Study on coal gasification with soot formation in two-stage entrained-flow gasifier

The coal gasifier (Fig. 1) considered here consists of a combustor stage and a reductor stage. Coal and char are injected into the combustor stage with O₂-rich gas mixtures. The gasifier has two levels of injectors that are positioned axisymmetrically at combustor and reductor stage. The combustor injectors are placed similar to a tangential firing system to create swirling flow inside the gasifier. The reductor injectors are directed towards the center of the gasifier. The diameter of the coal/char inlet zone is <10 mm which is very small compared to the height of the gasifier (4.94 m). Thus, we make the mesh with various size ranges, from 2 to 10 mm. Moreover, making a uniform mesh with 2 mm size will significantly increase the computational time. A three-dimensional mesh consisting of 247,818 computational cells is used with the small cell size being around 2 mm and the largest one around 10 mm. The near wall y ⁺ value is 250, which is appropriate (30 > y ⁺ > 300) to apply the standard wall functions in the standard k–ε turbulence model.

Soot formation in coal gasification is a very complicated process. This is due to the fact that the molecules of coal volatiles, particularly polycyclic aromatic hydrocarbons (PAHs), are much larger and more chemically diverse than those of simple hydrocarbon fuels. There are several soot models that have been proposed during the recent decades. Some of the most important empirical models are Khan and Greeves model [8], Tesner model [9] and Lindstedt model [10]. There are also some detailed models that take complex physical phenomena and detailed chemistry into account. One of the most comprehensive detailed models is that proposed by Frenklach [11]. A detailed kinetic model describing the formation and consumption of PAHs and soot in hydrocarbon combustion has also been developed by Richter et al. [12]. Although the detailed models have undergone remarkable development recently, these models are still computationally demanding and cannot be used for complex geometries. In our previous work [13], we investigated soot formation model in a plug flow reactor (PFR) and reported that the following two reactions are typically considered to be the main reaction path in soot formation: (a) particle nucleation—PAH of increasing size are mainly formed by sequence of chemical reactions between PAH and their radicals, and between PAH radicals. This process is repeated producing large PAHs that form soot particles [12, 13] and (b) soot/PAH oxidation—reaction of soot/PAH with oxygen/hydroxyl radicals that depletes PAH/soot [13]. Corresponding to these concepts, a one-step soot formation mechanism is proposed in the present study to investigate the effect of soot formation on product gas concentration and gas temperature in coal gasification. A schematic of soot formation mechanism is shown in Fig. 2. In the one-step soot formation mechanism, an aromatic hydrocarbon molecule benzene (C₆H₆) or naphthalene (C₁₀H₈) or phenanthrene (C₁₄H₁₀) or pyrene (C₁₆H₁₀) is considered as a precursor of soot formation. Before introducing the soot mechanism, the calculated results obtained using one-step soot formation mechanism are compared with those obtained using detailed soot formation mechanism under various gasification conditions [13].

For the fluid phase, the steady-state Reynolds Averaged Navier–Stokes (RANS) equations as well as the mass and energy conservation equations are solved for two-stage entrained-flow coal gasifier shown in Fig. 1. The governing equations for the conservation of mass, momentum, energy and species in 3D Cartesian coordinates are given as follows:

Turbulent flows are characterized by fluctuating velocity fields. Turbulence models seek to solve a modified set of transport equations by introducing averaged and fluctuating components. In Reynolds averaging, the solution variables in the instantaneous Navier–Stokes equations are decomposed into the mean (time-averaged) and fluctuating components. For the velocity component:where \( \overline{{u_{i} }} \) and \( u_{i}^{{\prime }} \) are the mean and fluctuating velocity components (i = 1, 2, 3). A standard k–ε model [14‐16] is used to solve the turbulence. The turbulence kinetic energy, k, and its rate of dissipation, ε, are obtained from the following transport equations: where G _k represents the generation of turbulence kinetic energy related to the mean velocity gradient, and µ _t is the turbulent viscosity. The turbulent model constants are C _1ε  = 1.44, C _2ε  = 1.92, C _μ  = 0.09, σ _k  = 1.0, and σ _ε  = 1.3 [15, 16].

The discrete ordinates (DO) radiation model is used to solve the radiative heat transfer equation. The DO radiation model considers the radiative transfer equation as:

In discrete phase modeling, pulverized coal particles are injected into the gasifier and tracked throughout the computational domain using a Lagrangian approach. The continuity and momentum of particles are expressed as follows: F _D(u–u _p ) is the drag force per unit particle mass and F _D is determined from Re _d is the relative Reynolds number based on the particle diameter and relative velocity.

The change of particle temperature during devolatilization is determined from the energy balance of particles governed by convective, latent heat and radiative heat transfer as follows [17, 18].

After the volatile species of the coal particle has evolved completely, char–O₂, char–CO₂ and char–H₂O surface reactions begin. During surface reaction, the following heat balance equation is used:

When the temperature of the coal particles reaches the vaporization temperature (600 K), chemical reactions occur producing various amounts of gases, tar, and coke. The tar and gases are usually referred as volatiles. The volatiles are released according to Kobayashi model [19]. This model assumes two kinetic rates, k _kin,1 and k _kin,2, which may control the devolatilization over different temperature ranges, and yields an expression for the devolatilization as:

For the gas phase reactions including soot formation (R1–R9 shown in Table 1), the smaller of the two reaction rates (finite rate and eddy dissipation) is used as the overall reaction rate (\( \hat{R}_{i,k} \)). The finite rate and the eddy dissipation models consider the reaction rate as follows:

The burning rate of the carbon in coal particle is calculated using the finite rate model proposed by Smith [6, 23, 24]. The rates of depletion of carbon due to surface reactions (R10–R12 shown in Table 1) are given as:

The kinetic reaction rate k _kin follows an Arrhenius expression as:and the values of the kinetic parameters that are used to determine k _kin for all reactions are also shown in Table 1.

Uniform distributions of inlet mass flow rate and temperature are given for all inlet boundary surfaces. The walls are assumed as stationary and smooth with no slip condition. A constant wall heat flux is assigned for wall boundary surfaces. The boundary condition of the discrete phase at walls is assigned as “reflect”, which means the discrete phase elastically rebound off once reaching the wall. At the outlet, the discrete phase exits the computational domain.

A bituminous-type CV coal (Coal Valley, Canada) is used to conduct the simulation of coal gasification. The proximate and ultimate analyses of coal are given in Table 2. The initial particle size distributions with a mean diameter of 60 μm are used in the calculation. The total mass inlet for experiment and calculation are kept same. The coal flow rates for combustor and reductor are set to 40 and 60 kg/h, respectively. The gas flow rates are adjusted in such a way that the inlet O₂ ratio and O₂ concentration become 0.528 and 23 wt%, respectively. The O₂ ratio is defined here as the ratio of the amount of O₂ fed into the gasifier to the amount of O₂ required for complete combustion of carbon present in coal. During devolatilization, all hydrogen, oxygen and nitrogen are assumed to be released as volatiles. Volatiles are considered as a single hypothetical component, C_α1H_α2O_α3N_α4. The values of α1, α2, α3 and α4 are calculated from the coal’s ultimate and proximate analyses. Once the volatile component is released, it is converted into CO, CO₂, H₂O, H₂, CH₄, C₆H₆ and N₂ according to reaction R1. The pyrolysis data obtained from previous experimental work [26] are used to calculate the β values. In the calculation, all aliphatic and aromatic compounds are lumped into CH₄ and C₆H₆, respectively (Table 3). Chen et al. [27] explained the gas evolution from rapid pyrolysis of a bituminous coal at various pyrolysis temperatures (500–900 °C). It was found that the ratio of CO to CO₂ yield does not change with increasing the pyrolysis temperature. They also showed that the yield of higher hydrocarbon is approximately three times higher than that of CH₄, which is very near to our previous works. Therefore, these two ratios (Y _d,CO /Y _d,CO2 = 1.27 and Y _d,C6H6 /Y _d,CH4 = 3.10) together with three elemental (C, H, O) mass balance equations are used to calculate the stoichiometric co-efficient (β) of product species for reaction R1 (see Table 3). Here Y _d represents the mass yield for the corresponding species.

To validate the one-step soot model, a tubular-type reactor of 0.28 m diameter and 48 m length with the inlet gas velocity of 26.4 m/s is used to conduct the simulation. Eight overall gas phase reactions (R2–R9 shown in Table 1) are considered in the calculation. Benzene (C₆H₆), naphthalene (C₁₀H₈), phenanthrene (C₁₄H₁₀) and pyrene (C₁₆H₁₀) are independently considered as a precursor of soot formation. The calculated outlet species concentration of soot and syngas using the proposed soot model are compared with those reported in our previous paper [13] under various gasification conditions. The comparisons are shown in Fig. 3. The trends in outlet species concentration with increasing temperature are found to be similar, in both: detailed mechanism and overall gas phase reactions with proposed one-step soot mechanism. Soot concentration decreases with increasing the temperature while syngas concentration gradually increases. Soot formation as well as soot oxidation tends to increase at higher temperatures, resulting in an increase in CO and H₂ concentration. A detailed explanation of the effect of temperature on soot and syngas concentration can also be found in Wijayanta et al. [13]. In addition, very similar results are obtained for various soot precursors (benzene, naphthalene, phenanthrene and pyrene) considered in the calculation. As increasing the molecular weight of species significantly increases the computational time, benzene is chosen as a soot precursor in the simulation of coal gasification in the two-stage entrained-flow gasifier shown in Fig. 1.

The comparison for two conditions of without soot and with soot shown in Fig. 4a indicates that there is a small change in outlet species concentration. An increase in H₂ concentration under soot formation condition suggests that the concentration of H₂ will be increased if the soot formation advances in the gasifier. This means that the formation of soot can increase the syngas heating value in this regard, despite having diverse effect of soot. Figure 4a also shows a comparison of outlet species concentration between experiment and calculation. Details of the experimental procedure and condition are described by Kidoguchi et al. [25]. A quite good agreement is obtained for main species CO, CO₂ and H₂. The agreement for the species H₂O is not good due to the lack of information of experiment. A large deviation for H₂O concentration between experiment and calculation is obtained due to the presence of some moisture in air during experiment which is ignored in the calculation.

The gas temperature profiles at centerline for experiment and calculations are shown in Fig. 4b. In both, trends of gas temperature are found to be similar for experiment and calculations. However, calculation without soot formation overestimates the experimental gas temperature. In contrast, calculation with soot formation provides better agreement with the experiment. In case of soot formation, the reaction R1 includes aromatic species C₆H₆ which is considered as a soot precursor. C₆H₆ is then accumulated to produce a larger species, Coronene (C₂₄H₁₂), which is referred here as soot. The gas temperature for calculation with soot decreases significantly because of reducing the heat of reaction (R1). The gas temperature also decreases due to the large heat capacity of aromatic species considered in the soot formation reaction mechanism.

The effect of O₂ ratio on soot concentration, gas temperature, carbon conversion, etc., is numerically investigated under CO₂-rich gasification condition (CO₂ concentration = 14 wt%). The contours of soot concentration and gas temperature under conditions of two different O₂ ratios at 0.528 and 0.7 are shown in Fig. 5a. A slight decrease in soot concentration at outlet from 1.79 to 1.73 wt% is observed if the O₂ ratio increases from 0.528 to 0.7 in the gasifier. On the other hand, the gas temperature at outlet significantly increases from 1352 to 1588 K with increasing the O₂ ratio. This is because under higher O₂ ratio, exothermic char–O₂ reaction tends to increase. The high gas temperature then advances the endothermic char–CO₂ and char–H₂O gasification reactions. This means increased O₂ ratio significantly enhances char–O₂ reaction as well char–CO₂ and char–H₂O reaction. Although, char–CO₂ and char–H₂O reactions are endothermic, the gas temperature increases due to significant rise in char–O₂ oxidation reaction under higher O₂ ratios. An increase in char–O₂, char–CO₂ and char–H₂O reactions under a high O₂ ratio at 0.7 results in an increase in carbon conversion and syngas concentration. Figure 5b shows that the carbon conversion gradually increases with increasing the O₂ ratio and reaches a complete (100 wt%) conversion at a O₂ ratio of 0.8. The soot concentration is also found to decrease at higher O₂ ratios. In contrast, syngas heating value initially increases with increasing the O₂ ratio, and reaches a maximum value of 3800 kJ/kg with 94 wt% carbon conversion at a O₂ ratio of 0.7 (Fig. 5b). Beyond this value (O₂ ratio = 0.7) heating value decreases with increasing O₂ ratio because of shifting the environment from gasification towards combustion. Therefore, if the target is to get a complete conversion of carbon, a lower heating value gas will be produced from the coal gasification. Considering the carbon conversion in real gasification process where unconverted carbon is recycled as char and the use of more O₂ where low heating value gas is produced, the O₂ ratio exceeding 0.7 is not recommended for getting efficient coal gasification. Therefore, to improve the gasification efficiency, the concentrations of other gasification agents (CO₂ and/or H₂O) need to be increased in coal gasification process keeping O₂ ratio under 0.7. With a target of reducing CO₂ release into the atmosphere from coal gasification, the effect of CO₂ concentration on soot concentration, syngas heating value and carbon conversion is numerically investigated in this study.

The effect CO₂ concentration on soot concentration, gas temperature, carbon conversion, etc., is numerically investigated by changing the inlet concentration of CO₂ at a constant O₂ ratio (=0.528). The contours of soot concentration and gas temperature under conditions of two different CO₂ concentrations at 14 and 50 wt% are shown in Fig. 6a. No significant difference in soot concentration is found for the two cases, although the gas temperature decreases with increasing the overall concentration of CO₂. The gas temperature decreases with increasing the CO₂ concentration due to increased char–CO₂ reaction rate. Under CO₂-rich concentration, endothermic reaction (backward reaction of R3) also increases, resulting in a decrease in gas temperature. The backward tendency of R3 on the other hand increases CO concentration and decreases H₂ concentration. However, the syngas heating value gradually increases with increase of the CO₂ concentration (shown in Fig. 6b) although H₂ concentration decreases at higher CO₂ concentrations. Soot concentration and carbon conversions for various cases are also shown in Fig. 6b. It is found that the soot concentration is nearly independent to CO₂ concentration. On the other hand, the reductor carbon conversion gradually increases with increasing the CO₂ concentration. A 15 % increase in carbon conversion of reductor coal is obtained if the inlet concentration of CO₂ is increased from 14 to 50 wt%. However, the same change in CO₂ concentration gives only a 1 % increase in overall carbon conversion. This is due to low carbon conversion of combustor coal at a high CO₂ concentration (50 wt%). Interestingly, the syngas heating value increases from 2717 to 3501 kJ/kg which corresponds to a 28 % increase in syngas heating value. This indicates that the carbon conversion is not directly related to the syngas heating value. Under higher CO₂ concentrations, char–CO₂ (C + CO₂ → 2CO) reaction dominates over char–O₂ (C + 1/2O₂ → CO) and char–H₂O (C + H₂O → CO + H₂) reactions. This results in an increase in CO concentration with a small increase in carbon conversion. Therefore, it can be concluded that the production of syngas heating value per unit weight of carbon conversion will be higher if CO₂ concentration increases in the gasifier.