energy resource, thermal treatment
Stanisław Gumuła*
Małgorzata Piaskowska-Silarska**
MUNICIPAL WASTE TREATMENT BY THERMAL
METHODS FOR GENERATION OF HEAT AND
ELECTRICITY
This study addresses the key issues associated with municipal waste disposal in the context of
energy generation. One ton can be transformed even into 2,6 MW∙h (0,6 MW∙h e; 2 MW∙ht).
Communal waste that is to be thermal utilized should have calorific value of not less than 5800 kJ/kg.
This quantity is the limit for self burning process of waste, that means without any additional fuel.
Most important characteristic of communal waste is the fact that it includes 50% of biodegradable
mass that comes from photosynthesis (CO2). The balance of carbon dioxide emission by creating and
burning of the mass is therefore environmentally neutral. Despite this each location offer of waste
incinerating plant in Poland causes public protests. Another hampering factor of development of
waste incinerating plants are high investment expenditure and utilisation. Very useful is UE founding
under Infrastructure and Environment Operational Programmes. This subsidy covers up to almost
60% of investment expenditure.
1. INTRODUCTION
Under the EC Directives, Poland ought to significantly limit the amounts of
municipal waste dumped on the dumping sites. Failure to comply with this regulation
will involve huge fines to be paid, amounting to 250 thousand euro daily. At the
moment as much as 95% of the 10-11 million tonnes of municipal waste produced
yearly in Poland is dumped on the dumping sites [4]. Waste decomposition is the
source of dump gas, containing about 50% of methane, which is a source of energy yet
at the same time an explosive gas, whose contribution to the greenhouse effect is
decidedly (20-fold) more significant that that of CO2.
Thermal treatment of solid waste is a more effective way of waste management
than production of methane gas, giving us both electricity and heat. In the old EU
countries, over 7% of energy from renewable sources is obtained in that manner.
Municipal waste becomes a valuable source of energy, one tonne of waste may yield
about 2.6 MW·h, including 0.6 MW·hc and 2 MW·ht. Of particular importance is the
composition of municipal waste. In some cases biodegradable substance, produced as
Wydział Inżynierii Mechanicznej i Robotyki, Akademia Górniczo-Hutnicza, al. A. Mickiewicza 30,
30-059 Kraków
** Wydział Matematyczno – Fizyczno – Techniczny, Uniwersytet Pedagogiczny, ul. Podchorążych 2,
30-084 Kraków
*
the results of photosynthesis of CO2, accounts for nearly 50% of waste. That means
that CO2 emissions during its combustion are balanced by CO2 loss during its
formation [3].
2. REQUIREMENTS AS TO WASTE QUALITY
There are several technologies of waste disposal by thermal methods. The most
widespread and well-tested technology uses the boilers fed with wastes only,
incorporating a moving grate, flat or sloping, air- or water-cooled. The similar
technology consists in co-incineration of waste and fuel of a higher calorific value to
improve the efficiency and smoothness of the combustion process. Apart from these
two basic technologies, there are several others, based on pyrolysis, gasification and
plasma processes.
In order that a municipal waste incineration plant can be effectively operated
without adding the fuel of high calorific value, it is required that the waste fed to the
boiler should have the calorific value no less than 5800 kJ/kg. This level is the limit
for autonomous waste incineration, where no additional fuel of higher calorific value
is required to support the combustion process. Typically, the calorific value of
municipal waste falls in the range 7000-8000 kJ/kg and tends to vary depending on the
waste type and composition. Municipal waste composition tends to vary between
individual countries and regions where the waste is produced. For example,
paper- a valuable component having a good calorific value accounts for nearly 38%
of the waste mass in highly-developed countries, 22% in other developed countries
and 2% in the developing ones. As regards the organic matter, the proportions are just
reverse. The organic matter accounts for 25% of the waste mass in highly-developed
countries, 42% in other developed countries and 65% in developing ones. In Poland
the average mass proportion of paper in the stream of municipal solid waste accounts
for about 20%, in tends to be much higher in big cities, approaching 20%. The mass
proportion of organic components in the entire country is similar, approaching 35%.
Furthermore, the data supplied by the Institute of Ecology of Industrialised Areas
indicate that the amount of waste per one inhabitant of rural areas is 2 times smaller
that the amount per capita in urban areas. It appears that the proportion of municipal
waste in large town agglomerations and in highly developed countries is larger and
their calorific value higher than in smaller towns and in developing countries [1,2].
3. CHARACTERISTIC OF WASTE TREATMENT BY THERMAL METHODS
Most technologies of municipal waste incineration utilise the following process
machines and installations:
- boilers where municipal waste is incinerated and steam produced;
- a turbine assembly incorporating a steam turbine and an electric generator;
-
heat exchangers transferring heat to the city mains;
flue gas treatment installations;
waste water treatment installations;
installations for separation of solid residues from the combustion processes to
be dumped on the dumping sites;
The most widespread installation in a municipal waste incineration plant is shown
in Fig 1.
Let us summarise briefly the process of thermal treatment of solid waste. The waste
supplied to the incineration plant is stored in the waste tank. Then the portion of waste
is taken out, mixed and fed onto the grate located in the incineration chamber. When
the process is activated and the incineration chamber is still too cold, the combustion
process is supported by gas burners. When the temperature in the incineration
chamber becomes sufficiently high to support the thermal processes, the gas burners
are switched off and the combustion process continues utilising only the chemical
energy contained in waste. The heat from waste incineration is then utilised to produce
steam of the temperature around 500 oC and pressure of the order of 3.5 MPa. On
leaving the boiler, the steam passes to the back pressure turbine where the energy of
steam is converted into mechanical energy and, via the generator, into electrical
energy. The heat of steam after passing through the turbine is utilised in further
processes. In the winter time it is sent to the city mains and in the summertime it
passes to the absorption cooler assembly which cools water to be distributed in the city
mains in order to reduce the temperature in residential and official buildings.
Slag is the residue of the incineration process. It is water-cooled to recover the
metals contained in the slag, to be further recycled.
The combustion process takes place in a manner to ensure the maximal oxidation
of flammable matter, followed by after-burning. Flue gas on leaving the boiler passes
through the electric filters to remove dust and then pass through the scrubbing stages.
Before the flue gas moves to the stack, it has to pass through the catalysts where the
nitrogen oxides and dioxins are decomposed.
The waste water treatment plant is another major installation in the incineration
plant. Waste water treatment involves several stages designed to neutralise and
remove all contaminants originating in the combustion processes. These contaminants
are finally removed from the waste water by sedimentation in settling tanks.
The thermal treatment of one tonne of waste yields 250 kg of solid particulate,
containing slag, ash, metal scraps and sediments.
4. ECONOMIC FACTORS
What remains to discuss, is the cost effectiveness of the whole undertaking: the
construction and operation of a waste incineration plant. The undertaking requires
huge capital investments at the stage of construction, often requiring also financial
support in the course of the plant operation as the revenues from sales of energy and
the fees for waste management may not be sufficient to cover the operating costs.
The amount of municipal waste, its type and composition, the size of waste
collection area, the number of inhabitants are the key parameters to be considered at
the starting point at the stage of design and when defining the capital investments and
operating costs of the plant. The experience gathered to date in EU countries teaches
us that the stream of waste of 100 thousand t/year is optimal for the waste incineration
plant, whilst the minimal level is 80 thousand t/year. However, there are plants where
the amounts of waste handled are entirely different. The incineration plant in Warsaw
handles 40 thousand tonnes of waste yearly, whilst the stream of waste handled by the
Moscow plant amounts to 700 thousand t/year. The number of inhabitants in the
waste collection area for one incineration plant should range from 300 000 to 400 000.
Nonconformity with those standards precludes the financial support under EU
programmes, which might cover up to 60% of costs of the enterprise, as mentioned
before.
Let us now compute the cost-effectiveness of the municipal waste incineration
plant, basing on the following assumptions: calorific value of the municipal waste
6000 kJ/kg, boiler efficiency 65%, investment cost 250 million PLN, amount of waste
100 thousand t/year, subsidies – none, fuel 0 PLN, revenues from the sales of heat 34
PLN/GJ (0.122 PLN/kW·h).
The amount of heat produced annually in these conditions is equal to:
6000 kJ/kg * 108 kg/year * 0.65 = 390 thousand million kJ/year
Let us first consider the case when the whole generated heat is sold in the form of
steam or hot water. We get:
390 * 109 kJ/year * 34 * 10-6 PLN/kJ = 13 thousand million PLN/year
Without taking into account any fluctuations in energy prices or inflation, the
payback period is derived by dividing the investment costs by the annual revenue: 250
million divided by 13 million PLN/year yields the payback period of about 19 years.
Let us now consider the case when the subsidies account for about 60% of the
investment costs, approaching 150 million PLN. Accordingly, the payback period
obtained by dividing 150 million PLN by 13 million PLN/year becomes 11 years. The
cost-effectiveness of the investment project can be still improved by utilising 25% of
heat in the production of electricity, whose price is three times higher. However, the
investment costs would increase too, as the plant would need a generator, a block
transformer, separating installations and other minor elements of the equipment. One
has to bear in mind that the presented calculation procedure ignores the current
operating costs and current revenues from waste collection fees. The proportion
between these quantities, (costs to revenues) will differ between countries.
5. CONCLUSIONS
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municipal waste may become a major resource for the production of
electricity and heat in the energy balance;
it is required that the stream of municipal waste to be handled should not be
less than 80 thousand t/year and the collection procedures should be specified;
the area from which the waste will be collected should be inhabited by
300 000 - 400 000 people;
the calorific value of municipal waste should not be less than 5800 kJ/kg;
the waste incineration technology is complicated and costly both at the stage
of investment and when the plant is already operational.
waste disposal by thermal methods as the source of energy has major
environmental advantages though in purely financial terms may prove
unprofitable;
the payback period should not be longer than the life of the plant, typically
taken to be 25 years.
REFERENCES
[1] PAJĄK T., Termiczne unieszkodliwianie odpadów w systemie gospodarki odpadami komunalnymi,
Wyd. AGH, Kraków 2001.
[2] PAJĄK T., Odnawialne i niekonwencjonalne źródła energii. Energetyczne wykorzystanie odpadów
komunalnych, Wyd. Tarbonus, Kraków 2008.
[3] Praca zbiorowa, Materiały Międzynarodowej Konferencji „Termiczne Przekształcania Odpadów”,
Kraków 2005.
[4] www.egospodarka.pl
TERMICZNE PRZEKSZTAŁCANIE ODPADÓW KOMUNALNYCH W CELU PRODUKCJI CIEPŁA
I ENERGII ELEKTRYCZNEJ
W referacie przedstawiono problemy związane z energetycznym wykorzystaniem odpadów
komunalnych. Jedna ich tona pozwala uzyskać energię w ilości około 2,6 MW∙h, w tym 0,6 MW∙h e
i 2 MW∙ht. Odpady komunalne poddawane termicznej utylizacji powinny mieć wartość opałową nie
mniejszą niż 5800 kJ/kg. Wielkość ta stanowi granicę autonomicznego spalania odpadów, tzn.
spalania nie wymagającego wprowadzenia do kotła dodatkowego paliwa (o wyższej wartości
opałowej) wspomagającego proces spalania. Niezwykle ważną i cenną cechą odpadów komunalnych
jest fakt, że aż 50% ich składu masowego mogą stanowić składniki ulegające biodegradacji,
powstające w wyniku fotosyntezy CO2. Oznacza to, że bilans emisji dwutlenku węgla przy ich
spalaniu i tworzeniu w procesie fotosyntezy wychodzi na zero, są więc neutralne w sensie emisji CO2.