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Jun-2016

Tackling tighter emissions rules

A review of the options to raise sulphur emissions performance to meet China’s tightening regulations.

FRANK SCHEEL
Jacobs Comprimo Sulfur Solutions

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Article Summary

All around the world, SO2 emissions are under pressure and becoming tighter. This is also true for China where new legislation was passed last year. This limits the SO2 emissions from sulphur recovery units (SRU) to 100 mg/Nm3 in highly populated areas, or 200 mg/Nm3 in less populated areas. The implication of the new rules is that almost all SRUs in refineries, gas plants and coal-to-chemical facilities will have to be reviewed. Additional technologies will have to be implemented to get these units ready to comply with the new regulations, and investments will have to be made.

The extra sulphur that will be produced has no value for the facility owners. It may already cost money to get rid of the sulphur, so extra produced sulphur is not paying for the required investments. The trick is to find the right technology that will meet the new legislation while minimising investment and operating costs. Preferably, also the required plot space is minimised, as most existing facilities do not have space for a lot of additional equipment. The best approach is to do a study of the available options and look into the pros and cons.

This article will list the available technologies to upgrade SRUs and limit SO2 emissions. It will also address how to control and manage existing SRUs: it makes no sense to invest heavily in additional installations when the existing facilities do not perform well and thus fail overall to meet the new regulations.

Available technologies
The first thing to do is have a look at what kind of sulphur recovery is being used at the facility. This can be a normal Claus unit with 
recovery of up to 98%. It is clear that some additional work needs to be done in this case. The easiest and cheapest solution would be to maximise the catalytic conversion by retrofitting the Claus unit to a SuperClaus (see Figure 1) or EuroClaus unit. This will already bring sulphur recovery into the range 99.0-99.5%: still not enough but getting closer to the target. The remaining 0.5 to 1.0% of sulphur species is now sent to the incinerator where all sulphur components are converted to SO2. To capture SO2 from the flue gas downstream of the incinerator, a scrubber column could be installed. The scrubber can utilise several chemicals as a scrubbing agent depending on what end product can be handled best. Most used is the caustic scrubber, which will be described in more detail. Caustic will react with SO2 in the scrubber column to form sodium sulphate. This sodium sulphate will be handled via the site waste water treatment system. If the SRU is already of a SuperClaus or EuroClaus design, only the scrubber system, consisting of a scrubber tower, a small blower, a caustic tank with pump and an effluent pump, is required. It can be even simpler if the site already has a caustic infrastructure. In that case, the caustic tank and pump can be deleted from the design.

This solution is ideal for SRUs of up to 400 or 500 t/d of sulphur production capacity: the investment costs are relatively low, the requirement for 200 and 100 mg/Nm3 will be met easily, and the operating cost will be low. The only downside is the small waste stream that needs to be handled; in coastal areas this should not be difficult.

But if handling the sodium sulphate stream is an issue, other scrubber chemicals can be used that produce gypsum, which might be easier to dispose of. If the SRU is much bigger in capacity, this automatically means that the waste stream gets bigger and the amount of chemicals to be used will increase. A detailed study into the alternatives and the possibilities of dealing with the wastes is then needed.

SCOT option

If the scrubber option is not possible, another well proven alternative is the installation of a SCOT unit (Shell Claus Off-gas Treatment). As with a scrubber, this technology can be installed behind the Claus unit (see Figure 2). The remaining sulphur species will be hydrolysed back to H2S over a catalyst. Via a quench step, the gas is cooled so that it can be handled in an amine wash. The amine will absorb the H2S, and clean gas is sent to the incinerator. The regenerator will release the H2S, routing it back to the main burner at the inlet of the SRU. With a SCOT unit, 200 mg/Nm3 SO2 should be achievable. For 100 mg/Nm3 SO2, this has to be checked carefully as the small amounts of COS, CS2 and mercaptans that pass the SCOT reactor unconverted might give too high emissions. For both the 200 and 100 mg/Nm3 cases, the sulphur degassing needs to be converted to a degassing under pressure. In this way, off-gas from the degassing vessel can be rerouted to the SRU main burner instead of to the incinerator to prevent it from contributing to the SO2 emission. Compared to the scrubber option, adding a SCOT unit will cost considerably more, and also the operating cost and required plot space will be higher, but the process is well proven. Some sites in China might already have a SCOT type unit installed. In this case, the performance needs to be verified and most likely the degassing needs to be changed to a degassing under pressure, to prevent degassing from being the reason for not achieving the emission limits.

In case the SCOT does not meet the 100-200 mg/Nm3 limit, some small adjustments can be made to allow lower emissions, such as introducing acid aided regeneration. If that still does not do the trick, there is no other choice than to add a small scrubber after the incinerator to capture the last milligrams of SO2. That may seem a bit over the top, but it will be the only solution to meet the very stringent emission rules.

Cansolv for SO2 capture
There is a third option that may be considered. This is the Cansolv process. Cansolv is also an amine system, similar to the SCOT process, but this system absorbs SO2 instead of H2S. Therefore a Cansolv unit is installed after the incinerator, capturing all SO2 that would otherwise go to the stack. The regenerator releases the SO2 and recycles it back to the main burner mixing chamber. The benefit of the Cansolv system compared to SCOT is that the degassing can remain as it is. As the sulphur species from the degassing will be captured when leaving the incinerator, no degassing under pressure is needed.

A second benefit of Cansolv is that, by routing SO2 back to the mixing chamber of the SRU, less air is needed in the main burner and additional sulphur recovery capacity is created. This would free up room to process more acid gas in the SRU. A downside is that the Cansolv process is high in investment and operating costs and also requires a large plot area.

Biological treatment
For new installations, the same considerations apply, but the outcome could be different. An option that is not suitable in existing situations but can be considered in new situations is the Thiopaque process. This is a biological treatment that can handle both high and low concentrations of H2S in its feed gas. Bacteria will absorb the H2S and digest it, producing sulphur. This product sulphur is contaminated with bacteria and contains a lot of water. But with additional handling steps, a sellable product results. This process is especially suitable for relatively small sulphur recovery capacities and where the feed gas does not contain components that can poison the bacteria. The Thiopaque process is totally different from the other processes mentioned, but it is certainly possible in some cases.

Study the options
Depending on the configuration of the existing SRU, one or more of the options described above will do the trick. One of them might be preferred over the other. What always pays off is to have a study executed by a party that has a broad portfolio of different technologies on offer. In this way, you can be sure that all technologies have been considered, and based upon the results you will have the best fit for your situation.


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