SPECIFYING PACKAGED BOILERS
The following process data should be specified as a minimum.
1. Steam parameters such as flow, pressure, temperature, and feedwater temperature. If saturated steam is taken from the boiler for deaeration or for NOx control, fuel oil heating, etc., it should be so stated. If the makeup water flow is 100%, the deaeration steam could be in the range of 15% of the steam generation and therefore not an insignificant amount.
2. If superheated steam is required, the steam temperature control range should be specified. Generally the steam temperature can be maintained from 50 to 100%. A larger range requires a larger superheater. Also, if several fuels were fired, the steam temperature would vary as discussed above.
3. Analysis of feedwater entering the economizer should be stated so that the blowdown requirements can be evaluated. An example is given in
Q5.17. In some refinery projects, I have seen very poor feedwater being used, which results in 10% to even 20% blowdown, which is a tremendous waste of energy; it also affects the boiler duty and heat input significantly. Heat input, in turn, affects the flue gas quantity and gas pressure drop.
4. Emission limits of NOx and CO should be stated up front because they affect the burner design as well as the furnace design, the flue gas recirculation rates, and therefore the entire boiler design and performance. The use of SCR may also have to be looked into, and the cost implications are significant.
5. Fuels used and their analysis should be stated. Standard natural gas or fuel oil may not have significant variations in analysis within the United States, but for projects overseas the fuel analysis is important. Some natural gas fuels overseas contain a large percentage of hydrogen sulfide, which can cause acid dew point problems. Gaseous fuels should have the analysis in percent by volume and not in percent by weight, whereas liquid and solid fuels should have the analysis in percent by weight.
6. Surface areas should not be specified, for reasons discussed earlier.
7. Operating costs such as the cost of fuel and electricity should be stated as well as the norm for evaluating operating costs. Ignoring operating costs and selecting boilers based on initial costs alone (which is unfortunately being done even today!) is doing a disservice to the end user.
8. Furnace area heat release rates are more important than volumetric heat release rates for clean fuels, as mentioned earlier, therefore specifying volumetric heat release rates is not recommended for gas and oil fuels.
9. Large fan margins should not be used, and efforts must be made to estimate the gas pressure drop accurately. Large margins on flow (such as 20%) and on heat (40%) not only increase the operating horsepower, which is a waste of energy, but also make it difficult to operate the fan at low loads. In boilers with single fans, the margins should be small, say 10-12% on flow and 20-25% on head. Those familiar with utility boiler practice where multiple fans are used try to apply the same norms to packaged boilers, which can lead to operating concerns at low loads unless variable-speed drives or variable-frequency drives are used. The ambient temperature variations and elevation at which the boiler is likely to be used are important because this information helps in the selection of appropriate fans.
These points along with information on mechanical requirements such as materials, corrosion allowances, and future operational considerations, if any, are
Important to the boiler designer. The proposal should also clearly state the
Required performance aspects.