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An innovative approach for controlling SOx and particulate flue gas emissions

Regulatory pressure to control emissions from refineries is rapidly increasing worldwide at the same time that refiners are under constant pressure to be more competitive by optimising staff utilisation, processing lower price higher sulphur feed stocks, increasing the time between FCCU turn-arounds and by adopting other cost cutting measures.

Nicholas Confuorto, Belco Technologies Corporation
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Article Summary
Furthermore, the skyrocketed price of emission credits for 
SO2 and NOx is causing many refineries (those using emission credits to meet their environmental requirements) to re-evaluate their strategy.

Faced with a considerable capital or operating costs, refiners are evaluating all options. In many cases addition of air pollution controls will be required. When adding air pollution control systems, refiners need to closely match the system performance not only to the refinery’s environmental requirements but also to their cost optimisation goals. Goals such as gaining the ability to process heavier/sour feed stocks, reducing maintenance costs, pushing FCCU capacity, increasing the time between scheduled FCCU turn-arounds, and gaining the flexibility to meet future regulatory requirements without having to replace the system.

Many refineries will gain the most benefit by controlling FCCU emissions. The FCCU regenerator flue gas usually represents the single largest air emission source. It emits particulate, SOx, NOx and other Hazardous Air Pollutants (HAPs). Because of the large gas volume involved, and the associated tons per year of pollutants generated, controlling emissions from the FCCU may allow the plant to avoid having to control multiple lower sources.

While control of particulate emissions is limited to addition of a control device, SOx and NOx emissions can be handled by various methods. For instance, one could consider adding an Electrostatic Precipitator (ESP) for particulate only and then for SOx opt to add a wet scrubber or use SOx/NOx reducing catalyst or purchase credits for SOx/NOx. Or one may chose to use a Wet Scrubber System such as Belco’s EDV Wet Scrubbing System, which can control both particulate and SOx and only be concerned with NOx as a separate device as regulations require it. Various technologies are presently being evaluated which may allow Belco to also offer soon NOx control as part of their EDV Wet Scrubbing System. Furthermore, for high SOx applications, the system can also be designed to regenerate the buffer used to scrub the SOx (Belco calls this option Labsorb).

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 effective in collecting catalyst for re-circulation, cyclones allow a significant amount of fine catalyst to escape. Typically uncontrolled catalyst emissions exiting cyclones used in the FCCU process range from 5 to10 kg of catalyst per 1,000 kg of regenerator coke burn-off (300 to 750 mg/Nm3). The particulate is usually less than 10 micron in size and contains a considerable amount of submicron size particles.

Sulphur emissions result from a portion of the feed stock sulphur being carried through the FCCU and emitted as SOx (SO2 and SO3) 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 volume basis (ppmv) (575 to 14,300 mg/Nm3) are not uncommon. A small percentage of the sulphur reported as SO2 will be found as SO3 (typically less than 15%).

Controlling emissions

The use of add-on control technologies is the likely solution for refineries being required to reduce particulate emissions. Additionally in many countries the refineries must also comply with limitations on SO2 and NOx emissions. It is this consideration that provides an opportunity for wet scrubbing systems.

When using a wet scrubber for particulate control the incremental cost of also controlling SOx is low and should be considered. In many cases, the cost of a wet scrubber is comparable to the cost of an ESP system, however, the wet scrubber can reduce both particulate and SO2 while the ESP can only reduce particulate. Once a wet scrubber is installed on an FCCU, a refinery may be able to take advantage of various cost reduction changes or profit enhancements to the FCCU without increasing emissions of pollutants.

Economics of controlling emissions
For combined control of particulate and SOx a refinery will most likely install either a wet scrubber or a combination ESP+ transfer catalyst for SOx. The amount of transfer catalyst additive used depends on many factors including whether the unit operates in a partial or full burn mode, the amount of SO2 reduction required, and other FCCU process variables.

The electrostatic precipitator system is normally configured using two separate precipitators and operating in parallel. This is a requirement due to reliability issues with electrostatic precipitators and the requirement for multi-year continuous FCCU operation.

In comparison the wet scrubbing approach, represented by a Belco Technologies Corporation’s EDV Wet Scrubbing System in this evaluation, can achieve the same level of particulate control as the electrostatic precipitator plus it has the ability to achieve SOx control in excess of 90% (most applications have shown that it can achieved in excess of 99% reduction of SO2). This EDV Wet Scrubbing System has been widely applied to control FCCU emissions worldwide and is well proven for providing continuous operation for periods in excess of 4 years.

Looking at a small to moderately sized FCCU (35,000 BPSD) with a 1% sulphur in the feed we evaluated the annualised cost for controlling particulate and SO2 emissions (60% and 90% reduction of SO2) by accounting for both capital and operating costs. The evaluation was based on a 15 year life and 10% interest. Both partial and full burn operations were evaluated.

Based on economic evaluations performed, we find that in most cases controlling particulate and SO2 emissions with a wet scrubbing system is less expensive than using an ESP plus transfer catalyst additive.
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