- Email: [email protected]
Simulation of processes for thermal treatment of wastes
Waste Management 20 (2000) 435±442
www.elsevier.nl/locate/wasman
Simulation of processes for thermal treatment of wastes Petr Stehlik a,*, Radim Puchyr a, Jaroslav Oral b a
Department of Process Engineering, Technical University of Brno Technicka 2, 61669 Brno, Czech Republic b EVECO Brno, Ltd, Brno, Czech Republic Accepted 6 December 1999
Abstract A software system for simulating processes for thermal treatment and/or incineration of various types of wastes has been developed. Developing this software was initiated by the need to support and facilitate designers' and operators' activities. The software product is based on modelling Ð performing a mass and energy balance. A structure of this system is explained as well as the principles of application. Using the simulation program is demonstrated through a case study (incinerator for thermal treatment of various kinds of both industrial and municipal wastes). Flowsheet generation, input data speci®cation, calculation and results of simulation are shown. Various ®elds of application (design, simulating existing operation, investigation of parametric sensitivity, supporting tool for decision-making) are mentioned. Using this system is comparable with using various professional packages for the simulation of chemical/petrochemical and other industrial processes. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Software; Thermal treatment; Incineration
1. Introduction Serious problems are currently solved in the ®eld of environmental protection. The challenge facing concerned citizens and decision-makers is a formidable one to identify and implement long-term solutions that are safe, socially acceptable, and cost-eective. Such amount of wastes, which is produced either by inhabitants or by industrial companies requires to use ecient ways of waste disposal. The recent focus on incineration has been on environmental consequences, not on performance. In particular, the limitations, as well as the advantages, of incineration are being increasingly recognized [1,2]. Incineration is not a waste disposal method but rather a waste processing technology. Modern incinerators, though simple in concept, are highly complex machines. The heart of any incinerator is the combustion chamber where waste is burned. However, the overall unit consists usually from various types of systems and equipment. Many types of wastes (like municipal, industrial, medical, hazardous, radioactive) have to be burned. Therefore, design and operation of incinerators is obviously a dicult task and eective supporting tools are very useful. * Corresponding author. Tel.: +420-5-411-2373; fax: +420-5-4112373. E-mail address: [email protected] (P. Stehlik).
We have developed a simulation program for simulating processes for thermal disposal and/or processing of wastes. Using this system it is possible to calculate heat and mass balance of various processes for thermal treatment technologies. This simulation system is highly versatile and can be considered as a powerful engineering tool. This is very important because there is currently a multitude of incinerators and other thermal treatment systems in existence worldwide. 2. Typical arrangement of incinerators and other thermal treatment systems Incinerator is in fact a system consisting of several subsystems (equipment) for ecient thermal processing of wastes. Let us consider a unit where the burned wastes would be changed into a product without toxic and environmentally harmful constituents. This product could be somehow utilized if its energy could be utilized. No additional raw materials (feed, ¯ue gas, etc.) would be used, no additional wastes would be generated. Such a unit can be called the theoretical ideal incinerator. The aim is to design incinerators the characteristics of which would be as close as possible to the ideal one. A multitude of various incinerators for various purposes has been developed. There are very similar
0956-053X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(00)00008-8
436
P. Stehlik et al. / Waste Management 20 (2000) 435±442
thermal, physical and chemical processes, similar systems and equipment, similar characteristic features in most of incinerators. A block diagram of a typical arrangement is in Fig.1. Incinerator consists in fact of the following systems: . storage and waste feeding system (enclosed tipping area, refuse pits for various types of wastes, feed bins, tanks for liquid wastes, etc.) . incineration (primary combustion chamber±± rotary kiln, furnace with moving grate etc., afterburner chamber) . heat recovery (boiler, superheater, economizer, waste heat boiler, air preheater) . o-gas cleaning (removal of particles: ®lter, baghouse, electrostatic precipitator, etc., cleaning system: o-gas scrubber, scrubber liquid treatment, adsorber, etc.) . auxiliary equipment (ash/slag transport, fans, etc.) . monitoring and control system From Fig. 1. it is obvious which systems of incinerator can be simulated using simulation program. A concrete example of an incinerator is shown in Fig. 2. This is a ¯owsheet of incinerator with capacity 10 kt/ year for combustion of industrial and/or municipal wastes with various composition. A rotary kiln K1 is used as the ®rst stage of combustion and gas burners B2 are installed in the afterburner chamber K2. The function of these burners is to stabilize the operation, i.e. to keep constant temperature of ¯ue gases in case of a change in wastes' net heating value. Heat recovery system consists of air preheater (E2), boiler (E4), superheater (E3) and heat exchanger keeping the required temperature of ¯ue gas in the o-gas cleaning system (E1). This system consists of a ®lter F1 for particulates removal, three stages of a scrubber system (A1 to A3) and adsorber A4. (Scrubber liquid treatment and wastewater treatment systems which are parts of incinerator are not considered for simulation.) It will be shown later how to generate the ¯owsheet from Fig. 2 with the aid of simulation program.
3. Simulation of wastes processing A software system for simulation is based on mathematical modelling. The principle is similar to the case of worldwide used software packages for simulating processes in chemical/petrochemical and other process industries (e.g. ChemCAD, ASPEN Plus, PRO II, HYSIM). However, there is a demand to have at one's disposal a software which would take into consideration speci®c features of thermal treatment and/or incineration processes and would be user-friendly. The primary purpose of modelling is to perform rapid and accurate heat and mass balances for each process equipment component of the user's desired processing scenario. The basic principles and purposes of modelling are similar, like for example in [3]. However, the software system has been developed and tailored in such a way to match most of incineration and thermal treatment systems in a professional way. 3.1. Structure of a system for simulation The thermal treatment and/or incineration process model has a modular design which allows evaluating various con®gurations of simulated facilities. The design consists in drawing ¯owsheet with individual equipment and/or apparatuses which are connected by streams. The incinerator is composed of several items (apparatuses) and piping in a real operation is represented by streams in a ¯owsheet. As already stated the model is based on heat and mass balance. Therefore it is necessary to set up appropriate procedures for all items of a ¯owsheet taken into consideration. A general representation of a model is obvious from Fig. 3a. Let us consider e.g. a primary combustion chamber where both solid and liquid wastes are burned and auxiliary fuel (e.g. natural gas) is used. A simpli®ed representation of this equipment is in Fig. 3b. The purpose of the primary combustion chamber modelling is to perform a heat and mass balance. This model accepts data of all input streams, calculates the elemental composition of each input stream, and converts the input streams through
Fig. 1. Block diagram of incinerator.
P. Stehlik et al. / Waste Management 20 (2000) 435±442
437
Fig. 2. Incinerator for thermal treatment of various types of wastes.
the proper reaction stoichiometry to either combustion product±±¯ue gas species or solid ash. The computer program balances the energy in the primary combustion chamber unit operation such that the output streams are at the user speci®ed temperature. This is done by adjustment of the input auxiliary fuel and combustion air ¯owrates. Heat losses (vessel surface losses) can also be speci®ed. For better understanding of a structure of the system simulated a concrete example is shown in Fig. 4. Let us consider a part of the incinerator from Fig. 2 Ð the primary and secondary combustion chambers Ð rotary kiln K1 and afterburner chamber K2, respectively. A
representation of this subsystem using the simulation program is as follows: The rotary kiln (primary combustion chamber) is comprised of two apparatuses K1S and K1L-B1 where solid and liquid wastes and oil from the auxiliary oil burner B1 are burned. Combustion air for the secondary combustion chamber is preheated by ¯ue gas in the heat exchanger (air preheater) E2. M1 is a mixer which enables to consider mixing of combustion air with recycled ¯ue gas. M2 and M4 are other mixers which are necessary for a convenient representation of the system for modelling. There is obvious from Fig. 4 how various parts of incinerator are represented in the ¯owsheet
Fig. 3. Simpli®ed representation of a model.
438
P. Stehlik et al. / Waste Management 20 (2000) 435±442
generated by computer program. Using similar symbols and representation as those in worldwide used and recognised software packages for simulation was one of the aims in developing this computer code. A model of the above subsystem consists of procedures for calculating combustion, evaluating combustion air ¯owrate, composition, temperature and ¯owrate of ¯ue gas, net heating value of fuel based on its composition and heat balance of the apparatus. It is necessary to emphasize that the approach like this is not an ambitious one as those based e.g. on CFD (computational ¯uid dynamics) but quite sucient for practical design in the sense of obtaining basic data about the process. All the procedures involved in the mathematical model (e.g. those for calculation of ¯ue gas ¯owrates and composition, net heating values of wastes, enthalpy of species and streams, heat transfer in heat exchangers etc.) are based on the theory generally available in the open literature, like e.g. [4±6]. A dierent situation occurs in case of nitrogen oxides (NOX) emissions. Kinetic eects along with other factors are more important. NOX emissions depend not only on the type of combustion technology but also on its size and the type of fuel used [7]. For a detailed investigation of combustion chambers some comprehensive models for simulation of ¯uid ¯ow, heat transfer, combustion and pollutant formation based on CFD are usually applied. The NOX formation can be considered in a post-processor routine. Another possible approach for NOX emissions prediction consists in
Fig. 4. Representation of primary and secondary combustion chambers using simulation program.
creating a simple semi-empirical kinetic mathematical model which is based on the evaluation of so-called equivalent temperature of NOX emissions formation using experimental data from testing facilities [7]. From the above aspects it can be stated that it is not convenient to combine the simulation program for thermal processing of wastes with procedures for evaluating NOX emissions concentration. 3.2. Simulation program Simulation software is an ecient tool for a rapid design and simulation of thermal treatment processes and incinerators. Similarly, like other professional computer programs, it is run under Windows thus it is a user-friendly product. The structure of this program is obvious from the example shown in Fig. 4 and explained above. A sequential method of calculation is used in simulation, i.e. output streams from one apparatus are simultaneously the input streams for the next one. This method is rather simple and straightforward. However, repeated computational runs have to be done e.g. in case of recycled ¯ue gas. The systems of an incinerator which are considered for simulation are obvious from Fig. 1 (they are surrounded by the dashed line). The apparatuses and/or pieces of equipment which are involved in the software are as follows: combustion chambers for burning solid, liquid and gas wastes, heat exchangers (air preheaters, economizers, coolers, etc.), boilers (steam generators), superheaters, fans, ®lters, o-gas scrubbers, adsorbers. Additional symbolic apparatuses as mixers, dividers and regulators enable to generate ¯owsheet of various types of incinerators. Their importance and application will be shown in case study. Apparatuses are connected by streams. Usually the streams like ¯ue gas, combustion air, solid or liquid wastes, natural gas, feed water, steam etc. are considered. Based on the representation of apparatuses and equipment described above a philosophy of the program can be explained using a very simpli®ed scheme from Fig. 5. The procedure starts with calculation of the ®rst apparatus for which input data are at our disposal. Thus we evaluate parameters of output streams for this apparatus (composition, ¯owrates, temperatures etc.). Then we continue with calculating next apparatus. However, if the set of required input data is not complete it is necessary to calculate a dierent apparatus. If all the input parameters are known calculation of the apparatus according to its type is performed. The procedure is repeated according to an internal algorithm (the description of which is outside the scope of this paper) until all the apparatuses are calculated. Usually a following apparatus is always calculated (sequential
P. Stehlik et al. / Waste Management 20 (2000) 435±442
method) except regulators. Therefore regulators must be conveniently and appropriately placed into the ¯owsheet including setting so called ``regulation path and/or loop'' from the starting point of calculation. For example. it is necessary to increase temperature of the ¯ue gas from the combustion chamber using an auxiliary burner. Then the regulator is placed after mixing of ¯ue gas from the combustion chamber and the auxiliary burner. The purpose of regulation is an automatic setting of fuel ¯owrate into this burner so as the required temperature could be achieved. 3.3. Using the software system The design of a unit consists in drawing a ¯owsheet. A new ¯owsheet can be generated and/or a previously generated one can be read and re-drawn. We can remove pieces of equipment or add new ones and/or solve various alternatives. Thus it is possible to design new units, to simulate existing ones or to answer questions connected with changes in the standard operating mode (e.g. evaluating the impacts to the rest of the system of changing the feed composition), with adapting the operation to reduced emission limit, etc. A ¯owsheet obtained by using the simulation program is similar as those obtained by professional software packages for simulation of processes. After a ¯owsheet generation input data for all the pieces of equipment have to be speci®ed. These items are accessible using the icon Apparatus in the main menu and then the icon Speci®cation in the related submenu. Similarly, input data of input streams are speci®ed and input data of those output streams where a speci®cation is required (e.g. temperature of exit ¯ue gas). An exam-
439
ple which shows ``solid waste input'' into a rotary kiln incinerator is shown in Fig. 6. After completing a ¯owsheet and input data speci®cation the computer program can be run. Results of simulation are accessible using the icon Output of results. It is possible to select the results either of only several apparatuses or streams or all of them. A detailed description of the software is in [8]. Using this software will be obvious from an example (case study). 3.4. Application The advantages of this software for simulation based on modelling mass and energy balance are many, including reproducibility, speed and accuracy. Using this system enables to obtain a better understanding of the process for thermal treatment. It is supported by the graphical representation (¯owsheet) where all the parts of a process including their connections are shown. As already mentioned above the computer program has been developed for the following purposes: . design of incinerators and/or thermal treatment processes (performing rapid and exact mass and heat balances for all process components and chemical constituents of a chosen processing scenario or of more alternatives) . simulation of existing operation (eective engineering tool in troubleshooting existing processes and a better understanding the processes) . investigation of parametric sensitivity (e.g. how a change in the air humidity in¯uences the process) . supporting tool for decision±making (assisting in answering questions ``what if'', evaluating parameters of a dierent regime of operation±dierent composition of wastes, required higher temperature of ¯ue gas, dierent parameters of steam, etc.)
4. Case study
Fig. 5. Algorithm of simulation program.
Simulating an incinerator for the thermal treatment of various types of wastes (see Fig. 2) can be considered as an instructive example of the bene®t of using the simulation software. Capacity of the incinerator is 10 kt/ year. It is designed for the thermal treatment of industrial wastes. However, municipal solid wastes can be processed in this unit, too. It is a typical design of incinerators for small to medium capacity for a regional use. Some basic information about the incinerator and representation of its primary combustion chamber using simulation program were mentioned in the previous paragraphs already. However, let us give a detailed and complete description of the process using a ¯owsheet
440
P. Stehlik et al. / Waste Management 20 (2000) 435±442
generated by the computer program. This ¯owsheet (see Fig. 7) is in agreement with the scheme in Fig. 2. An incinerator consists of a rotary kiln (K1S and K1L-B1) and after burner chamber with stabilizing burners (K2G-B2). To achieve a more eective operation an alternative with recycled ¯ue gas (stream 27) has been proposed. This stream is mixed with combustion air in the mixer M1. Alternatives with recycled ¯ue gas bring a bene®t as follows: overall volumetric ¯owrate of ¯ue gas at the outlet is substantially reduced (up to by one third), temperature of combustion air is increased after mixing and air is not necessary to be preheated and cleaning outlet ¯ue gas is easier because of higher concentration of pollutants in ¯ue gas. A regulator R1 follows the mixer (M2) where ¯ue gases from combustion of solid wastes (apparatus K1S) and liquid wastes and oil from the auxiliary oil burner (apparatus K1L-B1) are mixed. This regulator sets an optimum ¯owrate of liquid waste into the burner so as the required temperature of ¯ue gas in the R1 location would be achieved. Further, a gas burner is involved in the afterburner chamber (apparatus K2G-B2). Flowrate of natural gas is controlled by the regulator R2 in order that the ¯ue gas temperature in this point would be as required. Combustion air for the afterburner chamber is preheated by ¯ue gas in the exchanger E2. Air in®ltra-
tion through leakages and the kiln seals is simulated by the mixer M3. A mixture of ¯ue gas and air (stream 34) enters a waste heat boiler which consists of two apparatuses (heat exchangers) Ð steam generator (E4) and superheater (E3). The superheater is placed at the hot gases inlet. A textile ®lter (F1) for separating ash particles follows. A ¯ue gas divider (D1) for splitting the stream into recycled ¯ue gas and outlet ¯ue gas follows the ®lter. A correct ratio of splitting is controlled by the regulator R3 which also ensures stopping iterative runs. The exchanger E1 is placed after the regulator and enables ¯ue gas cooling before entering the three stages scrubber A1±A3. Flue gas from the scrubber is saturated by steam and leaves this unit with temperature of 60 C and it is heated up to 100 C in the exchanger E1 before entering the adsorber A4 (its relative humidity is around 20%). From the adsorber the ¯ue gas enters the stack (stream 33). As already stated, the above described arrangement of a unit for thermal disposal of wastes is typical for incinerators of industrial and municipal wastes with low to medium capacity. The ¯owsheet provides us with information which apparatuses and equipment can be used for a design of similar units for thermal disposal of wastes. The main results of a simulation are as follows:
Fig. 6. Example of input data speci®cation.
P. Stehlik et al. / Waste Management 20 (2000) 435±442
441
Fig. 7. Flowsheet of incinerator generated by simulation program.
heat and mass balance for all pieces of equipment, detailed process streams data (temperature, pressure, mass and volumetric ¯owrates, composition). Based on these data a design of a unit can be made (dimensions of equipment, piping, etc.). Required basic parameters important for heat and mass balance are given in Table 1. Four alternatives diering by types of wastes are considered for simulation. A mixture of municipal lump solid wastes and medical wastes with net heating value of 20 300 kJ/kg is marked as the alternative A, paste and loose type wastes mixed with a grinded lump waste with particles not exceeding 60 mm and relatively low net heating value (13 000 kJ/kg) caused by high humidity are considered in the alternative B. This is in fact a mixture of industrial wastes with high content of sludges from waste water treatment. A mixture of industrial liquid wastes with high content of crude oil compounds, ammonia water and rinsing liquid with net heating Table 1 Basic input parameters for case study Mass ¯owrate of wastes Flue gas temperature after the ®rst stage of incinerator (stream 7) Flue gas temperature at the after burner combustion chamber exit (stream 12) Flue gas temperature after waste heat boiler Boiler feed water temperature Boiler feed water pressure Temperature of generated steam Pressure of generated steam Temperature of ¯ue gas leaving the scrubber
ÿ1
(kgh ) ( C)
1429 900
( C)
950
( C) ( C) (kPa) ( C) (kPa) ( C)
220 115 250 400 4000 60
value of 31 300kJ/kg is marked as the alternative C while there is considered an average waste obtained by mixing of wastes A, B, C (with net heating value of 18 900kJ/kg) in the alternative D. Composition of wastes in mass percentage is speci®ed in Table 2 Selected result of heat and mass balance calculations are summarised in Table 3. However, detailed outlet data of all the apparatuses and streams are generated by the computer program. A qualitative evaluation is rather dicult because the composition of wastes in given alternatives (and also net heating values of wastes and ¯ue gas ¯owrates) is dierent. So that an approximate consistence of the alternatives would be kept, Table 2 Composition of wastes Waste A
Waste B
Waste C
Waste D
Wet waste (% mass)
Wet waste (% mass)
Wet waste (% mass)
Wet waste (% mass)
Components C H S A O Cl
53.0 3.6 0.8 0.0 1.9 0.6
34.4 3.1 0.4 3.0 2.4 0.0
66.0 12.4 0.4 0.8 2.8 0.1
49.5 3.4 0.1 1.1 2.6 0.1
Combustible
59.9
43.3
82.5
56.8
Ash H2O
14.1 26.0
8.8 47.9
2.0 15.5
18.5 24.7
Wet
100.0
100.0
100.0
100.0
442
P. Stehlik et al. / Waste Management 20 (2000) 435±442
Table 3 Selected results of simulationa Data of incinerator ( obtained from simulation using computer program)
Flue gas temperature after mixing M2 (at the rotary kiln exit) ( C) Flue gas temperature after mixing M4 (at the after burner chamber exit) ( C) Temperature of combustion air at the gas burner inlet ( C) Flue gas temperature at the superheater inlet ( C) Flue gas temperature at the superheater outlet ( C) Flue gas temperature at the steam generator outlet( C) Ash mass ¯owrate at the rotary kiln outlet (kghÿ1) Natural gas ¯owrate (m3Nhÿ1) Superheated steam mass ¯owrate (kghÿ1) Temperature of recycled ¯ue gas ( C) Volumetric ¯owrate of recycled ¯ue gas (m3Nhÿ1) Consumption of water in liquid scrubber (kghÿ1) Volumetric ¯owrate of ¯ue gas (at the exit) (m3Nhÿ1)
Stream No.
Waste A
Waste B
Waste C
Waste D
7 12 13 34 21 17 ± 9 20 27 27 29 33
900 950 200 935 834 220 206 139 12 806 202 8457 2931 29 012
900 950 200 930 830 220 128 98 8097 192 5465 1214 17 903
900 950 200 939 836 220 31 202 20 000 208 12 991 3896 43 818
900 950 200 934 833 220 265 131 11 877 200 7875 2713 27 000
a Air excess for combustion of natural gas a=1.05. Recycled ¯ue gas (stream 27) ¯owrates shares by 25% on the total ¯owrate at the divider inlet (stream 23).
equal parameters (i.e. excess air, heat losses, ¯ue gas temperature in important points etc.) of individual apparatuses as well as the amount of waste burned were considered. Selected alternatives can be evaluated according to various criteria. Based on the natural gas consumption the ¯owrate of this fuel in the alternative C is approximately twice higher than in other alternatives, however, there is much more steam generated in this alternative. If we consider the amount of ash, this alternative (C) is the most favourable one Ð mass of ash produced here is only 10% of that produced under the conditions of the alternative D which is the worst one. If we evaluate the volumetric ¯owrate of the outlet ¯ue gas we can ®nd the largest and lowest ¯owrates for the alternative C and B, respectively, and alternatives A and D are comparable. 5. Conclusion Developing the computer program for simulating processes for thermal disposal and/or processing of various types of wastes was initiated by the fact that such a software should exist to facilitate designer's and operators' eort. It would be possible to use some software products for simulating chemical/petrochemical and other processes. However, the simulation of incinerators can be considered as a speci®c one and the simulation program is tailored to match these systems. It is an ``open'' software product which can be extended according to further needs and requirements. A structure of the system for simulation and principles of modelling are brie¯y explained and various ®elds of application (design, simulating existing operation,
investigation of parametric sensitivity, supporting tool for decision-making) are mentioned. Using this software for simulation of incinerators is demonstrated through a case study (incinerator for thermal treatment of various kinds of wastes). The simulation program was used in many industrial cases (thermal disposal of exhaust gases from paint shops, incineration of volatile organic compounds, thermal treatment of waste gases from the polystyrene production, incineration of sludges in a pulp and paper company, etc.). It has practically no limitation for simulating the thermal treatment processes and it is only necessary to master its application.
References [1] Holmes G, Singh BR, Theodore L. Handbook of environmental management & technology. New York: John Wiley & Sons, 1993. [2] Demison RA, Ruston J, editors. Recycling & incineration Ð evaluating the choices. Washington (DC): Island Press, 1990. [3] Fisher JC, Vavruska JS, Thompson TKA. Mass and energy balance process model for thermal treatment processes. In: Proc. Int. Conf. Incineration and Thermal Treatment Technologies. Savannah (GA, USA) 6±10 May 1996, p. 747±53. [4] Perry RH, Chilton CH. Chemical engineers' handbook. New York: McGraw-Hill Inc, 1973. [5] Hewitt GF, coordinating editor. Handbook of heat exchanger design. New York: Hemisphere Publishing Corporation, 1992. [6] VDI-WaÈrmeatlas. 4th ed. DuÈsseldorf: VDI Verlag GmbH. 1984. [7] Stehlik P, Kana R, Puchyr R. Possible approach for NOx emissions prediction in process industry. In: Clean Air IV - Fourth International Conference on Technologies and Combustion for a Clean Environment, Lisbon, Portugal,. July 1997. p. 14.3. 11±6. [8] Puchyr R, Luksch A. TDW 1.0, Manual (user's guide) of software for simulation of units for thermal disposal of wastes. CZ: Technical University of Brno, 1997.
www.elsevier.nl/locate/wasman
Simulation of processes for thermal treatment of wastes Petr Stehlik a,*, Radim Puchyr a, Jaroslav Oral b a
Department of Process Engineering, Technical University of Brno Technicka 2, 61669 Brno, Czech Republic b EVECO Brno, Ltd, Brno, Czech Republic Accepted 6 December 1999
Abstract A software system for simulating processes for thermal treatment and/or incineration of various types of wastes has been developed. Developing this software was initiated by the need to support and facilitate designers' and operators' activities. The software product is based on modelling Ð performing a mass and energy balance. A structure of this system is explained as well as the principles of application. Using the simulation program is demonstrated through a case study (incinerator for thermal treatment of various kinds of both industrial and municipal wastes). Flowsheet generation, input data speci®cation, calculation and results of simulation are shown. Various ®elds of application (design, simulating existing operation, investigation of parametric sensitivity, supporting tool for decision-making) are mentioned. Using this system is comparable with using various professional packages for the simulation of chemical/petrochemical and other industrial processes. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Software; Thermal treatment; Incineration
1. Introduction Serious problems are currently solved in the ®eld of environmental protection. The challenge facing concerned citizens and decision-makers is a formidable one to identify and implement long-term solutions that are safe, socially acceptable, and cost-eective. Such amount of wastes, which is produced either by inhabitants or by industrial companies requires to use ecient ways of waste disposal. The recent focus on incineration has been on environmental consequences, not on performance. In particular, the limitations, as well as the advantages, of incineration are being increasingly recognized [1,2]. Incineration is not a waste disposal method but rather a waste processing technology. Modern incinerators, though simple in concept, are highly complex machines. The heart of any incinerator is the combustion chamber where waste is burned. However, the overall unit consists usually from various types of systems and equipment. Many types of wastes (like municipal, industrial, medical, hazardous, radioactive) have to be burned. Therefore, design and operation of incinerators is obviously a dicult task and eective supporting tools are very useful. * Corresponding author. Tel.: +420-5-411-2373; fax: +420-5-4112373. E-mail address: [email protected] (P. Stehlik).
We have developed a simulation program for simulating processes for thermal disposal and/or processing of wastes. Using this system it is possible to calculate heat and mass balance of various processes for thermal treatment technologies. This simulation system is highly versatile and can be considered as a powerful engineering tool. This is very important because there is currently a multitude of incinerators and other thermal treatment systems in existence worldwide. 2. Typical arrangement of incinerators and other thermal treatment systems Incinerator is in fact a system consisting of several subsystems (equipment) for ecient thermal processing of wastes. Let us consider a unit where the burned wastes would be changed into a product without toxic and environmentally harmful constituents. This product could be somehow utilized if its energy could be utilized. No additional raw materials (feed, ¯ue gas, etc.) would be used, no additional wastes would be generated. Such a unit can be called the theoretical ideal incinerator. The aim is to design incinerators the characteristics of which would be as close as possible to the ideal one. A multitude of various incinerators for various purposes has been developed. There are very similar
0956-053X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(00)00008-8
436
P. Stehlik et al. / Waste Management 20 (2000) 435±442
thermal, physical and chemical processes, similar systems and equipment, similar characteristic features in most of incinerators. A block diagram of a typical arrangement is in Fig.1. Incinerator consists in fact of the following systems: . storage and waste feeding system (enclosed tipping area, refuse pits for various types of wastes, feed bins, tanks for liquid wastes, etc.) . incineration (primary combustion chamber±± rotary kiln, furnace with moving grate etc., afterburner chamber) . heat recovery (boiler, superheater, economizer, waste heat boiler, air preheater) . o-gas cleaning (removal of particles: ®lter, baghouse, electrostatic precipitator, etc., cleaning system: o-gas scrubber, scrubber liquid treatment, adsorber, etc.) . auxiliary equipment (ash/slag transport, fans, etc.) . monitoring and control system From Fig. 1. it is obvious which systems of incinerator can be simulated using simulation program. A concrete example of an incinerator is shown in Fig. 2. This is a ¯owsheet of incinerator with capacity 10 kt/ year for combustion of industrial and/or municipal wastes with various composition. A rotary kiln K1 is used as the ®rst stage of combustion and gas burners B2 are installed in the afterburner chamber K2. The function of these burners is to stabilize the operation, i.e. to keep constant temperature of ¯ue gases in case of a change in wastes' net heating value. Heat recovery system consists of air preheater (E2), boiler (E4), superheater (E3) and heat exchanger keeping the required temperature of ¯ue gas in the o-gas cleaning system (E1). This system consists of a ®lter F1 for particulates removal, three stages of a scrubber system (A1 to A3) and adsorber A4. (Scrubber liquid treatment and wastewater treatment systems which are parts of incinerator are not considered for simulation.) It will be shown later how to generate the ¯owsheet from Fig. 2 with the aid of simulation program.
3. Simulation of wastes processing A software system for simulation is based on mathematical modelling. The principle is similar to the case of worldwide used software packages for simulating processes in chemical/petrochemical and other process industries (e.g. ChemCAD, ASPEN Plus, PRO II, HYSIM). However, there is a demand to have at one's disposal a software which would take into consideration speci®c features of thermal treatment and/or incineration processes and would be user-friendly. The primary purpose of modelling is to perform rapid and accurate heat and mass balances for each process equipment component of the user's desired processing scenario. The basic principles and purposes of modelling are similar, like for example in [3]. However, the software system has been developed and tailored in such a way to match most of incineration and thermal treatment systems in a professional way. 3.1. Structure of a system for simulation The thermal treatment and/or incineration process model has a modular design which allows evaluating various con®gurations of simulated facilities. The design consists in drawing ¯owsheet with individual equipment and/or apparatuses which are connected by streams. The incinerator is composed of several items (apparatuses) and piping in a real operation is represented by streams in a ¯owsheet. As already stated the model is based on heat and mass balance. Therefore it is necessary to set up appropriate procedures for all items of a ¯owsheet taken into consideration. A general representation of a model is obvious from Fig. 3a. Let us consider e.g. a primary combustion chamber where both solid and liquid wastes are burned and auxiliary fuel (e.g. natural gas) is used. A simpli®ed representation of this equipment is in Fig. 3b. The purpose of the primary combustion chamber modelling is to perform a heat and mass balance. This model accepts data of all input streams, calculates the elemental composition of each input stream, and converts the input streams through
Fig. 1. Block diagram of incinerator.
P. Stehlik et al. / Waste Management 20 (2000) 435±442
437
Fig. 2. Incinerator for thermal treatment of various types of wastes.
the proper reaction stoichiometry to either combustion product±±¯ue gas species or solid ash. The computer program balances the energy in the primary combustion chamber unit operation such that the output streams are at the user speci®ed temperature. This is done by adjustment of the input auxiliary fuel and combustion air ¯owrates. Heat losses (vessel surface losses) can also be speci®ed. For better understanding of a structure of the system simulated a concrete example is shown in Fig. 4. Let us consider a part of the incinerator from Fig. 2 Ð the primary and secondary combustion chambers Ð rotary kiln K1 and afterburner chamber K2, respectively. A
representation of this subsystem using the simulation program is as follows: The rotary kiln (primary combustion chamber) is comprised of two apparatuses K1S and K1L-B1 where solid and liquid wastes and oil from the auxiliary oil burner B1 are burned. Combustion air for the secondary combustion chamber is preheated by ¯ue gas in the heat exchanger (air preheater) E2. M1 is a mixer which enables to consider mixing of combustion air with recycled ¯ue gas. M2 and M4 are other mixers which are necessary for a convenient representation of the system for modelling. There is obvious from Fig. 4 how various parts of incinerator are represented in the ¯owsheet
Fig. 3. Simpli®ed representation of a model.
438
P. Stehlik et al. / Waste Management 20 (2000) 435±442
generated by computer program. Using similar symbols and representation as those in worldwide used and recognised software packages for simulation was one of the aims in developing this computer code. A model of the above subsystem consists of procedures for calculating combustion, evaluating combustion air ¯owrate, composition, temperature and ¯owrate of ¯ue gas, net heating value of fuel based on its composition and heat balance of the apparatus. It is necessary to emphasize that the approach like this is not an ambitious one as those based e.g. on CFD (computational ¯uid dynamics) but quite sucient for practical design in the sense of obtaining basic data about the process. All the procedures involved in the mathematical model (e.g. those for calculation of ¯ue gas ¯owrates and composition, net heating values of wastes, enthalpy of species and streams, heat transfer in heat exchangers etc.) are based on the theory generally available in the open literature, like e.g. [4±6]. A dierent situation occurs in case of nitrogen oxides (NOX) emissions. Kinetic eects along with other factors are more important. NOX emissions depend not only on the type of combustion technology but also on its size and the type of fuel used [7]. For a detailed investigation of combustion chambers some comprehensive models for simulation of ¯uid ¯ow, heat transfer, combustion and pollutant formation based on CFD are usually applied. The NOX formation can be considered in a post-processor routine. Another possible approach for NOX emissions prediction consists in
Fig. 4. Representation of primary and secondary combustion chambers using simulation program.
creating a simple semi-empirical kinetic mathematical model which is based on the evaluation of so-called equivalent temperature of NOX emissions formation using experimental data from testing facilities [7]. From the above aspects it can be stated that it is not convenient to combine the simulation program for thermal processing of wastes with procedures for evaluating NOX emissions concentration. 3.2. Simulation program Simulation software is an ecient tool for a rapid design and simulation of thermal treatment processes and incinerators. Similarly, like other professional computer programs, it is run under Windows thus it is a user-friendly product. The structure of this program is obvious from the example shown in Fig. 4 and explained above. A sequential method of calculation is used in simulation, i.e. output streams from one apparatus are simultaneously the input streams for the next one. This method is rather simple and straightforward. However, repeated computational runs have to be done e.g. in case of recycled ¯ue gas. The systems of an incinerator which are considered for simulation are obvious from Fig. 1 (they are surrounded by the dashed line). The apparatuses and/or pieces of equipment which are involved in the software are as follows: combustion chambers for burning solid, liquid and gas wastes, heat exchangers (air preheaters, economizers, coolers, etc.), boilers (steam generators), superheaters, fans, ®lters, o-gas scrubbers, adsorbers. Additional symbolic apparatuses as mixers, dividers and regulators enable to generate ¯owsheet of various types of incinerators. Their importance and application will be shown in case study. Apparatuses are connected by streams. Usually the streams like ¯ue gas, combustion air, solid or liquid wastes, natural gas, feed water, steam etc. are considered. Based on the representation of apparatuses and equipment described above a philosophy of the program can be explained using a very simpli®ed scheme from Fig. 5. The procedure starts with calculation of the ®rst apparatus for which input data are at our disposal. Thus we evaluate parameters of output streams for this apparatus (composition, ¯owrates, temperatures etc.). Then we continue with calculating next apparatus. However, if the set of required input data is not complete it is necessary to calculate a dierent apparatus. If all the input parameters are known calculation of the apparatus according to its type is performed. The procedure is repeated according to an internal algorithm (the description of which is outside the scope of this paper) until all the apparatuses are calculated. Usually a following apparatus is always calculated (sequential
P. Stehlik et al. / Waste Management 20 (2000) 435±442
method) except regulators. Therefore regulators must be conveniently and appropriately placed into the ¯owsheet including setting so called ``regulation path and/or loop'' from the starting point of calculation. For example. it is necessary to increase temperature of the ¯ue gas from the combustion chamber using an auxiliary burner. Then the regulator is placed after mixing of ¯ue gas from the combustion chamber and the auxiliary burner. The purpose of regulation is an automatic setting of fuel ¯owrate into this burner so as the required temperature could be achieved. 3.3. Using the software system The design of a unit consists in drawing a ¯owsheet. A new ¯owsheet can be generated and/or a previously generated one can be read and re-drawn. We can remove pieces of equipment or add new ones and/or solve various alternatives. Thus it is possible to design new units, to simulate existing ones or to answer questions connected with changes in the standard operating mode (e.g. evaluating the impacts to the rest of the system of changing the feed composition), with adapting the operation to reduced emission limit, etc. A ¯owsheet obtained by using the simulation program is similar as those obtained by professional software packages for simulation of processes. After a ¯owsheet generation input data for all the pieces of equipment have to be speci®ed. These items are accessible using the icon Apparatus in the main menu and then the icon Speci®cation in the related submenu. Similarly, input data of input streams are speci®ed and input data of those output streams where a speci®cation is required (e.g. temperature of exit ¯ue gas). An exam-
439
ple which shows ``solid waste input'' into a rotary kiln incinerator is shown in Fig. 6. After completing a ¯owsheet and input data speci®cation the computer program can be run. Results of simulation are accessible using the icon Output of results. It is possible to select the results either of only several apparatuses or streams or all of them. A detailed description of the software is in [8]. Using this software will be obvious from an example (case study). 3.4. Application The advantages of this software for simulation based on modelling mass and energy balance are many, including reproducibility, speed and accuracy. Using this system enables to obtain a better understanding of the process for thermal treatment. It is supported by the graphical representation (¯owsheet) where all the parts of a process including their connections are shown. As already mentioned above the computer program has been developed for the following purposes: . design of incinerators and/or thermal treatment processes (performing rapid and exact mass and heat balances for all process components and chemical constituents of a chosen processing scenario or of more alternatives) . simulation of existing operation (eective engineering tool in troubleshooting existing processes and a better understanding the processes) . investigation of parametric sensitivity (e.g. how a change in the air humidity in¯uences the process) . supporting tool for decision±making (assisting in answering questions ``what if'', evaluating parameters of a dierent regime of operation±dierent composition of wastes, required higher temperature of ¯ue gas, dierent parameters of steam, etc.)
4. Case study
Fig. 5. Algorithm of simulation program.
Simulating an incinerator for the thermal treatment of various types of wastes (see Fig. 2) can be considered as an instructive example of the bene®t of using the simulation software. Capacity of the incinerator is 10 kt/ year. It is designed for the thermal treatment of industrial wastes. However, municipal solid wastes can be processed in this unit, too. It is a typical design of incinerators for small to medium capacity for a regional use. Some basic information about the incinerator and representation of its primary combustion chamber using simulation program were mentioned in the previous paragraphs already. However, let us give a detailed and complete description of the process using a ¯owsheet
440
P. Stehlik et al. / Waste Management 20 (2000) 435±442
generated by the computer program. This ¯owsheet (see Fig. 7) is in agreement with the scheme in Fig. 2. An incinerator consists of a rotary kiln (K1S and K1L-B1) and after burner chamber with stabilizing burners (K2G-B2). To achieve a more eective operation an alternative with recycled ¯ue gas (stream 27) has been proposed. This stream is mixed with combustion air in the mixer M1. Alternatives with recycled ¯ue gas bring a bene®t as follows: overall volumetric ¯owrate of ¯ue gas at the outlet is substantially reduced (up to by one third), temperature of combustion air is increased after mixing and air is not necessary to be preheated and cleaning outlet ¯ue gas is easier because of higher concentration of pollutants in ¯ue gas. A regulator R1 follows the mixer (M2) where ¯ue gases from combustion of solid wastes (apparatus K1S) and liquid wastes and oil from the auxiliary oil burner (apparatus K1L-B1) are mixed. This regulator sets an optimum ¯owrate of liquid waste into the burner so as the required temperature of ¯ue gas in the R1 location would be achieved. Further, a gas burner is involved in the afterburner chamber (apparatus K2G-B2). Flowrate of natural gas is controlled by the regulator R2 in order that the ¯ue gas temperature in this point would be as required. Combustion air for the afterburner chamber is preheated by ¯ue gas in the exchanger E2. Air in®ltra-
tion through leakages and the kiln seals is simulated by the mixer M3. A mixture of ¯ue gas and air (stream 34) enters a waste heat boiler which consists of two apparatuses (heat exchangers) Ð steam generator (E4) and superheater (E3). The superheater is placed at the hot gases inlet. A textile ®lter (F1) for separating ash particles follows. A ¯ue gas divider (D1) for splitting the stream into recycled ¯ue gas and outlet ¯ue gas follows the ®lter. A correct ratio of splitting is controlled by the regulator R3 which also ensures stopping iterative runs. The exchanger E1 is placed after the regulator and enables ¯ue gas cooling before entering the three stages scrubber A1±A3. Flue gas from the scrubber is saturated by steam and leaves this unit with temperature of 60 C and it is heated up to 100 C in the exchanger E1 before entering the adsorber A4 (its relative humidity is around 20%). From the adsorber the ¯ue gas enters the stack (stream 33). As already stated, the above described arrangement of a unit for thermal disposal of wastes is typical for incinerators of industrial and municipal wastes with low to medium capacity. The ¯owsheet provides us with information which apparatuses and equipment can be used for a design of similar units for thermal disposal of wastes. The main results of a simulation are as follows:
Fig. 6. Example of input data speci®cation.
P. Stehlik et al. / Waste Management 20 (2000) 435±442
441
Fig. 7. Flowsheet of incinerator generated by simulation program.
heat and mass balance for all pieces of equipment, detailed process streams data (temperature, pressure, mass and volumetric ¯owrates, composition). Based on these data a design of a unit can be made (dimensions of equipment, piping, etc.). Required basic parameters important for heat and mass balance are given in Table 1. Four alternatives diering by types of wastes are considered for simulation. A mixture of municipal lump solid wastes and medical wastes with net heating value of 20 300 kJ/kg is marked as the alternative A, paste and loose type wastes mixed with a grinded lump waste with particles not exceeding 60 mm and relatively low net heating value (13 000 kJ/kg) caused by high humidity are considered in the alternative B. This is in fact a mixture of industrial wastes with high content of sludges from waste water treatment. A mixture of industrial liquid wastes with high content of crude oil compounds, ammonia water and rinsing liquid with net heating Table 1 Basic input parameters for case study Mass ¯owrate of wastes Flue gas temperature after the ®rst stage of incinerator (stream 7) Flue gas temperature at the after burner combustion chamber exit (stream 12) Flue gas temperature after waste heat boiler Boiler feed water temperature Boiler feed water pressure Temperature of generated steam Pressure of generated steam Temperature of ¯ue gas leaving the scrubber
ÿ1
(kgh ) ( C)
1429 900
( C)
950
( C) ( C) (kPa) ( C) (kPa) ( C)
220 115 250 400 4000 60
value of 31 300kJ/kg is marked as the alternative C while there is considered an average waste obtained by mixing of wastes A, B, C (with net heating value of 18 900kJ/kg) in the alternative D. Composition of wastes in mass percentage is speci®ed in Table 2 Selected result of heat and mass balance calculations are summarised in Table 3. However, detailed outlet data of all the apparatuses and streams are generated by the computer program. A qualitative evaluation is rather dicult because the composition of wastes in given alternatives (and also net heating values of wastes and ¯ue gas ¯owrates) is dierent. So that an approximate consistence of the alternatives would be kept, Table 2 Composition of wastes Waste A
Waste B
Waste C
Waste D
Wet waste (% mass)
Wet waste (% mass)
Wet waste (% mass)
Wet waste (% mass)
Components C H S A O Cl
53.0 3.6 0.8 0.0 1.9 0.6
34.4 3.1 0.4 3.0 2.4 0.0
66.0 12.4 0.4 0.8 2.8 0.1
49.5 3.4 0.1 1.1 2.6 0.1
Combustible
59.9
43.3
82.5
56.8
Ash H2O
14.1 26.0
8.8 47.9
2.0 15.5
18.5 24.7
Wet
100.0
100.0
100.0
100.0
442
P. Stehlik et al. / Waste Management 20 (2000) 435±442
Table 3 Selected results of simulationa Data of incinerator ( obtained from simulation using computer program)
Flue gas temperature after mixing M2 (at the rotary kiln exit) ( C) Flue gas temperature after mixing M4 (at the after burner chamber exit) ( C) Temperature of combustion air at the gas burner inlet ( C) Flue gas temperature at the superheater inlet ( C) Flue gas temperature at the superheater outlet ( C) Flue gas temperature at the steam generator outlet( C) Ash mass ¯owrate at the rotary kiln outlet (kghÿ1) Natural gas ¯owrate (m3Nhÿ1) Superheated steam mass ¯owrate (kghÿ1) Temperature of recycled ¯ue gas ( C) Volumetric ¯owrate of recycled ¯ue gas (m3Nhÿ1) Consumption of water in liquid scrubber (kghÿ1) Volumetric ¯owrate of ¯ue gas (at the exit) (m3Nhÿ1)
Stream No.
Waste A
Waste B
Waste C
Waste D
7 12 13 34 21 17 ± 9 20 27 27 29 33
900 950 200 935 834 220 206 139 12 806 202 8457 2931 29 012
900 950 200 930 830 220 128 98 8097 192 5465 1214 17 903
900 950 200 939 836 220 31 202 20 000 208 12 991 3896 43 818
900 950 200 934 833 220 265 131 11 877 200 7875 2713 27 000
a Air excess for combustion of natural gas a=1.05. Recycled ¯ue gas (stream 27) ¯owrates shares by 25% on the total ¯owrate at the divider inlet (stream 23).
equal parameters (i.e. excess air, heat losses, ¯ue gas temperature in important points etc.) of individual apparatuses as well as the amount of waste burned were considered. Selected alternatives can be evaluated according to various criteria. Based on the natural gas consumption the ¯owrate of this fuel in the alternative C is approximately twice higher than in other alternatives, however, there is much more steam generated in this alternative. If we consider the amount of ash, this alternative (C) is the most favourable one Ð mass of ash produced here is only 10% of that produced under the conditions of the alternative D which is the worst one. If we evaluate the volumetric ¯owrate of the outlet ¯ue gas we can ®nd the largest and lowest ¯owrates for the alternative C and B, respectively, and alternatives A and D are comparable. 5. Conclusion Developing the computer program for simulating processes for thermal disposal and/or processing of various types of wastes was initiated by the fact that such a software should exist to facilitate designer's and operators' eort. It would be possible to use some software products for simulating chemical/petrochemical and other processes. However, the simulation of incinerators can be considered as a speci®c one and the simulation program is tailored to match these systems. It is an ``open'' software product which can be extended according to further needs and requirements. A structure of the system for simulation and principles of modelling are brie¯y explained and various ®elds of application (design, simulating existing operation,
investigation of parametric sensitivity, supporting tool for decision-making) are mentioned. Using this software for simulation of incinerators is demonstrated through a case study (incinerator for thermal treatment of various kinds of wastes). The simulation program was used in many industrial cases (thermal disposal of exhaust gases from paint shops, incineration of volatile organic compounds, thermal treatment of waste gases from the polystyrene production, incineration of sludges in a pulp and paper company, etc.). It has practically no limitation for simulating the thermal treatment processes and it is only necessary to master its application.
References [1] Holmes G, Singh BR, Theodore L. Handbook of environmental management & technology. New York: John Wiley & Sons, 1993. [2] Demison RA, Ruston J, editors. Recycling & incineration Ð evaluating the choices. Washington (DC): Island Press, 1990. [3] Fisher JC, Vavruska JS, Thompson TKA. Mass and energy balance process model for thermal treatment processes. In: Proc. Int. Conf. Incineration and Thermal Treatment Technologies. Savannah (GA, USA) 6±10 May 1996, p. 747±53. [4] Perry RH, Chilton CH. Chemical engineers' handbook. New York: McGraw-Hill Inc, 1973. [5] Hewitt GF, coordinating editor. Handbook of heat exchanger design. New York: Hemisphere Publishing Corporation, 1992. [6] VDI-WaÈrmeatlas. 4th ed. DuÈsseldorf: VDI Verlag GmbH. 1984. [7] Stehlik P, Kana R, Puchyr R. Possible approach for NOx emissions prediction in process industry. In: Clean Air IV - Fourth International Conference on Technologies and Combustion for a Clean Environment, Lisbon, Portugal,. July 1997. p. 14.3. 11±6. [8] Puchyr R, Luksch A. TDW 1.0, Manual (user's guide) of software for simulation of units for thermal disposal of wastes. CZ: Technical University of Brno, 1997.