary
use. If one wished to develop this argument to the absurd, one can say that
one-third of the carbon in a rejected lettuce leaf is also derived from
fossil fuels because that is about the proportion of fossil fuel derived
carbon dioxide in the atmosphere. All the rail transport of fuel to the
plant is essentially carbon-free, mostly derived from hydroelectric sources,
with a proportion of nuclear. Much of the water used in the treatment of the
residual gases and solids is simply rainwater collected locally. The chimney
effluent is essentially nitrogen with a proportion of carbon dioxide and a
very small proportion of pollutants. The water effluent is reasonably pure,
probably chemically better than that from an average household. The heat and
electricity generated reduces the need for burning fossil fuels to produce
an equivalent energy (just think of the tonnes of fuel oil that would be
otherwise needed just to heat the water and space occupied by 18 000
persons!). Landfill volume is reduced by 90 per cent, compared to
landfilling the same amount of compacted rubbish. If the same rubbish were
landfilled, the emissions, notably methane, would amount to tens of thousand
of tonnes of carbon dioxide equivalent greenhouse gases, even after
subtracting the carbon dioxide emitted by the Tridel stack itself. There are
no ozone-depleting substances emitted.
On the negative side, there is
the fossil fuel required to transport waste by truck to Tridel; however,
would this be more than transporting the same waste to a landfill? Rather
doubtful! A small amount of fossil fuel is used for internal manipulations
within the factory itself. Otherwise, the production of other incoming
products, such as the acid, lime and caustic soda will require some fossil
fuel energy, possibly a small fraction of one per cent of the energy
produced. The construction of the plant will have required considerable
fossil fuel energy (and therefore carbon dioxide emissions) to produce the
concrete and glass for the building and metals for the equipment. It is not
possible to estimate this accurately but, as a guess, it is possible that
this could be amortised by the non-fossil fuel production of less than one
year (based on 40 per cent of the figures published for the construction of
a nuclear power station with its massive reactor).
Overall, the environmental
balance sheet is very positive.
Economic factors
(For reference,
costs are quoted in Swiss Francs. At the time of writing, CHF 1 = USD 0.91
and EUR 0.61)
The total capital cost of
Tridel was CHF 358.7 million. However, it should be realised that this
includes the cost of the rail tunnel to Sebeillon, the services tunnel to
Pierre-de-Plan and other factors specific to the site, such as a new road.
The cost of the buildings was CHF 111.6 million and the technical
installations CHF 119.2 million. The amortisation of the total cost and the
interest and capital repayment of the CHF 174.7 million bank loan represent
the heaviest exploitation charges.
Salaries are paid to 48
employees, some of whom are on 24/7 shift work.
The overall cost of treatment
works out at CHF 186.00/tonne, which compares favourably with the average in
Switzerland of CHF 230.00/tonne. The tariff charged for rubbish disposal
varies from CHF 215.00/tonne for household garbage to a maximum of CHF
450.00 for dangerous materials, such as medical refuse.
The energy sold in the form of
heat and electricity adds to the income, of course. Based on 50 per cent
efficiency at full capacity, this should amount to something like a maximum
of 250 GWh/year.
Overall, the financial balance
sheet has shown a profit, even for the first year of exploitation.
International, national and local policies and
incentives
The following table is an extract from Table
SPM.5 of the Summary for Policymakers of the IPCC Fourth Assessment
Report, Climate Change 2007: Synthesis Report (Draft, December
2007). I have underlined and posted in red some key phrases relating to the
technology in this essay (the bold type and italics are as in the original
document).
| Sector |
Key mitigation technologies and practices
currently commercially available.
Key mitigation technologies and practices
projected to be commercialised before 2030 shown in italics. |
Policies, measures and
instruments shown to be
environmentally effective |
Key constraints or
opportunities
(Normal font = constraints;
italics = opportunities) |
| Waste |
Landfill CH4 recovery;
waste incineration with energy recovery;
composting of organic waste;
controlled waste water treatment; recycling and waste minimisation;
biocovers and biofilters to optimise CH4 oxidation |
Financial incentives for
improved waste and wastewater
management |
May stimulate technology
diffusion |
| Renewable energy incentives
or obligations |
Local availability of
low-cost fuel |
| Waste management
regulations |
Most effectively applied at
national level with enforcement strategies |
The only comment worth mentioning is in the last
column where "Local availability of low-cost fuel" is seen as a constraint,
whereas it would seem to be more of an opportunity because of the amount of
garbage that is available in urban and suburban communities, even in
developing countries. The constraint may apply to effective collection.
Advantages and disadvantages of the technology
Advantages:
-
Much reduced greenhouse gas emissions
-
Virtually no pollution
-
Recovery of precious and semi-precious
metals
-
Landfill volume reduced to ~10 per cent
-
No landfill pests or emissions
-
Can recycle paper/plastics into energy where
direct recycling is impractical
-
Energy available as electricity and heat
-
Energy efficiency typically about 50 per
cent
-
Flexible, according to the type of available
waste
-
Can be adapted for large cities with economy
of scale
-
Very cost-effective
Disadvantages:
-
Relatively high capital cost (grants or
subsidies may be available)
-
Requires waste to be partially pre-sorted
-
Less suitable for sparsely populated rural
areas
Acknowledgements
I gratefully acknowledge the reception accorded
to us during our visit to the Tridel plant, the permission to photograph it
and for the permission to reproduce the copyright Tridel schematics on this
web page.
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