Energy production by means of

waste incineration

illustrated by the “Spittelau” plant

 

(Fachbereichsarbeit aus Geographie und Wirtschaftskunde 
mit Englisch als Arbeitssprache)

 

1.3. The energy situation in the future

 

 In this graph one can see very clearly the future trend, which leads to an increased use of renewable energy and away from non – renewable energy,

The use of fossil fuels will decrease (from 81% in 1993 to 61% in 2025) and therefore renewable energy will be used much more (from 7% in 1993 to 30% in 2025) in the future.

 

 This graph shows two interesting aspects. At first it shows which source of renewable energy will probably be most important in 2010 and secondly it points out that the figures, which are indicated in “million tons oil equivalent”, can give an idea of how many tons of oil, which is non - renewable would have been needed if the equivalent amount of energy had been produced with the help of oil instead of renewable resources. 

This future trend is extremely positive concerning the environmental situation we have to face on earth, as the increased use of renewable energy will lead to all the results which were already mentioned in “1.2.1 Environmental benefits”.

 One renewable energy producing technology is energy production by waste incineration.

3.2.1. Used techniques

 Waste delivery:

The Viennese municipal solid waste and non – hazardous commercial wastes are delivered to the Spittelau plant from Monday to Friday between 7.00 a.m. and 3.00 p.m. Up to 250 delivery vehicles daily pass over one of the two weighbridges first to establish the weight of the waste before emptying their loads into the 7.000m3 waste bunker (1) at one of the eight tipping points.

Following thorough mixing in the bunker (in order to keep the heating value constant) the waste is transferred to the two incineration lines by one of the two bridge cranes, which have a capacity of 4m3.

Waste to energy process:

The thermal waste treatment plant consists of two incineration lines, each with a flue gas treatment plant, with a SCR-DeNOx and dioxin destruction facility, serving both lines and a waste water treatment plant. Through the furnace feed chute (2), the waste passes from the bunker to the firing grate (3), situated at the lower end of the furnace. Up to 18 tons of waste per hour can be thermally treated on the sloping, 35 m2 stoker grate (3).

During the transient incinerator start-up and shut-down operational phases, two 9 MW gas burners ensure a furnace temperature of more than 800°C (4), and thus an achievement of a total burnout of the flue gas as required by law.

In normal operation the use of the auxiliary burners is not necessary because at 8.600 kJ/kg the lower heating value of the waste is absolutely sufficient to maintain an autogenous incineration process.

The incombustible components, slag (230 kg/t waste), arriving at the end of the firing grate are cooled by being dumped into the water-filled slag discharger (6). From there the cooled slag is transported to the slag bunker (16) by a conveyor belt and at the same time ferrous scrap is removed by overhead electromagnets (15). Then the slag is mixed up with water and cement and is used in landfill construction for borer walls as a slag-filter ash concrete. The ferrous scrap removed from the raw slag is returned to the material cycle (steel production).

The extraction of fresh air required for the incineration process from the waste bunker maintains the bunker in a constant state of partial vacuum, thus minimizing odour and dust emissions from the tipping points into the ambient air.

In addition the use of a highly developed computerized firing control system ensures optimum incineration along the grate and thus maximum slag and flue gas burnout.

In this way 5 MW of power for internal consumption and the public network as well as about 60 MW of district heating energy are produced from waste per hour. This is equivalent to the energy consumption of 15.000 homes with 80m2 floor space.

District heating and power generation:

The 850°C flue gas coming up from the incineration process gives off its heat at the surface of the waste heat boiler.  Both lines generate a total of 90 tons of saturated steam (33bar) per hour.

For power generation, this steam is first reduced to 4.5 bar in the steam turbine (13) before the heat is transferred to the returning water of the district heating network by means of condensation in the following heat exchanger bank (14).

 Flue gas treatment: Dust removal and wet scrubbing

In 1989 the plant was modernized. A flue gas cleaning system, consisting out of ESP (7), wet scrubber and Europe’s first SCR-DeNOx (10) facility were added to the already existing plant. The Spittelau became an international leader in flue gas cleaning and emission reduction for thermal waste treatment plants.

Therefore the plant is not just attractive for tourists because of its “special architectural features” but it is also a technological landmark. Experts, technicians, politicians and potential new investors are visiting Spittelau to learn about the new technology.

The flue gases leave the heat boiler (5) at a temperature of 180°C and enter the ESP (7) where it is cleaned to a dust content of less than 5mg/nm3 (cleaning efficiency of 99.9%).

The filter ash (15 kg/t waste) which is collected is transported into a 125m3 silo (18) and in the end mixed with the slag and used in concrete.

Then the almost fully dedusted flue gas enters the first scrubber (8/1), which cools it to saturation temperature of 60-65°C by open-circuit water injection.

The first scrubber removes hydrogen chloride (HCl), hydrogen fluoride (HF) and dust as well as particlebound and gaseous heavy metals through intensive gas-liquid contact.

The second scrubber (8/2) is responsible for the removal of sulphur dioxide (SO2) from the flue gas.

The next treatment stage is the electrodynamic Venturi (9), where any dust still existing is reduced to less than 1mg/nm3.

In the heat exchanger the flue gas is reheated to 105°C and fed to the DeNOx and dioxin destruction facility (10).

Flue gas treatment: Waste water plant

From the scrubbers, water flows together with flue gas pollutants HCl, HF, SO2, HMet to the waste water treatment plant (19-25). The water is cleaned by means of chemicals and physical effects (sedimentation…) and finally released into the river Danube.

The hazardous residues (24), filter cake (1,1 kg/t waste) are transported to the abandoned Heilbronner salt mines in Germany by rail in big bags and are used there as infill.

Flue gas treatment: DeNOxing and dioxin destruction

The DeNOx facility (10) is the final stage of the flue gas treatment process. The flue gas streams of both treatment lines are combined to one unit and heated to a reaction temperature of 280°C by a heating tube and gas duct burners.

Passing through 3 catalytic converter stages cause the nitrogen oxides (NOx) to react with the added ammonia and oxygen in the flue gas to form nitrogen and steam. Dioxins are destroyed. The remaining exhaust gas is then cooled to 115°C and finally released into the atmosphere through a 126 m high stack (11).


3.5. Advantages and disadvantages of waste incineration

 Advantages of incineration

          Minimum of land is needed compared to the dimensions of waste disposal sites.

          The weight of the waste is reduced to 25% of the initial value.

          The waste volume is reduced to almost 10% of the initial value.

          The flue gas, which is containing heavy metals and other harmful substances after the incineration process, is cleaned and emitted through the stack in environmentally friendly form. → If waste is dumped in untreated form, underground water can be poisoned and different gases are developing which can harm our environment very badly as they support the greenhouse effect.

          Incineration plants can be located close to residential areas, which are the centres of the production of waste, and this helps to reduce the volume of traffic, pollution, noise and of course the costs for the waste transportation.

          By using the ashes for environmentally appropriate construction, low costs are provided and furthermore the need for landfill capacity is reduced.

          The incineration of waste provides two possibilities of using the produced energy:

o         First of all district heating can be produced with the help of hot water.

o         Secondly current can be generated by means of steam turbines.

          By using district heating single heating systems in houses can be replaced which helps to reduce the pollution of the environment and greenhouse gas emissions are diminished.

          The produced residues, ash and slag as well as the developed flue gases, are odour-free compared to the partly offensive smells caused by dumps.

          As the raw material needed for waste incineration, which is municipal waste, is said to be kind of renewable it helps to reduce the use of fossil fuels or non – renewable resources.

Disadvantages of incineration

           The air pollution controls required in incineration plants are extremely expensive. Very often up to one half of the costs of a plant are due to air pollution control facilities. As the laws can change and maybe require updates in the air pollution controls this could lead to much higher costs in the future.

          Energy, produced by means of waste incineration is not likely to be practical for small communities. Therefore incineration plants have be situated in areas where the district heating network can easily be connected to very many households.

          The extremely high technical standards of the plants require skilled workers, which leads to the facts that rather high wages have to be paid.

          The residues from the flue gas cleaning can contaminate the environment if they aren’t handled appropriately and therefore they must be disposed of in controlled and well operated landfill to prevent groundwater- and surface pollution.

          Among the Austrian citizens the acceptance of waste incineration plants is very poor and therefore people are fighting hard to avoid the construction of a waste incineration plant in their neighbourhood.

          People’s efforts to avoid waste production are minimized when they know that that their waste is burnt in an incineration plant.

 

3.7.2. Highlights, taken from the results of the opinion poll

          Approximately one quarter of all the inhabitants of Vienna think of “heating system” when they hear the key word “district heating”.

          While clients associate a comfortable and environmentally friendly possibility for heating with the term “district heating, non-clients rather associate “waste incineration plant” and especially “Spittelau” with it.

          The high percentage of persons who didn’t give any answer leads to the conclusion that the subjective state of information is rather bad. But in general, district heating clients mostly consider themselves to be rather well informed, which in fact is true compared to non-clients.

          While clients get their information mainly from brochures and bulk mail, non-clients inform themselves mainly by watching TV and reading newspapers. Posters are more important for non-clients than for clients. The internet is not used very frequently by either group.

          The answers to the question, what district heating was produced with, show that clients are much better informed on this issue.

          Generally “Fernwärme Wien” has a very good image. In all the given criteria clients judged this business much more positively than non-clients did.

          The best marks were awarded for:           - Comfort

         - Environmentally friendliness

         - Modern technology

         - Cleanness

This leads to the conclusion that district heating fulfils these criteria to a high extend and moreover it shows that the clients keep an eye on these criteria, which is extremely positive because this shows that people are conscious of heating with an environmentally friendly system.

 

 

3.7.1. Graph analyses

On the following pages the different questions of the opinion poll are listed and interpreted with the help of graphs. The figures for the graphs are taken from the results of the opinion poll.

What are you associating with the term “district heating”?

When the questioned persons thought about “district heating”, 23% of them associated the term first with “heating system”, followed by 12% choosing “waste incineration”, 11% thought of Spittelau/plant and comfortable/easy and 10% associated “environmentally friendly”.

Just small percentages thought of terms like “modern”, “cheap”, “hot water”, “expensive” or “Hundertwasser” and 30% of the interviewed persons couldn’t give any answer to this question. This could lead to the conclusion that there are rather many people who actually aren’t informed at all about district heating as they can’t think of anything if they hear the term.