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Startup and Shutdown Scenarios for Solid Fuel Boilers

Controls Scenario Potential Compliance Issue Suggested Solution

Any Startup on clean fuel, 

engage non-listed controls 

prior to adding solid fuel.

May be an issue with clean fuel availability. May 

have CO issue with transition from gas or liquid fuel 

to solid fuel, may not be an issue for 30-day average 

depending on compliance margin.

Expand list of clean fuels to include all Other Gas 1 fuels, 

biodiesel, fuels that meet the HCl, Hg, TSM limits through 

fuel analysis, and other clean fuels as determined by 

permitting authorities. Provide adequate length of time 

for startup period to cover transition from startup fuel to 

solid fuel. (as defined below)

Wet Startup on solid fuel Shouldn’t be an issue for operating parameter 

monitoring – can start wet controls prior to startup

(depending on design of wet controls, may not be 

able to meet operating parameter limits 

instantaneously, but should be able to meet 30-day 

average). 

Need to address units that share a common control

device that startup separately. 

Could have issues meeting 30-day rolling CO CEMS 

limit if enough time is not given for startup. Need to 

differentiate between cold startup (need more time) 

and hot restart (need less time).

Startup ends either when the boiler or process heater has 

continuously maintained a steam production rate of at 

least 25% of maximum steam or heat output at normal 

operating pressure for 4 continuous hours after a cold 

startup or 1 hour after a hot restart and when all control 

devices are in stable operation or when the boiler or 

process heater is operating above an alternate minimum 

operationally stable output flow rate and pressure for a 

minimum time, as specified in a site-specific start-up plan. 

This would also apply to units sharing a common control 

device that start up at separate times (separate startup 

period occurs for each unit).

For units sharing a common control device that are 

starting up sequentially, startup ends either when the last 

boiler or process heater to start has continuously 

maintained a steam production rate of at least 25% of 

maximum steam or heat output at normal operating 

pressure for 4 continuous hours after a cold startup or 1 

hour after a hot restart and when all control devices are in 

stable operation or when the last boiler or process heater 

is operating above an alternate minimum operationally 

stable output flow rate and pressure for a minimum time, 

as specified in a site-specific start-up plan. 
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Controls Scenario Potential Compliance Issue Suggested Solution

Dry Startup on solid fuel Can’t startup ESP until certain flue gas temperature 

and oxygen level are reached. NFPA 85- Boiler and 

Combustion Systems Hazards Code, 2011 Edition, 

Section 6 Multiple Burner Boilers (firing oil, gas, or 

PC) requires purging of precipitators prior to initial 

firing to prevent a precipitator spark from igniting an 

explosive mixture of unburned fuel. 

Manufacturer and operating company procedures 

may include a requirement to be operating on main 

burners with adequate flue gas temperature and low 

enough oxygen levels prior to ESP energization to 

limit potential ignition of unburned fuel by a 

precipitator spark.

Need time within the “startup” period for the 

combustion unit firing primary fuel(s) and all 

exempted air pollution controls to reach stable 

operating conditions relative to the start of steam or 

process heater output to the distribution system.

Need to get better definition of end of startup if 

heat or steam is immediately supplied upon ignition 

of fuel. 

Need to address units that share a common control 

device that start up separately. Could have issues 

meeting 24-hour block opacity or 30-day rolling CO 

CEMS limit if enough time is not given for startup. 

Need to differentiate between cold restart (need 

more time) and hot restart (need less time)

ESPs must be included in the Table 3, item 5 list of 

exempted air pollution controls that must be started as 

expeditiously as possible. 

Startup ends either when the boiler or process heater has 

continuously maintained a steam production rate of at 

least 25% of maximum steam or heat output at normal 

operating pressure for 4 continuous hours after a cold 

restart or 1 hour after a hot restart and when all control 

devices are in stable operation or when the boiler or 

process heater is operating above a minimum 

operationally stable output flow rate and pressure for a 

minimum time, as specified in a site-specific start-up plan.

This would also apply to units sharing a common control 

device that start up at separate times (separate startup 

period occurs for each unit).

For units sharing a common control device that are 

starting up sequentially, startup ends either when the last 

boiler or process heater to start has continuously 

maintained a steam production rate of at least 25% of 

maximum steam or heat output at normal operating 

pressure for 4 continuous hours after a cold startup or 1 

hour after a hot restart and when all control devices are in 

stable operation or when the last boiler or process heater 

is operating above an alternate minimum operationally 

stable output flow rate and pressure for a minimum time, 

as specified in a site-specific start-up plan.
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Controls Scenario Potential Compliance Issue Suggested Solution

Any Unit begins startup, 

reaches a certain minimum 

stable load, then there is 

an equipment problem that 

results in “failed startup.” 

All of the above. Need to be sure that this entire period is considered 

startup, even if you reach the minimum load threshold. If,

for example, the end of startup is 25% load plus 4 hours, 

the unit has to be over 25% for 4 continuous hours.

Wet Lose solid fuel feed, takes 

time to get auxiliary fuel 

going to boiler or there is 

no auxiliary fuel available. 

Sometimes wet fuel can 

cause high O2 conditions, 

which will trip the ESP, 

which will trip the fuel 

feed. Sometimes there are 

problems with the 

equipment used to supply 

the solid fuel that cause 

loss of fuel feed. 

Losing fuel feed would trigger a shutdown –

however, shutdown as EPA currently interprets

would not be completed if the unit continues 

supplying useful steam until fuel feed is restored. 

We can’t classify these periods as malfunctions in all 

cases and these periods are not normal operation. 

We need to be able to call them shutdown, or we 

don’t know how to set up recordkeeping because 

we don’t have certainty on how to classify these 

periods. Operating parameters would probably be 

okay on a 30-day rolling average basis. Whether or 

not there was an issue meeting 30-day rolling CO 

CEMS limit if this is not shutdown would depend on 

compliance margin and length of event.

The shutdown definition should accommodate the 

scenario where fuel feed is reinitiated before the unit 

ceases to supply steam. Facilities need to be able to call it 

a shutdown, even if fuel supply is restored. Facilities 

should not be forced to take the boiler down fully before 

reinitiating a startup – this would be uneconomical and 

could result in more emissions. The end of this period is a 

hot restart, and then the startup work practices would 

apply until certain load and time conditions are met. 

Could revise end of shutdown definition: “Shutdown ends 

when there is both no steam or heat being supplied and

no fuel being combusted in the boiler or process heater or

when startup is initiated by reintroducing fuel to the boiler 

or process heater after fuel feed has been halted.”
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Controls Scenario Potential Compliance Issue Suggested Solution

Dry Lose solid fuel feed, takes 

time to get auxiliary fuel 

going to boiler or there is 

no auxiliary fuel available. 

Sometimes wet fuel can 

cause high O2 conditions, 

which will trip the ESP, 

which will trip the fuel 

feed. Sometimes there are 

problems with the 

equipment used to supply 

the solid fuel that cause 

loss of fuel feed. 

Losing fuel feed would trigger a shutdown –

however, shutdown as EPA currently interprets 

would not be completed if the unit continues 

supplying useful steam until fuel feed is restored. 

Also, NFPA 85 Boiler and Combustion Systems 

Hazards Code, 2011 Edition, Section 6 Multiple 

Burner Boilers (firing oil, gas, or PC) requires tripping 

of electrostatic precipitators as part of a master fuel 

trip (MFT) to prevent a precipitator spark from 

igniting an explosive mixture of unburned fuel. 

We can’t classify these periods as malfunctions in all 

cases and these periods are not normal operation. 

We need to be able to call them shutdown, or we 

don’t know how to set up recordkeeping because 

we don’t have certainty on how to classify these 

periods. Could have issues meeting 24-hour block 

opacity or 30-day rolling CO CEMS limit if this is not 

shutdown, depending on compliance margin, type of 

unit, length of event.

Need to be able to call it a shutdown, even if we get fuel 

back; emissions will be greater if we have to complete a 

shutdown before we can start back up. The end of this 

period is a hot restart, and then the startup work practices 

would apply until certain load and time conditions are 

met. Could revise end of shutdown definition: “Shutdown 

ends when there is both no steam or heat being supplied 

and no fuel being combusted in the boiler or process 

heater or when startup is initiated by reintroducing fuel to 

the boiler or process heater after fuel feed has been 

halted.”

ESPs must be included in the Table 3, item 6 list of 

exempted air pollution controls that are not required to be 

operated during shutdown. 

Any Begin shutdown upon 

halting fuel feed to the

boiler. For some designs, 

there is still fuel burning in 

the boiler.

Current definition says “Shutdown begins either 

when none of the steam and heat from the boiler or 

process heater is supplied for heating and/or 

producing electricity, or for any other purpose, or at 

the point of no fuel being fired in the boiler or 

process heater, whichever is earlier.” This could be 

interpreted that shutdown does not begin unless no 

fuel is burning or no steam/heat is being supplied. In 

some boiler designs, fuel continues to burn after fuel 

feed stops. 

EPA should clarify that fuel being fired in this case means 

fuel being fed to the combustion unit.
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Controls Scenario Potential Compliance Issue Suggested Solution

Wet More than one unit vents 

to a wet control device, 

one unit shuts down, the 

other unit stays operating.

For some wet control designs, the pressure drop 

depends on the flow through the scrubber. If only 

one unit is operating, the operating parameter limits 

established during the performance test with both 

units in operation may not be achievable.

Not sure if there is a way to address this in the rule, or if 

EPA would advise facilities to submit a request for 

alternate monitoring and establish different operating 

scenarios, such as an alternate operating limit with fewer 

units in operation. This is similar to the question of 

whether operating parameters must be maintained if a 

unit burns gas for an extended period of time and 

emissions controls for PM, HCl, Hg are not needed. This is 

probably covered under a request to permit alternate 

operating scenarios as well. 

For reference- Subpart DDDDD definitions:

Startup means either the first-ever firing of fuel in a boiler or process heater for the purpose of supplying steam or heat for heating and/or 

producing electricity, or for any other purpose, or the firing of fuel in a boiler after a shutdown event for any purpose. Startup ends when any of the 

steam or heat from the boiler or process heater is supplied for heating, and/or producing electricity, or for any other purpose.

Shutdown means the cessation of operation of a boiler or process heater for any purpose. Shutdown begins either when none of the steam from 

the boiler is supplied for heating and/or producing electricity, or for any other purpose, or at the point of no fuel being fired in the boiler or process 

heater, whichever is earlier. Shutdown ends when there is no steam and no heat being supplied and no fuel being fired in the boiler or process 

heater.

From Table 3:

5. An existing or new boiler or process 

heater subject to emission limits in Table 1 

or 2 or 11 through 13 to this subpart 

during startup

You must operate all CMS during startup.

For startup of a boiler or process heater, you must use one or a combination of the following clean 

fuels: natural gas, synthetic natural gas, propane, distillate oil, syngas, ultra-low sulfur diesel, fuel oilsoaked rags, kerosene, hydrogen, paper, cardboard, refinery gas, and liquefied petroleum gas.

 If you start firing coal/solid fossil fuel, biomass/bio-based solids, heavy liquid fuel, or gas 2 (other) 

gases, you must vent emissions to the main stack(s) and engage all of the applicable control devices 

except limestone injection in fluidized bed combustion (FBC) boilers, dry scrubber, fabric filter, selective 

non-catalytic reduction (SNCR), and selective catalytic reduction (SCR). You must start your limestone 

injection in FBC boilers, dry scrubber, fabric filter, SNCR, and SCR systems as expeditiously as 
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possible. Startup ends when steam or heat is supplied for any purpose.

 You must comply with all applicable emission limits at all times except for startup or shutdown periods 

conforming with this work practice. You must collect monitoring data during periods of startup, as 

specified in § 63.7535(b). You must keep records during periods of startup. You must provide reports 

concerning activities and periods of startup, as specified in § 63.7555.

6. An existing or new boiler or process 

heater subject to emission limits in Tables 

1 or 2 or 11 through 13 to this subpart 

during shutdown

You must operate all CMS during shutdown.

While firing coal/solid fossil fuel, biomass/bio-based solids, heavy liquid fuel, or gas 2 (other) gases 

during shutdown, you must vent emissions to the main stack(s) and operate all applicable control 

devices, except limestone injection in FBC boilers, dry scrubber, fabric filter, SNCR, and SCR.

 You must comply with all applicable emissions limits at all times except for startup or shutdown periods 

conforming with this work practice. You must collect monitoring data during periods of shutdown, as 

specified in § 63.7535(b). You must keep records during periods of shutdown. You must provide 

reports concerning activities and periods of shutdown, as specified in § 63.7555.