High Temperature, High Pressure
The Monash Energy project will utilise the proprietary Shell Coal Gasification Process (SGCP). This section contains generic information on the SCGP.
The Shell Group started in gasification technology in 1956 with the development of oil gasifiers. Since then, some 40 syngas projects involving more than 100 Shell gasifiers have been developed or at the planning stage (figures current as at 2006).
The SGCP is a dry-feed, oxygen-blown, entrained-flow slagging process that converts coal into clean, medium-calorific-value syngas.
Efficiency
The syngas leaving the gasification section contains approximately 80% of the energy of the original coal; this value is knows as the coal gas efficiency. High- and/or medium-pressure steam produced during the SCGP contains approximately 15% of the coal’s energy, which can be recycled for process use or power generation.
The coal (or dried coal) is pressurized via a lock-hopper system and is then fed into the gasifier with oxygen and steam through pairs of opposed burners. The oxygen is produced in an air-separation unit.
The combination of coal, oxygen and steam keeps the gasifier operating in the 1400 to 1600°C range, which means that the ash in the coal melts and runs down the wall to the bottom of the reactor, from where it is removed as slag. The high temperature also prevents the formation of undesirable pyrolysis by-products; only traces of methane remain.
By-products
The by-products of the SCGP are inert slag and fly ash. Sulphur compounds removed from the feedstock are converted into elemental sulphur. The waste water from the plant can be reused after treatment to remove relatively small amounts of salts.
About 99% of the gaseous sulphides are removed from the scrubbed syngas in the desulphurisation unit.
Shift System
For coal-to-liquids applications it is desirable to increase the H:CO ration in the syngas via what is known as a Shift process. The synthesis gas from the gasification area Sour Gas Scrubber is split with approximately one third entering the CO Shift Reactor. Medium pressure steam is added to the synthesis gas flow to the CO Shift Reactor where CO and H2O react to form CO2 and H2. The shift effluent is combined with the remaining bypass stream of synthesis gas to feed the hydrolysis reactor.
The combined stream enters the hydrolysis reactor where COS and HCN are hydrolysed to form H2S, NH3 and CO. The effluents from the hydrolysis reactor are cooled. The water containing some dissolved gases is directed to the Sour Water Stripper and the syngas exits to the Acid Recovery Unit.
