262399 Optimal Experimental Design of Chemical-Looping Combustion of Syngas in Fixed Bed Reactors

Monday, October 29, 2012
Hall B (Convention Center )
Lu Han1, Ari Fischer2 and George M. Bollas2, (1)Department of Chemical & Biomolecular Engineering, University of Connecticut, Storrs, CT, (2)Department of Chemical, Materials & Biomolecular Engineering, University of Connecticut, Storrs, CT

Optimal experimental design of chemical-looping combustion of syngas in fixed bed reactors

Lu Han, Ari Fischer, George M. Bollas

Department of Chemical, Materials & Biomolecular Engineering, University of Connecticut, Storrs, CT

Abstract

The objective of this work is to evaluate and explore current processes for chemical-looping combustion of synthesis gas and propose optimal experimental design (OED) algorithms to assist in reactor design. Chemical-looping combustion (CLC) is a novel technology for efficient and low cost CO2 separation. The basic concept of the process involves two reactors, a Reducer and an Oxidizer with an oxygen carrier (a metal oxide) circulating between the two (Figure 1). In the Reducer, synthesis gas (H2, CO) reacts with metal oxide, according to the Reactions 1 and 2. The relevant catalytic reactions are methanation (Reaction 3), water gas shift (Reaction 4), carbon gasification by CO2 (Reaction 5) or steam (Reaction 6), and dry reforming (Reaction 7). The reduced metal oxide, MyOx-1, is transferred to the Oxidizer, where it is oxidized back to MyOx with air, Reaction 8. The metal oxide is then returned to the Reducer and begins a new cycle of reactions. The flue gas leaving the Oxidizer contains N2 and unreacted O2. The exit gases from the Reducer contain CO2 and H2O, which are inherently separated from the rest of the flue gas. After condensation of the water almost pure CO2 is obtained, without significant energy penalties for separation.

Figure 1: Chemical-looping combustion.

Reducer:

MyOx + H2 MyOx-1 + H2O

(1)

 

MyOx + CO MyOx-1 + CO2

(2)

 

CO + 3H2 CH4 + H2O

(3)

 

CO + H2O CO2 + H2

(4)

 

C + CO2 2CO

(5)

 

C + H2O CO + H2

(6)

 

CH4 + CO2 2CO + 2H2

(7)

Oxidizer:

MyOx-1 + O2 MyOx

(8)

 

Application of CLC on the oxidation of various fuels is being researched in studies of the effect of experimental parameters including pressure, temperature, and oxygen carrier reactivity, selectivity and stability. CLC with coal-derived synthesis gas is of interest due to the high CO2 emissions from coal combustion. The effect of reactor pressure in syngas CLC is of interest due to the elevated pressure at which coal is typically gasified and the planned pressure for CO2 sequestration. Therefore, the effect of pressure on reaction kinetics is a concern to the development of CLC technology. A literature review of this effect on CLC reactions in fixed bed reactors using natural gas, solid coal, and coal derived syngas reveals inconsistencies in experimental observations. Positive effects of increased pressure are noted by Xiao et al. [1], who observed an increase in reactor efficiency with pressures up to 0.5MPa and Zhang et al. [2] who observed intensified reactions of coal gasification products with the oxygen carrier at 0.5MPa. On the other hand, negative effects are reported by Jin et al. [3] who observed lower reduction rates at elevated pressures due to pressure restrictions on the reforming reactions and Tian et al. [4] who noted reduced reduction rates due to slower bulk diffusion. Utilizing previous experimental studies and current modeling of CLC reaction kinetics at the University of Connecticut, we are designing and constructing a bench-scale fixed bed reactor for oxidation of syngas with metal oxides. This will enable a study for measuring metal oxidation and reduction kinetics at high pressures, furthering the research of pressure effects on CLC technology. Results will be translated into reaction mechanisms and kinetic constants to be used in modeling and scale-up studies. Overall, the effects of pressure on CLC reactors are undetermined and require further investigation to understand the reaction kinetics. This presentation will focus on the peculiarities of high-pressure chemical-looping combustion and showcase a novel method for optimal design of a bench-scale experimental apparatus to be used for the identification of the effect of operating conditions (e.g., reactor pressure) on the accuracy and consistency of experimental measurements.

References

1. Rui Xiao, Qilei Song, Shuai Zhang, Wenguang Zheng, Yichao Yang Energy & Fuels 2010 24 (2), 1449-1463

2. Shuai Zhang, Chiranjib Saha, Yichao Yang, Sankar Bhattacharya, Rui Xiao Energy & Fuels 2011 25 (10), 4357-4366

3. Hongguang Jin and Masaru Ishida Industrial & Engineering Chemistry Research 2002 41 (16), 4004-4007

4. Hanjing Tian, Karuna Chaudhari, Thomas Simonyi, James Poston, Tengfei Liu, Tom Sanders, Gtz Veser, and Ranjani Siriwardane Energy & Fuels 2008 22 (6), 3744-3755

 


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