- Email: [email protected]
Investigation on heat exchange feasibility of internal solids circulation for an ultra-supercritical CFB boiler
Accepted Manuscript Investigation on heat exchange feasibility of internal solids circulation for an ultra-supercritical CFB boiler
Sicong Sun, Xiaofeng Lu, Silin Zhou, Quanhai Wang, Jianbin Chen, Jianbo Li, Xiong Xie, Changxu Liu PII: DOI: Reference:
S0032-5910(18)30488-1 doi:10.1016/j.powtec.2018.06.048 PTEC 13481
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
1 February 2018 26 June 2018 29 June 2018
Please cite this article as: Sicong Sun, Xiaofeng Lu, Silin Zhou, Quanhai Wang, Jianbin Chen, Jianbo Li, Xiong Xie, Changxu Liu , Investigation on heat exchange feasibility of internal solids circulation for an ultra-supercritical CFB boiler. Ptec (2018), doi:10.1016/ j.powtec.2018.06.048
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Investigation on heat exchange feasibility of internal solids circulation for an ultra-supercritical CFB boiler Sicong Sun a, Xiaofeng Lu a,, Silin Zhou b , Quanhai Wang a, Jianbin Chen b , Jianbo Li a, Xiong Xie b , Changxu Liu a
b
Key Laboratory of Low-grade Energy Utilization Technologies and Systems of Min istry of Education,
Sichuan Baima CFB Demonstration Power Plant Co. Ltd., Neijiang 641005, P.R. China
AC C
EP T
ED
MA
NU
SC
RI
b
PT
Chong Qing University, Chong Qing 400044, P.R. China
Corresponding author. E-mail address: [email protected] (Xiaofeng Lu).
ACCEPTED MANUSCRIPT Abstract A novel ultra-supercritical circulating fluidized bed (CFB) boiler technology based on recycling the high-temperature internal solids in the dense-phase zone to heat final superheater and reheater was investigated in this paper. The merit of this technology was that it can guarantee the superheated and reheated steam outlet temperatures especially under the low load condition. The flow characteristics of internal solids circulation was investigated on a cold test rig using the
PT
circulating solids from the Baima’s 600MW supercritical CFB boiler and the valid internal solids
RI
circulation rate was measured by optical fiber probes. The effects of fluidizing air veloc ity, partition height and static main bed height on the valid internal solids circulation rate were studied.
SC
Moreover, the valid internal solids circulation rate of thermal state was measured on the Baima’s 600MW supercritical CFB boiler. By the cold and thermal tests, an empirical formula which could
NU
predict the valid internal solids circulation rate was summarized and refined. In addition, the heat exchange ability of internal solids circulation was analyzed and the needed internal solids
MA
circulation rate of actual boiler was calculated. Besides, practicable schemes of this technology were proposed in the end of the paper. Based on the above analysis, it was proved that the novel
ED
technology using the internal solids circulation was feasible to guarantee the temperatures of final superheated and reheated steam in an ultra-supercritical CFB boiler.
AC C
load condition.
EP T
Keywords : ultra-supercritical; CFB boiler; internal solids circulation; steam temperatures; low
ACCEPTED MANUSCRIPT 1. Introduction In the last three decades, circulating fluidized bed (CFB) boilers are widely used in the world because of their fuel flexibility, high combustion efficiency, and low emissions [1-3]. In order to face the shortage in energy resources and improve the efficiency of boilers, the steam parameters of CFB boilers are raised constantly. The outlet temperatures of superheated and reheated steam of
PT
subcritical CFB boilers were usually 540℃ [4]. They reached 560/580℃ in the world first supercritical CFB boiler (located in Lagisza, Poland) [5]. The designed outlet temperatures of
RI
superheated and reheated steam of the in-development ultra-supercritical CFB boiler in China would reach 605/623℃ [6]. However, the combustion temperature of CFB boilers is only
SC
800-900℃ [1]. Specifically, the outlet temperature of furnace may be down to 700℃ under the low load condition [7]. As a result, traditional superheating and reheating arrangements, which put the
NU
final superheater and reheater at the upper furnace, external heat exchangers, or rear flue [8-11], is difficult to guarantee the superheated and reheated steam temperatures of ultra-supercritical CFB
MA
boilers especially under the low load condition. Therefore, optimizing the arrangements of final superheater and reheater to guarantee the steam temperatures under different loads for
ED
ultra-supercritical CFB boilers become urgent.
Considering the stable bed temperature (about 850℃) during the operation of CFB boiler
EP T
[12-14], it is reasonable to utilize the internal solids circulation in the lower furnace for superheating and reheating of ultra-supercritical CFB boilers. Foster Wheeler has provided the integrated recycle heat exchanger (INTREX) which could utilize both internal and external solids
AC C
circulation to heat the superheater and reheater. The highest outlet temperatures of superheated and reheated steam of CFB boilers equipped with INTREX have reached 603/603℃ [15, 16]. In addition, more than a dozen supercritical CFB boilers have been put into use in China. They all used the traditional arrangements of final superheater and reheater and the outlet temperatures of the superheated and reheated steam were about 571/569℃ [6]. Therefore, in order to face the challenges of the higher steam parameters of ultra-supercritical CFB boilers, a technology of independently utilizing the internal solids circulation in the lower furnace for superheating and reheating is proposed in this paper. This technology can guarantee the steam temperatures under different loads for an ultra-supercritical CFB boiler. It is not only useful for newly built CFB boilers but also for improving outlet temperatures of steam in applied boilers.
ACCEPTED MANUSCRIPT Obviously, in this technology, enough and continuous solids which climb over the partition into the internally circulating chamber are the key to heat the final superheater and reheater of an ultra-supercritical CFB boiler. Many studies indicated that this kind of internal solids circulation [12, 17, 18] was widely applied in different aspects. Wang [19] studied the internally circulating gas-solid flow characteristics in a 2.5MW pilot internally circulating fluidized bed(ICFB) facility
PT
for a new kind of ICFB boiler. It was shown that the internal solids circulation was influenced by the static bed height, fluidizing air velocity, partic le diameter and arrangements of tubes. Huang et
RI
al. [20] investigated the particle flow characteristics in a clapboard-type internal circulating fluidized bed reactor for biomass gasification. It was discovered that the internal solids circulation
SC
was related to fluidization velocity, structure dimension and amount of side air. Fang et al. [21] introduced a double fluidized bed solid circulation system for producing the middle heating value
NU
town gas and did experiments on a small scale test facility using plastic pellets and CFB ash as the bed material. It was presented that the solid circulating rate between two beds was effected by the
MA
bed material, operation velocities and bed structure. However, few studies have been reported on the gas-solid flow and heat transfer of using the internal solids in the furnace to heat the
ED
high-temperature ultra-supercritical steam.
In order to prove the feasibility of this technology, the influences of static bed height, fluidizing
EP T
air velocity and partition height on internal solids circulation were investigated in a self -designed cold rig. The thermal characteristics of internal solids circulation were s tudied on the Baima’s 600MW supercritical CFB boiler. After these experiments, some discussions about the thermal
AC C
equilibrium of this technology and practicable schemes of this technology were conducted. The experimental results and discussion were given in this paper.
2.Experiments 2.1 Cold tests
2.1.1 Experimental apparatus The experimental apparatus of internally circulating fluidized bed cold test rig is illustrated in Fig.1. It mainly consisted of a furnace, two uniform-pressure windboxes, a loop seal and a cyclone. In order to directly observe gas-solid flow of the furnace, the cyclone and furnace were made of Plexiglas.
ACCEPTED MANUSCRIPT Lower furnace was constituted of two parts: a main bed with cross section of 390mm×265mm and an internally circulating chamber with cross section of 390mm×610mm. The size of main bed air distributor was 210mm×265mm.The cross section of upper furnace was 390mm×520mm. The height of the furnace was 3200mm and internally circulating chamber air distributor was 720mm higher than that of the main bed. The main bed and internally circulating
PT
chamber were separated by a partition which was formed by 6 Plexiglas bricks. Each Plexiglas brick height was 100mm. Therefore, the height of partition could be adjusted with experimental
RI
condition. Besides, there was a circulating channel at the bottom of the partition where the internal solids return to the main bed.
SC
The cold test rig fluidizing air was supplied by a centrifugal fan. After measured by gas flowmeters, the air passed through uniform-pressure windboxes to the main bed and internally
MA
2.1.2 Characteristics of bed material
NU
circulating chamber respectively.
The bed material of the experiments came from circulating solids from Baima’s 600MW
ED
supercritical CFB boiler. Its physical specification is listed in Table.1 and particle size distribution is demonstrated in Fig.2. As shown in Fig.3, the minimum fluidizing veloc ity of the bed
EP T
material(Umf ) was gotten by the relation curves of bed pressure difference and main bed fluidizing air velocity(Um ) [22, 23]. Besides, the bed material’s minimum fluidizing velocity was about
AC C
0.3m/s.
2.1.3 Measuring methods The internally circulating solids which climbed over the partition into internally circulating chamber was defined as valid internally circulating solids. So, the valid internal solids circulation rate could be gotten by measuring solids into the internally circulating chamber. It is depicted in Fig.4 that there were 3×3 measuring holes on the flank wall of internally circulating chamber which were used for inserting fiber-optic probes [24]. In addition, different height of the holes was serviced for different partition height during the experiments. As described in Fig.5, the cross section of the internally circulating chamber was divided into 15 areas and the solid points were measured by fiber-optic probes in the experiments.
ACCEPTED MANUSCRIPT The solid holdup (ε) and particle velocity (v) of the valid internally circulating solids were measured simultaneously by reflective-type fiber-optic probes whose model was PV6D [25]. As shown in Fig.5, rectangular coordinate system was established on the cross section of the internally circulating chamber and the length and width were presented as a and b respectively. The solid holdup and particle veloc ity of each measuring point were fitted into binary quadratic
PT
equations as ε(x, y) and v(x, y)[26, 27]. These equations represented the concentration and speed cross section distribution of the valid internally circulating solids in the internally circulating
RI
chamber. Thus, the valid internal solids circulation rate in cold state(Gvs ) can be described as follows [28]:
0
x, y v x, y dxdy b
0
ab
where ρs is the true density of the bed material.
SC
a
(1)
NU
Gvs
s
To ensure the experimental accuracy, measurements of the solid holdup and particle velocity
ED
2.1.4 Experimental arrangements
MA
of each point were repeated for 20 times and the average values were obtained.
The experimental conditions are listed in Table 2. In addition, the average experimental
EP T
temperature(tf ) was 30℃ and the average temperature of fluidizing air(tw) was 40℃. The fluidizing air velocity of the internally circulating chamber was 0.085m/s, which made it remain at the
AC C
bubbling state [29, 30].
2.2 Thermal tests
In order to get the internal solids circulation rate of the actual boiler in low load condition, measurements of the pressure profiles along the furnace height was conducted on the Baima’s 600MW supercritical CFB boiler under 60% load.
2.2.1 Main parameters of the boiler As shown in Fig.6, the height of the furnace was 55000mm. The width of the upper furnace was 15000mm. The size of air distributor was 4000mm×28000mm and the height of the breeches-legs(Hl) was 8600mm [11]. Besides, 4 pressure measuring ports were installed
ACCEPTED MANUSCRIPT symmetrically on the left and right walls of the boiler dense-phase zone. The lower two measuring ports were 1900mm higher than the air distributor and the upper two were 7100mm away from the air distributor. In addition, as shown in Fig.7, at the height of 25000mm and 38000mm away from the air distributor, 18 measuring ports were set uniformly around the furnace at each height. As shown in Fig.8, according to the transport disengaging height formula [31] and the operation
PT
parameters of the boiler, the solids whose diameter was less than 2mm can be entrained beyond 5m from the air distributor. These solids can be used in the internal solids circulation (72% of the
RI
bed material). Bes ides, the bed material height in the boiler operation was about 1000mm. Therefore, in the calculation of valid internal solids circulation rate of the thermal state, the static
SC
bed height was at least 720mm.
NU
2.2.2. Testing methods of the valid internal solids circulation rate in the furnace At 60% boiler load, the pressure values where were 38000mm, 25000mm, 7100mm and
MA
1900mm away from the air distributor were carefully measured. Besides, the pressure values at 55000mm and 400mm higher than the air distributor were directly obtained from the DCS system
ED
of the power plant. The fluidizing air velocity of the boiler (Ug ) could be calculated from the DCS system [9]. Therefore, the pressure distribution along the furnace height can be gotten from these
EP T
pressure values. So, the valid internal solids circulation rate in thermal state ( G’vs ) can be calculated as follows:
AC C
Gvs mU g
Lv Lt
(2)
where Lv and Lt were shown at Fig.6. Δρm was the gas-solid mixture density difference between two adjacent measuring areas. The measuring area was between two measuring ports. The mixture density (ρm ) was calculated as follows [32, 33]:
m
P g H
(3)
where ΔP was the pressure drop between two adjacent measuring ports and ΔH was the height between two adjacent measuring ports.
3. Results and discussion
ACCEPTED MANUSCRIPT 3.1 Results of cold tests 3.1.1 Effect of partition height on the valid internal solids circulation rate Figs.9 and 10 show dependence of the valid internal solids circulation rate (Gvs ) on the partition height (Hp ), under different main bed fluidizing air velocity (Um ) and static main bed height (Hs ). It is shown that Gvs reduced with the increment of Hp under the same Um. This is because at the
PT
same Um, the particle entrainment rate decreases along the vertical direction [34]. Hence with the increase in Hp, the partial amount of internally circulating solids which can climb over the
RI
partition became decreased. Moreover, the top of the partitions corresponds to the bottom of solids entrance of the internally circulating chamber in practical use. Therefore, the valid internal solids
NU
of internally circulating chamber and the air distributor.
SC
circulation rate can be controlled effectively by adjusting the distance between the solids entrance
3.1.2 Effect of main bed fluidizing air velocity on the valid internal solids circulation
MA
Figs. 11 and 12 show the effect of the main bed fluidizing air velocity (Um ) on the valid internal solids circulation rate (Gvs ) at different partition height (Hp ) and static main bed height (Hs ). It is
ED
indicated that Gvs increased firstly and then decreased later with the increasing Um , at various Hp. In addition, when Um was approximately 1.2 m/s, Gvs reached its peak value. This meant that
EP T
before Um grew up to 1.2m/s, increasing more internally circulating solids was entrained by the air and climbed over the partition upon the increase of Um. On the contrary, when Um surpassed 1.2 m/s, more internally circulating solids was taken into the cyclone. This made Gvs reduce with the
AC C
increasing Um [35]. Therefore, Gvs can reach its maximum at 1.2 m/s. As a result, during boiler operation, it can get higher valid internal solids circulation rate under the low load condition. On the contrary, the valid internal solids circulation rate will reduce in the full load condition, which is in favor of reducing the heat flux of the internally circulating chamber.
3.1.3 Effect of static main bed height on the valid internal solids circulation rate Figs.13 and 14 show the effect of static main bed height (Hs ) on the valid internally circulating rate (Gvs ) at different partition height (Hp ) and main bed fluidizing air velocity (Um ). It is shown that Gvs increased with the increment of Hs under the same Um . Furthermore, Gvs increased sharply when Hs reached 500mm under Um =0.6m/s. This is because at the lower Hs (300mm and 400mm),
ACCEPTED MANUSCRIPT the solids which can be entrained by Um have not been saturated and the core-annulus was unapparent. Most of the fine solids were carried into the upper furnace joining the external circulation. However, when Hs raised to 500mm, the solids which can be entrained by Um have approached saturation and the core-annulus appeared. Therefore, except the external circulation, there were also lots of fine solids joining the internal circulation. In addition, when Um was 1.2m/s,
PT
the solids that can be entrained by Um have approached saturation under different Hs and the core-annulus was obvious. As a result, Gvs almost linearly increased with increasing Hs .
RI
3.1.4 Empirical formula of valid internal solids circulation rate
Many empirical formulas were proposed in the literature to correlate the internal solids
SC
circulation rate with the relevant parameters. However, most of them were based on the hypothesis of ash flowing over an orifice [23, 36-38]. In this paper, an empirical formula about Gvs in the
NU
current type cold test rig was developed by the dimensionless parametrization and multi-variable regression analysis methods.
MA
In these cold tests, the independent variables were Hs , Hc, Um and dependent variable was Gvs . Four dimensionless variables (hb , hc, um and gvs ) were gotten by the following equations:
Hs Dt
(4)
hc
Hc Ht
(5)
um
Um U mf
(6)
gvs
Gvs bU mf
(7)
AC C
EP T
ED
hs
where Umf was the Minimum fluidizing veloc ity of the bed material. ρb was the bulk density of the bed material. Dt =243mm was the equivalent diameter of the main bed air distributor. Definitions of Hc and Ht are illustrated in Fig.1, and Ht =1615mm. An empirical formula of hs, hc, um and gvs was developed by using multi-variable regression analysis (using the data of Hs =300mm and Hs =400mm), as shown below:
gvs
0.035hs
0.046hc
0.015um3
0.148um2
0.447um
0.425
(8)
ACCEPTED MANUSCRIPT The correlation coefficient was 0.97, which indicates the regression correlation was superior [39]. In addition, the experiments with Hs =500 mm were conducted at this cold test rig, whose test conditions included Hp =200mm and 400mm as well as Um =0.6m/s and 1.2m/s. Comparison of the experimental and calculated data are shown in Fig.15. It presents that the experimental data had a
PT
good agreement with the calculated data.
RI
3.2 Results of thermal tests
3.2.1. Typical parameters and pressure distribution along furnace height under 60% boiler load
SC
The typical DCS parameters of the Baima’s 600 MW supercritical CFB boiler under 60% load are listed in Table 3. In addition, the vertical pressure distribution of left and right furnace is
NU
described in Fig. 16.
As shown in Fig. 16, at 60% boiler load, the pressure in furnace dropped rapidly within the
MA
dense-phase zone height and then remained nearly constant. That means at the bottom of the boiler, large quantities of solids were accumulating which resulted in a higher pressure gradient [32]. It
ED
indicated that there was a lot of internally circulating solids beneath the secondary air orifices. Therefore, under the low load condition of the boiler, the internal solids circulation rate at the
EP T
furnace bottom may be much enough to heat the final superheater and reheater.
3.2.2 Modification of the empirical formula in thermal state
AC C
In order to accurately calculate the valid internal solids circulation rate in thermal state, the empirical formula which is proposed in 3.1.3 is modified as follows [40, 41]:
gvs
(T f t f ) (Tw tw )
g vs
(9)
where Tf is the furnace temperature of boiler and Tw is the air supply temperature of boiler. In addition, the practical parameters of the boiler’s left and right dense-phase zone were normalized by equations (4) ~ (7) and then substituted into equation (9). The valid internal solids circulation rate where was 1900mm from the boiler air distributor was calculated by the modified empirical formula. Furthermore, the valid internal solids circulation rate which was 1900mm in height from the boiler air distributor was also calculated from equations (2) and (3) using the
ACCEPTED MANUSCRIPT measured pressure. As demonstrated in Fig.17, the calculated data coincided well with the measured data, indicating that the modified empirical formula (equation (9)) can be adequately applicable to the estimation of the valid internal solids circulation rate of this boiler.
3.3 Discussion
PT
3.3.1. Thermal equilibrium calculation of using internal solids circulation to heat final superheater and reheater
RI
Based on Baima’s 600MW supercritical CFB boiler, the outlet parameters of superheater and reheater listed in Table 3 was utilized as inlet parameters (569℃/566℃) of final superheater and
SC
reheater in ultra-supercritical CFB boiler at low load condition. In addition, ultra-supercritical CFB boiler should use internal solids circulation to heat its final superheater and reheater. Through
NU
this method, the ultra-supercritical CFB boiler superheater and reheater outlet temperatures should reach the designed parameters which are described in Table 4.
MA
Therefore, under low load condition of ultra-supercritical CFB boiler, the needed internal solids circulation rate could be calculated based on the principle of energy conservation and parameters
ED
listed in Table 3 and Table 4[8, 42]. Besides, as some parts of the furnace wall in the dense-phase may be used for returning external solids, only 50% of valid internal solids were considered
EP T
flowing into the internally circulating chamber. Thus, calculated results are presented in Table 5. According to Table 5, the practical needed internal solids circulation rate is 12.28kg/m2 s which can heat superheated and reheated steam to the parameters listed in Table 4. Besides, if the
AC C
ultra-supercritical CFB boiler is based on the Baima’s 600MW supercritical CFB, the modified empirical formula (equation (9)) can be used to predict the valid internal solids circulation rate of the boiler. As shown in Fig.18, when solids entrance of internally circulating chamber is 3.46m higher than the air distributor, the valid internal solids circulation rate can meet the requirement. As a result, if ultra-supercritical CFB boiler is based on Baima’s 600MW supercritical CFB boiler and set the solids entrance of internally circulating chamber less than 3.46m away from the air distributor, the valid internal solids circulation rate can heat its superheated and reheated steam to the required temperatures at 60% load condition.
3.3.2. Schemes of the technology for CFB boilers
ACCEPTED MANUSCRIPT At present, layout of large-scale CFB boilers can be usually divided into single furnace and double furnaces [2, 4, 43]. As described in Fig.19, internally circulating chamber can be sited on both sides of single furnace CFB boiler. As well as, at double furnaces CFB boiler, internally circulating chamber can be put between the legs, which is illustrated in Fig.20. In both arrangements, different heat exchange surfaces should be put into adjacent internally circulating
PT
chambers, which reduces solids outlet temperature difference of internally circulating chamber. Internally circulating chamber can be made up by membrane wall. An available structure of it is
RI
shown in Fig.21. The internally circulating chamber is divided into three rooms(C/D/E). Every room has its own windbox. Under different thermal load, if the heat absorbed proportion of final
SC
superheater/reheater need to be elevated, the room C and D will be fluidized and most of internally circulating ash will enter into room C to heat the final superheater/reheater. If the heat
NU
absorbed proportion of final superheater/reheater need to be reduced, the room D and E will be fluidized and little or no internally circulating ash will enter into room C. Therefore, in this case,
MA
the heat absorbed proportion of final superheater/reheater can be reduced to minimum or 0%. Railings and high-pressure air nozzles can be sited at solids entrances, which protect heat
ED
exchangers and solids entrances from blocking. The height of solids entrance from boiler air distributor can be determined according to the needed internal solids circulation rate. However,
EP T
because, during the operation of the internally circulating chamber, the fluidized air of three rooms need to be controlled respectively to adjust the heat absorbed proportion of final superheater/reheater, the complexity of controlling superheated and reheated steam temperature
AC C
will be elevated.
4. Conclusions
In order to make superheated and reheated steam reach designed temperature in low load condition of an ultra-supercritical CFB boiler, a technology of utilizing high temperature solids of dense-phase zone to heat final superheater and reheater is proposed in this paper. Based on the cold tests, thermal tests, empirical formula analysis, thermal equilibrium calculation and discussion of practicable schemes, the following conclusions can be summarized. (1) From cold tests, Gvs decreased with rising Hp under the same Um. As well as, it increased firstly and then decreased with growing Um under the same Hp . Moreover, it increased with the
ACCEPTED MANUSCRIPT increment of Hs under the same Um . According to the experiment condition, a dimensionless empirical formula can be achieved as follows:
gvs
0.035hs
0.046hc
0.015um3
0.148um2
0.447um
0.425
(2) According to thermal tests, valid internal solids circulation rates of the left and right 2
2
furnaces were 49.3kg/(m s) and 46.2kg/(m s), which was 1900mm away from air distributor. The
PT
measurement data were similar to the calculated data which were counted by modified empirical
(T f t f ) (Tw tw )
g vs
SC
gvs
RI
formula. In addition, the modified empirical formula is described as follows:
(3) Based on 60% load steam parameters of Baima’s 600MW supercritical CFB boiler, the 2
NU
practical needed internal solids circulation rate(12.28kg/(m s)) which was used for heating final superheater and reheater of ultra-supercritical CFB boiler was achieved by the thermal equilibrium.
MA
After analyzed by the modified empirical formula, when the distance between solids entrance of internally circulating chamber and the air distributor of the furnace was less than 3.46m, the valid
569/571℃ to 605/623℃.
ED
internal solids circulation rate shall meet the requirement which is heating the steam from
(4) Practicable schemes of this technology were carefully described, which can be used for both
EP T
signal furnace boilers and double furnaces boilers. The technology of using internal circulating solids for heating the final superheater and reheater of an ultra-supercritical CFB boiler was
AC C
proved to be feasible.
ACCEPTED MANUSCRIPT Nomenclature dp
mean size (μm)
Dt
equivalent diameter air distributor (mm)
g
acceleration of gravity (m/s )
G’vs
valid internally circulating rate in thermal state (kg/m 2 s)
Gvs
valid internally circulating rate in cold state (kg/m s)
gvs
dimensionless valid internally circulating rate in cold state
g'vs
dimensionless valid internally circulating rate in thermal state
Hs
static bed height (mm)
hs
dimensionless static bed height
hc
dimensionless solids entrance height
Hc
solids entrance height (mm)
Hl
breeches-legs height (mm)
Hp
partition height (mm)
Ht
total height(mm)
Lt
total length (m)
Lv
valid length (m)
tf
average experimental temperature (℃)
Tf
furnace temperature of boiler (℃)
Tw
air supply temperature of boiler (℃)
tw
average temperature of fluidizing air (℃)
Ui Um
EP T
ED
MA
NU
SC
RI
PT
2
AC C
Ug
2
fluidizing air velocity of Baima’s 600MW CFB boiler (m/s) internally circulating chamber fluidized air speed (m/s) main bed fluidizing air velocity (m/s)
um
dimensionless main bed fluidizing air velocity
Umf
minimum fluidizing velocity (m/s)
v
particle velocity (m/s)
ΔH
height between two adjacent measuring ports (m)
ΔP
pressure drop between two adjacent measuring ports (Pa)
ε
solid holdup
ACCEPTED MANUSCRIPT 3
ρb
bulk density (kg/m )
ρm
gas-solid mixture density (kg/m3 )
ρs
true density (kg/m3 )
Δρm
gas-solid mixture density difference between two adjacent measuring areas (kg/m )
3
PT
Acknowledgments The financial support from the National Key Research & Development Program of China (No.
AC C
EP T
ED
MA
NU
SC
RI
2016YFB0600201) is gratefully acknowledged.
ACCEPTED MANUSCRIPT References [1] P. Basu, Combustion of coal in circulating fluidized-bed boilers: a review, Chemical Engineering Science, 54 (1999) 5547-5557. [2] L.M. Cheng, X.L. Zhou, C.H. Zheng, et al., Development of large-scale circulating fluidized bed boiler, Journal of Power Engineering, (2008).
PT
[3] J. Koornneef, M. Junginger, A. Faaij, Development of fluidized bed combustion—An overview of trends, performance and cost, Progress in Energy and Combustion Science, 33 (2007)
RI
19-55.
[4] G.X. Yue, H.R. Yang, J.F. Lu, et al., Latest Development of CFB Boilers in China, in:
SC
Proceedings of the 20th International Conference on Fluidized Bed Combustion, Xi’an, China, 2009.
NU
[5] T. Jäntti, H. Lampenium, M. Ruuskanen, R. Parkkonen, Supercritical OT U CFB Projects Lagisza 460 MWe and Novercherkasskays 330 MWe, Russia Power, (2011) TP_CFB_11_04.
MA
[6] R.X. Cai, J.F. Lv, W. Ling, et al., Development of (Ultra-)Supercritical Circulating Fluidized Bed Boiler Technology, Electric Power, (2016) 1-7.
ED
[7] Y.H. Zhao, P. Han, Co-combustion tests under low load condition on supercritical circulating fluidized bed boiler, Clean Coal Technology, (2016) 76-81.
EP T
[8] H. Wang, X.F. Lu, W. Zhang, et al., Study on heat transfer characteristics of the high temperature reheater tube panel in a 300 MW CFB boiler with fluidized bed heat exchanger, Applied Thermal Engineering, 81 (2015) 262-270.
AC C
[9] J.Y. Lu, Study on Particle Population Balance and Heat Balance in Large-Scale Circulating Fluidized Bed Boiler Chongqing University, 2012. [10] H.B. Wu, M. Zhang, Y.K. Sun, et al., Research on the heat transfer model of platen heating surface of 300MW circulating fluidized bed boilers, Powder Technology, 226 (2012) 83-90. [11] C.H. Hu, X.F. Lu, Equipments and Operation of the 600MW Supercritical Circulating Fluidized Bed Boiler, Electric Power Press, Beijing, 2012. [12] P. Basu, Combustion and Gasification in Fluidized Beds, CRC Press, 2006. [13] X.F. Lu, Equipments and Operation of Large-Scale Circulating Fluidized Bed Boiler, Electric Power Press, Beijing, 2006. [14] P. Basu, Circulating Fluidized Bed Boilers. Design, Operation and Maintenance. Springer,
ACCEPTED MANUSCRIPT New York, 2015. [15] S.J. Goidich, T. Hyppanen, K. Kauppinen, CFB Boiler Design and Operation Using The INTREX™
Heat
Exchanger,
6th
International
Conference
on
Circulating
Fluidized,
BedsWürzburg, Germany, 1999. [16] T. Jäntti, Nuortimo, KalleRuuskanen, J. MarkoKalenius, Samcheok Green Power 4x550MWe
PT
Supercritical Circulating Fluidize Bed Steam Generators in South Korea, PowerGen Europe, (2012).
RI
[17] J.X. Bouillard, R.W. Lyczkowski, D. Gidaspow, POROSITY DISTRIBUTIONS IN A FLUIDIZED-BED WITH AN IMMERSED OBSTACLE, Aiche J., 35 (1989) 908-922. Y.T.
Choi, S.D. Kim,
BUBBLE CHARACTERISTICS IN AN INTERNALLY
SC
[18]
CIRCULATING FLUIDIZED-BED, J. Chem. Eng. Jpn., 24 (1991) 195-202.
NU
[19] J.L. Wang, Experimental and Numerical Investigations on Gas-Solid Flow in an Internally Circulating Fluidized Bed, Southeast University, 2013.
MA
[20] L.C. Huang, L.L. Ma, Z. Zhou, et al., Experimental study on particle flow characteristics in a clapboard-type internal circulating fluidized bed reactor, Acta Energlae Solaris Sinica, (2008)
ED
900-904.
[21] M.X. Fang, Z.Z. Shi, S.R. Wang, et, al., Experimental Research on Solid Circulation of a
54-57+61.
EP T
Double Fluidized Bed, Transaction of the Chinese Society for Agricultural Machinery, (2003)
[22] B. Zeng, X.F. Lu, L. Gan, et al., Development of a novel fluidized bed ash cooler for
AC C
circulating fluidized bed boilers: Experimental study and application, Powder Technology, 212 (2011) 151-160.
[23] B. Xiong, X.F. Lu, R.S. Amano, et al., Gas–solid flow in an integrated external heat exchanger for CFB boiler, Powder Technology, 202 (2010) 55-61. [24] J. Werther, Measurement techniques in fluidized beds, Powder Technology, 102 (1999) 15–36. [25] T. Hagemeier, M. Börner, A. Bück, E. Tsotsas, A comparative study on optical techniques for the estimation of granular flow velocities, Chemical Engineering Science, 131 (2015) 63-75. [26] C.X. Wang, J. Zhu, S. Barghi, et al., Axial and radial development of solids holdup in a high flux/density gas–solids circulating fluidized bed, Chemical Engineering Science, 108 (2014)
ACCEPTED MANUSCRIPT 233-243. [27] C.X. Wang, C.Y. Li, J. Zhu, Axial solids flow structure in a high density gas –solids circulating fluidized bed downer, Powder Technology, 272 (2015) 153-164. [28] W.B. Li, K. Yu, X.G. Yuan, et al., An anisotropic Reynolds mass flux model for the simulation of chemical reaction in gas-particle CFB risers, Chemical Engineering Science, 135 (2015)
PT
117-127. [29] N. Balasubramanian, Transition velocities in the riser of a circulating fluidized bed, Advanced
RI
Powder Technology, 16 (2005) 247-260.
[30] S.I. Ngo, Y.-I. Lim, B.-H. Song, et al., Song, Effects of fluidization velocity on solid stack
SC
volume in a bubbling fluidized-bed with nozzle-type distributor, Powder Technology, 275 (2015) 188-198.
NU
[31] S.T. Pemberton, J.F. Davidson, Elutriation from fluidized beds —II. Disengagement of particles from gas in the freeboard, Chemical Engineering Science, 41 (1986) 253-262.
MA
[32] J. Yong, J. Zhu, Z.W. Wang, et al., Fluidization Project Principle, Tsinghua University Press, Beijing, (2001).
ED
[33] J. Xu, X.F. Lu, Q.H. Wang, et al., Experimental study on gas-solid flow characteristics in a 60-meter-high CFB cold test apparatus, Proc. 22nd international conference on fluidized bed
EP T
conversion, Turku, Finland, 2015.
[34] D.C. Sau, S. Mohanty, K.C. Biswal, Critical fluidization velocities and maximum bed pressure drops of homogeneous binary mixture of irregular particles in gas –solid tapered fluidized
AC C
beds, Powder Technology, 186 (2008) 241-246. [35] K. Cen, Theories, Design and Operation of Circulating Fluidized Bed Boiler, Electric Power Press, Beijing, 1998.
[36] R. Korbee, O.C. Snip, J.C. Schouten, et al., Rate of solids and gas transfer via an orifice between partially and completely fluidized beds, Chemical Engineering Science, 49 (1994) 5819-5832. [37] W.C. Yeh, Measurement of the Solid Circulation Rate in an Interconnected Fluidized Bed, Chemical Engineering, (2001). [38] D.R.M. Jones, J.F. Davidson, The flow of particles from a fluidized bed through an orifice, Rheologica Acta, 4 (1965) 180-192.
ACCEPTED MANUSCRIPT [39] J. Peterson, Regression Analysis of Count Data, Technometrics, 41 (2014) 371-371. [40] M.H. Rahman, X.T. Bi, J.R. Grace, et al., Measurement of solids circulation rate in a high-temperature dual fluidized bed pilot plant, Powder Technology, 316 (2017) 658-669. [41] M.A. Habl, A. Frohner, G. Tondl, et al., Fluid dynamics study on a dual fluidized bed cold-flow model, Powder Technology, 316 (2017) 469-475.
PT
[42] J.Y. Lu, X.F. Lu, H.Z. Liu, H. et al., Calculation and analys is of dissipation heat loss in large-scale circulating fluidized bed boilers, Applied Thermal Engineering, 30 (2010) 1839-1844.
RI
[43] G.X. Yue, R.X. Cai, J.F. Lu, et al, From a CFB reactor to a CFB boiler – The review of R&D
AC C
EP T
ED
MA
NU
SC
progress of CFB coal combustion technology in China, Powder Technology, 316 (2017) 18-28.
ACCEPTED MANUSCRIPT
Figures Captions: Schematic diagram and picture of the experimental apparatus.
Fig. 2.
Accumulative particle size distribution of bed material.
Fig. 3.
Relationship between bed pressure difference and main bed fluidizing air velocity.
Fig. 4.
Layout of the measuring holes on the internally circulating chamber flank wall.
Fig. 5.
Distribution of the measuring points in the internally circulating chamber.
Fig. 6.
Arrangement of the measuring ports of Baima’s 600MW supercritical CFB boiler.
Fig. 7.
Layouts of measuring ports where were 25000mm and 38000mm away from the air
RI
PT
Fig. 1.
SC
distributor.
Particle size distribution of bed material in Baima’s 600MW supercritical CFB boiler.
Fig. 9.
Effect of partition height on the valid internally circulating rate (Hs =300mm).
NU
Fig.8.
Fig. 10. Effect of partition height on the valid internal solids circulation rate (Hs =400mm).
MA
Fig. 11. Effect of the main bed fluidizing air velocity on the valid internal solids circulation rate (Hs =300mm).
(Hs =400mm).
ED
Fig. 12. Effect of the main bed fluidizing air velocity on the valid internal solids circulation rate
Effect of static main bed height on the valid internally circulating rate (Um=0.6m/s).
Fig.14.
Effect of static main bed height on the valid internally circulating rate (Um=1.2m/s).
EP T
Fig.13.
Fig. 15. Difference between experimental data and calculated data (Hs =500mm).
AC C
Fig. 16. Pressure distribution profiles along furnace height of the Baima’s 600 MW supercritical CFB boiler under 60% boiler load.
Fig. 17. Comparison of valid internal solids circulation rate where was 1900mm from the air distributor.
Fig. 18. Position of solids entrance of internally circulating chamber. Fig. 19. Layout of internally circulating chamber in single furnace CFB boiler. Fig. 20. Two different layouts of internally circulating chamber in double furnaces CFB boiler. Fig. 21. An available structure of internally circulating chamber.
ACCEPTED MANUSCRIPT Table 1. Bed material physical specification. True density
Bulk density
M ean size
M inimum fluidizing velocity
ρs(kg/m )
ρb (kg/m )
dp(μm)
Umf (m/s)
2529.01
1133.41
382.56
0.30
EP T
ED
MA
NU
SC
RI
PT
3
AC C
3
ACCEPTED MANUSCRIPT Table 2. Experimental conditions. Static main bed
M ain bed fluidizing air
Partition height
Internally
circulating
height
velocity
H p(mm)
fluidizing air velocity
Hs(mm)
U m(m/s)
300
0.6,0.9,1.2,1.5
100,200,300,400,500,600
0.085
400
0.6,0.9,1.2,1.5
100,200,300,400,500,600
0.085
500
0.6,1.2
200,400
0.085
AC C
EP T
ED
MA
NU
SC
RI
PT
Ui(m/s)
chamber
ACCEPTED MANUSCRIPT Table 3. Typical DCS parameters of Baima’s 600MW supercritical CFB boiler under 60% load. Item
Parameter
Left dense-phase zone temperature/℃
848
Right dense-phase zone temperature /℃
843
Left air supply temperature/℃
293
Right air supply temperature /℃
279
3
246.5
Right primary air rate /( kNm3/h)
232.5
Left bottom pressure(400mm from air distributor)/kPa
5.5
PT
Left primary air rate/( kNm /h)
Right bottom pressure(400mm from air distributor)/kPa Superheated steam outlet temperature/℃
AC C
EP T
ED
MA
NU
SC
RI
reheated steam outlet temperature /℃
6.6 569 566
ACCEPTED MANUSCRIPT Table 4. Design parameters of superheater and reheater in ultra-supercritical CFB boiler [6]. Parameter
Superheated steam outlet pressure/M Pa
29.4
Reheated steam outlet pressure/M Pa
5.96
Superheated steam outlet temperature/℃
605
Reheated steam outlet temperature/℃
623
Superheated steam flowrate/(t/h)
1980
Reheated steam flowrate/(t/h)
1655.7
AC C
EP T
ED
MA
NU
SC
RI
PT
Item
ACCEPTED MANUSCRIPT Table 5. Calculation of needed internal solids circulation rate. Parameter
Final superheater needed heat /( kJ/s)
63580
Final reheater needed heat /( kJ/s)
61592
Internally circulating solids inlet temperature /℃
845.5
Internally circulating solids outlet temperature /℃
760
Internally circulating solids inlet enthalpy /(kJ/kg)
816.14
Internally circulating solids outlet enthalpy /(kJ/kg)
725.16
Total amount of calculated internally circulating solids heating final superheater and reheater /(kg/s)
1375.82
Calculated internal solids circulation rate /(kg/m2s)
6.14
PT
Item
2
AC C
EP T
ED
MA
NU
SC
RI
Needed internal solids circulation rate /(kg/m s)
12.28
AC C
EP T
ED
MA
NU
SC
Fig. 1. Schematic diagram and picture of the experimental apparatus.
RI
PT
ACCEPTED MANUSCRIPT
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.2. Accumulative particle size distribution of bed material.
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.3. Relationship between bed pressure difference and main bed fluidizing air velocity .
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.4. Layout of the measuring holes on the internally circulating chamber flank wall.
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.5. Distribution of the measuring points in the internally circulating chamber.
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
Fig.6. Arrangement of the measuring ports of Baima’s 600M W supercritical CFB boiler.
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
Fig.7. Layouts of measuring ports where were 25000mm and 38000mm away from the air distributor.
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.8. Particle size distribution of bed material in Baima’s 600MW supercritical CFB boiler.
SC
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
Fig.9. Effect of partition height on the valid internally circulating rate (H s=300mm).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.10. Effect of partition height on the valid internal solids circulation rate (H s=400mm).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.11. Effect of the main bed fluidizing air velocity on the valid internal solids circulation rate (Hs=300mm).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.12. Effect of the main bed fluidizing air velocity on the valid internal solids circulation rate (Hs=400mm).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.13. Effect of static main bed height on the valid internally circulating rate (Um=0.6m/s).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.14. Effect of static main bed height on the valid internally circulating rate (Um=1.2m/s).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.15. Difference between experimental data and calculated data (H s=500mm).
RI
PT
ACCEPTED MANUSCRIPT
SC
Fig.16. Pressure distribution profiles along furnace height of the Baima’s 600 M W supercritical CFB boiler under
AC C
EP T
ED
MA
NU
60% boiler load.
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.17. Comparison of valid internal solids circulation rate where was 1900mm from the air distributor.
AC C
EP T
ED
MA
NU
SC
Fig.18. Position of solids entrance of internally circulating chamber.
RI
PT
ACCEPTED MANUSCRIPT
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
Fig.19. Layout of internally circulating chamber in single furnace CFB boiler.
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
Fig.20. Two different layouts of internally circulating chamber in double furnaces CFB boiler.
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
RI
Fig.21. An available structure of internally circulating chamber.
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
RI
PT
Graphical abstract
ACCEPTED MANUSCRIPT Highlights A method to heat final heat exchanger of ultra-supercritical CFB boiler is proposed An empirical formula is achieved by cold and thermal tests Height of internally solid circulating chamber entrance is gotten
AC C
EP T
ED
MA
NU
SC
RI
PT
Practicable schemes of this method are carefully described
Sicong Sun, Xiaofeng Lu, Silin Zhou, Quanhai Wang, Jianbin Chen, Jianbo Li, Xiong Xie, Changxu Liu PII: DOI: Reference:
S0032-5910(18)30488-1 doi:10.1016/j.powtec.2018.06.048 PTEC 13481
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
1 February 2018 26 June 2018 29 June 2018
Please cite this article as: Sicong Sun, Xiaofeng Lu, Silin Zhou, Quanhai Wang, Jianbin Chen, Jianbo Li, Xiong Xie, Changxu Liu , Investigation on heat exchange feasibility of internal solids circulation for an ultra-supercritical CFB boiler. Ptec (2018), doi:10.1016/ j.powtec.2018.06.048
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Investigation on heat exchange feasibility of internal solids circulation for an ultra-supercritical CFB boiler Sicong Sun a, Xiaofeng Lu a,, Silin Zhou b , Quanhai Wang a, Jianbin Chen b , Jianbo Li a, Xiong Xie b , Changxu Liu a
b
Key Laboratory of Low-grade Energy Utilization Technologies and Systems of Min istry of Education,
Sichuan Baima CFB Demonstration Power Plant Co. Ltd., Neijiang 641005, P.R. China
AC C
EP T
ED
MA
NU
SC
RI
b
PT
Chong Qing University, Chong Qing 400044, P.R. China
Corresponding author. E-mail address: [email protected] (Xiaofeng Lu).
ACCEPTED MANUSCRIPT Abstract A novel ultra-supercritical circulating fluidized bed (CFB) boiler technology based on recycling the high-temperature internal solids in the dense-phase zone to heat final superheater and reheater was investigated in this paper. The merit of this technology was that it can guarantee the superheated and reheated steam outlet temperatures especially under the low load condition. The flow characteristics of internal solids circulation was investigated on a cold test rig using the
PT
circulating solids from the Baima’s 600MW supercritical CFB boiler and the valid internal solids
RI
circulation rate was measured by optical fiber probes. The effects of fluidizing air veloc ity, partition height and static main bed height on the valid internal solids circulation rate were studied.
SC
Moreover, the valid internal solids circulation rate of thermal state was measured on the Baima’s 600MW supercritical CFB boiler. By the cold and thermal tests, an empirical formula which could
NU
predict the valid internal solids circulation rate was summarized and refined. In addition, the heat exchange ability of internal solids circulation was analyzed and the needed internal solids
MA
circulation rate of actual boiler was calculated. Besides, practicable schemes of this technology were proposed in the end of the paper. Based on the above analysis, it was proved that the novel
ED
technology using the internal solids circulation was feasible to guarantee the temperatures of final superheated and reheated steam in an ultra-supercritical CFB boiler.
AC C
load condition.
EP T
Keywords : ultra-supercritical; CFB boiler; internal solids circulation; steam temperatures; low
ACCEPTED MANUSCRIPT 1. Introduction In the last three decades, circulating fluidized bed (CFB) boilers are widely used in the world because of their fuel flexibility, high combustion efficiency, and low emissions [1-3]. In order to face the shortage in energy resources and improve the efficiency of boilers, the steam parameters of CFB boilers are raised constantly. The outlet temperatures of superheated and reheated steam of
PT
subcritical CFB boilers were usually 540℃ [4]. They reached 560/580℃ in the world first supercritical CFB boiler (located in Lagisza, Poland) [5]. The designed outlet temperatures of
RI
superheated and reheated steam of the in-development ultra-supercritical CFB boiler in China would reach 605/623℃ [6]. However, the combustion temperature of CFB boilers is only
SC
800-900℃ [1]. Specifically, the outlet temperature of furnace may be down to 700℃ under the low load condition [7]. As a result, traditional superheating and reheating arrangements, which put the
NU
final superheater and reheater at the upper furnace, external heat exchangers, or rear flue [8-11], is difficult to guarantee the superheated and reheated steam temperatures of ultra-supercritical CFB
MA
boilers especially under the low load condition. Therefore, optimizing the arrangements of final superheater and reheater to guarantee the steam temperatures under different loads for
ED
ultra-supercritical CFB boilers become urgent.
Considering the stable bed temperature (about 850℃) during the operation of CFB boiler
EP T
[12-14], it is reasonable to utilize the internal solids circulation in the lower furnace for superheating and reheating of ultra-supercritical CFB boilers. Foster Wheeler has provided the integrated recycle heat exchanger (INTREX) which could utilize both internal and external solids
AC C
circulation to heat the superheater and reheater. The highest outlet temperatures of superheated and reheated steam of CFB boilers equipped with INTREX have reached 603/603℃ [15, 16]. In addition, more than a dozen supercritical CFB boilers have been put into use in China. They all used the traditional arrangements of final superheater and reheater and the outlet temperatures of the superheated and reheated steam were about 571/569℃ [6]. Therefore, in order to face the challenges of the higher steam parameters of ultra-supercritical CFB boilers, a technology of independently utilizing the internal solids circulation in the lower furnace for superheating and reheating is proposed in this paper. This technology can guarantee the steam temperatures under different loads for an ultra-supercritical CFB boiler. It is not only useful for newly built CFB boilers but also for improving outlet temperatures of steam in applied boilers.
ACCEPTED MANUSCRIPT Obviously, in this technology, enough and continuous solids which climb over the partition into the internally circulating chamber are the key to heat the final superheater and reheater of an ultra-supercritical CFB boiler. Many studies indicated that this kind of internal solids circulation [12, 17, 18] was widely applied in different aspects. Wang [19] studied the internally circulating gas-solid flow characteristics in a 2.5MW pilot internally circulating fluidized bed(ICFB) facility
PT
for a new kind of ICFB boiler. It was shown that the internal solids circulation was influenced by the static bed height, fluidizing air velocity, partic le diameter and arrangements of tubes. Huang et
RI
al. [20] investigated the particle flow characteristics in a clapboard-type internal circulating fluidized bed reactor for biomass gasification. It was discovered that the internal solids circulation
SC
was related to fluidization velocity, structure dimension and amount of side air. Fang et al. [21] introduced a double fluidized bed solid circulation system for producing the middle heating value
NU
town gas and did experiments on a small scale test facility using plastic pellets and CFB ash as the bed material. It was presented that the solid circulating rate between two beds was effected by the
MA
bed material, operation velocities and bed structure. However, few studies have been reported on the gas-solid flow and heat transfer of using the internal solids in the furnace to heat the
ED
high-temperature ultra-supercritical steam.
In order to prove the feasibility of this technology, the influences of static bed height, fluidizing
EP T
air velocity and partition height on internal solids circulation were investigated in a self -designed cold rig. The thermal characteristics of internal solids circulation were s tudied on the Baima’s 600MW supercritical CFB boiler. After these experiments, some discussions about the thermal
AC C
equilibrium of this technology and practicable schemes of this technology were conducted. The experimental results and discussion were given in this paper.
2.Experiments 2.1 Cold tests
2.1.1 Experimental apparatus The experimental apparatus of internally circulating fluidized bed cold test rig is illustrated in Fig.1. It mainly consisted of a furnace, two uniform-pressure windboxes, a loop seal and a cyclone. In order to directly observe gas-solid flow of the furnace, the cyclone and furnace were made of Plexiglas.
ACCEPTED MANUSCRIPT Lower furnace was constituted of two parts: a main bed with cross section of 390mm×265mm and an internally circulating chamber with cross section of 390mm×610mm. The size of main bed air distributor was 210mm×265mm.The cross section of upper furnace was 390mm×520mm. The height of the furnace was 3200mm and internally circulating chamber air distributor was 720mm higher than that of the main bed. The main bed and internally circulating
PT
chamber were separated by a partition which was formed by 6 Plexiglas bricks. Each Plexiglas brick height was 100mm. Therefore, the height of partition could be adjusted with experimental
RI
condition. Besides, there was a circulating channel at the bottom of the partition where the internal solids return to the main bed.
SC
The cold test rig fluidizing air was supplied by a centrifugal fan. After measured by gas flowmeters, the air passed through uniform-pressure windboxes to the main bed and internally
MA
2.1.2 Characteristics of bed material
NU
circulating chamber respectively.
The bed material of the experiments came from circulating solids from Baima’s 600MW
ED
supercritical CFB boiler. Its physical specification is listed in Table.1 and particle size distribution is demonstrated in Fig.2. As shown in Fig.3, the minimum fluidizing veloc ity of the bed
EP T
material(Umf ) was gotten by the relation curves of bed pressure difference and main bed fluidizing air velocity(Um ) [22, 23]. Besides, the bed material’s minimum fluidizing velocity was about
AC C
0.3m/s.
2.1.3 Measuring methods The internally circulating solids which climbed over the partition into internally circulating chamber was defined as valid internally circulating solids. So, the valid internal solids circulation rate could be gotten by measuring solids into the internally circulating chamber. It is depicted in Fig.4 that there were 3×3 measuring holes on the flank wall of internally circulating chamber which were used for inserting fiber-optic probes [24]. In addition, different height of the holes was serviced for different partition height during the experiments. As described in Fig.5, the cross section of the internally circulating chamber was divided into 15 areas and the solid points were measured by fiber-optic probes in the experiments.
ACCEPTED MANUSCRIPT The solid holdup (ε) and particle velocity (v) of the valid internally circulating solids were measured simultaneously by reflective-type fiber-optic probes whose model was PV6D [25]. As shown in Fig.5, rectangular coordinate system was established on the cross section of the internally circulating chamber and the length and width were presented as a and b respectively. The solid holdup and particle veloc ity of each measuring point were fitted into binary quadratic
PT
equations as ε(x, y) and v(x, y)[26, 27]. These equations represented the concentration and speed cross section distribution of the valid internally circulating solids in the internally circulating
RI
chamber. Thus, the valid internal solids circulation rate in cold state(Gvs ) can be described as follows [28]:
0
x, y v x, y dxdy b
0
ab
where ρs is the true density of the bed material.
SC
a
(1)
NU
Gvs
s
To ensure the experimental accuracy, measurements of the solid holdup and particle velocity
ED
2.1.4 Experimental arrangements
MA
of each point were repeated for 20 times and the average values were obtained.
The experimental conditions are listed in Table 2. In addition, the average experimental
EP T
temperature(tf ) was 30℃ and the average temperature of fluidizing air(tw) was 40℃. The fluidizing air velocity of the internally circulating chamber was 0.085m/s, which made it remain at the
AC C
bubbling state [29, 30].
2.2 Thermal tests
In order to get the internal solids circulation rate of the actual boiler in low load condition, measurements of the pressure profiles along the furnace height was conducted on the Baima’s 600MW supercritical CFB boiler under 60% load.
2.2.1 Main parameters of the boiler As shown in Fig.6, the height of the furnace was 55000mm. The width of the upper furnace was 15000mm. The size of air distributor was 4000mm×28000mm and the height of the breeches-legs(Hl) was 8600mm [11]. Besides, 4 pressure measuring ports were installed
ACCEPTED MANUSCRIPT symmetrically on the left and right walls of the boiler dense-phase zone. The lower two measuring ports were 1900mm higher than the air distributor and the upper two were 7100mm away from the air distributor. In addition, as shown in Fig.7, at the height of 25000mm and 38000mm away from the air distributor, 18 measuring ports were set uniformly around the furnace at each height. As shown in Fig.8, according to the transport disengaging height formula [31] and the operation
PT
parameters of the boiler, the solids whose diameter was less than 2mm can be entrained beyond 5m from the air distributor. These solids can be used in the internal solids circulation (72% of the
RI
bed material). Bes ides, the bed material height in the boiler operation was about 1000mm. Therefore, in the calculation of valid internal solids circulation rate of the thermal state, the static
SC
bed height was at least 720mm.
NU
2.2.2. Testing methods of the valid internal solids circulation rate in the furnace At 60% boiler load, the pressure values where were 38000mm, 25000mm, 7100mm and
MA
1900mm away from the air distributor were carefully measured. Besides, the pressure values at 55000mm and 400mm higher than the air distributor were directly obtained from the DCS system
ED
of the power plant. The fluidizing air velocity of the boiler (Ug ) could be calculated from the DCS system [9]. Therefore, the pressure distribution along the furnace height can be gotten from these
EP T
pressure values. So, the valid internal solids circulation rate in thermal state ( G’vs ) can be calculated as follows:
AC C
Gvs mU g
Lv Lt
(2)
where Lv and Lt were shown at Fig.6. Δρm was the gas-solid mixture density difference between two adjacent measuring areas. The measuring area was between two measuring ports. The mixture density (ρm ) was calculated as follows [32, 33]:
m
P g H
(3)
where ΔP was the pressure drop between two adjacent measuring ports and ΔH was the height between two adjacent measuring ports.
3. Results and discussion
ACCEPTED MANUSCRIPT 3.1 Results of cold tests 3.1.1 Effect of partition height on the valid internal solids circulation rate Figs.9 and 10 show dependence of the valid internal solids circulation rate (Gvs ) on the partition height (Hp ), under different main bed fluidizing air velocity (Um ) and static main bed height (Hs ). It is shown that Gvs reduced with the increment of Hp under the same Um. This is because at the
PT
same Um, the particle entrainment rate decreases along the vertical direction [34]. Hence with the increase in Hp, the partial amount of internally circulating solids which can climb over the
RI
partition became decreased. Moreover, the top of the partitions corresponds to the bottom of solids entrance of the internally circulating chamber in practical use. Therefore, the valid internal solids
NU
of internally circulating chamber and the air distributor.
SC
circulation rate can be controlled effectively by adjusting the distance between the solids entrance
3.1.2 Effect of main bed fluidizing air velocity on the valid internal solids circulation
MA
Figs. 11 and 12 show the effect of the main bed fluidizing air velocity (Um ) on the valid internal solids circulation rate (Gvs ) at different partition height (Hp ) and static main bed height (Hs ). It is
ED
indicated that Gvs increased firstly and then decreased later with the increasing Um , at various Hp. In addition, when Um was approximately 1.2 m/s, Gvs reached its peak value. This meant that
EP T
before Um grew up to 1.2m/s, increasing more internally circulating solids was entrained by the air and climbed over the partition upon the increase of Um. On the contrary, when Um surpassed 1.2 m/s, more internally circulating solids was taken into the cyclone. This made Gvs reduce with the
AC C
increasing Um [35]. Therefore, Gvs can reach its maximum at 1.2 m/s. As a result, during boiler operation, it can get higher valid internal solids circulation rate under the low load condition. On the contrary, the valid internal solids circulation rate will reduce in the full load condition, which is in favor of reducing the heat flux of the internally circulating chamber.
3.1.3 Effect of static main bed height on the valid internal solids circulation rate Figs.13 and 14 show the effect of static main bed height (Hs ) on the valid internally circulating rate (Gvs ) at different partition height (Hp ) and main bed fluidizing air velocity (Um ). It is shown that Gvs increased with the increment of Hs under the same Um . Furthermore, Gvs increased sharply when Hs reached 500mm under Um =0.6m/s. This is because at the lower Hs (300mm and 400mm),
ACCEPTED MANUSCRIPT the solids which can be entrained by Um have not been saturated and the core-annulus was unapparent. Most of the fine solids were carried into the upper furnace joining the external circulation. However, when Hs raised to 500mm, the solids which can be entrained by Um have approached saturation and the core-annulus appeared. Therefore, except the external circulation, there were also lots of fine solids joining the internal circulation. In addition, when Um was 1.2m/s,
PT
the solids that can be entrained by Um have approached saturation under different Hs and the core-annulus was obvious. As a result, Gvs almost linearly increased with increasing Hs .
RI
3.1.4 Empirical formula of valid internal solids circulation rate
Many empirical formulas were proposed in the literature to correlate the internal solids
SC
circulation rate with the relevant parameters. However, most of them were based on the hypothesis of ash flowing over an orifice [23, 36-38]. In this paper, an empirical formula about Gvs in the
NU
current type cold test rig was developed by the dimensionless parametrization and multi-variable regression analysis methods.
MA
In these cold tests, the independent variables were Hs , Hc, Um and dependent variable was Gvs . Four dimensionless variables (hb , hc, um and gvs ) were gotten by the following equations:
Hs Dt
(4)
hc
Hc Ht
(5)
um
Um U mf
(6)
gvs
Gvs bU mf
(7)
AC C
EP T
ED
hs
where Umf was the Minimum fluidizing veloc ity of the bed material. ρb was the bulk density of the bed material. Dt =243mm was the equivalent diameter of the main bed air distributor. Definitions of Hc and Ht are illustrated in Fig.1, and Ht =1615mm. An empirical formula of hs, hc, um and gvs was developed by using multi-variable regression analysis (using the data of Hs =300mm and Hs =400mm), as shown below:
gvs
0.035hs
0.046hc
0.015um3
0.148um2
0.447um
0.425
(8)
ACCEPTED MANUSCRIPT The correlation coefficient was 0.97, which indicates the regression correlation was superior [39]. In addition, the experiments with Hs =500 mm were conducted at this cold test rig, whose test conditions included Hp =200mm and 400mm as well as Um =0.6m/s and 1.2m/s. Comparison of the experimental and calculated data are shown in Fig.15. It presents that the experimental data had a
PT
good agreement with the calculated data.
RI
3.2 Results of thermal tests
3.2.1. Typical parameters and pressure distribution along furnace height under 60% boiler load
SC
The typical DCS parameters of the Baima’s 600 MW supercritical CFB boiler under 60% load are listed in Table 3. In addition, the vertical pressure distribution of left and right furnace is
NU
described in Fig. 16.
As shown in Fig. 16, at 60% boiler load, the pressure in furnace dropped rapidly within the
MA
dense-phase zone height and then remained nearly constant. That means at the bottom of the boiler, large quantities of solids were accumulating which resulted in a higher pressure gradient [32]. It
ED
indicated that there was a lot of internally circulating solids beneath the secondary air orifices. Therefore, under the low load condition of the boiler, the internal solids circulation rate at the
EP T
furnace bottom may be much enough to heat the final superheater and reheater.
3.2.2 Modification of the empirical formula in thermal state
AC C
In order to accurately calculate the valid internal solids circulation rate in thermal state, the empirical formula which is proposed in 3.1.3 is modified as follows [40, 41]:
gvs
(T f t f ) (Tw tw )
g vs
(9)
where Tf is the furnace temperature of boiler and Tw is the air supply temperature of boiler. In addition, the practical parameters of the boiler’s left and right dense-phase zone were normalized by equations (4) ~ (7) and then substituted into equation (9). The valid internal solids circulation rate where was 1900mm from the boiler air distributor was calculated by the modified empirical formula. Furthermore, the valid internal solids circulation rate which was 1900mm in height from the boiler air distributor was also calculated from equations (2) and (3) using the
ACCEPTED MANUSCRIPT measured pressure. As demonstrated in Fig.17, the calculated data coincided well with the measured data, indicating that the modified empirical formula (equation (9)) can be adequately applicable to the estimation of the valid internal solids circulation rate of this boiler.
3.3 Discussion
PT
3.3.1. Thermal equilibrium calculation of using internal solids circulation to heat final superheater and reheater
RI
Based on Baima’s 600MW supercritical CFB boiler, the outlet parameters of superheater and reheater listed in Table 3 was utilized as inlet parameters (569℃/566℃) of final superheater and
SC
reheater in ultra-supercritical CFB boiler at low load condition. In addition, ultra-supercritical CFB boiler should use internal solids circulation to heat its final superheater and reheater. Through
NU
this method, the ultra-supercritical CFB boiler superheater and reheater outlet temperatures should reach the designed parameters which are described in Table 4.
MA
Therefore, under low load condition of ultra-supercritical CFB boiler, the needed internal solids circulation rate could be calculated based on the principle of energy conservation and parameters
ED
listed in Table 3 and Table 4[8, 42]. Besides, as some parts of the furnace wall in the dense-phase may be used for returning external solids, only 50% of valid internal solids were considered
EP T
flowing into the internally circulating chamber. Thus, calculated results are presented in Table 5. According to Table 5, the practical needed internal solids circulation rate is 12.28kg/m2 s which can heat superheated and reheated steam to the parameters listed in Table 4. Besides, if the
AC C
ultra-supercritical CFB boiler is based on the Baima’s 600MW supercritical CFB, the modified empirical formula (equation (9)) can be used to predict the valid internal solids circulation rate of the boiler. As shown in Fig.18, when solids entrance of internally circulating chamber is 3.46m higher than the air distributor, the valid internal solids circulation rate can meet the requirement. As a result, if ultra-supercritical CFB boiler is based on Baima’s 600MW supercritical CFB boiler and set the solids entrance of internally circulating chamber less than 3.46m away from the air distributor, the valid internal solids circulation rate can heat its superheated and reheated steam to the required temperatures at 60% load condition.
3.3.2. Schemes of the technology for CFB boilers
ACCEPTED MANUSCRIPT At present, layout of large-scale CFB boilers can be usually divided into single furnace and double furnaces [2, 4, 43]. As described in Fig.19, internally circulating chamber can be sited on both sides of single furnace CFB boiler. As well as, at double furnaces CFB boiler, internally circulating chamber can be put between the legs, which is illustrated in Fig.20. In both arrangements, different heat exchange surfaces should be put into adjacent internally circulating
PT
chambers, which reduces solids outlet temperature difference of internally circulating chamber. Internally circulating chamber can be made up by membrane wall. An available structure of it is
RI
shown in Fig.21. The internally circulating chamber is divided into three rooms(C/D/E). Every room has its own windbox. Under different thermal load, if the heat absorbed proportion of final
SC
superheater/reheater need to be elevated, the room C and D will be fluidized and most of internally circulating ash will enter into room C to heat the final superheater/reheater. If the heat
NU
absorbed proportion of final superheater/reheater need to be reduced, the room D and E will be fluidized and little or no internally circulating ash will enter into room C. Therefore, in this case,
MA
the heat absorbed proportion of final superheater/reheater can be reduced to minimum or 0%. Railings and high-pressure air nozzles can be sited at solids entrances, which protect heat
ED
exchangers and solids entrances from blocking. The height of solids entrance from boiler air distributor can be determined according to the needed internal solids circulation rate. However,
EP T
because, during the operation of the internally circulating chamber, the fluidized air of three rooms need to be controlled respectively to adjust the heat absorbed proportion of final superheater/reheater, the complexity of controlling superheated and reheated steam temperature
AC C
will be elevated.
4. Conclusions
In order to make superheated and reheated steam reach designed temperature in low load condition of an ultra-supercritical CFB boiler, a technology of utilizing high temperature solids of dense-phase zone to heat final superheater and reheater is proposed in this paper. Based on the cold tests, thermal tests, empirical formula analysis, thermal equilibrium calculation and discussion of practicable schemes, the following conclusions can be summarized. (1) From cold tests, Gvs decreased with rising Hp under the same Um. As well as, it increased firstly and then decreased with growing Um under the same Hp . Moreover, it increased with the
ACCEPTED MANUSCRIPT increment of Hs under the same Um . According to the experiment condition, a dimensionless empirical formula can be achieved as follows:
gvs
0.035hs
0.046hc
0.015um3
0.148um2
0.447um
0.425
(2) According to thermal tests, valid internal solids circulation rates of the left and right 2
2
furnaces were 49.3kg/(m s) and 46.2kg/(m s), which was 1900mm away from air distributor. The
PT
measurement data were similar to the calculated data which were counted by modified empirical
(T f t f ) (Tw tw )
g vs
SC
gvs
RI
formula. In addition, the modified empirical formula is described as follows:
(3) Based on 60% load steam parameters of Baima’s 600MW supercritical CFB boiler, the 2
NU
practical needed internal solids circulation rate(12.28kg/(m s)) which was used for heating final superheater and reheater of ultra-supercritical CFB boiler was achieved by the thermal equilibrium.
MA
After analyzed by the modified empirical formula, when the distance between solids entrance of internally circulating chamber and the air distributor of the furnace was less than 3.46m, the valid
569/571℃ to 605/623℃.
ED
internal solids circulation rate shall meet the requirement which is heating the steam from
(4) Practicable schemes of this technology were carefully described, which can be used for both
EP T
signal furnace boilers and double furnaces boilers. The technology of using internal circulating solids for heating the final superheater and reheater of an ultra-supercritical CFB boiler was
AC C
proved to be feasible.
ACCEPTED MANUSCRIPT Nomenclature dp
mean size (μm)
Dt
equivalent diameter air distributor (mm)
g
acceleration of gravity (m/s )
G’vs
valid internally circulating rate in thermal state (kg/m 2 s)
Gvs
valid internally circulating rate in cold state (kg/m s)
gvs
dimensionless valid internally circulating rate in cold state
g'vs
dimensionless valid internally circulating rate in thermal state
Hs
static bed height (mm)
hs
dimensionless static bed height
hc
dimensionless solids entrance height
Hc
solids entrance height (mm)
Hl
breeches-legs height (mm)
Hp
partition height (mm)
Ht
total height(mm)
Lt
total length (m)
Lv
valid length (m)
tf
average experimental temperature (℃)
Tf
furnace temperature of boiler (℃)
Tw
air supply temperature of boiler (℃)
tw
average temperature of fluidizing air (℃)
Ui Um
EP T
ED
MA
NU
SC
RI
PT
2
AC C
Ug
2
fluidizing air velocity of Baima’s 600MW CFB boiler (m/s) internally circulating chamber fluidized air speed (m/s) main bed fluidizing air velocity (m/s)
um
dimensionless main bed fluidizing air velocity
Umf
minimum fluidizing velocity (m/s)
v
particle velocity (m/s)
ΔH
height between two adjacent measuring ports (m)
ΔP
pressure drop between two adjacent measuring ports (Pa)
ε
solid holdup
ACCEPTED MANUSCRIPT 3
ρb
bulk density (kg/m )
ρm
gas-solid mixture density (kg/m3 )
ρs
true density (kg/m3 )
Δρm
gas-solid mixture density difference between two adjacent measuring areas (kg/m )
3
PT
Acknowledgments The financial support from the National Key Research & Development Program of China (No.
AC C
EP T
ED
MA
NU
SC
RI
2016YFB0600201) is gratefully acknowledged.
ACCEPTED MANUSCRIPT References [1] P. Basu, Combustion of coal in circulating fluidized-bed boilers: a review, Chemical Engineering Science, 54 (1999) 5547-5557. [2] L.M. Cheng, X.L. Zhou, C.H. Zheng, et al., Development of large-scale circulating fluidized bed boiler, Journal of Power Engineering, (2008).
PT
[3] J. Koornneef, M. Junginger, A. Faaij, Development of fluidized bed combustion—An overview of trends, performance and cost, Progress in Energy and Combustion Science, 33 (2007)
RI
19-55.
[4] G.X. Yue, H.R. Yang, J.F. Lu, et al., Latest Development of CFB Boilers in China, in:
SC
Proceedings of the 20th International Conference on Fluidized Bed Combustion, Xi’an, China, 2009.
NU
[5] T. Jäntti, H. Lampenium, M. Ruuskanen, R. Parkkonen, Supercritical OT U CFB Projects Lagisza 460 MWe and Novercherkasskays 330 MWe, Russia Power, (2011) TP_CFB_11_04.
MA
[6] R.X. Cai, J.F. Lv, W. Ling, et al., Development of (Ultra-)Supercritical Circulating Fluidized Bed Boiler Technology, Electric Power, (2016) 1-7.
ED
[7] Y.H. Zhao, P. Han, Co-combustion tests under low load condition on supercritical circulating fluidized bed boiler, Clean Coal Technology, (2016) 76-81.
EP T
[8] H. Wang, X.F. Lu, W. Zhang, et al., Study on heat transfer characteristics of the high temperature reheater tube panel in a 300 MW CFB boiler with fluidized bed heat exchanger, Applied Thermal Engineering, 81 (2015) 262-270.
AC C
[9] J.Y. Lu, Study on Particle Population Balance and Heat Balance in Large-Scale Circulating Fluidized Bed Boiler Chongqing University, 2012. [10] H.B. Wu, M. Zhang, Y.K. Sun, et al., Research on the heat transfer model of platen heating surface of 300MW circulating fluidized bed boilers, Powder Technology, 226 (2012) 83-90. [11] C.H. Hu, X.F. Lu, Equipments and Operation of the 600MW Supercritical Circulating Fluidized Bed Boiler, Electric Power Press, Beijing, 2012. [12] P. Basu, Combustion and Gasification in Fluidized Beds, CRC Press, 2006. [13] X.F. Lu, Equipments and Operation of Large-Scale Circulating Fluidized Bed Boiler, Electric Power Press, Beijing, 2006. [14] P. Basu, Circulating Fluidized Bed Boilers. Design, Operation and Maintenance. Springer,
ACCEPTED MANUSCRIPT New York, 2015. [15] S.J. Goidich, T. Hyppanen, K. Kauppinen, CFB Boiler Design and Operation Using The INTREX™
Heat
Exchanger,
6th
International
Conference
on
Circulating
Fluidized,
BedsWürzburg, Germany, 1999. [16] T. Jäntti, Nuortimo, KalleRuuskanen, J. MarkoKalenius, Samcheok Green Power 4x550MWe
PT
Supercritical Circulating Fluidize Bed Steam Generators in South Korea, PowerGen Europe, (2012).
RI
[17] J.X. Bouillard, R.W. Lyczkowski, D. Gidaspow, POROSITY DISTRIBUTIONS IN A FLUIDIZED-BED WITH AN IMMERSED OBSTACLE, Aiche J., 35 (1989) 908-922. Y.T.
Choi, S.D. Kim,
BUBBLE CHARACTERISTICS IN AN INTERNALLY
SC
[18]
CIRCULATING FLUIDIZED-BED, J. Chem. Eng. Jpn., 24 (1991) 195-202.
NU
[19] J.L. Wang, Experimental and Numerical Investigations on Gas-Solid Flow in an Internally Circulating Fluidized Bed, Southeast University, 2013.
MA
[20] L.C. Huang, L.L. Ma, Z. Zhou, et al., Experimental study on particle flow characteristics in a clapboard-type internal circulating fluidized bed reactor, Acta Energlae Solaris Sinica, (2008)
ED
900-904.
[21] M.X. Fang, Z.Z. Shi, S.R. Wang, et, al., Experimental Research on Solid Circulation of a
54-57+61.
EP T
Double Fluidized Bed, Transaction of the Chinese Society for Agricultural Machinery, (2003)
[22] B. Zeng, X.F. Lu, L. Gan, et al., Development of a novel fluidized bed ash cooler for
AC C
circulating fluidized bed boilers: Experimental study and application, Powder Technology, 212 (2011) 151-160.
[23] B. Xiong, X.F. Lu, R.S. Amano, et al., Gas–solid flow in an integrated external heat exchanger for CFB boiler, Powder Technology, 202 (2010) 55-61. [24] J. Werther, Measurement techniques in fluidized beds, Powder Technology, 102 (1999) 15–36. [25] T. Hagemeier, M. Börner, A. Bück, E. Tsotsas, A comparative study on optical techniques for the estimation of granular flow velocities, Chemical Engineering Science, 131 (2015) 63-75. [26] C.X. Wang, J. Zhu, S. Barghi, et al., Axial and radial development of solids holdup in a high flux/density gas–solids circulating fluidized bed, Chemical Engineering Science, 108 (2014)
ACCEPTED MANUSCRIPT 233-243. [27] C.X. Wang, C.Y. Li, J. Zhu, Axial solids flow structure in a high density gas –solids circulating fluidized bed downer, Powder Technology, 272 (2015) 153-164. [28] W.B. Li, K. Yu, X.G. Yuan, et al., An anisotropic Reynolds mass flux model for the simulation of chemical reaction in gas-particle CFB risers, Chemical Engineering Science, 135 (2015)
PT
117-127. [29] N. Balasubramanian, Transition velocities in the riser of a circulating fluidized bed, Advanced
RI
Powder Technology, 16 (2005) 247-260.
[30] S.I. Ngo, Y.-I. Lim, B.-H. Song, et al., Song, Effects of fluidization velocity on solid stack
SC
volume in a bubbling fluidized-bed with nozzle-type distributor, Powder Technology, 275 (2015) 188-198.
NU
[31] S.T. Pemberton, J.F. Davidson, Elutriation from fluidized beds —II. Disengagement of particles from gas in the freeboard, Chemical Engineering Science, 41 (1986) 253-262.
MA
[32] J. Yong, J. Zhu, Z.W. Wang, et al., Fluidization Project Principle, Tsinghua University Press, Beijing, (2001).
ED
[33] J. Xu, X.F. Lu, Q.H. Wang, et al., Experimental study on gas-solid flow characteristics in a 60-meter-high CFB cold test apparatus, Proc. 22nd international conference on fluidized bed
EP T
conversion, Turku, Finland, 2015.
[34] D.C. Sau, S. Mohanty, K.C. Biswal, Critical fluidization velocities and maximum bed pressure drops of homogeneous binary mixture of irregular particles in gas –solid tapered fluidized
AC C
beds, Powder Technology, 186 (2008) 241-246. [35] K. Cen, Theories, Design and Operation of Circulating Fluidized Bed Boiler, Electric Power Press, Beijing, 1998.
[36] R. Korbee, O.C. Snip, J.C. Schouten, et al., Rate of solids and gas transfer via an orifice between partially and completely fluidized beds, Chemical Engineering Science, 49 (1994) 5819-5832. [37] W.C. Yeh, Measurement of the Solid Circulation Rate in an Interconnected Fluidized Bed, Chemical Engineering, (2001). [38] D.R.M. Jones, J.F. Davidson, The flow of particles from a fluidized bed through an orifice, Rheologica Acta, 4 (1965) 180-192.
ACCEPTED MANUSCRIPT [39] J. Peterson, Regression Analysis of Count Data, Technometrics, 41 (2014) 371-371. [40] M.H. Rahman, X.T. Bi, J.R. Grace, et al., Measurement of solids circulation rate in a high-temperature dual fluidized bed pilot plant, Powder Technology, 316 (2017) 658-669. [41] M.A. Habl, A. Frohner, G. Tondl, et al., Fluid dynamics study on a dual fluidized bed cold-flow model, Powder Technology, 316 (2017) 469-475.
PT
[42] J.Y. Lu, X.F. Lu, H.Z. Liu, H. et al., Calculation and analys is of dissipation heat loss in large-scale circulating fluidized bed boilers, Applied Thermal Engineering, 30 (2010) 1839-1844.
RI
[43] G.X. Yue, R.X. Cai, J.F. Lu, et al, From a CFB reactor to a CFB boiler – The review of R&D
AC C
EP T
ED
MA
NU
SC
progress of CFB coal combustion technology in China, Powder Technology, 316 (2017) 18-28.
ACCEPTED MANUSCRIPT
Figures Captions: Schematic diagram and picture of the experimental apparatus.
Fig. 2.
Accumulative particle size distribution of bed material.
Fig. 3.
Relationship between bed pressure difference and main bed fluidizing air velocity.
Fig. 4.
Layout of the measuring holes on the internally circulating chamber flank wall.
Fig. 5.
Distribution of the measuring points in the internally circulating chamber.
Fig. 6.
Arrangement of the measuring ports of Baima’s 600MW supercritical CFB boiler.
Fig. 7.
Layouts of measuring ports where were 25000mm and 38000mm away from the air
RI
PT
Fig. 1.
SC
distributor.
Particle size distribution of bed material in Baima’s 600MW supercritical CFB boiler.
Fig. 9.
Effect of partition height on the valid internally circulating rate (Hs =300mm).
NU
Fig.8.
Fig. 10. Effect of partition height on the valid internal solids circulation rate (Hs =400mm).
MA
Fig. 11. Effect of the main bed fluidizing air velocity on the valid internal solids circulation rate (Hs =300mm).
(Hs =400mm).
ED
Fig. 12. Effect of the main bed fluidizing air velocity on the valid internal solids circulation rate
Effect of static main bed height on the valid internally circulating rate (Um=0.6m/s).
Fig.14.
Effect of static main bed height on the valid internally circulating rate (Um=1.2m/s).
EP T
Fig.13.
Fig. 15. Difference between experimental data and calculated data (Hs =500mm).
AC C
Fig. 16. Pressure distribution profiles along furnace height of the Baima’s 600 MW supercritical CFB boiler under 60% boiler load.
Fig. 17. Comparison of valid internal solids circulation rate where was 1900mm from the air distributor.
Fig. 18. Position of solids entrance of internally circulating chamber. Fig. 19. Layout of internally circulating chamber in single furnace CFB boiler. Fig. 20. Two different layouts of internally circulating chamber in double furnaces CFB boiler. Fig. 21. An available structure of internally circulating chamber.
ACCEPTED MANUSCRIPT Table 1. Bed material physical specification. True density
Bulk density
M ean size
M inimum fluidizing velocity
ρs(kg/m )
ρb (kg/m )
dp(μm)
Umf (m/s)
2529.01
1133.41
382.56
0.30
EP T
ED
MA
NU
SC
RI
PT
3
AC C
3
ACCEPTED MANUSCRIPT Table 2. Experimental conditions. Static main bed
M ain bed fluidizing air
Partition height
Internally
circulating
height
velocity
H p(mm)
fluidizing air velocity
Hs(mm)
U m(m/s)
300
0.6,0.9,1.2,1.5
100,200,300,400,500,600
0.085
400
0.6,0.9,1.2,1.5
100,200,300,400,500,600
0.085
500
0.6,1.2
200,400
0.085
AC C
EP T
ED
MA
NU
SC
RI
PT
Ui(m/s)
chamber
ACCEPTED MANUSCRIPT Table 3. Typical DCS parameters of Baima’s 600MW supercritical CFB boiler under 60% load. Item
Parameter
Left dense-phase zone temperature/℃
848
Right dense-phase zone temperature /℃
843
Left air supply temperature/℃
293
Right air supply temperature /℃
279
3
246.5
Right primary air rate /( kNm3/h)
232.5
Left bottom pressure(400mm from air distributor)/kPa
5.5
PT
Left primary air rate/( kNm /h)
Right bottom pressure(400mm from air distributor)/kPa Superheated steam outlet temperature/℃
AC C
EP T
ED
MA
NU
SC
RI
reheated steam outlet temperature /℃
6.6 569 566
ACCEPTED MANUSCRIPT Table 4. Design parameters of superheater and reheater in ultra-supercritical CFB boiler [6]. Parameter
Superheated steam outlet pressure/M Pa
29.4
Reheated steam outlet pressure/M Pa
5.96
Superheated steam outlet temperature/℃
605
Reheated steam outlet temperature/℃
623
Superheated steam flowrate/(t/h)
1980
Reheated steam flowrate/(t/h)
1655.7
AC C
EP T
ED
MA
NU
SC
RI
PT
Item
ACCEPTED MANUSCRIPT Table 5. Calculation of needed internal solids circulation rate. Parameter
Final superheater needed heat /( kJ/s)
63580
Final reheater needed heat /( kJ/s)
61592
Internally circulating solids inlet temperature /℃
845.5
Internally circulating solids outlet temperature /℃
760
Internally circulating solids inlet enthalpy /(kJ/kg)
816.14
Internally circulating solids outlet enthalpy /(kJ/kg)
725.16
Total amount of calculated internally circulating solids heating final superheater and reheater /(kg/s)
1375.82
Calculated internal solids circulation rate /(kg/m2s)
6.14
PT
Item
2
AC C
EP T
ED
MA
NU
SC
RI
Needed internal solids circulation rate /(kg/m s)
12.28
AC C
EP T
ED
MA
NU
SC
Fig. 1. Schematic diagram and picture of the experimental apparatus.
RI
PT
ACCEPTED MANUSCRIPT
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.2. Accumulative particle size distribution of bed material.
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.3. Relationship between bed pressure difference and main bed fluidizing air velocity .
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.4. Layout of the measuring holes on the internally circulating chamber flank wall.
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.5. Distribution of the measuring points in the internally circulating chamber.
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
Fig.6. Arrangement of the measuring ports of Baima’s 600M W supercritical CFB boiler.
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
Fig.7. Layouts of measuring ports where were 25000mm and 38000mm away from the air distributor.
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.8. Particle size distribution of bed material in Baima’s 600MW supercritical CFB boiler.
SC
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
Fig.9. Effect of partition height on the valid internally circulating rate (H s=300mm).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.10. Effect of partition height on the valid internal solids circulation rate (H s=400mm).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.11. Effect of the main bed fluidizing air velocity on the valid internal solids circulation rate (Hs=300mm).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.12. Effect of the main bed fluidizing air velocity on the valid internal solids circulation rate (Hs=400mm).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.13. Effect of static main bed height on the valid internally circulating rate (Um=0.6m/s).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.14. Effect of static main bed height on the valid internally circulating rate (Um=1.2m/s).
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.15. Difference between experimental data and calculated data (H s=500mm).
RI
PT
ACCEPTED MANUSCRIPT
SC
Fig.16. Pressure distribution profiles along furnace height of the Baima’s 600 M W supercritical CFB boiler under
AC C
EP T
ED
MA
NU
60% boiler load.
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
Fig.17. Comparison of valid internal solids circulation rate where was 1900mm from the air distributor.
AC C
EP T
ED
MA
NU
SC
Fig.18. Position of solids entrance of internally circulating chamber.
RI
PT
ACCEPTED MANUSCRIPT
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
Fig.19. Layout of internally circulating chamber in single furnace CFB boiler.
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
Fig.20. Two different layouts of internally circulating chamber in double furnaces CFB boiler.
PT
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
RI
Fig.21. An available structure of internally circulating chamber.
ACCEPTED MANUSCRIPT
AC C
EP T
ED
MA
NU
SC
RI
PT
Graphical abstract
ACCEPTED MANUSCRIPT Highlights A method to heat final heat exchanger of ultra-supercritical CFB boiler is proposed An empirical formula is achieved by cold and thermal tests Height of internally solid circulating chamber entrance is gotten
AC C
EP T
ED
MA
NU
SC
RI
PT
Practicable schemes of this method are carefully described