(620c) Design and Performance of a Novel Reverse-Flow Fixed-Bed Reactor for Chemical-Looping Combustion

Design
and performance of a novel reverse-flow fixed-bed reactor for chemical-looping
combustion

Lu Han,
Zhiquan Zhou, George M. Bollas

Abstract

                The
reactor technologies for chemical-looping combustion (CLC) are categorized as circulating
fluidized bed reactors, rotating reactors, and alternating flow reactors (Fig.
1). The most widely used reactor type is based on the interconnected
fluidized-bed design, which has been successfully demonstrated in pilot plants
built in Chalmers University of Technology (Sweden), Institute of Carboquimica
(Spain), Korean Institute of Energy Research, and Vienna University of
Technology to name a few. Fluidization promotes excellent contact between the
gas and solids, which enables the process to achieve both high fuel conversion
and CO₂ selectivity, in addition to a uniform temperature distribution,
small pressure drop, and continuous operation. However, circulation of the
solids requires an energy-intensive cyclone system and loop seals to prevent air/fuel
slip. This harsh process makes particle attrition inevitable and creates fines
that can be entrained out of the fluidization vessel. These issues can severely
impact the process economics and safety of the CLC process, for example, if Ni
is used as the oxygen carrier. Alternatively, CLC can be implemented in a
transiently operated fixed-bed reactor where the gas flows alternate between
reducing and oxidizing environments. This design eliminates the need for
gas-solid separation and relieves some of the requirements of a potential
oxygen carrier. Fixed-bed reactors are in general easier to operate, scale-up
and pressurize and are flexible to handle a range of particle sizes. The design
is also more compact than a fluidized-bed reactor, which allows for better
utilization of the oxygen carrier, lower capital cost, and smaller process
footprint. However, the performance of the fixed-bed process exhibits lower CO₂
selectivity, greater carbon deposition, and much higher temperature
fluctuations compared to an equivalently designed fluidized bed reactor. Other
proposed reactor options for CLC include the moving bed reactor, rotating
reactor, and rotary reactor. These designs share in the technical difficulties
that arise from putting solids in motion, specifically related to gas leakage
and product gas mixing with air.

Figure 1. Reactor options
for CLC: (a) circulating fluidized-bed; (b) rotating reactor; and (c)
alternating flow over a fixed-bed.

In
this paper, a novel reverse-flow reactor is proposed for CLC of gaseous fuels, that
integrates gas-solid separation, reaction, and heat transfer in a single
fixed-bed unit. The periodic reversal of the fuel gas during the oxygen carrier
reduction cycle promotes more uniform contact between the fuel and the unconverted
solids and better utilization of the conductive heat of the packed bed, while
maintaining the intrinsic advantages of a stationary bed. Reverse-flow reactors
had been traditionally employed as a method to improve the heat integration for
many industrially relevant reactions, including SO₂ oxidation, total
oxidation of hydrocarbons in off-gases, and NO_x reduction.  When
applied to CLC of gaseous fuels, the reverse-flow reactor achieves higher CO₂
selectivity for all practical oxygen carrier conversions compared to a
traditional fixed-bed process, at no expense of the fuel conversion. Frequently
reversing the direction of the flow provides a uniform conversion profile
inside the bed and yields an even higher CO₂ selectivity. Observing
the CO₂ capture efficiency of the process (defined as time-integral
of the CO₂ product distribution over the total flow of fuel fed),
the traditional one-directional reactor quickly drops below 90% at an overall
bed conversion of 35% while the reverse-flow reactor can be continually
operated till a conversion of 60% to meet the same efficiency. These results
are comparable to a circulating fluidized bed unit, which achieves about 94% CO₂
gas yield for a solid conversion of 60%. Additional benefits to the heat transfer
characteristics are also realized with the reverse-flow design, where at least
a 50℃
reduction in temperature drop is exhibited. The results of this work
demonstrate that periodic reversal of the gas flow during CLC reduction leads
to significant improvements in reactor performance, in terms of CO₂
selectivity, CO₂ capture efficiency, and bed temperature
fluctuations. This fixed-bed design avoids the issue of attrition and gas-solid
separation commonly found in circulating fluidized-bed units. Furthermore, the
simplicity of the process makes it easily applicable to existing bench-scale
units without considerably complicated equipment, for which scale-up does not
entail significant challenges. Further design considerations and areas for
optimization will be discussed.

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