Advantages of Gasification
One major advantage of gasification is its ability to offer a variety of products. Many of these products are discussed, in detail, in the section Applications of Gasification Technology. This section discusses the advantages of such product flexibility.
A properly equipped gasification plant (co-producing, see below) can vary production of power, fuels, gases, chemicals, or other products to meet demand or need, or maximize profit. Synthesis gas (syngas), the intermediate product of gasification consisting of mainly hydrogen (H2) and carbon monoxide (CO), allows for this flexibility as it can be processed into a multitude of products: liquid fuels like diesel, gasoline, jet fuel, etc.; hydrogen or synthetic natural gas (SNG); a range of fertilizers and other high-value chemicals, like anhydrous ammonia, ammonium sulfate, sulfur, phenol, and naphtha. In addition, several useful byproducts of gasification can be sold, like carbon dioxide (CO2) for enhanced oil recovery (EOR) and slag from coal ash. Other salable gases can be recovered from the air separation process required by some gasifiers.
Worldwide, 45% of all gasification-produced syngas is used in the production of chemicals and 28% for the production of liquid transportation fuels (Fischer-Tropsch liquids). Power and gaseous fuels are produced from 19% and 8% of global syngas production, respectively. Fischer-Tropsch liquids were planned to make the largest increase in capacity by 2010, with 69% of new syngas production going toward the manufacture of liquid transportation fuel. The next largest expected growth in production capacity was chemicals (22%) and then power (9%), with no growth planned for gaseous fuels before 2010.
A gasification plant that can vary or produce multiple products is called a co-producing or polygenerating plant. For example, a co-producing gasification power plant can produce more electric power when prices are high (peak periods), but then during off-peak periods devote more syngas to another product (liquid fuels, for example). Co-production plants can also provide energy storage potential, as syngas, synthetic fuels, or other products produced during off-peak times to be stored for later use. Product flexibility helps manage investor risk, as plants can more easily respond to changes in product markets. A good example of co-production is the addition of ammonia-based fertilizer production at the Great Plains Synfuels Plant, discussed below.
Example: Great Plains Synfuels Plant
The Great Plains Synfuels Plant (GPSP) in Beulah, North Dakota, gasifies locally mined lignite coal and processes it into SNG. The plant’s management, the Dakota Gasification Company, has expanded into different products—ammonia and ammonia-based fertilizers—taking advantage of gasification’s product flexibility to reduce the potentially negative effects of a volatile natural gas market.
The plant uses a Rectisol unit to remove impurities like sulfur compounds and CO2 from the syngas. In 1996-97, the plant’s layout was altered because of a problematic sulfur recovery system (Stretford unit). Acid gases from the Rectisol unit’s waste stream were diverted to the plant’s boilers rather than to the Stretford unit and a Flue Gas Desulfurization (FGD) unit was installed to scrub the boiler flue gases. This is discussed in greater detail in the page on Useful Byproducts – Sulfur and Other Chemicals, but essentially the FGD unit scrubs sulfur dioxide (SO2) from the flue gas using ammonia. This produces ammonium sulfate as a byproduct. This can be sold as a fertilizer, and is specifically marketed as Dak-Sul 45 by the plant. Approximately 110,000 tons are produced and sold annually, which helps offset the cost of gas cleaning.
In order to meet the ammonia requirements of the FGD and diversify the plant's production, an ammonia plant was added in 1997. The unit produces anhydrous ammonia from syngas diverted just after the Rectisol unit. Anhydrous ammonia not needed for the FGD unit is sold for use as an industrial chemical or fertilizer.
Adding an additional “primary” product like anhydrous ammonia has allowed the GPSP to adapt to market changes. For example, the ammonia plant is used during periods that coincide with seasonal fertilizer markets, maximizing profits. During downtime, the full production of syngas can be put towards SNG manufacture. The production of ammonia can also be increased as ammonia and SNG prices fluctuate. For example, early in the plant's life, SNG was a more valuable commodity and so ammonia production was aimed at meeting the plant's needs, but recently ammonia prices have risen to equal or exceed equivalent production of SNG. This type of flexibility allows a gasification plant to minimize risk.
While SNG and the ammonia fertilizers are “primary” products, the plant also produces several useful byproducts, including chemicals like phenol and cresylic acid, rare gases krypton and xenon, and naphtha, much of which is burned in plant boilers. Carbon dioxide is also piped to a Canadian oil field for use in EOR. For more information on the GPSP, please visit the Great Plains Synfuels Plant section.
Many of the major products that can be produced by gasification are covered, in depth, in the section Applications of Gasification Technology, but one of the great advantages of gasification is the usefulness of its byproducts. To learn more about the useful byproducts of gasification, continue on to the next section. Some of the byproducts discussed include: