UPDRAFT GASIFIERS – Cold-Clean gas mode

Cold Clean Gas ModeIn ‘Cold-clean Gas Mode’, gas exiting from the gasifier is cooled and cleaned before being conveyed to the utilization equipment. Since the gas is to be cooled, it is attempted to utilize most of the sensible heat of gas within the gasifier reactor. Therefore, the gas is taken out of gasifier at minimum possible temperature. The gas is then cooled and cleaned utilizing an elaborate system consisting of coolers, scrubbers, filters, electrostatic precipitators, etc. The exact design and configuration of gas cooling-cleaning system depends upon the requirement of the application.

Tar is separated, collected and sold or used in-house. It can be used either as a fuel or chemical feedstock. Water treatment is designed as integral part of the entire system.

While the gasifier reactor efficiency is higher as compared to ‘hot gas mode’ due to internal heat utilization, overall system efficiency would be lower as sensible heat of gas is lost when it is cooled.

Calorific value of cold gas is higher as it contains larger fractions of hydrocarbons. It would be in the range of 1250 – 1400 kcal/Nm3. Total energy content in cold gas would be lower than that in hot gas.

ADVANTAGES & SUITABILITY
 Clogging of pipelines and other gas passages is insignificant as compared to ‘hot gas mode’
 Gas can be conveyed over long distances, even kilometers
 Long / convenient time intervals between shutdowns for pipeline maintenance (can be planned with rest of the plant shutdowns)
 Suitable for installations requiring long or distributed gas pipeline network
 Required when the gas utilization is likely to be affected even by small amounts of fine dust in gas
 Required when gas needs to be distributed to several usage points
 Suitable when fine controls are required at individual burners
 
DISADVANTAGES & LIMITATIONS
 It’s a higher cost option
 Internal power consumption, required for various gas clean-up stages, is higher
 Water consumption is significant
 Water treatment systems need to be incorporated in design
 Complex equipment, many subsystems are involved
 Overall system efficiency is lower as sensible heat of gases is lost
 Tar handling is an additional operation, which needs to be separated, collected and sold (consumed)