Keywords: 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 (CO 2). 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. There are two conditions of the subsidy: minimal mass of waste to be treated is 80 000
tons a year and coverage of area of 300 000 – 400 000 inhabitants.
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 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. SOCIAL AND ECONOMIC CONSIDERATIONS
The main aim of municipal waste incineration is to minimise their environmental impacts, so
environmental benefits come first. A further advantage of thermal methods is that municipal
wastes are the source of energy, so the combustion process becomes the source of energy and
energy generation is a profitable business. Those supplying the wastes will pay for their
disposal. However, the incineration technology has to meet several conditions, which may not
be an easy matter in Poland. That is illustrated by the fact that about 400 waste incineration
plants are already operated in the old EU countries whilst in Poland there is only one. There are
*
**
Wydział Inżynierii Mechanicznej i Robotyki,
30-059 Kraków
Akademia Górniczo-Hutnicza, al. A. Mickiewicza 30,
Wydział Matematyczno – Fizyczno – Techniczny, Uniwersytet Pedagogiczny, ul. Podchorążych 2,
30-084 Kraków
two main reasons for this state of affairs. The first reason is the deep distrust towards the written
and spoken word of state authorities and local governments. People fear hazardous atmospheric
emissions in their area. In this situation each proposed location of a waste incineration plant is
followed by a strong public outcry of local inhabitants. The other reason is strictly financial- the
investment costs and operating costs tend to be very high. The first problem may be overcome
by finding the plant location far from inhabited areas. However, that would involve higher
investment costs and lower effectiveness of heat utilisation. The financing of the plant can be
supported by the EU funds under the Operating Program: Infrastructure and Environment.
These subsidies can account for nearly 60% of investment expenditure. The actual costs depend
on the selection of incineration technology, the plant size and the yearly production capacity, the
size of the area from which the wastes are supplied and the land features in the area.
Basing on EU data, it seems reasonable to assume that the unit investment cost- i.e. the cost
of handling one 1 tonne per year ranges from 600 to 700 euro. Accordingly, the costs of
construction of a plant to handle 100 thousand t/year will approach about 240-280 million PLN.
3. 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 70008000 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].
4. ENVIRONMENTAL CONSIDERATIONS
It seems worthwhile to ask a question whether the waste disposal by thermal methods is
environmentally safe? Will hazardous solid waste not be transformed into still yet more
hazardous gas emissions to the atmosphere? The answer to this question is ‘decidedly not’,
provided that stringent EU regulations (Waste Disposal by Thermal Methods Directive
2000/76/WE) are fully complied with. One has to bear in mind that waste incineration plants
use very high and advanced technologies, where the energy generation unit (fuel storage,
boilers, turbine, heat exchangers for end users) is relatively small, accounting for about 25% of
machines and installations in the entire plant. The largest and more complicated elements of the
entire process lines include the flue gas treatment installations and segregation of residues from
the combustion processes. These installations account for the remaining 75% of the plant
machinery.
5. 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;
featuring 25 basic elements: 1- waste tank; 2- chute; 3- grate; 4- burner chamber; 5- heat
recovery boiler; 6- slag remover with water lock; 7- electric filters; 8- flue gas scrubbing (twostage process); 9- dust removal systems (Venturi jets); 10- nitrogen removal systems (the
system for catalytic reduction of SCR), 11- stack; 12- reservoir of boiler; 13- turbine and
generator; 14- group heat exchanger; 15- electromagnetic separator; 16- slag container;
17- scrapped metal container; 18- fly ash silo; 19- multi-station – recycling of waste water;
20- waste water treatment plant; 21- reservoir of treated water; 22- sediment tank; 23- filtering
press; 24- filtering cake container; 25- collector of treated process water.
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.
6. 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.
7. CONCLUSIONS
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municipal waste may become a major resource for the production of electricity and heat
in the energy balance;
the decision to erect a power plant supplied by municipal waste should be always
preceded by consultations with local inhabitants, particularly in the context of selecting
the plant location;
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
STRESZCZENIE
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.
Mimo to każda propozycja lokalizacji spalarni odpadów komunalnych w Polsce budzi liczne protesty
mieszkańców. Drugim czynnikiem hamującym budowę tego typu zakładów są wysokie nakłady inwestycyjne i
koszty eksploatacyjne. Bardzo pomocne mogą tu być fundusze z UE w ramach Programu Operacyjnego
Infrastruktura i Środowisko. Są to dotacje, które pozwalają pokryć blisko 60% nakładów na inwestycję. Jednak
warunkiem ich przyznania jest spełnienie dwóch warunków. Minimalny strumień odpadów powinien wynosić 80
tys. ton/rok, natomiast liczba mieszkańców, na terenie z którego pozyskiwane byłyby odpady dla jednego zakładu
termicznej utylizacji, to 300 000 – 400 000.