evaluation of the co2 re activity of chars obtained under conventional and oxyfuel atmospheres

8/3/2019 Evaluation of the Co2 Re Activity of Chars Obtained Under Conventional and Oxyfuel Atmospheres

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EVALUATION OF THE CO2 REACTIVITYOF CHARS OBTAINED UNDER

CONVENTIONAL (O2 /N2) AND OXY-FUEL(O2 /CO2) ATMOSPHERES

Juliana G. Pohlmann, Eduardo Osório, Antonio C.F. Vilela, Angeles G. Borrego**

Iron and Steelmaking Laboratory, UFRGS, P.O. Box15021, 91501-970, Porto Alegre, Brazil. E-mail

address: [email protected] **National Institute of Carbon (INCAR), CSIC, P.O. Box

73, 33080, Oviedo, Spain.

ABSTRACTNew technologies have been studied to improve

pulverized coal injection (PCI) and to make ironmakingprocess suitable to environmental requirements. Oxy-fuel combustion technology consists of burning coal inan N2-free atmosphere, which is exchanged by a CO2-

rich gas. This technology has already been studiedand developed for coal-fired power plants. It allowsCO2 concentration in the flue gas and a significantportion of it could be recycled. Aiming at a possibleapplication of oxy-fuel combustion in the blast furnacecontext, chars of three typical PCI coals of differentranks were obtained under conventional (O2/N2) andoxy-fuel (O2/CO2) atmospheres in a drop tube furnace(DTF). The aim of this work was to evaluate the CO2 reactivity of these chars via thermogravimetric analysisby isothermal method at 1000ºC. Coal burnouts, BETsurface areas and the morphologies of the charsbefore gasification gave support on the results of the

reactivity analysis. The chars behavior in all tests wasstrongly influenced by coal rank. In general, the higher reactivities were observed for the lower rank coalchars. The CO2 reactivity of chars increased when coalburnouts also increased. There was no effect inburnout because of the N2 replacement (conventionalatmosphere) by CO2 (oxy-fuel atmosphere). As for theCO2 reactivity results, differences were only foundbetween the conventional and oxy-fuel chars for thehighly burnt lower rank coal chars due to the higher BET surface area of the oxy-fuel char.

1. INTRODUCTION

Pulverized coal injection (PCI) is used in blastfurnace tuyeres aiming at coke substitution andattempting to provide energy and reducing gases tothe process. In high rates of PCI, due to extremelyshort residence time available for char combustion inthe blast furnace, unburnt char will be carried from thecombustion zone to the shaft, where it will competewith coke for CO2

1). Excessive amounts of unburnt

material, leads to problems in the furnace permeability,decreasing productivity

2). Nevertheless, high rates of

PCI are desirable to reduce coke utilization and pigiron costs

3). Then, an increase in the injection rate

must be related to higher combustion efficiency to

avoid an increase in unburnt char. Coal ranging fromhigh to low rank, different injection lances design,different blast conditions (flame temperature and

stoichiometry oxygen ratio) and the injection of other fuelshave been studied to improve combustion conditions

4,5).

There is international concern about substantialdecrease in greenhouse gases emissions in ironmaking

6).

Increasing combustion efficiency and unburnt char consumption via solution loss reaction in the blast furnaceshaft is important since it could lead to a decrease in fuelrate, increasing blast furnace productivity and releasing

less CO2 to atmosphere. In this way, the optimization of PCI could decrease the environmental impact caused byironmaking industries.

Oxy-fuel combustion technology consists of burning coalin an N2-free atmosphere and oxygen is usually diluted byrecycled flue gas rich in CO2

7). The characteristics of oxy-

fuel combustion differ from conventional combustion inseveral aspects

8). Studies about oxy-fuel technology have

mainly assessed the feasibility and economic aspects toapplications in retrofits and new power plants. In this way,many studies have been done comparing char differencesbetween conventional (O2/N2) and oxy-fuel atmospheres(O2/CO2).

The reactivity of australian coal chars obtained under O2/N2 and O2/CO2 environments in DTF at 1400°C andoxygen concentration ranging from 3 to 30% vol. wasmeasured in a thermobalance by Rathnam and others

9).

They observed similar and higher burnouts in DTF whencoals were produced under oxy-fuel conditions. The BETsurface area of chars was also higher in the charsobtained under oxy-fuel conditions and SEM analysis of chars indicated more reacted particles in the O2/CO2 chars, because of the particle reaction of the carbon withCO2. Borrego and Alvarez

10)have studied the influence of

gas composition and oxygen concentration during thepreparation of coal chars in a drop tube furnace. The

chars were prepared under different O2/N2 and O2/CO2 ratios (ranging from 3 to 21% oxygen) in DTF at 1300°C.Coal burnouts of both series (O2/N2 and O2/CO2) werehigher for the lower rank parent coal and the conventionalchars compared to the oxy-fuel chars obtained under thesame oxygen content. The authors also observed higher maceral reflectances and surface areas in O2/CO2 charscompared to the conventional ones obtained with a similar O2 content. The reactivity evaluated in thermobalancebetween the samples obtained under conventional andoxy-fuel atmospheres at the same O2 concentration wassimilar.

The literature on char characteristics and reactivity in

relation to its behavior in power station boilers is likely toprovide a substantial basis to investigate char reactivity ina blast furnace context. This approach could be useful if oxy-fuel PCI injection may be possible. Coal would beinjected in the tuyeres with a mixture of oxygen andrecycled CO2, thus avoiding N2 and minimizing the step of concentrating CO2 in the flue gases. Therefore CO2 capturing technologies could be implemented in the steelindustry

11).

The effects of CO2 injection in blast furnace tuyeres wasstudied using mathematical models. Austin and others

12)

found that to replace some of the blast gas volume withrecycled furnace top gas and additional enriching oxygen

could lead to an increase in the fuel rate. However, Castroand others

13)indicated that CO2 injection combined with



oxygen enrichment, in certain proportions, could leadto an increase in productivity of the blast furnaceprocess.

In order to apply oxy-fuel technology in PCI, researchis being done on char combustion in conventional andoxy-fuel atmospheres. The first step

14)comprised of

chars being prepared under different O2/N2 andO2/CO2 atmospheres in a DTF, simulating different

conditions within the combustion zone. Following thefirst research step, the aim of this work was to evaluatethe CO2 reactivity of these chars by thermogravimetricanalysis, simulating the solution loss reaction out of thecombustion zone. Coal burnouts, BET surface areasand the morphologies of the chars before gasificationgave support on the results of the reactivity analysis.

2. EXPERIMENTALThe three selected coals range from high to low

volatile bituminous and they are typical of imported PCIcoals used in Brazil: Guasare (GU) (Venezuela), BlackWater (BW) and Jellinbah (JB) (Australia). The

characterization of the coals is shown in Table 1.

Table 1 Proximate, ultimate and petrographic analysisof the individual fuels

GU BW JB

Ash

% db

6.9 9.5 9.8

VM 39.6 26.5 15.9

Cfix 53.5 64.0 74.3

C

% daf

80.9 83.4 87.2

H 5.1 4.3 3.8

N 1.6 2.0 1.9

O 11.3 10.6 5.6

S 1.1 0.7 0.7

Rr % 0.63 1.02 1.56

V% volmmf

77.2 2.4 20.4

L 61.8 2.8 35.4

I 69.3 - 30.7VM = volatile matter; Rr = random vitrinite reflectance; V = vitrinite; L= liptinite; I = inertinite; db = dry basis; daf = dry-ash-free basis; vol =volume; mmf = mineral matter-free.

As shown in Table 1, the coals ash and sulfur contents are suitable to blast furnace utilization. Thecoals have low liptinite content and moderate inertinitecontent.

Coal chars were prepared in a drop tube furnace

described elsewhere10)

at 1300°C. Drop tube furnaceswere developed to simulate coal reaction in power plants, but they can approach some conditions thatprevail in the combustion zone. The gas compositionsand sequence of experiments used in this study aresummarized in Fig. 1.

The flow rate was 900 l.h-1

plus 300 l.h-1

for thefeeder, the feeding rate 1 g.min

-1and the estimated

residence time around 200 ms. Coal was injectedunder highly sub-stoichiometric conditions (2.5% O2 inN2) to produce the release of volatiles and minimizesoot generation. Coal was also injected under conditions close to combustion reaction stoichiometry

(10% O2 in N2) to represent highly burned coal. Theexperiment under 10% O2 in CO2 was conceived to

Fig. 1 Scheme of the experimental approach followed for char preparation in this study

compare conventional and oxy-fuel conditions. Thesethree samples will be referred as one step chars. The2.5% O2 chars were again fed into the reactor using 5%O2 in both N2 and CO2 to obtain a result for char combustion without any interference from the volatilesemitted. An atmosphere rich in CO2 was selected torepresent the enrichment in CO2 occurring higher up theblast furnace.

Burnout (B) was calculated using the ash tracer expression showed in Equation 1

15):

100100

1001(%)

.

×⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟ ⎠

⎞⎜⎜⎝

⎛ −⎟⎟ ⎠

⎞⎜⎜⎝

⎛

−−=

−

−

comb char

comb char

coal

coal

Ash

Ash

Ash

Ash B (1)

in which, Ash coal is the ash content in parent coal andAsh char-comb. is the ash content in the combustion residue.The char samples were embeddeb in polyester resin andpolished for petrographic examination under incidentpolarized light.

The pore surface areas of carbon from gas adsorption

isotherms were evaluated using N2 at 77 K as adsorptive.The equipment used was a Micromeritics ASAP 2020. TheBrunauer-Emmett-Teller (BET) theory was applied to theN2 adsorption data to obtain the surface area. As some of the samples contained large amounts of mineral matter with different adsorption properties than the organicfraction, the isotherms were corrected for mineral effect.

The CO2 reactivity of the chars was evaluated bythermogravimetric analysis in a STA 409 PC Luxxapparatus. A small quantity of char (8mg) washomogeneously spread on the bottom of the crucible andheated up to 1000°C under nitrogen flow (60 ml.min

-1) at a

heating rate of 30°C.mim-1

. Immediately the reactant gas

was changed to CO2 at the same flow rate and thetemperature was maintained until weight stabilization. Theweights were corrected for buoyancy effects. Thereactivity at 50% of conversion on an ash-free basis wascalculated using the expression R 50% = 1/w 0 (dw/dt )50%,with w 0 being the initial sample weight, in milligrams, and(dw/dt)50% the reaction rate at 50% of conversion, in mg.s

-

1.

3. RESULTS AND DISCUSSION

3.1 Coal burnoutsCoal burnouts obtained in the drop tube furnace at

conventional and oxy-fuel atmospheres under differentoxygen contents are showed in Fig. 2.



0

20

40

60

80

100

Char

2.5%

Char

10%

Oxy-char

10%

Ref-char

5%*

Ref-oxy

5%

Ref-oxy

10%

B

u r n o u t ( % )

GU

BW

JB

Fig. 2 Coals burnouts in conventional and oxy-fuelatmospheres at different oxygen contents

The coal burnouts decreased as the rank of the coalinceased (Fig. 2). As expected, 2.5% O2/N2 chars(lower O2 content) had the lowest burnouts

9,10). For BW

and JB chars obtained under 1 step and the with the

same O2 content (Char 10% and Oxy-char 10%), theburnouts of oxy-chars were very similar to burnouts of conventional chars. There is no agreement about atrend in burnout behavior comparing chars obtainedunder O2/N2 and O2/CO2 and same oxygen content.Different research groups have also found lower

16),

similar 9,10)

or higher 9,10)

burnouts for chars obtainedunder oxy-fuel atmospheres compared to conventionalatmosphere. Moreover, differences between researchgroups also occur due to experimental conditions suchas equipment type, residence time and temperature.

Regarding the 2 steps oxy-chars prepared under different oxygen content, the samples obtained 10% O2

(Ref-oxy 10% O2) had higher burnouts than the onesprepared under 5% O2 (Ref-oxy 5%).The burnouts of the refired-chars and refired-oxy samples which wereprepared under the same oxygen content were verysimilar: for GU at 10% O2 and BW and JB at 5% O2.The samples refired in 10% O2, independent of atmosphere, showed the higher burnouts because theyexperienced a higher oxygen content (1

ststep 2.5% O2

+ 2nd

step 10% O2).The above observations indicate that for the three

coals studied here, the most important parameter inthe combustion efficiency was the oxygen content inDTF atmosphere, independent of conventional (O2/N2)or oxy-fuel combustion conditions (O2/CO2).

3.2 Chars appearence in optical microscopeThe chars appearence in optical microscope are

showed in Fig. 3, highlighting the differences mainlydue to the coal type and oxygen content in DTF.

Accordingly to Fig. 3, the chars had a cenosphericalshape, typical of high heating rates

17). Qualitatively, it

can be observed that as the rank of the parent coalincreases, the porosity of the produced chars decreasedue to the release of volatiles. In lower oxygenconcentration, the lower rank coal (GU) yielded charsconsisting of isotropic texture, expected for a low rankcoal, and thick-walled particles. The medium rank coal(BW) yielded chars of mainly anisotropic texture, but

Fig. 3 Appearence of the coal chars obtained under 2.5%and 10% O2 in N2 in DTF

small isotropic domains and tick-walled particles. Charsgenerated from the high rank coal (JB) showed mainlyanisotropic domains, due to the highly organized structureof this coal

18). When the oxygen concentration in the

reactor was 10% O2, the three coals showed texture andthickness similar to the 2.5% O2 chars, but, with morereacted surfaces. Differences between chars and oxy-chars obtained under the same oxygen content were notobserved.

3.3 Surface areaBET surface areas of chars are shown in Table 2. Due

to the high percentage of ash in the samples, mainly in thehigh conversion tests, some surface areas were notevaluated.

Table 2 BET surface areas of chars (m2.g

-1)

GU BW JB

Char 2.5% 13,08 12,65 12,16

Char 10% 48,48 40,66 23,59

Oxy-char 10% n.d.

n.d. 22,70Ref-char 5% 36,54

a20,30 13,97

Ref-oxy 5% 27,11 20,89 n.d.

Ref-oxy 10% 75,80 n.d. 20,06a

For Guasare, oxygen content was 10%.n.d. not determined.

Of the three coals, the BET surface area of Char 10%was higher than the one of Char 2.5% and this trend wasmore pronounced as coal rank decreased. The refiring of Char 2.5% in 5% O2/N2 led to the increase in BET surfacearea and this trend was less significant for the high rank

coal. Other authors10)

have also verified more significantdifferences in BET surface areas between high volatilecoal chars obtained under different O2 contents. It was

*For Guasare,oxygen contentwas 10%

GU 2.5

BW 2.5

JB 2.5

GU 10

BW 10

JB 10

50 µm



assumed that there was an increase in surface areawith combustion progress due to microporescoalescence or widening

19). Comparing the samples of

the JB, Char and Oxy-char, prepared under the sameoxygen content, no significant difference wasobserved. This result was also observed with the BWRef-char and Ref-oxy obtained under 5% O2.Regarding the GU refired samples obtained under

10%O2, the Ref-oxy sample showed higher BETsurface area than the Ref-char sample. This higher BET surface area for oxy-fuel samples can be theresult of internal surface area created by the char-CO2 gasification reaction

9).

3.5 Char reactivityThe CO2 reactivity at 50% conversion of the chars is

showed in Fig. 5.

0,0E+00

2,0E-04

4,0E-04

6,0E-04

8,0E-04

1,0E-03

1,2E-03

Char

2.5%

Char

10%

Oxy-char

10%

Ref-char

5%*

Ref-oxy

5%

Ref-oxy

10%

R 5 0 % C

O 2

( s - 1 )

GU

BW

JB

Fig. 5 CO2 reactivity at 50% conversion of chars

prepared in DTF at different atmospheres.

Similar to coals burnouts, there was a trend inreactivity increase with a decrease in coal rank for samples obtained under the same oxygen content inDTF (Fig. 5). It is also evident in Fig. 5 that reactivityof the lower rank coal char (GU) varied over a greater range when comparing all samples from the threecoals. In the case of the chars obtained under 2.5%O2, the low rank coal GU was less reactive thanexpected when compared to the medium rank coalchar BW. This may be due to the low ash deformationtemperature of this coal

20), 1280°C, which could lead to

a material deposition on the particle surface, blockingporosity and decreasing the active surface area andchar reactivity

21).

The reactivity of the GU Char prepared with 2.5% O2 was lower than the reactivity of the GU Char 10% O2.As for BW and JB, the reactivity did not change with O2 content in DTF, despite the higher surface area of the10% O2 samples.

For the BW and JB samples, the reactivity of theO2/N2 chars was similar to the reactivity of the O2/CO2 chars in both one and two steps (Fig. 7). In the case of the GU chars obtained under 10% O2 in two steps(Ref-char 10% O2 vs. Ref-oxy 10% O2), the oxy-fuel

sample was more reactive than conventional sample.The higher reactivity of this sample is associated withits higher BET surface area.

0,0E+00

2,0E-04

4,0E-04

6,0E-04

8,0E-04

1,0E-03

0,0E+00 2,0E-04 4,0E-04 6,0E-04 8,0E-04 1,0E-03O2/N2 chars R50%-CO2(s

-1)

O 2 / C O 2 c h a r s R 5 0 % - C O 2 ( s - 1 )

Fig. 7 Reactivity of chars obtained under conventionalcombustion (O2/N2) vs. Reactivity of chars of oxy-fuelconditions (O2/CO2) under the same oxygenconcentration. Solid symbols = 1 step; void symbols = 2

step (refired); ▲=GU; ■=BW; ♦=JB.

The burnout effect in the chars’ reactivity is showed inFig. 8.

0,0E+00

4,0E-04

8,0E-04

1,2E-03

20 40 60 80 100

Burnout (%)

R 5 0

% C

O 2

( s - 1 )

▲ GU

○ BW

■ JB

Figure 8 CO2 reactivity of the chars as a function of coalburnout

The CO2 reactivity of the three coals increased withincreasing burnout. This is due to the increase in BETsurface area of the samples. This trend indicates that thechar-CO2 reaction was controlled by both chemicalreaction and the diffusion of the gas into the particle,because the higher surface area favors gas diffusion,increasing reactivity

22).

The similar reactivities obtained for the different coalchars at conventional and oxy-fuel atmospheres and thepositive evolution of reactivity with burnout indicate thatthe chars tested have similar chances to completely reactin the blast furnace stack.

However, to better correlate the char behavior

(reactivity) in the blast furnace shaft, its necessary tomake combustion tests in conditions closer to those foundin the raceway (atmospheres and temperatures). To

*For Guasare,oxygen contentwas 10%



achieve this, the study will continue with new testsconducted in a raceway simulator (injection rig). Thisequipment is going to be built at LASID with thefinancial support of the Brasilian Coal Net.

5 CONCLUSIONS

• The chars behavior in all tests was strongly

influenced by coal rank. Higher burnouts, BET surfaceareas and reactivities were reached as the coal rankdecreased. Besides, among the three coals, the charsfrom the lowest rank coal had a greater rangevariance.

• Increasing oxygen content in DTF lead to anincrease in burnout and BET surface area

• For the three coals, there was no effect in burnoutbecause of the N2 replacement (conventionalatmosphere) by CO2 (oxy-fuel atmosphere).

• As for BET surface area results, differences wereonly found between the conventional and oxy-fuelchars for the highly burnt lower rank coal chars.

• The CO2 reactivity of chars increased with anincrease in burnout because of the greater surfacearea.

• The CO2 reactivity of conventional chars was similar to the CO2 reactivity of oxy-fuel chars for the mediumand high rank coal samples from both one and twosteps. For the greater burnt chars of the lower rankcoal, the oxy-fuel sample was more reactive than theconventional one. This was because of the higher BETsurface area of the oxy-fuel char.

ACKNOWLEDGEMENTS

The Brazilian team thanks CNPq and BrasilianCoal Net for the financial support.

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evaluation of the co2 re activity of chars obtained under conventional and oxyfuel atmospheres

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