Formic Acid
At higher partial pressures carbon monoxide will react with water to form formic acid according to the equation
CO + H20 S HCOOH -34.6 MJ/kmol (6-3)
The thermodynamics of the reaction favor formic-acid formation at lower temperatures so that this is particularly noticeable in the gas condensate.
At pressures up to about 60 bar there is usually sufficient ammonia formed to maintain a neutral pH in wash water. This is therefore seldom mentioned in connection with coal gasification because such pressures have only been seen in pilot plants operating under test conditions. It is, however, a phenomenon that has been observed in high-pressure oil gasification and requires consideration in material selection (Strelzoff 1974).
It is necessary to distinguish between two very different sources of carbon, which can occur in raw synthesis gas.
In coal gasification there is always a certain amount of the initial carbon feedstock, which is carried over unconverted in the form of char as particulate matter into the gas. Typically, this can be extracted from the gas in a particulate filter and—in case of low carbon conversions— recycled to the gasifier. In the case of slagging gasifiers, this form of recycling has the added advantage that this carbon is usually intimately mixed with dry ash, which can also be recycled for slagging.
In contrast to coal gasification, the carbon in synthesis gas leaving an oil gasifier is actually formed in the gasifier itself. The soot leaving an oil gasifier has an extremely high surface area of 200-800 m2/g, depending on ash content (Higman 2002).
An oil gasifier is deliberately operated to maintain a small quantity of this soot in the raw gas as an aid to sequestration of the ash from the reactor, whether of the quench type or with a syngas cooler. Typically, the soot make in modern plants is about 0.5% to 1.0% of the initial feed, although it could be as much as 3% in older plants. Removal of this carbon with a water wash is an integral part of all commercial oil-gasification processes.