Staged oxy-fuel natural gas combined cycle

Exhaust gas recirculation (EGR) in conventional natural gas-fired oxy-combustion cycles is required to maintain the combustion temperature at an allowable level. However, EGR is not beneficial from the system performance perspective. It is difficult to achieve in oxy-fuel cycles due to the high pressure and increased pressure drop in such systems. Consequently, alternative options to control the combustion temperature need to be considered. In this study, staged oxy-fuel natural gas combined cycle (SOF-NGCC) was proposed, which does not require EGR, and its feasibility was evaluated. A process model was developed in Aspen Plus in order to evaluate the thermodynamic performance of the proposed system and to benchmark it against the Allam cycle and conventional NGCC. The optimum net efficiency of the proposed cycle (47.63–51.32%) was shown to be lower than that for Allam cycle (54.58%) and the conventional NGCC without post-combustion capture (PCC) (56.95%). However, the SOF-NGCC is less complex and requires smaller equipment than the Allam cycle. This is mostly because the combined volumetric flow rate into expanders in both topping and bottoming cycles is approximately 25% of that estimated for the Allam cycle. Moreover, with a backpressure of 35 bar, no further compression is required prior to the CO2 purification unit

Gas-fired chemical looping combustion with supercritical CO2 cycle

Oxy-fuel combustion is currently gathering attention as one of the promising options for capturing CO2 efficiently, when applied to power plants, for subsequent carbon sequestration. However, this option requires a large quantity of high-purity oxygen that is usually produced in an energy-intensive air separation unit (ASU). Chemical looping combustion (CLC) is a technology with the potential of reducing the costs and energy penalties associated with current state-of-the-art cryogenic ASUs. In this work, the techno-economic performance of a natural gas-fired oxy-combustion cycle with cryogenic ASU is compared with that based on CLC. Two natural gas-fired cycles are considered: (i) staged oxy-fuel natural gas combined cycle as a reference; and (ii) gas-fired CLC with supercritical CO2 cycle. The process models were developed in Aspen Plus® in order to evaluate the thermodynamic performance of the proposed system and to benchmark it against the reference cycle. The results show that the net efficiency of the proposed cycle, including CO2 compression, is more than 51%, which is comparable to that of a conventional natural gas combined cycle with CO2 capture and 2.7% points higher than that of the reference cycle. Moreover, the economic evaluation indicates that a reduction in levelised cost of electricity from £38.3/MWh to £36.1/MWh can be achieved by replacement of the ASU-based oxy-fuel system with CLC. Hence, gas-fired CLC with a supercritical CO2 cycle has high potential for commercialisation

Mixture of piperazine and potassium carbonate to absorb CO2 in the packed column: Modelling study

This is an accepted manuscript of an article published by Elsevier in Fuel on 27/09/2021, available online: https://doi.org/10.1016/j.fuel.2021.122033 The accepted version of the publication may differ from the final published version.A rate-based non-equilibrium model is developed for CO2 absorption with the mixture of piperazine and potassium carbonate solution. The model is based on the mass and heat transfer between the liquid and the gas phases on each packed column segment. The thermodynamic equilibrium assumption (physical equilibrium) is considered only at the gas–liquid interface and chemical equilibrium is assumed in the liquid phase bulk. The calculated mass transfer coefficient from available correlations is corrected by the enhancement factor to account for the chemical reactions in the system. The Extended-UNIQUAC model is used to calculate the non-idealities related to the liquid phase, and the Soave-Redlich-Kwong (SRK) equation of state is used for the gas phase calculations. The thermodynamic analysis is also performed in this study. The enhancement factor is used to represent the effect of chemical reactions of the piperazine promoted potassium carbonate solution, which has not been considered given the rigorous electrolyte thermodynamics in the absorber. The developed model showed good agreement with the experimental data and similar studies in the literature.Accepted versio

Techno-economic-environmental assessment of biomass oxy-gasification staged oxy-combustion for negative emission combined heat and power

Climate change mitigation requires developing low-carbon technologies capable of achieving CO2 emission reductions at the gigatonne scale and affordable cost. Biomass gasification, coupled with carbon capture and storage, offers a direction to atmospheric CO2 removal. To compensate for the issues associate with the high-investment requirement of CO2 removal unit and lower efficiency compared to fossil-based power cycles, this study proposed a conceptual system for combined heat and power, based on biomass oxy-gasification integrated with staged oxy-combustion combined cycle (BOXS-CC). Aspen Plus® is used to develop the process model of the proposed cycle. The results obtained in the techno-economic analysis showed that the net power efficiency of the proposed concept with 50.2 kg/s biomass flowrate was 41.6%, and the heat efficiency was 27.4%, leading to a total efficiency of 69.0%, including CO2 compression. Moreover, the economic assessment of BOXS-CC revealed that it can achieve a levelised cost of electricity of €21.4/MWh, considering the heat and carbon prices of €46.5/MWh and €40/tCO2, respectively. Such economic performance is superior compared to fossil fuel power plants without CO2 capture. The environmental assessment shows that BOX-CC system results in net negative emissions of 766 kg CO2 eq./MWh

Modelling of photocatalytic CO2 reduction into value-added products in a packed bed photoreactor using the ray tracing method

This research suggests a comprehensive 3D model for modelling photocatalytic conversion of CO2 to methane, hydrogen and carbon monoxide in a packed bed reactor. This research includes two parts: designing the reactor's geometry using a new method in ''blender'' and using the computational fluid dynamics (CFD) technique to study and analyse the reaction, transport of phenomenon and light intensity through the reactor. Laminar flow, chemical reaction, mass transfer and optics physics were considered together to solve the equations. The surface reaction in the reactor follows a modified version of the Langmuir-Hinshelwood equation that evaluates the light profile in the reactor and the blockage of the catalyst's surface over time. Thus, a new method for 3D modelling light profiles in the reactor is introduced. The rate of reaction continues to increase with the pressure, and after 1 atm, the rate becomes steady. In the first 17 h, the methane rate is the highest, and then the carbon monoxide rate overcomes the methane rate. The rate of hydrogen is considerably lower than the other products. Changing pellets from spheres to Raschig rings causes growth in the probability density function (PDF) at the first moments. In methane's PDF, the amount of Raschig and sphere are 0.25 and 0.18, respectively, at the start of the reaction. Thus, the Raschig ring operates more effectively at the beginning moments of the process but eventually is outweighed after an hour by spherical particles. In the end, the validation of modelling and results were investigated with the aid of experimental data