Release and transformation of potassium in co-combustion of coal and wheat straw in a BFB reactor

Applied Thermal Engineering 144 (2018) 1010–1016

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Research Paper

Release and transformation of potassium in co-combustion of coal and wheat straw in a BFB reactor ⁎

T

⁎

Bo Zhanga, , Zhaoping Zhonga, , Zeyu Xuea, Jianming Xueb, Yueyang Xub a b

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing 210096, China Guodian Science and Technology Research Institute (State Power Environmental Protection Research Institute), Nanjing 210031, China

H I GH L IG H T S

of sulfation mainly took • Reaction place below 900 °C in co-combustion. mainly existed in the form of alu• Kminosilicates at higher combustion

G R A P H I C A L A B S T R A C T

Transfer pathway of K species in the combustion.

temperatures.

between bed material and • Reaction biomass was promoted with elevated temperature.

deposition during co-combustion • Ash at 900 °C can be divided into three stages.

A R T I C LE I N FO

A B S T R A C T

Keywords: Co-combustion Alkali metal Deposit Biomass Coal

Potassium transformation and migration characteristic during wheat straw-coal co-combustion was investigated in a lab-scale BFB reactor, with the emission of SO2 also discussed. Results indicated that the reaction of sulfation mainly took place below 900 °C in co-combustion. At higher combustion temperatures, K mainly existed in the form of aluminosilicates. In addition, reaction between bed material and wheat straw was promoted as the temperature increased, causing the enrichment of K in the bottom phase. Ash deposition during the co-combustion at 900 °C can be divided into three stages: (1) deposition of coal ash stuck to the probe, (2) deposition of WS ash adhered to the coal ash, (3) sulfation on the surface of the WS ash. Fluidized air velocity also made a difference to alkali distribution by changing heat and mass release of K into gas phase by improving the combustion. When the velocity was even higher, K content in the flue gas decreased due to the shorter residence time of fuel particles.

1. Introduction Biomass combustion has been put forward and already implemented in some areas to reduce emissions of CO2 and solve the energy shortage problems. However, in the combustion, alkali metals, especially potassium in the raw biomass is mainly released into the gas phase, leading to fouling, corrosion, deposit and slagging in power plants

⁎

[1,2]. Alkali-brought problems in biomass combustion has attracted more and more attentions and several methods such as leaching pretreatment, blending with additives, co-firing with coal have been proposed successively [3–6]. Coal-fired power still dominates the energy structure worldwide and the utilization of biomass by certain proportion is viable in the existing power plants. Co-firing biomass with coal can relieve the problems mentioned above effectively due to the

Corresponding authors. E-mail addresses: [email protected] (B. Zhang), [email protected] (Z. Zhong).

https://doi.org/10.1016/j.applthermaleng.2018.09.021 Received 29 September 2017; Received in revised form 24 July 2018; Accepted 3 September 2018 Available online 05 September 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.

Applied Thermal Engineering 144 (2018) 1010–1016

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relatively low alkali content in the coal and the stability of the minerals contained. Furthermore, addition of biomass could also improve combustion indices of coal and reduce the gaseous pollution [7,8]. In the coal K primarily existed in the feldspar and the content of activated K species (e.g. K2SO4, KCl, K2CO3) was negligible while K in the biomass mostly was combined with active functional groups or other inherent minerals which were of high mobility and activity [9]. Kassman and Hu reported that Cl could facilitate the release of K by the formation of KCl(g) in the biomass combustion, which was believed as the dominant existing form in the gas phase at high temperatures [10,11]. However it was in the stable Al-Si structure and barely transferred into gas phase in coal combustion [12,13]. In addition, content of S in the coal was also higher than that in the biomass. S in the combustion released in the form of SO2 and it can be captured by the alkali/ alkaline earth metals in the fuel or desulfurizer added, forming sulfates retaining in the solid phase. Furthermore, Ca also did impact on K for they had similar prosperities and relative content in the biomass fuel thus the transform of Ca is worthy researching [14,15]. The interaction between alkali and other related elements was rather complicated and the competitive effect of the elements to combine with K in combustion needed further studies. Several studies have indicated that increasing temperature promoted the release of K in the heat treatment in the biomass utilization. The release in the combustion could be divided into two stages: before 400 °C K released in the form of organic salt in the pyrolysis, which only accounted for a little part. After 500 °C the evaporation of KCl(g) and KOH(g) dominated K release. Earlier research also reported that the release increased with the temperature but saturated around 900 °C. For combustion of biomass, the release of K reached completion at 1150 °C [2,16,17]. Ash deposit can be sorted by the temperature and situation in the flue: cohesive deposit at high temperature and impact deposit at middle and low temperatures, respectively [18,19]. The former was closely related to sintering in the furnace while the latter was formed through the condensation of gaseous substance or crush effect of inert minerals in the fuel, which could be collected by the cyclone [20]. Wang analyzed biomass ash deposit and found alkali sulfate was the main chemical composition [21]. However, research combining the release of S and K in the cocombustion in the fluidized bed is seldom reported. This paper investigated the fate of K in the co-combustion, along with the element S. A comparative study of characteristics of the two elements were made in order to investigate sulfation occurred between the two fuels. Effect of combustion conditions on release and transformation K was also studied in this research. Considering the technology of co-firing of biomass with coal has already been implemented in many coal-fired power plants in Europe for a long time but the mass ratio was usually controlled below 20–30% [22,23], this research investigated the cocombustion in which the biomass mass fraction in the blends was set as 30% to simulate the fate of K at maximum biomass ratio in present cocombustion power plants.

Electric heater Primary Feeder

Absorption bottle

Gas analyzer Deposit probe

Secondary Feeder Cooling water Air compressor

Cyclone

Temperature controller

Air preheater

Fig. 1. Sketch of the lab-scale BFB combustion system used in the study.

zone and the temperature was controlled by heating procedure set in advance. In the upper part of the freeboard where the temperature was about 70 °C higher than dense phase zone, sampling probe made of heat resistant stainless steel 310S was fixed to collect the deposit formed at high temperatures. The outlet pipe was enwound by heating belt of which heating temperature was adjusted at 300 °C to avoid condensation of flue gas in flow and ensured gaseous fraction of potassium and sulfur could be captured to the greatest extent. Evaporated K in the flue gas (FG) was absorbed by 5% HNO3 + 10% H2O2 solution. The research focused on the potassium transfer characteristic influenced by (1) fuel composition, (2) combustion temperature and (3) fluidized bed velocity. In the experiment, the default fluidized velocity designed was set as 0.20 m/s and the feed quantity of the fuel was about 25 g/h, which was adjusted by the combustion efficiency monitored by flue gas analyzer to make excess air ratio fluctuate around 1.4. Each operating condition lasted for 2 h. 2.2. Materials In the study, wheat straw (WS) was cultivated in Hebei Province and the coal was collected from Shandong Province. To guarantee the normal operation of the BFB combustor, the bed material was finalized as bauxite from Henan Province, which was reported to prevent the agglomeration brought by biomass effectively [24,25]. To meet the set value of fluidizing velocity, different sizes of materials with different densities were sieved. The detailed information of the materials was listed in Tables 1–3.

2. Experimental 2.3. Data analysis 2.1. Test conditions in field study

The SO2 concentration detected by the flue gas analyzer in the experiment should be processed according to Eq. (1).

In the study, a lab-scale bubbling fluidized bed (BFB) was set up to conduct the combustion experiment. The BFB system was mainly consisted of a two-stage feeder, an air distributor, main combustion part and sample collection/analysis system. The second feeder was equipped with a water cooling tube to prevent the pre-pyrolysis of fuel inside and expedite feeding. At last, the exhaust air was analyzed by a MRU gas analyzer to determine the combustion conditions. In addition, pressure data was also referred to monitor the fluidization condition. The sketch of the BFB system is presented in Fig. 1. The main part of the BFB has an inner diameter of 32 mm and a height of 700 mm. The thermocouple was located in the dense phase

ρ = ρ′ ×

21−φ (O2 ) 21−φ′ (O2 )

(1)

where ρ and ρ′ represent the equivalent value and measured value of SO2 concentration in the flue gas, respectively, mg/m3, φ(O2) and φ′(O2) are the reference and measured value of O2 content in the flue gas, %. φ(O2) was set as 6 in this research [28]. After combustion and natural cooling, blends of bottom slag (BA) and bed material (BM), fly ash (FA) in the cyclone were collected and weighted. The absorption in the two bottles was blended and diluted in 1011

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WS ash

Bauxite

30.96 0.829 19.040 9.788 0.171 1.114 11.94 0.223 0.534 0.290 13.757 0.029

53.0 0.083 0.649 0.755 1.051 3.59 6.46 20.9 0.597 2.78 1.68 8.25

57.31 1.09 28.78 3.39 0.102 0.428 1.25 0.856 0.282 0.072 0.004 0.013

True density/g·cm Size/mm

ω×V × 100% A

RFA =

mFA × MFA × 100% A

RB =

coal

bauxite

0.52 1.4–1.8

1.21 0.8–1.2

2.64 0.25–0.4

75

-3

-3

70 65

800

coal co-firing WS

700 600

60 55 50

500

45 40

400 800

850

900

950

1000

T/ C

(2)

A = ω × V + mFA × MFA + mB × MB

WS

900

some proportion. To measure the total potassium content in the ash residue, EPA 3052 [29] was referred. Afterwards, the concentration of the digestion solution was analyzed by ICP-OES (Optima8000, PerkinElmer, USA). The distribution of K in each phase was calculated according to Eqs. (2)–(5).

RFG =

−3

1000

SO2 content/mg m

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO K2O Na2O P2O5 SO3 Cl

Coal ash

Table 3 Sizes of the materials applied in the research.

SO2 content/mg m

Table 1 Composition of fuel ash prepared in muffle furnace at 815 °C in air and bed material in the experiment.

Fig. 2. Effect of combustion temperature on SO2 concentration in the flue gas.

(3) adsorption in the flue. (4)

mB × MB × 100% A

4FeS2 + 11O2 (g) → 2Fe2 O3 + 8SO2 (g)

(6)

A couple of fluctuations were found in the other two fuel conditions. At lower temperature, WS and coal had the same SO2 release characteristic, which could be owing to the promotion of oxidization of sulfur compounds by high temperature. However, there was a slight decrease around 900 °C in both conditions. As for WS alone in the combustion, maximum value of SO2 content was achieved at 875 °C while for co-combustion was 850 °C in the temperature range from 800 to 900 °C, this could be owing to the sulfation reaction at this temperature point. When the oxidization is counteracted by the desulfurization rate, there could be a decrease in the SO2 content in theory. Sulfation reaction is listed in Eq. (7). This reaction was also affected by the H2O(g) and O2(g) concentration in the flue gas [32].

(5)

where A is the total content of K detected, mg. RFG is the K release ratio in the combustion, ω is the concentration of the absorbing solution, mg/ mL, V is the volume of the absorbing solution, mL, m is the weight of the ash/slag collected, g, M is the potassium content in the ash/slag, mg/g, subscript FG, FA and B represent the flue gas, fly ash and blends of bottom slag and bed material, respectively. The crystalline phase of the fly ash in the cyclone was determined by X-ray diffraction (SmartLab 3, Rigaku, Japan) over the ranger of 5–90° and the data generated was analyzed by MDI Jade 6.5 (Materials Data Incorporated, USA). To analyze the microstructure and element composition of the ash deposit on the probe, a scanning electron microscope (SU8010, HITACHI, Japan) and energy-dispersive spectroscopy (Bruker Quantax 400, Germany) were applied respectively.

4KCl + 2SO2 (g) + 2H2 O(g) + O2 (g) → 2K2SO4 + 4HCl(g)

(7)

After 900 °C, the sulfation was exceeded by oxidation again, thus causing the following increase in SO2 content.

3. Results and discussion 3.2. Effects of combustion conditions on K distribution 3.1. Effect of combustion conditions on S release Effect of temperature on K distributions in the combustion is shown in Fig. 3. Compared with coal, WS combustion appeared higher K release. In addition, trends can be seen that the volatilization of K was generally promoted by the rising combustion temperature. Firstly, higher temperature improved the combustion efficiency, thus facilitating the escape of K species from the fuel and ash particles. Secondly, the loss caused by adsorption and condensation was decreased naturally. Some research reported that at the evaporation rate of K may

Fig. 2 shows the effect of temperature on SO2 concentration in the flue gas. When combusting coal alone, SO2 content increased as the temperature rose on the whole and after 900 °C the growth tended to be slow. This result was in agreement with earlier studies [30]. Before 400 °C, released S mainly comes from decomposition of organic sulfur compounds, while after 400 °C, oxidization of pyrite dominated the generation of SO2 via the reaction in Eq. (6) [31]. Furthermore, high temperature could also reduce the loss caused by condensation and

Table 2 Proximate and ultimate analysis of fuels conducted following GB/T 212-2008 [26] and ISO 17247-2013 [27] separately, wt%. Sample

coal WS

Ultimate analysis (wt%, dry and ash-free basis)

Proximate analysis (wt%, dry basis)

N

C

H

O

S

Ash

Volatile matter

Fixed carbon

1.19 1.44

89.73 51.67

2.83 6.21

2.45 40.45

3.80 0.23

25.96 14.80

10.23 64.99

63.81 20.21

1012

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100

BS&BM FA FG

90

Relative content/%

80

material and bottom slag decreased. Higher velocity shortened the residence time of the fuel in the furnace, thus the contact time between the fuel and bed material were decreased, inhibiting the sufficient reaction of released K with the bed materials. However, K in the bottom phase only took a small part and content of K in the other two phases showed an opposite trend with the velocity. When the fluidized velocity increased from 0.14 to 0.18 m/s, K content in the fly ash decreased as the velocity increased and reached its minimum value of 45% at the velocity of 0.18 m/s. In this velocity range, higher velocity could promote the combustion of the WS, facilitating the transfer of K species from the inner part of the fuel particles. It is noteworthy that when the velocity was higher, the effect of velocity on gas–solid reactions became weaker and more K was retained in the unburnt fuel particles, thus improving the content in the fly ash. Additionally, shorter residence time led to higher residual carbon in the fly ash, which could adsorb released K in the flue gas via porous structure.

A-WS B-co-firing C-coal

70 60 50 40 30 20 10 0

A

B

C

800 C

A

B

C

A

850 C

B

C

A

B

900 C

C

950 C

A

B

C

1000 C

Fig. 3. Effect of temperature on K distributions in the combustion.

decrease after 900 °C due to the melting phenomenon in the ash particle during biomass combustion in a tube furnace [33]. However, the fluctuation did not appear in this research, which could be owing to the difference in the transfer conditions. Fluidized air could take the released K species away from the combustion zone and shorten the residence time in the furnace, avoiding its accumulation in the solid phase. Share of K in the bottom slag and bed material increased along with the temperature rising from 800 to 1000 °C. Rising temperature accelerated the combination reaction of alkali released and Al-Si structure in the bed materials. As a result, the K in the gas phase could be retained in the bed material to some degree and the fixation capacity appeared higher at higher temperatures in the combustion. K content in the bottom slag and bed material decreased with the increase of temperature during coal combustion, indicating that the extent of the reaction between coal and bed material was not sufficient enough and it became weaker with the temperature increasing. Considering that the effect of temperature on K evaporation was rather limited, the relative content of K increased in the fly ash in the coal combustion. Similar trends could be found in the co-firing. The fluidized velocity was set from 0.16 to 0.24 m/s as the variant to the function of K distribution. To ensure the constant excess air ratio, the feed rate was adjusted in proportion accordingly. For the co-combustion at 900 °C, experiments were conducted to investigate the effect of fluidized velocity on the migration of K. Effect of fluidized velocity on K transfer is shown in Fig. 4. Results indicated that as the velocity increased, the relative content of K in the bed

3.3. Ash deposit at high temperature The ash deposit attached on the sampling probe was scraped off from the probe and then investigated by SEM-EDS method. The morphology of ash deposits from burned coal, biomass and their blends are presented in Fig. 5. It is obvious that the shape of the deposit produced by combusting WS alone was irregular, the surface was rather rough and uneven. Actually, the particle was adhered together by aerosol particles of much smaller sizes. From the graph below, the aerosols were spherical and the interspaces between each two particles were filled with floccules. In addition, a slight melting phenomenon was observed, indicating the mere existence of alkali silicates with low-melting point in the deposit. For the deposit of coal combustion, it possessed a relatively more regular shape. The ash particle was consisted of many agglomerated bricks, appearing a compact surface. With the reference of the scale plate below, the bricks were, by and large, submicron particles, which deposited onto the sampling probe and then accumulated together into a block shape. Structure of the deposit of the blended fuels is illustrated in Fig. 5(c). Compared with the former two particle samples, the deposit particle produced in co-firing was a combination of the individuals, consisting of many brick shapes accumulated together with some floccules attached on them. It could be concluded that during co-firing, coal ash deposit firstly formed onto the probe and then bio ash deposit adhered to the former. The morphology indicated that the formation of ash deposit in cofiring was a two-stage process. WS ash was of low melting point and the coal ash could be integrated with the former by binding effect of the molten part. In other words, the coal ash was the skeleton of the deposits in co-firing. The main element composition of the ash deposits determined by EDS is illustrated in Fig. 6. As the share of wheat straw increased, the content of Al and Si in the ash deposit decreased while the content of K increased due to their relative contents in the initial fuels. When combusting the WS alone, It should be noticed that S was enriched in the co-firing condition and its content in WS ash was also larger than in coal ash in spite of its high content in the initial coal. The main compounds of sulfur produced in co-combustion were alkali metal sulfates produced due to the sulfation of the chlorine salts, indicating that sulfation took place in the co-firing. In the combustion of coal alone, S could partially combine with Ca, existing in the form CaSO4 in the bottom slag and absent in the fly ash. Taking the two-stage process assumption into consideration, this reaction mainly occurred at the contact surface between the flue gas and WS ash deposits. It could also be concluded that the noncohesive spherules on the surface of the ash deposit in co-firing were K2SO4 crystals and formed according to the following reaction.

FG FA BS+BM

60

Relative content/%

50

40

30

20

10

0 0.14

0.16

0.18

0.20

0.22

0.24

-1

Fluidized velocity/m s

Fig. 4. Effect of fluidized velocity on K distribution in co-combustion in BFB at 900 °C. 1013

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(a)×10k

(b)×10k

(a)×50k

(b)×50k

(c)×10k

(c)×50k

Fig. 5. SEM micrograph of ash deposit at 900 °C (a:WS, b:coal, c:co-firing, ×10k: enlarge 10,000 times, ×50k: enlarge 50,000 times).

Si in the ash, forming the alkali silicates with low melting point. As the result, KCl was absent in the bottom ash while it could be detected in the fly ash produced in the fluidized bed at low temperatures. However, at higher temperature of 900 °C, KCl was absent in the fly ash and K mainly existed in the form of potassium silicates, indicating the high temperature enhanced the evaporation of KCl. On the other hand, KCl could transform into K2SiO3 via reaction in Eq. (9). Moreover, K in the fly ash kept on reacting with Si by pathway listed in Eq. (10). It could be inferred that silicates with more Si atom is thermodynamically stable at higher temperature.

30 coal co-firing WS

Elemental wt.%

25 20 15 10

2KCl(g) + H2 O(g) + SiO2 → 2HCl(g) + K2SiO3

5

K2SiO3 + SiO2 → K2Si2 O5

0 Al

Si

S Elements

K

Ca

(10)

At 1000 °C, the main content of WS fly ash was SiO2 and KAlSiO4. It should be pointed out that the content of Al in the raw WS could almost be ignored and the Al was likely to be supplied by the bed material powder. The high temperature could promote the rate of the reaction and relieve the alkali-caused slagging and agglomeration. The reaction related is listed in Eq. (11). It could be concluded that the formation of alkali aluminosilicates in the combustion with the presence of Si in the fuel was a two-stage reaction and the alkali silicates were the intermediate products in the process of rising temperature.

Fe

Fig. 6. Composition of main elements in ash deposit at 900 °C.

2K2SiO3 + 2SO2 (g) + O2 (g) → 2K2SO4 + 2SiO2

(9)

(8)

Furthermore, no Cl was detected in the ash, which could be ascribed to desynchrony of releases of Cl and other metals at this temperature. In the co-firing, Cl mainly released in the form of HCl(g) while in the combustion of WS alone, Cl could bond with K actually but Cl was more stable in the gas phase and usually was a gaseous product in exhaust gas at high temperatures.

K2Si2 O5 + Al2O3 → 2KAlSiO4

(11)

For fly ash collected in coal combustion at 800 °C, the pattern consisted of intensity peaks of SiO2, CaSO4, CaCO3, Fe2O3, NaAl2(AlSi3) O10(OH)2 and so on. Alkali in the products of coal combustion was mainly incorporated into mica or feldspar minerals which inherited from the initial coal. When at the temperature of 900 °C, alkali sulfate was detected in the coal fly ash, confirming that the sulfation reaction took place. In addition, CaCO3 could transform into Ca2Al2O5 via the reaction in Eq. (12). This also suggested that Al2O3 was of chemical activity at this temperature.

3.4. Effect of combustion conditions on K transformation Fly ash was the result of the crush fuel particle and the condensation of the aerosols generated. Compared with the bottom slag and deposits at temperatures, it was loose and of smaller particles. Here, fly ash samples removed by the cyclone separator were collected and analyzed by XRD method. The patterns are shown in Fig. 7. As for combusting wheat straw at 800 °C, the main species in the fly ash were KCl, SiO2, K2CO3 and CaCO3. The ash composition was different from the result obtained in fixed bed at the same temperature in which Cl in the fuel mainly evaporated prior to K and released in the form of HCl(g), thus K cannot bind with Cl to release but combined with

2CaCO3 + Al2O3 → Ca2Al2O5 + 2CO2 (g)

(12)

The crystalline phase of coal fly ash detected at 1000 °C were SiO2, Na2SO4, Fe2O3, CaSO4, KAlSiO4 and CaCl2, indicating that CaSO4 could 1014

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WS

1800

Intensity

0 3000

J

A

G

G

900°C

C A ED B B E B E E C C E AB

0 3000

E

AD D AE

A

A

O O A EA EAE

A J E JLO

1000°C

AP

A

A

900°C

A

4000 3000 2000

P E

1000

Q

A R RL

0 4000

S

Q E L QA K E S K P

A

A Q

A

A

1000°C

3000

2000

2000

1000 0

L

0

F F D

A

1000

G

A

2000

E

2000

G

H A AJ H

600

1000

800°C

A

3000

I

Intensity

1200

coal

4000

800°C

G

K

15

L

K NL L N AAK

30

A

A

N

45

A

1000

A

60

75

0

90

E A

L

15

L U U S AVA KV S A

30

A KEA T K

45

A L L

60

A

75

90

2 theta/

2 theta/

(a)

(b) co-combustion 800°C

A

4000 3000 2000 A K VL

1000

E V ES X AEAE E A KE

Intensity

0 4000

K

A

A

A

900°C

3000 2000 1000

A E A Z Y K L Y E L LA KAL Z K

A

A

0

A

A

1000°C

3000 E

2000

A A1 B1

1000 0

15

E

30

A1 L LA E E B1 B1A A A1 E LE

45

2 theta/

A LL

60

A

75

90

(c)

Fig. 7. XRD patterns of the fly ashes produced by (a) WS, (b) coal and (c) blended fuel at different temperatures A-SiO2, B-K2Si2O5, C-MgCl2, D-Ca3(PO4)2, E-CaSO4, F-K2SiO3, G-KCl, H-K2H2SiO4, I-K2CO3, J-CaCO3, K-KAlSiO4, L-Fe2O3, M-Ca2Fe4AlSi6Al2O22(OH)2, N-NaAl5O8, O-NaAl2(AlSi3)O10(OH)2, P-TiO2,Q-Ca2Al2O5, RNaAl2(AlSi3)O10(OH)2, S-K2SO4, T-CaO, U-Na2SO4, V-K2Mg(SO4)2, W-Ca2SiO4, X-Mg3(PO4)2, Y-KAl(SO4)2, Z-CaAlSiO4(OH), A1-KAlSi3O8, B1-CaAl2Si2O8.

react with alkali chloride in the process of fluidized bed combustion. Taking the result obtained in a fixed bed reported before into consideration, this reaction may be influenced by the practical mass transfer condition [33,34]. The reaction is in Eq. (13). Fly ash in coal combustion almost kept constant from 900 to 1000 °C.

CaSO4 + 2NaCl(g) → CaCl2 + Na2SO4

thermogravimetry using raw and artificial ashes [35].

2KCl(g) + Al2 O3 + 4SO2 (g) + 2O2 (g) + H2 O(g) → 2KAl(SO4 )2 + 2HCl(g) Ca2SiO4 + Al2O3 + SiO2 + H2 O(g) → 2CaAlSiO4 (OH)

(13)

(16)

The crystalline phase of fly ash in co-combustion at 1000 °C was consisted of SiO2, CaSO4 and alkali/alkaline earth aluminosilicates. The result was the same as coal combustion, which also indicated that aluminosilicates were more thermodynamically stable than sulfates over 1000 °C. The reactions are listed in Eqs. (17)–(19).

Fig. 5(c) shows the XRD patterns of the fly ash produced in cocombustion at 800 °C. Similar to the coal fly ash, the main components detected were SiO2, CaSO4 and Fe2O3 too. However, K was in the form of sulfates, confirming that the sulfation of alkali species occurred in the co-combustion of coal and biomass. Reactions mentioned are listed in Eq. (7) and Eq. (14).

CaSO4 + K2SiO3 → CaSiO3 + K2SO4

(15)

(14)

At 900 °C, slight change took place in the fly ash and reaction in Eqs. (15) and (16) occurred. Similar results were also gained by

2K2SO4 + 2Al2O3 + 6SiO2 → 4KAlSi3O8 + 2SO2 (g) + O2 (g)

(17)

KAlSiO4 + 2SiO2 → KAlSi3O8

(18)

2CaSO4 + 2Al2 O3 + 4SiO2 → 2CaAl2 Si2 O8 + 2SO2 (g) + O2 (g)

(19)

It should be noticed that SO2 which has been captured by alkali 1015

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coal

K2SO4 SO2

co-firing

K2SO4

K2CO3 KCl

Al2O3 KAlSiO4

KAlSiO4

KAlSi3O8

SiO2

Al2O3

K2SiO3

KAlSiO4

WS Fig. 8. Transfer pathway of K species in the combustion.

metals could be released again via the two reactions at high temperatures in the combustion. The transfer from sulfates into aluminosilicates means the latter was more stable for potassium at high temperatures. Comparing the ash composition in the fluidized bed with fixed bed, it can be concluded that Ca was non-volatile and mainly existed in the solid phase. Results indicated that CaSO4 in the ash was formed in the combustion stage before the release of SO2. K2SO4 in the fly ash derived from the reaction between SO2 released and K in the flue gas or fly ash, thus it could not be detected in WS combustion for the lack of S. Combustion in fluidized bed also affected K transformation by enhancing gas-solid contact with SO2, which could not be detected in fixed bed. Transfer pathway of K in the combustion of coal, WS and their blends is concluded in Fig. 8. 4. Conclusion In this work, the influence of operating conditions on the release of K and S species during co-combustion of wheat straw and coal has been investigated in a lab-scale BFB from 800 to 1000 °C. The results indicate that sulfation reaction of alkali chloride takes place in the temperature range and can even affect the SO2 concentration in the flue gas around 900 °C with the presence of wheat straw. Fluidized velocity can also affect the K migration via the mass transfer effect. Rising temperature facilitates the migration of K species into gas phase while it could also improve the interaction of K and Si/Al species in the fly ash. Co-firing effectively restraints K release via the pathway to form sulfates or aluminosilicates, preventing the melting brought by silicates. Furthermore, in the co-firing, the interaction between the two fuels can be divided into two stages: (1) sulfation before 900 °C; (2) the reaction to generate aluminosilicates. However, in the co-firing ash deposition at 900 °C is also a step reaction. Acknowledgements This study was financially supported by the National Natural Science Foundation of China (51776042, 51706043, 51276040 & U1361115) and the Jiangsu Natural Science Foundation (No. BK20170679). The authors also gratefully acknowledge the financial support from Key Laboratory of Coal Science and Technology, Taiyuan University of Technology. References [1] B.M. Jenkins, L.L. Baxter, T.R. Miles Jr.et al., Combustion properties of biomass,

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