Thermodynamic Assessment of Membrane-Assisted Premixed and Non-Premixed Oxy-Fuel Combustion Power Cycles

Abstract This study focuses on the investigations of gas turbine power generation system that works on oxy-combustion technology utilizing membrane-assisted oxygen separation. The two investigated systems are (i) a premixed oxy-combustion power generation cycle utilizing an ion transport membrane (ITM)-based air separation unit (ASU) which selectively allows oxygen to permeate from the feeding air and (ii) a non-premixed oxy-fuel combustion power cycle, where oxygen separation takes place, with cogeneration of hydrogen in an integrated combustor. A gas turbine combined cycle that works on conventional air–methane combustion was considered as the base case for this work. Commercial software package Hysys V8 was utilized to conduct the process simulation for the proposed cycles. The two novel cycle designs were proposed and evaluated in comparison with that of the conventional cycle. The first law efficiency of the premixed combustion power cycle was calculated to be 45.9%, a loss of 2.4% as an energy penalty for the oxygen separation. The non-premixed cycle had the lowest first law efficiency of 39.6%, which was 8.7% lower than the efficiency of the base cycle. The lower effectiveness of the cycle could be attributed to the highly endothermic H2O splitting reaction for oxygen production. High irreversibility in the H2O-splitter and the reactor was identified as the main cause of exergy losses. The overall second law efficiency of the non-premixed power cycle was around 50% lesser than that of the other cycles. The energy penalty related to air separation is dominated as the parameter that reduces the efficiencies of the oxy-fuel combustion cycles; however, the premixed combustion cycle performance was found to be comparable to that of the conventional air-combustion cycle.</jats:p

Investigation of the Effect of the Top and the Bottom Temperatures on the Performance of Humidification Dehumidification Desalination Systems

Humidification dehumidification process is an attractive small scale water desalination technique in which desalinated water is produced by mimicking the nature’s water cycle. Various modifications to the basic HDH system can be vital in improving the productivity and reducing the production cost of the fresh water. In this study, a closed-air-open-water water-heated (CAOW-WH) cycle and a closed-air-open-water air-heated (CAOW-AH) cycle are modeled and optimized. Effects of mass flow ratio, humidifier and dehumidifier effectiveness, relative humidity, top and bottom temperatures (main concern of study) on the gain output ratio (GOR), the recovery ratio (RR), entropy generation in the system have been analyzed and presented. It has been observed that an optimal mass flow ratio exists for both the cycles, which maximizes the GOR of the system. Moreover, effectiveness of the humidifier and the dehumidifier is an important parameter, which determines the productivity of the systems. Furthermore, a higher GOR can be obtained at low Tmin and high Tmax and at high Tmin and low Tmax for systems heated by a water heater, whereas the GOR of the air heated HDH system increases with increasing both the Tmin and the Tmax for values of humidifier and dehumidifier effectiveness of 0.8. This study provide extended design charts for building an optimum HDH system to produce a pre-determined rate of desalinated water.</jats:p

Investigation of liquid ethanol evaporation and combustion in air and oxygen environments inside a 25 kW vertical reactor

The combustion characteristics of liquid ethanol in air and oxygen–carbon dioxide environments are investigated numerically inside a vertical reactor. Gambit 2.2 was used to construct the mesh and Fluent 12.1 was used to perform the calculations. Different oxidizer environments were considered including pure air, oxygen-enriched air, in addition to the cases of OF21 (21% O2 and 79% CO2) and OF29 (29% O2 and 71% CO2). Comparisons were performed between the different cases and the results were validated against wide range of the experimental data. Nonpremixed combustion model which utilizes probability density function to predict the scalar quantities was incorporated to simulate the combustion process. Two turbulence models, realizable k–ɛ (RKE) model and Reynolds stress model (RSM), were applied and their results were compared. The Euler–Lagrange approach was utilized to solve the discrete phase model. The results showed the ability of the RKE model to predict much closer data to the experimental data than the RSM which over predicts the temperature. For the case of OF21, the flame was lifted and the combustion temperature was reduced as compared to the air combustion case. However, the OF29 combustion case resulted in a very close performance to the case of air combustion. </jats:p