There have been many discussions regarding the economics of operating an endothermic gas generator as opposed to pre-mixed systems. It has been said of endothermic (endo) gas generators that they can be very temperamental during their operation. The reason that they can be perceived as temperamental is because of continually changing atmosphere and city gas conditions, including:
- Atmospheric pressure
- Atmospheric temperature
- Atmospheric humidity
Both oxygen and water vapor are always present in the atmosphere and will influence the gas-generation analysis.
Operating problems for endothermic gas generators include: sooting, variations in city gas analysis (hydrocarbon analysis), process gas-air ratio mix and gas cracking temperature. Sooting is caused by the failure to cool the cracked endo gas quickly enough, and a reversal of the formulation occurs.
2CO = C + CO?
This is generally caused by problems with the exit gas cooler before delivery of the cracked gas to the process furnace. So it is necessary to maintain a clean and functional exit-gas cooler.
The typical air-to-gas ratio of city gas/natural gas is 2.8:1. In other words, 2.8 volumes of air to 1 volume of gas. So it is necessary to monitor that relationship between gas and air.
The catalyst is simply porous refractory brick cubes soaked in a nickel-oxide solution. The objective of the catalyst is to enable the gas cracking reaction to occur without taking place in the reaction itself. The refractory brick cubes will act in the same manner that a sponge acts when placed into water. The sponge soaks up the water. The refractory brick cube soaks up heavy carbon ends contained in the incoming process gas from the mixing compressor. This is why it is necessary to have a gas chemistry that is as consistent as possible. With variations in the city gas/natural hydrocarbon content and analysis (particularly in winter operating conditions), variations in the gas chemistry occur even if the air-to-gas ratio is correct.
This is perhaps the most controversial of the endo generator maintenance requirements. Because the catalyst is soaking up heavy hydrocarbons (soot), it is necessary to burn out the generator. This is simply the action of burning the soot from the refractory brick cube in the presence of air and temperature. Many operators will burn out only once per month, while some endo gas systems do not get burned out at all (or only minimally).
The recommended practice by endo-gas manufacturers is once per week. This is not the writer recommending this practice. It is the generator equipment manufacturers that are recommending the procedure, which is very simple to conduct and occurs simply as a result of dropping the normal generator cracking temperature to a suitable temperature (approximately 1600°F), closing off the incoming process gas and blowing air through the process retort. This will cause the soot in the refractory brick cubes to ignite and burn.
A rise in temperature will be seen on the control instrument. When the temperature begins to reverse and go back to setpoint temperature (for burnout), burnout is complete.
Although the endogas generator is a very simple unit to operate, it can also be very sensitive. It does require an understanding of the generator operation by the operator, maintenance and management for the unit to be of 100% service to the heat-treatment operation.