CO2 Capture in Coal Combustion by Chemical-looping With Oxygen Uncoupling (CLOU)

7/24/2019 CO2 Capture in Coal Combustion by Chemical-looping With Oxygen Uncoupling (CLOU)

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CO2Capture in Coal Combustion by Chemical Loopingwith Oxygen Uncoupling (CLOU)

Iaki Adnez-Rubio, Pilar Gayn, Alberto Abad, Francisco Garca-Labiano, Luis F. deDiego, Juan AdnezInstituto de Carboqumica (C.S.I.C.), Dept. of Energy & Environment, Miguel Luesma

Castn, 4, Zaragoza, 50018, [email protected]; [email protected]

Abstract

Chemical Looping with Oxygen Uncoupling (CLOU) process is a Chemical Looping

Combustion (CLC) technology that allows the combustion of solid fuels using oxygencarriers with inherent CO2separation. As in CLC technology, in CLOU process the oxygennecessary for fuel combustion is supplied by a solid oxygen carrier, which contains a metaloxide. According to the CLOU process is intended to use the capability of some metallicoxide to generate gaseous oxygen at high temperature. The oxygen carrier circulates betweentwo interconnected reactors: the fuel reactor (FR) and the air reactor (AR). In the FR, theoxygen carrier gives the oxygen necessary for fuel conversion to CO2and H2O. In this reactoronly CO2and steam are produced. After steam condensation, a pure CO2stream is obtained.The reduced oxygen carrier is regenerated to the initial metal oxide in the AR, and it is readyto start a new cycle. The exit flow contain N2and unreacted O2. Thus, the CO2capture isinherent to the process, being low the energy penalty and economical costs for the CO2capture. This work shows the main characteristics of the CLOU process when different typesof solid fuels are used. The oxygen carrier was a Cu-based material selected from a previousscreening process with a 60 wt.% of CuO and a 40 wt.% of MgAl2O4as inert prepared byspray drying. As solid fuels, coals of different rank (a lignite, two bituminous and ananthracite) and biomass were used. The CLOU process was carried out in a continuous unitconsisting in two interconnected fluidized bed reactors with a thermal power of 1.5 kWth,where the proof of the concept for CLOU process was demonstrated. Combustion and CO2capture efficiencies were analyzed, as well as the oxygen carrier inventory needed to carryout the process. The results obtained show that CLOU technology has great possibilities forthe use of solids fuels in the CLC process, allowing full combustion and high CO2capture

efficiencies using low solid inventories.

1. Introduction

The Chemical Looping with Oxygen Uncoupling (CLOU) process is a Chemical LoopingCombustion (CLC) technology that allows the combustion of solid fuels with inherent CO2separation using oxygen carriers [1]. This technology has low energy penalty and thus lowCO2capture costs. Figure 1 shows scheme of the CLOU process design. The CLOUtechnology takes advantage of the property of some metal oxides which can generate gaseousoxygen at high temperatures. The oxygen generated by the oxygen carrier reacts directly withthe solid fuel, which is mixed with the oxygen carrier in the fuel reactor. Thus, the need of


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solid fuel gasification, that it is a limiting step in conventional CLC process with solid fuels,is avoided.

Coal

CO2+ H2ON2(+O2)

MexOy

H2O(l)

CO2

Fuel

Reactor

MexOy-1

Condenser

AshCO2

Air

Air

Reactor

CO2

Coal

CO2

Volatiles

O2

CO2

Oxygen

Carrier

H2O

Char

2

H2O

Figure 1: Scheme of the CLOU process, with the main processes happening in the fuel

reactor for coal combustion.

Three metal oxide systems have been identified to be used as oxygen carrier in CLOUprocess: CuO/Cu2O, Mn2O3/Mn3O4, and Co3O4/CoO [1]. After a thermodynamic and kineticstudy, it was found that Cu-based oxygen carriers fulfill all the requirements of the process.Reaction taking place in the fuel reactor with copper oxide as oxygen carrier are thefollowing:

2224 OOCuCuO + (1)

CoalVolatile Matter + Char (2)

Volatile Matter 2 2 2O CO H O+ + (3)2 2 2Char O CO H O+ + (4)

2 2 24 2CuO Coal Cu O CO H O+ + + (5)

The oxygen release reaction (r.1) is endothermic but the global reaction (r.5) in fuel reactor isexothermic due to the coal combustion (r.3 and r.4). This is an advantageous characteristic ofusing this metal oxide, as there can be a temperature increase in the fuel reactor.

The reduced oxygen carrier from the fuel reactor is regenerated in the air reactor:

CuOOOCu 42 22 + (6)

Figure 2 shows the equilibrium concentration of O2versus temperature for copper oxide,calculated using the HSC Chemistry 6.1 software [2]. The oxygen concentration atequilibrium conditions greatly depends on the temperature. Thus, an equilibriumconcentration of 1.5 vol% O2can be reached in the fuel reactor at 900 C, whereas theequilibrium concentration increases up to 12.4 vol% at 1000 C. Nevertheless it could be notneeded to reach the equilibrium concentration in the fuel reactor. Indeed it should bedesirable to get low concentration of oxygen from the fuel reactor in order to obtain a highpurity CO2stream. In the air reactor, the metal oxide is stable below 950 C if the maximumoxygen concentration from the air reactor is higher than 4.5 vol%.


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T (C)

850 900 950 1000 1050 1100

Oxygenequilibriu

m

(vol%)

0

5

10

15

20

CuO

2Cu O

Figure 2: Equilibrium oxygen concentrations over the CuO/Cu2O system as a function of

temperature, [2].

An analysis about the suitability of Cu-based materials was carried out previously at ICB-CSIC [3]. Particles prepared by several methods with different supporting materials anddifferent metal oxide contents were tested. From this work, it was shown that particlescontaining 60 wt% CuO and using MgAl2O4as supporting material are adequate for its use asoxygen carrier for the CLOU process. On the whole, this material shows adequate values ofreactivity and oxygen transport capacity, high attrition resistance and does not have tendencyto agglomerate during operation in a fluidized bed reactor.

The aim of this work was to investigate the performance of a CLOU system using a Cu-basedoxygen carrier. The combustion of coal was carried out in a continuously operated 1.5 kWthCLOU unit. The effect of operating conditions such as temperature of the fuel reactor,residence time and the rate of coal feeding on the combustion efficiency were investigated.The experiments were carried out at 900-960 C in the fuel reactor. The effect of coal rankwas studied in with four coals with different reactivity (a lignite, two bituminous and ananthracite). A renewable fuel, pine sawdust, was also used in the CLOU unit. The resultsobtained are analyzed and discussed in order to be useful for the scale-up of a CLOU processfuelled with coal.

2. Experimental section

2.1 The Cu-based oxygen carrier

The material used was a Cu-based oxygen carrier prepared by spray drying. Oxygen carrierparticles were manufactured by VITO (Flemish Institute for Technological Research,Belgium) using MgAl2O4as inert material. The CuO content was 60 wt%. Particles were

calcined for 24 h at 1100C and sieved (100200 m). From now on the oxygen carrier wasnamed as Cu60MgAl. Table 1 shows the main properties of this oxygen carrier. It has a lowporosity and superficial area. The mechanical strength of the particles after 24 h ofcalcination was adequate for its use in a fluidized bed. The compounds found by XRD

analysis were CuO and MgAl2O4.


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Table 1. Properties of the oxygen carrier Cu60MgAl (after 24 h of calcination).

CuOcontent (wt.%) 60

Oxygen transport capacity, Ro(wt.%) 6

Crushing strength (N) 2.4

Skeletal density (kg/m3) 4600

Porosity (%) 16.1

Specific surface area, BET (m2/g) < 0.5

XRD main phases CuO, MgAl2O4

2.2 Fuels

Different solid fuels have been used during the experimental work. Five coals of differentrank have been used: a Spanish anthracite from El Bierzo, three bituminous coals fromColombia, South African and Czech Republic, together with one Spanish lignite from Teruel.

According to ASTM characterization, Czech Republic coal is a low volatile bituminous coal(LV), South African coal is a medium volatile bituminous coal (MV) and Colombian coal is ahigh volatile bituminous coal (HV). Besides, a pine biomass, from Ans (Huesca, Spain), wasused. The main properties of selected fuels (proximate and ultimate analysis and LHV) areshowed in Table 2. The coal particle size used was +0.2-0.3 mm and the particle size usedwas +0.5-2.0 mm for the biomass.

Table 2. Properties of the fuels used in this work.

Anthracite LV Bitum. MV Bitum.HV Bitum.

Fresh

HV Bitum.

PretreatedLignite Biomass

Proximate Analysis (wt.%)Moisture 1.0 2.0 4.2 7.5 2.3 12.6 4.2

Volatile matter 7.5 17.1 25.5 34.0 33.0 28.6 81.0Fixed carbon 59.9 68.8 55.9 49.9 55.9 33.6 14.4

Ash 31.6 12.1 14.4 8.6 8.8 25.2 0.4

Ultimate Analysis (wt.%)

C 60.7 75.8 69.3 70.8 65.8 45.4 51.4H 2.1 3.7 3.9 3.9 3.3 2.5 6.5N 0.9 1.9 1.9 1.7 1.6 0.6 0.3S 1.3 0.4 0.9 0.5 0.6 5.2 0.0

O(1)

2.4 4.1 5.4 7.0 17.6 8.5 37.2

LHV (kJ/kg) 21900 28950 25500 25900 21900 16250 19200(1)Oxygen to balance

Agglomeration problems with pipes clogging were found when Colombian coal ElCerrejn was fed into the fluidized bed because this coal showed an important swellingbehaviour. In order to avoid coal swelling and bed agglomeration, the coal was subjected to athermal pre-treatment by pre-oxidation. Coal was heated at 180 C in air atmosphere for 28hours [4]. Proximate and ultimate analyses of the pre-treated coal are also shown in Table 2.

2.2 Experimental set-up

A schematic view of the CLOU unit is shown in Figure 3. The set-up was basically composedof two interconnected fluidized bed reactors the air and fuel reactors joined by a loop seal,and a riser for solids transport from the air reactor to the fuel reactor.


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Gas AnalysisGas Analysis

OO22, CO, CO22,, CO, HCO, H22, CH, CH44,SO,SO22

N2 CO2Air

Sec.Air

Fuel

Gas AnalysisGas Analysis

OO22, CO, CO22,, CO, SOCO, SO22

N2

AirReactor

FuelReactor

1.-Fuel Reactor, FR 7.- Solids reservoir

2.- Loop Seal 8.- Solids control valve

3.- Air Reactor, AR 9.- Fuel feeding system

4.- Riser 10.- Screw feeders

5.- Cyclone 11.- Furnace

6.- Diverting solids valve 12.- Filter

8

5

6

4

1

9

10

311

11

2

TorchTorch

TorchTorch

7

12

12

Tar AnalysisTar Analysis

Figure 3.Schematic view of the 1.5 kWthCLOU rig fuelled with coal.

Because of heat losses, the system is not auto-thermal and it is heated up by means of variousindependent ovens to get independent temperature control of the air reactor, fuel reactor, and

freeboard above the bed in the fuel reactor, which is actually kept constant at about 900 C inall the experiments. During operation, temperatures in the bed and freeboard of the fuelreactor, air reactor and riser were monitored as well as the pressure drops in importantlocations of the system, such as the fuel reactor bed, the air reactor bed and the loop seal.The fuel reactor consisted of a bubbling fluidized bed with 50 mm of inner diameter and 200mm bed height. N2was used as fluidizing gas. The gas flow was 186 LN/h. Coal is fed by ascrew feeder at the bottom of the bed right above the fuel reactor distributor plate in order tomaximize the time that the fuel and volatile matter are in contact with the bed material.The oxygen carrier is reduced in the fuel reactor, evolving gaseous oxygen to thesurroundings. The oxygen burns the volatiles and char proceeding from coal pyrolysis in thefuel reactor. Reduced oxygen carrier particles overflowed into the air reactor through a U-

shaped fluidized bed loop seal with 5 cm of inner diameter, to avoid gas mixing between fueland air. A N2flow of 60 LN/h was introduced in the loop seal.The oxidation of the oxygen carrier took place in the air reactor, consisting of a bubblingfluidized bed with 8 cm of inner diameter and 10 cm bed height, and followed by a riser. Theair flow was 1740 LN/h. In addition, a secondary air flow (240 LN/h) was introduced at thetop of the bubbling bed to help particle entrainment through the riser. N2and unreacted O2left the air reactor passing through a high-efficiency cyclone and a filter before the stack. Theoxidized solid particles recovered by the cyclone were sent to a solids reservoir, setting theoxygen carrier ready to start a new cycle. In addition, these particles avoid the leakage of gasbetween the fuel reactor and the riser. The regenerated oxygen carrier particles returned to thefuel reactor by gravity from the solids reservoir through a solids valve which controlled the

flow rates of solids entering the fuel reactor. A diverting solids valve located below thecyclone allowed the measurement of the solids flow rates at any time. Therefore, this design


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allowed us to control and measure the solids circulation flow rate between both reactors. Thetotal oxygen carrier inventory in the system was between 2 to 3 kg, being about 0.5-0.8 kg inthe fuel reactor. CO, CO2, H2, CH4, and O2were analyzed in the outlet stream from fuelreactor, whereas CO2, CO and O2were analyzed from the flue gases of the air reactor.

Data evaluationIn the continuous CLOU unit, to analyze the confidence of the results, mass balances tooxygen, carbon and sulphur were carried out using the measurements of the gas stream

coming from the air and fuel reactors. The CO2capture efficiency, CC , was defined as the

fraction of carbon present in the coal which is at the outlet of the fuel reactor. This is the realcarbon captured in the CLOU system, as the remaining carbon exits as CO2together withnitrogen at the air reactor outlet.

2CO ,outAR

CC 1

C SFC

F

fm

M

=

(7)

The CO2capture efficiency depends on the conversion of char in the fuel reactor,Xchar. Itsvalue was calculated considering that the carbon contained in the fuel reactor comes from thecarbon in volatiles (FC,vol) and carbon in char converted.

2 4

2 4 2

CO ,outFR CO,outFR CH ,outFR C,vol

char

CO ,outFR CO,outFR CH ,outFR CO ,outAR C,vol

+ +

+ + +

F F F F X

F F F F F

=

(8)

The combustion efficiency in the fuel reactor, comb,FR, evaluates the combustion degreeconsidering only the fraction of coal converted in the fuel reactor. Thus, the combustion

efficiency in the fuel reactor was calculated through the quotient between the oxygenrequired to fully burn unconverted gases (CH4, CO and H2) and the oxygen demanded for fullcombustion of the coal converted in the fuel reactor.

( )4 2

2

2

CH ,outFR CO,outFR H ,outFR

comb,FR

O SFCO ,outAR C,elut

O

2F + 0.5 F + F = 1

2 m-2F -2F

M

(9)

The results obtained were evaluated considering the fuel reactor temperature, the solidscirculation flow rate and the coal feeding rate. In this sense, the oxygen carrier to fuel ratio

was defined as the ratio of the oxygen transported by the oxygen carrier to the oxygendemanded by coal for complete combustion. A value of = 1 corresponds to the

stoichiometric flow of CuO needed to fully convert coal to CO2and H2O, being CuO reducedto Cu2O. Thus, was calculated with the following equation:

OC OC

O SF

R m

m =

(10)

OCm being the solids circulation flow rate in the completely oxidized state and SFm the mass-

based flow of coal fed in to the reactor; Ois the stoichiometric kg of oxygen to convert 1 kgof coal to CO2and H2O. This value was calculated from the proximate and ultimate analysis

of coal, see Table 1.2, by using the following equation:


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2

2

C S OHO O

C H S O

0.25 f f ff

MM M M M

= + +

(11)

fibeing the mass fraction of the element iin coal.

3. Result and discussion

3.1 Proof of the concept of the CLOU process in a 1.5 kWthcontinuousunit using coal

In this section the results obtained during coal combustion in a CLOU unit composed of twointerconnected fluidized beds are described. Until this work only experiments in batchfluidized bed reactor for the combustion of solid fuels at small scale were carried out [1, 5, 6]in the literature. Therefore, this extensive experimental campaign carried out in this work wasfundamental to understand the CLOU process, its flexibility depending on the type of fueland the effect of operating conditions, as well as its possible operational problems.

The effect of the operating conditions on the performance of the CLOU process was analyzedusing pre-treated Colombian coal El Cerrejn as fuel. Table 3 shows a compilation of themain variables used in each test. 30 h of hot fluidization conditions, whereof 18 hcorresponded to coal combustion were done.

Table 3.Main data for experimental tests in the continuous CLOU unit of 1.5 kWthwith ElCerrejn coal.

TestTFR(C)

OCm (kg/h)SFm

(kg/h)Power (W) mOC(kg) FR

m

(kg/MWth)

CLOU01 903 1.2 2.8 4.2 0.112 681 0.41 605CLOU02 917 1.2 2.8 4.2 0.112 681 0.41 605CLOU03 941 1.2 2.8 4.2 0.112 681 0.37 547CLOU04 960 1.2 2.8 4.2 0.112 681 0.39 577

CLOU05 924 4.3 4.7 9.0 0.67 410 0.47 1150CLOU06 929 3.2 3.5 9.0 0.89 541 0.45 835CLOU07 917 2.6 2.8 9.0 0.112 681 0.41 605CLOU08 920 2.1 2.3 9.0 0.135 821 0.37 454CLOU09 925 1.1 1.2 9.0 0.256 1560 0.37 235

CLOU10 901 1.1 2.8 3.7 0.112 681 0.45 663CLOU11 901 2.0 2.8 7.0 0.112 681 0.45 663CLOU12 898 4.0 2.8 13.9 0.112 681 0.49 721

The fuel reactor temperature (CLOU01-CLOU04), the coal feeding rate (CLOU05-CLOU09)and the solid circulation flow rate (CLOU01 and CLOU10-CLOU12) were varied duringexperimental work. The CLOU prototype was easy to operate and control, and steady statefor each operating condition was maintained for at least 60 minutes. Agglomeration ordefluidization was never detected with this Cu-based oxygen carrier, even though the fuelreactor temperature was as high as 960 C.

In all cases, CH4, CO or H2were not detected in the outlet gas stream of the fuel reactor. Thepossible presence of tars or light hydrocarbons (C2-C4) was also analyzed. For someexperiments, tar measurements were done using tar protocol [7] at the fuel reactor outlet

stream. Tar was not detected in the fuel reactor outlet flow, that is, no hydrocarbons heavierthan C5. In addition, gas from the fuel reactor outlet stream was collected in bags and


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analysed off line using a gas chromatograph. The analysis showed that there were no C2-C4hydrocarbons in the gases. Thus, CO2, H2O and O2were the only gases at the fuel reactoroutlet together with the N2 introduced as fluidizing gas. It must be pointed out that smallamounts of SO2and NOXwere also present at the fuel reactor gas stream. Therefore, volatileswere fully converted into CO2and H2O in the fuel reactor by reaction with the oxygen

released from the CuO decomposition. In addition, the oxygen release rate was high enoughto supply an excess of gaseous oxygen (O2) exiting together with the combustion gases. In theair reactor the CO2and O2concentration decreased when the temperature increased in thefuel reactor.

Figure 4(a), (b) and (c) show the CO2capture efficiency, char conversion and the combustionefficiency as a function of the fuel rector temperature, the coal feeding rate and the solidscirculation flow rate. Moreover, Figure 4 shows the variation of solids conversion in everycase. Complete combustion to CO2and H2O of the coal was observed in the fuel reactor, i. e

, 100%comb FR = . However, the char conversion in the fuel reactor determines the CO2

capture efficiency in the CLOU system by the undesirable CO2emissions in the air reactoroutlet. It can be seen that high values of CO2capture were obtained in all cases. It isnoteworthy the positive effect of fuel reactor temperature on the CO2capture efficiency, seeFigure 4(a). Thus, when the fuel reactor temperature was 960 C, 99 % of carbon in coal iscaptured, i.e. only 1 % of carbon is exiting in the air reactor outlet gas stream. The reason forthis high CO2capture efficiency is the fast conversion of char in the fuel reactor bycombustion with gaseous oxygen. Figure 4(a) also shows values of char conversion above 96%, which increases with the temperature.

Temperature (C)

900 920 940 960

Efficiency(%)

90

92

94

96

98

100

Coal feeding rate (g/h)

0 50 100 150 200 250

Solids flow rate (kg/h)

0 2 4 6 8 10 12 14 16

Xchar

(-)

0.90

0.92

0.94

0.96

0.98

1.00

XOC

(-)

0.0

0.2

0.4

0.6

0.8

1.0

(a) (b) (c)

Figure 4.CO2capture efficiency (----), char conversion (----), combustion efficiency ()and variation of the oxygen carrier conversion (--X--) at different (a) fuel reactor

temperatures; (b) coal feeding rates; and (c) solids circulation flow rate.

The fuel load had no relevant effect on the CO2capture efficiency; see Figure 4(b). Since thecirculation rate and the temperature are kept constant and there is an oxygen excess in allcases, the resulting CO2capture efficiency for the different coal feeding rates does not changesubstantially. Thus, at the conditions used in the CLOU system, the oxygen generated in thefuel reactor was not limited by the reactivity of this oxygen carrier, i.e. the more oxygen isdemanded, more oxygen is supplied. Figure 4(b) also shows that more oxygen is transferredbetween the two reactors, as it is indicated by the increase of the variation of solidsconversion between 23 to 90 %. Complete combustion was reached even with the highestcoal feeding rate uses, which corresponded to a solid inventory in the fuel reactor as low as

235 kg/MWth, see Table 3.


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Figure 4(c) shows that an increase in the solids circulation flow causes a decrease in the charconversion and in the CO2capture efficiency. The negative effect of the solids circulationrate on the char conversion was due to the decrease in the residence time of solids in the fuelreactor. A residence time above 300 s is needed to get a char conversion about 97 % at 900C with this coal.

In short, very good results during El Cerrejn coal combustion have been shown by theCLOU process. CO2capture efficiency can be close to 100 % and full combustion of fuel toCO2and H2O was obtained.

3.2 Effect of coal rank on the performance of CLOU process

In the previous section, full CO2capture and combustion efficiencies were obtained workingwith a highly reactive bituminous coal with high volatile content. However, it is expected thatcoal reactivity, which depends on coal rank, can affect to the efficiency of the process. Thisissue is analyzed in this section.

To investigate the performance of the CLOU system in the combustion of different coals,several tests under continuous operation were carried out in the CLOU unit of 1.5 kWth. TheCu60MgAl material was used as oxygen carrier. The effect of the coal rank and the fuelreactor temperature on the combustion efficiency and carbon capture efficiency wasinvestigated. A total of 40 h of operation were carried out. The temperature in the fuel reactorwas varied from 900 to 950 C. The temperature in the air reactor was maintained at 900 C.Table 4 shows operational variables used in this work for each coal. The solids circulationrate was maintained at a mean value of 3-4 kg/h, whereas the coal feeding rate was varied

from 0.08 to 0.13 kg/h depending on the coal type. The oxygen carrier to fuel ratio, , was

around 1.0-1.2. Steady state at constant temperature was maintained for 60 min in eachcondition.

Table 4. Operational conditions during experimental work in the CLOU continuous unit of1.5 kWthwith different coals.

Coal SFm

(kg/h)

OCm

(kg/h)

(-)

Power (W)mFR

(kg/MWth)

Anthracite 0.10 3.1 1.1 583 894LV Bituminous 0.10 3.7 1.0 805 709

MV Bituminous 0.08 3.2 1.1 592 1003

Lignite 0.13 3.0 1.2 582 845

Figure 5(a) shows the combustion and CO2capture efficiency obtained for different coals as afunction of the fuel reactor temperature. In all cases, no CH4, CO, H2or higher hydrocarbonswere detected in the gases exiting from the fuel reactor. Thus, CO2, H2O and O2were theonly gases, together with N2introduced as fluidizing gas. The oxygen release rate was highenough to supply an excess of gaseous oxygen (O2) exiting together with the combustiongases. Oxygen concentration was at equilibrium conditions for each temperature. Also, smallfractions of SO2and NO were present in the gases coming from sulphur and nitrogen presentin the coal. Therefore, complete combustion in the fuel reactor of coal to CO2and H2O wasfound for all the operating conditions and coals.


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Temperature (C)

900 920 940 960

CharConversion(-)

0.5

0.6

0.7

0.8

0.9

1.0

Temperature (C)

900 910 920 930 940 950 960

CO2CaptureEfficiency(%)

50

60

70

80

90

100

CombustionEff

iciency(%)

50

60

70

80

90

100

Lignite

MV Bituminous

LV Bituminous

Anthracita

Lignite

MV Bituminous

LV Bituminous

Anthracite

Comb.Capture

Figure 5.(a) Combustion efficiency in the fuel reactor and CO2capture efficiency, and (b)char conversion obtained with different coals, as a function of the fuel reactor temperature.

The type of coal affected to the CO2capture efficiency. The CO2capture efficiency decreasedin the order Lignite > Medium Volatile Bituminous > Low Volatile Bituminous > Anthracite.For all coals, CO2capture increased with temperature, being this effect more relevant forcoals with low volatile matter content as Anthracite and LV Bituminous coal. Similar CO2capture efficiencies were obtained for Lignite and Medium Volatile Bituminous coal attemperatures above 940 C. In these cases, the CO2capture efficiency was above 99%.However, these results clearly indicate the need of a carbon separation system to returnunconverted char to the fuel reactor when low reactive coals are used. This system is similarto the proposed for iG-CLC with solid fuels [8].

CO2capture efficiencies are dependent on the carbon transferred from the fuel to the airreactor. As all the carbon in volatiles was captured, the CO2capture depends on theunconverted char in the fuel reactor, i.e. the char conversion. Figure 5(b) shows the charconversion as a function of the fuel reactor temperature for different coals. Similar trends tothat found for the CO2capture efficiency were found for different coals. Char conversionsincreased when temperature increased for all coals and the increase was more relevant forlow rank coals.

3.3 Biomass combustion by CLOU process

If a CO2capture system is implemented in a biomass combustion process, the global carbonbalance to the atmosphere will be negative, because the carbon dioxide generated waspreviously removed from the atmosphere by the biomass. Consequently, biomass is aninteresting fuel for testing in CLOU process, because it would be possible to obtain negativeCO2 emissions. However, biomass has a higher volatile matter content than coals anddifferent behaviour could be expected during combustion. Therefore, the evaluation ofbiomass in a CLOU process must be analyzed.

To investigate the combustion of biomass by a CLOU system, different tests were carried outwith pine wood as fuel under continuous operation in the continuous CLOU unit of 1.5 kWth.Cu60MgAl material was used as the oxygen carrier. The effect of the fuel reactor temperature

on the combustion efficiency and CO2capture efficiency was investigated. Thus, the fuelreactor temperature was varied from 860C to 935C. The temperature in the air reactor was


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kept at 900C. A total of 10 h of continuous operation were carried out. The solids circulationrate was maintained at a mean value of 4.1 kg/h, while the biomass feeding rate was 0.22

kg/h, corresponding to a power of 1.2 kWth. The oxygen carrier to fuel ratio, , was around1.2.

Figure 6(a) shows the combustion and CO2capture efficiencies obtained in the CLOU unit asa function of the fuel reactor temperature. When the fuel reactor temperature was higher than900C, no CH4, CO, H2, tars or light hydrocarbons were detected in the gases exiting from thefuel reactor. Thus, CO2, H2O and O2were the only gases from biomass combustion by CLOUand complete combustion of biomass was found in the fuel reactor for the experiments attemperatures higher than 900C.

FR Temperature (C)

840 860 880 900 920 940

Charconversion(-)

0.2

0.4

0.6

0.8

1.0

FR Temperature (C)

840 860 880 900 920 940

Combustionefficiency(%)

75

80

85

90

95

100

CarbonCaptureefficiency(%)

75

80

85

90

95

100

(a) (b)

Figure 6.(a) Combustion efficiency in the fuel reactor and CO2capture efficiency, and (b)char conversion, as a function of the fuel reactor temperature

At a lower temperature, i.e. 860C, 82% of combustion efficiency was obtained, and therewere CO and tars presents at the outlet of fuel reactor. Tar compounds were detected at aconcentration of 0.31 g/Nm3(dry basis). The composition of these tars was mainlynaphthalene (61.7 wt.%), phenantrene (13.1 wt.%), acenaphthylene (12.5 wt.%) indene (7.2wt.%) and stryrene (5.5 wt.%). The high volatile matter content of the biomass is responsiblefor the low combustion efficiency showed at the lowest temperature.

Regarding the CO2capture efficiency, an increase with the fuel reactor temperature was

measured. Thus, high CO2capture efficiency values were found at fuel reactor temperatureshigher than 900C because of the high char conversion, reaching 100% at 935C. Figure 6 (b)shows char conversion increased with the fuel reactor temperature because of the higher charcombustion rates. However, low char conversion can be seen at the lowest temperature, whenonly 15 % of char conversion is reached in the fuel reactor. At this temperature, there is nogaseous O2in the fuel reactor exiting flow. However, the 80% of CO2capture efficiencyobtained was due to the high amount of volatiles in the biomass.

To understand the differences on performance at temperatures lower or higher than 900C, itshould be considered the increase in the reaction rate and in the equilibrium O2concentration.It is necessary to take into account that O2generated from CuO increases with temperature. In

this way, at 860C, the O2generated is very low (equilibrium O2concentration is 0.5 vol.%).At this temperature, oxygen carrier reduction to generate gaseous O2is extremely slow [9].


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At temperatures 900C, O2released from the oxygen carrier should be of great relevancefor the conversion of gaseous compounds proceeding from coal devolatilization. Thus,volatile matter reacts with gaseous O2from the oxygen uncoupling mechanism in ahomogeneous reaction. In order to obtain complete combustion, an oxygen uncoupling effectis needed. Thus, it was found that volatiles were fully converted into CO2and H2O in the fuel

reactor at temperatures higher than 900C by reaction with the oxygen released from CuOdecomposition. Thus, it was concluded that at the lower temperature, 860C, the oxygenuncoupling effect is low because the O2equilibrium concentration is low and the oxygengeneration by the oxygen carrier particles is slow. So the conversion rate of char bycombustion reaction should be low. Char conversion is mainly driven at this temperature bythe slower gasification reaction with the low H2O or CO2concentration present inside the fuelreactor (since N2is used as fluidization media in these tests).

4. Conclusions

The proof of the CLOU concept in a continuous 1.5 kWthCLOU unit system was carried outduring 18 h of bituminous coal combustion with the Cu60MgAl oxygen carrier. The effect ofthe operating conditions such as temperature of the fuel reactor, solids circulation rate andthe coal feeding rate on the CO2capture efficiency, and the combustion efficiency wereinvestigated. In all cases, unburnt compounds were not present in the fuel reactor outlet,being CO2and H2O the only products of combustion even if the oxygen carrier particles werehighly converted. Also, high CO2capture efficiencies were measured.

Moreover, the performance of the CLOU process for combustion of coals of different rank(one lignite, two bituminous coals and one anthracite) was determined in the 1.5 kWthacontinuous CLOU unit with the Cu60MgAl oxygen carrier. With all the coals used, unburnt

compounds were not present in the fuel reactor outlet. The coal rank showed an importanteffect on the CO2capture efficiency. Very high CO2capture efficiencies were obtained withthe lignite and the medium volatiles bituminous coals, reaching values of 99% with thelignite at 950 C. It can be said that with all the coals used, unburnt compounds were notpresent in the fuel reactor outlet. CO2, H2O and O2were the only products present at the fuelreactor outlet. Coal rank showed an important effect on CO2capture efficiency. Very highcarbon capture efficiencies were obtained for Lignite and MV Bituminous coals, reachingvalues of 99% with Lignite at 950 C. The CO2capture efficiency increased with fuel reactortemperature and this increase was more relevant for Anthracite and LV Bituminous. For lowreactivity coals it would be necessary the use of a carbon separation system to reach high CO2capture efficiencies.

Combustion of biomass by CLOU process to obtain negative CO2emissions wasdemonstrated for the first time in the continuous CLOU unit of 1.5 kWthduring 10 h ofoperation. A fuel reactor temperature higher than 900C was required to exploit the oxygenuncoupling benefits and resulting in no unburnt compounds at the fuel reactor outlet.Complete combustion and 100% CO2capture efficiency were achieved at a fuel reactortemperatures higher of 935C using low solids inventory in the fuel reactor.

Acknowledgement

This work was partially supported by the European Commission, under the RFCS program

(ECLAIR Project, Contract RFCP-CT-2008-0008; and ACCLAIM Project, Contract RFCP-CT-2012-00006), ALSTOM Power Boilers (France) and by the Spanish Ministry of Science


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and Innovation (PN, ENE2010-19550 ENE2011-26354). I. Adnez-Rubio thanks CSIC forthe JAE fellowship.

Additional information

This work was compiled in the Thesis for the degree of Doctor by the Zaragoza University ofIaki Adnez-Rubio (2015). More information can be found in the bibliography: [3, 9-17]

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[9] I. Adnez-Rubio, P. Gayn, A. Abad, F. Garca-Labiano, L.F. de Diego, J. Adnez,Kinetic analysis of a Cu-based oxygen carrier: Relevance of temperature and oxygen partialpressure on reduction and oxidation reactions rates in Chemical Looping with OxygenUncoupling (CLOU), Chem. Eng. J. (Lausanne), 256 (2014) 69-84.

[10] I. Adnez-Rubio, P. Gayn, A. Abad, L. de Diego, F. Garca Labiano, J. Adnez,Evaluation of a Spray-Dried CuO/MgAl2O4Oxygen Carrier for the Chemical Looping withOxygen Uncoupling Process, Energy Fuels, 26 (2012) 3069-3081.


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CO2 Capture in Coal Combustion by Chemical-looping With Oxygen Uncoupling (CLOU)

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