Chemical looping combustion Basic principle

Chemical looping combustion (CLC) is an indirect combustion method. The fuel is not directly reacted with air but instead reacted with a solid oxygen carrier that transfers oxygen from the air to the fuel and then transports the chemical energy released by the fuel. CLC is divided into two reactions that take place in separate reactors as shown in Figure 2-12 which also details the chemical formulas that describe the process.

One reactor is a reduction unit (fuel reactor) in which the oxygen carrier, a metal oxide MyOx, oxidises the fuel and the released energy is stored in the reduced metal oxide, MyOx-1 (as shown in Figure 2-12 with nickel oxide as an example). The reduced metal oxide is then transported into the second reactor (termed the oxidation unit or air reactor) in which the reduced metal oxide is re-oxidised by air (Equation 2) and the energy stored in the reduced metal oxide is released in the reactor. The metal oxide is then again

transferred to the fuel reactor and the cycle repeated. The overall reaction in the two reactors is equal to the combustion of fuel with oxygen instead of with air (Equation 3).

Depending on the oxygen carrier, reaction (1) is often endothermic while reaction (2) is exothermic. The combined net heat of reaction is the same as that of the fuel combusting directly with air. A full conversion between MyOx

and MyOx-1 in reactors (1) and (2) may not be necessary because the

conversion rates of the oxygen carrier in the reactors may be more important for this type of CLC process.

Figure 2-12 The concept of chemical looping combustion (CLC) with nickel oxide as the oxygen carrier

As shown in Figure 2-12, the products of combustion from the two reactors are separated into two streams. The stream from the reduction reactor mainly contains CO2 and water vapour. Because water can be easily separated by condensation, CO2 can then be captured with a much lower energy penalty than for other capture concepts. Theoretical studies indicate an efficiency penalty of 2-3 percentage points, of which most is related to the compression of CO2.

Components and special considerations

Although there are various ways to perform CLC, a fluidised-bed combustion system has some advantages over other alternatives, e.g., good heat

transfer between the gases (air or fuel) and the solid oxygen carriers and also a mechanism for the physical transfer of the solid oxygen carriers between the two reactors. The major components of a chemical looping process are: solid oxygen carriers; a chemical looping system; fuel and air supplies; heat utilization/recovery and CO2 capture.

Oxygen carriers

The solid oxygen carriers are generally metal-oxide particles combined with support materials that improve the reactivity and mechanical strength. The oxygen carriers should have: sufficient capacity for oxygen transfer; high enough reaction rates for conversion of metal oxides from the reduced state to the oxidised state (and vice versa); sufficient mechanical strength for attrition resistance and durability and they should also be inexpensive. In particular, many transition metals such as Fe, Ni, Co, Cu, Mn and Cd based

Oxidation unit Reduction unit

NiO

oxides are candidates for oxygen carriers (Lyngfelt et al., 2001, Mattisson et al., 2001, Jin et al., 1999, Ishida et al. 1999 and 2002).

Figure 2-13 Chemical-looping combustion systems CLC reactor systems

Figure 2-13 shows a typical CLC system with two interconnected fluidised beds (A) (Lyngfelt et al., 2001) and a CLC system integrated with the thermal cycle (gas turbine) of power generation (B) (Copeland et al., 2002).

In the first concept (Figure 2-13 (A)), one bed is used as a reduction reactor with a low fluid velocity and the other one (oxidation reactor) has a high fluid velocity. The high-temperature nitrogen/residual oxygen, arising from the reaction between air and metal oxide particles in the oxidation reactor, are supplied to a thermal cycle for power generation or heat recovery.

From a thermodynamic point of view, one of the major differences between CLC and conventional combustion is that the oxygen carrier materials effectively limit the maximum combustion temperature. Currently, there are experimental results that indicate that the combustion temperature is limited to about 800°C when using Fe based oxygen carriers and 1050°C when using Ni based oxygen carries (Copeland et al., 2001). The temperature limitations affect the efficiency of the power cycle and, by way of comparison, modern, high efficiency gas turbines generally have turbine inlet

temperatures in the region of 1200°C. In order to address this, additional fuel could be burned to raise the temperature of the gas steam leaving the

Air

oxidation reactor, as shown in Figure 2-13. However, this solution would increase the uncontrolled CO2 emissions.

Fuel

In general gaseous fuel would be used for chemical-looping combustion.

The fuels that have been tested are currently limited to H2, CH4, natural gas, and CO. Although solid fuel such as coal could also be considered for CLC, this is at the conceptual stage and no experimental data are available. The major challenges of using solid fuel for chemical-looping combustion are the separation of the solid fuel and solid oxygen carrier to prevent transfer of fuel to the air reactor, and separation of the ash and solid oxygen carrier. The direct reduction reaction of the solid fuel with the oxygen carrier is also more difficult when compared with gas/solid reactions.

Power plant integration

Results presented up until today consider integration of the CLC system into a natural gas combined cycle, see for instance Wolf et al., 2001. Overall electric efficiency has been estimated to about 52 % including CO2

compression compared to 56 % for the reference plant without chemical looping when using natural gas as the fuel. However, this requires pressuration of the reactor system. Using a steam turbine based power generation system would enable the reactor system to operate at

atmospheric pressure and thereby simplify the design and operation of the reactor system.

Technology status and R&D needs

The technical development of CLC started during the late 1980’s. Initial research activities were focused on the material development of oxygen carriers and power plant process integration studies. Laboratory-scale and pilot scale tests have also been conducted to evaluate the reactivity of Fe and Ni based oxygen carriers in a fluidised-bed CLC system. Tests of a reactor system at a 10 kW-scale are ongoing at Chalmers University of Technology in Sweden. Theoretical studies have been carried out on the design of fluidised-bed combustion, system analysis, and process simulation.

However, further research and development is needed in, for example, the following areas:

• High temperature oxygen carrier development to improve the thermal efficiency and the suitability of CLC systems for power cycles

• Reactivity of different oxygen carries used for various fuels

• Pilot scale experiments to obtain engineering data for performance evaluation and CLC system design

• Demonstration of CLC - specially the integration with various power cycles

• Economic evaluations (based on large-scale pilot performance)

• Solid fuel chemical-looping combustion system development

• Development of highly reactive particles that are not prone to fragmentation.