Ammonia in Syngas
Posted by Andy Olds on Fri, Aug 14, 2009 @ 08:00 AM
The primary products of synthetic gas (syngas) production from biomass are hydrogen gas, carbon monoxide, and methane. Unfortunately, those are not the only compounds formed. Other compounds form depending on the elemental chemistry of the biomass. One of the more common byproducts is ammonia, released from organically-bound nitrogen.
Why ammonia rather than NOx
Organically-bound nitrogen converts to ammonia, rather than NOx, in a gasifier. Gasifiers are typically oxygen-starved environments. Conversion of organic nitrogen to NOx requires oxygen. Without oxygen, combustion thermodynamics favor the production ammonia. In fact, ultra-rich combustion environments produce reduced compounds, like ammonia, rather than oxidized compounds.
Why it is still NOx
The main goal in the production of syngas is to eventually burn it! Ammonia left in the syngas WILL convert to NOx in a rich combustion environment, like an internal combustion engine or a syngas-fired boiler. NOx is a heavily controlled environmental pollutant and it is important to minimize its production.
How do I remove it
Fortunately, ammonia is significantly easier to remove than NOx. Ammonia can be removed with an ammonia scrubber using water and sulfuric acid. Ammonia reacts with the sulfuric acid to form ammonium sulfate. Provided any particulate is removed upstream, an ammonia scrubber can produce a very high concentration of liquid ammonium sulfate that has commercial value as a fertilizer. Since sulfuric acid is relatively cheap, the operating costs of an ammonia scrubber are minimal.
Ammonia gas can also be formed using a scrubber/stripper approach, first removing the ammonia in an ammonia scrubber, and then liberating the ammonia as a gas in an ammonia stripper.
Photo Credit: ajturner