(41c) Operational Experience with a Furnace Optimizing Tool for Burners Adjustment in Ethylene Cracking Furnaces and Refinery Fired Heaters in Order to Improve the Air-to-Fuel Ratio on Every Individual Burner, Increasing Both Efficiency and Safety and Lowering Emissions

Heide Refinery (HR) is a former Shell oil refinery located in the North of Germany. HR operates about thirty combustion units comprising fired heaters, boilers and incinerators. With increasing demands in respect to higher profit, higher efficiency and lower emissions (CO, NO_x, etc.), HR was obliged to improve operation of their fired equipment. One key problem was the adjustment of air flow to each individual burner so as to achieve uniform air-to-fuel ratios in each burner. This is a real challenge for all furnaces with a multiple burner configuration, because the air flow to each burner is normally not measured. Specifically during the start-up of furnaces, but also during normal operation, keeping the air-to-fuel ratio within a specific threshold range is crucial for safe operation. Furthermore, it allows achieving lower excess air levels without CO breakthroughs and, sometimes, also lowering NO_x emissions.

The adjustment of the individual burner air dampers and, therefore, of the air flows to each burner is normally the task of field operators. But operator experience differs from person to person and flame shape or appearance are judged differently in each case. Consequently, some burners operate closer to stoichiometric conditions than others. Operating these burners on lower excess air levels tends to generate higher flue gas temperatures and to increase the heat transfer to the product tubes. Over time, this causes overheating in some tubes or zones (“hot spots”) and makes internal coke formation more likely. As a consequence, coke reduces the heat transfer, resulting in even higher tube surface temperatures. In ethylene cracking furnaces the tubes are normally monitored by manual pyrometer measurements on a daily or half-daily basis. The longer the running length of a furnace, the more likely it is that one or more tubes will exceed a critical surface temperature. When this happens, producers are forced to either reduce the feed flow rate or lower the feed outlet temperature. Both options normally lead to lower yield and lower total production. Finally, the furnace needs to be decoked. It is therefore an important goal of producers to heat the product tubes as uniformly as possible in order to achieve the longest running time between two decoking phases.

To improve the subjective individual burner air damper setting by the field operators, HR has equipped five furnaces (three ethylene cracking furnaces, the main crude oil distillation furnace and the vacuum distillation furnace) with a furnace optimizing tool that measures the air flow to each burner indirectly through the pressure drop that the air causes while flowing through the burner throats. Comparing the different pressure drops allows the operators to easily see which burner receives more or less air and to adjust the air damper accordingly. The tool is installed in furnaces with forced draft burners, although it also works perfectly well in natural draft burners.

Besides the need to reduce the hot spots, the three ethylene furnaces at HR had a severe air distribution problem. Air duct design to the 16 burners (per furnace) was poor and led to some burners operating on high excess air while others were starving on air, thus producing CO emissions. As emissions (NO_x, CO) are measured in the common stack, it was difficult to figure out which furnace and which burner were causing the CO problems. While CO was high, HR was not allowed to proceed increasing the feed throughput due to environmental permit restrictions. Therefore, CO had to be reduced first, and it took HR one to three days of intensive try-and-error seek until the problem was solved and throughput could be put to normal.

With the help of the furnace optimizing tool, this time was cut back to only 1-2 hours, leading to higher production and compliance with emission regulations at all times. In addition, the cycle time between two decoking phases increased, even though this was not the primary goal for these three specific furnaces.

The major goal of installing the furnace tool in the crude oil distillation unit (CDU) with 16 burners was not to prolong the running time between two decoking cycles, as CDU normally operates non-stop between 3 and 5 years. The challenge here was the fact that the CDU furnace operates on four different fuels at the same time: two different refinery fuel gases, a low-BTU off-gas and a heavy fuel oil. Maintenance requirements (e.g. of oil guns) had caused burners to operate on different heat releases. Burner air damper settings were based purely on visual flame observation, although the physical settings were kept almost always the same for all burners. It is obvious that different heat releases require different amounts of combustion air and, therefore, the air damper settings must be in accordance to the heat released by every burner. On top of that, air distribution throughout the air ducts was poor, leading to different air-to-fuel ratios in every burner.

By using the furnace optimizing tool, HR was able to adjust air-to-fuel ratio individually and to finally reduce the overall excess air of the furnace to levels as low as 1.5 Vol% O₂, with CO well below 10 ppmv and an improved flame shape.

On the vacuum distillation furnace, the goal was NO_x reduction. The furnace operates with four forced draft burners. Air distribution was also poor here. The optimizing tool allowed trimming primary and secondary air in every burner so that each became the same in every burner, even when they differed from each other. This has led to a NO_x reduction of ~22%, falling from 110 mg/Nm³ (~54 ppmv) to 85 mg/Nm³ (~41 ppmv) without any costly measures, just through burner damper adjustment.

The implementation of this furnace optimizing tool has multiple advantages over traditional operation. Depending on the specific needs and the equipment installed, it can achieve a shortening of cycle times between decoking phases (in ethylene cracking furnaces!), a reduction of the overall excess air with the subsequent increase in efficiency, a reduction of hot spots or flame impingement, a lowering of emissions, etc. Above all, operation becomes safer as burners can be kept within certain air-to-fuel ratio envelopes.

The investment is returned in 2 to 3 months. Furthermore, the tool can be installed and commissioned with the furnaces running, meaning there is no need for a shut-down. Operators are quickly convinced to use this tool when they realize that operation improves and they spend less time and make less adjustments in the furnace than before.