Enhancement of the value of waste
A visit to Tridel SA, Lausanne.
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 2˝ 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).
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