Inherent CO2 Capture Using Chemical Looping Combustion in a Natural Gas Fired Power Cycle

Abstract

In this paper an alternative to the so-called “oxy-fuel” combustion for CO2 capture is evaluated. “Chemical looping combustion” (CLC), is closely related to oxy-fuel combustion as the chemically bound oxygen reacts in a stoichiometric ratio with the fuel. In the CLC process the overall combustion reaction takes place in two reaction steps in two separate reactors. In the reduction reactor, the fuel is oxidized by the oxygen carrier, i.e., the metal oxide MeO. The metal oxide is reduced to a metal oxide with a lower oxidation number, Me, in the reaction with the fuel. In this manner, pure oxygen is supplied to the reaction with the fuel without using a traditional air separation plant, like cryogenic distillation of air. The paper presents a thermodynamic cycle analysis, where CLC is applied in a humid air turbine concept. Main parameters are identified, and these are varied to examine the influence on cycle efficiency. Results on cycle efficiency are presented and compared to other CO2 capture options. Further, an evaluation of the oxygen carrier, metals/oxides, is presented. An exergy analysis is carried out in order to understand where losses occur, and to explain the difference between CLC and conventional combustion. The oxidation reactor air inlet temperature and the oxidation reactor exhaust temperature have a significant impact on the overall efficiency. This can be attributed to the controlling effect of these parameters on the required airflow rate. An optimum efficiency of 55.9% has been found for a given set of input parameters. Crucial issues of oxygen carrier durability, chemical performance, and mechanical properties have been idealized, and further research on the feasibility of CLC is needed. Whether or not the assumption 100% gas conversion holds, is a crucial issue and remains to be determined experimentally. Successful long-term operation of chemical looping systems of this particular type has not yet been demonstrated. The simulation points out a very promising potential of CLC as a power/heat generating method with inherent capture of CO2. Exergy analysis show reduced irreversibilities for CLC compared to conventional combustion. Simulations of this type will prove useful in designing CLC systems in the future when promizing oxygen carriers have been investigated in more detail .

Modeling of Cu oxidation in adiabatic fixed-bed reactor with N2 recycling in a Ca/Cu chemical loop

“…As mentioned above, the use of packed bed reactor technology in processes intended for power generation makes it possible to operate at high pressure and therefore achieve a high overall efficiency [4,20,21]. In the case of the Ca/Cu looping process, performance at high pressure leads to faster reaction rates during the Cu oxidation stage and also reduces the loss of carbon through CaCO 3 calcination, even at relatively high operating temperatures [22].…”

“…With this technology, it is possible to generate a product gas at a high temperature and pressure that can be expanded in a gas turbine for power generation with high overall energy efficiency [4]. Cu-based oxygen carriers have a number of favorable features for chemical looping applications.…”

Chemical looping combustion process in fixed-bed reactors using ilmenite as oxygen carrier: Conceptual design and operation strategy

“…Blowers (46) and (42) are open in order to allow the syngas produced in (B) to be fed into the reactor that is operating as stage (A). Subsequently, valves (31), (26) and (21) are also open to regulate the fraction of the CO 2 and H 2 O produced in stage (A) that is destined for transport and storage and the fraction that is recirculated into the process. Finally, valves (47) and (48) are open so that the outlet N 2 (that is being discharged from stage (D) at 1200 ºC) can be sent to the combined cycle while part of the exhaust N2 is being recirculated to the system via the compressor (109).…”

“…3 A maximum temperature of 1200 ºC is assumed in order to maintain the chemical and mechanical stability of the carrier after multiple redox cycles [43][44][45]. An operating pressure of about 20 bar has been selected in order to achieve the highest energy efficiency in the downstream gas turbine [21]. Table 2 summarizes the input operating conditions chosen for the case study and Table 3 indicates the temperature, flow rate and composition of the gas streams obtained by solving the mass and heat balances at each stage of the CLC process.…”

“…However, the use of interconnected fluidized-beds in CLC has several drawbacks. Operation at high pressure is required to enable a CLC system to be integrated in a combined cycle to increase the energy efficiency [21]. Several works have been published in recent years about CLC performed in fluidized-beds at high pressure [22][23][24], but this concept still faces critical challenges, such as the difficulty in maintaining a stable circulation of solids between the pressurized reactors, in order to demonstrate its feasibility on a large scale.…”