Selection Criteria
As part of the selection process it is necessary to consider the following criteria:
Gas purity. The demands on syngas purity can vary extremely with application. A chemical application such as ammonia, methanol, or SNG can require desulfurization to lOOppbv or lower. For an IGCC power application with a limit of 5ppmv S02 in the flue gas, about 40ppmv at the outlet of the AGR is satisfactory. But desulfurization is not the only purity criterion. Ammonia syngas requires lOppmv max C02. Other limitations may arise indirectly; in a hydrogen application for instance, any sulfur entering the PSA unit becomes concentrated in the tail gas, where emissions regulations may also create a sulfur slip limitation.
Raw gas composition. The washing solution must be able to cope with impurities in the raw gas. Most gasification processes leave about 5% of the sulfur as COS, which in many AGR processes is not absorbed as well as H2S, if at all, so where deep sulfur removal is required, it may be necessary to convert it to H2S prior to the main sulfur removal stage. But the raw gas composition issue is not just a matter of the previous “gas purity criterion”; the action of minor impurities on the solution itself also have to be considered: for example, HCN in the raw gas reacts, particularly with primary amines causing solution degradation.
Selectivity. The selectivity of a gas separation process is the ability, for example, to remove H2S while leaving C02 in the synthesis gas. There can be a number of motivations leading to a desire for high selectivity.
о The capital and operating expense of most gas-washing systems correlates well with the amount of solvent in circulation. A nonselective wash that also washes out large quantities of C02, may (in the case of a chemical wash will) inflate the solution rate and therefore costs considerably. A good selectivity can therefore make a contribution to good economics.
о The acid gas removed from the syngas is usually processed in a Claus sulfur recovery unit (SRU). It is important for the Claus process that the H2S stream to the SRU not be too dilute. For a typical air-blown SRU, the lower limit for practical operation is about 30mol%. In the context of gasification, it is usually possible to operate the SRU in an oxygen-blown mode, since the SRU oxygen requirement is generally small compared to that necessary for the gasifier. An oxygen-blown SRU can operate with as little as 10mol% H2S in the sour gas. Either way, excess C02 in the sour gas will inflate the cost of the SRU. When looking at the selectivity of an AGR system, it is therefore necessary to consider the effect on the SRU.
о For an IGCC application C02 in the synthesis gas contributes to the total mass flow through the gas turbine and so to the power output. It is therefore in principle desirable to leave any C02 in the synthesis gas rather than washing it out. For many gasification processes with a low content of C02 (<5%) in the syngas this is not usually decisive, but with a higher C02 content it can be important.
Other issues that need to be reviewed as part of the selection process are corrosion (often the practical limit on higher solvent loading), co-absorption of the useful gas components (in syngas application hydrogen and carbon monoxide, but in general this would include hydrocarbons), solvent losses through degradation or vapor
Sulfur in product gas, [ml/m3 ]
pressure, opportunity for waste heat integration, particularly for the solvent regeneration in chemical washes, and availability of the solvent and toxicity.
• Economic boundary conditions. In particular the depreciation rate or pay-out time specified for a project may influence the process selection. Typically, chemical washes will tend to require less capital investment than a physical wash, but at the expense of a higher utility demand for solvent regeneration.
Examination of the above criteria in any particular case will probably narrow the field down to three or four serious contenders, sometimes even less. The chart in Figure 8-3 provides assistance in this. Selection from this short list is then generally a matter of pure economics.