This article focuses on carryover at a paper mill that had been solved using computational fluid dynamics (CFD) to visualize flow within the boiler. Technicians had tried adjusting airflow and firing arrangements without success. They turned the problem over to analysts who simulated the airflow within the boiler using CFD. An animated sequence of streamlines showing airflow provided engineers with a clear understanding of exactly what was happening inside the boiler, making it relatively easy to adjust operating conditions and solve the problem. McDermott analysts use FIELDVIEW, a commercial post-processing program from Intelligent Light in Lyndhurst, NJ. With the software, the analyst can create three-dimensional perspective views with hidden-line removal and light shading. She or He can trace the path of a marker traveling along with the fluid through a series of animated views. The analysts made a second FIELDVIEW movie of the airflow conditions with the new arrangement, showing the elimination of the center core. They played the two movies simultaneously on two monitors set side-by-side to demonstrate for the customer’s engineers how the recommended changes would solve the problem.
Power boilers are commonly used in paper mills to burn industrial wood waste to generate process steam and electric power. A situation may arise in which a considerable amount of unburned wood is being picked up by the gas and carried out of the boiler; this is known in the industry as carryover. As a result, unburned wood fuel may be blown out of the boiler, causing efficiency and emissions problems.
Carryover at a paper mill was solved using computational fluid dynamics (CFD) to visualize flow within the boiler. Technicians had tried adjusting airflow and firing arrangements without success. They turned the problem over to analysts who simulated the airflow within the boiler using CFD. An animated sequence of streamlines showing airflow provided engineers with a clear understanding of exactly what wa happening inside the boiler, making it relatively easy to adjust operating conditions and solve the problem.
The 80-foot-high, 22-foot-wide, 25 foot-deep stoker-fired power boiler is used to generate process steam and electric power. Wood waste and air are injected at several points on the sides of the boiler. About one-third of the wood is burned in suspension above the stoker grate, and what remains burns on the grate. Residual ash is dumped at the end of the grate into an ash pit, and combustion gases go out the top of the boiler. The problem in thus application was that a considerable amount of unburned wood was being entrained by the gas and carried through the steam-generating banks.
Technicians at the plant tried adjusting the boiler's operating parameters. At the. bottom of the boiler, just above the grate, wood waste (chips, chunks, and dust) is blown into the boiler through four windswept spouts on the front wall. Above the windswept spouts, the combustion air is introduced into the boiler through four rows of overfire air ports, two on the front wall and two on the rear wall. Each row has seven or eight ports with 4-inch nozzles, with the front wall ports opposing the rear wall ports. Undergrate air is introduced under the stoker grate through a compartmentalized wind box. The original distribution was 60-percent undergrate air to 40-percent overfire air. Technicians at the boiler site tried changing the airflow distribution as well as increasing and decreasing the fuel intake through different spouts without any success.
They turned the problem over to analysts at McDermott Technology 1ne. in Alliance, Ohio, a unit of McDermott International Inc., a leading provider of engineering, procurement, and project management services. (The Babcock & Wilcox Power Generation Group, which is also a unit of McDermott, designs, manufactures, and builds steam-generation systems.)
McDermott analysts have used CFD for the last decade to analyze airflow and combustion inside power boilers. A CFD analysis provides fluid velocity, pressure, and temperature values throughout the solution domain for problems with complex geometries and boundary conditions. As part of the analysis, a researcher may change the geometry of the system or boundary conditions such as inlet velocity and flow rate, then view the effect on fluid-flow patterns or concentration distributions. CFD can also provide detailed parametric studies that can significantly reduce the amount of experimentation necessary to develop prototype equipment, thus reducing design cycle times and costs.
Pressure, flow, and temperature measurements are difficult to perform inside an actual boiler because of the heat. When technicians are making adjustments to a boiler, they are forced to rely on time-consuming trial and error, an often unsuccessful process. The technicians responsible for operating and maintaining the boilers were originally skeptical that computer simulation would provide practical benefits. They teased a visiting analyst, asking if he saw any arrows (such as those used to visualize flow patterns in the output of a simulation) inside the boiler.
The technicians changed their minds after seeing that CFD simulations could provide a clear picture of what is happening at any point within the boiler, making it easy in most cases to identify the problem and develop a solution. They were also impressed by a long series of experiments that showed a very high degree of correlation with the simulation results.
In this application, the analysts used a CFD program developed by McDermott staff particularly for combustion applications. This code, the combustion model for wood and refuse (COMO WR), includes wood combustion in suspension and on the stoker grate-something commercial CFD codes are unable to provide. To the best of the McDermott analysts' knowledge, it is the only code capable of accurately simulating wood combustion in a stoker-fired power boiler.
The analysts began by modeling the power boiler using the existing operating conditions. They used an in-house grid-generator program to develop a finite-difference model with about 90,000 grid points. Flow-boundary conditions were applied to the air inlets that defined the pressure, temperature, and composition. The McDermott team entered the chemical analysis, density, and temperature of the fuel at each fuel spout. The heat-transfer properties of each surface of the boiler were defined by entering convective coefficients and surface emissivity. The temperature and flow rate of the cooling water were also included.
Used by itself, COMO WR produces tabular output that does not make it easy to visualize what is happening inside the boiler. For that reason, McDermott analysts use FIELDVIEW, a commercial postprocessing program from Intelligent Light in Lyndhurst, N.J. With the software, the analyst can create three-dimensional perspective views with hidden-line removal and light shading. He or she can trace the path of a marker traveling along with the fluid through a series of animated views. Engineers can depict an isosurface, a surface running through the points where a numeric value such as fluid velocity is constant, colored with contour plots depicting another numeric value such as temperature. These and the many other visualization techniques provided by the program can also be combined, such as by plotting pressure contours on various surfaces while streamlines illustrate the flow.
FIELDVIEW is compatible with most leading commercial CFD codes and is available for all major Unix, Windows NT, and Windows 95 platforms. McDermott runs it on a Silicon Graphics Indigo workstation. The latest release of the program, version 5.5, contains breakthrough technology for processing large transient CFD problems.
The visualization technique that was most useful in this application used streamlines to trace the flow path through the boiler. The clearest picture of the problem came from using FlELDVIEW's flipbook capability, which allows movies to be created from a time sequence of flow plots and either played on the workstation monitor or converted to other formats used to create a VHS video. Analysts used the scripting language provided by the software to automate the presentation. They rotated the view of the boiler during the time sequence to visualize the boiler depth.
In the resulting movie, they saw the air exiting the front wall's overfire air ports, flowing toward the center of the boiler, and colliding with the streams from the opposite wall. This generated a high-vertical-velocity center core that lifted the wood up to the exit before it had a chance to be burned. The movie provided a clear understanding of exactly what was happening inside the boiler.
Another FIELDVIEW visualization technique used in this application was defining horizontal planes at intervals through the length of the boiler and superimposing parameters that indicated the state of combustion, such as temperature, percentage of oxygen, and percentage of carbon monoxide at each point on the plane. These plots indicated precisely where combustion was occurring and helped point out areas where oxygen needed to be directed to increase wood burning and reduce carryover.
To reduce vertical velocity, analysts tried offsetting the air ports and increasing the distribution of airflow to the overfire air ports. After only three additional iterations of changing the model and rerunning the analysis, they discovered a new arrangement that reduced the carryover to lower levels. This arrangement involved changing the airflow distribution to 40-percent undergrate air and 60-percent overfire air. The effect of this arrangement was to reduce vertical velocity and create an air curtain that blocked unburned particles from moving up toward the exit.
The analysts made a second FIELD VIEW movie of the airflow conditions with the new arrangement, showing the elimination of the center core. They played the two movies simultaneously on two monitors set side-by-side to demonstrate for the customer's engineers how the recommended changes would solve the problem. Based on this presentation, the customer authorized the changes. Once they were n1ade, they reduced the carryover problem by 20 percent.
These analysis results, visualized with FIELDVIEW, show predicted gas-flow patterns and a surface of constant gas temperature for a flow and combustion model of a stoker-fired power boiler burning wood waste.
Computer simulations provide a clear picture of what is happening at any point within the boiler.
Technicians responsible for skeptical that CFD would the boilers were originally provide practical benefits.