Plasma gasifiers use a plasma gun to create an intense electric arc between two elec­trodes. The temperature of the arc can reach extremely high (> 13,000 °C). Biomass, on the other hand, is fed at lower temperatures (2,700-4,500 °C), but still sufficiently high to crack heavy hydrocarbons. Due to its high operating temperature, the plasma

Flow meter

Fig. 10.2 Sketch of a (a) conventional rotating fluidized bed and (b) novel rotating fluidized bed. (Reprinted from ref. [74], Copyright 2008, with permission from Elsevier) gasifier can crack harmful products, such as furan and dioxin, making it suitable for MSW and other types of waste products.

10.6.5 Rotating Fluidized Bed Gasifier

As previously discussed, heat and mass transfer play an important role in the gasi­fication process. As such, fluidized beds are advantageously characterized by high heat and mass transfer rates that will result in temperature homogeneity and the rapid mixing of particulate materials. Heat and mass transfer are maximized by increasing the gas fluidization velocity, but the gas velocity cannot be indefinitely increased. With increasing superficial gas velocity the solid hold up in the bed region decreases causing a short solid residence time and a low conversion or reactor volume increase. To overcome these limitations of conventional fluidized bed gasifiers, the concept of a rotating fluidized bed (RFB) has been introduced. RFB reactors were first patented by Horgan and Morrison in 1979 for a coal combustion application in a centrifu­gal fluidized bed [73]. RFB consists of a cylindrical gas distributor chamber rotating around its axis (shown in Fig. 10.2). The rotating motion of the cylinder is transferred to the particles via fraction and gas is injected inward through the gas distributor. The particles are fluidized uniformly under the action of two opposite forces: the radially inward drag force exerted by the injected gas and the radially outward centrifugal force. The minimum fluidization velocity increases with increasing the reactor rota­tion speeds (rising centrifugal force magnitude). The magnitude of the forces, which can be much higher than gravity, depends on the operating conditions: solid rotating velocity and gas injection velocity.

The rotating motion of RFBs may cause difficulties in design and operation, like severe vibrations of the reactor during operation. To overcome these difficulties, a new concept of a rotating fluidized bed has been proposed where the geometry of the reactor is fixed and the fluidizing gas is injected tangentially via multiple gas inlet slots at the fluidization chamber wall. As a result, the tangential drag force will induce the solid particles into a rotating motion as well as produce a radially outward centrifugal force [74]. The RFB reactor can operate at much higher gas velocities and solid hold-up compared to the fluidized bed. Due to high attrition the novel RFB reactor is suitable for processes where solid is a reactant, like biomass gasification [75].

As discussed above there are several options when choosing and designing a gasifier. The choice of one type of gasifier over another is, however, determined by the type of fuel, its size, the moisture content, the physical limitations of the reactor, its production capacity and the final use of the product gas since one type is not necessarily suitable for the full range of capacities. The different gasifier characteristics are summarized in Table 10.3.