Reducing CO2 and NOx while Increasing efficiency from fired heaters without selective catalytic reduction
Fired heaters, also called furnaces, use the heat from combustion of hydrocarbons to increase the temperature of a fluid. Direct-fired heaters are common in refining and petrochemicals.
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In these heaters, flames produced by burners directly irradiate a series of tubes called a coil, which carries the fluid to be heated. Depending on the design and operation, these heaters vary in fuel efficiency from approximately 50% to 95%. A low fuel efficiency causes unnecessary waste of fuel and high carbon dioxide emissions (CO2).
The fuel efficiency of fired heaters can be increased by preheating the air used for combustion. Increased combustion air temperature reduces CO2 but also increases oxides of nitrogen (NOx). NOx causes smog, acid rain and ground-level ozone that damages ecosystems. NOx emissions can be greatly reduced by Selective Catalytic Reduction (SCR) which reacts NOx with ammonia (NH3). SCRs may be costly to install, particularly where there is no ammonia infrastructure in place, and they only address NOx emissions from fired heaters.
Many heaters are operating at a higher capacity than the original design which may result in unacceptably high tube metal temperatures leading to thermal degradation of the process fluid, fouling of the tubes reducing the heat transfer, and potentially a rupture of the tube itself. Additionally, greater heat input often results in higher temperatures everywhere leading to additional stress on other components of the heater such as tube supports.
Changing the fuel from those with higher carbon-to-heat content ratios to those with lower carbon-to-heat content ratios will reduce CO2 emissions. Switching to inorganic fuels, such as hydrogen or ammonia, can eliminate CO2 emissions from the heater stack. However, both of these fuels will change the heat transfer characteristics of the heater, increase NOx emissions, and have other potentially negative effects on heater components.
An ideal solution would increase efficiency, reduce NOx emissions, allow for fuel flexibility, and increase capacity. This solution should also use less space and not require ammonia for NOx reduction. An integrated system for mixing and dispersing fuel within the fired heater can deliver these results.
Heat transfer in fired heaters
Heaters are designed with component sections based on the principal mode of heat transfer from the flue gas to the coil. There are a wide variety of shapes for fired heaters and burners can be placed in the floor, walls, or roof. Figure 1 shows a schematic view of a common fired heater known as a vertical cylindrical. In this design, the process fluid flows from the top of the heater to the bottom, while the combustion products flow from the bottom of the heater to the top.
The process first passes through the convection section. Both the process fluid and the flue gas are at the lowest temperature while in this section. The primary mode of heat transfer within the convection section is convective. Attaching fins or studs to the tubes in this section will increase the available surface area for convection heat transfer. Heat transfer in the convection section can be increased by adding more rows of tubes or more surface area to existing tube rows.
The process fluid then flows through the coil into the radiant section, where tubes are typically located against the wall and burners are located in the centre. In this section of the heater over 80% of the heat transfer comes from the flames and hot flue gas irradiating the tubes, but the flames cannot radiate to the tubes uniformly. Flames transfer more heat to the flame-facing surface of the tube than to the backside of the tube. Even if burners were placed on both sides of the tubes, the adjacent tube will always shadow the next one, resulting in non-uniform heat flux. More space between the tubes reduces the shadowing but spacing is limited by practical size constraints.
This ratio of peak-to-average flux around the circumference of the tube is known as the Circumferential Flux Factor (CFF). Figure 2 shows the CFF taken from API Standard 530 and the work of Hottel (Hottel, 1983). This flux factor is not dependent on the shape of the heater or the style of the burner. The factor is derived by calculating the shadowing effect on adjacent tubes from a perfectly uniform infinitely long plane radiating to the tubes. The CFF is often the limiting factor for peak tube metal temperature and in turn, the duty of a heater when everything functions as designed. It is often the case, however, that everything does not function exactly as designed.
The burners in fired heaters often produce flames from which the radiation varies significantly from the idealised infinite plane used to derive the CFF. The reasons for this are many, but the most common cause is that the burner flame is distorted by the flue gas flow patterns within the fired heater. The flames are then either pushed into the tubes or into each other. When the flue gas pushes the flames to the tubes, the local heat flux increases due to hot flue gas impinging directly onto the coil surface. Alternately, the flue gas pushes the burner flames into each other and the fuel and air can no longer mix as designed. Instead, the flames become long resulting in a reduced radiant section heat transfer or the extreme case of flame impingement on the shock tubes at the entry to the convection section.
To calculate the heat transfer to the tubes, the degree of maldistribution within the radiant section that is not attributable to the CFF can be represented by a second factor, the Longitudinal Flux Factor (LFF). Figure 3 shows an example computational fluid dynamics (CFD) simulation where the flames impinge on the radiant coil. In this case, the CFD calculated CFF is 1.85 and the LFF for this case is 1.12, resulting in a combined peak-to-average heat flux ratio of 2.07. The combined peak value is critical because it sets the limiting tube metal temperature for the entire coil design.
Figure 4 shows the normalised heat flux within the radiant section versus elevation. As expected, 50% of the heat flux is greater than the mean. However, the extent to which the flux is locally greater than the average represents an opportunity to reduce tube wall temperature and increase coil utilisation. A reduction of the LFF to 1.00 results in more of the coil surface being available for heat transfer, up to the CFF limit of 1.85. Reducing the CFF allows higher heat transfer to even more of the coil surface without exceeding the maximum tube metal temperature or film temperature of the process. Figure 4 shows that if the heat transfer is shifted from the 1.9 m to 4.9 m elevation to the 0.0 m to 1.9 m, 4.9 m to 6.1 m, and 8 m to 9.2 m elevations, the peak tube metal temperature can be reduced while maintaining or even increasing the heat input.
Increasing efficiency to reduce CO2 emissions and fuel costs
Additional recovered heat is often used to preheat the combustion air. Every unit of energy added to the combustion air is a unit of energy not needed from the fuel gas. Preheating the combustion air reduces fuel consumption and carbon dioxide emissions. When using purchased gas, the savings from reduced fuel consumption can be significant. One downside to preheated combustion air is the increased system complexity and maintenance requirements. The other major downside is that preheating the combustion air increases the NOx emissions from the burner flames.
Figure 5 shows the change in CO2 emissions and NOx when increasing efficiency by increasing the combustion air temperature. The baseline efficiency is assumed to be 80% while firing methane with 15% excess air. When increasing efficiency from 80% to 95%, CO2 emissions are reduced by 16%. However, due to the exponential dependence of thermal NOx production on temperature, NOx increases by 213% with the same change in combustion air temperature. For heaters operating near the permitted NOx limit, the available efficiency increase is limited by NOx emissions rather than heat transfer.
An increase in efficiency using combustion air preheating does not necessarily lead to an increase in the available heater capacity. Increasing the firing rate of burners in conjunction with air temperature can still result in increased LFF and a tube metal temperature exceeding the maximum allowable temperature.
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