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Jul-2009

FCC flue gas scrubber alternatives: part I

The merits of combining additives and barrier filters as a cost-effective and highly efficient way of meeting SOx, NOx and particulate matter emissions targets

John Sawyer, Hanif Lakhani, Kurt Schuttenberg and Lindsay, McRae Pall Corporation
Ray Fletcher and Martin Evans, Intercat (Johnson Matthey)
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Article Summary
Refinery emissions of pollutants such as SOx, NOx and particulate matter (PM) are coming under increased scrutiny in all parts of the world. Tighter emissions limits have recently been imposed in many western countries. Regulations can soon be expected to follow suit globally. In most refineries, the fluid catalytic cracking unit (FCCU) is the major source of these airborne pollutants. There are several technologies available for reducing emissions of these materials from the FCCU process. In this endeavour, the merits of combining additives and barrier filters as a cost-effective and highly efficient alternative to current options are proposed. Both have been proven separately, and the combination now offers a Best Available Control Technology (BACT) that can meet SOx (and NOx) targets of 25–200 ppm and particulate matter emissions to levels below 10 mg/Nm3.

Refiners today are faced with the challenge of converting heavier and increasingly sour feeds at a time when protection of the environment and natural ecosystems is critical. Many environmental regulations have started in California, moved to the rest of the US, then to Europe and the rest of the world. Beginning in the West, significantly stricter limits have been imposed on FCC emissions of SOx, NOx and particulate matter. Just recently, the South Coast Air Quality Management District (SCAQMD) has regulated PM10 (particulate matter below 10 ppm) from FCCUs (Rule 1105.1). In addition, the US’s EPA has published a New Source Performance Standard (NSPS) for FCCUs as well as other refinery processes. It can therefore be expected that limits on these pollutants will change everywhere, and significantly, over the coming five to ten years. In many regions, reductions in emissions are legislated as a condition of obtaining permits for unit modifications and capacity expansions.

The FCC process is the major source of airborne emissions at many refineries. The FCC regenerator is a large combustion unit and can therefore emit significant quantities of most regulated pollutants, including but not limited to:
• SOx
• NOx
• Particulate (both total PM and micro-particulate: ie, PM2.5 and PM10).

Some of these pollutants can be reduced only by installing additional hardware. When this is required, capital investments can be significant in many cases. Some emission-control hardware solutions can reduce several pollutants simultaneously, but the capital and operating cost implications can be very high. For example, in the case of wet scrubbers, the cost of the additional infrastructure required for the handling of the high flow rates of wastewater and solids can be considerable. The optimum solution for many refiners is likely to be a hybrid approach requiring some capital investment and some additive use. This permits the handling of any emitted materials in the dry state, thus eliminating the need for expensive infrastructure and secondary treatment systems, and the high operating costs that accompany these traditional technologies.

In this two-part article — part II to be published in the Q4 2009 issue of PTQ — a novel approach to controlling emissions of SOx, NOx and PM using the integrated approach of additives combined with FCC offgas filtration is discussed. Compared to wet scrubbing technology, this approach is relatively low in initial capital expenditure, while being significantly lower in 
both operating and maintenance 
costs. It is a cost-effective approach to meeting the current and future emissions limits required by environmental regulations around the world. This article will place particular emphasis on the technical and commercial merits of the approach. Examples are given of how some refineries in different parts of the world have chosen to meet their emission limits, and the rationale behind the choices to minimise capital outlay, while allowing for upgrades to meet future requirements, are explained.

Current emissions regulations for FCC emissions
This is not an exhaustive review of the emissions regulations governing FCC emissions around the world, but is a summary of Pall’s experience in providing equipment designed to meet emissions regulations. The US EPA recently proposed an NSPS, which would tighten and add to the existing regulations governing FCC emissions. The proposed PM emissions limits are 0.5 kg/Megagram (0.5 lb/1000 lb) of coke burn-off in the regenerator.1 Our experience equates this to emission limits of approximately 50 mg/Nm3. In addition, we have seen the start of PM10 regulations in some parts of the US. SCAQMD has promulgated Regulation 1105.1, which regulates PM10 emissions from FCCUs to approximately 
10–12 mg/Nm3. In other parts of the world, we typically see PM regulations of 50–150 mg/Nm3.

In the proposed NSPS, the US EPA has set the SO2 emission limit to 50 ppm on a seven-day rolling average, and 
25 ppm on a 365-day rolling average.1 In the same document, the proposed NOx emission limits are 80 ppm on a seven-day rolling average.1

From discussions with environmental regulators, it is believed that PM2.5 can be composed of a significant portion of condensables. However, these condensables are primarily thought to be composed of sulphate condensables, and would therefore be controlled by the removal of SOx before the acid gas dew point is reached.

Review of available hardware technologies
The following technologies have demonstrated the ability to 
substantially meet the previously discussed emission limits:
• FCC feed pretreatment The FCC feed pretreatment option has the highest capital costs (by several orders of magnitude in some cases), but it does have the advantage of improving 
FCC yields or allowing heavier feeds to be run, thus improving operating margins. FCC feed pretreatment will, however, only reduce SOx emissions, so refiners still need to take additional measures to reduce CO, NOx and particulates
• Flue gas scrubbers This is normally the highest capital cost option for downstream flue gas treatment. Scrubbers are usually the most appropriate solution when the FCCU is running very high sulphur feeds, especially when the regenerator is operating in partial-burn mode. Today’s scrubbers are also capable of treating several pollutants at the same time, such as SOx, PM and NOx. Some of the proprietary methods of reducing NOx in scrubbers are effective. However, the costs of operation can also be very high, as increased wastewater treatment, high utilities usage and sodium hydroxide consumption all contribute to the expense of operation
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