Evaluation of CO2-reactivity patterns in cokes from coal and woody biomass blends

Fuel 113 (2013) 59–68

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Evaluation of CO2-reactivity patterns in cokes from coal and woody biomass blends M.A. Diez ⇑, A.G. Borrego Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain

h i g h l i g h t s  CO2-gasification of biomass-doped metallurgical cokes was assessed by TG analysis.  Cokes were more reactive and gasification started at a lower temperature.  Different gasification patterns of cokes were identified in the early stages.  Effects of adding wood and charcoal were compared to those of woody constituents.  Contribution of chemical and physical factors on CO2-reactivity was established.

a r t i c l e

i n f o

Article history: Received 9 January 2013 Received in revised form 12 March 2013 Accepted 17 May 2013 Available online 7 June 2013 Keywords: Biomass Charcoal Metallurgical coke CO2 gasification Thermogravimetry

a b s t r a c t Cokes produced at 1000 °C from blends made up of a coking coal and lignocelulosic biomass -Eucalyptus and Olive woods and their charcoals- were evaluated under dynamic and isothermal gasification conditions in a CO2 environment. The effects of adding different types of biomass to coal in quantities as low as 2 wt% were compared with the effects caused by the addition of wood constituents -xylan, cellulose and lignin-. Not only were cokes more reactive than the coke produced without the addition of biomass, but they also exhibited a lower threshold temperature during the Boudouard reaction. Comparison of the CO2-reactivity profiles of the cokes showed to be a suitable protocol for detecting differences in low-temperature gasification caused by the residual biomass present in the matrix of high-temperature coke and also for selecting biomass to be blended with coking coals. An additional CO2-gasification cycle of the partially-gasified cokes showed that the inner core of the coke exhibited a lower reactivity and conversion degree than the original parent cokes. Differences in CO2-reactivity between the partially-gasified cokes were found to be negligible. The effects of the oxygen content and the ash chemistry of the biomass, the Gieseler fluidity of its blend with coal, the presence of char particles in the coke matrix and the CO2 surface area of the cokes were shown to contribute to explain the differences found in coke reactivity. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Carbon is the most suitable agent for reducing iron oxides in pig iron production in a blast furnace (BF). Metallurgical coke obtained by carbonizing selected coking coals at temperatures in the range of 950 and 1100 °C, is the source of carbon used for this purpose. In the middle section of a blast furnace, coke is in a practically isothermal zone at a temperature of around 950 °C. Referred to as the thermal reserve zone (TRZ), it has been directly linked to the production of reducing gases such as carbon monoxide as a result of the gasification of coke with carbon dioxide. Carbon reacting is limited by the supply of CO2 from the inner chemical reactions in a BF and it is always dependent on the strongly endothermic Boudouard reaction, also called the carbon solution-loss reaction: ⇑ Corresponding author. Tel.: +34 985119090; fax: +34 985297662. E-mail address: [email protected] (M.A. Diez). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.05.056

C þ CO2 ! 2CO  172:4 J=mol

ð1Þ

The rate and the amount of coke that is gasified in the thermal reserve zone (TRZ), -before it reaches the lower zones of the blast furnace where coke is needed as a carburizing agent, fuel and permeable support-, is considered as one of the most important criteria for evaluating coke quality. The Japanese method developed by the Nippon Steel Corporation (NSC) [1] and adopted by the ASTM and ISO standard Institutions (ASTM D5341:93a and ISO 18894:2006) is universally accepted for measuring the CO2-reactivity of cokes produced in industrial ovens. In this test, the percentage of mass loss in a dried coke sample (200 g, 19–22.4 mm) gasified at 1100 °C for 2 h in a constant stream of CO2 (5 l/min flow rate) is taken as the coke reactivity index (CRI). Coke reactivity assessment expands CSR with the measurement of the mechanical strength of the partially-gasified coke in an I-type drum device (CSR index). Although the experimental conditions are more extreme than inside the blast furnace and the effects of other gases

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present in the cohesive zone are not taken into consideration, the CRI and CSR indices provide a useful guide of coke behavior for steelmakers. In general, there is a high degree of correlation between the two indices [1–3], both of which show that if coke reacts excessively with carbon dioxide, coke will weaken and will disintegrate into smaller particles. In other words, as the CRI increases, the CSR decreases. The operation of a blast furnace is affected by two factors that influence coke reactivity: (1) the strength of the coke arriving in the tuyere zone -which is reflected by the CSR index- and a mass loss of 25–30% (CRI) [3,4], and (2) the temperature at which carbon gasification starts (threshold or onset temperature) [4]. Recently, the Japanese steel industry introduced an innovative blast furnace operation in order to reduce CO2 emissions, to improve the efficiency of the reduction reaction and to decrease the consumption of the reductant [5]. This approach is based on controlling and lowering the thermal reserve zone temperature (TRZT), which in turn will decrease the threshold or onset temperature of coke gasification represented by the Boudouard reaction (Eq. (1)). As a consequence, the temperature interval in which this reaction occurs will be extended, causing an increase in the gasification rate. Naito et al. [5] have demonstrated the effectiveness of this technology by using highly reactive cokes and carbon-containing ore agglomerates in blast furnace simulators. They found that by employing highly-reactive lump cokes, the rate of consumption of the reducing agent can be decreased by approximately 25– 35 kg/t pig iron. Nomura et al. [6] investigated the behavior of cokes produced by adding a Ca-based catalyst to coal (Ca pre-addition) or by impregnating the coke surface with a Ca catalyst (Ca post-addition) in order to increase coke reactivity towards CO2 and to decrease the threshold gasification temperature, while maintaining the coke strength. Industrial trials in the Muroran blast furnaces with pre-addition Ca-doped cokes revealed a 10 kg decrease in the rate of consumption of reducing agent per ton of pig iron. The production of a highly reactive and, at the same time, highly resistant coke presents a great challenge in R&D cokemaking because of the well-known relation between the CRI and CSR of cokes produced from coal blends, (i.e. the higher the CRI, the lower the CSR and viceversa). The above theory is based on the control of the equilibrium point of the FeO–Fe reduction in RIST diagram, with the subsequent lowering of the temperature in the thermal reserve zone, increasing the reduction potential of the gas (CO/CO2 ratio) and acceleration of the reduction of iron oxides by CO-enriched gas. Babich et al. [7] have reported, however, that a shift towards low temperature of the FeO–Fe reaction does not automatically decrease carbon consumption in a BF. Several factors such as extension of direct reduction, kinetic and reduction behavior of iron oxides can hinder it. On the other hand, in modern BFs using pulverized coal injection (PCI) they require a low reactive coke which increases the gasification temperature. In order to increase productivity, the use of small-sized coke (so-called nut coke) would have some beneficial effects. Based on the effects of alkalies, char and pulverized coal ash on the behavior of nut coke for increasing the CO2 reactivity of the coke and for decreasing coke consumption, the authors concluded that nut coke would be more susceptible to react with CO2 and it can protect blast-furnace coke of degradation in the shaft. At the same time, carbon–neutral feedstocks from renewable and sustainable sources are considered an attractive alternative in the iron and steel industry for partially replacing fossil fuels and for reducing the emissions of CO2 [8–26]. Carpenter [27] recently reviewed methods of abating CO2 emissions from raw materials preparation (coking, sintering and pelletising plants) in the production of liquid steel in basic oxygen furnaces and electric arc furnaces. In the steel industry, most of the research has been focused on replacing the coal injected or coke charged into the

blast furnace with biomass. Less attention has been paid to the use of biomass as an additive to coal blends. In this context, Hanrot et al. [14] reported that the amount of charcoal added to a sufficiently fluid coal blend is limited to a small quantity (63 wt%) in order to produce a more reactive coke and a decrease in the threshold gasification temperature by about 100 °C. Nevertheless, using such cokes will reduce the amount of carbon required in a BF and contribute to a mitigation of CO2 emissions. MacPhee et al. [15] have also established a limit to the amount and the particle size of charcoal that can be added to coal blends. They observed that the addition of coarse-sized charcoal does not have the strong negative effect on CRI and CSR as that of fine charcoal. One of the factors responsible for the drastic increase in CRI, according to these authors [15] is that finely dispersed calcium may act as a catalyst agent in coke gasification, whereas in cokes to which coarse charcoal is added the Ca-mineral species would be dispersed among discrete areas and would experience less catalytic activity. More detailed results on coke reactivity have been reported by Ng et al. [19,20]. They found that the addition of 5 wt% charcoal produced a more reactive coke and that, at the same time, there was a shift to a lower value in the threshold gasification temperature (approx. 50–70 °C). Similar effects on coke reactivity and strength were found by Thomas et al. [21], when adding 4 wt% of torrefied wood chips to a coal blend. They observed that the increase in CRI was influenced by the amount of the added biomass with a particle size of less than 3.35 mm. From the results outlined above, it can be deduced that careful control of the coking characteristics of the coal blend as well as those of the biomass (type, quantity, particle size distribution) is essential for producing cokes with slightly increased CO2 reactivity without causing any serious deterioration in their mechanical properties. The main aim of this study is to investigate the effect that adding various types of biomass -Eucalyptus and Olive woods and their corresponding charcoals- to a coking coal has upon the CO2 reactivity of cokes produced at a laboratory scale. To this end, thermogravimetry was used to perform coke gasification under dynamic and isothermal conditions in order to obtain relevant information about any developments and changes occurring in the course of the Boudouard reaction.

2. Experimental 2.1. Raw materials A coking coal A with a volatile matter content of 21 wt% db and a Gieseler maximum fluidity of almost 400 ddpm was used as the base coal for blending with two types of wood (Eucalyptus and Olive) and their corresponding charcoals obtained at 415 and 450 °C, respectively. Coal A gives a coke with very low CRI and high CSR (18% and 69%, respectively). Eucalyptus was selected because of its rapid growth and adaptability to different soil qualities as well as its widespread use as a charcoal feedstock in metallurgy, especially in mini blast-furnaces. Olive, however, is a fruit tree of slow growth and of great agricultural relevance in the sunny coastal regions of the Mediterranean Basin. The Olive wood used in this study was obtained from the pruning process. By TG analysis, it can be observed that Eucalyptus and Olive woods differ in thermal stability. Thermal decomposition of Olive wood components occurs at a temperature much higher than Eucalyptus. For this reason, the Olive charcoal was produced at 450 °C, whereas Eucalyptus at 415 °C. Commercial cellulose (powder, 20 lm), xylan as being representative of hemicellulose (powder, <600 lm) and lignin (alkali lignin with an average molecular weight of 28,000, particle size <212 lm) from Sigma Aldrich were also employed. The following

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instrumental equipments were used for the proximate (LECO TGA601) and elemental (LECO CHN-2000 for carbon, hydrogen and nitrogen, LECO S-144DR for sulfur) analyses of the coal and biomasses [28]. The oxygen content was calculated from the mass balance. The thermoplastic properties of coal A and its blends with biomass were tested in a constant-torque Gieseler plastometer, R.B. Automazione PL2000, following the ASTM D2639 standard procedure. The parameters derived from this test are: (i) coal softening temperature (Ts); (ii) temperature of maximum fluidity (Tf); (iii) resolidification temperature (Tr) of the fluid mass into a semicoke; (iv) the plastic or fluid range (Tr–Ts); (v) maximum fluidity (Fmax), expressed as dial divisions per minute (ddpm). Table 1 shows the results of the proximate and ultimate analyses of coal A and the additives (wood, charcoal and woody components) together with the Gieseler fluidity parameters of the blends. The ash composition of coal A was determined by X-ray fluorescence (Siemens-Bruker SRS3000), according to the ASTM D4326-04 standard; while that of woods was determined by ICP-MS (Agilent 7700x series), previous a digestion process of the ash samples. The major elements analyzed were those generally present in coals and used to define the basicity or alkali index (Si, Al, Na, K, Ca, Mg, Fe). This index is taken as an indicator of the catalytic activity of the inorganic components of coke during CO2 gasification and it is widely used in prediction models of CRI and CSR [3]. The basicity index is defined by the ratio between the basic and acid components in the coal ashes (A) [29] and can be expressed as:

BI ¼ A

  Na2 O þ K2 O þ CaO þ MgO þ Fe2 O3 SiO2 þ Al2 O3

ð2Þ

The basicity index modified by Price et al. [30] for Western Canadian coals, which considered the ash content (A) and chemistry and the volatile matter content (VM), was also calculated.

MBI ¼



100 A 100  VM



Na2 O þ K2 O þ CaO þ MgO þ Fe2 O3 SiO2 þ Al2 O3



ð3Þ

300 ml/min to sweep the volatiles evolved during carbonization and to ensure inertness during the coke quenching. In all the tests, the amount of sample used was about 25 g and the particle size less than 1 mm, except in the case of the model compounds. The addition rate of each renewable additive was 2 wt%. Each coke is referred to by the first letter of the coal (A), followed by the addition rate and the two or three initial letters of the biomass added to the coal. 2.3. Coke reactivity The gasification experiments were carried out in a thermogravimetric analyzer (SDT2960 TA Instruments) under dynamic (with a linear increase in temperature) and isothermal conditions. A representative sample of about 7 mg of coke with a particle size <0.212 mm was placed into an open platinum crucible of approx.110 ll capacity (3.65 mm height  6.15 mm diameter). Before gasification with CO2, a pyrolysis step from room temperature up to 1000 °C at a nitrogen flow rate of 100 ml/min was programmed to eliminate any moisture and residual volatile matter from the cokes. Afterwards, the sample was cooled down to 750 °C in the nitrogen atmosphere and a short period of stabilization was programmed to ensure thermal stability. Next CO2 was supplied at a flow rate of 90 ml/min and the gasification curves were obtained by heating the coke sample from 750 to 1000 °C at 10 °C/min. The sample was then isothermically maintained at this temperature (1000 °C) for 2 h. For selected samples, gasification tests were carried out, at least twice, in order to obtain reliable results with a good repeatability. Partially-gasified cokes (A, A2CEu and A2COl) were recovered after each test in order to form a global sample of almost 7 mg, which was subjected to a second cycle of gasification in CO2 under the same experimental conditions as those described above. The reactivity towards CO2 (R) and the conversion degree (X) were calculated from the mass loss of the sample as a function of time and temperature, using the following expressions:

R ¼ 2.2. Co-carbonization of coal and woody biomass Cokes were produced from coal A and its blends with the biomasses in a horizontal electric furnace heated from room temperature up to 1000 °C at a rate of 5 °C/min [23]. The final temperature was maintained for 15 min. Nitrogen was supplied at a flow rate of

X¼

1 dm dX ¼ mo dt dt

ð4Þ

ðmo  mÞ 100 mo

ð5Þ

where R is expressed in %/min, mo is the initial mass of coke in mg, measured after removing the moisture and volatiles and after

Table 1 Proximate and elemental analysis (expressed on a dry basis) of coal, wood, charcoals and woody components and Gieseler fluidity parameters of the coal A and its blends with a 2 wt% biomass addition. Coking coal

Eucalyptus

Olive

Wood

Charcoal

Wood

Charcoal

Xylan

Cellulose

Lignin

Code Ash (wt% db) VM (wt% db) Fixed C (%) C (wt% db) H (wt% db) N (wt% db) S (wt% db) O (wt% db) H/C atomic ratio O/C atomic ratio

A 9.0 21.2 69.8 82.46 4.52 1.67 0.6 1.7 0.65 0.015

Eu 0.55 83.7 15.8 46.73 5.88 – 0.04 46.80 1.54 0.75

CEu 0.37 30.4 69.2 83.30 3.07 0.08 – 13.18 0.44 0.12

Ol 0.71 84.8 14.5 52.11 6.04 0.23 – 40.91 1.39 0.59

COl 2.48 24.4 73.1 82.54 3.20 0.59 0.01 11.18 0.47 0.117

XYL 11.7 70.3 18.0 39.43 5.54 0.15 0.03 43.15 1.69 0.82

CEL 0.0 95.8 4.2 44.30 6.53 0.18 0.02 48.87 1.77 0.83

LIG 14.3 50.6 35.1 56.48 5.02 0.24 2.49 21.47 1.07 0.39

Gieseler fluidity parameters Fmax (ddpm) Ts (°C) Tf (°C) Tr (°C) Plastic range (°C)

389 410 463 497 87

180 412 465 495 83

321 411 462 498 87

233 420 465 499 79

334 425 467 497 72

388 406 460 495 89

200 410 464 494 84

271 410 461 497 87

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switching from N2 to CO2, m is the coke mass at any time or temperature, dm/dt is the rate of mass loss per unit of time expressed in mg/min and X is the conversion in per cent. 2.4. CO2 surface areas of cokes Surface areas were determined in a Micromeritics ASAP2020 instrument by CO2 adsorption at 273 K using the Dubinin–Radushkevich equation [31]. Coke particles <0.212 mm in diameter were degasified at 350 °C for 4 h, before analysis. 2.5. Optical microscopy To identify how residual biomass was incorporated into the coke matrix, polished coke surfaces, previously mounted in epoxy resin, were examined in a polarized-light Leica DM4500 P microscope with a one-wave retarder plate and at an overall magnification of 500 and 1000 in oil immersion. 3. Results and discussion 3.1. Effects of biomass addition on coke CO2-reactivity Fig. 1 shows the conversion degree and mass loss rate curves for the two-stage CO2-gasification obtained from the different runs of cokes produced from coal A and its blends with the 2 wt% additions of different biomasses. Relevant parameters from the thermogravimetric curves are shown in Table 2 along with the standard deviations resulting from four TG runs in the case of coke A and those produced by adding raw biomass and charcoal. A good repeatability with standard deviations of on average 2.6% of conversion and 3.3% of reactivity can be observed (Table 2). From a qualitative point of view, the reactivity profiles of the cokes produced from coal A and 2 wt% addition of the different types of biomass seem to undergo a similar evolution, with reactivity towards CO2 reaching a maximum value (Rmax) and then decreasing during conversion and tending towards a stability (Fig. 1). The most important part of the conversion is always initiated from prolonging the action of CO2 under isothermal conditions. A visual inspection of the reactivity and conversion patterns of the cokes produced by adding biomass to coal reveals some remarkable features that reflect both similarities and differences in their response to the action of CO2: (a) the initial stage of reaction, where reactivity reaches to a maximum value, differs from one coke to another, specially during the first 20 min in the dynamic regime of gasification; (b) the coke from Eucalyptus charcoal clearly exhibits a higher reactivity and achieves a higher final

Fig. 1. Evolution of the conversion degree and reactivity to CO2 of the cokes produced from coal A and its blends with biomass (wood and charcoal) added at a rate of 2 wt%.

conversion than the other cokes tested; (c) the two cokes produced by adding charcoal from Eucalyptus and Olive are more reactive at any specific value of conversion than the cokes produced by adding the parent woody biomass; (d) the different wood species influence reactivity only slightly, the most remarkable difference occurring in the low temperature region; (e) after Rmax has been reached at the end of the isothermal conditions at 1000 °C, reactivity seems to remain within a very narrow domain rather than undergo any further decrease with time (0.150–0.179%/min); and (f) the final conversion is greatly influenced by the early gasification step. The overall gasification behavior of the cokes, reflected by the reactivity profiles, clearly conforms to the well-known patterns of porous coal-based materials [29,32–40]. Reactivity depends on the access the reactant gas has to the internal surface area of the coke. This is influenced by the continuous evolution of the carbon structure with increasing temperature and the residence time in a CO2 environment as well as the presence of catalytic species. In the initial stage of the gasification reaction, the more disorganized carbon, -where the structural defects and active sites reside-, is gradually activated by the adsorption of the gasifying agent. Consequently, reactivity up to a certain limit that is characteristic of each coke is enhanced. Cokes from coals, when examined under a polarized-light microscope are found to consist of isotropic and anisotropic areas, the latter differing in size and shape [29,32– 35]. It is widely accepted that the isotropic carbon texture originates mainly from the inertinite present in the parent coal and partly from very reactive macerals which react preferentially with CO2 [29,32–35]. Charcoal as a non-graphitizable carbon produced by solid–gas carbonization, is isotropic in nature and maintains the porous cellular wall structure in the wood precursor. In general, isotropic charcoal can be considered as a type of coal inert. Hence, the addition of charcoal to coal provides more accessible sites for CO2 as is apparent from the reactivity profiles in Fig. 1. However, it is important to note that not all kinds of isotropic material within the coal-derived cokes and wood/charcoal respond to CO2 in the same way. As the reaction progresses (Fig. 1), the available active surface area diminishes due to the reduction in the number of potential gasification sites per residual mass and the accumulation of more ordered carbon structures and more resistant sites to CO2 under the isothermal conditions tested (1000 °C) [36]. As a result, reactivity decreases and thereafter tends to be fairly constant. Furthermore, the influence of mineral species from wood acting as a catalyst of carbon gasification under controlled conditions cannot be excluded. These impurities undergo significant changes at the beginning of the reaction and become less catalytically active agents in the course of the Boudouard reaction [37–40]. A more detailed inspection of the shape of the CO2-reactivity in the non-isothermal stage reveals a first region up to around 15 min (915 °C) in which the reactivity increases slowly, the coke undergoes surface activation and the conversion is negligible (<1%). This period is followed by a continuous increase in reactivity up to a maximum value (Rmax) with a consequent increase in the degree of conversion (X). In this gasification stage, the continuous temperature increase is the main factor responsible for the increase in coke reactivity. The Rmax value lies at 0.442 ± 0.015 for coke A and varies from 0.426 ± 0.004 to 0.548 ± 0.041%/min for the different additives (Table 2). For coke A, Rmax occurs at the beginning of the isothermal conditions at 1000 °C after a holding period of 10 min (global gasification time of 34 min), where the conversion degree is still low (almost 7%). When raw biomass is added to the coal, the coke conversion is slightly higher (7.4% and 8.8%) and the time to reach Rmax slightly longer (35–37 min). However, in the case of charcoal addition, Rmax is in the vicinity of the end of the dynamic conditions, after 23 and 29 min of global reaction, and the conversion degree is 4.8% and 5.8% for A2CEu and A2COl,

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M.A. Diez, A.G. Borrego / Fuel 113 (2013) 59–68 Table 2 Some gasification parameters and CO2 surface areas corresponding to the cokes produced from coal A and its blends with different biomasses. Coke

A

A2Eu

A2CEu

A2Ol

A2COl

A2XYL

A2CEL

A2LIG

Xf (%) Rmax (%/min) X at Rmax (%) t at Rmax (min) T at Rmax (°C)

35.1 ± 0.9 0.442 ± 0.015 6.8 34 1000

36.6 ± 0.9 0.438 ± 0.010 7.4 35 1000

39.2 ± 1.0 0.548 ± 0.041 4.8 23 996

36.6 ± 0.9 0.454 ± 0.010 8.8 37 1000

37.4 ± 1.1 0.426 ± 0.004 5.8 29 1000

39.0 0.496 6.3 29 1000

40.0 0.598 7.7 28 1000

43.6 0.689 6.4 26 999

2.6 15.2 24.4 nd

4.9 19.1 27.9 87

2.5 15.7 24.8 nd

3.4 15.5 24.6 54

3.7 17.2 26.3 41

3.9 20.0 28.9 87

4.7 21.8 31.4 82

Conversion at 1000 °C and different times X1000/0 (%) 2.2 X1000/30 (%) 14.9 X1000/60 (%) 23.8 CO2 surface area (m2/g) 25

Xf: final conversion at 1000 °C and 120 min; Rmax: maximum reactivity; X1000/t: conversion at 1000 °C and different times (t); nd: not determined.

respectively. Longer periods of exposure to CO2 at 1000 °C produce a gradual decrease in reactivity which is accompanied by a significant increase in coke conversion. Such differences and similarities are reflected in a condensed form in Table 2 in several gasification parameters such as: final conversion at 1000 °C and 120 min of reaction (Xf), maximum reactivity (Rmax) and the conversion degree, time and temperature at which Rmax takes place. The results show that, over the whole two-step gasification process, the final conversion order for cokes produced by adding raw biomasses and charcoal to coal A is:

A < A2Eu  A2Ol < A2COl < A2CEu The same trend is not so clearly observed in the case of the Rmax values. The most striking difference corresponds to the most reactive coke A2CEu, the other cokes varying within a very narrow window. The conversion degree at maximum reactivity is always lower for cokes obtained by adding two charcoals and greater for cokes obtained by adding raw wood to the coal. Reference coke A experienced an intermediate level of carbon gasification. Thus, the active sites in the carbon network of cokes A2Eu and A2Ol seem to be more easily accessible (greater conversion at Rmax), although the preferential attack by CO2 is slower (lower Rmax and longer times) than for the charcoal-derived cokes. The specific surface areas available for CO2 attack were higher for the cokes prepared with charcoal than for the reference coke (Table 2). The CO2 surface area of A2CEu (87 m2/g) is greater than that of A2COl (54 m2/g) despite the former is expected to yield less amount of char than the latter (see fixed carbon in Table 1). The variations in reactivity can be initially explained in terms of the amount of char formed from wood (nearly 15 wt%) and the maximum fluidity during the co-carbonization of the feedstocks as this is a major factor in controlling the size and shape of the optical texture of the resultant cokes [29,41]. It has been demonstrated that Eucalyptus wood is a stronger inhibitor of Gieseler fluidity than charcoal (180 vs. 321 ddpm for blends A2Eu and A2CEu, respectively). The same trend is found for the pair made up of the parent Olive wood and the charcoal (233 vs. 334 for A2Ol and A2COl, respectively). The large reduction in fluidity caused by the wood is further supported by the results obtained for a series of twelve different wood species and their corresponding charcoals [42]. This reduction might be due to: (1) the formation of highly-oxygenated degradation compounds in the early stages of co-carbonization where the chemical reactivity of the coal increases and, so early polymerization leads to the formation of a more disordered carbon; and (2) the formation of porous char particles that may become encapsulated inside the coke matrix [22–24]. In both cases, the carbon forms are more susceptible to attack by carbon dioxide. In summary, the data collected show that a common feature of all the reactivity profiles of the cokes obtained by adding biomass

to coal is that there is a characteristic pattern in the low-temperature region which will be responsible of a decrease in the threshold or onset temperature of gasification as described below. 3.2. Influence of adding woody constituents to coal on coke CO2reactivity To investigate the different gasification patterns of cokes produced by adding the different types of biomass, an additional series of cokes produced from blends of the base coal A and the individual wood constituents, xylan -as being representative of hemicellulose-, cellulose and lignin, were subjected to the two-stage gasification process. Fig. 2 displays the conversion and reactivity profiles of this series of cokes. For comparison purposes, coke A is also included in the graph. Each coke undergoes a very rapid increase in reactivity as in the case of A2CEu and A2COl, reaching maximum values in a shorter time than coke A. Table 2 contains the numerical values corresponding to several conversion degrees and parameters which define the maximum reactivity for the cokes produced from coal A plus wood constituents. Lignin has the most marked effect on coke reactivity. Coke A2LIG is the most reactive; it reaches the highest Rmax in a shortest reaction time and has the highest final conversion (43.6%). Cellulose exhibits an intermediate Rmax, while xylan has a moderate effect on coke reactivity. The CO2 specific surface areas of cokes with model compounds (Table 2) indicate that despite the low char yield of the model compounds, particularly cellulose and xylan (Table 1), their presence favor the development of surfaces available for CO2 reaction. In view of the results obtained for model woody components, the following Rmax increasing order can be established:

Fig. 2. Evolution of the conversion degree and reactivity to CO2 of the cokes produced from coal A and its blends with wood constituents (xylan, cellulose and lignin) at an addition rate of 2 wt%. Coke A is shown as reference.

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A  A2Ol  A2Eu  A2COl < A2XYL < A2CEu < A2CEL < A2LIG 3.3. Overall effects of biomass on coke CO2-reactivity To provide an overview of how different chemical, structural and thermochemical properties of biomass and its blends with the coking coal A affect the reactivity of the cokes studied, their different relationships are analyzed (Fig. 3). The number of samples is limited, but the relationships detected are important for understanding the reactivities of the cokes studied. Fig. 3 clearly shows a relationship between the oxygen content of wood and other forms of biomass and the rheological properties of the blend, e.g. a decrease in Gieseler fluidity with the increasing oxygen content of the biomass. The outlier corresponds to xylan which has a high oxygen content, but fluidity development is not inhibited by a 2 wt% addition of this biomass. The lack of any fluidity suppression by xylan at this low addition rate has been previously attributed to its low thermal stability and the type of oxygen functionalities present in this woody component [22,24]. Xylan decomposes at a temperature below 300 °C, before coal undergoes any softening or chemical transformation, and evolves oxygenated compounds such as furfural and its derivatives, which have less influence on fluidity [22,24].

Fig. 3. Relationships between the oxygen content of the biomass added, the Gieseler fluidity of the blends and the final conversion (Xf) of the CO2-gasification of the cokes.

It is well known that the oxygen content in different rank coals and the Gieseler fluidity affect the carbonization chemistry and, then, the development of the anisotropic textures of the resultant cokes [29,35,42]. Fig. 4 displays optical micrographs of selected cokes A2LIG, A2CEu and A2COl. Charcoal particles of different sizes and morphologies are easily recognized in the coke matrix. Some of them retain their original morphology, i.e. open circular or elongated pores arranged approximately in parallel and vertical alignment (position Z) with or without anisotropic carbon filling cells (positions Y and Z, respectively). Additionally, some few particles present multiple elongated and parallel porosity with bar-like isotropic carbon between the openings (position X). Not only is the char morphology different, but also the interaction with anisotropic carbon from the coal. There is a clearly defined boundary with a high degree of fissuring or a good bonding between the particle components of the two materials (the charcoal and coke matrix). The A2LIG coke also presents some isotropic porous particles, but its morphology is different from that of the charcoals from Eucalyptus and Olive. They appear as massive particles with irregular pores of different sizes. Due to the small amount of biomass added to the coal, the quantitative assessments of the optical texture are similar for all the cokes, with differences close to those of the experimental error. In such cases, fluidity seems to be very sensitive to even small additions of biomass, reflecting the thermochemical history of the blend. Coke reactivity as reflected by the final conversion achieved is dependent on the oxygen content of the biomass added to the coal as well as the Gieseler fluidity of the blend (Fig. 3). Two different populations can be distinguished. The first group is formed by the two charcoals and lignin, while the second includes the two woods and their major constituent, cellulose. In the first group, the final coke conversion increases with increasing oxygen content and decreasing fluidity of the charcoal-lignin series. To this respect the Eucalyptus and Olive charcoals differ. However, this difference cannot be appreciated from the parent woods. Equivalent relationships are also found between the different parameters reflecting reactivity, i.e. conversion degrees at 1000 °C and different times. It has been widely recognized that the mineral matter of coal and the way it is dispersed in the inert material affect fluidity and the coke properties [43]. In prediction models of CRI and CSR, the chemistry of coal ash is reflected by the basicity indices defined by Eqs. (2) and (3). In general, coals enriched in acid oxides, Al2O3 and SiO2, produce less reactive coke than those enriched in basic oxides Fe2O3, CaO and MgO, or alkalis Na2O and K2O. The latter catalyze carbon consumption by CO2 and, then, increase the coke reactivity. Table 3 shows the ash composition of the coal A and the two woods together with the basicity index values. The two parent woods differ in the major elements that are found in their ashes. Eucalyptus wood is characterized by a high proportion of iron, while the major element in Olive wood is calcium. Thus, it is not surprising that, even with the addition of a small amount of wood, iron- and calcium-bearing minerals contribute to increasing the reactivity of the cokes towards CO2 through a catalytic action. If the additivity law for components of the blend to be carbonized is applied, the basicity indices (BI) of the blends containing wood and charcoal are 1.18 and 1.22 for the Eucalyptus and Olive blends, respectively; slightly higher than that of coal A (1.12). MBI values follow the same trend, 1.52–1.58 vs 1.42 for coal A. On the other hand, an increase in microporosity as measured by CO2 adsorption (Table 2) will also contribute to increasing coke reactivity. Indeed, the highest reactivities were achieved for cokes having the highest CO2 surface areas (A2CEu, A2CEL and A2LIG) although this factor by itself does not explain the differences in reactivity observed.

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Fig. 4. Optical micrographs of selected cokes A2LIG, A2CEu and A2COl (500 oil immersion). Z, Y, X and V: residual biomass with different morphology in coke matrix.

Table 3 Major elements present in coal and wood ashes. Element oxides

Coal A

Eu

Ol

SiO2 Al2O3 Na2O K2O MgO CaO Fe2O3 Basicity index (BI) Modified basicity index (MBI)

51.3 34.3 0.5 2.2 0.8 2.6 4.7 1.12 1.42

35.2 19.2 1.7 2.0 3.7 7.1 30.4 0.45 2.78

14.0 2.6 3.4 9.4 3.0 39.1 5.4 2.59 17.01

In summary, the oxygen content and functionality of the additive (wood, charcoal, woody components), the ash composition of the raw materials, the inhibition of fluidity development in the carbonized blend, the presence of porous isotropic particles from the biomass and the microporosity of the cokes all contribute to the increase in CO2-reactivity of the cokes. Furthermore a certain degree of interrelation between these factors helps to explain the differences in the gasification behavior of the cokes. 3.4. The early non-isothermal reactivity stage Not only are the cokes more reactive, but also the CO2-gasification commences at lower temperatures when biomass is added.

This is evident from a comparison of the reactivity profiles (Fig. 1 and 2) where certain relevant events during the first 24 min of the dynamic gasification process merit special attention. At low temperature and after a short gasification time, cokes from blends with raw woody biomass (A2Eu and A2Ol) and Olive charcoal (A2COl) display a major and broad peak preceded by a shoulder located at a low temperature. This shoulder differs in its relative intensity and is more pronounced for the coke derived from the addition of Olive charcoal. The characteristic shoulders are located in a temperature interval between 900 and 950 °C. In the case of coke A2CEu, however, the hump seems to be obscured by the broad peak, due to the greater slope in this region and the small temperature gap between them. This can be attributed to the contribution of structural imperfections, the high surface area of low-temperature charcoal and the catalytic inorganic species which survives the co-carbonization with coal at 1000 °C. When a slow heating rate (3 °C/min) is applied in the non-isothermal gasification step of the A2CEu coke, it is easier to separate the two overlapping peaks (Fig. 5). In order to quantify the temperature changes, the temperature at which the reactivity reaches a value of 0.1%/min is taken as the threshold or onset temperature of gasification. This criterion is totally arbitrary and only one condition is imposed: the reactivity value should be set somewhere before the shoulder. To illustrate more clearly at what temperature the carbon gasification starts, the reactivity as a function of temperature is displayed on a larger

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Fig. 5. Evolution of the reactivity to CO2 of the cokes produced from coal A and charcoals added at a rate of 2 wt% at a low heating rate (3 °C/min).

scale in Fig. 6 together with the variation of the estimated temperatures. All the cokes obtained by the addition of biomass exhibit a lower temperature (Tcoke) than the reference coke (Tr = 927 °C). The order of the threshold gasification temperature becomes:

Xylan evolves the largest amount of volatile products in the early stages of carbonization, maintains enough fluidity during co-carbonization and gives less char than lignin. Cellulose is the strongest modifier of the chemistry in coal carbonization and most of it decomposes into tar and permanent gases. Only a very small amount of char is produced [23]. The lowering of the threshold carbon-gasification temperature or the Boudouard reaction caused by the addition of biomass to coke ranges between 29 and 43 °C for the additives yielding low residue (wood, cellulose and xylan) and between 49 and 60 °C for charcoals and lignin (Fig. 6). This is in good agreement with previous results by other authors reporting differences between 50 and 100 °C [14,19,20] for cokes produced by adding charcoal, although in these studies the onset temperatures were generally lower than those of Fig. 6 [19,20]. It was also reported that the addition of charcoal to coal A has a similar effect to that observed for cokes produced from blends containing large proportions of high-volatile coals, cokes with a low degree of carbonization or semicokes produced from a low-temperature carbonization [4]. In all cases, a TRZ temperature below 950 °C was reported by Loison et al. [4]. This benefit, however, is accompanied by a loss of coke strength.

A > A2Eu > A2CEL > A2XYL > A2Ol > A2LIG > A2COl > A2CEu The temperature variations are consistent with the carbonization behavior of the wood components and the contribution of the derived-char to the structure of the high-temperature coke. The most significant changes correspond to the lignin and charcoals. Lignin decomposes slowly over a very broad temperature interval during co-carbonization and it is the wood component with the highest fixed carbon content (Table 1). It contributes to a greater extent to the formation of charcoal. The two components based on polysaccharides (xylan and cellulose) cause a smaller decrease in temperature and the cokes A2XYL and A2CEL derived from them are located between the Eucalpytus and Olive woods.

Fig. 6. CO2-reactivity as a function of the programmed temperature and variation of the threshold gasification temperature for the cokes calculated at a reactivity value of 0.1%/min. Tr and Tcoke are is the temperatures calculated for the reference coke A and the cokes produced by adding any biomass, respectively.

3.5. CO2-reactivity of partially-gasified cokes In the gasification profiles presented in Figs. 1 and 2, different types of available active sites which differ in CO2 activity as the temperature increases can be outlined. The optically isotropic carbon into the coke matrix is a combination of isotropic material formed from the incorporation of residual biomass and/or from the modifications induced in the coke structure as well as the unfused inertinite components existing in coals that undergo limited transformation during carbonization can be expected to react more intensively during CO2 gasification at low temperatures than the anisotropic units of the coke. This means that after CO2 treatment isotropic carbon and other reactive units from coal and biomass will be preferentially consumed while the remaining mainly anisotropic carbon textures of the partially-gasified coke will be more resistant to the action of CO2. The partially-gasified cokes from coal A and its blends with charcoals (A2CEu and A2COl) were subjected to a second CO2-gasification cycle under the same reaction conditions (heating up to 1000 °C at 10 °C/min followed by a 120 min holding period). Fig. 7 contrasts the evolution of conversion and reactivity of the partially-gasified cokes A, A2CEu and A2COl during the second gasification cycle with those of the parent coke. The reactivity profiles fulfill the general conditions of gasification behavior established for the original cokes, one of which was that reactivity first reaches a maximum value after which it declines as gasification progresses. It seems, however, that the differences in reactivity and conversion of the inner core of the cokes are not very pronounced, especially in the case of the coke produced from the addition of Olive charcoal to the coal (A2COl). This partially-gasified coke shows similar reactivity profiles to that of the reference coke A, since it has a final conversion degree of around 20 wt% and a maximum reactivity rate of 0.23%/min (Table 4). The temperature at the onset of the reaction is nearly the same (915 °C) and it is always higher than those of the parent cokes. All three cokes show a converging tendency in their reactivity profiles after being subjected to gasification at 1000 °C for 120 min, with values of the order of 0.131–0.137%/min. Consequently, it is reasonable to conclude that after a ‘‘peeling effect’’ on the coke surface during the first cycle of gasification, the carbon that remains inside the inner core of the coke particles reacts in a similar way towards the gasifying gas under the same experimental conditions.

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produced from the single coal in terms of maximum reactivity and final conversion degree. Overall, the cokes obtained by charcoal addition were more reactive than those prepared by the addition of raw biomass. The wood constituents -xylan, cellulose and ligninused as model additives, helped to shed more light on the different gasification characteristics of the woody species remaining inside the matrix of high temperature cokes. Such species could be responsible for the presence of a low-temperature shoulder that precedes the main reactivity peak of the coke. On the basis of the shape of the reactivity profiles and the intensity of the low-temperature shoulder, reactivity patterns for the cokes can be established, making it easier to explain the differences between the cokes obtained from coal with added biomass. The oxygen content and functionality, the ash composition and the isotropic porous nature of the biomasses, the inhibition of fluidity development in the carbonized blend and the microporosity of the cokes, all contribute to explain the increase in CO2-reactivity of the cokes. Furthermore a certain degree of interrelation between these factors helps to explain the differences in the gasification behavior of the cokes. Acknowledgments The financial support from the Spanish MCINN (research project PIB2010BZ-00418) is gratefully acknowledged. The authors also thank Dr. M. Fernández from CENIM-CSIC for providing the charcoals for this study. Fig. 7. Evolution of the conversion degree and reactivity to CO2 of the partiallygasified cokes produced from the coal A and its blends with Eucalyptus and Olive charcoals -A2CEu and A2COl- at an addition rate of 2 wt%. Parent cokes A, A2CEu and A2COl are shown as reference.

Table 4 Some gasification parameters corresponding to the partially-gasified cokes.

Onset temperature (°C) Xonset (%) R at X = 10 (%/min) R at X = 15 (%/min) Rmax (%/min) T at Rmax (°C) X at Rmax (%)

A-cycle 2

A2CEu-cycle 2

A2COl-cycle 2

913 0.7 0.141 0.134 0.228 993 2.0

914 0.7 0.157 0.148 0.260 994 2.2

914 0.7 0.142 0.138 0.229 994 2.1

2.4 8.4 12.7 21.2

2.3 7.7 11.9 19.9

Conversion at 1000 °C and different times 2.2 X1000/0 (%) X1000/30 (%) 7.4 X1000/60 (%) 11.8 X1000/120 (%) 19.7 Footnote: see Table 2.

4. Conclusions A combination of dynamic and isothermal gasification carried out in a CO2 environment in a thermobalance was found to be a suitable protocol for establishing differences in the gasification patterns of cokes produced from blends of coal with different types of woody biomass, charcoals from Eucalyptus and Olive and their parent woods. Cokes produced by adding 2 wt% biomass were more reactive than the coke derived from the single coal and, what is more, a decrease in the threshold temperature of the Boudouard reaction was observed. The extent of these modifications was dependent on the type of charcoal and parent wood. Eucalyptus charcoal produced the most reactive coke and the largest decrease in threshold carbon gasification temperature, whereas the other cokes seemed to behave in a way more similar to that of the coke

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