Positive Flow Response Characteristic in Vertical Tube Furnace of Supercritical Once-Through Boiler

Positive flow response characteristic in vertical tube furnace of supercritical oncethrough boiler provides a feasible and effective way to alleviate the thermal maldistribution to some extent. In the present paper, theoretical derivation and simulations were performed to investigate the mechanism and forming conditions of positive flow response characteristic systematically. Two types of transition criterions were built up to judge positive or negative flow response characteristic. Parametric study was carried out to analyze the influencing factors. Both the operating conditions (mass flux, pressure, temperature or quality, heat flux) and the geometric parameters (inside diameter, length, pitch and type of tube) were investigated. In addition, thermal-hydraulic simulation of a 600 MW supercritical W-flame boiler has been carried out to take all these factors into account simultaneously. The conclusions drawn from this study will be of help to the design of large capacity supercritical once-through boilers as well as the refurbishment of existing supercritical or subcritical boilers

Thermodynamic analysis of an LNG fuelled combined cycle power plant with waste heat recovery and utilization system

SUMMARY This paper has proposed an improved liquefied natural gas (LNG) fuelled combined cycle power plant with a waste heat recovery and utilization system. The proposed combined cycle, which provides power outputs and thermal energy, consists of the gas/steam combined cycle, the subsystem utilizing the latent heat of spent steam from the steam turbine to vaporize LNG, the subsystem that recovers both the sensible heat and the latent heat of water vapour in the exhaust gas from the heat recovery steam generator (HRSG) by installing a condensing heat exchanger, and the HRSG waste heat utilization subsystem. The conventional combined cycle and the proposed combined cycle are modelled, considering mass, energy and exergy balances for every component and both energy and exergy analyses are conducted. Parametric analyses are performed for the proposed combined cycle to evaluate the effects of several factors, such as the gas turbine inlet temperature (TIT), the condenser pressure, the pinch point temperature difference of the condensing heat exchanger and the fuel gas heating temperature on the performance of the proposed combined cycle through simulation calculations. The results show that the net electrical efficiency and the exergy efficiency of the proposed combined cycle can be increased by 1.6 and 2.84% than those of the conventional combined cycle, respectively. The heat recovery per kg of flue gas is equal to 86.27 kJ s À1 . One MW of electric power for operating sea water pumps can be saved. The net electrical efficiency and the heat recovery ratio increase as the condenser pressure decreases. The higher heat recovery from the HRSG exit flue gas is achieved at higher gas TIT and at lower pinch point temperature of the condensing heat exchanger

Heat and mass transfer characteristics of simulated high moisture flue gases

Abstract The heat transfer process occurring in a condensing heat exchanger where noncondensible gases are dominant in volume is different from the condensation heat transfer of the water vapor containing small amount of noncondensible gases. In the process the mass transfer due to the vapor condensation contributes an important part to the total heat transfer. In this paper, the Colburn-Hougen method is introduced to analyze the heat and mass transfer process when the water vapor entrained in a gas stream condenses into water on the tube wall. The major influential factors of the convective-condensation heat transfer coefficient are found as follows: the partial pressure of the vapor p v , the temperature of the outer tube wall T w , the mixture temperature T g , Re and Pr. A new dimensionless number Ch, which is defined as condensation factor, has been proposed by dimensional analysis. In order to determine the relevant constants and investigate the convectioncondensation heat and mass transfer characteristics of the condensing heat exchanger of a gas fired condensing boiler, a single row plain tube heat exchanger is designed, and experiments have been conducted with vapor-air mixture used to simulate flue gases. The experimental results show that the convection-condensation heat transfer coefficient is 1.532 times higher than that of the forced convection without condensation. Based on the experimental data, the normalized formula for convention-condensation heat transfer coefficient is obtained

Carbon emissions from in-situ pyrolysis of tar-rich coal based on full life cycle analysis method

The control of carbon emissions has already become a great social strategic problem in China which must be solved at present and in the future. It is imperative to carry out safe, efficient, and low-carbon utilizations in the coal industry under the target of achieving carbon emission peak. Tar-rich coal is abundant in western China. It is mostly combusted for power generation, which results in the wastage of valuable resources and serious environmental pollution. The in-situ pyrolysis process of tar-rich coal provides a new method for generating oil from coal. This method is to produce oil without mining coal while alleviating damage and pollution to geological formations. Compared with traditional coal mining methods, it can reduce the size of goaf section and minimize the damage to rock structure. As a new coal-to-oil route, the in-situ pyrolysis of oil-rich coal is still at an initial stage for research, for which there are still few carbon emission evaluations from the perspective of the full life cycle analysis. Based on the carbon emission accounting method widely adopted, the life cycle analysis (LCA) is employed to analyze the carbon dioxide emission in the whole process of an in-situ tar-rich coal pyrolysis project, including coal seam modification, in-situ heating, product processing, product transportation and terminal consumption. A lateral comparison of greenhouse gas inventory with indirect coal liquefaction and direct coal liquefaction is also carried out. At the same time, the greenhouse gas emission from the in-situ pyrolysis of tar-rich coal is analyzed systematically. The results show that it is necessary to adopt low-carbon energy in the development of in-situ pyrolysis of oil-rich coal. With power grid as the energy source, the LCA carbon emission of in-situ pyrolysis is about 2.234 5 t CO2 for each tonne of coal treated, while with wind power as the energy source, merely 0.608 6 t CO2. The in-situ pyrolysis of tar-rich coal has an obvious advantage in carbon emission reduction over indirect or direct coal liquefaction process. To reduce carbon emissions effectively, several mitigation measures need to be combined, including promoting energy efficiency, optimizing heat sources, and increasing the proportion of clean energy

Numerical simulation on H2S distribution characteristics of tangentially coal-fired boiler under wide loads

Air staged combustion reduces NOx emissions, but increases the H2S concentration in primary combustion zone. The excessive H2S concentration near the furnace wall is an important factor that aggravates the high-temperature corrosion of the water-cooled wall. With large-scale renewable power integrated into the grid, the demand for flexible peak-shaving operation of traditional thermal power units has increased. The H2S concentration distribution near the furnace wall under different boiler loads deserves attention. Hence, the influence of different operating parameters of tangentially coal-fired boiler on H2S concentration distribution near the furnace wall was investigated by orthogonal test. A supercritical 600 MW tangentially coal-fired boiler was selected to establish a numerical model. The L16(45) orthogonal numerical conditions was designed to cover four boiler loads, including 100% BMCR, 75% THA, 50% THA and 35% BMCR. A user-defined SOx generation model was employed to calculate the H2S concentration distribution inside the furnace. The release of fuel sulfur and mutual transformation of sulfur components were considered. At the same time, the model included a multiple surface reaction model for describing the heterogeneous reaction between coke and O2/CO2/H2O, and the ratio of the gasification rate to the consumption rate for char particle was calculated. The results show that the high H2S concentration areas are mainly located below the bottom burner that on operation and between the top burner and SOFA nozzle. The main reason for the high H2S concentration in the latter area is that the tangential circle of flue gas increases gradually along the furnace height. The orthogonal analysis indicates that the average H2S concentration in the key area of the furnace wall under 35% BMCR load is 364 μL/L, which is significantly lower than that under the other loads. In addition, the impact of operating parameters on the H2S concentration in key areas follows the order of boiler load > primary air rate > excess air coefficient of main combustion zone > imaginary tangent circle diameter > vertical swing angle of burner

Radiative properties of flue gas under high-altitude sub-atmospheric pressure

The special geography of the plateau area leads to a series of problems in boiler operation. In this study, the absorption coefficients and total emissivity of flue gas were determined under different air pressures during air combustion using the Line-By-Line (LBL) method based on the HITEMP2010 database (High-temperature molecular spectroscopic database). The effects of pressure, temperature, and molar fraction (H2O and CO2) on the radiative properties of flue gas were analyzed. An improved Weighted-Sum-of-Gray-Gases (WSGG) correlation, which relates the absorption coefficients to temperature and total pressure, was proposed. The results show that the reduced total pressure diminishes the total emissivity of flue gas. The maximum differences in total emissivity along the path lengths for the four working conditions with a pressure drop from 0.101 325 to 0.061 655 are 0.093 4, 0.084 5, 0.091 1, and 0.084 3, respectively. For a larger molar fraction, the effect of pressure on the total emissivity is greater for shorter path lengths but not for longer ones. Similarly, the higher temperature would reduce the total emissivity of flue gas. The maximum differences in total emissivity along the path lengths for the four working conditions with a temperature increase from 1 000 K to 2 500 K are 0.273 6, 0.270 5, 0.251 5, and 0.250 5, respectively. For a larger molar fraction, temperature has a greater effect on the total emissivity for shorter path lengths but not for longer ones. Furthermore, increasing the molar fraction enhances the total emissivity of flue gas. The maximum differences in total emissivity along the path lengths for the four working conditions with a molar fraction increase from 1 to 2 are 0.088 1, 0.100 4, 0.088 9, and 0.100 6, respectively. For a higher temperature or lower pressure, the effect of molar fraction on the total emissivity is smaller for shorter path lengths but greater for longer ones. The maximum relative error of the improved WSGG model for the total emissivity of flue gas under different working conditions is 3.67%. It is a significant reduction in the error compared to that of the existing WSGG model. Therefore, the improved WSGG model is more accurate for air combustion atmosphere and sub-atmospheric pressure

Evaluation of retrofitting a conventional natural gas fired boiler into a condensing boiler. Energy Conversion and Management

Abstract The exit flue gas temperature of a conventional gas fired boiler is usually high and a great amount of heat energy is lost to the environment. If both sensible heat and latent heat can be recovered by adding a condensing heat exchanger, the efficiency of the boiler can be increased by as much as 10%. In this paper, based on combustion and heat transfer calculations, the recoverable heat and the efficiency improvement potential of different heat recovery schemes at various exit flue gas temperatures are presented by performing design calculations. The payback period method has been used to analyze the feasibility of retrofitting a conventional gas fired boiler into a condensing boiler in a heating system in detail. The results show that the most economical exit flue gas temperature is 40-55°C when a conventional natural gas fired boiler is retrofitted into a condensing boiler simply by adding a condensing heat exchanger. It is feasible to use the return water of a heating system as the cooling medium of the condensing heat exchanger because the return temperature varies with the ambient temperature and is lower than the dew point of the water vapor in the flue gas in most periods of a heating season in some regions, which has been verified by retrofitted case

Research on 3D Simulation Modeling Method of Faults Based on Coal Stratum Floor TIN

The research on 3D simulation technology for complex geological body according to geological and remote sensing data is a hot issue in current geosciences research field. The simulation and expression of coal mine structures containing faults are the main bottleneck. A new 3D interactive simulation modeling method is proposed and a relevant code is developed for the method. By using the system, the fault data can be gained from the coal stratum floor triangulated irregular network (TIN) interactively, and the 3D solid models of the fault are generated according to the results of computing the fault/coal seam intersection line, fault modeling centre lines and fault modeling influence domains. Moreover, the floor contours of coal stratum including faults can be updated quickly and accurately, which lay the foundation for precise modeling of coal body and digital mine construction

HY-2A Altimeter Data Initial Assessment and Corresponding Two-Pass Waveform Retracker

The accuracy and resolution of the marine gravity field derived from multisatellite altimeter data sets mainly depend on the corresponding range precision and spatial distribution. Here, we preliminarily investigate the performance of HY-2A altimeter data by analyzing cross-mission sea surface height discrepancies with SARAL/AltiKa and calculating correlation coefficients with respect to tide gauge measurements. We also explore the improved range precision that can be achieved using a two-pass weighted least squares retracker which was proposed for the purpose of optimal gravity field recovery. Firstly, both the exact repetitive mission and the geodetic mission for HY-2A provide new track orientations and different data coverage for recovering the marine gravity field, and these dense geographical distributions are more greatly attributed to the geodetic mission in recent years. Secondly, HY-2A provides reliable sea surface height measurements based on exterior verifications by SARAL/AltiKa geophysical data records and tide gauge measurements, although the accuracy level is slightly lower than SARAL/AltiKa. Another more exciting finding is that the statistics of along-track sea surface heights in one-second intervals show that the two-pass retracking does further improve the range precision by a factor of 1.6 with respect to 20 Hz retracked results in sensor data records. In conclusion, the HY-2A mission can substantially improve the global accuracy and resolution of the marine gravity field and will reveal new tectonic features such as microplates, abyssal hill fabric, and new uncharted seamounts on the ocean floor