Jun-2003

Clean generation

How new wet NOx technology has responded to increasing 
environmental pressures. One of the major concerns for refineries worldwide is nitrogen oxide emissions, commonly known as NOx.

Nicholas Confuorto Belco Technologies Corp
Michael Barrasso and Naresh Suchak The BOC Group, Inc

Article Summary

These emissions contribute significantly to global environmental problems including ozone (photochemical smog), acid rain and elevated fine particulate levels, which cause various respiratory illnesses. To protect human health and the environment, most countries either have regulations for NOx or are currently considering instituting them. In the USA, many federal and state regulations address this issue. As such, the market for NOx control systems has been growing steadily over the past few years. Until recently, the primary focus for the regulations was the electric power industry. The power industry proceeded to comply by installing selective catalytic reduction (SCR) systems where higher levels of reduction were required, and selective non catalytic reduction (SNCR) systems where low to medium levels of reduction were required.

Recently, however, the refining industry has also been drawn under regulatory scrutiny. This is now forcing refiners to investigate various options to meet NOx reduction requirements. In addition, the low to medium reduction efficiency systems such as SNCRs have become much less appealing under today’s high reduction requirements. This leaves the refinery manager with only two options for high NOx reduction: SCR and low temperature oxidation (LoTOx).

SCRs can easily be applied to power boilers, incinerators and heaters, however FCCU operators rightfully fear potential plugging during FCCU reversals. In addition, gas temperatures need to be closely controlled to avoid potential ammonium sulfate/sulphite depositions at lower temperatures, and sintering of the SCR catalyst surface at higher temperatures. An SCR for an FCC application requires an operating temperature of approximately 340-400ËšC. These are approximate temperatures; specific temperatures need to be calculated based on gas composition and type of catalyst used. Unfortunately, this range of temperature is usually too low for an FCCU regenerator temperature and too high for a typical CO boiler outlet. Modifications to the existing CO boiler or heat recovery are usually required to apply an SCR in this application.

Many refiners are also concerned about the inherent conversion of some of the SO2 to SO3 with SCRs; some catalyst manufacturers are better than others. The range of conversion varies between 1% and 5% depending on the catalyst used. However, even the smallest conversion can contribute to a blue plume where the natural level of stack SO3  is already elevated.

On the other hand, the LoTOx process, when applied with the EDV wet scrubbing system, can provide similar NOx reductions to an SCR, but without obstruction in the gas stream or concerns with temperature excursions or conversion to SO3 . This combined process system is marketed worldwide by Belco Technologies, in cooperation with The BOC Group, for applications in the refining industry.
 
Process stages
When using the LoTOx process, ozone is injected into a flue gas stream to oxidise insoluble NOx to highly soluble compounds. The ozone is produced on-site and on demand by flowing oxygen through an ozone generator. The process is ideally suited to FCCU applications (Figure 1). LoTOx is a low temperature process that does not require heat input to maintain operational efficiency or to prevent treatment chemical slip common with SCR and SNCR systems (known as ammonia slip).

Ozone is produced on demand in response to the amount of NOx generated by the upstream processes. To produce ozone, an ozone generator (Figure 2) is included as a standard component. The ozone generator has a set of dielectric tubes through which electric current is passed. As oxygen is passed over the energised tubes, the electric field oxidises part of the oxygen (O2) to ozone (O3). The O2 can either be generated on-site, or brought in from outside (either by pipeline or shipped as liquid). If on-site generation is chosen, gas supply companies such as The BOC Group can provide complete packaged supply options. The ability of LoTOx to work at low scrubber operating temperatures allows stable and consistent control regard-less of variation in flow, gas temperature, load or NOx content. There are no known adverse effects of acid gases or particulate on the LoTOx system. Ozone rapidly reacts with relatively insoluble NO and NO2 molecules to form soluble NO3 and N2O5. These highly oxidised species of NOx make it very soluble, and mean that it reacts with moisture in the gas stream to form nitric acid. The conversion of NOx into the aqueous phase in the scrubber is rapid and, in presence of an alkali, results in an irreversible capture of NOx, allowing almost complete removal from the flue gas.

The rapid reaction rate of ozone with NOx makes it highly effective for treatment of NOx in the presence of other compounds such as CO and SOx, resulting in high ozone utilisation efficiency for NOx removal with no reaction with CO and SOx at the design conditions. Figure 1 depicts typical LoTOx system setup employing a low temperature oxidation process on an FCCU with a BELCO EDV wet scrubbing system.

Process chemistry The LoTOx process uses ozone to oxidise NO and NO2 to NO3 and N2O5, which are highly soluble, and by wet scrubbing N2O5 is easily and quickly converted to HNO3 and finally to NaNO3. Many reactions occur, but for sake of brevity, the following reactions can summarise the LoTOx process chemistry:

NO + O3 → NO2 + O2
NO2 + O3 → NO3 + O2
NO2 + NO3 →N2O5
N2O5 + H2O → 2 HNO3
HNO3 + NaOH → NaNO3 + H2O

In addition to NOx removal, other pollutants present in flue gas, such as SO2 and HCl, can be simultaneously removed in the wet scrubber, which can be simplistically described by the following reactions:

SO2(g) → SO2(l)
SO2(l) + H2O(l) → H2SO3(l)
H2SO3(l) + 2NaOH → Na2SO3(l) + 2H2O
2Na2SO3(l) + O2 → 2Na2SO4
HCl + NaOH → NaCl + H2O