A. Sharma, Contributing Author, Jaipur, India
The world is experiencing a consistent, daily increase in carbon dioxide (CO2) due to industrial emissions causing global warming—by the year 2022, energy-related CO2 emissions will surpass 35 billion tons per year (Btpy).1 Most energy-related CO2 is produced by fire heaters or furnaces within the refining and petrochemical sectors.
Because furnaces are fuel guzzlers, even the smallest improvement in furnace efficiency (FIG. 1) can save money in terms of fuel costs, resulting in greater profits. Additionally, each enhancement of furnace efficiency reduces carbon emissions, which also helps to protect and preserve the environment.
Efficiency calculation. The performance of any equipment can be evaluated based on its efficiency. The loss of heat in a furnace or fire heater is calculated using thermal efficiency: the ratio of heat absorbed by the process to the heat input. Heat losses increase as process heat absorption decreases, and vice versa.
Thermal efficiency can be calculated by either direct or indirect methods. When the absorbed heat is known, the direct method is used to calculate the thermal efficiency.
When only the amount of heat loss is known, the indirect method is used. Most often, process engineers use the indirect method to calculate furnace performance. The method is simple and does not require a large amount of furnace data.
Estimating the thermal efficiency of a furnace can be estimated by the energy balance formulas in Eqs. 1–4:
HInput = HAbsorbed + HLoss (1)
Thermal efficiency (%) = HAbsorbed × 100/HInput (2)
Thermal efficiency (%) = (HInput – HLoss) × 100/HInput (3)
Thermal efficiency (%) = 100% z – ( HLoss × 100/HInput ) (4)
where HInput, HAbsorbed and HLoss are the heat of input (by fuel gas and air), heat absorbed by the process, and heat loss (by stack and radiation), respectively.
HLoss = Radiation/convection loss + stack loss (hot flue gas)
There are two major losses in the thermal efficiency calculation: stack loss (hot flue gas) and radiation/convection loss. Any deviations or assumptions in the loss calculation infer the thermal efficiency of the furnace.
Radiation/convection loss. This is the rate of heat loss through the surface wall or body of the furnace. The furnace internal surface is lined with insulation and refractory that hold the maximum heat inside the furnace. Furnace refractory is designed so that the temperature of the furnace casing should not exceed 82°C (180°F) at the outer surface wall, and 90°C (194°F) at the radiant floor outer surface to minimize radiation loss.
As described in API 560, “The rate of heat loss from the exterior surfaces of the furnace; along with heat loss from associated ducts, fans, air preheater and selective catalytic reduction (SCR); to cooler surroundings is typically in the range of 1.5%–2.5% of the calculated normal fuel heat release, based on the fuel’s lower heating value.”2
This rule to measure the radiation loss (1.5%–2.5%) is satisfactory if the refractory is well-designed and maintained with a surface temperature of 82°C (180°F) and an ambient temperature of 27°C (81°F).
In a normal run, the actual radiation loss, connective loss and the efficiency of the furnace are affected by parameters such as outer wall temperature, ambient air temperature, ambient wind speed, total surface area of the walls and the surface emissivity of the wall.
The empirical formula3 to calculate the actual radiation/convection loss is shown in Eq. 5:
Hradiation loss = [e*{(Tw )4 – (Ta )4 }*10-8 + w(Tw –Ta )1.23] (5)
*Total surface area
where e is the surface wall emissivity factor, w is the wind velocity factor, Tw is the surface wall temperature (K) and Ta is the ambient temperature (K).
This empirical relation clearly shows that the radiation loss is directly dependent on Tw, Ta, the emissivity and wind velocity.
When the surface wall temperature goes higher, heat loss increases in quartic power. “Every 1°C increase in furnace exterior wall temperature over design increases the radiation loss by 2%” (Eq. 5). This increment in furnace wall or surface temperature can result in refractory damage, blanket module damage, porosity change of the refractory, uneven heat distribution within the furnace firebox and over-firing of burners.4
A decrease in ambient temperature also causes an increase in furnace heat loss. This is caused by the delta (Tw –Ta ) increment. “Every 1°C decrease in ambient temperature increases the exterior surface radiation loss by 2%” (Eq. 5). Heaters lose relatively more radiation heat through exterior surface operating in the winter than in the summer.
Furnaces operate from turndown (some 40%–50%) to rated capacity. Fire heaters or furnaces run on temperature control of the process stream, so the fire box temperature is well maintained at every load/capacity.5 This means that the furnace radiation/convection loss would be the same at all runs as running at design capacity (100%).
About 2.5% of the rated capacity heat input is lost in a well-designed refractory furnace, as per API 560.2 “Heat loss from a furnace operating at a turndown or lower capacity would be 3%–4% of the actual heat input because outside surface radiation loss would equal the rated capacity of heat input” (an approximate calculation from Eq. 5). At lower loads, the furnace's efficiency decreases because heaters emit the same amount of radiation heat as the design due to reduced load or turndown rates.
Wind velocity also increases the heat loss through the convection at the outer surface of the furnace. Higher wind velocity increases the heat transfer coefficient outside the surface wall that reduces the wall temperature—this reduction in surface wall temperature takes more heat from the furnace due to the temperature difference. This wind velocity factor (w) varies from 1.5–3.3, which means at a higher wind velocity the heat would be higher.
Over time, the furnace's external surface wall's emissivity also varies. Damage can also increase the furnace's emissivity and roughness due to oxidation and corrosion of the coating or surface wall, which would increase radiation heat loss. This aspect should be considered if the furnace has been in operation for longer than 2 yr. This emissivity plays a pivotal role in the surface temperature measurement as well as actual wall temperature. Generally, measurement of the wall temperature by thermometer/pyrometer/infrared guns uses a fixed emissivity.1 These temperature measurement sensors read/show a lower temperature than the actual temperature, as emissivity of the wall is lower than 1.
To calculate radiation loss, the actual wall temperature should be calculated by taking the correction of the emissivity.6
Stack loss. This major loss of heat, calculated using Eq. 6, affects the efficiency of the furnace. Heat, in terms of temperature, is carried by the flue gas to the atmosphere, which reduces furnace efficiency and increases the heat in the environment.
Hstack Loss = Wfluegas Cp (Tstack – Treferance ) (6)
where Wfluegas is the mass flowrate of the flue gas and Cp is the specific heat of the flue gas.
An increase in mass flowrate of the flue gas through the stack significantly increases the heat loss of the furnace and reduces furnace efficiency. The total mass flow of the flue gas is the sum of the fuel gas and total air. Excess air increments and tramp air (leakage) contribute to the mass flowrate of the flue gas. As a rule of thumb,7 a 10% increase in excess air reduces the heater efficiency by almost 1%.
The arch section poses the greatest risk of tramp air leakage as tubes penetrate the firebox. The roof penetration tube seal system reduces the air infiltration around the penetrations, improving heater efficiency and performance.
An increase in the percentage of the absolute humidity of air decreases the adiabatic flame temperature and excess moisture results in an increase in flue gas and a reduction in the efficiency of the furnace.6,7 This can be caused by steam traps near any water sprinklers or nearby cooling tower.
Each 1°C increment in the temperature increases the stack loss by 1%, whereas the general assumption is that every 18°C reduction in stack flue gas temperature increases efficiency by 1%.7 If the used fuel contains less sulfur, then the stack temperature can be reduced to improve the efficiency of the furnace.
Takeaways. As global energy consumption increases daily, the world shares the responsibility to produce that energy using the most efficient techniques. This is particularly true for furnaces used in the refining and petrochemical sectors.
A furnace's efficiency drops with lower loads because, as a result of lower load or turndown rates, heaters continue to emit the same quantity of radiated heat of the rated capacity through the surface of the wall.
The actual radiated heat loss of an operating furnace should be calculated based on actual conditions.
Control excess air and minimize tramp air and heat losses to conduction to achieve maximum furnace efficiency.
Any hot spot on the exterior wall surface should be checked frequently and the refractory should be changed, if required.
The actual emissivity of the wall should be used for the furnace wall temperature measurement.
The stack temperature should be optimized based on the sulfur content in the fuel gas and the dewpoint of water and sulfur oxides (SOx).
Bad firing, such as flame rollover, flame impingement and flame interaction of the burners, must be checked frequently and corrected.
Distribute the heat (heat flux) within the firebox by manipulating burner operation, and burner cleaning ameliorates furnace performance and efficiency.
NOTE
This article is based on author’s experience and is not affiliated with any company.
LITERATURE CITED
International Energy Agency (IEA), “World Energy Outlook 2022 – Analysis,” 2022.
American Petroleum Institute (API) Standard 560, “Fired heaters for general refinery service,” 5th Ed., February 2016, online: 560_e5 pa.pdf (api.org)
Perry, R. H., C. H. Chilton, et al., Chemical Engineers’ Handbook, 5th Ed., McGraw-Hill Book Co., New York, N.Y., 1973.
Arora, V. K., “Check fired heater performance,” Hydrocarbon Processing, May 1985.
Beareu of Energy Efficiency, “Energy performance assessment of furnaces,” online: Ch-02_gopsons.qxd (beeindia.gov.in)
Clausing, L. T., “Emissivity: Understand the difference between apparent, actual IR temperatures,” Fluke Corporation.
Garg, A., “Optimize fire heater operation to save money,” Hydrocarbon Processing, June 1997.
Abhishek Sharma works at an olefin production plant as a process engineer in India. He has 7 yr of experience working in steam crackers and olefin units. Sharma earned a B.Tech degree in chemical engineering with honors from the National Institute of Technology Raipur in India and finished a process equipment design course at the Indian Institute of Technology Roorkee. He is an associate member of the IChemE, an active professional member of the American Institute of Chemical Engineers, and the author of eight technical articles in industry publications.