Saving fuel makes perfect sense; if you use less you pay less. That applies to filling up your car, and it applies to fuel consumption in a process heater. 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 actually quite tempting for an operator to use a little extra air for the combustion process for a number of 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. These and other variables may cause substantial variation in the firebox oxygen level. Now, 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; we certainly don’t want to go below zero because the combustion process would not receive enough air and we’d risk filling the combustion chamber with unburned hydrocarbons. Keeping it at exactly zero is not feasible either due to the aforementioned fluctuations in the system, but also because it is not easy to design a combustion process with perfect mixing of air and fuel. So, we need to provide some “excess” air to the system.
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 temperature. In those cases, 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.
By the way, the operator of the furnace typically only knows the firebox oxygen level. To convert from oxygen level to excess air percentage, the following simple formula can be used:
Excess air = 92 O2 / (21 – O2)
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.
Okay, so what is the cost of “excess” excess air?
Let us first discuss some firebox fundamentals that few people know or care about; excess air affects the firebox radiant thermal efficiency (unless you don’t care either – in that case skip to The End Result.
Air consists almost exclusively of nitrogen and oxygen. Since they are diatomic, neither gas participates in the radiation energy transport. The only gases that cooperate in a meaningful manner are the water vapor and carbon dioxide that are formed during combustion. If the firebox operates at high excess air level, the concentration of H2O and CO2 is diluted, which lowers the effective emissivity of the flue gas. With the flue gas becoming a less effective emitter of radiant energy, the firebox thermal efficiency drops. Running a firebox on 35% excess air instead of 15% excess air lowers the flue gas emissivity by 5%.
The second problem is that every excess pound of air “steals” heat from the combustion process. It effectively lowers the equilibrium temperature, also known as the adiabatic flame temperature. Radiation depends on temperature to the fourth power, so radiant heat transfer drops tremendously when the firebox temperature drops because of all the extra air baggage. Running a firebox on 35% excess air instead of 15% excess air lowers the adiabatic flame temperature by a whopping 400°F.
The End Result: the radiant thermal efficiency drops significantly at high levels of excess air. For the example case of 15% excess air versus 35% excess air, the difference is about 7%. The firebox needs to be fired proportionally harder to compensate and is less energy efficient.
Using a fuel cost of $3 per MMBtu, efficiency losses are easily calculated. For a process heater operating at 100 MMBtu/h, each 1% reduction in fuel efficiency costs $26,300 per year. For a typical 300,000-bpd refinery each % energy gain or loss represents around $1 MM.
In addition to fuel costs, lower energy efficiency also increases the greenhouse gas emissions. In a 100 MMBtu/h heater, each % efficiency corresponds to 550 tpy CO2.
Running at a higher excess air level changes the duty split between radiant and convection section. The combination of higher firing rate and lower radiant efficiency leads to a significant increase in convection duty. This will impact tube metal temperatures, tube support temperatures and fin tip temperatures and could shorten the lives of each of these components.
Then there is the additional fan power in forced draft or induced draft fans and the loss of furnace capacity.
Finally, running at a high excess air level can significantly increase emissions of nitrogen oxides. Using the same example of running at 35% excess air compared to 15% could increase emissions to 150 – 200% of the design values.
Although air is free, running at high excess air is not! Besides having a direct impact on operating cost through fuel efficiency, excess air affects furnace reliability and stack emissions. Running at high excess air may buy some improved resistance to fluctuations, but too much will adversely impact profitability.