Ammonia, chemical formula NH3, is a caustic (but not toxic) gas at room temperature and pressure but it is easily stored under moderate pressure just as propane is. It is used as an industrial chemical, both directly and indirectly as a fertilizer, and there is a growing movement to use ammonia as a fuel; it is the only carbon free hydrogen carrier we can make and handle with today's technology.
I've been working in this area for the last ten months and I thought I'd provide a little overview of how the stuff is made and where we need to go next to ensure our food supply and begin the transition to this clean hydrogen carrier as a fuel.
Ammonia today is made using the Haber-Bosch process invented in 1910. Pure nitrogen and hydrogen are reacted at high temperature and pressure producing the ammonia. The process itself actually produces energy as the hydrogen and nitrogen are being combined, but the entire synthesis process, which includes separation of nitrogen from the air and the various means of obtaining hydrogen, all require energy from either electricity or fossil fuels.
Most of the hydrogen used in ammonia production today is made via two reactions that begin with natural gas. The first uses steam to strip the four hydrogen atoms from the methane molecule's carbon atom and the second uses the carbon monoxide produced in that reaction to strip two more hydrogen atoms from a water molecule. More than 70% of ammonia production uses this method, with the rest being coal synthesis and that is happening primarily in developing countries.
Both coal and petroleum coke can be used in a gasifier to produce the same effect as the reaction done with methane. Coal and petroleum coke have a formula of roughly CH, as opposed to methane's CH4, and thusly much more CO2 is produced. There are a few plants in the U.S. that do this now and only one of the U.S. based operations sequesters their CO2 by selling it to oil producers for well pressurization.
Hydrogen may also be produced by stripping it from water molecules using electrolyzers. These systems consume enormous amounts of electricity and produce a great deal of heat as well as hydrogen. Electrolysis was the first hydrogen production technology used but it has always been a relatively small component. The last electric powered plant belonging to Norsk Hydro was shut down just seventeen years ago in response to low cost ammonia based on North Sea gas.
There are exciting developments in the area of ammonia synthesis that are currently at the bench top stage and need to be taken to pilot stage next.
Drs. John Holbrook and Jason Ganley of NHThree, LLC have created a method which is called solid state ammonia synthesis. The reaction chamber is an array of electric powered ceramic tubes into which water and nitrogen are fed and ammonia is produced. The technology is basically a fuel cell made to run in reverse, turning water, nitrogen and electric energy into ammonia. Capital costs and power usage will be dramatically lower with this method compared to Haber-Bosch. The biggest benefit will be that this is a modular technology with no lower bound to its size, unlike the Haber-Bosch systems which have a minimum reactor and support equipment size that runs into the millions. Solid state ammonia synthesis can start with a single tube and a wall power socket.
Dr. Ed Cussler and Ph.D. candidate Mark Huberty are working on a modification to the original Haber-Bosch method that would allow batch production using a simple salt to absorb the ammonia. Traditional Haber-Bosch based plants and very large and must be run continuously but wind power is only available intermittently. If this approach is proven to production scale a single wind turbine could be found driving a electrolyzer, nitrogen separation unit, and a small reactor with the production being sufficient to support corn crops on several square miles of land.
Dr. Keith Lovegrove and Ph.D. candidate Rebecca Dunn are working on a solar thermal system using ammonia as the working fluid. This scheme will permit overnight storage of solar energy and when excess power is being made the system will serve as an ammonia production source as well as a storage system.
The trend to smaller operations might surprise some people but there are several drivers for this. U.S. ammonia plants have declined in recent years as local gas prices and labor costs climbed until we reached the point where 75% of production is offshore. Prices then spiked to triple their 2006 level. Farmers are the primary user of ammonia and for some crops they've simply stopped fertilizing and accepted the yield losses. The smaller ammonia plants of all types are amenable to use with renewable resources such as hydroelectric, wind, or solar, and this plays well with the carbon taxes anticipated to start with the new U.S. administration.
Right now our combined production and importation of ammonia is not sufficient for the crops we grow. Given the dollar's long term weakness, decreasing gas supplies, and the potential to use ammonia as a liquid fuel we can not build new plants of all types too quickly.