LHV Efficiency Research Articles

Direct Ammonia SOFC-Gas Turbine Hybrid Systems Modeling for Heavy-Duty Transportation Sector

Ammonia currently stands as being the second most produced chemical in the world [1]. While it is prominently used in the manufacturing of fertilizers, increase in production capacity through renewable sources such as green hydrogen and its potential as a carbon-neutral energy carrier make it a promising candidate to replace fossil fuels for power generation in the future [2]. Moreover, when compared to liquid hydrogen, ammonia can be stored at more practical temperatures (-253⁰C vs -33⁰C at 1 bar) and has a higher volumetric energy density (8.5 MJ/L vs 12.7 MJ/L) [3]. Though the toxicity of ammonia as a fuel is well known, there are well established safety and handling procedures for storage and transport, given its ubiquitous existence as an essential commodity required for agriculture.From a systems perspective, residual chemical exergy available in the SOFC anode gas exhaust can be utilized through a gas turbine to enable increased efficiency [4]. Direct ammonia SOFCs (DASOFCs) can eliminate the need for external cracking to hydrogen, thereby reducing system complexity. In addition, heat loss and thermal management in the stack periphery may be effectively managed via in-situ endothermic decomposition of ammonia [5]. Integration in a hybrid system is also beneficial to SOFCs due to the associated heat of compression for pressurized operation. The different physicochemical timescales of electrochemical fuel cells and mechanical turbomachinery are advantageous to dynamic operation which is important because of fluctuating power loads in a mobile application.In this work, the potential of an ammonia-based hybrid SOFC/GT system concept is assessed. A multiscale stack simulation using constitutive relations for heat, mass and charge transfer based on high power density SOFCs is integrated with turbomachinery and other auxiliary system components within the gPROMS process simulation framework. State variables for each point can then be defined to estimate the operating boundaries of the system.This presentation discusses the thermodynamics of SOFC operation and system concepts to utilize ammonia in a hybrid system for a kW-scale transportation application. This involves evaluation of system indicators such as thermodynamic efficiency, fuel utilization and specific power which can help determine optimal performance. The efficiency and specific power are dictated by the operating temperature of the fuel cell, preheating requirements and the relative utilization of fuel between the SOFC and gas turbine. The proposed system concept is shown to achieve LHV efficiencies of up to 60% (LHV) within reasonable operating limits. The presentation includes an examination of the prospects of ammonia as a power source for the mobility sector in comparison to contemporary and future renewable fuels, and concludes with practical challenges for deployment.

Cooling analysis of an axial turbine for a direct fired sCO2 cycle and impacts of turbine cooling on cycle performance

In this paper, the details of the cooling analysis and design process of a 950 MW rated sCO2 turbine for a direct-fired sCO2 power plant are presented. An analytical cooled turbine algorithm for combustion gas turbines is modified for sCO2 coolant and mainstream flows to determine the coolant fractions. Inputs of the cooling technologies were determined from internal NETL studies and publicly available data for prototype axial turbines. A simplified thermal stress analysis method is used to down-select and determine the cooling configurations to be used by considering the disk and material properties and candidate stream coolant temperatures. The cooled turbine algorithm was first used to eliminate configurations that would require too much coolant from the cycle. The best cooling configurations for a single flow and a double flow turbine were determined by testing in a direct-fired sCO2 power plant performance model in Aspen Plus. A weighed scoring criteria was developed to help select amongst closely related performance outcomes. Results of the scoring gave the best option as a single flow turbine with a calculated turbine efficiency of 88%, and a resulting cycle LHV efficiency of 65.73% and plant LHV efficiency of 54.43% for a 601.58 net MW direct-fired sCO2 power plant (952 MW turbine).

Process modeling of a hybrid-sulfur thermochemical cycle combined with solid oxide fuel cell/gas turbine system

This work proposes a novel design of the hybrid sulfur (HyS) thermochemical water-splitting cycle for hydrogen production. The hydrogen production plant is combined with a solid oxide fuel cell/gas turbine (SOFC/GT) system to satisfy the demand for the high-temperature heat from the HyS cycle by exploiting the heat loss of the SOFC stack and the demand for electric power for SO2-depolarized electrolysis. A detailed thermodynamic analysis of the HyS cycle and the SOFC/GT system was conducted on a flowsheet basis to determine the energy requirement and to estimate the effects of the operating conditions on the overall efficiency. As a result, it was found that the LHV efficiency of the combined HyS-SOFC/GT system can reach > 29% (34.32% HHV efficiency), with a net electric efficiency of the SOFC/GT system of 38.83% and consumption of 340.3 kJ molH2-1 high-temperature heat, 148.77 kJ molH2-1 low-temperature heat, and 134.35 kJ molH2-1 electric power.

Performance evaluation, Optimization and exergy analysis of a high temperature co‐electrolysis power to gas process using Aspen Plus®‐a model based study

AbstractIn this paper, a high temperature coelectrolysis power to gas (LTE PtG) setup was developed that was fed a stereotypical feed in CO2 one could acquire from carbon capture and sequestration (CCS) retrofit. The electrolyzer was fed an equimolar 10 kmol/s feed in water and CO2 and the electrolyzer product was composed of 38.80 mole% CO2, 30.96 mole% CO, and 30.24 mole% H2 for a syngas conversion of 43.8%. The product from the methanation unit was only composed of 21.38 mole% CH4, with the rest (76.53 mole%) being predominantly CO2. The methanation unit was composed of only one Sabatier reactor operating at 313°C, which had 90% of its product recycled back to the front end of the unit. The electrolyzer was found to have a LHV efficiency of 31.49%, and the entire HTCE PtG process was found to have an efficiency of 74.31% with methane storage and 76.49% without methane storage. A novel adaptation was developed on previous work on the exergy analysis for flow‐sheet simulators which can be used for cyclic and noncyclic processes. The procedure was determined to be accurate, with irreversibilities determined across all the major subunits found to equal the irreversibility around the processes a whole. The electrolyzer was found to be responsible for the majority of the irreversibility within the setup, and this was attributed to the fact that water electrolysis is a very energy intensive process. The exergy efficiency of the electrolyzer was found to be 87.07% and that of the entire LTE PtG processes was found to be 84% with methane storage, and 87% without methane storage. In the end, it was determined that HTCE is unsuitable for the production of methane via retrofit with a methanation unit, and that it would be better suited for the production of liquid hydrocarbon fuels with much higher molecular carbon to hydrogen ratios.

Performance Analysis of Ammonia in Solid Oxide Fuel Cells

The transition of the marine propulsion system towards alternative fuels is mandatory to offset carbon emission. Among the alternative fuels, ammonia is carbon-free and can be produced in sustainable ways. Ammonia has 17.8% hydrogen (wt %) and is easily liquified at 25°C and 8 bar pressure. The two-stroke internal combustion engines currently used in the marine sector reach efficiencies of about 50% but generate substantial polluting emissions. Solid oxide fuel cells generate electricity with efficiencies greater than 50 % and can use ammonia as fuel.In this work, a single-cell SOFC was characterised using in-situ ammonia decomposition reaction (Int-ADR) and compared with ex-situ ammonia decomposition reaction (Ext - ADR; composition - N2:H2 = 1:3), and pure hydrogen (H2 100%; composition - H2 = 100%), between temperatures of 750 °C to 850°C. The constant load tests performed at 750°C with 84% fuel utilization revealed that the LHV efficiency of the ammonia fed SOFC was 58%. The J-V characteristics showed similar performances for the three compositions at different temperatures and flowrates. The open-circuit voltage (OCV) of Int-ADR was similar to that of Ext- ADR, confirming that usage of ammonia as fuel in the Ni-YSZ anode involved two steps, (i) decomposition of ammonia into nitrogen and hydrogen, followed by (ii) electrochemical conversion of the hydrogen into steam.The electrochemical impedance spectroscopy (EIS) results showed that the polarisation resistance (Rp) decreased by 15% over this temperature range. The total flowrate of 80 Nml min -1 of ammonia for Int-ADR, 160 Nml min -1 for Ext ADR and 120 Nml min -1 for H2 100% were chosen after performing distribution of relaxation time (DRT) analysis on changing total flowrates at 775 °C. The DRT coupled with complex nonlinear least-square (CNLS) analysis for the temperature range of 750-850°C revealed that the charge transfer resistance reduced by 66%, while the gas conversion resistance increased by 12% due to better evacuation of steam. The Arrhenius plot of DRT peak resistances revealed that the polarisation activation energy was the same for the three gas compositions, at 16 kJ mol -1 . Figure 1

Investigation of a hybrid solar thermochemical water-splitting hydrogen production cycle and coal-fueled molten carbonate fuel cell power plant

Increasing electricity demand in the world’s energy portfolio and the focus on reducing carbon emission footprints has led to broad research into flexible and efficient energy conversion systems. The integration of various energy conversion systems is an efficient approach to meet sustainable energy technologies’ demand in the future. In this study, a brainchild integration process of the solar thermochemical water-splitting hydrogen production cycle and coal-fueled molten carbonate fuel cell coupled with a gas turbine is proposed, modeled, and simulated in AspenTech v9.1. The Zinc/Zinc-oxide thermochemical cycle directly utilizes the solar high-temperature reactor thereby avoiding the need for non-renewable power, thus increasing the efficiency of the system. The molten carbonate fuel cell feeds on the syngas produced by the coal gasification and the oxygen and hydrogen of the thermochemical cycle. The heat of the fuel cell is further exploited by a gas turbine. A sensitivity analysis is conducted to appraise cardinal parameters on system performance indicating the fuel cell pressure, voltage, and current density as the most influential parameters. The concentrated solar power provides 5.88 MW of heat for the thermochemical cycle encompassing 434 parabolic dishes. The molten carbonate fuel cell has a 63% and 61% LHV and HHV efficiency, respectively, and the system has a total efficiency of approximately 85%, producing 13.63 MW of electricity.

Performance Analysis of Ammonia in Solid Oxide Fuel Cells

Ammonia (NH3) has 17.8 wt% hydrogen and is easily liquified at 25°C and 8 bar pressure. Ammonia is carbon-free and can be produced sustainably at large scale and low cost. Solid oxide fuel cells generate electricity with efficiencies greater than 60% and can use ammonia as fuel without the need for external cracking. In this work, a single-cell SOFC was characterized using the in-situ ammonia decomposition reaction (Int-ADR) and compared with the ex-situ ammonia decomposition reaction (Ext-ADR), and pure hydrogen (H2 100%), between temperatures of 750°C and 850°C. Constant load tests performed at 800°C with 84% fuel utilization reached the LHV efficiency of 58%. The open-circuit voltage (OCV) of Int-ADR was similar to that of Ext-ADR, confirming that usage of ammonia as fuel in the Ni-YSZ anode involved two steps (i) ammonia decomposition into nitrogen and hydrogen and (ii) electrochemical conversion of hydrogen into steam.

(Invited) Applications of Protonic Ceramics for Electrochemical Energy Conversion and Storage

Recent exciting advancements in proton-conducting ceramic materials and devices suggest that they are approaching a technology readiness level that rivals more well-established polymeric and oxygen-ion conducting counterparts. Because they can enable proton-mediated electrochemistry under both dry and wet environments at moderate temperatures, protonic ceramics provide unique opportunities to enhance or synergize a diverse range of complementary electrochemical and thermochemical processes. Because of this potential, significant efforts have been devoted to advancing numerous energy-related applications using these materials. Here, we will provide an overview of recent research efforts at the Colorado School of Mines focused on developing protonic ceramics for a number of applications, including: Hydrocarbon-tolerant protonic ceramic fuel cells for electricity generation (PCFCs)Protonic ceramic electrolyzers for fuel synthesis (PCECs)Reversible protonic-ceramic electrochemical cells for energy storage (RePCECs). Recent progress has lead to remarkably high-performance H2 and hydrocarbon-fueled PCFCs; exceptionally efficient (>97% LHV efficiency) PCECs for H2 production and for the co-conversion of steam and carbon dioxide to renewable methane; H2/H2O-based RePCECs with >75% round-trip efficiency (cell-level) for seasonal energy storage, and a reversible ammonia fuel cell for ammonia synthesis/power production. In addition, ongoing collaboration with industrial partners demonstrates promising scale-up and durability progress for protonic ceramic cells and stacks that underscores their potential for eventual commercial application.Acknowledgements: This work was supported by the Advanced Research Projects Agency-Energy (ARPA-E) through the REFUEL (Award No. DEAR0000808) and REBELS programs (Award No. DE-AR0000493) and the Office of Fossil Energy (Award No. DE-FE0031716). Additional support was provided by the Army Research Office under Grant No. W911NF-17-1-0051, the Office of Naval Research via Grant No. N00014-16-1-2780, and faculty research funding from Kansas State University.

Numerical study on solar spouted bed reactor for conversion of biomass into hydrogen-rich gas by steam gasification

A solar-powered biomass steam gasification system was developed, in which heat transfer model, flow model and chemical model were constructed to predict the distributions of temperature, pressure, mole fraction of syngas, and solar incident flux. Several key parameters of gasifier were designed to ensure the fluidization stability. Based on the model validation, gasifier performance simulations in the design working conditions were obtained. The effects of the key variable parameters, including the rim angle of the dish collector, steam-to-biomass mass flow ratio, biomass feeding rate and the solar irradiance in the different operation working conditions on the composition of syngas, lower heating value, and efficiencies were investigated. The results reveal that the coupled system implements the best gasification performance in the design conditions which the rim angle, steam-to-biomass mass flow ratio, and biomass feeding rate are set at 60°, 0.4, and 2.5 g/min, while the LHV, carbon conversion, and gasification energy efficiencies are 11.51 MJ/m3, 78.17%, and 93.01%, respectively. The overall energy efficiency considering solar energy is 30.79%.

Integration of gas switching combustion and membrane reactors for exceeding 50% efficiency in flexible IGCC plants with near-zero CO2 emissions

• Gas switching combustion and membrane reactors are integrated in an IGCC plant. • The proposed plants produce flexible electricity/H 2 with CCS to balance renewables. • IGCC benchmarks with/without CCS incorporate hot gas clean-up and H-class turbines. • Electric/H 2 LHV efficiencies up to 50.3%/62.4% with >98% CO 2 capture are achieved. • The energy penalty is less than 2%-points relative to the unabated IGCC plant. Thermal power plants face substantial challenges to remain competitive in energy systems with high shares of variable renewables, especially inflexible integrated gasification combined cycles (IGCC). This study addresses this challenge through the integration of Gas Switching Combustion (GSC) and Membrane Assisted Water Gas Shift (MAWGS) reactors in an IGCC plant for flexible electricity and/or H 2 production with inherent CO 2 capture. When electricity prices are high, H 2 from the MAWGS reactor is used for added firing after the GSC reactors to reach the high turbine inlet temperature of the H-class gas turbine. In periods of low electricity prices, the turbine operates at 10% of its rated power to satisfy the internal electricity demand, while a large portion of the syngas heating value is extracted as H 2 in the MAWGS reactor and sold to the market. This product flexibility allows the inflexible process units such as gasification, gas treating, air separation unit and CO 2 compression, transport, and storage to operate continuously, while the plant supplies variable power output. Two configurations of the GSC-MAWGS plant are presented. The base configuration achieves 47.2% electric efficiency and 56.6% equivalent hydrogen production efficiency with 94.8–95.6% CO 2 capture. An advanced scheme using the GSC reduction gases for coal-water slurry preheating and pre-gasification reached an electric efficiency of 50.3%, hydrogen efficiency of 62.4%, and CO 2 capture ratio of 98.1–99.5%. The efficiency is 8.4%-points higher than the pre-combustion CO 2 capture benchmark and only 1.9%-points below the unabated IGCC benchmark.

(Invited) Electrification of the Chemicals Industry Using Soec - Status of Activities at Haldor Topsoe

Haldor Topsoe considers electrolysis and solid oxide electrolysis cells (SOEC) in particular an enabling technology for the electrification of the chemical industry. We will present status updates on our various stepping stone projects: CO2 electrolysis for CO production and H2O electrolysis for biogas upgrading, as well as the demonstration project for the electrification of ammonia production.Compared to other electrolysis technologies, SOEC allows for the conversion of CO2 into useful chemicals at very high efficiencies and at a faradaic yield of 100%. Haldor Topsoe is commercializing the CO2 electrolysis technology as eCOs™: a platform for on-site on-demand CO generation from CO2 feedstock for customers requiring a reliable and safe feed of carbon monoxide at a scale of up to thousands of Nm3 CO/h.The use of SOEC for the production of H2 is also interesting, not merely due to the inherently high conversion efficiencies that can be achieved, but also due to system-level synergies found in integrating the endothermal electrolysis process with an exothermal chemical synthesis process. The pilot plant for upgrading CO2 in biogas into pipeline quality synthetic natural gas (SNG), located at Foulum, Denmark, provides an example of such integration. Long-term operation of the 16 Nm3/h H2 SOEC module integrated with a methanation unit will be presented.Rapidly dropping renewable energy (RE) prices have created a unique opportunity space for introducing new technology based on electricity and RE, over the more conventional fossil-based processes. Electrolytic hydrogen production is a key technology for the transition towards a 100 % sustainable energy system, as it can both facilitate large-scale storage of renewable electricity (RE) and provide the necessary supply of CO2-free H2 for industrial use and for the production of sustainable fuel and chemicals. One of the most essential of the latter in our modern society is ammonia (NH3), which is used for fertilizer production and thus vital for feeding the world’s population.Haldor Topsoe is a market leader in providing technological solutions and catalysts for the production of ammonia. Since 2000, Topsoe has commissioned >60 ammonia plants across the world with an accumulated capacity of almost 100 000 metric tonnes of NH3 per day. It is Haldor Topsoe’s ambition to become a technology provider for future fully electrified ammonia plants and a roadmap highlighting the key milestones towards SOEC + Haber-Bosch plants in 2030 will be presented. One scenario in that vision is to utilize the ability of SOEC stacks to operate in endothermal mode. A novel process is being developed where the nitrogen for the ammonia synthesis is provided by burning air between the SOEC stacks and utilizing steam generated in the Haber-Bosch loop as feedstock. This results in a very efficient plant (71 % LHV efficiency) and eliminates investment in an air separation unit.Common for the applications mentioned above is that the SOEC stacks must have high robustness to handle dynamic operation, as well as long lifetimes, to lower the overall cost of stacks and systems. The technical features of Haldor Topsoe’s SOEC stack platform will be presented, along with lifetime test results of multi-stack configurations in different electrolysis operations.

Effect of catalyst to lignite ratio on the performance of a pilot scale fixed bed gasifier

Gasification of lignite in the presence of MgCO3 as a catalyst was carried out in 10 kWe pilot scale (atmospheric pressure) downdraft gasifier. The main disadvantage with lignite is the clinker formation at high temperature. To overcome this problem, MgCO3 lumps were used as a catalyst with lignite. The experiments were carried out with four different catalysts to lignite (C/L) ratios (wt%) viz. 0%, 3%, 5% and 7%. The performance of the gasifier was evaluated on various parameters such as specific fuel consumption (SFC), gas yield, producer gas composition, producer gas heating value (LHV), cold gas efficiency (CGE), tar and particulate matter (PM). The Energy balance, mass balance, and exergy analysis were also carried out for different C/L ratios. The studies reveals with an increase in the C/L ratio, clinker formation reduced and became almost negligent with 7% C/L ratio. Furthermore, the gas yield, H2/CO ratio, LHV, CGE and exergy efficiency also increased by 2.52%, 9.94%, 22.22%, 20.24%, and 29.04%, respectively with the same ratio. Whereas, SFC, tar and PM concentration were reduced by 16.48%, 41.61%, and 33.09%, respectively. The study concludes that the 7% loading of MgCO3 with lignite offered the best results amongst most of the reported feedstock.

Reversible Solid Oxide Cell Systems for Grid Energy Management

The present study summarizes optimization studies aimed at identifying the most effective techno-economic pathways of deploying reversible solid oxide cell (ReSOC) based systems for energy storage. Carbon dioxide and water are co-electrolyzed in the stack to produce methane and hydrogen, which can be injected directly into a natural gas pipeline or further refined into a purer stream of methane. We explore performance and cost of a 50 MWe ReSOC system for system design concepts that evaluate different levels of methane purification. Results indicate that without external methanation, the system can produce a synthetic gas mixture of roughly 42% H2 and 55% CH4 at a cost of 72 $/MWh (20 $/GJ) and an 82.7%-LHV efficiency. Adding both a methanation reactor and a membrane increases methane purity to 92%, while only increasing cost by about 5% to 77 $/MWh (21.4 $/GJ). Over half of this cost is associated with the cost of carbon dioxide and electricity.

Thermodynamic Quasi-Equilibrium Model for the Energetic Performances Analysis of the Air and Air–Steam Gasification of Raw and Pretreated Cotton Stalks by Torrefaction and Carbonization

Numerical and experimental study of the gasification of raw and pretreated (torrefied and carbonized) cotton stalks were investigated. The experimental results were used to calibrate the quasi-equilibrium model by multiplying the equilibrium reaction constant with coefficient varying from 0.01 to 10 and by fixing the methane molar fraction. Values varying from 0.15 to 0.7, 0 to 0.8 and 400 to 900 K were considered for air equivalence ratio (ER), the ratio of steam to fuel (SF) and the temperature of steam respectively. LHV and energy efficiency decreased from 4528 kJ Nm− 3 and 73.5% for the raw cotton stalks to 3984 kJ Nm− 3 and 68.9% for torrefied cotton stalks and to 3815 kJ Nm− 3 and 69.6% for carbonized cotton stalks. The energy efficiency was improved by steam injection to 80, 78 and 75% for the gasification of raw, torrefied and carbonized CS respectively due to the increase of H2 and CH4. The air–steam gasification was more efficient to air gasification at a temperature of steam over than 500 K for raw cotton stalks. However, only the air–steam gasification of pretreated cotton stalks was recommended since it has the net energy efficiency (which consider the steam energy) superior to 60%.

Renewable Energy-Driven Reversible Solid Oxide Cell Systems for Grid-Energy Storage and Power-to-Gas Applications

Reversible solid oxide cells are an electrochemical energy technology capable of providing high roundtrip efficiency and cost-effective electrical energy storage (EES). This presentation discusses the thermodynamics of cell operation and system concepts for leveraging C-H-O chemistry for energy storage and Power-to-Gas production options. Strategies for effective thermal management and balance-of-plant systems integration are highlighted, including distributed energy systems. This involves analyzing the off-design behavior of balance-of-plant components employed in both operating modes, which is necessary to reduce capital cost. The roundtrip efficiency, energy density, and capital cost tradeoffs of these configurations are quantified through computational modeling and production cost estimates for the technology. At the distributed scale, system performance of nearly 70% roundtrip efficiency and cost of storage at 6-7 ¢/kWh are possible. Power-to-gas ReSOC systems can achieve nearly 70% LHV efficiencies and reasonable costs of syngas and electricity. The presentation concludes with challenges for technology deployment.

Predicting the ultimate potential of natural gas SOFC power cycles with CO2 capture – Part B: Applications

An important advantage of solid oxide fuel cells (SOFC) as future systems for large scale power generation is the possibility of being efficiently integrated with processes for CO2 capture. Focusing on natural gas power generation, Part A of this work assessed the performances of advanced pressurised and atmospheric plant configurations (SOFC + GT and SOFC + ST, with fuel cell integration within a gas turbine or a steam turbine cycle) without CO2 separation. This Part B paper investigates such kind of power cycles when applied to CO2 capture, proposing two ultra-high efficiency plant configurations based on advanced intermediate-temperature SOFCs with internal reforming and low temperature CO2 separation process. The power plants are simulated at the 100 MW scale with a set of realistic assumptions about FC performances, main components and auxiliaries, and show the capability of exceeding 70% LHV efficiency with high CO2 capture (above 80%) and a low specific primary energy consumption for the CO2 avoided (1.1–2.4 MJ kg−1). Detailed results are presented in terms of energy and material balances, and a sensitivity analysis of plant performance is developed vs. FC voltage and fuel utilisation to investigate possible long-term improvements. Options for further improvement of the CO2 capture efficiency are also addressed.

Predicting the ultimate potential of natural gas SOFC power cycles with CO2 capture – Part A: Methodology and reference cases

Driven by the search for the highest theoretical efficiency, in the latest years several studies investigated the integration of high temperature fuel cells in natural gas fired power plants, where fuel cells are integrated with simple or modified Brayton cycles and/or with additional bottoming cycles, and CO2 can be separated via chemical or physical separation, oxy-combustion and cryogenic methods. Focusing on Solid Oxide Fuel Cells (SOFC) and following a comprehensive review and analysis of possible plant configurations, this work investigates their theoretical potential efficiency and proposes two ultra-high efficiency plant configurations based on advanced intermediate-temperature SOFCs integrated with a steam turbine or gas turbine cycle. The SOFC works at atmospheric or pressurized conditions and the resulting power plant exceeds 78% LHV efficiency without CO2 capture (as discussed in part A of the work) and 70% LHV efficiency with substantial CO2 capture (part B). The power plants are simulated at the 100 MW scale with a complete set of realistic assumptions about fuel cell (FC) performance, plant components and auxiliaries, presenting detailed energy and material balances together with a second law analysis.

Supercritical CO2 Brayton cycles for coal-fired power plants

This paper investigates the supercritical CO2 cycles performance, from thermodynamic consideration and within realistic industrial modeling hypotheses, for coal power plant application. It proposes a design of such a power cycle for a first step implementation of the technology. Main findings are the following: even with the low temperature heat available in the coal combustion flue gas, a recompression cycle is mandatory for this application: the difference between this option and a standard Brayton cycle is more than 4.5%pt efficiency. Compared to no-reheated cycle, single reheat is an effective configuration with 1.5%pt efficiency increases. Another process improvement such as double reheat cycle, double recompression cycle and an advanced flue gas economizer configuration induce efficiency gain between 0.3 and 0.5%pt. Influence of the heat sink temperature stability has been quantified: 1.5%pt reduction for 5 °C increase with a minimal cycle pressure of 7 MPa; however, performance stability could be improved by adapting the main compressor inlet pressure. As a conclusion, CO2 supercritical coal-fired power plant theoretically offers interesting performances, of 47.8%-LHV efficiency, with existing materials at current operating conditions in a relatively near timeframe.

Synthetic natural gas via integrated high-temperature electrolysis and methanation: Part I—Energy performance

In this two-part paper the production of synthetic natural gas (SNG) through integrated plants featuring high temperature electrolysis and subsequent syngas methanation is analyzed. Part one focuses on plant configuration and performance evaluation. Part two focuses on cost for an economic assessment.Two different ways to produce SNG have been analyzed: the first option features a plant that integrates steam electrolysis with methanation (Sabatier reaction); the second one considers co-electrolysis of water and carbon dioxide coupled with TREMP™ (Topsøe recycle energy-efficient methanation process). In both cases high temperature electrolysis with solid oxide cells (SOEC) technology has been employed.A power input of 10MWe was taken as the DC electricity input for both plant SOEC generators, based on power-to-gas plants now under construction in Europe. Sensitivity analyses were used to evaluate the impact of selected operating parameters on the plant performance. Especially the pressurization of the SOEC brings a reduction of the over all electrical input required to run the plant. By operating a pressurized SOEC, post-electrolysis syngas compression (that would be required otherwise because methanation takes place at ≈33bar) is partly replaced by liquid water pumping before electrolysis. Thermal integration based on the pinch analysis methodology was also applied in order to calculate the minimum external (thermal) energy requirement. Notably, most of the heat required for vaporizing and super-heating the electrolysis water can be recovered from the exothermic methanation section. Hence, a good thermal integration is available between SOEC and syngas upgrade catalytic section. This boosts the electricity-to-SNG efficiency to values as high as 80%.The co-electrolysis plant shows a LHV efficiency of 81.4% that is more than five percentage points higher than the steam electrolysis case (76%): notably exothermic in-stack methanation – that occurs in the case of high pressure co-electrolysis assessment – allows for a reduction of the electricity input to the SOEC for the same amount of syngas produced among the two plants.

Comparison on process efficiency for CLC of syngas operated in packed bed and fluidized bed reactors

Chemical-looping combustion can be carried either in circulating fluidized bed reactors or in dynamically operated packed bed reactors. In this work, the influence of the reactor selection on the overall process efficiency has been estimated. The CLC reactors are fed with syngas at 20bar, which is produced from coal gasification.In the fluidized bed system, the oxygen carrier particles are transferred from the air to the fuel reactor and vice versa. Uniform reactor temperatures can be maintained due to internal solids circulation and vigorous mixing. Due to the large solids circulation and low reaction enthalpy in the syngas-fed fuel reactor, small temperature differences result between the reactors. A case where NiO/Al2O3 is used as oxygen carrier has been considered and an LHV efficiency of 41.4% has been calculated, assuming the fuel conversion at chemical equilibrium, leading to some unconverted CO and H2 in the reaction products. If full gas conversion is obtained by using another oxygen carrier operating at the same maximum temperature of 1200°C, this efficiency can be increased to 41.8%. In the packed bed case, large temperature gradients inside the reactors are obtained and the CO2 and steam is produced at a lower (time-)average temperature. In addition, thanks to the lower average reaction temperature during fuel oxidation, the selectivity of the reduction reactions with nickel is not an issue. On the other hand, in this case a significant amount of steam has to be added or recycled to avoid carbon deposition, leading to an LHV efficiency of 41.1%. If a carrier is selected with a low kinetic activity for the Boudouard reaction, the efficiency can be increased by 1.1% point up to 42.2% of LHV. Other differences between the two systems are related to the number of vessels needed and the volume of the solids inventory, which is expected to be lower in the circulating fluidized bed system. An initial investment cost estimation demonstrates that the oxygen carrier costs are important, so that a low cycle time and a cheap oxygen carrier is preferred.The process efficiency for both reactor configurations is at the same level, 41–42% of LHV. Therefore, it can be concluded that the reactor selection will not be made based on the process efficiency, but on the availability, operability and cost of high temperature and high pressure reactor systems, that are still under development and both present pros and cons.