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Solution

The conservation equations (i.e., mass, momentum, and species) and constitutive equation presented in Tables 2.1 and 2.2, for both phases, were solved on a threedimensional (3D) Cartesian domain. It is assumed that the process is isothermal and particle size is constant and uniform. The Syamlal et al. (1993) drag model was used as the drag force between phases (Abbasi et al. 2015). The Syamlal-O’Brien drag expression contains adjustable parameters that can be used to tune the drag to match the theoretical minimum fluidization velocity to experimentally observed values. It

Table 5.1 Summary of simulation cases

Case

Inlet gas velocity (m/s)

Solid circulation rate (g/s)

Baseline

0.15

44

Case 1

0.15

220

Case 2

0.15

440

Case 3

0.1125

220

Case 4

0.0975

220

Fig. 5.3 NETL carbon capture unit (C2U) experimental setup (Source: Shadle et al. 2010)

should be noted that the original drag correlation was derived for homogeneous gas-solid flows and the adjustment reduces the drag to partially account for the heterogeneous gas-solid structure in the fluidized bed.

A summary of all of the simulations performed at different solid circulation rates and gas inlet velocities is presented in Table 5.1. These variables were varied within a range that ensures the fast fluidization regime in the riser. To study the effect of the solid circulation rate, the inlet solid mass flow rate was increased by factors of 5 and 10, while the inlet gas velocity was kept constant at the baseline value. Furthermore, at the inlet solid mass flow rate of 220 g/s (Case 1), the inlet gas velocity was decreased by 25 % and 35 % (Case 3 and Case 4, respectively) to investigate the effect of gas residence time on CO2 removal. In these simulations, a 50/50 (mole fraction) mixture of CO2 and N2 was used as the inlet gas.

Fig. 5.4 Schematic of the reacting particle (This figure was originally published in Powder Technol 286, 2015 and has been reused with permission)

 
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