Optimal Design of a Fixed-Bed Ilmenite-Based Chemical-Looping Combustion Process

Global warming is a serious threat to the environment and human life. Inordinate emission of greenhouse gases as a result of increasing global energy demand is a major factor contributing to global warming. Although renewable energy sources are actively being developed, fossil fuels are expected to continue being a predominant part of the global energy mix in the next several decades. Emitted carbon dioxide from fossil fuels combustion is the one of the most influential greenhouse gas which leads to global warming. Chemical-looping combustion (CLC) has been suggested as one of the most efficient methods for carbon capture. CLC is a nonconventional unmixed combustion process where the fuel and the air reactions occur in separate reactors. In this manner, the dilution of combustion products with nitrogen is avoided and the exhaust gas is highly concentrated in CO₂.

Fluidized-bed reactors are commonly used for CLC processes, since the small size of the particles used in fluidized beds provides a good contact between the solid and gas phase. In addition, the rapid mixing of solids ensures adequate temperature control. Fluidized-bed reactors for CLC applications, however, face a number of challenges such as the availability of high temperature and high pressure solids filtering systems that can remove the fines produced from particle attrition. More importantly, using interconnected fluidized-beds at high pressure pose some challenges such as difficulty in finding a stable circulation of solids between the pressurized reactors. At high operative pressure, fixed-bed reactors might be preferable as the attrition of the oxygen carriers will be negligible and hence cyclones and filters downstream will not be required. Heat management and temperature control in fixed-bed systems, however, can be challenging since oxygen carriers are stationary with no mixing and hence bed temperature will be varying, which might lead to process instability.

Nickel and cooper based synthetic minerals (e.g. NiO and Cu₂O supported in alumina) are commonly chosen as oxygen carriers due to their high reactivity with methane and high oxygen carrying capacity (0.21 kg O₂/kg of Ni carrier). However, the cost of these synthetic carriers can be very high (around $19/kg for Ni carriers), limiting the economic attractiveness of the CLC process. Ilmenite is a natural mineral commonly found in igneous rocks and composed of iron and titanium oxide (FeTiO₃). Due to its relative natural abundance, ilmenite has a lower cost than synthetic carriers at around $0.3/kg. Furthermore, it has a low tendency towards carbon deposition, increasing the number of cycles that the carrier can be used without regeneration. It also has a relatively high oxygen carrying capacity at around 0.05 kg O₂/kg of carrier.

High pressure chemical-looping combustion of methane using ilmenite as oxygen carrier was investigated in a set of five dynamically operated fixed-bed reactors. Each reactor cycles through four operating stages: reduction, reforming, oxidation, and heat removal. The steam reforming stage allows the bed to be cooled down before the next oxidation stage. Reactor geometry (L/D ratio), catalyst load, and particle size diameter were selected as model parameters; input operating conditions of 10 kg/s of methane at 350^oC and 20 bar were assumed. To maintain the long-term chemical and mechanical stability of the ilmenite oxygen carrier, gas temperature along the reactor was kept below 1200^oC.

A dynamic model accounting for the change in the area of the intraparticle reaction front as reaction proceeds, intraparticle diffusion, heat transfer limitations inside the oxygen carrier, and spatial variations along the reaction bed was developed. The dynamic model was solved numerically, and the temperature and compositions profiles were obtained at each stage of the CLC process. Sensitivity analysis showed that by adjusting flue gas recycle rates the temperature profiles and heat evolution can be managed in an optimal way. It was concluded that during the oxidation stage, which is highly exothermic, the relevance of heat transfer limitations is negligible but internal diffusion effects should be accounted for. In the reduction stage, where high CO₂capture efficiency is sought, it is necessary to take intraparticle mass transfer limitations into account. The use of ilmenite -a low cost, naturally occurring, oxygen carrier- has a significant potential to improve the performance and feasibility of chemical-looping combustion systems.