Elsevier

Energy

Volume 160, 1 October 2018, Pages 245-256
Energy

Heat transfer analysis of 5kWth circulating fluidized bed reactor for solar gasification using concentrated Xe light radiation

https://doi.org/10.1016/j.energy.2018.06.212Get rights and content

Highlights

  • Presents performance of spouted fluidized bed reactor for solar gasification.

  • Presents the interaction of concentrated radiation with fluidized particles.

  • The effect of gas flow rate on particulate flow behavior at fountain is analyzed.

  • Provides the influence of porosity of the bed on radiation diffusion processes.

Abstract

A combined numerical and experimental investigation of hydrodynamics and heat transfer of internally circulating fluidized bed reactor for solar gasification of coal coke to produce syngas is reported. As the main objective of this study is to study the thermal performance of the reactor, chemically inert quartz particles have been used. A numerical model has been developed by the combined approach of computational fluid dynamics and discrete element method (CFD-DEM). The radiation transfer equation has been solved by the discrete ordinate (DO) model. The experimental data have been used for model validation. The thermal performance of the reactor and the spout-annulus flow have been predicted by the model and other hydrodynamic parameters such as internal solid volume fraction, velocity and temperature of the particles are analyzed as a function of superficial gas velocity (gas flow rate) at different initial conditions.

Introduction

Concentrated solar radiation has been used as a heat source to perform thermochemical conversions such as gasification, combustion and pyrolysis. Solar gasification is one of the promising techniques to convert the carbonaceous materials to clean chemical fuels, which offer the advantages of being transportable as well as storable for extended periods of time [[1], [2], [3], [4]]. Thus, for solar gasification processes, various thermochemical reactors have been developed and demonstrated by various concepts including fluidized and fixed beds [5,6], vortex flow reactors [7] and molten salt pool [8]. These reactors are broadly classified as directly and indirectly irradiated reactors. In directly irradiated solar gasifiers, the concentrated solar radiation enters through a transparent window (usually quartz glass) so that the gasification process is closely controlled. However, the reactor should be carefully designed to prevent the particles, tars, and/or ash adhere to the window surface. Otherwise, which may contaminate and break the glass due to overheating [2].

Solar gasification of carbonaceous materials was initially demonstrated by a packed bed reactor in a solar furnace using steam/CO2 [9]. Coal, coke, and mixtures of coal and biomass were used as feedstocks. The bed in a 30 cm diameter L-shaped stainless tube reactor was directly irradiated through quartz window to produce CO and H2. Thermocouples were placed at different locations of the bed to temporally monitor the temperatures. Gasification occurred at temperature range between 1175 and 1575 K. The gas compositions were then measured with two online IR gas detectors and two online gas chromatographs. The performance of the reactor was investigated from 4 to 23 kWth solar power. Up to 40% of the sunlight arriving at the focus was chemically stored as product gases. Then using 2 kWth solar furnace, gasification of charcoal and wood was conducted by direct irradiation [10]. The steam was generated by spraying water directly on the surface of the charcoal. During experimentation, a plunger was used to continuously move the packed bed into the focus. Various experiments were carried out using CO2 and steam and up to 30% of the incident solar energy was stored by chemical reaction.

Since irradiation of suspended particles in the gas stream provides efficient heat transfer directly to the reacting particles, fluidized bed reactors were developed [[10], [11], [12], [13], [14], [15], [16], [17], [18]]. On-sun testing of steam/CO2 gasification of charcoal and wood was assessed in a 2 kWth solar furnace and 10% of the solar energy was stored as chemical enthalpy [10]. Kodama et al. [5] demonstrated the CO2 gasification of Australian bituminous coal using directly irradiated solar gasifier. The peak energy flux density of the incident beam was varied up to 1270 kW/m2. The powder (5.0 g) was filled in the quartz tube reactor with an inner diameter of 20 mm. The static bed height was about 20 mm. The concentrated irradiation was introduced horizontally, resulting in a maximum energy conversion of 8%. Steam gasification of coal in a fluidized bed contained in a quartz tubular reactor was performed by directly exposing to concentrated thermal radiation [6,12]. Numerically computed temperature profiles, product gas composition, and conversions were compared to experimentally measured values. High quality syngas was observed at temperatures above 1450 K. Although the temperature of the fluidized bed rector depends on various parameters including particle size, superficial gas velocity and thermophysical properties of particles and gas, irradiation power plays an important role. The incident radiation flux distribution based on concentration ratio determines the temperature of the bed. Solar gasification process required relatively less temperature (>700 °C) than the thermal reduction process of cerium oxide (>1300 °C) for water splitting cycles to produce hydrogen [2]. Thus, the irradiation flux should be optimized according to the required temperatures and thermochemical processes. To get a specific peak flux with gaussian irradiation profile for different sizes (diameter) of fluidized bed reactors, various solar simulators were developed from 3 to 30 kWth in beam-down orientation [2,[14], [15], [16], [17], [18]]. For CO2 gasification of coal cokes, two types of reactors were tested; internally circulating type and conventional type without draft tube and the production rates of CO, H2 and CO2 were reported [[14], [15], [16]]. The hydrodynamics, particle mixing and heat transfer determine the reaction rate and efficiency of these reactors. Hence, various numerical studies have been performed to assess the behavior and performance of the fluidized bed reactors for solar thermochemical conversions.

Von-Zedtwitz et al. [6] developed a one dimensional numerical model to solve the energy and mass conservation of a directly exposed tubular fluidized bed reactor for solar gasification. In spite of particle shrinking, the steady state assumption was made by justifying the low rate of coal conversion. The Monte Carlo method was applied for solving the radiative exchange within the reactor quartz walls, the bed particles, and the gas phase. Predicted values were compared with experimental measurements for model validation. Gordillo et al. [19] developed a numerical model to examine a solar downdraft gasifier, utilizing high carbon content biomass char (biochar) with steam, based on the systems kinetics. The transient temperature and concentration distributions of solid and gas phases were predicted based on the heat and mass balances. High quality syngas was produced by the down draft gasifier and the hydrogen was the principal component followed by carbon monoxide. The system efficiency was reported to be as high as 55% for low steam velocities. The model predictions were in very good agreement with the experimentally reported measurements. A three-dimensional steady state model coupling radiative transfer with fluid flow, heat transfer and chemical reaction kinetics was developed to investigate a solar receiver consisting of an array of five tubes enclosed within a specularly reflective cylindrical cavity with a windowed aperture [20]. Steam gasification of entrained 42 nm acetylene black particles was performed. Experimentally measured carbon conversion was compared with predictions. Loutzenhiser et al. [21] reviewed the recent developments of solar-driven gasification processes with carbonaceous materials.

Eulerian-Eulerian approach, based on the kinetic theory of granular flow, has been predominantly used in the numerical models developed for solar gasification by assuming uniform particle size. For the fluidized beds filled with range of particles sizes, Eulerian-Lagrangian approach has been used since it allows to define the particle size distribution. A few studies were investigated by CFD-DEM model based on Eulerian-Lagrangian approach for solar thermal applications [[22], [23], [24], [25]]. Recently, we have developed a CFD-DEM model to investigate the granular flow and heat transfer characteristics of a two-tower fluidized bed reactor, which consisted of two rectangular chambers placed side by side and connected by two interaction ports [25]. The large tower in the left hand side was irradiated by solar radiation and the heated particles were moved to the small tower through top interaction port by drag force of gas stream. One of the purposes of the two-tower reactor was to carry out two-step water splitting cycles concurrently to produce hydrogen; the left tower for thermal reduction and the right tower for hydrolysis. This reactor can also be simultaneously used as particle receiver (large tower) and storage system (small tower). The particle flow between these two towers and the circulation pattern, clockwise or anticlockwise, were analyzed for different gas flow rates. In order to simplify the heat transfer model, the uniform particle size was used and the heat transfer between the particles and wall was not considered.

In this study, we have performed a heat transfer analysis of a spouted fluidized bed reactor for solar gasification. In which, the gas velocity at the spout inlet is significantly higher than the annulus region. Accordingly the particles in the spout and annulus region move up and down respectively. Moreover, the incident radiation flux depends on the position. Peak flux occurs at the focal point around the axis and gradually decreases along the axial and radial direction. Thus, the resident time and flow behavior of particles at the fountain (top part of the bed), spout and annulus play an important role on the heat transfer characteristics of the bed. Thus to analyze the particulate flow and heat transfer characteristics of the bed, a CFD-DEM model has been developed and coupled with the discrete ordinate radiation model. To the best of our knowledge, CFD-DEM modeling of directly irradiating spouted fluidized bed reactor for solar thermal applications has not been reported so far. The fluidization behavior and heat transfer characteristics of the bed composed of different size particles have been analyzed for different operating conditions to understand the system and enhance the performance.

Section snippets

Experimental setup

Fig. 1 shows the schematic and snapshot of the internally circulating fluidized bed reactor experimental set up. A sun simulator, consists of three 7 kW Xe lamps, is used to generate the concentrated Xe light radiation in beam-down orientation. The reactor is placed below the sun simulator by keeping the top surface of the static bed at the focal point of the concentration. The incident concentrated radiation covers the top surface of the bed about 0.08 m diameter with average heat flux and

Governing equations

In the developed model, the gas phase and discrete phases are governed by Navier-stokes equation and Newton's second law respectively. To predict the hydrodynamics of gas-particle flow, the conservation of mass, momentum, species and energy equations of the gas phase are formulated as follows;t(αfρf)+·(αfρfuf)=0t(αfρfuf)+·(αfρfufuf)=αfp+·(αfτ=f)+αfρfg+FDEMt(αfρfYi)+·(αfρfufYi)+·(αfJi)=αfSit(αfρfCp,fTf)+·(αfρfufCp,fTf)=·(αfkf,effTf)+Qp+Qradwhere αf, ρf, uf, p, g, τ=f C

Model validation

In order to validate the model, a simulation was performed for the operating conditions given in Table 1. Fig. 4 compares the temperature distribution of the bed at two different locations measured by experiment and predicted by the model. As the velocity of the fluid in the central axial region of the bed is high, the heat transfer rate is high, consequently, the temperature gradient between positions b and d is very low. As can be seen in the figure, a good agreement has been found between

Conclusions

The solid-gas hydrodynamics and heat transfer of a windowed internally circulating fluidized bed reactor has been studied using chemically inert particles. A three dimensional CFD-DEM model has been developed and validated using the experimental results. The effect of annulus and spout flow rate on thermo-fluid flow characteristics of the spout, fountain and annulus regions has been analyzed. The important results obtained from this investigation are summarized as follows:

For the given loaded

Acknowledgement

This study has been supported by the Tenure Track Program, The Ministry of Education, Culture and Sports, Science and Technology of Japan.

Nomenclature

a
absorption coefficient [1/m]
Ap
surface area of particle [m2]
Cp,f
specific heat capacity of fluid [J/kg/K]
Cp,s
specific heat capacity of solid [J/kg/K]
dp
particle diameter [m]
Faero
aerodynamic forces [N]
Fconn
normal contact force [N]
Fcont
tangential contact force [N]
Fg
volume force due to gravity [N]
G
incident radiation [W/m2]
g
acceleration due to gravity [m/s2]
h
heat transfer coefficient [W/m2/K]
I
radiative intensity [W/m2/sr]
Ip
moment of inertia [kg·m2]
K
spring constant [N/m]
kf,eff
effective thermal

References (38)

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