There are many factors that affect the gasification process, producer gas quality ( composition, production of H2, CO, CO2 and CH4, free tar content and heating value) and the performance of gasification ( represented by gas yield, carbon conversion efficiency, and total heating value efficiency). Generally, these factors are reactor design, origin feedstock of fuel, operating conditions such as equivalence ratio, temperature, pressure and gasifying agent (medium). In addition, for each type of gasifier design, there are additional factors that affect the gasification process and reactor performance.
2.3.1 Gasifier design
Reactor design is crucial for gasification efficiency, composition and heating value of the product gas, and also for tar formation. Practically, according to the gas-solid conversion, but their operations and performances are different.
2.3.1.1 Fluidised bed gasifiers:
As mentioned in Chapter 1-Section 1.3, fluidised bed reactors can be used for thermochemical gas-solid reaction processes. For a fluidized bed gasifier the fuel, solid fuel biomass or coal, is gasified in a bed of small particles (inert or catalytic bed material or both) and fluidized by a suitable gasification medium gas (Basu 2010). There are two principal types of fluidised bed gasifier.
19 I. Bubbling fluidized bed gasifier (BFBG):
The oldest bubbling fluidised bed gasifier for coal gasification was developed by Fritz Winkler in 1921 (Basu 2010). Basically, as shown in Figure 2.1 a and b, the BFBG simply comprised of an air blower, gas plenum (gas box), the distributor plate, screw feeders for solid fuel and bed material feed, fluidized bed with freeboard column unit and cyclone separator unit (Engineering 2011).
When the agent gas introduces through the distributor plate and passes upward through the gasifier bed, at a specific velocity bubbles are formed within the bed which rise and grow in size until they reach the surface material bed where they burst (Sadaka 2010).
The little amount of solid materials are carried along with bubbles in wake, and when the bubbles burst at the surface of the bed, the carried particles fall downward by gravity. They flow again upward along with newly formed bubbles. Just this portion of the process, wake portion movement, is responsible for mixing the bed materials, fuel particles, and gasifying agent. This boiling state gives nearly uniform temperature, high heat and mass transfer that is characteristic of fluidized bed reactors (Harriott 2003)(Tzeng 2007)(Basu 2006).
Figure 2. 1 A simple sketch of bubbling fluidised bed gasifier: a) (Sadaka 2010), b) (Patel 2014)
Characteristics of the bubbling fluidised bed gasifier
This design operates at low fluid superficial velocity, typically less than 1m/sec.
Although it is often operated at atmospheric pressure, but also can operates under pressurized conditions, which will further increase the throughput. Owing to the low residence, time of the fuel and low char’s reactivity the char conversion is low.
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The uniform temperature distribution, high mixing and high heat and mass transfer are applicable throughout the gasifier. Carbon with some fine bed material and ash are entrained in the gas product, trapped, and separated out in a cyclone. BFBG has high flexibility and suitability for the gasification of solid biomass fuel regarding both particle size and different types of materials. This design results in lower cost and less maintenance. It is suitable for scaling up. (Gautam 2010)(Tzeng 2007) (Ciferno and Marano 2002)(Brown 2006).(Siedlecki et al. 2011)(Puig-Arnavat et al. 2010).
Drawbacks of the bubbling fluidised bed gasifier
Particulates elutriate as the product gas increases the solid load in the cyclone and filter.
The issue of the weakness of the interaction and mixing of the species when the conversion of the char is low due to the low residence time of the fuel, the slow reactivity of the char and un-recycled trapped solid materials.
At higher temperatures above 900-950oC and when the biomass fuels have a high content of ash, potential ash melting will occur causing stickiness of particles leading to the agglomeration phenomena causing bed de-fluidization and thereby the gasifier.
(Cirad 2009) (Puig-Arnavat et al. 2010) (Siedlecki et al. 2011).
II. Circulating fluidized bed gasifier (CFBG):
Simple (classical) circulating fluidized bed
This general circulating fluidized bed abbreviated as CFB, has been used as a common term since the 1970s and for gas-solid process applications CFB technology dates back to the 1960s (Yang 2003). This gasifier type works at a high superficial gas velocity beyond bubbling and turbulent fluidization regimes. It is also known as fast fluidization under certain conditions (see Section 2.5.3.5). At this critical point, known as the transition boundary velocity, the bed particle entrainment occurs. This is called a transport or transition velocity. Beyond this point, the bed fluidisation cannot be continued without entrained solids recycling. The typical gas velocity range is 2-12 m/sec and particle rate flux range is 10-1000 kg/m2.sec, so that there is not an interface distinguishing between a dense bed and a dilute region above. By this point, a CFB is differentiated from a bubbling fluidized bed BFB (Siedlecki et al. 2011)(Yang 2003) (Ciferno and Marano 2002)(Klein 2002).
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Figure 2.2 shows a schematic diagram of CFBG gasifier. As a result of gas high-velocity the entrained solid particles will separate, re-circulate and return back to the reactor through an external particle flow system, which usually consists of one or more cyclones, a standpipe and a valve or seal (Yang 2003) (Ciferno and Marano 2002)
Figure 2. 2 Classical circulating fluidized bed gasifier-direct heating: a) Lurji Gasifier (Christopher Higman 2003), b) Classical type (Siedlecki et al. 2011)
There are two types of circulating fluidized bed (Siedlecki et al. 2011) (Christopher Higman 2003):
Fast circulating fluidized bed FICFB (Indirectly Heated Unit):
Sometimes is called Dual or Twin Circulating Fluidized Bed Reactor DCFB. The operation of this gasifier is based in which the gasifier vessel is divided into two distinct fluidization; reactors, which are operated at two different gas velocities as shown in Figure 2.3, one of them is a bubbling fluidized gasifier BFBG, where usually the steam is an agent gas while the other reactor (combustor) is a simple circulating fluidized bed combustor CFBC, usually air is an agent gas. Some of its features are available in (Puig-Arnavat et al. 2010). The design aimed to avoid mixing of gasification products with those from the combustion in order to obtain high purity hydrogen. In the combustor heat generated, due to char combustion raises the bed material temperature. After leaving combustor, it is captured by a cyclone and then recirculated into the BFBG to supply the required heat for char gasification endothermic reactions using steam as an agent gas (Siedlecki et al. 2011) (Brown 2006)(Christopher Higman 2003).
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Figure 2. 3 Twin (dual) fluidized bed gasifier (Basu 2010)
Characteristics of the circulating fluidised bed gasifier:
If height of the bed is significantly high, then long and controllable residence time of particles can be achieved. It can be operated at pressurized conditions. Also at higher velocities, typically 2-12 m/sec leading to higher velocities of the recirculation and violent gas-solid contact and mixing. This will give high heat and mass transfers and reaction rates which causing higher overall carbon conversion. This is suitable for large-scale systems and has very good large-scale-up potential. Its ability and flexibility to gasifying different types and particle sizes of feedstocks with different compositions and moisture content, especially biomass and wastes, of which the size, shape, and fluidizing characteristics, are harder to control than coal. The energy throughput per unit cross-sectional area of gasifier is higher than for BFBG.
Drawbacks of circulating fluidising bed:
The reactor height significantly increases their cost. The process control mechanism is more complex in comparison to its BFB counterpart. As with BFB, because the ash content and temperature limitation, bed agglomeration is a possibility. As a result of long circulation loop, gradients of temperature occur in the solid flow axis direction.
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Tar conversion is still not high, but it is little higher than BFBG gasifier. (Sadaka 2010) (Tzeng 2007) (Siedlecki et al. 2011)(Klein 2002)(Ciferno and Marano 2002) (Yang 2003).