MW Boiler Research Articles

A method to target and correct sources of unburned carbon in coal-fired utility boilers

This study presents a novel method to examine particle burnout in coal-fired utility boilers by extracting information from computational fluid dynamics (CFD) model results. The direct targeting of underlying causes enabled by this method can provide dramatic improvement in the unburned carbon in fly ash (carbon-in-ash or CIA), improving the commercial value of the ash as an additive in the production of concrete and cement and improving boiler efficiency. Data for thousands of particles are extracted and summarized for engineering characteristics such as injection burner, particle size, residence time, and exposure to oxygen. As an example of the method, unburned carbon is studied for a 200MWe tangentially-fired utility boiler with CIA issues. The analysis, which is consistent with boiler operation data, reveals that a majority of CIA can be contributed by a disproportionately small number of sources, hence the advantage of a targeted approach. The analysis of the 200 MW boiler reveals that fewer than half of the burners contribute about 80% of the CIA and the two largest coal particle size classes contribute 70% of the CIA. Most importantly, however, the oxygen availability for the coal particles is found to be the key factor for coal burnout. Based on this result, a simple and targeted strategy to improve burnout by improving oxygen availability is designed. This method is predicted by the model to reduce CIA from 3.27% to 1.3% and NOx from 588 ppm to 503 ppm.

Highly efficient and economical nitrogen oxides controlled by an in‐furnace urea solution pyrolysis coupled with SCR system for a coal‐fired utility boiler

ABSTRACTAn in‐furnace urea pyrolysis coupled with selective catalytic reduction (SCR) system was applied on a 125 MW tangentially coal‐fired utility boiler in China. The coupling system was well‐designed to distribute NO and NH3 more uniformly in the flue gas at the inlet of the SCR reactor. The experimental results showed that the homogeneous NO and NH3 concentrations distribution could result to better performance of the SCR reaction. A 15 wt% urea solution directly injected into the furnace at the same flow rate of 1.2 t/h in Cases 4–6, corresponded to the normalized stoichiometric ratio of 1.40, 1.47 and 1.35. The coupling system showed a high NO reduction efficiency of up to 85–89% and could control NO emission to as low as 40, 34 and 27 ppm, respectively, with NH3 slip less than 5 ppm. The calculated urea utilization efficiency was above 58% in the cases. The high utilization efficiency of the urea solution would significantly reduce the running cost of the system. There were only five injectors at the boiler's front wall, and one layer of total 90.3 m3 catalysts in the two SCR reactors, which would drastically reduce the initial investment. The effects of the urea solution flow rates were studied at 100 and 125 MW boiler loads; it was found that 1.2 t/h was the suitable flow rate of the urea solution, and the NO emission could be controlled lower than 40 ppm. The coupling system was stable, efficient and cost‐efficient in industrial operation. © 2012 Curtin University of Technology and John Wiley & Sons, Ltd.

A Multizone Model of an Economizer in a 600 MW Boiler Unit

In this paper, a multizone model is developed to investigate the performance of an economizer under all conditions. The model primarily determines the economizer’s distribution parameters under all conditions with a small computational cost. Both the steady-state and dynamic behavior are calculated. These results are shown to be accurate and reliable using a computational fluid dynamic (CFD) model and the operational data obtained from a 600 MW boiler unit in Hubei province, China. Additionally, the model is used to predict the distribution characteristics during some fault conditions.

Technology of oxygen production in the membranecryogenic air separation system for a 600 MW oxy-type pulverized bed boiler

Abstract In this paper the results of the thermodynamic analysis of the oxy-combustion type pulverized bed boiler integrated with a hybrid, membrane- cryogenic oxygen separation installation are presented. For the calculations a 600 MW boiler with live steam parameters at 31.1 MPa /654.9 oC and reheated steam at 6.15 MPa/672.4 oC was chosen. In this paper the hybrid membrane-cryogenic technology as oxygen production unit for pulverized bed boiler was proposed. Such an installation consists of a membrane module and two cryogenic distillation columns. Models of these installations were built in the Aspen software. The energy intensity of the oxygen production process in the hybrid system was compared with the cryogenic technology. The analysis of the influence of membrane surface area on the energy intensity of the process of air separation as well as the influence of oxygen concentration at the inlet to the cryogenic installation on the energy intensity of a hybrid unit was performed.

Application of Fluidization Reconstruction Energy-Saving Combustion Technology on 300MW CFB Boiler

Aiming at high power consumption by auxiliaries of circulating fluidized bed (CFB) boiler, combustion tests were carried out on a 300MW inferior anthracite fired CFB boiler to decrease the bed material inventory via energy saving combustion technologies for CFB boiler based on fluidization reconstruction. The effects of fluidization reconstruction on boiler performance improvement were analyzed. Application practices show that after fluidization reconstruction, the power consumed by auxiliaries of the CFB boiler decreases obviously. Auxiliary power ratio of the 300MW boiler unit has been reduced from 6.1% to 4.3%. The erosion of boiler heating surface has been alleviated obviously.

Characterizing and modeling of an 88 MW grate-fired boiler burning wheat straw: Experience and lessons

Grate-firing is one of the main technologies currently used for biomass combustion for heat and power production. However, grate-firing is yet to be further developed, towards a better technology for biomass combustion, particularly towards higher efficiency, lower emissions, and better reliability and availability. To better understand grate-firing of biomass and to establish a reliable but relatively simple Computational Fluid Dynamics (CFD) modeling methodology for industrial applications, biomass combustion in a number of different grate boilers has been measured and modeled. As one of the case studies, modeling effort on an 88 MW grate-fired boiler burning wheat straw is presented in this paper. Different modeling issues and their expected impacts on CFD analysis of the kind of grate boilers are discussed. The modeling results are compared with in-flame measurements in the 88 MW boiler, which shows an acceptable agreement. The discrepancies are analyzed from different aspects. The lessons learned and experience gained from this and other case studies are summarized and discussed in detail, which can facilitate the modeling validation effort as well as improve grate-firing technology. Some of the addressed measures will be tested in a modern 500 kW grate boiler rig.

Superheater Steam Temperature Control Based on the Expanded-Structure Neural Network Inverse Models

In order to improve the control effect of the Superheater Steam Temperature (SST) for a 300MW boiler unit, this paper presents an inverse compensation control scheme based on the expanded-structure neural network inverse models. The input and output variables of the expanded–structure neural network Inverse Dynamic Process Models （IDPMs） for the superheater system are determined from understanding of the boiler operating characteristics. Then, two neural network (NN) inverse controllers are designed with the IDPMs as on-line output compensators for the original cascade PID controllers in order to improve the control effect. Detailed simulation tests are carried out on the full-scope simulator of the given 300MW power unit. It is shown by tests that the control effects of the NN-compensated control on the SST are significantly improved compared with the case of the original cascade PID control scheme.

Modeling of Power Plant Superheated Steam Temperature Based on Least Squares Support Vector Machines

Aiming at the strong nonlinearity and large time-varying characteristics in controlling of super-heater temperature in plant, the method of LS-SVMs(Least Squares Support Vector Machines) based on radial basis function are used to model. Under the condition of modeling approximating to performance, the sparse modeling is gotten by the pruning algorithm. The merits of the algorithm are conforming to the least structural risk in training process and hardly leading to over-fitting. The simulation of a superheating system, in one supercritical concurrent 600MW boiler in one power plant, is taken. The result shows that the controlling system can be adapt to the variation of the object characteristic well with strong nonlinearity and large time-varying characteristics rapidly.

Numerical and Experiment Research for Soft Coal under Condition of Blending Lignite

The paper adopts standard k−ɛ turbulence model and other mathematical models, and completing the blending lignite combustion numerical simulation of 600 MW boiler of one power plant. Three conditions were simulated in this paper, and these three conditions were compared and analyzed, the basic conclusion is consistent with the test. The result shows that the combustion character of blending lignite is lower than burning soft coal. Burning lignite shows a decline furnace temperature, and the simulation results are consistent with the experimental results. Pay attention to adjust the reasonable operating manner, and the operation is safety and economic. At the same time this paper also provides a useful reference for researching tangentially fired boiler furnace process.

An On-Line Fatigue Life Monitoring System for Boiler Drums Based on the Inverse Problem of Heat Conduction Method

A method based on solution of the inverse heat conduction problem was presented for online stress monitoring and fatigue life analysis of boiler drums. The mathematical model of the drum temperature distribution is based on the assumptions that the difference of temperature along the longitudinal axis of the boiler drum is negligible with changes only in the radial direction and the circumferential direction, and that the outer surface of drum is thermally insulated. Combining this model with the control-volume method provides temperatures at different points on a cross-section of the drum. With the temperature data, the stresses of the boiler drum are derived according to the FEM and the life expectancy of it is calculated based on the rain flow method. Applying this method to the cold start-up process of a 300 MW boiler demonstrated the absence of errors caused by the boundary condition assumptions on the inner surface of the drum and an applicable technique for the online stress monitoring and fatigue life analysis of boiler drums.

Burning Blended Coal of Indonesia Lignite and Vietnam Anthracite on a Face-Fired 600MW Boiler Original Designed for Bituminous Coal

Elemental research of burning blended coal of Indonesia Lignite and Vietnam anthracite on a face-fired 600MW supercritical boiler is performed, which is original designed for burning high-quality bituminous coal. Maximum ratio of Vietnam anthracite can be 40 percent. With the increase of the ratio of the anthracite, the size of residue particles and the ratio of unburned carbon in residues (UBCRs) rise up markedly, the slag formation of Indonesia Lignite alleviate. The results reveal: the burnout of anthracite is more difficult than its ignition, the key way to solve this problem is to control Fineness of Pulverized Coal Powder (FPCP) of the blended coal according requirements of anthracite, but to control the Primary Air (PA) outlet temperature of mill and the mass ratio of PA to coal as flammable coal Lignite. Besides, Secondary Air (SA) Distribution mode of Centralized Oxygen Feeding(COF), Big Swirl Intensity also have obvious effects the combustion of blended coal.

Investigation of fuel lean reburning process in a 1.5 MW boiler

This paper examines a detailed study of fuel lean reburning process applied to a 1.5MW gas-fired boiler. Experimental and numerical studies were carried out to investigate the effect of the fuel lean reburning process on the NOX reduction and CO emission. Natural gas (CH4) was used as the reburn as well as the main fuel. The amount of the reburn fuel, injection location and thermal load of boiler were considered as experimental parameters. The flue gas data revealed that the fuel lean reburning process led to NOX reduction up to 43%, while CO emission was limited to less than 30ppm for the 100% thermal load condition. The commercial computational fluid dynamics code FLUENT 6.3, which included turbulence, chemical reaction, radiation and NO modeling, was used to predict the fluid flow and heat transfer characteristics under various operational conditions in the boiler. Subsequently, predicted results were validated with available measured data such as gas temperature distributions and local mean NOX concentrations. The detailed numerical results showed that the recirculation flow developed inside the boiler was found to play an important role in improving the effectiveness of fuel lean reburning process.

CFD based identification of clinker formation regions in large scale utility boiler

Pulverised coal combustion continues to be one of the main conventional methods of producing electricity over the last several decades. Mineral matter present in coal may occur as free ions, salts, organically associated inorganic elements and crystalline minerals. During coal combustion this mineral matter may partly vapourise, fragment or coalesce. The mineral matter in coal transforms into ash during combustion which may result in ash deposition causing problems such as fouling and slagging. Slagging deposits forming in the radiation zone directly exposed to the flame radiation are generally called clinkers whereas sintered deposits formed in the convection zone not directly exposed by flame radiation are referred to as fouling deposits. The paper deals with clinker formation in a typical 330 MW boiler using the commercial computational fluid dynamic code FLUENT and several available empirical indices, the latter indices being used to calculate the slagging propensity of the feed coal. The temperature distribution, velocity profiles and particle trajectories were analysed and utilised to predict likely zones of clinker formation. The most probable locations for clinker formation were found to be in the radiation zone near to nose of the furnace and the left-top side of the superheater tube sections which agrees closely with empirical plant observations. However, empirical slagging indices indicate a low to medium slagging propensity. Comparisons between the clinker formation temperatures based on the ash analysis (corresponding phase diagram) of selected clinkers from large scale boiler and CFD based temperatures for selected regions showed good agreement suggesting that the modelling based identification is reliable and can be valuable for power utilities in the development of soot blowing strategies to minimise the formation of clinker deposits.

EVALUATION AND APPLICATION OF SEMI-DRY FUEL GAS DESULFURIZATION (FGD) WITH FABRIC FILTER

Semi-dry FGD with fabric filter has been extensively studied in recent years. After a series of industrial tests on 300MW boiler with semi-dry FGD and fabric filter, some operation notes were obtained. The test results show that CFB-FGD with fabric filter can be run longer and stable, and its actual performance is even higher than the design parameters. Moreover, the pressure drop of fabric filter after CFB-FGD is 600~800Pa (higher than normal). The fabric filter, which contributes to the desulfurization efficiency of the whole system about 5%, increasing the cleaning period, is benefit to the desulfurization efficiency, and the operation environment of fabric filter is that the relative humidity (RH) is below 30% or the flue gas temperature is 20oC higher than the dew point (DP).

Evaluation of mercury speciation and removal through air pollution control devices of a 190 MW boiler

Air pollution control devices (APCDs) are installed at coal-fired power plants for air pollutant regulation. Selective catalytic reduction (SCR) and wet flue gas desulfurization (FGD) systems have the co-benefits of air pollutant and mercury removal. Configuration and operational conditions of APCDs and mercury speciation affect mercury removal efficiently at coal-fired utilities. The Ontario Hydro Method (OHM) recommended by the U.S. Environmental Protection Agency (EPA) was used to determine mercury speciation simultaneously at five sampling locations through SCR-ESP-FGD at a 190 MW unit. Chlorine in coal had been suggested as a factor affecting the mercury speciation in flue gas; and low-chlorine coal was purported to produce less oxidized mercury (Hg 2+) and more elemental mercury (Hg 0) at the SCR inlet compared to higher chlorine coal. SCR could oxidize elemental mercury into oxidized mercury when SCR was in service, and oxidation efficiency reached 71.0%. Therefore, oxidized mercury removal efficiency was enhanced through a wet FGD system. In the non-ozone season, about 89.5%–96.8% of oxidized mercury was controlled, but only 54.9%–68.8% of the total mercury was captured through wet FGD. Oxidized mercury removal efficiency was 95.9%–98.0%, and there was a big difference in the total mercury removal efficiencies from 78.0% to 90.2% in the ozone season. Mercury mass balance was evaluated to validate reliability of OHM testing data, and the ratio of mercury input in the coal to mercury output at the stack was from 0.84 to 1.08.

E311 OPTIMIZATION OF BOILER START-UP PROCESS BASED ON FUZZY REASONING AND PROCESS SIMULATION(Boiler-2)

This paper presented a method for optimization of boiler start-up process based on fuzzy reasoning and process simulation. The boiler parameters are calculated by the simulation program of boiler start-up process, and the drum stress and fatigue life are analysed according to the ASME Boiler and Pressure Vessel Code. The fuzzy relationship between the firing rate and the drum stress is established, and the fuzzy reasoning rules are instituted. The firing rate curve and the pressure-raising curve of the start-up process are optimized by fuzzy reasoning. Using this method, the start-up process of a 600MW boiler from the time of boiler light-off to the time of turbine rolling is optimized. The results show that using the optimized starting curves the start-up time can be reduced greatly without increasing the loss of drum life. Simultaneity, the optimum firing rate and pressure-raising curve provided by the method can help the operators of power plant regulate the start-up process conveniently and intuitively.

Creep Behaviour and Stress Analysis of a Seventeen-Year Service-Exposed Primary Super Heater Tube in a Thermal Power Plant

The elastic, inelastic deformation and stress distribution pattern of a seventeen year service-exposed primary super heater tube in a 120 MW boiler of a thermal power plant have been evaluated by using both analytical and numerical techniques. The commercial finite element computer code ANSYS® was used for stress analysis. The methodology is a valuable design tool for development of new boiler tubes as well as for considering the effects of numerous operating variables on creep life. The health of the tube was also assessed based on microstructure, hardness and a few conventional creep tests carried out at 500 °C at various stress levels (40 to 177 MPa). Results revealed that there was not much variation in the microstructure and hardness of the service-exposed tubes compared to the virgin material. Creep deformation behaviour of the service-exposed and virgin tubes of the same material at 40 MPa revealed that deterioration of the creep properties fell within the 20% scatter band, which is well within the specified limits of ASTM standard. The service-exposed primary super heater tube is thus in a good state of health.

Analytical and experimental research for decreasing nitrogen oxides emissions

This paper presents analytical as well as experimental research methods focused on decreasing nitrogen oxides formation while burning natural gas in a furnace of an 80 MW boiler. For this work different methods are investigated such as: the recirculation of smoke gases in the burners both in hot air and fuel; injection of water in the burning zone with the simultaneous decrease of the excess air coefficient into the burning zone including the organization of two stage burning in a furnace. The effects of combining these methods are investigated in order to obtain the maximum reduction in the concentration of nitrogen oxides exiting the burning zone. Operating modes of the boiler with the recirculation of smoke gases in hot air are used as a reference in calculating NOx formation and NOx emissions are measured from the stack of the boiler.

A failure analysis and remnant life assessment of boiler evaporator tubes in two 250 MW boilers

It has been established from (a) the tube thermal flux calculations, (b) the failure surface fractographic features, (c) the data sheets on evaporator tube temperatures from both Mitsubishi and Seimens, (d) the reported data from the literature and (e) to a certain extent, the tube strain gauge data that the recent waterwall tube failures in two 250 MW boilers were the result of high stresses being active at certain boiler locations. At start-up these stresses were estimated from fractographic features to be around 370–420 MPa in magnitude, which resided between the tube materials yield and ultimate tensile strengths. The defect extension process in the present waterwall tube failures was shown to be fatigue crack growth, which is the most common failure mode in waterwall boiler tubes. By adopting two separate approaches, viz., an “upper bound” ASME XI design line and an influence diagram model approach which was designed specifically for waterwall tubes, detailed remnant life values could be estimated for a number of different situations. In the case of new replacement tubes, both approaches estimated a remnant life of about 9 years. Note that the failure probability of the upper bound ASME XI predictions was demonstrated to be around 10 −5. Also the influence diagram model approach was extremely helpful in identifying the probable root cause of corrosion fatigue failures in waterwall boiler tubes. With respect to the remnant life estimates two separate cases were considered, viz., (1) tubes with defect populations as recorded during the recent NDT study and (2) new replacement evaporator tubes which contained zero defects.

Estimation of furnace exit gas temperature (FEGT) using optimized radial basis and back-propagation neural networks

The boiler is a very important component of a thermal power plant, and its efficient operation requires continuous online information of various relevant parameters. Furnace exit gas temperature (FEGT) is one such important design/operating parameter. Knowledge of FEGT is not only useful for design of convective heating surface but also helpful for operating actions and decision making. Its online information ensures improvement in economic benefit of the power plant. Non-availability of FEGT on the operator desk greatly limits efficient operation. In this study, a novel method of estimating FEGT using neural network is presented. The training data are first generated by calculating FEGT using heat balances through various heat exchangers. Prediction accuracy and fast response are major advantages in using neural network for estimating FEGT for operator information. Two types of feed forward neural modeling networks, radial basis function and back-propagation network, were applied and compared based on their network simplicity, model building and prediction accuracy. Results are verified on practical data obtained from a 210 MW boiler of a thermal power plant.