Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed Gasifier

Abstract

Bubbling Fluidized Bed Gasifier (BFBG) technology is an efficient and economical way of producing syngas from various feedstocks, such as coal, biomass, and municipal waste. However, the prediction of the gasification process inside the BFBG is quite complex due to many factors, including multiphase flow hydrodynamics. This study analyzed the hydrodynamics of a bench-scale top-fed bubbling fluidized bed coal gasifier with sand or glass beads used as bed materials at different bed aspect ratios. Two separate test rigs were built with the same dimensions for cold flow (without reaction) and hot flow (with reaction) studies, respectively. The cold flow test rig was used to investigate the hydrodynamics of BFBG fluidization. Bed pressure drop, minimum fluidization velocity, and mixing were analyzed in the test room conditions. Following that, gasification tests were carried out in the hot flow BFBG test rig with a novel feeding system using the optimum hydrodynamical parameters determined from cold flow analyses. Results showed that syngas was successfully produced at an adequate composition. This study contributes to a better understanding of the fluidization hydrodynamics of the binary coal and bed material mixtures in a top-fed BFBG for a more optimum gasification process and easier operation of the BFBG.

1. Introduction

The demand for energy has been growing due to the increase in human population and industrialization. However, the greater need for fuel and power generation brings more greenhouse gases and, hence, more environmental pollution, mostly because of the dominance of fossil energy sources being used as primary fuels for transportation and power generation. According to the International Energy Agency (IEA) and British Petroleum (BP), coal was the primary energy source, accounting for 38.5% of electricity production from 1971 to 2021 and 36% in 2021. Gasification can be an environmentally friendly alternative way of generating fuel (synthetic gas, syngas) from coal. Syngas produced by coal gasification can be used in various applications, such as transportation fuel [1], electricity production, heating, etc. Besides, the use of syngas can significantly lower greenhouse gas emissions. Bubbling fluidized bed gasifiers are a type of fluidized bed reactor that can significantly increase the gasification reaction efficiency compared to fixed-bed reactors. Also, it is more economical to operate and maintain compared to Circulating Fluidized Beds (CFB). However, the BFBG fluidization process is quite complex, and it has a direct impact on the reaction process and its efficiency. There are many parameters affecting the fluidization hydrodynamics inside the BFBG. Particle characteristics such as size, sphericity, density [2, 3], bed aspect ratio (the ratio of the bed height to the bed diameter) [3, 4], fluidizing gas and feedstock moisture content [5], temperature, and feeding location have strong effects on fluidization hydrodynamics. Besides, fluidizing gas velocity is a crucial parameter that mainly regulates and determines the fluidizing regime according to the particle characteristics and other factors. For an efficient gasification reaction inside a BFBG, all these parameters should be considered and adjusted interactively.

Particle size and density affect the bubble-induced particle mixing in a gas–solid fluidized bed. Because, particle shape affects the bubble formation and dynamics in fluidized beds. Spherical particles tend to form small and uniform bubbles, while non-spherical particles tend to form large and irregular bubbles [6]. Larger and denser particles tend to mix slower than smaller and lighter particles. Si and Guo [3] studied the fluidization behavior of binary mixtures of quartz sand with sawdust or wheat stalk in an acoustic bubbling fluidized bed. They found that the addition of sand improved the fluidization quality of the biomass particles. The authors suggested that the improved fluidization behavior was due to the increased particle density and reduced voidage caused by the addition of sand. They also observed that the fluidization behavior of the binary mixtures was affected by the particle size ratio and the proportion of sand in the mixture.

Higher aspect ratio beds have higher reaction efficiencies due to increased interparticle attraction and gas residence time, resulting in a longer reaction time. However, they lead to poorer mixing due to the transition from single bubble regime to slug flow regime [7]. Feedstock particles need to be delivered homogeneously along the bed height for optimum particle interaction and heat transfer rates. Thus, homogeneous mixing is suggested to obtain better syngas composition and higher reaction efficiencies. Most of the applications deliver the feedstock particles into the bed mainly by pushing the feedstock directly into the bed or on top of the bed by using a screw driver. However, driving a screwdriver directly into the reactor bed through the reactor wall can be expensive, and it is prone to mechanical and operational problems such as leaks. Zhijie Fu et al. [8] studied the particle mixing and segregation behavior in an Air Dense Medium Fluidized Bed (ADMFB) with binary mixtures of solid particles for dry coal beneficiation. The study examined the effects of various operating parameters such as particle density ratio, particle size ratio, mixture composition, superficial gas velocity, and fluidized bed height on the mixing and segregation pattern. The results of the study show that the degree of segregation increases with increasing density difference of binary mixtures and partial segregation can occur with an increase in particle size ratio. However, the mixing and segregation of binary systems are almost independent of lower excess gas velocity and initial bed height when it is over 15 cm. The study also employs a mixing index to evaluate the mixing and segregation performance and identifies criteria for good mixing to achieve the bed density adjustment.

During the operation of the BFBG, bed pressure drop and minimum fluidization velocity are the two main parameters analyzed to control the fluidization behavior inside the fluidized bed. Particle density affects the minimum fluidization velocity and bed expansion of fluidized beds. Gao et al. [9] showed that higher particle density leads to higher minimum fluidization velocity and lower bed expansion. Abdullah et al. [10] investigated the effect of mixture bulk density and bed voidage on the minimum fluidization velocity ($U_{\mathit{mf}}$) of various materials, including Geldart B-group materials. They found that both mixture bulk density and bed voidage had a significant effect on the minimum fluidization velocity of the materials. In addition, they found that Geldart B-group materials exhibited better fluidization behavior compared to other groups of materials in Geldart’s classification. On the other hand, many studies, conducted to analyze the effect of the bed aspect ratio on the $U_{\mathit{mf}}$, show that higher bed aspect ratios do not affect the $U_{\mathit{mf}}$ significantly [11, 12]. Many predictions have been made to predict the $U_{\mathit{mf}}$, but many of them do not agree, particularly for the binary mixtures (feedstock and inert materials). Besides, studies made in lab-scale applications can generate results that differ significantly in real (larger-sized) applications. In addition, most of these correlations have been generated in ambient conditions, excluding the temperature effect on fluidization hydrodynamics [13].

Temperature and pressure affect the hydrodynamics of gas–solid fluidized beds by changing the gas properties (density and viscosity) and the interparticle forces (van der Waals, electrostatic, etc.). Higher temperature and pressure increase the interparticle forces and cause agglomeration and defluidization of fine particles. The fluidization behavior of different Geldart groups of particles (A, B, and D) can vary significantly under extreme conditions [14]. Higher temperature leads to lower gas density and higher gas viscosity, which decrease the minimum fluidization velocity and increase the drag force [14, 15].

Overall, the mixing characteristics and quality of binary mixtures in fluidized beds can be an important aspect to consider in various industrial applications such as coal combustion, gasification, and catalytic reactions. According to the literature review, the theory behind fluidization hydrodynamics is not fully developed and understood. Hence, further research is needed to fully understand and optimize the mixing behavior of these complex mixtures under different operating conditions. Few studies considering the temperature effect on the fluidization characteristics and mixing behavior of binary mixtures of coal and inert materials in a fluidized bed have been found in the literature. As a result of the complexity of the fluidization and gasification theories, as well as the difficulties in their application, this technology is not widely used in both large- and small-scale applications.

This study aims to contribute to a better understanding of the coal and inert material (glass beads or sand) fluidization behavior in a top-fed deep-bed application of a BFBG by considering the particle characteristics, temperature, and bed aspect ratio.

1.1 Gasification and bubbling fluidized bed gasifier

Gasification is also known as “partial combustion” or “oxidation” due to the lower oxygen requirement (25 to 40%) compared to the amount of oxygen used in the stoichiometric combustion reaction. As a result, in addition to syngas, gasification generates some carbonaceous by-products such as ash, tar, and char (Figure 1). A series of reactions that take place interactively in a gasification reaction are shown in Table 1.

Figure 1.

Gasification reaction diagram.

Reaction name	Reaction formula	Enthalpy values
Water-gas shift reaction	CO + ${\mathrm{H}}_2\mathrm{O}$ $\rightleftharpoons$ ${\mathrm{CO}}_2$ + ${\mathrm{H}}_2$	$\mathrm{\Delta }$H${}^{\circ }$298K = -41 kJmol${}^{-1}$
Steam methane reforming reaction	${\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}$ $\rightleftharpoons$ $\mathrm{CO}+3{\mathrm{H}}_2$	$\mathrm{\Delta }{\mathrm{H}}^{\mathrm{o}}$298K = 206 kJmol${}^{-1}$
Boudouard reaction	C + ${\mathrm{CO}}_2$ $\rightleftharpoons$ 2CO	$\mathrm{\Delta }{\mathrm{H}}^{\mathrm{o}}$298K = 171 kJmol${}^{-1}$
Methanation reaction	C + $2{\mathrm{H}}_2$ $\rightleftharpoons$ ${\mathrm{CH}}_4$	$\mathrm{\Delta }{\mathrm{H}}^{\mathrm{o}}$298K = -75 kJmol${}^{-1}$
Water-gas reaction	C + ${\mathrm{H}}_2\mathrm{O}$ $\rightleftharpoons$ CO + ${\mathrm{H}}_2$	$\mathrm{\Delta }{\mathrm{H}}^{\mathrm{o}}$298K = 131 kJmol${}^{-1}$

Table 1.

Major $1^{\mathrm{st}}$ step gasification reactions.

A bubbling fluidized bed gasifier is a type of Fluidized Bed Reactor (FBR) in which gasification takes place with a complex multiphase fluidization process of gas and solid particles. Fluidized bed reactors are more efficient in terms of heat transfer and carbon conversion rates compared to fixed-bed reactors, in which there is no particle motion or mixing [16, 17]. Fluidization can be described as the state of the solid particles that are suspended or gained motion (like fluidized) with the lift force exerted by an upcoming fluid (fluidizing fluid or agent) such as gas or liquid in a vertical column. If the flow rate of the gasifying agent is not enough for the desired fluidization operation, additional inert fluidizing gas can be used to obtain the required fluidizing gas flow rates. The mixing and the heat transfer rates are directly related to the bubble dynamics inside the BFBG. Here, besides the bubble dynamics, the inert (bed) material plays a significant role in providing the necessary heat transfer rates to the feedstock particles. Hence, inert material characteristics such as diameter, sphericity, density, and thermal conductivity affect fluidization hydrodynamics, and therefore, heat transfer rates and reaction rates are affected. The term “bed” is used to refer to the mixture inside the reactor. A sample fluidized bed reactor and 3D CAD model illustration are shown in Figure 2a, b, respectively. The main parts of the BFBG are: a plenum for fluidizing agent intake, a distributor plate to distribute the flow uniformly above it and provide the bed pressure drop for bubble formation, the reactor bed where the reaction takes place, and a freeboard to decrease the gas velocity to allow the particles to fall back to the reactor bed.

Figure 2.

Fluidized bed reactor diagram.

The operation of the BFBG can be described by the following procedures: Typically, a screw feeder mechanism is used to supply feedstock particles to the reactor bed from the bottom, side, or top. The gasification reaction starts when the carbonaceous particles are added to the oxygen-rich atmosphere at the required reaction temperatures. During the gasification reaction with the fluidization process, the produced ash particles sink to the bottom of the bed. On the contrary, light particles such as char can leave the reactor at high gas velocities. To increase the gasification reaction efficiency, particularly in circulating fluidized bed (CFB) applications, a cyclone is used to transfer the leaving particles back to the reactor bed. The operation of the BFBGs is more economical compared to CFB applications. Besides, BFBG can use a wider range of materials as feedstocks [18]. Before being used in the different applications previously described, the syngas is cooled and filtered.

1.2 Bed pressure drop and minimum fluidization velocity

The pressure drop between any points along the reactor bed height is equal to the weight of the particles between the measurement points per unit cross-sectional area of the fluidized bed. Thus, the bed pressure drop term is used for the pressure drop of the whole bed weight at the fluidization state. The bed pressure drop can be calculated by using the formula:

$$\mathrm{\Delta }{P}_{b}A=W,\text{where}W=\mathit{mg}={\mathit{AH}}_{\mathit{mf}}\left(1-{\epsilon }_{\mathit{mf}}\right)\left({\rho }_p-{\rho }_g\right)g$$E1

where, $\mathrm{\Delta }{P}_b$ is the bed pressure drop, $A$ is the cross-sectional area, $m$ is the mass of the bed, $g$ is the gravitational acceleration, $H_{\mathit{mf}}$ is the bed height at minimum fluidization, ${\epsilon }_{\mathit{mf}}$ is the bed voidage at minimum fluidization, ${\rho }_p$ is the particle density, and ${\rho }_g$ is the fluidizing gas density. The bed voidage, $\epsilon$, is the ratio of the void volume to the bulk volume of the bed, can be calculated as:

$${\epsilon }_1=1-\frac{{\rho }_b}{{\rho }_s}$$E2

where ${\rho }_b$ is the bulk density, and ${\rho }_s$ is the bed skeletal density.

The minimum fluidization velocity, $U_{\mathit{mf}}$, is the gas velocity required to balance the bed weight and initiate fluidization. And, theoretically, as the gas velocity increases, the pressure drop per bed weight remains constant. Many correlations have been developed to predict the minimum fluidization velocity; however, most of these correlations, particularly for binary mixtures, do not agree on the prediction results [19]. One of the well-known correlations to predict the minimum fluidization velocity derived by Ergun [20] is:

$$\frac{{\rho }_g\left({\rho }_p-{\rho }_g\right)\left({\mathit{gd}}_p^3\right)}{{\mu }_g^2}=\frac{150\left(1-{\epsilon }_{\mathit{mf}}^2\right)}{{\varphi }^2{\epsilon }_{\mathit{mf}}^3}\frac{{\rho }_g{U}_{\mathit{mf}}{d}_p}{{\mu }_g}+\frac{1.75}{{\mathit{\varphi \epsilon }}_{\mathit{mf}}^3}\frac{{\rho }_g^2{U}_{\mathit{mf}}^2{d}_p^2}{{\mu }_g^2}$$E3

where $d_p$ is the mean particle diameter, ${\mu }_g$ is fluidizing gas viscosity, and $\varphi$ is the average particle sphericity.

Furthermore, the minimum fluidization velocity can be determined graphically by measuring the bed pressure drop as the fluidizing gas velocity increases. The intersection of the lines of the slopes in the fixed-bed state and the complete fluidization state, respectively, gives the minimum fluidization velocity. Fluidization starts at the initial fluidization velocity and reaches a complete fluidization state at the complete fluidization velocity. The graphical illustration of the determination of the initial ($U_{\mathit{if}}$), minimum ($U_{\mathit{mf}}$), and complete ($U_{\mathit{cf}}$) fluidization velocities is illustrated in Figure 3.

Figure 3.

Graphical solution to determine $U_{\mathit{\text{if}}}$, $U_{\mathit{mf}}$, and $U_{\mathit{cf}}$ for increasing superficial gas velocity (fluidization).

1.3 Fluidization regimes

The potential fluidization regimes within a fluidized bed with increased fluidizing gas velocity ($U_g$) are shown in Figure 4. The bed retains its shape and bulk density as long as the gas velocity remains below the minimum fluidization velocity ($U_{\mathit{mf}}$). This regime of fluidization is called “packed bed” or “fixed bed,” which is illustrated in Figure 4a. With the rise in the gas velocity, initially smaller particles in diameter are suspended by the upcoming fluidizing gas. Later, with an adequate flow rate, all particles become lifted, and the weight of the bed is balanced with the force exerted by the fluidizing gas on the bed’s cross-section area. The minimum fluidization velocity, $U_{\mathit{mf}}$, is the gas velocity required to balance the bed weight and initiate fluidization. And, theoretically, as the gas velocity increases, the pressure drop per bed weight remains constant. The shift in the bed height (bed expansion) due to the minimum fluidization condition is demonstrated in Figure 4b. As $U_g$ continues to rise, the bed gains momentum, and motion starts with the bubbles emerging, as seen in Figure 4c. Maintaining a homogeneous bubble distribution, bubble formation, and bubble frequency is critical for better mixing, which leads to higher heat transfer rates and, consequently, faster reaction rates. As the gas velocity increases, the bubbles condense into larger bubble formations known as “slugs” with a diameter similar to the bed diameter (Figure 4d). Bubble formations break up as $U_g$ increases, resulting in rapid particle mixing (Figure 4e). A higher increase in the gas velocity causes a transition to a fast fluidization regime (Figure 4g). A further increase in the gas velocity can transport the particles outside of the bed, as seen in Figure 4g. This regime is called pneumatic transport.

Figure 4.

Schematic illustration of the fluidization regimes.

1.4 Geldart’s particle classification

The multiphase flow fluidization process is influenced by particle diameter, sphericity, and density. Geldart [21] classified four particle groups according to their fluidization behavior by measuring the variations in gas and solid phase densities with the mean particle diameter. Geldart group particles are classified as follows:

Group A: Particles in Group A, such as cracking catalysts, have a tiny diameter (20 to 100 μm) and/or low density. After the minimum fluidization condition, dense phase expansion is visible. As a result, Group A particles require higher gas velocities for bubble formation compared to Group B particles.

Group B: Group B particles have mean diameters and densities ranging from 40 to 500 μm and 1.4 to 4 g/cm³, respectively. A good illustration of this category of particles is sand. Bubbles are visible shortly after the minimum fluidization velocity. This group of particles shows the best fluidization characteristics.

Group C: Group C refers to particles having a diameter (10 to 80 μm) and a high degree of cohesion. Due to the increased interparticle forces caused by their high cohesive nature, they mix and fluidize poorly. Bubbles can be seen shortly after the lowest fluidization velocity, with a slight bed expansion.

Group D: Group D particles often have high particle densities and have diameters greater than 600 μm. Hence, they require higher gas flow rates to fluidize. Compared to Group B particles, they show poorer fluidization behavior.

2. Experimental setup and methodology

2.1 Cold flow and BFBG test rigs

The experimental setup consists of the cold flow (Figure 5a) and BFBG (Figure 5b) test stands. The cold flow test rig, made of transparent acrylic, allows the visualization of the fluidization behavior and the measurement of the pressure drop at ambient conditions. The main parts of the cold flow test rig are a plenum, a distributor plate, the bed section, and the freeboard (Table 2). A stainless steel distributor plate with a 10-μm pore size and 39% total porosity was used in the test rig. A mass flow controller was used to control and regulate the flow rate of the fluidizing gas (nitrogen, air). Pressure was measured at nine different points aligned vertically with 3.81 cm increments along the bed height, starting just below the distributor plate (measurement point 1) up to the measurement point 9 just below the transition cone between the reactor bed and the freeboard. Pressure signals were recorded at a 10 Hz sampling rate using a data acquisition system. Later, the obtained data was analyzed with Python-based software. Figure 5a depicts the alignment of the pressure taps. Fluidization behavior was studied by using the images captured by a high-speed camera for each test case. Images were processed with an open-source image processing tool called Python-Scikit to improve their qualities and the contrast between the inert and feedstock materials to better visualize the mixing condition. Cold flow experiments were conducted for the total mixture masses of 100, 200, and 300 g with the feedstock (coal) on top (segregated state) with a weight ratio of 4% to simulate the actual BFBG hydrodynamical behavior at the elevated temperatures. For each test case, measurements were taken after waiting at least another 30 seconds to stabilize the test case and avoid transition data. Tests were repeated three times, and the results are shared in the Cold Flow Analysis section.

Figure 5.

a) Cold flow and b) BFBG experimental setups.

Material	Polymethyl-methacrylate
Reactor internal diameter (cm)	3.81
Reactor tube height (cm)	40.64
Transition cone height (cm)	2.54
Freeboard internal diameter (cm)	7.62
Freeboard height (cm)	25.4

Table 2.

Cold flow rig characteristics.

The BFBG test stand consists of the BFBG reactor, a top-load furnace, a screw feeder, and a micro gas chromatograph to analyze the gas composition of the synthetic gas acquired from the gasification tests. A BFBG reactor was installed inside the furnace, which can reach temperatures of 1500°C. The temperatures inside the reactor, just above the inert material, and on the wall were measured with K-type thermocouples. The maximum temperature measured during the experiments was around ${820}^{\mathrm{o}}\mathrm{C}$, which is adequate to provide the required heat transfer rates for a successful gasification reaction. The BFBG reactor was made of inconel steel with similar dimensions as the cold flow test rig. The BFBG test stand also measured pressure drop to investigate the effect of elevated temperatures on bed pressure drop. Tests were conducted at the same time interval for the 200 and 300-g unary sand mixtures. The study conducted by Sivri [3] contains a detailed description and information about the design of the test stands.

2.2 Material analysis and preparation

In this study, the cold flow and actual BFBG tests used coal as the feedstock and sand or glass beads as the bed material, respectively. It was Pittsburgh coal seam number eight that was used as feedstock. Glass beads from Ballotini and commercial-grade fine silica sand from Quikrete brands were used as bed materials. The results of moisture, volatile, ash, and elemental analyses of the biomass and coal are displayed in Table 3. Further, the size and sphericity analyses with their distributions were conducted with a dynamic image analysis method (Sympatec GbmH, Model QICPIC) and skeletal densities were analyzed by a gas pycnometer (AccuPyc, Model 1330 Helium Pycnometer). The results of the size, sphericity, and density analyses for the feedstock and inert materials are summarized in Table 4. Glass beads showed the highest mean sphericity of 0.93 compared to 0.85 for coal, and 0.86 for sand, respectively. Sand and glass beads have a narrower sphericity (90% between 0.85 and 0.95) and size (90% between 235 and 347 μm) distribution compared to coal as well. Coal has a bigger mean diameter of 362 μm compared to glass beads (271 μm) and sand (324 μm). Hence, the coal and glass beads mixture has better packing, which enhances the fluidization characteristics (Figure 6).

	Carbon (C), (%)	Hydrogen (H), (%)	Oxygen (O), (%)	Sulfur (S), (%)	Moisture (%)	Ash (C), (%)
Coal, Pittsburgh #8	73.62	4.38	7.83	2.59	3.69	7.89

Table 3.

Elemental and proximate analysis (by mass) of bituminous coal.

Material	Average of sphericity	Sauter mean diameter (μm)	Bulk density ($\mathrm{g}/{\mathrm{cm}}^3$)	Skeletal density ($\mathrm{g}/{\mathrm{cm}}^3$)	Geldart’s group
Coal	0.85	362	0.7	1.36	B
Sand	0.86	324	1.43	2.64	B
Glass beads	0.93	271	1.46	2.48	B
Coal (wt%4)and Sand	0.86	325	1.56	2.59	B
Coal (wt%4)and Glass beads	0.93	282	1.61	2.44	B

Table 4.

Material size, density, sphericity, and Geldart’s group analysis.

Figure 6.

Particle size and sphericity distributions.

3. Cold flow analyses

BFBG deep-bed applications have a higher gasification efficiency due to improved heat transfer rates and a longer gas residence time. However, top-fed deep-bed application of the BFBG is quite complicated, not only because of the requirement to deliver the feedstock particles homogeneously on top of the bed, but also because of the requirement for an optimum homogeneous binary mixture to obtain the most efficient gasification process. Hence, the observation of the fluidization process in cold flow conditions is required to better understand the intricate hydrodynamics of coal-top-fed deep-bed $\left(H_p/D_b\gtrsim 2\right)$ binary mixtures.

3.1 Mixing and fluidization behavior of coal with sand or glass beads mixtures

The mixing and fluidization behavior of binary mixes of coal with two separate inert components (glass beads and silica sand) was analyzed in this section. The total masses of the binary mixtures investigated were 100, 200, and 300 g, respectively, including coal, which made up 4% of the total mass and was spread on top of the bed material. Bed pressure drop, $\mathrm{\Delta }{P}_b$, and minimum fludization velocity, $U_{\mathit{mf}}$, were analyzed as a function of superficial fluidizing gas velocity with a 0.0146 m/s (1 SLM) increments. The bed fluidization and mixing behavior were also observed with the high-speed camera at each flow rate after attaining fluidization. In Figures 7–12, each column picture under the pressure drop curve represents the bed behavior at the corresponding fluidizing gas velocity. With the use of these images, the ideal fluidizing-gas velocity interval was estimated to obtain an almost homogeneous (quasi-homogeneous) binary mixture with reliable fluidization behavior for the BFBG operation.

Figure 7.

Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and glass beads with the total mixture mass of 100 g.

Figure 8.

Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and sand with the total mixture mass of 100 g.

Figure 9.

Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and glass beads with the total mixture mass of 200 g.

Figure 10.

Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and sand with the total mixture mass of 200 g.

Figure 11.

Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and glass beads with the total mixture mass of 300 g.

Figure 12.

Bed pressure drop and fluidization behaviors with increasing superficial gas velocity for the mixtures of coal and sand with the total mixture mass of 300 g.

Coal and glass beads binary mixtures have higher bulk density ($1.61\mathrm{g}/{\mathrm{cm}}^3>1.56\mathrm{g}/{\mathrm{cm}}^3$) and higher average sphericity (0.93 $>$ 0.86) (Table 4) compared to coal with sand mixtures. Hence, better fluidization and mixing behaviors were expected for the coal and glass beads mixtures. Later on, Case I and Case II represent the coal and glass beads mixture and the coal and sand mixture, respectively. The results obtained during the investigation of the mixing and fluidization behavior of the binary mixtures are shared in this section.

The bed pressure drop and fluidization behaviors of the binary mixtures of Case I and Case II for the total mixture mass of 100 g with increasing fluidizing gas velocity are shown in Figures 7 and 8, respectively. The initial static bed aspect ratio ($H_p/D_b$) measured as $\approx 1.83$ for Case I, and slightly higher $\approx 2$ for Case II due to the lower average sphericity and bulk density. In Case I, there was a smooth transition from the minimum fluidization to complete fluidization after the pressure curve climbed linearly up to the initiation of the fluidization. However, the transition was not smooth in Case II. The abrupt drop in bed pressure drop was followed by an increase in $U_g$, and $\mathrm{\Delta }{P}_b$ rose until all particles fluidized as a result of Case II’s greater particle size distribution and channeling. Tiny bubble formations due to channeling before complete fluidization were visible in Case II at the speeds between 0.117 and 0.146 m/s (Figure 8b columns 5–7). Besides, wider particle size distribution and less sphericity encoupled with the humidity effects of Case II caused to reach minimum fluidization velocity at a higher speed of 0.06 m/s compared to Case I with a $U_{\mathit{mf}}\approx 0.05$ m/s. As seen in Figure 7b, except for a tiny layer of coal on top of the glass beads in Case I, the bed was almost in a well-mixed state for the fluidization gas velocities of 0.088 and 0.103 m/s (Figure 7b third and fourth columns, respectively). A further increase in the flow rate resulted in smooth bubble formations and eventually a well-mixed state at around $U_g=0.12$ m/s. However, Case II required a higher $U_g$ of 0.16 m/s to achieve a well-mixed state (Figure 8b eighth column) due to the same reasons mentioned in the $U_{\mathit{mf}}$ comparison for both cases. In Case II, mixing happened just after reaching the complete fluidization velocity ($U_{\mathit{cf}}$) of $\approx 0.14$ m/s because of the sudden breakdown of the interparticle and cohesive forces. At higher gas velocities after reaching the complete fluidization, flat slug formations were not observed due to the low bed aspect ratios, which were around two for both cases.

Cases I and II were also tested for the total mixture mass of 200 g with the coal making up 4% of the total mass. Figures 9 and 10 demonstrate the fluidization behavior and pressure drop changes with increasing gas velocity for Cases I and II, respectively. Static bed aspect ratios $\left(H_p/D_b\right)$ were 3.5 and 3.66 for Cases I and II, respectively. In both Cases, the bed pressure drop showed a linear increase during the fixed-bed state. As in the previous test, the transition from fixed bed state to fluidization was smoother for Case I compared to Case II due to the reasons mentioned earlier. Furthermore, in Case I, mixing began earlier at $U_g\approx 0.074$ m/s, just after reaching the complete fluidization, with the penetration of glass beads into the coal layer. $U_g\approx 0.088$ m/s achieves a nearly well-mixed state, except for the tiny coal layer on top of the bed. But, in Case II, complete fluidization was achieved relatively late at a gas velocity of $U_g\approx 0.09$ m/s, and complete mixing could be achieved at the gas velocity of 0.103 m/s with a narrow coal layer on top. Another factor, except the sand characteristics, that contributed to the delay in reaching the complete fluidization and well-mixed state was the higher relative humidity of the fluidizing gas (air), which strengthened the interparticle forces and cohesiveness. In both cases, $U_{\mathit{mf}}$ was measured graphically around 0.05 m/s. Further increase in $U_g$ caused slug formations, which can be seen in both Cases. Despite the slug formations, the well-mixed states achieved in both Cases (Figure 9b and 10b, columns 4–11), supported by the same images, demonstrate almost a homogeneous distribution of the coal particles inside the bed material. Some of the coal dust particles were stuck on the bed wall in Case II due to the higher ambient humidity.

Later on, experiments were conducted to test the fluidization behaviors for a deeper bed with bed aspect ratios of 5.16 and 5.66 and a total mass of 300 g, with the coal making up 4% of the total mass for Cases I and II, respectively. Fluidization behaviors and pressure drop attitude with increasing gas velocity for Cases I and II with the total mass of 300 g are shown in Figures 11 and 12, respectively. Similarly, bed pressure drop increased linearly during the fixed-bed state for both Cases as in the previous test conducted with the total masses of 100 and 200 g, respectively. Despite the higher bed aspect ratio compared to the previous tests, Case I still showed a smooth transition from static state to fluidization as seen in Figure 11. On the contrary, transition was relatively stiff for Case II. Due to the stronger interparticle forces and cohesiveness in Case II peak pressure drop value obtained was higher ($\approx 3.3$ kPa) compared to Case I ($\approx 2.4$ kPa). After reaching the peak pressure drop in Case II, there was an abrupt fall to $\approx 2.4$ kPa. Case I and II reached the complete fluidization states at the approximate gas velocities of $\approx 0.07$ m/s and $\approx 0.075$, respectively. For both Cases after reaching the complete fluidization, coal particles started mixing with the sand particles at the contact of the segregated layers. Complete mixing is achieved at an approximate gas velocity of 0.09 m/s for Case I, and 0.1 m/s for Case II. As in the previous experiments conducted for 100 and 200 g total masses, a well-mixed state was achieved at a higher gas velocity in Case II compared to Case I for 300 g due to the same reasons mentioned before. Slug formations were visible for both Cases after the gas velocity of $\approx 0.1$ m/s (Figure 11b and 12b, columns 4–11). For both Cases, the bed could preserve its well-mixed state at higher gas velocities despite the narrow coal layer on top of the bed. Elutriation was observed at a gas velocity of 0.175 m/s and higher for Case II, and 0.207 m/s for Case I.

3.2 Summary and conclusions

The fluidization and mixing behaviors were investigated for Cases I (coal and glass beads) and II (coal and sand) mixtures with different total masses of 100, 200, and 300 g, with coal making up 4% of the total mass. Initially, the mixtures were in a segregated state in which coal was placed on top. The fluidization and mixing behaviors of the mixtures were observed as the gas velocity increased. The main conclusions drawn from this study were:

• Coal and glass beads mixtures showed better fluidization and mixing characteristics compared to coal and sand mixtures at each total mass tested due to their higher bulk density and average sphericity, which led to better packaging.

• Higher average sphericity and bulk density led coal and glass bead mixtures to reach complete fluidization and well-mixed conditions at lower gas velocities.

• Strong interparticle interactions between sand particles may result in higher peaks of bed pressure drop and cohesiveness, particularly when the fluidizing gas is air with a higher relative humidity than usual.

• Slug formations, which decrease mixing and fluidization quality, were more likely to be formed with an increasing bed aspect ratio greater than two.

• With increased gas velocity, both mixtures in Cases I and II could achieve and maintain a well-mixed state at bed aspect ratios greater than two.

4. Fluidization and gasification analyses at elevated temperatures

The effect of temperature on fluidization characteristics such as bed pressure drop and minimum fluidization velocity was investigated, and the results are shared in this section for sand material. Besides, gasification test results for the binary mixture of coal and sand are shared.

4.1 Effect of temperature on the bed pressure drop and minimum fluidization velocity

The bed pressure drop versus gas velocity was studied for unary sand mixtures of 200 and 300 g at elevated temperatures ranging from ${200}^{\mathrm{o}}\mathrm{C}$ to ${805}^{\mathrm{o}}\mathrm{C}$ (Figure 13). The bed aspect ratios measured were 3.3 and 4.83 for the total masses of 200 and 300 g, respectively, in the cold flow test rig, with similar dimensions to the BFBG. Bed pressure drop increased linearly at all temperatures for both total bed masses of 200 and 300 g, respectively. A linear increase in the pressure drop was followed by a smooth transition to complete fluidization in all cases. However, for the mass of 300 g, small pressure drop peak formations were visible due to higher wall effects. After complete fluidization, the pressure drop continued to increase with the increasing gas velocity due to three main factors: wall effects, increased bed voidage [22], and stronger interparticle forces particularly for this narrow-sized (3.81 cm diameter) deep-bed reactor. Similar results of increasing bed pressure drop due to wall effects were also reported by Srivastava and Sunderasan [23] and Olatunde et al. [24]. While the minimum fluidization condition’s bed pressure drop increased with temperature, $U_{mf}$ decreased for both total masses of 200 and 300 g (from 0.086 to 0.063 m/s for 200 g, and from 0.096 to 0.062 m/s for 300 g). The decrease in the $U_{\mathit{mf}}$ can be attributed to the higher interparticle forces [22], and lower $\mathit{Re}$ numbers at higher gas temperatures, which lead to higher gas viscosity and increase the drag coefficient.

Figure 13.

Bed pressure drop versus fluidization gas velocity at different temperatures. G.

4.2 Gasification analysis

The findings of the gasification of coal with 10% steam and air in the BFBG are presented in this subsection. The elemental composition of coal (Table 3) reveals that it has a low oxygen content, demanding the use of an outside oxygen source to create the appropriate syngas composition. As a fluidizing agent, a mixture of air and steam was used. And sand was preferred due to its lower thermal conductivity for a better thermal management of the reactor. When 10% steam was added to the coal feed, an average ratio of ${\mathrm{H}}_2/\mathrm{CO}=3.23$ was observed ignoring the nitrogen content. Gasification tests were performed in a slightly fast fluidization regime to assure sudden mixing and enhanced particle-particle and particle-gas interaction, which generates better heat transfer between the bed material (sand) and the feedstock (coal) particles and improves reaction kinetics. The complex gasification reaction kinetics interact strongly with fluidization hydrodynamics, particularly bubble formations; shape, frequency, and so on. As a result, fluidization hydrodynamics should be studied in greater depth in order to better understand the complicated process of gasification and its interaction with fluidization hydrodynamics. Initially, the temperature of the bed medium was elevated to the desired temperatures (around ${800}^{\mathrm{o}}\mathrm{C}$) to achieve an effective gasification process. Temperature was measured on the reactor wall, at the level of the bed material, and inside the bed, just above the bed material, to assure an accurate temperature measurement to start the gasification process. Later, for each supply, 2 g of coal were fed by the supply line using pressurized nitrogen. Coal flow rate was maintained at the rate of 2 g/min. The average syngas composition at the initial stage of the pyrolysis, that liberates hydrogen component in coal first before proceeding the carbon gasifying status, and its low heating value obtained during the tests are shared in Table 5.

H${}_2$, (%)	CO, (%)	${\mathrm{CO}}_2$, (%)	${\mathrm{CH}}_4$, (%)	LHV, (${\text{MJNm}}^{-3}$)
50	16	17	15	11.72

Table 5.

Syngas composition and low heating value.

4.3 Summary and conclusions

The effect of the temperature on the BFBG fluidization hydrodynamics was analyzed and the initial gasification test results were shared in Chapter 4. The main conclusions are:

• The minimum fluidization velocity decreases with the increasing temperature. It can be related to the stronger interparticle forces and lower wall effects with the increasing temperature.

• The gasification results show that syngas can be generated successfully by using the top-fed feeding system. And the product gas can be used as primary or dual fuel in internal combustion engines and solid oxide fuel cell applications.

Acknowledgments

This work was supported by US Department of Energy’s Fossil Energy Advanced Gasification Program. The Research was executed through NETL Research and Innovation Center’s Advanced Gasification effort within the Advanced Reaction Systems FWP under the RSS contract 89243318CFE000003. The author gratefully acknowledge WVU’s CIGRU, MAE, and CBE departments for their assistance with comprehensive mechanical, electronic, and data-acquisition hardware/software systems, respectively. The research was conducted at the WVU’s Advanced Combustion Laboratory in Morgantown, WV.

Disclaimer

This work was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with Leidos Research Support Team (LRST). Neither the United States Government nor any agency thereof, nor any of their employees, nor LRST, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.