Steam gasification of coal cokes in an internally circulating fluidized bed of thermal storage material for solar thermochemical processes

Steam gasification of coal cokes in an internally circulating fluidized bed of thermal storage material for solar thermochemical processes

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Steam gasification of coal cokes in an internally circulating fluidized bed of thermal storage material for solar thermochemical processes Nobuyuki Gokon a,*, Takuya Izawa b, Takehiko Abe b, Tatsuya Kodama b,c a

Center for Transdisciplinary Research, Niigata University, 8050 Ikarashi 2-nocho, Nishi-ku, Niigata 950-2181, Japan b Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan c Department of Chemistry & Chemical Engineering, Faculty of Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan

article info

abstract

Article history:

A laboratory-scale prototype windowed internally circulating fluidized-bed reactor made of

Received 19 March 2014

quartz sand and coal coke particles was investigated for steam gasification using

Received in revised form

concentrated Xe-light radiation as the energy source. The quartz sand was used as a

16 May 2014

chemically inert bed material for the fluidized bed, while the coal coke particles functioned

Accepted 19 May 2014

as the reacting particles for the endothermic gasification reaction. The advantages of using

Available online 13 June 2014

quartz sand as the bed material for the directly irradiated gasification reactor are as follows: (1) The bed height is maintained at a constant level during the gasification. (2) The

Keywords:

quartz sand functions as a thermal transfer/storage medium inside the reactor. The

Solar reactor

gasification performances such as the production rates of CO, H2, and CO2; carbon con-

Fluidized bed

version; and light-to-chemical energy conversion were evaluated for the fluidized-bed

Coal cokes

reactor with a thermal transfer/storage medium (quartz sand). The effects of using the

Steam gasification

bed material (quartz sand) on the gasification performance are described in this paper.

Thermal storage

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Beam-down optics

Introduction Concentrated solar radiation can be utilized to provide the heat necessary to drive various highly endothermic chemical reactions for renewable fuel production including direct or multistep thermochemical water-splitting cycles using redox metal oxide, steam/CO2 reforming of methane/hydrocarbon, steam/CO2 gasification of coal, cokes, biomass, or other carbonaceous materials. The coal gasification process

involves two basic stages: pyrolysis of coal and gasification of char [1,2]. The pyrolysis stage can be approximately shown by the following reaction: Coal / coke (carbon) þ CO þ CO2 þ H2 þ CH4 þ tars

(1)

The pyrolysis stage needs a smaller amount of heat than the gasification stage. Conversion kinetics of gasification of char, for which the reaction time is an order of magnitude larger than that of pyrolysis, controls the performance of the

* Corresponding author. Tel./fax: þ81 25 262 6820. E-mail address: [email protected] (N. Gokon). http://dx.doi.org/10.1016/j.ijhydene.2014.05.124 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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gasifier [2]. Thus, solar gasification is a promising key technology for thermochemical conversion, which can produce clean chemical fuels by using high-temperature solar heat. The greatest advantage of solar-driven gasification is the storage of a significant fraction of solar energy as the chemical energy of the synthesized fuel molecules, and the fuels can reduce the net CO2 emissions to the environment and conserve fossil fuels [3e6]. The main reactions for the steam and CO2 gasification of coke (carbon) are shown as follows: C (coal) þ H2O(l) ¼ CO þ H2 C (coal) þ CO2 ¼ 2CO

DH298 K ¼ 175 kJ mol1

DH298 K ¼ 172 kJ mol1

(2) (3)

The calorific value of coal or carbon feed can be theoretically upgraded by ~45% when the process heat required to drive the reactions, as shown in Equations (2) and (3), is provided by concentrated solar radiation. Syngas obtained using solar energy can be thermochemically transformed to hydrogen by the wateregas shift reaction, to liquid hydrocarbon fuels such as diesel, kerosene, and gasoline via FischereTropsch synthesis, or directly used as a combustion fuel for power generation. Moreover, ammonia, methanol, and dimethyl ether (DME) can be used as transportable liquid fuel. From the sunbelt countries (Australia, India, and China, etc.) with much solar energy and abundant coal reserves, the solar energy in the form of fuels can be stored and imported to Japan and other countries by tankers. Several kinds of reactor concepts have been proposed and demonstrated for the solar gasification process of carbonaceous materials. These solar reactors for the gasification process can be divided into two types in terms of the methods for heat supply to the particulate solid feedstock: indirectly irradiated reactors and directly irradiated reactors. For the indirectly irradiated reactors, packed-bed reactors [7e10], indirectly irradiated fluidized-bed reactors [11], entrainedflow reactors [12,13], serpentine-tubular reactors [14], and molten-salt reactors [15,16] have been proposed and investigated for solar gasification. Alternatively, for directly irradiated reactors in which the solid reactants are directly exposed to the concentrated solar radiation through the transparent window, packed-bed reactors [17e19], vortex-flow reactors [6,20e23], and fluidized-bed reactors [18,24e26] were developed and investigated for the solar gasification of coal and petroleum cokes. Among the directly irradiated reactors in which the solid reactants can easily reach high temperatures in comparison with the indirectly irradiated reactors, a fluidized-bed reactor can essentially overcome the limitations of mass and heat transfer, long solid residence times, and ash buildup that slow the reaction. Taylor et al. [18] demonstrated CO2 gasification in a fluidized-bed reactor consisting of coconut charcoal using a vertical silica-glass tube reactor and a 2 kW solar furnace. Murray and Fletcher [24] investigated the steam gasification in a fluidized bed of cellulose using a quartz reactor and a solar furnace. Von Zedtwitz and Steinfeld [25] modeled and investigated a quartz tube reactor containing a fluidized bed of coal particles for steam gasification. Muller et al. [26] studied the reaction kinetics for the steam gasification of a fluidized bed of coal using a quartz tube reactor.

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A newly developed solar reflective tower or beam-down optics was proposed as a promising solar concentrating system for solar fuel production [27e30]. The optical path of a beam-down system consists of a heliostat field that illuminates a hyperboloidal/ellipsoidal-shaped secondary reflector placed on a tower and directs the beams downward. In fact, this beam-down arrangement is advantageous over standard tower-top reactor arrangements because it allows a largescale reactor to be built closer to the ground; the solar radiation enters the reactor chamber through a transparent quartz window in the ceiling of the reactor. Both the upward and downward focal points of the concentrated solar radiation are essentially fixed, irrespective of the Sun's trajectory over time. Therefore, a solar gasification reactor utilizing a beam-down arrangement enables a large heliostat field compared to that in a conventional tower system; thus, a much higher concentration of solar energy can be achieved in one receiver. We developed fluidized-bed reactors containing coal coke particles for the thermochemical solar gasification with steam or CO2 [31e34]. A laboratory-scale prototype windowed reactor, designed in combination with a solar reflective tower or beam-down optics, was designed and demonstrated using concentrated Xe-light radiation as the energy source. In this study, to enhance the gasification rate for the internally circulating fluidized-bed reactor, quartz sand was investigated as a chemically inert bed material for the fluidized bed and as a thermal transfer/storage medium inside the reactor for coal coke gasification under direct light irradiation. A laboratoryscale prototype windowed internally circulating fluidizedbed reactor consisting of quartz sand and coal coke particles was investigated for steam gasification using concentrated Xelight radiation as the energy source. The gasification performances: production rates of CO, H2, and CO2; carbon conversion; light-to-chemical energy conversion were evaluated for the use of a thermal transfer/storage medium (quartz sand). The effects of using the bed material (quartz sand) on the gasification performance are described in this paper.

Experimental procedures and performance evaluation Preparation of coal coke and quartz sand Coal coke was gratuitously provided by Nippon Steel Corporation (currently, Nippon Steel & Sumitomo Metal Corporation). Table 1 shows the proximate and ultimate analyses of the coal coke. The density of the coke was 1.76 g cm3, and its calorific value was 28,600 kJ kg1. The coke was ground and sieved using mesh screens into four particle size ranges: <300 mm, 300e500 mm, 500e710 mm, and 710e1000 mm; coke particles of <300 mm and 300e500 mm were used as the carbonaceous resource for investigation. Alternatively, quartz sand was purchased from Japan Pure Chemical Co., Ltd. The quartz sand was sieved using mesh screens into four particle size ranges: 100e200 mm, 200e300 mm, 300e500 mm, and 500e700 mm; the quartz sand of 300e500 mm size was used as the chemically inert bed material for investigation. Real and bulk densities of quartz sand at 25  C are 2.6 g cm3 and 1.3 g cm3, respectively.

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Table 1 e Proximate and ultimate analyses of coal coke. Proximate analysis/wt%

Ultimate analysis/wt%, daf (dry, ash free)

Moisture

Ash

Volatile material

Fixed carbon

C

H

O

N

S

1.4

12.5

0.9

86.6

83.19

0.13

2.35

1.17

0.66

Internally circulating fluidized-bed reactor Fig. 1 shows an internally circulating fluidized bed (ICFB) of reacting particles in a reactor with a transparent quartz window at its top. A windowed fluidized-bed reactor can prevent direct contact, ensuring an interspacing gap between the coal coke particles and window. Concentrated solar radiation passes downward through the window and directly heats the ICFB of reacting particles. A draft tube is located at the center of the fluidized particle bed inside the reactor. Separate inlets allow gases to flow into the draft tube and annulus region between the internal tube and reactor wall. In this reactor design, the reacting particles are always transported upward in the draft tube and move downward in the annulus region. This forced circulation pattern enables solar energy to be transferred from the top to the bottom of the fluidized particle bed. The bed temperature remains high and homogeneous, thus preventing localized overheating of some areas within the bed. Therefore, by directly heating the bed, the solar gasification occurs within all the bed layers at temperatures in excess of 1000  C. The reactor consists of a stainless steel (SUS310S) fluidized-bed reactor tube (length 420 mm, inner diameter 62.3 mm, and thickness 7 mm) and a metal foam-distributor fixed in the reactor tube. The internal centrally located draft tube of the reactor has an inner diameter of 20 mm and is 3 mm thick and 44 mm long. The bottom of the draft tube was

Concentrated Xelight radiation

Quartz window

R type thermocouple

Purge gas (Ar)

Gas outlet

K type thermocouple

Quartz sand (bed material) and coal coke particles

Tmiddle

Insulation Metallic sintered plate Inconel tube Steam inlet

K type thermocouple

Fig. 1 e Experimental setup for gasification in an internally circulating fluidized-bed including quartz sand and coal coke particles.

positioned 28 mm above the porous stainless steel frit of the distributor. The steam was allowed to flow upwards through the tubes to generate an ICFB of reacting particles. The top of the reactor tube was equipped with a diverging conical funnel for mounting a quartz window in front of the focal plane.

Temperature measurement of quartz sand bed material under concentrated Xe-light radiation Quartz sand (320 g) was loaded as the bed material of the fluidized bed without coal coke particles to the reactor tube. The static bed height prior to gasification was ~110 mm. In addition, to avoid the deposition of quartz fine powders on the window, a stream of Ar gas was blown across the window from a window-purging nozzle at flow rate of 0.25 dm3 min1 under normal conditions. A direct steam-type spiral tube steam generator that does not require a carrier gas was used to generate steam [34]. A stream of steam was introduced into the reactor tube to produce a fluidized bed of quartz sand without gasification reaction. The reactor was preheated to 500  C by a cylindrical electric furnace (preheater) under a steam flow. The preheater was controlled using a K-type thermocouple contacting the exterior reactor wall. After the temperature of the reactor reached 500  C under the steam flow, the preheater was turned off before initiating the radiation of concentrated Xe light, and the concentrated Xe-light radiation was applied to heat the fluidized bed. As the steam flowed (Ftotal ¼ Fd þ Fa) through the draft tube (Fd) and annulus region (Fa), the ICFB was directly heated for 60 min by concentrated Xe-light irradiation, thus increasing the bed temperature inside the reactor. The steam flow varied in Ftotal ¼ 1.0e12.0 dm3 min1 at 373 K, and the gas flow ratio (Fd/ Fa) was Fd/Fa ¼ 2:1e16:1. A high-flux sun simulator [33,34] (three 6-kW Xe lamps; designed and fabricated by Nihon Koki: SFS-6003A) was used to generate concentrated Xe-light radiation and heat the quartz sand bed material in the reactor by the irradiation. The reactor was placed below the sun simulator's lamp chamber. The concentrator of the sun simulator reflected the lamp's beam downward to the focal spot. The top of the static bed was at the same level as the focal spot, and the focal diameter of the spot was set to ~6 cm. The intensity of the concentrated Xe-light beam on the spot can be varied by changing the power supply to the Xe arc lamp. The energy flux distribution of the concentrated Xe-light beam on the spot was previously measured using a heat flux transducer with a sapphire window attachment (Medtherm, 64-100-20/SW-1C150). Fig. 2 shows the energy flux distribution of the light from the three Xe lamps focused on the irradiated surface of the bed. The power input of the incident concentrated Xe light (Qinput) was estimated at the irradiated surface of the coke bed. The peak or central flux density was 2085 kW m2, and the mean flux density was 1122 kW m2. The total power input of

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incident Xe light (Qinput) was 3.2 kW. The fraction of the light energy stored by the reaction of steam with coke particles was calculated from the measured light energy and the production rates of CO, H2, and CO2 during the gasification. The temperature distribution of the fluidized-bed material during Xe-light irradiation was measured by K-type thermocouples. The thermocouples were inserted from the bottom of the reactor to measure the temperature at three different positions along the central axis of the static coke bed: Ttop was measured at a location 2 mm below the top of the static bed, Tmiddle was measured 5 cm below the top, and Tbottom was measured at the bottom of the coke bed. The thermocouples were shielded by quartz tubes to prevent their direct irradiation by the concentrated Xe light from the sun simulator.

Steam gasification of coal coke particles in a quartz sand bed material Coke particles (32e112 g) were loaded as the reacting particles of the fluidized bed to the reactor tube, while 96e320 g quartz sand was added as the bed material to the reactor tube. The static bed heights composed of mixture of coke particles and quartz sand were set to ~110 mm prior to gasification for all experimental conditions. In addition, to avoid dust deposition on the window, a jet of Ar gas was blown across the window from a window-purging nozzle at flow rate of 2.0e2.5 dm3 min1 under normal conditions. A stream of steam, generated by the direct steam-type spiral tube steam generator, was introduced to the reactor tube to produce a fluidized bed of the particle mixture and perform the gasification reaction. The reactor was preheated to 500  C by a preheater under a steam flow. The preheater was controlled using a K-type thermocouple contacting the exterior reactor wall. After the temperature of the reactor reached 500  C under the steam flow, the preheater was turned off before initiating the radiation of concentrated Xe light to start the gasification reaction, and the concentrated Xe-light radiation was applied to provide energy for the gasification reactions. As the steam flowed (Ftotal ¼ Fd þ Fa) through the draft tube (Fd) and annulus region (Fa), the ICFB was directly heated for ~120 min by concentrated Xe-light irradiation, resulting in the

Evaluation of steam gasification performance According to the thermodynamic calculation of the steam gasification for a carbonesteam system, steam supply is one of the important parameters governing the steam gasification processes involving gasification (eq. (1)), wateregas shift (eq. (4)), and methanation reactions (eq. (5)): CO (g) þ H2O (g) ¼ H2 (g) þ CO2 (g)

DH298 K ¼ 41 kJ mol1(4)

CO (g) þ 3H2 (g) ¼ CH4 (g) þ H2O (g)

DH298 K ¼ 206 kJ mol1(5)

0 -30

y axis [mm]

+30

Diameter of the reactor (60mm)

gasification of coal coke particles. Steam was passed at Ftotal ¼ 6.0e12.0 dm3 min1 at 373 K, and the gas flow ratio (Fd/ Fa) was Fd/Fa ¼ 6:1 and 12:1. The steam in the effluent gases from the reactor was condensed using a cooling trap connected to the outlet of the reactor. After the steam removal, the effluent gases were analyzed by gas chromatography (GC, Shimadzu, GC-8A) equipped with a thermal conductivity detector (TCD). The production rates of H2, CO, and CO2 that evolved during the gasification process were measured against the reaction time by the GC. Their production profiles were integrated over the reaction time, and the corresponding amounts of evolved H2, CO, and CO2 were calculated. The test campaign was performed for two operation modes of the reactor. The first operation mode was a batch-type reaction mode: (1) Steam was passed into the bed of a solid mixture of coal coke and quartz sand to prepare a fluidized bed and perform the gasification reaction by heating the reactor, namely, the start of fluidization and radiation of concentrated Xe light. This operation mode may involve a batch-type gasification reaction of coal cokes. This also simulates reactor startup in the morning and reheating of the reactor by concentrated solar radiation after brief periods of cloud passage. (2) The second operation mode was based on the assumption that coal coke particles were continuously fed to the reactor: A solid mixture of coal coke and quartz sand was first fluidized by passing N2 gas into the reactor. After stabilizing fluidization, the passing gas was switched from N2 to steam to start the gasification reaction at high temperatures. This operation mode was based on the assumption that coal coke particles were continuously fed to the quartz sand functioning as the bed material and heat transfer/thermal storage for solar gasification. The experimental conditions for the two operation modes are listed in Table 2. Experiment Nos. 1 and 2 are those for the first operation mode, and experiment Nos. 3e5 are those for the second operation mode. The gasification of the ICFB reactor was investigated on the two operation modes, and the performances were evaluated.

Flux density [kW/m ]

-30

0

+30

x axis [mm]

Fig. 2 e Energy flux distribution of the incident Xe light on the irradiated surface of the bed.

The thermodynamic equilibrium calculation for the carbonesteam system under various carbon/steam ratio at 1 bar pressure is shown in Fig. 3. When the C:H2O ratio is relatively low, as shown in Fig. 3(a) and (b), the gasification reaction occurs under high H2 and CO yields and low CO2 emission, while an unreacted carbon remains. In addition, with increasing reaction temperature, the selectivity of H2 and CO increases and the remained carbon is gasified. Alternatively, when the C:H2O ratio is relatively larger as shown in

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Table 2 e Experimental conditions for internally circulating fluidized-bed reactor containing quartz sand and coal coke particles in this study. No.

1 2 3 4 5

Loading amount of cokes (g)

Particle size of cokes (mm)

80 112 32 64 64

300e500 300e500 300e500 300e500 Less than 300

Loading Particle Window Steam flow Steam flow N2 gas flow N2 gas flow rate ratio (Fd/Fa) amount of size of ratio (Fd/Fa) purge gas rate (dm3 min1) quartz quartz (dm3 min1) (dm3 min1) sand (g) sand (mm) 160 96 320 288 288

350e550 350e550 350e550 350e550 350e550

Fig. 3(c)e(f), the CO yield decreases, CO2 emission increases and all carbon is gasified. Moreover, although the H2 selectivity increases at higher bed temperatures above 900  C, an excess steam input is attributed to high CO2 emission, thus decreasing solar-to-chemical energy (H2 þ CO) conversion. The calculations indicate that in the steam gasification of coal cokes at the low C:H2O ratio and at higher bed temperatures above 900  C, as shown in Eq. (1), H2 and CO are mainly present in the product gas, and the H2/CO ratio significantly exceeds the stoichiometric ratio of 1. Thus, a steam input needs to be decreased to the extent possible for the reactor operation. In this study, the energy flux density of the Xe-lamp beam on the bed surface was measured and adjusted before the

12 12 6 12 12

4:1 4:1 12:1 12:1 12:1

e e 6 12 12

e e 12:1 12:1 12:1

2.5 2.0 2.0 2.0 2.0

gasification to ensure high temperatures, as much as possible for the gasification. The production rates of CO, H2, and CO2, RCO (t), RH2 ðtÞ, and RCO2 ðtÞ, were calculated by the following equations: FinAr  XCO ðtÞ   1  XCO ðtÞ þ XH2 ðtÞ þ XCO2 ðtÞ

(6)

FinAr  XH2 ðtÞ   RH2 ðtÞ ¼  1  XCO ðtÞ þ XH2 ðtÞ þ XCO2 ðtÞ

(7)

FinAr  XCO2 ðtÞ   RCO2 ðtÞ ¼  1  XCO ðtÞ þ XH2 ðtÞ þ XCO2 ðtÞ

(8)

RCO ðtÞ ¼ 

Fig. 3 e Thermodynamic equilibrium calculations for carbonesteam system at 1 bar: (a) C:steam ¼ 1:0.5; (b) C:steam ¼ 1:1; (c) C:steam ¼ 1:3; (d) C:steam ¼ 1:5; (e) C:steam ¼ 1:10.

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where FinAr is the molar flow rate (mol s1) of Ar carrier gas in the reactor inlet; XCO, XH2 , and XCO2 are the molar fraction of CO, H2, and CO2, respectively, in the effluent gas from the reactor. The light-to-chemical energy (enthalpy) conversion (hchem) was estimated according to the following equation: hchem ðtÞ ¼

DH0298 K ðtÞ  Rgas ðtÞ Q input

(9)

where Qinput (kW) is the power input of incident concentrated Xe light; DH0298 K (t) (kJ mol1) is the standard enthalpy change in the overall reaction; Rgas (t) (mol s1) is the total flow rate of production gas for the overall reaction:

Rgas (t) ¼ (Fout (t), flow rate of outlet gas)  (FinAr, flow rate of inlet Ar carrier gas) (10) Herein, Ar carrier gas did not chemically react with all the chemical species in the steamecoke system. Thus, Fout (t) (mol s1) can be calculated from the mass balance as follows:    FinAr ¼ FoutAr ¼ FoutðtÞ  1  XCO ðtÞ þ XH2 ðtÞ þ XCO ðtÞ 2

FinAr   FoutðtÞ ¼  1  XCO ðtÞ þ XH ðtÞ þ XCO ðtÞ 2

(11)

2

The carbon conversion (Xcarbon) was estimated as follows: Z Xcarbon

Wreaction ¼ ¼ W0

t

Z

t

RCO ðtÞdt þ

0

0

W0

RCO2 ðtÞdt (12)

where W0 (mol) and Wreaction (mol) are the initial and reacted amounts of carbon in the coke used, respectively, and t (min) is the reaction time. The integral on the right hand side of Eq. (12) was evaluated graphically as the area under RCO (t) and RCO2 ðtÞ the t curve. The temperature distribution of the fluidized bed during Xe-light irradiation was measured using R-type thermocouples. The thermocouples were inserted from the bottom of the reactor to measure the temperature at different positions along the central axis of the static coke bed: Tmiddle was measured 5 cm below the top and Tbottom was measured at the bottom of the coke bed.

Results and discussion Fig. 4 shows the time variation in the bed temperature for the ICFB of quartz sand without coal cokes at three different depths, for thermocouples Ttop, Tmiddle, and Tbottom. The flow rate of steam varied in the range Ftotal ¼ 1.0e12.0 dm3 min1 at 373 K, while the gas flow ratio (Fd/Fa) was fixed at Fd/Fa ¼ 4:1. Because of preheating the reactor prior to irradiation, the temperatures for Tmiddle, Tbottom, and Ttop were initially in the range 400e600  C at the middle and bottom layers of the bed. As shown in Fig. 4(a), the temperatures for Tmiddle and Tbottom were almost constant during the irradiation when the flow rate of steam was 1.0 dm3 min1. Ttop rapidly increased to above 1000  C at 10 min irradiation time, and then gradually increased to over 1400  C at 30 min. The results for the steam flow rate of 1.0 dm3 min1 indicate that the Xe-light

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irradiation overheats the bed surface region because the quartz bed temperature increased only in the bed surface region, which was near the focus of the incident beam. When the flow rate increased to 4.0e12.0 dm3 min1, a temperature gap between the Ttop and Tmiddle, Tbottom decreased, and these temperatures remained nearly constant without local overheating. The results indicate that the circulation of quartz sand particles homogeneously distributes the temperature across all the bed layers at ~1000  C. For the steam flow rate of 8.0 and 12.0 dm3 min1, the temperatures of the bed were almost the same at all the depths. As the flow rate increased from 1.0 dm3 min1 to 12.0 dm3 min1, the maximum temperature of the bed gradually decreased. Fig. 5 shows the time variation in the bed temperature for the ICFB of quartz sand without coal cokes at three different depths. The gas flow ratio (Fd/Fa) varied in the range Fd/ Fa ¼ 2:1e16:1, while the flow rate of steam was fixed at 6.0 dm3 min1. As shown in Fig. 5(a), for the gas flow ratio Fd/ Fa ¼ 2:1, the temperature for Ttop rapidly increased just after illuminating the bed and reached >1200  C during the first 5 min of irradiation. The temperatures for Tmiddle and Tbottom gradually increased with irradiation time; however, both the temperatures were <1000  C. As shown in Fig. 5(b)e(e), the difference in the temperature between Ttop and Tmiddle or Tbottom gradually decreased as the gas flow ratio Fd/Fa increased. The temperatures, 1000e1200  C, were preserved for all the bed during the irradiation. The results for the measurements of bed temperatures in Figs. 4 and 5 indicate that the particle circulation homogeneously distributed the temperature across all the bed layers at higher temperatures above 1000  C, where the steam flow rates were >4.0 dm3 min1 and the gas flow ratio Fd/Fa was >4:1. Thus, under the flow conditions, these temperatures for all the bed layers remained nearly constant without local overheating during the irradiation, leading to superior gasification performance, i.e., gasification reaction may occur during the entire bed layer if coal coke particles were continuously fed to the reactor with a fluidized bed of quartz sand particles as the thermal transfer/storage medium. The test campaign for coal coke gasification by the fluidized bed of quartz sand particles was performed for two operation modes of reactor. The first operation mode was a batch-type reaction mode: Steam was passed into the bed of a solid mixture of coal coke and quartz sand to prepare a fluidized bed and perform the gasification reaction by heating the reactor. The second operation mode involved a series of processes: A solid mixture of coal coke and quartz sand was fluidized by passing N2 gas into the reactor to facilitate particle circulation before gasification reaction inside the reactor during irradiation; after stabilizing fluidization and reaching higher temperatures above 1000  C in all the layers of the bed, the passing gas was switched from N2 to steam to start the gasification reaction at high temperatures. The experimental conditions are listed in Table 2. Nos. 1 and 2 are for the 1st operation mode, whereas Nos. 3e5 are for the 2nd operation mode. Figs. 6 and 7 show the corresponding experimental results for the 1st operation mode, as listed in Table 2. First, the results for the gasification performance during 120 min irradiation for experiment No. 1 are shown in Fig. 6: (a) The time

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(a)

(b) 4.0 dm3min-1

1.0 dm3min-1

1600

1600

1400

Temperature/

1400

Ttop

1200

1200

1000

1000

800 600 400

Tmiddle

800

Tbottom

400

600

200

200

0

0

0

(c)

5

10

15

20

25

30

35

40

0

10

20

30

(d) 8.0 dm3min-1

6.0 dm3min-1

40

50

(e)

12.0 dm3min-1

1600

1600

1600

1400

1400

1400

1200

1200

1200

1000

1000

1000

800

800

800

600

600

600

400

400

400

200

200

200 0

0

0 0

10

20

30

40

50

60

60

0

10

20

30

40

50

60

0

10

20

30

40

50

60

Time /min Fig. 4 e Time variations of bed temperature at three different depths (Ttop, Tmiddle, and Tbottom) for internally circulating fluidized-bed of quartz sand without coal coke particles at various flow rate of passing gas: (a) 1.0 dm3 min¡1; (b) 4.0 dm3 min¡1; (c) 6.0 dm3 min¡1; (d) 8.0 dm3 min¡1; (e) 12.0 dm3 min¡1.

variations for the production rates of CO, H2, and CO2, RCO (t), RH2 ðtÞ, and RCO2 ðtÞ, and the bed temperature of Tmiddle in the middle depth; and (b) the time variations for carbon conversion (Xcarbon) and light-to-chemical energy conversion (hchem). As shown in Fig. 6(a), the bed temperature of Tmiddle rapidly increased to 1000  C under the steam flow at 10 min irradiation; subsequently, the bed temperature rapidly decreased to ~700  C and remained at nearly the same temperature. Alternatively, the rates of CO and H2 production rapidly reached 80e110 and 65e145 mmol min1, respectively, and then gradually decreased with time. The decrease in the RCO (t) and RH2 ðtÞ over time is due to the batch-type operation without continuous input of new coal coke particles into the reactor. However, the rate of CO2 production remained very low and did not increase during the gasification reaction. The resulting low rate of RCO2 ðtÞ indicates that the production of CO and H2 gases dominated in the reactor during concentrated Xe-light irradiation; however, the production of CO2 associated with the wateregas shift reaction, CO(g) þ H2O(g) ¼ H2(g) þ CO2(g), was minimal. As shown in Fig. 6(b), the peak carbon conversion Xcarbon was 64% after 120 min irradiation, while the conversion of light energy to chemical energy (henergy) varied during the irradiation, with a minimum conversion of ~9%. Fig. 7 shows the results of the gasification performance for experiment No. 2. In this experiment, compared to experiment No. 1, the loading amount of coal cokes increased by 1.4 times, while that of quartz sand decreased by 0.6 times. As shown in Fig. 7(a), the bed temperature, Tmiddle, rapidly increased to ~900  C; however, it then decreased as the

production rates of CO and H2 gradually increased. The production rates were greater for experiment No. 2 than those for experiment No. 1. This behavior for CO and H2 production can be attributed to dominant endothermic gasification reaction (eq. (1)). The rate of CO2 production remained very low during the gasification reaction. Moreover, the carbon conversion (Xcarbon) after 120 min irradiation reached 88%, which greatly exceeds that for experiment No. 1. The maximum light-tochemical energy conversion (henergy) was ~13% during 120 min irradiation. These results for experiment Nos. 1 and 2 indicate that the gasification rates, carbon conversion, and efficiency increase in the case of a large loading ratio of coal coke/quartz sand, and the steam flow supplied to the reactor, which is not the rate-determining step, was sufficient to proceed the gasification reaction. Thus, the ICFB reactor with quartz sand bed material as the thermal storage and heat transfer medium can achieve greater coal coke gasification by combined irradiation and steam. The results for experiment Nos. 3e5 in the 2nd operation mode are shown in Figs. 8e10. Fig. 8 shows the results of gasification performance during 120 min irradiation for experiment No. 3. After 19 min, the passing gas was changed from N2 to steam to perform the gasification reaction. The production of CO, H2, and CO2 started from the steam flow into the reactor along with decreasing bed temperature. However, the rate of gasification reaction was not high, because the loading amount of coal coke was insignificant, by the tenth of quartz sand. As shown in Fig. 8(b), the carbon conversion was ~60% after 120 min irradiation, and the light-

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Fd/Fa = 2:1

(a)

(b)

1400

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Temperature/

1200

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800

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600 400

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Time /min Fig. 5 e Time variations of bed temperature at three different depths (Ttop, Tmiddle, and Tbottom) for internally circulating fluidized-bed of quartz sand without coal coke particles at various flow ratio of Fd/Fa: (a) Fd/Fa ¼ 2:1; (b) Fd/Fa ¼ 4:1; (c) Fd/ Fa ¼ 8:1; (d) Fd/Fa ¼ 12:1; (e) Fd/Fa ¼ 16:1.

to-chemical energy conversion (henergy) was ~5% at a maximum, which were the lowest values obtained among these tests. This result for experiment No. 3 indicates that the smallest loading ratio of coal coke/quartz sand negatively affects the gasification performance. In addition, probably the production rates of CO, H2, and CO2 were too slow for the small steam flow rate of 6 dm3 min1. To enhance the

(b)

Steam S ea flow o 1200

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Temperature / Coke conversion Xn / %

Production rate / mmol min-1

(a)

gasification performances, the loading ratio of coal cokes and the steam flow rate were increased in experiment No. 4, as listed in Table 2. The results of the gasification performance during 120 min irradiation are shown in Fig. 9. After 54 min, the passing gas was changed from N2 to steam. Immediately after the change, the bed temperature, Tbottom, at the bottom layer increased, followed by the significant increase in CO, H2,

CO2

0 0

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Fig. 6 e Gasification performances for the experimental condition of No. 1 listed in Table 2: (a) H2, CO, and CO2 production rates (RH2 , RCO, and RCO2 ) and bed temperature of Tmiddle; (b) Carbon conversion (Xcarbon) and light-to-chemical energy conversion (henergy). Steam was flowed into internally circulating fluidized-bed reactor with quartz sand and coal coke particles.

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(a)

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0 120

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Fig. 7 e Gasification performances for the experimental condition of No. 2 listed in Table 2: (a) H2, CO, and CO2 production rates (RH2 , RCO, and RCO2 ) and bed temperature of Tmiddle; (b) Carbon conversion (Xcarbon) and light-to-chemical energy conversion (henergy). Steam was flowed into internally circulating fluidized-bed reactor with quartz sand and coal coke particles.

irradiation are shown in Fig. 10. Other experimental conditions are the same as experiment No. 4. After 55 min, the passing gas was changed from N2 to steam. Immediately after the change, the bed temperature, Tmiddle, at the middle layer significantly increased, while the gasification reaction started and produced H2, 95e115 mmol min1; CO, 70e90 mmol min1; and CO2, 15e30 mmol min1 during 25 min irradiation. Furthermore, when the bed temperature exceeded 1000  C during the irradiation by the internal circulation of the particles from the surface to the bottom of the bed, extremely high rate of coal coke gasification was observed in the experiment. The peak rate of production of each gas species is listed in Table 3. The results for the sudden increase in the gasification rate indicate that small

and CO2 production. The peak rate of production of each gas species is listed in Table 3. The bed temperature decreased with the progress in the gasification reaction and time. The sudden increase in the bed temperature indicates that the particle circulation of coal coke/quartz sand mixture may increase by switching gas. As shown in Fig. 9(b), the carbon conversion was ~93% after 120 min irradiation, and the light-to-chemical energy conversion (henergy) reached ~13% at a maximum, which was the highest among the experiments in this test campaign. Finally, to investigate the effect of the particle size of coal cokes on the gasification performance, a coal coke with <300 mm particle size was used in experiment No. 5. The results of the gasification performance during 120 min

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Fig. 8 e Gasification performances for the experimental condition of No. 3 listed in Table 2: (a) H2, CO, and CO2 production rates (RH2 , RCO, and RCO2 ) and bed temperature of Tmiddle; (b) Carbon conversion (Xcarbon) and light-to-chemical energy conversion (henergy). The passing gas was switched from N2 into steam stream at high temperatures over 1000  C in order to simulate continuous coal coke supply into the reactor.

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Fig. 9 e Gasification performances for the experimental condition of No. 4 listed in Table 2: (a) H2, CO, and CO2 production rates (RH2 , RCO, and RCO2 ) and bed temperature of Tmiddle; (b) Carbon conversion (Xcarbon) and light-to-chemical energy conversion (henergy). The passing gas was switched from N2 into steam stream at high temperatures over 1000  C in order to simulate continuous coal coke supply into the reactor.

coal coke particles are rapidly gasified at high temperatures above 1000  C mainly on the upper level of fluidized bed, where the particles are directly subjected to the concentrated Xe-light irradiation from above. Because coal coke particles (1.76 g cm3) are lighter than quartz sand (2.6 g cm3), they relatively accumulate more on the upper layer of the bed by the particle circulation. Thus, the coal coke particles smaller and lighter than quartz sand are subjected to higher temperatures. On the other hand, the use of light coal coke particles together with dense quartz sand can envision a negative factor for the gasification process; the small and light particles is liable to flow out along with the produced gases. This means that it is difficult to maintain the same hydrodynamic conditions with these two materials in the reactor. If the small coke particles are significantly entrained with time from the reactor, both the rate of gasification reaction and carbon conversion will decrease. However, the value of carbon conversion was found to be relatively higher than that for 300e500 mm particle size coal cokes. Thus, it can be concluded that the effect of particle entrainment is minor in the test campaign.

Table 3 e Peak rate of the product gases, carbon conversion and light-to-chemical energy conversion efficiency. No.

1 2 3 4 5

Peak rate of each gas species (mmol/min) CO

H2

CO2

113.0 170.6 28.4 173.1 207

145.1 207.8 51.73 234.4 320.9

18.8 28.1 12.5 37.6 83.0

Carbon conversion (%)

Light-to-chemical energy conversion efficiency (%)

64.3 88.2 59.4 92.7 94.7

9.1 12.6 5.3 13.3 11.5

During the investigation in this study, as shown in Figs. 6e10, the bed temperature tends to decrease while the gasification rate significantly increase by passing steam inside the reactor. This result indicates that the stored heat of quartz sand is thermochemically utilized by endothermic steam gasification reaction and transformed into gaseous chemicals, CO, CO2, and H2. The result also indicates that the power input is relatively low compared to the power needed for the consecutive gasification; nevertheless, coke particles still remain in the reactor. If the high temperatures of bed, such as that before Xe-light irradiation on the reactor, remain constant during the gasification reaction, a higher gasification rate and energy conversion will be favored and promising for the fluidized-bed reactor. Furthermore, if coal coke particles can be continuously fed to the reactor externally from an outside source, for example, by using a screw feeder and hopper, the peak rate of production of gases may remain nearly constant without lowering the gasification rate during the gasification under the full course of irradiation. The ICFB reactor combined with coke particle feeding technology is a promising technology for producing solar hydrogen or syngas from coal cokes by thermochemical coal gasification.

Summary A windowed internally circulating fluidized-bed reactor made of chemically inert bed materials such as quartz sand was investigated for the solar steam gasification of coal coke. The quartz sand was utilized as a heat storage and transfer medium for coal gasification. The test campaign for coal gasification using the fluidized bed of quartz sand particles was performed by two operation modes of solar reactor. The first operation mode was a batch-type reaction mode: Steam was passed into the bed of a solid mixture of coal coke and quartz

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Fig. 10 e Gasification performances for the experimental condition of No. 5 listed in Table 2: (a) H2, CO, and CO2 production rates (RH2 , RCO, and RCO2 ) and bed temperature of Tmiddle; (b) Carbon conversion (Xcarbon) and light-to-chemical energy conversion (henergy). The passing gas was switched from N2 into steam stream at high temperatures over 1000  C in order to simulate continuous coal coke supply into the reactor.

sand to prepare a fluidized bed and simultaneously perform the gasification reaction by heating the reactor. The second operation mode involved a series of processes: A solid mixture of coal coke and quartz sand was fluidized by passing N2 gas into the reactor to facilitate particle circulation before gasification reaction inside the reactor during irradiation. After stabilizing fluidization and reaching 1000  C in all the layers of the bed, the passing gas was switched from N2 to steam to start the gasification reaction at the high temperature. The latter operation mode involved the continuous input of coal coke particles using a screw feeder with hopper. For the first operation mode of the reactor, the production rates of CO and H2 were higher than that of CO2 during the gasification reaction. The carbon conversion (Xcarbon) after 120 min irradiation reached 60e88%. The light-to-chemical energy conversion (henergy) was ~5e13% during 120 min irradiation. In the case of a large ratio of coal coke/quartz sand, the gasification rates, carbon conversion, and efficiency increased. Thus, the ICFB reactor with quartz sand bed material, as a thermal storage and heat transfer medium, achieved greater coal coke gasification by combined irradiation and steam. For the second operation mode of the reactor, when the passing gas was changed from N2 to steam inside the reactor, the gasification rate significantly increased and the bed temperature rapidly decreased. This result indicates that the stored heat in quartz sand is thermochemically utilized by the endothermic steam gasification reaction and converted into gaseous chemicals, CO, CO2, and H2. In the case when small coal coke particles were utilized for the steam gasification in this reactor, the peak rates increased by 1.2-fold for CO production, 1.4-fold for H2 production, and 2.2-fold for CO2 production, compared to those obtained with 300e500 mm particle size of coal cokes. If a high bed temperature can be maintained by Xe-light irradiation during the gasification reaction and coal coke particles can be continuously fed to the

reactor, the peak rate of production of gases would remain nearly constant without lowering the gasification rate during gasification under the full course of irradiation.

Acknowledgments This research was partially supported by the Ministry of Education, Science, Sports, and Culture, Grant-in-Aid for Scientific Research (B), JSPS KAKENHI Grant number 24360406, 2012.

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