Specifying Waste Heat Boilers

While developing specifications for waste heat boilers, consultants and plant engineers should have in mind the following points regarding thermal design and performance aspects.

1. The type of boiler is based on several aspects discussed earlier. While these are general guidelines, experience is the key. Though steam and gas parameters often dictate the type, whether fire tube or water tube, it is likely that even with small gas flow, a water tube may be economical, while even with large gas flows, a fire tube may be a better option. In special cases, a discussion with various boiler sup- pliers will help.

2. The process that generates the hot flue gas should be described and the resulting

nature of the waste gas stream. With clean gas, finned tubes can be used, but if it is a dusty gas with high fouling tendency, plain tubes have to be used with a high fouling factor. A fire tube boiler with dusty (but not slagging) flue gas may be easier to clean if flue gas is inside the tubes. Some process boilers such as those in hydrogen plants require exit gas temperature control, and hence, this information should be made available to the boiler designer.

3. If the flue gas has particulates that can cause slagging of salts, then the melting temperatures of the ash should be determined in a laboratory and results conveyed to the boiler company. If the flue gas temperature entering the boiler is higher than the ash melting temperature, then the front end of the water tube boiler should be carefully designed with wide-spaced plain tubes followed by single-spaced tubes. Fire tube boilers are better avoided as the tubes will be difficult to clean if slag forms on the tube sheet refractory or over tubes. Superheater if required is preferably located after a furnace section and screen tubes to avoid fouling and corrosion, and provision should be made for cleaning the tube bundles. If dust content is high, then erosion of tubes is likely, and gas velocities in the boiler must be low on the order of 8–12 m/s. With clean flue gas such as gas turbine exhaust or flue gas from incineration of gases or fumes, gas velocity is limited by gas pressure drop considerations and can be even in the range of 20–40 m/s.

4. Desired steam purity should be mentioned, particularly if steam generated is used in a gas or steam turbine. Boiler water must be maintained within limits as discussed in Chapter 6. If steam at low pressure (1000–3000 kPa) is used for process and if the gas flow is not large, a fire tube boiler with integral steam space may be adequate. However, even with fire tube boilers, if steam is used in a turbine, an elevated drum boiler is a better option.

5. The extent of optimization required and the cost of fuel, electricity, and steam should be indicated, particularly with large gas flow units. Simply stating that energy recovery should be maximized is not adequate. If Supplier A cools the gas stream to, say, 250°C and supplier B to 200°C, the plant must know how to evalu- ate the options. Also the difference in gas pressure drop should be evaluated and annualized.

6. If steam for deaeration is taken from the boiler, then it should be so mentioned as

it will affect the superheater size. Similarly, if import steam is likely to be super- heated in the waste heat boiler, it should be specified as it will affect the superheater size and cost.

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7. Flue gas analysis including any sulfur compounds must be specified as it affects

the economizer or air heater if used; the minimum feed water temperature to be used is also determined by corrosion considerations. The exit gas temperature or efficiency of energy recovery is affected by the feed water inlet temperature, and hence, all these issues are interlinked. The presence of hydrogen chloride can cause high-temperature corrosion, and hence, superheater steam temperature selected should preferably be below 400°C. The duty of the boiler is also impacted by the flue gas analysis as the specific heat is a function of gas constituents such as water vapor, sulfur dioxide, and hydrogen.

8. Flue gas flow should be clearly stated in mass units such as kg/s or lb/h and not in volumetric units as this can lead to some differences in the estimation of gas den- sity and mass flow. The energy balance or duty recovered is dependent on mass flow of flue gas, and any misunderstanding in its value can lead to differences in boiler duty and size.

9. Emission regulations and limits on CO, NO x , SO x , if any, should be stated in the

specifications itself so the boiler can be designed appropriately and cost estimation can be reasonably accurate.

10. Any cycling requirements should be stated upfront as this has an impact on tube failures and method of welding tubes to headers, drums, and use of refractory in the boiler. Also if the gas flow and inlet gas temperature are likely to fluctuate, then this should be discussed with the boiler supplier. A large screen section and

a furnace will help dampen the effect of these fluctuations.

11. Feed water used for steam temperature control should be demineralized and preferably have zero solids. If not, a sweet water condenser system as described in Chapter 3 may be required adding to the cost of the boiler.

12. The type of casing for the boiler is important whether refractory lined or mem- brane wall. Start-up time and casing corrosion issues are dependent on the type of casing used.

Specifying Waste Heat Boilers