Gasification

Gasification rigiems

There are basically two regimes of gasification,

1. Low Temperature Gasification (LTG) of around 650-700°C and 
2. High Temperature Agent Gasification (HTAG) of above 700^oC regimes

(a)                Low Temperature Air Gasification (LTAG)

It  involves gasification process at temperature levels below 700^oC hence some tars remain in the syngas as residue as they do not crack into more volatile component. It is a product of highly irreversible process taking place in the pyrolysis zone.  The presence of tars in the syngas renders the gas suitable for use in most engines and gas turbines. The other main problems associated with low temperature gasifiers is the low calorific value of the gas of about 3-5MJ/kg.

 Syngas has also the characteristics of poor operational performance due to high ash content of more than 20%, the original tar solid content is great at 10g/m³  for some biomass.  The poor performance of low temperature gasifiers in terms of low syngas quality and low yield can partly be overcomed by the deployment of high temperature gasification process technology. 

(a)                High Temperature Agent Gasification (HTAG)

The High temperature agent gasification (HTAG) was developed in recognition of the emerging environmental concerns, the need of energy efficiency and sustainability of energy supply. A relatively clean gas is possible to be produced by increasing the gasification temperature. The HTAG technology uses a highly preheated air/steam as a gasifying media. Preheated air in excess of 1000^oC is usually utilized, and is exceeding the ash melting point of gasified materials. In this way, ash disposal problems and trace metal limits are avoided as they are included in sludge.

During the HTAG gasification process , a highly preheated gasifying agent, normally air is produced by means of a pre-heater, which is equipped with a pair of ceramic honeycomb heat storage beds.  The honeycomb heat storage beds are used as regenerator media. The operating principle of the device is that flue gas and air pass through the honeycomb heat storage beds alternatively, set at short switching periods, of 45 seconds.  In this way, highly preheated gasifying agent is generated for use as the biomass gasification agent. While in operation, part of the produced syngas is burnt to enhance the gasifying agent temperature.

As the syngas exits the gasifier at higher temperature in the range of 1200°C, a low-pressure boiler can be utilized to use the syngas thermal energy.  Before use, the syngas is passed through a gas cleaning system comprising of cyclones and filters. Here the clean syngas is also cooled further to about 25°C, ready for combustion in a furnace, gas engine or turbine depending on the desired thermal energy or for electricity production.