Apr-2007

Advanced combustion system for cracking furnaces

A discussion of design considerations for burner technology used in cracking furnaces. Significant improvements include more uniform heat flux profile, wider turndown, stable flame, high combustion efficiency, and low CO and NOx emissions

Roger Poe, Ashok Patel, Charles E Baukal and Daniel Wright
John Zink Company LLC

Article Summary

Cracking furnaces often present significant burner challenges compared to process heaters, because of higher temperatures and more specific heat flux profile requirements. Minimising pollution emissions is a general requirement for all industrial combustion applications. New generations of burner technology continue to reduce emissions, but often impose operational limitations. For example, low NOx emissions may only be achievable over a relatively narrow range of operating conditions. New burner technology has been developed using some advanced fluid dynamic mixing and control techniques, which minimises both NOx and CO emissions over a wide range of operating conditions. This new burner has a much wider turndown ratio and a more compact flame, which are both important advantages in cracking furnaces.

Burner design challenges
Some of the more important challenges for burners used in cracking furnaces include heat flux distribution along the furnace wall, burner turndown and draft variation, flame quality, ease of maintenance and emissions performance.¹

One of the most important design considerations for burners used in cracking furnaces is the heat flux distribution to the process tubes.2 The total firing rate or capacity alone is not a sufficient parameter to determine throughput and run length. The heat must be properly distributed in the furnace to maximise the heat transfer to the tubes, without overheating the tubes, which causes accelerated coking, leading to reduced run length. Furnaces are designed around a target heat flux profile. The trend in cracking furnaces is toward fewer but larger burners. This makes it difficult to uniformly distribute the heat, as there are fewer burners to adjust along the height of the furnace to produce the desired flux profile.

Another important challenge with burners is achieving a wide turndown range. Changes in production, startup and shutdown all require burners capable of operating at firing rates below the normal operating rates. A burner with a wider turndown range allows operators to reduce furnace temperatures for decoking without having to take burners offline, which is labour intensive. However, wide burner turndown has become increasingly difficult for many reasons. Some burners in cracking furnaces use at least some premixing of the fuel and combustion air, so the burner is referred to as a premixed burner. While there are some advantages to a premixed burner, one of the disadvantages is reduced turndown due to the possibility of flashback. Another related disadvantage of premixed burners is that significant hydrogen content in the fuel further limits turndown due to flashback concerns, because of the exceptionally high flame speed of hydrogen flames. Ultra-low NOx burners use a variety of techniques to minimise NOx emissions, such as fuel staging, furnace gas recirculation and ultra-lean premixing. These burners generally have lower turndown due to the more closely controlled mixing of the fuel and air, and the closer operation to the limits of flammability.
Burners must have acceptable performance over a wide range of operating conditions. The furnace draft can vary considerably and change quickly. During a cold startup, the draft is low compared to the draft during normal operation. In natural draft furnaces, the draft can fluctuate due to wind conditions blowing across the stack outlet. These changes in draft directly impact the flow of combustion air to the burners. This means the burner must be capable of safely operating at both low and high O2 conditions.

Flame quality refers to the shape and stability of the flame. While it is important for any application, it is particularly important in cracking furnaces for several reasons. If the flames are too wide, they can interact with each other and adversely affect performance. Coalescing flames can increase the flame length, causing flames to rollover into the tubes, changing the heat flux profile, and causing both NOx and CO emissions to increase. When CO and unburned hydrocarbons mix with oxygen in the vicinity of the process tubes, localised combustion can create hot spots that lead to premature coking in the tubes. Controlled mixing is important to ensure burning occurs at the burner rather than downstream in the furnace.

Maintenance is important for any technology because new equipment will not be effective if it does not have adequate on-stream performance. Equipment that will be used in a continuous high production environment must have good reliability, require relatively little maintenance and be easy to maintain whenever service is required. There are several aspects of burners that need to be considered relative to maintenance.

Cracking furnaces have relatively high temperatures (>1200°C or 2200°F), which means burners must be capable of withstanding the higher heat levels. Any metal parts must be made of the proper alloys and have minimal exposure to the heat. This is especially important if a burner needs to be shut down suddenly while the furnace is still hot. Bringing this same burner back online when the burner tips (fuel injectors) have had no convective cooling allows the gas to be subjected to temperatures that promote coking of the fuel gas if the burner is not designed to minimise this effect.

The higher propensity for coking in cracking furnace service mandates that fuel injectors be designed to minimise the potential for coking leading to plugging. Fuel-gas injectors must be easy to clean or to replace while the burner is in service, ensuring the furnace does not need to be shut down for maintenance. The burners also need to be capable of handling some degree of thermal cycling as furnaces are ramped up and down at various times for the decoking cycle, for example. Maintenance should be simple, without the need for specialised tools or numerous replacement parts. Ideally, maintenance can be performed quickly and easily by site maintenance personnel or furnace operators.

Most petrochemical plants have regulations limiting their pollutant emissions. Of particular interest with burners are NOx and CO emissions. Thermal NOx is exponentially dependent on temperature, as shown in Figure 1. NOx is also dependent on the fuel/air mixture composition, as shown in Figure 2 and the volume of combustion products recirculated back through the flame, as shown in Figure 3. These three figures are based on adiabatic equilibrium conditions where there is no heat loss from the flame. The temperatures are higher and therefore the NOx levels are much higher than for actual conditions where there is significant heat loss from the flame. The higher temperatures associated with cracking furnaces naturally generate high NOx levels compared to lower-temperature process heaters. The higher levels of hydrogen in the fuels used for cracking furnaces also tend to increase NOx emissions, because of the higher flame temperatures associated with hydrogen. These factors make minimising NOx emissions especially challenging in cracking furnaces. CO emissions are typically low at normal operating conditions where the furnace is hot. However, CO emissions can be fairly high during startup conditions when the furnace is relatively cold.3 While CO rapidly decreases once the furnace is heated, the transient CO spike during startup can cause emissions to significantly exceed permitted levels. Some burner designs have added operational complexity to minimise problems associated with high CO emissions during startup. This needs to be considered in the control of pollutant emissions from the process.

Burner development
A new burner (patent pending) has been developed that incorporates the Coanda principle for controlling fluid flow, mixing and stability. This aerodynamic effect is sometimes referred to as boundary layer attachment. Figure 4 shows a diagram of the Coanda principle. A fluid is supplied to a chamber with a small opening. A curved surface with a radius “R” within a specified range causes the fluid to be drawn against the surface instead of being injected straight out of the nozzle. The fluid outlet velocity must be within a certain range and the surface must have the appropriate characteristics for this principle to apply. If the fluid velocity is too low or too high, the fluid will not attach to the surface.