Feb-2023

Ten easy hacks for heater and combustion engineers

It’s not possible to run heater simulations out in the field. Here we highlight rules, formulas and easy hacks to help fired heater engineers in their day-to-day work.

XRG Technologies

Article Summary

Since you can’t run a heater simulation every time you’re out in the field, here are some easy hacks, rules of thumb, and formulas to help fired heater engineers in their daily routine, especially when it comes to estimating the effect of fuel and air changes on burner capacity and heater efficiency.

Convert per cent excess air to per cent excess O₂
Process heaters do not operate at exactly the right amount of air, so we need to provide ‘excess’ air to the system to ensure complete combustion of the fuel.

The recommended excess air level for a gas fired process furnace is 15%, according to industry recommended practices, like API 535. In certain process plants, such as ethylene and hydrogen production, furnaces operate steadily at high temperature.

Here, the industry norm is an excess air level of 8-10%. Combustion of liquid fuels, on the other hand, requires excess air levels of 20-25% to prevent soot formation. Since the operator of the furnace typically only knows the firebox oxygen level, use the following formula to convert to excess air (EA) percentage.

with O2 expressed in vol% (dry). The other way around:

The equations work well for typical refinery fuel gas mixtures but deviate for fuels that are very high in inerts, hydrogen or carbon monoxide.
Example:

arch O2 = 5 vol%(dry) γ Excess Air = 92 x 5 / (21 – 5) = 29%

Excess Air = 15% γ Excess O2 = 21 x 15 / (92 + 15) = 3 vol% (dry)

Fired heater efficiency
Fuel efficiency of a fired heater is an important indicator. It tells you how close the heater runs to the design sheet conditions, if there is fouling or damage causing excess fuel use, and ways to improve capacity or fuel consumption. The precise calculation of the fuel efficiency is a bit of an undertaking, as shown in Annex G of API 560, which requires the fuel composition, excess air, stack temperature, fuel temperature, combustion air temperature, etcetera.

However, it is possible to get a good estimate of efficiency just from excess air and stack temperature. See Figure 1 for the heater efficiency when operating on natural gas and ambient air. The graph can be used for other fuels as well, but accuracy will drop if the amount of hydrogen or inert components is high. Also note that the graph does not account for external heat sources like preheating the combustion air or fuel with waste steam.

If you do not know the excess air but only the stack O₂ content, an even faster method is available:
Metric:

Imperial:

These linear equations are only valid for O₂ < 5%; above 5% the efficiency drops exponentially.
Example:

Stack temperature = 300°C and stack oxygen is 2%: Efficiency = 100 – 300/20 – 2/2 = 84%

Firebox draft
Fired heaters are typically controlled to a draft of around 0.05 – 0.15 in.H2O at the exit of the firebox. Sometimes we need to know what the draft is in other locations, for example if we want to estimate the pressure drop over a natural draft burner. Since draft varies linearly with height inside the firebox, this can be easily estimated with the vertical distance (in feet) between the location of the draft measurement and the point of interest. By multiplying this number by 0.01 we get the draft difference expressed in inches of water column:

Draft = 0.01∙HFB

For example, if a firebox is 45 ft high and the arch draft is 0.15 in H₂O, the draft at the firebox floor is

0.15 + 45 x 0.01 = 0.60 in. H₂O

For natural draft burners, this is the maximum available pressure drop for the combustion air.

Estimate burner air capacity
If the performance of a burner is known at one condition (i.e. the design conditions on the data sheet), you can estimate the effect of changes, like air temperature, on capacity.
Change in burner duty as a function of air temperature (with constant pressure drop):

Change in burner pressure drop as a function of air temperature (with constant duty):

where T is the absolute temperature (in K or °R).

If a burner is designed for a certain pressure drop with 250°F air temperature, and the new air temperature drops to 150°F, we gain 8% extra capacity:
Using the same example but keeping the duty constant, we get 14% lower pressure drop:

Volumetric air to fuel ratio for any hydrocarbon
The volumetric air requirement for any hydrocarbon can be calculated from the ratio of carbon to hydrogen in the molecule (CxHy) and the fraction of excess air (EA) :

For example, methane is CH4, so x=1 and y=4. For an excess air of 20%, the air to fuel volume ratio is:

Which means that we need 11.424 ft3 (or m3) of air to combust 1 ft3 (or m3) of methane.
For hydrogen (H2), x=0 and y=2. So, hydrogen combustion with 10% excess air yields: