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Effect of air preheated temperature on sodium transformation during Zhundong coal gasification in circulating fluidized bed Guoliang Song, Weijian Song, Xiaobin Qi, and Shaobo Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03209 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 11, 2017
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Effect of air preheated temperature on sodium transformation during Zhundong coal gasification in circulating fluidized bed
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Song Guoliang*, a ,b, Song Weijiana,b, Qi Xiaobina,b, Yang Shaobo a,b
1 2
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a
People’s Republic of China
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Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190,
b
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
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Abstract
2
Preheated air could improve the coal gas heating value and gas quality, meanwhile may
3
cause slagging in the gasifier due to the enrichment of sodium in the ash. In this work, the
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migration and transformation of sodium during the Zhundong coal circulating fluidized bed
5
gasification with different air preheated temperature were investigated. During the Zhundong
6
coal gasification at 940°C, with the air preheated temperature increased from 20 °C to 600 °C,
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the air equivalent ratio decreased from 0.52 to 0.38, the sodium content in the bottom ash
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increased 62.99%, 425.58% for circulating ash and 44.02% for fly ash due to the effect of
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carbon inhibition. Accordingly, the release fraction of gaseous sodium decreased 42.82%.
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The condensation of gaseous sodium started at 800°C of the coal gas temperature and
11
reached maximum at about 700°C. The turning point of temperature increased with the
12
increase of air preheated temperature. The air preheated temperature had little influence on
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the sodium occurrence of bottom ash. As the increase of air preheated temperature, the
14
fraction of H2O-soluble sodium in fly ash and circulating ash increased, the ash fusion
15
temperature of both fly ash and bottom ash dropped. Slagging bocks appeared during the
16
gasification when the air preheated temperature was up to 600°C. Slagging bocks mainly
17
contain SiO2 and Na2O. Sodium in the slagging blocks mainly exists as Na2Si2O5, insoluble
18
sodium. Air preheated temperature should be less than 600 °C to ensure the stable operation
19
when the content of Na2O in the Zhundong coal ash is up to above 7.28%.
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Key words: CFB gasification; high-sodium Zhundong coal; air preheated temperature;
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slagging
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1 Introduction
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Zhundong coalfield is the biggest coal reserve newly found in China. It is located in the
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east of the Junggar Basin in Xinjiang. Zhundong coalfield contains more than 164Gt of
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lignite as is explored
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fouling occurred during Zhundong coal combustion in the pulverized coal furnace due to the
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high content alkali mental content in the coal ash
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gasification reactivity due to its high content of sodium 6-9. Gasification is an optional choice
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for Zhundong coal utilization. The circulating fluidized bed gasifier operates at 850oC ~
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950oC. Research results show that the appropriate bed materials and suitable operating
10
parameters could effectively inhibit fouling and slagging 10. During circulating fluidized bed
11
gasification, air preheating could increase H2 and CO production and gas heating value 11. Air
12
preheating is widely used to improve overall operational efficiency of gasifier and to improve
13
the coal gas heating value and gas quality. However, with the increasing of air preheated
14
temperature, extra heat was introduced into the gasifier. To keep the heat balance of the
15
whole system, the amount of fed coal was increased. Accordingly, the air equivalent ratio
16
(ER, the ratio of the amount of oxygen used for gasification to the amount of oxygen required
17
for complete combustion of the coal) decreased. Preheated air influenced the reaction
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atmosphere and influenced the release and transformation of sodium further. Preheated air
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may cause slagging and defluidization in the gasifier due to the enrichment of sodium in the
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ash. The appropriate air preheating temperature to ensure the stable operation is not clear.
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Understanding the release and transformation behavior of sodium during CFB gasification
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with preheated air is highly significant in the practical utilization of Zhundong coal.
1, 2
. Zhundong coal belongs to low rank coal. Severe slagging and
3-5
. Zhundong coal also shows high
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The release and transformation behavior of sodium during Zhundong coal thermal
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conversion is affected by many factors, such as coal type, sodium occurrence, reacting
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temperature, reacting atmosphere and operating parameters. Lots of researches have been
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carried out on sodium behaviors in Zhondong coals
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occurrences in coals. The H2O-soluble sodium is prone to be the most active13-17. Reacting
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temperature mainly influences the equilibration of solid phase sodium and gas phase sodium 5,
7
18
8
temperature 19. Li et al.
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and parts transformed to HCl-soluble and insoluble sodium during Zhundong coal
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combustion. The release ratio and conversion rate both increased with the increase of
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combustion temperature. Manzoori et al.
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process. Reacting atmosphere and O2 concentration have great influence of the retention of
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sodium during the thermal conversion. Song et al.
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oxidizing atmosphere and reducing atmosphere. The results revealed that, the release of
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sodium was inhabited due the bound role of carbon chain in the reducing atmosphere. Higher
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O2 concentration would promote the release of sodium during combustion process
17
whereas would promote the reaction of sodium with alumina silicates during gasification
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process
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sodium behavior and slagging tendency is not clear.
12-17
. Sodium exists as different
.The interaction of sodium with inherent minerals is also significantly affected by reacting 18
reported that parts of H2O-soluble sodium released into gas phase
20
got the similar conclusions during pyrolysis
21
compared the sodium behaviors under
22-24
25
. The effect of operating parameters, such as air preheated temperature, on the
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The study aims to reveal the effect of air preheated temperature on the release and
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transformation characteristics of sodium in Zhundong coal during circulating fluidized bed
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gasification. The appropriate preheated air temperature for Zhundong coal gasification in
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circulating fluidized bed gasifier was proposed. The effects of air preheated temperature on
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the ash fusion temperatures (AFTs) and slagging characteristic were also discussed in this
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paper. The experiments were carried out in a 0.4 t/d CFB gasifier with air cooled slagging
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probes.
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2 Experimental
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2.1 Experimental system and operating conditions
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Figure 1 shows the experimental system for Zhundong coal gasification. Detailed
8
descriptions could be found in our other works 25-27. In addition, an air preheater was added to
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the system especially to control the air temperature for gasification. The CFB gasifier is
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equipped with 7 slagging probes (named A~G) for ash deposition. The slagging probes were
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cold by compressed air. The air flow rate was set to 3 Nm3/h for every single slagging probe.
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The staring and operating procedures were also shown in our previous works
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During the gasification experiments, the gasification temperature was kept at 940°C±5°C.
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The air preheated temperatures were set at 20°C, 200°C, 400°C and 600°C, respectively.
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Each run was kept for 8 hours to obtain effective and enough samples.
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2.2 Fuel and bed material
25-27
.
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The experiment coal was exploited from Mulei kazakh autonomous county, Xinjiang
18
province of China. The coal was called as Mulei coal（ML）. Mulei coal is one typical
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Zhundong coal. The size range of coal used in the experiments was from 0.1 mm to 1 mm.
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The coal basis properties are presented in Table 1. The ash content of ML is 3.16%, whereas
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the content of Na2O in the ash is ultra high, reaches 7.28%. The contents of CaO and SO3 in 5
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the ash are also very high. The occurrence of sodium in ML was shown in table 2. It can be seen that sodium in ML mainly exists as H2O-soluble and NH4Ac-soluble occurrence.
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Corundum was chosen as the bed material. The bed material particle size ranged from
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0.38mmto 0.84 mm and the average diameter is 0.57mm. The composition of bed material is
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shown in table 3. The bed material mainly composes of Al2O3 and SiO2.
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2.3 Sampling and analysis methods
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Three sampling tubes were designed to collect the ash samples. Bottom ash was
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collected from the Sample point 1 at the bottom of the gasifier. Sample point 2 and 3 were
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used for the collecting of fly ash and circulating ash respectively. Sample point 2 was located
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at the cyclone outlet and Sample point 3 was located at the loop seal outlet.
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All the fly ash during the experiment of 8 hours was collected for the calculating of mass
13
balance. The gas composition was analyzed by infrared gas analyzer (Gasboard 3100P,
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Sifang, China).
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The analyzing method of sodium content and occurrence was chemical fractionation
16
analysis (CFA)28, 29. Briefly speaking, the samples were leached by water, ammonium acetate
17
and hydrochloric acid sequentially and the content of sodium was analyzed by Ion
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Chromatography (IC, IC-900, Thermo Fisher, America). Our previous works give the
19
detailed procedures of leaching and analyzing 25-27.
20 21 22
The ashes collected from the slagging probes were digested by microwave digestion (Tank, Haineng, China) and the digested solutions were analyzed by IC. Samples were analyzed by X-ray powder diffraction (XRD, Empyrean, PANalytical, 6
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Netherlands) to determine the mineral compositions in the ashes. The samples were ashed in
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muffle furnace before the XRD analysis and the ashing temperature is 575oC 19.
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The ash fusion temperatures were tested in ash fusion analyzer (AF-700, Leco, America). Tests followed the Chinese standard procedures (GB/T219-2008). The compositions of slagging blocks were determined by the X-ray fluorescence (Axios, PANalytical, Netherlands). The microstructure and elements distribution of slagging blocks were analyzed by
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SEM-EDX (SU8020, Hitachi, Japan).
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3 Results and discussion
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3.1 Bed temperatures at different air preheated temperatures
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The experimental conditions were shown in table 4. The bed temperatures along the
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gasifier at different air preheated temperatures were shown in figure 2. It can be seen that the
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gasification temperature and air flow rate of each experimental condition were kept almost
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the same. With the increasing of air preheated temperature, extra heat was introduced into the
15
gasifier. To keep the heat balance of the whole system, the amount of feed coal was increased
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with the air preheated temperature, from 16.00 kg/h to 22.07 kg/h. Accordingly, the air
17
equivalent ratio decreased from 0.52 to 0.38 as the air preheated temperature increased from
18
20 °C to 600 °C. The superficial gas velocity was about 3.3m/s during the gasification
19
experiments. The surface temperature of each slagging probes under different experimental
20
conditions was shown in figure 3. The coal gas flow velocities near the slagging probes were
21
shown in table 5.
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3.2 Distribution of sodium in ash
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The distribution of sodium in the bottom ash, circulating ash and fly ash after
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gasification with different air preheated temperatures was shown in figure 4. The content of
4
sodium was calculated at the ash basis. It reveals that the sodium content in the fly ash was
5
higher than that in the bottom ash and circulating ash at the same gasification condition. This
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could be explained by the followings: the carbon content in the fly ash reaches 70% to 80%,
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higher than in the bottom ash and circulating ash. The carbon left in the ashes would inhibit
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the release of sodium, leading to the high sodium content in the fly ash. The carbon inhibition
9
effect was also reported in our previous work
21
and the work of Takuwa et al.
30
as well as
10
Manzoori et al. 20. During the gasification, the coal particle experienced devolatilization and
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carbonation. The specific surface area of fly ash decreased as the increase of preheated air
12
temperature. The decrease of fly ash specific area leads to the decrease of sodium release.
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The specific surface area of fly ash is shown in table 6. The residence time of fly ash was
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shorter, which also decreased the release of sodium. The effect of carbon inhibition and
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shorter residence time led to the high content of sodium in the fly ash. The content of sodium
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in the bottom ash was higher than that in the circulating ash. The bottom ash was consisting
17
of gasification char and the bed material (mainly as Al2O3 and SiO2). The interaction of
18
sodium in the bottom ash with the bed material would retain the sodium in the bottom ash 9,
19
31-35
.
20
The sodium content in the bottom ash, circulating ash and fly ash exhibited the same
21
variation with the air preheated temperature. The sodium content in the bottom ash increased
22
62.99%, 425.58% for circulating ash and 44.02% for fly ash as the air preheated temperature 8
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increased from 20 °C to 600 °C. The increase of air preheated temperature led to the drop of
2
air equivalent ratio. The air equivalent radio reflects the fraction of burned carbon. Lower air
3
equivalent radio indicates more carbon was left in the ash and the carbon inhibit effect was
4
enhanced. More sodium was retained in the ash at higher air preheated temperature.
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According to the mass balance during the gasification experiments, namely the total mass of
6
fed coal and air is equal to the total mass of bottom ash, fly ash and coal gas, the mass of
7
bottom ash could be calculated. Combining the sodium content in the fly ash and bottom ash,
8
the distribution of sodium in the solid phase (bottom ash and fly ash) and gas phase could be
9
got. The formulas are as follows:
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mc + ma = m fa + mba + mcg
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p fa (ba ) =
12
w fa (ba ) * m fa (ba ) wc * mc
*100%
(1) (2) (3)
pcg = 100 − p fa − pba
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Here, ma, mc, mfa, mba, and mcg represent the mass of air, coal, fly ash, bottom ash and
14
coal gas, respectively. wc, wfa, wba represent the content of sodium in coal, fly ash and bottom
15
ash, respectively. pcg, pfa, pba represent the fraction of sodium distributed in coal gas, fly ash
16
and bottom ash, respectively.
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Figure 5 gives the results of sodium distribution in the solid phase (bottom ash and fly
18
ash) and gas phase. There lacks the data of experiment 4 because of the occurrence of
19
slagging during experiment 4. More discussion about slagging was present in section 3.4.
20
Figure 5 shows that the fractions of sodium in the fly ash and coal gas were almost the same.
21
Sodium mainly exists in the bottom ash, reaches 41.31% to 64.18%. Turn et al. 36 investigated
22
the alkali mental retention of biomass gasification in a bench scaled fluidized bed gasifier and 9
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found that 40% to 80% alkali was retained in the bed as bottom residence. The enrichment of
2
sodium in the bottom ash may cause slagging easily. As the increase of air preheated
3
temperature, the fractions of sodium in the fly ash and coal gas dropped evidently whereas
4
the enrichment of sodium in the bottom ash was enhanced. As shown in figure 5, the release
5
fraction of gaseous sodium was decreased 42.82% with the increase of air preheated
6
temperature. Although the relative content of sodium in the fly ash increased with the air
7
preheated temperature (shown in figure 4), the fraction of sodium distributed in the fly ash
8
decreased. This could be explained by the following reason, the volume of coal gas increased
9
obviously as the increase of air preheated temperature, leading to the increase of coal gas
10
flow velocity at the inlet of cyclone. The increase of coal gas flow velocity would lead to the
11
increase of cyclone separation efficiency further and the amount of fly ash decreased from
12
2.98 kg/h to 1.96 kg/h. Finally, combined of amount of fly ash and the sodium content in the
13
fly ash, the fraction of sodium distributed in the fly ash decreased with the air preheated
14
temperature. According to the calculation results of Gabra et al.
15
should be in gas phase and the remainder was retained in liquid phase during the gasification
16
of bagasse at 950°C.
37
, about 15% of the alkali
17
Figure 6 describes the sodium content in the ashes on slagging probes along the coal gas
18
flow direction. The temperature range of coal gas covered from 632°C to 893°C. In the flow
19
direction of coal gas, the temperature could be divided into three temperature range by two
20
temperature turning points. With the decrease of coal gas temperature in the flow direction at
21
different air preheated temperature, the variation trend of sodium content in the slagging
22
probe ash with the decrease of coal gas temperature in the flow direction at different air
10
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preheated temperature was similar. The turning points of the temperature were about 800°C
2
and 700°C respectively and were affected by the air preheated temperature. When the coal
3
gas temperature was above 800°C (Section 1), the sodium content in the slagging probe ashes
4
decreased with the decrease of coal gas temperature. The melting point of NaCl is 801°C.
5
When the coal gas temperature was above 800°C, sodium in the ash kept relasing into
6
gaseous phase from the ash, and led to the decrease of sodium in the ashes. The sodium
7
contents in the slagging probe ashes show an evident increase when the coal gas temperature
8
decreased from 800°C to 700°C. This variety of sodium content indicated that large amount
9
of sodium in gaseous phase condensed to the ashes. The condensing of gaseous sodium
10
reached maximum when the coal gas temperature was about 700°C. As the further decreasing
11
of the coal gas temperature, the content of sodium in the slagging probe ashes would decrease.
12
This could be explained by the followings: most of gaseous sodium had condensed on the
13
slagging probes in section 2. As the further decreasing of the coal gas temperature, the
14
gaseous sodium decreased, and the condensed sodium decreased, as a result, the content of
15
sodium in the ashes decreased. The similar conclusion was got by Wang et al.
16
investigation of Zhundong coal combustion.
17
3.3 Transformation characteristics of sodium
5
during the
18
Figure 7 gives the occurrence of sodium in the ashes. The results of bottom ash,
19
circulating ash and fly ash were present in figure 7(a), (b) and (c), respectively. The
20
occurrence of sodium in the fly ash and circulating ash were similar. When the air preheated
21
temperature was 20°C, the fraction of NH4Ac-soluble, HCl-soluble and insoluble sodium
22
were about the same. At the range of 200°C to 600°C, the H2O-soluble sodium is the 11
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predominant occurrence. As to the bottom ash, the sodium existence was different
2
remarkably with that in the fly ash and circulating ash. Sodium in the bottom ash mainly
3
exists as insoluble sodium. The air preheated temperature had little influence on the sodium
4
existence in the bottom ash. The fraction of sodium of different occurrence in the bottom ash
5
kept stable with the variance of air preheated temperature. The fraction of insoluble sodium
6
was maintained at about 60%. As the increase of the preheated air temperature, the fraction of
7
HCl-soluble sodium kept increasing. The H2O-soluble sodium increased slightly whereas the
8
NH4Ac-soluble sodium decreased slightly when the preheated air temperature increased from
9
20 °C to 200°C. With the further increase of preheated air temperature, the H2O-soluble
10
sodium kept decreasing. It could be indicated that the transformation of H2O-soluble sodium
11
to HCl-soluble sodium was promoted with the increase of air preheated temperature at the
12
temperature range of 200°C to 600°C. The influence of air preheated temperature on the
13
occurrence of sodium in the fly ash and circulating ash was pronounced. As the increasing of
14
air preheated temperature, namely the drop of air equivalent ratio, the fraction of H2O-soluble
15
sodium in the fly ash and circulating ash increased。The increase of H2O-soluble sodium
16
indicates that the release of H2O-soluble sodium was inhibited as the drop of air equivalent
17
ratio. The carbon inhibition effect works mainly on H2O-soluble sodium. Li et al.
18
Wang et al. 12 found that H2O-soluble sodium was the most active sodium during gasification.
19
H2O-soluble sodium was the main source of sodium which released into gaseous phase. The
20
decrease of fly ash specific surface area reduced the release of H2O-soluble sodium.
18
and
21
The XRD patterns of gasification ashes at different air preheated temperature were
22
present in figure 8. The mineral compositions in the bottom ash and fly ash were different
12
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evidently. In the bottom ash, Al2O3 and SiO2 are the main mineral compositions. The
2
detection of NaAlSiO4illustrated that sodium alumina silicate is the main occurrence of
3
sodium and the reaction of sodium with alumina silicate did occurred during gasification
4
process. This conclusion is in agreement with the result of CFA. Piotrowska et al.
5
investigated the mechanism of agglomeration and deposition in CFB boiler and found that the
6
interactions between alkali metals and aluminum silicates were in favor for mitigate the
7
agglomeration or deposit. Arromdee et al.
8
combustion of peanut shells in a bubbling fluidized-bed combustor and no evidence of bed
9
agglomeration was found.
40
38, 39
used alumina as the bed material during the
10
Great differently, CaSO4 is the main mineral composition in the fly ash. The existence of
11
sodium in the fly ash was more complex. Na2SO4, NaAlSi3O8 and NaAlSiO4 were detected in
12
the XRD patterns of fly ash, which was similar with that in the raw coal ash. Na2SO4,
13
H2O-soluble sodium, could promote the fouling and ash deposition. Combining the results of
14
CFA, the content of Na2SO4 in the fly ash increased as air preheated temperature. In the
15
temperature range of 20°C to 600°C, air preheated temperature has little influence on the
16
mineral compositions of gasification ashes.
17
3.4 Ash fusion characteristics
18
Ash fusion temperature (AFT) is an important factor to the behavior of coal ash in coal
19
combustion and gasification 41. The content of sodium in the ashes has great effect on the ash
20
fusion temperature. Figure 9 gives the ash fusion temperature of the bottom ash and fly ash at
21
different air preheated temperature. It can be seen that the air preheated temperature has
22
evident effect on the ash fusion temperature of gasification ashes. As the air preheated 13
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temperature increased, the ash fusion temperature of both fly ash and bottom ash dropped.
2
Considering the content of sodium in the ashes, it could be concluded the effect mechanism
3
of air preheated temperature on the ash fusion temperature. The increasing of air preheated
4
temperature led to the retention of sodium in the ashes. Sodium in the ashes could lower the
5
melting point of ashes. The effect of sodium on the ash fusion temperature have been proven
6
by previous works 41, 42. Yao et al.
7
(wt.), the softening temperature (ST) would drop 17.7°C and the flowing temperature (FT)
8
would drop 15.6°C. Figure 9(a) and (b) shows that the effect air preheated temperature on the
9
fly ash fusion temperature was more evident compared to that on the bottom ash fusion
10
temperature. This could be explained by the different existence of sodium in the fly ash and
11
bottom ash. The CFA results and XRD analysis illustrate the occurrence of sodium in the
12
ashes. In the bottom ash, the sodium feldspar (melting point is about 1100 °C) is the main
13
sodium occurrence. Na2SO4 (melting point is 884°C), as H2O-soluble sodium, is the main
14
sodium occurrence in the fly ash. The existence of Na2SO4 affects the melting behavior of
15
ashes to a great extent.
16
3.5 Slagging behaviors during gasification
43
found that as the Na2O in the coal ash increased by 1%
17
The photos of bottom ash and slagging blocks at different air preheated temperature are
18
presented in figure 10. Slagging bocks appeared during the gasification when the air
19
preheated temperature was 600°C. Figure 10(d) shows the photo of slagging blocks collected
20
from the gasifier. Figure 11 presents the microstructure of the slagging blocks cross-section.
21
The diameter of the biggest slagging block reached 50 mm. XRF was applied to determine
22
the ash composition of slagging blocks. The result of XRF was present in table 7. It can be 14
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seen the slagging blocks mainly contain SiO2, Na2O, Al2O3 and CaO. The XRF results were
2
in keeping with the EDX results presented in table 8. The enrichment of SiO2 and Na2O
3
would reduce the ash fusion temperature for the formation of low-temperature-melting
4
eutectics, namely sodium silicates. The ash fusion temperature of slagging blocks was test in
5
theash fusion analyzer. The softening temperature of slagging blocks was 1120°C, lower than
6
the bottom ash of about 300°C. Figure 12 and figure 13 illustrate the mineral compositions
7
and sodium occurrence in the slagging blocks. The fraction of insoluble sodium in the
8
slagging blocks reached 70.05%. The XRD analysis proved the existence of Na2Si2O5 in the
9
slagging blocks as shown in figure 12.
10
Kosminski et al.
44
reported that the reaction of sodium with silica did occur during
11
gasification process. It could be indicated that as the increasing of air preheated temperature,
12
the air equivalent ratio dropped. More sodium was retained in the ash due to the effect of
13
carbon inhibition. The enrichment of sodium in the ash promoted the reaction of sodium with
14
silica, leading to the formation of lots of sodium silicate. Finally, the slagging blocks were
15
formed.
16
Air preheated temperature should be less than 600 °C to ensure the stable operation
17
when the content of Na2O in the Zhundong coal ash is up to above 7.28%.
18
4 Conclusions
19
Sodium migration and transformation during the gasification of Zhundong coal
20
circulating fluidized bed gasification with different air preheated temperature were
21
investigated in this work. The main conclusions are as follows:
22
(1) During the Zhundong coal gasification at 940°C, the air equivalent ratio decreased 15
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1
from 0.52 to 0.38 as the air preheated temperature increased from 20 °C to 600 °C. The
2
sodium content in the bottom ash, circulating ash and fly ash increased with the air preheated
3
temperature due to the effect of carbon inhibition. Accordingly, the release fraction of
4
gaseous sodium was decreased. The condensing of gaseous sodium started at 800°C of the
5
coal gas temperature and reached maximum about 700°C. The turning point of temperature
6
would increase with the increase of air preheated temperature.
7
(2) The air preheated temperature has little influence on the occurrence of bottom ash.
8
As the increasing of air preheated temperature, the fraction of H2O-soluble sodium in fly ash
9
and circulating ash increased.
10
(3)As the air preheated temperature increased, the ash fusion temperature of both fly ash
11
and bottom ash dropped. Slagging bocks appeared during the Zhundong coal gasification
12
when the air preheated temperature was up to 600°C. Slagging bocks mainly contain SiO2
13
and Na2O. Sodium in the slagging blocks mainly exists as Na2Si2O5, insoluble sodium. At the
14
excremental condition, the preheated air temperature during gasification should be less than
15
600 °C to ensure the stable operation of the gasifier.
16 17
ACKNOWLEDGMENT
18
This work was financially supported by the Strategic Priority Research Program of the
19
Chinese Academy of Sciences (No.XDA07030100) and the International Science &
20
Technology Cooperation Program of China (No. 2014DFG61680).
21 22
References
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Energy & Fuels
(1) Zhou, J.; Zhuang, X.; Alastuey, A.; Querol, X.; Li, J., Int. J. Coal Geol. 2010, 82 (1–2), 51-67. (2) Shouyu, Z.; Chuan, C.; Dazhong, S.; Junfu, L.; Jian, W.; Xi, G.; Aixia, D.; Shaowu, X., Proceedings of the CSEE 2013, 33 (5), 1-12. (3) Wu, X.; Zhang, X.; Yan, K.; Chen, N.; Zhang, J.; Xu, X.; Dai, B.; Zhang, J.; Zhang, L., Fuel 2016, 181, 1191-1202. (4) Li, J.; Zhu, M.; Zhang, Z.; Zhang, K.; Shen, G.; Zhang, D., Fuel Process. Technol. 2016, 144, 155-163. (5) Wang, X.; Xu, Z.; Wei, B.; Zhang, L.; Tan, H.; Yang, T.; Mikulčić, H.; Duić, N., Appl. Therm. Eng. 2015, 80, 150-159. (6) Yang, J. M.; Xu, J. N.; Du, B. In Effect of Alkali Metal on Gasification of Coke, International Conference on Electrical, Automation and Mechanical Engineering, 2015; 2015. (7) Yang, T.; Kai, X.; Sun, Y.; He, Y.; Li, R., Fuel 2011, 90 (7), 2454-2460. (8) Yang, T.; Kai, X.; Li, R.; Sun, Y.; He, Y., Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 2014, 36 (1), 15-22. (9) Kosminski, A.; Ross, D. P.; Agnew, J. B., Fuel Process. Technol. 2006, 87 (11), 943-952. (10) Linjewile, T. M.; Manzoori, A. R., Role of additives in controlling agglomeration and defluidization during fluidized bed combustion of high-sodium, high-sulphur low-rank coal. In Impact of Mineral Impurities in Solid Fuel Combustion, Springer: 2002; pp 319-331. (11) Doherty, W.; Reynolds, A.; Kennedy, D., Biomass Bioenergy 2009, 33 (9), 1158-1167. (12) Wang, C. a.; Jin, X.; Wang, Y.; Yan, Y.; Cui, J.; Liu, Y.; Che, D., Energy Fuels 2014, 29 (1), 78-85. (13) Li, X.; Bai, Z.-Q.; Bai, J.; Han, Y.-N.; Kong, L.-X.; Li, W., Energy Fuels 2015, 29 (9), 5633-5639. (14) Zhang, J.; Han, C.-L.; Yan, Z.; Liu, K.; Xu, Y.; Sheng, C.-D.; Pan, W.-P., Energy Fuels 2001, 15 (4), 786-793. (15) He, Y.; Qiu, K.; Whiddon, R.; Wang, Z.; Zhu, Y.; Liu, Y.; Li, Z.; Cen, K., Science Bulletin 2015, 60 (22), 1927-1934. (16) van Eyk, P. J.; Ashman, P. J.; Alwahabi, Z. T.; Nathan, G. J., Combust. Flame 2011, 158 (6), 1181-1192. (17) van Eyk, P. J.; Ashman, P. J.; Nathan, G. J., Combust. Flame 2011, 158 (12), 2512-2523. (18) Li, G.; Wang, C. a.; Yan, Y.; Jin, X.; Liu, Y.; Che, D., J. Energy Inst. 2016, 89 (1), 48-56. (19) Zhang, X.; Zhang, H.; Na, Y., Procedia Eng. 2015, 102, 305-314. (20) Manzoori, A. R.; Agarwal, P. K., Fuel 1992, 71 (5), 513-522. (21) Song, G.; Song, W.; Qi, X.; Lu, Q., Energy Fuels 2016, 30 (4), 3473-3478. (22) Wang, H.; Zheng, Z.; Yang, L.; Liu, X.; Guo, S.; Wu, S., Fuel Process. Technol. 2015, 132, 24-30. (23) Wang, L.; Mao, H.; Wang, Z.; Lin, J.-Y.; Wang, M.; Chang, L., J. Energy Chem. 2015, 24 (4), 381-387. (24) Zhou, B.; Zhou, H.; Wang, J.; Cen, K., Fuel 2015, 150, 526-537. (25) Song, W.; Song, G.; Qi, X.; Lu, Q., Fuel 2016, 182, 660-667. (26) Song, G.; Qi, X.; Song, W.; Yang, S.; Lu, Q.; Nowak, W., Fuel 2016, 186, 140-149. (27) Qi, X.; Song, G.; Song, W.; Yang, S.; Lu, Q., Appl. Therm. Eng. 2016, 106, 1127-1135. (28) Benson, S. A.; Holm, P. L., Industrial & Engineering Chemistry Product Research and Development 1985, 24 (1), 145-149. (29) Yang, Y.; Wu, Y.; Zhang, H.; Zhang, M.; Liu, Q.; Yang, H.; Lu, J., Fuel 2016, 181, 951-957. (30) Takuwa, T.; Mkilaha, I. S. N.; Naruse, I., Fuel 2006, 85 (5–6), 671-678. (31) Masnadi, M. S.; Grace, J. R.; Bi, X. T.; Lim, C. J.; Ellis, N.; Li, Y. H.; Watkinson, A. P., Renewable Energy 2015, 83, 918-930. (32) Masnadi, M. S.; Grace, J. R.; Bi, X. T.; Lim, C. J.; Ellis, N., Appl. Energy 2015, 140, 196-209. (33) Ellis, N.; Masnadi, M. S.; Roberts, D. G.; Kochanek, M. A.; Ilyushechkin, A. Y., Chemical Engineering
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Journal 2015, 279, 402-408. (34) Habibi, R.; Kopyscinski, J.; Masnadi, M. S.; Lam, J.; Grace, J. R.; Mims, C. A.; Hill, J. M., Energy Fuels 2013, 27 (1), 494-500. (35) Wang, L.; Skreiberg, Ø.; Becidan, M.; Li, H., Appl. Energy 2016, 162, 1195-1204. (36) Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M.; Zhou, J., Fuel 1998, 77 (3), 135-146. (37) Gabra, M.; Nordin, A.; Öhman, M.; Kjellström, B., Biomass Bioenergy 2001, 21 (6), 461-476. (38) Piotrowska, P.; Zevenhoven, M.; Davidsson, K.; Hupa, M.; Åmand, L.-E.; Barišić, V.; Coda Zabetta, E., Energy Fuels 2010, 24 (8), 4193-4205. (39) Piotrowska, P.; Zevenhoven, M.; Davidsson, K.; Hupa, M.; Åmand, L.-E.; Barišić, V.; Coda Zabetta, E., Energy Fuels 2010, 24 (1), 333-345. (40) Arromdee, P.; Kuprianov, V. I., Appl. Energy 2012, 97, 470-482. (41) Vassilev, S. V.; Kitano, K.; Takeda, S.; Tsurue, T., Fuel Process. Technol. 1995, 45 (1), 27-51. (42) Vorres, Quaternary International 1977, 371 (2), 197-208. (43) Yao, R. S.; Xiao-Hong, L. I.; Zuo, Y. F.; Fan, L. I., Meitan Xuebao/Journal of the China Coal Society 2011, 36 (6), 1027-1031. (44) Kosminski, A.; Ross, D.; Agnew, J., Fuel Process. Technol. 2006, 87 (12), 1037-1049.
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C
B A
Sample 2
Chimney
D E
Slagging probe
F G Bag-filtering dust precipitator
Coal Hopper
Loop seal
Screw Feeder
Sample 3
Air
Sample 1
Ash can
Air in
Air out
Slagging probe
Air preheater
1 2
Figure 1 Schematics of 0.4t/d fluidized bed experimental system
4000 Gasifier height (mm)
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3000 2000 1000
20°C 200°C 400°C 600°C
0 800 3 4
850 900 950 Bed temperature (°C)
1000
Figure 2 Bed temperatures along the gasifier at different air preheated temperatures
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900 800
20°C 200°C 400°C 600°C
700 600 500 400 300 200 B
C
D
E
F
G
Position
1
Figure 3 Surface temperature of slagging probs
2
12
Content of Na (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Surface temperature difference (°C)
Energy & Fuels
9
6
3
0 3 4
20°C 200°C 400°C 600°C
Bottom ash
Circulating ash
Fly ash
Figure 4 Sodium content in the ashes at different air preheated temperatures
20
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70 Fly ash Bottom ash Coal gas
Fraction of Na (%)
60 50 40 30 20 10 1 2
20
200 400 Air preheated temperature (°C)
Figure 5 Sodium distribution at different air preheated temperature
50
Content of Na (mg/g)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
40 30 20 10 0
3 4
Coal gas direction 20°C 200°C 400°C 600°C
Section 2 Section 3 Section 1 900 850 800 750 700 650 600 Gas Temperature (°C)
Figure 6 Sodium content in the slagging probe ashes at different air preheated temperature
21
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Energy & Fuels
120 H2O-soluble
NH4Ac-soluble
HCl-soluble
insoluble
Content of Na (%)
100 80 60 40 20 0 20 1
200 400 600 Preheated air temperature (°C) (a) Bottom ash
2
120 H2O-soluble
NH4Ac-soluble
HCl-soluble
insoluble
Content of Na (%)
100 80 60 40 20 0 20 3
200 400 600 Preheated air temperature (°C) (b) Circulating ash
4
120 H2O-soluble
NH4Ac-soluble
HCl-soluble
insoluble
100 Content of Na (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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80 60 40 20 0
5
20
200 400 600 Preheated air temperature (°C)
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1
(c) Fly ash
2
Figure 7 Occurrence of sodium by CFA at different air preheated temperature 9000 600°C 6000
ab
e e kk
3000
v
v
vv
0 8700
400°C ev
Diffracted intensity (cps)
5800 2900
a beka k
0 9000
a
6000
0 15000
v vv
v
20°C
v
u
v
30
vv
v
e k
b 20
200°C
v
10000 5000
v v
v
a
0 10
v v
v
v e bk k v ea e
u
3000
40
50
60
70
80
90
2θ (°)
3
(a) Bottom ash
4 a
600°C
9000 6000 3000
a a o k eo da a a a a a k
0 9000 Diffracted intensity (cps)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
a
a
6000
400°C
a k o a o ek a a d a a aa
3000
a
0 a 200°C
9000 6000
a a o k eok da a a a a a
3000
a
0 a
15000
20°C
10000 5000
a
a k o aa ek d a a a a
a
0 10
5 6
20
30
40
50
60
70
80
2θ (°)
(b) Fly ash
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90
Energy & Fuels
1
a-CaSO4; b-SiO2; d-CaO; e-Ca2SiO4; h- Na2SO4; k-NaAlSiO4; o-KAlSi3O8; u-Al2O3·SiO2; v-Al2O3
2
Figure 8 XRD patterns of ashes at different air preheated temperature
1600
Temperature (°C)
1500 1400 DT ST HT FT
1300 1200 1100 20
3
200 400 600 Slagging Preheated air temperature (°C) (a) Bottom ash
4
1600 1500 Temperature (°C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1400
1200 1100
5 6 7 8
DT ST HT FT
1300
20
200 400 600 Preheated air temperature (°C)
(b) Fly ash Figure 9 Fusion temperatures of ashes at different air preheated temperature
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1 2
(a) 20°C
(b) 200°C
(c) 400°C
(d) 600°C
3 4 5
Figure 10 Photos of bottom ash and slagging blocks at different air preheated temperature
6 7
Figure 11 Microstructure of slagging blocks（600°C）
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Energy & Fuels
10000 Diffracted intensity (cps)
b 8000 6000 4000
v
a u
2000
ke io v k
v v
v
50
60
vv
0 10
20
30
40
70
80
90
2θ (°)
1 2
a-CaSO4; b-SiO2; e-Ca2SiO4; i-Na2Si2O5; k-NaAlSiO4; o-KAlSi3O8; u-Al2O3·SiO2; v-Al2O3
3
Figure 12 XRD patterns of ashes（600°C）
50 Insoluble 40 Content of Na (mg/g)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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30 20 HCl soluble 10 0
4 5
HN4Ac soluble H2O soluble Speciation of Na
Figure 13 Occurrence of sodium in the slagging blocks（600°C）
6
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Table1Properties of ML
1
Proximate analysis (ad,wt%) Water content Ash Volitile matter Fixed carbon Lower heating value (MJ/kg) Ultimate analysis (ad, wt%) C H N O St Cl Ash fusion temperature (oC) DT ST HT FT Chemical components in ash (wt%)
ML 14.34 3.16 27.02 55.48 23.70 64.54 3.02 0.52 13.97 0.45 0.058 1360 1370 1370 1380
SiO2
3.73
Al2O3
6.16
Fe2O3 CaO MgO TiO2
5.37 33.45 5.42 0.41
SO3
29.34
P2O5
0.00
K2 O
0.45
Na2O
7.28
2 3
Note: ad-as air dried basis; DT-deformation temperature; ST-softening temperature; HT-hemispherical temperature; FT-flowing temperature.
4
Table 2 The occurrence of sodium in ML Occurrence
H2O-soluble Na
NH4Ac-soluble Na
HCl-soluble Na
insoluble Na
Content (wt.%)
56.96
36.22
2.42
4.40
Table 3 Composition of bed material
5 Components
Al2O3
SiO2
Na2O
K2O
Fe2O3
CaO
SO3
Others
Content (wt.%)
90.05
7.69
1.27
0.30
0.17
0.14
0.18
0.20
6 7
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Table 4 Experimental conditions
1 Experimental number
1
2
3
4
Air preheated temperature (°C) Gasification temperature (°C) Coal feed rate (kg/h) Air flow rate (Nm3/h) Air to coal rate (Nm3/kg) Air equivalent ratio Superficial gas velocity (m/s)
20 937 16.00 50.21 3.14 0.52 3.26
200 935 17.14 51.68 3.02 0.50 3.35
400 944 19.00 50.29 2.65 0.43 3.29
600 936 22.07 51.55 2.34 0.38 3.34
Table 5 Coal gas flow velocity near the slagging probes
2
Coal gas flow velocity (m/s)
Preheated air temperature (°C) 20 200 400 600
B
C
D
E
F
G
14.02 14.47 14.52 15.23
13.05 13.57 13.59 14.31
12.62 13.05 13.17 13.82
12.39 12.73 12.76 13.64
11.96 12.29 12.53 13.25
11.29 11.51 11.81 12.41
Table 6 Specific surface area of fly ash
3
Preheated air temperature (°C)
20
200
400
600
Specific surface area (m2/g)
686.65
590.58
515.29
429.95
Table 6 Ash composition of slagging blocks
4 Composition
SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
SO3
K2O
Na2O
Cl
Content(wt.%)
47.53
20.18
2.90
11.33
1.70
0.24
1.08
0.18
13.88
0.14
Table 7 Element distribution of slagging blocks cross-section
5 Element
C
O
Na
Mg
Al
Si
S
Cl
K
Ca
Fe
Content%
3.77
38.18
4.69
2.00
6.47
25.87
0.24
0.20
0.22
14.55
3.81
6
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