Experimental investigation of fluidized bed chemical looping combustion of Victorian brown coal using hematite

Journal of Environmental Chemical Engineering 2 (2014) 1642–1654

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Experimental investigation of fluidized bed chemical looping combustion of Victorian brown coal using hematite Chiranjib Saha 1, Sankar Bhattacharya * Faculty of Engineering, Department of Chemical Engineering, Monash University, Clayton Campus, Victoria 3800, Australia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 25 November 2013 Accepted 22 February 2014

An experimental investigation on chemical looping combustion (CLC) of Victorian brown coal is carried out in a fluidized bed of hematite. The aim of this study is to understand the feasibility of Victorian brown coal CLC as very little technical information is currently available on the process using these coals. The in situ CLC experiments are first performed using a thermogravimetric analyzer (TGA). The TGA results show good performance of hematite as oxygen carrier over five multiple re-dox cycles under CO2 gasification environment. Therefore, further investigation is performed using a bench-scale fluidized bed that operates in a batch mode cyclically with reduction in CO2 environment, and oxidation in air. Several tests have been conducted to assess the impact of different temperatures, particle size of hematite and CO2 concentration in a flowing fluidizing gas medium. It is observed that the hematite particles of 100– 150 mm performed best with respect to carbon conversions that show an increasing trend with increasing temperature and CO2 concentration in the fluidizing gas. ß 2014 Elsevier Ltd. All rights reserved.

Keywords: Fluidized bed combustion Thermogravimetric analyzer Victorian Brown coal Chemical looping combustion Hematite CO2 capture

Introduction Chemical looping combustion – general description Coal currently accounts for 40% of the world’s electric power generation, a role that is expected to continue in the foreseeable future [1]. Being carbon-intensive, conventional coal-fired power generation also results in large CO2 emissions. Therefore, there is a strong incentive to develop technologies that would allow easier capture of CO2 emissions from coal-fired power plants in a carbon constrained world. Three main options are under research and development for easier CO2 capture. These are post-combustion, pre-combustion and oxy-combustion. But all these suffer from high energy requirement either due to handling of large volume of flue gas or requirement of energy intensive oxygen separation [2]. Chemical looping combustion (CLC) technology has evolved as a promising technology for easier CO2 separation [3]. In this process two interconnected fluidized bed reactors, air and fuel reactors, are

* Corresponding author. Tel.: +61 3 9905 9623; fax: +61 3 9905 5686. E-mail address: [email protected] (S. Bhattacharya). 1 Current address: Natural Resources Canada, CANMET Energy Technology Center (CETC) – Ottawa, Clean Electric Power Generation, 1 Haanel Drive, Ontario K1A 1M1, Canada. http://dx.doi.org/10.1016/j.jece.2014.02.014 2213-3437/ß 2014 Elsevier Ltd. All rights reserved.

used for combustion of fossil fuels [4]. The oxygen from oxygen carriers, preferably oxides of metal, is used instead of gas phase oxygen from air to convert carbonaceous fuels into CO2 and steam. As a result, an easily sequestrable CO2 stream, free from dilution with N2 can be produced with less energy consumption. Also the exergetic efficiency of CLC process is higher compared to conventional combustion [5]. Metal oxide oxygen carriers play the most important role in CLC and their properties are the key to this process [6]. Significant research efforts have been devoted to oxygen carrier particle development and bed hydrodynamics [7]. Iron oxide is preferable in CLC application because of low cost, minimum environmental impact, good mechanical and chemical stability with high reactivity [8]. Iron also possesses three oxidation states and hence can be used for hydrogen generation along with heat and power during a chemical looping combustion process [9]. The redox potential of iron oxide has been investigated for direct coal and syngas CLC. Iron based ores are also investigated for coal CLC [10]. Moreover iron oxide used with different binders is used for novel chemical looping concepts for di-methyl ether combustion [11]. However, performance of iron oxide for CLC using Victorian brown coal is yet unknown. Therefore this work relates to the applicability of hematite for Victorian brown coal CLC process using in a fluidized bed.

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Notation Symbols XC fractional carbon conversion carbon conversion rate (1/min) rC

Literature review – chemical looping combustion of coal using iron oxide in fluidized bed reactors Coal can be utilized in a CLC process either by combustion of syngas from coal or directly in situ for gasification and combustion [12]. The in situ CLC of coal can be described as a three step process: devolatilization, gasification and reactions between gases and metal oxides [13]. In situ CLC of Hambach German lignite in presence of steam and CO2 in a fluidized bed reactor with Fe2O3 as oxygen carrier is reported in literature [14,15]. It was concluded that kinetics of gasification was observed to be faster in presence of Fe2O3. There was no agglomeration between Fe2O3 and ash particles. Two interconnected fluidized beds of thermal power between 100 and 300 W was used by Abad et al. [16] for syngas (derived from coal) combustion using iron oxide. Around 99% of combustion efficiency was achieved and no sign of agglomeration was observed in the particles. The loss in mass and reactivity of the carrier particles over repeated cycle operation was minimal. The CLC and hydrogen production from a char of Chinese lignite using Fe2O3 as oxygen carrier was investigated by Yang et al. [17] in a fluidized bed reactor. The oxygen carrier was reduced to a mixture of FeO and Fe at a temperature of 800 8C. Very good char conversion rate was achieved with high CO2 concentration in the exhaust gases. Leion et al. [18,19] investigated Fe2O3 supported by MgAl2O4 and ilmenite (FeTiO3) in a laboratory fluidized bed reactor system for combustion of pet coke and syngas derived from coal. The increase in temperature and concentration of steam in the fluidizing gas enhances the gasification rate of petcoke in the presence of the metal oxide. The high reactivity of ilmenite was maintained after 37 consecutive redox cycles when syngas was combusted. A 10 kWth chemical looping circulating fluidized bed reactor for solid fuel combustion is developed at Chalmers University. Berguerand and Lyngfelt [20–25] investigated the applicability of ilmenite (FeTiO3) as oxygen carrier for combustion of Mexican Petroleum coke and South African Coal. The CO2 capture for different fuel ranged between 60% and 96% and average solid fuel conversion ranged between 65% and 70%. South African coal was observed to be more reactive with Ilmenite compared to petcoke. Li and Fan [26] and Gnanapragasam et al. [9] investigated Fe2O3 for the direct coal and syngas chemical looping combustion of Pittsburgh #8 coal. It was concluded that direct coal combustion is advantageous over syngas combustion. Considerably higher energy conversion efficiency can be achieved for direct coal CLC process. Therefore it can be concluded that iron oxide has the potential to be used in in situ direct coal CLC process. Objective of this work This study aims to investigate the Victorian brown coal CLC using hematite. The effect of coal devolatilization, gasification and combustion of gases in presence of hematite is investigated in situ using a thermogravimetric analyzer (TGA) and a fluidized bed. The flue gases are analyzed on-line using a gas analyzer and temperature of the system was monitored over time. Initial screening tests were performed using a TGA. This provided information on reactivity and regeneration ability of

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hematite during redox operation. Subsequent experiments were performed in a small fluidized bed at different temperatures with different concentrations of gasification agents at inlet, and by varying the oxygen carrier particle size. The innovative part of this study is application of CLC with Victorian brown coal using hematite as oxygen carrier in a fluidized bed concept for the first time. This will help for scientific understanding of this new concept with Victorian brown coal. Materials and methods Experimental set-up As already indicated, a TGA with steam injection capability (TGDTA/DSC, NETZSCH 449 F3) was used first. Then a batch fluidized bed quartz reactor (length 920 mm, diameter 30 mm and 45 mm respectively), as shown in Fig. 1, is used to investigate the applicability of hematite for CLC of Victorian brown coal. The flow of the fluidizing gases, during reduction and reoxidation, into the reactor are measured and controlled by mass flow controllers. The gases at the inlet are distributed through the bed by a porous quartz plate. Although the rig has capability for steam gasification, in this work only CO2 is used as a gasification environment. K-type thermocouple is used to measure the gas temperature. The reactor was filled with iron oxide and coal mixtures in batches for every experiment. The mass ratio of hematite and coal was 6:1 for stoichiometric oxygen supply. It corresponds to a bed height of 8 cm in fluidized conditions. It is important that coal is always at the dense phase of the bed to ensure a good contact between released volatile matter and oxygen carrier during reduction period. An expanded section in the reactor is designed to prevent elutriation of any fine particles. Samples of flue gases, after moisture removal and filtration, are pumped to a gas analyzer to analyze the concentrations of different gases (CO2, CO, H2, CH4 and O2). The H2 analyzer is a thermal conductivity analyzer. All the other gases were analyzed using an infrared gas analyzer. LABVIEW software package was used to create an inter phase for gas and temperature data acquisition systems. This coded interphase was used to monitor the experimental data on-line by using a computer. Coal and oxygen carrier The experiments have been performed with a dried Victorian brown coal – Loy Yang (LY). Drying was done in an oven for 6 h at 105 8C. The oxygen carrier used in this study is hematite powder (>99% purity). These were thoroughly mixed with the dried coal particles prior to the experiments. The size of the coal and hematite particles varied from less than 100 mm to greater than 250 mm in different experiments. Similar particle sizes have been used by Stainton et al. and Leion et al. [13,18] in their experiments. The coal-ash analysis and metal oxide properties are shown in Table 1. Experimental procedure The effect of consecutive reduction–oxidation cycles on the reactivity of iron oxides in CO2 gas composition was first assessed in the TGA. The coal and metal oxide mixture was heated from ambient temperature to 950 8C at a heating rate of 10 8C/min under the CO2 gas compositions. During the experiment, the sample was heated to 950 8C and maintained at that condition for 1 h, following which air was introduced to oxidize the reduced particles for another hour. The CO2 flow rate during reduction was 3 ml/min that corresponds to 20% CO2 by mass and remaining N2.

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Fig. 1. Fluidized bed chemical looping combustion of Victorian brown coal experimental set-up.

The bed of the quartz reactor was filled with hematite and coal mixture in a mass ratio of 6:1. The inlet gas velocity varied between 0.4 and 0.5 m/s during reduction and oxidation period depending upon the operating conditions. This velocity was well above the minimum fluidization velocity of 0.051 m/s for particle sizes of 125 mm and 0.131 m/s for particle sizes of 200 mm. The bed was first fluidized in reduction condition with a mixture of N2 and CO2, and heated to the desired temperature range. In this reducing environment the coal–metal oxide mixture was kept at the isothermal condition until the CO2 release (as measured by the gas analyzer) was close to zero. Once this condition was reached, the CO2 flow was replaced by air. So the oxidation started at the same isothermal condition and it was continued until the O2

concentration at the gas outlet was stabilized to the concentration of feed air. This condition was indicating to the fact that metal oxide particles were fully oxidized. The concentration of CO2 and air varied between 20 and 60 mass percent (remaining N2) in different experiments. Three different temperatures (950 8C, 900 8C and 850 8C) and particle sizes (<100 mm, 100–150 mm, 150–250 mm) were used in different experiments. Fresh and spent hematite particles were characterized using Scanning electron microscopy (SEM) equipped with energy dispersive Xray (EDX) system. These analytical techniques were used for identification of changes in particle morphologies during the cyclic tests and to characterize the elemental distribution on the surface of the samples.

Table 1 Analysis of Loy Yang coal, ash and hematite. Coal

Ash (wt.% db)

Proximate analysis (wt% db) Volatile matter Fixed Carbon Ash Ultimate analysis (wt% db) Ash Carbon Hydrogen Sulphur Nitrogen Oxygen

SiO2 Al2O3 Fe2O3 TiO2 K2O MgO Na2O CaO SO3

51.3 47.2 1.5 1.5 68.3 4.3 0.4 0.5 25.0

Hematite 21.2 26.7 5.5 0.8 0.3 9.3 9.4 5.0 21.8

Purity (Fe2O3) BET area (m2/gm) Density (kg/m3) Fusion temperature (8C)

>99.9% 1.08 5120 1538

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100

oxygen carrier for CLC of Victorian brown coal in a fluidized bed reactor system.

Reduction and Oxidation of Fe2O3 and Loy Yang

Percent combustion ¼

95

Actual weight loss of coal from TGA Theoretical weight loss based on the carbon content in the coal sample  100

90

(1) 85

Fluidized bed re-dox test The outlet gas concentration during combustion of Loy Yang coal in presence of hematite in the fluidized bed is shown in Fig. 3. This experiments are performed in 20% (by mass) CO2, and with the remaining gas being N2. The particle size of both coal and Hematite used were between 100 and 150 mm. The fluidized bed was heated from room temperature to 950 8C with an isothermal hold of 30 min. The rate of carbon conversion, as defined by Zhang et al. [27], is plotted in Fig. 5b shown later on. As shown in Fig. 3, the initial concentration of CO2 increases between first 20 and 40 min of combustion not only due to the release of CH4 but also due to the CO and H2 released in this stage at a temperature range of 400–650 8C. However, CO and H2 were not observed in presence of hematite mainly due to the better reactivity of CO and H2 than that of CH4. This is expected as the volatile matters are released and in presence of oxygen from hematite it is converted to CO2. Similar results were observed in our previous TGA tests [28], where it was reported that the reactivity of hematite shows a peak in the same temperature range. After that the concentration drops and as the temperature approaches 815 8C, the concentration increases and is almost stabilized after 97 min. After 40 min, CO concentration increases and the CO is combusted in presence of hematite in this temperature range. After 97 min, CO concentration also decreases and hence the decrease in CO2 concentration is justified. Very negligible amount of H2 was detected throughout the analyzed combustion period. Fig. 4 shows the gas concentrations measured during oxidation period following the reducing period shown in Fig. 3. During oxidation, the bed was fluidized by 20% (by mass) air and remaining N2 for 1 h. It can be seen that during initial 10 min the oxygen concentration measured is almost zero. It is due to the fact

0

200

400

600

800

1000

1200

Time (Min) Fig. 2. Five cycles TGA redox test of Loy Yang coal with hematite in N2–CO2 environment.

As the hematite used is of high purity (>99.9%) Fe2O3, the term hematite and Fe2O3 are used synonymously. Results and discussion TGA multicycle experiments

Outlet gas concentrations CO2 and CO - Dry basis (vol%)

A five-cycle TGA test was conducted with hematite–Loy Yang coal mixtures in N2–CO2 environment to evaluate the coal combustion and metal re-oxidation process. After each reoxidation process at 950 8C, reacted metal oxide/ash was mixed with same amount of coal used in previous cycles. Ash could not be separated after each cycle and hence accumulated in the TGA crucible. The results of redox cycles in CO2 gas are shown in Fig. 2. The extent of coal combustion and re-oxidation of metal oxides, as shown by the weight loss and weight gain respectively, during TGA tests show a very small decrease for the Hematite-coal mixture. The percentage of combustion, as defined in Eq. (1), at fifth cycle is 89%. The reactivity of the iron oxide particles did not deteriorate after five consecutive cycles of reduction and oxidation. Based on these performance tests, hematite is investigated as

0.20 25

0.15 20

CO2

15

0.10

CO CH4 Temperature

10

0.05 5

0.00

0 0

20

40

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80

1000

800

600

400

200

0

100

Time (min) Fig. 3. Outlet gas concentrations during combustion of Loy Yang coal with hematite in a fluidized bed.

O

75

Temperature ( C)

80

Outlet gas concentrations CH4 - Dry basis (vol%)

Weight Loss (%)

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The reactivity of iron oxide observed during TGA tests also provided an indication of high reactivity during initial oxidation period. A very low concentration of CO2 can be observed during initial oxidation period which reduces significantly as the time progresses. This can be due to the CO2 trapped in the gas line and combustion of the carbon left in the bed after reduction period. Initially a very small amount of CO was also detected and it is due to combustion of left over carbon in the bed after reduction.

6

O2

4

CO2 CO

Effect of temperature 2

0 0

10

20

30

40

50

60

Time (min) Fig. 4. Gas concentrations during oxidation period in air with hematite in a fluidized bed.

1000

0.035

800

0.030

20

600

15

400

10

CO2 (900ºC) CO2 (850ºC)

5

200

Temperature

0

0 0

20

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100

950º C 900º C 850º C

0.025

0.020

0.015

0.010

0.005

1000

0.20

0.000 0.0

800 0.15

600 0.10 400

CH4 (900ºC) CH4 (850ºC) Temperature

0.05

Temperature (ºC)

Outlet CH4 gas concentration-Dry basis (vol%)

Time (min)

Carbon conversion rate, rC (1/min)

25

Temperature (ºC)

Outlet CO2 gas concentration-Dry basis (vol%)

that the oxygen from air is consumed by the spent hematite and after that the O2 concentration starts to show an increasing trend and eventually reaches a stable value and the experiment is terminated.

The effect of temperature on CLC of Victorian brown coal with hematite in the fluidized bed is investigated. Three different temperatures, 950 8C, 900 8C and 850 8C are investigated with coal and oxygen carriers particle size of 100–150 mm. The results of one redox cycle are reported in this section for each of this operating temperature. Inlet gas concentration during reduction period was 20% (by mass) CO2 and remaining N2. The gas concentrations and rate of carbon conversion with respect to fractional conversion Rt (X C ¼ 100  ðC Consumption =Total CÞ ¼ ð 0 CO2 ; CO; CH4 =Total CÞ, where CConsumption = integration of outlet gas concentration during experiment, Total C = cumulative amount of carbon consumed during reduction and during oxidation [13]) are plotted in Fig. 5(a) and (b) respectively. As the gas concentrations of 950 8C operating

0.2

0.4

0.6

(b)

200

0.00

0 0

20

40

60

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Outlet CO gas concentration-Dry basis (vol%)

Time (min) 1000

3.5

CO (900ºC) CO (850ºC)

3.0

Temperature

800

2.5 600

2.0

1.5

400

1.0 200 0.5

0.0

0 0

20

40

60

80

0.8

Fractional carbon conversion (XC)

Temperature (ºC)

Outlet gas concentration - Dry basis (vol%)

8

100

Time (min)

(a) Fig. 5. (a) Gas concentration and (b) carbon conversion rate at different conversion during CLC at different temperatures.

1.0

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temperature case are already plotted in Fig. 3, these are not added in Fig. 5(a). As discussed in Fig. 3, during the first 20– 40 min the concentration of CO2 increases from its initial values due to the combustion of volatile matters. The similar trend can also be observed in Fig. 5(a). After that the steady release of CO due to gasification of char particles and its subsequent conversion in presence of oxygen from hematite helps to maintain a steady release of CO2 in both the 950 8C and 900 8C cases. However, when the operating temperature is 850 8C the release of CO is relatively less due to the low operating temperature. The rates of carbon conversion plots as shown in Fig. 5(b) indicate a maximum value at 80% conversion for 950 8C operating temperature. The rate decreases with decreasing temperature as expected. It can be seen that within fractional carbon conversion of 60–80%, the maximum rate of conversion can be achieved. During initial pyrolysis stage the conversion rate is suppressed and in all the three cases similar rate of conversion is achieved. This observation is expected and as the temperature starts increasing the difference in rate of conversion can be clearly observed.

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Effect of hematite particle size Fig. 6 shows the effect of hematite particle size on CLC of Victorian brown coal. Three different particle sizes, <100 mm, 100–150 mm and 150–250 mm, of hematite are investigated at operating temperature of 950 8C and 20% (by mass) CO2 concentration during combustion and remaining N2. Fig. 3 represents the second case here (950 8C, 100–150 mm, 20% CO2) and therefore it is not added in Fig. 6(a). A comparison (Figs. 3 and 6a) of CO2 gas concentration reveals that during pyrolysis period the maximum concentration values are very similar in all the three cases. It is also observed in pyrolysis period that the concentration of CH4 released is very similar, as detected by the gas analyzer, in the three cases analyzed. This means that as expected variation in particle sizes during low temperature pyrolysis do not affect the performance of the particles. However, it can be observed that during initial char gasification and combustion period (upto 60 min) the concentration of CO2 in the case of 100–150 mm particle size, is around 3 percentage point higher compared to other two cases. As the experimental time progresses, the CO2

Fig. 6. (a) Gas composition and (b) carbon conversion rate at different conversion during CLC at different particle sizes.

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concentration drops for 150–250 mm particle size. The main reason being its reducing capability of converting char to gasified products CO, as can be seen from the CO concentration curves. Among the other two cases, the CO concentration in outlet gases detected are similar but the higher values are still achieved for 100–150 mm particle size case. These results also impact the rate of carbon conversion curves. It can be seen from Fig. 6(b) that variation of particle size does not impact on the rate of the carbon conversion during coal devolatilization period. However, during in situ char gasification and combustion, maximum carbon conversion is achieved for 100–150 mm case with maximum rate and as the particle size decreases there is slight decrease in carbon conversion rate. But when particle size is increased, the carbon conversion decreases drastically. The analysis of gas composition support this observation related to carbon conversion. Therefore it is clear that among the investigated size ranges the optimized particle size of hematite is between 100 and 150 mm for fluidized bed CLC of Victorian brown coal in a dual CFB concept to get the best carbon conversion under the conditions tested. Dual CFB is the

concept of chemical looping combustor where two fluidized bed reactors (air and fuel reactors) will be used in a commercial CLC based power plant with oxygen carrier circulating between them. Effect of CO2 concentration Fig. 7(a) and (b) represents the gas concentrations and the carbon conversions respectively for three CO2 concentrations (20%, 40% and 60% by mass and remaining N2) in the fluidizing gas at inlet. The operating temperature is 950 8C and particles sizes of coal and iron oxides are between 100 and 150 mm. It can be seen from Fig. 7(a) that the CO2 concentration profiles for all the three cases are very identical, although the fact remains that with the increase in initial concentration at inlet, the CO2 concentration at outlet increases by an equivalent amount and the difference is maintained throughout the combustion period. This behavior is expected, however the important thing is that the concentration profile is similar. First of all due to release of CH4

Fig. 7. (a) Gas composition and (b) carbon conversion rate at different conversion during CLC at different CO2 concentration at inlet.

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(which is expected to convert to CO2 in presence of O2 from iron oxide) the CO2 concentration increases in flue gas during coal devolatilization. The first peak in CO2 concentration profile is observed after 25 min in all the three cases. CH4 is released during coal devolatilization and its subsequent conversion in presence of oxygen from hematite causes this CO2 peak at outlet gases. The CH4 release curve is very identical for all the cases, indicating the fact that increase in CO2 concentration has very little impact on coal devolatilization. Moreover, this peak in CO2 concentration is achieved within similar time difference and with similar maximum value. The rate of conversion curves (Fig. 7b) for all three cases during coal devolatilization show an increasing trend with increase in the amount of CO2 mainly because of more CO2 present at the inlet. However the initial CO concentrations, after coal devolatilization period, are almost identical in all the three cases. Therefore, a steady CO2 concentration at outlet for all the three cases can be observed. With time the CO concentrations show an increasing trend with increasing CO2 content. This is expected as in presence of more CO2, coal is gasified more completely and produce more CO. During this period, the conversion of CO in presence of oxygen from metal oxide produces CO2. However, the difference in CO concentration narrows down as the inlet CO2 concentration is varied between 40% and 60%. It can be seen from rate of carbon conversion curves that as the CO2 concentrations at inlet increases the maximum rate and overall conversions are also increased. However, the difference is narrowed down between 40% and 60% CO2 concentration cases. This means as expected carbon conversion is faster with more CO2 as gasification agent at inlet. It is already established that the char gasification is the limiting step during coal CLC [4]. Therefore high CO2 concentration via recirculation of produced CO2 in the fluidized bed chemical looping fuel reactor can be utilized for higher carbon conversion at a higher rate. But it should be noted that, in case of Victorian brown coals, more than 40% of CO2 present in the fluidizing gas will not improve the gasification rate by a large amount. This information is important from practical application point of view to decide the amount of recirculated CO2 from flue gases.

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Effect of hematite on coal devolatilization and gasification The effect of hematite on coal devolatilization and gasification has been investigated experimentally and the results are discussed in this section. Low rank coals, such as Victorian brown coals, are high in volatile matter. Therefore it is important to investigate the capability of hematite to convert volatile matter during CLC of Victorian brown coals. The major aim of in situ CLC from practical application point of view is that a concentrated stream of CO2 should be achieved with higher conversion of coal gasification products. Therefore, the higher conversions of volatile matter and char gasification products from low rank coals are very important steps in this process. The performance of hematite to convert volatile matter is first investigated. In this regard two sets of experiments are performed, one with silica (Sic)-coal mixture and the other with hematite-coal mixture in the fluidized bed, under identical experimental conditions. Coal: Sic and coal: hematite mass ratio used were 1:12 and 1:6 to keep the bed height same at atmospheric condition. The particle sizes varied between 100 and 150 mm as it was observed earlier that these particles showed best performance in terms of carbon conversion. The particles are fluidized in pure nitrogen to a temperature of 950 8C and the isothermal condition was maintained for 30 min. No CO2 is injected to avoid gasification and to keep the focus on volatile matter conversion with and without hematite. The results are shown in Fig. 8. It should be noted here that as we are considering the effect on devolatilization (that typically occurs between 400 8C and 650 8C, Fig. 3), the gas concentrations are plotted until the temperature point when it reached 950 8C. The remaining isothermal period is excluded from the analysis in Fig. 8 as it is of least interest in this case. The major peaks are identified for CO2, CO, CH4 and H2 with both the Sic and hematite. It can be noted that with Sic in bed, the CO2 concentration from coal conversion is very low compared to the fluidized bed that contained hematite-coal mixture. In between 400 8C and 700 8C, mostly CO and CH4 are detected with Sic. In this similar temperature range CH4 can be detected with hematite in the bed but the concentration of CO is almost zero in this range. This clearly indicates that the CH4 and higher hydrocarbons evolved during devolatilization of coal has been converted to CO2 more in presence of the oxygen in hematite than

1000

5 Sic-CO2 Fe2O3-CO

4

800

Sic-CO Fe2O3-CH4 Sic-CH4

O

Fe2O3-H2

3

600

Sic-H2 Temperature (OC)

2

400

1

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20

Temperature ( C)

Outlet gas concentration-Dry basis (Vol%)

Fe2O3-CO2

30

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Time (min) Fig. 8. Impact of hematite on coal devolatilization.

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Sic. The difference in the magnitude of CO2 peak is the result of the phenomena that coal volatile matter is less converted in the silica bed compared to the bed of hematite. CO peak can be observed in presence of hematite at around 850–900 8C due to the partial conversion of char left in the bed in presence of oxygen from hematite. The amount of H2 detected in the gas phase is less in the case of hematite fluidized bed compared to the Sic fluidized bed. This is also expected as the oxygen in hematite can convert the H2 more into steam compared to the bed consisting Sic. Moreover, during both the experiments it was clearly observed that the moisture traps contained more water in case of experiments with hematite than experiments with Sic. The silica balls in the silica ball trap turned green from orange (original color) color relatively quickly in case of experiments with hematite indicating the presence of more steam in the outlet gases. This observation also supports the fact that in presence of hematite more H2 is converted to steam compared to the case of Sic in the fluidized bed. Therefore, it can be concluded that conversion of volatile matter can be achieved in presence of hematite. A gasification experiment was performed with Sic-coal mixture in the fluidized bed. The operating temperature was 950 8C with isothermal hold of 60 min to allow sufficient time for conversion of coal gasification products. The bed was fluidized by a mixture of CO2 and N2 (20% and 80% by mass ratio) to investigate the effect on gasification of coal in presence of silica. This experiment was somewhat similar to the experiment described in fluidized bed redox test section with hematite. But the difference was that in this experiment silica was used instead of hematite in the bed. Coal: Sic and coal: hematite mass ratio used were same as mentioned earlier. The particle sizes were same as described above. The CO/ CO2 ratios between these two are compared in Fig. 9. It can be noted that during coal devolatilization period (400–650 8C) the CO/ CO2 ratio is almost zero in case of the experiment performed with hematite. It is expected as more CO2 (also very low CO) is produced due to combustion of volatile matters as described above. It can be observed that during char gasification period (that starts from 850 8C) a constant CO/CO2 ratio can be achieved with hematite meaning that the rate of conversion of gasified products is almost uniform until it drops to almost zero at 100 min. However, the CO/CO2 ratio is higher when the fluidized bed consists of Sic, compared to hematite, throughout the experiment and it takes

prolonged time to reach a minimum conversion value. From these experiments it can be concluded that the in situ gasification and combustion of coal is enhanced by the presence of hematite. Micrographs and structural analysis Fig. 10 shows the SEM images of fresh and used hematite particles. The particles shown here are at two magnification ranges – 500 and 8000. The ‘‘used particle’’ mentioned in this section and Figs. 10 and 11 denotes hematite particles used in reduced and re-oxidized cycles. It can be seen that the fresh iron oxide particles consist of granules of less than 0.1 mm. The fresh particles are of spherical shape and the outer surface is relatively smooth compared to the used particles. It can be seen from Fig. 10(b)–(d) that the used particles that were exposed to operating temperature of 850 8C, 900 8C and 950 8C, have rough surface compared to the fresh particles. It can also be seen that the particles that were exposed to relatively low operating temperature of 850 8C and 900 8C have smoother surface compared to the particles exposed to high operating temperature of 950 8C. The magnified images of these particles show that the small grains of the fresh particles increased in size in the used particles and are agglomerated. The agglomerated structure also changes with increase in operating temperature and particles at 850 8C have the least porous structure. These results are expected and mean that increasing temperature also increases rate of in carbon conversion as the transfer of O2 at high temperature is easier due to these structural changes observed. It can be again seen from Fig. 10(e) and (f) that the surface of the particles used at different CO2 concentrations have rough surface compared to the fresh ones. The higher magnification images in Fig. 10(d)–(f) show that there is similar agglomerated structure of the small grains. These structural information lead to conclusions that the high CO2 concentration and carbon conversion rate observed in Fig. 7 is mainly due to the presence of more CO2 in the fluidizing medium that helps to convert carbon in coal more easily. The oxygen release behavior from the oxygen carriers can be similar. The lower and higher magnification images in Fig. 10(d), (g) and (h) show similar structural changes in the used particles compared

4

1000 CO/CO2 - Sic CO/CO2 - Fe2O3 Temperature

800

2 400 1 200

0

0 0

15

30

45

60

75

90

105

120

135

Time (min) Fig. 9. CO/CO2 ratio (by volume) during gasification of Loy Yang coal with silica and hematite.

O

600

Temperature ( C)

CO/CO2 ratio

3

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Fig. 10. SEM images of used hematite particles (1) 500 and (2) 8000 magnification.

to the fresh ones. However, the higher magnification images show that in the largest particle, the surface is perforated and the porosity has been increased. In case of <100 mm and 100–150 mm particle sizes, the agglomerated structure is similar. That can be one of the reasons that these particles showed less difference in carbon conversion rates throughout the experimental period. From EDX analysis it was observed that the weight percentage of Fe and O in the used particles shown in Fig. 10(b)–(d) are 69.64% and 30.26%, 68.48 and 31.52 and 68.31% and 31.39%

respectively. These ideal values in a hematite particle should be 69.94% and 30.05% for Fe and O respectively. The values for Fe and O in fresh particles are 69.97% and 30.03% indicating to the fact that high purity of hematite is used. Therefore it can be concluded that the overall structural characteristics of the particles are not changed after reduction and re-oxidation cycle. The EDX analysis of the particles in Fig. 10(e) and (f) quantified the Fe and O percentages to be 69.77% and 30.23% and 69.57% and 30.43% respectively.

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Fig. 11. Comparison of compounds in fresh and used hematite.

These values are close to the values of fresh particles and used particles at high temperature. Therefore it can concluded that the difference in carbon conversion rate due in fluidizing gas concentrations is not mainly due to particle’s structural degradation, but due to presence of more favorable experimental environments. The EDX analysis of the particles in Fig. 10(g) and (h) show that the weight percentages of Fe and O are 67.93% and 32.07% and 67.54% and 32.46% respectively. This means the oxygen concentration at surface is relatively high in these particles. Therefore less oxygen is transferred during reduction and this can cause a potential decrease in carbon conversion rate as observed in Fig. 6. Fig. 11 shows the XRD patterns of the fresh and used (in fluidized bed redox cycle under different experimental conditions) hematite particles. The fresh particles contain mainly Fe2O3 as expected the hematite used in this study is greater than 99.9% pure Fe2O3 as reported in Table 1. The reduced and reoxidized particles under different experimental conditions mainly consist of Fe2O3 with presence of Fe3O4 in three cases. No ash containing minerals and their compounds are formed. EDX analysis also detected no ash containing elements. These results indicate that there was no interaction between ash and hematite particles. It also can be seen from Fig. 11 that the

relative intensity of Fe2O3 between fresh and different used particles in redox cycles varies. This confirms the fact that the particles are able to re-oxidize to original form during oxidation after reduction period with coal in CO2 environment. However, the internal structure of the particles has been changed. Similar observation is made from SEM images in Fig. 10. The reduced and re-oxidized particles of less than 100 mm and in between 150 and 250 mm sizes show the presence of Fe3O4 along with the redox particles operated at 900 8C. The carbon conversions during these three experimental conditions were relatively less as described above. In a separate study using TGA the authors identified that the hematite particles reduced in CO2 with coal mainly consists of FeO and Fe3O4 and during oxidation the only compound detected was Fe2O3 [28]. Similar results are observed here. Therefore from these results it can be concluded that no major ash containing compounds were formed in CLC of Victorian brown coals with no carbon deposition and good oxidation conversion capability of hematite particles were observed. Table 2 lists the BET surface area, total pore volume and average pore diameter of three different sizes fresh and used hematite particles in a typical redox cycle under similar experimental conditions. Similar values can be observed between fresh and used

C. Saha, S. Bhattacharya / Journal of Environmental Chemical Engineering 2 (2014) 1642–1654 Table 2 Pore structure analysis of fresh and used hematite particles. Samples

BET surface area (m2/gm)

Total pore volume (cm3/gm)

Average pore diameter (mm)

Fresh hematite <100 mm Fresh hematite 100–150 mm Fresh hematite 150–250 mm Redox used hematite <100 mm Redox used hematite 100–150 mm Redox used hematite 150–250 mm

358 158 175 378 148 186

0.032 0.015 0.023 0.037 0.014 0.029

35.82 38.68 53.25 45.67 38.83 62.36

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The used particles changed in structure during the reduction and re-oxidation process. But the major components in fresh and used particles were the same (Fe and O, and no ash containing elements). Similar to the TGA experiments, used particles from the fluidized bed also maintained the structural integrity. From the above discussions it can be concluded that CLC of Victorian brown coal is feasible in a fluidized bed. A 10 kWth CLC rig is currently being built for further investigation of the issues pertaining to low rank coal CLC identified from these studies. Acknowledgements

particles. This confirms the XRD results that the major compound in fresh and used particles is Fe2O3. The surface area and total pore volume of the used particles have been increased. This means that the porosity of the particles is increased with rougher surface in used particles compared to fresh ones. This is also confirmed by SEM images in Fig. 11. The average pore diameter is also increased in used particles as expected due to sintering of small grains in the fresh particles to relatively larger size in used particles as described above in SEM images. But it is important to notice that there is no significance difference in structure between fresh and used particles.

Conclusions This study investigates the application of CLC with Victorian brown coal using hematite as oxygen carrier in a TGA and in a fluidized bed. It is observed in the TGA tests that multiple cycle reduction and re-oxidation of Hematite is feasible in the CLC process to combust Victorian brown coal. Very low mass loss percentage (less than 0.1% per cycle) of hematite confirmed that the mechanical and chemical integrity of the particles are maintained in repeated cycles’ operation. High coal combustion percentage is achieved at fifth cycle (more than 89%). These results inspired to investigate further the applicability of Hematite in brown coal CLC using a bench-scale batch fluidized bed. Several tests have been conducted using the fluidized bed to assess the effect of temperatures, hematite particle size and CO2 concentration in fluidizing gas. It is observed that the release of volatile matter and their subsequent combustion is relatively less dependent on different operating conditions. The concentration of CH4 is maximum between 400 and 650 8C for all the operating conditions. Negligible amount of CO and very little CO2 is detected during reoxidation that gives an indication of high coal conversion percentages during reduction. It is observed that among the investigated particle sizes, the hematite particles of 100–150 mm performed best with respect to carbon conversion that showed an increasing trend with increasing temperature and CO2 concentration in fluidizing gas. However, beyond 40% CO2 concentration at inlet during reduction there is very little impact on carbon conversion. The effect of hematite on coal devolatilization and gasification has also been investigated separately. A sharp peak of CO2 was observed in presence of hematite in the bed as compared to silica in bed during the coal devolatilization process. A relatively low CO/ CO2 ratio was observed during in situ gasification and combustion of coal when the hematite bed was fluidized by a mixture of CO2 and N2. However, the similar experiment with silica in bed showed high peaks of CO/CO2 curve. This difference in peak values proved that carbon conversion in in situ CLC of coal is enhanced by the presence of hematite, a desirable outcome.

The authors acknowledge the financial support of Brown Coal Innovation Australia (BCIA) and Monash University, and the Monash Centre for Electron Microscopy for the scanning electron microscopy work.

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