When excess air becomes too much
Excessive use of excess air impacts operating costs through fuel efficiency, furnace reliability, and stack emissions
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Saving fuel makes perfect sense; when you use less, you pay less. This applies to filling up your car and to fuel consumption in a process heater alike. How much money you save is easily calculated by multiplying the fuel savings by the fuel price per gallon. With combustion air, it is not so clear. Air is free, so why do you need to save on combustion air?
It is quite tempting for an operator to use a little extra air for the combustion process for several reasons. Oxygen requirements can vary because of fluctuations in the process, such as changing feed rates and feed quality. On top of that, the combustion side of the heater can be impacted by changes in fuel composition and ambient conditions. A notorious problem is that draft and air distribution inside natural draft heaters are impacted by wind gusts or rain. These and other variables may cause substantial variation in the firebox oxygen level. Any smart operator wants to keep that level well above zero, and if the fluctuations can be severe the cautious operator adds a good margin on top of the recommended level.
So, how much extra excess air is reasonable? To answer that question, we need to look at the cost of excess air. There is no simple gallon price but there are hidden costs that can be substantial.
What is optimum?
From an efficiency point of view, the theoretical optimum excess air level is zero percent. Providing a flame the exact required amount of air for combustion ensures that all available heat is released from the fuel. This ratio of air to fuel is called the stoichiometric ratio.
We certainly do not want to use less than the stoichiometric ratio because the combustion process would not receive enough air and risk filling the combustion chamber with unburned hydrocarbons. This is called firebox ‘flooding’ and the uncontrolled reaction of these hydrocarbons with any leakage air is a serious safety risk.
Keeping it at exactly zero is not feasible either. The typical heater instrumentation and control system is not able to keep up with any of the aforementioned fluctuations in the system due to response lag. Even if it could, it is very difficult to design a combustion process with perfectly mixed air and fuel. So, we need to provide some ‘excess’ air to the system to provide margin against fluctuations and ensure complete oxidation of the hydrocarbons.
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, the furnaces operate very steadily and at high temperatures. In those cases, the industry norm is an excess air level of 8-10%. Combustion of liquid fuels, on the other hand, typically requires excess air levels of 20-25% to prevent soot formation.
The operator of the heater measures excess air indirectly by checking the firebox oxygen level. To convert from oxygen level to excess air percentage, use the following simple formula:
with O2 expressed in vol% (dry). Using this equation, we see that 3% O2 translates to 15% excess air, and 5% O2 is equal to 35% excess air.
The cost of excess air
Let us first discuss some firebox fundamentals that few people know or care about. Air consists almost exclusively of nitrogen and oxygen. Since they are diatomic, neither gas participates in the transportation of radiation energy. The only gases that cooperate in a meaningful manner are the water vapour and carbon dioxide that form during combustion (see Figure 1). If the firebox operates at a high excess air level, the concentration of H2O and CO2 is diluted, which lowers the effective emissivity of the flue gas. As the flue gas becomes a less effective emitter of radiant energy, the firebox thermal efficiency drops.
The second problem is that every excess pound of air ‘steals’ heat from the combustion process. Each excess pound of air entering the heater is an extra pound that must be heated to the furnace temperature. It effectively lowers the equilibrium temperature, also known as the adiabatic flame temperature. Since radiation heat transfer is proportional with absolute temperature to the fourth power, the radiant efficiency of a firebox drops tremendously when its temperature drops because of all the extra air.
Table 1 lists the properties of flue gas from the combustion of natural gas with varying levels of excess air. The table clearly shows a strong dependence of emissivity and adiabatic flame temperature on flue gas composition. Between 15% and 25% excess air, the dry oxygen level only increases from 3.0 to 4.6 vol%. However, due to the drop in CO2 and H2O concentration the flue gas emissivity drops 3% and the adiabatic flame temperature drops by an astounding 200°F (93°C). In a typical firebox, this combination of lower emissivity and lower adiabatic flame temperature reduces the radiant thermal efficiency by about 5%. The firebox needs to be fired proportionally harder to compensate and is less energy efficient.
The convection section is where the residual heat in the flue gas is used for feed preheating. The convection section will compensate for some of the loss of firebox radiant efficiency but not completely.
The cost of ‘excess’ excess air
One can use Figure 2 and Figure 3 to calculate the cost of too much excess air. Use Figure 2 to determine the fuel efficiency of a fired heater as a function of excess air and stack gas temperature and Figure 3 to find the cost of natural gas around the world, expressed in $/MMBtu. An example calculation follows.
In Q3 of 2019, the US natural gas cost was approximately $3 per MMBtu (see Figure 3). For a process heater operating at 100 MMBtu/h, the total fuel cost is then 100 MMBtu/h x 8760 h/year x $3 per MMBtu/h = $2.63 million. That means each 1% reduction in fuel efficiency costs $26300/y. For a typical 300000 b/d refinery each percent energy gain or loss represents around $1 million.
A train of four identical heaters runs at an average of 5.5 vol% O2 (dry) at the arch, due to various design and operational issues. A change in the downstream process reduced the heat requirement from the heaters by 40%, which dropped the firebox temperature well below 1200°F (650°C). The floor-mounted burners are of the latest generation ultra low NOx design. Burners of this type reduce NOx emissions using internal flue gas recirculation. The dilution of the flame with inert gas causes a delay in combustion reactions and a reduction in thermal NOx. This approach works well at typical firebox temperatures of 1400-1600°F (760-870°C) but flame quality and stability deteriorate significantly when the firebox temperature becomes too cold. The only remedy available to the operator is to operate the heaters at higher oxygen levels.
Additional problems are caused by operating the burners at duties well below their optimal design point. In this case, the 40% reduction in heat liberation creates a soft and lazy flame with a tendency to roll into the coils. This is due to a lack of airside pressure drop used for fuel-air mixing. High wind speeds in the summer create large swings in air flow through the natural draft burners, occasionally producing high amounts of carbon monoxide. Again, the only remedy is to operate at higher excess air levels to increase the mixing rate and create a stiffer flame.
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