Compact PEMFC grade NH3 based hydrogen generation system

Ammonia is stored as a liquid under pressure (10 bar) in puncture-resistant aluminum tanks lined with a layer of UHMWPE.
The ammonia is then sent to the cracker heated by combusting a small amount of hydrogen. This hydrogen is combusted directly inside a module consisting of the reactor surrounded by high temperature vacuum panel insulation manufactured by Nanopore™ in New Mexico. The use of low thermal conductivity insulation allows for minimal heat loss, maximing reactor efficiency. Very low nitrogen oxide emissions are generated from the combustion due to the relatively low flame temperature required, the gas is passed to a micro three-way catalytic converter since the combustion is stoichiometric. NOx emissions are near zero. Air is supplied to the combustor via a small electrically driven rotary compressor, similar to a turbocharger. The cracker operates at 600-650 C, leaving 5-500 ppm of residual ammonia depending on the exact operating temperature, GHSV, and operating pressure. The catalyst used is a commercially available cesium promoted ruthenium on alumina micro powder catalyst marketed as HYPERMEC™ 10010 and manufactured by Acta S.P.A in Italy. The reactor utilizes proven microstructured technology manufactured by Fraunhofer ICT-IMM in Germany. The stability of the catalyst is over 99% over 900 hours.

The high-temperature gas exiting the reactor consisting of 75% hydrogen and 25% nitrogen by volume is sent to a liquid-cooled heat exchanger, similar to an EGR cooler on a diesel engine. The temperature is reduced to 80 C. At this point, the cool gas is sent to an acid impregnated carbon-based absorbent (Calgon Carbon Ammonasorb™) to purify to below 10 ppb. At this point, only nitrogen remains, which is reduced to less than 1% concentration using a polymeric membrane separator. Preferably, the polymeric membrane separation occurs prior to the ammonia purification, reducing the volume of the gas inside the purifier module. The polymeric membrane is mature technology, currently widely used for nitrogen production. Manufactueres include Ube and AIRRANE.
The now highly purified cool gas stream can enter the proton exchange membrane fuel cell, generating electricity at an efficiency of 59%. 25% of the hydrogen is required to provide the heat to reform the ammonia, the resultant net system efficiency is 45%. The entire system operates at 3.5 bar, this pressure is chosen to provide sufficient pressure to operate the nitrogen membrane separator, which is designed for pressures of 7 bar, the very high sensitivities of N2/H2 allow for easy separation. A lower pressure is favored in the decomposition reactor, thus a pressure below that ideal for gas separation is chosen as a compromise.
Since ammonia is caustic and poses major health risks if released in high concentrations, the tanks are puncture-resistant, which depending on the liner thickness, can prevent a .50 caliber bullet from penetrating from a 15-meter distance. The puncture-resistance comes from the high molecular weight polyethylene liner.
The power system has a wide range of applications, ranging from small generators to large scale heavy-duty propulsion.

Leave a Reply