Scholarly article on topic 'Combined manganese/iron oxides as oxygen carrier for chemical looping combustion with oxygen uncoupling (CLOU) in a circulating fluidized bed reactor system'

Combined manganese/iron oxides as oxygen carrier for chemical looping combustion with oxygen uncoupling (CLOU) in a circulating fluidized bed reactor system Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Magnus Rydén, Anders Lyngfelt, Tobias Mattisson

Abstract Two kinds of particles consisting of mainly manganese and iron oxides have been examined as oxygen carrier for chemical-looping combustion with O2 uncoupling in a circulating fluidized-bed reactor. The first was produced by spray drying and consisted of 66.8 wt% iron oxide and 33.2 wt% manganese oxide. The second was a manganese ore which also contained iron oxide and silica oxide. During O2 uncoupling experiments, both materials were found to release O2 in gas phase at temperatures above 850 °C, when fluidized with CO2. 7–8 h of continuously operating experiments were recorded for each oxygen carrier, and it was found that the O2 release increased with increased reactor temperature. At 1000 °C, the O2 concentration in the outlet from the fuel reactor was in the order of 7.5 vol% for the synthetic particles. For the ore, the O2 concentration was roughly 0.7 vol% at 990 °C. Further, chemical-looping combustion experiments with natural gas as fuel were carried out. While the conversion of fuel to CO2 and H2O initially was very high (96%) for the synthetic particle and decent (75%) for the ore, both oxygen carriers were found to erode into dust during combustion experiments. Some of the ore particles also swelled greatly. The solids circulation stopped abruptly after 4 h of combustion experiments for the synthetic particle, and after 2 h for the ore. In both cases, the stoppage was likely associated with the physical breakdown of the particles. It is concluded that combined oxides of manganese and iron have very interesting thermodynamical properties and could potentially be suitable for chemical-looping applications. The physical and chemical stability of such materials will have to be further studied and improved though.

Academic research paper on topic "Combined manganese/iron oxides as oxygen carrier for chemical looping combustion with oxygen uncoupling (CLOU) in a circulating fluidized bed reactor system"

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Energy Procedió 4 (2011) 3441-3548

Energy Procedía

www.elsevier.com/locate/procedia

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Combined manganese/iron oxides as oxygen carrier for chemical looping combustion with oxygen uncoupling (CLOU) in a circulating fluidized bed reactor system

Magnus Rydena*, Anders Lyngfelta and Tobias Mattissona'

aDepartment of Energy and Environment, Chalmers University of Technology, SE-412 96, Gothenburg, Sweden

Abstract

Two kinds of particles consisting of mainly manganese and iron oxides have been examined as oxygen carrier for chemical-looping combustion with O2 uncoupling in a circulating fluidized-bed reactor. The first was produced by spray drying and consisted of 66.8 wt% iron oxide and 33.2 wt% manganese oxide. The second was a manganese ore which also contained iron oxide and silica oxide. During O2 uncoupling experiments, both materials were found to release O2 in gas phase at temperatures above 850°C, when fluidized with CO2. 7-8 h of continuously operating experiments were recorded for each oxygen carrier, and it was found that the O2 release increased with increased reactor temperature. At 1000°C, the O2 concentration in the outlet from the fuel reactor was in the order of 7.5 vol% for the synthetic particles. For the ore, the O2 concentration was roughly 0.7 vol% at 990°C. Further, chemical-looping combustion experiments with natural gas as fuel were carried out. While the conversion of fuel to CO2 and H2O initially was very high (96%) for the synthetic particle and decent (75%) for the ore, both oxygen carriers were found to erode into dust during combustion experiments. Some of the ore particles also swelled greatly. The solids circulation stopped abruptly after 4 h of combustion experiments for the synthetic particle, and after 2 h for the ore. In both cases, the stoppage was likely associated with the physical breakdown of the particles. It is concluded that combined oxides of manganese and iron have very interesting thermodynamical properties and could potentially be suitable for chemical-looping applications. The physical and chemical stability of such materials will have to be further studied and improved though. (©5 2011 Published by Elsevier Ltd.

Keywords: Chemical-looping combustion; Chemical-looping with oxygen uncoupling; Manganese oxide; Iron oxide; Combined oxides

1. Introduction

A majority of the scientific community now concludes that global CO2 emissions would need to be reduced greatly in the future. One way to reduce CO2 emissions that is receiving increasing interest is carbon capture and storage, which involves capturing of CO2 at emission sources and storing it where it is prevented from reaching the atmosphere. For example, CO2 could be captured in flue gases from combustion or industrial processes, and stored in geological formations such as depleted gas fields or deep saline aquifers. Chemical-looping combustion with

Corresponding author. Tel.: (+46) 31 772 1457; fax: (+46) 31 772 3592.

E-mail address: magnus.ryden@chalmers.se.

doi:10.1016/j.egypro.2011.01.060

oxygen uncoupling (CLOU) involves oxidation of a fuel using gas-phase O2 released from a solid oxygen carrier. In this way the products are not diluted with N2 and CO2 for sequestration is obtained without the need for costly gas separation. Due to this favourable characteristic, chemical-looping combustion with oxygen uncoupling could have an important role to play in the global task to reduce anthropogenic CO2 emissions.

2. Background

2.1. Chemical-looping combustion

Chemical-looping combustion (CLC) is an innovative method to oxidize fuels which has attained considerable interest. Two separate reactors are used, one air reactor (AR) and one fuel reactor (FR). A solid oxygen carrier performs the task of transporting oxygen between the reactors. Direct contact between fuel and air is avoided. Therefore, the combustion products are not diluted with N2, see Figure 1.

The oxygen carrier circulates between the reactors. In the fuel reactor, it is reduced by the fuel, which in turn is oxidized to CO2 and H2O according to reaction (1). In the air reactor, it is oxidized to its initial state with O2 from the combustion air according to reaction (2). Since many oxygen carriers are metal oxides, the abbreviation Me-O/Me is often used to describe the oxygen carrier in its oxidized/reduced form.

CnHm + (2n+/m) Me-O ^ n CO2 + /m H2O + (2n+/m) Me (1)

Me + / O2 ^ Me-O (2)

The net energy released in the reactor system is the same as in ordinary combustion. This is apparent since combining reaction (1) and reaction (2) yields reaction (3), which is complete combustion of the fuel with O2.

CnHm + (n+^m) O2 ^ n CO2 + /m H2O (3)

Most often, it is suggested to design chemical-looping combustion processes as circulating fluidized beds with oxygen-carrier particles used as bed material. Commonly proposed oxygen carriers include metal oxide particles, typically with NiO, Fe2O3, CuO or Mn3O4 as active phase. Chemical-looping combustion has several attractive features. There should be no thermal formation of NOX and since the gas from the fuel reactor consists essentially of CO2 and H2O, cooling in a condenser is all that is needed to obtain almost pure CO2 for sequestration. The progress within the area of chemical-looping combustion has been reviewed recently by Lyngfelt et al [1, 2], Fang et al [3], and Hossain and de Lasa [4].

2.2. Chemical-looping combustion with oxygen uncoupling (CLOU)

In reaction (1), it was assumed that the fuel is in gas phase and that it reacts with the oxygen carrier in a solid-gas reaction. However, with some oxygen carrier gas-phase O2 can be released directly in the fuel reactor according to reaction (4).

Figure 1. Schematic description of chemical-looping combustion.

2 Me-O ^ 2 Me + O2 (4)

Released O2 can react directly with the fuel in the fuel reactor according to reaction (3). The reduced oxygen carrier is then recirculated to the air reactor where it is reoxidized according to reaction (2). The overall reaction is identical to the one for chemical-looping combustion. This concept is referred to as chemical-looping with oxygen uncoupling (CLOU) in a patent application by Lyngfelt and Mattisson [5], and later in a publication by Mattisson et al [6]. However, the mechanism of O2 uncoupling using CuO as oxygen carrier was investigated already by Lewis et al [7] in the 1950's. The main difference compared to chemical-looping combustion is the mechanism by which the fuel is oxidized. In ordinary chemical-looping combustion, the oxidation takes place mainly via gas-solids reactions. So if the fuel is a solid such as coal or biomass, it has to be gasified in order to be able to react with the solid oxygen carrier, which is a slow process that requires high temperature. By contrast, in chemical-looping with oxygen uncoupling the oxidation of the fuel can proceed by direct combustion. Mattisson et al [8] and Leion et al [9] show that oxidation of petroleum coke can be 45 times faster with chemical-looping with oxygen uncoupling using a CuO-based oxygen carrier, compared to a chemical-looping combustion process which relies on char gasification.

A feasible oxygen-carrier material for chemical-looping with oxygen uncoupling should 1) be thermodynamically capable to take up and release gas-phase O2 at relevant conditions, 2) provide sufficiently fast reaction kinetics for the O2 uncoupling and the oxidation reactions, 3) have a decently high content of active oxygen, 4) have low tendency for mechanical, chemical and thermal degeneration, 5) not promote extensive formation of solid carbon in the fuel reactor, 6) preferably be cheap and environmentally sound. Most oxygen carriers for chemical-looping combustion fail to satisfy the first of these requirements, i.e. they can not release gas-phase O2 at relevant conditions. As was mentioned above, CuO-Cu2O works for this application. Unfortunately, the low melting point of metallic copper could be an obstacle, and CuO is also fairly costly. In theory, Mn2O3-Mn3O4 could also work, but only if operated at inappropriately low temperature levels. However, it is possible to alter the thermodynamical properties of manganese oxides by addition of other anions, thus forming combined oxides of spinel structure, as has been examined by Shulman et al [10], or perovskite structure, as has been examined by Ryden et al [11] and Leion et al [12]. Here the former of these two possibilities is explored.

3. Experimental

3.1. Oxygen carrier particles

Two oxygen carrier materials were examined within the scope of this study. The first material was a synthetic particle that consisted of 66.8 wt% iron oxides and 33.2 wt% manganese oxides. Its oxidized form is supposed to be a solid solution of a-Fe2O3 and a-Mn2O3, while the reduced form should be a solid solution of mostly cubic spinel Fe2MnO4, possibly with some Fe3O4 and Mn3O4. The particles were produced by spray drying, heat treated for 4 h at 1100°C, and finally sieved to a size range of 90-212 ^m. The bulk density of fresh particles was 1500 kg/m3. Similar particles have been studied by Azimi et al [13]. The second material was a manganese ore from Kerametal in Slovakia that consisted of manganese oxides (>55 wt%), with iron oxide (<6.5 wt%) and silica oxide(<7 wt%) as main impurities. Due to the presence of iron and silica, as well as other impurities, the ore is expected to have complex phase chemistry. The ore was not pre-treated in any particular way, except crushing and sieving to a size range of 90-212 ^m, followed by oxidation in air at 950°C. The bulk density of the ore was 1730 kg/m3.

3.2. The fluidized-bed reactor

The experiments were carried out in a small-scale laboratory reactor for gaseous fuels. The reactor has been used earlier for similar experiments by Ryden et al [11, 14, 15]. A schematic picture of the reactor is shown in Figure 2. Suitable flows are 0.15-0.75 Ln/min natural gas and 3-10 Ln/min air. The height of the reactor is 200 mm. The base of the fuel reactor measures 25x25 mm. The air reactor is 25x40 mm in the bottom and 25x25 mm in the upper narrow part. Fuel and air enter the system through separate wind boxes, located in the bottom of the reactor. Porous quartz plates act as gas distributors. Particles are thrown upwards in the air reactor, and a fraction enters the downcomer, which is a J-type loop-seal. From here particles enter the fuel reactor via the return orifice. Particles return to the air reactor through a U-type slot, and this way, a continuous circulation of oxygen-carrier particles is

obtained. The downcomer and the slot are fluidized with inert gas. In order to make it possible to reach a suitable temperature, the reactor is placed inside an electrically heated furnace. The temperature in each reactor section is measured with thermocouples located 70 mm above the distributor plates. Downstream, filters and water seals are used to catch elutriated particles. The water seals are also used to apply an overpressure of 250 Pa to the fuel reactor, with respect to the air reactor. This is done in order to prevent leakage of air into the fuel reactor. Along the reactor sections there are thirteen pressure measuring taps, which makes it possible to estimate where particles are located in the system, and to detect disturbances in the fluidization. Prior to analysis, the gas from the reactor passes through particle filters, coolers and water traps, so all measurements are made on dry gas. CO2, CO and CH4 are measured using infrared analyzers, O2 with paramagnetic sensors. A gas chromatograph is used to measure H2 and N2.

Return Orifice

Fuel Reactor

3.3. Experimental procedure

The aim of the O2 uncoupling experiments was to establish that it is possible to use the oxygen carrier particles to transfer O2 from the air reactor and release it in gas phase in the fuel reactor. The furnace was heated to the desired fuel-reactor temperature, which was 800-1000°C. During this period, both reactor sections were fluidized with air and a continuous circulation of solids between the fuel reactor and the air reactor was established. When the desired temperature was reached, the air to the fuel reactor was replaced with CO2. The air reactor continued to be fluidized with air. The particle seals were fluidised with argon or CO2. In the bottom of the fuel reactor, the O2 concentration should be zero, so the oxygen carrier could be expected to release gas-phase O2. In the outlet from the air reactor, the O2 concentration was slightly below 21 vol%, so here the oxygen carrier could be expected to take up O2.

Traditional chemical-looping

combustion experiments were also conducted. The purpose was to examine the reactivity between the oxygen carrier and natural gas, and to establish whether the material is stable during more extensive

reduction than what could be achieved with O2 uncoupling in inert atmosphere. The experimental procedure was similar to that for the O2 uncoupling experiments, except that natural gas with a composition equivalent of C1.14H4.25O0.01N0.005 was added to the fuel reactor rather than CO2. The natural gas reacted with the oxygen carrier and formed mostly CO2 and H2O. The combustion efficiency for the chemical-looping combustion experiments was defined simply as 100% minus as the lower heating value of the gas leaving the fuel reactor divided by the lower heating value of the fuel added to the fuel reactor.

In both kinds of experiments, minor gas leakage between the reactor parts was a possibility. Further, the downcomer was fluidized with 0.30 Ln/min inert gas, while 0.15 Ln/min was added to the slot. The numbers presented in this paper have been slightly modified from the measured data, in order to make up for this. See Ryden et al [11] for a detailed description of the methodology used. A detailed description for how data from the combustion experiments have been evaluated can be found in earlier work by Ryden et al [14, 15].

Figure 2. The two-compartment fluidized-bed reactor. During operation, the downcomer and a third of the air and fuel reactor is filled with oxygen carrier particles.

4. Results

4.1. O2 uncoupling experiments

A range of process parameters were examined during the course of the O2 uncoupling experiments. A summary can be found in Table 1. Totally, about 8 hours of O2 uncoupling experiments were conducted with the synthetic oxygen carrier, and 7 hours with the manganese ore.

Table 1. Summary of conducted O2 uncoupling experiments. FCO2fr is the flow of CO2 to the fuel reactor, Far is the flow of air to the air reactor and T is the reactor temperature.

Experiment Oxygen carrier Bed mass Operation F CO2,fr F ar T

(g) (min) (Ln/min) (Ln/min) (°C)

CLOU: Fe/Mn-synthetic 200 60 0.45-0.67 4.0-7.0 900-950

CLOUn Fe/Mn-synthetic 250 255 0.45 4.0 850-1000

CLOUm Fe/Mn-synthetic 250 45 0.60 4.0 850-900

CLOUIV Fe/Mn-synthetic 250 125 0.60 4.0 850-1000

CLOUV Manganese ore 250 90 0.67 5.0-6.0 950

CLOUvi Manganese ore 250 305 0.67 5.0 780-980

CLOUvii Manganese ore 250 35 0.67 5.0 850-990

In table 1, it can be seen that the main parameter which was varied during the experiments was the fuel reactor temperature. This could be expected to affect the level of O2 uncoupling in two ways. Firstly, the expected equilibrium pressure of O2 over the particles could change. Secondly, high temperature could improve the reaction kinetics.

In Figure 3, the concentration of O2 in the fuel reactor gas and the approximate volumetric O2 uncoupling, is expressed as function of the fuel reactor temperature for experiments with the synthetic particles and the manganese ore. It can be seen that there was no uncoupling of O2 below 850°C. When the reactor temperature was increased, so did the O2 uncoupling. For the ore the increase was rather modest, but for the synthetic particles it was considerable. Other parameters, such as bed mass and the flows of air and inert, were not examined extensively enough to draw any clear conclusions. The impact of these parameters appears to have been much smaller than for the reactor temperature though. The main conclusions that can be drawn from the O2 uncoupling experiments is that both materials had the desirable thermodynamical properties, i.e. were capable to release O2 in inert atmosphere at relevant temperatures. The effect was several orders of magnitude greater for the synthetic particles than for the ore. High temperature clearly favours O2 uncoupling for both materials.

0.06 u

0.05 O

0.04 5'

0.02 5)

Reactor temperature (°C)

Figure 3. O2 concentration in the fuel reactor gas and estimated O2 uncoupling as function temperature for CLOUm and CLOUXi.

4.2. Chemical-looping combustion experiments

A summary of the chemical-looping combustion experiments conducted can be found in Table 2. About 4 hours of combustion experiments were conducted with the synthetic particles as oxygen carrier, and merely 2 hours with the manganese ore. This was due to practical problems with the experiments, as will be explained below.

Table 2. Summary of conducted chemical-looping combustion experiments. Fngjr is the flow of natural gas to the fuel reactor, Far is the flow of air to the air reactor and Tfr is the temperature in the fuel reactor. The temperature in the air reactor was 5-10°C higher due to the exothermic oxidation reaction. * Marks that the natural gas flow was diluted with 0.30 Ln/min argon.

Experiment Oxygen carrier Bed mass Operation Fngfr F ar Tfr

(g) (min) (Ln/min) (Ln/min) (°C)

CLCi Fe/Mn-synthetic 250 50 0.30* 5.0 950

CLCii Fe/Mn-synthetic 250 80 0.30-0.40 5.0 950

CLCiii Fe/Mn-synthetic 250 105 0.30 5.0 900-950

CLCiv Manganese ore 250 25 0.30 6.0 950

CLCV Manganese ore 250 95 0.30 6.0 950

The combustion experiments differed from the O2 uncoupling experiments in several ways. Notably, the O2 uncoupling experiments produced at most 0.08 Ln/min O2, as can be seen in Figure 3. By comparison, combustion of 0.30 Ln/min natural gas required more than 8 times as much. In practice, this means that the oxygen carrier was reduced much more extensively in the combustion experiments, compared to the O2 uncoupling experiments. It also means that reaction mechanisms associated with conventional chemical-looping combustion, i.e. gas solid reactions between metal oxide and fuel gas, could be expected to dominate.

Although the chemical-looping combustion experiments eventually had to be discontinued, the solids circulation worked properly for a few hours and it was possible perform experiments with both oxygen carriers, as is shown in Figures 4-5 below. It can be seen that the synthetic particles were superior to the manganese ore in terms of fuel conversion. The combustion efficiency was roughly 96% at 950°C with 0.30 Ln/min natural gas for the synthetic particles, but only about 75% for the ore. This could be expected. Not only did the synthetic particles have superior O2 uncoupling properties, but spray dried particles also are more porous and have larger active area than an ore. It can also be seen that the CO2 concentration was only about 30-40%. Instead, the main gas in the outlet from the fuel reactor was argon, which is used as fluidization gas in the slot and the downcomer. Due to the large amounts of argon present in the fuel reactor gas, it can be concluded that most of the gas added to the slot and downcomer ended up in the fuel reactor, which was unexpected. Due to the practical problems surrounding both particles, this phenomenon has not been further examined.

As indicated above, both materials collapsed after a few hours of operation with fuel. The erosion of the particles may very well have started immediately, as dust was seen to accumulate early in the filters between the reactor and the gas analysers. Further, the pressure measurements drifted considerably, particularly the last hour of operation with the synthetic particles. The circulation of particles worked throughout the

0 15 30 45 60 75 90 105 Time (min)

Figure 4. Dry gas composition for chemical-looping combustion experiment with synthetic Fe/Mn-particles (CLCm).

experiments though, as it could be seen that the correct amount of O2 was consumed in the air reactor.

After the experiments, the reactor was disconnected and the remaining particles examined. It was found that much of the synthetic particles turned into a fine dust, which blew out of the air reactor and eventually got stuck downstream in system, mainly in the particle separation box directly above the reactor. This created a complete stop in the process after 4 hours. In the fuel reactor, collapsed material formed very soft agglomerations. Sieving of the remaining material revealed that the size distribution of the particles had changed completely, and that only about 50 wt% remained in the desired size span of 90-212 ^m, while most of the rest was dust below 90 ^m. The used particles had only about 30% of the density of fresh synthetic particles. The manganese ore was also found to have turned partly into dust, but here a large share of the particles had actually swelled to a diameter 3-4 times as large as the original. The direct reason for the stop in the particle circulation, which for the ore happened already after 2 hours, appears to have been dust that was blown downstream in the reactor system, thus plugging the outlet pipes. Used ore had about 60% of the density of fresh.

5. Discussion

From a thermodynamic point of view, the simple oxide system Mn2O3/Mn3O4 has the ability to release O2 in gas phase. However, the equilibrium partial pressure of O2 at the most desirable temperature level, i.e. =850C-1000°C, is too high to be applicable to practical chemical-looping with oxygen uncoupling. Further, in preliminary experiments, the release of O2 from Mn2O3 has been found to be very slow. By contrast, as has been shown in this work, it is clear that combined oxides of Mn/Fe can release and take up O2 reasonably fast. The two examined oxygen carriers worked very well during the O2 uncoupling experiments. Here the experiments had high repeatability and the measured pressure drops indicated stable operation. This suggests that the particles could withstand minor changes in its phase composition introduced by O2 uncoupling in inert atmosphere without falling apart. The synthetic Fe/Mn-particles released a lot of O2at a temperature perceived as attractive for chemical-looping applications. The ore also released O2 and could be of interest for many applications due to its low cost. Fe/Mn-materials could prove to be especially interesting for solid fuel applications, since they both release gas phase O2 and is expected be resistant to impurities such as sulphur.

However, it is clear that the physical stability of the examined materials was low. Once natural gas was added to reactor, something went wrong in both experimental series. While there was a solids circulation and the required amount of O2 clearly was transferred from the air reactor to the fuel reactor, several other indicators pointed towards a process not working as intended. Accumulation of dust downstream, large amounts of argon present in the fuel reactor gas, loss of pressure drop in parts of the reactor etc. The reactor system has been used for several successful earlier series of chemical-looping combustion experiments with no or only minor problems see Ryden et al [11, 14, 15]. So this type of oxygen carrier will need to be studied further. It seems reasonable to believe that improved stability somehow could be achieved, perhaps by addition of an inert carrier material.

950°C

CH4 H2

0 15 30 45 60 75 90 Time (min)

Figure 5. Dry gas composition for chemical-looping combustion experiment with manganese ore (CLCV).

6. Conclusions

Two particles with manganese oxide and iron oxide as active phases, one synthetic and one ore, have been examined as oxygen carrier for chemical-looping combustion with O2 uncoupling. During O2 uncoupling experiments, both materials released O2 in gas phase at temperatures above 850°C, when fluidized with CO2. The release of O2 increased with increased reactor temperature. At 1000°C, the O2 concentration in the outlet from the fuel reactor was roughly 7.5 vol% for the synthetic particles. For the manganese ore, the O2 concentration was much lower, about 0.7 vol% at 990°C. The chemical-looping combustion experiments with natural gas as fuel were not as successful. With both oxygen carriers, the combustion experiments had to be discontinued after a few hours of operation due to collapse of the particles. It is concluded that combined oxides of manganese and iron have interesting thermodynamical properties and could be feasible as oxygen carriers for chemical-looping applications. The phase chemistry and physical stability of the materials need to be better understood though.

7. Acknowledgements

This publication was based on work supported by Award No KUK-F1-023-02, made by King Abdullah University of Science and Technology (KAUST).

8. References

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