Dealing with more sulphur post-IMO 2020
Combinations of revamping opportunities enable SRUs to deal with higher throughput arising from lower limits for sulphur in marine fuels.
DEBOPAM CHAUDHURI and SRINIVASA ORUGANTI
Fluor Daniel India
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The global sulphur cap on marine fuels is 0.5 wt% from 1 January 2020. Since the sulphur cap on bunker fuels up to that date is a much less stringent 3.5%, the effect of this change is global and far reaching. It is estimated that around 320 million t/y of fuel will be required for the shipping industry in 2020. Figure 1 shows the distribution of sulphur content in residual fuel oil across the globe. The average sulphur content of residual fuel oil currently sold is about 2.45%. Thus, nearly all currently sold fuel oil has a sulphur content far above the 0.5% limit. Based on IMO data, this is greater than 98%.
Hence, this change impacts practically all users of bunker fuel oil. Additionally, this also incurs a very high reduction for most residual fuel oil to meet the 0.5% limit (an 80% reduction from 2.5% to 0.5%). This translates to an immense amount of sulphur that will have to be removed from fuel oil. The estimated amount is 6-7 million t/y. This sulphur will need to be converted to H2S, with the H2S subsequently needing to be converted to elemental sulphur in Claus units. This is equivalent to almost 20000 t/d of sulphur handling capacity on a global scale in all sulphur recovery units (SRU).
Beyond the IMO 2020 deadline, the sulphur specification is expected to become more and more stringent with each passing year. This will continue to pinch the sulphur handling capability of refiners across the globe, and the search to achieve the optimum sulphur recovery methodology for all such future changes of standards will be a continual effort for refiners.
One possible solution to meet the IMO 2020 target is to implement the ‘end of pipe’ solution in the form of a flue gas scrubber on board ships. This solution is seemingly simple, not too costly, and allows ships to continue to burn high sulphur fuels. But given the enormous number of ships globally, it is expected that no more than about 10% of vessels would have scrubbers fitted before the 2020 deadline.
Hence, the problem actually needs to be addressed at its source, meaning that refineries should modify their configurations and operations to ensure a steady supply of low sulphur marine fuel oil beyond 2020.
Depending on the present capacity (size) and complexity of the refinery, any one or a combination of the following approaches is being adopted: • Switch to a ‘sweeter’, low sulphur crude diet, such that the fuel oil produced will automatically meet IMO 2020 regulations. This seems like a logical option, and is very attractive in that it does not require capital investment. However, for crudes to yield a sulphur content in the neighbourhood of 0.5 wt% in vacuum residue (the main component of fuel oil) requires a crude sulphur content in the order of 0.2 wt%. Such very sweet crudes are likely to command high price premiums post-2020, which is likely to depress margins for refiners relying on this option. While this option will be attractive to some refiners, it is not the most sought after option on a global scale in view of the gradual depletion of sweeter crudes complemented by the gradually increasing cost differential between sweet and sour crudes.
• Generate low sulphur marine fuel oil simply by desulphurising the existing high sulphur marine fuel oil in a hydroprocessing unit. While requiring less capital outlay than conversion of the fuel oil (the following option), this also would require high investment. Furthermore, it leaves the refiner somewhat exposed to possible future tightening of standards to well below 0.5%. Such a development is credible; standards, once introduced, become stricter and stricter, as has been observed with transportation fuels (notably gasoline and diesel).
• Modify the refinery configuration to upgrade residual fuel oil to lighter transportation fuels such as middle distillates (diesel, jet) and gasoline. This will involve investing in conversion units like hydrocrackers, fluid catalytic crackers, and cokers. The attractiveness of this option lies in its beneficial effect on refinery margins, given that transportation fuels such as gasoline, jet, and diesel command significantly more attractive prices than marine fuel oil. On the down side, this option requires very high capital investment and a long implementation time.
The sulphur balance
There cannot be a single solution for the refining sector to meet the 0.5% IMO deadline. Especially for large new ships, a scrubber may be an attractive option. As ships with scrubbers will continue to use high sulphur fuel oil, the market for high sulphur fuel oil will not entirely disappear. This in turn alleviates the pressure on refineries somewhat. Still, it is certain that a lot of heavy, high sulphur fuel oil is going to disappear – either converted to lighter, cleaner transportation fuels or desulphurised. This translates to a lot of work to be done by refineries, to ensure that they produce heavy fuel oil that is in line with market requirements (see Figure 2). This clearly shows that even though there will still be a small market for high sulphur fuel oil, the shift from the present high sulphur fuel to low sulphur fuel is overwhelming.
Apart from the required investment scope in core process units to meet the new limit – be it in process units that convert heavy residues into lighter materials (delayed coker, residue hydrocracker, residue FCC), or in units that desulphurise the heavy fuel oil (residue desulphurisation) – there will be a big impact on refinery hydrogen and sulphur balances. To focus on this, global sulphur recovery capacity was just above 128000 t/d in 2016,1 whereas the expected extra sulphur recovery capacity just to meet the IMO deadline is estimated to be about 20000 t/d, which translates to about an additional 15% over present global sulphur handling capabilities.
The sudden demand for additional SRU capacity that arises at rather short notice for refineries represents a real challenge. With typical build time for new SRUs in the order of 2-3 years, revamping existing SRUs is in many cases the most attractive option.
Managing additional sulphur load
The sulphur load to be processed will increase irrespective of the design modification route adopted by a refinery. The sulphur block, especially the SRU, will experience major changes to be capable of handling this additional load.
Oxygen enrichment route
Most refineries are still using the traditional modified Claus process to recover sulphur, using air as the source of oxygen in the reaction furnace. Oxygen enrichment provides an easy way to increase the sulphur handling capacity of a Claus unit. Table 1 shows the impact of oxygen enrichment on sulphur handling capabilities. Oxygen enriched operation reduces the amount of nitrogen entering the process and hence allows acid gas to replace it while keeping the total throughput of gases the same, and hence the pressure drop in the system comparable. The actual increase will, however, be impacted by other factors in the Claus unit, which will be unique and typical for each SRU.
Thus, shifting from a standard straight through process to low level oxygen enrichment technology (oxygen <28%) will significantly increase plant throughput (10-30%). The additional advantage of low level oxygen enrichment is that it can be easily implemented with little to no revamp required elsewhere on the SRU; only arrangements to supply oxygen to the Claus unit need to be made.
The typical challenge associated with implementing medium or high level oxygen enrichment is that the reaction furnace temperature increases very rapidly as the concentration of oxygen increases. The burner design, and the refractory system as well as the downstream waste heat boiler design, need to be evaluated carefully and modified in most cases. But still, the overall cost for these modifications vis-à-vis installing a new SRU train is much less (around 20%). The requirement for managing oxygen as a feed to the SRU would of course be the additional constraint on this change.
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