CFD validation of optimized compact heat exchanger designs

In offshore oil and gas production gas turbines are used for both power production and to provide process heat. CO2 emissions from the gas turbines accounts for about 25 % of the total Norwegian emissions and installing a bottoming cycle to produce power by recovering heat from the gas turbine exhaust is one way to reduce these missions. When installing a steam bottoming cycle offshore, the total weight and size will be important, and there is a need for a compact heat recovery steam generator (HRSG). A compact HRSG will often need to be designed with smaller tube diameters than conventional on-shore steam generators. To increase confidence in the compact design, the heat transfer and pressure loss models need to be accurate for the relevant geometry ranges. In this work, a compact Once Through Steam Generator (OTSG) is designed using optimisation procedures where the total weight of the steam generator has been minimised for a desired duty with restrictions for pressure losses. A range of correlations from the literature were used for the calculation of the performance. The results from the optimisation show that the ’heaviest’ results were about three times the minimum weight than the ’lightest’. To increase confidence in the results, and to provide arecommendation for design models, a validated CFD model was used to perform a numerical analysis of the optimised geometry and compare this with the correlations

Steady State and Transient Modelling of A Three-Core Once-Through Steam Generator

To reduce emissions and save fuel in offshore power production using gas turbines, one can use the gas turbine exhaust as a heat source for a bottoming cycle for heat and power production. This can replace about one in four gas turbines. In offshore applications weight and size become more important and thus a once-through steam generator (OTSG) is a way to achieve low weight for the bottoming cycle. To reduce the size and weight of the OTSG further, one can reduce the tube diameter in the tube bundles. In this work a three-core OTSG, representing the economizer, evaporator, and superheater, was modelled and the design optimized to achieve minimum weight, while producing a certain amount of power and keeping within constraints of flue gas and steam pressure losses. This was done for varying tube diameters in each of the cores, in steady state. Afterwards transient simulations were performed for each optimized design to find their response times to a step change in the gas turbine load. The evaporator has the biggest impact on both the weight and the response time, while the superheater and economizer had similar and smaller impacts on both the weight and response time

Steady State and Transient Modelling of A Three-Core Once-Through Steam Generator

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CFD validation of optimized compact heatexchanger designs

In offshore oil and gas production gas turbines are used for both power production and to provide process heat. CO2 emissions from the gas turbines accounts for about 25 % of the total Norwegian emissions and installing a bottoming cycle to produce power by recovering heat from the gas turbine exhaust is one way to reduce these missions. When installing a steam bottoming cycle offshore, the total weight and size will be important, and there is a need for a compact heat recovery steam generator (HRSG). A compact HRSG will often need to be designed with smaller tube diameters than conventional on-shore steam generators. To increase confidence in the compact design, the heat transfer and pressure loss models need to be accurate for the relevant geometry ranges. In this work, a compact Once Through Steam Generator (OTSG) is designed using optimisation procedures where the total weight of the steam generator has been minimised for a desired duty with restrictions for pressure losses. A range of correlations from the literature were used for the calculation of the performance. The results from the optimisation show that the ’heaviest’ results were about three times the minimum weight than the ’lightest’. To increase confidence in the results, and to provide arecommendation for design models, a validated CFD model was used to perform a numerical analysis of the optimised geometry and compare this with the correlations.CFD validation of optimized compact heatexchanger designspublishedVersio

Numerical modelling of fin side heat transfer and pressure loss for compact heat recovery steam generators

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Comparing exergy losses and evaluating the potential of catalyst-filled plate-fin and spiral-wound heat exchangers in a large-scale Claude hydrogen liquefaction process

Detailed heat exchanger designs are determined by matching intermediate temperatures in a large-scale Claude refrigeration process for liquefaction of hydrogen with a capacity of 125 tons/day. A comparison is made of catalyst filled plate-fin and spiral-wound heat exchangers by use of a flexible and robust modeling framework for multi-stream heat exchangers that incorporates conversion of ortho-to para-hydrogen in the hydrogen feed stream, accurate thermophysical models and a distributed resolution of all streams and wall temperatures. Maps of the local exergy destruction in the heat exchangers are presented, which enable the identification of several avenues to improve their performances. The heat exchanger duties vary between 1 and 31 MW and their second law energy efficiencies vary between 72.3% and 96.6%. Due to geometrical constraints imposed by the heat exchanger manufacturers, it is necessary to employ between one to four parallel plate-fin heat exchanger modules, while it is possible to use single modules in series for the spiral-wound heat exchangers. Due to the lower surface density and heat transfer coefficients in the spiral-wound heat exchangers, their weights are 2–14 times higher than those of the plate-fin heat exchangers. In the first heat exchanger, hydrogen feed gas is cooled from ambient temperature to about 120 K by use of a single mixed refrigerant cycle. Here, most of the exergy destruction occurs when the high-pressure mixed refrigerant enters the single-phase regime. A dual mixed refrigerant or a cascade process holds the potential to remove a large part of this exergy destruction and improve the efficiency. In many of the heat exchangers, uneven local exergy destruction reveals a potential for further optimization of geometrical parameters, in combination with process parameters and constraints. The framework presented makes it possible to compare different sources of exergy destruction on equal terms and enables a qualified specification on the maximum allowed pressure drops in the streams. The mole fraction of para-hydrogen is significantly closer to the equilibrium composition through the entire process for the spiral-wound heat exchangers due to the longer residence time. This reduces the exergy destruction from the conversion of ortho-hydrogen and results in a higher outlet mole fraction of para-hydrogen from the process. Because of the higher surface densities of the plate-fin heat exchangers, they are the preferred technology for hydrogen liquefaction, unless a higher conversion to heat exchange ratio is desired.publishedVersion©2019 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy PublicationsLLC. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/

Effect of use of zero-carbon and low-carbon fuels on the performance of compact combined cycles for power generation

Offshore electrical power is normally generated with several gas turbines running on natural gas produced on-site. These account for about 85% of the CO2 -emissions from the oil and gas sector in Norway. About 24% reduction of these emissions can be achieved by installing a steam bottoming cycle. Switching to zero-carbon fuels can be a future option to fully remove these emissions. Only a few offshore steam bottoming cycles are installed today, and gas turbines running on non-carbon fuels are technology under development. This study aims to quantify and analyze the effects on the performance of an offshore combined cycle after switching the gas turbine fuel from natural gas to alternatives with lower environmental impact. The fuels and fuel blends considered include hydrogen, ammonia, bio-methanol, and mixtures thereof — all of which have the potential to reduce overall CO2 intensity of power production. Hydrogen and ammonia, in particular, offer direct reductions in CO2 emissions. In our analysis we want to answer the following: - (1) will an installed steam bottoming cycle produce the same amount of power and operate under similar conditions when switching fuels? (2) For the different fuels, how much fuel would a combined cycle save with the same total power output? and, (3) What is the estimated total emission reduction potential for different fuel alternatives? The study is based on a common offshore configuration composed of four 40 MW gas turbines operating in simple cycle. In our analysis we replace one of them with a steam bottoming cycle and the results show that the net power output from the steam cycle across the fuels varied from 42.7 to 46.4 MW using equal steam cycle assumptions, with the ammonia-dominated mixtures showing the best performance. A once-through steam generator (OTSG) was designed for natural gas exhaust and applied in an improved cycle optimization aimed at maximizing steam bottoming cycle net power output while maintaining a fixed total power production. Results indicate significant fuel-saving potential ranging from 157 t/day for natural gas to 462 t/day for ammonia as fuel. These fuel-saving potentials were further used to estimate the potential for greenhouse gas emissions. The results showed that the reductions would be in the range of 85%–91% for the zero-carbon fuels. For bio-methanol, the emission reduction could vary from 4% to 70% depending on the production pathway.publishedVersio

Heat transfer and pressure drop of supercritical CO2 in brazed plate heat exchangers of the tri-partite gas cooler

The heat transfer characteristic of supercritical CO2 is an essential research topic due to its significant influence on the performance of heat exchangers and systems. In this paper, the heat transfer and pressure drop of supercritical CO2 in the brazed plate heat exchangers are experimentally researched. The heat exchangers belong to a tri-partite gas cooler which can simultaneously fulfill the demands of domestic hot water and space heating. The results demonstrates that the thermal resistance in the CO2 side is the main factor that influences the total heat transfer. The increase of CO2 inlet pressure can reduce the heat transfer coefficients except at the high temperature region. The improvement of heat transfer coefficient by increasing the CO2 mass flow rate is more significant in the space heating (SH) and domestic hot water (DHW) preheating gas coolers, and is lowest in the DHW reheating gas cooler. The influence of DHW inlet temperature is more obvious in the DHW preheating gas cooler that connected to the water inlet. The influence of water mass flow rate is different in the DHW and SH operation modes. Moreover, the effects of CO2 pressure and mass flow rate on the buoyancy force are discussed and the influence of buoyancy force on heat transfer is verified. The inaccuracy of the correlations from the literature is proved and then new correlations are established. The mean absolute relative errors of the new correlations are 11.61% and 12.82% for the one-pass and two-pass configurations, respectively. Furthermore, the frictional pressure drop in the heat exchangers is low (up to 36.51 kPa) and basically increases as the Reynolds number increases.publishedVersio

Thermodynamic models to accurately describe the PVTxy-behavior of water / carbon dioxide mixtures

Carbon dioxide/water (CO2/H2O) mixtures are of much interest in carbon capture and storage, atmospheric science, in the description of human lungs and in the processing of food and beverages. We present a comprehensive comparison of thermodynamic models for describing their PVTxyPVTxy behavior, i.e. densities and phase compositions. The most accurate experimental data in the temperature range 273–478 K and at pressures below View the MathML source61MPa are selected after a critical data evaluation. The most reliable phase equilibrium data are used to fit the binary interaction parameters of a wide range of thermodynamic models: cubic equations of state (EoS) with quadratic/Wong–Sandler/Huron–Vidal mixing rules, CPA, PC-SAFT and PCP-SAFT with different association schemes, and corresponding states models with various reference fluids. We test the predictive ability of the models by comparing to data outside of the region used in the parameter-fit. All of the thermodynamic models are fitted with the same experimental data and compared on the same basis, facilitating a general discussion about their strengths and weaknesses. As a benchmark for the performance of the models, we compare with the performance of two multiparameter EoS: GERG-2008 and EoS-CG. At least three fitting parameters are needed to represent the PVTxyPVTxy behavior of CO2/H2O mixtures within an accuracy of 10%. By including a fourth parameter, it is possible to significantly improve the accuracy for phase compositions, where the Peng–Robinson cubic EoS with the Huron–Vidal mixing rule and volume shift gives the best results with an average accuracy of 4.5% and 2.8% for phase compositions and densities respectively. In comparison, the most accurate multiparameter EoS, EoS-CG, exhibits an average accuracy of 8.0% and 0.6% for phase compositions and densities respectively.publishedVersion© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/

Achieving 50% weight reduction of offshore steam bottoming cycles

Adding a bottoming cycle to the gas turbines powering offshore oil and gas production plants allows additional power to be produced from recovered excess heat. Hence, the power demand of the platform can be met by burning less natural gas, and the CO2 emissions reduced by up to 25%. However, the weight of the current bottoming cycles must come down to enable widespread implementation. This work presents a thorough weight minimization of a steam bottoming cycle utilizing gas turbine exhaust heat. Unconventional, but feasible designs of heat exchangers, ductwork and structural components are considered along with materials switching. Overall weight reductions of 38% and 52% were achieved for a 16 MW and a 12 MW offshore bottoming cycle respectively when compared to a 16 MW reference system. Key factors in achieving the weight reduction were the use of small steam generator tubes with an inner diameter of only 10 mm, improved condenser design and the use of aluminium structural framework replacing steel. By more than halving the weight of the bottoming cycle, it's implementation potential on offshore platforms has been greatly improved and can move the oil and gas industry towards significantly reduced CO2 emissions.publishedVersio