Sep-2014

Retrofitting lower emission burners into existing ethylene cracking furnaces

The increasing supply of global shale gas feedstocks has rekindled interest worldwide in olefins production. Many petrochemical companies are restarting once-idled olefins facilities or planning plant expansions.

Rex K Isaacs
Zeeco Inc

Article Summary

Since these facilities may have been out of commission for several years, operators should be aware that existing ethylene furnace equipment might not meet today’s emissions requirements. This article will discuss the challenges facility operators face when retrofitting furnaces with the latest burner technology to meet required emissions levels.

Retrofitting ethylene furnaces with the latest burner technology can be difficult due to burner spacing issues and their resulting effect upon burner flame quality. As existing furnaces are upgraded to achieve higher capacities, more floor burners are added. The addition of more burners can result in smaller distances between burners and more potential for adverse flame conditions such as burner-to-burner flame interaction and flame rollover.

Flame interaction between burners can cause flame impingement upon the process tubes and increased emissions. Flame impingement can also be problematic for prolonged ethylene production, resulting in shorter run lengths and higher tube metal temperatures, which can cause premature coking and lead to shorter periods between de-coking, reducing ethylene production and outputs.

One of the other primary concerns for the end user operating company is flame rollover. When the momentum of the hot gases moving upwards from the burner becomes less than the momentum of the colder gases moving down the furnace tubes, flame rollover occurs. One product developed to address these concerns is the patented ultra low NOx GLSF Min-Emissions Enhanced Jet Flat Flame burner. In this article, we will review the design details for this type of burner, provide specific retrofit installation details in a case study format, review lessons learned during the retrofit, and include results for several of the retrofit applications.

An inherent design aspect of the burner is the fact that the fuel gas is introduced between the furnace wall and the air stream. Consequently, flame interaction between burners is minimised due to the location of the burner tip and the very compact design. Since the gas does not cross the air stream, the tip drilling design can be modified to achieve better heat flux profiles without adversely effecting the thermal NOx emissions.

Application background
Since the early 2000s, Zeeco and a petrochemical facility have worked together on many ethylene furnace revamp applications where the number of floor burners increased and the burners had to be moved closer. Even in these situations, using the right technology helps avoid problematic occurrences such as flame rollover and interaction.

To achieve the improved flame quality without any flame rollover or flame interaction, the customer selected Zeeco’s GLSF Min-Emissions flat flame floor mounted burners. As shown in Figure 1, the GLSF burner is designed to entrain the unburned gas next to the wall to prevent flame rollover as the furnace currents pull the air and products of combustion toward the tubes. As the unburned gas moves up the wall, it mixes with the inert flue gas products of combustion, burning directly below the unburned gas. As the mixture of unburned gas and products of combustion continues to 
move up the wall, it then combines with air and burns. Since the unburned gas is mixed with some of the products of combustion before burning, peak flame temperature is lowered, producing lower thermal NOx emissions. Therefore, not only are flame rollover and flame interaction problems solved, but ultra low emissions can be achieved.

Comparison to Low Emissions Burners
Low emission burners found in ethylene cracking units typically use staged fuel technology. These particular burners have staged fuel tips strategically positioned for fuel to exit the orifices and pass over the combustion air stream before reaching the furnace wall. In order to modify the flame pattern to achieve an even heat flux in the lower portions of the flame envelope, the orifices must be drilled at increasingly abrupt angles toward the furnace wall. These orifice angles cause the air and fuel gas to mix at a faster rate, thus increasing thermal NOx, and requiring a compromise between the heat flux profile and thermal NOx production. As the heat flux profile is made more uniform, with an average above 90%, the NOx emissions typically increase along with the increase in flux percentage. In the same respect, as the NOx is decreased, the heat flux percentage is also decreased.

It is important to note that the location of the staged gas tips also affects the flame quality of the burner. For example, if the burner is required to make very low NOx emissions, the staged orifices must be aimed in a more vertical direction. This vertical direction, coupled with the combustion air stream located in between the unburned gas and the furnace wall, increases the likelihood of flame impingement. The mixing of the fuel gas energy becomes so reduced at higher elevations that the furnace currents can easily influence the flame toward the tubes. In general, the lower the NOx emissions when the staged gas has to pass completely over the combustion air opening, the higher the tendency for flame impingement or hot spots on the tubes.

The latest in low emission burner technology, such as Zeeco’s GLSF Min-Emissions Flat Flame Burner, incorporates staged gas tips with gas ports aimed toward the wall in a pattern that provides a very uniform heat flux profile in the middle to upper regions of the flame envelope. The mounting of the burner’s flame shaping tips on the side of the combustion air stream allows the gas to avoid passing directly over the combustion air stream. The primary gas tips and gas lance provide the necessary heat distribution in the lower regions of the flame envelope. Since the burner mixes internally inert flue gases, the flame is stretched over a longer distance, enabling a more uniform heat flux distribution on the furnace wall. With the flame shaping tips located on the side of the combustion air stream, the burner heat flux profile can be changed without adversely affecting NOx or causing flame impingement on the tubes. The burner can then evenly transfer heat to the process tubes, and reduce the possibility of localised hot spots while producing lower NOx emissions.

Figure 2 shows a comparison between the GLSF Min-Emissions burner and a typical staged fuel burner. The typical staged fuel burner shown on the left, with staged gas tips that normally comprise 70% of the heat release positioned on the back side of the tile, requires that the unburned gas cross the combustion air stream before reaching the furnace wall. The Min-Emissions Burner, shown on the right, illustrates that the staged gas does not directly cross the combustion air stream. This type of burner design allows the angle of the staged gas port to be changed without adversely affecting the thermal NOx emissions. Since the volume of air can be around nine (9) times greater than gas, it is very important that the gas be injected between the furnace wall and the combustion air stream to keep furnace currents from affecting the flame quality.

Small Burner Size for Retrofit Applications
Another critical design detail when retrofitting for emissions control without sacrificing flame quality is the compactness of the burner design. Operators should seek a combustion company who engineers a burner that is compact in design with no metal located in the throat of the burner, excluding the gas tips. Gas tips are required on each burner to distribute the fuel and mix it with the combustion air stream so that it burns completely. Eliminating any other metal from the burner throat results in a design with fewer items that can fail and require replacement. In addition, by simply eliminating metal from the burner throat, the burner can be designed with a smaller external dimension. When the external dimensions of the burner are smaller, it can normally replace conventional NOx and staged fuel NOx burners with only minor furnace modifications. This compact footprint allowed the petrochemical plant operator to install more burners mounted more closely together in revamped furnace applications without adverse flame impact or major modifications to the furnace.