NOx, VOC and particulate abatement from refineries
Refinery NOx and VOC emissions primarily originate from combustion processes in heaters, crackers, FCC regenerators, reformers, gas turbines and boilers.
NOx emissions from refineries can be removed by a variety of methods. The primary methods are combustion modifications, burner operation and maintenance optimization, reburning, flue gas recirculation or use of low-NOx burners. When the primary methods are not sufficient, the secondary methods Selective Non-Catalytic Reduction (SNCR), or SCR are applied. The SCR method is widely acknowledged as the BAT, making it possible to achieve more than 95% NOx removal.
At some plants, both secondary methods SNCR and SCR are used. The advantage of this hybrid solution is that a high NOx removal degree and a low ammonia slip can be achieved, while the needed SCR catalyst volume can be minimized relative to a pure SCR installation.
Implementation of SCR DeNOx technology on refinery applications has been increasingly applied over the last two decades. Initially, the SCR DeNOx technology was implemented in the USA, but later refineries in Europe and Asia have also adopted the technology. Often the SCR systems are made as retrofits on existing combustion equipment. For new units however, space is often reserved for the SCR reactor already in the design phase.
The SCR reactor can be placed in a variety of places downstream of the combustion unit. The main requirement is a flue gas temperature in the interval 180°C to 500°C, and optimally 350-420°C. For boilers, the SCR reactor is normally placed immediately after the boiler and economizer sections. For FCC units, heaters, crackers, reformers and gas turbines, the SCR reactor will typically be placed in between the heat exchangers of a waste heat recovery section, where there is sufficient space and a good temperature window.
SCR DeNOx catalysts installed in refinery applications typically have long lifetimes compared to other industries. Guaranteed lifetimes of 3 to 5 years are standard whereas actual lifetimes may very well be longer than 10 years. These catalysts are designed for a desired end of run activity, meaning that the initial catalyst activity will be higher than the required activity and then slowly decrease to the required activity after several years of operation.
The SCR DeNOx catalysts will experience a slow decline in performance during their lifetime for one or more of the following reasons: chemical poisoning, thermal sintering, fouling of catalyst surface, erosion or plugging of catalyst channels.
Particularly, SCR catalysts in reformer and ethylene cracker applications experience high deactivation due to chromium poisoning of the SCR catalysts. The catalyst poison chromium arises from the reformer tubes when these are heated to above 700°C.
The table below provides several examples of SCR DeNOx installations in reformers, FCC units, crackers, heaters, gas turbines and boilers at different refineries. NOx removal efficiencies are typically in the range of 75% to 97%. NOx and NH3 emissions are typically 2-100 ppmvd @3% O2 and 2-15 ppmvd @3% O2, respectively. An estimate for the total number of SCR installations in refinery applications is more than 1,000 SCR applications globally. The SCR DeNOx technology is thus a well proven technology for refinery applications.
Removal of Volatile Organic Compounds such as CO, propane, butane, and toluene may be done catalytically. The catalyst is essentially a SCR DeNOx catalyst with palladium (Pd) added. The removal efficiencies are different from each VOC species. But generally the catalyst can be designed for high removal efficiencies well above 90% of most VOC species. The lower alkanes methane and ethane are exceptions; they cannot be removed catalytically but require elimination by thermal oxidation.
VOC-oxidation catalysts are so far primarily used in gas turbines and a few reformer installations. An example is Celanese Clear Lake methanol plant in USA, see Table below.
VOC oxidation catalysts may be placed before the injection of ammonia and SCR DeNOx catalyst. Or the catalysts may be designed as one dual functionality catalyst, able to remove both VOC’s and NOx. The dual function catalysts have the obvious advantages of minimal space required, lower pressure drop and reduced SCR catalyst volume.