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 . 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 . 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 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  PAJĄK T., Termiczne unieszkodliwianie odpadów w systemie gospodarki odpadami komunalnymi, Wyd. AGH, Kraków 2001.  PAJĄK T., Odnawialne i niekonwencjonalne źródła energii. Energetyczne wykorzystanie odpadów komunalnych, Wyd. Tarbonus, Kraków 2008.  Praca zbiorowa, Materiały Międzynarodowej Konferencji „Termiczne Przekształcania Odpadów”, Kraków 2005.  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.