Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 22.2.4.2 Gasification of heavy hydrocarbonsThe gasifier consists of an empty pressure vessel lined with heat resistant alumina bricks.Hydrocarbons, oxygen and steam are introduced through nozzles and react non-catalytically toyield carbon monoxide and hydrogen at elevated pressures up to 80 bar:2CH n + O 2 f 2CO + nH 2Apart from CO and H 2 , the reaction gas (raw gas) contains approximately 3 – 5 % CO 2 ,approximately 0.2 % CH 4 and 0.5 % soot depending on the quality of the feedstock. The CO/H 2ratio depends on the feedstock composition and the quantity of steam added to atomise thefeedstock and to moderate the reaction, which raises the temperature in the gasifier up to1400 °C.The hot raw gas containing soot is cooled either by water quench or in a waste heat boiler. Thecarbon is subsequently removed by water scrubbing in a packed tower.2.2.4.3 Soot removalDifferent methods can be applied for recovering and recycling the soot. In one process, it isextracted with naphtha. After separation from the water, the soot-naphtha suspension is mixedwith the hydrocarbon feedstock and the naphtha is topped off in a distillation column. Thetopped naphtha is recycled to the extraction section. The resulting carbon/heavy hydrocarbonmixture is recycled to the partial oxidation reaction. Another variant extracts the soot with lightgas oil in the form of carbon pebbles. These are screened off and recycled to the heavyhydrocarbon feed. The extracted water is returned to the soot scrubbing section. Heavy metalssuch as nickel and vanadium (introduced with the feedstock) are suspended as oxides and arepartially also present as salts in the soot water circuit. To prevent an accumulation of thesecompounds in the water circuit, some of the extracted water has to be drained. The drainedwater is cleaned by flocculation, applying settlers and/or filters, and disposed of after abiological treatment. A paste containing vanadium and nickel is recovered and sold to themetallurgical industry.2.2.4.4 Gasification of coalFor ammonia production, there are presently two gasifier types commercially used, the“entrained flow gasification” and the “moving bed gasification”. The applied pressures rangefrom 30 to 80 bar.The entrained flow gasification corresponds basically to the partial oxidation concept of heavyhydrocarbons and takes place in an empty pressure vessel. The differences from the partialoxidation route are mainly in the method of introduction of the feedstock into the gasifier. Thecoal is fed to the gas generator either as dry dust via lock hoppers and rotary feeders orintroduced as concentrated water/coal slurry by means of a reciprocating pump. Flow directionand use of waste heat boilers or quench and their degree of process integration may differ in theindividual processes as well as the provisions to collect and remove the slag at the bottom of thegasifier. The separation of entrained coal dust from the raw gas, containing some fly ash, is verysimilar to carbon removal in the gasification of heavy hydrocarbons. Reaction temperatures arearound 1500 °C. The raw gas has a low methane content (0.4 %), a moderate CO 2 content andCO/H 2 ratios larger than 1.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 45