CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 52, 2016 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Petar Sabev Varbanov, Peng-Yen Liew, Jun-Yow Yong, Jiří Jaromír Klemeš, Hon Loong Lam 
Copyright © 2016, AIDIC Servizi S.r.l., 

ISBN 978-88-95608-42-6; ISSN 2283-9216 
 

Control of Solid Mass Flow Rate in Circulating Fluidized Bed 

by a Pulsed Gas Flow 

Masanori Ishizuka, Hiroyuki Mizuno, Yasuki Kansha, Atsushi Tsutsumi* 

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan 

a-tsu2mi@iis.u-tokyo.ac.jp 

As a coal-fired power generation technology for further improvement of power generation efficiency of coal-

fired power generation, exergy recuperation type coal gasification power generation technology, a triple-bed 

circulating fluidized bed, has been proposed. The authors analysed the flow characteristics of the triple-bed 

circulating fluidized bed, it has the flow characteristics of the riser and downer perform the proposed approach 

to representation by the equivalent circuit model. This equivalent circuit has the nature of the low-pass filter. A 

combination of the low-pass filter and the pulse voltage is used as a switching power supply. Then, we applied 

that the pulsed gas supply to the riser combined with a low-pass filter characteristics to control the particle 

circulation rate of the triple-bed circulating fluidized bed. We used an electric circuit simulator SPICE to 

calculation of circuit behaviour. Circuit constant is to use the value from the experiment, the input pulse height 

is set to 80 V. When the input pulse width is changed, the output current is changed depending on the pulse 

width. Moreover, when changing the density of the pulse, the output current is changing depending on the 

pulse density. This result by giving a pulsed gas supply to the riser, it shows the possibility controlling the 

particle circulation rate of the triple-bed circulating fluidized bed. 

1. Introduction 

It is necessary to improve response the power generation efficiency of thermal power plants to the increase of 

power demand, thermal power plants based on coal gasification technology have been developed worldwide 

(Henderson). Assuming a next-generation high-efficiency coal gasification combined cycle power generation 

(Tsutsumi), a triple-bed circulating fluidized bed (TBCFB) consists of riser as a combustion furnace, downer as  

pyrolysis furnace, bubbling fluidized bed as steam gasification reactor, has been proposed and developed as 

coal gasification reactor (Guan et al., 2010). The proposed triple bed circulating fluidized bed is a system 

which has downer, bubble fluidized bed, riser, moving bed, particle distributor, gas seal bed, gas-solid 

separator, etc. The TBCFB is a very complex system. Then, this system has a number of parameters to 

control, it is expected to show a complex flow behaviour of fluidized bed (Guan et al, 2010b). It was reported 

that the flow characteristics of large scale cold model of this system (Fushimi et al.,2011), additional flow 

analysis is required to control the particle circulation for stable operation of the gasifier system for practical 

usage. 

2. Measurement of particle circulation rate in the triple-bed circulating fluidized bed 

Figure 1 is a schematic image of a large TBCFB cold model (Guan et al, 2011) . This cold model consists of 

riser (0.1 m ID × 16.6 m height), downer (0.1 m ID × 6.5 m height), gas-solid separation equipment, bubbling 

fluidized bed (BFB, 0.75 m × 0.27 m × 3.4 m), gas seal bed (GSB, 0.158 m ID × 5.0 m). In this cold model, it 

has been measured the particle circulation rate using silica sand particles, which density and average 

diameter is 2,600 kgm-3 and 83 m (minimum fluidization velocity (Umf) = 0.0058 ms-1). The superficial gas 

velocity of riser is in the range of from 5 to 12 ms-1 and of gas-seal bed is from 0 to 0.094 ms-1, the superficial 

gas velocity of the BFB is fixed to 0.0041 ms-1. Pressure was measured by pressure sensors (Keyence, AP48) 

placed along to the whole TBCFB system. The output signals of the sensor were acquired through the data 

                                

 
 

 

 
   

                                                  
DOI: 10.3303/CET1652022 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Ishizuka M., Mizuno H., Kansha Y., Tsutsumi A., 2016, Control of solid mass flow rate in circulating fluidized bed by 
a pulsed gas flow, Chemical Engineering Transactions, 52, 127-132  DOI:10.3303/CET1652022   

127



acquisition system (CONTEC, AIO-163202FX) and a laptop computer at 100 Hz sampling frequency. A solid 

mass flux (Gs) as particle circulation rate was measured using a valve based on the time required to 

accumulate the amount of certain particles. 

 

Figure 1: Schematic image of the triple-bed circulating fluidized bed gasifier (TBCFB) 

 

Figure 2: Effect of the gas velocity in the riser on the solid mass flux 

128



Figure 2 shows a solid mass flux(Gs) as the particle circulation rate as a function of the riser the superficial 

gas velocity (Ugr). At this time, the superficial gas velocity of GSB (Ugg) was set to 0.094 ms-1. When Ugr = 5 

ms-1, Gs was 210 kgm-2s-1. In Ugr = 12 ms-1 has reached to 546 kgm-2s-1. In this system, along with the 

increase of the riser gas superficial velocity, a total particle circulation rate was increased. However, the 

lowest rate of the riser gas superficial velocity of particles circulation is established in this system was present. 

In this experimental system, a Ugr = 5 ms-1, in which less than the gas superficial velocity, not performed the 

particle circulation. In this report, by performing the pulsed gas supply to the riser for the control of the particle 

circulation, aimed at the particle circulation rate control, which corresponds to the gas superficial velocity of 

less than 5 ms-1. 

 

 

Figure 3: Equivalent circuit model of the riser/downer flow 

3. Equivalent circuit model of the triple-bed circulating fluidized bed 

The authors have, so far, to analyse the flow characteristics of the triple-bed circulating fluidized bed, it has 

been proposed a method that the flow characteristics of the riser and downer is expressed by an equivalent 

circuit model (Ishizuka et al., 2015). The equivalent circuit model of the riser and the downer is shown in 

Figure 3. Modelling was based on the similarity between the flow characteristics of the fluidized bed and an 

electrical properties of electrical circuit. The basic similarity between the electrical characteristics and flow 

properties is defined by the following equation. 

 

(1) 

 

 

(2) 

 

 

(3) 

 

 

Eq(1) is Ohm's law, it expressed for fluid resistance. Eq(2) is the inductance of the fluidised bed. It was 

obtained from the definition of the inductance of the electrical circuit. Further, Eq(3) is a fluid compliance, 

which were obtained from the definition of capacitance in electronic. 

This equivalent circuit has the nature of the low-pass filter. Output control by a combination of the low-pass 

filter and a pulse voltage is used as a switching power supply. Thus, we were proposed that apply a pulsed 

gas supply to the riser for the particle circulation rate control of the TBCFB combined with a low-pass filter 

characteristics of the riser. 

 

FsRaP 

dt

dFs
MaP 



Fs Ca
dP

dt

129



 

Figure 4: Input-output characteristic of pulse and low pass filter. Solid line: input pulse voltage. Dot line: output 

current 

4. Control of particle circulation rate by the pulsed gas supply 

The behaviour supplied pulsed gas to the riser of the TBCFB was studied using an equivalent circuit model of 

the riser and circuit simulation software. Figure 4 shows the relationship between input and output applied the 

input pulse voltage in the equivalent circuit of the riser. Output currents were compared by the waveform 

difference of the input pulse voltage. Input pulses were assumed two types pulse modulation. One is in the 

case of changing the pulse width fixing the repetition frequency of the pulse (Pulse Width Modulation) and 

other is to fix the width of the pulse when changing the repetition frequency of the pulses (Pulse Density 

Modulation). The calculation of the pulse input to these equivalent circuits were carried out using the electric 

circuit simulator (LTspice, Liner Technology). In the equivalent circuit as shown in Figure 3, circuit constants 

were used in the value of a reference (Ishizuka et al., 2015). The input pulse height was 80 V, resistance was  

2 k, capacitance was 0.5 mF and inductance was 1.0 mH. It is shown the voltage of the pulse input in solid 

line, output current as a dotted line in the Figure 4. 

5. Result and discussion 

Figure 5 (a) - (d) show calculation result of the pulse width modulation, and (e) ~ (g) shows the results of a 

pulse density modulation. In case of pulse width modulation, when changing by increasing the pulse width of 

the pulse voltage to be input, the output current depending on an increase in the pulse width is increased. In 

case of pulse density modulation, when performing pulse density modulated by changing the period of the 

pulse, the output current depending on the density increase of the pulse is increased. The upper limit of the 

output current is when the pulse is continuous in all of the modulation. Further, it is able to control up to about 

approximately one third with respect to the upper limit of the output current. From these results, particle 

circulation rate conditions corresponding to the gas superficial velocity of less than 5 ms-1 may be possible to 

achieve applied a pulse modulation and pulse gas supply. However, when the actually performing pulsed gas 

supply, it is expected that there is the particle circulation rate of lowest as a threshold value. Due to the 

decrease in the pulse density and pulse width, a large fluctuation has appeared in output current, thereby 

there is a threshold that particle circulation is not satisfied. In the Figure 5, the output current from the start of 

the pulse is seen an increase in the output current until stabilize with time delay. Because the output current 

changes with a delay time when changing the modulation of the pulse. Therefore, to control of the particle 

circulation rate by the pulse, it is necessary the actual dynamic characteristics of the flow properties of the 

circulating fluidized bed. Then, there is a possibility of control in the start and stop procedure of the particle 

circulation by pulse method. 

6. Conclusions 

In this report, to control the particle circulation rate of the TBCFB, we have proposed a method of performing a 

pulsed gas supply to the riser. Denoting the flow characteristics of the riser portion of the circulating fluidized  

130



 

 

Figure 5: Input-output characteristic of equivalent circuit model. Solid line: input pulse. Dot line: output current. 

Pulse width modulation: a) 0.05 s, b) 0.1 s, c) 0.2 s, d) 0.25s. Pulse density modulation e), f), g) 

131



bed in the equivalent circuit, since the low-pass filter, to consider the possibility of circulation quantity control 

using the equivalent circuit model and an electric circuit simulation. Results, a pulse input by changing the 

PDM or PWM, that is by changing the output, it was possible as an electrical circuit. By providing a pulsed gas 

supply to the riser, it indicated the possibility of controlling the particle circulation rate of the triple-bed 

circulating fluidized bed (TBCFB). 

Acknowledgments 

This work was supported by the Japan Society for the Promotion of Science KAKENHI under grant number 

25420798, 15H05554 and a Grant-in-Aid for Scientific Research. 

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