Chemical Looping with Oxygen Uncoupling (CLOU) Studies at the University of Utah

JoAnn S. Lighty, Professor and Chair (jlighty@utah.edu) Asad H. Sahir, Ph.D. Candidate

Kevin Whitty, Associate Professor Chris Clayton, Ph.D. Candidate

Department of Chemical Engineering, Institute for Clean and Secure Energy, University of Utah,

50 S. Central Campus Drive, Room 3290 Salt Lake City, UT 841129203 Tel: (801) 5816915, Fax: (801) 5859291

ABSTRACT:

Chemical-looping with oxygen uncoupling (CLOU) is one of the emergent fuel combustion technologies being currently investigated which has the potential to assist with CO2 capture from coal-fired power plants. CLOU involves the combustion of fuel in the presence of gaseous-phase oxygen released from the decomposition of an oxygen carrier (OC) metal oxide (e.g. CuO). Compared to Chemical-looping Combustion (CLC), the CLOU process has the promise of reducing the fuel reactor volume and the OC inventory.. The CLC process requires slower pre-gasification reaction of the solid fuel into synthesis gas, which is eventually oxidized by the circulating oxygen carrier.

The presentation discusses components of the program at the University of Utah including laboratory-scale fluidized bed experiments, process modelling, and construction of a new 100-200 kW process development unit (PDU). The goal of the laboratory-scale experiments is to derive kinetics for the reduction and oxidation of the OCs. The process model is being used to explore material and energy balance scenarios. These scenarios are looking at the amount of OC circulated and, given the kinetics, OC inventories needed. The process model also shows potential heat recovery. Finally, the PDU design considerations are discussed and updates on the construction given.

INTRODUCTION:

Coal-fired power plants contribute to significant CO2 emissions, which has encouraged investigation of processes that can capture CO2 with reduced energy penalty. Lab-scale and pilot scale chemical-looping combustion (CLC) studies have demonstrated the possibility of facilitating carbon capture [1,2]. A chemical-looping combustion system typically involves two interconnected fluidized-bed reactors with a metal oxide circulating between them. One of the reactors serves as a fuel reactor, in which the fuel is combusted with the help of oxygen supplied by the circulating metal oxide. After being reduced, the metal oxide is regenerated by reaction with atmospheric oxygen in the air reactor. Chemical-looping combustion (CLC) for solid fuels involves the following reactions in the fuel reactor: drying (a rapid reaction), devolatilization (occurring within a second), gasification (on a time scale of minutes) and the reduction of oxygen carrier by the gases produced by devolatilization and gasification of coal [3]. The slow gasification reactions limit the rate of fuel conversion, resulting in comparatively large reactors.

Researchers at Chalmers University of Technology in Sweden recognized the limitation of using CLC technology for processing coal, and several years ago began developing a new generation of oxygen carriers (OCs). Specifically, they focused on carriers that could spontaneously release the oxygen in the fuel reactor as gaseous O2. Through thermodynamic analysis, three metal oxides were identified which have a suitable oxygen partial pressure at fuel reactor

temperatures: CuO, Mn2O3 and Co3O4 [4]. These materials will release, or uncouple, oxygen under fuel reactor conditions:

4 CuO 2 Cu2O + O2(g) (1)

6 Mn2O3 4 Mn3O4 + O2(g) (2)

2 Co3O4 6 CoO + O2(g) (3)

Because it is in gaseous form, the oxygen released by these reactions can react with solid coal (char) in the same manner as in a conventional combustion system; therefore, it is not necessary to have the solid carbon reaction with CO2 and/or water vapor to form a gaseous fuel to react with a solid metal oxide. This concept, chemical-looping with oxygen uncoupling, or CLOU, is schematically illustrated in Figure 1 for the CuO/Cu2O system. Solid coal can be introduced directly into the fuel reactor.

Thermodynamic analysis indicates that the partial pressure of oxygen over the uncoupling carriers is sufficient for them to release oxygen at conditions of the fuel reactor (low O2 partial pressure, temperature 900-1100C), and for them to re-oxidize at conditions in the air reactor (relatively high O2 partial pressure, temperature 800-950C).

Figure 1. Schematic of the CLOU process for using a copper oxide-based oxygen carrier and coal as the fuel. Oxygen (O2) is spontaneously released (uncoupled) in the fuel reactor and subsequently reacts with carbon and hydrogen in the coal. The fuel reactor is fluidized by steam and/or recirculated CO2.

Of the proposed metal oxide systems above, the copper oxides are most promising as the overall reactions in the fuel reactor are exothermic and the rates of reaction are faster for the uncoupling reaction. Further, the exothermic nature of the reaction adds flexibility with respect to recirculation rates of the solid material between the air and fuel reactor since it is not necessary to provide sensible energy from the air reactor to the fuel reactor. For example, compared to a CLC process, a 45-60 fold increase in reaction rate, for a petcoke with a CuO-based carrier, was found using CLOU at temperatures between 950-985C [4,5].

UNIVERISTY OF UTAH WORK

The University of Utah has been researching the CLOU process since 2007 with support from the U.S. Department of Energy. Efforts have focused on the CuO/Cu2O system (Figure 2), since the copper-based carriers offer the best reactivity and economics. In this case the reactions are:

4CuO 2 Cu2O + O2 and C + O2 CO2 in the fuel reactor (4)

2 Cu2O + O2 4 CuO in the air reactor (5)

To date, we have focused on OC kinetics, lab-scale studies, and process modeling.

The work on OC kinetics has been completed in both TGA studies and lab-scale fluidized bed reactors. The kinetics of both uncoupling oxygen (reduction) and oxidation are of importance. To avoid agglomeration of the OC, copper oxide must be on a support material. A variety of supports and copper concentrations have been studied. Recently a copper-oxide on silicon carbide has been studied and has been shown to be stable. Materials from Chalmers have also been investigated.

Oxygen Carrier Kinetics and Lab-Scale Studies

Experimental data from these studies, and lab-scale fluidized bed, work have resulted in the determination of kinetic parameters. This is shown in Figure 2.

Figure 2. Kinetics for a variety of materials in TGA, fluidized bed, and fixed bed.

Other values have ranged from 280-300 kJ/mol. These data are important to determine the times required for reduction, carbon oxidation, and copper oxide oxidation. Figure 4 illustrates the case for a PRB coal. As seen in this figure, for the fuel reactor, the limiting reaction is the reduction of the copper oxide. Interestingly, the rate for the copper oxide oxidation decreases with increasing temperature, resulting in an increase in residence time. This is currently under further investigation but it has been shown that diffusion versus kinetics might be controlling at these higher temperatures.

-110 kJ/mol

Figure 3. Residence times for 40% CuO/60% ZrO2 particles in a batch fluidized bed

[Size= 125-180 m]. Times are based on 60% and 64% conversion for reduction and oxidation, respectively.

The results above determine the amount of material needed to recycle between the two reactors and the amount in the reactor at any given time. It was desired to keep the residence times below 100 seconds, which determine the temperatures of the reactors. Table 1 lists the parameters that were used in an ASPEN Plus model of the process for a feed rate of 100 kg/hr of a PRB coal.

Process Modeling

Table 1. Parameters in the ASPEN Plus Simulation

Air Flow Rate 895 kg/h Temperature of Fuel Reactor investigated 950C Temperature of Air Reactor investigated 935C Amount of Cu circulating in the system

(represents 40% CuO on ZrO2)

3262 kg/h

Amount of ZrO2 circulating in the system 4579 kg/h

Fraction of flue gas stream recycled for fluidization for fuel reactor particles

0.69

Particle density 2140 kg/m3

Design superficial velocity for fuel reactor calculated based on particle properties

2.1 m/s

Design superficial velocity for air reactor calculated based on particle properties

2.4 m/s

The ASPEN Plus process model results in various heat resources. These are individually listed in Table 2. The entire process, assuming no heat losses, releases approximately 510 kW of

energy. The process model can now be used to investigate different kinetics, conversion rates, and scenarios.

Table 2. Results from ASPEN Plus Modeling, Energy Sources/Sinks

ENERGY Fuel reactor 109 kW Air reactor 394 kW Cool air reactor exhaust from 935C to 150C 174 kW Cool flue gas from 950C to 150C 272 kW Cool OC for air reactor operation 22 kW Reheat OC for fuel reactor operation -23 kW Reheat recycle gas from 150C to 950C -185 kW Heat air from 25C to 935C -241 kW PUMPING/COMPRESSION Fluidize particles in fuel reactor -3 kW Fluidize particles in air reactor -6 kW

A final step is the construction of a 100-200 kW process development unit (PDU). The PDU is comprised of two circulating fluidized beds. This unit can further develop the material and energy balances and begin to investigate operating conditions such as attrition and fluid dynamics.

Future Work

REFERENCES:

[1] Adanez J. et al. (2012), Progress in Chemical-Looping Combustion and Reforming technologies, Progress in Energy and Combustion Science 38(2), pp. 215-282.

[2] Fan L.S. et al. (2012), Chemical looping processes for CO2 capture and carbonaceous fuel conversion prospect and opportunity, Energy Environ. Sci. 5, pp. 7254-7280.

[3] Linderholm C. et al. (2012), Chemical-looping combustion of solid fuels Operation in a 10kWth unit with two fuels, above-bed and in-bed fuel feed and two oxygen carriers, manganese ore and ilmenite, Fuel, DOI:10.1016/j.fuel.2012.05.010.

[4] Mattison, T., Lynfelt, A., Leion, H., (2009), Chemical-Looping with Oxygen Uncoupling Using CuO/ZeO2 with Petroleum Coke, Fuel 88: 683-690.

[5] Abad, A., et al., Demonstration of Chemical Looping with Oxygen Uncoupling Process in a 1.5 kWth Continuously Operating Unit using a Cu-Based Oxygen Carrier, International Journal of Greenhouse Gas Control, 6:189-200 (2012).