Reduction kinetics of ilmenite ore for pressurized chemical looping

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Reduction kinetics of ilmenite ore for pressurized chemical looping combustion of simulated natural gas Yewen Tan, Firas Nor Ridha, Dennis Y. Lu, and Robin W Hughes Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02648 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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Energy & Fuels

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Reduction kinetics of ilmenite ore for pressurized

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chemical looping combustion of simulated natural

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gas

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Yewen Tan*, Firas N. Ridha, Dennis Y. Lu, Robin W. Hughes

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Natural Resources Canada, CanmetENERGY-Ottawa

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1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1

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KEYWORDS

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chemical looping combustion; ilmenite; elevated pressure; thermogravimetric analysis; natural

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gas

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ABSTRACT

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Reduction kinetics of ilmenite ore as an oxygen carrier for the chemical looping combustion of a

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simulated natural gas mixture under elevated pressure was studied using a pressurized

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thermogravimetric analyzer (PTGA). The fuel gas is a mixture of hydrocarbon, carbon dioxide

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and nitrogen to simulate an actual combustion environment. The oxidation phase of the

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experiments was carried out in air. Effects of temperature (1023 – 1223 K), total pressure (0.6 –

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1.6 MPa), fuel partial pressure (0.126 – 0.34 MPa) as well as CO2 partial pressure *

Corresponding author, [email protected] 1 ACS Paragon Plus Environment

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(pFuel:pCO2=0.5–1) were studied. The results showed that the presence of small amounts of

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ethane and propane clearly led to a higher ilmenite reactivity at temperatures below 1123 K, but

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this effect became less significant as temperature increased and completely disappeared above

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1123 K. The results also showed that increasing CO2 partial pressure had little effect on ilmenite

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conversion rate, though it did have some slightly negative influence on ilmenite oxygen carrying

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capacity that was especially noticeable at lower total pressure. A higher fuel partial pressure

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appeared to have a slightly negative impact on ilmenite oxygen carrying capacity, especially at

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higher temperature. A kinetic model, based on a phase-boundary controlled mechanism with

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contracting sphere, was developed by incorporating the total pressure, fuel and CO2 partial

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pressures and the temperature and it was able to satisfactorily reproduce most of the test results

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with a conversion ratio of up to 70%. This model predicted that the ilmenite conversion rate had

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a strong positive correlation with the temperature and fuel partial pressure, and a relatively

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weaker negative correlation with the total pressure and CO2 partial pressures. Overall conversion

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rate will increase when total pressure increases, which justify the pressurized chemical looping

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combustion technology.

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1. Introduction

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Chemical-looping combustion (CLC) is a technology for the conversion of fossil fuels with

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inherent separation of CO2 and efficient use of energy. In a typical CLC setup, two reactors are

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used. One of them is a fuel reactor and the other is an air reactor. A solid, called an oxygen

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carrier (OC), is used as an oxidant for the fuel in the fuel reactor. A fuel, such as natural gas,

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reacts with the OC in the fuel reactor, producing a flue gas composed mostly of CO2 and H2O,

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thus making CO2 capture very effective. The reduced OC is then sent back to the air reactor

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where it reacts with air for regeneration before being sent back to the fuel reactor. As a result, 2 ACS Paragon Plus Environment

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CLC does not require an air separation unit as the oxy-fuel combustion does nor does it need

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expensive post-combustion CO2 capture units. According to the literature, improved efficiency

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can be obtained by employing a CLC-integrated power generation system.1-3 CLC processes can

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be also used for hydrogen and steam production. A comprehensive review of the CLC

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technology and systems by Adánez et al. can be found in the literature.4

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Recently, pressurized CLC (PCLC) technology has attracted the attention of some research

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groups.5-10 The major advantages of pressurized CLC are: high pressure CO2 will be produced

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from the fuel reactor thereby reducing the power and associated capital required to compress

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CO2 to pipeline pressure (~13.8 MPa); meanwhile the latent heat of condensation of flue gas

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moisture can be more effectively integrated into the steam cycle due to high dew point

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temperature at high pressure; and the pressurized system provides improved hydrodynamics in

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the fluidized bed resulting in better mixing of reactants. Other noted advantages of PCLC include

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a more compact system for transport to remote areas, and the improved reaction rate with higher

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reactant partial pressures.

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CanmetENERGY-Ottawa has been studying CLC technology, including pressurized CLC, for

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the past several years.11-14 The works have focused on using a commercial ilmenite ore as oxygen

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carrier. Ilmenite ore is mainly composed of FeTiO3 (FeO·TiO2). It is an attractive oxygen carrier

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due to its wide availability and low cost. There have been numerous studies on its characteristics

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as an oxygen carrier.13, 15-19 Most of these studies were conducted using methane, CO or coal. For

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natural gas or syngas as fuel, very few studies were done with ilmenite.20, 21 A few were done

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with iron ore such as in the study of Wang et al.22

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Natural gas is mainly composed of methane with some higher hydrocarbons. According to

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Union Gas, a typical natural gas can be composed of 87-97% CH4, 1.5-7% of C2H6, 0.1-1.5%

3 ACS Paragon Plus Environment

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C3H8, 0.01-0.3% iso-butane and other minor components. Literature shows that these small

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amounts of higher hydrocarbons can have significant impact on the rate of reaction of the

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mixture compared to pure CH4 alone.23-25 It is hypothesized then that the presence of these higher

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hydrocarbons may have an impact on the reactivity of ilmenite as oxygen carrier in chemical

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looping combustion.

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To the best of the authors’ knowledge, no prior research has been done to study the reduction

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kinetics of ilmenite using natural gas or simulated natural gas, whether at atmospheric or

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elevated pressures. The intent of this work is to fill this knowledge gap at elevated pressure. To

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this end, the effects of total pressure, fuel and CO2 partial pressures and temperature were

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studied and a preliminary model was derived based on the experimental data. The conversion

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ratio, conversion rate and oxygen transport capacity of ilmenite ore were evaluated.

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2. Materials and methods

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Pressurized thermogravimetric analyzer (PTGA). The PTGA used in this work was

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manufactured by Linseis GmbH and is described elsewhere.13 This facility operates from

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atmospheric pressure up to 10.1 MPa at temperatures of up to 1873 K. The reaction gases,

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provided by the gas cylinders, are mixed with steam before entering the PTGA from the top. The

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balance itself is purged with nitrogen during the operation. The PTGA is connected to a data

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acquisition system controlled by a computer. The test data are collected at 1 s intervals. A

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schematic of the facility is given in Figure 1.

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Figure 1. A schematic diagram of the PTGA


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Energy & Fuels

Sample preparation and fuel mixture. The fuel mixture used for this work was provided by Linde with a certified gas composition of 95.02% CH4, 3.96% C2H6 and 1.02% C3H8. The oxygen carrier (OC) used in this work is UKTO ilmenite ore provided by Rio Tinto

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Canada. It is an upgraded roasted ilmenite ore (concentrate produced from raw massive ilmenite

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ore after removing most of its gangue material by gravity separation and sulphur by roasting at

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1073 K. 11 The material was crushed and sieved to 106-212 µm and was then calcined in a muffle

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furnace for 2 hours at 1173 K before it was used in the PTGA tests. Its composition, analyzed by

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X-ray fluorescence, is presented in Table 1.

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Table 1. UKTO ilmenite ore composition (wt.%)

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Test procedure. The test procedure has been described in detail in a previous publication13

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and is only briefly described here. For each test, 95-100 mg of OC sample was evaluated and

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each test consisted of 8 redox cycles. Our previous study using methane has shown that, after an

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activation period of 3 redox cycles under elevated pressures, the reactivity of ilmenite stayed

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constant for at least 40 cycles. Nitrogen was used to pressurize the PTGA to the test pressure and

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as purge gas. The total flow rate was 0.5 normal L/min. Previous work13 has shown that this flow

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rate was enough to eliminate mass transfer effects.

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Data repeatability of the PTGA was very good as can be seen from the previous study.

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3. Reactions of ilmenite with hydrocarbons

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As stated above, most of the previous studies focused on using pure methane as fuel. The

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main reactions between methane and ilmenite ore are those involving pseudobrookite and

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hematite and can be described by the following overall reactions:18

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4Fe2TiO5+4TiO2+CH4 → 8FeTiO3+CO2+2H2O

(R1)

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12Fe2O3+CH4 → 8Fe3O4+CO2+2H2O

(R2)


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The oxidation stage can be described by the following reactions:18

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4FeTiO3+O2 → 2Fe2TiO5+2TiO2

(R3)

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4Fe3O4+O2 → 6Fe2O3

(R4)

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With the higher hydrocarbons under consideration here, we can write the reactions as below.

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7Fe2TiO5+7TiO2+C2H6 → 14FeTiO3+2CO2+3H2O

(R5)

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21Fe2O3+C2H6 → 14Fe3O4+2CO2+3H2O

(R6)

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10Fe2TiO5+10TiO2+C3H8 → 20FeTiO3+3CO2+4H2O

(R7)

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30Fe2O3+C3H8 → 20Fe3O4+3CO2+4H2O

(R8)

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However, it should be noted that ilmenite ore reactions in CLC are in fact more complex than

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the above reactions. This is because iron in the ilmenite ore can exist in several different oxide

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forms and combine with titanium in various ways.26

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In the literature, the reactivity of oxygen carriers is typically assessed using the conversion

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ratio Xr, which is expressed as equation (1):

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ܺ௥ =

120 121 122

௠೚ ି௠೟

(1)

௠೚ ି௠ೝ

Here, mo is the mass of the ilmenite ore sample after oxidation; mt is the mass at time t and mr is the sample mass at the end of the reduction stage. Conversion rate Ẋr is also used to assess the reactivity of oxygen carriers and is calculated

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using the equation (2):

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ௗ௑ ܺሶ௥ = ௗ௧ೝ

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where t is time in seconds.

(2)

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Since the conversion ratio, Xr is a normalized term from which the conversion rate Ẋr can be

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determined, it is important to note that it is possible to misinterpret OC performance test results

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because an oxygen carrier with a very low oxygen carrying capacity may appear to have a faster 6 ACS Paragon Plus Environment

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reaction rate than one with a high oxygen carrying capacity. To compensate for this, oxygen

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carrying capacity Ro is used in combination with Ẋr in evaluating oxygen carrier performance. A

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better oxygen carrier would be the one with a higher Ẋr as well as a higher Ro. The oxygen

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transport capacity Ro, in percentage, can be calculated using equation (3):

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ܴ௢ =

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௠೚ ି௠ೝ ௠బ

× 100%

(3)

In this paper, ilmenite reactivity is often presented with a weight change (%) vs. time curve as

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we feel that this can lead to easier data interpretation and comparison. The weight change C in

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percentage, is calculated using equation (4):

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‫=ܥ‬

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௠೟ ି௠೚ ௠బ

× 100%

(4)

With this formula, the weight change is negative during the reduction stage and positive

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during the oxidation stage. Equation (4) shows that the absolute value of C is the same as that of

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oxygen carrying capacity at the end of the reduction stage.

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Carbon deposition is a common concern in chemical looping combustion with hydrocarbons.

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Our previous study with methane has shown that, under the test conditions in Table 2, carbon

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deposition was not observed.13 The fuel used in this work was a mixture of methane, ethane and

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propane. In theory, ethane and propane are more likely to lead to carbon deposition because of

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their higher carbon content. However, due to their low concentrations in the mixture, carbon

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deposition was not observed in any of the tests presented here.

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4. Test results and discussion

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Table 2 summarized the test conditions where total pressure and partial pressures are given as

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absolute pressure. Nitrogen was used as balance gas. Carbon dioxide was used to simulate a

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realistic operating environment. In commercial operations, it may not be practical to use pure

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natural gas to fluidize the fuel reactor, so a recycled flue gas (mainly CO2) or a mixture of CO2 7 ACS Paragon Plus Environment

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and steam may be used. For this reason, the partial pressure of CO2 varies for some tests in order

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to evaluate its effects on oxygen carrier reactivity. The test data from the 8th redox cycle was

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used in results evaluation.

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Table 2. Test conditions for PTGA experiments Under elevated pressure conditions, gas dispersion in the gas supply system of the

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experimental device becomes pronounced and results in the sample being exposed to a gas

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mixture of varying composition when passing from one stage to the next (i.e. oxidation to

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reduction and vice versa). This effect of gas dispersion can be seen in Figure 2 showing oxygen

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carrier reduction for tests T915a and T505a. Gas dispersion effect becomes more pronounced

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when total pressure increases. This phenomenon has been observed also by other researchers.7

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Increasing gas flow rate can reduce the gas dispersion effect but cannot eliminate it. Our tests

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showed that increasing gas flow rate from 0.5 to 1 normal L/min had no influence on ilmenite

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reactivity and only marginally reduced the gas dispersion effect.

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Figure 2. Ilmenite weight change (T=1223 K, pHC=0.34 MPa, pFuel:pCO2=1) for tests T915a

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(dashed line) and T505a (solid line)

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In order to assess the conversion ratio Xr and the conversion rate Ẋr across different total

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pressures, the data in period of pronounced gas dispersion effect was removed from the data sets.

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This was achieved by assessing the minimum value of the derivative

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after that point in determining the reaction kinetics. On the other hand, the criteria

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used in assessing Ro since the oxygen carrier was reacting with fuel while the fuel partial

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pressure was still rising during the gas dispersion period.

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ௗమ ௠ ௗ௧ మ

and then selecting data ௗ௠ ௗ௧

< 0 was

Effect of temperature and total pressure. Figure 3 provides plots of ilmenite conversion ratio, Xr, for the oxidation of the CH4-C2H6-C3H8 mixture as a function of time at temperatures 8 ACS Paragon Plus Environment

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from 1023 to 1223 K, at total pressure of 0.9 MPa and fuel-CO2 partial pressure ratio

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pFuel:pCO2=1, where pFuel= pCH4+ pC2H6 + pC3H8.. As such, the curves represent the

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conversion ratio at each temperature as a function of time.

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Figure 3. Conversion ratio at 0.9 MPa for pFuel:pCO2=1. Tests T511a, T531a, T513a, T504a,

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T503a and T505a

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Figure 3 shows that, up to 70% conversion, the rate of reduction of ilmenite increases with

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increasing temperature from 1023 to 1173 K. However, increasing temperature from 1173 K to

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1223 K did not increase the conversion rate. In fact, increasing temperature beyond 1173 K led

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to a decrease in oxygen carrying capacity, Ro, by ~6%, as can be seen from Figure 4.

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Figure 4. Oxygen carrying capacity at 0.9 MPa for pFuel:pCO2=1. Tests T511a, T531a, T513a,

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T504a, T503a and T505a

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Even though this paper focuses on the reduction of ilmenite, it is interesting to see what

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happens at 1223 K during the oxidation stage as it can help us to interpret our observations

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during the reduction period. The negative effect of high temperature on ilmenite reduction

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conversion rate and capacity, i.e. the effect noted in the previous paragraph at 1223 K, was

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observed in a more pronounced way during the re-oxidation of the sample where the oxidation of

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the reduced ilmenite was considerably slower at 1223 K as shown in Figure 5. At the 120 s mark,

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the total weight change (%) of the reduced ilmenite sample at 1223 K was comparable to that at

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1023 K, even though its oxidation rate was comparable to that at 1173 K in the first 10 s. It

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should be noted that, over an extended period of oxidation, the ilmenite ore was still fully

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oxidized at 1223 K with a weight gain of ~4.3%.

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Figure 5. Weight change at 0.9 MPa for oxidation stage. Tests T511a, T531a, T513a, T504a,

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T503a and T505a


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SEM images for the reduced samples of tests T503b (T=1173 K) and T505b (T=1223 K) are

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presented in Figure 6. These images show that the surface morphology of the oxygen carrier

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underwent significant changes when temperature rose from 1173 to 1223 K. At 1173 K, ilmenite

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particles preserved their structure with no major cracks or defects apparent on the surface, as

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shown in image 6a. At higher magnification, as shown in image 6b, the surface seems to exhibit

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a granular morphology with grain size ranges from 1 to 5 µm and pores apparent on the surface.

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When the temperature was increased to 1223 K, the surface morphology of the particle

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experienced significant changes. Large agglomerates, with size of ~500 µm, were formed as

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shown in image 6c. The reason for agglomerates formation is the development of fused regions

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on the surface of particles that tend to form at points of contact. Fused regions are shown in

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image 6d that presents a magnified image of the surface of particles in image 6c. Furthermore, a

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closer examination of this image reveals the disappearance of grain boundaries as they merged

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into each other. Such agglomeration was noted to take place during and after oxidation of deeply

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reduced particles.

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The above discussion is consistent with our previous study from this group that showed that

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larger grains were formed with increasing reaction temperature.11 The same study also showed

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that total pressure did not have any noticeable effect on surface morphology.

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Figure 6. SEM images for test T503b (T=1173 K, top) and T505b (T=1223 K, bottom) b

The effect of total pressure at 0.9 MPa (T505a) and 1.6 MPa (T915a) can be seen from Figure

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2. It shows that, as has been previously demonstrated in the literature,7, 12 increasing total

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pressure while maintaining constant fuel partial pressure led to a clear decrease in the rate of

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weight loss of ilmenite ore. This figure also shows that no significant effect on the oxygen

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carrying capacity of ilmenite ore was observed with the increase of total pressure as the final c

d 10

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weight loss was the same at both pressures. It is important to point out here that, even though

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total pressure by itself is a negative factor in terms of reaction kinetics, with increasing total

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pressure in practical operation, fuel partial pressure can be increased, which will lead to a higher

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reaction rate as shown later in the modeling section of this paper. This, combined with other

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benefits associated with high-pressure systems, makes PCLC an overall attractive option.

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Comparison with pure methane. Tan et al.25 showed that, in a jet-stirred reactor,

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combustion kinetics of methane could be significantly accelerated when a small amount of

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ethane and propane was added. It was thus reasonable to hypothesize that, when small amounts

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of ethane and propane were present, the kinetics of the hydrocarbon mixture could also be

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accelerated. In the TGA studies for chemical looping combustion scenarios, this would manifest

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itself as improved reactivity of the oxygen carrier. This is shown to be true in Figure 7. At 1023

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K, the reactivity of ilmenite for the oxidation of the hydrocarbon mixture was significantly

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higher than for pure methane, and its oxygen carrying capacity was also much higher for the

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mixture. For example, after 50 sec, ilmenite weight loss with the simulated natural gas mixture

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was ~2% compared to ~1.1% for pure methane. Not surprisingly, the reactivity of ilmenite was

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the highest for pure ethane. At 50 sec, the ilmenite weight loss with ethane was at ~3.7%. The

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final ilmenite weight loss was ~3.2% for the simulated natural gas mixture and ~2.3% for pure

238

methane. A much deeper reduction, over 5% weight loss, was obtained when pure ethane was

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used.

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Figure 7. Ilmenite weight loss for pure methane, the simulated natural gas mixture and pure

241

ethane. Tests T511a, T512 and T628. T=1023 K, P=0.9 MPa, pHC 0.34 MPa, pFuel:pCO2=1


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As temperature rose, the difference between the simulated natural gas mixture and pure

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methane became gradually smaller and at 1173 K, no difference between the two was observed

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as can be seen in Figure 8.

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Figure 8. Ilmenite weight loss for pure methane and simulated natural gas mixture. Tests T503a

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(solid line) and T922 (dotted line). T=1173 K, P=0.9 MPa, pHC=0.34 MPa, pFuel:pCO2=1

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The first step in high temperature alkane oxidation is unimolecular breakdown of the parent

248

molecule. The rate constant for methane decomposition is considerably smaller than that of

249

ethane and propane. Because of this, in a methane-ethane-propane mixture, the initial radical

250

production will be exclusively from the breakdown of ethane and propane at a rate orders of

251

magnitudes faster than that of methane.

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According to Tan et al.,25 when ethane and propane are added to methane, the higher

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hydrocarbons start to decompose first,

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C2H6 ↔ CH3 + CH3

(R9)

255

C3H8 ↔ CH3 + C2H5

(R10)

256

Methyl radicals then mainly react with molecular oxygen to produce OH radicals while ethyl

257

radicals mainly decompose and become major H-atom source,

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CH3 + O2 ↔ CH2O + OH

(R11)

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C2H5 ↔ C2H4 + H

(R12)

260

H + O2 ↔ OH + O

(R13)

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Finally, H and OH radicals react with methane, increasing the radical pool,

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OH + CH4 ↔ H2O + CH3

(R14)

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H + CH4 ↔ H2 + CH3

(R15)


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Energy & Fuels

As temperature rises, methane becomes increasingly less stable and starts to decompose more

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easily, so that the difference between the simulated natural gas mixture and pure methane

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becomes increasingly smaller and eventually disappears.

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While the above mechanism was well established for combustion of hydrocarbons in gaseous

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environment, it is likely also valid for chemical looping combustion and can be invoked to

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explain the increased ilmenite reactivity for the combustion of simulated natural gas mixtures

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compared to pure methane. In this case, due to the nature of gas-solid reactions, reactions

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involving oxygen must be rewritten. For example, R11 can be rewritten as

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CH3 + 2FeTiO5 + 2TiO2 ↔ CH2O + OH + 4FeTiO3

(R11a)

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CH3 + 6Fe2O3 ↔ CH2O + OH + 4Fe3O4

(R11b)

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For R13, it is more complicated as an extra O radical is produced when H reacts with the

275

ilmenite:

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H + FeTiO5 + TiO2 ↔ OH + 2O + FeTiO3

277 278

(R13a)

On the other hand, one less O radical is produced by reaction R13b: H + 3Fe2O3 ↔ OH + 2Fe3O4

(R13b)

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The exact elementary reaction mechanism of hydrocarbons reacting with ilmenite oxygen

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carrier is unknown at this time. This will require more fundamental research. However, the lack

281

of understanding of this mechanism does not affect this work, as we are only interested in the

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overall kinetics of reactions between ilmenite and hydrocarbons.

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The negative effect of high pressure on hydrocarbon oxidation is consistent with our previous

284

findings12, 13 as well as findings of other researchers7 and it was explained before using other gas-

285

solid systems. From a kinetics perspective, it is well known that radical recombination reactions

286

become increasingly important as system pressure increases. Reactions such as H + O2 + M ↔


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287

HO2 + M are increasingly favored in the recombination direction, thus reducing concentrations

288

of the hyperactive H atoms, leading to slower and steadier reactions. Dagaut and Cathonnet

289

argued27 that, under high pressure conditions, the oxidation route of ethyl radical oxidation

290

(producing C2H3 + O2) started to compete with the thermal decomposition route (forming H2).

291

Thus, the net production of H atoms decreases reducing the rate of the main hydrogen production

292

channel. Considering the presence of ethane in the fuel mixture, this is a plausible explanation.

293

The above discussion shows that the radical formation theories based on gas phase chemical

294

kinetics could be used to explain some of the observations seen in this study and that it could be

295

interesting to pursue research in this area to understand interactions between the gas phase fuel

296

and the solid phase ilmenite.

297

Effect of CO2 partial pressure. Most of the practical chemical looping systems have been

298

based on dual fluidized bed concepts. In such configurations, a fluidizing gas is needed to keep

299

the solids looping between two reactors to maximize combustion efficiency. For the fuel reactor,

300

in order to produce a flue gas composed of only CO2 and H2O to avoid any need for flue gas

301

separation, the options for fluidizing gas will be limited to either fuel gas or recycled flue gas

302

(mostly CO2) or a mixture of the two. It is therefore necessary to understand the effect of CO2

303

partial pressure on ilmenite reactivity.

304

To this end, several tests were done with different fuel-CO2 partial pressure ratios:

305

pFuel:pCO2 =1 and pFuel:pCO2 = 0.5, where pFuel = pCH4+ pC2H6 + pC3H8. The results are

306

shown in Figures 9-11.

307

Figure 9. Weight loss curves at T=1173 K, P=0.6 MPa, pHC=0.126 MPa. T920a, pFuel:pCO2=1

308

(solid line) and T920b, pFuel:pCO2=0.5 (dotted line)


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309

Figure 10. Weight loss curves at T=1173 K, P=0.9 MPa, pHC=0.126 MPa. T923a,

310

pFuel:pCO2=1 (solid line) and T923b, pFuel:pCO2=0.5 (dotted line)

311

Figure 11. Weight change at T=1173 K, P=1.6 MPa, pHC=0.126 MPa. T825a, pFuel:pCO2=1

312


313

These figures, obtained for tests done at 1173 K, show that increasing CO2 partial pressure

314

had a slightly negative effect on the conversion rate of ilmenite from 0.6 to 1.6 MPa. Increasing

315

CO2 partial pressure also led to lower oxygen carrying capacity. At 0.6 MPa at 140 s, doubling

316

CO2 partial pressure led to a decrease of ~13.7% in ilmenite oxygen carrying capacity; at 0.9

317

MPa at 200 s, doubling CO2 partial pressure led to a decrease in oxygen carrying capacity by

318

~7.8% and at 1.6 MPa at 150 s, doubling CO2 partial pressure led to a decrease of about ~5%. It

319

can thus be seen that the effect of CO2 on ilmenite oxygen carrying capacity diminished as total

320

pressure increased. These results show that CO2 partial pressure is a factor that should be taken

321

into account for fuel reactor design, especially at lower total pressure conditions. The effect of

322

the observed CO2 partial pressure can be explained by the fact that, as CO2 is a product of

323

reactions R1, R2, R5-R8, when CO2 partial pressure increased, the equilibrium of these reactions

324

was favored toward their left. At higher total pressures, the increased fuel partial pressure was

325

able to compensate for the negative effects of pCO2 as the model shows in a later section.

326

Effect of fuel partial pressure. The effect of hydrocarbon partial pressure is shown in

327

Figures 12-14. As expected, higher fuel partial pressure led to higher rate of ilmenite reduction.

328

However, it is interesting to note that oxygen carrying capacity was slightly higher at lower fuel

329

partial pressure, especially as temperature increased. For example, at 1123 K, ilmenite weight

330

loss was ~9% higher at fuel partial pressure of 0.126 MPa than at 0.34 MPa; at 1173 K, the

331

weight loss was ~10% higher at fuel partial pressure of 0.126 MPa than at 0.34 MPa. At 1223 K,


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332

the difference was ~13%. Probably, at higher fuel partial pressure, the reaction proceeded so fast

333

that the active sites of the oxygen carrier surface were quickly covered so that further reaction

334

was hindered. With higher temperature, the reaction proceeds even faster so this effect became

335

more pronounced.

336

Figure 12. Weight loss curves for tests T927a and T504a. T=1123 K, P=0.9 MPa, pFuel:pCO2=1.

337

pHC=0.34 MPa (solid line) and pHC=0.126 MPa (dotted line)

338


339


340


341


342

5. Kinetic modeling

343

The approach for kinetic modeling of the experimental data was described in detail in a

344

previous publication.13 Essentially, the Hancock and Sharp method28 was used to establish

345

kinetic models for ilmenite ore reduction with methane. This approach has also been successfully

346

used by several research groups to model ilmenite ore redox at atmospheric pressure

347

conditions.29, 30

348 349

Following Hancock and Sharp’s method, ln(-ln(1-Xr)) against ln(t) is plotted, where t is the time of reaction in seconds, and a linear equation is fitted to the data.

350

In Hancock and Sharp’s model, the phase-boundary controlled mechanism with contracting

351

sphere, which is the adopted model for this work, could be described with the following equation:

352

݇ × ‫ = ݐ‬1 − (1 − ܺ௥ )య

భ

(5)


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Energy & Fuels

353

where t is reaction time (s), Xr is ilmenite ore’s conversion ratio and k is a variable determined by

354

total pressure, fuel partial pressure, pCO2 and temperature. In the Arrhenius form, k can be

355

expressed as

356

݇ = ‫(݌ݔ݁ܣ‬− ோ்)

357

where A is the pre-exponential factor (s-1), E the activation energy (J mol-1 K-1), T the reaction

358

temperature (K) and R the universal gas constant (J mol-1 K-1).

359 360 361 362

ா

(6)

The conversion ratio, Xr, can then be calculated as ܺ௥ = 1 − (1 − ݇ × ‫)ݐ‬ଷ

(7)

By plotting 1-(1-Xr)1/3 against time t using experiments data, it is possible to derive k from the slope of the resulting line as shown in Figure 15.

363

Figure 15. Data (test T504a) fitting to obtain k. T=1123 K, P=0.9 MPa, pHC=0.34 MPa,

364

pFuel:pCO2=1

365

From equation (6), the activation energy can be calculated using the experimental data. In the

366

temperature range from 1023 – 1173 K, at total pressure of 0.9 MPa and partial pressure pCH4 of

367

0.34 MPa, the activation energy was 69 kJ mol-1 K-1 for pure methane reduction and 56 kJ mol-1

368

K-1 for the hydrocarbon mixture reduction.

369

Compared to our previous work,13 the activation energy for pure methane is quite a bit higher

370

in this work, 69 kJ mol-1 K-1 instead of 28 kJ mol-1 K-1. This is because we extended the

371

temperature range from the previous 1123 K-1173 K to 1023 K-1173 K with a lower temperature

372

boundary. This indicates the significant effect of temperature on ilmenite activation energy,

373

which in fact can be expected as literature has already shown that the final products of ilmenite

374

oxidation depend greatly on the temperature. It is generally acknowledged that complete


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Page 18 of 42

375

oxidation to pseudobrookite (Fe2TiO5) occurs above 1123 K.18 Below this temperature, a mixture

376

of different oxidized compounds can exist in the product.

377

With the data from this work, in the temperature range of 1123 – 1173 K where complete

378

oxidation of ilmenite can be expected, the activation energy obtained for the hydrocarbon

379

mixture was 31 kJ mol-1 K-1, close to the 28 kJ mol-1 K-1 obtained for pure methane. On the other

380

hand, in the lower temperature range of 1023 – 1123 K, the activation energy for the

381

hydrocarbon mixture reduction was 67 kJ mol-1 K-1 while that for pure methane reduction was

382

much higher at 90 kJ mol-1 K-1.

383

In order to easily evaluate the effect of various parameters studied in this work on the kinetics

384

of ilmenite for hydrocarbon reduction, k was fitted using the experimental results obtained under

385

various conditions and the following expression for k was obtained:

386

݇ = 5.263 × 10ିଶହ × ܲି଴.ସଶହ × ‫ ݈݁ݑܨ݌‬଴.ଽଷହ × ‫ܱܥ݌‬ଶ ି଴.ସ଺ × ܶ ଻.ଷ଺

387

(8)

Here, the pressure is in MPa(a) and the temperature T in K; P is total pressure, pCO2 is CO2

388

partial pressure and pFuel is fuel partial pressure. This expression applies to a total pressure

389

range between 0.9 and 1.6 MPa(a). Equation (8) shows that temperature T has by far the most

390

significant and positive influence on conversion rate and that k has an almost linear relationship

391

with fuel partial pressure. It also shows that the negative impact of pCO2 on k was even stronger

392

than the total pressure P. Further, equation (8) reveals that the negative effects of increasing P

393

and pCO2 could be compensated for by the positive effect of increasing pFuel. For example, for

394

a reaction gas mixture of 20 vol. % fuel and 20 vol. % CO2, when total pressure P increases from

395

0.9 to 1.6 MPa(a), k will decrease by ~21% due to increased P and ~23% due to increased pCO2,

396

but will increase by 70% due to increased pFuel. Overall conversion rate k will increase when P


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Energy & Fuels

397

increases, as can be seen in Figure 16. This justifies our research in improving reaction rates by

398

pressurizing chemical looping combustion.

399 400

Figure 16. Individual effects on k by total pressure P, fuel partial pressure pFuel and CO2 partial pressure pCO2, assuming 20 vol.% fuel and 20 vol.% CO2 in the reaction gas mixture

401

Using equations (7) and (8), most of the experimental results can be satisfactorily reproduced

402

with up to 70% conversion, for some as high as 90% conversion, as shown in Figure 17. The

403

model does have a tendency to overestimate the negative effect of pCO2 by underestimating the

404

conversion ratio at a higher pCO2 (T919a) when conversion ratio was above 50%. Overall, the

405

model proposed in equation (8) has a strong confidence level for temperature, as well as for total

406

pressure and fuel partial pressure, but a relatively lower confidence level for CO2 partial pressure.

407

Figure 17. Comparison between model predictions (solid lines) and experimental data (symbols).

408

o: T825a; ∆: T919a; +: T503a; *: T927a

409

6. Conclusions

410

Reduction kinetics of a commercial natural ilmenite ore were studied using a pressurized

411

thermogravimetric analyzer with a simulated natural gas as fuel. The results showed that,

412

compared to reaction with pure methane, the reduction kinetics of ilmenite ore were considerably

413

improved with the hydrocarbon mixture for temperatures below 1123 K, but no significant

414

difference was observed for temperatures above 1123 K. The higher ilmenite reactivity at lower

415

temperature was attributed to the presence of ethane and propane in the hydrocarbon mixture

416

because these higher hydrocarbons are easier to decompose than methane. The results also

417

showed that increasing CO2 partial pressure had a minor negative effect on ilmenite reduction

418

kinetics but a more pronounced negative effect on its oxygen carrying capacity and this effect

419

diminishes at higher total pressure. While increasing hydrocarbon partial pressure was beneficial

420

to ilmenite reduction kinetics, it had slight negative impact on its oxygen carrying capacity, 19 ACS Paragon Plus Environment

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Page 20 of 42

421

especially at higher temperature. The conclusion of this paper would draw is that the presence of

422

small amounts of higher hydrocarbons in the natural gas can have significant effect on the

423

reduction kinetics of an oxygen carrier at lower temperature so that they should be taken into

424

account when designing fuel reactors for such conditions. Finally, a kinetic model was developed

425

as a function of the total pressure (0.9-1.6 MPa(a)), fuel and CO2 partial pressures and the

426

temperature. The model showed that the ilmenite conversion rate had a strong positive

427

correlation with the temperature and fuel partial pressure, and a relatively weaker negative

428

correlation with the total pressure and CO2 partial pressures.

429

430

The authors wish to acknowledge funding from Natural Resources Canada’s Program of

431 432

Acknowledgments

Energy and Research Development.

References

433

(1) Ishida M, Zheng D, Akehata T. Evaluation of a chemical-looping combustion power-generation system by

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graphic energy analysis. Energy 1987;12(2):147-154.

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(2) Anheden M, Svedberg G. Exergy analysis of chemical-looping combustion systems. Energy Conversion &

436

Management 1998;39(16-18):1967–1980.

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(3) Brandvoll O, Bolland O. Inherent CO2 capture using chemical looping combustion in a natural gas fired power

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cycle. J. Eng. Gas Turbine Power 2004; 126(2):316–321.

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(4) Adánez J, Abad A, García-Labiano F, Gayán P, de Diego LF. Progress in chemical-looping combustion and

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reforming technologies. Prog. Energy & Comb. Sci. 2012; 38(2):215-282.

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(5) Xiao R, Song Q, Song M, Lu Z, Zhang S, Shen L. Pressurized chemical-looping combustion of coal with an iron

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ore-based oxygen carrier, Comb. & Flame 2010: 157(6):1140-1153.

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(6) Xiao R, Chen L, Saha C, Zhang S, Bhattacharya S. Pressurized chemical-looping combustion of coal using an

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iron ore as oxygen carrier in a pilot-scale unit. Int. J. of Greenhouse Gas Control 2012; 10:363-373.

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(7) García-Labiano F, Adánez J, de Diego LF, Gayán P, Abad A. Effect of pressure on the behavior of copper-, iron-,

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and nickel-based oxygen carriers for chemical looping combustion. Energy Fuels 2006; 20(1):26-33.


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(8) Xiao R, Song Q, Zhang S, Zheng W, Yang Y. Pressurized chemical-looping combustion of Chinese bituminous

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coal: cyclic performance and characterization of iron ore-based oxygen carrier. Energy Fuels 2010; 24(2):1449-1463.

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(9) Zhang S, Saha C, Yang Y, Bhattacharya S, Xiao R. Use of Fe2O3-containing industrial wastes as the oxygen

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carrier for chemical-looping combustion of coal: effects of pressure and cycles. Energy & Fuels 2011; 25(10):4357-

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4366.

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(10) Zhang S, Xiao R, Zheng, W. Investigation of chemical-looping combustion of coal with iron ore as oxygen

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carrier under pressurized condition. In: CFB-11: Proceedings of the 11th International Conference on Fluidized Bed

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Technology 2014:855-860.

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(11) Ridha FN, Duchesne MA, Lu, X, Lu DY, Filippou D, Hughes RW. Characterization of an ilmenite ore for

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pressurized chemical looping combustion. Applied Energy 2016; 163:323-333.

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(12) Lu X, Rahman RA, Lu DY, Ridha FN, Duchesne MA, Tan Y, Hughes RW. Pressurized chemical looping

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combustion with CO: reduction reactivity and oxygen-transport capacity of ilmenite ore, Applied Energy 2016; 184:

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132-139.

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(13) Tan Y, Ridha FN, Duchesne MA, Lu DY, Hughes RW. Reduction kinetics of ilmenite ore as an oxugen carrier

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for pressurized chemical looping combustion of methane. Energy&fuels, DOI:10.102 1/acs.energyfuels.7b01038.

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(14) Ashrafi O, Navarri P, Hughes R, Lu D. Heat recovery optimization in a steam-assisted gavity drainage (SAGD)

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plant. Energy 2016; 111:981-990.

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(15) Miller DD, Siriwardane R, Poston J. Fluidized-bed and fixed-bed reactor testing of methane chemical looping

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combustion with MgO-promoted hematite, Appl. Energy 2015; 146:111-121.

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(16) Cuadrat A, Abad A, Adánez J, de Diego LF, García-Labiano F, Gayán P. Behavior of ilmenite as oxygen

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carrier in chemical looping combustion. Fuel Processing Technology 2012; 94(1):101-112.

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(17) Campos DC, Belkouch J, Hazi M, Ould-Dris A. Reactivity investigation on iron-titanium oxides for a moving

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bed chemical looping combustion implementation. Adv. Chem. Eng. & Sci. 2013; 3(1):47-56.

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(18) Abad A, Adánez J, Cuadrat A, García-Labiano F, Gayán P, de Diego LF. Kinetics of redox reactions of

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ilmenite for chemical-looping combustion. Chem. Eng. Sci. 2011; 66(4):689-702.

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(19) Bao J, Li Z, Cai N. Reduction kinetics of foreign-ion-promoted ilmenite using carbon monoxide (CO) for

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chemical looping combustion. I&EC Research 2013; 52(31):10646-10655.


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(20) Pröll T, Mayer K, Bolhàr-Nordenkampf J, Kolbitsch P, Mattisson T, Lyngfelt A, Hofbauer H. Natural minerals

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as oxygen carriers for chemical looping combustion in a dual circulating fluidized bed system. Energy Procedia

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2009; 1(1):27-34.

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(21) Bidwe AR, Hawthorne FM, Charitos A, Schuster A, Scheffknecht. Use of ilmenite as an oxygen carrier in

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chemical looping combustion-batch and continuous dual fluidized bed investigation. Energy Procedia 2011; 4:433-

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440.

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(22) Wang B, Yan R, Lee DH, Liang DT, Zheng Y, Zhao H, Zheng C. Thermodynamic investigation of carbon

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deposition and sulfur evolution in chemical looping combustion with syngas. Energy & Fuels. 2008;22:1012-1020.

482

(23) Lifshitz A, Scheller K, Burcat A, Skinner B. Shock-tube investigation of ignition in methane-oxygen-argon

483

mixtures. Combust. Flame 1971; 16(3):311-321.

484

(24) Crossley RW, Dorko EA, Scheller K, Burcat A. The effect of higher alkanes on the ignition of methane-

485

oxygen-argon mixtures in shock waves. Combust. Flame 1972; 19(3):373-378.

486

(25) Tan Y., Dagaut P., Cathonnet M, Boettner J.C. Natural gas and blends oxidation and ignition: experiments and

487

modeling. In: Symp. (Int.) on Combustion 1994; 25(1):1563-1569.

488

(26) Den Hoed P, Luckos A. Oxidation and reduction of iron-titanium oxides in chemical looping combustion: A

489

phase-chemical description. Oil Gas Sci. Technol. 2011; 66(2):249-263.

490

(27) Dagaut P, Cathonnet M. Kinetics of ethane oxidation in a high pressure jet-stirred reactor: experimental results.

491

J. Chim. Phys. 1990:87:1173-1185.

492

(28) Hancock JD, Sharp JH. Method of comparing solid-state kinetic data and its application to the decomposition of

493

kaolinite, brucite, and BaCO3. J. Am. Ceram. Soc. 1972; 55(2):74-77.

494

(29) Nasr S, Plucknett KP. Kinetics of iron ore reduction by methane for chemical looping combustion. Energy

495

Fuels 2014; 28(2):1387-1395.

496

(30) Jeong MH, Lee DH, Bae JW. Reduction and oxidation kinetics of different phases of iron oxides. Int. J.

497

Hydrogen Energy 2015; 40(6):2613-2620.

498


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Energy & Fuels

499

Table 1. UKTO ilmenite ore composition (wt.%)

500

Al2O3

Fe2O3

TiO2

MgO

Others

0.36

60.90

36.50

1.72

0.52

501 502


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 42

Table 2. Test conditions for PTGA experiments

503

Total

Partial

Partial

Partial

Partial

Partial

Pres.

Pres.

Pres.

Pres.

Pres.

Pres.

Test

P,

pCH4,

pC2H6,

pC3H8,

pFuel,

pCO2,

ID

MPa

MPa

MPa

MPa

MPa

MPa

TemperatureK

T920a

0.6

0.120

0.005

0.001

0.126

0.126

1173

T920b

0.6

0.120

0.005

0.001

0.126

0.252

1173

T923a

0.9

0.120

0.005

0.001

0.126

0.126

1173

T923b

0.9

0.120

0.005

0.001

0.126

0.252

1173

T926a

0.9

0.120

0.005

0.001

0.126

0.126

1223

T927a

0.9

0.120

0.005

0.001

0.126

0.126

1123

T922

0.9

0.340

0.000

0.000

0.340

0.340

1173

T503a

0.9

0.320

0.013

0.003

0.336

0.336

1173

T503b

0.9

0.320

0.013

0.003

0.336

0.560

1173

T504a

0.9

0.320

0.013

0.003

0.336

0.336

1123

T505a

0.9

0.320

0.013

0.003

0.336

0.336

1223

T505b

0.9

0.320

0.013

0.003

0.336

0.560

1223

T511a

0.9

0.320

0.013

0.003

0.336

0.336

1023

T512

0.9

0.340

0.000

0.000

0.340

0.340

1023

T513a

0.9

0.340

0.000

0.000

0.340

0.340

1073

T531a

0.9

0.320

0.013

0.003

0.336

0.336

1048

T628

0.9

0.000

0.34

0.00

0.340

0.340

1023

T825a

1.6

0.320

0.013

0.003

0.336

0.336

1173


Page 25 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

T825b

1.6

0.320

0.013

0.003

0.336

0.672

1173

T915a

1.6

0.320

0.013

0.003

0.336

0.336

1223

T919a

1.6

0.320

0.013

0.003

0.336

0.336

1123

504 505


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 26 of 42

506

CO2

Air

CH4

N2

507 508

Figure 1. A schematic diagram of the PTGA

509


Page 27 of 42

0 -0.5 -1 weight change, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

-1.5 -2 -2.5 -3 -3.5 -4 -4.5 0

20

40

60

80

100

Time, s

510 511

Figure 2. Ilmenite weight change (T=1223 K, pHC=0.34 MPa, pFuel:pCO2=1) for tests T915a

512

(dashed line) and T505a (solid line)

513


Energy & Fuels

1 0.9 0.8 Conversion ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 42

0.7

1023 K

0.6

1048 K

0.5

1073 K

0.4

1123 K

0.3 0.2

1173 K

0.1

1223 K

0 0

50

100

Time, s

514 515

Figure 3. Conversion ratio at 0.9 MPa for pFuel:pCO2=1. Tests T511a, T531a, T513a, T504a,

516

T503a and T505a

517


Page 29 of 42

5 4.5 Oxygen carrying capacity, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

4 3.5 3 2.5 2 1.5 1 0.5 0 1023 1048 1073 1123 1173 1223 Temperature, K

518 519

Figure 4. Oxygen carrying capacity at 0.9 MPa for pFuel:pCO2=1. Tests T511a, T531a, T513a,

520

T504a, T503a and T505a

521


Energy & Fuels

5 4.5 4 weight change, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 42

3.5 3 2.5

1023K 1048K 1073K 1123K 1173K 1223K

2 1.5 1 0.5 0 0

50

100

Time, s

522 523

Figure 5. Weight change at 0.9 MPa for oxidation stage. Tests T511a, T531a, T513a, T504a,

524

T503a and T505a

525


Page 31 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

a

b

c

d

526

527 528

Figure 6. SEM images for test T503b (T=1173 K, top) and T505b (T=1223 K, bottom)

529


Energy & Fuels

0 -1

Weight change, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 42

-2 -3 CH4

-4

Mixture -5 C2H6 -6 0

50

100

150

200

Time, s

530 531

Figure 7. Ilmenite weight loss for pure methane, the simulated natural gas mixture and pure

532

ethane. Tests T511a, T512 and T628. T=1023 K, P=0.9 MPa, pHC 0.34 MPa, pFuel:pCO2=1

533 534


Page 33 of 42


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

-1.5 -2 -2.5

mixture

-3

CH4

-3.5 -4 -4.5 -5 0

20

40

60

80

100

Time, s

535 536

Figure 8. Ilmenite weight loss for pure methane and simulated natural gas mixture. Tests T503a

537

(solid line) and T922 (dotted line). T=1173 K, P=0.9 MPa, pHC=0.34 MPa, pFuel:pCO2=1

538 539


Energy & Fuels


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 42

-1.5 -2 -2.5 -3 -3.5 -4 -4.5 -5 0

50

100

150

Time, s

540 541

Figure 9. Weight loss curves at T=1173 K, P=0.6 MPa, pHC=0.126 MPa. T920a, pFuel:pCO2=1

542


543


Page 35 of 42

0 -1 Weight change, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

-2 -3 -4 -5 -6 0

50

100

150

200

Time, s

544 545

Figure 10. Weight loss curves at T=1173 K, P=0.9 MPa, pHC=0.126 MPa. T923a,

546

pFuel:pCO2=1 (solid line) and T923b, pFuel:pCO2=0.5 (dotted line)

547


Energy & Fuels


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 42

-1.5 -2 -2.5 -3 -3.5 -4 -4.5 -5 0

50

100

150

Time, s

548 549

Figure 11. Weight change at T=1173 K, P=1.6 MPa, pHC=0.126 MPa. T825a, pFuel:pCO2=1

550


551


Page 37 of 42

0 -0.5 -1 weight loss, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

-1.5 -2 -2.5 -3 -3.5 -4 -4.5 -5 0

50

100

150

200

Time, s

552 553


554


555


Energy & Fuels


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 42

-1.5 -2 -2.5 -3 -3.5 -4 -4.5 -5 0

50

100

150

Time, s

556 557


558


559


Page 39 of 42


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

-1.5 -2 -2.5 -3 -3.5 -4 -4.5 -5 0

50

100 Time, s

560 561


562


563


Energy & Fuels

0.4 0.35 0.3 1-(1-Xr)1/3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 42

0.25 0.2 y = 0.0112x R² = 0.9948

0.15 0.1 0.05 0 0

10

20

30

40

Time, s

564 565

Figure 15. Data (test T504a) fitting to obtain k. T=1123 K, P=0.9 MPa, pHC=0.34 MPa,

566

pFuel:pCO2=1

567


Page 41 of 42

80 P

k value change, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60

pFuel

40

pCO2

20 0 0.9

1.1

1.3

1.5

-20 -40

Total pressure, MPa(a)

568 569 570

Figure 16. Individual effects on k by total pressure P, fuel partial pressure pFuel and CO2 partial pressure pCO2, assuming 20 vol.% fuel and 20 vol.% CO2 in the reaction gas mixture

571


Energy & Fuels

1 0.9 0.8 0.7 Conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 42

0.6 0.5 0.4 0.3 0.2 0.1 0 0

50

100

Time, sec

572 573

Figure 17. Comparison between model predictions (solid lines) and experimental data (symbols).

574

o: T825a; ∆: T919a; +: T503a; *: T927a

575


Reduction kinetics of ilmenite ore for pressurized chemical looping

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