LabNotes - January 2014

Chemical Looping 101: The Basics

NETL is investigating advanced combustion technologies that have the potential to significantly reduce CO2emissions from fossil fuel applications. Chemical Looping (CL) is an indirect combustion technology in which the air and fuel do not mix and is one of several combustion technologies that may be a viable option for reducing greenhouse gas emissions.

NETL/URS scientist Steve Carpenter in the Chemical Looping laboratory.
NETL/URS scientist Steve Carpenter in the Chemical Looping laboratory.

In conventional fossil fuel combustion, fuel and air are directly mixed in a combustion reaction, yielding water vapor, CO2, N2, and excess O2 as combustion products. When combustion temperatures exceed 1500°C, nitrogen oxides (NOx) may also be formed; NOx are pollutants contributing to acid rain and are regulated in most of the US. Water vapor can be condensed to liquid water and removed from the combustion gas stream, but the other products require significant amounts of energy for separation.

In CL technology, a “carrier” material transports oxygen from air to the fuel. Combustion products are water vapor and CO2; a pure stream of CO2 is produced by condensing the water vapor, significantly reducing the cost of recovering carbon dioxide from the combustion of fossil fuels. This technology was originally proposed in 1950 as a method to produce high purity CO2 from a hydrocarbon fuel source, but fifty years later, it has attracted international interest as an advanced low-GHG emission combustion approach.

A CL process typically operates in a temperature range of 700 – 1000°C, significantly lower than conventional combustion. Temperatures are high enough to generate steam for power production, but low enough to virtually eliminate production of NOx pollutants. For a steam turbine power plant, the lower operating temperature could eliminate the need for after-treatment NOx control devices.

There are two separate reactions that must occur in chemical looping to convert the chemical energy in the fuel to heat. In equation 1a below, in which MeO represents a metal-oxide carrier material, the fuel (CnHm) is converted to CO2 and water. In Equation 1b, the metal-oxide carrier is regenerated.

(1a) CnHm + 2(n+m/4)MeO→nCO2 + m/2 H2O + 2(n+m/4) Me
(1b) 2(n+m/4) Me + (n+m/4)(O2 + 3.76N2)→2(n+m/4)MeO + 3.76(n+m/4)N2

Theoretically, the total amount of heat generated from a CL process (Equations 1a and 1b) is the same as the amount of heat generated from a conventional combustion process.

Currently, chemical looping research at NETL is a 4-year effort funded by the American Recovery and Reinvestment Act (ARRA). Researchers at NETL are studying the hydrodynamics of chemical looping systems, and the physical and chemical behaviors of carrier materials. Research facilities include a 50kWth Chemical Looping Reactor (CLR) designed to circulate up to 1000 lbs/hr of carrier material at temperatures up to 1000°C, and a cold-flow CLR replica built of clear polycarbonate pipe that allows researchers to visually observe flow patterns.

Steve Carpenter, a chemical engineer working for URS and NETL, says, “NETL is the first to initiate a coordinated effort of CL research that incorporates experimental testing, CFD simulation, and techno-economic studies to accelerate technology development. ”

Contacts: Doug Straub, 304-285-5444 and Steve Carpenter, 304-285-1312

NETL’s Chemical Looping Research Facilities

NETL’s chemical looping research is a coordinated effort across the NETL-Regional University Alliance that combines experimentation with CFD simulation and techno-economic studies to accelerate technology development. The experimental arm of NETL’s research effort provides the key data and validating information needed to assess chemical looping technology and to build and compare simulations.

NETL’s Chemical Looping Reactor research facility.
NETL’s Chemical Looping Reactor research facility.

The flagship of the NETL chemical looping research facility is a 20-foot tall, 50kWth Chemical Looping Reactor (CLR) designed to circulate up to 1000 lbs/hr of carrier material at temperatures up to 1000°C (1830°F). In this facility and in a cold-flow replica of the CLR, researchers are developing solutions to technical problems to accelerate successful development and future deployment of CL technology for industrial steam boiler applications, such as the processing and production of crude oil from oil sand formations.

Both reactors comprise a fuel section and an air section. In the CLR fuel reactor section, the fossil fuel mixes with the metal-oxide carrier and combusts, and in the air reactor section, the used carrier (now deficient in oxygen) is regenerated by mixing with air. Repeatedly reduced and oxidized, the carrier is continuously circulated through the CLR; fuel and air are introduced to the two sections, and combustion products are removed. The cold-flow replica, which is constructed with clear polycarbonate piping sections, circulates solid carrier materials at the same scale as the CLR, allowing visual observations of the flow patterns.

As might be expected in such a complex system, understanding processes and conducting successful operations offer challenges. Oxygen carriers must have sufficient reactivity to completely oxidize the fuel, while being durable enough to survive many cycles of reduction and oxidation. Trade-offs must be made between carrier reactivity and solids circulation rate to maintain temperatures throughout the process. Cross-mixing of combustion products into the air reactor and nitrogen into the fuel reactor occurs, complicating reactions and separations.

Operating the reactors in the NETL facility is helping researchers to understand and address the many challenges associated with this high-temperature, multiphase flow system.

In addition to the CLR, other workhorse analytical techniques and smaller experiments are used to address key issues, such as oxygen carrier reactivity and durability. Thermogravimetric analysis (TGA) is done to characterize carrier properties and to provide important input parameters for numerical reaction models. Attrition studies have evaluated the physical endurance of the carriers, and fixed-bed reaction tests are used to evaluate the reactivity of carrier materials independent of forced particle movement. Other tests are used to evaluate carrier reactivity under fluidized conditions without the complication of solids circulation. All of these experimental activities are coordinated with a computational modeling effort. Many of these smaller experimental rigs are modeled as an intermediate calibration point prior to modeling the 50kWth CLR facility.

Computational Fluid Dynamic (CFD) models are also used to address operational issues that are common to many different chemical looping reactor configurations. For example, CFD models have been used to design a novel, compact particle separation device. This device virtually eliminated carrier loss from a conventional cyclone when installed in the CLR test facility. This device is roughly one-half the volume of a conventional cyclone and has other potential advantages that are attractive for a chemical looping application. See below for more on NETL’s chemical looping modeling and simulation research.

Contact: Doug Straub, 304-285-5444 and Steve Carpenter, 304-285-1312

Oxygen Carriers in Chemical Looping Combustion

Tom Simonyi tests oxygen carriers by TGA (thermogravimetric analysis). Left to right:  Dr. Hanjing Tian, Dr. Duane Miller, and Dr. Ranjani Siriwardane are behind him.
Tom Simonyi tests oxygen carriers by TGA (thermogravimetric analysis). Left to right: 
Dr. Hanjing Tian, Dr. Duane Miller, and Dr. Ranjani Siriwardane are behind him.

Development of efficient oxygen carriers is key to the success of chemical looping combustion (CLC). The oxygen carrier, usually a metal oxide, transports oxygen to the fuel. An effective carrier will react readily with the fuel, oxidizing methane to CO2 and H2O, and will fully reoxidize upon contact with oxygen. Reactivity and durability of oxygen carrier materials are significant issues being addressed at NETL and by many other research organizations around the world. Greater rates of reaction and better utilization of the oxygen in the carrier add up to more efficient combustion. Physical stability of the oxygen carrier is important, because reusing oxygen carrier materials reduces the costs of operating the CLC system. A material that breaks down in the process would need to be replaced frequently, and one that melts or agglomerates would quickly lose its effectiveness and even clog up the system.

Oxygen carriers tested at NETL for chemical looping: 1) hematite, 2) MgO-promoted hematite, and 3) CuO/Fe<sub>2</sub>O<sub>3</sub>.
Oxygen carriers tested at NETL for chemical looping: 1) hematite, 2) MgO-promoted hematite, and 3) CuO/Fe2O3.

NETL has tested many different materials, focusing on development of enhanced metal oxide carriers with greater oxygen transfer capacities, faster reaction kinetics, and improved stability during many cycles of oxidation and reduction. Iron-oxide based oxygen carriers, in particular, showed promise, particularly when enhanced by additions of other substances. The reaction rates for iron oxide (Fe2O3) by itself are too slow for use in a commercial process, but experiments showed that adding cerium oxide (CeO2), magnesium oxide (MgO), or copper oxide (CuO) has a synergistic effect, with the combination of iron oxide and one of these ‘promoters’ tripling the carrier’s ability to transfer oxygen and increasing reaction rates up to five times.

Iron oxide is also a common substance in nature, found in the rusty-looking brownish mineral, hematite. Hematite, known as a ‘commodity carrier,’ was also evaluated as an oxygen carrier material and it was found that its performance was also enhanced by adding these promoters.

The research at NETL is striving to meet other criteria for oxygen carriers, including low cost and good availability, in addition to efficiency and effectiveness. Natural iron minerals like hematite are advantageous because of their low cost and good availability. Patent applications have been filed on oxygen carriers invented by NETL researchers, an MgO-promoted hematite carrier and a synthetic CuO-Fe2O3-alumina carrier, as well as on the process for addition of metal oxides to improve carrier performance.

Three carriers are currently slated for testing in the NETL pilot-scale 50 kWth chemical looping combustion reactor (CLR). NETL researchers with chemical engineering expertise have prepared small batches of the carriers for developmental testing, but testing in the CLR will require much more material, hundreds of pounds. NETL is partnering with NexTech Materials to prepare sufficient quantities of the carriers chosen for testing.

Contact: Ranjani Siriwardane, 304-285-4513

Chemical Looping Modeling and Simulation Research at NETL

The primary goal of the modeling and simulation portion of the chemical looping project is to develop and apply numerical models to predict chemical looping system performance. This is aligned with the overall focus of the NETL computational modeling area which is the advancement of simulation and modeling technologies applied for fossil energy systems.

Reacting multiphase computational fluid dynamics (CFD) is the primary tool used for this activity, though some work is done in developing lower fidelity models. Four computational fluid dynamic codes (CFD) were used to perform simulations, two open source: MFIX and OpenFOAM and two commercial: Barracuda and Fluent. MFIX and Barracuda are specialized tools for reacting gas-solid whereas OpenFOAM and Fluent are more general purpose tools and can be used for a wider application range. The activity is strongly linked with the chemical looping experimental program. Sub-models for oxygen-carrier reactivity are calibrated from onsite experiments and the larger scale chemical looping systems experiments are used for validation. It is anticipated that tools validated as part of this activity would be used to assess chemical looping as a general technology and eventually to design reactors for chemical looping systems.

(1) NETL cold flow chemical looping reactor simulations show the evolution of the solid volume fraction. Red areas contain 64 vol% solids, blue 10 vol% solids, and clear < 0.01 vol% solids.
NETL cold flow chemical looping reactor simulations show the evolution of the solid volume fraction. Red areas contain 64 vol% solids, blue 10 vol% solids, and clear < 0.01 vol% solids.

Modeling and simulation activities can be loosely classified into three categories: (1) Model Development, (2) Validation & Calibration, and (3) Application. Model Development includes formulation of mathematical models to describe a physical process and implementing these mathematical models to a computer code and testing (verifying) the consistency of the implementation. For this project we used computer codes which were written and verified by other projects or organizations and thus our model implementation and verification was focused on sub-models to describe the carrier reduction and oxidation. Decisions to implement and test sub-models for other processes such as gas-solid drag and particle momentum are made by weighing the implementation and the possible impact on prediction.Validation is the process of calculating the accuracy of a computational model using experimental data or a more accurate model of the physical process of interest. If accuracy is insufficient for the intended use then additional model development would be pursued. If the accuracy is sufficient, then the validation provides an error estimate, along with the prediction when applying the computation model. Validation has been the largest modeling and simulation activity thus far. Researchers currently have performed simulations of most of the NETL chemical looping related systems. For example, the figure shows the evolution of the volume fraction of the cold flow chemical loop reactor. Application activities include both performing CFD simulations to directly support the associated experimental activity and chemical looping technology exploration.

The simulations group recently helped design an additional cyclone which was subsequently installed on the chemical looping reactor. In the original system, the experimentally observed cyclone efficiency was lower (~95%) than estimated during the design (99.99%), resulting in a significant mass loss of particles during operating requiring shutdown every 15 minutes to empty filters.

Starting from a basic configuration several CFD simulations were performed adjusting different geometrical parameters to improve the collection efficiency and reduce the pressure drop. Based on the CFD analysis, the new cyclone was fabricated and installed downstream of the existing cyclone. This configuration significantly reduced the particle accumulation in the filters allowing the system to run continuously. A second study to support the chemical looping system has begun to address concerns that a high temperature region near the fuel injector is causing agglomeration of solid carriers during system heat up. The goal is to suggest operational changes to composition and location of gas injector to more evenly distribute the heat from the heating flame.

On the technology exploration side, NETL researchers have identified three areas of interest: (1) innovative compact natural gas reactor, (2) sub-pilot scale solid fuel systems, and (3) industrial-plant scale natural gas systems. All represent extrapolation outside the validation domain and represent a significant modeling challenge and thus as needed we will draw on previous studies of similar systems. The results of these simulations should provide an illustration of possible future fossil energy systems and a demonstration of the impact of modeling and simulation for chemical looping.

Contact: Dave Huckaby, 304-285-5457