Feb-2023

Unfire your heater for better performance

The benefits of implementing dispersed combustion in fired heaters in order to align with current and future NOx requirements using a practical technology.

Matthew Martin
XRG Technologies

Article Summary

Nitrogen oxides (NOx) are a harmful byproduct of high-temperature combustion used in refining and petrochemical heaters. Refiners and petrochemical producers have made large reductions in the amount of NOx emitted over the last 20 years. Between 2008 and 2017, refineries and chemical producers in the US have reduced their NOx emissions by 20%.1 This reduction is in addition to significant reductions in the previous decades.

When faced with lower environmental targets or increased demand from existing equipment, it can be difficult to meet NOx requirements for fired heaters. Increasing the combustion air temperature is one of the most effective ways to reduce carbon dioxide (CO₂) emissions, but this, in turn, increases NOx emissions. Modern ultra-low NOx burners often produce more NOx than is allowed as the combustion air temperature increases to enable heater efficiencies greater than 90%.

Limitations on peak radiation heat flux often limit heat transfer to the tubes. As the process feed rate to the heater increases, the heat flux must also increase to achieve the desired outlet temperature. If one does not decrease the peak-to-average heat flux ratio, the tubes will overheat; the inside convection heat transfer cooling the tube increases proportionally to 0.8 power with flow rate, while the outside radiation heat transfer increases linearly.

It is known – and is now part of the API 560 standard – that too much heat released with a given cross-section of a fired heater results in low-quality flames that may impinge on the heater tubes. Impinging flame increases the local tube temperature much more than an increase in firing rate. This high temperature can cause coking of petroleum feedstocks, oxidation/carburisation of the heater tubes, and may result in mechanical failure of the tube itself.

High-temperature flames produce NOx, increase localised radiation heat flux, and can be carried by furnace currents to impinge on tube surfaces. Low-temperature combustion without flame can eliminate these issues.

For example, in 1993, Japan’s New Energy and Industrial Technology Development Center demonstrated highly-efficient combustion using 1,000°C combustion air temperature while reducing NOx and CO₂ emissions.²

When properly designed, flameless combustion in a fired heater will limit NOx, reduce the peak-to-average heat flux ratio, allow for wide fuel gas variability, and enable higher combustion air temperature. However, many practical considerations have inhibited the use of flameless combustion in fired heaters: the substantial number of fired heaters already in service, requirements for flame sensing in those heaters, potential changes in operation, and concerns about unknown interaction between flameless combustion and the heater. A dispersed combustion system – a system that combines flames and flameless operation – practically resolves these issues.

Worldwide fired heaters and the state of technology
The estimated number of fired heaters in refineries and petrochemical plants worldwide is more than 10,000. It is not practical or cost-effective to replace these heaters with purpose-built flameless heaters. Any practical solution should be able to be applied to existing assets.

Selective catalytic reduction systems (SCR) reduce NOx but have not been cost-effective for smaller heaters, which represent a large majority of the heaters that have not already been retrofitted for improved NOx performance. In addition to the cost barrier, there may simply not be space available to place an SCR on a heater.

Preheated combustion air typically improves the heat flux uniformity in the radiant section, but only to a limit. To debottleneck a heater and further increase the heater capacity, another technology is required.

System overview
Figure 1 shows an overview of a dispersed combustion system inside a cut-away view of a fired heater.

Conventional or ultra-low NOx burners remain in place. The radiant section exit temperature thermocouple is used to measure the temperature of the flue gas exiting the radiant section. Most heaters have a radiant section exit temperature thermocouple. It is increasingly common to find a CO or combustibles analyser installed, but not all fired heaters have this equipment. Dispersed combustion nozzles are installed some distance away from the burners, near the top of the radiant section. Many heaters have tube skin thermocouples used to measure the metal temperature of the process tubes. The process temperature is measured at the exit of the radiant section. Many heaters now have flame sensing, which is typically a UV/infrared (IR) scanner, built into the burners.

The dispersed combustion nozzles are not burners. These nozzles are supplied with fuel that mixes with flue gas both internal to the nozzle and in the radiant section.
Figure 2 shows fuel issuing from the nozzles and mixing with flue gas inside a fired heater. No fuel is supplied to these nozzles during start-up, turndown, or shutdown of the heater. In these states, the heater is instead heated by combustion from the burners alone. There is no change to the current start-up, shutdown, or emergency stop procedures.

During normal operation, when the temperature at a specified location is greater than the temperature required to completely oxidise the fuel, the nozzles are placed into service, and a large portion of the fuel is diverted from the burners to these nozzles. The fuel gas from these nozzles burns diffusely in the radiant section, reducing NOx emissions and the peak-to-average heat flux ratio at the heater tubes.

The reduction in NOx means that heaters that are limited in capacity by their air permit limits can increase capacity. The reduction in peak-to-average heat flux to the tube enables increased capacity for heaters that are limited by high tube metal temperature.