In the June 2002 issue of Boiler Systems Engineering, we wrote an article titled “Boiler Turndown.” Contrary to conventional wisdom, the article stated that high turndown ratios are not always good — higher is not necessarily better in every case. Several boiler manufacturers incorporated the article into their training programs, and at least one major boiler manufacturer still uses it as a technical reference. There was, however, one significant point of post-publication controversy: We wrote that, in most cases, there was no significant efficiency advantage to operating a test boiler at a high (10-1) turndown ratio vs. a low (4-1) turndown ratio. We still believe that to be the case, but we recognize that the total energy capacity of a boiler — whether achieved by increased steam/water storage or a wider range of allowable operating pressures — has to be sufficient to allow the shorter cycle time associated with a lower turndown ratio. This can result in a larger boiler/footprint and, therefore, higher first cost. This obviously runs contrary to manufacturers' goals to build smaller and less costly boilers.

It also should be noted that, in our original article, we ignored so-called “shell losses” because we were considering one boiler with differing burner turndowns. If a larger boiler is chosen to provide more energy storage, convection and radiation losses — which are directly proportional to the effective surface area of the boiler shell — must be considered. These losses will, of course, adversely impact the boiler's overall efficiency, but can be addressed with insulation and surface-area design.

An alternative to a larger boiler is a shorter cycle time, which incurs an efficiency penalty because of the safety and code requirements needed for a burner purge cycle. A purge cycle must be initiated as part of each ignition sequence; because the burner is being purged with relatively cold inlet air to remove combustible-gas accumulation, heat is removed from the boiler. This reduces radiation heat-transfer efficiency and initial flame temperature. The lower flame temperature results in a decrease in flame stability. In our original article, we took into account the efficiency loss that occurs when a boiler is purged before startup.

Although efficiency and emissions were important when the original article was written, they did not have the overarching significance they do now. With the passage of the Energy Policy Act of 2005 and the increased boiler-efficiency requirements of such standards as ANSI/ASHRAE/IESNA Standard 90.1-2007, Energy Standard for Buildings Except Low-Rise Residential Buildings, energy efficiency has become a critical consideration in HVAC-equipment selection. Even more-stringent nitrogen-oxide- (NOx-) emissions standards continue to affect burner design, particularly for boilers in states such as California.

For low- and ultralow-NOx burners, regardless of turndown, fuel selection is critical. Most (or all) commercial low-NOx burners will fire a hydrogen-rich gas. Although long understood and accepted anecdotally, recent work at Turkey's Selcuk University has evidenced the directly proportional relationship between a fuel's hydrogen content and its lower heating value (LHV).¹ Table 1 shows common hydrocarbon fuels and their LHVs.²

Burning No. 2 oil naturally will be more efficient than burning a natural gas simply because of the amount of hydrogen in the fuel. Water vapor has significantly more heat capacity than carbon dioxide (CO₂) or nitrogen (N₂). This difference can account for more than 3 percentage points of efficiency, depending on operating conditions and boiler efficiency.

Because emissions now are one of the largest issues facing fossil-fired boilers/heaters, several emission-control strategies have been developed:

• Fuel and air staging.
• Premixed combustion.
• Flue-gas recirculation.
• Water/steam injection.
• Cooling/heated-flame surfaces.
• Catalytic combustion.
• Oxygen firing.

Each of these methodologies requires some type of alteration to the combustion process and affects efficiency and combustion in a different way. Strategies for one type of fuel may not be as effective for another. For example, premixed oil combustion is difficult to achieve because the oil must be atomized/vaporized before it is mixed with oxygen for combustion. The same is true of fuel and air staging. Each fuel has different reaction rates that control the combustion and emission-formation processes, and staging the fuel for emission reduction should optimally take this into consideration.