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.
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