Dual fluidized bed systems

The dual fluidized bed (DFB) reactor system is a preferred reactor system for the HTSLC processes. A DFB system is a combination of two fluidized bed reactor systems with an interlinking for solid transport between the two fluidized beds. The DFB system offers a convenient transport of the solid reactants from one reactor to the other without mixing the gases between the two reactors. Figure 10 shows a typical example of DFB system. Theoretically FCC systems also use a DFB system, and are in use since the 1940`s. Every catalytic reaction with catalyst regeneration is carried out in a DFB system. As per Corella et al. [60] the DFB systems of FCC reactors or catalytic reaction system typically use the CFB as one reactor and downcomer-seal pot arrangement as the second reactor, while the gasification DFB or HTSLC DFB require a separate FB reactor for different reaction steps. Table 6 shows the number of DFB facilities all over the world currently under operation for different HTSLC processes.

A classification of DFB systems is not reported in the literature; however DFB classification is suggested in this work based on the type of FB reactors used and the

∆𝑝_𝑠𝑡𝑝= (𝑝 𝑑𝐻)

𝑠𝑡𝑝𝐻_𝑠𝑡𝑝= (150𝜇(1 − 𝜀)²

(𝜑𝑑_𝑝)²𝜀³ |𝑈_{𝑠𝑙 𝑠𝑡𝑝}| +1.75𝜌_𝑔(1 − 𝜀)

(𝜑𝑑_𝑝)𝜀³ |𝑈_{𝑠𝑙 𝑠𝑡𝑝}|²) 𝐻_𝑠𝑡𝑝 (21)

𝑈_{𝑠𝑙 𝑠𝑡𝑝}= 𝑈_{𝑠 𝑠𝑡𝑝}− 𝑈_{𝑔 𝑠𝑡𝑝} (22)

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type of interlinking used. Three combinations of FB reactors are possible in DFB systems and are shown in Figure 11.

• BFB – BFB

• BFB – CFB or CFB – BFB

• CFB – CFB

In BFB-BFB type system both reactors are operating in a bubbling regime or may be operable in a turbulent regime. BFB-BFB systems are easy to operate and are suitable for small scale or a lab scale installation or where the use of BFB reactors is required. Since BFB do not generate large enough solid entrainment, the solid transfer between the two beds is achieved with the help of pneumatic transport or with the use of an extra CFB riser.

BFB-CFB type is the most widely used DFB system, suitable for small to large scale systems. The CFB risers act simultaneously as reactor and generate solid circulation required for solid looping. The solid transport from BFB to CFB is generally under gravity with a loop seal (in between) to prevent gas mixing. It is also a preferred combination for steam gasification system, where BFB is gasifier and CFB is combustor [60]. The largest DFB system based on this type is 8 MWth in Güssing, Austria [69]. The CFB-CFB type is the preferred choice for large and commercial scale installations due to the inherent advantages offered by the CFB. Till date the largest

Figure 10 – 10 kWth electrically heated bench scale test plant facility at University of Stuttgart, (DIVA-ELWIRA)

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installation for CFB-CFB type is 1.7 MWth CaL pilot plant at La-Pereda. Compared to other two types of DFB systems the operation of this system could be challenging. Two approaches are used presently to carry out solid looping. In the first approach both CFB operate independently, with a diversion (complete or partially) of solid flow from both the loop seals takes place. This approach is used in the present work, as well as at INCAR-CSIC [47], TU Darmstadt [50,135] and La-Pereda unit [11]. In the second approach as used by TU Wien [136], both the CFBs are interlinked at the bottom via loop seal and solid circulation generated from one of the CFB is transported to the neighboring CFB.

The interlinking of the fluidized bed is very important in DFB systems; its primary function is to transport solid particles from one fluidized bed to other, without mixing the gases between them, or in other words solid looping between them. The solid flow rate between the reactors is called as the ´solid looping rate` denoted as ´𝐺<sub>𝐿𝑖</sub>`, where 𝑖 is the type of interlinking used. Several types of interlinking are in use namely loop seals, double exit loop seals with a mechanical valve, L-valve, pneumatic transport and mechanical conveyor. Figure 12 shows the schematics of different types of interlinking applied in various DFB facilities across the world.

Loop seals are commonly used in single loop CFB systems and also most commonly used for interlinking between the two fluidized beds [47,53,137–140], for DFB applications the supply side of the loop seal receives the particles from FB1 and recycle side of the loop seal is directed towards FB2. The main advantage of loop seal is that there are no mechanical or moving parts; it is simple in construction and operation; and mainly provides a very good pressure sealing between the reactors.

Figure 11 – Types of DFB system based on the combination of fluidized beds (a) BFB - BFB (b) BFB – CFB (c) CFB - CFB

31 Table 6- DFB facilities across the world

Sr Location Process Size Type and interlinking Ref

1 University of Stuttgart

CaL, SER, CLC 10 kWth CFB- BFB

Double exit loop seal and cone valve

[34,43,63 ,141]

CaL SER

200 kWth CFB-CFB (Car- Reg) CFB-BFB (Ga- Comb) Cone valve, L-valve

This work

2 SPE-TUHH,

Hamburg

CLC CFB-BFB, (AR- FR)

Loop seal

[142]

3 Güssing, Austria Gasification SER

8 MWth BFB – CFB (Ga- Comb) Loop seals

[69]

4 TU Wien, Austria CLC 120 kWth CFB- CFB (AR – FR) Loop seals

[136]

Gasification SER

100 kWth BFB-CFB (Ga- Comb) Loop seals

[53,62]

6 TU Darmstadt, Germany

CaL and CLC 1 MWth CFB – CFB

(Car- Reg and AR- FR)

[50,135]

7 La Pereda, Spain CaL 1.7 MWth CFB- CFB (Car- Reg) Double exit loop seal

[11]

8 Chalmers, Sweden

CLC 10 kWth CFB- BFB (AR – FR) [138]

CLC (coal) 100 kWth CFB-BFB (AR-FR) Loop seals

[143]

11 ICB-CSIC, Spain

CLC 0.5 kWth BFB-BFB (AR-FR) [144]

Gasification CLC

10 kWth BFB-BFB (AR-FR) 12 INCAR circe

Oviedo, Spain

CaL 30 kWth CFB- CFB (Car-Reg)

Loop seals

[47]

13 CANMET,

Canada

CaL 75 kWth CFB – BFB (Car-Reg)

Loop seals

[42]

14 Ohio State University, USA

CaL 25 kWth Interconnected Moving bed-Entrained bed

[38]

15 IFP-Total, France CLC 10 kWth BFB – BFB (AR-FR) Pneumatic transport

[145]

16 Dalhousie

University, Halifax

SER 0.2 kg/h CFB-BFB (Ga- Comb) [67]

17 Southeast University, China

CLC, Gasification

10 kWth CFB-spouted bed [146]

18 KITECH

Cheonan, South Korea

Gasification 200 kWth BFB – CFB (Ga- Comb) Loop seals

[137]

19 Cranefield, UK CLC, CaL 25 kWth CFB-BFB (Car- Reg) [53]

AR: Air reactor, Car: carbonator, Comb: combuster, FR: Fuel reactor, Ga: Gasifier, Reg: Regenerator

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However, there are few disadvantages of using loop seals in a DFB system.

• Loop seals offer no or very little control over global circulation rate, the control of global circulation rates should be initiated by other means

• No internal circulation with FB1

• Sealing is ineffective in highly fluctuating FB operation

• The aeration requirement is high therefore may become source of dilution (in small scale units) for product gases in FB2.

The double exit loop seal with a mechanical valve (Figure 12b) is an effective solution to deal with the disadvantages of the loop seal. This type of interlinking consists of a normal CFB loop seal for FB1 with a small variable orifice (valve e.g. cone valve) on the supply side of the loop seal to facilitate particle flow to the FB2 as shown in Figure 12b. In industry this configuration is used in FB combustors for solid supply to external heat exchangers[147]. University of Stuttgart has been using this type of interlinking [34,43,52,59,71,72] and aims to develop it further.

The cone valve can control the solid flow rate precisely. Cone valve characteristics are studied in [71] whereas Eq. (23) shows that the cone valve flow rate is linear to the pressure drop across the valve [71] while some other studies show it as a square root of the pressure drop [148].

𝐺_𝐿𝐶𝑉 = 𝑘 𝐴_𝐶𝑉∆𝑝_𝐶𝑉^𝑛 (where n = 1 [71], n = 0.5 [148]) (23)

Figure 12 – Types of interlinking (a) loop seal (b) loop seal with cone valve (c) double exit loop seal (d) L-valve

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Unlike an L-valve and a loop seal the cone valve is a type of mechanical valve and commonly used in CFB combustors for the application of external heat exchangers. L-valves are commonly used in single loop CFB systems. L-valves in DFB application are seldom but recently cited [149,150]. The application of L-valves in DFB systems has a considerable potential mainly because

• they allow very precise and repeatable control of solid looping rates

• they require less aeration therefore less dilution of the product gases

The use of high superficial gas velocity (𝑢₀ > 10 m/s) namely in pneumatic transport regime as a means of solid conveying is common in process industry. A similar principle is used in DFB application as a means of interlinking. This mechanism is suitable for BFB-BFB units where fluidized beds cannot generate enough global circulation rates due to lower superficial velocities [42,49,149,151]. Screw conveyors are widely used for solid conveying and are primarily used in fluidized beds for feeding fuel and bed material. However screw conveyors as a means of interlinking are rare and cited only at 1 MWth DFB facility at Darmstadt [50].

The concept of double exit loop seal (Figure 12c) is inspired from the multiple exit loop seals used in FBCs to ensure uniform distribution of recycled particles. For DFB application this type of interlinking is cited in the literature [136] interestingly at La-Pereda CaL DFB facility in Spain [11,152]. The two exits in the loop seal are basically to facilitate global circulation and internal circulation without the application of moving parts. However, the success of this configuration is questionable, because in practice the pressure between two fluidized beds is different unlike FBC where multiple exits enter in the same FB. The control of global circulation rates will be a major concern in such configuration.

Im Dokument Hydrodynamic studies of the dual fluidized bed reactor systems for high temperature solid looping cycles (Seite 46-51)