Process heaters are the largest consumers of energy in most plants. A refinery, on average, burns approximately 2 billion Btu/hr of fuel in fired heaters. The total quantity of fuel burned (heat released) is so high that any improvement will result in significant fuel savings. Although there are many ways to improve heater performance, including better design, operation and maintenance, excess air is the No. 1 contributor to poor heater efficiency and must be addressed.
High energy costs and tighter emissions regulations require increased understanding and control of excess air. Any reduction in excess air will raise the efficiency of a heater and reduce total emissions. NOX emissions are of the highest concern in a fired heater, although excess air control will also reduce refinery CO2 emissions and boost heater efficiency.
Fuel efficiency in a fired heater is a function of heater design, maintenance and operating parameters. Heaters must be designed for optimum efficiency, and providing an adequate heat-transfer area at the design stage will ensure better efficiency. It is recommended that the flue gas temperature approach (defined as flue gas temperature leaving convection, minus process inlet temperature) be between 50°F and 100°F, depending on heater tube material and the cost of fuel. However, heater efficiency may decline with the degradation of heater components. The degree of degradation is dependent on the quality of the maintenance program implemented at the refinery.
Excess air is defined as the amount of air above the stoichiometric air requirement that is needed to complete the combustion process. Excess oxygen (O2) is the amount of O2 in the incoming air not used during combustion.
In an operating plant, the airflow rate can be adjusted at a fixed absorbed-heat duty (constant feed flowrate and inlet/outlet conditions) until an optimum fuel-to-air ratio is achieved. It is important to note that there is a limit on minimum possible excess O2. Below this level, combustibles can enter the flue gas, which poses a safety hazard. Heater and burner manufacturers establish this minimum limit during the design stage. Operators should also keep a safe margin for upset conditions.
A frequently asked question is, Why do operators often run heaters with higher excess air? The answer is that additional excess air reduces flame temperature, shortens flame length and decreases tube flame impingement, thereby making it easier for workers to operate the heater without overheating the tube.
Excess O2 can be measured in flue gases, which can be correlated with excess air. Fig. 1 provides a correlation between excess air and flue gas O2 for a typical natural gas. Also, the following equation can be used to calculate the excess air based on flue gas O2:
EA = excess air, %
O2 = vol% of flue gas oxygen (dry).
Higher excess O2 helps achieve a stable flame in the firebox. At the same time, it reduces the efficiency of the heater. As a general rule, 3% O2 in flue gas is equivalent to 15% excess air.
Flue gas quantity
Flue gas quantity increases with a rise in excess air, which lowers heat and increases the fuel requirement. Fig. 2 provides a correlation between flue gas generated during combustion, and excess air.
Fig. 1. Correlation between excess air and flue
gas O2 for a typical natural gas.
Fig. 2. Correlation between flue gas generated
during combustion, and excess air.
The following equation can be used to calculate approximate flue gas quantity for natural gas:
qf = 1 + 0.167 3 (100 + EA)
EA = excess air, %
O2 = vol% of flue gas oxygen (dry)
qf = flue gas quantity in lb/lb of fuel.
As a general rule, flue gas quantity is approximately 20 times the fuel quantity at 15% excess air.
The net efficiency
The net eddiciency of a fired heater is equal to the total heat absorbed, divided by the total heat input. The heat absorbed is equal to the total heat input, minus the total losses. The net thermal heater efficiency can be calculated using the following equation:
η = Net thermal efficiency, %
LHV = Lower heating value of the fuel, Btu/lb of fuel
Ha = Sensible heat of air, Btu/lb of fuel
Hf = Sensible heat of fuel, Btu/lb of fuel
Hm = Sensible heat of atomizing media, Btu/lb of fuel
Hs = Stack heat losses, Btu/lb of fuel
HL = Setting loss, Btu/lb of fuel.
For all practical purposes, we can assume Ha and Hf to be negligible. Hm is applicable for fuel oil firing. Setting (casing) heat losses are in the range of 1.5%2.5%, depending on the capacity, design and size of the heater. Given these assumptions, there are two parameters for the estimation of efficiency: excess air/flue gas O2 and stack temperature.
Fig. 3 depicts a graph for the estimation of fired heater efficiency, based on flue gas O2 and stack temperature for a typical natural gas with a setting heat loss of 1.5%. For heat losses higher than 1.5%, additional heat loss should be reduced from the calculated efficiency.
For example, consider a heater operating at a stack temperature of 400°F, with 4% O2 (dry), and a 1.5% setting loss. Using the graph in Fig. 3, the efficiency can be estimated at 89%. For the same heater with a higher setting loss of 2.5%, the efficiency is 88%.
Fig. 3. Estimation of fired heater efficiency.
Knowledge of efficiency loss will clarify economic incentives to lower the stack temperature or the percentage of excess O2. As a general rule, every 35°F increase in flue gas temperature reduces the heater efficiency by 1%.
Natural draft heaters
Natural draft heaters use the draft (buoyancy) effect of hot flue gases to draw combustion air into the heater. The net draft available is the draft created by the stack effect, minus frictional and velocity losses. The net draft should be sufficient to obtain a negative pressure along the heater flue gas path.
It is important to maintain a safe draft level in a fired heater to achieve the best possible efficiency and operation. The target draft of 0.1 inchWC is set at the heater arch. A higher value of draft will result in ingress of tramp air into the heater. Tramp air takes heat from the combustion process and exits the stack, reducing heater efficiency. The flue gas sample taken from the stack does not represent the actual volume of O2 available for combustion. It is the sum of unused O2 from the firebox (actual excess O2) and O2 from tramp air.
A positive draft value will result in the leakage of hot flue gases through openings in the heater. This is a hazardous operation that can overheat the steel structure, refractory and heater supports, and, consequently, shorten heater life.
Fig. 4. Value of draft generated in the heater
for flue gas and ambient air temperatures.
Fig. 4 provides the value of draft generated in the heater for flue gas temperature and ambient air temperature. It should be noted that stack effect decreases with an increase in site altitude. The calculated draft should be amended using the correction factor for site altitude. The following equation can be used to calculate the draft generated in a heater:
H = Height, ft
Patm = Atmospheric pressure, psia
Tamb = Ambient air temperature, °R
Tfg = Flue gas temperature, °R.
As a general rule, for every 10 ft of firebox height, the draft increases by 0.1 inchWC:
Draft at burner (inchWC) ≈ 0.1 + Hfb ÷ 100 (5)
Hfb = Firebox height, ft.
As an example, for a 50-ft-tall firebox, the draft at burner is 0.6 inchWC (0.1 inchWC at arch, plus 0.5 inchWC as a stack effect). Generally, the stack effect decreases with an increase in site elevation. For example, a taller stack will be needed for a heater operating in Wyoming (altitude ~ 5,000 ft) than for one operating along the Texas Gulf Coast (altitude ~ 0 ft).
The typical combustion air preheater
The typical combustion air preheater (APH) will increase the heater efficiency by approximately 10%. Fuel gas generally contains H2S or sulfur, which convert into SO2 and then into SO3. The APHs heat-transfer surface is subject to cold-end corrosion caused by condensation of sulfur trioxide (SO3), which results in APH leakage. Air preheater leakage is one of the most common APH operating problems, and any such leakage results in a reduction in the overall efficiency of the heater.
In APH operation, the flue gas is generally at negative pressure, and the air is at positive pressure. Therefore, leakage occurs from air to the flue gas side. This reduces the quantity of air available for combustion, and it increases the quantity of flue gas leaving the APH.
This leakage can be detected by measuring the flue gas O2 content at the APH inlet and outlet. Any leakage will result in higher flue gas O2 at the APH exit, compared to the APH inlet. Generally, the APH is not equipped with a flue gas O2 analyzer at the inlet and the outlet; however, the inclusion of 2-in. connections at the APHs inlet and outlet will enable operators to measure O2 levels using a portable analyzer.
Another method of measuring leakage involves heat balance. The flue gas/air heat balance across the APH can be described as follows:
(m 3 Cp ∆T)flue gas = (m 3 Cp ∆T)air (6)
For a typical fuel gas at 15% excess air:
mflue gas ≈ 1.05 3 mair
Cpflue gas ≈ 1.15 3 Cpair (7)
∆Tair ≈ 1.2 3 ∆Tflue gas
m = Flowrate
Cp = Specific heat
∆T = Temperature difference across the APH.
Any leakage in the APH will reduce the ratio of ∆Tair to ∆Tflue gas. For example, for a 10% leakage in the APH, the ratio of temperature difference will be around 1.1.
Fig. 5 indicates the percentage of air leakage based on the ratio of ∆Tair to ∆Tflue gas for a typical natural gas firing.
Fig. 5. Percentage of air leakage for a typical
natural gas firing.
Air leakage through openings
A fired heater is not a 100% sealed unit; there are always openings through which air ingress (tramp air) can move. The volume of tramp air depends on the opening size and the draft at the location of the opening.
After the draft at the opening location is estimated, the following equation can be used to estimate the air leakage through an opening:
∆P = C 3 0.003 3 ρ 3 v2 (8)
This equation can be simplified for the leakage calculation purpose based on the following data:
- Molecular weight (MW) of air = 28.96
- Atmospheric pressure (psia) = 14.7
- Velocity head (C) = taken as 1
The simplified equation reads:
ΔP = Draft at opening location, inchWC
ρ = Density of air at ambient temperature, lb/ft3
v = Velocity of air through opening, ft/s
C = Velocity head
ql = Air leakage, lb per ft2/s
T = Ambient air temperature, °R.
Fig. 6 provides the quantity of air leakage per ft2 of opening size. This figure is based on an ambient air temperature of 60°F. Once the opening size is known, the amount of air leakage can be estimated. The estimated air can be translated into the additional firing rate required.
Fig. 6. Quantity of air leakage per ft2 of opening
A commonly asked question in heater discussions is, How much fuel can be saved if excess air is optimized? The efficiency chart in Fig. 3, which shows operating O2 and target O2, helps calculate the savings.
However, there is a drawback. The absorbed heat duty of the fired heater is constant. Any increase in the O2 level will reduce the efficiency, resulting in a higher firing rate. This increase in the firing rate will lead to a rise in stack temperature, which results in another reduction in efficiency. This reduction, in turn, demands a further increase in the firing rate.
For example, a 100-MM-Btu/hr fired heater is designed for operating at a stack temperature of 600°F, with 84% efficiency at 3% O2. The operating efficiency at 6% O2 is around 80% (and not 82%, as shown in the efficiency chart).
The method of efficiency calculation for off-design operating conditions presented in API-560 Appendix G can be used to estimate the stack temperature when excess air is present. This method can be simplified for excess air as follows:
Ts = Flue gas stack temperature, °R
EA = Excess air, %
Tf = Feed inlet temperature, °R (Tf1 = Tf2)
Φ = Excess air correction factor (subscripts 1 and 2 refer to design and operating conditions, respectively).
Once the new flue gas stack temperature at excess air is known, then the heater efficiency can be estimated. Fig. 7 shows the estimated fuel savings for a reduction in the O2 level to 3%. This graph is based on a fuel price of $6/MMBtu. The design flue gas temperature lines indicate the baseline stack temperature (i.e., the flue gas stack temperature at 3% O2).
Fig. 7. Estimated fuel savings for a reduction
in the O2 level to 3%.
CO2 emissions reduction
The volume of CO2 emissions generated in a fired heater is directly proportional to the firing rate. In combustion processes, fuel carbon converts into CO2. Therefore, excess air reduction will lower CO2 emissions.
Fig. 8 provides estimated decreases in CO2 emissions through a reduction in the O2 level to 3%. The basis for this graph is the same as that in Fig. 7.
Fig. 8. Estimated decreases in CO2 emissions
through a reduction in the O2 level to 3%.
Heater excess air control starts at the design stage. Well-designed heaters have low tramp air. There are three stages of excess air control:
- Design stage
The following methods can be used to reduce excess air and tramp air in the heater.
Design stage. A heater has many potential leak points for air ingress:
- Clearance around the bottom coil guide (spigots)
- Sight doors and peepholes
- Header boxes, manholes and other openings for viewing and access
- Modules and duct splice joints
- Terminals and crossover tubes
- Weld joints on the heater casing
- Soot-blower sleeves
- The APH.
These leak points must be designed for the lowest possible leakage. Suggestions for designing a low-leakage heater include the following:
- Seal the clearance space around the bottom tube guides by using a floor sleeve with an end cap, or seal boots
- Use sight doors, with safety glass, that are equipped with an interlock cover or flapper
- Use a self-closing peephole cover in the heater floor
- Ensure that header box panels and other openings are airtight, and use gaskets between the gaps
- Seal-weld all splice joints between modules from the inside, or use high-temperature sealant; also, use closer-bolt spacing (6 in. from center to center)
- Seal all terminals and crossover openings with flexible seals
- Ensure that all header box drain points are plugged
- Ensure that no leakage is occurring through instrument mountings
- Limit leakage through the APH during the design stage, and perform an air-leakage test in the shop.
Maintenance. Routine maintenance of the heaters is essential, since corrosive agents can be present in flue gases. Deterioration from sulfur oxides occurs mostly on cold sections of the steel casing. Climate conditions can also lead to rusting on exposed surfaces of the heater casing. Suggested inspection and maintenance methods include the following:
- Check for heater casing corrosion; if any leaks are discovered, they should be sealed to stop air ingress
- Ensure that observation doors (generally located in the bottom section of the radiant box) are closed after technicians inspect the heater flame
- Check peepholes, access doors, etc., for proper closing
- Check flue gas O2 content in the convection section and on the APH; if there is any increase in O2 content across the flue gas path, it indicates leakage
- Use a smoke test during heater shutdown to detect leakage
- Use infrared scanning, while the heater is in operation, to pinpoint locations with air leakage; these will have localized, lower heater casing temperatures
- To reduce leakage in burners, keep all burners in operation, even during lower operating loads; and close the air register when a burner is taken out of service.
Excess air control. Knowing the target flue gas O2 content is the first step in excess air control. Each heater is unique in its design. The O2 level required to achieve ideal combustion may be anywhere from 1%4% or higher, depending on the design and operating characteristics of the heater.
The following two instruments are necessary to control excess air:
- Flue gas O2 analyzer. This is the most important instrument on the heater. It is recommended to install an O2 analyzer at the radiant section arch.
- Draft gauge. A draft gauge should be installed at the heater arch. The arch is the point of the highest flue gas pressure in the heater.
Heater O2 and draft at the radiant arch should be checked and, if necessary, adjusted at least once per shift and whenever there is a change in process load. All operators should be familiar with the heater controls. Often, heaters with air registers and stack dampers become jammed simply because they are not used. Fig. 9 provides tactics for controlling excess air in a natural draft heater. For controlling excess air in other types of heaters, Table 1 and Table 2 can be used alongside Fig. 7.
Fig. 9. Tactics for controlling excess air in a
natural draft heater.
The basics of automatic heater control are similar to those described in the manual control method. In automatic control, reliable instrumentation is key. One reason for the small number of heaters with automatic control is a lack of confidence in reliable instrumentation. Also, artificial intelligence can be built into the control system to account for all operating cases.
A remote manual control for both O2 and draft is best suited for the natural draft heater. A fully automated control can be safely implemented on balanced draft heaters. Optimum heater performance can be achieved by controlling O2 and combustibles in flue gas using an O2/CO analyzer and automatic dampers.
Case Study 1
In this case study, a vertical, cylindrical, natural draft heater with an absorbed heat duty of 100 MMBtu/hr included the following design parameters:
- Flue gas stack temperature: 600°F
- Fluid inlet temperature: 300°F
- Design excess air: 15%
- Design heat loss: 2%
- Firebox height: 55 ft
- Number of radiant tubes: 64
- Flue gas O2 content at operating conditions: 6 vol% (dry)
- Efficiency of heater at design conditions: 84.2% at 1.5% heat loss (see Fig. 3)
- Efficiency at 2% heat loss: 83.7%
- Firing rate: 100/0.837 = 119.5 MMBtu/hr
- Fuel gas flowrate: 5,770 lb/hr (using a fuel gas LHV of 20,700 Btu/lb)
- Flue gas flowrate: 5,770 3 [1 + 0.167 3 (100 + 15)] = 116,582 lb/hr
- Air flowrate (flue gas flowrate − fuel flowrate): 116,582 − 5,770 = 110,812 lb/hr.
The heater operated under the following conditions:
- Flue gas O2: 6 vol% (dry)
- Excess air: (92 3 6)/(21 6) = 36.8% (also see Fig. 1)
- Efficiency at higher excess air, design stack temperature of 600°F and 2% heat loss = 81.3%; however, the revised efficiency must be calculated based on operating stack temperature, and if this measurement is not available, then the revised stack temperature can be estimated as follows:
- Efficiency at revised stack temperature and operating flue gas excess air: 80.5% (from Fig. 3)
- Corrected efficiency for 2% heat loss: 80%
- Operating firing rate: 100 ÷ 0.8 = 125 MMBtu/hr
- Fuel savings potential: (125 119 .5) 3 24 3 365 = 48,180 MMBtu/yr; at a fuel price of $6/MMBtu/hr, the annual fuel savings potential = $289,000 (this can also be estimated using Fig. 7).
Case Study 2
In this case study, the heater experienced leakage through the bottom guide and an open peephole. The coil guide and sleeve size were 2-in. Nominal Pipe Size (NPS) Schedule 80 and 3-in. NPS Schedule 80, respectively. The heater was not provided with a cap on the sleeve. The open area between the sleeve and the guide included an inside cross-section of 3-in. NPS sleeve and an outside cross-section of 2-in. NPS guide (equal to 2.96 in.2). Other design parameters included:
- Number of guides: 32 (one guide for two radiant tubes)
- Total opening area at guides: 32 3 2.96 ÷ 144 = 0.66 ft2
- Size of radiant section observation door: 5 in. 3 9 in.
- Opening area of observation door: 45 in.2 (0.31 ft2)
- Total open area: 0.97 ft2
- Firebox height: 55 ft
- Draft at opening (draft at heater floor): 0.1 + 55 ÷ 100 = 0.65 inchWC
- Ambient air temperature: 60°F
- Air leakage per ft2 of opening:
- Total air leakage: 4.07 3 0.97 3 3,600 = 14,212 lb/hr (this can also be estimated using Fig. 6)
- Excess air for the burner: 15%
- Actual excess air, including leakage, can be calculated as follows:
Based on revised excess air, a fuel savings can be calculated as in Case Study 1:
- Calculated efficiency: 81.1%
- Loss in efficiency due to air leakage: 83.7 − 81.1 = 2.5%
- Annual fuel savings potential = $201,320.
Controlling excess air has many benefits, and opportunities exist to save fuel regardless of whether the heater is old or new. Day-to-day monitoring significantly improves heater operation. The first visible benefit of excess air reduction is a decrease in fuel consumption. Reduction of emissions, including CO2, is another benefit.
The heater must be provided with at least two instruments (an O2 analyzer and a draft gauge at the arch), both of which are important for energy improvement. Additionally, proper training should be provided for heater operators, and a simple and straightforward heater tuning program must be implemented. It is unlikely that an operator will voluntarily adjust the heater unless a plan is in place to do so. HP
||Sultan Ahamad is a fired-heater equipment engineer at Bechtel Corp. in Houston, Texas. He has more than 14 years of experience in the design, engineering and troubleshooting of fired heaters and combustion systems for the refining and petrochemical industries. He graduated from the Indian Institute of Technology in Roorkee, India, with a degree in chemical engineering. He worked for eight years at Engineers India Ltd. in New Delhi, India, and for five years at Furnace Improvements in Sugar Land, Texas.|
||Rimon Vallavanatt is the senior principal engineer at Bechtel Corp. in Houston, Texas. He has more than 37 years of experience in the design, engineering and troubleshooting of fired heaters, thermal oxidizers, boilers and flares. He graduated from the University of Kerala in India with a degree in mechanical engineering. He also received a degree in industrial engineering from St. Marys University in San Antonio, Texas. Mr. Vallavanatt is a registered professional engineer in the state of Texas, and he has served on the American Petroleum Institutes subcommittee on heat transfer equipment for the past 27 years.|