Experimental research on two-stage desulfurization technology in traveling grate boilers

Energy 26 (2001) 759–774www.elsevier.com/locate/energy

Experimental research on two-stage desulfurizationtechnology in traveling grate boilers

Junhu Zhou, Jun Cheng*, Xinyu Cao, Jianzhong Liu, Xiang Zhao, ZhenyuHuang, Kefa Cen

Clean Energy and Environment Engineering Key Lab of Ministry of Education, Zhejiang University, Hangzhou310027, People’s Republic of China

Received 4 April 2000

Abstract

In order to promote the desulfurization efficiency of calcium-based sorbents during coal combustion intraveling grate boilers, the influences on sulfur removal of the thermal conditions and the sorbents werediscussed in this paper. It was found that the SO2 concentration first rises, then declines along the travelinggrate and reaches the peak near the midpoint of the grate. The fluctuation of the SO2 concentration overtime in the flue gas is mainly affected by the flame temperature. When the particle size of the sorbentsdecreases from 75 to 0.1µm, the sulfur removal efficiency will increase slightly. A reasonable Ca/S molarratio is about 2 when sorbents are blended with the coal on the grate and its further increase has littlebenefit to desulfurization. A new, so-called two-stage desulfurization process — sulfur capture firstly inthe coal bed and secondly in the combustion gas — is suggested as it can greatly promote the sulfurremoval efficiency up to 70�80%. By X-ray powder diffraction analysis, some thermal stable phases wereidentified in the sulfur retention cinder obtained from the on-grate process. 2001 Published by ElsevierScience Ltd.

1. Introduction

Desulfurization during coal combustion is an interesting subject for industrial boiler operators,because of the low capital and operating costs in contrast with various kinds of gas desulfurization(FGD) technologies [1]. However, desulfurization technology in furnaces has been very difficultto commercially popularize in the field of engineering because of the low sulfur removal efficiencywhich has not been improved upon in a long time.

* Corresponding author. Fax:+86-571-795-1616.E-mail address: [email protected] (J. Cheng).

0360-5442/01/$ - see front matter 2001 Published by Elsevier Science Ltd.PII: S0360-5442(01)00035-4

760 J. Zhou et al. / Energy 26 (2001) 759–774

For conventional grate furnaces, when ordinary calcium-based sorbents (CaCO3, CaO, etc.) aredirectly injected into the combustion gas, the sulfur removal efficiencies are only 40�60% [2,3].When sorbents are physically blended with the dispersed coal, the sulfur removal efficienciesduring coal combustion are generally below 40% [2,4,5]. In recent years, a coal briquette,especially biobriquette which is produced from a mixture of coal and biomass under high-com-pression pressure, has been developed as a technique for coal combustion in small boilers andstoves which can limit the dust emission to some extent. It is found that calcium hydroxide hasbetter desulfurization capabilities than does limestone because of its lower calcination temperature,as does scallop shell because of its larger porosity after calcination [6]. The desulfurizationefficiency of the biobriquette is not sensitive to the temperatures ranging from 700 to 900°C, butit is strongly affected by coal type and changes from 25 to 67% for eight different experimentalcoals under the same conditions [7].

For pulverized coal fired boilers, the furnace sorbent injection process (FSI) can yield a SO2

reduction of 30–40% by injecting powdered sorbents into the upper furnace at Ca/S=2. In alimestone injection multi-stage burner process (LIMB), limestone added to the periphery of a“ low NOx” burner typically reduces SO2 emissions by 30–50% when Ca/S=2 [2]. A LIFAC pro-cess (limestone injection into the furnace and activation of unreacted calcium) has been developedby Tempella Company in Finland and it gives a SO2 reduction of about 70% [8]. Haojie et al.[9] investigated a combined process composed of furnace limestone injection, in-duct humidifi-cation and bag-filter capture in a pilot-scale pulverized coal fired furnace which can yield a SO2

reduction of about 80%. It should be noted that even for the same in-furnace technology, differ-ences in SO2 removal efficiencies among pilot facilities probably occur due to variations in thefollowing parameters: quench rate of the combustion gas, temperature of the injection location,residence time and mixing, and sorbent properties [10].

It was reported [11,12] that the use of alkali metal compounds such as NaCl, KCl as promotersof furnace SO2 capture can produce large cracks on the surfaces of sorbent particles and reducesorbent particle sizes. The extent of crack formation and reduction in particle size is found toincrease with increasing cationic radius. The transition metal compounds such as Fe2O3, Cr2O3

can also promote the sulfation reactions. Desal et al. [13] reported that Fe2O3 catalyzes the conver-sion of SO2 to SO3, which is a rate-limiting step in the sulfation process. Slaughter et al. [12]found that Cr2O3 reacts with CaO to form a low-melting eutectic mixture which would be a liquidat temperatures above 1000°C. The ability of Cr2O3 to promote the reaction of CaO with SO2 isapparently related to the more rapid diffusion through this liquid phase. In addition, the use ofcalcium lignosulfonate or ethanol–water solutions during the CaO hydration process can producean increase in SO2 retention, due mainly to the decrease in the particle size and the modificationin the porous structure of the calcium hydroxide [14].

It is now generally recognized that smaller sorbent particles result in higher SO2 removal andCaO conversion. In the pilot or full scale tests of dry furnace injection, the limestone particlesize is in the range of 5–100 µm and the SO2 removal efficiency is about 50%. The grinding costand destruction of pore volume, if sorbents are ground too fine, dictate a minimum mean diameterof approximately 5 µm. [15]. Naiyi and Scaroni [16] investigated eight geologically dissimilar,commercially available calcium-based sorbents with particle sizes of 37–105 µm. It was foundthat the fragmentation behavior is a function of sorbent type, particle size and particle temperatureover the range of 600–1600°C, with the sorbent type being the dominant parameter. Dolostones

761J. Zhou et al. / Energy 26 (2001) 759–774

are more susceptible to breakage than limestones. It was reported that decreasing the mass meandiameter of the limestone particles from 10 to 1 µm increases the SO2 capture from 40 to 50%at Ca/S=2 [10]. Masayoshi et al. [17] investigated the reaction rate of ultrafine CaO particles (dp�0.1 µm) with SO2 at temperatures of 700–1000°C in a tube heater and found that it is5×102–5×103 times higher than that of other CaO particles (dp�1 µm) used in the conventionaldry processes. However, it is speculated that diffusion resistances within the particle are eliminatedat particle sizes of 1–2 µm, and further size reductions offer no benefit [10,18].

During coal combustion, not all of the sulfur in coal is emitted as gases because of the sulfur-retaining property of the coal ash. Yan et al. [19] investigated the behaviors of ten minor coalelements during coal combustion in the temperature range of 127–1727°C. It was found that Al,Ca, Fe, K, Mn and Na sulfates are dominant at low temperatures under oxidizing conditions,whereas under reducing conditions most of them are sulfides. Changdong et al. [20] reported thatcalcium plays a dominant role in sulfur retention of laboratory-prepared ash and the sulfur reten-tion percentage increases with the increase in Ca/S molar ratio of the parent coal, while thecontributions of other elements are limited. However, in a pulverized coal-fired combustor(PCFC), the contribution of calcium is reduced markedly, while the roles of other alkalineelements are obviously enhanced. The sulfur retention efficiency of the coal ash in PCFC isgenerally below 25%.

In fact, the effects of sorbents on sulfur removal performance during coal combustion stronglydepend on the thermal conditions, especially the furnace temperature. If only the chemical compo-sitions or physical characteristics of sorbents are changed, the ability to promote sulfur removalefficiency above 1200°C is very limited. It was pointed out that the poor contacts between sizedcoal and pulverized sorbents, as well as the oxidation of sulfur retention product CaS, are thekey factors limiting the sulfur retention efficiency in a stoker fuel bed [5]. Thus, in order topromote the sulfur removal efficiency, it is essential to study not only the thermodynamics andkinetics of the desulfurization reactions, but also the influences of thermal conditions in the fur-nace. In this paper, the influences on sulfur removal of the thermal conditions and the sorbentswere discussed. A new, so-called two-stage desulfurization process — sulfur capture firstly in thecoal bed and secondly in the combustion gas — was put forward and it can greatly promote thesulfur removal efficiency to 70�80% in the traveling grate furnace.

2. Experiments

2.1. Materials

Xinwen coal, Baoji coal and a blended coal (composed of Yima coal and Wannan coal witha weight ratio of 4:1) were used in the experiments. The heating value and sulfur content of thecoals are shown in Table 1.

Natural limestone, lightweight CaCO3, ultrafine CaCO3, CaO and calcium carbide residue wereused as the sorbents for sulfur removal. The particle size and source of the pulverized sorbentsare shown in Table 2.


Table 1Analysis of experimental coals and the self-desulfurization efficiency in the traveling grate boiler

Coal sample Heating value Qnet,ar Sulfur content St,ar Theoretical sulfur Self-desulfurization(kJ/kg) (%) dioxide efficiency h0 (%)

concentration in theflue gas SO0

2 (ppm)

Blended coal 25,080 1.20 872 16.0(Yima:Wannan=4:1)Xinwen coal 25,080 0.85 617 8.30Baoji coal 22,990 0.45 354 11.97

Table 2The particle size and source of the pulverized sorbents

Pulverized sorbents Particle size (µm) Source of the sorbents

Natural limestone 75 Pulverized natural limestoneLightweight CaCO3 38 Firstly the calcined limestone is hydrolyzed to form

Ca(OH)2 solution; secondly CO2 gas is infused into thesolution and then reacts with Ca(OH)2; thirdly thedeposition out of the solution is dried to form lightweightCaCO3

Ultrafine CaCO3 0.1 Precisely grinded natural limestoneCaO 38 Calcined limestone at high temperatureCalcium carbide residue 150 The residue remained in the procedure of producing PVC

with calcium carbide

2.2. Experimental apparatus and method

The pilot-scale experiments for sulfur removal were conducted in a 0.5 t/h traveling grate boiler,as shown in Fig. 1. The flue gas compositions of O2, CO2, CO, SO2 and NO were measured bya portable MSI-2000 multi-function combustion analyzer. The flame temperature was measuredby an IR-QIT bi-color pyrometer. The coal bed temperature was measured by three ϕ0.5 mmplatinrhodium–platinum thermocouples embedded in the coal bed. The three thermocouples fixedon an iron support were placed 6, 45 and 85 mm, respectively, above the grate surface, travelingalong with the grate, as shown in Fig. 2. The signal of electronic potential difference producedby the thermocouples was recorded by a computer and the dynamic curve of the coal bed tempera-ture was synchronically displayed on the screen.

Five different approaches by which the sorbents were added into the coal-fired traveling-grateboiler were compared in the experiments. (1) The blending approach had the sorbents mechan-ically blended with the dispersed coal in a mixer before entering the furnace. (2) The injectingapproach had the sorbents directly injected into the combustion gas through a screw feeder. (3)The combination of blending and injecting approaches (B&I) had 50 wt% sorbents blended withthe dispersed coal and the rest directly injected into the combustion gas. (4) The combination of


Fig. 1. Schematic of the 0.5 t/h traveling grate boiler.

Fig. 2. Schematic of the thermocouples fixed on an iron support.

blending and puffing approaches (B&P) had all of the sorbents blended with the dispersed coalin advance and some of them puffed into the combustion gas from the coal bed during combustionby two air fans symmetrically installed outside the two walls of the heating zone. (5) The combi-nation of covering and puffing approaches (C&P) had all of the sorbents evenly covering the coalbed surface by a reformed feeder and some of them puffed into the combustion gas from the coalbed surface during combustion by two bundles of air. The last three combination approaches areall termed as two-stage desulfurization technology during coal combustion.


3. Results and discussions

The self-desulfurization efficiencies of the experimental coals without adding sorbents areshown in Table 1. A series of comparative experiments for sulfur removal conducted in thetraveling grate boiler are shown in Tables 3 and 4.

3.1. The dynamic coal bed temperature

The coal bed temperature was measured according to the Lagrangian method. A cubic pieceof as-fired coal was considered as a volume unit for research. Assuming that the coal bed tempera-ture was uniform in the cross direction of the grate, the two-dimensional bed temperature in thetraveling direction of the grate and vertical direction of the coal bed was studied. The dynamiccoal bed temperature as a function of experimental time and height of the coal bed, when Xinwencoal was blended with limestone, is shown in Fig. 3. It is indicated that in the initial heatingstage the coal bed temperature gradually increases as the experimental time passes and the bedheight rises; about 25 min later, the whole bed temperature from the surface to the bottom turnsalmost uniform at about 1200°C; in the last burnout stage the coal bed temperature slightly fluctu-ates in a narrow range of about ±100°C. The coal bed gradually burns out from the surface tothe bottom and the stratified combustion phenomenon is distinct, as is that reported by Zhonghuand Yongzhao [21].

3.2. Distribution of the SO2 concentration along the traveling grate

The SO2 concentration along the traveling grate was measured by a water-cooled combustiongas extraction probe which was constructed of stainless-steel. It was successively placed at severalpoints on the coal bed surface along the traveling grate, which were 1000, 1400, 1800, 2200 and2600 mm, respectively, from the coal feeder. The gas samples collected by the probe were tempor-arily preserved in small ballonets and then analyzed by a pulsed fluorescent SO2 analyzer producedby Thermo Environmental Instruments Inc. The experimental coal was a blended coal (composedof Baoji coal and Changguang coal with a weight ratio of 3:1) with a total sulfur content of1.23%. As shown in Fig. 4, the SO2 concentration firstly rises and then declines along the travelinggrate. It reaches the peak near the midpoint of the grate where the coal violently burns. Whenthe feed coal is blended with some sorbents (composed of limestone, calcium carbide residualand Na2SiO3 with a molar ratio of Ca/S=2), the distribution of the SO2 concentration is quitesimilar to that without sorbents, except that the former SO2 concentration value is correspondinglylower than the latter. It is found that some SO2 gas has been adsorbed by the sorbents in thecoal bed.

3.3. The dynamic flue gas compositions

It is found that there is a close relationship between SO2 and CO concentration in the flue gas,as shown in Fig. 5. The SO2 concentration will decrease when CO concentration increases, andvice visa. This may be the result of the fact that the higher CO concentration in the flue gas, theless sufficiently coal burns in the furnace and the less sulfur is released during coal combustion.


Tab

le3

The

sulf

urre

mov

alef

ficie

ncy

ofso

rben

tsph

ysic

ally

blen

ded

with

the

disp

erse

dco

al

Con

ditio

nsN

o.1

No.

2N

o.3

No.

4N

o.5

No.

6N

o.7

No.

8

Coa

lsa

mpl

eX

inw

enco

alSo

rben

tsN

oN

atur

alL

ight

wei

ght

Ultr

afine

Cal

cium

Cal

cium

CaO

CaO

limes

tone

CaC

O3

CaC

O3

carb

ide

carb

ide

resi

due

resi

due

Ca/

S/

22

22

66

8.5

Add

ing

appr

oach

/B

lend

ing

Stea

mlo

ad(t

/h)

0.42

0.52

0.36

0.36

0.23

0.47

0.46

0.33

Stea

mpr

essu

re(M

Pa)

1.32

1.08

1.26

1.31

�1.

62–

–Fl

ame

tem

pera

ture

1370

1367

1370

1387

1332

1333

1298

1261

(°C

)T

rans

form

edSO

256

650

849

647

443

743

040

138

8co

ncen

trat

ion

ata

=1.4

(ppm

)Su

lfur

rem

oval

010

.25

12.3

716

.25

22.7

924

.03

29.1

531

.45

effic

ienc

y(%

)


Tab

le4

The

sulf

urre

mov

alef

ficie

ncy

oftw

o-st

age

desu

lfur

izat

ion

tech

nolo

gy

Con

ditio

nsN

o.9

No.

10N

o.11

No.

12N

o.13

No.

14N

o.15

Coa

lsa

mpl

eB

lend

edco

al(Y

ima:

Wan

nan=

4:1)

Bao

jico

alSo

rben

tsN

oC

aOC

aOC

aON

oU

ltrafi

neU

ltrafi

neC

aCO

3C

aCO

3

Ca/

S/

22

4/

44

Add

ing

appr

oach

/B

lend

ing

Inje

ctin

gB

&I

/C

&P

B&

PSt

eam

load

(t/h

)0.

330.

290.

290.

400.

650.

42–

Stea

mpr

essu

re(M

Pa)

1.14

1.33

1.34

1.37

1.52

1.17

1.35

Flam

ete

mpe

ratu

re(°

C)

1407

1428

1308

1443

1305

1053

1010

SO2

conc

entr

atio

nas

α=1.

4(p

pm)

732

537

317

194

311

7389

Sulf

urre

mov

alef

ficie

ncy

(%)

026

.64

56.6

973

.50

076

.53

71.3

8


Fig. 3. The dynamic coal bed temperature as a function of experimental time and height of the coal bed.

Fig. 4. Distribution of the SO2 concentration along the traveling grate.


Fig. 5. The relationship between SO2 and CO concentration in the flue gas.

3.4. Effects of the furnace temperature on the SO2 concentration in the flue gas

The temperatures close above the coal bed, on the coal bed surface and in the coal bed wererespectively measured every 20 min by a water-cooled platinrhodium–platinum thermocoupleprobe which was placed near the midpoint of the grate. The flame temperature was measured bya bi-color pyrometer. The temperatures of the furnace exhaust gas and the economizer exhaustgas were measured by two nickel chromium–nickel silicon thermocouples. A blended coal(composed of Yanzhou coal and Changguang coal with a weight ratio of 2:1) with a total sulfurcontent of 1.54% was used in the experiments, which was blended with some sorbents (composedof white mud and calcium carbide residual with a molar ratio of Ca/S=2). The effects of thefurnace temperature on the SO2 concentration in the flue gas are shown in Fig. 6. It is found thatthe flame temperature is the highest followed by the temperature in the coal bed. The change ofthe temperature close above the coal bed is nearly the same as that on the coal bed surface, whileboth temperatures remain low. It is implied that in the vertical direction the temperature of thecoal bed firstly rises and then decreases, as does the temperature of the combustion gas abovethe coal bed. The change of temperature in the vertical direction is consistent with that of CO2

concentration reported in the classical literature on grate furnaces [21]. However, the change inthe temperature versus time, whether in the coal bed, close above the coal bed or on the coal bedsurface, is quite different from that of the SO2 concentration in the flue gas. Thus it is believed


Fig. 6. Effects of the furnace temperature on the SO2 concentration in the flue gas.

that there is no close relation between the three temperatures and the SO2 concentration in theflue gas. Meanwhile, it should be noted that the change in the SO2 concentration versus time inthe flue gas is quite similar to the changes of temperature of the flame, the furnace exhaust gasand the economizer exhaust gas. It is known that the temperature changes of the furnace exhaustgas and the economizer exhaust gas follow the change of the flame temperature, so it can beconcluded that the fluctuation of the SO2 concentration versus time in the flue gas is mainlyaffected by the flame temperature. Sage and Ford reported that the combustion conditions in thecoal bed are not conducive to efficient sulfur capture and more suitable conditions for sulfurcapture exist in the combustion gas above the coal bed [2]. Furthermore, considering the in-furnace technologies for sulfur removal, it can be inferred that injecting sorbents into the furnace


to react with SO2 in the combustion gas is more beneficial than blending sorbents with the feedcoal on the grate.

3.5. Effects on desulfurization of particle size of sorbents

The sulfur removal efficiency of natural limestone, lightweight CaCO3 and ultrafine CaCO3

added in the same proportion of 7 wt% was respectively 10.25, 12.37 and 16.25%, as shown inTable 3. When the particle size of the sorbents decreased from 75 to 0.1 µm, the sulfur removalefficiency slightly increased in a limited range of 6%. This result agrees with previous work whichhas shown that a decrease in particle size below 1–2 µm is believed to have a limited effect onsulfur removal [15].

3.6. Effects on desulfurization of adding proportion of sorbents

Table 3 shows that the sulfur removal efficiency of the blended calcium carbide residue onlyincreased from 22.79 to 24.03% when the mole ratio of Ca/S increased from 2 to 6. In addition,the sulfur removal efficiency of the blended CaO only increased from 29.15 to 31.45% when themole ratio of Ca/S increased from 6 to 8.5. It was found that when calcium-based sorbents areblended with the coal on the grate, a reasonable Ca/S molar ratio is 2–2.5, and if the ratio ismore than 4, its increase will hardly promote the desulfurization efficiency and may have harmfuleffects on the combustion condition. These findings are consistent with those reported by Guoqinget al. [7]

3.7. Effects on desulfurization of adding approach of sorbents

On the basis of many experiments, it was found that the most important factor causing the lowsulfur removal efficiency of the blended sorbents in the traveling grate furnace should be thethermal instability of the sulfation product CaSO4 at high temperature (above 1200°C) during along residence period (about 1 h) in the coal bed. Analyzing the limitation of the single blendingor injecting approaches, we put forward a so-called two-stage desulfurization technology, includ-ing three approaches of B&I, B&P and C&P. Some of the sorbents were added into the dispersedcoal before entering the furnace and a portion of the sorbents were injected from a nozzle outsideor puffed from the coal bed into the combustion gas. The SO2 gas that was released during coalcombustion and not captured by the sorbents in the coal bed would react further with the sorbentsin the combustion gas. Thus the reaction time and temperature necessary for desulfurization wereapplied and the sulfur removal efficiency was greatly promoted.

When CaO was blended with the dispersed coal or injected from a nozzle into the combustiongas, the sulfur removal efficiency was 26.6 and 56.7%, respectively. The sulfur removalefficiencies of the on-grate process and the in-furnace process are consistent with those reportedby Sage and Ford [2] and Muzio and Offen [10]. However, by the approach of B&I, the sulfurremoval efficiency of CaO was greatly promoted to 73.5%, as shown in Table 4. It was foundthat the sulfur removal efficiency of the two-stage desulfurization technology nearly equals theefficiency obtained by the blended sorbents in the coal bed plus that obtained by the injected


sorbents in the combustion gas. This finding is valuable in developing high-efficiency and low-cost desulfurization technology during coal combustion.

It has been learned from condition nos 4, 14 and 15 that the sulfur removal efficiency ofultrafine CaCO3 added using three approaches — single blending, C&P and B&P — was 16.25,76.53 and 71.38%, respectively, when added in the same proportion of 7 wt%. During the processof C&P and B&P, some of the desulfurization reactions were transferred to the combustion gaswhile others continued in the coal bed. Thus the problems of the sulfation product CaSO4 beingdifficult to produce and easily decomposed in the coal bed were solved. Using two-stage desulfur-ization technology the sulfur removal efficiency can be greatly promoted, but whether the combus-tion efficiency is promoted or not depends on the structure of the flow field in the furnace. If theflow field is not well organized, the flame temperature will decline and the burnout efficiency ofthe coal will drop; these effects were noticed especially when the C&P or B&P approach wasapplied. How to efficiently control the flow field needs further research. However, it has beenproved by experiments that the B&I approach not only achieves a high sulfur removal efficiencyof about 75%, but also there is little negative influence on the combustion condition. Althoughthe injection process increases the dust burden and has an impact on the particulate collectionequipment, the B&I approach is still very competitive when applied to small and medium indus-trial coal-fired boilers, introducing high-efficiency and low-cost desulfurization technology in fur-naces.

3.8. X-ray powder diffraction analysis of the sulfur retention cinder

The crystal phases of the sulfur retention cinder obtained from the pilot-scale experiments wereanalyzed by the X-ray powder diffraction (XRD) technique, as shown in Fig. 7. The experimentalcoal was a blended coal (composed of Baoji coal and Changguang coal with a weight ratio of3:1) with a total sulfur content of 1.23%, which was blended with some sorbents (composed oflimestone and Na2SiO3 with a molar ratio of Ca/S=2). The flame temperature was about 1325°Cand the SO2 concentration at the air excess coefficient a=1.4 in the flue gas was 525 ppm. It wasfound that the crystal phases of the cinder consist of Ca5(SiO4)2SO4, CaSO4, CaS,3CaO·3Al2O3·CaSO4, α-SiO2 and mullite. According to the semi-qualitative analysis, the contentsof the four crystal phases containing both sulfur and calcium elements can be arranged in orderas follows: Ca5(SiO4)2SO4�CaSO4�CaS�3CaO·3Al2O3·CaSO4. The crystal phases of the sulfurretention cinders obtained from the on-grate process are dissimilar to those from a furnace lime-stone injection process (comprising calcite, calcium oxide, anhydrite, mullite, α-SiO2, etc.)reported by Gomes et al. [22].

In terms of the powder diffraction file published by JCPDS, the characteristics of the thermalstable phases in the sulfur retention cinders are described as follows [23–25]:

1. Ca5(SiO4)2SO4 is synthesized in a lime kiln at approximately 1100°C by combination of sil-iceous lime sand feed with sulfur trioxide. It can also be prepared from CaCO3, SiO2 andCaSO4·2H2O by ignition at 1150°C for 150 h.

2. 3CaO·3Al2O3·CaSO4 is a ternary compound existing as the main constituent in sulfo-aluminousclinker used as expansive agent for the manufacture of expansive cement. It can be synthesizedfrom firing a mixture of lime, alumina and CaSO4 to 1350°C.


Fig. 7. XRD analysis of the sulfur retention cinder.

3. In general, CaSO4 decomposes at high temperatures above 1200°C, but α-CaSO4 remains ther-mally stable from 1210°C up to melting point at 1495°C. However, the kind of CaSO4 whichcan be produced during clean coal combustion needs further research.

4. CaS can be prepared from the reaction of CaO and H2S at high temperature under reducingconditions [26]. It is known that CaS remains thermally stable at high temperatures up to2400°C [2].

The identification of the thermally stable phases in the sulfur retention cinder is beneficial tofurther improve the in-furnace desulfurization technologies. But how to prepare suitable conditionsto promote the production of the special phases during coal combustion needs further research.

4. Conclusions

The most important factor causing the low sulfur removal efficiency of the blended sorbentsin the traveling grate furnace is that the sulfation product CaSO4 is difficult to produce or easyto decompose at high temperatures (above 1200°C) during a long residence period (about 1 h)in the coal bed. No matter whether the sorbents are physically blended with the dispersed coalor directly injected into the furnace, the sulfur removal efficiency of the ordinary calcium-basedsorbents is generally below 40%.

The thermal conditions in the grate furnace were investigated and it was found that the coalbed gradually burns out from the surface to the bottom, and the stratified combustion phenomenon


is distinct. The SO2 concentration firstly rises, then declines along the traveling grate and it reachesa peak near the midpoint of the grate where the coal violently burns. The fluctuation of the SO2

concentration with time in the flue gas is mainly affected by the flame temperature. Injectingsorbents into the furnace to react with SO2 in the combustion gas is more beneficial than blendingsorbents with the feed coal on the grate.

When the particle size of the sorbents decreases from 75 to 0.1 µm, the sulfur removal efficiencyslightly increases in a limited range. A reasonable Ca/S molar ratio is about 2 when sorbents areblended with the coal on the grate and its further increase has little benefit to desulfurization. Anew so-called two-stage desulfurization technology was put forward, including three approachesof B&I, B&P and C&P. Through the two-stage desulfurization process — sulfur capture firstlyin the coal bed and secondly in the combustion gas — the sulfur removal efficiency can be greatlypromoted to 70�80% during coal combustion in the traveling grate furnace.

By XRD analysis, it was found that the crystal phases of the sulfur retention cinder from theon-grate process are mainly composed of Ca5(SiO4)2SO4, CaSO4, CaS, 3CaO·3Al2O3·CaSO4, α-SiO2 and mullite. Identification of the thermally stable phases in the sulfur retention cinder isbeneficial to further improve the in-furnace desulfurization technologies.

Acknowledgements

Subsidized by the Special Funds for Major State Basic Research Projects (G1999022204) andsupported by the Foundation for University Key Teachers by the Ministry of Education.

References

[1] Maibodi M. Implications of the Clean Air Act acid rain title on industrial boilers. Environ Prog 1991;10(4):307–13.[2] Sage PW, Ford NWJ. Review of sorbent injection process for low-cost sulphur dioxide control. Proc Inst Mech

Eng A: J Power Energy 1996;210(3):183–90.[3] Holder RG, Milner CN, Minchener AJ. SO2 control on stoker-fired industrial boilers. In: Institution of Chemical

Engineers Symposium series no. 123. Rugby, (UK): Institute of Chemical Engineers. p. 227–237.[4] Ford NWJ, Cooke MJ, Sage PW, Gibbs BM. SO2 emissions control by on-grate sorbent addition on industrial

stoker-fired plant. Process safety and environmental protection. Trans Inst Chem Eng B 1995;73(1):59–69.[5] Ford NWJ, Cooke MJ, Gibbs BM. Control of SO2 emissions from stoker-fired coal combustion by on-grate

addition of limestone. In: Institution of Chemical Engineers Symposium series no. 123. Rugby, UK: Institute ofChemical Engineers, 97–108.

[6] Ichiro N, Heejoon K, Guoqing L, Jianwei Y, Kazutomo O. Study on characteristics of self-desulfurization and self-denitrification in biobriquette combustion. In: Proceedings of the 27th Symposium (International) on Combustion.Pittsburgh: The Combustion Institute, 1998:2973–9.

[7] Guoqing L, Heejoon K, Jianwei Y, Ichiro N, Kazutomo O, Mitsushi K. Experimental study on self-desulfurizationcharacteristics of biobriquette in combustion. Energy Fuels 1998;12(4):689–96.

[8] Ball ME, Smith DW, Koskinen JJ, Enwald TA. LIFAC-economical solution to SO2 control. CIM Bull1993;86(968):122–5.

[9] Haojie F, Guoxin W, Mingchuan Z, Qiang Y, Xinyu C, Kefa C. Study on SO2 removal during pulverized coalcombustion. In: Proceedings of the International Conference on Energy and Environment, ICEE 1998. Beijing:China Machine Press, 1998:336–41.


[10] Muzio LJ, Offen GR. Assessment of dry sorbent emission control technologies, part 1: fundamental processes.JAPCA 1987;37(5):642–54.

[11] Paolo D, Gennaro DM, Pado G. An investigation of the influence of sodium chloride on the desulphurizationproperties of limestone. Fuel 1992;71(7):831–4.

[12] Slaughter DM, Chen SL, Seeker WR, Pershing DW. Increased SO2 removal with the addition of alkali metalsand chromium to calcium-based sorbents. In: Proceedings of the 22th Symposium (International) on Combustion.Pittsburgh: The Combustion Institute, 1988:1155–64.

[13] Desal NJ, Yang RT. Catalytic fluidized-bed combustion: enhancement of sulfation of calcium oxide by iron oxide.Ind Eng Process Des Dev 1983;22(2):119–23.

[14] Juan A, Vanessa F, Francisco GL, Jose MP. Study of modified calcium hydroxides for enhancing SO2 removalduring sorbent injection in pulverized coal boilers. Fuel 1997;76(3):257–65.

[15] Zhicheng Y, Wuyin W, Qin Z, Ingemar B. High temperature desulfurization using fine sorbent particle underboiler injection conditions. Fuel 1995;74(5):743–50.

[16] Naiyi H, Scaroni AW. Fragmentation of calcium-based sorbents under high heating rate, short residence timeconditions. Fuel 1995;74(3):374–82.

[17] Masayoshi S, Takashi S, Azuchi H, Hiroshi Y, Hi JK. Removal of SO2 from flue gas using ultrafine CaO particles.J Chem Eng Japan 1994;27(4):550–2.

[18] Wei SH, Mahuli SK, Agnihotri R, Fan LS. High surface area calcium carbonate: pore structure properties andsulfation characteristics. Ind Eng Chem Res 1997;36:2141–8.

[19] Yan R, Gauthier D, Flamant G, Badie JM. Thermodynamic study of the behavior of minor coal elements andtheir affinities to sulfur during coal combustion. Fuel 1999;78:1817–29.

[20] Changdong S, Minghou X, Jun Z, Yiqian X. Comparison of sulfur retention by coal ash in different types ofcombustors. Fuel Process Technol 2000;64:1–11.

[21] Zhonghu L, Yongzhao Z. Boiler handbook. Beijing: China Machine Press, 1995.[22] Gomes S, Francois M, Pellissier C, Evrard O. Characterization and comparative study of coal combustion residuals

from a primary and additional flue gas secondary desulfurization process. Cement Concrete Res1998;28(11):1605–29.

[23] JCPDS. Powder diffraction file, sets: 16 to 18. Swarthmore, PA: Joint Committee on Power Diffraction Stan-dards, 1974.

[24] JCPDS. Powder diffraction file, sets: 25 to 26. Swarthmore, PA: International Center for Diffraction Data, 1984.[25] JCPDS. Powder diffraction file, sets: 33 to 34. Swarthmore, PA: International Center for Diffraction Data, 1989.[26] Mattisson T, Lyngfelt A. The reaction between sulphur dioxide and limestone under periodically changing oxidis-

ing and reducing conditions — the effect of reducing gases. J Inst Energy 1998;71:190–6.

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