Numerical Investigation of a Portable Incinerator: a Parametric Study

processes

Article Numerical Investigation of a Portable Incinerator: A Parametric Study

Mohsen Saﬀari Pour 1,2 , Ali Hakkaki-Fard 1,* and Bahar Firoozabadi 1 1 Department of Mechanical Engineering, Sharif University of Technology, Tehran 79417, Iran; [email protected] (M.S.P.); ﬁ[email protected] (B.F.) 2 KTH Royal Institute of Technology, 114 28 Stockholm, Sweden * Correspondence: [email protected]; Tel.: +98-216-616-5525

Received: 18 June 2020; Accepted: 21 July 2020; Published: 2 August 2020

Abstract: The application of incinerators for the municipal solid waste (MSW) is growing due to the ability of such instruments to produce energy and, more specifically, reduce waste volume. In this paper, a numerical simulation of the combustion process with the help of the computational fluid dynamics (CFD) inside a portable (mobile) incinerator has been proposed. Such work is done to investigate the most critical parameters for a reliable design of a domestic portable incinerator, which is suitable for the Iranian food and waste culture. An old design of a simple incinerator has been used to apply the natural gas (NG), one of the available cheap fossil fuels in Iran. After that, the waste height, place of the primary burner, and the flow rate of the cooling air inside the incinerator, as the main parameters of the design, are investigated. A validation is also performed for the mesh quality test and the occurrence of the chemical reactions near the burner of the incinerator. Results proved that the numerical results have less than 5% error compared to the previous experimental and numerical approaches. In addition, results show that by moving the primary burner into the secondary chamber of the incinerator, the temperature and the heating ability of the incinerator could be affected dramatically. Moreover, it has been found that by increasing the flow rate of the cooling air inside the incinerator to some extent, the combustion process is improved and, on the other hand, by introducing more cooling air, the evacuation of the hazardous gases from the exhaust is also improved.

Keywords: portable incinerator; computational ﬂuid dynamics (CFD); combustion; parametric study

1. Introduction The history of burning waste is narrated from many years ago up to modern human life. In the previous era, people burned waste, usually various kinds of woods and animal wastes, to produce heat for boiling water or cooking foods [1,2]. This procedure is still popular in some rural areas of Iran and some other countryside due to the less populated area and difficulties of preparing/transportation of other kinds of fuels for their domestic use. Of course, waste burning and other related technologies have been improved during the past decades, but still these techniques are far from being fully implemented worldwide. Some of the main obstacles in front of their advances can be pointed out as; cultural issues, the countries’ directives and priorities, waste types, and sufficient places for landfills. It is noteworthy that all the progress in this way is related to many trials and errors and numerous devoted strategic studies, which are rarely publicly available for all researchers [2]. Access to all data related to the progress of this subject due to the vast usages, different applications, and various kinds of wastes are the main debates between the researchers in this field. In the last few decades, many countries tried to have some good ideas to manage their wastes [3]. For example, during this period, many advances happened in medical applications; the medical

Processes 2020, 8, 923; doi:10.3390/pr8080923 www.mdpi.com/journal/processes Processes 2020, 8, 923 2 of 19 wastes, how to treat them environmentally, and the use of them as a source of energy become a headache for all battle sides [3,4]. Moreover, it was essential for some troops to reduce their track by completely eliminating their wastes. Therefore, several countries have tried to apply some traditional methods together with novel research to invent new methods to use wastes efficiently [2]. In the available literature, the prior usage of the incinerators to the modern techniques with their current definition was first reported for the Czech Republic and the UK [4,5]. After that, recent instruments and advances in combustion techniques helped researchers and many other countries to be sure to make huge investments on this topic. Such efforts have paid off for advanced countries like Sweden, Germany, and Denmark. Nowadays, these countries use their wastes, and even import wastes from other countries to produce the domestic heat and electricity using the knowledge and techniques of district heating (DH) [6–9]. The progress in this topic is not only focused on the big incinerators but also on the small and home application of incinerators. Nowadays, small incinerators and portable ones are becoming popular in marine, military and hospital applications, and a few samples exist for home applications [10]. In advancing countries like Iran and many other regional countries, due to the vast and low-cost sources of fossil fuels, other resources received less attention [9]. For instance, renewable energies, like solar, hydropower, geothermal and incineration, were not that successful in competing with fossil fuels [11]. Renewable energies could be completely independent of fossil fuels, but incineration is a good sample way, which helps countries to think about a transition technique to partially/completely reduce their dependencies to fossil fuels. In Iran, traditionally, many applications of incineration were reported. Recently in Iran, some municipalities and some little companies tried to implement incineration to produce heat. Reports show that to some extent, by helping companies abroad and importing their techniques, some progress has been achieved. All of their attempts were mainly on the building of big incinerator facilities to produce heat and electricity [12–17]. Meanwhile, many research projects have been introduced at university levels, which are focused on the environmental concerns and not on the techniques [18]. Therefore, less attention to the combustion techniques and small portable incinerators is a matter of concern in this research. In such studies, it has been attempted to improve the combustion knowledge in incinerators as well as the application of portable incineration for small rural areas [19–22]. During the last year, many countries focused on research with special attention to the incinerators. The main principal behind using the incineration technique is to reduce the volume and toxicity level of the municipal solid wastes, chemical and other biomaterial wastes. As a sample of implementation of the incinerations, the United States could be considered. In this country, more than 100 active facilities exist for the solid wastes. In addition, 1600 smaller size incinerators are acting to reduce the medical wastes. These are not the only application of such chambers, around 200 similar combustion chambers, industrial boilers and furnaces are dealing with hazardous and nonhazardous fuel sources [23]. Therefore, many papers have been published, but their main matter of concern was about thermo-economic and environmental aspects. It means that a narrow gap exists regarding the full understanding of the combustion behavior inside incinerators [24,25]. More specifically, when pandemic dieses endanger human beings, the knowledge of incineration is of importance to deal with reducing the landfilling of the untreated medical wastes [26,27]. In this paper, with the help of Computational Fluid Dynamics (CFD) and combustion knowledge, a portable home incinerator, which can be used in the rural areas of Iran, has been simulated. A parametric study is done to investigate the most involved parameters on the working of a portable incinerator. Moreover, the ability of the incinerator to burn various volumes of wastes has been studied numerically.

2. Geometry Creation In order to simulate the combustion process inside the incinerator, it is necessary to follow several steps. In the ﬁrst step, the geometry of the incinerator is created by using the ANSYS WORKBENCH ProcessesProcesses 2020, 20208, x FOR, 8, x PEERFOR PEER REVIEW REVIEW 3 of 223 of 22

2. Geometry2. Geometry Creation Creation Processes 2020, 8, 923 3 of 19 In orderIn order to simulate to simulate the combustionthe combustion process process insi deinsi thede incinerator,the incinerator, it is itnecessary is necessary to follow to follow severalseveral steps. steps. In the In firstthe firststep ,step the, geometrythe geometry of the of incineratorthe incinerator is created is created by usingby using the ANSYSthe ANSYS 19.2WORKBENCH (AnsysWORKBENCH Inc., 19.2 Canonsburg, 19.2(Ansys (Ansys Inc., PA, Inc.,Canonsburg, USA) Canonsburg, [28]. AccordingPA, PA,USA) USA) [28]. to the [28].According ﬁrst According design to ofthe to the firstthe incinerator, firstdesign design of thethe of the geometryincinerator,incinerator, is the depicted geometrythe geometry schematically is depicted is depicted in schematicall Figures schematicall1 andy in2. yFigures In in FigureFigures 1 2and, the1 and2. consideredIn 2. Figure In Figure 2, incinerator the 2, consideredthe considered is presentedincineratorincinerator in is a presented side is presented view in to a show sidein a othersideview view importantto show to show other aspects other important ofimportant such aspects geometry. aspects of such Basedof such geometry. on geometry. the schematic Based Based on on ofthe the schematicthe incinerator, schematic of the theof incinerator, the computational incinerator, the computational the domain computational is created domain do inmain Figure is created is3 created. Later in Figure on,in Figure the 3. cratedLater 3. Later on, geometry the on, crated the is crated preparedgeometrygeometry by is aprepared networkis prepared ofby mesh, aby network a which network isof veriﬁed mesh, of mesh, andwhich validatedwhich is verified is byverified temperature and andvalidated validated variations by temperatureby along temperature the centerline.variationsvariations along Such along validationthe centerline.the centerline. is done Such to Such showvalidation validation the independence is done is done to show to of show thethe results independencethe independence to the element of the of numbers.resultsthe results to to Morethe elementthe details element numbers. of validations numbers. More More are details reported details of validations inof thevalidations results are andreported are discussionreported in the in part.results the results and discussionand discussion part. part.

FigureFigureFigure 1.1. A schematic1. A schematic 3D view 3D view of the of considered the considered portableportable portable incinerator.incinerator. incinerator.

FigureFigureFigure 2.2. Side 2. viewSide view ofof thethe of portableportable the portable incinerator.incinerator. incinerator.

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Processes 2020, 8, x FOR PEER REVIEW 4 of 22

Figure 3. TheFigure created 3. The computational created computational domain domain for doing for doing combustion combustion simulation simulation of theof the portable portable incinerator incinerator (all values are in mm—A: Gas exhaust, B: Burner inlet in primary chamber, C: Fresh air (all values are in mm—A: Gas exhaust, B: Burner inlet in primary chamber, C: Fresh air inlet, D: Air inlet, D: Air inlet, E: Burner inlet in secondary chamber, F: fresh air inlet). inlet, E: Burner inlet in secondary chamber, F: fresh air inlet). Generally, the powers of the portable incinerators are in the range of 1000 kW to 1500 kW with Generally,the burning the powers rate of 60–80 of the kg/h portable [10,29]. incineratorsAccording to the are principal in therange geometry, of 1000 the important kW to 1500 parts kW of with the burning ratethe ofportable 60–80 incinerator kg/h [10 ,are29]. identified According as the to primary the principal combustion geometry, chamber theand important the secondary parts of the combustion chamber. It is noteworthy to mention that for doing a simulation, it would be very portable incinerator are identified as the primary combustion chamber and the secondary combustion difficult to consider all geometry details. Consequently, the most involved details in the simulations chamber. Itare is introduced noteworthy to tothe mentionmodels. Figure that for 3 is doing presented a simulation, to show the it entire would computational be very diffi domaincult to consider all geometryaccording details. to the Consequently, simplified model. the As most it is shown involved in this details figure, the in thebiggest simulations chamber on arethe right introduced side to the models. Figureis the primary3 is presented one, and the to smallest show the one entire left under computational the exhaust port domainis the secondary according chamber. to the simplified In Figure 4a–c, the network of the created computational elements is presented. As it is shown model. As it is shown in this figure, the biggest chamber on the right side is the primary one, and the in this figure, a network of mesh elements is generated in triangular form for the surface mesh and smallest onetetrahedral left under and triangular the exhaust prism port for volume is the mesh. secondary The total chamber. number of elements after doing the mesh In Figurequality4a–c, test theis set network to be 6,665,341 of the elements. created In computationalFigure 4a, a cut plane elements in the X–Z is direction presented. is depicted, As it is shown in this figure,which a networkpassed through of mesh the upper elements limit of is the generated rubbish volume in triangular to show the form volume for themeshes surface in this mesh and region. In Figure 4c, a cut plane in the X–Y direction is passed through the center line of the main tetrahedral and triangular prism for volume mesh. The total number of elements after doing the mesh fresh air inlet to show the volume mesh in another view. quality test is set to be 6,665,341 elements. In Figure4a, a cut plane in the X–Z direction is depicted, which passed through the upper limit of the rubbish volume to show the volume meshes in this region. In FigureProcesses4c, a 2020 cut, 8 plane, x FOR PEER in the REVIEW X–Y direction is passed through the center line of the main fresh5 of 22 air inlet to show the volume mesh in another view.

(a)

Figure 4. Cont.

(b) (c)

3. Waste Properties The rubbish (waste) composition and their properties are completely different according to many factors and the food culture. Table 1 is prepared according to the average material properties, which are prepared from the Iranian food and waste culture. In this study, the MSW involves approximately 38% paper, 25% food waste (solid material), 10% plastics, 19% car tire, and 8% glass.

Table 1. Normalized Iranian waste properties. All numbers are reported as their lower heating values (LHV).

Solid Materials MJ/kg Car tire 37 Food waste 4.2 Glass fiber-reinforced polyester (GRP) 15 Leather 18.9 Newspaper 18.6 Paraffin wax 42

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Processes 2020, 8, 923 5 of 19 (a)

(b) (c)

Figure 4. (a) The created mesh domain for the portable incinerator; (b) The cut plane from the above of rubbish height in (X–Z) direction to show the volume meshes; (c) The cut plane in (X–Y) direction to showFigure the involved 4. (a) The volume created mesh meshes domain in the for computational the portable incinerator; domain. (b) The cut plane from the above of rubbish height in (X–Z) direction to show the volume meshes; (c) The cut plane in (X–Y) direction 3. Waste Properties to show the involved volume meshes in the computational domain. The rubbish (waste) composition and their properties are completely diﬀerent according to many 3. Waste Properties factors and the food culture. Table1 is prepared according to the average material properties, which are preparedThe rubbish from the (waste) Iranian composition food and waste and their culture. proper In thisties study,are completely the MSW different involves according approximately to 38%many paper, factors 25% foodand the waste food (solid culture. material), Table 1 is 10% prep plastics,ared according 19% car to tire, the andaverage 8% material glass. properties, which are prepared from the Iranian food and waste culture. In this study, the MSW involves approximatelyTable 1. Normalized 38% paper, Iranian 25% wastefood waste properties. (solid material), All numbers 10% are plastics, reported 19% as car their tire, lowerand 8% heating glass. values (LHV). Table 1. Normalized Iranian waste properties. All numbers are reported as their lower heating values (LHV). Solid Materials MJ/kg

SolidCar Materials tire MJ/kg 37 FoodCar wastetire 4.2 37 Glass ﬁber-reinforced polyester (GRP) 15 Food waste 4.2 Leather 18.9 Glass fiber-reinforcedNewspaper polyester (GRP) 18.6 15 ParaLeatherﬃn wax 18.9 42 NewspaperSawdust 18.6 19 ParaffinPlastics wax MJ 42/kg

Acrylonitrile butadiene styrene (ABS) 39 Cellulose 16 Nylon, polyamide (PA) 28 Polyester (PET), textiles, bottles 23 Polyethylene (PE) 46 Polypropylene (PP) 43 Polystyrene (PS) 46 Polyurethane (PU) 36 Polyvinyl chloride (PVC) 18.9 Liquid MJ/kg Acetic acid 15.8 Acetone 22.4 Kerosene/Paraﬃn 34.8 White spirit 34.8

4. Governing Equations In order to calculate the flow field and the combustion of the waste burning inside an incinerator, it is necessary to have a very complicated system, which considers all of the involved phenomena. In the first step of simulations, for geometry a simplification is performed to consider the most involved boundaries. Now, it is the time to solve the equations according to the geometry and the created computational domain. Therefore, it is very common to use some simplifications in assumption Processes 2020, 8, 923 6 of 19 to consider the most involved reactions as well. Accordingly, the governing equations are divided into two parts. The first part is related to the solid combustion and the respective reactions, and the second part is related to the gas-related equations. These two parts, with their corresponding nomenclatures, are reported in Tables2 and3. Moreover, Iranian natural gas (NG) is used as the primary fuel. Therefore, in this case, all of the composition is normalized and given to the general combustion model of ANSYS FLUENT 19.2 [28]. The general combustion model is used according to the combustion theory for the hydrocarbons. This equation can be written as the following [30–32]:

y z y CxHyOzN Sn + x + + n + l O xCO + H O + lNO + nSO (1) l 4 − 2 2 2 2 2 2 2

Table 2. Governing equations and the chemical reactions related to the solid combustion [29–31], reproduced with permission from M. Saﬀari Pour, Producer Gas Implementation in Steel Reheating Furnaces from Lab to Industrial Scale: A Computational Fluid Dynamics and Thermodynamics Approach, Doctoral Thesis, 2016 [32].

Equations Name Material Phase Governing Equations Solid Devolatilization and char burnout Continuity (Mass balance) Fluid ∂ (ρ Y ) + ρ U→ Y = →J + R + S (EDC model) ∂t i i ∇ i f i −∇ i i i

dup g(ρp ρ f ) Solid = FD u f up + − + Fex dt − ρp Momentum ∂ → → → 2→ Fluid ρ U + ρ U U = P + µ U u u + Fext ∂t f f ∇ f f f −∇ f ∇ f − ∇ 0f 0f dT Solid p = + 4 4 mpcp dt hAp T Tp εpApσ T f Tp Energy ∞ − − ∂ → Fluid ρ e + ρ U e = (k T) + Eex ∂t f f ∇ f f f ∇ ∇

Table 3. Governing equations [30,31] and the chemical reactions [33] related to the gas combustion, reproduced with permission from M. Saﬀari Pour, Producer Gas Implementation in Steel Reheating Furnaces from Lab to Industrial Scale: A Computational Fluid Dynamics and Thermodynamics Approach, Doctoral Thesis, 2016 [32].

Equations Name Material Phase Governing Equations ∂ ∂ ∂Yek . Continuity ρeuiYek = µ ρu00 Y00 + ωk Gas Mixture ∂xi ∂xi ∂xi − i k (Mass balance) k = 1, 2, 3, ... , N

dup g(ρp ρ f ) Ash Solid = FD u f up + − + Fex dt − ρp Momentum ∂ ∂P ∂ Gas Mixture ρeuieuj = + τij ρu00 u00 + ρg ∂xi − ∂xj ∂xi − i j ∂ ∂ µ ∂h Energy Gas Mixture ρeuieh = ρu00 h00 + Sh ∂xi ∂xi Pr ∂xi − i

In the above chemical reaction, x, y, z, l, and n are the coefficients that can be derived from the stoichiometric balance of the reaction. As the gas and air mixture velocity is high enough, the turbulent model has been used for the flow fields. The common k ε model is used for the turbulent modeling [30,31,33]. − ! ! ∂ ∂ ∂ µt ∂k (ρk) + (ρeuik) = µ + + Gk ρε (2) ∂t ∂xi ∂xi σk ∂xi −

! 2 ∂ ∂ ∂ µt ∂ε ε ε (ρε) + (ρeuiε) = µ + + C1 Gk C2ρ (3) ∂t ∂xi ∂xi σε ∂xi k − k Processes 2020, 8, 923 7 of 19

! ∂euj ∂eui ∂eui Gk = µt + (4) ∂xi ∂xj ∂xi k2 µt = C ρ (5) µ ε

In the above equations, the constants are deﬁned as: C1 = 1.44 C2 = 1.09, Cµ = 0.09, σk = 1.0, and σε = 1.3. Moreover, the radiative heat transfer is modeled by discrete ordinate (DO). The heat transfer equations are fully discussed by Khodabandeh et al. [29]. The governing boundary conditions for solving the previous problem are given in Table4 as:

Table 4. The governing boundary condition applied in this study.

Location Type Value A Pressure gauge P = 0 kPa mass flow rate of air = 8 (kg/s) B Burner mass flow rate of fuel = 5 (kg/s) C Air inlet V = 1.75 (m/s) V = 1.5 (m/s) D Air inlet This value for all air inlets is the same. mass flow rat of air = 6 (kg/s) E Burner mass flow rate of fuel = 3 (kg/s) F Air inlet V = 3 (m/s) Numbering is presented according to Figure3.

5. Results and Discussion For doing a reliable simulation of an incinerator, it is completely essential to test the numerical procedures from computational and chemical error aspects. In order to pave the way of checking the simulation procedure, in the first step, three different numbers of mesh elements have been Processes 2020, 8, x FOR PEER REVIEW 8 of 22 tested with the temperature gradient along with the considered incinerator. A line is passed from the centerwith of the the bottomtemperature of the gradient main along fresh with air inlet the consider and it isedcontinued incinerator. alongA line is the passed incinerator’s from the center full length. Theseof three the bottom different of the numbers main fresh of mesh air inlet elements and it is arecontinued denoted along by the cases incinerator’s 1 to 3 with full 5,969,969, length. These 6,665,341, 77,723,753three elements,different numbers respectively. of mesh Moreover, elements are the deno maximumted by cases skewness 1 to 3 with for cases 5,969,969, is 0.847, 6,665,341, 0.791, and 77,723,753 elements, respectively. Moreover, the maximum skewness for cases is 0.847, 0.791, and 0.621, respectively. The curvature normal angle for cases is 18◦. Figure5 shows the results of these 0.621, respectively. The curvature normal angle for cases is 18°. Figure 5 shows the results of these three cases. As it is clearly illustrated in this figure, the temperature of cases 2 and 3 are very close, three cases. As it is clearly illustrated in this figure, the temperature of cases 2 and 3 are very close, and theyand reportedthey reported almost almost similar similar results. results. Therefore, Therefore, for for saving saving the computational the computational time and time due andto the due to the similarsimilar results results derivedderived by by cases cases 2 2and and 3, 3, case case 2 2is isselected selected as the as thereference reference mesh mesh case for case further for further thermo-chemicalthermo-chemical calculations. calculations.

Figure 5. A validation for 3 different mesh element numbers with temperature variation along the Figure 5. A validation for 3 diﬀerent mesh element numbers with temperature variation along the incinerator length. incinerator length. A combustion process verification could be achieved through the validation of chemical reactions. A literature survey leads us to a very similar case, which is done by Alessio Frassoldati et al. [34]. They did the simulation for a 2D axisymmetric burner, which worked with methane/hydrogen as fuel. This selection is chosen due to their comparison with the experiments, and in their numerical work, different turbulent models were considered. Hence, in this case, a direct comparison is done for both numerical simulations and experiments in Figure 6. The compared results show that the oxygen concentration on the centerline is very close to those reported by Alessio Frassoldati et al. [34] and the overall error is less than 5 percent.

Processes 2020, 8, x FOR PEER REVIEW 8 of 22 with the temperature gradient along with the considered incinerator. A line is passed from the center of the bottom of the main fresh air inlet and it is continued along the incinerator’s full length. These three different numbers of mesh elements are denoted by cases 1 to 3 with 5,969,969, 6,665,341, 77,723,753 elements, respectively. Moreover, the maximum skewness for cases is 0.847, 0.791, and 0.621, respectively. The curvature normal angle for cases is 18°. Figure 5 shows the results of these three cases. As it is clearly illustrated in this figure, the temperature of cases 2 and 3 are very close, and they reported almost similar results. Therefore, for saving the computational time and due to the similar results derived by cases 2 and 3, case 2 is selected as the reference mesh case for further thermo-chemical calculations.

ProcessesFigure2020, 5.8, 923A validation for 3 different mesh element numbers with temperature variation along the8 of 19 incinerator length.

A combustioncombustion process process veriﬁcation verification could could be achieved be achieved through through the validation the validation of chemical of reactions.chemical Areactions. literature A surveyliterature leads survey usto leads a very us to similar a very case, similar which case, is which done byis done Alessio by FrassoldatiAlessio Frassoldati et al. [34 et]. Theyal. [34]. did theThey simulation did the for simulation a 2D axisymmetric for a 2D burner, axisymmetric which worked burner, with which methane worked/hydrogen with as fuel.methane/hydrogen This selection isas chosenfuel. This due selection to their comparisonis chosen due with to thetheir experiments, comparison andwith in the their experiments, numerical work,and in ditheirﬀerent numerical turbulent work, models different were considered.turbulent models Hence, were in this considered. case, a direct Hence, comparison in this case, is done a direct for bothcomparison numerical is done simulations for both and numerical experiments simulati in Figureons 6and. The experiments compared results in Figure show 6. thatThe the compared oxygen concentrationresults show that on thethe centerlineoxygen concentration is very close on to the those cent reportederline is by very Alessio close Frassoldati to those reported et al. [34 by] andAlessio the overallFrassoldati error et is al. less [34] than and 5 the percent. overall error is less than 5 percent.

Processes 2020, 8, x FOR PEER REVIEW 9 of 22

Figure 6. Oxygen concentration along the centerline of the primary burner of the incinerator Figure 6. Oxygen concentration along the centerline of the primary burner of the incinerator compared compared with Reference [34]. with Reference [34].

In order to present a visual visual sense sense of of the the differences diﬀerences of the the compared compared cases, cases, Figure Figure 7 is presented. In this ﬁgure,figure, aa cut cut plane plane has has been been shown shown in in the the X–Y X– planeY plane with with its normalits normal vector vector in the in z-directionthe z-direction at Z at Z = 0.98 m. All of the contours in this part are depicted based on this cut plane. The reason for = 0.98 m. All of the contours in this part are depicted based on this cut plane. The reason for choosing choosing this cut plane is to explain the most involved boundaries of waste to show the chemical this cut plane is to explain the most involved boundaries of waste to show the chemical reactions at reactionsthese boundaries. at these boundaries.

Figure 7. The place of cut plane for the computational contours.

5.1. Effects of Cooling Air The first parameter in focus in this research is the mass flow rate of the cooling air. To investigate the effects of the cooling air, other parameters are kept constant. It means that the rubbish height is set to be 800 mm, and the primary burner position is set to be 1500 mm. Therefore, the flow rate of the cooling air varies from 0.07 kg/s up to 0.21. Table 5 is prepared to show a summary of the investigated cases and their conditions.

Table 5. Different mass flow rate of the cooling air for the incinerator.

Case Numbers Rubbish Height Burner Position Mass Flow Rate

Case 1 800 mm 1500 mm 0.07 kg/s

Case 2 800 mm 1500 mm 0.14 kg/s

Case 3 800 mm 1500 mm 0.21 kg/s

In Figure 8, three different cases for variation of the mass flow rate of the cooling air are presented. One of the famous contours, which are related to the production of gasses in the incinerator, is presented in this figure. As it can be seen from this figure, by increasing the mass flow of the cooling air, carbon monoxide (CO) production has been increased. This is due to the injection of more air with higher velocities, which can produce higher suction power for the evacuation of off- gases from the portable incinerator system.

Processes 2020, 8, 923 9 of 19

5.1. Effects of Cooling Air The first parameter in focus in this research is the mass flow rate of the cooling air. To investigate the effects of the cooling air, other parameters are kept constant. It means that the rubbish height is set to be 800 mm, and the primary burner position is set to be 1500 mm. Therefore, the flow rate of the cooling air varies from 0.07 kg/s up to 0.21. Table5 is prepared to show a summary of the investigated cases and their conditions.

Table 5. Diﬀerent mass ﬂow rate of the cooling air for the incinerator.

Case Numbers Rubbish Height Burner Position Mass Flow Rate Case 1 800 mm 1500 mm 0.07 kg/s Case 2 800 mm 1500 mm 0.14 kg/s Case 3 800 mm 1500 mm 0.21 kg/s

In Figure8, three di fferent cases for variation of the mass flow rate of the cooling air are presented. One of the famous contours, which are related to the production of gasses in the incinerator, is presented in this figure. As it can be seen from this figure, by increasing the mass flow of the cooling air, carbon monoxide (CO) production has been increased. This is due to the injection of more air with higher velocities, which can produce higher suction power for the evacuation of off-gases from the portable incineratorProcesses system.2020, 8, x FOR PEER REVIEW 10 of 22

Case 1 Case 2 Case 3

FigureFigure 8. Mass 8. Mass fraction fraction of of carbon carbon monoxide monoxide (CO) for for different diﬀerent mass mass flow ﬂow rates rates of the of thecooling cooling air. air.

A combustionA combustion reaction reaction is alwaysis always going going in in aa direction to to produce produce carbon carbon dioxide dioxide (CO (CO2). Therefore,2). Therefore, havinghaving the contoursthe contours of COof CO2 during2 during a a combustion combustion process could could help help to toidentify identify the theraterate of pollutant of pollutant emissions.emissions. In Figure In Figure9, the 9, the contours contours of of the the mass mass fraction fraction of CO2 2onon the the representative representative plane plane similar similar to to CO areCO depicted. are depicted. From From these these contours, contours, it it can can bebe inferred that that by by introducing introducing more more air the air hot the spots hot spots in in the production of CO2 have been reduced from case 1 to case 3. In addition, an even dispersion of the production of CO2 have been reduced from case 1 to case 3. In addition, an even dispersion of CO2 in theCO entire2 in the chamber entire chamber could be could expected. be expected. Figure 10 is presented to show the total pressure during the combustion of the waste inside the incinerator. It can be seen that by increasing the mass ﬂow rate of the cooling air from 0.07 to 0.21 kg/s, the pressure inside all chambers has been reduced. It means that the suction power is increased and the evacuation of the oﬀ-gases would be much faster than previously.

Case 1 Case 2 Case 3

Figure 9. Mass fraction of CO2 for different mass flow rates of the cooling air.

Figure 10 is presented to show the total pressure during the combustion of the waste inside the incinerator. It can be seen that by increasing the mass flow rate of the cooling air from 0.07 to 0.21 kg/s, the pressure inside all chambers has been reduced. It means that the suction power is increased and the evacuation of the off-gases would be much faster than previously.

Processes 2020, 8, x FOR PEER REVIEW 10 of 22

Case 1 Case 2 Case 3

Figure 8. Mass fraction of carbon monoxide (CO) for different mass flow rates of the cooling air.

A combustion reaction is always going in a direction to produce carbon dioxide (CO2). Therefore, having the contours of CO2 during a combustion process could help to identify the rate of pollutant emissions. In Figure 9, the contours of the mass fraction of CO2 on the representative plane similar to CO are depicted. From these contours, it can be inferred that by introducing more air the hot spots in Processesthe2020 production, 8, 923 of CO2 have been reduced from case 1 to case 3. In addition, an even dispersion of10 of 19 CO2 in the entire chamber could be expected.

Case 1 Case 2 Case 3 Processes 2020, 8, x FOR PEER REVIEW 11 of 22 Processes 2020, Figure8, x FOR 9.PEERMass REVIEW fraction of CO for diﬀerent mass ﬂow rates of the cooling air. 11 of 22 Figure 9. Mass fraction of CO2 2 for different mass flow rates of the cooling air.

Case 1 Case 2 Case 3 Case 1 Case 2 Case 3

FigureFigure 10. Total 10. Total pressure pressure inside inside the the primary primary andand secondarysecondary chambers chambers of ofthe the incinerator incinerator (kPa). (kPa). Figure 10. Total pressure inside the primary and secondary chambers of the incinerator (kPa).

AnotherAnother important important characteristic characteristic of of a a combustion combustion processprocess is is the the temperature. temperature. As Asit is itdepicted is depicted in in Another important characteristic of a combustion process is the temperature. As it is depicted in FigureFigure 11, more 11, coolingmore cooling air results air inresults reducing in reduci the totalng the temperature total temperature inside the inside chambers. the chambers. Furthermore, Figure 11, more cooling air results in reducing the total temperature inside the chambers. Furthermore, by comparing the above-mentioned cases, it can be observed that the hot spots reduced by comparingFurthermore, the by above-mentioned comparing the above-mentioned cases, it can be cases, observed it can thatbe observed the hot that spots the reduced hot spots from reduced case 1 to from case 1 to case 3. In addition, the temperature has a smooth distribution by increasing the cooling case 3.from In addition,case 1 to case the 3. temperature In addition, the has temperatur a smoothe has distribution a smooth distribution by increasing by increasing the cooling the air. cooling It means air. It means that a dilution happened inside the incinerator. that aair. dilution It means happened that a dilution inside happened the incinerator. inside the incinerator.

Case 1 Case 2 Case 3 Case 1 Case 2 Case 3

Figure 11. Temperature contours inside the primary and secondary chambers of the incinerator (K). FigureFigure 11. Temperature 11. Temperature contours contours inside inside the the primaryprimary and secondary secondary chambers chambers of the of theincinerator incinerator (K). (K).

For a full investigation of the flow field inside an incinerator, it is essential to study the velocity For aFor full a investigationfull investigation of of the the flow flow field field insideinside an incinerator, incinerator, it itis isessential essential to study to study the velocity the velocity fields, vectors and contours. As it is shown in Figure 12, the velocity magnitude near the exhaust port fields,fields, vectors vectors and and contours. contours. As As it it is is shown shownin in FigureFigure 12,12, the the velocity velocity magnitude magnitude near near the exhaust the exhaust port port and inside the second chamber has been increased from case 1 to case 3. It means that the suction and inside the second chamber has been increased from case 1 to case 3. It means that the suction process creates more negative pressure and helps to evacuate the off-gases faster from the incinerator process creates more negative pressure and helps to evacuate the off-gases faster from the incinerator system. Moreover, the contours of velocity vectors from case 1 to case 3 illustrate that the suction system. Moreover, the contours of velocity vectors from case 1 to case 3 illustrate that the suction process causes an amplitude in the velocity vectors near the outlet zone and consequently a better process causes an amplitude in the velocity vectors near the outlet zone and consequently a better evacuation of off-gases from the secondary chamber has been achieved. evacuation of off-gases from the secondary chamber has been achieved.

Processes 2020, 8, 923 11 of 19 and inside the second chamber has been increased from case 1 to case 3. It means that the suction process creates more negative pressure and helps to evacuate the oﬀ-gases faster from the incinerator system. Moreover, the contours of velocity vectors from case 1 to case 3 illustrate that the suction process causes an amplitude in the velocity vectors near the outlet zone and consequently a better evacuationProcesses of2020 o,ﬀ 8-gases, x FOR PEER from REVIEW the secondary chamber has been achieved. 12 of 22

Case 1 Case 2 Case 3

FigureFigure 12. Velocity 12. Velocity contours contours inside inside the the primary primary andand se secondarycondary chambers chambers of ofthe the incinerator incinerator (m/s). (m /s).

5.2. E5.2.ﬀects Effects of Primary of Primary Burner Burner Position Position The primaryThe primary burner burner has has the the main main responsibilityresponsibility of of burning burning most most of the of waste the waste volume. volume. Hence, Hence,it it is quiteis quite essential essential to to be be exact exact and and inin placeplace to re reduceduce the the volume volume of ofthe the waste waste and and produce produce the most the most energyenergy and heatand heat from from the waste.the waste. In thisIn this part, part, the the ﬂuid fluid dynamics dynamics of of the the placeplace ofof the primary burner burner are in focus.are in Two focus. cases Two have cases been have prepared. been prepared. For theFor previousthe previous case, case, the the place place of of the the primary primary burnerburner was 1500 mmwas 1500 from mm the from main the fresh main air fresh inlet. air Ininlet. this In section,this section, the the minimum minimum and and the the maximum maximum place for for the installationthe installation of this burner of this from burner the from main the air main inlet air are inlet considered are considered to be 1000 to be and 1000 2000 and mm, 2000 respectively. mm, respectively. Besides, other parameters have been kept constant, according to the summary presented Besides, other parameters have been kept constant, according to the summary presented in Table6. in Table 6.

Table 6. TableDi 6.ﬀ erentDifferent places places of theof the primary primary burner burner installationinstallation in in the the incinerator incinerator (Shown (Shown in Figure in Figure 13). 13).

Case NumbersCase RubbishRubbish Height BurnerBurner Position Mass Mass Flow Flow Rate CaseNumbers 4 800Height mm Position 1000 mm Rate 0.07 kg/s CaseCase 1 4 800 800 mmmm 1000 1500 mm mm 0.07 0.07kg/s kg/s CaseCase 5 1 800 800 mmmm 1500 2000 mm mm 0.07 0.07kg/s kg/s Case 5 800 mm 2000 mm 0.07 kg/s Figure 13 shows the differences between the total pressure inside the incinerator when the burner Figure 13 shows the differences between the total pressure inside the incinerator when the is in 1000burner mm is andin 1000 2000 mm mm and from 2000 the mm main from air the inlet. main It air is clearlyinlet. It presentedis clearly presented that by increasing that by increasing the distance fromthe the distance main air from inlet, the the main pressure air inlet, has the been pressure decreased. has been This decreased. phenomenon This phenomenon is due to producing is due to the mostproducing heat and flowthe most right heat near and the flow secondary right near chamber the secondary and under chamber the and exhaust under port.the exhaust So, the port. incinerator So, couldthe evacuate incinerator the could off-gases evacuate faster the and off- moregases efasterfficiently. and more efficiently.

Processes 2020, 8, 923 12 of 19 ProcessesProcesses 2020 2020, ,8 8, ,x x FOR FOR PEER PEER REVIEW REVIEW 1313 of of 22 22

CaseCase 4 4 Case 1 Case Case 5 5

Figure 13.FigureFigureTotal 13. 13. pressure TotalTotal pressurepressure inside insideinside the incinerator thethe incinerator for thefor the place place of of installationof installationinstallation of ofof the thethe primaryprimary primary burner burnerburner (kPa). (kPa).(kPa). In Figure 14, it is attempted to show the temperature contours for the position of the primary In Figure 14, it is attempted to show the temperature contours for the position of the primary burner. As canIn Figure be seen 14, fromit is attempted the temperature to show the contours, temperature for case contours 5, the for burner the position is so close of the to primary the wall that burner.burner. As As can can be be seenseen fromfrom thethe temperaturetemperature contours,, forfor casecase 5,5, thethe burnerburner is is so so close close to to the the wall wall it separatesthat theit separates primary the and primary the secondary and the secondary chamber. cham Beingber. Being adjacent adjacent to thisto this wall wall results results in in anan uneven that it separates the primary and the secondary chamber. Being adjacent to this wall results in an temperatureuneven distribution temperature in distribution the primary in the chamber primary and chamber the creation and the ofcreation hot spots of hot in thespots secondary in the one. uneven temperature distribution in the primary chamber and the creation of hot spots in the secondary one. Likewise, the burner at the position of 2000 mm is very far from the waste volume Likewise,secondary the burner one. Likewise, at the position the burner of 2000at themm position is very of 2000 far mm from is thevery waste far from volume the waste and volume is unable to and is unable to provide enough heat for them. provideand enough is unable heat to forprovide them. enough heat for them.

Case 4 Case 1 Case 5 Case 4 Case 1 Case 5 Figure 14.FigureTemperature 14. Temperature contours contours inside inside the the incinerator incinerator fo forr the the place place of installation of installation of the ofprimary the primary burner (K).Figureburner 14. (K). Temperature contours inside the incinerator for the place of installation of the primary burner (K). For theFor velocity the velocity ﬁeld field presented presented in in Figure Figure 15 15,, thethe same same principles principles are arestill stillvalid valid in the insame the way same way For the velocity field presented in Figure 15, the same principles are still valid in the same way for the temperaturefor the temperature and the and pressure. the pressure. The The closer closer thethe burner is isto tothe the exhaust exhaust port, port,the more the velocity more velocity formagnitude the temperature can be observed and the pressure.at the vicinity The closerof the theboundary burner ofis theto the primary exhaust and port, secondary the more chamber. velocity magnitude can be observed at the vicinity of the boundary of the primary and secondary chamber. magnitudeThisProcesses will 2020 result can, 8, x beFORin observedthe PEER evacuation REVIEW at the of vicinity more heat of th rathere boundary than off-gases. of the primary and secondary chamber.14 of 22 This willThis result will inresult the in evacuation the evacuation of moreof more heat heat rather rather thanthan off-gases. oﬀ-gases.

Case 4 Case 1 Case 5

Figure 15.FigureVelocity 15. Velocity contours contours inside inside the the incinerator incinerator forfor the place place of of installation installation of the of primary the primary burner burner(m/s). (m/s).

5.3. Effects of Rubbish Height The other important parameter to be studied in this research is the waste volume’s effect on the incinerator operation condition. Such evaluation is done by changing the waste height of the rubbish inserted to the incinerator. In order to study these effects, it is crucial to keep other parameters constant. A summary of the compared conditions in this part is reported in Table 7.

Table 7. Different rubbish heights inside the primary chamber of the incinerator.

Case Rubbish Burner Mass Flow Rate Numbers Height Position

Case 6 700 mm 1500 mm 0.07 kg/s

Case 1 800 mm 1500 mm 0.07 kg/s

Case 7 900 mm 1500 mm 0.07 kg/s

In Figure 16, it is attempted to show the pressure contours inside the incinerator versus variations in waste volume. As it can be observed, the pressure is not that affected by the waste volume changes. It means that such an incinerator is able to provide a reliable pressure range inside the chambers due to an efficient installed ejector system in the exhaust port to regulate this pressure.

Case 6 Case 1 Case 7

Figure 16. Total pressure inside the incinerator for the variations in waste volume (kPa).

Processes 2020, 8, x FOR PEER REVIEW 14 of 22

Case 4 Case 1 Case 5

Processes 2020Figure, 8, 923 15. Velocity contours inside the incinerator for the place of installation of the primary burner (m/s). 13 of 19

5.3. Effects of Rubbish Height 5.3. Effects of Rubbish Height The other important parameter to be studied in this research is the waste volume’s effect on the Theincinerator other importantoperation condition. parameter Such to beevaluation studied is in done this by research changing is the the waste waste height volume’s of the erubbishffect on the incineratorinserted operation to the incinerator. condition. In Such order evaluation to study thes is donee effects, by changingit is crucial the to waste keep heightother parameters of the rubbish insertedconstant. to the A incinerator. summary of In the order compared to study conditions these eff inects, this itpart is crucial is reported to keep in Table other 7. parameters constant. A summary of the compared conditions in this part is reported in Table7. Table 7. Different rubbish heights inside the primary chamber of the incinerator.

Case Rubbish Burner Table 7. Diﬀerent rubbish heights inside the primary chamberMass ofFlow the Rate incinerator. Numbers Height Position Case Numbers Rubbish Height Burner Position Mass Flow Rate Case 6 700 mm 1500 mm 0.07 kg/s Case 6 700 mm 1500 mm 0.07 kg/s CaseCase 1 1 800 800 mm mm 1500 1500 mm mm 0.07 0.07kg/s kg/s Case 7 900 mm 1500 mm 0.07 kg/s Case 7 900 mm 1500 mm 0.07 kg/s

In FigureIn Figure 16, it 16, is attempted it is attempted to show to theshow pressure the pressure contours contours inside inside the incinerator the incinerator versus versus variations in wastevariations volume. in waste As it canvolume. be observed, As it can the be pressureobserved,is the not pressure that aﬀ ectedis not bythat the affected waste by volume the waste changes. It meansvolume that changes. such an It incinerator means that issuch able an toincinerator provide ais reliableable to provide pressure a reliable range insidepressure the range chambers inside due to anthe eﬃ chamberscient installed due to ejector an efficient system installed in the ejector exhaust syst portem in to the regulate exhaust thisport pressure.to regulate this pressure.

Case 6 Case 1 Case 7

Figure 16. Total pressure inside the incinerator for the variations in waste volume (kPa). Processes 2020Figure, 8, x FOR 16. PEER Total REVIEW pressure inside the incinerator for the variations in waste volume (kPa). 15 of 22

TheThe temperature temperature contours contours forfor two two samples samples of the of thewaste waste volume volume are presented are presented in Figure 17. in FigureFrom 17. Fromthese these contours, contours, it itis isobserved observed that that by byincreasing increasing the theheight height of the of thewaste, waste, the hot the spots hot spots are reduced are reduced drastically.drastically. In addition, In addition, the total the temperaturetotal temperature inside inside the primarythe primary combustion combustion chamber chamber has beenhas been reduced; it meansreduced; that it less means heat that can less be providedheat can be for provided the lower for the parts lower of theparts waste. of the waste.

Case 6 Case 1 Case 7

FigureFigure 17. Temperature17. Temperature contours contours inside inside thethe incineratorincinerator for for the the variation variation of waste of waste volume volume (K). (K).

5.4. Comparison of All Cases by Oxygen Concentration For a complete comparison of the previously mentioned cases, the oxygen concentration inside the incinerator chamber is selected. Such a selection is made for more detailed investigation of the all-chemical involved reactions. On the other hand, for safety and according to the environmental regulations, the oxygen concentration at the exhaust of the incinerators must be around 3–5% [35]. Consequently, to have a good combustion system, keen attention must be devoted to the oxygen concentration. The oxygen data extraction is done on a line at Y = 1.09 m, which starts from the center of the main air inlet and passes directly along with the incinerator to the end of it. For the sake of clarity, this line is visualized in Figure 18.

Figure 18. The place of the line at Y = 1.09 m for the numerical data extraction.

Figure 19 shows the effects of the cooling air mass flow rate on the mass fraction of oxygen inside the considered incinerator. The plotted lines show that by increasing the mass flow of the cooling air up to 0.14 kg/s, the same trend follows for the oxygen concentration. On the other hand, when the mass flow of oxygen is increased to an amount of more than 0.14 kg/s the trend does not follow as it did previously. As the oxygen for the mass flow of 0.21 kg/s is high enough, it starts to be consumed immediately. Simultaneously, other air inlets on both sides of the incinerator increase the oxygen

Processes 2020, 8, x FOR PEER REVIEW 15 of 22

The temperature contours for two samples of the waste volume are presented in Figure 17. From these contours, it is observed that by increasing the height of the waste, the hot spots are reduced drastically. In addition, the total temperature inside the primary combustion chamber has been reduced; it means that less heat can be provided for the lower parts of the waste.

Case 6 Case 1 Case 7 Processes 2020, 8, 923 14 of 19 Figure 17. Temperature contours inside the incinerator for the variation of waste volume (K).

5.4. Comparison Comparison of of All All Cases Cases by by Oxygen Concentration For a complete comparison of the previously mentioned cases, the oxygen concentration inside the incinerator chamber is selected. Such Such a a selection selection is is made made for for more detailed investigation of the all-chemical involved reactions. On On the the other other ha hand,nd, for for safety safety and according to the environmental regulations, the the oxygen oxygen concentration concentration at at the the exhaus exhaustt of of the the incinerators incinerators must must be be around around 3–5% 3–5% [35]. [35]. Consequently, to have a good combustioncombustion system, keen attention must be devoted to the oxygen concentration. The oxygen data extraction is done on a line at Y = 1.091.09 m, m, which which starts starts from from the the center center of the main air inlet and passes directly along with the incinerator to the end of it. For the sake of clarity, this line isis visualizedvisualized inin FigureFigure 1818..

Figure 18. The place of the line at Y = 1.091.09 m m for for the the numerical numerical data data extraction. extraction.

Figure 19 shows the effects effects of the cooling air mass flow flow rate on the mass mass fraction fraction of oxygen oxygen inside inside the considered incinerator. The plo plottedtted lines lines show show that that by by increasing increasing the mass mass flow flow of the cooling cooling air air up to 0.14 kgkg/s,/s, the same trend followsfollows for thethe oxygenoxygen concentration.concentration. On the other hand, when the mass flow flow of oxygen is increased to an amount of more than 0.14 kg/s kg/s the trend does not follow as it did previously. As the oxygen for the mass flow flow of 0. 0.2121 kg/s kg/s is high enough, it starts to be consumed immediately. Simultaneously, Simultaneously, other other air air inlets inlets on on both both sides sides of the incinerator increase the oxygen content, which can be observed in the middle of incinerator (X = 1.5). Consequently, due to the high airflow rate, the velocity of oxygen has been increased, and all of the injected oxygen is consumed at the primary chamber, and not enough oxygen exists at the secondary chamber. Lack of oxygen at this chamber could be a reason that more pollutants are produced when continuously working with this condition. Figure 20 shows the effects of the primary burner position on the oxygen concentration inside the portable incinerator. As it is depicted by the plotted lines, the oxygen is injected from the main air inlet, and by reaching the burner position, it starts to be consumed. In this way, it can be seen that by increasing the primary burner distance from the main air inlet, more oxygen can be introduced to the system, but this oxygen behind the secondary chamber is suddenly consumed. This could lead to an unbalanced oxygen concentration inside the chamber and consequently, uneven heat release from the primary burner. In addition, the peak points right after X = 2 m correspond to the oxygen concentrations in the secondary combustion chamber. Processes 2020, 8, x FOR PEER REVIEW 16 of 22

content, which can be observed in the middle of incinerator (X = 1.5). Consequently, due to the high airflow rate, the velocity of oxygen has been increased, and all of the injected oxygen is consumed at the primary chamber, and not enough oxygen exists at the secondary chamber. Lack of oxygen at this chamber could be a reason that more pollutants are produced when continuously working with this condition.

Processes 2020, 8, 923 15 of 19

Processes 2020, 8, x FOR PEER REVIEW 17 of 22

Figure 19. Mass fraction of oxygen on the line at Y = 1.09 m with the rubbish height = 700 mm and the primaryFigure burner 19. Mass position fraction= 1500 of oxygen mm. on the line at Y = 1.09 m with the rubbish height = 700 mm and the primary burner position = 1500 mm.

Figure 20 shows the effects of the primary burner position on the oxygen concentration inside the portable incinerator. As it is depicted by the plotted lines, the oxygen is injected from the main air inlet, and by reaching the burner position, it starts to be consumed. In this way, it can be seen that by increasing the primary burner distance from the main air inlet, more oxygen can be introduced to the system, but this oxygen behind the secondary chamber is suddenly consumed. This could lead to an unbalanced oxygen concentration inside the chamber and consequently, uneven heat release from the primary burner. In addition, the peak points right after X = 2 m correspond to the oxygen concentrations in the secondary combustion chamber.

Figure 20. Mass fraction of oxygen on the line at Y = 1.09 m with the rubbish height = 700 mm and the coolingFigure air with20. Mass the mass fraction flow of rate oxygen= 0.07 on kg the/s. line at Y = 1.09 m with the rubbish height = 700 mm and the cooling air with the mass flow rate = 0.07 kg/s. Figure 21 shows the effect of the waste volume on the oxygen concentration inside the primary and secondaryFigure chambers.21 shows the For effect all of of the the rubbishwaste volume volumes on asthe the oxygen cooling concentration air mass flow inside rate the is keptprimary and secondary chambers. For all of the rubbish volumes as the cooling air mass flow rate is kept constant, the same trends could be expected. On the other hand, as the rubbish with a height of 700 mm is the minimum loading of the wastes, the total amount of oxygen would be higher compared to other rubbish heights. This means that the oxygen would react in a faster way and keep some unused oxygen in the primary chamber. Again, similar behavior for the second chamber could be expected in the same way as the primary chamber. Moreover, comparing all the results, whether in terms of temperature or other main factors including pressure and oxygen mass fraction, shows that the optimum place of the primary burner at the ceiling of the primary chamber is found to be 1500 mm from the main air inlet.

Processes 2020, 8, 923 16 of 19 constant, the same trends could be expected. On the other hand, as the rubbish with a height of 700 mm is the minimum loading of the wastes, the total amount of oxygen would be higher compared to other rubbish heights. This means that the oxygen would react in a faster way and keep some unused oxygen in the primary chamber. Again, similar behavior for the second chamber could be expected in the same wayProcesses as the primary2020, 8, x FOR chamber. PEER REVIEW Moreover, comparing all the results, whether in terms of temperature or18 of 22 other main factors including pressure and oxygen mass fraction, shows that the optimum place of the primary burner at the ceiling of the primary chamber is found to be 1500 mm from the main air inlet.

Figure 21. Mass fraction of oxygen on the line at Y = 1.09 m with the primary burner position = 1500 mm and theFigure cooling 21. Mass air with fraction the mass of oxygen flow rate on =the0.07 line kg at/s. Y = 1.09 m with the primary burner position = 1500 mm and the cooling air with the mass flow rate = 0.07 kg/s. 6. Conclusions 6.In Conclusions this numerical work, an investigation is done through CFD for a portable incinerator. The waste inside theIn considered this numerical incinerator work, isan selected investigation according is done to Iranian through waste CFD compositions. for a portable A incinerator. parametric The studywaste has been inside proposed the considered for the most incinerator involved factorsis selected for the according design of suchto Iranian an incinerator. waste compositions. Accordingly, A the followingparametric remarks study canhas bebeen concluded: proposed for the most involved factors for the design of such an incinerator. Accordingly, the following remarks can be concluded: By introducing the higher mass flow rate of the cooling air, the hot spots inside the combustion • •chamber By introducing reduced, and the anhigher even mass temperature flow rate distribution of the cooling has air, been the achieved. hot spots inside the combustion By introducingchamber reduced, the higher and an mass even flow temperat rate ofure the distribution cooling air, has not been only achieved. the air velocities inside • •the combustionBy introducing chambers the higher have mass been flow improved, rate of butthe co alsooling the air, negative not only pressure, the air whichvelocities helps inside the the evacuationcombustion of hazardous chambers gases, have decreased been improved, drastically. but also the negative pressure, which helps the By increasingevacuation the of rubbishhazardous volume, gases, the decreased incinerator drastically. is still able to burn the waste but this burning • •would By faceincreasing some limitations, the rubbish which volume, could the a incineratorffect the fluid is still dynamics able to parameters burn the waste of the but incinerator. this burning Accordingwould toface the some investigations, limitations, the optimumwhich could place affect of the the primary fluid burnerdynamics at the parameters ceiling of theof the • primaryincinerator. chamber is found to be 1500 mm from the main air inlet. •By increasingAccording the to burnerthe investigations, distance from the the optimum main air place inlet, of all the thermo-chemical primary burner parameters at the ceiling of the of the • incineratorprimary are chamber affected. is found to be 1500 mm from the main air inlet. • By increasing the burner distance from the main air inlet, all thermo-chemical parameters of the incinerator are affected. The worst case of the primary burner could be the closest one to the secondary chamber. In this case, there would be a high risk of unburned wastes in the main chamber as well as high-temperature concentrations near the boundary of these two chambers, which could damage the separation wall between the two chambers.

Processes 2020, 8, 923 17 of 19

The worst case of the primary burner could be the closest one to the secondary chamber. In this case, there would be a high risk of unburned wastes in the main chamber as well as high-temperature concentrations near the boundary of these two chambers, which could damage the separation wall between the two chambers.

Author Contributions: M.S.P. performed the research, calculated all data, analyzed the data, and mainly wrote the paper. A.H.-F. suggested the structure of the research, edited, revised, and partially wrote the paper. B.F. is the principal supervisor of the research, suggested the main idea of the research, and numerical analysis has been performed under her supervision. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Elites Foundation of Iran for supporting the young researchers through the Postdoctoral research grant issued for Mohsen Saffari Pour. Acknowledgments: The authors would like to express their gratitude to the National Elites Foundation of Iran for preparing the Postdoctoral research grant for Mohsen Saffari Pour. The authors are also very thankful for the Sharif University of Technology (SUT) staff, more specifically the Faculty of Mechanical Engineering for creating supportive equipment and cooperative atmosphere to fulfill this research. Conflicts of Interest: The authors declare that they have no conflict of interest.

Nomenclature

Notations 2 Ap Particle surface area [m ] J cp Specific heat capacity of particle [ kg K ] w Eex Volumetric heat sources [ m3 ] J e f Total internal energy [ kg ] FD Inverse of relaxation time [1/s] 2 Fex Other forces per unit mass of particle [m/s ] F Other forces per unit volume of gas kg ext [ m2s2 ] g Gravity [m/s2] w h Convective coefficient [ m2K ] J eh Favre-Averaged enthalpy [ kg ] → Diffusive flux of species kg J i [ m2s ] w k Thermal conductivity [ m K ] mp Mass of particle [kg] P Pressure kg [ m s2 ] P Reynolds-Averaged pressure kg [ m s2 ] Pr Prandtl number [ ] − R Net production rate of species kg i [ m3s ] S Additional created source rate of species kg i [ m3s ] w Sh Other volumetric heat sources [ m3 ] T f Temperature of gas mixture [◦C] Tp Temperature of particle [◦C] T Temperature far from surface of particle [◦C] ∞ → m U f Gas mixture velocity [ s ] m eui Favre-Averaged gas mixture velocity [ s ] m u f Fluid flow velocity [ s ] u m 0f Gas mixture velocity fluctuations [ s ] m up Particle velocity [ s ] Y Local mass fraction of each species [ ] i − Ye Favre-Averaged mass fraction of species [ ] k − Processes 2020, 8, 923 18 of 19

Greek symbols ε Particle emissivity [ ] p −kg µ f Dynamic gas viscosity ms kg ρ Reynolds-Averaged density of species m3 kg ρi Density of species i m3 kg ρ f Density of gas mixture m3 kg ρp Density of particle m3 w σ Stefan-Boltzmann constant [ m2K4 ] τ Viscous diﬀusion tensor kg ij [ m2s2 ] . Net production rate of species kg ωk [ m3s ] Subscripts f Gas p Particle Abbreviations CFD Computational ﬂuid dynamics NG Natural Gas DO Discrete ordinate LHV Lower heating value MSW Municipal solid waste

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