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Effect of Chemical Reaction Mechanisms and NOx Modeling on AirFired and Oxy-Fuel Combustion of Lignite in a 100-kW Furnace Audai Hussein Al-Abbas and Jamal Naser* Faculty of Engineering and Industrial Science, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia ABSTRACT: In the present paper, a three-dimensional numerical investigation of pulverized dry lignite was undertaken, integrating the combustion of four different scenarios adopted experimentally in a 100-kW Chalmers laboratory-scale furnace. A hybrid unstructured grid computational fluid dynamics (CFD) code was used to model and analyze: an air-fired, oxy-fuel OF25 (25 vol % O2 concentration), oxy-fuel OF27 (27 vol % O2 concentration), and oxy-fuel OF29 (29 vol % O2 concentration). The appropriate mathematical models with the related kinetics parameters were implemented to calculate the temperature distributions, species concentrations (O2, CO2, CO, H2O, and H2), NOx emission concentrations, and the radiation heat transfer. The multistep chemical reaction mechanisms were conducted on the gas phase and solid phase of coal reaction in one-, two-, and three-step reaction schemes. The predicted results showed reasonably good agreement against the measured data for all combustion cases; however, in the three-step scheme, the results were highly improved, particularly in the flame envelope zone. For the NOx calculations, the obvious differences between the air-fired and oxy-fuel (OF27 and OF29) cases were evident. In the OF27 and OF29 cases, the expected increase in the flame temperatures and CO2 and H2O concentrations led to a slight increase in the radiative heat fluxes on the furnace wall, with respect to the air-fired case. As a continuation of improvement to the oxy-fuel combustion model, this numerical investigation might probably provide important information toward future modeling of a 550-MW, large-scale, brown coal oxyfuel tangentially fired furnace.

1. INTRODUCTION In recent years, the emissions of fossil fuel combustion (particularly coal-firing) have been highly increased in the atmospheric zone. This increase of greenhouse gas (GHG) emissions leads negatively to the global climate change. Carbon dioxide (CO2) and nitric oxides (NOx) are being considered as the main contribution gases to the emission of GHGs from the energy production units. General awareness from these environmental problems, particularly in the developed countries, led to the establishment of some strict policies and legislations1 against the conventional electricity power generations. In order to avoid these emissions and to keep the continuous supply to the electricity demand online in the foreseeable future, new climate protocols have been undertaken to prevent climate changes run out, such as Kyoto Protocol.2,3 Several CO2 capture technologies have been developed for continuing use of fossil fuel sources such as precombustion capture, post-combustion capture, and oxy-fuel combustion. Oxy-fuel combustion technology has been extensively considered as one of the most competitive options among the abovementioned technologies to control and reduce several types of gaseous emissions such as CO2, NOx, and SOx from pulverized coal (PC) power plants.1,4,5 The basic principle of oxy-fuel combustion is to increase the partial pressure of CO2 in the exhaust gases in order to make its sequestration and compression processes chemically easier and more economical. This approach can be performed using a mixture of pure oxygen (produces in air separation units) with part of recycled flue gas (RFG) (mostly CO2), instead of air in the combustion chamber, in order to dilute the high combustion temperature.6 In this case of combustion, a higher concentration (partial pressure) of CO2 can be achieved in the flue gas stream; therefore, the © 2012 American Chemical Society

higher cost of its capturing processes can be avoided in the oxy-fuel combustion (challenging combustion). In oxy-fuel (O2/CO2) combustion, many changes could happen inside the furnace, in terms of flame temperature levels, species concentrations, and radiation heat transfer, with respect to the conventional air-fired combustion characteristics. These changes on the combustion characteristics are due to the higher volumetric heat capacity of CO2 relative to that of nitrogen in the air-fired case, radiative properties of gas mixture, and other gas properties such as kinetic viscosity, thermal diffusivity, and gas-phase chemistry.1,5,7 The precise investigations on the combustion characteristics of O2/CO2 furnaces are of great importance, because of the significant increase in the partial pressures of CO2 and H2O, and the noticeable decrease of NOx in the flue gas recycling. As the oxy-fuel combustion campaign was started in the past decade, several experimental and numerical studies have been conducted on the different coal types in laboratory-scale and pilot-scale furnaces. These studies could definitely provide relevant information to maintain combustion characteristics similar to those of the conventional combustion case. Thereby, the cost-effective basis can be achieved for retrofitting the equipments of existing power plant or to build a new power plant unit under oxy-fuel combustion conditions. However, the recent studies6,8−12 are concluded that the oxy-fuel combustion technology is technically feasible. It certainly represents a competitive approach and can demonstrate the potential application in a large-scale pulverized coal-fired power plant. As a result, Received: March 6, 2012 Revised: May 16, 2012 Published: May 17, 2012 3329

dx.doi.org/10.1021/ef300403a | Energy Fuels 2012, 26, 3329−3348

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the conventional boiler upstream, such as a flue gas recycle system and an air separation unit. On this basis, Zhou and Moyeda20 conducted process analysis and calculations on an 820-MW facility to compare the furnace temperature profiles for the air-fired and oxy-fuel combustion with both wet and dry recycled flue gas modes. The authors proposed several design criteria for both oxy-fuel combustion cases as follows: the same total heat fuel, the same boiler exit O2, and the same gas flow rate in order to reduce the retrofit impact on the conventional boiler performance. They showed that the flue gas recycle ratio depends on the stoichiometric (air-to-fuel) ratio and the ash content in the coal. Nikolopoulos et al.14 performed CFD simulations for a 330-MW facility tangentially fired pulverized-lignite boiler located in Greece under three different combustion conditions: air firing (reference), partial oxy-fuel, and full oxy-fuel scenarios. The Discrete Ordinates (DO) model, with the Exponential Wide Band Model (EWBW) for the absorption coefficient of the gas mixture, was used in the radiation model for accurate calculations in both air-firing and oxy-fuel combustions cases. Regarding the coal combustion, reaction of the volatile matter (VM) was modeled by means of one- and two-step reaction schemes, while char combustion was modeled using a threestep semiglobal intrinsic kinetic law mechanism. NOx formation mechanisms were modeled in terms of thermal and fuel NOx mechanisms as source terms of NO, NH3, and HCN in transport equations. The main comparison results among the combustion scenarios showed that the higher emissivity of the flue gas (H2O and CO2), in the partial and full oxy-fuel combustion environments, resulted in an increase in the radiation heat fluxes of the furnace wall, but it is still within the allowable operating limits of the boiler materials. The gaseous- and solid-phase chemistry mechanisms were presented and discussed in this paper. Multistep chemical reaction mechanisms and nitric oxide formation mechanisms were performed in four different combustion scenarios on a laboratory-scale 100-kW firing lignite unit (Chalmers furnace). The air-fired case (the reference combustion case) and three different oxy-fuel combustion cases (known as OF25, OF27, and OF29) were modeled using AVL Fire ver.2008.2 CFD code. The subroutines required for coal devolatilization, multiple reactions of coal combustion, and NOx models were written and incorporated to the CFD code. The temperature distributions and species concentrations inside the furnace were presented and validated against the measurements.21

investigation, development, innovation, and research into this approach are significantly important to provide confidence and operational experience in laboratory-scale and pilot-scale furnaces, and then gradually proceed to large-scale oxy-fuel power plants. Coal combustion is considered to be a complex process, with respect to the combustion of liquid and gaseous fuels, because it includes several complex physical and chemical processes. During the thermal decomposition of the pulverized coal particles, the volatile matter (VM) and moisture content (MC) are released, and the char remains as a solid fuel in this pyrolysis process. As a result, in the gas-phase process, several radical species can be formed in the reaction zone such as total hydrocarbons (THC), hydrogen cyanide (HCN), ammonia (NH3), N, CO, H2, etc. In order to be closer to the real reaction mechanisms and achieve an accurate reaction modeling, these intermediate species require special treatments in the combustion zone. This can be practically performed by applying multistep chemical reaction mechanisms on these aforementioned species and residual char, in order to predict the important species and the thermodynamic equilibrium temperature accurately enough. Currently, to our knowledge, very little research work has been conducted on the dry lignite oxy-fuel combustion using multistep (one-, two-, and three-step) chemical reaction mechanisms in detail for VM and residual char combustion, and global NOx formation mechanism in numerical simulation fields. However, some experimental and numerical works have been conducted in laboratory-scale and pilot-scale oxy-fuel furnaces,12,13 and very few numerical simulation studies were performed on a large-scale oxy-fuel combustion furnace.14,15 Liu et al.16 experimentally investigated the combustion of UK bituminous coal in a 20-kW down-fired test facility. Under different combustion media, in air and various O2/CO2 mixtures, their main comparisons were focusing on the gas composition, gas temperature, char burnout, and NOx and SO2 formations. The results showed that there was a noticeable decrease in the gas temperatures and char burnouts in O2/CO2 combustion cases due to the higher thermodynamics properties (specific heat) of CO2, with respect to N2 in air firing. This reduction in the gas temperatures and the char burnouts can be addressed by applying 30 vol % O2 and 70 vol % CO2 in oxycoal combustion in order to maintain the similar combustion characteristics of corresponding air-coal firing. NOx formation from the coal-nitrogen is gradually increased with the oxygen concentration in the O2/CO2 combustion cases. A 0.3 MW pilot scale of CANMET Vertical Combustor Research Facility (VCRF) in Canada was used with two different burner configurations in order to evaluate burners and combustor design concepts in both the experimental investigations and numerical simulations of O2/CO2 combustion.17−19 Tan et al.19 investigated the combustion characteristics, using three different Canadian coals: Eastern bituminous (EB), subbituminous (SB), and lignite (LN). The authors noticed that, in O2/CO2 combustion, there is a slight increase in the heat flux values inside the furnace with the 35 vol % O2 concentration and 65 vol % CO2, compared to the air-firing (reference) case. Meanwhile, with 28 vol % O2 concentration and 72 vol % CO2, the heat flux values were slightly lower than the reference combustion. A good understanding of oxy-fuel combustion on the thermal boiler performance can increase the potential for CO2 capture and provide key parameters to the necessary modifications of

2. FACILITY DESCRIPTION OF COMBUSTION SCENARIOS Four different combustion scenarios have been conducted on a 100-kW laboratory-scale oxy-fuel facility in Göteborg, Sweden.21 The air-fired combustion case (21 vol % O2/79 vol % N2) was used as a reference case to three oxy-fuel combustion cases: OF25 (25 vol % O2 and 72 vol % CO2), OF27 (27 vol % O2 and 71 vol % CO2), and OF29 (29 vol % O2 and 69 vol % CO2). The furnace is a drop-tube cylindrical refractory-lined facility, the dimensions of which are 80 cm in diameter and 240 cm in height. The furnace was designed to be suitable for both gas and pulverized-coal combustion. Regarding the burner configuration, the fuel (lignite) was injected in the central lance (34 mm diameter), while the primary (52 mm diameter) and secondary (92 mm diameter) swirl registers were used to feed oxidizer gases (air or RFG). The burner exit plane was lined with the combustor ceiling to improve the flame temperature 3330


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Figure 1. Grid system of computational domain at the near-burner region.

level. In all oxy-fuel cases, the composition of the carrier gas was 30 vol % O2 and 68 vol % CO2 under dry basis. The primary and secondary registers were equipped with fin angles of 45° and 15°, respectively, in order to improve the flame stability inside the furnace. For the oxy-fuel combustion scenarios, there was a gradual reduction in the volumetric flow rates of feed oxidizer gases, compared to the reference combustion scenario, by ratios of 17%, 23%, and 28% for the OF25, OF27, and OF29, respectively. More-detailed information about the experimental setup and operating conditions can be found in the preliminary studies.7,22

Table 1. Inlet Flow Field Parameters Used for Primary and Secondary Registers of the Burner in All Combustion Cases Combustion Cases inlet flow field parameter volume flow rate (m3/h) mean velocity (m/s) angular velocity (rad/s) volume flow rate (m3/h) mean velocity (m/s) angular velocity (rad/s)

3. NUMERICAL DESCRIPTION AND METHODOLOGY 3.1. Description of Numerical Models. A commercial computational fluid dynamics (CFD) code, AVL Fire ver.2008.2,23 was used in this numerical simulation. Appropriate subroutines were written and coupled with the CFD code to account for devolatilization and char burnout reactions, mass heat-transfer processes between particles and gases, and NOx modeling in term of thermal and fuel NO formation. For the homogeneous phase, Eulerian partial differential conservation equations (PDEs) are solved for mass, momentum, turbulence kinetic energy, turbulent dissipation rate, enthalpy, and a number of species mass fractions. Several submodels are coupled via the source terms of PDEs to calculate the heat transfer (convection and radiation), turbulence, coal particle temperatures and trajectories, coal devolatilization, and char combustion models. The general form of the non-steady-state Eulerian transport equation used is given by24 ∂ ∂ ∂ ⎛ ∂Φ ⎞ (ρΦ) + (ρUi Φ) = ⎜Γ ⎟ + SΦ + SpΦ ∂t ∂xi ∂xi ⎝ ∂xi ⎠

air

OF25

Primary Register 34.87 28.94 7.966 6.612 433.293 359.615 Secondary Register 81.37 67.54 4.995 4.146 41.14 34.152

OF27

OF29

26.85 6.134 333.645

25.11 5.737 312.052

62.65 3.845 31.673

58.59 3.596 29.608

boiler and addressed the effect of flow rates, through the primary and secondary registers, on the overall flow aerodynamics. The pulverized coal (lignite) particles are modeled using the Discrete Droplet Method (DDM).27 Particle trajectories of the lignite particles are assumed to have a spherical shape in which each group of identical noninteracting particles (termed parcels) and tracked through the computational grid used. The differential equation of motion for a lignite particle is given by mp

duid ⃗ = Fidr⃗ + Fig⃗ + Freact dt

Fidr⃗ =

(1)

Additional sources for k and ε are used to calculate the effect of particle-to-gas turbulence flow. The boundary conditions of the computational domain used in the present furnace have been divided into three main boundaries: the inlet, the wall, and the outlet. The fuel inlet and feed oxidizer gases are located in the central position of the top wall of the furnace (see Figure 1). The inlet flow field parameters at the primary and secondary registers of the burner, for all the combustion cases, are summarized in Table (1). The standard k−ε turbulent model is used, together with a near-wall treatment (wall function). Hart et al.25 and Ahmed et al.26 also investigated the cold flow in a small-scale furnace of a utility

⎛1⎞ ⎜ ⎟ρ A C | u | u ⎝ 2 ⎠ g p D rel rel

(2)

(3)

Fig⃗ = Vp(ρp − ρg )gi

(4)

⎛ −mvp ⎞ ⃗ = Vp⎜ ⎟ Freact ⎝ dt ⎠

(5)

The right side of eq 2 illustrates the magnitudes of all external forces (the drag force, the gravitational force, and the reaction force in eqs 3−5, respectively) that are acting on the particle phase. The diameter and density of solid particles decrease after each iteration of the calculation. In this heterogeneous phase, the direction of reaction force is predicted using the stochastic tracking means. Regarding the combustion of dry coal particles, there are two complex reaction processes: devolatilization of the dry coal particle and oxidation of the residual char. The reaction model of Badzioch and Hawksley28 is implemented to simulate the 3331


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Weighted Sum of Gray gases Model (WSGGM),36 which is given in the following equation:

devolatilization of the pulverized lignite particles. The production rate of volatile is given by the first-order reaction

dV = K v(Vf − V ) dt

I+1

ε(T , xi) =

(6)

i=1

The production rate constant (Kv) of volatile matter is given by the Arrhenius form

where Av and Ev are the pre-exponential factor and the activation energy constants, respectively (Av = 0.2 × 105 s−1 and Ev = 5941 J kmol−1 K−1).29 In the heterogeneous reaction, the oxidation rate of the char is governed by the diffusion of oxygen to the external surface of the particle. This is considered to be a more-suitable reactive model, relative to other char oxidation models,7 especially for lignite that has high porosity and surface area.30 The diffusion rate of oxygen is given by the following expression: Kd(Pg − Ps), where Pg and Ps are the oxygen partial pressures in the bulk phase of the furnace and at the external surface of the particle, respectively. Kd is expressed as follows:31 0.75 2.53 × 10−7 ⎛ Tp + Tg ⎞ PA ⎟ ⎜ Rp ⎝ 2 ⎠ P

(7)

Q c = πDpλNu(Tg − Tp)

The char oxidation rate per unit area of the particle surface is described by KcPs. The kinetic rate is given by the following Arrhenius expression: ⎛ E ⎞ Kc = Ac exp⎜⎜ − c ⎟⎟ ⎝ Tp ⎠

·

⎛ Cprypr̅ ⎞ y̅ ⎟⎟ ρ ̅ min⎜⎜yfu̅ , Ox , S 1 + S⎠ τR ⎝

Q r = εσπDp2(T g4 − T p4)

(8)

(13)

where ε represents the particle emissivity and the recommended value from refs 38−40 (which is 0.7) is used in this study. The temperature distribution within particles is neglected in this calculation in order to simplify the problem. Based on this thermal assumption, the Biot number (Bi) is assumed to be small (Bi < 1). It is a dimensionless number and can be defined as a ratio of the convective heat transfer and characteristic length divided by the thermal conductivity of the coal particle. This means that the heat conduction within particles is much faster than the heat convection away from particle’s surface. Therefore, the uniform temperature distribution is assumed, and temperature gradients are neglected inside of particles in this simulation. More details about the abovementioned numerical models can be found in the previous similar simulation study.7 3.2. Chemistry Reaction Mechanisms. In this section, eight speciesfuel (volatile and char), O2, N2, CO2, CO, H2, and H2Owere considered and taken into account in one-, two-, and three-step chemical reaction schemes. Although, there were some good predictions, in terms of temperature distributions and species concentrations in a one-step reaction, some discrepancies were visible in those numerical results, relative to the experimental data.7 As described previously, those discrepancies in the predicted results might have been caused by the usage of a single-step chemical reaction. Therefore, it is appropriate to consider these species, especially in the flame envelope zone (hottest region in the furnace), in order to predict the important intermediate species such as CO and H2, and the thermodynamic equilibrium temperature accurately enough. In addition, the one-step chemical mechanism used to

Cfu

(9)

where Cfu and Cpr are empirical coefficients, and the correct values for these coefficients can be determined based on the fuel and the turbulent flow field. Therefore, in this study, the values of Cfu and Cpr were kept at 3.0 and 0.5, respectively, for both the air-fired case and for the oxy-fuel combustion cases. Regarding the heat-transfer models, the discrete transfer radiation method (DTRM) has been used, because of its ability for better prediction in participating media, especially in furnaces.35 Generally, the total radiation intensities at intersections of a ray with control volume faces can be computed over the computational domain as follows: i′n + 1 = i′n (1 − ε(T , xi)) + i′b (T )ε(T , xi)

(12)

However, the radiative heat-transfer equation is used to calculate the heat transfer between the particles and gases, as follows:

where Ac and Ec are the pre-exponential factor and the activation energy, respectively (Ac = 497 kg m−2 s−1 ((497 N m−2)−1) and Ec = 8540 J kmol−1 K−1, which are recommended in ref 32 for low-rank coal). The Eddy Breakup (EBU) Turbulence combustion model is used for these combustion simulations. The EBU model was first proposed by Spalding33 and later modified by Magnussen and Hjertager.34 The equation of the mean reaction rate of fuel is given by ρr fu =

(11)

where αε,i(T) is the emissivity weighting factor for the ith gray gas, s the path length, ai the absorption coefficient of the gas mixture, and p the sum of the partial pressures (mass concentrations) of absorbing species such as H2O and CO2. The WSGGM is one of the possible ways to solve eq 10. It is based on the presumption that the total emissivity of the gaseous composition may be represented by the sum of gray gas emissivity weighted by temperature-dependent factors. The absorbing model selected (WSGGM), in this study, is of great importance in the gas radiation calculation, particularly in oxyfuel combustion scenarios, because of the higher increase of the species concentrations (H2O and CO2) in the flue gas.37 The number of gray gases used in this absorbing model was three, and third-order polynomials of weighting factors were used. Regarding the mass heat transfer, the rate of change of temperature between the particles and gases inside the furnace is conducted using the convection and radiation heat transfer processes. The convective heat transfer between the particles and gases is given by the following expression:

⎛ E ⎞ K v = A v exp⎜⎜ − v ⎟⎟ ⎝ Tp ⎠

Kd =

∑ αε ,i(T ){1 − exp[−(aip + BTc)s]}

(10)

where ib is the directional blackbody emissivity in the control volume (ib = σTg4/π), and ε(T,xi) represents the emissivity and depends on the local temperature and gas mixture. Thus, the gas mixture emissivity (ε) can be represented through the 3332


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have been widely used by many numerical simulation references in the literature.43−46 3.2.3. Three-Step Chemical Reaction. As previously stated, the flame envelope zone is considered the higher-temperature region, compared to other locations inside the furnace. As a result, the dissociation of the main gaseous product (water vapor and carbon dioxide) into many radical species (carbon monoxide and hydrogen) in the flame region should be taken into account during the numerical modeling of both the homogeneous and heterogeneous reactions of the pulverized coal particles. Three-step reaction mechanism has the ability to predict these species (CO and H2) well, in addition to the main dominant species. Therefore, the hydrocarbon fuel (methane) and residual char (Cchar), in this reaction mechanism, are presented in a three-step chemical reaction, in order to explain the dissociation processes accurately, as illustrated in the following chemical equations. The chemical equation for the devolatilized methane burned with oxygen can be expressed by these three chemical reactions:

describe the fuel conversion cannot capture the CO2/CO ratio and the equilibrium between H2 and H2O. Moreover, regarding the char burnout, these multistep chemical reaction mechanisms can provide good information to clarify the CO/CO2 production rate with the temperature and oxygen partial pressure.14,41 3.2.1. One-Step Chemical Reaction. The one-step irreversible reaction of solid fuel (lignite) with air or O2/CO2 was considered in term of a hydrocarbon fuel (CnHm) and residual char (Cchar) for all the combustion cases. For coal combustion, there are two main reaction processes. The first process is homogeneous reactions of volatile matter (VM). In this reaction, methane (CH4) is considered as a sample of the hydrocarbon fuel, because no crucial differences could be distinguished between methane and other gases that are released during the thermal decomposition process of coal particles.42 The second process is the heterogeneous reaction of residual char particles. During the oxy-fuel combustion, the following chemical equations were considered as shown below. The chemical equation for the devolatilized methane burned with oxygen (homogeneous phase) can be expressed as follows: CH4 + 2O2 → CO2 + 2H 2O + heat

(14)

The chemical equation for char burned with oxygen (heterogeneous phase) can be expressed as follows: Cchar + O2 → CO2 + heat

CO +

3 O2 → CO + 2H 2O + heat 2

1 O2 ⇆ CO2 2

(15)

(16)

(17)

CO +

1 O2 → CO + heat 2 1 O2 ⇆ CO2 2

CO + H 2O ⇆ CO2 + H 2

(21)

O2 + 2H 2 ⇆ 2H 2O

(22)

Cchar +

The heats of reaction of eqs 16 and 17 are 307 880 and −1 110 190 kJ/kmol, respectively. However, the chemical equation for char burned with oxygen can be expressed by these two chemical reactions: Cchar +

(20)

The heats of reaction of eqs 20, 21, and 22 are 549 710, −868 360, and −483 660 kJ/kmol, respectively. Regarding the char burnout, because of the increasing carbon dioxide (CO2) concentration (partial pressure) in the flue gas, particularly in oxy-fuel combustion cases, the effect of the Boudouard reaction is of great importance under conditions that promote oxy-fuel combustion. Therefore, further char reaction with CO2 should be taken into consideration in the numerical model, as shown in eq 24 (that represents Boudouard reaction, ΔH = 172 464 kJ/kmol). In contrast, eq 23 is only relevant for the air-fired case. Although the gasification process of coal particles has lower importance on the char reaction (i.e., heat of combustion, ΔH = 131 298 kJ/kmol) for both the air-fired and oxy-fuel combustion cases, eq 25 is also presented and taken into consideration in order to increase the certainty of accuracy of the present simulation. Detailed information of the final char reaction (eq 25) can be found in the recent numerical simulation work.47 The chemical equation for char burned with oxygen can be expressed by these three heterogeneous chemical reactions:

The heats of combustion (ΔH) of the methane and residual char are considered and, based on the combustion model, are equal to −802310 and −393520 kJ/kmol, respectively. 3.2.2. Two-Step Chemical Reaction. Because of the limitation of single-step reaction mechanism on the important intermediate species (CO) and flame temperature, a twostep reaction mechanism can provide partial information on the carbon monoxide (CO) in the hydrocarbon reaction and the char burn-out schemes, which are based on the following reactions. The chemical equation for the devolatilized methane burned with oxygen can be expressed by these two chemical reactions: CH4 +

CH4 + O2 → CO + H 2 + H 2O + heat

1 O2 → CO + heat 2

(23)

Cchar + CO2 → 2CO

(24)

Cchar + H 2O → CO + H 2

(25)

The heats of combustion (ΔH) of the above-mentioned species are taken into account and incorporated as a source term in the enthalpy equation for all combustion cases as separate subroutines. Detailed numerical information regarding the multiple reaction mechanism can also be found in the recent simulation study of Al-Abbas and Naser.22 3.3. NOx Modeling. Generally, the formation of nitric oxides (NOx) can be simulated via three principal sources: thermal NO, which is formed by the dissociation of the molecular air-nitrogen; prompt NO, which is formed from the attack of hydrocarbon fragments on the air-nitrogen; and fuel NO,

(18)

(19)

From the interesting point of investigation, the reverse chemical reactions of eqs 17 and 19 have a significant effect on the equilibrium concentration of CO and the heat release intensity in the flame envelope, which considered the higher temperature level inside the furnace. These aforementioned chemical reactions 3333


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⎛ 3.53362425 × 106 ⎞ K1b = 3.8 × 1010 exp⎜ − ⎟ RT ⎝ ⎠

which is formed from nitrogen-containing components in the coal for both the volatile matter and residual char. In the present study, the only main sources of NOx formation (thermal NO and fuel NO) were used, while the formation of prompt NO was ignored, because it is only important at low temperature (below 1000 K), and in fuel-rich mixtures.48 For a realistic turbulent combustion model of NOx formation in a reactive system, a simplified approach of the chemical kinetics is more appropriate than other approaches (i.e., such as detailed chemistry and advanced engineering correlations), because of the complexity of the interacting processes, such as turbulence, heterogeneous reactions, heat transfer, etc. Therefore, this simplified approach used in the model of NOx formation must be considered for complex combustion applications, particularly furnaces.49 The NOx model involved solving three extra transport equations for the mass fraction of NO, HCN, and NH3 as source term values (kg/(m3 s)) in eq 1 as SNO, SHCN, and SNH3 respectively. Regarding thermal NO formation, the extended Zeldovich mechanism is used as follows: N2 + O ⇆ NO + N

(26)

N + O2 ⇆ NO + O

(27)

N + OH ⇆ NO + H

(28)

⎛ 173.1060162 × 106 ⎞ K 2b = 3.81 × 106 exp⎜ − ⎟ RT ⎝ ⎠ ⎛ 204.2019096 × 106 ⎞ K3b = 1.7 × 1011 exp⎜ − ⎟ RT ⎝ ⎠

In the fuel-lean zones, a quasi-steady state consumption and formation of free nitrogen atoms can be established, particularly when there is an adequate amount of oxygen in the combustion zones. Therefore, the overall NO formation rate for the three above-mentioned equations (eqs 26−28) can be written as follows: dc NO dt

dc NO = K1f cOc N2 + K 2f c NcO2 + K3f c NcOH − K1bc NOc N dt

⎛ 27123 ⎞ ⎟ cO = 36.64T 0.5cO0.52 exp⎜ ⎝ T ⎠

(29)

where c is the concentration of the species in mol/m3; K1f, K2f, K3f are the kinetic rate constants for the forward reactions; andK1b, K2b, K3b are the kinetic rate constants for the backward (reverse) reactions. The expressions for the kinetic rate constants (m3/(kmol s)) used in the thermal NO formation are given as follows, according to Hanson and Salimian:50

(31)

(32)

where all species concentrations are given in units of mol m−3 and temperature (T) is given in Kelvin. The fuel nitrogen (nitrogen-containing compounds) in both the VM and char can be contributed to the total NOx formed during the reaction pathways. Although the route leading to the formation/destruction of fuel NOx is still under investigation, the conversion of the fuel nitrogen to NO is highly dependent on two main concepts: the initial concentration of nitrogencontaining compounds and the local combustion characteristics. Generally, during the devolatilization process of the pulverized coal particles, several secondary intermediate nitrogen species are released into the gas phase, such as hydrogen cyanide (HCN), ammonia (NH3), nitrogen atom (N), etc. The first two species (HCN and NH3) are commonly considered the most dominant species from the thermal decomposition of the coal particles in the reaction zone. Therefore, the global simplified fuel NO formation/destruction

⎛ 319.0239117 × 106 ⎞ K1f = 1.8 × 1011 exp⎜ − ⎟ RT ⎝ ⎠ ⎛ 38.9114388 × 106 ⎞ K 2f = 1.8 × 107 exp⎜ − ⎟ RT ⎝ ⎠ ⎛ 173.1060162 × 106 ⎞ K3f = 7.1 × 1010 exp⎜ − ⎟ RT ⎝ ⎠

2 ⎡ ⎛ K1bK 2bc NO ⎞ ⎤ ⎢ 1 − ⎜⎝ K1f c N K 2f cO ⎟⎠ ⎥ 2 2 ⎥ = 2K1f cOc N2⎢ ⎛ K1bc NO ⎞ ⎥ ⎢ ⎜ ⎟ + 1 ⎢⎣ ⎝ K 2f cO2 + K3f cOH ⎠ ⎥⎦

It could be seen that, in eq 31, the thermal NO formation is completely independent of the coal as a fuel, but it is strongly dependent on the concentration of the oxygen atoms (oxygen radicals). Therefore, precise calculation of the concentration of oxygen atoms is very important, particularly with the concentration increasing in the combustion zones. All species concentrations in eq 31, except concentrations of the oxygen atoms, are obtained from the combustion simulation. However, the overall NO formation rate mechanism is slow, compared to the fuel oxidation processes;51 as a result, the thermal NO is formed after completion of combustion process. Therefore, the thermal NO formation mechanism can be decoupled from the fuel oxidation processes and the concentrations of oxygen atoms can be calculated by either the equilibrium approach or the partial equilibrium approach. The partial equilibrium approach is used in this simulation study according to the recommended literature.52,53 However, it was noticed that, in turbulent diffusion flames, the concentrations of oxygen radicals were not in equilibrium with the oxygen molecules. The partial equilibrium oxygen atom concentration can be given by the following expression:

The thermal NO reaction mechanism is defined based on the chemical equilibrium assumption, which means that only atomic nitrogen (N) is used as an extra intermediate species. Equations 26 and 27 were first proposed by Zeldovich and extended later to include eq 28 in order to take into account the hydrogen radical’s effect on the formation of nitric oxide during the oxidation processes under near-stoichiometric conditions. This reaction mechanism is called the extended Zeldovich mechanism, which is strongly dependent on the flame temperature. The net rate of NO formation via eqs 26−28 can be given as follows:

− K 2bc NOcO − K3bc NOc H

(30b)

(30a) 3334


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where R1−4 are the conversion rates of eqs 33−36 (in units of s−1), X is the mole fraction of the aforementioned species (in mol m−3), R5 is the coefficient rate for the NO/char reaction (in mol s−1), AE is the external surface area of the char (AE ≈ 20.3 m2/g-mol in this study), PNO is the bulk pressure of nitric oxide in the atmosphere, and a is the oxygen reaction order that is derived based on oxygen mole fraction in the flame, as given in the following mathematical expression:

pathways are presented and taken into consideration in this numerical calculation, as shown in the schematic diagram of Figure 2.

⎧1.0 ⎪ ⎪ ⎪−3.95 − 0.9 ln XO2 a=⎨ ⎪−0.35 − 0.1 ln X O2 ⎪ ⎪0 ⎩

XO2 ≤ 4.1 × 10−3 4.1 × 10−3 ≤ XO2 ≤ 1.11 × 10−2 1.11 × 10−2 < XO2 < 0.03 XO2 ≥ 0.03

(39) Figure 2. Schematic diagram of the formation/destruction of fuel NOx.

The source terms of NO, HCN, and NH3 must be incorporated in the transport equation for predicting the formation of fuel NO, as given in the following expressions:

As stated earlier, the routes leading to fuel NO formation/ destruction are under development, and they are still not completely understood, because of the complexity of reaction, in real-life reaction, that might take place through several chemical intermediate routes. However, the following five simplified models of oxidation and reduction processes (R1−R5) can sufficiently simulate the NOx model, which is used in many similar calculation efforts:14,48,54 HCN + 1.25O2 → NO + CO + 0.5H 2O

(33)

HCN + 1.5NO → 1.25N2 + CO + 0.5H 2O

(34)

NH3 + 1.25O2 → NO + 1.5H 2O

(35)

NH3 + 1.5NO → 1.5N2 + 1.5H 2O

(36)

Cchar + NO → 0.5N2 + CO

(37)

SHCN = SHCN,P + SHCN,1b + SHCN,2b S NH3 = S NH3,P + S NH3,1b + S NH3,2b S NO = Schar,NO + S NO,1 + S NO,2 + S NO,3

Based on the literature of the NOx formation mechanism,14,48,57 as for the gas−solid reaction, coal-nitrogen is supposed to be equally divided by the VM and the char, and its amount is usually obtained from the proximate analysis of the coal. The nitrogen bound of the char is directly converted to NO, while a fraction of nitrogen in the VM is commonly released in a mixture of NH3 and HCN with a NH 3 /HCN ratio (α) of 9:1. This ratio (α) of the intermediate nitrogen compounds released in the devolatilization stage is only applicable for low-rank coals (lignite in this study). This means that the production level of NH3 is 9 times higher than that of HCN, which could provide better numerical results of NOx formation. Therefore, the abovementioned ratio is presented and taken into account in the source terms of production rates of HCN and NH3, as shown below:

As for the above reaction mechanisms, eqs 33−36 represent the oxidation and reduction processes and the depletion rates of HCN and NH3 are given below, according to De Soete.55 Equation 37 represents additional heterogeneous reduction of NO to N2 due to reacting between the char particles and the nitric oxide and its reaction rate is approximately equivalent to the carbon consumption rate that is given by Levy et al.:56

SHCN,P =

⎛ 33730.829 ⎞ ⎟ R1 = 1.0 × 1010XHCNXOa 2 exp⎜ − ⎝ ⎠ T

S NH3,P =

⎛ 30206.713 ⎞ ⎟ R 2 = 3.0 × 1012XHCNXNO exp⎜ − ⎝ ⎠ T

(1 − α)SvolYN,fuelMHCN MNΔV αSvolYN,fuelM NH3 MNΔV

+

+

ScharYN,fuelMHCN MNΔV

ScharYN,fuelM NH3 MNΔV (41)

⎛ 16110.247 ⎞ ⎟ R3 = 4.0 × 106X NH3XOa 2 exp⎜ − ⎝ ⎠ T

where SHCN,P and SNH3,P are the sources of HCN and NH3 from the thermal decomposition of the coal (in units of kg/(m3 s)), Svol and Schar are the rates of coal devolatilization and char burnout (in units of kg/s), YN,fuel is the mass fraction of nitrogen in the coal (see Table 2), MHCN, MNH3, and MN present the molecular weight of species HCN, NH3, and N respectively, and ΔV is the control volume (in units of m3).

⎛ 13593.021 ⎞ ⎟ R 4 = 1.8 × 108X NH3XNO exp⎜ − ⎝ ⎠ T ⎛ 34.70 ⎞ ⎟A P R 5 = 4.8 ×104 exp⎜ − ⎝ RT ⎠ E NO

(40)

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Table 2. Proximate and Ultimate Analysis of the Lignite Coal Particle Ultimate Analysis (wt %, d.a.f.a)

Proximate Analysis (wt %, as received)

a

ash

combustibles

moisture fraction

volatile matter, VM (%, d.a.f.a)

C

H

O

N

S

high heating value, HHV (MJ/kg)

5.0

84.8

10.2

59.4

69.9

5.4

23.1

0.6

1.0

20.9

Dry ash-free basis.

NH3 in eqs 35 and 36, P is the pressure, and T̅ represents the mean temperature. The latter (T̅ ) is used under the turbulent combustion conditions due to the combination of NOx model with the EBU combustion model in this study. Finally, as shown in eq 40 the source term of NO can be accurately calculated via four different terms and can be written as follows:

For the mass consumption rates of HCN and NH3, the following expressions are used in this simulation: MHCNP RT̅ M P SHCN,2b = −R 2 HCN RT̅ M NH3P S NH3,1b = −R3 RT̅ M NH3P S NH3,2b = −R 4 RT̅ SHCN,1b = −R1

Schar,NO =

ScharYN,fuelMNO MNΔV

MNOP M P − R 2 NO RT̅ RT̅ MNOP MNOP = R3 − R4 RT̅ RT̅

S NO,1 = R1 (42)

S NO,2

where SHCN,1b and SHCN,2b are the consumption rates of HCN in eqs 33 and 34, SNH3,1b and SNH3,2b are the consumption rates of

where Schar,NO is the rate of production of NO from the char reaction, SNO,1 and SNO,2 present the production and destruction rates of NO via eqs 33 and 34 for HCN and via eqs 35 and 36 for NH3, respectively. The fourth term of NO (SNO,3) in eq 40 is the reduction rate of NO due to its reaction with char particles, as explained in eq 37. 3.4. Numerical Description of the Furnace. A commercial CFD code (AVL Fire ver. 2008.2) has been used to

Table 3. Locations of Port Number in the Furnace port number

distance from burner exit (mm)

1 2 3 4

215 384 553 800

(43)

Figure 3. Flame temperature distributions (K) on the vertical cut along the furnace axis and at the horizontal cuts through four different locations (ports 1, 2, 3, and 4) from the burner exit for the air-fired, oxy-fuel (OF25), oxy-fuel (OF27), and oxy-fuel (OF29) combustion scenarios. 3336


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Figure 4. Temperature distribution profiles (in Kelvin) at port 1 of the furnace in one-, two-, and three-step chemical reactions for the (a) air-fired, (b) oxy-fuel (OF25), (c) oxy-fuel (OF27), and (d) oxy-fuel (OF29) combustion scenarios.

simulate the combustion, gas flow, particle flow, heat transfer, and mass transfer using the finite-volume technique in an unstructured grid system, as a first step of calculation. A SIMPLE algorithm was used to calculate the combination between the velocity and the pressure. For the convergence criterion, the numerical calculations are repeated for all variables in each cell until the accepted convergence limit of 1500 K in this study) and oxygen concentration.41,58 Hence, in the threestep numerical model, most CO2 concentrations are supposed to be coming from the gas-phase reactions (homogeneous reaction). At port 4 of the furnace, Figures 9a−d shows further investigation on the carbon dioxide mass fraction (kg/kg) profiles for the air-fired, OF25, OF27, and OF29 combustion scenarios, respectively, in one-, two-, and three-step reactions schemes. It can be seen that the numerical result compared well with the experimental data for the two- and three-step numerical models in all combustion scenarios, but the one-step model shows overprediction due to ignoring the thermal dissociation as previously mentioned. It is interesting to note that the Boudouard reaction used in the three-heterogeneous-chemistry model (eq 24) was improved the three-step numerical results, especially in oxy-fuel combustion cases. On this basis, the threestep chemistry mechanism model has been used to visualize the carbon dioxide concentration, as already shown in Figure 7. The distributions of carbon monoxide (CO) concentrations are presented in Figure 10 on a vertical plane through the

sides), respectively. As seen, for the air-fired and OF25 cases, the distributions of CO concentrations showed some similarities in the near-burner region. This similar trend of distribution between the two aforementioned combustion cases is definitely caused due to the similar distributions of the flame temperatures in the upper part of the furnace, as already shown in Figure 3. In contrast, OF27 and OF29 cases show higher CO concentration compared to the reference firing and OF25 cases, particularly near the furnace centerline. This increase in CO concentrations might be due to an increase in the O2 concentration in the feed oxidizer gases and the residence time, in OF27 and OF29 cases, which also leads to increase the combustion temperature. The higher flame temperature generally leads to enhancement of the chemical dissociation of main species in the flame envelope zone, and therefore increases the intermediate species concentrations, as recently investigated by Kim et al.59 They predicted the structure of natural gas flames in both air-firing and oxy-fuel combustion scenarios by coupling a conditional moment closure reactive model with the flow solver. The detailed chemistry model was used to precisely calculate the intermediate species formed due to thermal dissociation at higher temperature. Figures 11a−d show the carbon monoxide (CO) mass fraction (kg/kg) profiles at port 1 of the furnace for the airfired, OF25, OF27, and OF29 combustion scenarios, respectively, in two- and three-step chemistry mechanisms. For all of the combustion cases, a good agreement is achieved between the physical data and the predicted results for both the two- and three-step reaction schemes. As already stated in sections 3.2.2 and 3.2.3, eqs 17, 19, and 21 strongly show the clear connection between the CO concentration and CO2 in the reverse reactions. This means that the two- and three-step chemistry models used to describe the coal conversion can highly capture the equilibrium between CO and CO2 in the flame zone. A slight overprediction, in Figure 11d, is observed close to the furnace centerline, which represents the rich-fuel mixture region in the furnace. This discrepancy might be caused due to an increase in the CO/CO2 production rate in the OF29 case. This combustion case has the higher residence time for burning coal particles, compared to the other combustion cases (see Table 1). As a result, the thermal dissociation of the main species is largely enhanced in that region of the furnace to produce a high amount of CO. In addition, the higher O2 concentration in OF29 case could also have a clear contribution to this increase in CO concentration. Figures 12a and 12b present the water vapor mass fraction (kg/kg) profiles at port 4 of the furnace for the air-fired and oxy-fuel (OF29) combustion scenarios, respectively, in oneand three-step chemistry models. The numerical results obtained with the three-step model are more similar to the experimental data for both combustion cases, while the onestep result showed overprediction. The improved predicted results for the three-step model are due to the use of additional chemistry reactions in both combustion phases. In the homogeneous gas-phase reaction, two reverse reactions (eqs 21 and 22) participated to improve the balance between H2O and H2 in the near-burner region for both the air-fired and OF29 cases. In addition, compared to the one-step reaction results, a clear reduction in the water vapor is achieved in the three-step model, because of the reaction of available H2O with the residual char particles, as displayed in the gasification process (eq 25) of the heterogeneous solid-phase reaction. To elaborate improvements in the main and intermediate species in the

Figure 10. Carbon monoxide concentration (kg/kg, dry) on a vertical plane through the furnace axis for the reference air-fired case (left-hand sides) and oxy-fuel combustion environments OF25, OF27, and OF29 (right-hand sides) respectively; all dimensions are given in millimeters.

furnace axis for the air-fired case (left-hand sides) and oxy-fuel combustion scenarios OF25, OF27, and OF29 (right-hand 3342


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Figure 11. Carbon monoxide mass fraction (kg/kg) profiles at port 1 of the furnace for the (a) air-fired, (b) oxy-fuel (OF25), (c) oxy-fuel (OF27), and (d) oxy-fuel (OF29) combustion scenarios, respectively in two- and three-step chemical reactions.

exit plane. The highest predicted values of NOx concentration inside the furnace were 455, 374, and 421 ppm for the air-fired, OF27, and OF29 combustion cases, respectively. As seen in Figure 14, the distribution of the NOx concentration inside the furnace was noticeably different between the air-fired and oxyfuel (OF27 and OF29) cases. The NOx emissions distribution of OF25 combustion case was not presented, because it is approximately similar to the reference case. For the air-fired case, the concentration of NOx emissions started at ∼400 mm in the axial distance from the burner exit plane. In contrary to the air-fired case, the concentrations of NOx emissions for the OF27 and OF29 cases were initiated further downstream, in which the predicted emissions of NOx were started at ∼800 mm and 900 mm from the burner exit plane for the OF27 and OF29 cases, respectively. In addition, there is another difference in the NOx distribution between the reference case and oxy-fuel combustion cases. In the air-fired case, a very high concentration of NOx along the furnace centerline, especially in the region close to the near-burner region, was observed. This higher value of nitric oxides may be caused by the activation of a thermal mechanism of NO formation, which is completely dependent on the flame temperature and oxygen concentration.60,61 In fact, the formation of thermal NO is strongly dependent on the temperature distributions and O2 concentration (see Figure 3).

three-step chemistry mechanism, hydrogen concentration (kg/kg) profiles at port 1 of the furnace for the air-fired, OF25, OF27, and OF29 combustion scenarios are presented in Figure 13. It can be observed that the H2 concentration is higher in the fuel-rich mixture for all the combustion cases. As expected, there is an increase in the H2 concentration for the OF27 and OF29 cases, but the air-fired and OF25 cases showed lower production levels of H2, and the two latter cases have approximately similar distribution trends. This result has been given a clear explanation to the connection between the O2 concentration and the production rates of intermediate species, especially the related H2 and CO formation, as previously illustrated in the hot flame zone. Although the numerical results of these species are in reasonable qualitative and quantitative agreement with the experimental data, further chemistry reaction schemes are required to describe the thermal pyrolysis of coal particles and the related intermediate species more precisely. 4.3. NOx Concentrations. The NOx emissions distribution is presented on a vertical cut through the furnace axis for the air-fired, OF27, and OF29 combustion cases, as shown in Figure 14. The images of Figure 14 show the full width of the furnace (i.e., from −400 mm to +400 mm), while the height is limited to the axial distance: 0.0 to 1200 mm from the burner 3343


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Figure 14. NOx emissions (ppm) on a vertical cut through the upper part of the furnace axis for the air-fired, oxy-fuel (OF27), and oxy-fuel (OF29) combustion scenarios; all dimensions are given in millimeters.

This is due to ignorance of the thermal NO, i.e., there is no atomic nitrogen (N) entering the furnace except that from the air leakage. The above-mentioned region showed low O2 concentrations in the oxy-fuel combustion scenarios, as shown in ref 7. On the same trend of connection between NOx and O2 in the furnace, for oxy-fuel cases, there is an elevated concentration of NOx near the wall, compared to the air-fired case. This aspect can be attributed to the higher concentration of oxygen in that region of the furnace; as a result, the conversion of fuel nitrogen to NO is increased. In fact, this process of conversion to NO is strongly dependent on the local combustion characteristics, as well as the nitrogen-bound compounds of the coal. However, from the numerical results of all of the combustion cases, it can be noticed that the concentration of NOx emissions for the air-fired case is commonly higher than those of oxy-fuel cases, because there is no production to the thermal NO in oxy-fuel scenarios. The overall reduction ratios of the NOx emissions for the oxy-fuel cases, compared to the air-fired case, were ∼18% and ∼9%, in total, for the OF27 and OF29 cases, respectively. The increase of the NOx concentration in the OF29 case compared to the OF27 case, probably resulted from the higher O2 concentration in the combustion environment, and thus enhanced the conversion of fuel nitrogen to NO. These findings are very consistent with the previous experimental19,62 and numerical works.14,17 Figures 15a−d present the comparisons and validations of the NOx emissions between the predicted results and the measured data at port 1 for all of the combustion scenarios. Generally, the predicted results of the NOx were in good agreement with the measured data for all combustion cases. As was discussed in the species concentration subsection, in the air-fired and OF25 cases, the O2 concentration was higher than those of OF27 and OF29 cases along the centerline of the furnace. This higher value of oxygen affected the NOx emissions in that region of the furnace. As seen in Figure 15a and 15b, the concentrations of NOx were higher at the centerline of the furnace, compared to the other locations along port 1. In contrast, the higher values of NOx, in the OF27 and OF29 cases were far from the centerline of the furnace at port 1, as shown in Figures 15c and 15d. In the latter two combustion cases, the maximum predicted values of NOx emissions were noticed from 200 mm away from the center, measured in the radial direction, which also represented the region of higher

Figure 12. Water vapor mass fraction (kg/kg) profiles at port 4 of the furnace for the (a) air-fired and (b) oxy-fuel (OF29) combustion scenarios, in one- and three-step chemical reactions.

Figure 13. Hydrogen concentration (kg/kg) profiles at port 1 of the furnace for the air-fired, oxy-fuel (OF25), oxy-fuel (OF27), and oxyfuel (OF29) combustion scenarios at the three-step reaction model.

In contrast, it can be seen that, in the OF27 and OF29 cases, the concentration of NOx emissions is very low in the vicinity of the burner, compared to other locations inside the furnace. 3344


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Figure 15. NOx concentration (ppm) profiles at port 1 of the furnace for the (a) air-fired, (b) oxy-fuel (OF25), (c) oxy-fuel (OF27), and (d) oxyfuel (OF29) combustion scenarios.

oxygen concentration (see Figure 6). In OF29 case, there was overprediction in the numerical results of NOx emission with respect to the experimental data in the above-mentioned region (i.e., from 200 mm away from the center, measured in the radial direction). This overprediction could be due to the rapid reaction of oxygen with the intermediate species HCN and NH3 (see eqs 33 and 35) during the oxidation process, which leads to the formation of higher fuel NO concentrations in that location of the furnace. In contrast to this discrepancy, it can be seen that there was good agreement of the numerical results with the measured data at the centerline of the furnace (see Figure 15d). Although there was overprediction in the OF29 case, the numerical results for all of the cases maintained the same trend with the experimental data at that critical port. The maximum differences of the predicted results with the measured data at port 1 were ∼14%, ∼7%, ∼13%, and ∼8% for air-fired, OF25, OF27, and OF29 cases, respectively. These numerical differences from the measured data can also be partially attributed to the uncertainties of measured data or to the lack of measurements calibration. Therefore, the numerical difference values can be accepted up to 20% from the experimental data as described in ref 14. Finally, the predicted results of the present modeling study could capture the main

qualitative and quantitative aspects of the measured data in all the combustion environments, especially at that location (port 1) of the furnace. 4.4. Wall Radiative Heat Transfer. In this subsection of the discussion, the radiative heat transfer on the furnace wall is presented for all of the combustion scenarios, as shown in Figure 16. As already observed in the species concentrations section, the CO2 and H2O concentrations in the flue gas increased in the oxy-fuel combustion cases. Under this specific condition of oxy-fuel combustion characteristics, the effect of gas mixture on the calculation of the radiative heat transfer should be considered. The change in the radiative properties of the gas mixture under oxy-fuel conditions is taken into account in the present study. The specific heat capacity of CO2, which is dominant in the gas mixture of oxy-fuel combustion, is high with respect to that of N2. The WSGGM used in this study accounted for the effect of higher CO2 and H2O concentrations in the calculation of gas absorption coefficient for the radiative heat-transfer model used. In their recent study, Zhou and Moyeda20 used the WSGGM for the oxy-fuel combustion purpose. As seen in Figure 16, there is a slight increase in the radiative heat transfer on the furnace wall in the oxy-fuel cases, compared to the air-fired case at the same fuel input power 3345


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measured data, particularly under oxy-fuel combustion conditions. The simplified approach of the chemical kinetics has been modeled to calculate the formation of fuel and thermal NO, decoupled from the main fluid flow computations. The present simulation results showed a very reasonable agreement with the measured results for all combustion scenarios, especially with the three-step reaction scheme. Generally, the predicted results of the latter chemistry reaction mechanism illustrated improved agreement compared to those of the one- and two-step reaction schemes. The improvements were particularly observed in the flame envelope zone in terms of the temperature distributions and species concentrations. This is obviously due to the adoption of thermal dissociation of chemical species in the three-step reaction scheme. Furthermore, the three-step chemical reaction mechanism used to describe the fuel conversion can precisely capture the CO/CO2 production rate and the equilibrium between H2 and H2O in the near-burner region (fuel-rich zone). A clear difference was observed in the NOx distribution between the reference case and oxy-fuel (OF27 and OF29) combustion cases at the centerline of the furnace in the near-burner region. This is principally due to absence of thermal NO formation under oxy-fuel combustion conditions (i.e., there is no dissociation process used to the air-nitrogen and only the conversion of the nitrogen-bound compounds of the coal to fuel NO was adopted during oxy-fuel scenarios). Finally, the elevated values in the flame temperatures and CO2 and H2O concentrations under oxy-fuel combustion led to increased radiation heat flux on the furnace wall. This is important and should be taken into consideration during the design of new oxy-fuel tangentially fired furnaces.

Figure 16. Wall radiative heat flux for air-fired, oxy-fuel (OF25), oxyfuel (OF27), and oxy-fuel (OF29) combustion scenarios.

(76 kW). Furnace temperature and radiative properties of gas mixture are the main reasons for that increase on the furnace wall under oxy-fuel conditions. The net radiative heat fluxes on the furnace wall were 70.77, 70.2, 74.55, and 77.91 kW for the air-fired, OF25, OF27, and OF29 cases, respectively. Similar investigations14,47 are confirmed with the trend of the present predicted results. Although a higher CO2 concentration was observed in the OF25 case, with respect to the air firing, a similar radiative heat transfer value is achieved for both of the aforementioned combustion cases. This is due to the similarity in flame temperatures, as shown in Figure 3. It can also be seen that the radiative heat flux values of the OF27 and OF29 cases are not too far from the air-firing values. This may be due to the reduction in the flue gas velocity, which has significant influence on the heat transfer distribution of the furnace wall (see Table 1 for more details about the inlet flue gas velocities for different combustion cases). However, the oxygen concentrations in the feed oxidizer gases and the flue gas volume rates are also other relevant parameters, in oxy-fuel cases, that could be used to match the corresponding heat flux of the air-fired combustion. This is of a great importance in the design of boiler material under the oxy-fuel combustion conditions, as investigated experimentally by Tan et al.19

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +613-9214-8655. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the Iraqi Ministry of Higher Education and Scientific Research (www.mohesr.gov.iq) for financial support during the scholarship period (2009-2012) in Australia.

■

5. CONCLUSION The commercial computational fluid dynamics (CFD) program, AVL Fire ver. 2008.2, was used to predict and analyze four different combustion scenarios (air-fired, OF25, OF27, and OF29), simulating the experiments on a 100 kW Chalmers laboratory-scale furnace. The temperature distributions, species concentrations (O2, CO2, CO, H2O, and H2), and NOx emission concentrations were investigated at different locations inside the furnace, as well as the radiation heat transfer on the furnace wall. The multistep chemical reaction mechanisms were carried out on the homogeneous and heterogeneous reactions of the pulverized lignite particles in terms of one-, two-, and three-step reaction schemes. The appropriate mathematical models with the related constants and kinetics parameters were used to increase the certainty of predicted results against the 3346

NOMENCLATURE ρ = gas density (kg/m3) xi = spatial distance in the ith direction Γ = diffusion coefficient of the variable Φ SΦ = source term for the variable Φ SpΦ = additional source term CD = drag coefficient Ap = cross-sectional area of the particle urel = relative velocity (m/s) mvp = mass of coal particles (K) V = product of volatiles Vf = ultimate product of volatiles Tp = temperature of coal particle (K) Rp = radius of the particle Tp = temperature of the particle (K) Tg = gas temperature (K) PA = atmospheric pressure τR = turbulent time scale (s−1) dx.doi.org/10.1021/ef300403a | Energy Fuels 2012, 26, 3329−3348

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S = stoichiometric (oxygen-to-fuel) ratio σ = Stephan−Boltzmann constant; σ = 5.67 × 10−8 W/(m2 K4) S = path length ai = absorption coefficient of the gas mixture Dp = diameter of the particle λ = thermal conductivity Nu = Nusselt number Tg = temperature of the gas (K) Tp = temperature of the particle (K) X = mole fraction of species (mol m−3) a = oxygen reaction order Svol = rate of coal devolatilization (kg/s) Schar = rate of char burnout (kg/s) T̅ = mean temperature (K) Schar,NO = rate of production of NO Abbreviations

GHG = greenhouse gases PC = pulverized coal RFG = recycled flue gas VM = volatile matter VCRF = vertical combustor research facilities EWBM = exponential wide band model WSGGM = weighted sum gray gas model

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