Performance of CLOU process in the combustion of different types of coal with CO 2 capture

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1 Performance of CLOU process in the combustion of different types of with CO capture I. Adánez-Rubio*, P. Gayán, A. Abad, L. F. de Diego, F. García-Labiano, J. Adánez Instituto de Carboquímica (ICB-CSIC), Miguel Luesma Castán 4, Zaragoza, Spain * Abstract: The Chemical-Looping with Oxygen Uncoupling (CLOU) process is a Chemical-Looping Combustion (CLC) technology that allows the combustion of solid fuels using oxygen-carriers with inherent CO separation. The heat involved in the global process is the same that in usual direct combustion with air but the CO is captured with low energy penalty in the process. Thus, the cost of CO capture is maintained low. As in the CLC technology, in the CLOU process the oxygen necessary for the fuel combustion is supplied by a solid oxygen-carrier, which contains a metal oxide. The oxygen-carrier circulates between two interconnected fluidized reactors: the fuel- and the air-reactor. In the CLOU mode, the oxygen-carrier releases gaseous oxygen in the fuel-reactor, which reacts with as in common combustion with air. CO and H O are combustion products, and a pure CO stream can be obtained after steam condensation. The reduced oxygen-carrier is regenerated to the initial metal oxide in the air-reactor, then being ready to start a new cycle. The exit flow contains N and unreacted O. The aim of this work is study the performance of the CLOU process using different type of s. Experiments were carried out in a continuously operated 1.5 kw th unit, and the temperature in the fuel-reactor was varied between 900 and 950 ºC. Four types of s with different range (anthracite, low volatile bituminous, medium volatiles bituminous and lignite) were used as fuel. Particles prepared by spray drying containing 60 wt.% CuO were used as oxygen-carrier for the CLOU process. In all the experiments there was complete combustion of the to CO and H O, without unburnt products. The reactivity of affected to the CO capture efficiency. So, as more reactive is the the CO capture was higher. In addition, the CO capture increased with fuel-reactor temperature with all tested. The maximum capture efficiency was 99.3% at 950 ºC with lignite. Keywords: Carbon capture, combustion,, CLOU, copper, chemical-looping. INTRODUCTION The Chemical-Looping with Oxygen Uncoupling (CLOU) process is a Chemical-Looping Combustion (CLC) technology that allows the combustion of solid fuels with inherent CO separation using oxygen-carriers. This technology has low energy penalty and thus low CO capture costs. A CLC technology system is composed of two interconnected fluidized-bed reactors joined by a loop seal, a riser for solids transport from the air-reactor to the fuel-reactor, a cyclone to recover the entrained solids, and a solid valve to control the solids circulation flow rate between both reactors (Adánez et al. 011). Solid fuels are considerably more abundant and less expensive than natural gas, and it would be highly advantageous if the CLC process could be adapted for these types of fuels. The CLOU technology may be especially suitable for solid fuels, such as, petroleum coke or biomass. The CLOU technology takes advantage of the property of some metal oxides which can generate gaseous oxygen at high temperatures. The oxygen generated by the oxygen-carrier reacts directly with the solid fuel, which is mixed with the oxygen-carrier in the fuel-reactor. The oxygen-carrier for CLOU needs to have special characteristics and needs to react reversibly with oxygen at high temperatures. The oxygen-carriers for CLOU must have the ability to react both with oxygen in the air-reactor and then also to release this oxygen through decomposition in the fuel-reactor. Thus CLOU utilizes the fact that some metal oxides have suitable equilibrium oxygen partial pressure at temperatures of interest for combustion, i.e ºC. Three such metal oxide systems have been

2 identified: CuO/Cu O, Mn O 3 /Mn 3 O 4, and Co 3 O 4 /CoO (Mattisson et al. 009). N (+O) CuO MexOy CO + H O CO Condenser CO Air Reactor Fuel Reactor H O(l) Cu O MexOy-1 Coal Air CO Ash Fig. 1. Schematic design of the CLOU system using copper oxide as oxygen-carrier. Fig. 1 shows a schematic design of the CLOU system using copper oxide as oxygen-carrier. If copper oxide is used as active compound, reaction taking place in the fuel-reactor are the following: 4CuO Cu O + O C + O CO 4CuO + C Cu O + CO H = 63. kj / molo (1) 850º C r H = 397 kj / molo () 850º C r H = kj / molo (3) The oxygen release reaction (r.1) is endothermic but the global reaction (r.3) in fuel-reactor is exothermic due to the combustion (r.). This is an advantageous characteristic of using this metal oxide, as there can be a temperature increase in the fuel-reactor. 850º C r The reduced oxygen-carrier from the fuel-reactor is regenerated in the air-reactor: Cu O + O 4CuO H = 63. kj / molo (4) An analysis about the suitability of Cu-based materials was carried out previously at ICB-CSIC, (Gayán et al. 011). Particles prepared by several methods with different supporting materials and different metal oxide contents were tested. From this work, it was shown that particles prepared by extrusion containing 60 wt.% CuO and using MgAl O 4 as supporting material are adequate for its use as oxygen-carrier for the CLOU process. On the whole, this material shows adequate values of reactivity and oxygen transport capacity, high attrition resistance and does not have tendency to agglomerate during operation in a fluidized-bed reactor. Similar particles prepared by spray drying were tested in a 1.5 kw th unit with Colombian. Complete combustion and high carbon capture efficiency were obtained (Abad et al. 011). The aim of this work was to investigate the performance of the developed oxygen-carrier in a continuously operated CLOU plant for different type of s ranging from lignite to anthracite. In addition, the effect of temperature of the fuel-reactor on the combustion efficiency and carbon capture efficiency was investigated. The results obtained are analyzed and discussed in order to be useful for the scale-up of a CLOU process fuelled with. 850º C r EXPERIMENTAL SECTION The Cu-based oxygen-carrier The material used was a Cu-based oxygen-carrier prepared by spray drying. Oxygen-carrier particles

3 were manufactured by VITO (Flemish Institute for Technological Research, Belgium) using MgAl O 4 spinel from Baikowski, and CuO from PANREAC as raw materials. The CuO content of particles was 60 wt.%. After particles formation by spry drying the particles were calcined for 1 h at 1100ºC and sieved ( μm). Nevertheless, after material reception particles were calcined for a second time to increase the mechanical strength. The final oxygen-carrier had a total calcination time of 4 h at 1100 ºC. From now on the oxygen-carrier was named as Cu60MgAl. Table 1 shows the main properties of this oxygen-carrier. It has a low porosity and superficial area. The mechanical strength of the particles after 4 h of calcination was adequate for its use in a fluidized bed. The compounds found by XRD analysis were CuO and MgAl O 4. In order to have good fluidizing behaviour and enough high solids circulation rate, only the lower size fraction ( μm) was used in the continuously operated CLOU prototype Table 1. Properties of the oxygen-carrier Cu60MgAl (after 4 h of calcination). CuO content (wt.%) 60 Oxygen transport capacity, Ro (wt.%) 6 Crushing strength (N).4 Real density (g/cm 3 ) 4.6 Porosity (%) 16.1 Specific surface area, BET (m /g) < 0.5 XRD main phases CuO, MgAlO4 Coals Four different s were used for CLOU experiments with Cu60AlMg oxygen-carrier. These fuels cover the rank from lignite to anthracite. Properties of these s are showed in Table 3. The particle size used for this study was μm. Table. Properties of s used in this work. Anthracite Proximate Analysis (wt.%) Low Volatile Bituminous Medium Volatile Bituminous Lignite Moisture Volatile matter Fixed carbon Ash Ultimate Analysis (wt.%) C H N S LHV (kj/kg)

4 Experimental set-up A schematic view of the plant is shown in Fig.. The set-up was basically composed of two interconnected fluidized-bed reactors the air- and fuel-reactors joined by a loop seal, a riser for solids transport from the air-reactor to the fuel-reactor, a cyclone and a solids valve to control the solids circulation flow rate in the system. A diverting solids valve located below the cyclone allowed for the measurement of the solids flow rates at any time. Therefore, this design allowed us to control and measure the solids circulation flow rate between both reactors. Because of heat losses, the system is not auto-thermal and is heated up by means of various independent ovens to get independent temperature control of the air-reactor, fuel-reactor, and freeboard above the bed in the fuel-reactor. During operation, temperatures in the bed and freeboard of the fuel-reactor, air-reactor bed and riser were monitored as well as the pressure drops in important locations of the system, such as the fuel-reactor bed, the air-reactor bed and the loop seal. Torch 5 Gas Analysis O, CO, CO FR 1 11 Torch Gas Analysis O, CO, CO, H, CH 4 Sec. Air AR Fuel Fuel Reactor, FR 7.- Solid reservoir 3.- Loop Seal 8.- Solids control valve Air Reactor, AR 9.- Fuel feeding system 4.- Riser 10.- Screw feeders 5.- Cyclone 11.- Furnace Air N N CO 6.- Diverting solids valve Fig.. Schematic view of the 1.5 kw th CLOU rig fuelled with. The fuel-reactor consisted of a bubbling fluidized bed with 5 cm of inner diameter and 0 cm bed height. N can be used as fluidizing gas. The gas flow was 186 L N /h. Coal is fed by a screw feeder at the bottom of the bed right above the fuel-reactor distributor plate in order to maximize the time that the fuel and volatile matter are in contact with the bed material. A small N flow (4 L N /h) is introduced at the beginning of the screw feeder to avoid any possible volatile reverse flow or entrance of steam. The oxygen-carrier is reduced in the fuel-reactor, evolving gaseous oxygen to the surroundings. The oxygen burns the volatiles and char proceeding from pyrolysis in the fuel-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 fuel and air. A N flow of 60 L N /h was introduced in the loop seal. The oxidation of the carrier took place in the air-reactor, consisting of a bubbling fluidized bed with 8 cm of inner diameter and 10 cm bed height, and followed by a riser. The air flow was 1740 L N /h. In addition, a secondary air flow (40 L N /h) was introduced at the top of the bubbling bed to help particle entrainment. N and unreacted O left the air-reactor passing through a high-efficiency cyclone and a

5 filter before the stack. The oxidized solid particles recovered by the cyclone were sent to a solids reservoir, which acts as a loop seal, setting the oxygen-carrier ready to start a new cycle. In addition, these particles avoid the leakage of gas between the fuel-reactor and the riser. The regenerated oxygen-carrier particles returned to the fuel-reactor by gravity from the solids reservoir through a solids valve which controlled the flow rate of solids entering the fuel-reactor. A diverting solids valve located below the cyclone allowed the measurement of the solids flow rates at any time. The total oxygen-carrier inventory in the system was.0 kg, being about 0.5 kg in the fuel-reactor. CO, CO, H, CH 4, and O were analyzed in the outlet stream from fuel-reactor, whereas CO and O were analyzed from the flue gases of the air-reactor. In some selected experiments, the tar amount present in fuel-reactor product gases was determined following the tar protocol (Simell et al., 000), as well as higher C, C 3 and C 4 hydrocarbons were analyzed by GC. The oxygen carrier to fuel ratio (Φ) was defined by the following equation: 0.5F φ = Ω m ɺ CuO F CuO being the molar flow rate of CuO and mɺ the mass-based flow of fed to the reactor. Ω is the stoichiometric mols of oxygen as O to convert 1 kg of to CO and H O. This value was calculated from the proximate and ultimate analysis of the, see Table. f f f f f Ω = M M M M M C H N S O C H N S O A value of Φ = 1 corresponds to the stoichiometric flow of CuO to fully convert to CO and H O. Air flow into the air-reactor was maintained constant for all tests, always remaining in excess over the stoichiometric oxygen demanded by the fuel. The air excess ratio, λ, was defined in equation (7). Depending on the fuel flow, the value of λ ranged from 1.1 to 1.. Oxygen flow Oxygen demanded 0.1F m λ = = air (7) Ω ɺ (5) (6) RESULTS AND DISCUSSION To determine the behaviour of the Cu60MgAl oxygen-carrier in a CLOU system during, several tests under continuous operation were carried out in the experimental rig using as fuel. The fuel-reactor temperature was varied from 900 ºC to 950 ºC. The temperature in the air-reactor and in the freeboard was maintained at around 900 ºC in all experiments. The solids circulation rate was maintained at a mean value of 4. kg/h, whereas the feeding rate was varied between 0.09 and 0.1 kg/h depending on the type. The oxygen-carrier to fuel ratio, Φ, was 1.1, as defined by equation (5). The gas composition at the exit gases of fuel- and air-reactors was determined. N instead CO was used as fluidizing gas in order to improve the accuracy for calculation of C burnt in the fuel-reactor. In a previous work it was determined that the fluidization agent does not have any influence on the oxygen-carrier behaviour [Gayán et al., 011]. During 40 h of hot operation, the oxygen-carrier never showed agglomeration of particles. As example, Fig. 4 shows the concentration of gases (dry basis) measured as a function of the operating time for experiments carried out with lignite. At steady state, the gas outlet concentration and temperature were maintained uniform during the whole combustion time. When temperature was varied, a transition period appeared and stable combustion was reached usually in less than 10 min. In all cases, no CH 4, CO or H were detected in the gases exiting from the fuel-reactor. The possible

6 presence of tars or light hydrocarbons was also analyzed. For one experiment with conditions kept constant for longer times that two hours, tar measurements in the fuel-reactor were done using tar protocol. The results showed that there were not tars in the fuel-reactor outlet flow, that is, no hydrocarbons heavier than C 5. In addition, in some selected experiments, gas from the outlet stream was collected in bags and analysed with a gas chromatograph (GC). The analysis proved that there were no C -C 4 hydrocarbons in the gases. Thus, CO, H O and O were the only gases, together N introduced as fluidizing gas. Also, small fractions of SO and NO were present in the gases coming from sulphur and nitrogen present in the. However, these components were not evaluated in this work. Therefore, volatiles were fully converted into CO and H O in the fuel-reactor by reaction with the oxygen released from the CuO decomposition. In addition, the oxygen release rate was high enough to supply an excess of gaseous oxygen (O ) exiting together the combustion gases. As temperature was increased more O is released from the fuel-reactor according to the equilibrium of CuO decomposition. In the other hand, the O concentration in outlet gases from the air-reactor decreased with the temperature due to the oxygen-carrier comes more reduced from the fuel-reactor because more char was burnt. This fact do decreased the CO concentration from the air-reactor outlet. CO or O (vol.%) CO O CO FR O AR T T Temperature (ºC) Time (min) Fig. 4. Evolution of the gas composition in the air- and fuel-reactor as temperature in the fuel-reactor was varied. Coal: lignite, mɺ = 0.10 kg / h The performance of the CLOU process with different s was analyzed using the combustion and carbon capture efficiency. The combustion efficiency in the fuel-reactor is calculated as the volatile matter and char being converted in the fuel-reactor by the O supplied by the oxygen-carrier. The oxygen supplied by oxygen-carrier in the fuel-reactor is calculated through the flow of oxygen containing species in the fuel-reactor product gas, excepting gaseous oxygen, i. e. O. The oxygen demanded by volatile matter and char in the fuel-reactor is calculated as the oxygen demanded by the minus the oxygen from air reacting with char in the air-reactor, calculated as the flow of CO from the air reactor, Therefore, the combustion efficiency in the fuel-reactor was calculated as: F. CO,AR

7 f H O f O FCO, outfr + FCO, outfr + FH O, outfr + mɺ M H O M O comb = (8) Ω mɺ FCO, outar M The carbon capture efficiency, O CC, was defined as the fraction of carbon initially present in the fed in which is actually at the outlet of fuel-reactor as CO. This is the actual CO captured in the CLOU system, the rest is exiting together nitrogen from the air-reactor. CC M F = f mɺ C CO,outFR C (9) f C being the carbon content in. Fig. 5(a) shows the combustion and carbon capture efficiency obtained for different s as a function of the fuel-reactor temperature. Full combustion of to CO and H O was found for all s tested. However, the type of affected to the carbon capture efficiency. The carbon capture followed the order Lignite > Medium Volatile Bituminous > Low Volatile Bituminous > Anthracite. Fig. 5(b) shows the char conversion for different s. So, less reactive s gave lower carbon capture because the char conversion was slower. Lignite showed the highest carbon capture efficiency. The effect of the fuel-reactor temperature was evident for all s tested. Thus, the carbon capture was higher when the temperature was increased. Similar carbon capture efficiency was obtained for Lignite or Medium Volatile Bituminous at temperatures above 940 ºC. In these cases the carbon capture efficiency was above 99%, but lower values were obtained for Low Volatile Bituminous (90%) or Anthracite (83%). Therefore, the necessity for a carbon separation system will depend mainly on the reactivity of. CO capture efficiency (%) (a) Comb. Capture Lignite MV Bituminous LV Bituminous Anthracite Temperature (ºC) Combustion efficiency (%) Char conversion (b) Lignite MV Bituminous LV Bituminous Anthracite Temperature (ºC) Fig. 5. (a) Combustion efficiency in the fuel-reactor and carbon capture efficiency and (b) char conversion, as a function of the fuel-reactor temperature obtained with different types.

8 CONCLUSIONS The CLOU process proved with four s of different rank in a continuously operated facility using a Cu-based oxygen-carrier prepared by spray drying. The oxygen-carrier contained 60 wt.% CuO and MgAl O 4 was used as material. During 40 h of hot operation, the oxygen-carrier never showed agglomeration of particles. Different types of s were used as solid fuel. In all cases, unburnt compounds were not present in the fuel-reactor outlet. CO, H O and O were the only products of reactions. The carbon capture efficiency depended on the fuel-reactor temperature and the type of. Thus, higher temperature increased the carbon capture efficiency because the char combustion in the fuel-reactor was improved. In addition, as the reactivity of increased, the carbon capture was also higher. Lignite showed the highest carbon capture (97-99%), whereas the lowest was for the anthracite (75-85%). Therefore, the necessity for a carbon separation system will depend mainly on the reactivity of. NOTATION Symbols F i Molar flow of compound i mol/s f i Mass fraction in of element or compound i. - mɺ Mass-based flow of fed-in to the fuel-reactor kg/s M i Molecular weigh of i compound kg/mol Greek letters H r Enthalpy of reaction kj/mol CC Carbon capture efficiency - comb Combustion efficiency - λ Air excess ratio - φ Oxygen-carrier to fuel ratio - Ω Stoichiometric mols of O to convert 1 kg of Subscripts outfr outar mol/kg Outlet stream from fuel-reactor Outlet stream from air-reactor ACKNOWLEDGEMENT This work was partially supported by the European Commission, under the RFCS program (ECLAIR Project, Contract RFCP-CT ), ALSTOM Power Boilers (France) and by the Spanish Ministry of Science and Innovation (PN, ENE ). I. Adánez-Rubio thanks CSIC for the JAE fellowship. REFERENCES Abad, A; Adánez-Rubio, I.; Gayán, P.; García-Labiano, F.; de Diego, L.F.; Adánez, J.: Int J Greenhouse Gas Control (011), accepted to publication. Adanez, J., Abad, A., Garcia-Labiano, F., Gayan, P., de Diego, L.F.: Prog. En. Comb. Sci. (011) doi: /j.pecs Mattisson, T., Lyngfelt, A., Leion, H.: Int J Greenhouse Gas Control 3 (009), pp Gayán, P., Adánez-Rubio, I., Abad, A., de Diego, L.F., García-Labiano, F., Adánez, J.: Submitted to Fuel (011) Simell, P., Stahlberg, P., Kurkela E., Albretch J., Deutch S., Sjostrom K.: Biomass and Bioenergy 18 (000), pp