How to Test for Relative Operating Efficiency

The aforementioned items are general symptoms that are suspect in causing a high unburned carbon loss from the combustion of coal. However, the magnitude of the problem often goes undetected and remedial action is never taken because of the difficulty in establishing and quantifying this loss while relating it to the overall unit operating efficiency.

In light of this, it is recommended that the boiler manufacturer be consulted to establish the magnitude of this operating loss as well as to review the system equipment and operating methods.

The general test procedure to determine the un-burned carbon loss requires manual sampling of the ash (refuse) in the ash pit, boiler hopper(s), air heater hopper, and dust collector hopper and performing a laboratory analysis of the samples. For reference, detailed methods, test procedures, and results are outlined in the ASME Power Test Codes, publications PTC 3.2 (Solid Fuels) and PTC 4.1 (Steam Generating Units), respectively. In addition to the sampling of the ash, a laboratory analysis should be performed on a representative raw coal sample and pulverized coal sample.

Results and analysis of such a testing program will:

1. Quantify the unburned carbon loss.

2. Determine if the unburned carbon loss is high for the type of coal fired, unit design, and operating methods.

3. Reveal a reasonable and attainable value for un-burned carbon loss.

4. Provide guidance for any corrective action.

5. Allow the plant, with the aid of Figure 5.18, to assess the annual loss in dollars by operating with a high unburned carbon loss.

6. Suggest an operating mode to reduce the excess air required for combustion.

Figure 5.18 can be used to determine the approximate energy savings resulting from reducing unburned coal fuel loss. The unburned fuel is generally collected with the ash in either the boiler ash hopper(s) or in various collection devices. The quantity of ash collected at various locations is dependent and unique to the system design. The boiler manufacturer will generally specify the proportion of ash normally collected in various ash hoppers furnished on the boiler proper. The balance of ash and unburned fuel is either collected in flue-gas cleanup devices or discharged to the atmosphere. A weighted av erage of total percentage combustibles in the ash must be computed to use Figure 5.18. To further illustrate the potential savings from reducing combustible losses in coal-fired systems using Figure 5.18, consider the following example.

Example: Determine the benefits of reducing the combustible losses for a coal-fired steam generator having a maximum continuous rating of 145,000 lbs/hr at an operating pressure of 210 psig and a temperature of 475°F at the desuperheater outlet, feedwater is supplied at 250°F.

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