Riser-Based Carbon Stripper for Coal-Fueled Chemical Looping

Article pubs.acs.org/IECR

Riser-Based Carbon Stripper for Coal-Fueled Chemical Looping Combustion Hongming Sun, Mao Cheng, Zhenshan Li,* and Ningsheng Cai Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Beijing Municipal Key Laboratory for CO2 Utilization & Reduction, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Carbon stripping is critical to preventing char particles from reaching air reactors. In this study, a riser-based carbon stripper is proposed to separate the lighter char particles from the heavier oxygen carrier particles. The performance of the proposed riser-based carbon stripper in the separation of plastic beads from ilmenite particles was experimentally investigated, where plastic beads were used to simulate the behavior of char particles. The effects of gas velocity and solids circulation rate on the separation characteristics were studied. The results showed that 2−63 wt % mixtures can be entrained, containing 17−100 wt % plastic beads. An appropriate gas velocity range for separating plastic beads from ilmenite particles is 2−2.25 m/s. The appropriate mixture feeding rate is below 23.7 kg/m2·s. The power cost analysis shows that the riser-based carbon stripper is more efficient than the larger fuel reactor strategy in increasing CO2 capture efficiency when using solid fuels that require long residence times. (1) In terms of fuel conversion,11−13 methods are still needed to increase the fuel combustion efficiency by minimizing the losses due to unconverted char and unconverted fuel gases. When solid fuels are well mixed with oxygen carrier particles, char gasification and the release of syngas and volatiles occurs in the entire volume of the fuel reactor, in contrast to the CLC of gaseous fuels where the gaseous fuels are fed from the bottom of the fuel reactor. Therefore, an amount of gaseous fuels inevitably escape from the fuel reactor. (2) Addressing the problem of ash discharging14 entails preventing ash from accumulating in the system. Unlike in traditional fluidized combustors where most of the bed material is ash, in a CLC system, the ash is mixed with the oxygen carrier particles, and most bed material is oxygen carrier. Therefore, an efficient method is needed to separate ash from oxygen carriers to reduce the losses of oxygen carriers caused by ash removal. Few studies have investigated this issue to date. (3) The issue of carbon stripping11−13,15,16 involves minimizing the amount of char particles slipping into the air reactor. Usually, char particles require longer residence times for complete conversion than is needed for the reduction of oxygen carriers. Therefore, char particles will reach the air reactor with the transfer of the oxygen carriers without carbon stripping, resulting in the burning of char in the air reactor and reduced carbon capture. The slipping of char into the air reactor should be avoided because the CO2 produced by char

1. INTRODUCTION Chemical looping combustion (CLC) is regarded as an effective means of reducing greenhouse-gas emissions because of the reduced costs due to its inherent CO2 separation. A typical CLC process employs a dual fluidized-bed system in which oxygen carriers, often in the form of metal oxide particles, transfer oxygen from one fluidized bed (the air reactor, AR) to the other fluidized bed (the fuel reactor, FR). The CLC process using gaseous fuels has been successfully demonstrated with several different reactor prototypes.1−6 Significant progress toward industrial demonstration has been made.7 In recent years, CLC with solid fuels has attracted a great deal of attention because of the lower cost and abundance of solid fuels. There are generally three approaches to employ solid fuels in CLC. The first approach is to gasify the char directly in the fuel reactor [in situ gasification (IG)-CLC]. The second strategy is to implement first a solid-fuel gasification unit to convert solid fuels to syngas and then introduce the syngas into the CLC unit. The last strategy is referred to as chemical looping with oxygen uncoupling (CLOU).8 CLOU can be realized with only certain oxygen carriers, such as Cubased,9 Co-based, and Mn-based10 materials, within certain temperature ranges. Considering that oxygen carriers might be contaminated by the ash of solid fuels, oxygen carriers that have low costs and are suitable for CLOU need to be screened out. IG-CLC has a higher efficiency comparing to implementing a solid-fuel gasification unit because of the elimination of an air separation step which is high energy-consuming. In contrast to the successfully demonstrated CLC of gaseous fuels, IG-CLC still involves a number of unsolved problems, including fuel conversion, ash discharging, and carbon stripping. © XXXX American Chemical Society

Received: October 21, 2015 Revised: December 25, 2015 Accepted: February 12, 2016

A

DOI: 10.1021/acs.iecr.5b03970 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Table 1. Summary of Carbon Stripper Configurations with Thermal Input Higher than 10 kWth

fluidization regime. Sun et al.23 studied the separation efficiency and power costs of a carbon stripper operated in both the bubbling and turbulent fluidization regimes using a mixture of Plexiglas beads and ilmenite particles in a cold flow model. Their results showed that the separation efficiency of the Plexiglas beads was in the range of 0−60%. The carbon stripper resulted in 160% additional residence time in the reducing atmosphere for the fuel particles. Calculation of the power costs showed that the CS strategy was less competitive than the FR strategy in this case because of the low separation efficiency of the designed carbon stripper. In terms of fundamental fluidization principles, a carbon stripper operated in the bubbling or turbulent regime involves a binary mixture of particles, namely, lighter char particles and heavier oxygen carrier particles, and therefore, mixing and segregation phenomena are important for the separation of the lighter particles from the heavier particles. Keller24 investigated the mixing and segregation phenomena and concluded that a low superficial gas velocity (i.e., just above the minimum fluidization velocity) causes significant segregation whereas a high superficial gas velocity (i.e., in the bubbling regime) mixes the two components. Increasing the superficial gas velocity increases the degree of mixing of the bed. Therefore, in the turbulent fluidized regime, the bed tends to be well mixed by the bubbles, whereas when solid particles are thrown up into freeboard area, the lighter particles are entrained. In other words, separation occurs in the freeboard zone above the bed surface. The gas velocity of the carbon stripper should be increased to realize particle separation by using the different terminal velocities of different particles. Therefore, in this study, a riserbased carbon stripper was proposed and experimentally studied. First, a cold model of the riser with a solids mixture feeding system and a cyclone was constructed. Then, plastic beads were used to simulate char particles. A mixture of plastic beads and ilmenite particles was used as the bed material. The

burning will be diluted by N2 and pollutants such as SOx, NOx, heavy metals, and particle matter (PM2.5) will also be produced by char burning, so that additional expensive units would be required to reduce these pollutants and meet environmental regulations. Several different fuel-reactor prototypes have been proposed to improve the gas conversion of the fuel reactor. A two-stage countercurrent moving bed,17 a two-stage bubbling fluidized bed,18 and a bubbling fluidized bed coupled with a downer reactor19 have been studied experimentally. Apart from these two-stage reducers, a postoxidation chamber (POC) projected downstream of the fuel reactor to fully convert the combustible gases was proposed by Ströhle et al.20 However, the estimated cost of oxy-polishing is 6.5 €/ton of CO2, approximately onethird of the total estimated cost of 20 €/ton of CO2 avoided.21 A comparison of the additional costs needed for creating the oxidizing zones in different ways still remains to be investigated. To prevent the slipping of solid fuels, one strategy is to separate char particles from oxygen carrier particles with a carbon stripper (referred to as the CS strategy in this article). Another strategy is to guarantee sufficient residence time for char particles in the fuel reactor, which implies a larger fuel reactor (referred to in this article as the FR strategy). The FR strategy is relatively straightforward, although a detailed investigation of the separation characteristics of a carbon stripper (CS) is needed before the implementation of a CS in an industrial demonstration. Kramp et al.15 and Mendiara et al.22 suggested that a carbon stripper reduces CO2, SO2, and NO emissions in the air reactor and concluded that a carbon stripper is a critical unit for solid-fuel CLC. Table 1 lists the different carbon strippers that have been designed and experimentally investigated for chemical looping combustors, in hot or cold models. As shown in Table 1, all of the carbon strippers have been run with gas velocities (ug,CS) in the range of 0.15−0.4 m/s. For typical oxygen carriers used in CLC, the carbon strippers are run in the bubbling or turbulent B


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Industrial & Engineering Chemistry Research

therefore, the distribution of the plastic beads fraction could be obtained along the riser height. At the same heights as the sampling ports, 11 pressure ports were mounted to measure the pressure drops. These 11 pressure taps, pressure difference transducers, an A/D card, and a computer made up an online system measuring pressure differences. Based on the measured pressure drops, the solids volume fraction along the riser height could calculated. 2.2. Materials. In the cold model experiments, ilmenite particles were used as oxygen carriers, and plastic particles were used to simulate the behavior of char particles. The detailed properties of the ilmenite particles and plastic particles are listed in Table 2. ω indicates the weight loss ratio after burning

concentrations of plastic beads in the mixture were measured to study the separation characteristics of the riser-based carbon stripper. Finally, the power costs of the CS strategy were calculated and compared with those of the FR strategy.

2. EXPERIMENTAL SECTION 2.1. Riser-Based Carbon Stripper. The experimental setup used in this study was made of transparent Plexiglas. Figure 1 shows a schematic of the setup. It consisted of a feed

Table 2. Properties of Ilmenite Particles and Plastic Beads Used in This Study particle type

dp(0.5) (μm)

size (μm)

ρp (kg/m3)

ut (m/s)

ω (%)

ilmenite plastic beads coal

257 94 −

140−440 70−130 −

4260 960 80016

5.65 0.39 −

1 100 −

at 1073.15 K. The particle density was measured using a true density meter (Micromeritics, Norcross, GA). Particle size distribution analyses were conducted using a Malvern Mastersizer (Malvern Instruments Ltd., Malvern, U.K.). 2.3. Experimental Procedures. For a given mixture composition, the two kinds of particles were first mixed in the feed hopper through turbulent fluidization. Then, the mixture flow rate was calibrated by adjusting the opening of the gate valve. Second, the feed hopper was hung so that it was connected with the riser inlet. Air at a particular flow rate and pressure (20 kPa) was introduced into the riser. Third, the ball valve was switched on, resulting in mixture injection. Fourth, after the system had reached a steady state, which was confirmed by a steady pressure difference at the bottom of the riser (ports 1−3), all of the valves of the sampling ports were turned on, starting the collection of mixture samples. Fifth, after the completion of the sampling process, 15 mixture samples (11 samples at different heights of the riser, 2 samples from the top tank, and 2 samples from the bottom tank) were collected and ready for composition analysis by the combustion method. Finally, this case was finished with cleaning of all of the sampling ports. Table 3 shows the ranges of the operating variables used in the present work.

Figure 1. Schematic of the experimental setup of the riser-based carbon stripper.

hopper (5), a riser with a diameter of 0.07 m and height of 4.03 m (from the gas distributor to the top mixture exit), a top solids collection tank (6), and a bottom solids collection tank (8). The gas distributor had 41 holes with a diameter of 3 mm, accounting for a free area of 7.5%. Below the gas distributor was a wind box with a height of 0.15 m. Compressed air was used as the fluidizing agent of the riser. The flow rate and pressure (20 kPa) of the inlet air were regulated by two gate valves (2), a rotameter (3), and a pressure gauge (4). A cylindrical fluidized bed (5) was used as the feed hopper, which controlled the solids circulation rate in the riser with a precalibrated gate valve (11) and a ball valve (2). The mixture entrained out the top of the riser was separated from the air by a cyclone (7) and collected in the top solids collection tank (6). The remaining particle stream was diverted into the bottom solids collection tank (8) through a bottom discharging pipe. The electrostatic forces generated among the particles and on the reactor were eliminated by grounding the reactor and the particles with copper wire. The solids concentrations were sampled with 11 particle sampling ports mounted axially on the riser. Because the system had a positive pressure, when the ball valve was switched on, mixtures were ejected, and the filter bags separated the solids from the air. In this way, particle mixtures at different heights of the riser were sampled. The solids mixture contained both ilmenite and plastic beads. The mass fraction of plastic beads in the mixture could be measured by the combustion method, and

Table 3. Ranges of Operating Parameters Used in the Present Work

a

variable

units

range

Gmix,in cpb,in Hinlet ug

kg/m2·s % m m/s

12.2 ± 1.6, 23.7 ± 2.6, 34.8 ± 3.4a 5 1 1.5, 1.75, 2, 2.25, 2.5, 2.75

Referred to as Gmix,in1, Gmix,in2, and Gmix,in3, respectively.

The concentration of plastic beads in each mixture sample was determined by the combustion method. Specifically, the mixture of ilmenite and Plexiglas particles was heated in a muffle oven at 1073.15 K for 30 min, after which the plastic beads were completely burned to form CO2 and H2O whereas the weight loss of ilmenite particles was about 1%. Therefore, C


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Figure 2. Effects of gas velocity on the solids volume fraction along the riser height for different solids injection rates: (a) 12.2 ± 1.6, (b) 23.7 ± 2.6, and (c) 34.8 ± 3.4 kg/m2·s.

the fraction of plastic beads could then be calculated according to the different weight loss ratios c pb = (ωmix − ωilm)/(1 − ωilm)

η=

mmix,upc pb,top mmix,topc pb,top + mmix,btc pb,bt

(3)

3. RESULTS 3.1. Distribution of the Solids Mixture along the Riser. The particle distribution was analyzed by calculating the solids volume fractions from the experimental data on pressure drop. It was assumed that the contributions to the pressure drop from particle acceleration and frictional resistance were negligible. Such assumptions are not valid in all parts of the riser, especially the part below the solids feeding point (port 5) where particles are accelerated. However, the calculated solids volumes are good first approximations. With this assumption, the solids volume fraction between two pressure measuring ports was calculated as

mmix,top mmix,top + mmix,bt

mpb,top + mpb,bt

=

(1)

The separation ratio is defined as the ratio of the amount of mixture collected from the top solids tank to the total amount of the mixture collected from both the top and bottom tanks

α=

mpb,top

(2)

where mmix,top is the mass of the mixture collected from the top tank and mmix,bt is the mass of the mixture collected from the bottom tank. The separation efficiency of lighter particles is defined as the ratio of the mass of plastic beads collected from the top tank to the mass of plastic beads collected from both the top and bottom tanks

εs, i − j ≈ D

Δpi − j ρp g Δhi − j

(4) DOI: 10.1021/acs.iecr.5b03970 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


Figure 3. Effects of gas velocity on the axial concentration of plastic beads at different solids circulation rates: (a) Gmix,in1, (b) Gmix,in2, (c) Gmix,in3.

where Δpi−j is the pressure difference between port i and port j. The pressure differences were averaged after the system had stabilized, and then the mixture volume fractions were calculated according to eq 4. Figure 2 shows the concentrations of beads in the mixture along the riser. The solids volume fraction (εs) decreased with increasing height. The solids distribution can be divided into two ranges according to the different values. Below port 5, εs decreased dramatically with increasing height (ε1−3 to ε3−5). However, εs remained almost constant from ε5−8 to ε8−11. For the part of the riser above port 3, as ug increased from 1.5 to 2.75 m/s, more particles could be entrained; therefore, ε3−5, ε5−8, and ε8−11 increased. ε5−8 and ε8−11 increased from 0.001 to 0.005 with the increase in ug. However, for the part of the riser below port 3, the fluidizing gas, on one hand, elutriated the solids above port 3 and, on the other hand, prevented the solids from venting out from the bottom discharging pipe. Therefore,

ug did not have a significant effect on εs in this section. As the solids injection rate increased from 12.2 ± 1.6 to 34.8 ± 3.4 kg/ m2·s, ε5−8 and ε8−11 remained almost constant, whereas ε3−5 did not show much difference and ε1−3 increased significantly from 0.018 to 0.03 and then to 0.09. Therefore, it can be concluded that ε3−5, ε5−8, and ε8−11 are more affected by ug whereas ε1−3 is more affected by Gmix,in. Consider the particles enclosed in a cube with a side length of L. The volume fraction of the dispersed phase is

εs =

πd p 3 6L3

(5)

Therefore, L/dp can be calculated from the solids volume fraction E


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Figure 4. Effects of solids feeding rate on the axial concentration of plastic beads.

⎛ π ⎞1/3 L =⎜ ⎟ dp ⎝ 6εs ⎠

to the bottom of the bed, and the lighter plastic beads are entrained upward. This phenomenon occurs in the entire volume of the riser, leading to an increase in the concentration of plastic beads with riser height. The concentration of plastic beads near the feeding point is higher than those at the neighboring points. With an increase in ug, the concentration of lighter particles decreases, because more ilmenite particles than plastic beads are entrained upward to the top tank with increasing ug. Figure 4 shows the effects of the solids feeding rate on the axial plastic beads concentration. It is evident that the solids feeding rate does not affect the concentration of plastic beads significantly. However, the concentration of plastic beads first increases with the feeding rate and then decreases. Based on this analysis, 23.7 kg/m2·s should be an appropriate solids feeding rate. 3.3. Phase Diagram of Lighter and Heavier Particles. Figure 5 shows the concentration−velocity phase diagram for the plastic bead−ilmenite system. The cpb,bt−ug line shows the effects of the gas velocity on the concentration of plastic beads at riser bottom, and the cpb,top−ug line shows the effects of the

(6)

The ranges for dilute and dense flows can be defined by the spacing between two individual particles: For L/dp ≥ 10, individual particles can be treated as “isolated” with little influence of the neighboring particles on the drag. For 10 ≥ L/ dp ≥ 3, the flow can be considered “collision-dominated”, with both collisions and the drag of the fluid affecting the trajectory of the particles. Finally, for 3 ≥ L/dp ≥ 1, collisions play a more important role than drag, as the particles have no time to respond to the fluid dynamic forces before the next collision, and the flow is dense;26 in this case, the system is considered “contact-dominated”. Therefore, the solids volume fraction ranges for isolated, collision-dominated, and contact-dominated systems are 5 × 10−4 ≥ εs, 2 × 10−2>εs ≥ 5 × 10−4 and εs ≥ 2 × 10−2, respectively. As shown in Figure 2a, the entire riser is in the collisiondominated range. With the increase in the solids flow rate from Gmix,in1 (12.2 ± 1.6 kg/m2·s) to Gmix,in2 (23.7 ± 2.6 kg/m2·s), εs,1−3 is in the contact-dominated range. When the solids inlet flow rate is Gmix,in3 (34.8 ± 3.4 kg/m2·s), the region below port 5 is in the contact-dominated range, whereas the region above port 5 is in the range of 0−0.008, which is in the collisiondominated range. Only in a few experimental cases of this work did εs fall into the isolated range. The fact that εs does not change much above port 5 indicates that solids entrained above port 5 do not settle down below port 5. Otherwise, εs decreases with height above port 5. Therefore, collision and contact between the particles play important roles in the present riser. 3.2. Distribution of the Plastic Beads along the Riser. The preceding section discusses the distribution of the solids mixture along the riser height. It should be noted that the solids represent a binary mixture of particles and that what is more important is the distribution of lighter particles along the riser. Figure 3 shows the influence of the riser gas velocity on the axial solids concentrations of the lighter particles. The concentrations of the plastic beads increase with increasing riser height, which is actually the fundamental principle of the riser-based carbon stripper: The heavier ilmenite particles settle

Figure 5. Effects of gas velocity and the solids flow rate on the phase diagram. F


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Figure 6. Effects of operating velocity on the (a) entrainment flow rate and (b) separation ratio.

velocity. In these cases, the solids supply, which is characterized by Gmix,in, is much greater than Gmix,top. Therefore, the effect of increasing the solids feeding rate on the solids entrainment rate is not significant. Therefore, the separation ratio decreases with unchanging Gmix,top and increasing Gmix,in, as shown in Figure 6b. From cpb,top, cpb,bt, and the solids separation ratio, the separation efficiency can be calculated. Figure 7 shows the

gas velocity on the concentration of plastic beads at the riser top. As shown in Figure 5, cpb,bt, cpb,top, and (cpb,top − cpb,bt) all decrease with increasing ug. This is because, as ug increases, more ilmenite particles are entrained toward the top solids tank, leading to a decrease of cpb,top. Meanwhile, a higher gas velocity indicates a stronger carbon stripping effect, so that more plastic beads are entrained to the top, resulting in a decrease in cpb,bt even to zero. For cases with gas velocities higher than 2.25 m/s, cpb,bt does not change much with increasing gas velocity. cpb,bt for Gmix,in1 is near zero, cpb,bt for Gmix,in2 is around 0.15, and cpb,bt for Gmix,in3 is around 0.25. As shown in Figure 5, the solids feeding rate Gmix,in does not significantly affect cpb,top. However, the concentration of plastic beads at the bottom is greatly affected by Gmix,in. Because the solids mixture is fed into the system from near port 5 with a downward velocity, particles enter the region of ports 1−5, where they are accelerated. On one hand, most of the particles that can be entrained in the region of ports 5−11 will not settle down. On the other hand, those particles that cannot be entrained stay in the region of ports 1−5, where most of the separation occurs. Therefore, the fluidization status of the particles in the region of ports 1−5 is decisive to cpb,bt, whereas the fluidization status of the particles in the region of ports 5− 11 is decisive to cpb,top. From the previous analysis of L/d, as the solids feeding rate increases from Gmix,in1 to Gmix,in2 and then to Gmix,in3, the region of ports 1−5 becomes denser and denser, from collision-dominated to contact-dominated, whereas the region of ports 5−11 remains almost unchanged in the collision-dominated regime. Therefore, the solids feeding rate Gmix,in does not significantly affect cpb,top, whereas cpb,bt is greatly affected by Gmix,in. 3.4. Particle Separation Ratio and Separation Efficiency. The previous study showed how the solids distribution in the riser is affected by ug and Gmix,in. As a result, the solids entrainment will also be affected by ug and Gmix,in. Figure 6a shows the effects of the gas velocity on the entrainment rate, Gmix,top. The entrainment rate increases as ug increases. This is because the driving force of the entrainment is the carrying ability of the fluidizing gas, which is characterized by the gas

Figure 7. Effect of operating velocity and solids inlet flow rate on the separation efficiency.

effects of the gas velocity on the separation efficiency. The separation efficiency increases almost linearly as ug increases from 1.5 to 2.25 m/s, and then the separation efficiency remains almost unchanged. Therefore, 2.25 m/s is an appropriate operating gas velocity for the riser. When Gmix,in equals 12.2 kg/m2·s, almost 100% of the plastic beads are separated from the solids mixture when ug exceeds 2.5 m/s. The separation efficiency decreases as ug increases from 1.5 to G


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Industrial & Engineering Chemistry Research 2.25 m/s. When Gmix,in equals 23.7 kg/m2·s, 88% of the plastic beads are separated when ug exceeds 2.5 m/s, and when Gmix,in equals 34.8 kg/m2·s, only 60% of the plastic beads are separated. This is because the upward motion of the plastic beads is hindered by collisions with ilmenite particles. As Gmix,in increases from 12.2 to 34.8 kg/m2·s, εs,1−5 increases from the collision-dominated to the contact-dominated regime, resulting in more intense collisions between the ilmenite particles and the plastic beads. Therefore, the separation efficiency decreases with increasing solids mixture feeding rate.

where QFR is the mass flow rate of fluidization gas in the fuel reactor, ΔpFR is the pressure difference of the fuel reactor, and ηfan is the overall efficiency of the fan. For the CS strategy, additional power is needed for carbon stripper fluidization. Therefore, the additional power cost is the fan power cost PCSstrategy = Pfan,CS = Q CSΔpCS /ηfan = Q CSΔpCS /ηfan (12)

Thus, a normalized power cost index for a carbon stripping strategy, defined as the ratio of the residence time increase ratio per additional power cost, can be calculated as

4. DISCUSSION The purpose of the two strategies of carbon stripping, namely, the FR strategy and the CS strategy, is to increase the residence time of the fuel particles in the reducing atmosphere. Because the FR strategy uses a higher fuel reactor bed mass to increase the residence time of the solid particles, the increasing ratio of the residence time can be defined as λFR =

tr,fp,1 tr,fp,0

−1=

(mFR + ΔmFR ) Fmix,CS2AR mFR Fmix,CS2AR

ψFR =

ψCS = −1

λCS =

tr,fp,0

−1=

c pb,av(FR − CS) c pb,CS2AR

−1≈

Cpb,av(FR) Cpb,CS2AR

γPsystem

dp ′ ug,FR dh (V FR

− VFR )VFR

=

ηfanPsystem Q FR ΔpFR

PCSstrategy

⎛c ⎞ η Psystem mix,in fan = ⎜⎜ − 1⎟⎟ c Q ⎝ pb,bt ⎠ CSΔpCS

which equals 7 × 10−4

(14)

ṁ mix,in(1 − α) ṁ mix,70kW

. Therefore, ψCS equals

⎛c ⎞ η Psystem mix,in fan ψCS = ⎜⎜ − 1⎟⎟ c Q ⎝ pb,bt ⎠ CSΔpCS

(

ṁ

(1 − α)

−4 mix,in ⎛c ⎞ ηfan 7 × 10 ṁ mix,70kW mix,in = ⎜⎜ − 1⎟⎟ uCSA CSΔpCS ⎝ c pb,bt ⎠

−1 (8)

) (15)

Figure 8 shows the effects of the gas velocity on ψ/ηfan. The purple dash-dotted line shows that the ψFR/ηfan ratio equals 3080. It is evident that ψ/ηfan is significantly affected by the

where cpb,av(FR−CS) is the average concentration of plastic beads in the fuel reactor and the carbon stripper, cpb,av(FR) is the average concentration of plastic beads in the fuel reactor, cpb,CS2AR is the concentration of plastic beads in the solids stream from the carbon stripper to the air reactor, and tr,fp,2 is the residence time of the fuel particles in the fuel reactor and the carbon stripper due to the application of the CS strategy. In the present study, mCS is negligible compared to mFR. Therefore, it is assumed that (mFR + mCS)/m ≈ 1 and cpb,av(FR−CS) ≈ cpb,av(FR), and we can write λCS ≈

′ − VFR )Psystem ηfan(V FR

where Psystem is the thermal input power of the CLC system and VFR is the solids volume in the fuel reactor. ψFR is calculated from the experimental results for ΔpFR and Psystem in the cold model of a 70-kW chemical looping combustor of solid fuels.23 In the designed combustor, the solids circulation rate is 0.2 kg/ s. In the meantime, the solids circulation rate, ṁ mix,in, is linearly proportional to the power of the system. Therefore, the psystem value used to calculate ψFR is calculated linearly proportionally,

(7)

(mFR + mCS)c pb,av(FR − CS) Fmix,CS2AR c pb,CS2AR mFR Fmix,CS2AR

PFRstrategy

≈

(13)

where mCS and mFR are the bed masses of the carbon stripper and the fuel reactor, respectively; Fmix,CS2AR is the solids circulating rate, mFR is the originally designed bed mass of the fuel reactor, ΔmFR is the additional bed mass of the fuel reactor due to the application of the FR strategy, tr,fp,0 is the originally designed residence time of the fuel particles in the fuel reactor, and tr,fp,1 is the residence time of the fuel particles in the larger fuel reactor due to the application of the FR strategy. The CS strategy separates the fuel particles from the ilmenite particles, decreasing the concentration of the fuel particles. Therefore, the increasing ratio of the residence time can be defined as tr,fp,2

λPsystem

−1 (9)

The fan power of the fuel reactor can be calculated as Pfan,FR = Q FR ΔpFR /ηfan

(10)

For the FR strategy, because the bed mass of the fuel reactor is increased, additional fan power is needed for fuel-reactor fluidization, which can be calculated as ′ ′ ΔpFR ′ − Q FR ΔpFR ) PFRstrategy = Pfan,FR − Pfan,FR = (Q FR /ηfan

(11)

Figure 8. Effects of gas velocity and solids feeding rate on ψ/ηfan. H



■

solids feeding rate: An increase of Gmix,in leads to a decrease of ψ/ηfan. With Gmix,in3, ψCS/ηfan is less than ψFR/ηfan, which means that the FR strategy is more efficient in carbon stripping in these cases. However, when the separation efficiency approaches 1, cpb,bt approaches 0, leading to a greatly increased ψCS/ηfan ratio approaching infinity. Therefore, when the solids feeding rate equals Gmix,in1, ψCS/ηfan is much greater than ψFR/ ηfan, and when the solids feeding rate equals Gmix,in2, ψCS/ηfan is close to ψFR/ηfan. Let ψCS/ηfan equal ψFR/ηfan. Then, λ can be calculated to be in the range of 1−4. Therefore, the following guidelines can be concluded ⎧ FR strategy λ

Riser-Based Carbon Stripper for Coal-Fueled Chemical Looping

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