By far the most widely and extensively used intermediate for making nitrogen fertilizers is ammonia, with over 80% of world-wide ammonia production going towards synthesis of fertilizers such as urea. And by most measures, ammonia production is the largest of any bulk-produced chemical.
Most ammonia is manufactured by steam reforming of natural gas, followed by water gas shift, CO2 separation to isolate pure hydrogen, which is then reacted with nitrogen to form ammonia in the Haber-Bosch process. Because reformed natural gas results in syngas with relatively high hydrogen content, it is well-suited for ammonia synthesis. However, in regions lacking inexpensive natural gas, such as China, coal gasification is a very important route for ammonia synthesis for production of fertilizers. In 2008, out of global ammonia production of 146 million tonnes/year, 27% used coal as the feedstock; 97% of China's ammonia production used coal in that year, a trend which is expected to continue and increase.
Ammonia and Urea Synthesis
Figure 1 shows a simplified BFD of the Farmland Coke-to-Ammonia plant, to illustrate the typical flow scheme of a gasification-based ammonia and urea production process. All component technologies are extensively, commercially proven. Per Figure 5, high purity H2 is produced from coke gasification. It is combined with a high purity nitrogen (N2) stream extracted from the air separation unit (ASU) to produce ammonia according to the following reaction:
||3 H2 +
||- 23,400 Btu/lbmol NH3
The reaction is carried out typically between 370 and 540°C, and at high pressure (> 2,000 psi) over an iron-based catalyst to achieve reasonable conversion per pass. Ammonia synthesis is highly exothermic, and the reaction is equilibrium limited. Per pass conversion is increased by allowing the reaction to take place in successive catalyst beds, arranged as two to four adiabatic conversion stages. Some form of intercooling and/or dilution is applied between stages to allow the reaction to continue.
As an alternative to iron-based ammonia synthesis catalysts, ruthenium on graphite catalysts are offered (e.g. KBR's KAAP™ catalyst) that have intrinsic activity ten to twenty times higher than conventional magnetite (or wustite) catalyst, and which allow ammonia synthesis operation at lower pressure, i.e. 1300 psia as opposed to > 2,000 psi, which reduces design complexity and capital equipment costs. The main drawback is the relatively rapid deactivation of the catalyst from consumption of the catalyst’s carbon support.
Companies that supply the proprietary commercial technology and/or processes for ammonia synthesis include Haldor Topsoe, KBR/Haliburton, Linde/Lurgi and Uhde.
With the Farmland ammonia plant, part of the CO2 captured from the Selexol acid gas removal process is purified and reacted with a stream of the ammonia product to produce urea, according to the following reaction:
||2 NH3 +
Technology licensors offering ammonia-to-urea technology include: Snamprogetti, Stamicarbon, and Toyo Engineering Corp.
Figure 1: Farmland Coke-to-Ammonia Block Flow Diagram1
The SINOPEC Hubei Chemical Fertilizer Company is located on the Changjang River in the Hubei Province of China, adjacent to the city of Yichang. The plant has operated for 30 years producing more than 12 million tons of urea for use in agricultural production. Gasification was introduced to the plant after three years of construction with completion in December of 2006.
The SINOPEC Baling Company is located in Yueyang City of Hunan Province, China. This facility began using Shell’s gasification technology in 2007 to produce chemical fertilizers.
The SINOPEC Anqing Company is located in Anqing City in Anhui Province, China. As a means to decrease urea production costs, Shell gasification technology was used to switch the raw material from oil to coal. This project was initiated in 2001, with construction beginning in 2004. Operation was commenced in late 2006.
Ube Ammonia Co., Ltd.
DOE R&D Advantages
Ube Ammonia Co., Ltd. in Japan owns and operates a coal to ammonia plant which uses GE gasification technology (formerly Texaco). Production on the plant began in August 1984. When constructed, plant capacity was 350 tons of ammonia per day.
Considering that the building block of nitrogen fertilizers is ammonia, which in turn is synthesized from hydrogen and nitrogen, DOE's R&D programs in the area of low-cost and efficient hydrogen production and air separation (which produces nitrogen as a major by-product), can be thought to be the fundamental R&D that will have the incidental benefits of improving cost-effectiveness of fertilizer production as one of many downstream benefits. Given that fertilizer synthesis technology is mature, technological advance to lower cost of its primary feedstocks is the route towards overall improvement in this area.