Thermal Waste Treatment

Transcription

1 OTTO-VON-GUERICKE-UNIVERSITY MAGDEBURG Faculty of Process and System Engineering Institute of Fluid Dynamics and Thermodynamics Prof. Dr.-Ing. E. Specht Thermal Waste Treatment July 2007

2 Thermal Waste Treatment Contents: 1. Characterisation of Residues 1.1 Composition 1.2 Mechanical Separability 1.3 Thermal Decomposition 1.4 Behavior in the Combustion Chamber 2. Thermal Processes 2.1 Sub-Processes 2.2 Process Factors 2.3 Apparatuses 2.4 Examples of Processes 2.5 Stoker Firing Plant for Household Waste 3. Cost-Effectiveness

3 1. Characterization of Residues 1.1 Composition Residues are essentially characterized by a composition which fluctuates with the time of day a variable composition which depends on the season (e.g. in autumn a high proportion of wet leaves in municipal wastes, other bacterial conversion in sewage sludge) a composition difficult to measure and thus only imprecisely known a mostly high humidity a high ash content a low calorific value very diverse component parts many component parts capable of causing harmful emissions many component parts capable of generating low-melting ash mostly lumpy, heterogeneous and difficult to reduce in size The following provides the composition of residues for several examples. Figure 1-1 shows the composition of household waste. The proportion of water amounts to 30%. Hence, without drying beforehand, the household waste is difficult to reduce in size and to pulverize. Approximately 25% of the waste is inert and must be disposed of after incineration. The dry substance (DS) exhibits a wide range of variations of organic components. Attention should be given to the relatively high proportion of components which can cause harmful emissions, such as, for example, nitrogen with NO x, sulfur with SO 2, chlorine with dioxins and furans as well as lead, cadmium and mercury with emissions of heavy metal. The proportions of the types of waste are also supplied in the partial Figure below. These can fluctuate according to the refuse collection of the community. Thus, in the case of separate waste collection, the proportions of vegetable residue, of glass, of paper and of cardboard, and also of plastics turn out lower. Figure 1-2 shows the composition of a sewage sludge in relation to the dry substance. The volatile matter amounts to 60 to 75% according to rate of digestion. The inert proportion is thus approximately between one third and one fourth. The calorific value of the dry substance is around 20,000 kj/kg DS. Sludges are characterized by an extremely high humidity. After mechanical drying this amounts to up to 3 kg water per kg DS. Hence in the case of isolated incineration the sludges must be dried in advance. A greater part of the calorific value is required for drying, according to the drying process. In Figure 1-3 provides the average proportions of materials in European mid-sized automobiles. Around half is steel. The proportion of plastic of 12% will climb in the coming years. The plastics are in turn composed of many different thermoplastics. Figure 1-4 shows the composition of a gas of disposal. It mainly consists of methane. The relatively high proportion of CO 2 and H 2 S deserve emphasis. A particular feature of gas of disposal is the composition which fluctuates over time.

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5 Further examples for waste from industrial processes or manufacture processes are: gaseous waste - drying gas from painting containing organic solvents - gases from chemical processes containing components with nitrogen - gases from metallurgy (converter, cupola furnaces, electric arc furnaces) containing CO, C x H y, dust - exhaust gases from ceramic production (e.g. tunnel kilns for bricks) containing SO 2 and HF liquid waste - contaminated mineral oil - old fried fat (from food) - chemical residues - paints - sewage sludge solid waste - ground wood from paper manufacturing - plastic coated wood from furniture manufacturing - old timber, coated wood, rail sleeper - animal fat, animal meal - textiles, leather - tyres - plastics - electronic scrap - sand from metal casting industries - biomass, leaves, straw, corn - ash from coal power plants 1.2 Mechanical Separability In principle solid residues can be mechanically separated to a great extent. To this end however these must be reduced in size to relatively small particles. Moist residues and residues with plastic constituent parts are, as a rule, difficult to pulverize since these tend to smear. Thus residues must be thermally pretreated before mechanical recycling. The principle of mechanical separation will be outlined before the thermal treatment is dealt with. In accordance with Figure 1-5 the (dry) residue is first reduced in size, for example, with a shredder. An air classification blows off paper, fibers, textiles and similar materials. In principle these can be recycled. In light of inadequate separation by grading and in light of impurities, this is however only possible to a limited degree. Hence these fractions are often processed into pellets and offered as solid fuel rich in calorific value. This carries the designation RDF (Refuse Derived Fuel), i.e. fuel from waste. Figure 1-6 shows a shredder schematically.

6 A further stage of separation isolates the metals. These in turn are separated by magnets into iron and non-ferrous metals. The fraction free of metal can again be separated into plastics as well as glass, ceramic, stones, etc. The reusability of the individual materials depends on the purity of the grading. Especially in the case of plastic grading and steel grading the degree of purity is relatively small. The recycling of old automobiles in corresponding to Figure 1-7 should be regarded as an example. After the removal of recyclable parts and the draining off of liquid residues the old automobiles are shredded. Automobiles can be crushed in it within one minute. Other scrap is also crushed in such shredders. After the shredder the metal is isolated by air classification and in turn is separated into iron and non-ferrous metals. The metals are reused in remelting processes. The non-metal fraction is designated as light refuse from the shredder. At present this cannot be recycled and therefore is disposed of. The problems with old automobile recycling thus substantially lie in the processing of the 27 to 28% light refuse of the shredder. 1.3 Thermal Decomposition During heating, residues run through different stages of decomposition according to the type of constituent matter. First the water evaporates at temperatures around 100 C. As soon as the temperature exceeds 100 C and the steam pressure thus becomes higher than the pressure of 1 bar in the apparatus, the steam can flow off continuously. The transfered heat is then completely converted into evaporation enthalpy during this time until the residue is dry. As a result the temperature remains nearly unchanged during this time. Figure 1-8a depicts the composition of different plastics under the exclusion of air (pyrolysis) as a function of the temperature. In the temperature range between 250 C and 300 C the chlorine escapes from the polyvinyl chloride (PVC). The organic constituent parts decompose in the temperature range of 350 C to 500 C. Figure 1-8b separately depicts the decomposition of the PVC. The two stages of decomposition are clearly discernible. In the first the chlorine escapes; in the second the hydrocarbon. The loss in weight amounts to c. 90%. The residual 10% consists of carbon and inert material. After pyrolysis of the organic constituent parts, as a rule, a small residue of carbon, the socalled pyrolysis coke still remains. This coke can then be extracted by combustion from a residue. The reaction with oxygen begins at temperatures above 500 C. During the heating of residues, many other constituent parts, such as halogens, chlorides, sulfides and metals, also evaporate. Figure 1-9 shows the saturation pressure curves of a number of metals, metal oxides, metal sulfides and metal chlorides. According to that, the heavy metals Hg, Cd, Pb as well as most of the combinations of metals evaporate up to temperatures of 1200 C. 1.4 Behavior in the Combustion Chamber Decomposition of Organic Substances The complete destruction of organic, gaseous substances must be ensured in the combustion chamber, i.e. they must be oxidized into CO 2 and H 2 O. The Figures 1-10 and 1-11 show the decomposition of different organic substances in air as a function of the temperature at

7 selected residence times. According to that at least 1000 C is required, in order to completely destroy the substances. In general 2 seconds are suggested as residence time. Hence the following is required as the directive for most combustion chambers: A residence time longer than 2 seconds at temperatures higher than 1200 C. Shorter residence times and lower temperatures can also be sufficient in individual cases. Corrosion Evidence of corrosion can appear especially through the chlorine compounds in the gas. Figure 1-12 shows as an example an attack of corrosion as a function of the temperature. It is obvious that a great threat of corrosion arises particularly in the temperature range of 500 C to 800 C. In Figure 1-13 several rates of corrosion for stainless steel sheets are cited as an example. According to that pure Cl 2 -gas and HCl-vapor are very corrosive with calcium chlorides as well as with natrium chlorides. Condensates A number of various vapors condense during the cooling from the high combustion temperatures. In the process the finest drops, so-called aerosols, form. These possess many unpleasant properties. They can seal filters and cause corrosion and contamination on the walls. Condensates from metals and combinations of metals can set on the walls and thus form deposits, encrustations and so forth. Figure 1-14 gives the classification of elements in waste according to their behavior in the combustion chamber. Hence hot combustion gas with constituent parts that form condensate cannot be sent through recuperators for cooling, which would be desirable for heat recovery, for example, through air preheating. In such cases the recuperator tubing would be damaged or become so thoroughly clogged in short time, that the heat transfer would be considerably reduced. As a rule, water is sprayed in with a jet for the cooling of such combustion gases Ash During incineration of solid waste ash remains. This ash consists of a lot of different oxides. Some oxides react to a product which can have a very low melting temperature. Figure 1-15 shows as an example the two phase diagram of NaO and SiO 2. Both pure oxides have a very high melting temperature. But if a mixture forms with a concentration of 30 % NaO and 70 % SiO 2 the melting temperature decreases to 800 C. These mixtures gives liquid drops which freeze at walls and forms layers of slag. As a result the cross section area and the heat transfer will be reduced. It is always a problem to predict the mixtures which can occur and their melting temperatures because of the high number of oxides in the ash. So a lot of testes with samples of waste are necessary before burning in an existing plant.

8 Net calorific value kj/kg Combustible material 45 Mass-% Ash component 25 Mass-% Water component 30 Mass-% Quantity of large iron bars 4-5 Mass-% Organic substances (TOC) Mass-% dry matter Inorganic substances Mass-% dry matter Carbon C Mass-% dry matter Hydrogen H 4-5 Mass-% dry matter Nitrogen N 1-2 Mass-% dry matter Chlorine Cl 0,1-1,0 Mass-% dry matter Sulphur S 0,03-0,5 Mass-% dry matter Lead Pb 0,7-2,0 g/kg dry matter Cadmium Cd 0,01-0,05 g/kg dry matter Mercury Hg 0,001-0,05 g/kg dry matter Total amount 14 Mio. Mg Minerals 2% Compound materials 1,1 % Disposable diapers 2,8 % Problem waste 0,4 % Textiles 2 % Plastics 5,4 % Glass 9,2 % Non-ferrous metals 0,4 % Ferrous metals 3,5 % Packing material 1,9 % Fine waste (<8 mm) 10,1 % Paper 12 % Medium waste (8 to 40 mm) 16 % Cardboard 4 % Vegetable waste 29,9 % Fig. 1-1: Reference values for the composition of household waste

9 Parameter Units Raw Sludge Digested Sludge Glowing loss kg/kg C kg/kg H kg/kg O kg/kg N kg/kg S kg/kg Cl kg/kg F mg/kg 1 1 As mg/kg 2.6 to 25 Cd mg/kg 1.6 to 21 Co mg/kg 3.2 to 5 Cr mg/kg 19.6 to 1,200 Cu mg/kg 87.9 to 860 Hg mg/kg 1.6 to 4.7 Mn mg/kg 74 to 260 Ni mg/kg 94 to 340 Pb mg/kg 52 to 124 Sb mg/kg 1.8 to 10 Sn mg/kg 54 to 673 Tb mg/kg 0.6 to 19 V mg/kg 9.5 to 25.5 Zn mg/kg 657 to 2,500 AOX mg/kg 192 to 287 PCB µg/kg 80 to 846 PCDD/F TE ng/kg 15 to 470 Fig. 1-2: Composition of Sewage Sludge

10 Operating media Glass 6 % Elastomers 4 % 5 % Cast iron 8 % Paint other 4 % Non-ferrous metal 2 % Aluminium 7 % Plastics 12 % Steel 52 % , ,5 3,8 3,7 1,5 PMMA PE PPO PP PVC ABS POM PA PC Fig. 1-3a: Percentage of material in European mid-size autos in wt.-percentage

11 Eco-charge Computer Components of a PC and the contained materials - 50 % Metal e.g. inner casing, parts of the picture tube, aluminium, barium, lead, cadmium, iron, zinc, tin - 23 % Plastics e.g. casing: PVC, PE,ABS, Flame inhibitor - 15 % Glass - 12 % Electronic e.g. circuit board, batteries: cadmium, lead oxide, components flame inhibitor, furan/dioxin, lithium, mercury, copper Amount of Electronic scrap in EU - year million tons - year 2006 (forecast) 7.6 million tons - year 2012 (forecast) 12 million tons Fig. 1-3b: Fig. 1-4: Chronological progression of gas concentration in a landfill

12 Dry Residue Shredder Dust Air Classification Paper Fibers Textiles Liquid Classification ρ < 2 g/cm 3 ρ > 2 g/cm 3 (Metal) Fine Classification Magnetic Seperation ρ > 1.2 Glass Ceramic Stone ρ < 1.2 Plastics Iron Non-ferrous Metals (Cu, Al,...) Fig. 1-5: Principle of mechanical Seperation of Residues

13 Fig. 1-6: Principle of a shredder Old Cars Mill. Wordwide 20 Mill. West Europe 3 Mill. Germany Motor Vehicles Recycling Assembly Part Spare parts Operating fluids Combustion Thermal treatment Light Waste Shredder Non-ferrous metals 2-3 % % Stell 70 % Metal Industry t/a ^ 3 % Houesehold Waste ^ ~ 10 % Steel Scrap ~ 3 % Steel production Fig.: 1-7: Disposal Route of Old Cars

14 Fig. 1-8a: Dynamic TG and DTG curves of PVC, PA 6, PE, PP and PS in helium with a heating rate of 10 K min -1 Fig. 1-8b: Thermogram of polyvinyl chloride for different heating rates

15 Fig. 1-9: Steam pressures of metals, oxiding, chlorides and sulphides Fig. 1-10: Thermal destructibility of organic substance in air

16 Fig. 1-11: Thermal Decomposition of PCB and formation/decomposition of PCDF (gas phase) Fig. 1-12: Rate of chlorine corrosion Taken from F. Vollhardt, Chem. Ing. Techn. (1987)

17 Analyzed samples: metal sheets mad of 18 Cr/8 Ni/Ti (50 mm x 10 mm x 1 mm) Temperature: 600 C Impact HCI-vapor Cl 2 -gas+steam HCI-vapor + 90 % CaCl 2 /10 % NaCl HCI-vapor + 90 % CaCl 2 /10 % NaSO4 Cl 2 -gas Corrosion rate 1 mm in 1.2 x 10 7 h 1 mm in 3.2 x 10 7 h 1 mm in h 1 mm in 500 h 1 mm in 100 h Fig. 1-13: Qualitative Comparison of Corrosion Rates (from Bayer, 1982) Class 1: Class 2: Class 3: Fig. 1-14: Al, Ba, Be, Ca, Co, Fe, Mg, Mn, Si, Sr and Ti Elements which do not volatize noticably in the combustion chamber As, Cd, Cu, Ga, Pb, Sb, Zn, Se, Na and K Elements which are mainly volatilised in the combustion chamber and in the quenching of the firing flue gas condense on the flue dust Hg, Cl and Br Elements which are mainly volatilised in the combustion chamber and in the quenching of the firing flue gas only condense slightly. Elements which are mainly volatilised in the combustion chamber and in the quenching of the firing flue gas condense on the flue dust Classification of elements in waste according to their behaviour in the combustion chamber

18 Fig. 1-15: Two phase Diagram for melting