NFPA 85 - Is there a Low Air Flow Trip at < 25% full-load air flow

[CFSE001]

As late as 2001, the boiler code trip diagram clearly indicates a low air flow trip at < 25% full-load air flow in Figure 4.9.3(b) Boiler Trip Logic. However, the 2004 and current 2007 code is much less clear about this issue stating “low combustion air flow below the permitted limits shall activate the master trip relay”. It appears that the prior continuous requirement of 25% air flow is being relaxed due to newer burners with higher turndown requirements. For example, I am currently working a new boiler with a 100:1 turndown in standby mode where the core burner has a dedicated blower. In this case, a 25% full-load air flow trip wouldn’t make any sense since the inner and outer burners are supplied by an FD fan which (in theory) could be turned off in standby mode. It seems that NFPA is moving away from a definitive trip requirement at < 25% MCR air flow with new generation burners, but there are still conflicting statements within the document.

2001 version - Appendix A Explanatory Material

A.1.8.5 The minimum airflow value (25 percent for most boilers) is based on historical experience in reducing the occurrence of explosions. This value is based on safety considerations and could be in conflict with economic considerations or emission limits. Factors considered in establishing the minimum airflow include the following:
(1) Removal of combustibles and products of combustion
(2) Cooling requirements for burners out of service
(3) Accuracy of total burner airflow, individual burner airflow, and other airflow measurements
(4) Accuracy of burner air and main burner fuel distribution
(5) Effect of thermal and pressure transients within the combustion chamber on the air and main burner fuel flows
(6) Impact of air leakage
(7) Wear and deterioration of the unit and equipment
(8) Operational and control margins

A.2.3.9.1 Low NOx operation — special problems.
(a) Air pollution control regulations require that new installations meet NOx emission limits that are lower than emissions now obtained from many of the presently installed firing systems and furnace designs, which are using past operating procedures. In addition, air quality regulations in some local areas require a reduction of NOx emissions from existing boilers.
(b) To achieve these reductions, one or more of the following methods should be used:
(1) Low excess air firing (i.e., less than the “normal” 10 percent to 25 percent excess air)
(2) Multistage air admission, involving the introduction of combustion air in two or more stages partly at the fuel nozzle, which could be less than stoichiometric air, and partly by independent admission through special furnace ports; and a second stage of air admission within the same burner housing
(3) Flue gas recirculation into all or a portion of the secondary air
(4) Reduced secondary air temperature
(5) Fuel staging
(6) On new units, introduction of new burner and furnace designs by equipment manufacturers
(c) Generally, the effect of all of these methods is to produce lower flame temperatures and longer, less turbulent flames, which result in lower NOx.

 

[davefitz]

OK, what is the question?

at 25% MCR heat input, the furnace appears to be oversized, there will be a very low flame temp and very low thermal NOx regardless of XS air, so there is nothing (NOx) lost by maintaining 25% MCR air flow ( except boiler efficiency).

If one is to operate for a long period at such a low load (<25% MCR) and you wish to minimze airflow below 25% MCR, then the specific air flow to the active burner will need to be measured and controlled reliably to sufficient XS air to ensure flame stability, including stability affects of pilot burners .

Other secondary effects that need to be approved by the boiler oem is that

(a) at low heat input to only a few burners, low excess air operation will imply high local flame temperature and high spot heat flux to the tubes local to that active burner, with negative affects on furnace water circulation- some tubes get all the nat circ water flow, and some tube may actually get reverse flow ( corner tubes)

(b) insufficient gas side turbulence and friction to overcome bouyancy effects- leads to pockets of zero gas flow and accumulated CO and unburned fuel as well as stratifeid gas flow re: O2 , CO ,and temperature and

(c) the air flow monitoring device will need to reliablily measure the very low total air flow rate- use of a full load sized flow element is inviting disaster- the very low DP is so small that only a few drops of accumulated condensation will lead to 100% error in flow readings- other technologies are required for reliable flow measurement- it is incorrect to pretend that a DP Xmitter sensitivity and accuracy of 0.5% will solve this problem- there is more to the total monitoring system than just the Xmitter.

As human history shows, we will push the envelope using new technolgy plu loss of memeory of past failures, until the next failure wakes us up.