Incorporation of Charcoal in Coking Coal Blend – A Study of the Effects on Carbonization Conditions and Coke Quality

Ka Wing Ng

CanmetENERGY 1 Haanel Drive, Ottawa, ON, Canada, K1A 1M1

Phone: (613) 996 8712 Fax: (613) 995 9728

Email: kng@nrcan.gc.ca

Louis Giroux CanmetENERGY

1 Haanel Drive, Ottawa, ON, Canada K1A 1M1 Phone: (613) 996 7638 Fax: (613) 995 9728

Email: lgiroux@nrcan.gc.ca

Tony MacPhee CanmetENERGY

1 Haanel Drive, Ottawa, ON, Canada, K1A 1M1 Phone: (613) 996 4440 Fax: (613) 995 9728

Email: tmacphee@nrcan.gc.ca

Ted Todoschuk ArcelorMittal Dofasco Inc.

1390 Burlington Street East, Hamilton, ON, Canada, L8N 3J5 Phone: (905) 548 4796 Fax: (905) 548 4653

Email: ted.todoschuk@arcelormittal.com

Key words : Biomass, Charcoal, Cokemaking, GHG mitigation, Bio-coke

INTRODUCTION

Incorporation of biomass in coking coal blend is one of the potentially effective approaches for reducing GHG emissions in blast furnace ironmaking. Experimental work in the pilot-scale movable wall coke oven at CanmetENERGY was performed to examine the effect of charcoal incorporation to coal blends on carbonization conditions and on properties of resultant bio-coke. Substituting coal with charcoal significantly reduced oven wall pressure during coking. Moreover, ash content and particle size of charcoal were found to play important roles on the properties of resultant bio-coke. Substitution of coal with fine charcoal preserved the stability and hardness of the resultant bio-coke. However, fine charcoal had a stronger negative impact than coarse charcoal on reactivity and CSR of resultant bio-coke. In order to produce a bio-coke suitable for blast furnace ironmaking and, at the same time, maximising the amount of charcoal in the blend, charcoal mineral content and its localization in the blend are essential factors requiring careful control.

BACKGROUND

Blast furnace ironmaking is currently the main hot-metal producing technology. In 2010, blast furnaces in Canada produced 7.7 Mt of hot metal compared to 0.6 Mt by direct reduced iron (DRI) pathway1. However, the blast furnace is also the most intensive emitter of greenhouse gas (GHG) in integrated steelmaking. Blast furnace ironmaking relies heavily on gasification of coke to provide reducing gases for reduction of iron ore and the necessary energy for melting metallic iron. All of the gasified carbon is eventually released to the atmosphere in the form of CO2 after retrieval of residual energy. As production of 1 tonne of hot

metal generates about 1.5 t of CO2 2, CO2 emission associated with blast furnace ironmaking operation in

Canada generated roughly 12 Mt in 2010. Global awareness of the adverse effects of atmospheric CO2 concentration on climate change has imposed significant pressure on the steel industry to reduce emissions associated with its manufacturing processes. The Canadian steel industry has demonstrated continuous effort to improve the performance and energy efficiency of its manufacturing process and, as a result, has achieved significant advances. Compared to 1990, the energy intensity of Canadian iron and steelmaking in 2008 was reduced by 26% and absolute GHG emission by 17%. On a per tonne basis of shipped steel, GHG emissions were reduced by almost 30%3. To further lower GHG emission associated with blast furnace ironmaking, substitution of fossil carbon by bio-carbon to support the process is proposed. The Canadian Carbonization Research Association (CCRA)4 in partnership with CanmetENERGY conducts research on bio-carbon application in iron and steel production. Bio-carbon refers to carbon sources originating from recent biological materials. As for fossil carbon sources, combustion of bio-carbon also releases CO2. However, CO2 released by combustion of bio-materials from renewable sources is balanced by the CO2 absorbed during its growth. Since the duration of this natural carbon cycle is relatively short compared to that of fossil fuel, CO2 originating from renewable bio-carbon sources is considered not contribute to the increase in atmospheric GHG concentration. Hence, process emission can be reduced by replacing fossil carbon with bio-carbon without affecting the furnace productivity. Bio-carbon can be introduced into the existing blast furnace ironmaking process via two ways. It can be incorporated in the coal blend for cokemaking and as an auxiliary fuel by directly injecting it into the hearth of the furnace, Figure 1.

Figure 1. Bio-Carbon Application in Cokemaking-Ironmaking System The effects of direct injection of bio-carbon from various sources on furnace behavior have been studied in detail by conducting both numerical modeling and pilot-scale experiments5-7. These investigations concluded that substitution of PCI by charcoal injection does not affect the operating conditions of the furnace and the chemistry of the hot metal produced. However, GHG emission on per tonne basis of hot metal produced can be reduced by as much as 25%. The effects of incorporating charcoal into a coking coal blend were studied by performing carbonization trials in the 350 kg capacity pilot-scale moveable wall oven at CanmetENERGY (460 mm wide, 0.405 m3 internal volume). The coke produced from the blend containing charcoal was termed “bio-coke” to distinguish it from coke produced only from coal. The objective of the experimental work is to develop an understanding on the effect of charcoal incorporation into a coking coal blend for maximizing the amount of coal substituted by charcoal while producing a bio-coke suitable for blast furnace ironmaking. The outcomes of the experimental efforts are summarized in the following sections.

EFFECT OF CHARCOAL SUBSTITUTION ON COKE BLEND PROPERTIES

A total of nine (9) pilot-scale carbonization trials were performed with charcoal substitution ranging from 2% to 5%. The charcoals used were obtained from commercial market and were prepared by slow pyrolysis

of hardwood from Eastern Canada. The coals used in the tests were ground to 80% less than 3.35 mm and charcoal in three different size ranges (6.4-9.5 mm, 2.4-3.4 mm and <0.07 mm) was used for examining effect on oven conditions and bio-coke properties, Table I. Table I: Chemical Analysis of Blends for Carbonization Test

Test No. 1 2 3 4 5 6 7 8 9

Coal identifier

Blend 1 Blend 1 Blend 2 Blend 2 Blend 2 Blend 3 Blend 3 Blend 3 Blend 3

Charcoal substitution

Nil 5% Nil 2% 5% Nil 2% 3% 5%

Charcoal Size

-- 6.4-9.5mm

-- 2.4-3.4mm

2.4-3.4mm

-- <0.07mm <0.07mm <0.07mm

Proximate Analysis, db

% Ash 8.32 7.79 8.06 8.36 8.06 7.30 7.09 7.01 7.13

% VM 26.41 26.05 26.25 26.20 25.76 29.26 29.44 28.95 29.44

% FC 65.27 66.16 65.69 65.44 66.15 63.44 63.47 64.04 63.43

Ultimate Analysis, db

% C 81.30 81.60 81.53 81.21 81.70 81.50 81.61 81.73 81.80

% H 4.74 4.61 4.51 4.42 4.36 4.90 4.90 4.91 4.89

% N 1.43 1.39 1.38 1.31 1.32 1.52 1.52 1.51 1.49

% S 0.80 0.74 0.68 0.67 0.65 0.88 0.88 0.89 0.90

% O(diff) 3.41 3.87 3.84 4.03 3.91 3.90 4.00 3.95 3.79

Ash Chemistry

%SiO2 54.21 53.03 56.66 56.43 54.91 51.68 51.03 49.80 50.36

%Al2O3 30.25 29.98 29.68 29.13 28.95 30.77 30.59 29.97 29.93

%Fe2O3 6.71 6.25 5.32 5.57 5.18 8.63 8.76 9.06 8.83

%TiO2 1.54 1.49 1.50 1.39 1.39 1.62 1.62 1.63 1.60

%P2O5 0.77 0.77 0.54 0.51 0.60 0.39 0.39 0.44 0.43

%CaO 1.78 2.22 1.27 1.60 2.05 1.67 1.76 2.02 2.36

%MgO 0.76 0.74 0.66 0.76 0.83 0.86 0.85 0.88 0.90

%SO3 1.13 2.28 0.89 1.42 2.01 1.01 1.29 1.52 1.71

%Na2O 0.36 0.38 0.36 0.29 0.42 0.56 0.62 0.70 0.62

%K2O 1.55 1.60 1.54 1.63 1.78 1.97 2.07 2.08 2.26

%BaO 0.20 0.20 0.14 0.14 0.14 0.16 0.16 0.16 0.17

Basicity Index

0.132 0.135 0.106 0.115 0.122 0.166 0.172 0.185 0.186

Table I also lists Proximate, Ultimate and Ash chemistry Analyses of the blends. The most noticeable consequence of substituting coal by charcoal was the increase in Ca content and the basicity of the blend. For comparison, the elemental Ca content in the blends was expressed as g/kg Blend and shown in Figure 2. The Ca content in the blend increased as the degree of substitution increased. Moreover, the ash basicity index also increased as the degree of charcoal substitution increased, Figure 3.

Figure 2. Calcium Content in Blends

Figure 3. Basicity Index of Blends

Table II lists thermal rheology properties of the blends. The small decrease observed in log10 Maximum Fluidity with incorporation of charcoal is also supported by comparable variation in blend dilatation, expressed as SD2.5 (ASTM D5515-97(2010)). FSI, being a much less sensitive indicator than either Maximum Fluidity and Dilatation, is found to remain unchanged with charcoal addition. This suggests that charcoal, present in low amount in the blend, is not an incompatible material within the blend and actually mixes well with the coal. The presence of charcoal in the blend therefore did not affect the development of a plastic layer in coal during coking.

Table II: Thermal Rheology Properties of Blends for Carbonization Tests

Test No. 1 2 3 4 5 6 7 8 9

Coal identifier Blend 1 Blend 1 Blend 2 Blend 2 Blend 2 Blend 3 Blend 3 Blend 3 Blend 3

Charcoal substitution

Nil 5% Nil 2% 5% Nil 2% 3% 5%

Charcoal Size -- 6.4-9.5mm

-- 2.4-3.4mm

2.4-3.4mm

-- <0.07mm <0.07mm <0.07mm

Gieseler Fluidity

Initial Softening Temp, oC

408 416 415 418 418 396 400 n/a n/a

Fusion Temp, oC

422 429 427 429 429 410 413 n/a n/a

Max. Fluid Temp, oC

447 454 452 454 453 448 450 n/a n/a

Final Fluid Temp, oC

481 482 487 485 487 486 484 n/a n/a

Solidification Temp, oC

488 489 492 493 495 491 490 n/a n/a

Melting Range, oC

80 73 77 75 77 95 90 n/a n/a

Log Max. Fluidity, ddpm

2.72 2.26 2.68 2.47 2.42 3.55 3.80 n/a n/a

Ruhr Dilatation

Softening Temp, oC

388 393 391 394 399 370 366 n/a n/a

Max. Contraction Temp, oC

425 432 430 432 436 417 416 n/a n/a

Max. Dilatation Temp, oC

467 466 468 466 466 466 470 n/a n/a

% Contraction 30 30 29 26 24 26 26 n/a n/a

% Dilatation 29 19 18 20 15 95 101 n/a n/a

SD2.5, % 32 27 22 21 17 100 106

Free Swelling index

6.5 7 5 6.5 5 5 5 n/a n/a

Table III lists the petrographic analysis of the blends. In comparison to the base blend, substitution of coal by charcoal in other blends leads to increased inerts. As a consequence, the vitrinite content in the blend, and the total reactives for that matter, was subsequently decreased by diluting with inert material (pyrolysed wood). The increase in blend inert content is reflected by a corresponding decrease in the mean maximum vitrinite reflectance of the blends, Romax. The maceral analysis also indicates that the finer the charcoal added to the blend, the better it mixes in with the coal, thus making it the more difficult via optical microscopy to distinguish it from coal particles. Table III: Petrography Properties of Blends for Carbonization Tests

Test No. 1 2 3 4 5 6 7 8 9

Coal identifier

Blend 1 Blend 1 Blend 2 Blend 2 Blend 2 Blend 3 Blend 3 Blend 3 Blend 3

Charcoal substitution

Nil 5% Nil 2% 5% Nil 2% 3% 5%

Charcoal Size

-- 6.4-9.5mm

-- 2.4-3.4mm

2.4-3.4mm

-- <0.07mm <0.07mm <0.07mm

Maceral Analysis

Vitrinite, %

68.1 63.5 57.1 58.2 56.9 62.1 62.9 59.9 61.0

Reactive Semi-Fusinite, %

5.3 5.3 6.6 6.0 6.1 3.8 3.9 4.8 4.9

Exinite, % 2.0 2.8 2.0 3.0 1.3 3.8 4.3 4.1 3.6

Total Reactive,%

75.4 71.6 65.7 67.2 64.3 69.7 71.1 68.8 69.5

Inert Semi-Fusinite, %

10.6 10.6 13.2 12.0 12.3 7.5 7.7 9.5 9.7

Micrinite, %

6.3 4.5 11.7 9.3 7.4 15.4 12.8 14.3 13.0

Fusinite, % 2.9 2.5 4.8 4.3 6.0 3.2 4.0 2.2 2.1

Mineral Matter, %

4.8 4.4 4.6 4.7 4.6 4.2 4.1 4.0 4.1

Other, % 0.0 6.4 0.0 2.5 5.4 0.0 0.3 1.2 1.6

Total Inert,%

24.6 28.4 34.3 32.8 35.7 30.3 28.9 31.2 30.5

Romax 1.17 1.13 1.23 1.18 1.16 1.11 1.09 1.16 1.09

EFFECT OF CHARCOAL SUBSTITUTION ON PILOT OVEN WALL PRESSURE

Carbonization of the nine (9) blends listed in this paper was carried out in the 350 kg capacity pilot-scale moveable wall oven at CanmetENERGY. Blend was gravity fed into the oven via a charging chute positioned above the oven. The two vertical walls, fixed and moveable, respectively, were maintained at 1200oC throughout the test and the pressure exerted on the moveable wall during the coking test was continuously monitored. Figure 4 shows the wall pressure profiles recorded during the carbonization of the blends. Substitution of coal by charcoal significantly reduced the blend pressure. Moreover, the degree of reduction in wall pressure appeared to be related to the size of the charcoal. As the charcoal particle size decreased, the

reduction in wall pressure also decreased. Inert materials such as charcoal or coke breeze are known to act as pressure modifiers enabling the gas to escape more freely from the plastic layer of a coking coal blend8.

Figure 4. Pilot Oven Wall Pressure Evolution Profiles during Bio-Cokemaking

EFFECT OF CHARCOAL SUBSTITUTION ON COKE COLD AND HOT STRENGTH PROPERTIES

Table IV lists coke properties obtained for the different blends. Table IV: Coke Properties

Test No. 1 2 3 4 5 6 7 8 9

Coal identifier Blend 1 Blend 1 Blend 2 Blend 2 Blend 2 Blend 3 Blend 3 Blend 3 Blend 3

Charcoal substitution

Nil 5% Nil 2% 5% Nil 2% 3% 5%

Charcoal Size -- 6.4-

9.5mm --

2.4-3.4mm

2.4-3.4mm

-- <0.07mm <0.07mm <0.07mm

Carbonization Conditions

Coking Time, h:min

18:17 19:20 17:30 17:40 18:00 17:15 17:55 17:15 17:55

Final Centre Temp, oC

1095 1070 1094 1085 1100 1095 1081 1094 1082

Max Wall Pressure, psi

1.12 0.49 0.93 0.88 0.78 0.96 0.73 0.80 0.73

Coke Yield, % 76.0 75.2 76.2 77.7 75.7 74.5 74.8 74.5 74.4

Coke Size

+50 mm, % 52.1 27.0 35.9 37.6 43.8 33.6 33.6 36.4 36.7

-12.5 mm, % 3.2 8.7 4.7 7.2 10.5 5.2 4.9 4.8 4.3

Mean, mm 54.6 41.8 48.1 46.2 47.7 47.1 46.6 47.4 48.0

Cold Strength

ASTM Tumbler

Stability 59.1 26.8 59.1 44.6 18.3 60.0 61.0 62.6 59.9

Hardness 71.0 66.6 70.3 68.5 66.0 69.2 68.9 68.7 69.2

IRSID Tumbler

I40 40.6 5.4 35.1 22.0 6.8 37.4 38.1 38.0 39.9

I20 78.0 59.7 77.6 71.6 54.0 77.8 77.6 78.6 77.6

I10 19.7 26.8 19.6 21.9 27.8 19.8 19.6 19.4 20.3

Density

ASG 0.964 0.909 0.963 0.946 0.912 0.883 0.948 0.947 0.950

Hot Strength

CRI 30.8 34.3 25.4 30.3 33.0 28.8 36.9 33.6 41.0

CSR 54.6 45.8 63.5 56.6 49.7 58.3 44.5 49.0 34.2

The presence of coarse charcoal, 2.4-9.5 mm, in Blends 1 and 2 lead to significant increase in level of small size coke, < 12.5 mm. Figure 5 and 6 respectively show ASTM stability and hardness of the bio-cokes produced in the carbonization tests as per ASTM D3402/D3402M-93(2008). Stability and hardness of bio-cokes produced from Blends 1 and 2,, were decreased significantly with respect to coke produced from the base blends. The lower IRSID I40 and higher I10 indices for Blend 1 and 2 bio-cokes also indicate the drastic reduction in cold strength with respect to those of the base blends. The apparent specific gravity (ASG) of Blend 1 and 2 bio-cokes is lower than those of the corresponding base blends. On the other hand, substitution of coal by fine charcoal for Blend 3 (<0.07 mm) showed minor changes on the stability and hardness and IRSID of the bio-coke produced.

Figure 5. ASTM Stability of Bio-Cokes

Figure 6. ASTM Hardness of Bio-Cokes Optical microscope images of bio-cokes produced with 5% charcoal substitution but with different particle sizes, 2.4-3.4 mm and <0.07 mm, respectively, are shown in Figure 7 and 8. From microscopic examination, charcoal particles were identified by the cellular structure inherited from the raw woody biomass material. As illustrated in Figure 7, vitrinite penetration took place into the cellular structure of the charcoal. However, a significant portion of the large charcoal particles was not surrounded by the strong coke. Hence, this resulted in weak points in the bio-coke matrix and causing the stability and hardness to be significantly reduced.

Figure 7. Microscopic View of Bio-Coke with Coarse Charcoal (Blend 2)

Figure 8. Microscopic View of Bio-Coke with Fine Charcoal (Blend 3)

When fine charcoal was incorporated in the coal blend, the small particles of this inert material were completely embedded within the coke structure, Figure 8. This eliminated the weak points observed upon addition of coarse charcoal. Stability and hardness of Blend 3 bio-coke is comparable to the coke produced from coal only. Figure 9 and 10 respectively show the effect of charcoal substitution on Coke Reactivity Index (CRI) and Coke Strength after Reaction (CSR) of the bio-cokes determined as per ASTM D5341-99(2010). Reactivity of bio-coke was significant increased compared to that of the coke from the base blend. As a result of the increase in reactivity, the CSR was reduced by partially substituting coal by charcoal.

Figure 9. CRI of Bio-Cokes

Figure 10. CSR of Bio-Cokes

Moreover, CSR of bio-coke was found to be more significantly reduced when fine charcoal was used, Blend 3, <0.07 mm. Figure 11 compares the reduction in CSR at 5% charcoal substitution relative to cokes produced from the base blends. The enhanced reactivity of bio-coke was caused by the high mineral content, in particular Ca, in charcoal. In fact, the Ca present acts as a catalyst to promote the reaction of coke carbon with CO2, Boudouard or solution loss reaction. When fine charcoal was used to partially replace coal, the minerals were more or less evenly dispersed in the resultant bio-coke. For coarse charcoal addition, the catalytic effect is relatively localized. As a consequence, the reduction in CSR is found to be less severe when coarse charcoal is used for coal substitution.

Figure 11. Reduction in CSR with 5% Charcoal Substitution

As discussed above, substitution of coal by fine charcoal is able to preserve the cold strength of bio-coke compared to coke from the base blend. However, incorporation of fine charcoal in the coal blend has a stronger negative impact on the CSR and CRI than coarse charcoal. For producing a bio-coke with properties suitable for blast furnace application and maximizing the degree of charcoal substitution, the pre-processing of charcoal for mineral matter removal prior to its incorporation into the coal blend is a necessary step. In addition, it is also extremely important that the charcoal incorporated into the coal blend be localized via briquetting. Further research efforts in these two areas are needed to maximize the degree of charcoal substitution to be able to achieve a successful implementation of bio-cokemaking.

CONCLUSIONS The effects of incorporating charcoal in coal blends for cokemaking were studied. In total, nine (9) carbonization tests were performed in a 350 kg pilot-scale coke oven. The effects of charcoal substitution on pilot oven conditions and properties of bio-cokes produced are summarized as follows:

1. Substitution of coal by charcoal in the blend significantly reduced the oven wall pressure during coking.

2. The presence of coarse charcoal, 2.4-9.5 mm, in Blends 1 and 2 lead to significant increase in level of small size coke, < 12.5 mm and to lower Apparent Specific Gravity of coke.

3. Cold strength (ASTM stability and hardness and IRSID I40 and I10) of bio-coke can be maintained similar to coke produced from base blend by using fine charcoal.

4. Substitution of coal by fine charcoal has a much stronger negative impact on CSR and CRI of the resultant bio-coke than does coarse charcoal.

Further research on the removal of minerals in charcoal and the utilization of briquetting will be performed in order to maximize the degree of charcoal in a coal blend while, at the same time, maintaining suitable properties of bio-coke produced for blast furnace application.

ACKNOWLEDGEMENTS We would like to express thanks to the Canadian Carbonization Research Association (CCRA) and to the Canadian Federal Government ecoETI program for supporting this work.

REFERENCES 1 worldsteel Committee on Economic Studies, ‘Steel Statistic Yearbook 2011’, worldsteel Association,

Brussels, 2011.

2 K.W. Ng, L. Giroux, J.A. MacPhee and T. Todoschuk, ‘Biofuel Ironmaking Strategy from a Canadian Perspective: Short-Term Potential and Long-Term Outlook’, EECRsteel, METEC INSTEELCON 2011, Jun 27- July 1, 2011, Düsseldorf, Germany.

3 Environmental Performance Report, Canadian Steel Producers Association, http://www.canadiansteel.ca/media/2010/cspa-environmental-performance-report-en.pdf, Retrieved on January 11, 2012.

4 Canadian Carbonization Research Association, http://www.cancarb.ca/

5 K.W. Ng, W.P. Hutny, J.A. MacPhee, J.F. Gransden, and J.T. Price, “Bio-fuels Use in Blast Furnace Ironmaking to Mitigate GHG Emission”, Proceedings of the 16th European Biomass Conference and Exhibition, Valencia, Spain, June 2008, pp. 1922-1928.

6 K.W. Ng, L. Giroux, J.A. MacPhee and T. Todoschuk, ‘Direct Injection of Biofuel in Blast Furnace Ironmaking’, AISTech 2010, May 3-6, 2010, Pittsburgh, PA.

7 K.W. Ng, L. Giroux, J.A. MacPhee and T. Todoschuk, ‘Combustibility of Charcoal for Direct Injection in Blast Furnace Ironmaking’, AISTech 2011, May 2-5, 2011, Indianapolis, IN.

8 S. Nomura, M. Mahoney, K. Fukuda, K. Kato, A. Le Bas, S. McGuire, ‘The mechanism of coking pressure generation I: Effect of high volatile matter coking coal, semi-anthracite and coke breeze on coking pressure and plastic coal layer permeability’, Fuel, 89 (2010), p. 1549.