Combustion conditions and design control of a two-stage pilot scale starved air incinerator by CFD F. Kokalj, N. Samec & L. Škerget University of Mariboru, Faculty of Mechanical Engineering, Slovenia Abstract Contemporary development of successful incineration devices is based on computational fluid dynamics (CFD) simulation of reacting flow. Thus, the appropriate numerical simulation is engaged as a tool to develop new or to upgrade old incinerators. State-of-the-art commercial software packages enable reliable and accurate simulation of combustion processes including the ultimate analysis of the physical complete combustion conditions. Turbulence, temperature and residence time can be calculated and predicted by applying various macroscopic empirical fluid flow and combustion models. Direct accurate insight into processes inside the combustion chamber are however possible only through the practical CFD application. In our case the study of achievement difficulties of the complete combustion conditions in a two stage pilot incinerator has been numerically investigated by applying the CFD software package CFX 5.6. Initially all available combustion models were examined and the most appropriate model giving most accurate results chosen for further numerical investigations. Results obtained by different combustion models have been compared at different combustion and flow conditions in the secondary chamber of the two-stage pilot incinerator. The most suitable combustion model has been used for the ultimate analysis of the complete combustion conditions considering real boundary condition values. The influences of the way of secondary and tertiary air inflow, flue gases mass flow rate and changes of the waste calorific value on the physical complete combustion conditions (i.e. temperature, residence time and turbulence) have been investigated. The original second chamber pilot incinerator design modifications were submitted. New secondary and tertiary air intake design proposals were made in accordance with the primary incineration goal of achieving complete combustion of waste. Modifications are small-scale low cost interventions. Keywords: numerical simulation, combustion models, reactive flow, waste incineration, incinerators.
26 Waste Management and the Environment II 1 Introduction The work is based on extensive and thorough numerical simulations of combustion in secondary combustion chamber of pilot scale waste incinerator with use of various available versions of CFX program package software. Different meshing methods and techniques were used with different types of program package versions. Produced results were compared. Latest version of program package CFX has two different combustion models convenient for desired numerical simulation of pilot scale incinerator. Unstructured meshing of geometry enables more detailed numerical design of the model with no simplifications compared to real incinerator. The combustion models employed allow non-premixed (diffusion) combustion in turbulent multi component flow field. They are well suited for numerical simulation of real combustion conditions taking place in secondary combustion chamber of waste incinerator (thermoreactor). Modeled numerical results are introduced and compared with experimental data gathered in secondary combustion chamber through the employment of test trials. The evaluation was made and best accurate model was chosen for further analysis. Real life employability of numerical combustion modeling has been many times confirmed by several authors, presented in papers and on conferences [6], [7], [8]. This also applies for experimental evaluation of numerical results in incineration science and research field [3], [4], [5]. 2 Combustion models Four combustion models were tested for best suitability: mixed is burnt, eddy break up, eddy dissipation and finite rate chemistry. First two models are available in older CFX version [1], later two in newer version [2]. Newer version also offers various advantages for the user that will be discussed in next section. There is also a chance to produce combined simulation with finite rate chemistry and eddy dissipation combustion model. One step combustion models determine the rate of production or consumption while multi step combustion models include further reactions and combined reaction rate. For improved convergence and employment of longer time steps in transport equations of chemically active components coupled solver is used. 2.1 Mixed is burnt combustion model This model is widely used for non-premixed combustion modelling. For such modelling two separate entering flows of fuel and oxidant are necessary. Fuel is defined as a hydrocarbon or carbon monoxide (and nitrogen), oxygen is oxidant, and products are carbon dioxide, water (and nitrogen). User can decide to run the simulation with single delta, double delta or beta function that represents the statistical distribution of mixture fraction in space. For single delta function only mean fraction differential transport equation is
Waste Management and the Environment II 27 solved and for the rest two functions the additional differential transport equation for mean fraction variance is solved. Mass fractions are solved with algebraic equations. For presented results the beta function was used. 2.2 Eddy break up combustion model This combustion model is frequently used for furnace combustion modelling. The model solves the differential transport equation for mean mixture fraction and fuel mass fraction. The rest of mass fractions are computed using algebraic equations. 2.3 Eddy dissipation combustion model Eddy dissipation combustion model is based on presumption that chemical reactions are fast compared to diffusion processes in reactive flow field. When reactants are mixed at the molecular level, they produce products of combustion. Combustion model suggests a direct connection of reaction velocity and time needed for reactant mixing at the molecular level. In turbulent flows the mixing time basically depends on the eddy properties. The reaction velocity is therefore proportional to the mixing time, determined with turbulent kinetic energy k and its dissipation ε. 2.4 Finite rate chemistry model Finite rate chemistry model available is designed for various chemical reactions and may run both ways forward and backward. For computation of chemical reactions velocity constants the Arrhenius equation is used. In case of ideal gases the reactive flow only depends of temperature and activation energy is a single component. 2.5 Combined eddy dissipation combustion model and finite rate chemistry model For combined eddy dissipation combustion model and finite rate chemistry model the reaction velocities are computed for both models separately and compared. Lower value of the two is used for further numerical simulation. This procedure is repeated on every reaction step. Thus combustion can be limited in one computation step with chemical kinetics and in other step with turbulent mixing at the same time and same place. 3 Discrete numerical model used Geometry modeling and its meshing was performed with program package CFX build. The use of different solver packages created a need for two slightly different geometry models prompted by meshing and convergence problems. Older program package operated with structured mesh and not even very dense mesh could not produce convergence. Thus modification took place and
28 Waste Management and the Environment II geometry had undergone smaller changes. Newer program package runs with unstructured mesh and detailed geometry model was made and calculation was successful with less and more dense mesh. The decision for final mesh density was made on the analyses of several meshes and produced results. Unstructured meshing model has 123.273 control volumes and unstructured meshing has 275.674 control volumes. This meshing and geometry were the basis for all further analyses and results presented and discussed in the following pages of this paper. Figure 1 shows both meshes of the pilot scale waste incinerator geometry. Figure 1: Unstructured (left) and structured geometry meshes (right). 4 Comparison analyses and selection of combustion models The goal was to compare available models and select best suitable combustion model for further analyses. Accuracy of produced numerical results was compared as well as computational time spent, results of pollutants formed and robustness of the models tested. The study was also interested in multi step models that predict carbon monoxide formation. Quantitative analyses and result comparison was performed. Because of high number of produced results the analyses has been reduced to longitudinal middle plane of the secondary combustion chamber of pilot scale incinerator. All computational results at this plane were adequately averaged on the single height of thermoreactor and then compared. This algorithm was established to avoid comparison problem due to different density and form of meshing. First presented is the comparison of average values alongside the thermoreactor as presented in Figure 2. Great differences are emerging among different combustion models. The secondary and tertiary air inlets and their influence are clearly seen on the picture. There are almost no differences between eddy dissipation combustion model and combined combustion model. There are also no differences between various single or multi step chemical reaction mechanisms. The results for different models are almost the same and chart lines are covered and all cannot be seen in the figure.
Waste Management and the Environment II 29 Figure 2: Average velocity result comparison. 2100 EBUM MIB 1900 EDCM (1 step) Temperature [K] 1700 1500 EDCM (2 step) EDCM (5 step) EDCM/FCR (1 step) EDCM/FCR (2 step) EDCM/FCR (5 step) 1300 1100 900 0-0,5-1 -1,5-2 -2,5 Longitudinal axle of thermoreactor [m] Figure 3: Average temperature results comparison. Temperature in thermoreactor is one of the key parameters since it provides conditions for efficient decomposition of organic components and its minimum value for waste combustion is set in directives and regulations. Great differences
30 Waste Management and the Environment II can be seen between mixed is burnt and eddy break-up combustion models. On Figure 3 results of eddy dissipation and combined combustion model are also presented. Their average values are around 1300 K and are between results of mixed is burnt and eddy break up model. Experimental temperature data for average operating conditions is around 1000 C. It is very interesting that single and multi step chemical reaction models produced same results as it was the case in velocities results. On Figure 3 lines are again covered by each other. Results on Figure 4 are also very insightful since all models predict almost the same course of fuel combustion along the flow of thermoreactor and decrease of fuel mass fraction. Full covering of the results of single and multi step chemical reactions is present again. Fuel mass fraction [CH4] 0,6 0,5 0,4 0,3 0,2 EBUM MIB EDCM (1 step) EDCM (2 step) EDCM (5 step) EDCM/FCR (1 step) EDCM/FCR (2 step) EDCM/FCR (5 step) 0,1 0 0-0,5-1 -1,5 Longitudinal axle of thermoreactor [m] -2-2,5 Figure 4: Average fuel mass fraction results comparison. Eddy dissipation and combined combustion model are offering the ability to compute the quantity of carbon monoxide if it is decided to calculate by the use of multi step chemical reactions (two or five step) model. On Figure 5 the results demonstrate significant difference between two and five step models but there is almost no difference between both combustion models. Five step chemical reaction mechanism models produce relatively high concentration of carbon monoxide and they were not measured in such quantity, when incinerator had run stable and with same boundary conditions. The difference between both 5 step combustion models shows where finite rate chemistry model was used in combined model and the difference toward eddy dissipation combustion model has been produced. The use of combined combustion model is disputable since it produces only slightly different results (see Figures 1 to 3) but consumes more computational time. Based on experimental data and simulation results, the best suitable model for further analyses is the eddy dissipation combustion model. If carbon monoxide production analyses are necessary then two-step model should be used. It should
Waste Management and the Environment II 31 be reminded that multi step chemical reaction models cost more computational time and shall be used only if really needed. Carbon monoxide mass fraction [CO] 0,02 0,018 0,016 0,014 0,012 0,01 0,008 0,006 0,004 EDCM (2 step) EDCM (5 step) EDCM/FCR (2 step) EDCM/FCR (5 step) 0,002 0 0-0,5-1 -1,5 Longitudinal axle of thermoreactor [m] -2-2,5 Figure 5: Carbon monoxide mass fraction results comparison. 1,9 1,7 1,5 1,3 1,1 0,9 0,7 Tmaks (1000 K) 0,5 Taverage (1000 K) 100 kj/s 150 kj/s Heating value 200 kj/s Products (relative value) Figure 6: Calorific values influence on temperatures and combustion products. 5 Thermoreactor design and operating conditions recommendations 5.1 Heating value influence Heating value of volatile gases, coming from primary chamber strongly influences the temperature in secondary chamber. Heating value of gases was
32 Waste Management and the Environment II simulated with various concentrations of methane, totalling from 100 over 150 to 200 kj/s. The velocity field for all three tested cases remained almost the same and only insignificant differences were noticed. It is interesting that the increase in calorific value has not produced expected rise of the highest temperatures but only moderate increase of average temperature of 100 degrees in thermoreactor for each calorific value increase. All the remaining boundary values remained unchanged. The maximal temperatures in thermoreactor also shifted with calorific value increase along the flow. The amount of combustion products formed can be also seen on Figure 6. The relative values are results at the outlet of the thermoreactor. The combustion and formation of products is almost equally good with lower two calorific values and decreases considerably for highest calorific value. This indicates too high amount of fuel introduced to thermoreactor that cannot be completely burnt in this geometry with unchanged amount of air and mixing. The upper and most appropriate calorific value for thermoreactor considering temperature and combustion products together is the middle calorific value of 150 kj/kg. 5.2 Influence of primary chamber volatile gases amount The quantity of gases beside others very much influence on the residence time of gases in the thermoreactor and thus influence combustion and destruction of waste. The simulation with the volatile gases flow velocity of 0,25 m/s or 30 m 3 per hour, 0,38 m/s (45 m 3 /h) and 0,5 m/s (60 m 3 /h) was made. The increase of volatile gases velocity raised the average and top velocities in the thermoreactor. 2 1,8 1,6 1,4 1,2 1 0,8 0,6 0,4 0,25 0,38 Velocity [m/s] 0,5 Tmaks (1000 K) Taverage (1000 K) Products (relative value) Figure 7: Volatile gases velocity influence on temperature and products. Presented temperatures in thermoreactor clearly demonstrate their influence on combustion. With higher velocity more fuel enters the thermoreactor. The
Waste Management and the Environment II 33 temperature is considerably raised and completely changes the temperature field. Maximal temperature has amplified for round 100 degrees and average for over 200. The simulation shows that the quantity of gases induced into thermoreactor in case of middle and highest velocity is too big and raises temperatures well over designed values. The quality of combustion decreased significantly beyond acceptable values. 5.3 Influence of secondary and tertiary air inlet velocity and direction Conducted simulations included simultaneous change of velocity of air at inlets and angle of induced air. The changes were made according to the Table 1. All other boundary condition values remained the same. Table 1: Projected and changed air inlets velocity and direction. Quantity Secondary air inlets Tertiary air inlets Velocity = 6 m/s = 4 m/s 4 m/s 6 m/s 4 m/s 8 m/s Direction + 10 o + 10 o + 15 o + 15 o velocity (s=6;t=4) changed velocity (s=4;t=6) changed velocity (s=4;t=8) direct. +15, combustion products (relative velue) direct. +15, temperature (1000K) direct. +10, combustion products (relative velue) direct. +10, temperature (1000K) direction, combustion products (relative velue) direction, temperature (1000K) 1,2 1,15 1,1 1,05 1 0,95 0,9 0,85 0,8 Figure 8: Air inlets velocity and direction influence on temperature and products. Results show that the change of velocity and direction of air induced into the thermoreactor has great influence on temperature field. The influence can also be seen on the flow and products field. Already the change of velocity but not the total amount of air induced into thermoreactor caused the average temperature rise for 50 degrees and further improved quality of combustion. The amount of final combustion products has magnified for round 10 %. Additional tertiary air
34 Waste Management and the Environment II induced has lowered average temperature but increased the combustion products quantity. These simulations have shown that slight and inexpensive changes are advisable at the pilot incinerator to further improve complete combustion. 6 Conclusion Computational fluid dynamics is continuously gaining in its applicability at solving real problems from the field of combustion and is increasingly used by broader range of users. This tool is more and more reliable and precise and this is the key reason of all for the wide employability. This was also shown in the course of the work presented. New possibilities of geometry modeling and meshing enabled precise geometry model, build without modification, the same as the real model. The eddy dissipation combustion model has proven as very robust and accurate when modeling thermoreactor of pilot incinerator. For detailed geometry analyses and its performance the interesting multi step chemistry models should be used. They have the needed capacity to predict carbon monoxide formation. Carbon monoxide is created at incomplete combustion. This is offering a possibility to locate parts of geometry where incomplete combustion in taking place, analyze causes and abolish them. The work presented demonstrates the applicability of eddy dissipation combustion model for determining the appropriate working conditions for thermoreactor and enables to test and predict better secondary and tertiary air inlet velocities and directions to achieve more complete combustion. References [1] CFX 4.4, (2000), Users Manual, AEA Technology. [2] CFX 5.5.1, (2002), Users Manual, AEA Technology. [3] Chen, K., S., Tong, C., H., (1997), Modeling of Turbulent Burning Flow and Performance Evaluation of Municipal Solid Incinerator, Journal of Environmental engineering. [4] Epelbaum, Greg, (2003), Simulation and Validation of a Mass Burn WTE Boiler using the FLIC and Fluent Programs, Waste 2003. [5] Ficarella, A., Laforgia D., (2000), Numerical simulation of flow field and dioxins chemistry for incineration plants and experimental investigation, Waste Management. [6] Niessen, Walter, R., (1995), Combustion and Incineration Processes: Applications in Environmental Engineering, Second Edition, Revised and Expanded, Marcel Dekker Inc., New York. [7] Yang, Y., B., Nasserzadeh, V., Swithenbank, J., (2003), Development and application of FLIC (Fluid Dynamics of Incinerator Combustion), Waste 2003. [8] Yang, Y., B., Yamauchi, H., Ryu, C., Nasserzadeh, V., Swithenbank, J., (2003), Integrated FLIC/FLUENT Modeling of large scale MSW Incinerator Plants Waste 2003.