Ceria based thermochemical cycles
[2] These type of thermochemical cycles are mainly studied for concentrated solar applications.
In the first step, a metal oxide, such as ceria, is reduced by providing heat to the material, liberating oxygen.
In the second step, a stream of steam oxidises the previously obtained molecule back to its starting state, therefore closing the cycle.
[4] The stoichiometric ceria cycle uses the cerium(IV) oxide (
And an oxidation step, to split the water molecules into hydrogen (
In order to enhance the reduction of the material, low partial pressures of oxygen are required.
To obtain these low partial pressures, there are two main possibilities, either by vacuum pumping the reactor chamber, or by using an chemically inert sweep gas, such as nitrogen (
[5] On the other hand, the oxidation step is an exothermic reaction that can take place at a considerable range of temperatures, from 400 °C up to 1,000 °C.
[6] In this case, depending on the fuel to be produced, a stream of steam, carbon dioxide or a mixture of both is introduced to the reaction chamber for hydrogen, carbon monoxide or syngas production respectively.
The temperature difference between the two steps presents a challenge for heat recovery, since the existing solid to solid heat exchangers are not highly efficient.
[7][8] The thermal energy required to achieve these high temperatures is provided by concentrated solar radiation.
Due to the high concentration ratio required to achieve this high temperatures, the main technologies used are concentrating solar towers (CST) or parabolic dishes.
[3] The main disadvantage of the stoichiometric ceria cycle lies in the fact that the reduction reaction temperature of cerium(IV) oxide (
) is at the same range of the melting temperature (1,687–2,230 °C) of cerium(IV) oxide (
),[5] which in the end results in some melting and sublimation of the material, which can produce reactor failures such as deposition on the window or sintering of the particles.
The non-stoichiometric ceria cycle uses only cerium(IV) oxide, and instead of totally reducing it to the next oxidation molecule, it performs a partial reduction of it.
In this way, by partially reducing ceria, oxygen vacancies are created in the material.
Oxidation reaction: The main advantage of this cycle is that the reduction temperature is lower, around 1,773 K (1,500 °C) which alleviates the high temperature demand of some materials and avoids certain problems such as sublimation or sintering.
[9] Temperatures above these would result in the reduction of the material to the next oxidation molecule, which should be avoided.
In order to reduce the thermal loses of the cycle, the temperature difference between the reduction and oxidation chambers need to be optimized.
Oxidation reaction: The main disadvantage of these cycles is the low reduction extent, due to the low non-stoichiometry, hence leaving less vacancies for the oxidation process, which in the end translates to lower fuel production rates.
[10] Due to the properties of ceria, other materials are being studied, mainly perovskites based on ceria, to improve the thermodynamic and chemical properties of the metal oxide.
[11][12] Since the temperatures needed to achieve the reduction of the material are considerably high, the reduction of the cerium oxide can be enhanced by providing methane to the reaction.
The main disadvantages of this cycle are the carbon deposition on the material, which eventually deactivates it after several cycles and needs to be replaced, and the acquisition of the methane feedstock.
There are two main types of reactors for these specific cycles: These type of reactors consist on a piece of solid material, which is shaped as a reticulated porous foam (RPC) in other to increase both the surface area and the solar radiation penetration.
[15][16] This reactors are shaped as a cavity receivers, in order to reduce the thermal losses due to reradiation.
They usually count with a quartz (fused silica) window in order to let the solar radiation inside the cavity.
[17] Since the metal oxide is a solid structure, both reactions must be done in the same reactor, which leads to a discontinuous production process, carrying out one step after the other.
The intention is to always have one or multiple reactors operating in the oxidation step at the same time, hence always generating hydrogen.
Many types of reactors work with solid particles, from free falling receivers, to packed beds, fluidized beds or rotary kilns.
