Ethylene from gasification

One can envisage the future production of liquid fuels and commodity chemicals in a biorefinery Biomass is first subjected to extraction to remove waxes and essential oils. Various options are possible for conversion of the remaining biofeedstock, which consists primarily of lignocellulose. It can be converted to synthesis gas (CO + H2) by gasification, for example, and subsequently to methanol. Alternatively, it can be subjected to hydrothermal upgrading (HTU), affording liquid biofuels from which known transport fuels and bulk chemicals can be produced. An appealing option is bioconversion to ethanol by fermentation. The ethanol can be used directly as a liquid fuel and/or converted to ethylene as a base chemical. Such a hiorefinery is depicted in Fig. 8.1. 

Derivation (1) Air oxidation of ethylene followed by hydration of the ethylene oxide formed (2) ace-toxylation (3) from carbon monoxide and hydrogen (synthesis gas) from coal gasification (4) Oxirane process. 

The pyrolysis of hydrocarbons follows the thermal cracking mechanism (4). Apart from the pressure, the conditions in the tubular steam reformer and in the preheater are not far from that of a steam cracker in an ethylene plant. With low catalyst activity, the pyrolysis route may take over. This is the situation in case of severe sulphur poisoning or in attempts to use non-metal catalysts so far showing very low activity (1). Non metal catalysts have mainly been based on alkaline oxides being active for gasification of coke precursors. However, it has been difficult to avoid the formation of olefins and other pyrolysis products (1,2,5). In fact, it was demonstrated (2,4) that co-production of syngas and light olefins was possible from heavy gas oil and naphtha over a potassium promoted zirconia catalyst. 

The initial results published indicated that a clay-reinforced carbonaceous char formed during combustion (or pyrolysis) of nanocomposites. This is particularly significant for systems whose base resin normally produces little, or no, char when burned alone (PS, PP-g-MA, PA6, and EVA [poly(ethylene-co-vinyl acetate)]). The char formation in PS clay nanocomposites is easily seen in the images from nitrogen atmosphere gasification shown in Figure 3.2. 

Figure 6.1 Schematic process of fermentation and gasification routes for ethylene production from renewable feedstock.

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