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

Chapter 2In the moving bed process, coarse grained coal (4 to 30 mm) enters at the top of the gasifier viaa lock hopper and is evenly distributed over the cross-section of the surface of the coal bed. Thecoal moves very slowly downward, and the gas leaves at the top of the gasifier at flowratessmaller than the minimum fluidising velocity. Ash is removed at the bottom of the gasifier usinga revolving grid with slots through which the gasifying agents, oxygen and steam, areintroduced. The temperatures are lower than in the entrained flow gasification: 1000 °C in thelower section of the bed and around 600 °C at the top where the gas leaves. Compared to theentrained flow gasification, the raw gas contains a higher amount of CH 4 and CO 2 and a lowerCO/H 2 ratio because of the higher amount of steam, which causes the water gas reaction toproceed parallel to the partial oxidation reaction:C + ½ O 2 f COC + H 2 O f CO + H 2gH o = -100.6 kJ/molgH o = 175.4 kJ/molThe hot reaction gas (raw gas) is first quenched with recycled gas condensates from thesubsequent waste heat boiler. Because of the lower gasification temperature (a feature whichsaves oxygen) the raw gas contains a higher amount of impurities, e.g. tars, phenols and somehigher hydrocarbons, which are normally recovered from the gas condensates. Additionaladsorptive pre-cleaning is necessary before the gas can be fed to subsequent process steps suchas sulphur removal.2.2.4.5 Sulphur removalSulphur originating from the feedstock (up to 7 %) is present in the raw gas, mainly as H 2 S.Depending on the process configuration, the raw gas is cooled further under waste heat recoveryand scrubbed with a solvent, usually chilled methanol at -30 °C, whereby a CO 2 /H 2 S fraction isseparated and fed to a Claus plant. In this unit, H 2 S is converted to elemental sulphur bycombustion with air using alumina catalysts. Claus plants need an abatement system to reduceSO 2 emissions.In an alternative process, the raw gas is sent directly to the subsequent shift conversion withoutprior sulphur removal. In this case, H 2 S is removed after the shift conversion together with thetotal CO 2 formed there.2.2.4.6 Shift conversionDepending on the gasification design, i.e. waste heat boiler or quench, additional steam has tobe supplied ahead of the shift conversion by saturation and direct injection. The conversiontakes place step-wise over iron-chromium oxide shift catalysts with intermediate heat removal.A residual CO content of between 2 and 3 % can be attained. Over the past twenty years,sulphur resistant catalysts containing cobalt-molybdenum have in part replaced iron-chromiumoxide catalysts. These catalysts are active at temperatures ranging from 230 to 500 °C and allowshift conversion without prior sulphur removal. In this case, the sulphur is recoveredsimultaneously with the CO 2 after the shift conversion. For the performance of the cobaltmolybdenumcatalyst, the presence of sulphur compounds in the gas is essential.2.2.4.7 CO 2 removalAfter cooling the exit gas from the shift conversion, the process condensate is separated. Thegas is chilled and scrubbed with chilled methanol, which absorbs CO 2 and H 2 S. Strippingregenerates the loaded methanol. In the process version without sulphur removal ahead of theshift conversion, two fractions are received in the regeneration: one is pure CO 2 , which may beused for urea or other purposes, the other contains H 2 S/CO 2 and is fed to a Claus plant.46 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers