1. Introduction
Generally, an entrained flow gasifier (EFG) uses finely pulverized coal with steam and oxygen co-current to make syngas. This design forms a uniform internal temperature, and has a residence time of only a few seconds [
1]. The coal conversion reaches approximately 100%, because the gasifiers use pulverized coal at high temperature. An EFG is not affected by the rank of the coal [
2]. Currently many commercial EFGs are operated by enterprises such as (General Electric) GE, Shell, Siemens, CB&I, MHI and ThyssenKrupp [
3]. These types of gasifiers are operated at a temperature higher than the AFT. The ash, which is a coal residue, is discharged in the form of molten slag. The slag that is discharged to the bottom has a considerable amount of sensible heat. The design of the gasifier should consider this heat, because it affects the internal maximum temperature.
Existing EFG models [
4,
5,
6] focus on calculating the composition of the gas. To improve the model, representations of the reaction mechanism of coal have been improved. Previous models have proposed various reaction kinetic models, such as random pore model [
7], shrinking core model [
8,
9], and shrinking sphere model [
10]. In addition, equilibrium models have been suggested calculating the reaction between gases [
11,
12,
13,
14]. Most of the studies [
7,
8,
9,
10,
11,
12,
13,
15] have focused on developing reaction models and adjusting parameters. The energy balance is not considered sufficiently to find the optimal gasifier design. They have developed heat balance with two variables: (i) input and output heat flow; and, (ii) reaction heat.
The Texaco pilot plant is a typical EFG. Several studies (
Table 1) [
4,
5,
6,
16,
17] have attempted to model it based on experimental data [
18] acquired from the Electric Power Research Institute (EPRI). Wen et al. (1979) proposed a model that uses three reaction zones: (i) pyrolysis and volatile combustion zone; (ii) combustion and gasification zone; and, (iii) only gasification zone; the model applies a Stokes’ law approximation instead of momentum balance [
16]. Govind and Shah (1984) used the same kinetics as those of Wen et al., but neglected the momentum balance [
17]. Vamvuka et al. (1995) used thermogravimetric analysis data to develop the kinetics based on bituminous coal, but does not consider the momentum balance [
4,
5]. Hwang et al. (2015) expressed two reaction zones without considering the ‘pyrolysis and volatile combustion’ zone; this model applies the Stokes’ law approximation, and adjust parameters, such as outer wall temperature and the reaction rate constant [
6].
All of these models of the EFG have limitations. For energy balance, they all consider only input and output heat flow, and reaction enthalpy; they neglect energy that is absorbed by the melting of ash, and therefore do not accurately represent the inside of the gasifier in the real world. As a result, the calculated temperature is too high. For this purpose, previous papers introduce an additional term that represents heat loss to the outer wall. This calculation of heat loss requires assumptions about variables such as wall temperature, overall heat transfer coefficient, and thermal conductivity. These assumptions decrease the accuracy of the models.
The objective of this study is to improve the existing one-dimensional EFG model by including the ash-melting phenomenon instead of approximating it as heat loss at the outer wall. We propose a model to increase the accuracy of the temperature profile. The resulting model can predict the composition change of the product gas. We apply a shrinking sphere model to consider the combustion reaction, and then suggest reaction kinetics to calculate the amount of ash. We also design a new algorithm to consider the melting phenomenon of ash to improve the accuracy of the predicted temperature profile. We discuss the energy balance equation in three cases, according to the temperature: (i) temperature is lower than AFT; (ii) temperature in the first cell is higher than AFT; and, (iii) temperature in the second cell is higher than AFT. Finally, we compare the simulation results with the experimental results.
5. Results and Discussion
5.1. Model Valdation
To validate the model, several main variables were selected: final coal conversion rate; major product gas composition at exit; and, the hydrogen-to-carbon monoxide ratio (H
2/CO). The results (
Table 7) of this simulation are presented together with the results of the previous researchers and experimental data. We did not arbitrarily adjust the kinetic parameters that were used in this study. We applied the same reactor size and operating conditions of the existing pilot gasifier. For this reason, results of this work were similar to results of previous studies. In addition, we considered the melting phenomenon of ash that was not considered in the existing one-dimensional (1-D) model and reduced the estimated heat loss in the previous model. As a result, the modeling results are improved.
5.2. Simulation Results
5.2.1. Coal Conversion
Most of the reaction proceeded rapidly at the front of the gasifier (
Figure 4). Only 10% of coal was reacted at around 0.15 m of the gasifier, but 80% had reacted at 0.24 m. This increase occurred because the combustion reactions were accelerated. The reaction rate of coal decreased as the size of coal particles decreased. Especially after oxygen was completely consumed, the reaction rates were very slow. This trend is characteristic of typical EFGs; it is consistent with the results in past research [
5,
6,
16]. The final coal conversion was 98.8%.
5.2.2. Gas Composition
The mole fractions of the major gases changed over the reactor length (
Figure 5). Oxygen was abruptly consumed near 0.25 m; this change is the result of rapid combustion. Syngas was generated in an oxygen-free environment. Steam was slightly generated in the front of the gasifier; afterwards, the proportion of steam was controlled by WGS equilibrium. After all of the oxygen was consumed, the gas composition did not change significantly. CH
4, H
2S, and N
2 were < 1% of the product; they are not represented in
Figure 5. Trends in the graph agreed with trends that were reported in previous research [
5,
6,
16]. The exit gas consists mainly (mole fraction 98.9%) of carbon dioxide, hydrogen, steam, and carbon monoxide. All of these compositions are determined by WGS equilibrium, which is a function of temperature and is closely related to the outlet temperature of the gasifier. The attempt to calculate the temperature is the first step in predicting the composition of the gas, and this study is significant in that respect.
5.2.3. Temperature and Heat Flow
Simulations were used to calculate two temperature profiles (
Figure 6). The heat balance of the first case (
Figure 6, blue line) considered the sensible heat of the slag and the latent heat of the ash. The second case (
Figure 6, red line) neglected these phenomena; both were set to 0. Below AFT, the temperatures of the two cases were the same, but the peak and outlet temperatures differed between the two cases. The same results were obtained in the temperature range below AFT; they are acceptable because the melting of ash had not yet been considered. The maximum temperature was calculated as 2112 K when sensible heat of the slag and the latent heat of the ash were considered, but 2155 K when they were neglected. The exit temperatures were 1464 K when the sensible heat of the slag and the latent heat of the ash were considered, and 1521 K when they were neglected; the difference between the two outlet temperatures was 57 K. The difference can be explained, as follows. The heat capacity of ash and slag is not taken into account in the system, and the corresponding energy was transferred to the gas, so the calculated temperature increased. Previous models have released heat to outside. EFGs generally use thick refractories. The assumption that a significant amount of heat escapes to the outside must be modified: these two temperature profiles show that the ash melting phenomenon must be considered when the internal temperature of the gasifier is calculated. This is a reasonable conclusion, given the fact that the internal peak temperature of the gasifier is higher than the AFT.
In this study, ash accounted for 15% of the mass of the coal and 7% of the total mass. The effect of sensible heat of slag is evident when the heat flows are divided into the solid and gas (
Figure 7). Solids include coal, ash, and slag. After most of the reaction has proceeded, the solids are mainly in slag form. Especially at the exit condition, >99% of the solid was slag, which accounted for 5.0% of the total heat. In the coal that was used in this study, the ash was not negligible. As a result, our new attempt was meaningful. This phenomenon must be considered, especially for coal that has high ash content.
5.3. Consideration of Ash Melting Effect
The melting of ash has a similar effect on energy balance, as does heat loss from the outer wall. Because the ash melts and absorbs energy, the temperature of the system is lowered. In contrast, the loss to the outer wall lowers the temperature because heat escapes from of the system. These are different phenomena that occurred inside the gasifier, and were considered independently.
We quantitatively calculated the melting effect. This approach used the same method that was used to consider the heat loss. Based on the coal heating value, the latent heat of the ash and the sensible heat of the slag were calculated. The melting effect at the outlet was ~1%. This is not negligible given that heat loss is assumed to be 4% in this study. We calculated the melting effect at the exit as
In this way, we corrected the overestimate of heat loss that occurred in previous studies. In addition, we could explain some of the uncertainty of heat loss.
5.4. Applicability of High Ash Content Coal and Limitations
Coal containing a large amount of ash can be applied in the same manner. If information about the ash component is available, then it can be used to calculate the slag properties and incorporate thermal properties in the heat balance. When the ash content is high, modeling errors can be reduced by considering the fusion heat of ash and the sensible heat of the slag.
However, the reaction kinetic parameters that are cited in this study represent only coal combustion and gasification, so the model is only applicable to the entrained coal gasifier. In addition, lab-scale or pilot-scale data of other coal that has high ash content should be used to further assess applicability of this model.
6. Conclusions
This study suggests that a heat-balance model of an EFG must consider the effects of melting of ash. We attempted modeling based on kinetic models similar to those of previous researchers. Our proposed 1-D model is the only one that includes ash in the heat-balance and temperature-calculation algorithm. Gas production and coal conversion trends were similar to those of existing ones, and the results at the exit were mostly consistent with the experiments.
This result is meaningful in that it reflects actual phenomena occurring inside the EFG. Ash melts in any slagging-type gasifier. We can expect to calculate the internal temperature more accurately based on this study. These results can be used to guide the choice of design elements of EFG, such as material and thickness of the refractory wall.
One limitation of this model is that information on the ash component was not available. We used minerals to express ash components. This demerit must be eliminated. For further research, advanced modelling should use thermal properties based on ash analysis data. This model can be extended to account for radial direction temperature distribution.