Photocatalytic CO2 reduction seems potential to mitigate greenhouse gas emissions and produce renewable energy. A new model of photocatalytic CO2 reduction in optical fiber monolith reactor with multiple inverse lights was developed in this study to improve the conversion of CO2 to CH3OH. The new light distribution equation was derived, by which the light distribution was modeled and analyzed. The variations of CH3OH concentration with the fiber location and operation parameters were obtained by means of numerical simulation. The results show that the outlet CH3OH concentration is 31.1% higher than the previous model, which is attributed to the four fibers and inverse layout. With the increase of the distance between the fiber and the monolith center, the average CH3OH concentration increases. The average CH3OH concentration also rises as the light input and water vapor percentage increase, but declines with increasing the inlet velocity. The maximum conversion rate and quantum efficiency in the model are 0.235 μmol g−1h−1 and 0.0177% respectively, both higher than previous internally illuminated monolith reactor (0.16 μmol g−1h−1 and 0.012%). The optical fiber monolith reactor layout with multiple inverse lights is recommended in the design of photocatalytic reactor of CO2 reduction.
With global energy demand growing and fossil fuels consumption, the level of carbon dioxide (CO2) in the atmosphere has been rising, which results in the greenhouse effect to cause environmental hazards [1], [2] and [3]. In the past decades, the conversion of CO2 to value-added chemicals and renewable fuels has been investigated by various methods such as thermal conversion, plasma conversion and photoreduction [4] and [5]. CO2 reduction using solar light in a photocatalytic reactor is promising to relieve greenhouse effect and energy crisis [6]. In this way, solar energy can be converted and stored as chemical energy without producing greenhouse gases, similar to photosynthesis [7]. However, the conversion of CO2 via H2O splitting is relatively low [8], many studies focus on the field to improve the conversion, especially the efficient reactor design [9], [10], [11] and [12].
The photoreactor design is of great importance in the photocatalytic CO2 reduction. Various reactors were summarized by Tahir and Amin [1] and [4], including optical fiber reactors [13], [14] and [15], monolith reactors [16], [17] and [18], slurry reactors [19], annular reactors [20], etc. Some of them are limited to the laboratory scale due to the layout of light resources and the long distance between the light and catalyst [17]. Comparatively speaking, optical fiber and monolith photoreactors are more efficient due to higher reaction surface area, efficient light harvesting, and uniform light distribution [1] and [9]. In order to further improve the illuminated surface area and light utilization, a distributive optical fiber monolith reactor (OFMR) was proposed by Lin and Valsaraj [21], which is made up of lots of parallel channels. On the condition that the variables such as flow flux and radiance flux are uniformly distributed at the monolith inlet and outlet, these reaction channels could be presumed identical [22]. The OFMR has some advantages over other reactors, first of which is that a honeycomb monolith substrate has 10–100 times greater specific surface area than that of other types of reactor substrates with the same external size [23]. Besides, the monolithic reactor is easy of scale and industrialization by increasing the number and dimension of channels, so the OFMR has been widely applied to the photocatalytic field.
However, the spectral range used in the CO2 reduction is under 400 nm, only possessing nearly 3% of sunlight [24], so making full use of limited UV light attracts more attention in the photocatalytic research. Previous studies focused on selecting efficient catalyst and advanced methods, such as montmorillonite modified TiO2 proposed by Tahir and Amin [9] and [25], and a simple method studied by Yang and Liu [26]. Except the synthesis of new catalysts and theoretical methods, lights layout is equally crucial to improve the utilization ratio of energy input, but seldom taken into consideration.
In the present work, the model with four fibers inverse layout in a reaction unit was developed on the basis of OFMR. A new light distribution equation was derived using the geometrical methods. By means of the multiphysics software COMSOL, which has great advantages over the traditional physical experiments, the fiber location, inlet reactants velocity, reactants concentration and light input were investigated. This new optical fiber monolith reactor layout with multiple inverse lights can improve the conversion of CO2 to CH3OH and has a high quantum efficiency, which is of benefit to the design of the photocatalytic reactor of CO2 reduction.
A new model of photocatalytic reduction of CO2 with multiple and inverse lights was proposed. The light intensity distribution was calculated by geometric relations and the CH3OH concentration was obtained by simulation experiments. The results show that the four fibers and inverse light source layout is superior to previous models in the light utilization, and CH3OH concentration at the outlet reaches 2.32 × 10−5 mol/m3.
The effects of the optical fiber position and operation conditions on the CO2 reduction were studied. The greater the distance between one fiber and channel center is, the higher the outflow CH3OH concentration is, showing that the fibers position is a key issue to the reaction conversion. It is recommended that fibers should be close to the monolith wall as much as possible. The CH3OH concentration is almost in linear with the water vapor ratio and the initial light input. The increase of inflow velocity results in a higher production rate, but leads to the decrease of outlet CH3OH concentration. At the velocity of 1 mm/s, the highest production rate reaches 0.235 μmol g−1 h−1, and the quantum efficiency arrives at 0.0177%.
Performance analysis of photocatalytic CO2 reduction in optical fiber monolith reactor with multiple inverse lights can be of benefit to the optimized design and application of larger scale photocatalytic reactors.
