SOLAR RECEIVER/REACTOR OF FLUIDIZED BED WITH MIYAZAKI BEAM-DOWN SOLAR CONCENTRATING SYSTEM

Transcription

1 Proceedings of the Asian Conference on Thermal Sciences 217, 1st ACTS March 26-3, 217, Jeju Island, Korea ACTS-P SOLAR RECEIVER/REACTOR OF FLUIDIZED BED WITH MIYAZAKI BEAM-DOWN SOLAR CONCENTRATING SYSTEM Kazuya Senuma 1, Nobuyuki Gokon 2*, Hyun-Seok Cho 2, Selvan Bellan 2, Tsuyoshi Hatamachi 3, Tatsuya Kodama 3 1 Graduate School of Science and Technology, Niigata University, Japan 2 Center for Transdisciplinary Research, Niigata University, 85 Ikarashi 2-nocho, Nishi-ku, Niigata , Japan 3 Dep. of Chem. & Chem. Eng., Faculty of Engineering, Niigata University, Japan Presenting Author: F16B53K@mail.cc.niigata-u.ac.jp * Corresponding Author: ngokon@eng.niigata-u.ac.jp ABSTRACT Mitaka Koki Co., Ltd., Miyazaki prefecture, Miyazaki University and Niigata University build a beam-down (BD) solar concentrating system in August 212 at Miyazaki University for solar demonstration of fluidized bed receiver/reactor. Both universities started an R&D joint project since 211 to demonstrate the up-scaled solar receiver/reactor under a concentrated solar radiation. In this study, first demonstration results of the fluidized bed receiver/reactor using the BD solar concentrating system is reported. Chemically-inert quartz sand is used as a fluidization particle to examine performances of the receiver/reactor, while air is used as a passing gas to make a fluidization of quartz sand inside the reactor. Firstly, a flux distribution at a lower focal point in the system was experimentally examined and evaluated in comparison to the numerical simulation. Compound Parabolic Collector (CPC) was designed, manufactured and installed below the lower focal point in order to enhance solar flux intensity. Finally, a solar receiver/reactor of fluidized bed was designed, fabricated and examined for the receiver/reactor performances (temperatures of outlet gas and quartz sand). The temperature of quartz sand at the center position of the bed was about 11 C, and that of outlet air was 112 C. KEYWORDS: Beam down optics, Solar receiver/reactor, Fluidized bed, CPC, Flux distribution, High temperature 1. INTRODUCTION In concentrating solar power (CSP) systems, which can be used for cost-competitive power generation, especially in the sun-belt region where direct solar light is abundant [1], solar energy is concentrated by a solar collector and transported by a heat transfer fluid (HTF) into a thermal energy storage (TES) system [2]. However, the intrinsic low density and intermittent nature of solar radiation are major barriers to achieving economic viability [4]. Recently, particle receiver system attracts attention for CSP society to obtain high temperature solar heat by solar power tower or beam down concentrating systems. The particles have a functional role as a fluidization medium/thermal storage/heat transfer medium to make a solar thermal power generation via heat exchange process in CSP [3]. Similarly, a solar chemical reactor, where concentrated solar radiation is used as an energy source to drive a thermochemical reaction for productions of hydrogen and synthetic gas to produce liquid fuels such as a kerosene, must also effectively receive concentrated solar radiation and convert the high-temperature solar heat into chemical fuels by means of endothermic thermochemical processes [4-6]. It is well known that the incorporation of TES systems into CSP plants and solar chemical reactors can increase the operational periods of both systems [7]. Various solar thermochemical processes, for example, thermochemical water splitting cycle, solar gasification of carbonaceous material and solar reforming of methane et al. have been proposed and demonstrated for the purpose of converting concentrated solar high-temperature heat into clean hydrogen in sunbelt or solar belt regions. Among the processes, a solar thermochemical two-step water-splitting is a simple method, operating at technically 1

2 manageable temperatures by use of a metal oxide as a redox medium for the cycle, which is capable of producing hydrogen from water. Also, a solar gasification of carbonaceous material contains highly endothermic chemical reactions [8]. Thus, the solar thermochemical process 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 CO 2 emissions to the environment and conserve fossil fuels [8]. Niigata University is developing solar chemical reactor of fluidized bed for solar thermochemical processes of twostep water-splitting cycle and solar gasification process. Their solar chemical reactors require beam-down solar optics to receive concentrated solar radiation and obtain high temperature solar heat to operate chemical reaction. In the present study, a newly developed solar reflective tower or beam-down optics was designed and constructed in Miyazaki prefecture in Japan. A solar chemical reactor of 1 kw th fluidized bed was design and constructed by Niigata University, and tested with combination of Miyazaki BD for solar demonstration. 2. FLUIDIZED BED SOLAR REACTOR 2.1 FLUIDIZED-BED REACTOR FOR SOLAR DEMONSTRATION BY USING BD SOLAR CONCENTRATING SYSTEM AT MIYAZAKI Fig. 1 shows a fluidized bed (FB) of reacting/unreacting 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 particles and window. Concentrated solar radiation passes downward through the window and directly heats the FB of particles. In order to make an internal-circulation of particle inside the reactor, a distributor made by stainless steel is located at the bottom of the reactor. Gases are flowed through the distributor into the FB of particle. A number of through-holes are located in the center area of the distributor, while few through-holes are in the circumference for the distributor. In this reactor design, the fluidizing particles are always transported upward in the center area of FB and move downward along with reactor wall. 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, Compound Parabolic Concentrator (CPC) Fuels (Hydrogen, synthetic gas)/high temperature air Secondary reflective mirror Quartz window Fluidizing particles Solar reactor by directly heating the bed, the solar thermochemical process occurs within all the bed layers at temperatures in excess of 1 C. The reactor consists of a stainless steel (SUS31S) and Inconel 6, and fluidized-bed reactor tube is length 25 mm, inner diameter 25 mm, and thickness 7 mm. The steam was allowed to flow upwards through the tube to generate an ICFB of 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. 2.2 TEMPERATURE MEASUREMENT OF QUATRZ SAND BED MATERIAL UNDER SOLAR RADIATION Quartz sand (loading amount of 17 kg, particle size of 1-5 m) was loaded as the bed material of fluidizing particles to the reactor tube. The static bed height was ~25 mm. A stream of air was introduced as a carrier gas into the reactor tube to produce a fluidized bed of quartz sand without chemical reaction. The FB of quartz sand was preheated to 6-7 C by cylindrical electric furnace (preheater) composed of two semicircular columns under an air flow. The preheater was controlled using a K-type thermocouple contacting the exterior reactor wall. After the temperature of the reactor reached 6-7 C under the steam flow, the preheater was turned off before initiating the Gas flow Distributor Heliostat Fig. 1 Schematic system of solar fluidized bed reactor combined with beam-down optics. 2

3 radiation of concentrated Xe light, and the concentrated Xe-light radiation was applied to heat the fluidized bed. The bed temperature was measured at the center position of the FB by using a R-type thermocouple inserting from a bottom of the reactor. As the air flowed (F air ) through the distributor, the FB was directly heated for 2-4 h by concentrated solar radiation, thus increasing the bed temperature inside the reactor. The air flow varied in F air = 18-4 dm 3 min -1 at ambient temperature. 3. BD SOLAR CONCENTRATING SYSTEM AND COMPOUND PARABOLIC CONCENTRATOR 3.1 BD SOLAR CONCENTRATING SYSTEM A solar reflective tower or beam-down optics was already proposed as a promising solar concentrating system for solar fuel production [9]. The optical path of a beam-down system consists of a heliostat field that illuminates a 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 large-scale 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 thermochemical 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. Fig. 2 shows 1kW th beam-down solar concentrating system constructed in Miyazaki, Japan. The ellipsoidal-shaped secondary reflector was designed and manufactured by Mitaka Koki Co. Ltd. The BD system was built in Miyazaki University by co-funding of Mitaka Koki Co. Ltd., Miyazaki Prefecture, Miyazaki University and Niigata University, and then has 88 heliostats; 176 m 2 total reflective mirror area; 16 m tower height; 4.6 m 2 area of secondary reflective mirror. 3.2 COMPOUND PARABOLIC CONCENTRATOR Beam-down solar concentrating system 1. Numbers of heliostat (Mirror area of 2 m 2 / heliostat) A high-flux solar radiation is focused on the upper and lower focal spots in the beam-down arrangement. The reactor was placed below the lower focal spot. The concentrated solar radiation was received by the FB of particles, and a high-temperature solar heat generated by concentrated solar radiation can be used for solar thermochemical processes. It is well-known for thermochemical two-step water-splitting cycle to produce hydrogen/oxygen from water that a temperature for thermal reduction (TR) step is generally higher than that for water-decomposition (WD) step of two-step reaction. In the case that cerium oxide is used as a redox particle for a thermochemical water splitting cycle, high-temperatures of above13 C or favorably 15 C is required to perform an endothermic TR step. On the other hand, in the case of solar gasification of coal cokes, high-temperatures of above 8 C or favorably 1 C is required to perform an endothermic gasification process. In order to obtain such high-temperature solar heat at the lower focal spot of BD solar concentrating system, a compound parabolic concentrator (CPC) was designed and manufactured by Mitaka Koki Co. Ltd. Fig. 3 shows photographs of CPC installed on the experimental stage of the system. The CPC is 758 mm inlet diameter; outlet diameter 447 mm; 15 mm height. Flat mirrors in the CPC are 1 number of columns; 24 mirrors at a total. The solar radiation entering into the CPC is reflected by the mirror, and the solar flux is enriched at the CPC outlet Total reflective area of mirror 176 m 2 3. Tower height 16 m 4. Area of secondary reflective mirror 4.6 m Secondary reflective mirror Fig. 2 Principle of beam-down solar concentrating system, specifics and photographs of beam-down optics built at Miyazaki in Japan. 3

4 Beam-down concentrating system Overall view Inside CPC Fig. 3 photographs of overview and side view of CPC installed in the Miyazaki BD. Fig. 4 Solar thermal flux distribution below 25 cm from the bottom of CPC. The top of the static bed is located lower than the outlet. The focal diameter of the spot enlarges as the distance between the CPC outlet and the top position of FB increases. In this study, the flux distribution of the concentrated solar radiation on the lower focal spot and 25cm below the CPC outlet is previously measured using a heat flux transducer with a sapphire window attachment (Medtherm, /SW-1C15). Fig. 4 shows the energy flux distribution of the concentrated solar radiation focused on the irradiated surface of the bed (25cm below the CPC outlet). The flux distribution was measured where the CPC was tilted toward south direction by 12 degree. The power input of the incident concentrated solar radiation (Q input ) was estimated from an integration of flux distribution. The peak or central flux density was 157 kw/m 2, and the mean flux density was kw/m 2. The total power input Q input was 1 kw th. The results for flux distribution mean that the BD solar concentrating system can work as planned. It is expected that high temperature solar heat over 1 C can be obtained at below lower focal spots. 4. PERFORMANCE TESTS OF SOLAR REWACTOR BY BD SOLAR CONCENTRAITNG SYSTEM In this experimental campaign, position of reactor and superficial air velocity were varied as a parameter to measure temperatures of fluidizing particles at the middle height of bed in the reactor and outlet air from the reactor. The experimental conditions and results for the reactor testing performed using a windowed fluidized bed reactor are listed in Table 1. The temperature variations of the fluidized-bed and outlet air were measured during solar radiation by R-type thermocouples. The thermocouples were inserted from the bottom of the reactor to measure the temperature at middle position along the central axis of the static bed, while inserted into the outlet tube in order to measure the outlet gas. Fig. 5 shows the results for case No. 1 in Table 1. Time variations for bed and air temperatures at air flow rate F air Table 1 Series of reactor testing on solar demonstration Date of testing Flow rate (m/sec) Preset temperature of external heater (ºC) Position of reactor below the bottom of CPC (cm) DNI at maximum (kw/m 2 ) DNI at minimum (kw/m 2 ) Bed temperature at a maximum (ºC) Outlet air temperature at a maximum (ºC) Case1 Case2 Case3 Case4 215/12/8 215/12/11 215/12/16 215/12/18 2~4 35~2 3~38 18~4 95~ =2-5 N dm 3 min -1, and for direct normal irradiation (DNI) measured at Miyazaki University on the day are shown. Before inputting concentrated solar radiation into the reactor, air was passed through the particle bed to make fluidization during external heating from ambient temperature into 9 C by using an electrical heater. The air 4

5 temperature gradually increases as a flow rate of airstream increases from 35 to 5 N dm 3 min -1 (.12 to.17 m/sec). On the one hand, the bed temperature drastically rises at 5 N dm 3 min -1 (.17m/sec). The results indicate that a fluidization of particle bed can be successfully made at the flow rate in the reactor. Subsequently, when the flow rate decreased from 5 to 3 N dm 3 min -1 (.17 to.12m/sec), both temperatures was stabilized and remained at the temperatures of C for a particle bed and C for a airstream. The result means that a particle bed will be preferentially heated by the external heater, leading to relatively lower temperature of an airstream due to ineffective heat transfer under the experimental conditions. Concentrated solar radiation was incident through the transparent window into the particle bed. After solar radiation started to enter into the heating of fluidized bed, a temperature of airstream dominantly increased in comparison to the bed temperature. Thus, the solar reactor operated in the fluidization mode is available as a solar receiver to make high temperature airstream above 85 C by using beam-down solar concentrating system. DNI [kw/m 2 ] DNI Solar power input Temperature [ºC] Preset temperature of external heater Flow rate Bed temperature Air temperature 12:16 12:59 13:42 14:25 15:8 Time of day Flow rate [m/sec] 9 Preset temperature of external heater [ºC] DNI [kw/m 2 ] Temperature [ºC] Fig. 5 Results for reactor testing of No. 1. Solar power input DNI Bed temperature 15.4 Air temperature Preset temperature of external heater Flow rate 8:3 9:42 1:54 12:6 13:18 Time of day Flow rate [m/sec] 1 95 Preset temperature of external heater [ºC] Fig. 6 Results for reactor testing of No. 3. Fig. 6 shows the results for case No. 3 in Table 1. Before entering concentrated solar radiation, air was initially flowed into the reactor at 11 N dm 3 min -1 (.37m/sec) in this time. The bed and air temperatures rapidly increased at the air flow rate. Thus, we gradually decreased air flow rate to 6-4 N dm 3 min -1 ( m/sec) in order to effectively heat a particle bed and airstream. Then, the temperatures of particle bed and sir stream reached 83 C and 52 C, respectively. The result means that a heat transfer between a particle bed and airstream will be improved under the experimental conditions in comparison to the case No.1. A concentrated solar radiation was entered into the reactor at the flowrate of 4 N dm 3 min -1 (.14 m/sec). After the incident solar radiation, a temperature of 5

6 airstream dominantly increased in comparison to the bed temperature as well as the case No. 1. In addition, the bed temperature gradually rose so that both temperatures were synchronously elevated under the solar radiation. The maximum temperatures of a particle bed and airstream were 995 and 992 C, respectively. The results indicate that an operation of solar reactor under the case No. 3, which includes an external heating mode and subsequent solar mode, will prove useful for solar thermochemical process such as a solar gasification of coal cokes because a particle bed attains to high temperatures of 1 C. Finally, the experimental campaign is a preliminary phase of solar reactor testing for solar thermochemical process to produce hydrogen from water via two-step water-splitting cycle and synthetic gas (a gas mixture of hydrogen and carbon mono-oxide) via pyrolysis/gasification of carbonous materials including coal, cokes and biomass. It is expected from the present study that solar gasification process operating at high temperatures of 8-1 C works well by a solar reactor using fluidized bed of quartz bed as a fluidization medium/thermal storage/heat transfer medium in the Miyazaki BD. 5. SUMMARY A newly developed solar reflective tower or beam-down optics was designed as a promising solar concentrating system for solar fuel production. The ellipsoidal-shaped secondary reflector was designed and manufactured by Mitaka Koki Co. Ltd. The BD system was built in Miyazaki University by co-funding of Mitaka Koki Co. Ltd., Miyazaki Prefecture, Miyazaki University and Niigata University, and then has 88 heliostats; 176 m 2 total reflective mirror area; 16 m tower height; 4.6 m 2 area of secondary reflective mirror. In order to obtain high-temperature solar heat, which can works on thermochemical processes to convert it into fuels, at the lower focal spot of BD solar concentrating system, a compound parabolic concentrator (CPC) was designed and manufactured by Mitaka Koki Co. Ltd. The CPC is 758 mm inlet diameter; outlet diameter 447 mm; 15 mm height. Flat mirrors in the CPC are 1 number of columns; 24 mirrors at a total. The solar radiation entering into the CPC is reflected by the mirror, and the solar flux is enriched at the CPC outlet. The peak or central flux density at 25 cm below from the bottom of CPC outlet was 157 kw/m 2, and the mean flux density was kw/m 2. The total power input Q input was 1 kw th. The results for flux distribution mean that the BD solar concentrating system can work as planned. High temperature solar heat over 1 C can be obtained at below lower focal spots. In this experimental campaign, the fluidized bed was tested for solar demonstration by Miyazaki BD. The temperatures of bed and air steam reached high temperatures of 1 C in the reactor. Some operation modes of solar reactor were proposed for solar thermochemical process and air-receiver to make high temperature air by Miyazaki BD for CSP. ACKNOWLEDGMENT 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 16H4645. REFERENCE [1] Concentrated Solar Power Heats Up. Emerging Energy Research (EER); 26. [2] M. Liu, W. Saman, F. Bruno, Review on storage materials and thermal performance enhancement techniques for high temperature phase change thermal storage systems, Renewable and Sustainable Energy Reviews, 16 (212) [3] C. Tregambi, F. Montagnaro, P. Salatino, R. Solimene, Directly irradiated fluidized bed reactors for thermochemical processing and energy storage: application to calcium looping, SolarPACES216, Abu Dhabi, United Arab Emirates, October 11-14, 216. [4] C. Agrafiotis, M. Roeb, C. Sattler, A review on solar thermal syngas production via redox pair-based water/carbon dioxide splitting thermochemical cycles, Renewable and Sustainable Energy Review, 42 (215) [5] T. Kodama, N. Gokon, Thermochemical cycles for high-temperature solar hydrogen production, Chemical Reviews, 17 (27) [6] N. Gokon, T. Izawa, T. Abe, T. Kodama, Steam gasification of coal cokes in an internally circulating fluidized bed of thermal storage material for solar thermochemical processes, International Journal of Hydrogen Energy, 39 (214) [7] F. Manenti, A. R. Leon-Garzon, Z. Ravaghi-Ardebili, C. Pirola, Assessing thermal energy storage technologies of concentrating solar plants for the direct coupling with chemical processes. The case of solar-driven biomass gasification. Energy, 75 (214) [8] A. Steinfeld, Solar thermochemical production of hydrogen a review, Solar Energy 78 (25) [9] A. Segal, M. Epstein, Solar ground reformer, Solar Energy, 75 (23)