Pilot plant experience with calcium looping

Pilot plant experience with calcium looping

9

H. Dieter, A.R. Bidwe, G. Scheffknecht Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Stuttgart, Germany

9.1

Introduction

One of the most important steps in the development of calcium looping is the demonstration of process feasibility in a pilot plant suitable to operate under process conditions that are close to those of large-scale facilities. Since its conceptualization (Shimizu et al., 1999), the calcium looping process has progressed steadily. In the initial phase lab-scale studies, especially by means of TGA (thermogravimetric analysis), provided the basis for the basic process design and sorbent behavior. In the later stages, bench-scale plants were built, the feasibility was proven, and important parametric studies were performed. These bench-scale plants provided a very important breakthrough for further research at pilot scale. In recent years, several pilot plant projects were undertaken and the calcium looping process has been demonstrated successfully. This chapter highlights the operation of these pilot plants and their results. The experience gained is shared and could be utilized in designing future largerscale or commercial-scale plants for calcium looping.

9.2

Calcium looping facilities around the world

Several lab-scale facilities and pilot-scale plants for calcium looping have been built. The lab scale and pilot scale are distinguished on the basis of their scale and heating mode. The scale of the lab-scale plants is within the low kilowatt range and they are generally heated by external means, such as by electrical heating systems. The pilotscale plants are larger in scale, and the process heat is generated by combustion of fuel inside the regenerator.

9.2.1

Lab-scale plants

During the period from 2005 to 2010, several projects were carried out to demonstrate the calcium looping process principle in lab-scale setups. Table 9.1 gives an overview of the lab-scale plants used for the demonstration of calcium looping. Initially the 30 kWth in Oviedo, Spain, at INCAR-CSIC (Alonso et al., 2010), 75 kWth in Ottawa, Canada, at CANMET (Lu et al., 2008), and 10 kWth at the University of

Calcium and Chemical Looping Technology for Power Generation and Carbon Dioxide (CO2) Capture http://dx.doi.org/10.1016/B978-0-85709-243-4.00009-4 Copyright © 2015 Elsevier Ltd. All rights reserved.

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Calcium and Chemical Looping Technology for Power Generation and CO2 Capture

List of calcium looping lab-scale and bench scale plants around the world

Table 9.1

Type (carbereg), interlinking

Location

Size

References

1

University of Stuttgart

10 kWth

CFBeBFB Double exit loop seal and cone valve

Charitos et al. (2010, 2011) and Bidwe et al. (2011)

2

INCAR-CSIC, Oviedo, Spain

30 kWth

CFBeCFB Loop seals

Alonso et al. (2010)

3

CANMET, Ottawa, Canada

75 kWth

CFBeBFB Loop seals

Lu, Hughes, and Anthony (2008)

4

Ohio State University, USA

120 kWth

Entrained flowerotary kiln Moving bed/entrained flow

Wang et al. (2010)

5

ITRI, Hsinchu, Taiwan

3 kWth

BFBerotary kiln Pneumatic transport

Chang et al. (2013)

6

Tsinghua University, China

BFBeBFB Pneumatic transport

Fang, Li, and Cai (2009)

Stuttgart (Charitos et al., 2008, 2010, 2011) contributed to process demonstration and later detailed characterization of calcium looping. The INCAR-CSIC system consists of two circulating fluidized bed (CFB) reactors connected with loop seals. CANMET and the University of Stuttgart used a combination of a CFB and a bubbling fluidized bed (BFB) reactor. The University of Stuttgart facility additionally offers a solid split device to control sorbent circulation by means of a cone valve. The Tsinghua facility, in China, consists of two BFB reactors interconnected with a pneumatic transport system (Fang et al., 2009). Ohio State University, in the United States, and ITRI, in Taiwan, follow a different calciner principle. The 3 kWth facility at ITRI and 120 kWth at Ohio State use a rotary kiln for sorbent calcinations, as commonly used in cement plants for clinker production. The carbonator at ITRI is a BFB. The 120 kWth at Ohio State additionally contains a steam hydration reactor to hydrate the freshly calcined sorbent before it is supplied to an entrained flow carbonator. Table 9.1 summarizes all the different facilities, their specific features, and differences between the setups. All facilities have a common feature that they are electrically heated. As a result of the lab-scale activities, calcium looping was proven to be feasible, with high CO2 capture efficiency. The INCAR-CSIC facility reported a maximum capture efficiency of 90% (Alonso et al., 2010), while the CANMET facility and Stuttgart facility reported 94% (Lu et al., 2008) and 93% (Charitos et al., 2010), respectively.

Pilot plant experience with calcium looping

173

Later, Ohio State University (Wang et al., 2010), Tsinghua University (Fang et al., 2009), and ITRI (Chang et al., 2013) successfully demonstrated process feasibility. As a result of their simplicity, lab-scale plants were used for detailed parametric studies. This database was the basis for validation of various models to describe the calcium looping process (Alonso et al., 2010; Charitos et al., 2011; Rodriguez et al., 2011). Nevertheless, the influence of real conditions such as combustion (and fuel/carbonate interactions) could not be investigated appropriately in such small-scale facilities. Therefore, pilot-scale studies have been and are still necessary.

9.2.2

Pilot-scale plants

The success at lab scale initiated a strong momentum to demonstrate calcium looping under realistic conditions at pilot scale. Three calcium looping pilot-scale plants have been built, commissioned, and operated (in Europe) for demonstration of calcium looping CO2 capture from power plant flue gases at pilot scale: the 1.7 MWth at La Pereda by the CaOling consortium in Spain (2011) (S
anchez-Biezma et al., 2012; Arias et al., 2013), the 1 MWth at TU Darmstadt, Germany (2012) (Pl€otz et al., 2012; Str€ohle, Junk, Kremer, Galloy, & Epple, 2014), and the 200 kWth at the University of Stuttgart, Germany (2010) (Dieter et al., 2014; Dieter, Hawthorne, Bidwe, Zieba, & Scheffknecht, 2012; Hawthorne et al., 2011, 2012). Table 9.2 gives a detailed comparison of these three pilot plants. Another 1.9 MWth (Chang et al., 2013) pilot plant for cement application was built and commissioned at ITRI, Taiwan in 2013. The facility design at ITRI is based on the 3 kWth lab-scale plant, where the carbonator is a BFB and the calcination is achieved in a rotary kiln.

9.2.2.1

The 1.7 MWth pilot plant, La Pereda

The La Pereda pilot plant is the largest calcium looping plant at present for power plant applications. The decision to build the 1.7 MWth pilot plant was made in 2009. The plant is located in Asturias (northern Spain) and placed at the Hunosa power plant. It is operated with a slip stream of the 50 MWel coal-based power plant. As shown in the schematic of the pilot plant in Figure 9.1, the pilot plant consists of two interconnected CFB reactors forming a dual fluidized bed (DFB) system. Both reactors have a height of 15 m while the internal diameter of the carbonator is 0.65 and 0.75 m for the regenerator. The reactor exits are equipped with high-efficiency cyclones. The cyclone of the carbonator separates the CO2-lean flue gas from the partially carbonated solids; the calciner cyclone separates the concentrated CO2 stream from the oxy-fired CFB combustor from the calcined sorbent. Below, the solids fall into double loop seals, which are operated as BFBs, enabling the control of the solid circulation between reactors. Part of the solids coming to each loop seal is circulated internally to maintain the operating stability of the CFB system; the rest of sorbent particles are circulated toward the other reactor (from the carbonator to calciner or from the calciner to carbonator). The calciner can be operated in air and oxy-fuel combustion conditions, using O2 and CO2 from gas storage tanks. Removable cooling bayonet tubes in the carbonator are installed to remove heat from the reactor at different

174

Table 9.2

Calcium and Chemical Looping Technology for Power Generation and CO2 Capture

Comparison of calcium looping pilot plants La Pereda

TU Darmstadt

Stuttgart University

Capacity

1.7 MWth

1 MWth

200 kWth

Type

CFBeCFB

CFBeCFB

CFBeCFB TFBeCFB

Solid looping mechanism

Double exit loop seals

Screw conveyor and loop seal

Loop seal with cone valve L-valve and loop seal

Reactor dimensions

Carbonator D: 0.65 m, H: 15 m Regenerator D: 0.75 m, H: 15 m

Carbonator D: 0.6 m, H: 8.6 m Regenerator D: 0.4 m, H: 11.5 m

CFB carbonator D: 22 cm, H: 10 m CFB regenerator D: 21 cm, H: 10 m TFB carbonator D: 33 cm, H: 6 m

Regenerator firing

Coal oxy-fuel

Propaneecoal oxy-fuel

Coal, biomass, oxy-fuel

Regenerator flue gas recirculation

No

No

Yes

Carbonator flue gas feeding

Real flue gases

Synthetic

Synthetic, real

Operational velocities

Carbonator 2e5 m/s Regenerator 3e6 m/s

Carbonator 2.2e3.3 m/s Regenerator 3e4.1 m/s

Carbonator A 4e6 m/s Carbonator B 1e4 m/s Regenerator 4e6 m/s

Make-up

Continuous

Continuous

Continuous

Solid looping measurement

N/A

N/A

Discontinuous and online

Postprocessing units

Electrostatic precipitator

Gas coolers, fabric filters

Gas coolers, fabric filters

Other processes under study

N/A

Chemical looping combustion

Sorption Enhanced Reforming, gasification, oxy-fuel CFB combustion

Pilot plant experience with calcium looping FCO

Tcarb

Coal F0

Xcarb

Xsulf Xash

calc

Calciner

out

Carbonator

FCO

175

Tcalc Xcalc

FCa Xcalc Internal circulation

FCO

in

nCa Xave Air, CO2/O2/H2O F0

Figure 9.1 Schematic of 1.7 MWth La Pereda pilot plant. Arias et al. (2013).

temperatures levels. This is required when different operating points are operated and parameter variations, such as solid circulation flows and different intensity of the exothermic carbonation reaction of CaO, make-up rates, etc., are performed. Flue gas from the power plant is blown to the carbonator with a fan (Arias et al., 2013; S
anchez-Biezma et al., 2012). Continuous limestone and coal feeding is realized by a feeding system connected to the calciner, which enables stable operation of combustion and sorbent make-up. In order to remove the purged sorbent, a continuous solid removal system in the calciner with a water-cooled screw feeder is installed. To monitor the process, analytical instrumentation including temperature, pressure, and continuous gas analysis from different points is installed. Furthermore, different ports are available for solid sampling and local solid circulation rate measurement with isokinetic probes. Subsequent analysis of solid samples for particle size distribution, chemical composition, and reactivity with respect to CO2 and SO2 capture are carried out in the lab, using equipment such as thermogravimetric analyzers. Important parameters that have been monitored at the La Pereda plant are as follows: (1) The inventory of solids in the carbonator is calculated continuously from the measurement of pressure differences between reactor bottom and exit of the reactor. (2) The average carbonator reactor temperature

176

Calcium and Chemical Looping Technology for Power Generation and CO2 Capture

measuring an axial temperature profile along the reactor height which indicates the effect of the bayonet tubes, the temperature effect of the exothermic carbonation reaction which is more intense in the dense bottom part of the carbonator, and the feed of hot sorbent from the calciner at a temperature level between 830 and 920
C. (3) The molar flow of CO2 entering the carbonator with the flue gas is measured continuously, as well as the mass flow of flue gas fed to the carbonator. (4) Solid samples are taken periodically to monitor the composition of the solids arriving to the carbonator with respect to CO2 and SO2 capture. (5) The circulation rate between carbonator and calciner is estimated by two methods in parallel: the closure of the carbon balance and the closure of the heat balance in the carbonator. The total solid circulation rate through the risers can also be measured using a suction probe in isokinetic conditions at the exit of the riser since the upward solid circulation at this point is close to the total solid circulation (Arias et al., 2013). The calciner was operated with the goal of full sorbent conversion to CaO. This has been achieved by operating sufficiently high calcination temperatures of 20e30 K above the equilibrium concentration of the calcination reaction and an O2 excess of 5 vol% at the exit of the regenerator reactor to ensure high coal combustion efficiencies in the calciner. Since its commissioning in 2011, the La Pereda pilot plant has been operated for 1800 h with interconnected reactors in combustion mode. In total, 380 h have been achieved in CO2 capture mode with capture efficiencies between 40% and 95%. A total of 170 h of operation have been performed in CO2 capture mode with stable oxy-fuel combustion of coal in the calciner (Arias et al., 2013).

9.2.2.2

The 1 MWth pilot plant at TU Darmstadt

The 1 MWth TU Darmstadt pilot plant was commissioned in the year 2012. This pilot plant consists of two interconnected CFB reactors. The carbonator has a height of 8.6 m and an internal diameter of 0.6 m. The regenerator is 11.5 m high and has an internal diameter of 0.4 m. The whole reactor system including circulation ducts is refractory lined. The flue gas supplied to the carbonator is a synthetic mixture of air and CO2 from gas tanks. After the absorption of CO2 in the carbonator, a decarbonized flue gas leaves the system through a heat exchanger and filter for dust removal. Gas compositions and flows of the flue gas are continuously measured. Make-up limestone is fed into the carbonator by means of a gravimetric dosing system. The carbonator is equipped with an adjustable, internal bed material cooler in order to remove the reaction heat and to control the temperature in the reactors. The solid looping from carbonator to regenerator is carried out by means of a screw conveyor while the circulation from regenerator to carbonator is achieved by a loop seal (Pl€otz et al., 2012). Different fuels can be burned in the regenerator to provide the heat for sorbent regeneration. The reactor can be fired with propane, either by a burner or by a bed lance. The bed lance allows the introduction of propane into the bottom zone of the reactor. Alternatively, the reactor can be fired with pulverized coal up to 150 kg/h, corresponding to approximately 1 MWth. In order to vary the oxygen content in the reactor, pure oxygen can be mixed into the primary air. Analogously to the carbonator,

177

CO2 + H2O

Filter

Filter Heat exchanger

ID fan Fly ash

Gas analysis

Coal

CaCO3

FI Air Synthetic flue gas

CaO

Fluidized bed calciner T = 900 ºC

Make-up CaCO3

Ash + spent sorbent Preheating

Cyclone

Cyclone

Fluidized bed carbonator T = 650 ºC

Flow control

CO2

ID fan Fly ash

CO, CO2, NO, SO2, O2

Flow measurement FI

Heat exchanger

FI

Propane Propane

FI Air

FI Preheating

Oxygen enriched air

Air Primary air fan

Air

Air

O2

Stack

Decarbonized flue gas

Stack

Pilot plant experience with calcium looping

Primary air fan

Figure 9.2 Flow chart of the 1 MWth calcium looping plant at TU Darmstadt. Str€ohle et al. (2014).

the calciner flue gas is released via a heat exchanger and a filter to the environment. Flow and gas composition of the calciner flue gas are continuously monitored. Additionally, the pilot plant is equipped with pressure transducers and thermocouples along the reactor height and in the peripheral components. Details of the facility are shown in the flow sheet given in Figure 9.2 (Pl€ otz et al., 2012). The first experiments of the facility have demonstrated CO2 capture rates up to 80%. Besides calcium looping, the TU Darmstadt pilot plant is used for the demonstration of chemical looping combustion (Pl€ otz et al., 2012).

9.2.2.3

The 200 kWth pilot plant at the University of Stuttgart

The University of Stuttgart pilot plant was commissioned in the year 2010. The size of 200 kWth was chosen to follow a consecutive development strategy from lab to pilot scale. On the one hand, the size of the 200 kWth pilot avoids the limitation of scaling risks and process demonstration with realistic parameters. On the other hand, the advantages of high plant flexibility and reasonable costs are ensured. To enable different carbonator fluidization regimes and hydrodynamic designs, the facility consists of two carbonators, a circulation fluidized bed reactor for high gas throughput, and a turbulent fluidized bed reactor to enable flexible flue gas load (Dieter et al., 2012, 2014; Dieter, Hawthorne, Zieba, & Scheffknecht, 2013; Hawthorne et al., 2011, 2012).

178

Table 9.3

Calcium and Chemical Looping Technology for Power Generation and CO2 Capture

Details of the 200 kWth Stuttgart pilot plant Unit

Regenerator R1

CFB carbonator R2

TFB carbonator R3

Internal diameter

m

0.21

0.22

0.33

Height

m

10

10

6

Firing capacity

kWth

150e330

e

e

Flue gas equivalent

kWth

e

170e200

50e200

Operational velocity

m/s

4e6

4e6

1e4

Regime

e

Fast

Fast

Bubbling-turbulent

The Stuttgart pilot plant consists of three fluidized bed reactors: a CFB regenerator (R1), a CFB carbonator (R2), and the BFB/TFB (turbulent fluidized bed) carbonator (R3). Details of these fluidized bed reactors are given in Table 9.3. The reactors and cyclones are refractory lined and designed for low heat loss. Parts of the fluidized bed system, such as standpipes, loop seals, return legs, cone valves, and pipes to interconnect reactors, are constructed in steel to allow flexibility for modifications. The pilot plant can be operated in two separate modes, shown in Figure 9.3: (1) configuration A, consisting of two interconnected CFB reactors; and (2) configuration B, with interconnected TFB and CFB reactor. In configuration A, both carbonator and regenerator are CFB reactors. Both CFB reactors consist of a riser, cyclone, standpipe, and loop seal arrangement. To realize solid looping between the reactors, cone valves are employed, which are situated at the bottom of the loop seal, as shown in Figure 9.3. The cone valve diverts the solid flow between the opposed reactor and the internal circulation via the loop seal overflow. This design of the solid split was found to be optimal within hydrodynamic cold model studies but also in process operation in the 10 kWth and 200 kWth facilities. The carbonator can be fed with a synthetic flue gas or real flue gases generated in the TFB when the CFBeCFB mode is operated. In configuration B, consisting of a TFB carbonator and the CFB regenerator, solid looping between the two reactors is achieved by an L-valve and bottom loop seal arrangement at the TFB. The calciner is operated as an oxy-fuel-fired CFB combustor with flue gas recycle. The combustion can be controlled by staged oxidant supply and fuel feeding rate. Different fuels, such as coal and biomass, can be used. The circulation rate between both reactors, but also in the internal loop, is measured by measurement ports at the return leg of the CFBs, where the solid flow is stopped and monitored through a sight glass. The ports are designed not to influence the DFB hydrodynamics. In addition, microwave sensors, adapted for high-temperature applications, are installed in the return legs. With the help of these sensors operators can continuously monitor the riser circulation rates. Cooling of the carbonator

Configuration B

Configuration A CO2-lean flue gas

CO2-rich gas

CO2-lean flue gas

(b)

Configuration A

Fly ash

Filter

CaO

CaCO3

CaCO3

Cyclone

Solid valve

Carbonator

CaO

Fly ash

Gas cooler

Solid valve

CaO

Filter

Gas cooler

Cyclone

L-valve

ID fan

Regenerator

O2/CO2

Flue gas

Loop seal

Fuel Make up Purge

O2

Pilot plant experience with calcium looping

(a)

Configuration B ID fan

Filter

Fly ash

Filter

Fly ash

R3

O2 + CO2

Flue gas

Gas cooler

Gas cooler

Fuel CaCO3

O2 + CO2

CaO Carbonator

R2

R1

Primary air / O2+CO2

Solid valve

Fuel

Regenerator

R1

Make up Purge

Flue gas

Flue gas

Cyclone

Cyclone

O2/CO2

O2

179

Figure 9.3 The 200 kWth calcium looping pilot plant at University of Stuttgart with its two operational configurations (Dieter et al., 2012). Configuration A: CFB Carbonator (R2)eCFB Regenerator (R1). Configuration B: TFB Carbonator (R3)eCFB Regenerator (R1).

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Calcium and Chemical Looping Technology for Power Generation and CO2 Capture

is achieved by a bed cooler in the bottom region of the reactor and bayonet coolers at the top of the carbonator. A block diagram of the 200 kWth reactor is shown in Figure 9.3. It shows the peripheral components of both configurations. After each of the fluidized bed reactors, a secondary cyclone for dust removal, a gas cooler, and a bag filter for final gas cleaning are installed. The pressure level of the single reactor trains is controlled by pressure control valves after the filters, before the ID fan. The regenerator (R1) additionally consists of a flue gas recirculation train to recycle the CO2-rich flue gas at temperatures of 200
C for oxy-fuel combustion. All three downstream lines are connected to a common ID fan. The initial heat-up of the facility is conducted by means of an external gas burner, which provides hot flue gas to all reactor trains. Further equipment installed at the facility is the dosing system, consisting of loss-of-weight feeders that enable automatically controlled fuel and limestone dosing. Both feeds are introduced to the reactor system by a rotary valve. For flue gas and oxidant supply to the carbonator and calciner, water-cooled side channel blowers are used. Both streams are enriched with oxygen for oxy-fuel combustion in the calciner and carbon dioxide required for carbonator synthetic flue gas. Loop seals and L-valves are fluidized with CO2 and air, respectively. The technical gases are supplied from the tanks, which are part of the oxy-fuel infrastructure at Stuttgart University. The steam required for the simulation of realistic power plant flue gas with vapor concentrations of 10e20% after wet flue gas desulfurization is supplied by a steam generator. The pilot plant is equipped with numerous instruments and a control system. All 600 parameters from measurement and control instruments are stored in the process control system at intervals of 1 s. The plant is equipped with mass flow controllers (MFCs) for CO2 and oxygen supply, gas flow sensors, thermocouples, and pressure transducers. The system pressure is controlled by control valves after the filters. Flow rates are controlled by MFCs or control valves in combination with gas blowers. The temperature in the carbonator is controlled by coolers, the regenerator temperature by the fuel supply. To assure operational safety, an additional safety process control system is installed that automatically turns the facility into a safe mode if critical parameters are exceeded (e.g., if oxygen concentrations in the calciner flue gas exceed 21 vol% or if reactor temperatures exceed the temperature limit). As a result, fuel feeding, oxygen, CO2 supply, etc. are stopped. Up to now, the pilot plant has been operated 1400 and 700 h in CO2 capture mode. Stable oxy-fuel regeneration with flue gas recycle in the calciner was operated for more than 300 h.

9.2.2.4

The 1.9 MWth pilot plant at ITRI, Taiwan

At ITRI, Taiwan, a 1.9 MWth pilot plant was planned and erected in 2013 for CO2 capture from cement plant flue gases (Chang et al., 2013). The pilot plant is based on ITRI’s design of a 3 kWth lab-scale pilot plant. Unlike the commonly used DFB design, the ITRI design consists of a BFB carbonator and a rotary kiln regenerator, similar to the design used at Ohio State University (Wang et al., 2010). The transport of solid material takes place via a pneumatic conveying link. To ensure smooth solid flow and prevent mixing of gases between the two reactors, a series

Pilot plant experience with calcium looping

181

of tanks are used (Chang et al., 2013). Lab-scale experiments with this design have shown promising results, with high CO2 capture, close to the equilibrium capture efficiency. The operation of the rotary kiln regenerator was studied in detail and has shown full calcination. However, the major challenge for calcination in a rotary kiln is to achieve homogeneous temperature distributions and avoid hot spots in the burner zone. The 1.9 MWth carbonator has a diameter of 3.3 m and a height of 4.5 m. The rotary kiln has a diameter of 0.9 m and a length of 5 m. The system is fired with fuel oil. The flue gas recirculation in the rotary kiln is expected to improve the temperature distribution (Chang et al., 2013). First results from the ITRI pilot plant are planned for 2014.

9.3

Plant operating experiences

The complexity of the calcium looping process and pilot plants require detailed preparations of the experiments and plant operation. Existing pilot plants are designed for operation times of one to a few weeks.

9.3.1

Start-up of a calcium looping plant

The start-up of a calcium looping facility will be explained using the Stuttgart pilot plant as an exemplar. The start-up phase begins with heating up the reactors and their refractory with the assistance of internal or external start-up burners. The heating-up process must follow the guidelines of refractory manufacturers to meet specified heating rates. The first heating phase usually requires one shift. Once the ignition temperature for solid fuel (e.g., wood pellets or coal) is reached, additional solid fuel can be fed to the reactors to increase heating rates. In a second heating phase of usually one shift, the reactors can be filled with bed material and a lean CFB operation started at simultaneous combustion of fuel. The third phase starts when the refractory lined reactors are at a sufficiently high temperature of around 800
C. The regenerator temperature can then be increased further, and calcination of limestone bed inventory starts until full calcination of the limestone is achieved. While the regenerator temperature is kept constant, the carbonator can be filled with limestone and set into CFB operation. The main requirement for process investigations is to reach steady-state operation, where temperature in the reactors, reactor inventories, average pressure drops in both reactors with uniform fluctuations, inlet and outlet flows, gas concentrations, and circulation rates remain constant. For each steady state, parameters are recorded, solid circulation rates measured, and solid samples taken. The aforementioned parameters can usually be in a steady state within half an hour. However, for sorbent-specific parameters like the ash content or the degree of sulfation, which directly influences the average sorbent capacity and CO2 capture, longer operating times are required. To reach a full steady state, the whole bed inventory has to be exchanged by the addition of constant sorbent make-up and purge removal at least once within a long-term experiment.

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9.3.2

Hydrodynamic behavior of dual fluidized bed systems

To achieve stable and sufficient sorbent circulation between the fluidized beds is the most important requirement for calcium looping processes. The required solid circulation is generated from the CFB riser entrainment. Part of the riser entrainment is diverted to the other reactor with a known rate, with the help of, for example, cone valves or double loop seals, depending on the individual design. In a loop sealoperated CFB system, the riser entrainment rates are dependent on the riser velocity. Additionally, for the regenerator, air staging influences the entrainment of particles within the CFB. Therefore, to satisfy the solid looping rate, an appropriate riser reactor has to be designed. Since the hydrodynamic behavior of a DFB system is of such high complexity, cold model investigations are one possibility to learn and understand the hydrodynamic behavior. Additionally, cold model systems enable design improvements and operational training for the operating personnel. Detailed cold model studies of the plant were performed in Stuttgart and Darmstadt to design and optimize the pilot plant designs (Bidwe et al., 2014). In particular, the dimensions and geometry of the fluidized beds, cone valve performance, and loop seal depth could be improved before the pilot plant was erected (Bidwe et al., 2014). A detailed understanding of pressure balances, developed in such tests, is the key for the operator. The pressure balance equations of a CFBeCFB system with loop seal/cone valve arrangement are given in the equations 9.1, 9.2 and 9.3. For a single-loop CFB, for example, the carbonator pressure balance is defined as: Dpstp;Ca ¼ DpLS;Ca þ Dpriser;top;Ca þ Dpcyc;Ca

(9.1)

where Dpstp,Ca is the carbonator standpipe pressure drop, DpLS,Ca, the pressure drop of the carbonator loop seal, Dpriser,top,Re, the pressure drop in the riser of the regenerator, and Dpcyc,Ca, the carbonator cyclone pressure drop. The second loop in this DFB system links both CFBs. It describes the loop standpipe(Ca) / conevalve(Ca) / risertop(Re) / cyclone(Re) in case of the carbonator to the regenerator and vice versa for the regenerator to the carbonator, i.e. standpipe (Re) / cone valve (Re) / riser top (Ca) / cyclone (Ca). The pressure balance for both loops are given in Equations (9.2) and (9.3). Thus, it is implied that the cone valve pressure drop is the difference between the exit pressure drops between the two reactors, the difference between the standpipe pressure drop Dpstpi, the sum of the pressure drops in the riser top and the cyclone of the other reactor. pCa and pRe are the absolute pressures of the carbonator and the regenerator cyclone exit respectively. For an interconnected CFBeCFB loop the pressure balance is as follows: pCa þ Dpstp;Ca ¼ DpCV;Ca þ Dpriser;top;Re þ Dpcyc;Re þ pRe

(9.2)

pRe þ Dpstp;Re ¼ DpCV;Re þ Dpriser;top;Ca þ Dpcyc;Ca þ pCa

(9.3)

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183

where pCa is the carbonator pressure, Dpstp,Ca, the pressure drop of the carbonator standpipe, DpCV,Ca, the carbonator cone valve pressure drop, Dpriser,top,Re, the pressure drop in the riser of the regenerator, Dpcyc,Re, the regenerator cyclone pressure drop and DpRe, the regenerator pressure. In the second look, Dpstp,Re is the regenerator standpipe pressure drop, DpCV,Re, the pressure drop of the regenerator cone valve, Dpriser,top,Ca, the carbonator riser pressure drop and Dpcyc,Re, the regenerator cyclone pressure drop. To characterize the hydrodynamic conditions in a pilot plant, characteristic pressure plots are necessary. The pressure profile of the DFB is shown for the Stuttgart pilot plant, for both configurations, in Figure 9.4, as an example. Configuration A represents the CFBeCFB configuration where the representative reactor pressures are shown over the reactor height with full lines. Moreover, cyclone pressure drop, standpipe, loop seal pressure drop, and the pressure drop across the cone valve can be observed (gray lines). This diagram represents the pressure conditions at a steady operating point. Since the pressure conditions of a CFB are sensitive to any change of process parameter—especially the riser velocity, solid inventory, air staging, and cone valve opening—significant attention has to be paid to process control. For constant and stable sorbent circulation, it is of particular importance to achieve similar pressure profiles and pressure differences over the cone valves. Changes of the reactor pressure level of the different fluidized bed reactor trains will change the inventory distribution. An increase of the reactor pressure level in one reactor, for example, will shift bed inventory to the opposed reactor. The system will self-stabilize at a different steady state. This effect can be used to influence the bed distribution between the reactors. However, large fluctuations should be avoided. Pilot plant configuration B offers different hydrodynamic and operational possibilities. The DFB system consists of a turbulent carbonator, which also demands a different design of the sorbent looping interconnection. While the sorbent regenerator works as described before, the carbonator is shorter, has a bigger diameter, and is operated at a lower fluidization velocity. This corresponds to a turbulent fluidization pattern with minor entrainment but still good contact between gases and solids. The interconnection of carbonator and regenerator with a bottom loop seal enables highly flexible operation of the operating velocity in the carbonator, compared to a CFB carbonator, which requires constant entrainment. The carbonator pressure profile has a sharp increase in the lower region, which represents a carbonator inventory distribution shifted toward a dense region at the reactor bottom. Similar to configuration A, the inventory distribution can be adjusted through an increase in the overall reactor pressure toward the other reactor. For example, an increase of the regenerator pressure results in increased carbonator bed inventory, since the bed mass must accumulate on the carbonator side to increase the standpipe pressure enough to overcome the increased pressure resistance at the bottom loop seal discharge side. The sorbent looping rate between the regenerator and the carbonator can be controlled by the degree of gas supplied to the L-valve. Experiments over several hours showed that stable operation of the system with constant sorbent looping ratios can be achieved, given that the pressure between both reactor trains remains constant. The comparison of the different carbonator designs indicated that the CFB carbonator will be useful in large-scale setups, whereas a TFB carbonator will have advantages in small- to medium-scale setups. It has been seen that the turbulent

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(a)

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Connection carb -> reg Circulation regenerator

Figure 9.4 Characteristic pressure profiles of (a) configuration A (CFBeCFB) and (b) configuration B (TFBeCFB) of the 200 kWth pilot plant at University of Stuttgart. Dieter et al. (2012).

Pilot plant experience with calcium looping

185

carbonator can operate with a broader range of flue gas streams corresponding to 1e4 m/ s of fluidization velocity in the riser. CFB reactors have shown advantages with respect to flue gas throughput and lower cross-sections of the reactor. In general, achieving hydrodynamic stability in the start-up phase is an important issue and various operational aspects have to be taken into account simultaneously. When solid looping is initiated, carbonator and regenerator temperatures change rapidly. To adjust the temperatures, fuel and oxidant flow in the regenerator must be changed accordingly. Owing to these changes, the reactor pressure is also influenced significantly. In order to minimize the influences on the whole system, slow parameter changes are of importance so that automatic controllers (e.g., for the overall reactor pressure level in the reactors) can follow and do not affect other parameters.

9.3.3

Process demonstration

The main purpose of pilot plant operation is to demonstrate the calcium looping process under realistic process conditions. The main result of the activities in La Pereda, Darmstadt, and Stuttgart is that CO2 capture efficiencies of over 90% can be achieved. Various results of the different facilities will be shown within this chapter. Figure 9.5 includes operating results of all pilot plants demonstrating high capture efficiencies over several hours of operation. The results of the Stuttgart pilot plant (a) show curves for CO2 capture efficiency, temperature, and gas concentrations over 5 h of operation. Temperatures were varied between 580 and 680
C. The average CO2 outlet concentration over the whole duration could be kept below 2%, which corresponds to a CO2 capture efficiency of more than 90%, while the flue gas inlet CO2 concentrations of 14% were introduced to the carbonator. The La Pereda results (b) show a typical steady-state period with respect to flue gas velocities, carbonation and calcination temperatures, and solid circulation between the reactors. The average carbonator temperature was kept constant at around 660
C. The activity of the solids decreased from 0.3 to 0.2 during this period, since no fresh limestone was fed to the system. The CO2 capture efficiency was observed to be above 90% for the whole duration of the experiment, except for an intermediate drop due to an alteration of the sorbent looping rate, which increased the carbonator temperature (S
anchez-Biezma et al., 2013). The third part of Figure 9.5 (c) shows experimental results obtained at TU Darmstadt with a coal-fired carbonator. It displays the measured profiles for CO2 flow, temperature, pressure, and CO2 capture in the carbonator for a period of 22 h. After the start of CO2 feeding to the primary air flow of the carbonator to produce a synthetic flue gas, which is introduced into the carbonator, an immediate temperature increase to 670
C occurs due to the exothermic carbonation. In order to perform the CO2 capture at the desired temperature level of 650
C, heat was removed from the carbonator with an internal heat removal system. The pressure increase in the system is partially caused by the CaCO3 formed in the reactor, which increases the weight of the solids. Moreover, fresh limestone was fed to the carbonator during this operating period to increase the sorbent inventory (Str€ ohle et al., 2014).

186

Calcium and Chemical Looping Technology for Power Generation and CO2 Capture 1000

CO2 capture (%) Temperature (ºC)

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Figure 9.5 (aec) Three long-term operating points of the 200 kWth plant at University of Stuttgart, at the 1.7 MWth plant at La Pereda with CO2 capture efficiency above 90%, and at TU Darmstadt. Dieter et al. (2014); S
anchez-Biezma et al. (2013); Str€ohle et al. (2014).

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187

Generally, homogeneous temperatures in the reactors are of major importance, especially for oxy-fuel combustion in the sorbent calciner, to avoid hot spots and accelerated sorbent sintering. In order to reduce hot spots, staged oxidant supply to the calciner is recommended. At the Stuttgart pilot plant, oxy-fuel combustion with up to 50 vol% oxygen inlet concentration has been conducted. A typical temperature profile of the calciner shows a homogeneous temperature in the riser and a temperature drop of 50e100
C in the bottom zone due to the incoming colder sorbent from the carbonator. The upper part of the reactor should be controlled at temperatures of 900e920
C in order to achieve full sorbent calcination. The required temperature thereby depends on the CO2 partial pressure in the calciner, which varies depending on the reactor type, since oxy-fuel combustion can be realized in different ways. While the La Pereda and Darmstadt plants feed an O2/CO2 mixture from gas tanks to the system, the Stuttgart pilot plant operates with recycled CO2. As a result, the water vapor concentration in the Stuttgart pilot plant is higher. Depending on the moisture and hydrogen concentration of the coal, the flue gas water vapor concentration is between 20 and 30 vol%, while without recycle (representing a calcium looping plant with a flue gas condenser) the water vapor content is between 5 and 10%. Accordingly, the CO2 partial pressure varies between 70 and 95%. In comparison, the temperature in the carbonator behaves in an opposite fashion to that in the calciner. Due to the hot incoming sorbent from the calciner at the bottom, the temperature in this region is higher and decreases over the height. For CO2 capture, higher bottom temperatures of 650e680
C are beneficial, since a high percentage of the total capture takes place in this region with fast reaction kinetics (Charitos et al., 2011). In order to achieve optimum capture, the riser temperatures should be lower and taken as the target temperature to realize a good equilibrium outlet concentration. The evaluation of optimum temperature profiles in the carbonator can be addressed as future research goals.

9.4

Parametric studies in pilot plants

The major goal of pilot plant testing is the demonstration of the process. Therefore, stable plant operation at constant process parameters was aimed at the first experimental campaigns at all facilities. In a second step, process optimization was the focus and parametric studies were carried out in the pilot plants. Primarily, the repeatability of the lab-scale results should be confirmed at higher scale. Secondly, the influence of typical pilot-scale conditions such as the combustion in the regenerator, which could not be achieved in electrically heated lab-scale plants, was investigated. At all three pilot plants, parametric studies such as the influence of temperature, sorbent circulation rate, and specific sorbent inventory have been carried out. Depending on the pilot plant design, different studies such as the influence of sorbent make-up, the effect of realistic flue gas containing water vapor, the effect of sulfur and ash from coal combustion, or the influence of oxy-fuel conditions on calcination can be carried out. Up to now, these specific process investigations have been studied only partially and will thus be addressed in ongoing and future projects.

188

9.4.1

Calcium and Chemical Looping Technology for Power Generation and CO2 Capture

Carbonator performance

Carbonator temperature is an important parameter that affects CO2 capture efficiency significantly. Lab-scale studies (Charitos et al., 2010) have identified an optimum temperature range of 630e650
C. Above these temperatures, the CO2 capture is restricted by the equilibrium and at lower temperatures the reaction is kinetically limited. Figure 9.6 shows a variation of temperature from the La Pereda plant. Carbonator temperature was varied between 650 and 700
C. A high temperature of 700
C in the carbonator resulted in lower CO2 capture efficiencies ðECO2 < 70%Þ and improved with lower temperature at 650
C to 90% CO2 capture, very close to the chemical equilibrium (S
anchez-Biezma et al., 2013). At TU Darmstadt the temperature of 660
C was found to be the optimum for kinetic and thermodynamic conditions. Temperatures less than 620
C were kinetically unfavorable for the process (Pl€otz et al., 2012).

1.0

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CO2 capture efficiency

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90

80 Dry flue gas (0 vol.-% H2O)

70

Realistic flue gas (15 vol.-% H2O)

60 580

600 620 640 660 680 Carbonator temperature (ºC)

700

Figure 9.6 Effect of carbonator temperature on the CO2 capture efficiency investigated at the (a) La Pereda and (b) Stuttgart pilot plant. Dieter et al. (2014); S
anchez-Biezma et al. (2013).

Pilot plant experience with calcium looping

189

Investigations at the Stuttgart pilot plant have shown an optimum capture temperature between 640 and 660
C for a simulated dry flue gas without water vapor, which is generally present in real flue gases (Figure 9.6). Below 640
C and above 660
C the capture efficiency decreases, due to the decreasing reaction rate and limiting equilibrium, respectively. However, for a realistic flue gas with 15 vol% water vapor, representative for flue gases after a wet flue gas desulfurization plant, the capture efficiency follows the chemical equilibrium concentration over the complete temperature range. Detailed investigation with a TGA led to the explanation that the water vapor catalyzes the reaction and an increased carbonate conversion can be observed. In real facilities, flue gases contain water vapor in a range of 10e30 vol%, depending on the flue gas treatment. This outcome therefore opens the potential for further efficiency improvements of calcium looping.

9.4.2

Sorbent degradation and deactivation by sulfur capture

The capture capacity of calcium looping sorbents is one of the most investigated issues in calcium looping development. Grasa and Abanades (2006) investigated the degradation behavior of different limestones over multiple carbonation/calcination cycles (and see also Chapter 6 of this book). With increasing cycle number, the sorbent loses its high initial capture capacity and decreases to a residual capture capacity due to a reduction of active surface area caused by sintering. So far, most of the sorbent-related studies have been based on TGA, and limited results are available from actual plant operation. Comparison of thermogravimetric measurements and real process operation was carried out in the Stuttgart 10 kWth test plant (Charitos et al., 2011). In contrast to thermogravimetric measurements, in a continuous pilot plant operation it is difficult to estimate an exact cycle number for a given sorbent, since fluidized beds provide a well-mixed system. Therefore the carbonationecalcination cycle number was defined using the following calculation (Arias et al., 2013; Charitos et al., 2011). The approximate cycle number Nth is given as the integral over the molar flow of CO2 into the carbonator ðFCO2 Þ and the instantaneous CO2 capture efficiency (Ecarb(t)) over the total inventory of sorbent in the system (nCa,total) and the average CO2 carrying capacity (Xave): Zt Nth ¼

FCO2 Ecarb ðtÞ dt: nCa;total Xave

(9.4)

0

In addition to sintering and the respective reduction of active surface, sorbent deactivation by sulfur from coal combustion has to be considered in pilot-scale experiments. Figure 9.7 shows the results for sorbent deactivation from the La Pereda plant. The loss of CO2 carrying capacity (Xave) from the pilot plant is consistent with the TGA observation, shown as a solid black line. The increasing difference between the measured carrying capacity and the TG curve can be considered to be deactivation due to sulfation. Xsulf represents the fraction of total limestone converted to sulfate. The addition of sulfation (Xsulf) and carbonation (Xave) is called Xef and represents the total capacity for both CO2 and SO2 in the fast reaction regime.

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Calcium and Chemical Looping Technology for Power Generation and CO2 Capture

0.6

Xave XN Xsurf exp Xsurf calc Xef

0.5

Xave, XN, Xsulf

0.4 0.3 0.2 0.1 0.0 0

5

10

15

20

25

30

35

Nth

Figure 9.7 Comparison of sorbent degradation in a TG and sorbent from La Pereda pilot experiments. Arias et al. (2013).

Even though the degree of sulfation is relatively high after the 35th cycle, the sorbent is able to maintain a capture capacity for CO2 of 0.1 molCO2 =molCaO . At this level, the plant is still able to capture high rates of CO2 for a specific sorbent looping ratio and sufficient sorbent inventory. The sulfur introduced into the pilot plant came from two sources: the flue gas of the CFBC power plant and the coal burned in the calciner. More than 95% of the SO2 from the carbonator flue gas and the SO2 generated from the coal combustion in the regenerator could be captured.

9.4.3

Sorbent attrition

Sorbent attrition has been identified as a potentially major bottleneck in the development of the calcium looping process. This is in common with chemical looping e see Chapter 11. Various attrition measurement methods have been observed to evaluate suitable limestones for calcium looping (Coppola, Montagnaro, Salatino, & Scala, 2012; Materic, Holt, Hyland, & Jones, 2014). However, attrition is very difficult to simulate in a test rig, and therefore the experience from pilot plant operation is crucial. At the Stuttgart pilot plant, attrition was monitored in various experimental campaigns, yielding in total more than 600 h of operation (Dieter et al., 2014). Precise mass balances were carried out over the whole campaign duration. Figure 9.8 shows the particle size distribution of the raw limestone and bed material sampled during the campaign after two days of constant operation. After this time, the sorbent is considered to be in a steady state, since the whole bed inventory has been exchanged several times due to constant make-up supply and purge rates. The limestone used was from the Swabian Alb region

Pilot plant experience with calcium looping

191

Cumulative particle size (%)

100

75 Raw limestone Bed material

50

25

Fines

0 0

250

500 Particle size (µ µm)

750

1000

Figure 9.8 Measurement of sorbent attrition from a steady-state operation after several days of continuous operation in comparison with the raw Swabian Alb limestone. Dieter et al. (2014).

in southern Germany. As shown, a particle size reduction of the mean diameter d50 from 420 to 350 mm was measured. At the same time, an increase of fines indicated attrition during operation. The average loss of bed material was observed to be 3 wt% of the total solid inventory (TSI) per hour. Steady operating periods with less than 2 wt% of TSI per hour were monitored over several hours of operation. The value obtained is lower than the required make-up ratios to maintain the bed material activity. Therefore, attrition and bed material loss with this specific limestone can be considered as not critical for calcium looping operation. However, limestones with lower hardness show higher attrition tendencies. First results with comparably weak limestones have shown that calcium looping operation is possible. However, special attention has to be paid to plant operation. Minimizing the thermal and mechanical stress to the sorbent is necessary and can be realized by reduced fluidization velocities in the reactor and operating points closer to the minimum calcination temperatures. Also, the design improvement of reactor components with a large impact on sorbent attrition, such as cyclones or fluidization nozzles, should be considered. During operation, fines with a particle size less than 20e30 mm resulting from attrition are removed from the CFB through the primary cyclone. To trap these fines, the Stuttgart pilot plant is equipped with secondary cyclones and bag filters. Larger fines were separated by the secondary cyclone. Bag filters capture dust particles below 10 mm. The major amount of fines was captured from the regenerator flue gas stream, which identifies calcination as a major cause of attrition. The comparison of fines generation at different make-up rates has shown a strong dependency on the limestone feed rate. This indicates that the initial calcination of the fresh limestone is responsible for a significant part of the overall amount of attrition. In order to reduce attrition and avoid operational problems such as deposition of fines in the reactor system, cyclones should be designed carefully so that fines are not kept in the system.

192

9.4.4

Calcium and Chemical Looping Technology for Power Generation and CO2 Capture

Regenerator operation under oxy-fuel conditions

Full calcination of sorbent is one key issue to achieve high CO2 capture with calcium looping. Oxy-fuel combustion, where the coal is burned with oxygen, produces a concentrated CO2 flue gas. In this case, usually a recycled CO2 stream is added to the inlet oxygen to avoid high adiabatic temperatures and hot spots at the oxygen supply. Oxy-fuel combustion for sorbent calcination of calcium looping is one further challenge, since the sorbent has to be treated with care in order to avoid sintering but still full calcination has to be achieved. All three pilot plants possess the ability to operate the regenerator at oxy-fuel conditions using O2/CO2 mixtures for combustion. In each, the oxygen is supplied from tanks. At the La Pereda and Darmstadt plants the CO2 required for dilution of the oxygen is also supplied by storage tanks. The Stuttgart pilot plant uses a flue gas recycle as would be realized in a full-scale process. Here, the flue gas is cooled down, filtered, and recycled with a high-temperature blower at temperatures of 200
C. As a result, the vapor content in the calciner flue gas is in the range of 20e25 vol% compared to 8e10 vol% when dry CO2 from the tank is utilized. The CO2 partial pressures, as a consequence, are in the range of 75% for the wet recycled flue gas, which enables calcination at comparatively lower temperatures. The regenerator operation under oxy-fuel conditions has been demonstrated successfully in all three pilot plants. The La Pereda plant has demonstrated oxygen combustion of 35 vol%,dry. The Stuttgart facility operated inlet oxygen concentration of maximum 55 vol%,dry without temperature hot spots. Further increase of the CO2 inlet concentration is one goal of future investigations in order to reduce the fuel consumption for reheating of recycled flue gas. In long-term operation, La Pereda has shown steady regenerator operation with maximum CO2 exit concentrations of 85 vol%,dry at 950
C (S
anchez-Biezma et al., 2013). The Stuttgart plant achieved a maximum CO2 exit concentration of 92 vol%,dry using recycled flue gases and an excess oxygen concentration of 3 vol%,dry.

9.5

Summary

The calcium looping process has been demonstrated successfully in a number of bench-scale and pilot-scale plants ranging from 3 kWth up to 1.7 MWth. These plants made it possible to prove the calcium looping process to be feasible and to gain a data basis for further demonstration at larger scales in the future. The operational experience gained at bench scale and pilot scales will be crucial in designing the plants at higher scales and commercial scales. The operational experiences and results gained from pilot plant tests at three calcium looping pilot plants were presented in this chapter. Cold model studies can provide important know-how for the hydrodynamic feasibility and improvement of the design. The influences of the main operational parameters such as temperature, space time, and looping rates were in accordance with previous studies carried out in bench-scale

Pilot plant experience with calcium looping

193

units. Investigations such as those of the influence of water vapor in a realistic flue gas were found to increase CO2 capture efficiency significantly (see also discussion in Section 6.5.2 in the context of extended carbonation). Oxy-fuel conditions in the regenerator with O2 inlet concentrations up to 55 vol%,dry could be realized without temperature hot spots and full sorbent calcination. The attrition rate at pilot plant operation could be kept lower than 2 wt% per hour, which is less than the make-up required for stabilization of the sorbent capture capacity. If limestones with low attrition tendency are used or plants are operated smoothly, attrition will not be an obstacle for calcium looping operation.

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Dieter, H., Hawthorne, C., Bidwe, A. R., Zieba, M., & Scheffknecht, G. (2012). The 200 kWth dual fluidized bed calcium looping pilot plant for efficient CO2 capture: plant operating experiences and results. Naples (Italy). In 21st international conferance on fluidized bed combustion (Vol. 1, pp. 397e404). Dieter, H., Hawthorne, C., Zieba, M., & Scheffknecht, G. (2013). Progress in calcium looping post combustion CO2 capture: successful pilot scale demonstration. Energy Procedia, 37, 48e56. Fang, F., Li, Z., & Cai, N. (2009). Continuous CO2 capture from flue gases using a dual fluidized bed reactor with calcium-based sorbent. Industrial & Engineering Chemistry Research, 48, 11140e11147. Grasa, G. S., & Abanades, J. C. (2006). CO2 capture capacity of CaO in long series of carbonation/calcination cycles. Industrial & Engineering Chemistry Research, 45, 8846e8851. Hawthorne, C., Dieter, H., Bidwe, A., Schuster, A., Scheffknecht, G., & Unterberger, S. (2011). CO2 capture with CaO in a 200 kWth dual fluidized bed pilot plant. Energy Procedia, 4, 441e448. Hawthorne, C., Poboss, N., Dieter, H., Gredinger, A., Zieba, M., & Scheffknecht, G. (2012). Operation and results of a 200 kWth dual fluidized bed pilot plant gasifier with adsorption enhanced reforming. Biomass Conversion and Biorefinery 2.3, 217e227. Lu, D. Y., Hughes, R. W., & Anthony, E. J. (2008). Ca-based sorbent looping combustion for CO2 capture in pilot-scale dual fluidized beds. Fuel Processing Technology, 89, 1386e1395. Materic, V., Holt, R., Hyland, M., & Jones, M. I. (July 1, 2014). An internally circulating fluid bed for attrition testing of Ca looping sorbents. Fuel, 127, 116e123. Pl€ otz, S., Bayrak, A., Galloy, A., Kremer, J., Orth, M., Wieczorek, M., et al. (2012). First carbonate looping experiments with a 1 mwth test facility consisting of two interconnected CFBs. Naples (Italy). In 21st international conference on fluidized bed combustion (Vol. 1, pp. 421e428). Poboss, N., Swiecki, K., Charitos, A., Hawthorne, C., Zieba, M., & Scheffknecht, G. (2010). Experimental investigation of the absorption enhanced reforming of biomass in a 20 kWth dual fluidized bed system. In 23rd ECOS conference, Lausanne, Switzerland. Rodriguez, N., Alonso, M., Abanades, J. C., Charitos, A., Hawthorne, C., Scheffknecht, G., et al. (2011). Comparison of experimental results from three dual fluidized bed facilities capturing CO2 with CaO. Seite(n) Energy Procedia, 4, 393e401. S
anchez-Biezma, A., Diaz, L., L
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