In ‘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.

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

INDIRECT TYPE POLLUTION-FREE TAR SEPARATION
In this innovative gas cooling & tar separation system, gas is cooled through indirect water circulation. Tar is separated through centrifuging. Water and gas do not come in contact. So, there is no phenolic water generation – no water pollution.Tar comes out through the bottom of the separator and is collected. Almost 90% tar is separated in this simple process. Separated tar can be either collected for use/sale or recirculated back to the gasifier. For removal of residual tar, if the application so desires, electrostatic precipitators are employed.

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 or consumed