A comparative simulation study of power generation plants involving chemical looping combustion systems
Abstract This work presents a simulation study on both energy and economics of power generation plants with inherent CO 2 capture based on chemical looping combustion technologies. Combustion systems considered include a conventional chemical looping system and two extended three-reactor alternatives (exCLC and CLC3) for simultaneous hydrogen production. The power generation cycles include a combined cycle with steam injected gas turbines, a humid air turbine cycle and a simple steam cycle. Two oxygen carriers are considered in our study, iron and nickel. We further analyze the effect of the pressure reaction and the turbine inlet temperature on the plant efficiency. Results show that plant efficiencies as high as 54% are achieved by the chemical looping based systems with competitive costs. That value is well above the efficiency of 46% obtained by a conventional natural gas combined cycle system under the same conditions and simulation assumptions.
Kinetics of the Reactions Prevailing during Packed‐Bed Chemical Looping Combustion of Syngas using Ilmenite
Chemical looping combustion has been identified as a very promising technology for power production with integrated carbon dioxide capture. The objective of this work is to study the kinetics of all reactions that are important in the combustion of syngas in a packed-bed chemical looping reactor by using ilmenite as an oxygen carrier. The reaction kinetics of reduction, with hydrogen and carbon monoxide, and oxidation, with oxygen, of ilmenite-based pellets have been determined in a thermogravimetric analyzer. The shrinking core model for a spherical geometry was used to obtain the kinetic parameters by considering mixed control by chemical reactions and diffusion through the product layer for all reaction involved. The reduction rate of ilmenite by hydrogen was at least an order of magnitude higher than that by carbon monoxide (especially at higher particle conversions) and the activation energies were found to be 45 and 30 kJ mol−1 for reduction with hydrogen and carbon monoxide, respectively. For the oxidation reaction, a much lower apparent activation energy of 5 kJ mol−1 was found, which indicated that the temperature did not have an important effect on the oxidation kinetics of ilmenite. In addition, the catalytic activity of ilmenite for water gas shift (WGS) was measured in a micro fixed-bed reactor. An empirical power-law model was used to describe the kinetics of the WGS reaction over ilmenite.
Investigation of NiO-based mixed oxides in a 300-W chemical-looping combustor
Chemical-looping combustion, CLC, is a novel combustion concept with inherent separation of CO2. This study evaluates the performance of spray-dried nickel-based oxygen carrier particles prepared from commercially available materials. The possibility to optimize the methane conversion while retaining the oxygen transport capacity by mixing different NiO-based oxygen carriers was evaluated and results showed that such optimization was indeed possible. Experiments were carried out in a batch reactor as well as in a continuous 300-W unit. Experiments in the batch reactor evaluated the performance of two different spray-dried particles, individually and mixed, at two different temperatures, 850°C and 950°C. The reference particle, referred to as N-VITO in this study, contained only NiO and NiAl2O4 while the other spray-dried particle was similar to the reference but contained a small amount of MgO as additive in the starting material. It was found that the reference particle had good oxygen-transport characteristics, but that methane conversion left room for improvement. The particle with MgO addition, on the other hand, showed excellent methane conversion, but poor oxygen transport capability, especially at the lower temperature. The 50/50mass-mixture of the two particles resulted in a potent oxygen-carrier batch with the desired qualities. Three experimental series were conducted in the 300-W CLC-unit; (i) using only the reference particle, (ii) using a mixture of the reference particle and the particle with MgO-addition, and (iii) using the previous mixed oxide system together with a small quantity of a high-surface impregnated oxygen-carrier based on NiO and Al2O3. It is shown that the methane conversion to CO2 is dependent primarily on the solids flux in the reactor system, but also on the temperature in the fuel reactor. Results showed that the operation was more stable and that it was possible to obtain better fuel conversion when the mixtures of two or three different oxygen carrier particles were used. With these mixtures, the methane fraction could be brought down to < 0.1% while still maintaining a low CO fraction.