Gas Sides Research Articles

Numerical simulations and an experimental study for optimal design of a 1500 [formula omitted] water-tube condensing boiler

The present study is focused on investigating the condensation process and numerical simulation for optimal design of a 1500 kW water-tube condensing boiler. The specific geometry of the heat exchanger of this boiler involves compact arrangement of spiral coils to enhance heat transfer efficiency and achieve high capacity. Experimental validation is conducted on a 580 kW condensing boiler. Numerical simulations of the flow for gas and water sides of the boiler are performed in parallel using the ANSYS-FLUENT software with the k-omega (SST) turbulence model. Twelve models with different combustion chamber dimensions, coil numbers and revolutions are considered. Important design criteria include flue gas temperature, combustion chamber pressure and water pressure drop in the coils. The results indicate that increasing effective heating surface area and reducing coils gap decrease flue gas temperature to the dew point. Increasing coil revolutions has a greater impact on flue gas temperature and boiler weight compared to increasing coil numbers. With the same number of coils, by adding an extra revolution the weight is increased approximately 17 % and the effective heating surface area about 21 %. On the other hand, for the same number of revolutions, each additional spiral coil increases both of the weight and the effective heating surface area approximately 4 %. Moreover, with an increase in the number of the coils and a decrease in the number of revolutions, the pressure within the combustion chamber is decreased. By reducing the gap size, the flue gas temperature is reduced approximately 2 °C and the combustion chamber pressure is increased about 18 %. Base on the design criteria, the model six with 27 spiral coils and 7–8 revolutions, is identified as the optimal design. This progress and capability allows the design and manufacture of high-capacity boilers for residential sector as well as for industrial applications.

A coupled modeling framework for investigating flow maldistribution in cross-flow-corrugated plate heat exchangers

Accurate prediction of the impact of flow maldistribution on the thermo-hydraulic behavior of plate heat exchangers is important for thermal design and optimization in a number of practical applications. In this paper, a coupled modeling framework, consisting of a heat transfer model and a hydraulic model, is proposed for quantitatively evaluating the impact of maldistribution on the thermo-hydraulic performance of a cross-flow-corrugated plate heat exchanger. The numerical results of the proposed approach are validated by comparison with experimental data. Using the coupled model, the flow distribution nonuniformity, heat exchanger effectiveness deteriorations, and pressure drop deviations are analyzed. Furthermore, the combined effects of side stream maldistribution are investigated for different inlet Reynolds numbers and inlet temperatures. The results demonstrate that when the inlet gas Reynolds number is in the range of 1100–2700, nonuniform distribution of the gas side will cause air maldistribution, and a 10%–30% effectiveness deterioration rate. It is found that air maldistribution will not result in gas maldistribution when the inlet air Reynolds number is in the range of 6000–18 000, but increasing air nonuniformity. Will lead to increasing deterioration in heat exchanger effectiveness. In the presence of air and gas flow maldistribution, the pressure drops on the gas and air sides increase by 5% and less than 12.4%, respectively. This work provides a computational framework for optimization of cross-flow-corrugated plate heat exchangers to achieve desired thermo-hydraulic performance.

CFD modelling of the air preheater fired with gas mixture to find the cause of the preheater damage

Since various fuels, such as natural gas, blast furnace gas, coke oven gas, and a mixture of the three, are burned in the boiler, there is a danger of corrosion caused by sulphuric acid. Sulphur dioxide produced after combustion and condensed water vapour from flue gases on cold tube surfaces form sulphuric acid. Sulphuric acid solution flowing down the vertical tubes of the air heater causes corrosion not only of the air heater tubes but also of the horizontal water heater tubes located below. A simulation of the flow and heat exchange on the flue gas and air sides was performed to determine the areas of the air heater where the temperature of the air heater tubes is low and condensation may occur. Based on the measured air and flue gas mass flows, the measured flue gas temperatures behind the heater, and the air temperature at its inlet, the flue gas temperature before the air heater was determined using the ε-NTU (Effectiveness-Number of Transfer Units) method. Based on CFD modelling and experimental research, the areas in the heater where sulphuric acid can form at given outside air temperatures were determined.

Integration of thermoelectric generators in a biomass boiler: Experimental tests and study of ash deposition effect

Thermoelectric generators (TEGs) are based on Seebeck effect and produce electricity from a heat flow. They require low maintenance and provide quiet operation. However, they have low efficiency and need suitable temperature values. Accordingly, the integration of TEGs in boilers is a promising approach, since these equipment present high temperature differences between hot gases and water and allows the use of waste heat for water heating. There are experiences of the integration of TEGs in biomass boilers, but almost none with unconventional biomass with a high tendency to ash deposition. The integration of TEG modules in a commercial pellet-fired 25 kWth boiler is described and experimental results, working with pine pellets and with an agropellet (70% of vineyard pruning and 30% of barley straw), are presented. Power produced with only 6 modules can reach 60 W. After quantifying the effect of the temperature on both gas and water sides, the study focuses on analyzing the ash deposition on the exchange surfaces and their effect on TEG modules operation (e.g. power reduction of 0.24 W per hour with pine and 0.78 with agropellet). This effect can be minimized by working with high excess of air or by integrating a cleaning system.

Dynamics of large-scale bubbling fluidized bed combustion plants for heat and power production

Bubbling fluidized bed combustion (BFBC) plants for combined heat and power (CHP) production have traditionally been dispatched under slow load changes. As the amount of variable renewable electricity increases in energy systems worldwide, knowledge regarding the transient capabilities of the gas and water-steam sides of BFBC plants is required. The aim of this work is to investigate the dynamic performance of large-scale BFBC plants when accounting for both the gas and water-steam sides. To do so, this paper presents a dynamic model of BFB-CHP plants that result from connecting a model of the gas side to a process model of the water-steam side. The plant model output is validated by comparisons with operational data measured in a 130-MWth BFBC plant that produces electricity, district heating (DH) water and steam for industrial clients. The validation shows that the model can satisfactorily describe both multi-load steady-state operation and load transients. The simulation results highlight the fact that the water-steam cycle achieves stabilization more rapidly after changes in the DH line and steam delivered to customers, as compared to changes in the combustor load. The timescales of the plant outputs for different changes have been calculated, with stabilization times ranging from 2 to 15 min for the power production versus 2–25 min characterizing the DH production. Compared to the stabilization times of the gas side, the water-steam side is an order of magnitude slower, thereby limiting the transient operation capabilities of BFB-CHP plants.

A coupled combustion and hydrodynamic model for the prediction of waterwall tube overheating of supercritical boiler

Tube overheating is the leading cause of boiler tube failures that had led to tremendous replacement and maintenance costs. Boiler tube temperature is simultaneously affected by the heating effect of hot combustion gas in the furnace and the cooling effect of steam flow in boiler tubes. Thus, prediction of tube temperature needs to include the simulations of both the gas and steam flows of the boiler in the model. To compromise the significant scale difference between the boiler furnace and tubes, a coupled model that integrates a three-dimensional (3D) CFD model describing the gas flow and combustion processes in the furnace with a one-dimensional (1D) hydrodynamic model describing the steam flow in the boiler waterwall tubes was developed in this paper. The 3D CFD and 1D hydrodynamic models are coupled by iterative data exchange of boiler heat flux and steam temperature between the two models. In this way, this coupled model can incorporate all the key parameters that strongly affect the waterwall tube temperature, including the distributions of boiler heat flux, steam temperature and the convective heat transfer coefficient of steam flow in boiler tubes. In particular, the critical effects of steam maldistribution caused by both the structure of tube network system and uneven heat absorption of parallel tubes on tube temperature can be resolved by this model. This coupled model provides the capability to predict the detailed distribution of waterwall tube temperature and locate the areas of tube overheating under different boiler operating parameters. This coupled model was employed to predict the tube temperature distribution of the spiral waterwall of a 350 MW supercritical boiler. The predicted results show that the areas having higher risk of tube overheating are located near the outlet of spiral waterwall tubes where the steam temperature is the highest and on the wall areas where the local wall heat flux is the highest. The results strengthen the necessity of incorporating both the gas and steam sides of boiler into the model for accurate prediction of waterwall tube temperature.

Dynamics and control of large-scale fluidized bed plants for renewable heat and power generation

As the share of variable renewable electricity increases, thermal power plants will have to adapt their operational protocols in order to remain economically competitive while also providing grid-balancing services required to deal with the inherent fluctuations of variable renewable electricity. This work presents a dynamic model of fluidized bed combustion plants for combined heat and power production. The novelty of the work lays in that (i) it provides an analysis of the transient performance of biomass-based fluidized bed combustion plants for combined heat and power production, (ii) the dynamic model includes a description of both the gas and water-steam sides and (iii) the model is validated against operational data acquired from a commercial-scale plant. The validated model is here applied to analyze the inherent dynamics of the investigated plant and to evaluate the performance of the plant when operated under different control and operational strategies, using a relative gain analysis and a variable ramping rate test.The results of the simulations reveal that the inherent dynamics of the process have stabilization times in the range of 5–25 min for all the step changes investigated, with variables connected to district heating production being the slowest. In contrast, variables connected to the live steam are the fastest, with stabilization times of magnitude similar to those of the in-furnace variables (i.e., around 10 min). Thus, it is concluded that the proper description of the dynamics in fluidized bed combustion plants for combined heat and power production requires modeling of both the gas and water sides (which is rare in previous literature). Regarding the assessment of control strategies, the boiler-following and hybrid control (combined fixed live steam and sliding pressure) strategies are found to be able to provide load changes as fast as −5%-unit/s, albeit while causing operational issues such as large pressure overshoots. The relative gain analysis outcomes show that these control structures do not have a steady-state gain on the power produced, and therefore it is the dynamic effect of the steam throttling that triggers the rapid power response. This study also includes the assessment of a turbine bypass strategy, the results of which show that it enables fast load-changing capabilities at constant combustion load, as well as decoupling power and heat production at the expense of thermodynamic losses.

Self-optimizing control and safety assessment to achieve economic and safe operation for oxy-fuel combustion boiler island systems

Oxy-fuel combustion is a promising carbon capture, utilization and storage (CCUS) method to reduce CO2 emissions in power plants. However, it faces with the issues of high energy-cost penalties and operating risks. In this work, a combined control optimization and safety analysis are applied to optimize the control and operation of oxy-fuel combustion boiler island system. To maximize the specific operating profit, a new self-optimizing control structure is proposed to optimize the selection of desirable controlled variables. This control structure shows a robust controllability and maintains a maximum specific operating profit around 28.7 $/tCO2. Moreover, the operating profit exhibits an opposite tendency to load change, increases with the increasing oxygen supply purity and keeps constant in presence of air leakage. The failure of oxygen concentration controller causes some fluctuations on the dynamic behavior of operating parameters at water-steam and flue gas sides. The loss of oxygen flow control loop leads to a larger effect on the dynamic responses of operating parameters than the fault of oxygen concentration control loop during load change and oxygen supply purity change operating scenarios. The results provide a new route to improve control reliabilities, reduce operating costs and guide safe operations for oxy-fuel combustion systems.

Comparison of the Transient Behaviors of Bubbling and Circulating Fluidized Bed Combustors

This work compares the transient behaviors of the flue gas sides of large-scale bubbling and circulating fluidized bed (BFB and CFB, respectively) boilers. For this purpose, a dynamic model of the in-furnace side of fluidized bed combustors presented and validated by the authors in a former work is used to simulate two industrial units. The results show that for load changes the heat transfer to the waterwalls stabilizes more rapidly in BFB units. Differences in stabilization time between the dense bed and the top of the furnace are observed in both units, caused by the distribution of solids along the combustor: the dense bed contains more solids than regions located higher up in the furnace and, therefore slower to respond, with stabilization times of around 15 minutes, as compared to stabilization times in the range of 1–8 minutes for the upper furnace. This behavior is accentuated in the BFB, where all the solids remain in the dense bottom region. The effect of the characteristic times of the main in-furnace mechanisms (fluid-dynamics, fuel conversion, and heat transfer) on the dynamic performance of BFB and CFB units has been explored and expressed through proposed mathematical relationships.

The solution of shear stress discontinuity in film drainage model

The Stokes–Reynolds–Young–Laplace (SRYL) model of film drainage for pure liquid and surfactant solution systems have been numerically solved with a simplified Young-Laplace equation in several literatures. However, with this simplification, the calculated profile of the tangential shear stress τb at the gas sides had an unphysical increment at the boundary of the computational domain, which violated the continuity in shear stress especially when inhibitory effect of surfactants on film drainage was considered. In this work, an improved model of film drainage was proposed by modifying the simplified Young-Laplace equation and artificial initial shape of the liquid film. The underlying reason for unreasonable τb caused by the simplified Young-Laplace equation was analyzed. This improvement avoided the unphysical sharp increase of τb at the boundary of the computational domain for both pure liquid and surfactant solution systems, which was critical to extend the SRYL model of film drainage to some complex systems.

Experimental Study of a System for Hydrogen Permeation from Gas Phase to Liquid Sodium

In the framework of sodium fast reactors, the management of tritium contamination in the sodium secondary circuit and the control of its release into the atmosphere is fundamental. In order to capture and recover tritium by coprecipitation worth hydrogen in cold traps, it is necessary to maintain a certain amount of hydrogen dissolved in the liquid sodium stream. Hydrogen injection by permeation through nickel membranes has been proposed to provide a continuous hydrogen intake to a liquid sodium stream, allowing the desired hydrogen concentration to be reached. A permeator prototype and the related process have been designed. Permeation tests have been carried out in an experimental facility set up at CEA Cadarache at sodium temperatures from 375°C to 450°C and hydrogen partial pressures from 5 × 103 to 3 × 104 Pa in order to quantify their influence on hydrogen permeation flux. Measurements on both the gas and sodium sides provide a complete hydrogen content observability over the system. Experimental results show a good agreement with the theoretical permeation laws for hydrogen pressures below 2 × 104 Pa and provide an estimation of the temperature dependency of the permeability coefficient, which will be useful for the industrial scale-up of the process.

ОПТИМИЗАЦИЯ СТЕПЕНИ РЕГЕНЕРАЦИИ ДЛЯ ЦИКЛОВ ГАЗОТУРБИННЫХ УСТАНОВОК С КОЖУХОТРУБНЫМИ РЕГЕНЕРАТОРАМИ

The subject of consideration in the article is a mathematical model of the pressure recovery factors of gas turbine plants (GTP) regenerators, which takes into account the dependence of the hydraulic resistance of the heat exchanger on its surface area and the coefficient of compactness. Optimization of the GTP regenerative cycle and the cycle with a turbocompressor utilizer and regeneration was carried out in order to further increase their efficiency. It has been established that the use of the proposed model of the pressure recovery factors for the air and gas sides makes it possible to find the regeneration ratio that is optimal from a thermal engineering point of view. This model can be used for simplified and preliminary calculations of the GTP.

Evolution of Surface Density Function in an Open Turbulent Jet Spray Flame

A three-dimensional Direct Numerical Simulation of an open turbulent jet spray flame representing a laboratory-scale burner configuration has been used to analyse the statistical behaviours of the magnitude of reaction progress variable gradient left| {nabla c} right| [alternatively known as the Surface Density Function (SDF)] and the strain rates, which affect its evolution. The flame has been found to exhibit fuel-lean combustion close to the jet exit, but fuel-rich conditions have been obtained further downstream due to the evaporation of fuel droplets, which leads to the reduction in the mean value of the SDF in the downstream direction. This change in mixture composition in the axial direction has implications on the statistical behaviours of the SDF and the strain rates affecting its evolution. The mean value of dilatation rate remains positive, whereas the mean normal strain rate assumes positive values where the effects of heat release are strong but becomes negative towards both unburned and burned gas sides. The mean values of dilatation rate, normal strain rate and tangential strain rate decrease downstream of the jet exit. However, the mean behaviours of displacement speed and its components do not change significantly away from the jet exit. The mean values of normal strain rate arising from flame propagation remain positive and thus act to thicken the flame. The mean tangential strain rate due to flame propagation (alternatively the curvature stretch rate) remains negative throughout the flame at all axial locations investigated. The mean effective normal strain rate assumes positive values throughout the flame and it increases in the downstream direction for the present case, which is consistent with the reduction in the peak mean value of the SDF in the axial direction. The mean effective tangential strain rate (alternatively stretch rate) assumes negative values throughout the flame at all axial locations.

Kinetic Study of Reactive SO2 Absorption in a Jet Bubbling Reactor: Experimental Determination of Mass Transfer Coefficients and Absorption Rate

The reactive absorption kinetics for SO2 removal from flue gas was studied by the use of a saturated solution of Ca(OH)2 in a bench-scale jet bubbling reactor (JBR). Gas and liquid sides mass trans...

Effect of ion concentration and multivalence on methane-brine interfacial tension and phenomena from molecular perspectives

Gas-brine interfaces play an important role in natural gas recovery, which can be negatively affected by capillary pressure, induced by the interfacial tension (IFT) between gas and brine phases. Therefore, understanding about the gas-brine IFT is of great importance for natural gas production. While experiments can measure the IFT, the underlying mechanisms and interfacial phenomena remain less clear at molecular level. Previous molecular simulation works are limited to narrow salt concentrations and types. In this work, we use molecular dynamics (MD) simulations to study IFT between methane and brine containing various monovalent and divalent cations (Na+, K+, Mg2+, and Ca2+) over wide range of salt concentrations (∼0.05 to ∼4.53 M, i.e. between ∼3.0 and ∼23.6 wt% in terms of NaCl solution) under reservoir conditions. We find that methane accumulates at interfaces, resulting in the decrease of IFT as pressure increases, but becomes insignificant at high pressures. On the other hand, pressure has minor effects on water and ion distributions. Cation type has a negligible effect for given anion (Cl−) concentrations, indicating that the charge molarity is the dominant factor to determine the gas-brine IFT. In addition, while both cations and anions deplete from the gas-brine interfaces, divalent cations are more devoid from the interface than monovalent cations, showing strong electrical double layers. The electrostatic potentials on the gas and brine sides are positive and negative, respectively. Our study should provide fundamental understandings on the gas-brine interfacial properties, and important insights into natural gas production.

New methodology for CFD simulations of compact evaporators used in automotive ORC systems

Waste heat dissipated in exhaust systems of a combustion engine represent a major source of energy that can be recovered and converted into useful work. Waste heat recovery systems based on the organic Rankine cycle (ORC) have already proven their efficiency in recovering energy from different heat sources, thus, achieving a significant reduction in fuel consumption and exhaust emissions. This study aims to evaluate an engine's main component, an evaporator, using computational fluid dynamics (CFD). This study implements a coupling methodology between a one-dimensional discretised model of a working fluid (WF) and a three-dimensional solution for the gas and metal sides in the CFD software ANSYS Fluent. Computational results were compared with experimental test-bench data and with wall temperature contours registered using infrared thermography, which elicited good agreement regarding the evaporator performance, outlet temperature responses of the gas and WF, and wall temperatures. The evolution along the evaporator for all the parameters of interest, such as vapour quality, Reynolds or Nusselt numbers, and local thermal properties, were also predicted. The presented methodology represents a valuable tool for aiding the design and enhancement of compact evaporators used in automotive ORC systems.

Numerical investigation of combustion optimization in a tangential firing boiler considering steam tube overheating

A comprehensive numerical study of the combustion optimization of a 660-MW tangentially fired boiler under the constraint of steam tube overheating is presented. To obtain the chemical products and the heat flux at the heating surfaces, a three-dimensional (3D) combustion model was established using computational fluid dynamics (CFD) software. In addition, a 1D + 1D thermal hydraulic model was developed using MATLAB to describe the heating process inside the tubes. Detailed mapping methods were also provided to connect the mesh systems in CFD and MATLAB. As a result, the tube wall temperature can be calculated by considering the coupled heat transfer on the gas and steam sides. The simulation results reveal that under full load conditions, the upward tilting of the separated over-fire air (SOFA) burners can reduce the formation of CO and NOx, which helps achieve higher boiler efficiency. However, when the SOFA tilt angle increases from +20° to +30°, 0.16% of the outer surfaces of the final re-heater would exceed the allowable temperature of the TP347H material; additionally, the improvement in the boiler efficiency and decrease in NOx formation are much less. Thus, considering the unit’s safe operation, the optimal SOFA tilt angle for combustion optimization should be +20°.

Determination of the work life of low-temperature heating surfaces

Prolongation of heat and power equipment resource, which for the time being, is the only way out of the critical situation, gives a significant economic effect. Considering the uniform nature and low rate of low-temperature corrosion when burning water-fuel emulsions with water content of 30%, method for estimating service life of metal low-temperature heating surfaces at pipe metal surfaces temperature above 60°C has been proposed, at which uniform corrosion is observed. In the published literature, there are practically no data of impact on service life of low-temperature heating surfaces simultaneous influence of corrosion rates on gas and water sides of heating surfaces. The reliability of conclusion about the reliability of pipe heating surfaces largely depends on validity of conclusions made on the basis of calculations and especially taking into account experimental data of processes affecting reliability. It is on the basis of calculation results that it is possible to identify most tense elements that need to be subjected to detailed control. Therefore, in essence, reliability of entire methodology for assessing achieved and remaining resources depends on them. The paper presents a method for conducting computational studies of simultaneous influence of corrosion processes on gas and water sides of pipes and comparative assessment of service life of low-temperature heating surfaces when the limit states of metal are reached at acceptable and limit values of tensions and minimum possible thickness of pipe wall under influence of gas-chemical and water-chemical regimes and considering the data of experimental studies of processes affecting the reliability of work. To determine the time of calculated and residual service life of metal under uniform corrosion, a method for determining minimum residual thickness of metal has been presented, which still provides predestructive maximum allowable tension for remaining pipe wall thickness

Design and optimization of a novel high temperature heat exchanger for waste heat cascade recovery from exhaust flue gases

The waste heat of high temperature exhaust flue gases is widely distributed in many industrial processes. Recovery of waste heat is of great significance to energy saving and sustainability. In this paper, a novel high temperature heat exchanger with hybrid enhancement technologies is proposed to improve waste heat recovery efficiency based on the cascade recovery and utilization method. Algorithm for HTHE structural design and optimization is developed and verified according to the experimental results. Heat transfer and pressure drop performance of the proposed HTHE are estimated by using the algorithm. The results show that the effectiveness of the proposed HTHE increases as the gas temperature increases and mass flow rate decreases. Average effectiveness of the proposed HTHE and temperature of preheated air are 12.5% and 85.8 °C higher than those of traditional HTHE with additional 70.0% and 22.0% pressure drop on air and gas sides, respectively. The structural optimization of the proposed HTHE is carried out and it shows that the optimized HTHE has better heat transfer capacity and comprehensive performance under identical pressure drop, increasing effectiveness by 12.6% without enlarging pressure drop compared with the non-optimized HTHE.

Modelling of a Superheater Heat Exchanger with Complex Flow Arrangement Including Flow and Temperature Maldistribution

ABSTRACTA network approach was used to develop a thermofluid process model of a cross-flow primary superheater heat exchanger with complex flow arrangement in the convective pass of a coal-fired boiler. The model solves the transient one-dimensional forms of the conservation equations for mass, energy, and momentum, combined with the applicable closure relations, boundary values, and initial values. The dual-tube 12-pass superheater was discretized along the flue gas flow path as well as along the steam flow path. The model accounts for the convective thermal resistance on the steam side, the conductive thermal resistances of the tube wall, and scaling or fouling on the tube walls, as well as the convective and radiative thermal resistances on the flue gas side. The model was qualitatively validated using real plant data and for reference purposes also systematically compared to conventional lumped parameter models. The ability of the model to analyze the effect of the ramp rate during load changes on the tube metal temperature was demonstrated, as well as the ability to determine the maldistribution of flow and temperature on the steam and flue gas sides. Models such as this can be employed to study complex thermofluid process phenomena that may occur during intermittent, transient, and low-load operation of power plants. This could form the basis for improving operations and for the development of advanced tools for online process condition monitoring.