IFRF Combustion Journal Article Number , November 2006 ISSN X

Transcription

1 IR Combustion Journal Article Number 63, November 6 ISSN X CD as applied to high temperature air combustion in industrial furnaces Yang Weihong and Blasiak Wlodzimierz Division of Energy and urnace Royal Institute of Technology (KTH) Stockholm, Sweden

2 -- ABSTRACT Combustion in a high temperature and oxygen deficient atmosphere shows different characteristics compared to combustion in a normal atmosphere. This is known as high temperature air combustion or HiTAC. The existing mathematical models have proven not to be suitable for simulation of HiTAC. It is a challenge of numerical simulation to be able to reflect the characteristics of HiTAC. The objective of this study is to develop and experimentally verify a mathematical model. We also expect to develop some parameters to classify the characteristics of HiTAC, which are different from normal combustion. In this work, the available mathematical models were investigated and developed. The numerical simulations undertaken here include numerical calculation of a single fuel jet in HiTAC conditions (including both cross-flow and co-flow of the fuel jet and air flow) and the modelling of a HiTAC test furnace with two different High Cycle Regenerative Systems (one flame and two flame systems). The results show that the combustion model used for simulation of HiTAC must be capable of expressing precise reaction rates in a high-temperature and low oxygen partial pressure atmosphere. Concepts including the oxidation mixture ratio, furnace-gas-temperatureuniformity-ratio, the furnace flame occupation coefficient and the flame entrainment ratio were defined to describe the characteristics of HiTAC, which provides help for optimal design of a HiTAC furnace and burner. Additionally, the benefits of HiTAC technology are quantitatively demonstrated by mathematical models. These benefits are: lower peak temperature, larger flame volume, more uniform thermal field, lower local firing rate, higher heat transfer, higher energy utilizing efficiency and lower combustion noise. The operating parameters, including the oxygen concentration and the temperature of the preheated combustion air, the fuel temperature, the fuel flow rate, the excess air ratio and flame locations are shown to have stronger influences on combustion and NO emission in the HiTAC furnace. The optimum combination of these parameters should be considered. NO emissions formed by N O-intermediate mechanism are very important during HiTAC operation. The approximate percentage of NO production by nitrous oxide according to the Zeldovich and prompt mechanism varies from 5:95 at 1% oxygen concentration to 95:5 at 5% oxygen concentration. The critical diameter and the length of the furnace fitted with HiTAC technology are proposed for an optimum design for HiTAC operation. The numerical simulation results and are very encouraging and can be used as an analytical or a design tool of an industrial furnace Keywords: CD, High temperature air combustion, furnace, flame, heat transfer, NO emission. Contents Nomenclature 1. Introduction 1.1 High temperature air combustion 1. Mathematical modelling for an industrial furnace. Objectives 3. Mathematical model for HiTAC 3.1 Turbulent combustion model 3. Other models used 4. Developed concepts for describing HiTAC characteristics 5. Gas jet combustion study 6. HiTAC test furnace study 6.1 Comparison of predicted and measured data 6. eatures of combustion and flow of HiTAC 6.3 Heat transfer elevation 6.4 Simulation of moving slab 6.5 Effect of flame locations on the flame properties from the HiTAC 7. NOx emissions 7.1 NO models 7. NO emissions for gas jet combustion study 7.3 NO emissions for HiTAC test furnace study 8. Optimal design of HiTAC furnace 8.1 lame entrainment ratio 8. Optimal design of HiTAC furnace 9. Conclusions Acknowledgments Reference Nomenclature A empirical coefficient A r area of flue gas recirculation A cr criteria area of flue gas recirculation B empirical coefficient C pre-exponential factor E a activation energy k turbulent kinetic energy (m s - ) k a absorption coefficient of flue gas m mass fraction M molecular weight (kgmol -1 ) n stoichiometric coefficient (number of moles) q c local heat release of difference fuel species, (kw/m 3 ) Q f the heat release in flame zone, (kw). q HR average lame Heat releasing ( kw/m 3 ) R universal gas constant, (Jkmol -1 K -1 ) R EBU fuel consumption rate for eddy break up model (kgm -3 s -1 ) R OC furnace occupation coefficient R HiTAC rate of high-temperature air combustion (kg/m 3 s) R KIN Arrhenius reaction rate (kgm -3 s -1 ) R o oxidation mixture ratio R tu furnace temperature uniformity ratio R fe flame entrainment ratio T temperature (K) T average temperature in furnace (K) υ the specie rate exponent v velocity V f flame volume (m 3 ). V furnace volume ( m 3 ) Z horizontal distance from burner face, (m) β temperature exponent ε turbulent kinetic energy dissipation (m s -3 ) Subscripts: initial state out final state c species

3 -3- f l i O r flame urnace fuel calculation cell number oxygen recirculation flue gas gas and cold combustion air flows alternatively in the regenerator in contact with the regenerator material. The furnace gas transmits heat to the storage material during the heating period. The combustion air absorbs heat from the storage material, cooling down the regenerator during the cooling period. 1 INTRODUCTION 1.1 High Temperature Air Combustion The general objectives of the technical improvement over the last decades of the performance of a fuel fired industrial furnace have been to achieve a more uniform and better controlled temperature distribution in all furnace zones, high and efficient energy utilization as well as lower pollutant emissions. Combustion with hightemperature and dilution of air with combustion products is one of the most promising technologies to meet these new targets over the recent years. This technology is referred to as high-temperature air combustion (HiTAC) [1] or flameless oxidation (LOX) []. Due to the design of the HiTAC burners, to control the mixing of the fuel and air jets with the furnace gases, the chemical reaction rate in these burners is lower than in conventional combustion. The typical characteristic of HTAC technology is its capacity to generate larger flame volumes than conventional combustion, which results in an increased heat transfer. Additionally, the more homogeneous reaction associated with HiTAC technology also implies that the heat release in the reaction zone is more widespread, leading to a more even and moderate temperature rise. Consequently, the emission of NO x can be kept low. To maintain a continuous operation at least two regenerators are required. Continuous operation is achieved in the model regenerator by switching periodically from hot, furnace gas to cold air in the regenerator with a short switching interval. It was found that a shorter switching time results in a more efficient waste-heat recovery rate and a more uniform and high temperature [3]. When more than one pair of burners are used, different firing configurations can be used to obtain a better heating performance that is desired in special applications [4]. The switching interval used can be varied from 4-5 seconds [4] to 6 seconds, and corresponds to known applications of HiTAC that are referred to as highcycle regenerative systems or HRSs. The two main HRS solutions currently in use feature either one or two-flame burner systems. A one-flame HRS is characterized by a single flame created by one fuel nozzle surrounded by air inlets and flue gas outlets [, 3]. The scheme is shown in igure 1. The single flame develops along the axis of the fuel-jet nozzle during the cooling and heating periods. uel is supplied continuously through the same nozzle. In this way a single flame can be formed with a permanent position. This position remains almost unchanged between heating and cooling periods, as the regenerators are located around the fuel jet nozzle. or its industrial application, the fuel and high temperature combustion air nozzles are separated from each other on the burner and are injected directly into the furnace at high velocities. Because of their momentum the two jets entrain in-furnace gas, the zone near the burner is thoroughly mixed and the partial pressure of oxygen is reduced. Stable combustion of fuel injected directly into this zone of oxygen deficient combustion air is possible if the combustion air is preheated to a temperature that exceeds the auto ignition temperature of the fuel. In an industrial furnace, combustion air preheat temperatures can be obtained in the range of o C. The exhaust gases can be at a temperature as low as 5 o C, when the energy in the flue gas is captured by a high performance heat exchanger. or example, a regenerative heat exchanger working in the high cycle mode, can recover as much as 9% of the waste heat in the flue gases, resulting in a large energy savings. Also, CO emissions are reduced dramatically since for a fixed system less fuel can be used. Moreover, the temperature of the fuel and air are raised well above the auto-ignition temperature for most fuel gases, indicating that the conditions for flame stabilization are very favourable. Results from previously published studies of HiTAC applications in industrial furnaces demonstrate that hightemperature air can be obtained using a regenerative heat exchanger. In this heat exchanger, which can be made with ceramic balls or honeycomb, the heat is periodically stored and withdrawn from the heat storage material. Hot furnace

4 -4- resh Air lue gas igure 1 Scheme of one-flame HRS In a two-flame HRS, there are two separated high-cycle regenerative burners. This scheme is shown in igure. The two burners are located in the walls of the furnace and work in pairs by a set of valves that change the direction of the air and the flue-gases according to the required switching time. Normally there are a few pairs of burners working together. Each burner has a preheated air outlet located centrally, and two fuel nozzles located laterally. When the hot furnace gas passes out through the regenerator of one of burners (heating period), the fuel nozzle of this burner is closed, and the combustion air and flame are switched off. During this time another burner operates in the combustion mode, or cooling period of the regenerator. That means the air is preheated via cooling of the regenerator, and both fuel nozzles of the fired burner are on and two flames can be created. In this way, the flame can be shifted from one burner to another in accordance with the switching time between the heating and cooling periods of the regenerator.. H.A..G. lue Gas H.A...G. resh Air.G. igure Scheme of two-flame HRS In order to understand the characteristics of this novel combustion technology, HTAC has been modelled by a single jet gas combustion in a low-oxygen partial pressure and high-temperature air environment. However progress in the use of different applications of HiTAC technology has increased the need for more information and data for furnace and process designers. In particular, it is very important to specify optimal conditions for installation of HiTAC in industrial furnaces. or these reasons, studies are performed at a larger scale where at least one set of regenerative burners systems is installed. 1. Mathematical Modelling of Industrial urnace Despite its shortcomings, computational fluid dynamic (CD) modelling has become widely accepted as being a valuable and cost-efficient tool to aid designers and operators of industrial furnace equipment. Mathematical models used in CD must be tested against experimental measurements when they are applied to new problems. urthermore, it requires professional experience in order to effectively solve a problem as well as to analyse the results of CD modelling. Industrial furnaces and burners are designed to quickly raise the temperature of products and to ensure a high production capacity. Different furnaces have different characteristics. or example, the heating features of heat treatment furnaces are dependent on the heating process and heated material. There is also a diversity of furnace constructions and operation in different companies even for the same furnace steps. In the scheme of activities for the application of HiTAC in industrial furnaces mathematical modelling can be found in the following steps: Mathematical model studies include assessing existing models, developing new models and verifying the models used. ield studies yield information on for example the: velocity field, temperature distribution, species concentration levels, heat transfer distribution, burnout levels, pressure gradients, etc. eature studies satisfy the various special needs for chosen cases. or example, the characteristics of the combustion and flame of HiTAC are expected to be classified. Parametric studies investigate the influences of different parameters on combustion, NO emissions and heat transfer. These parameters for HiTAC include oxygen concentration, the temperature of the preheated combustion air, the fuel temperature, fuel flow rate, excess air ratio and the flame location.. OBJECTIVES The general objective of this study is to develop and experimentally verify a mathematical model. Modelling will be verified against experimental results. We will also develop some parameters in order to classify the characteristics of HiTAC that are different from conventional combustion and from these parameters assess the optimal design of a HiTAC furnace. In order to meet the above objectives, mathematical modelling is based on two main steps: 1. Modelling of a single fuel jet in conditions of HiTAC including cross-flow and co-flow of fuel and air.. Modelling of a HiTAC test furnace for two different High Cycle Regenerative Systems (one flame and two flames systems). Simulation of the small scale test stand is used for validation of the mathematical models. The model data is used to verify the numerical results of the specific phenomena of HiTAC, such as NOx emissions, the flame shape and size, flame radiation and the effects of fuel and oxidizer composition and velocity.

5 -5- In modelling the HiTAC test furnace, we continue to verify the numerical modelling used, basing the comparison on measurements of a number of parameters including temperature, gaseous species concentration and heat flux on the furnace-wall. urthermore, the benefits of HiTAC are demonstrated by comparison with conventional combustion technology. The heat transfer inside the HiTAC test furnace equipped with (HRS) was evaluated. The heat sink includes a stationary and a moving slab. A model of the formation and destruction of NO in combustion of LPG with high temperature air was developed. The formation of NO via the N O-intermediate mechanism was used to interpret the NO emissions during HiTAC. inally, an optimal design guideline for a HiTAC furnace is proposed. 3. MATHEMATICAL MODEL OR HiTAC 3.1 Turbulent Combustion Model or HiTAC, the incoming preheated air is diluted with combustion products that are recirculated inside the furnace before the preheated and diluted air jet makes contact with the fuel. The temperature of the air is normally higher than the auto-ignition temperature of the fuel. The combustion can take place immediately after the air and fuel are mixed. However, this rate of combustion is slower due to the lower partial pressure of oxygen in the combustion air [5, 6]. The zone of chemical reaction tends to be larger, which is quite different from conventional combustion. Studies [7] have shown that the characteristic time of kinetics and turbulence are comparable, (Damköhle number 1) and the two are coupled with each other. Therefore, the combustion rate is controlled by both chemical kinetics and by the turbulent-mixing. As a consequence, the combustion model that is based on the assumption that mixing is burned is not suitable for predicting HiTAC. Attention should be paid to the rate constant of a reaction during simulation of HiTAC. These constants are commonly obtained for normal combustion using ambient temperature air. The same problem exists when a full reaction mechanism is used, even for the elementary reactions, since the accuracy of all the associated rate constants have not been confirmed. Therefore, the constants in models have to be optimized on the assumption that the air temperature and oxygen concentration are variable. The combustion model used for HiTAC simulation must be a model capable of expressing precise reaction rates in a high-temperature and low oxygen partial pressure atmosphere. In this study, the combustion model involves both chemical-kinetic and turbulent-mixing based models. This entails evaluation of both rates locally and then taking the slower of the two as the controlling rate according to the following: [ R R ] R min, HiTAC = (1) EBU KIN where R EBU is the corresponding turbulence-controlled rate, determined from the eddy-break-up model, and R KIN is the kinetic rate. The kinetically controlled reaction rate of the fuel R KIN is defined as: R KIN = CM T β allj m ( ρ M j j ) v j e Ea / RT In the context of the Magnussen and Hjertager model [8], the kinetic rates are deliberately set very high so that turbulent mixing is guaranteed to be the controlling rate. Mathematically, these statements translate into the following equation: ρ ε mo m R EBU = Amin m,, B k so s Where, s = n M / n M, O P P O O s = n M / n M, A and B are empirical P coefficients. To be able to accurately simulate HiTAC, when using the full reaction mechanism it is indispensable to consider all the intermediates. However, a practical simulation of an industrial furnace including a three dimensional flow with full reaction mechanism is far beyond the capability of present computers. Therefore, the most realistic solution would be to adopt a set of greatly simplified reaction mechanisms covering some intermediates. In this study, a two-step reaction with CO as an intermediate is used since it is one of the most commonly used in CD applications. The reaction constants used were taken from [9]. In order to decide on a suitable combustion model for HiTAC, besides the eddy-break-up model, a PD-mixture fraction model with chemical equilibrium was also used. The mixture is assumed to obey the ideal gas law. The viscosity, thermal conductivity and specific heat of the mixture have been computed from the properties of individual species, and are all functions of temperature 3. Other Models Used The flows in the industrial furnace are turbulent. It follows that the performance predictions of combustors depend very much on the turbulence model adopted. With the requirement of a million nodes per cubic millimeter, it is clear that the application of DNS (direct numerical simulation) for engineering purposes is not practical for application here. LES (Large eddy simulation) seems to have a bright future, but more research on LES is required. Dong [1-11] has carried out a simulation of a single fuel jet flow in high-temperature diluted air combustion. It was found that advanced turbulent models, such as LES and RSM, gave small differences in the near field when predicting the flow. However, the empirical constants, for example C s in the LES model, have a significant influence on the predictions. This implies that the empirical constants in traditional models must be adjustable to be able to obtain the best performance for HiTAC simulations. P P () (3)

6 -6- The k-ε model remains the obvious starting point, especially for diffusion flames for engineering calculations. It has been verified that it is robust and efficient for most engineering calculation purposes in the range it can be used. or flows in HiTAC, due to the larger reaction zone and the similarities to a well-stirred reactor, the assumption of non isotropy of the turbulence is weak. urthermore, the buoyancy in the furnace is relatively small compared to conventional combustion due to both the preheated air and lack of a distinct flame zone. Consequently the k-ε model was chosen to be used in this paper. Additionally, radiation was handled using the discrete transfer method [1] 4. DEVELOPED CONCEPTS OR DESCRIBING HiTAC CHARACTERISTICS HiTAC has many characteristics that are completely different from conventional combustion. or example, the HiTAC flame is less visible than the flame from conventional combustion where there is a high concentration of oxygen by volume (more than or equal to 1%, i.e air). Therefore it is generally accepted that the flame length is not a suitable parameter for characterizing flame size for HiTAC. Instead, it is necessary to characterise the flame shape and size using a comprehensive numerical simulation. To describe the flame under HiTAC conditions, the oxidation mixture ratio is used in this work [13]. The oxidation mixture ratio allows the combustion progress to be estimated and to be calculated as the mass fraction of oxygen divided by the mass fraction of oxygen plus the amount of oxygen needed to achieve complete combustion at any point in the combustion chamber, as follows: R o = m s = n M / n O mo + scm M c, c where O O. This ratio has a value of RO =1 at the air inlet or when the combustion is completed, and a value of R O = at the fuel inlet. The lean flammability limit for different fuel species has been used to indicate the outside border of the flame, and the rich flammability limit is used to define the inside border of the flame. R O =.99 is assumed to indicate a flame border [13]. The flame volume can be approximately defined when (4) R O.99 (5) In order to describe the overall radiation field of a flame, the radiant fraction (f rad ) is used. It is defined as the ratio of the net radiative heat loss from the flame (Q rad ) to the total heat released during combustion (Q ) as follows: Q rad f rad = (6) Q To evaluate the gas temperature field uniformity inside the furnace, a furnace-gas-temperature uniformity-ratio, R tu, is defined as follows [13]: ( T ) R tu = T T i (7) where T i [K] is the temperature of the calculated cell number and T [K] is the average temperature in the furnace. When R tu gradient inside the furnace. =, there is no gas temperature or HiTAC flames, to be able to characterize the flame volume in relation to the volume of the combustion chamber, a dimensionless coefficient called urnace lame Occupation Coefficient (OC), R OC is defined as the ratio between the flame and furnace volume [14]: V f R OC = (8) V where, V f [m 3 ] is the flame volume calculated according to the relationship in equation 5, and V [m 3 ] is the furnace volume calculated from the geometric dimensions of the furnace. Combustion intensity is also a very important parameter for designing the furnace and the burner. To evaluate quantitatively the chemical reaction intensity in the furnace and especially in the chemical reaction zone (flame), two parameters are used. One is lame Heat Release (HR), which is defined as the ratio of heat released inside the flame zone (Q f ) to the flame volume (V f )[14]: where Q f is obtained as follows: f Q f q HR = (9) V Q = q dv (1) f V f Here q c (kw/m 3 ) is local heat release of different fuel species. Another parameter used in this work is the ratio between the heat released by the flame zone (Q f ) and total heat released inside the combustion chamber, (Q ). It is named lame Heat Occupation Coefficient (HOC), and defined as follows [14]: c Q f R HOC = (11) Q where, Q is calculated according to: Q = q dv (1) V c

7 -7- urthermore the entrainment ratio of the nozzle is a good parameter for the description of the internal recirculation of the flue gas which plays an important role in the HRS systems. Due to the interaction between fuel and air nozzles for one-flame HRS, the entrainment of a single nozzle is not a suitable parameter for the characterization of the internal flue gas recirculation. Therefore, the entrainment ratio must include the interactions between the nozzles. In order to describe the interactions between the fuel and air nozzles, the flame entrainment ratio is more efficient. The flame entrainment ratio (R fe ) is defined as the following [14]: to be larger when the PD model is used. The lower peak temperature seems more realistic when the oxygen concentration is only 1%. The detailed discussion can be found in reference [13]. Similar conclusions are made in [6] and the authors give a full theoretical analysis.,,3 R m f fe = (13) m,8 uel,75 Here, m (kg/s) and m f (kg/s) represent the initial total mass flow rates and mass flow rates through the cross section of the flame respectively. Diluted and Preheated Air Diluted and Preheated Air uel 5. GAS JET COMBUSTION STUDY The combustion of a propane gas jet in the laboratory furnace was studied numerically. Both cross-flow and coflow of fuel and air nozzles were considered. The objective of the study was to investigate the interactions between the fuel jet and the diluted air in a steady state condition. The schematics of the combustion chamber are shown in igure 3. The combustion air for the first test case is preheated by an electrical heater, and diluted by nitrogen. The fuel and air is injected with a cross-flow arrangement. The combustion air for the second test case is preheated and diluted by a flue gas generator. In this case, the composition of the oxidizer is close to what can be found in a real industrial furnace. The fuel jet was injected in a co-flow arrangement to the main flow of the hot flue gases. The details of the test cases can be seen in [15, 16]. The fuel studied in this work is LPG (98.5% propane). The ranges for the parameters studied are as follows: 1. low rate of the fuel was varied from kg/s (.13 nl/min) to kg/s (.53 nl/min),. low rate of the oxidizer was from 1.1kg/s to 1.3 kg/s, 3. uel preheat temperature was varied in the range between 388 K and 173 K, 4. Air preheat temperature was in the range from 141 K to 1573 K 5. Oxygen concentration in the preheated air was varied from % to 3.% (mass%) The gas flame zones are shown in igures 4 for the crossflow arrangement. It can be seen that the flame volume as defined by means of the oxidation mixture ratio was predicted to be smaller when the PD model was used. Also it can be noticed that the flame size and shape predicted by the EBU model is visually similar to the flame shown in the photograph (igure 4a) reported by Lille et al. [15]. urthermore the calculation results indicate that the maximum gas temperature predicted when using the PD combustion model is higher than that predicted by the EBU combustion model. Also the areas of maximum gas temperature within the flame are predicted igure 3 Schematic of the combustion chamber for single fuel jet test furnace (a) Cross flow (b) Co flow (a) (b) igure 4 Predicted distributions of oxidation mixture ratio for 1% oxygen in the air preheated up to 141K and at fuel inlet temperature equal to 473 K. (a) flame photograph; (b) PD model; (c)ebu model. Based on these facts, it was assumed that the HiTAC flame predictions with the EBU model are more realistic and the EBU model was further studied as more applicable to HiTAC flame modelling. It has often been necessary to adjust the two empirical coefficients A and B in the reaction rate equation (3) of the EBU model to obtain good performance for a particular application. The coefficient B is adjusted to inhibit reactions where the temperature is low. However in HiTAC combustion, the air temperature is higher than the fuel self-ignition temperature thus the influence of the empirical constant B is not important. Therefore, B is set at a constant value of.5. Concerning A, it is known that the fuel consumption decreases when A decreases according to the reaction equation, and the reaction rate will also decrease. Because of the diluted nature of the jets in HiTAC, the combustion rate is slower than traditional combustion [6]. As such A should be less than the nominal value of 4. Simulations were therefore performed using the value of A set at 4.,. and 1.. (c)

8 -8- The numerical results [13] show that the predicted temperature field is strongly influenced by the value of the A coefficient. A lower value of the A coefficient results in a smaller temperature gradient in the flame. Clearly it is seen that area of flame with the highest temperature is reduced when the lower value of the A coefficient is used. urthermore, the numerical predictions demonstrate that the flame volume increases with the reduction in the A value. The results also show a more uniform distribution of the fuel inside the flame when smaller values of A are used. The larger A value is more suitable for simulation of HiTAC because of the HiTAC flame features. 5.1 Characteristics and Parameter Study: The combustion model employed is the eddy dissipation concept with a two/three-step mechanism to describe the reactions. The flame characteristics of HiTAC studied include the flame volume, flame temperature and the gas temperature uniformity. The influence of the combustion parameters on these features were investigated numerical simulation. The combustion parameters include oxygen concentration in the air, and the temperature of the air and fuel. igure 5 shows that the flame volume ratio R f depends very much on the oxygen concentration in the preheated combustion air, the temperature of the combustion air and the fuel inlet temperature. The flame volume ratio always increased when the oxygen concentration in the preheated air was reduced. This trend is more pronounced when the oxygen concentration in the preheated air is below 1%. Rf Oxygen Concentration (mass%) Ta=141,T=88 Ta=141,T=573 Ta=1173,T=573 Ta=173,T=88 Ta=173,T=573 Ta=141,T=473 Ta=1173,T=88 Ta=1173,T=873 Ta=173,T=473 Ta=173,T=873 igure 5 R f versus oxygen concentration for various air and fuel temperatures (K) in gas jet combustion with cross-flow Increasing the fuel temperature leads to a reduction of the flame volume ratio R f at a constant oxygen concentration. The reason for this is a decrease in the fuel density with the increased temperature and an increase in the fuel inlet velocity at constant fuel fluxes. Changes in the density and the fuel inlet velocity are proportional to the fuel temperature. The increase of the initial velocity of the fuel jet improves the mixing between the fuel and the preheated air which results in a decrease in the flame volume. The improvement of the mixing is approximately proportional to the fuel inlet velocity. Therefore, a reduction of R f is also proportional to the fuel inlet temperature. The combustion air temperature has a much less significant influence on the flame volume at constant oxygen concentrations and fuel temperatures. or the investigated temperature range (141 K 173 K) of the preheated air, the flame volume was found to be almost constant at fixed oxygen concentrations and fuel inlet temperatures. Generally it can be concluded that the largest flames are obtained at the lowest oxygen concentrations in the combustion air and at the lowest fuel and preheated air temperatures. These numerical experiments show that the HiTAC process is spread over a much larger volume than in conventional turbulent diffusion flames. It confirms that the HiTAC is a large volume combustion with a reduced combustion rate. The influence of the oxygen concentration on the flame volume in the case of gas jet combustion with co-flow is shown to give the similar trends as shown in igure 6. The differences in flame volume between these two cases are much smaller, as shown in igure 7. This implies that the influences of diluents and mixing processes between fuel and diluted air on flame volume are small. Radiant raction, % lame Volume,m3,x1-3 lame volume 1,6x1-3 1,x1-3 8,x1-4 4,x1-4 Peak temperature,,,3,6,9,1,15,18,1 Oxygen Concentration, (vol%) Radiant fraction igure 6 lame volume, peak temperature and radiant fraction versus oxygen concentration for 1173K air temperature and 99 K fuel temperature for gas jet combustion with cross-flow lame volume, m3 8,E-4 6,E-4 4,E-4,E-4,E+ co-flow cross-flow 1 3 Oxygen concentration(mass%) igure 7 The influence of the oxygen concentration on the flame volume under gas jet combustion with co-flow and cross-flow igure 6 also demonstrates the influence of the oxygen concentration on the radiant fraction. The radiant fraction remains almost constant when the oxygen concentration is varied. This indicates that the increasing of flame volume compensates for the influence of the decrease in flame temperature on radiation Tpeak,K

9 -9- It is obvious as shown in igure 8 that the flame peak temperature approaches maximum for the highest oxygen concentration, and minimum for the lowest oxygen concentration. The peak temperature decreases linearly with the reduction of oxygen concentration. At any preheated air temperature the peak temperatures fall slightly for a constant oxygen concentration and at a reduced fuel inlet temperature. The variations of the maximum temperature with preheated air temperature are also linear. Tmax, K Oxygen Concentration (mass%) Ta=1173,T=88 Ta=1173,T=573 Ta=1173,T=873 Ta=173,T=88 igure 8 Peak flame temperature (T max ) versus oxygen concentration for various combustion air and fuel temperature (K) at the case of gas jet combustion with cross-flow oxygen concentration of 5 % and a fuel inlet a temperature of to 88 K R tu is two times lower than for the case of an oxygen concentration of 18%. The increase in the temperature field uniformity results from the reduction of flame peak temperature and from the increase of the flame volume R f at lower oxygen concentrations. 6. HiTAC TEST URNACE STUDY The HiTAC test furnace, built at KTH, has internal furnace body dimensions of m. This furnace is shown schematically in igure 9. our tubes with an external diameter of 11 mm each, one cooled with air, have been installed horizontally in each corner of the furnace to remove heat from the combustion chamber. The cooling air flows from one side of the burner to the opposite side. On the opposite side of the furnace to the burner face, there are two flue gas ducts of 11 mm external diameter for exhausting hot flue gases from the furnace. The walls of the test furnace consist of two layers: an outer steel cover 5. mm thick and an inner layer of fibrous ceramic insulation 3 mm thick with a thermal conductivity of.14 W/m K. The emission factor of the insulation was set to.5 according to the manufacturer s specification, and the emission factor of the air cooled tubes was set to.8 to represent an oxidized steel surface. The details of this test furnace are available in reference [17]. The maximum flame temperature for both co-flow and cross-flow configurations depict the same variation. The maximum flame temperature in the case of cross-flow is around 4 K higher than that in the case of co-flow if the temperature and oxygen content of combustion air are equal. One should remember that the diluents in the case of cross-flow were assumed to be nitrogen and the diluents in the case of co-flow were assumed to be combustion products. As an example, when the oxygen concentration is 1.%, the diluents include 1.1% CO, 74.1% N and 5.6% H O. The difference in maximum temperature should result from the difference of heat capacity and/or the difference of mixing processes between fuel and diluted air. The maximum furnace temperature for common fuels during preheated and diluted air combustion can be estimated as: T = T Tad + 1 /[ O ] + 1 f _ max a _ in (14) Here, [O ] is oxygen concentration of air. Other similar definitions are defined in the literature [, 6]. The gas temperature uniformity ratio, R tu is used to describe the influence of the oxygen concentration in the combustion air on the temperature profile. R tu decreases with reduction of the oxygen concentration, which indicates a more uniform temperature field at a lower oxygen content. or example, the simulation of the gas jet combustion in a cross-flow shows that for the case of an

10 -1-14 D (e) (a) igure 9 HiTAC test furnace and burner (a) HiTAC test furnace at KTH (b) Configuration of HiTAC test furnace with one-flame HRS (c) One-flame HRS (d) Configuration of HiTAC test furnace with two-flame HRS (e) Two-flame HRS The furnace is designed such that two different HRSs can be used. The first HRS is attached to the front of the furnace. It is a so-called one-flame system and has a thermal capacity of kw. igure 9 (b) represents the computational domain of the HiTAC test furnace. The other system which is composed of two pairs of HRSs is installed on the left and right sides of the furnace as shown in igure 9 (c). 11 D= 35 3 (b) (c) Air-nozzle Exhaust flue nozzle The kw one-flame HRS with honeycomb regenerative burners was the first used in this project. The ceramic honeycomb regenerators, through which the exaust gas and combustion air are vented, are an intergral part of the burner body. igure 9 shows the dimensions of the burner and locations of fuel and air injection ports. There are 1 regenerators in total, working in pairs and organised into two groups separated by intervals. 8% of flue gases are vented through the burner outlets, which is sufficient to preheat the combsution air for the desired fuel. The remainder of the exhaust gases flow out from the furance through the chimney located on the rear wall of the furnace. The second calculated case assumes that the furnace is equipped with four high-cycle regenerative burners with a capacity of 1 kw each. The burners (four burners marked A, B, C, D) are placed on the sidewalls of the furnace as it is shown in igure 9(d). Each burner consists of one injection port for combustion air and two nozzles for fuel injection. Combustion air and fuel are injected separately. The combustion air injection port is located in the centre of the burner. The fuel nozzles are placed in the same plane on both sides of the combustion air port as shown in igure 9 (d). This type of regenerator allows preheating of combustion air up to 1537 K. The fuel used in the study was LPG with a flow rate of 7.7 Nm 3 /h for all the studied cases. The composition of the fuel used was.% CH 4,.95% C H 6, 98.35% C 3 H 8, and.67% C 4 H 1. The air flux was around Nm 3 /s (d)

11 Comparison of Predicted and Measured Data The modelling validation is performed by the comparison of the following computed and measured data: - Energy balance, - Wall temperature profiles, - In-furnace gas species These are done using tests based on the HiTAC test furnace equipped with one-flame High-cycle regenerative System (HRS) [18-]. Energy Balance: The overall thermal energy input to the test furnace in this study was 18 kw. The sensible heat flow rate at the fuel inlets was zero since the reference temperature was set to T=98 K. The thermal input of fuel for simulation was calculated according to the chemical reaction steps for the fuel given above. The heat of exhaust leaving the burner for the simulation was approximately calculated as the value of heat of flue gas entering the burner minus the sensible heat of the combustion air. A figure of 54.65% of the predicted fuel thermal input passed through the burner outlets, and 83.6 percent of the sensible heat carried by the flue gases through the burner outlets was used to preheat the combustion air from 3 to 111 K. This is about % of the total thermal input. This implies that a very high level of energy utilization efficiency can be achieved. 8.94% of the predicted total fuel thermal input was removed by flue gas through the burner. This value is higher than the measured value of 5.1%. Possible reasons for this could be one or a combination of the following: heat loss in the burner, or the measurement point being on the outside of the burner. The predicted amount of heat removed by the air-cooling tubes accounts to % of total fuel thermal input. Results also indicate that 97.4% of heat transferred to the air-cooling tube was due to radiation, and.6% was due to convection. The heat absorbed by the air-cooling tubes was measured to be about 54.7% of total fuel thermal input. Therefore, the predicted and measured amounts of heat removed by air-cooling tubes were in reasonable agreement within the margin of the measurement error of 6.44%. urthermore, the predicted value of energy carried by the flue gas out the main chimney was 9.78% of the fuel thermal input, while the actual value measured was 11.5%. Again, the agreement was acceptable, falling within the measurement error limits (6.44%). The predicted heat loss through the furnace walls was found to be 9.61% of the thermal input, compared with an actual measurement of 9.49%. Thus, they were also in good agreement. Temperature ield: urnace temperatures in the HiTAC test furnace were measured at various positions along the left-hand-side wall of the test furnace (viewed from the burner). igure 1 presents a comparison of temperature predictions and measurements showing reasonable agreement, with a maximum difference of about 1 K with a range of error of.8%. These values were obtained from a stationary thermocouple located on the furnace wall. Temperature, oc Measured Prediction Z (mm) igure 1 Temperature distribution on the side wall of the furnace at x =.8 m, and y = -.3m Gas Species: igures 11 and 1 present the measured and predicted concentrations of O and CO across the furnace chamber at specific vertical distances from the burner face. The x-axis represents the vertical distance from the centreline of the burner. Details of the validation for a HiTAC test furnace equipped with one-flame HRS can be found in references 18-. rom igure 11, it can be seen that the calculated O levels are in good agreement with that measured. Both the measured and predicted curves show the same locations and magnitudes for the maximum and minimum O and CO concentrations. urthermore, the measured CO concentration agrees with that predicted. The predicted CO concentrations on, or close to, the burner centerline are lower than the measured values, however, the relative difference decreases with increasing distance from the burner. Meanwhile, the modelling underestimates the fuel consumption in the front part of furnace. rom this data, the conclusion can be drawn that the flame diffusion in the furnace is well predicted since there is good agreement of the locations and magnitudes of the combustibles, including hydrocarbon, and CO and O. It is possible to improve the efficiency of the furnace by further increasing the efficiency of the heat recovery from waste flue gases from the current value of 8% to 1%. The extra heat recovery from the flue gases can be used to preheat the fuel, which can bring benefits, such as an even greater reduction in NO emissions [13]. This is very important for combustion stability when using low and medium caloric value fuels in HiTAC technology as in these cases, the fuel volume is larger. This method features preheating of both fuel and air, and is referred to in the field as twin-preheating [1]. As such the method involving preheating of the combustion air only can be referred to as single-preheating.

12 -1- (a) O(dry%) O (%dry) O (%dry) 1 1 Measured 8 predicted Y (mm) (a) -8-4 Y (mm) Measured predicted (b) Measured predicted -8-4 Y (mm) 4 8 (c) igure 11 Predicted and measured O profiles in the furnace (a) x =, z =.3 m (b) x =, z =.6m (c) x =, z = 1. m CO (ppm) M. P Y (mm) CO (ppm) CO (ppm) CO (ppm) 5 M P -8-4 Y (mm) M. P. (b) Y (mm) (c) M P Y (mm) (d) igure 1 Predicted and measured CO profiles in the furnace (a) x =, z =.3 m, (b) x =, z =.6 m, (c) x =, z = 1. m, (d) x =, z =.15 m 6. eatures of Combustion and low of HiTAC Simulation studies were performed and the differences between the heat transfer and combustion features between a conventional high velocity turbulent jet flame and HiTAC flame were found. The influence of a heat sink on furnace heat transfer with these two types of burner systems was investigated as well. References [14, 19-] give all the details of the systems studied. Vectors of the in-furnace gas velocity for HiTAC with one-flame HRS are shown in igure 13. A cross section through the fuel and one of air inlets is presented in order to clearly show the flow characteristic of HiTAC. The combustion air was injected into the furnace with a

13 -13- velocity as high as 1 m/s. This large injection velocity leads to strong internal recirculation zones (IRZ). On the other hand, flue gas flows to the root of flame on its way out through the burner located at the root of flame as shown in figure 9(c). Recirculation allows good mixing of the combustion air with the flue gases before ignition occurs. Because of this, the HiTAC mode leads to a lower peak temperature and a larger combustion volume and consequently lower NO emissions. Thus, the HiTAC flame stability depends on the existence of strong internal recirculation zones. igure 14 Predicted temperature profile at a cross section through the fuel and one of the air inlets at HiTAC mode [K] The predicted gas temperature field uniformity is higher for the HiTAC mode, which is a known and expected advantage of the HiTAC technology. However, one should be very careful when using more uniform temperature distribution to describe HiTAC performance. This is demonstrated by the fact that the temperature near the burner zone is very low, for example, 87K for the oneburner regenerative burner system studied here. This is very much lower than the furnace temperature, thus implying that temperature distribution is not uniform. igure 13 Predicted velocity vectors at a cross section through the fuel and one of the air inlets in HiTAC mode [m/s] The highest temperature zone was found along the central axis of the furnace for both analyzed cases (igure 14). The peak temperature zone is further away from the burner face at a distance of about 1. m. The maximum gas temperature for the HiTAC mode is lower than that for the turbulent jet flame mode although the combustion air for HiTAC firing mode is preheated up to 13 K. or example, for the design operation of burner, comparing HiTAC and conventional firing modes, the maximum temperature difference is equal to 361K. This is the result of a very intensive flue gas recirculation created by internal recirculation zones in HiTAC firing mode. igure 15 shows the typical differences between the flame shapes and sizes for a HiTAC burner and a conventional high velocity turbulent jet burner. rom the figure it is very clear that the HiTAC flame spreads over a much larger volume than the conventional flame. The furnace flame occupation coefficient in the case of HiTAC is 15.8 times larger than for the conventional flame mode. The predicted maximum normalized flame length for the oneflame HRS is 1.95 at the design condition and the predicted maximum normalized flame diameter is.4. The heat release zone (chemistry reaction zone) for the HiTAC mode is significantly larger than for the conventional flame mode. By implication, the firing rate in the HiTAC flame is much smaller than that for the conventional flame mode. The maximum firing rate for the flame mode is kw/m 3, which is 48.9 times higher than for the HiTAC mode where the maximum firing rate is kw/m 3. The average flame heat release for the flame mode is also much greater than that for HiTAC mode. This proportion is consistent with the flame volume as shown above. It is a known fact that the flame volume is significantly larger in HiTAC mode for the same type of fuel and the same firing rate. Moreover, the value of the firing rate also implies that the combustion noise in HiTAC mode is much lower than in the flame mode since any approach that reduces combustion intensity within a combustion reaction may be expected to reduce the sound power produced by a flame. urther analysis can be found in references made in [14].

14 -14- range 18.7 to kw/m. The average difference of total radiation for these two firing modes is the same proportion as the net heat flux as shown above. Total radiation heat flux density along the furnace wall depends also very much on the combustion mode. or the HiTAC mode the value of the heat flux density was around kw/m. This value is similar to the total radiation heat flux on the top of the stationary heat sink. or the conventional mode it was in the range of 138 to 151 kw/m. (a) (b) igure 15 Predicted flame shape for HiTAC and Conventional burner (a) HiTAC burner (b) Conventional burner 6.3 Heat Transfer Elevation Heat transfer was evaluated in a test furnace equipped with the one-flame HRS as shown in references [14, 19, ], which include a stationary sink and a moving sink. igure 16 summarizes some results. Various heat flux densities were obtained depending on the type of charge used. The highest values were obtained for the stationary heat sink. or the HiTAC mode with a stationary sink the value of the heat flux density was on average 16.9 kw/m. or the flame mode it was 91.4 kw/m. This indicates that the heat flux density for HiTAC mode is 1.78 times that for the conventional flame mode on the same type of sink. The air cooling tubes characterize another distribution of the heat flux density. or the HiTAC mode the value of the heat flux density was in the range of 36.8 to 4.8 kw/m. or the flame mode it was in the range of 1. to 7. kw/m. A 59 % greater average heat flux density for Case was demonstrated. The total radiation heat flux density for a stationary heat sink depends very much on the combustion mode [1]. or the HiTAC mode with the stationary heat sink, the value of the radiation heat flux density was in the range 191. to 5.4 kw/m. or the conventional mode it was in the Net heat flux for sink, kw/m Distance from burner face, mm Case Case 1 Case Case 3 Case 4 igure 16 Predications of heat flux absorbed by the charge along central line on the surface of sink Case : Test furnace with a one-burner HRS without any charge or heat sink. Case 1: Test furnace with conventional turbulent jet flame without any charge or heat sink. Case : Test furnace with a one-burner HRS and with a stationary heat sink whose surface temperature is equal to O C and constant, Case 3: Test furnace with conventional turbulent jet flame with a stationary heat sink whose surface temperature is equal to O C and constant. Case 4: Test furnace with a one-burner HRS with a moving steel slab which initial surface temperature is equal to O C. 6.4 Simulation of a Moving Slab In preparation for calculations for a real industrial furnace with a moving heat sink, a moving slab is assumed in HiTAC test furnace equipped with a one-burner HRS. The slab s initial surface temperature was equal to o C. The simulation results can be found in [14]. The moving slab was assumed to be made of a low carbon steel in the form of a moving plate. The moving slab was treated as a charge and its heating was calculated for one side facing the in-furnace processes. The total heat transfer surface of the moving slab was equal to.945 m and the heating capacity of the furnace was 1.5t/h. The steel slab moves with velocity equal to.833 m/s along the furnace length beginning from the furnace inlet located below the burner.

15 -15- The distribution of the surface temperature of the moving slab heated under the one-flame HRS is shown in igure 17. The slab s surface temperature increases gradually and is fairly uniform across the furnace width. It should be noticed that the slab end temperature is 68.5 K. This is due to the limited length of the test furnace and due to the initial surface temperature, which is equal to o C. The test furnace can be treated as a short section of a real heating furnace. A A.G..G. A.G..G. A.G. A.G. A (a) (b) (c) igure 18 Two-flame HRS firing configuration for uniformity temperature profile in the furnace (a) single side firing configuration (b) stagger firing configuration (c) counter firing configuration igure 17 Predicted temperature distribution of moving slab with one-flame HRS 6.5 Effect of flame locations on the flame properties in HRS To be able to produce a uniformly heated product, the furnace itself must have a uniform temperature profile. The control of the thermal profiles in a furnace with twoflame HRS is based on the configuration of burners and different switching times. Temperature control across the furnace width is accomplished through three basic flame configurations namely: a) Single side firing Mode (firing by combining burner A and B or Burner C and D), b) Counter Mode (firing by combining burner B and D or Burner A and C) and c), Stagger Mode (firing by combining burner A and D or Burner B and C). These are investigated through simulation. igure 19 shows the temperature distributions on the horizontal plane, including the central plane, of injection ports of combustion air for different flame locations used in this study. They are Counter mode, fired by combining burner B and D, Single-side mode, fired by combining burner C and D and Stagger mode, fired by combining burner A and D. The peak temperatures of the gases are 1856, 181 and 19 K for different combustion modes of counter, singleside and stagger, respectively. The maximum flame temperature occurs in the stagger mode case. This happens because the recirculation formed by burner locations for stagger mode decreases the heat transfer from the flame. urthermore, the highest temperature zones were found to occur in the middle of the combustion chamber at the burner level, further away from the burner face. The counter mode gives the longest distance from the burner face at which the maximum temperature appears, while the staggered mode gives the shortest. (a) (b) (c) igure 19 Temperature profiles (K) at various firing locations (a) Counter mode (b) Single-side mode, (c) Stagger mode igure shows the influences of the flame locations on flame volume, which was defined in previous work [13] and is based on the flammability limit of the fuels. The details of this study can be found in [3]. Due to the difference in the burner locations, the flame shape and size

16 -16- are different. The Counter flame occupies the smallest flame volume and the Single-side flame occupies the largest fraction of the furnace volume. Consistent with this conclusion, the Counter mode has the highest local peak firing rate ( kw/m 3 ) and the highest flame combustion intensity (681.6 kw/m 3 ). The Single-side mode has the lowest local peak firing rate ( kw/m 3 ) and the lowest flame combustion intensity (349.3 kw/m 3 ). One of the possible formation mechanisms is the nitrous oxide mechanism. The emphasis of this study is to predict the NO formation by this mechanism. The high temperature zones of a flame are the principal sources of NOx, mainly consisting of NO formed by the following mechanisms: Thermal NO formation: the formation rate of thermal NO is significantly enhanced by the existence of oxygen rich pockets at high temperature within a flame. Prompt NOx route: this mechanism is enhanced in the presence of radicals in the flame front. N O route: in some circumstances, a NOx producing mechanism via formation of NO is also active. NO reburning (a) (b) Computations of NO formation rates and concentrations were done using a post-processor based on previously calculated velocities, turbulence, temperature, and chemistry fields. (c) igure lame shape for difference firing modes (a) Counter mode (b) Single-side mode (c) Stagger mode The effects of the elevated fuel temperature on the combustion and flame properties in the furnace equipped with two-flame HRS were also investigated numerically [3]. Simulations were performed for fuel temperatures ranging from ambient to 173K. It can be seen that the flame temperature decreases as the preheated fuel temperature increases, and the temperature distribution tends to more uniform. The explanation for this phenomenon is the effect of fuel injection at a higher temperature, thus a higher velocity which limits the mixing of the fuel with the combustion air in the primary combustion region. This creates a more uniform distribution of the reactants inside the flame and a larger flame volume. 7. NOx EMISSIONS 7.1 NOx models Due to the large quantities of recirculated combustion products that are entrained into the fresh reactants before combustion, the maximum temperature after the combustion reaction is consistently limited with respect to the adiabatic flame temperature of the pure reactants. Consequently, this means that the temperatures in the reaction zone are close to the temperature of reaction products, i.e. of the furnace or process temperature. Turbulent fluctuations of temperature and oxygen concentration are inherently limited and the free radical formation is also hampered. Because of a lack of higher peak temperatures (T<18 K), thermal NOx formation is suppressed and much of the NO is formed by mechanisms that are insignificant in most conventional combustors. The N O-intermediate mechanism is important in fuellean, low temperature conditions. The N O-intermediate NOx mechanism was proposed first by Malte and Pratt [4] for NO formation from molecular nitrogen (N ) via nitrous oxide (N O). The N enters combustion systems mainly with the combustion and the dilution air. Under favourable circumstances, this mechanism may contribute to as much as 9% of the NOx formed in the combustion process. There is no available mathematical model for the calculation of NO formation via N O-intermediate route. Thus a N O-intermediate NO model is developed in this work. ollowing Löffler et al [5], this kinetic pathway has been reduced to the following main reactions: N + O + M N O + M (R6) N O + O N + (R7) O O + H N N + OH (R8) N O + OH N + HO (R9) N O + O NO (R1) N O + H NO + NH (R11) Table 1: Arrhenius kinetic coefficients used for nitrous oxide mechanism reactions (Units: cal, mol, cm3, sec) Reaction K Arrhenius =AT b exp(-e a /RT) Reference A b E a 4.7e CECR[6] R6 M body collision efficiencies: H=, O=.4, Ch4=, CO=, CO=1.5, HO=6, GRI 3.[7] other species=1. R7 1.4e1 181 GRI 3.[7] R8 3.87e GRI 3.[7] R9.e1 16 GRI 3.[7] R1.9e GRI 3.[7] R e GRI 3.[7]