Modeling Chemical-Looping Combustion in Bubbling Fluidized Bed Reactors

Chemical-looping combustion (CLC) is typically realized as a dual fluidized bed technology. Most current CLC reactor designs for gaseous fuels processing comprise riser and bubbling fluidized beds [1], two bubbling fluidized beds [2], or dual circulating fluidized beds [3]. Because of the higher space times required for complete metal oxide reduction (or fuel oxidation), when compared with the metal oxidation reactions, fuel reactor designs have focused on bubbling bed technologies. The objective of this work is to analyze existing bubbling bed reactors for chemical-looping reduction through dynamic modeling. Experimental data from published bubbling bed chemical-looping reactors are used for model development and validation. The reaction scheme and kinetics used in this study have been developed and validated against published fixed bed chemical-looping units operating with CH₄ and NiO/support (oxygen carrier) [4].  The proposed reaction scheme includes heterogeneous reactions of NiO reduction by CH₄, H₂ and CO, gas phase reactions including reforming reactions (steam, dry and overall steam), water gas shift reaction, methane decomposition and carbon gasification by CO₂ and H₂O. The two-phase (emulsion and bubble) flow model is used to predict bubbling bed hydrodynamics [5,6]. It considers gas bubbles flowing through a dense emulsion phase at a relative velocity given by minimum fluidization with gas percolating through the bed of solids (shown in Figure 1). Using this model the gas profiles predicted for the data published by Iliuta et al. [7] are shown in Figure 2. Gas composition profiles are in good agreement with experiment data.

Figure 1: Bubbling fluidized bed reactor physical model	Figure 2: Chemical-looping selectivity using Ni/NiO and CH4 in bubbling fluidized bed at 645°C

A summary of the bubbling bed experimental data analyzed and modeled in this work, including all the relevant published data for CLC with CH₄ and NiO as the oxygen carrier, is presented in Table 1.

Table 1:  CLC bubbling bed units

Author	Iliuta et al. [7]	Hoteit et al. [8]	Jerndal et al. [9]	Gayán et al. [10]	Ryu et al. [11]
T(°C)	623 – 810	800 – 900	950	950	900
NiO/Support	15% NiO/Al2O3	60% NiO/NiAl2O4	40% NiO/αAl2O3	18% NiO/ αAl2O3 28% NiO/ γAl2O3	60% NiO/bentonite 70% NiO/NiAl2O4
OC load (kg)	0.3	2.5 – 4	0.015	0.3	2.3
Particle size (µm)	140	171	125 – 180	100 – 300	106 – 212
Specific surface area (m2/g)	102	7	0.91	7, 77.5	-
CH4 composition	10%, 50% in Ar	100%	100%	25% in N2	45% syngas in N2 H2: CO2: CO = 30:10:60
Gas flow rate (m3/s)	2E-05	2E-05 – 2E-04	7.5E-06	2E-04	3.33E-05
I.D. (mm)	46	96	22	54	50
Bed height(m)	0.23	1	0.0179	~0.1	0.4
Space time (s gNiO0 /gCH4)	5297	9747	1254	1643	-
Bulk density (kg/m3)	785	2200	2400	2500, 1800	1172

In this presentation, implementation of this model in different bubbling fluidized bed units of the various CLC laboratories will be shown. A comparison of the predicted results with the experimental results for the different reactors will be illustrated and discussed. Dynamic parameter estimation is performed to re-optimize the kinetics of Ni-based oxygen carriers for chemical-looping reduction in bubbling fluidized beds. Best fit kinetic parameters between different bubbling bed units will be presented, along with the consistency and discrepancy of the kinetic parameters estimated for their fixed bed reactor equivalents.

Acknowledgement: This material is based upon work supported by the National Science Foundation under Grant No. 1054718.

References:

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