March 31, 2011

Flue Gas Analysis to Maximize Furnace Efficiency

Doug Simmers here again, discussing flue gas analysis, and it’s operational value for boilers and industrial furnaces. Controlling the amount of air going into any combustion process is important in maximizing the efficiency of the furnace. 

It’s pretty easy to see why a fuel-rich mixture is inefficient, since unburned fuel goes out the stack without giving up its heat value. Besides being inefficient, the accompanying black smoke also draws the neighborhood’s attention, and operation in this mode is also unsafe.

The disadvantages of operating with too much air (lean) is not as obvious. After all — air is free, isn’t it? Since air is invisible, it’s easy to forget that it has mass, as sticking your hand out the window of a moving car demonstrates. The energy required to heat up air is called its specific heat (0.24 BTU/lb/degree F), and any air that is not used for burning the fuel merely cools off the flame. Granted, this excess heated air gives back some of its heat in to the boiler tubes, but it almost always exits the stack at a temperature significantly above the temperature it goes into the burner. This heat is lost forever, and if one considers the significant volumes passing through the furnace, this loss can be significant. Further, an excess oxygen reading of 1% is the smaller amount of gas that is being heated up, since it’s only about a fifth of the total volume of air (20.95% O2). So a small increase in excess O2 increases the total air going through the furnace significantly. Additionally, it costs money for the fan blowing air into the burner to move this excess air, and it also reduces the total amount of steam the boiler can produce.

In the previous blog, we discussed how the ideal O2 setpoint is arrived at by detecting the point of CO breakthrough, but how do we determine how important running at the optimum level is? A blog is not the ideal place to run down the ASME short form calculations, but our Jim Thompson has developed a neat program that calculates this out for you (note that most utilities use more comprehensive calculations for determining heat rate). http://www2.emersonprocess.com/en-US/brands/rosemountanalytical/Gas/combustion-flue-gas-analyzers/OXT5A/Pages/O2_TrimCalculator.aspx

The procedure is to determine the “as found” operational condition of the boiler, and then determine how much lower in oxygen the boiler or furnace can operate. The payback is the final output — a great tool for justifying a project.

Next time we’ll discuss how to use the oxygen measurement to minimize the thermal NOx produced in a burner.

Until then, let me know what you think! Post any comments or questions here!

March 15, 2011

Temperature Compensation Algorithms for Conductivity

Pete Anson here. In today’s blog, I’m discussing temperature correction in conductivity measurements. A few customers have inquired about this topic, so thought it might be useful to discuss. Conductivity depends on both ion concentration and temperature. To reduce the influence of temperature on the measurement, common practice is to measure the raw conductivity and temperature and calculate what the conductivity would be at a reference temperature, typically 25°C. Clearly, the calculation requires making some assumption about the liquid being measured.

Below, I describe the temperature compensation algorithms commonly available in process conductivity analyzers. I also give examples where the correction is appropriate and some of the pitfalls associated with each algorithm.

  • Linear temperature correction. The linear temperature correction is an empirical correction used to convert the conductivity at the measured temperature to the conductivity at 25°C. It has the form: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
    C25 is the calculated conductivity at 25°C, Ct is the measured conductivity at t°C, and a is the linear temperature coefficient expressed as a decimal fraction. The linear temperature coefficient is user selectable between 0 and 5% per °C (a = 0.00 to 0.05). For dilute solutions of most salts the linear temperature coefficient is about 2% per °C. For acids the temperature coefficient is typically less, and for bases it is typically greater. The temperature coefficient is also a function of the concentration and temperature. Thus, a single temperature coefficient will rarely be suitable over a broad range of concentration or temperature. Common practice is to use an average temperature coefficient, but doing so is likely to introduce errors.
    .
  • High purity water correction. The high purity water correction is also called the dilute sodium chloride correction. The model assumes the sample is pure water contaminated with a trace amount of sodium chloride. It also assumes (correctly) that the presence of the salt does not influence the conductivity of the water. This means that the conductivity of the solution is simply the sum of the conductivity of water and the conductivity of sodium and chloride ions. The correction can be best understood by reference to the graph below.
    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . .
    The pink line is the conductivity of pure water, and the blue line is the conductivity of sodium and chloride ions. The total conductivity of the solution, the green line, is the sum of the two. Point 1 on the green line is the raw conductivity at the measurement temperature. To perform the correction, the analyzer subtracts the conductivity of pure water at the measurement temperature from the raw conductivity. The result, Point 2, is the conductivity of sodium and chloride at the measurement temperature. Next, the analyzer converts the conductivity of sodium and chloride to the conductivity at 25°C, Point 3. Finally, the analyzer adds the conductivity of pure water at 25°C to the conductivity of sodium and chloride at 25°C to give the corrected conductivity of the solution at 25°C, Point 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . . . . . . . . . . .
    The basic assumption behind the model is that contribution of water and the contaminating salt to the total conductivity are independent of one another. Therefore, the model works very well when the contaminant is a neutral salt. However, if the contaminant is a slightly acidic or basic salt, like ammonium chloride or sodium acetate, small errors begin to creep in because the fundamental assumption is no longer met. Have you encountered these circumstances, and if so, did you see these errors? The presence of the acidic or basic salt suppresses the dissociation of water, so the contribution of water to the total conductivity is no longer independent of the salt. If the contaminant is a strong or weak acid or base, the suppression of the dissociation of water is even greater, and large errors result. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
    Generally, the neutral salt correction should be used when the conductivity is less than about 5 uS/cm and there is reasonable expectation that the major contaminant in the sample is a neutral salt. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
    At higher conductivity, about 6 uS/cm at 25°C, the neutral salt correction behaves like the linear temperature correction model, albeit a correction in which the slope is temperature dependent. At 5°C the coefficient is 0.019, and at 90°C the coefficient is 0.025.
    .
  • Cation conductivity correction. The cation conductivity temperature correction is also called the dilute hydrochloric acid correction. It is used exclusively in steam power plants where the cation conductivity measurement is used to detect contamination in steam condensate and boiler feedwater. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
    The model assumes the sample is pure water contaminated with trace amounts of hydrochloric acid. It is considerably more complicated than the dilute sodium chloride model. Hydrochloric acid is a much stronger acid than water. It strongly suppresses the dissociation of water and thus has a profound effect on the contribution of water to the overall conductivity. The contribution of water also depends on the acid concentration. Increasing the acid concentration decreases the dissociation of water, and reduces its contribution to the total conductivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
    The correction algorithm involves two steps. First, using the measured conductivity and temperature, the analyzer calculates the concentration of hydrochloric acid. Next, the analyzer calculates the conductivity produced by the same concentration of hydrochloric acid at 25°C. Although the algorithm is easy to describe, the algebra involved is extensive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
    In the cation conductivity measurement, the typical contaminants are hydrochloric and sulfuric acids and carbonic acid. Hydrochloric and sulfuric acids are strong acids, and the correction algorithm works very well in samples where they are dominant. Carbonic acid is a weak acid, and, depending on its concentration relative to the strong acid, an error can result. Typically, the errors are small unless the sample is very cold (<10°C).

Using these guidelines, users can make reasonably accurate conductivity measurements in most applications. What have been your experiences with temperature compensation?

Post a comment.

March 2, 2011

Wow. Online Determination of Sample System Performance.

G’day y’all – Shane Hale here again from Gas Chromatographs. At the end of January, I had the honor of presenting at the Natural Gas Sampling Technology conference (NGSTech) held in New Orleans (they even have a photo of me speaking in 2008). This two-day conference focused exclusively on the latest developments and challenges in the field of natural gas sampling for both spot samples and online analyzers.

Fig. 1 – Calculating HCDP at pipeline pressure to provide an early warning of two-hase flow.

Just about every single paper presented at the conference mentioned the importance of the hydrocarbon dew point of the sample gas when designing or operating a sample system. (This being in New Orleans, there was even a suggestion to start a drinking game around the term!) My paper dealt with the issues that the hydrocarbon dewpoint of the sample can cause when using a gas chromatograph. I discussed how you can use the hydrocarbon dew point calculation in our C9+ gas chromatographs to calculate the hydrocarbon dewpoint at the pipeline pressure to provide an early warning of two-phase flow and avoid inaccurate flow measurement (see Fig. 1).

Fig. 2 – The effect on the energy value of the gas being measured when heavy components condense in the sample lines.

I then discussed the effect that poor sample handling can have on the analysis. I have a real-life example that I use all the time that shows the effect of the analysis when the heavy hydrocarbons drop out as the sample line temperature drops at nighttime (see Fig. 2). In this example, the heat trace was turned off.
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And this is where it got interesting. I’m standing there on stage, in front of over 180 people, and an idea comes to me. We can calculate the hydrocarbon dew point at up to four pressures. The regulated sample pressure is controlled to around 20PSI G/1.4BarG. What if we calculate the hydrocarbon dew point of the sample at the regulated sample pressure, and compare this to the ambient temperature? If the hydrocarbon dew point is the same as (or very close) the ambient temperature, it means that the sample has dropped out some of the heavy components into the sample system. (see Fig. 3)

Fig. 3 – HCDP at the sample pressure tracking the ambient temperature, indicating the heavy components are dropping out in the sample lines.

If the heavies have dropped out, then the GC is no longer analyzing the same gas as what is in the pipeline. If the GC is not analyzing the same gas that is in the pipeline, then energy calculations and flow calculations will be incorrect, and thus the custody transfer measurement will be incorrect. This must have been a good idea, as the audience clapped at the end of my presentation, and quite a few people came up to me to discuss this in the exhibition hall later that day.

Wow. What a concept: online determination of the sample system performance. After the conference, I went back to my office and fleshed this out a bit. By entering the nominal regulated sample pressure into hydrocarbon dew point calculation as a fixed value, the C9+ Gas Chromatograph will provide the dew point of the gas in the sample lines. An ambient temperature transmitter can be connected to the analog inputs of the 700XA C9+ GC (for the Model 500, the second detector on the C9+ HCDP application uses the analog inputs) or the ambient temperature downloaded to the controller via modbus. A user calculation can then compare the values and if they are within a certain tolerance (e.g. 10°F), the controller can raise an alarm to highlight an issue in the sample system (for example, the heat-trace is turned off or not installed).

In practice, the comparison of the sample pressure hydrocarbon dew point and the ambient temperature is best done in the flow computer or the SCADA system, as they have better alarming and trending capabilities than the gas chromatograph, but the concept is the same. The ability of a C9+ gas chromatograph to calculate the hydrocarbon dew point on multiple pressures give us new online diagnostics to help us operate our custody transfer metering systems.

Do you think you could use this at your locations? Leave a comment here and tell us your thoughts. And if you’d like a copy of my paper, just click here.

’till next time,

Shane

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