STUDY OF GASIFICATION REACTION OF COKES EXCAVATED FROM PILOT BLAST FURNACE

V. Sahajwalla*, T. Hilding**, *S. Gupta, B. Björkman**, R. Sakurovs#, * M. Grigore and N. Saha-Chaudhury*

*University of New South Wales, Sydney, NSW 2052, Australia ** Luleå University of Technology, S-971 87 Luleå, Sweden # CSIRO, Newcastle, Australia

ABSTRACT Fundamental understanding of coke reactions with gas, metal and slag phases is essential for ensuring smooth operation and optimisation of coke performance in existing and advanced blast furnace process, and is dictated by coke properties and blast furnace process conditions. In this study, coke samples excavated from LKAB’s Experimental Blast Furnace (EBF) at MEFOS in Luleå, Sweden were collected. The centreline quenched coke samples from different zones of this EBF were used to observe the influence of in-furnace reactions on the evolution of coke properties and their associations with CO2 reactivity. Carbon structure of coke was found to increasingly ordered, silicon and iron concentration in the coke samples decreased, while alkali concentration particularly potassium and sodium were found to increase as the coke descended towards lower part of the EBF. Both isothermal and non-isothermal reactivity based on TGA measurements showed that coke reactivity towards CO2 is increased as coke descends towards cohesive zone despite increasing order of carbon structures. Increased reactivity of cokes at lower parts of EBF was related to alkali enrichments of cokes. The study further shows that increased alkali components in cokes have a strong impact on CO2 gasification in EBF such that influence of coke graphitisation could be compensated by catalytic influence of alkalis. To further assist with development of understanding of reactivity of coke, gasification studies were also conducted in a fixed bed reactor at 900ºC using a series of cokes made from Australian coals (varying in rank, maceral and mineral matter). The CO2 reactivity of cokes in a fixed bed reactor was also found to be strongly influenced by the coke minerals compared to carbon structure. Further studies are required to provide a critical insight into the influence of key parameters such as coke graphitisation and mineral reactions on coke gasification particularly at higher temperatures.

1 INTRODUCTION Coke has been used as a sole source of fuel in a blast furnace from early 18th century. Since then, lots of studies have been conducted on coke and its function in a blast furnace. However, still there are many questions to be answered when it comes to coke and its behaviour and degradation in a blast furnace. One of the major developments in the blast furnace operation is the introduction of pulverized coal technology in which coke is substituted by pulverized coal injection through tuyeres. Economic and environmental pressures are the primary driving force behind the promotion of PCI technology. The old coking plants are gradually closing while few new plants are being built to replace the coke supply particularly in developed countries including Europe. New coke plants are extremely expensive due to stringent environmental regulations. Therefore, in any future, blast furnace operations have to rely on less coke per unit metal production. At low coke rate operation, less amounts of coke in the burden is available in the blast furnace shaft, but it still has to maintain the bed permeability for reducing gases upwards, and liquids passing downwards. Therefore high quality coke is essential for future blast furnace operations.

Coke quality is often characterized by measuring cold and hot strength, ash composition and chemistry which are largely dictated by coal properties. A range of laboratory tests and procedure have been developed to characterize physical and chemical properties of coke and their potential impacts in blast furnaces. The unspeakably most used and well-known tests are the so-called Coke Reactivity Index and the Coke Strength after Reaction developed by Nippon Steel Corporation (NSC) in Japan in early seventies to assess the effect of CO2 reactions on coke. There is no universally accepted standard procedure however NSC/CRI test is widely recognized around the world and was adopted by ASTM as a standard procedure. The test is currently under draft stage for becoming an ISO standard [1, 2].

CRI and CSR have quite different judgements in the interpretation of coke performance in a blast furnace. Generally high CSR coke is believed to prevent the coke from breaking down, improve the permeability of gas and liquid in the wet part of the blast furnace and increase the productivity as well as decrease the specific coke consumption [3]. However, no international agreement of an ideal way to determine the quality exists as each industry relies on their empirical experience for the interpretation. These laboratory tests are designed to test the coke properties under specific set of conditions which might not be universally suitable. The reproducibility of CRI/CSR values among different laboratories also varies a lot [4]. Therefore, in addition to bench-scale testing, a more comprehensive approach is the pilot- scale testing of materials under more realistic industrial environment. Even though these tests are time consuming and very expensive, data generated in these tests is critical to further improve the interpretation of bench-scale tests.

In the present, a campaign was conducted in LKAB’s EBF at MEFOS in Luleå, Sweden to test the performance of raw materials including coke. The test was conducted using relatively good quality coke i.e. low CRI and high CSR. A large number of samples and data were collected during this campaign. However, a limited part of study is presented here to illustrate the variation in coke properties and its reactivity. Coke samples excavated from the centreline of EBF are used to investigate some of the changes that coke undergoes as it descends into lower part of furnace. The primary focus of this study is to investigate the effect of in-furnace reaction on coke and their consequences on CO2 reactions using weight loss in a TGA reactor. In addition, CO2 reactivity measurements of custom made coke using Australian coals are also presented to compliment the EBF findings in order to develop an understanding of

critical parameters influencing coke reactivity under both lab-scale and industrial environments.

2 EFFECT OF COKE PROPERTIES ON REACTIVITY WITH GASES

Reactivity towards oxidizing gases including CO2, oxygen, air and steam play an important role in many metal-smelting processes. As the primary focus of this study is the gasification, it becomes imperative to discuss some of the factors that influence the coke reactivity with CO2. Combustion/gasification of carbonaceous materials can be divided into three different regimes depending on the steps limiting the reaction rate. Combustion is controlled by chemical reaction at low temperatures (regime I), by pore diffusion at moderate temperatures (regime II) and by gas phase mass transfer at high temperatures (regime III) [5]. Coke properties such as porosity, chemical structure and minerals could influence the coke reactivity in different regions to different extent. For example, atomic structure plays an important role under regime I and II. However, reactivity under combustion regime II, where external or internal diffusion becomes rate limiting is also affected by particle size, porosity.

Porosity of Coke: Reactions with oxidising gases change porous carbon matrix during combustion/gasification. The evolution of pore structure by growth and coalescence leads to increasing or decreasing available surface areas, changes in pore structure/distribution, gas diffusion and reactivity. Porous structure of coke is governed by the coking properties of coals, particularly by maximum fluidity and swelling number [6].

Chemical Composition of Minerals: Transformations of inorganic matter upon heat treatment include changes in chemical bonding, sintering, melting and vaporization as well as mutual interactions with organic matter. In addition to catalytic affect on reactivity of carbonaceous materials, mineral matter affects the thermal behaviour of char, and aggregation and particle size of mineral matter affect the fragmentation and mechanical stability of the carbonaceous material. Hermann [7] has evaluated the effect of chemical composition of coal ash on coke reactivity such that CaO and SO3 are gasification stimulating, Fe2O3 an Al2O3 have an intermediate effect, and P2O5, TiO2, MgO are gasification-inhibiting. Feng et al [8] have observed that iron is a major catalyst during gasification of bituminous coal as well organised crystalline structures of carbon were found predominantly in the vicinity of the carbon/iron interface. Moreover, an increasing burnout mineral matter could have inhibiting effect by forming a barrier for oxidizing gases [9].

Carbon Structure: During its descent through a blast furnace, coke is exposed to extreme conditions. The prevailing high temperatures in the cohesive zone areas lead to a more graphitized coke i.e. a more ordered structure. Synthetic graphite has a highly ordered structure, high fixed carbon content with low levels of ash and volatile matter. Graphite structure can be described by a regular, vertical stacking of hexagonal aromatic layers with the degree of ordering characterised by the vertical dimension of the crystallite Lc (Figure 1). Each C atom within the aromatic layer (basal plane) is linked through covalent bonds to three C atoms. However, bonding between the layers is very weak and can easily be broken by external forces. Natural graphite has highly ordered structure like synthetic graphite but contains high level of impurities. Lc for coal/char /coke has been often calculated from X-ray diffraction profiles using Scherrer equation [10].

Figure 1: A schematic of crystal structure of graphite.

3 EXPERIMENTAL

3.1 Pilot-Scale Furnace (LKAB) and Coke Samples The current study is based on coke samples from an Experimental Blast Furnace (EBF) situated in Luleå, Sweden which was designed in designed in 1995. Further details and its unique features are described elsewhere [11]. At the end of each campaign, the furnace is quenched with nitrogen for about ten days in order to stop the prevailing reactions and to cool down the furnace. Within about a minute, the reducing gases are removed and the chemical reactions stop. As the quenching gas is added from the top, an upward moving heat wave, is avoided, thus limiting reactions caused by heat such as further smelting and changes of slag composition. The dissection takes about two weeks to complete. Each burden layer is carefully removed and sampled according to a fixed sampling routine, after observing and documenting the shape, position and any anomalies in the layers that might occur.

The coke samples used in current study are based on cokes excavated from the tenth campaign which lasted for 8 weeks. All coke layers were sampled at three positions; centre line, middle (in-between the centre and the wall) and wall. Figure 2a illustrates the locations of coke bed layers and codes of excavated coke samples. The drawing of the furnace and the layers are according to scale. The reactivity measurements are carried out only on the centre line coke samples. A simple attempt to estimate the temperature profile was done. Temperatures were estimated from trend lines from a graph based on plot using measured temperatures. Figure 2b illustrates the coke layers of interest as a function of tentative temperature estimations. Samples 5 and 10, 15 & 20 are considered to represent coke in stock line and thermal reserve zone while samples 25 & 30 represent coke from the cohesive zone. Sample 35 represents bosh coke.

>700 (C5)

4.0 5

5.0 900 – 1200 10 (C10, C15, C20) 15 20 6.0 25 30 35 7.0

1200 –1500 (C30) Distance from top (meters)

8.0 600 800 1000 1200 1400 1600 1800 Temperature of furnace zone (oC) > 1500 (C35)

(a) (b) Figure 2 a) Schematic of EBF illustrating the locations of coke samples; b) Temperature profiles of coke samples from EBF.

3.2 Coke Analysis and TGA Measurements Figure 3 shows the schematic of Netzsch STA 409 instrument at Luleå University of Technology, which can be used for simultaneous Thermal Gravimetric and Differential Thermal Analysis. Non-isothermal reactivity was measured by using a small amount of coke powder (60 ~ 80 mg) in an Al2O3 crucible in TGA/DTA equipped with a mass-spectrometer with the setting to detecting ions with mass of 1 to 65. The loss in sample weight is recorded by a very accurate balance. All samples of interest have however been reacted under dynamic heating up to 1300 °C with a heating rate of 10K/minute.

A TGA at UNSW was used to measure the weight loss in coke sample during isothermal ° heating at 900 C for 2 hours under CO2 and at various flow rates ranging from 1.5 to 2.0 l/min. The TGA furnace consists of recrystallised vertical alumina (60 mm ID) tube. Sample temperature is controlled by an internal thermocouple located close to the sample holder. Approximately 0.2 g sample was placed on a square alumina crucible (30X 30 mm) holder at room temperature. Alumina sample assembly is suspended by a high temperature stainless wire which is connected to a balance that can measure weight changes of the order of 1 micro gram (Precisa® 1212 M SCS). The assembly was kept at low temperature zone in the furnace o o followed by heating up to 900 C at the rate of 2 C/minute while N2 (4-6 l/m) was continuously purged through the furnace which were regulated by Brooks 5850E mass flow controller. As the furnace reaches the required reaction temperature, the furnace chamber is raised to move the sample in the reaction zone followed by switching on CO2. Dilution with N2, and various total flow rates were also tested (up to 10 l/min).

Coke samples were analysed by XRF both at SSAB’s laboratories and the University of New South Wales (UNSW), Australia while carbon content was measured using LECO analyser. XRD was used to measure carbon structural parameters. Siemens 5000 X-ray diffractometer

at UNSW was used to record scattering intensities of samples by using Copper Kα radiation (30 kV, 30 mA) as the X-ray source. Samples were packed into an aluminum holder and scanned over an angular range from 5-105° by using a step size of 0.05° and collecting the scattering intensity for 5 seconds at each step. The XRD data has been processed to obtain crystallite dimensions, e.g. Lc values, in carbonaceous materials. The average stacking height of 002 carbon peak is calculated using Scherrer’s equation. A sharper 002 peak will indicate a larger crystallite size and a greater degree of ordering in the carbon structure.

4 RESULTS & DISCUSSION

4.1 Coke Gasification in Experimental Blast Furnace (EBF) Coke gasification is influenced by coke porosity, carbon structure and its minerals. Preliminary porosity measurements of the coke samples used in this study indicated no significant variation in the porosity of coke samples from different locations in the furnace. Therefore, most of the discussion is limited to variation in carbon structure and coke ash chemistry.

4.1.1 Evolution of Carbon Structure of Coke in EBF

As the coke descends into blast furnace, it reacts with upcoming CO2 gases and loses its carbon. Figure 4 shows that carbon content of coke samples is decreasing such that around bosh region (sample 35) coke loses approximately 5% carbon content resulting in increased concentration of ash content in a similar proportion. On the other hand, the nature of carbon phases also gets modified such that less ordered carbon material is lost. Figure 5a shows that carbon atoms become more ordered as coke passes from thermal reserve zone to bosh region as indicated by higher Lc values. Higher Lc value indicates increased number of ordered carbon layers in the coke. Figure 5a plots the calculated Lc values, generated from XRD measurements on the cokes of interest against the distance. Figure 5b plots the change in Lc values against the estimated temperatures of coke layers. The linear correlation suggests that increase in Lc value is primarily determined by temperature experienced by coke at a given location. Generally, higher ordered carbons are believed to be less reactive towards oxidising gases including CO2.

2000 87 Central Layer Temperature Coke carbon content (wt%)

C) 1800 Pl (C lL T ) o C

( 86 r o ke car

aye 1600 l

d e b

85 o

1400 n ke b co co n f t

1200 en 84 e o r t ( u w at t r 1000 . % e p 83 ) m e 800 T

600 82 45678 Distnace from top of furnace (meters)

Figure 4 Variation in carbon content of EBF centreline coke samples plotted against distance from top of EBF, tentative associated temperatures in EBF are also indicated.

60 2000 40 Central Layer Temperature Lc values T 1800 e Pl (C t lL T t ) m ) p m er o ) r 1600 at st om ur r g

45 t e n

s 30

g A o n ( f

1400 co e A k ( e ke b k co

o

1200 c e d of es of s l u

30 ayer l

ue 20 a 1000 l a v ( o Lc v C) Lc 800

15 600 45678 10 600 800 1000 1200 1400 1600 1800 Distnace from top of furnace (meters) Temerature of coke bed layer (oC)

(a) (b) Figure 5 a) Increase in Lc values of coke during its journey towards cohesive zone in the EBF and associated temperatures; b) Effect of coke layer temperature on Lc value of cokes.

4.1.2 Evolution of Coke Ash Chemistry in EBF The XRF analyses of coke samples from centreline position are indicated in Table 1. Figure 6a plots the alkali in coke ash against the furnace depth and shows that alkali content (K2O and Na2O) in coke increases as the coke moves from shaft to cohesive zone. It appears that alkali present in recirculation gases inside the blast furnace might be condensed on coke surface or penetrated on the outer surface. Micro structural examination of coke surface will clarify the impact of alkali on coke structure and will be reported later. Figure 6b illustrates the variation in silica and iron in coke ash. The results are consistent in the sense that silica and iron reduction is believed to increase at higher temperatures. It may be noted that generally, the presence of high alkali could enhance coke reactivity towards oxidising gases including CO2.

Table 1: Chemical composition of EBF coke samples and coke used in fixed bed reactor. XRF(SSAB) Ash SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 P2O5 SO3 Coke samples from Pilot-scale Furnace (LKAB) KL05C 9.83 5.76 2.63 0.42 0.01 0.06 0.17 0.10 0.16 0.03 0.59 KL10NC 10.89 6.39 2.77 0.46 0.04 0.06 0.35 0.16 0.17 0.03 0.50 KL15NC 11.66 6.30 2.65 0.37 0.00 0.06 1.24 0.43 0.15 0.02 0.54 KL20NC 11.52 5.58 2.57 0.32 0.02 0.07 1.78 0.61 0.14 0.02 0.51 KL25NC 12.39 5.83 2.61 0.32 0.02 0.08 2.31 0.67 0.14 0.02 0.47 KL30NC 12.39 5.97 2.66 0.34 0.04 0.08 2.07 0.68 0.14 0.02 0.50 KL35C 13.34 5.81 2.64 0.27 0.00 0.08 3.21 0.85 0.12 0.22 0.44 Cokes used in fixed reactor (UNSW) Coke-A1 9.0 5.12 1.65 1.15 0.33 0.14 0.08 0.04 0.10 0.12 0.18 Coke-A2 11.8 5.70 4.47 0.63 0.30 0.07 0.06 0.08 0.17 0.22 0.04 2 *CO2 surface areas of coke A-1 and coke A-2 were 142 and 74 m /g respectively at 15% burn out. KL05C represents the feed coke sample in EBF.

40 2000 70 1800 Central Layer Central Layer Temperature Temperature T

1600 ) K2O % ) e 60 C )

1800 % ° m

Na2O % t ( % . p r w

t 1400 ) e er y w

30 % . at 50 a ( 10x t 1600 SiO2 % l ( u w 1200 d h r e e o b as

40 e ash (

ke ash 1000 f k

1400 co ke o o c co

c 20 ke be n 800 n coke i 30 i of n e O 1200 Iron i 2 es 2 ur a d l t

d 600 O a N r

Si 20 ayer e d oxi p n 1000 400 m 10 on r ( I O a o

10 Te 2 C) 200 K 800 0 0 0 600 45678 45678 Distance from top of furnace (meters) Distnace from top of furnace (meters) (a) (b) Figure 6(a) Variation in sodium and potassium concentration in coke ash samples in EBF. Figure 6(b) Loss of silica and iron content in coke ash in EBF coke samples.

4.1.3 Evolution of Coke Reactivity with CO2 Figure 7a compares the non-isothermal reactivity of coke samples from different locations in the EBF. This demonstrates that as the coke descends in the blast furnace, its reactivity increases. Similar trend was also observed during isothermal reactivity measurements. Figure 7b compares the isothermal reactivity of coke samples collected from top (5C) and bottom zone (35C) indicating that CO2 reaction is faster for cokes from lower parts of the furnace. Interestingly, this reactivity trend could not be explained on the basis of carbon structure which is found to increase as the coke moves down the furnace. Most likely, the increase in coke reactivity is related to alkali enrichments. This means that in the EBF, the coke reactivity can increase due to catalytic influence of alkali even the carbon structure becomes ordered.

TGA results of centre line cokes

105 0.8 KL01C 95 o KL35C 900C-100% CO2 w

KL05C )/

85 -w o 0.6

KL10C (w

75 KL20C n, o y = 6E-05x + 0.013 %

s 0.4 s 65 KL15C lo

t KL25C h

ig 55 e

W 0.2 KL30C y = 2E-05x - 0.0106 45 Carbon conversi 35 KL35C 0.0 25 0 1200 2400 3600 4800 6000 7200 Time (seconds) 15 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 Temperature °C

(a) (b) Figure 7 a) Loss in wt. of EBF coke samples with increasing temperature in a TGA/DTA furnace; b) Loss in weight of two coke samples (5C & 35C) with time at 900oC in a TGA furnace at UNSW.

4.2 Coke Gasification in a Fixed Bed Reactor

Ina pilot-scale facility, it is difficult to isolate the effect of various factors of coke properties on coke reactivity. Fundamental aspects of coke reactivity can be investigated under laboratory conditions by selecting suitable coke properties. For this purpose, CCSD is conducting a comprehensive research program under which a range of coke samples were prepared using Australian bituminous coals in the 8 kg capacity ovens at 1000°C and heating rate of 3°C min-1 in nitrogen environment. Coke samples were tested for their reactivity towards metal and CO2 and H2O. Gasification results of two cokes (similar rank, ash content and carbon structure) are presented to demonstrate the effect of coke minerals on their reactivity differences under laboratory conditions. Coke analysis and chemical composition of its ash is provided in Table 1. Coke reactivity was measured in a fixed bed reactor using 1- -1 2 g of sample at atmospheric pressure and flow rate of 0.700 L min of CO2. Two infrared analysers were used to determine the concentration of CO and CO2 in exhausted gases, which were used to calculate the reaction rate. Further details can be found elsewhere [21].

Figure 9a compares the apparent reaction rate at 900 ºC with increasing burn off. At 15% conversion, the CO2 surface areas of coke A-1 and coke A-2 increased to 142 and 74. The apparent reaction rates were divided by surface are to calculate the specific reaction rates in order to exclude the effect of surface area. Figure 9b shows that specific reaction rate of coke A-1 is higher than that of coke A-2. In this study, it was shown that higher CO2 reactivity of coke A-1 could not be attributed to carbon structure [21]. Authors proposed that differences in coke reactivity could be attributed to the presence of presence of sulfide and oxide mineral in cokes as illustrated in Figure 10. Coke A-1 which indicated higher frequency of sulfide minerals such as pyrrhotite (Fe1-xS)/troilite (FeS) was found to more reactive when compared to coke A-2 which contained higher amounts of iron oxides. The observations were consistent with previous researchers who also noticed the catalytic effect of iron on coke gasification with CO2 [22-24]. Comparison of coke gasification in both laboratory and EBF cases highlights the significance of coke mineral reactions on coke gasification. Further research is required to understand the exact mechanisms of coke mineral reactions during different conditions of gasification and their potential influence on CO2 reactivity.

) 50 0.4 /s /g Coke A-1 ) s g / g ( 40 / 6 0 g 1 m x

30 ( te te a a 0.2 20 R t R

n Coke A-2 e fic i r c

a 10 e p p p S A 0 0.0 0.00 0.04 0.08 0.12 0.16 Fractional Weight Loss Coke A-1 Coke A-2

(a) (b)

Figure 9 a) Weight loss in cokes with increasing burnout during CO2 gasification in a fixed bed reactor at 900oC; b) Specific reactivity of the same cokes at 15 % burn off.

(a) (b) Figure 10 a) SEM images of typical sulfides and b) oxides mineral phases of iron in both cokes. Coke A appears to have higher iron sulphide minerals.

5 CONCLUSIONS

The coke samples from an experimental blast furnace were reacted with CO2 to monitor weight loss at 900oC in TGA furnace while at the same temperature, custom made coke from Australian coals were reacted in a fixed reactor. The study showed that coke properties in experimental blast furnace continuously change as coke moves down from thermal reserve zone towards cohesive zone which influences the CO2 reactivity. In experimental blast furnace, carbon structure of coke becomes increasingly ordered leading to increased graphitisation, which is attributed to increasing temperatures experienced by coke as it descends into the blast furnace. In the same coke samples, silicon and iron concentration was decreased while alkali concentration, particularly potassium, was found to increase. Both isothermal and non-isothermal reactivity based on TGA measurements clearly established that coke reactivity towards CO2 is improved as the coke moves from thermal reserve zone to cohesive zone and onward in the EBF despite increasing order in their carbon structures. Increased reactivity of cokes at lower parts of furnace could be related to increased concentration of alkali components in cokes. The study suggests that the potential adverse effect of coke graphitisation due to thermal annealing could be compensated by catalytic influence of alkali components of coke in the present test campaign. Most likely, the increased alkali components in coke might be related to condensed alkali from recirculating gases, however microstructural examination of coke will confirm the mechanisms of alkali enrichment in the EBF samples.

Australian research shows that coke gasification in a fixed bed reactor at 900ºC is also strongly influenced by coke mineral rather than degree of ordering of carbon layers in cokes. In both laboratory and industrial and environments of coke gasification, coke minerals were found to have a significant influence on CO2 gasification. Further studies are required to provide a critical insight into the key factors that affect the CO2 gasification of coke particularly the mechanisms of coke mineral reactions and coke graphitisation during different environments of gasification.

6 REFERENCES

[1] ASTM Standard D 5341-93. Standard Method for Measuring Coke Reactivity Index (CRI) and Coke Strength After Reaction (CSR).

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