Enhanced bioconversion of hydrogen and carbon dioxide to methane using a micro-nano sparger system: mass balance and energy consumption

Simultaneous CO2 removal with renewable biofuel production can be achieved by methanogens through conversion of CO2 and H2 into CH4. However, the low gas–liquid mass transfer (kLa) of H2 limits the commercial application of this bioconversion. This study tested and compared the gas–liquid mass transfer of H2 by using two stirred tank reactors (STRs) equipped with a micro-nano sparger (MNS) and common micro sparger (CMS), respectively. MNS was found to display superiority to CMS in methane production with the maximum methane evolution rate (MER) of 171.40 mmol/LR/d and 136.10 mmol/LR/d, along with a specific biomass growth rate of 0.15 d−1 and 0.09 d−1, respectively. Energy analysis indicated that the energy-productivity ratio for MNS was higher than that for CMS. This work suggests that MNS can be used as an applicable resolution to the limited kLa of H2 and thus enhance the bioconversion of H2 and CO2 to CH4

Effect of Gas Velocity Distribution on Heat Recovery Process in Packed Bed of Plate-Shaped Slag

A new twin-roll continuous slag solidification process and heat recovery process from a slag packed bed was developed for utilization of the waste heat of steelmaking slag. Plate-shaped slag with the thickness about 7 mm was successfully produced in a pilot plant, and the sensible heat of the slag was recovered by blowing air into the slag chamber. However, the gas distribution inside the slag packed bed was unclear because of the unique shape of the slag plates, and this remained a concern for further scale-up designing of the slag chamber. Therefore, in order to estimate the gas distribution in the packed bed, a simple computational fluid dynamics (CFD) model which considers the wall effect around the inner wall of the chamber was developed, and this model was fitted to the results of laboratory-scale velocity distribution measurements. The results showed that the gas velocity distribution was properly estimated, and the intensity of the wall effect was similar in both cases. As the next step, the gas velocity distribution and its effect on the slag heat recovery process in a pilot-scale slag chamber were evaluated with the assistance of the CFD simulation model. The simulation results were compared with the measured data obtained in a pilot-scale test, and as the result, a similar wall effect was also observed in the pilot-scale chamber. However, the intensity of the wall effect was limited enough to prevent serious deterioration of the uniformity of the gas distribution

Reduction of Electric Power Consumption in CO2-PSA with Zeolite 13X Adsorbent

Reduction of CO2 from waste gases from various industries is now desired worldwide. As a technology for separating CO2 from mixed gases, Pressure Swing Adsorption (PSA) is one of the practical processes that are widely used at present owing to the simplicity of its gas separation mechanism. In order to reduce the cost of CO2 separation, a further reduction of the running cost of CO2-PSA operation is required. Among all the utilities used in CO2-PSA, electric power consumption has the greatest impact, especially in cases where the pressure swing range between gas adsorption and gas desorption is large. Electric power consumption increases significantly when the pressure loss inside the adsorber has reached a non-negligible level. Changing the adsorbent pellet size is a convenient method for reducing pressure loss, but its effect on CO2-PSA performance was unclear. Therefore, in this work, the effects of the size of the adsorbent pellets on both the gas adsorption behavior and the electric power consumption in CO2-PSA were evaluated experimentally. From the results of laboratory-scale CO2-PSA experiments and gas adsorption rate measurements, it was observed that the effect of the pellet size appeared only in the early stage of the gas adsorption step and was not dominant when the cycle time was sufficiently long. Subsequently, pilot-scale CO2-PSA experiments with the same CO2 throughput were also conducted, and as a result, the electric power consumption of a vacuum pump was lowered by 15% in case of using d = 3.0 mm larger adsorbent pellets compared to the results with d = 1.5 mm smaller adsorbent pellets