Controlling FCCU emissions

More stringent refinery air quality regulations are evolving worldwide, while at the same time refiners are looking to process heavy and sour FCCU (Fluid Catalytic Cracking Unit) feed stocks. These trends, coupled with the FCCU’s important contribution to refinery profitability, are bringing a great deal of attention to approaches for controlling FCCU emissions.

Scott T Eagleson and Edwin H Weaver
Belco Technologies Corporation (Now BELCO Clean Air Technologies)

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Article Summary

For many years this focus was limited to new FCCUs, while older units received little regulatory attention. This is changing. In the USA, recently proposed regulations require older units to control particulate emission to the same outlet levels as new units. For compliance, the addition of add-on control devices will likely be required to reduce current particulate emissions.

This article investigates the addition of add-on particulate control technologies for regulatory compliance while considering the costs/benefits of also controlling SOx emissions. The economics, performance, flexibility and reliability of add-on control solutions currently in use on FCCUs are examined. Positive and negative features of each are discussed.

FCCU emissions
For many refineries, the FCCU regenerator flue gas represents the single largest air emission source, contributing particulate, SOx, NOx and CO emissions. This article discusses particulate and SOx emissions. A brief discussion will help characterise these.

Particulate emissions result from catalyst escaping the FCCU regenerator in the flue gas. Cyclones, incorporated within the FCCU process, remove a large portion of catalyst from the flue gas and return it to the process. While affective in collecting catalyst for recirculation, cyclones allow a significant amount of fine catalyst to escape. Typically uncontrolled catalyst emissions exiting cyclones used in the FCCU process range from 5–10 pounds of catalyst per 1,000 pounds of regenerator coke regenerator coke burn-off.

Sulphur emissions result from a portion of the feed stock sulphur being carried through the FCCU and emitted as SOx (SO2 and SO3, typically expressed as SO2) in the FCCU regenerator flue gas. Levels vary significantly. Sulphur distribution within the FCCU, as depicted in Figure 1, depends on many factors. In a FCCU reactor, typically 70% to 95% of incoming feed sulphur is transferred to product and fuel gas. The remaining 5% to 30% is deposited with coke on catalyst. During catalyst regeneration, the coke and sulphur are burned off. The sulphur is oxidised to SOx and emitted in the flue gas. SO2 emissions from 200 to 5000 parts per million dry volume basis (ppmdv) are not uncommon. A small percentage of the sulphur reported as SO2 will be found as SO3.

World Regulatory Environment
Regulations covering FCCU emissions vary worldwide and are summarised for a number of countries in Figure 2. These values represent the minimum control requirements for new FCCUs. Emissions for individual units may be controlled to lower levels to minimise overall refinery emissions or meet more stringent local standards. This article focuses on the current regulatory environment in the USA. The ideas and concepts easily translate to situations throughout the world.

USA Regulatory 
Since 1984, emissions from new FCCUs in the USA have been governed by the U.S.E.P.A.’s New Source Performance Standard (NSPS) outlined in Figure 3. This requires particulate emissions controlled to below a set level (1 pound per 1,000 pound of regenerator coke burn-off) for all units.

Options are provided for control of SOx emissions. Without the use of add-on control devices for SOx control, feed stock sulphur must either be limited to below 0.3% by weight or SO2 emissions must be limited to below 9.8 pounds per 1,000 pounds of regenerator coke burn-off. With the use of add-on control devices for SOx control, SO2 emissions must be controlled to below an outlet limit of 50 ppmdv or 90% removal, which ever is less stringent.

Units installed prior to 1984 are unaffected by NSPS and considered “grandfathered”, until such time there is an increase in capacity, a significant revamp or changes to higher sulphur feed stocks. That will soon change. On September 11, 1998 the U.S.E.P.A. published proposed rules in the Federal Register that addresses the control of certain hazardous air pollutants from a number of refinery sources including FCCUs. Also known as MACT II, this proposed rule sets a minimum particulate control level for all existing FCCUs at 1 pound per 1,000 pounds of regenerator coke burn-off. This is the same level of particulate control required for new units under NSPS. As proposed, existing FCCUs not meeting the 1 pound per 1,000 pounds of regenerator coke burn-off will need to reduce particulate emission levels. Many “grandfathered” units may be affected.

Other regulations may also impact FCCU emission control requirements. While it is difficult to predict what might be required in future years, issues related to Title V, regional haze, compliance assurance monitoring (CAM), and P.M. 2.5 could all have a significant impact.

Controlling emissions
Add-on control technologies will be the likely solution for units that need to reduce particulate emissions for MACT II compliance. The use of electrostatic precipitators and wet scrubbers is specifically addressed in the proposed MACT II rules. Both technologies are in widespread use for particulate control compliance at NSPS emission level requirements. The use of cyclones, third stage separators in particular, may also be a consideration.

Although MACT II does not require reductions in SO2 emissions, some consideration of adding SO2 control should be evaluated. The incremental cost of adding SO2 control at the same time as adding particulate control is low and provides a number of advantages, both economical and environmental. The addition of SO2 control may enable processing of higher sulphur feeds or pushing the feed rate beyond the “grandfathered” limits. Reductions in FCCU SO2 emissions may also compensate for sulphur emissions from other refinery areas. Adding only the capability to control particulate control for MACT II compliance may severely limit a FCCU’s future operational flexibility, expandability, and profitability.

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