Investigation on Ash Deposition Behavior During Biomass ... · PDF fileInvestigation on Ash...

PACE RGDTE, THU, Beijing

Investigation on Ash Deposition Behavior During Biomass-coal

Co-Combustion

Gengda Li, Shuiqing Li, Xiaoguang Xu, Ming Dong, Qiang YAO

Key lab of Thermal science and power engineering, MOE Department of Thermal Engineering,

Tsinghua University, Beijing

Symposium on Global Energy Future Oct.1-5,2010Washington University in St.Louis,


• 1. Backgrounds

• Experimental

• Results and discussion

• Conclusion remarks

Outline


Biomass fuels near zero CO2 emissions


Biomass boilers

nation Plants Capacity Type Steam Tem.Operation

DEN Rudkobing 10MW Grate 450 ℃ Pure

USA Wood land 25MW FBC 513 ℃ Pure

DEN Slagelse 30MW Grate ~510℃ Pure

FIN Alholmens 550MW CFB 545 ℃ Co-firing

SWD Idbacken 100MW CFB 540 ℃ Co-firing

UK Drax 6×660MW PC 570 ℃ Co-firing

Serious Ash deposit problems: (Particularly) Wheat straw and other herbaceous fuels rich in alkali and chlorine

Reference：Sjaak et al，2003；Michelsen et al，1996；Nielsen HP et al，1998；Thomas RM et al，1995；Li D.K.，2008；

Investigator Sampling Temperature

Flemming,2005 400~550 ℃

Baxter,2001,2005 400~500 ℃

Interest 1：high temperature

for ash deposit sampling is

needed in next-step work


J.K. Zhuo, S.Q. Li & Q.Yao, 2009, Proc. Combust. Inst.

Interest 2：the relation between the ash particle deposition and PM formation mode (ultra-fine, intermediate or coarse ones)


Different conclusions were drawn by:

Dayton et al (1999);

Wei et al.(2002);

Robinson & Baxter (2002)

1. The effect of Alkali-aluminum silicate: two-folds

Interest 3：key factors of ash deposition on a probe

Alkali elements, particularly potassium, releasedfrom the straw may easily form fine particulate orvapors, then collide and react with the silicatesfrom the coal and finally create alkali silicatesand alkali-aluminum silicates

2. The effect due to the Sulfation of Alkali chloride

1) Condensation and reaction mechanisms;

2) Stickiness of ash layer

Two kinds of coals, YK and WN, are selected for co-firing studies!

d C RIgm T= + + +


• Backgrounds

• 2. Experimental



Outline


25 kWHeight: 3.6m;Diameter: 150mmSecondary air: 380℃

2.1 One-dimensional down-fired combustor


Temperature profiles (steady-state)

1140.5

974

871

785.5

471.5

0

500

1000

1500

2000

2500

3000

3500

400 600 800 1000 1200

Hei

ght,

mm

Tempt. ℃


2.2 Ash deposit sample sytem

structure

Deposit

Baxter (1993);

Aho (2005)

X.G. Xu (2008)


2.3 potassium liquor spraying system –controlling K content in gas

Picture of metering pump Schematic picture of spraying process


2.3 Fuel samples

SiO2 Al2O3 Fe2O3 CaO M gO TiO2 SO3 P2O5 K2O Na2O0

10203040506070

Percentage / %

Sawdust Straw Corn straw YK W N

1) K:10%

2). YK coal: Kaolinite (Al-silicate)

3). WN coal: High S


2.4 Definition of collection efficiency

d p

f a r

m A

m X Aδ =

——deposition rate，g/h

——Fuel feeding rate, g/h

——Ash content in fuels，%；

——projected area of sampling probe，mm2

——Cross-sectional area of combusto，mm2

dm

fm

aX

pA

rA

Fly ash

detachment

deposition Probe

Gη=

2

9p p p

g c

d VStk

dρµ

=

1 2

3 1

( ) [1 ( ) ( )( ) ]

Stk b Stk a c Stk ad Stk a

η − −

− −

≅ + − − −

+ −

Baxter (1993); Lokare (2006)


• Backgrounds

• Experimental

• 3.Results and discussion


Outline


3.1.1 Effect of sampling tube wall temperature on the ash deposition

550℃ 600℃ 650℃0

2

4

6

capt

ure

effic

ienc

y / %

wheat straw corn straw

550℃ 600℃ 650℃0.00

0.05

0.10

0.15

0.20

0.25

Depo

sitio

n m

ass

/ g


550℃ 600℃ 650℃0

2

4

6

Col

lect

ion

effic

ienc

y /%



3.1.1 Effect of sampling tube wall temperature on the ash deposition

550℃ 600℃ 650℃0

100

200

300

400

500

Deposition area /m

m2

Surface temperature / ℃


550℃ 600℃ 650℃0

1

2

3

4

5

Deposition height /m

m

Surface temperature / ℃



3.1.2 Effect of sampling time on ash deposition

0 30 60 90 120 150 180 210 240 2700.00

0.08

0.16

0.24

0.32

Deposition mass /g

Sampling time /min

sawdust

wheat straw

corn straw

mg/min sawdust wheat corn0-30min 0.46 2.62 3.02

30-60min 0.31 0.27 0.28

60-120min 0.57 2.21 3.03

120-240min 0.56 0.73 0.19

Collection efficiency: herbage fuel >

woodiness fuel.

Collection efficiency: wheat straw > corn

straw.

In 4hs, the deposition rate shows trends of “fast-slow-fast-slow”

0 30 60 90 120 150 180 210 240 2700

1

2

3

4

5

Collection efficiency / %

Sampling time /min

sawdust

wheat straw

corn straw


3.1.2 Effect of sampling time on ash deposition

30min 60min 120min240min0

1

2

3

4

Deposition height

/mm

sawdust wheat straw corn straw

30min 60min 120min240min0

200

400

600

800

Deposition area /m

m2

sawdust

wheat straw

corn straw


3.1.3 Enrichment of Ash-forming Elements in Deposition

0- 30min：more K contained in ash deposition

30-240min：

1. K concentration increased as time goes

in sawdust and corn deposition，

2. K concentration decreased as time goes

in wheat deposition ；

Si/10 Al K Na Ca Mg S P Fe0

5

10

15

20

25

The mass in deposition /

% 30m in 60m in 120m in 240m in

wheat


5

10

15

20

25

The

mass in deposition /

% 30m in 60m in 120m in 240m in

corn


5

10

15

20

25

The

mass

in

depo

siti

on /

% 30m in 60m in 120m in 240m in

sawdust


3.2.1 Effect of potassium(K) content in gas on ash deposition

K content has great effect on the collection efficiency; as more potassium Contained, higher collection efficiency can be got.


3.2.2 Effect of K on deposition area and height


3.2.3 SEM images to present K effect (saw dust as example)


3.3.4 Detailed infomations from SEM/EDX results

saw dust + 10K

More sintering bond K

Enhance impaction

mechanism

More fine mode K

Enhance condensation and

nucleation mechanism


As for combustion of unblended fuels, the collection efficiency of wheat straw (ST) is about 1.5 times as much as that of YK, and about 2.7 times of WN.

As for co-firing, the deposit mass increases an increasing mixing ratio of biomass from 25 to 75% (ash basis);

3.3.1 Deposit mass and Collection efficiency


1. When co-firing ST and WN, the measured collection efficiencies are obviously higher than the predictions. low-kaolinite content of WN coal is not enough to capture alkali metal.

2. When co-firing ST and YK, neither apparently enhanced nor inhibited.

Different to previous work. Ig increases with Tempt. (600)

while T/C/R decreases

d C RIgm T= + + +

Pure summation model without straw-coal interaction

3.3.1 Deposit mass and Collection efficiency


3.3.2 Deposit MorphologyStraw /YK coal Shadow effect

Ash particles from pure Straw combustion tend to deposit a narrow region (shadow eff.); Ash particles from YK coal nearly deposit on the whole upstream surface.

Straw

YK


3.3.2 Deposit MorphologyStraw /WN coal

WN

WN vs. YK

Deposit Area: narrow , Deposit height: small


3.3.3 Enrichment of Ash-forming Elements in Deposition

Straw /YK coal

The apparent enrichment of K in ash deposit further verifies the formation of alkali-aluminum silicates in the deposited ash layers.


Straw /WN coal

The apparent enrichment of K in ash deposit is not found in these cases


Alkali-aluminum Silicate

Alkali-aluminum Silicate

Adhesive role by fine particles: Alkali Al-silicate vs. Al-silicate

3.3.4 SEM pictures—— Straw /YK coal


3.3.4 SEM pictures—— Straw /YK coal

23

41

23

41

Na Mg Al Si S K Ca Fe0

20

40

60

80

100

质量分数 / % (a).1


20

40

60

80

100

质量分数 / % (a).2


20406080

100

质量

分数

/ % (a).3


20

40

60

80

100

质量

分数

/ % (a).4

11


20

40

60

80

100

质量分数 / % (b).1

Alkali-chlorides

Calcium carbonate/sulfate

Adhesive role by fine particles: different


• Backgrounds

• Experimental


• 4. Conclusion remarks

Outline


4. Conclusion• Temperature effect: leads to the competition of nucleation mechanism and

impaction mechanism; As temperature goes up, nucleation is inhibited and impaction is enhanced.

• In the initial time of ash deposition, the depositing rate has trend of “fast-slow-fast-slow”;

• Potassium has great effect on ash deposition, it enhances both nucleation and impaction mechanisms

• The high-kaolinite content in YK captures alkali from straw and forms stickyalkali-aluminum silicates. The ash deposition during co-firing is in a balance between

inhibiting the condensation/reaction mechanism and enhancing the inertial-impactmechanism.

• When co-firing ST with WN, the collection efficiencies are higher than thepredictions by “rule of mixtures” for all ratios. The low-kaolinite content of WNcannot effectively capture alkali metals from straw and counteract thecondensation/reaction mechanism. The effect of sulfation of alkali chlorides onash layers in a high-sulfur environment can be ignored.


Acknowledge• Supported from the Consortium for

Clean Coal Utilization funded collaborative Project:” Air-Fired and Oxy-Combustion of Coal and Biomass”

• Supported from the national Natural Science Foundation


Thanks!

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