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RULE 4352 SOLID FUEL FIRED BOILERS, STEAM GENERATORS AND PROCESS 

HEATERS (Adopted September 14, 1994; Amended October 19, 1995; Amended 

May 18, 2006) 

1.0 Purpose 

 The purpose of this rule is to limit emissions of oxides of nitrogen (NOx) and carbon 

monoxide (CO) from solid fuel fired boilers, steam generators and process heaters. 

2.0 Applicability 

 

 This rule applies to any boiler, steam generator or process heater fired on solid fuel. Heat 

may be supplied by liquid or gaseous fuels for start-ups, shutdowns, and during other 

flame stabilization periods, as deemed necessary by the owner/operator. 

3.0 Definitions 

 3.1 Air Pollution Control Officer (APCO): as defined in Rule 1020 (Definitions). 

 3.2 ARB: California Air Resources Board. 

3.3 Block 24-hour Average: the arithmetic average of the hourly NOx emission rates 

of a unit as measured over 24 one-hour periods, daily, from 12:00 AM to 11:59 

PM, excluding periods of system calibration.

 3.4 Boiler or Steam Generator: any combustion equipment fired directly or indirectly 

with any solid fuel used to produce hot water or steam. 

 3.5 British Thermal Unit (Btu): the amount of heat required to raise the temperature of 

one pound of water from 59°F to 60°F at one atmosphere. 

3.6 EPA: United States Environmental Protection Agency. 

3.7 Gaseous Fuel: any fuel which is a gas at standard conditions. 

 3.8 Heat Input: the heat of combustion released due to burning a fuel in a unit, based 

on the higher heating value of the fuel, not including the sensible heat of incoming 

combustion air and fuel. 

 3.9 Higher Heating Value (hhv): the total heat liberated per mass of fuel burned (Btu 

per pound), when fuel and dry air at standard conditions undergo complete 

combustion and all resulting products are brought to their standard states at 

standard conditions. 
 

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3.10 Liquid Fuel: any fuel which is a liquid at standard conditions. 

 3.11 Potential to Emit: as defined in Rule 2201 (New and Modified Stationary Source 

Rule). 

 3.12 Process Heater: any combustion equipment fired on solid fuel, which transfers heat 

from combustion gases to water or process streams. Process heaters exclude kilns 

or ovens used for drying, baking, cooking, calcining, heat treating or vitrifying. 

 3.13 NOx Emissions: the sum of oxides of nitrogen (NO) in the flue gas, collectively 

expressed as nitrogen dioxide. 

 3.14 Rated Heat Input: Rated Heat Input (million Btu per hour): the heat input capacity 

specified on the nameplate of the unit. If the unit has been physically modified 

such that its maximum heat input differs from what is specified on the nameplate, 

the modified maximum heat input shall be considered as the rated heat input and 

made enforceable by Permit to Operate. 

 3.15 Shutdown: the period of time during which a unit is taken from operational to nonoperational status by allowing it to cool down from its operating temperature and 

pressure to an ambient temperature. 

 3.16 Solid Fuel: any fuel which is a solid at standard conditions. 

 

 3.17 Standard Conditions: defined in Rule 1020 (Definitions). 

 3.18 Start-up: the period of time during which a unit is heated to the operating 

temperature and pressure from a shutdown status. 

 3.19 Stationary Source: as defined in Rule 2201 (New and Modified Stationary Source 

Review Rule). 

 3.20 Unit: any boiler, steam generator or process heater as defined in this rule. For the 

purpose of this rule, two boilers, two steam generators, or two process heaters 

may be considered as one unit, if, they are operated as one single unit sharing a 

single common stack and have been issued only one District Permit to Operate 

(PTO). 

4.0 Exemptions 

Except for complying with the recordkeeping requirements of Section 6.2 and applicable 

compliance schedule in Section 7.0, this rule shall not apply to units operated at a 

Stationary Source that has a potential to emit less than 10 tons per year of oxides of 

nitrogen (NOx) or volatile organic compounds (VOC). 
 

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5.0 Requirements 

5.1 The owner/operator of a boiler, steam generator or process heater shall not operate 

such a unit in a manner that results in NOx and CO emissions exceeding the limits 

specified in Table 1. The emission limits measured in parts per million by volume 

(ppmv) are referenced at dry stack gas conditions and shall be corrected to the 

applicable percent O2 or CO2 specified in Table 1 in accordance with EPA Method 

19. 

Table 1 - NOx and CO Emission Limits 

Tier 1 Emission Limits effective until 

December 31, 2006 

Tier 2 Emission Limits effective 

on and after January 1, 2007 

Fuel Type 

NOx Limit CO Limit NOx Limit CO Limit 

Municipal Solid 

Waste 

200 ppmv 

corrected to 12% 

CO2 

200 ppmv 

corrected to 

12% CO2

Biomass using 

Multiple Hearth 

Furnace 

0.35 lb/MMBtu 

of heat input 

115 ppmv 

corrected to 

3% O2

All Others 0.20 lb/MMBtu 

of heat input 

400 ppmv corrected 

to 3% O2, or 

310 ppmv corrected 

to 7% O2, or 

310 ppmv corrected 

to 12% CO2

115 ppmv 

corrected to 

3% O2

400 ppmv 

corrected to 3% 

O2 

 

 5.2 All NOx and CO emission limits shall be based on a block 24-hour average. A 

violation of the emission limits as measured by the test methods listed in Section 

6.4 shall constitute a violation of this rule. 

 5.3 Start-up and Shutdown Provisions 

 The applicable emission limits of Section 5.1 shall not apply during start-up or 

shutdown provided an operator complies with the requirements specified below. 

5.3.1 The duration of each shut down shall not exceed twelve (12) hours, except 

as provided in Section 5.3.4. 

5.3.2 Except as provided in Section 5.3.4, the duration of each start-up shall not 

exceed 96 hours. If curing of the refractory is required after a 

modification to the unit is made, the duration of start-up shall not exceed 

192 hours, except as provided in Section 5.3.4. 
 

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5.3.3 The emission control system shall be in operation and emissions shall be 

minimized insofar as technologically feasible during start-up or shutdown. 

5.3.4 Notwithstanding the requirements of Section 5.3.1 or Section 5.3.2, the 

APCO, ARB, and EPA may approve a longer start-up or shutdown 

duration, if an operator submits an application for a Permit to Operate 

which provides a justification for the requested additional duration. 

5.3.4.1 The maximum allowable duration of start-up or shutdown will be 

determined by the APCO, ARB, and EPA. 

5.3.4.2 At a minimum, a justification for increased start-up or shutdown 

duration shall include the following: 

5.3.4.2.1 A clear identification of the control technologies or 

strategies to be utilized; and 

5.3.4.2.2 A description of what physical conditions prevail 

during start-up or shutdown periods that prevent the 

controls from being effective; and 

5.3.4.2.3 A reasonably precise estimate as to when the 

physical conditions will have reached a state that 

allows for the effective control of emissions; and 

5.3.4.2.4 A detailed list of activities to be performed during 

start-up or shutdown and a reasonable explanation 

for the length of time needed to complete each 

activity; and 

5.3.4.2.5 A description of the material process flow rates 

and system operating parameters, etc., the 

owner/operator plans to evaluate during the 

process optimization; and an explanation of how 

the activities and process flow affect the operation 

of the emissions control equipment; and 

5.3.4.2.6 Basis for the requested additional duration of startup or shutdown. 

5.4 Units operated at less than 50 percent of rated heat input (RHI) 

 Notwithstanding the requirements of Section 5.1, the APCO, ARB, and EPA may 

approve an increased emission limit if the owner/operator submits an application 
 

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for a Permit to Operate, which provides a justification for the requested limit. 

Operations above 50 percent of RHI shall comply with the emission limits in 

Section 5.1. 

5.4.1 The maximum NOx or CO emission limit for operations less than 50 

percent of RHI will be determined by the APCO, ARB, and EPA. 

5.4.2 At a minimum, a justification for an increased emission limit shall include 

the following: 

 5.4.2.1 The basis for the requested increased emission limit; and 

5.4.2.2 A clear identification of the control technologies or strategies to 

be utilized; and 

5.4.2.3 A description of what physical conditions prevail at less than 50% 

of RHI that prevent the controls from being completely effective; 

and 

5.4.2.4 A reasonably precise estimate as to when the physical conditions 

must be reached that allows for the effective control of emissions; 

and 

5.4.2.5 A detailed list of activities to be performed to control emissions 

and a reasonable explanation for the length of time needed to 

complete each activity; and 

5.4.2.6 A description of the material process flow rates and system 

operating parameters, etc., the owner/operator plans to evaluate 

during the process optimization; and an explanation of how the 

activities and process flow affect the operation of the emissions 

control equipment. 

5.5 Monitoring Provisions 

 The owner/operator of any unit using ammonia injection as a NOx control 

technique, shall operate a Continuous Emissions Monitoring system (CEM) to 

monitor and record NOx concentrations, CO2 or O2 concentrations, as well as the 

NOx emission rate. Continuous Emission Monitoring systems shall be operated, 

maintained, and calibrated pursuant to the requirements of 40 CFR 60.7 (c) and 

60.13. CEMs must also satisfy the Performance Specifications of 40 CFR 60 

Appendix B and the Relative Accuracy Test Audit of Appendix F. 
 

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6.0 Administrative Requirements 

 6.1 Emission Control Plan 

 The owner/operator of any unit subject to the provisions of this rule shall submit to 

the APCO an emissions control plan of all actions to be taken to satisfy the 

requirements of Section 5.0. Such plan shall identify the type of emission controls 

to be applied to each unit and a construction schedule, or shall provide source 

tests, in accordance with Section 6.4, sufficient to demonstrate that the unit is 

currently in compliance with the emission limits of this rule. 

 6.2 Recordkeeping 

 6.2.1 Except for municipal solid waste (MSW) fired units; the owner/operator of 

any unit subject to the requirements of this rule shall maintain, on a 

monthly basis, an operating log for each unit that includes the following 

information: 

 6.2.1.1 type and quantity of fuel used. 

 6.2.1.2 the higher heating value (hhv) of each fuel as determined by 

Section 6.4, or as certified by a third party fuel supplier. 

 6.2.2 The records required by Section 6.2.1 shall be retained on site for a period 

of five years, and shall be made available to the APCO, ARB, and EPA 

upon request. 

 6.3 Compliance Source Testing 

 6.3.1 Each unit subject to the requirements of this rule shall be tested at least 

once every twelve (12) months, to determine compliance with the 

applicable requirements of Section 5.0. 

 6.3.2 All emission measurements shall be made with the unit operating either at 

conditions representative of normal operations or conditions specified in the 

Permit to Operate. 

 6.3.3 No compliance determination shall be established within two hours after a 

period in which fuel flow to the unit is zero, or is shut off for 30 minutes 

or longer. 
 

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 6.4 Test Methods 

6.4.1 Compliance with the requirements of Section 5.0 shall be determined in 

accordance with the following source test procedures unless otherwise 

approved by the APCO, ARB, and EPA: 

 6.4.1.1 Oxides of nitrogen (ppmv) – EPA Method 7E, or ARB Method 

100. 

 6.4.1.2 Carbon monoxide (ppmv) - EPA Method 10, or ARB Method 

100. 

 6.4.1.3 Stack gas oxygen - EPA Method 3 or 3A, or ARB Method 100. 

 6.4.1.4 NOx emission rate (Heat input basis) - EPA Method 19. 

 6.4.1.5 Stack gas velocities - EPA Method 2. 

 6.4.1.6 Stack gas moisture content - EPA Method 4. 

 6.4.1.7 Solid fuel higher heating value (hhv) - ASTM Method D 2015-

96, or 

 6.4.1.8 Solid fuel higher heating value (hhv) - ASTM Method E 711-87. 

6.4.1.9 ASTM D 1826-94 or D 1945-96 in conjunction with ASTM D 

3588-98 for gaseous fuels. 

7.0 Compliance Schedule 

 7.1 Owners/operators of each unit subject to the provisions of this rule shall 

demonstrate compliance in accordance to the following schedule: 

 7.1.1 For units at a stationary source that has the potential to emit 10 to 50 tons 

per year of NOx or VOC: 

 On and after January 1, 2007, demonstrate compliance with the Table 1 

Tier 2 emissions limits and all applicable requirements of this rule. 

 7.1.2 For units at a stationary source that has the potential to emit greater than or 

equal to 50 tons per year of NOx or VOC: 
 

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 7.1.2.1 By March 14, 1995, submit to the APCO an emission control 

plan pursuant to Section 6.1 and a complete application for an 

Authority to Construct (ATC), if necessary. 

 7.1.2.2 By May 31, 1995, demonstrate full compliance with Table 1 Tier 

1 emission limits and all applicable requirements of this rule. 

 7.1.2.3 On and after January 1, 2007, demonstrate compliance with Table 

1 Tier 2 emissions limits and all applicable requirements of this 

rule. 

7.1.3 For units exempted pursuant to Section 4.0, on and after January 1, 2007 

comply with the recordkeeping requirements of Section 6.2.