RULE 425.2 - Boilers, Steam Generators, and Process Heaters (Oxides of Nitrogen) - Adopted 10/13/94, Amended 4/6/95

I. Purpose

The purpose of this Rule is to limit oxides of nitrogen (NOx) emissions from boilers, steam generators, and process heaters to levels consistent with Reasonably Available Control Technology (RACT) to satisfy California Health and Safety Code Section 40918(b) and 1990 Federal Clean Air Act Amendments, Section 182(f). Carbon monoxide emissions are also limited to insure efficient combustion at reduced NOx levels.

II. Applicability

This Rule shall apply, as specified, to any boiler, steam generator or process heater with rated heat input of 5 million Btu per hour or more and fired with gaseous and/or liquid fuels.

III. Definitions

A. Annual Heat Input - total heat released (therms) by fuel(s) burned in a unit during a calendar year as determined from higher heating value and cumulative annual fuel(s) usage.

B. Boiler or Steam Generator - any external combustion unit fired with liquid and/or gaseous fuel used to produce hot water or steam, but not including gas turbine engine exhaust gas heat recovery systems.

C. British Thermal Unit (Btu) - amount of heat required to raise temperature of one pound of water from 59°F to 60°F at one atmosphere.

D. Gaseous Fuel - any fuel existing as gas at standard conditions.

E. Heat Input - total heat released (Btu's) by fuel(s) burned in a unit as determined from higher heating value, not including sensible heat of incoming combustion air and fuel(s).

F. Higher Heating Value (HHV) - total heat released per mass of fuel burned (Btu's per pound), when fuel and dry air at standard conditions undergo complete combustion and all resulting products are brought to standard conditions.

G. Liquid Fuel - any fuel, including distillate and residual oil, existing as liquid at standard conditions.

H. Natural gas curtailment - loss of natural gas supply due to action of PUC -regulated supplier. For Section V curtailment limit to apply, curtailment must not exceed 168 cumulative hours of operation per calendar year, excluding equipment testing not to exceed 48 hours per calendar year.

I. Oxides of Nitrogen (NOx) - total nitrogen oxides (expressed as NO2).

J. Process Heater - any external combustion unit fired with liquid and/or gaseous fuel used to transfer heat from combustion gases to liquid process streams.

K. Reasonably Available Control Technology (RACT) - lowest emission limitation a particular source is capable of meeting by application of control technology reasonably available considering technological and economic feasibility.

L. Rated Heat Input - heat input capacity (Btu's/hr) specified on nameplate of unit or by manufacturer for that model number, or as limited by District permit.

M. Standard Conditions - as defined in Rule 102, Subsection DD.

N. Therm - 100,000 British thermal units (Btu's).

O. Unit - any boiler, steam generator or process heater as defined in this Rule.

IV. Exemption

This Rule shall not apply to any unit with rated heat input less than 5 million Btu's per hour.

V. Requirements

A. An owner/operator of any unit subject to this Rule with annual heat input of 90,000 therms or more during one or more of the three preceding years of operation shall comply with following applicable NOx emission limit(s):

Gaseous Fuel

Liquid Fuel

During Normal


70 ppmv, or


115 ppmv, or

0.15 lb/MMBtu

During Natural




150 ppmv, or

0.19 lb/MMBtu

For units subject to this Subsection, carbon monoxide (CO) emissions shall not exceed 400 ppmv.

NOx emission limit for any unit fired simultaneously with gaseous and liquid fuels shall be heat input-weighted average of applicable limits. Calculations shall be performed as prescribed in Section VIII.

NOx and CO emission limits in ppmv are referenced at dry stack gas conditions, adjusted to 3.00 percent by volume stack gas oxygen in accordance with Section VIII., and averaged over 15 consecutive minutes from no less than 5 data sets, recorded from sampling of no more than 3 minutes.

B. An owner/operator of any unit subject to this Rule with annual heat input rate of 90,000 therms or more shall comply, until November 30, 1997, and any unit with annual heat input rate of less than 90,000 therms shall comply with one of the following NOx minimization procedures:

  1. Tune each unit at least once per year in accordance with Section IX.;

  2. Operate each unit in manner maintaining stack gas oxygen at no more than 3.00 percent by volume on dry basis; or

  3. Operate each unit with automatic stack gas oxygen trim system set at 3.00 (+0.15) percent by volume on dry basis.

C. Monitoring Requirements

  1. An owner/operator of any unit simultaneously firing a combination of different fuels shall install and maintain totalizing mass or volumetric flow rate meters in each fuel line.

  2. An owner/operator of any unit equipped with equipment intended to reduce or control NOx shall install and maintain appropriate provisions to monitor operational parameters of unit and/or NOx control system that correlate to NOx emissions.

D. Compliance Demonstration

  1. An owner/operator of any unit subject to Subsection V.A. shall have option of complying with either concentration (ppmv) emission limits or heat input basis (lb/MMBtu) emission limits as specified in Subsection V.A. All compliance demonstrations shall be performed using applicable test method(s) specified in Subsection VI.B. and methods selected to demonstrate compliance shall be specified in Emission Control Plan required by Subsection VI.D.
  2. All emission measurements shall be made with unit operating at conditions as close as physically possible to maximum firing rate allowed by KCAPCD Permit to Operate.

VI. Administrative Requirements

A. Recordkeeping and Reporting

  1. An owner/operator of any unit subject to this Rule or limited by permit condition to firing less than 5 million Btu's/hr shall monitor and record HHV and cumulative annual use of each fuel.

  2. An owner/operator of any unit operated under natural gas curtailment limit of Subsection V.A. shall monitor and record cumulative annual hours of operation on liquid fuel during curtailment and during testing.

  3. An owner/operator of any identical units wishing to limit emissions testing to one unit per group of units pursuant to Subsection VI.C. shall establish correlation of NOx emissions and key operating parameters and keep records of these data for each affected unit.

  4. Records shall be maintained for at least two calendar years on site and shall be made readily available to District personnel.

  5. Compliance test data and results collected to satisfy Subsection VI.C. shall be submitted to District within 60 days of collection.

B. Test Methods

  1. Fuel HHV shall be certified by third party fuel supplier or determined by:

  2. a. ASTM D 240-87 or D 2382-88 for liquid fuels; and

    b. ASTM D 1826-88 or D 1945-81 in conjunction with ASTM D 3588-89 for gaseous fuels.

  3. Oxides of nitrogen (ppmv) - EPA Method 7E, or CARB Method 100.

  4. Carbon monoxide (ppmv) - EPA Method 10, or CARB Method 100.

  5. Stack gas oxygen - EPA Method 3 or 3A, or CARB Method 100.

  6. NOx emission rate (heat input basis) - EPA Method 19, or CARB Method 100 and data from fuel flow meter.

  7. Stack gas velocity - EPA Method 2.

  8. Stack gas moisture content - EPA Method 4.

C. Compliance Testing

  1. An owner/operator of any unit subject to requirements of Subsection V.A. shall be tested to determine compliance with applicable requirements not less than once every 12 months. An owner/operator of gaseous fuel-fired units demonstrating compliance for two consecutive years can, if desired, demonstrate compliance once every thirty-six months.

  2. An owner/operator of any unit subject to Subsection V.B.2. shall sample and record stack gas oxygen content at least monthly.

  3. Test results from an individual unit may be used for other units at the same location provided manufacturer, model number, rated capacity, fuel type, and emission control provisions are identical and key operating parameters such as stack gas oxygen, fuel consumption, etc. are monitored and established to correlate with NOx emissions from unit tested.

D. Emission Control Plan

An owner/operator of any unit subject to this Rule shall submit to Control Officer an Emission Control Plan including:

  1. List of units subject to Rule, including rated heat inputs, anticipated annual heat input, applicable Section V. requirements, and control option chosen, if applicable;

  2. Description of actions to be taken to satisfy requirements of Section V. Such plan shall identify actions to be taken to comply, including any type of emissions control to be applied to each unit and construction schedule, or shall include test results to demonstrate unit already complies with applicable requirements; and

  3. Specification of proposed test methods.

VII. Compliance Schedule

A. An owner/operator of any unit subject to Section V. shall comply with following schedule:

  1. By March 1, 1995, submit to Control Officer an Emission Control Plan pursuant to Subsection VI.D., and a complete application for Authority to Construct, if necessary;

  2. By May 31, 1995 demonstrate full compliance with Subsection V.B.; and

  3. By November 30, 1997 demonstrate full compliance with all additional and applicable provisions of this Rule.

B. An owner/operator of any unit becoming subject to requirements of Subsection V.A. by exceeding annual heat input exemption threshold shall comply with following increments of progress:

  1. On or before December 31st of calendar year immediately following year annual heat input threshold was exceeded, submit an Emission Control Plan containing information prescribed in Subsection VI.D.; and

  2. No later than three calendar years following submission of Emission Control Plan, demonstrate final compliance with all applicable standards and requirements of this Rule.

VIII. Calculations

A. All ppmv emission limits specified in Section V.A. are referenced at dry stack gas conditions and 3.00 percent by volume stack gas oxygen. Emission concentrations shall be corrected to 3.00 percent oxygen as follows:

B. All lb/MMBtu NOx emission rates shall be calculated as pounds of nitrogen dioxide per million Btu's of heat input (HHV).

C. Heat input-weighted average NOx emission limit for combination of natural gas and liquid fuel shall be calculated as follows:

IX. NOx Minimization Tuning Procedures

A. Purpose

The purpose of this procedure is to provide a reasonable, cost-effective method to minimize NOx emissions from smaller, or low-fire/low use-rate combustion units subject to this Rule. These procedures not only minimize NOx emissions, but also result in reduced operating costs.

B. Equipment Tuning Procedure1 for Mechanical Draft Boilers, Steam Generators, and Process Heaters

Nothing in this Tuning Procedure shall be construed to require any act or omission that would result in unsafe conditions or would be in violation of any regulation or requirement established by Factory Mutual, Industrial Risk Insurers, National Fire Prevention Association, California Department of Industrial Relations (Occupational Safety and Health Division), Federal Occupational Safety and Health Administration, or other relevant regulations and requirements.

  1. Operate the unit at the firing rate most typical of normal operation. If the unit experiences significant load variations during normal operation, operate it at its average firing rate.

  2. At this firing rate, record stack gas temperature, oxygen concentration, and CO concentration (for gaseous fuels) or smoke spot number2 (for liquid fuels), and observe flame conditions after unit operation stabilizes at the firing rate selected.
  3. If the excess oxygen in the stack gas is at the lower end of the range of typical minimum values3; and if CO emissions are low and there is no smoke, the unit is probably operating at near optimum efficiency - at this particular firing rate. However, complete the remaining portion of this procedure to determine whether still lower oxygen levels are practical.

  4. Increase combustion air flow to the furnace until stack gas oxygen levels increase by one to two percent over the level measured in Step 2. As in Step 2, record the stack gas temperature, CO concentration (for gaseous fuels) or smoke spot number (for liquid fuels), and observe flame conditions for these higher oxygen levels after boiler operation stabilizes.

    1. This tuning procedure is based on a tune-up procedure developed by KVB, Inc. for EPA.

    2. The smoke-spot number can be determined with ASTM Test Method D-2156 or with the Bacharach method.

    3. Typical minimum oxygen levels for boilers at high firing rates are:

    4. For natural gas: 0.5% to 3%

      For liquid fuels: 2% to 4%

  5. Decrease combustion air flow until the stack gas oxygen concentration is at the level measured in Step 2. From this level gradually reduce the combustion air flow, in small increments. After each increment, record the stack gas temperature, oxygen concentration, CO concentration (for gaseous fuels) and smoke-spot number (for liquid fuels). Also, observe the flame and record any changes in its condition.

  6. Continue to reduce combustion air flow stepwise, until one of these limits is reached:

  7. a. Unacceptable flame conditions- such as flame impingement on furnace walls or burner parts, excessive flame carryover, or flame instability.

    b. Stack gas CO concentrations greater than 400 ppm.

    c. Smoking at the stack.

    d. Equipment-related limitations- such as low windbox/furnace pressure differential, built in air-flow limits, etc.

  8. Develop an O2/CO curve (for gaseous fuels) or O2/smoke curve (for liquid fuels) similar to those shown in Figures 1 and 2 on Page 425-10 using the excess oxygen and CO or smoke-spot number data obtained at each combustion air flow setting.

  9. From the curves prepared in Step 6, find the stack gas oxygen levels where the CO emissions or smoke-spot number equal the following values:

  10. Fuel Measurement Value
    Gaseous CO Emissions 400 ppm
    #1 and #2 Oils smoke-spot number number 1
    #4 Oil smoke-spot number number 2
    #5 Oil smoke-spot number number 3
    Other Oils smoke-spot number number 4

    The above conditions are referred to as the CO or smoke thresholds, or as the minimum excess oxygen levels.

    Compare this minimum value of excess oxygen to the expected value provided by the combustion unit manufacturer. If the minimum level found is substantially higher than the value provided by the combustion unit manufacturer, burner adjustments can probably be made to improve fuel and air mix, thereby allowing operations with less air.

  11. Add 0.5 to 2.0 percent to the minimum excess oxygen level found in Step 7 and reset burner controls to operate automatically at this higher stack gas oxygen level. This margin above the minimum oxygen level accounts for fuel variations, variations in atmospheric conditions, load changes, and nonrepeatability or play in automatic controls.

  12. If the load of the combustion unit varies significantly during normal operation, repeat Steps 1-8 for firing rates that represent the upper and lower limits of the range of the load. Because control adjustments at one firing rate may affect conditions at other firing rates, it may not be possible to establish the optimum excess oxygen level at all firing rates. If this is the case, choose the burner control settings that give best performance over the range of firing rates. If one firing rate predominates, setting should optimize conditions at that rate.

  13. Verify that the new settings can accommodate the sudden load changes that may occur in daily operation without adverse effects. Do this by increasing and decreasing load rapidly while observing the flame and stack. If any of the conditions in Step 5 result, reset the combustion control to provide a slightly higher level of excess oxygen at the affected firing rates. Next verify these new settings in a similar fashion. Then make sure that the final control settings are recorded at steady-state operating conditions for future reference.