Furnace Waterwall Research Articles

First results gained from mastering the operation of a KV-GM-100-150S gastight hot-water boiler constructed using all-welded membrane panels for waterwalls and convective banks

The results obtained from studies of the temperature operating conditions and stressed state of furnace waterwalls and sections of convective banks are analyzed, and technical solutions for improving the reliability of subsequent boilers of this type are recommended on the basis of this analysis.

Soft-computing models for soot-blowing optimization in coal-fired utility boilers

Fouling and slagging are classical difficulties in pulverized fuel utility boilers, which cause important degradation and dramatic reduction of efficiency. Current cleaning devices, traditionally based on prefixed soot-blowing manoeuvres, mitigate the problem only partially and offers clear optimization possibilities. But the development of predictive control is far from easy: soot-blowing represents a significant fraction of efficiency, the effectiveness has an intrinsic level of randomness and the fouling dynamics involves nonlinear feedback loops. The complexity of the phenomenon makes not applicable theoretical models or statistical analysis. The problem requires the application of alternative methodologies as expert systems or soft-computing based techniques. The present paper aims to develop a probabilistic model to predict the effectiveness of soot-blowing based in Artificial Neural Networks and Adaptive Neuro-Fuzzy Inference Systems. The validity of the model has been illustrated in a real case-study boiler, a 350 MW e Spanish power station. Training and test data for these models are provided by a monitoring system based on heat-flux measurements in the furnace water-walls. For evaluation and comparison purposes, the quality of prediction obtained for the mentioned algorithms is analyzed in terms of performance indices. The connection weight approach reveals the relative importance of input variables in each soft-computing model. Finally, the integration of these models into an advisory tool is discussed.

Burnout in the horizontal tubes of a furnace waterwall panel

An experimental study of heat transfer that occurs in tubes nonuniformly heated over the perimeter at low velocities of subcooled water flowing in them is presented. Experiments with unsteady supply of heat made it possible to determine heat fluxes under burnout conditions. Unusually low values of critical heat fluxes were obtained under such conditions.

Origins of tensile stress-induced circumferential cracking of waterwall tubes in boilers

The leakage and rupture of boiler tubes in power plants is a serious problem that can lead to unscheduled and costly outages. The predominant failure location of current concern is circumferential cracking on the fireside of waterwall tubes in the furnace waterwall section of a boiler. Although there is basic agreement that cracking results from a combination of thermal fatigue and corrosion, a complete explanation of the basic phenomena needed to establish the root causes of this problem is lacking. The purpose of the present study was to analyse the sources of the tensile stress responsible for initiating circumferential cracking and to identify the key parameters controlling this tensile stress. The results of analytical modelling suggested that a combination of increasing tube wall temperature with increasing thickness of internal oxide layers, and temperature spiking due to deslagging events eventually may result in tensile stresses sufficient to crack the fireside oxide and initiate the development of circumferential cracks. This scenario also led to suggestions for reducing tensile stresses in waterwall tubes which, in turn, would be expected to delay/avoid circumferential cracking and improve the reliability of waterwall tubes.

Reliability of furnace waterwalls of hot-water boilers

It is shown that scale formation plays a deciding role among the factors that cause the wall temperature and axial thermal stresses to rise to limiting values at which waterwall tubes lose their strength. Conditions under which hydraulic nonuniformity of waterwall tube panels, a phenomenon stimulating the growth of scale in misadjusted tubes, occurs are considered. Ways for achieving better operational reliability of waterwall tubes are discussed.

The thermal efficiency factor of furnace waterwalls as applied to the standard method for thermal calculation of boilers

A justification is given for the possibility of improving the accuracy with which heat transfer in a furnace is calculated in accordance with the third edition of the standard method without making any changes to it. A better accuracy is achieved solely by varying the empirical coefficient ζ taking into account the ideas regarding the effect of different factors on the processes through which the waterwalls are contaminated and covered with slag.

Development of a Thermal-Hydraulic Analysis Code in the Distributed Parameter System for Furnace Water Wall Tubes of Fired Boilers

For design optimization of the furnace water wall of pulverized coal-fired boilers, a thermal-hydraulic analysis code to calculate the flow rate and wall temperature distribution in the water wall has been developed. The code can be applied to complex flow networks for two-phase flow consisting of many tubes and headers for both steady and transient states in the distributed parameter system. The detailed flow conditions at any tube position, such as temperature, quality, velocity, etc. can be calculated stably and quickly by the developed algorithm using a matrix solver. The results of 3-D combustion analysis are used as one of the boundary conditions for this code. The performance of the code was verified using steady and transient states data of temperature distribution in the furnace water wall tubes of actual boilers. The results of both steady and transient states analyses showed good agreement with measured data. The new algorithm decreased the calculation time for steady-state analysis significantly compared with the previous algorithm. For transient-state analysis with a high performance computer, it can calculate tube temperature faster than the real transient time.

Quasi-constant temperature combustion for improving the overall performance of a coal-fired boiler

After reviewing the current combustion technologies for burning pulverized coal with frequent and large fluctuations in coal quality and load demand, a new concept of quasi-constant temperature combustion for pulverized coal is proposed. In this method, combustion temperatures near the burners are maintained almost constant using a moveable multilayer heat-insulation device, which is installed on the fire-side of the furnace water-wall and can be moved up and down during operation. The results obtained from numerical modeling and experimental investigation show that such an insulation device can greatly improve the distribution of the flue gas temperature and can maintain the temperature effectively constant near the burner for a wider range of boiler load if the heat-insulation area of the MHID is sufficiently large. In addition, such quasi-constant temperature combustion can remarkably decrease losses of unburned materials at low loads. It can be inferred from the experiments that MHID has a good chance of being applied in practice.

Development of a Steady and Transient States Thermal-Hydraulic Analysis Program for Furnace Water Wall Tubes of Fired Boilers.

A thermal-hydraulic analysis program that can be applied to the complex flow network consisting of tubes and headers in the furnace water wall of fired boilers has been developed. This program can evaluate the flow distribution in the tubes at both steady and transient states. Heat flux distribution in the furnace, which determines each tube's heat absorption, is given by the threedimensional combustion analysis. By using this program, detailed information about the water flowing in the tube, such as temperature, quality, mass flux, etc. is obtained and the temperature of the furnace water wall is calculated. This program is applicable to most operating conditions including slagging, soot blowing and water filling at boiler start up. Both steady and transient states analyses show satisfactory agreement with measured values. This program has already been used to design fired boilers, and good performance has been obtained.

Experimental investigation on the feasibility of a movable heat-insulation device

Currently, many boilers in China can not burn low rank coals and low-calorific-value gases (blast-furnace gas, converter gas, etc.) stably and/or efficiently and have a poor adaptability to frequent variation of load and fuel rank. Moreover, the temperature and pressure of the generated steam deviate a lot from the nominal value and can not meet the users’ demand. In order to resolve these problems, a microporous ceramic-board-based movable heat-insulation device is devised. The board is installed on a furnace waterwall tube and can be moved up or down during operation so as to maintain a properly high and quasi-constant temperature in the near burner zone in the case of low load or low rank fuel. The structure, working principle and feasibility of the device are introduced and analyzed. Finally, encouraging calculation and experiment results are demonstrated. The results show that the device can work safely and reliably in low-calorific-value gas fired furnaces and all kinds of stoker-based furnaces and has very good potential for improving the overall performance of a boiler in terms of the combustion stability and efficiency, the adaptability to load and fuel.

回収ボイラーの能力向上への改造とその後の操業経験

In 1989 p Cariboo Pulp & Paper Co., upgraded the existing recovery boiler capacity including an expanded composite tube furnace upgrade, a three level air system, radiant secondary superheater section, new economizer and others.In 1994 serious furnace waterwall leaks were experienced due to water circulation problems. Circulation modifications to the boiler resulted in a 24% capacity increase over the original design.

96/06313 A 1000 MW coal fired supercritical sliding pressure operation boiler with vertical furnace waterwall

96/06313 A 1000 MW coal fired supercritical sliding pressure operation boiler with vertical furnace waterwall

Heat transfer correlations of rifled tubing for boilers under sliding pressure operating condition.

A heat transfer experiment has been carried out under the same conditions as the furnace water wall tubes of an actual sliding pressure operation boiler, using a rifled tube of 28.6 mm in outer diameter with four spiral 0.83 mm-height ribs on its inner surface, and the data was systematically correlated in order to predict the heat transfer coefficients in each region of subcritical and supercritical pressures. In the subcritical pressure single phase region, Bishop's correlation, which is usually applied to smooth tubing in a supercritical pressure region, was applicable to the rifled tubing by use of hydraulic diameter. At the post-CHF region, the heat transfer coefficient could be correlated by the Groeneveld-type expression, but its value was enhanced more than that for the smooth tubing, with a decrease in steam quality. A new correlation was developed for the supercritical pressure region.

Some Fundamental Considerations in the Design of Boiler Furnaces

Abstract This paper deals with the influence of furnace water walls on the furnace volume requirements in so far as flame temperature is the limiting factor in the allowable rates of energy release. It is shown that the totally cold furnace envelope provides for the highest permissible energy-release rate at a given flame temperature. The permissible load range in a given furnace for given high and low limits of flame temperature is, however, greatest in the refractory-lined water-wall furnace.