Chlorine Compounds
Chlorine compounds are present in most coals. They will react with ammonia in the raw gas to form ammonium chloride (NH4C1). At high temperature this is (dissociated) in the vapor phase, but below 250-280°C it becomes solid and presents a fouling risk to the gas cooling train. At lower temperatures still, below the water dewpoint of the gas, it goes into solution and is highly corrosive. These aspects have to be considered in the design of the cooling train.
Metals in the feedstock will also form chlorides (e. g., sodium chloride). Many of these have melting points in the range 350-800°C and represent a fouling risk in heat exchangers.
Note also that chlorine is a catalyst poison for ammonia and methanol syntheses as well as for the low temperature shift.