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While developing specifications for packaged boilers, emphasis is often placed on aspects such as painting, insulation or structural details, while the process specifications that are important for boiler performance are not elaborated. I have seen specifications that are a stack of papers a few inches high, with only one page devoted to process information! Also, important data would be missing. Hence presented below is a checklist of process considerations. These should be of interest to consultants and plant engineers who are involved in developing packaged boiler specifications.
Steam parameters such as flowrate, pressure, steam and feedwater temperatures, process steam (steam for deaeration or heating oils, feedwater outside boiler, etc.) should be clearly stated.
If the boiler were to operate at a different steam pressure, then this should also be stated. Often engineers assume that a 100,000-lb/h unit designed for, say, 600-psig operation can generate 100,000 lb/h at 100 psig! The specific volume of steam increases significantly at lower pressures. This increases the steam velocity in the main steam line and in the superheater by about six times. The performance of the steam line valves could also be affected. The steam pressure drop in the superheater could be 30 times higher! Hence the boiler may have to operate at a much lower capacity at lower pressure.
Though the boiler is mechanically designed for a higher pressure, it does not mean one can generate the same steam quantity at any lower pressure. The superheater design and steam line sizes have to be reviewed carefully in such cases. Probably a different superheater may have to be used at higher pressures. The steam drum internals also will not operate well due to the higher steam velocities at lower pressures.
The author also remembers a case where consultants did not specify that the steam for deaeration would come from the boiler until it was fully designed. The steam for deaeration added 15 percent to the capacity and hence the boiler had to be redesigned.
Required steam temperature variations with load should be clearly stated.
For example steam temperatures can generally be controlled between 60 to 100 percent load. However some boiler specifications call for 10 to 100 percent load range, which is a difficult task due to flow maldistribution problems discussed earlier and accuracy of predicting thermal performance at low loads. Also fan operability must be taken into account. Variable speed drives or similar facility must be incorporated to achieve stable fan operation at low loads. In some cases, allowing the steam temperature to float may not be a problem and the consultant must explore this option along with the plant engineers. Controlling the steam temperature over a very large load range complicates the design of the boiler. Remember that with radiant superheaters, tube failures are more likely due to flow maldistribution concerns at lower capacities.
Feedwater quality should be stated in the specs.
If feedwater is used for temperature adjustment, then the water must be of demineralized quality. Low-quality feedwater also results in high blowdown losses as discussed earlier.
CO and NOx emission requirements must be stated clearly.
When an SCR (selective catalytic reduction) system is required for low NOx, it affects the boiler design as the gas temperature to the SCR must be maintained between 600-750?F or as required by the SCR supplier. Thus, provision must be made for gas bypass in the boiler, to increase the gas temperature to the SCR at lower loads. Fluegas recirculation techniques used for NOx control also affect the cost and size of the boiler. Superheater performance is also impacted by excess air and fluegas recirculation.
Fuel analysis should be stated clearly.
The furnace exit gas temperature is lower when firing oil compared to, say, natural gas. Therefore, the temperature for superheated steam will be lower on oil firing unless the boiler is designed to achieve a temperature setpoint with oil. Low-Btu fuels have lower adiabatic combustion temperatures, which decreases the furnace exit gas temperature and affects superheater performance. The fluegas quantity through the boiler will also be higher, causing larger gas-pressure drop. Fuels containing ash with salts, such as sodium or vanadium, require careful review with regard to high-temperature corrosion effects. The presence of sulfur in the fuel affects low-temperature corrosion. Boiler feedwater temperature must be high enough to avoid this concern.
Surface areas should not be specified
See Boiler Surface Area: More is not Necessarily Better.
A boiler can be designed in several ways, and the right method to compare different options is to look at the installed cost and capitalized operating costs.
Furnace-area heat-release rates are more important than volumetric heat-release rates.
Volumetric release rates are meaningful for fuels that are difficult to burn, such as solid fuels, which require longer residence times in the furnace. Area heat-release rate is an indication of heat flux inside the furnace tubes, which is related to DNB (departure from nucleate boiling) concerns. Hence specifying volumetric heat-release rates is not meaningful, though unfortunately several engineers use a number such as 60,000-100,000 Btu/ft3h, without realizing its significance. Area heat-release rates up to 200,000 Btu/ft2h have been used in packaged boilers.
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