Industrial burner

01

Set and hold the air-fuel ratio

The US DOE process heating tip sheet Check Burner Air to Fuel Ratios says periodic checking and resetting of burner air-fuel ratios is one of the simplest ways to get maximum efficiency from fuel-fired furnaces, ovens, heaters and boilers. The tip sheet says "most high temperature direct-fired furnaces, radiant tubes, and boilers operate with about 10% to 20% excess combustion air at high fire" to prevent dangerous carbon monoxide and soot; for natural gas, propane and fuel oils roughly one cubic foot of air releases about 100 Btu in complete combustion, and efficiency falls considerably if air is well above or below the theoretical requirement. For "most systems", it gives 2–3% oxygen in the flue gas with only 10–50 ppm of combustibles indicates ideal operation. [1]

Air-gas ratio can be found by metering air and fuel, by flue gas analysis, or both. The tip sheet's example: a furnace with 2,400°F flue gas running at 25% excess air (4.5% O2) has 22% available heat against 29% at the optimum 10% (2.1% O2), so fuel savings are 100 × (29 − 22) ÷ 29 = 24%. It warns that processes evaporating moisture or solvents need large amounts of excess air to dilute vapours and carry them away, and cutting it too far can slow the process and create an explosion hazard. [1]

Editorial note: ask the supplier how its control keeps excess air at your target across the turndown range, and whether oxygen trim is offered.

Give the supplier the application (furnace, oven, heater, boiler), heat input range and turndown, fuel(s) and supply pressure, combustion air temperature and source, process atmosphere and any solvent loads, target excess air or flue O2, flame shape and length limits, NOx and CO limits and the reference conditions, ratio control and oxygen trim, flame safeguard and safety shutoff requirements, and the code under which the burner management system must be approved.

02

Meet NOx and CO limits

EPA's AP-42 Section 1.4 (Natural Gas Combustion) says "the two most prevalent combustion control techniques" for NOx from gas-fired boilers are flue gas recirculation (FGR) and low-NOx burners. FGR returns part of the flue gas to the burner windbox, lowering peak flame temperature and oxygen; AP-42 says it "is normally used in combination with specially designed low NOx burners" designed to hold a stable flame with the extra inert gas, and together they reduce NOx by 60–90%. Low-NOx burners stage combustion (staged air or staged fuel), giving a cooler flame that suppresses thermal NOx, with reductions of 40–85% observed. It also notes that NOx controls can lower combustion efficiency and raise CO, and that poorly tuned boilers emit more CO. [2]

Its Table 1.4-1 gives NOx emission factors per million standard cubic feet of gas: for small boilers under 100 MMBtu/h, 100 lb uncontrolled, 50 lb with low-NOx burners and 32 lb with low-NOx burners plus FGR; for large wall-fired boilers, 280 lb pre-NSPS uncontrolled, 190 lb post-NSPS and 140 lb with low-NOx burners; CO is 84 lb for these boilers. Factors are based on a gas heating value of 1,020 Btu/scf. [2]

Editorial calculation: dividing by 1,020 Btu/scf, 100 lb/10⁶ scf is about 0.098 lb/MMBtu, 50 lb is about 0.049 and 32 lb about 0.031 lb/MMBtu; compare these with the limit in your permit and ask for a guarantee at your firing rate and excess air. [2]

03

Read a low-NOx burner data sheet line by line

Cleaver-Brooks' product page for its ProFire SBR-30 low-NOx burner shows what a maker's data sheet states. It lists emissions of less than 30 ppm NOx without FGR, about 3% O2 across a wide load range, up to 10:1 turndown when firing gas, and #2 fuel oil as a back-up fuel. The technical data give a natural gas input of 16.8 to 54.6 MMBtu/hr, boiler sizes of 400–1,300 horsepower, an oil input of 120–390 US GPH, full modulation, and parallel positioning as standard. [3]

Cleaver-Brooks describes parallel positioning, standard on this burner, as controls that use dedicated motorized actuators for the fuel and air valves so the burner can optimise its fuel-to-air ratio. The page also says the firing head design achieves controlled combustion leading to low emissions with 3% O2. [3]

Editorial note: the page states NOx in ppm but no reference O2 or test load for that figure. Ask the supplier for the NOx and CO guarantee in ppm at a stated O2 reference, at both maximum and minimum firing, and confirm whether the turndown figure applies to gas, oil or both.

A second maker's pages show how ultra-low-NOx figures are stated. Power Flame lists its NP2 burner for gaseous fuels only, from 400,000 to 8,600,000 Btu/hr, using fully premixed, surface-stabilised combustion on an all-metallic porous-mesh firing head to give sub 9 to 20 ppm NOx without flue gas recirculation, with a washable stainless steel filter, parallel positioning control and a UL-listed, factory-tested package, for firetube and watertube boilers and process heaters. [4] Its range page treats sub 30 ppm as the low-NOx class and lists other ultra-low-NOx models at sub 7–12 ppm (NVC) and sub 9 ppm on natural gas (UCM). [5] Editorial calculation: 400,000–8,600,000 Btu/hr is about 117–2,520 kW across the model range, which is not the same as one burner's turndown; and a sub-9 ppm guarantee is 70% below a 30 ppm one, so state the ppm limit, its O2 reference and the firing range it must hold over. [4] [5]

04

Trace the burner system

This is a functional reference built from the sources cited; it is not one maker's design.

05

Fill the specification fields with values for your application

A burner quotation needs values for your process. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the process, fuel and emission data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Application: [furnace; oven; heater; boiler]
Input: [max; min; turndown]
Fuel: [type; pressure]
Air: [temperature; excess air target]
Atmosphere: [solvents; moisture]
Emissions: [NOx; CO; reference O2]
Control: [ratio; O2 trim; flame safeguard]
Chamber: [size; flame limits]

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