By Dave Earley, Combustion Technologies Corp., and Bill Kirkenir, Progress Energy Corp.
At a time when combustion optimization seems to be such an important part of reducing emissions and improving efficiency, in part because of carbon constraints, we have to ask ourselves, how do we balance coal flow to the burners? For dozens of years utilities have spent countless dollars on a variety of techniques and services aimed at balancing their coal pipes. So why is combustion still so bad?
The reality is that manual coal sampling in real world power plant conditions is challenging. Call it dirty airflow or rotorprobing, ASME sampling or isokinetic sampling; it does not work for the purpose of accurately measuring coal flow. This should come as no surprisethe procedure is inherently flawed.
Manual Sampling Not Correct
In a typical coal mill, primary air (PA) is used to both dry the coal and transport it to the burner(s). Figure 1 depicts a two-pipe coal milling system showing the PA that is used to transport pulverized coal to the burners through both outlet pipes.
Most original equipment manufacturers (OEMs) supply the user with a PA:feeder curve. One challenge in creating these curves is determining and maintaining optimum transport velocity throughout the entire range of mill loading/operation. The velocity is a function of coal piping geometry, coal type, moisture content, coal fineness and more. The value is important because at lower velocities, coal flow cannot be sustained and coal layout, pipe pluggage and fires can occur.
Through years of trial-and-error testing, these curves have often been changed in an effort to improve combustion and to optimize low NOX burner performance. Several factors are crucial to creating these curves.
Primary air is essential for proper drying of the coal. The mill’s outlet temperature is used to determine this and the hot and tempering air dampers are controlled to allow for desired mill outlet temperature, but it is the volume of the primary air that must deliver the coal properly to the burners. Too little and coal layout, mill spillage or pipe pluggage can occur. Too much, and coal pipe erosion, long flames/waterwall impingement, high loss on ignition (LOI), high NOX and more may result.
Another area of concern is with the air distribution to the pipes and whether or not the air (PA) is distributed equally to each of the pipes. Field testing the airflow in the coal pipes with no coal in thema “clean air” testis performed with Pitot tubes. The desired outcome is equal air velocities in the coal pipes. If clean air velocities are not equal, an orifice or restriction is sometimes located in the pipe(s) with the higher velocity to “balance” the velocities.
Testing is usually performed closer to the burners to accommodate for piping losses; longer pipes may need more air to achieve equal velocities near or at the burner entrance. But if the major job of the PA is to get the coal to the burners, why should any orificing or restricting be done prior to knowing how the coal is distributed among the pipes?
When coal mixes with PA in the milling system in addition to PA, it becomes a two-phase flow instead of single phase, with air as the gas phase and coal as the dense phase. Based on typical PA:feeder curves (approximately 2:1 mass basis), there is significantly more air in a coal pipe by volume than there is coal. If the coal enters the pipes such that the mass of coal to each of the pipes is equal and air velocities are equal as determined by clean air testing as described above, then all other things being equal (such as coal quality and distribution of the coal in the pipes) the pipes should again be in balance and equal.
However, since friction between the air and coal actually transports the coal to the burners, then the coal must travel more slowly than the air, so they are not equal. Because of this, measuring dirty air flow or the air in a coal pipe laden with coal is of no real use in sampling coal. If coal is to be extracted isokinetically from the coal pipe, it must be extracted at the rate at which the coal is traveling (not the rate that air is traveling) and then weighed.
This is one reason why manual sampling yields erroneous data and results worsen if the coal mill does not distribute its coal equally to the coal pipes.
In Figure 2, the coal mill delivers a higher amount of coal to pipe 1. This creates two problems:
- The air velocities in the two pipes have no reason to change (based upon the mill’s output of solids), though they may. But the coal velocity in Pipe 1 will be much slower than in Pipe 2. In fact, the extreme example of this is when a pipe is overloaded by three to four times the normal loading. Then the velocity will drop to zero, coal transport stops and the pipe plugs. When coal pipe sampling uses the air velocity to determine the extraction rate of coal for these pipes, it yields incorrect results. Extractive sampling as performed by industry standards will be wrong.
- Clean air testing in the example above showed higher velocity in Pipe 1 that was corrected with the addition of an orifice/restriction. But if Pipe 1 has more coal delivered to it, the pipe restriction will serve to take away air that would be needed to transport the coal to the burner, so it is likely that the results of such clean air testing would cause coal layout in Pipe 1.
Moving coal and transport air between the two pipes can be complex. Without the ability to accurately measure the coal mass and velocity, this “blind” movement with orifices or other adjustable valves often causes more imbalances and other combustion problems.
New Test Methods
Although coal milling systems have not made many advancements in the past 50 years other than capacity and the use of wear-resistant alloys, coal measurement technologies have. We can now see the adverse conditions created through the use of “old” test methods.
Using technologies such as microwave and mathematical cross-correlation methods, we can measure today what was not possible 50 or even 15 years ago.
A four-pipe mill delivering a greater amount of coal to one pipe than the others, with improper orificing caused by using manual sampling and other improper control strategies, could result in a lack of transport air in the heavily loaded pipe making it much more unstable.
The massflow measurements are made using a microwave mass measurement to measure coal density and an electrostatic cross-correlation to measure the velocity of the moving coal particles. (See Figure 3.) The massflow is the product of the two. The mill delivers a much higher loading (density) to pipe 2-1 LR. Lacking more transport air in this pipe, the coal particles move much more slowly. But previous dirty air tests show the air velocity is at or near that of the other pipes, so extractive sampling did not derive accurate results.
Figure 3 A typical coal pipe arrangement
The velocity data is that of the moving coal particles. (See Figure 3.) Air velocity in these pipes ranges from 110 to 120 fps. The difference between the air and coal velocities is the coal slip. If isokinetic sampling were to be accurately performed, the coal would have to be extracted at the rate at which the coal is moving in the pipesthe coal velocity. Because the coal velocity cannot be measured with existing sampling methods, the industry practice is to measure the air velocity and then extract coal at that rate. As can be seen, the rates are not the same and any data generated with such methods will be incorrect.
Some field testing methods take into consideration that slip exists between the air and coal and adjust their sampling devices with this in mind. But the slip varies with coal loading, so correct slip factors cannot be determined without knowing the coal loading. Therefore, random slip values are used. The end result is that there is no way for these manual sampling devices to generate accurate coal flow measurements.
Not only is manual sampling inherently inaccurate, it is resource- and time-consuming. And this slow sampling adds to the inherent inaccuracy.
While air flow/velocity may be somewhat “stable,” the movement of the coal particles in coal pipes can be “erratic.” The coal particles have size and weight. The pressure in a coal pipe is low and the result is erratic movement of the coal. The higher the air concentration, the more stable the movement of the particles. But excessive air in the coal pipes will lead to pipe errosion, impingement on waterwalls, higher NOX and poor combustion related to “throwing” the coal through the flame (poor, incomplete combustion). Extracting coal at individual points in a coal pipe via a manual sampling procedure takes time. By the time the sample is obtained at one point, the coal flow at the next point has changed. Similarly, when the sampling of the last pipe of a mill is being completed, the massflow in the first one(s) is likely to have changed.
Coal flow is dynamic and constantly changing. Today’s coal flow measurement technologies are real-time, generating coal flow data in each pipe as fast as every second. Only by using real time measurements can what the system is doing be seen and coal pipe balancing performed.
Benefits of More Accurate Measurements
Adjusting coal between the coal pipes of a mill using orifices (fixed or adjustable) is complex because of the two-phase flow environment. A restriction in the pipe’s cross-section will affect both the air and the coal. Continuous coal velocity measurement is critical when using restrictive devices for balancing pipe coal flows.
Adjusting coal valves (at the mill outlet) can improve coal distribution while still maintaining proper coal velocity. The ability to continuously measure both the massflow and the velocity of the moving coal is critical to balancing coal pipes with valves and orifices.
Many operators choose to increase PA because of spillage or high mill differential due to wet coal, mill wear or other reasons. This higher PA results in high NOX, high LOI, high carbon moNOXide (CO), excessive slagging and more. Immediate identification of excessive coal velocity, through real time measurement, can lead to the reduction of PA. This in turn can improve coal distribution.
Why continue to perform or contract for manual sampling just because it’s been done that way for 50-plus years? The results will be the same as they were 50 years ago. Obtaining erroneous data results in changes that may hurt, not help, the combustion process.
Today’s real-time measurement methods lead to correct data collection and then improved combustion. These instruments can be used in your distributed control systems (DCS) for on-line control of PA, coal feed, fuel:air ratio control using burner secondary air and more. All of this will result in reduced emissions, improved efficiency, fewer tube leaks and more.
While we stressed the main difficulty of manual samplingextraction of coal at erroneous ratesthere are several other challenges to manual coal sampling, including variations in coal distribution in each pipe and the inability of an extraction probe to detect this; effects of angular flow on Pitot tubes; difficulty with using manual probes such as holding the probe in proper position in each pipe; difficulty in reading dirty air flows (dP) with an anemometer and more. Some manual testing personnel do a more thorough job of obtaining data with sampling instruments than others. But true accuracy cannot be obtained with the flawed methods.
Authors: Dave Earley is with Combustion Technologies Corp. in Apex, N.C. He is a mechanical engineer with more than 20 years of industry experience, focused on combustion measurements and optimization for the past 15 years. Bill Kirkenir is the lead combustion engineer with Progress Energy Corp. He is a mechanical engineer with 30 years of experience in power plant operations and combustion optimization.