FEV Engine Technology has a more compact system compared with liquid urea NOx exhaust aftertreatments.  

    Exhaust emissions is not a very fun topic. No one will ever write a song about their catalytic converter with as much feeling as one about a fine 409. Let's wallow through this subject together. Misery loves company.  If we must have an aftertreatment for our exhaust one like FEV's might be the easiest to swallow. 

Instead of using a solution of urea and distilled water ( base) to quench the NOx (acid) the FEV's system uses ammonia carbamate. This results in a volume reduction of 70% compared to liquid urea. Plus the solid is more stable over time. Refilling the replaceable solid cartridge seems easier than dealing with liquids. Another benefit is the solid provides an inherently pre-heated gas to be introduced to the exhaust. Sublimation (solid to gas) occurs at 60 degrees Celsius this means heating systems are not as crucial compared with liquid urea that freezes easily.