Advantages of Gasification - Product Flexibility - Useful Byproducts
Sulfur and Ammonia
In the course of cleaning synthesis gas (syngas) in a variety of gasification applications, several chemicals can be removed and processed into useful products. This page first looks at the chemical byproducts of the Great Plains Synfuels Plant (GPSP) and then sulfur and ammonia in general.
Example: Great Plains Synfuels Plant
Run by the Dakota Gasification Company, the GPSP in Beulah, North Dakota, gasifies locally mined lignite coal and processes it into synthetic natural gas (SNG). It has taken advantage of gasification’s product flexibility to adapt to changing product markets, by adding an ammonia synthesis plant in the late 1990s.
The GPSP also produces a number of saleable byproducts, in particular ammonium sulfate. The syngas produced by the gasifiers is cleaned in a Rectisol unit (see Gas Cleaning for more information). The acid gas streams from this unit are sent to a boiler that has its flue gas scrubbed. A flue gas desulfurization (FGD) unit uses ammonia to remove sulfur (in the flue gas as sulfur dioxide [SO2]), producing ammonium sulfate [(NH4)2SO4], a fertilizer. The sale of ammonium sulfate helps to recoup some of the cost of cleaning the syngas—required for further processing and by environmental regulations.
Sulfur is a component of coal and other gasification feed stocks. The sulfur typically must be removed from the gasifier syngas product prior to additional processing.
There are many methods for syngas clean-up, and specifically for sulfur removal, which is normally separated from the syngas stream as hydrogen sulfide (H2S) and processed into elemental sulfur. These options are detailed in the sections on Syngas Clean-Up and Sulfur Removal, but are discussed in general here.
In a typical design, a process called MDEA/Claus/SCOT is used. Each of these processes can be used independently, but many plant designs (e.g., NETL IGCC Base Cases) run similar sulfur removal systems. Methyl diethanolamine (MDEA) is a chemical solvent that is used as an absorber. In the gas cleanup process, cooled syngas is contacted with the MDEA in an absorber unit, where, as the gas passes through, almost all of the H2S and some carbon dioxide (CO2) are removed. The MDEA is now rich with H2S and it moves on to a stripping unit to clean and recycle the MDEA and separate the H2S.
The syngas that passes through the MDEA absorber still contains some sulfur compounds and so this gas continues on to the Claus/SCOT unit. In the Claus unit, the century-old Claus process turns H2S and SO2 into elemental sulfur. A catalyst, typically titanium or alumina based, is used to facilitate the reaction.
The last commonly used unit in this desulfurization process example is the Shell Claus Off-gas Treating (SCOT) unit. This process removes most all of the remaining sulfur—in the form of a tail-gas stream of unreacted sulfur, H2S, SO2, and carbonyl sulfide (COS)—that the MDEA and Claus units miss. The SCOT unit uses a cobalt-molybdenum catalyst to convert SO2 to H2S, which is then removed in an absorber. The H2S rich stream continues on to another stripping unit, while the remaining acid gas that passes through the absorber is recycled to the Claus unit.
Finally, the H2S-rich streams are stripped from their solvents. The stripper units remove the H2S, and the solvent is regenerated and stored for reuse. The H2S then continues to a sulfur plant, where it is converted to elemental sulfur and stored with the sulfur generated in the Claus unit. Alternatively, sulfur compounds can be converted to SO2 then to sulfur trioxide (SO3) and finally to sulfuric acid (H2SO4) in a sulfuric acid reactor.
Source: Simplified Diagram of the AGR Process
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. Being able to recover saleable sulfur is an important economic benefit.
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 similar MDEA system to remove CO2, byproduct ammonia can be recovered. Following the MDEA absorber, some of the CO2 gas contains what is called “sour water.” This sour water condenses as it is cooled. It dissolves almost all the nitrogen compounds and any chloride and fluoride present. It also dissolves smaller amounts of H2S, COS, CO, and CO2. The water is removed in low temperature syngas coolers and a knock-out drum (vapor-liquid separator), where it is then sent to a sour water treatment system.
Ammonia is recovered as anhydrous ammonia which can then be used in selective catalytic reduction (SCR), converted, or sold as is for fertilizer or other chemical markets.
A general overview of the CO2 scrubbing process is given in the 2006 paper, Impact of CO2 Capture on Transport Gasifier IGCC Power Plant, commissioned by DOE and partners.