Sulfur Recovery and Tail Gas Treating
Sulfur is a component of coal and other gasification feed stocks. Sulfur compounds need to be removed in most gasification applications due to environmental regulations or to avoid catalyst poisoning. Whether it is electricity, liquid fuels, or some other product being output, sulfur emissions are regulated, and sulfur removal is important for this reason, along with the prevention of downstream component fouling. In addition to these constraints, recovering saleable sulfur is an important economic benefit for a gasification plant.
To illustrate the previous point, in 2011 8.1 million tons of elemental sulfur was produced, with the majority of this coming from petroleum refining, natural gas processing and coking plants. Total shipments were valued at $1.6 billion, with the average mine or plant price of $200 per ton, up from $70.48 in 2010. The United States currently imports sulfur (36% of consumption, mostly from Canada), meaning the market can support more domestic sulfur production.
- USGS Mineral Commodity Information: Sulfur
In a typical design, a series of process steps is used to recover sulfur as a useful by-product from gasification processes. First, cooled syngas/feed gas is contacted with a chemical solvent in an absorber unit, where, as the gas passes through, almost all of the H2S and some CO2 are removed (this is the basic AGR process discussed at length elsewhere). The solvent is now rich with H2S and it moves on to a stripping unit to clean and recycle the solvent and separate the H2S. The basic stripping process and solvent stripping/regeneration are depicted below.
|Simplified Diagram of the AGR Process
The sulfur-containing acid gases removed by the AGR process consist primarily of a mixture of hydrogen sulfide (H2S) and carbon dioxide, which is inevitably captured along with the H2S to some degree. This serves as feed gas to a sulfur recovery process, in which sulfur is recovered as either liquid or solid elemental sulfur, or as sulfuric acid, depending on market demands. For an elemental sulfur product, a Claus sulfur recovery unit produces elemental sulfur from H2S in a series of thermal stage followed by multiple catalytic stages, achieving about 98% recovery of the sulfur in the syngas. Part of the H2S is oxidized to produce sulfur dioxide (SO2), which is then reacted with the remaining H2S to give elemental sulfur and water. Tail gas from the Claus process is sent for further treatment to a Tail Gas Unit (TGU), which may be exemplified by the amine-based Shell Claus Off-gas Treatment (SCOT) unit. The SCOT unit removes nearly all of the remaining sulfur—in the form of a tail-gas stream of unreacted sulfur, H2S, SO2, and carbonyl sulfide (COS)—that the Claus unit misses. The SCOT unit uses a cobalt-molybdenum catalyst to convert SO2 to H2S, which is then removed in an absorber. Altogether, this process series approach can achieve a typical overall sulfur recovery of 99.8%.
Sulfuric acid synthesis is an alternative to sulfur recovery via the Claus process. This may be preferred if the plant is located close to a sizeable market for sulfuric acid. In this case, H2S is first burned in a furnace to form SO2, which is then converted to sulfur trioxide (SO3), which is then scrubbed with water or a recycled weak sulfuric acid stream to yield saleable 98% sulfuric acid. Typically, 99.8% of the H2S can be recovered in a sulfuric acid plant.
More detail on the Claus, SCOT tail gas treatment, and sulfuric acid production processes are provided here.
Syngas Clean Up