SPECIFYING WASTE HEAT BOILERS
The following points may be considered while developing specifications for heat recovery applications.
1. Because there are numerous applications of heat recovery, it is always good practice to start off the specifications by describing the process that generates the flue gases, because that gives an idea of the nature of the gas stream. With a clean gas stream, finned tubes could be used to make the boiler design compact, whereas a dirty gas with slagging potential must have bare tubes, with provision for cleaning the surfaces. Process gas applications such as hydrogen plants or sulfuric acid plant boilers require exit gas temperature control systems.
2. Desired steam purity should be mentioned, particularly if the steam generated is used in a gas or steam turbine. Also, based on load swings, one could arrive at the proper size for the steam drum.
3. The extent of optimization required and the cost of fuel, electricity, and steam should be indicated. For example, simply stating the inlet gas conditions and steam parameters may not be adequate. If design A cools the gas to, say, 450°F and design B cools it to, say, 400°F by using a larger boiler at higher cost, how is this to be evaluated? Also, if for the same steam parameters, one design has 6 in. WC pressure drop and another has 4 in. WC, is there any way to evaluate operating costs? Such an indication in the specifications will help the designer to review the design and balance the installed and operating costs.
4. Space availability and layout considerations should be indicated. Sometimes a boiler is built before the builder finds out that it has to be located inside a building that has already been constructed.
5. The steam system should be clearly described. Often only the makeup water conditions are given without an indication of where the steam to
The deaerator comes from. If the steam is taken from the boiler itself, then the design is likely to be affected, particularly if a superheater is present. Hence a scheme showing the complete steam-water system for the plant will be helpful. In waste heat boilers, sometimes import steam from another source is superheated in the boiler. This affects the superheater and boiler performance, particularly when the import steam supply is reduced or cut off.
6. Often feedwater is used for desuperheating steam to control its temperature. This water should have zero solids and should preferably be demineralized. Softened water will add solids to the steam if used directly as spray, so one may have problems with solid deposits, fouling, and overheating of superheater tubes and possible deposition of solids in the steam turbine blades. If demineralized water is not available and that is so stated up front, the designer could come up with a sweet water condensing system to obtain the desired spray water for steam temperature control (see Chap. 3). The feedwater analysis is also important because it affects blowdown rates.
7. Gas flow should be stated in mass units. Often volumetric units are given and the writer of the specifications has no idea if it is actual cubic feet per minute or standard cubic feet per minute; then without the gas analysis, it is difficult to evaluate the density or the mass flow. The ratio between standard and actual cubic feet per minute of flue gas could be nearly 4 depending on the gas temperature. The problem is resolved if the flue gas mass flow is given in pounds per hour or kilograms per hour.
8. Flue gas analysis is important. We have seen that the presence of water vapor or hydrogen in flue gases increases the heat transfer coefficient and also affects the specific heat and temperature profiles of the gas. The presence of corrosive gases such as hydrogen chloride, sulfur trioxide, and chlorine suggests the possibility of corrosion. The boiler duty for the same gas temperature drop and mass flow could be different if one designer assumes a particular flue gas analysis and another designer assumes another. Hence flue gas analysis should be stated as well as the gas pressure. High gas pressure, on the order of even 1-2 psi, affects the casing design and cost.
9. With HRSGs, one should perform a temperature profile analysis before arriving at the steam generation values. As shown in Q8.36, assuming an exit gas temperature and computing HRSG duty or steam generation on that basis can lead to errors.
10. Emission levels of NOx, CO, and other pollutants required at the exit of the HRSG or waste heat boiler should be stated. In such cases, information on pollutants in the incoming gases should also be given.
11. Fuel analysis should be provided for a fired HRSG or boiler. Also, the cost of fuel helps to determine if a design can be optimized by using a larger boiler and smaller fuel consumption or vice versa.
12. If the boiler is likely to operate for a short period only or weekly or is being cycled, then this information should also be given. Frequent cycling requires some considerations in the design to minimize fatigue stresses. Provisions for keeping the boiler warm during shutdown may also be necessary.
In addition, local code requirements, site ambient conditions, and constructional
Features, if any, should be mentioned.