Wet scrubbing-based NOx control using LoTOx technology - first commercial FCC start-up
Wet scrubbing based NOx control had been successfully applied to various industries for several years. Refineries, however, had not used such an approach until Belco Technologies Corporation (Belco) introduced it to the refining industry in 2003.
Nicholas Confuorto, Belco Technologies Corporation
Jeffrey Sexton, Marathon Petroleum Company LLC
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At that time Belco acquired a license from the BOC Group to supply the LoTOx technology to refineries worldwide. Since that time three refineries have purchased Belco’s EDV wet scrubbing systems with LoTOx technology and two refineries purchased LoTOx technology to add to their existing EDV wet scrubbing systems. All five of these systems are applied to Fluid Catalytic Converter Units (FCCU). The first of the LoTOx systems started up operation during February of 2007 at Marathon’s Texas City, Texas, refinery.
In the fall of 2002, Marathon conducted a review of several options to reduce nitrogen oxides (NOx) from its 52,000 barrels per day FCCU. The LoTOx technology was identified as a potential NOx control technology. Unfortunately, the LoTOx technology had never been used on an FCCU. Marathon needed more assurance that applying it to an FCCU would result in similar performance to that experienced in the other application. To that end, Marathon, BOC and Belco conducted a demonstration of Belco’s EDV wet scrubbing system with LoTOx technology. The demonstration was conducted October through November 2002. Following review of the results, Belco finalised its proposal to Marathon for the system and eventually concluded negotiations for the design and supply of the system for the Texas City refinery. This commercial system successfully started operating in February 2007, achieving all established goals and confirming the results of the demonstration. The following primary conclusions were drawn from that demonstration:
1) The EDV wet scrubbing system with LoTOx technology can reliably reduce FCCU NOx emissions to meet stringent regulations.
2) The LoTOx technology does not convert sulphur dioxide (SO2) to sulphur trioxide (SO3).
3) NOx removal efficiency can be instantaneously varied up or down by adjusting the ozone injection rate.
4) High NOx removal efficiency can be maintained during start-up and FCC upset conditions.
5) The system modulates well-fluctuating inlet NOx levels while steadily maintaining the desired stack NOx level.
The LoTOx system
A preferred approach to NOx control
LoTOx is a patented technology sold to refinery clients worldwide by Belco Technologies Corporation under an exclusive license from the BOC Group. It is a Low Temperature Oxidation process where ozone is injected into the flue gas stream to oxidise insoluble NOx to soluble oxidised compounds.
LoTOx is a selective, low temperature oxidation technology that uses ozone to oxidise NOx to water soluble nitric pentoxide (N2O5), which, inside the wet scrubber, forms nitric acid that is subsequently scrubbed by the scrubber nozzles and neutralised by the scrubber’s alkali reagent. The oxidation of NOx with ozone that creates the N2O5 and the subsequently produced dilute nitric acid is similar to the process that occurs in the atmosphere when smog reacts with moisture, resulting in acid rain.
Since the process is applied at a controlled temperature zone within the EDV wet scrubbing system, it can be used at any flue gas temperature. This means that the refinery can recover as much energy as possible from their boilers without maintaining a temperature range as required by SCR systems. Additionally, extensive testing indicates that the LoTOx technology does not generate measurable amounts of SO3 and, in fact, is beneficial in oxidising some of the hazardous, heavy metals particulate to facilitate their collection in the downstream devices. The LoTOx technology is integrated within Belco’s EDV Wet Scrubbing System for simultaneous control of NOx, SOx and particulate matter.
The LoTOx technology is robust and capable of delivering the desired NOx removal, irrespective of fuel changes or load swings. Tested and proven in conjunction with the industry-leading EDV wet scrubber technology, LoTOx is an excellent solution for the control of NOx from FCC operations.
Some of the benefits of the LoTOx technology include the following:
• The LoTOx technology does not interfere with FCC chemistry.
• The LoTOx technology does not utilise a fixed catalyst bed and does not impact system hydraulics. As a result, the process is robust and can handle unit upsets without impacting overall reliability and mechanical availability.
• Performance of the LoTOx technology is insensitive to O2 concentration in the FCC off gas.
• The LoTOx technology provides another degree of freedom for FCC unit operating flexibility while controlling environmental emissions.
The operating costs for the LoTOx system are derived from the power, oxygen and cooling water for the generation of ozone. Ozone is the oxidising agent used in the LoTOx technology. Each of these operating costs is directly proportional to the levels of NOx treated. Unlike ammonia storage and handling issues associated with SCR or SNCR technologies, the LoTOx technology generates ozone on demand as required. Ozone is produced on site by passing oxygen through a conventional industrial ozone generator. This means that there is no storage of ozone and the generation of ozone can be stopped by simply switching off the ozone generator. The other operating cost is related to the addition of caustic solution for neutralisation of nitric acid produced by the process of reducing NOx. This slightly increases the caustic solution used by the scrubber for removing particulates and SOx.
The LoTOx process
The LoTOx process operates optimally below 300°F. Therefore, it does not require heat input to maintain operational efficiency (no flue gas re-heating), and can enable maximum heat recovery from hot flue gases. Ozone is produced in response to the amount of NOx present in the flue gas generated by the combustion process and the final NOx emission required. The low operating temperature allows stable and consistent control regardless of variation in flow, load or NOx content. There are no adverse effects of acid gases or particulates on the performance of the LoTOx process. Ozone, once mixed with flue gas, rapidly reacts with insoluble NO and NO2 molecules to form higher oxides such as N2O5. These higher oxides of nitrogen are highly soluble and rapidly react with moisture present in the flue gas stream to form dilute oxy acids such as nitric acid. The conversion of higher oxides of nitrogen into the aqueous phase in the scrubber is rapid and irreversible, allowing nearly complete removal of NOx.
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