Page 158 - Advances in bioenergy (2016)

Amount of methane and higher hydrocarbons

Catalyst poisons like sulfur-, nitrogen-, and chlorine components

Operation pressure

Synthesis gas consists mainly of hydrogen and carbon monoxide, in some cases small amounts
of carbon dioxide (methanol synthesis) or methane (BioSNG) are also present. For most
syntheses a H :CO ratio of 2 is required. This ratio is normally adjusted in a separate catalytic
2
reactor before the synthesis reactor, where some CO is converted to hydrogen by the water-gas
shift reaction. If the gasifier produces the correct H :CO ratio, the exothermal water-gas shift
2
reaction can be avoided, which reduces investment and operation costs and also increases the
efficiency.

Impurities like nitrogen act as inert during the synthesis and their concentration has to be as
low as possible. The inerts reduce the partial pressure and by this effect reduce the
conversion. Especially for synthesis reactions, where the product is separated as a liquid and
where a recycle of remaining unconverted gas is done (e.g., methanol), the inerts have to be
bled off as they would otherwise be accumulated. Also for production of BioSNG the inerts
have to be below 1 vol%, as otherwise the heating value of the BioSNG will not fulfill the
requirements of natural gas.

Methane and higher hydrocarbons normally act as inert during the synthesis reaction, so they
have to be treated similar to inerts. These components are mainly present in fluidized bed
reactors and not in high temperature gasification (Table 7.1). As the heating value of the

hydrocarbons is much higher than that of H and CO, a small amount of hydrocarbons already
2
present in the gas can contain most of the energy of the synthesis gas. So the overall conversion
from biomass to final product (e.g., FT liquids) is reduced by the small amounts of
hydrocarbons that are already produced in the gasifier. So in most cases the hydrocarbons are
converted to H and CO in a reformer to maximize the conversion from biomass to the final
2
desired product. The only exemption is the production of BioSNG. Here a high content of
methane and nontar hydrocarbons in the synthesis gas are favored, as then the highest
conversion efficiency to BioSNG is achieved.

Catalyst poisons deactivate the synthesis catalyst and have to be removed to very low levels.
The most well-known poison is sulfur, which can be in form of H S, COS, mercaptans, or
2
thiophens in the synthesis gas. The organic sulfur components are mainly present in fluidized
bed gasifiers, and not in high temperature gasification. So the removal technology has to be
adapted to the type of the sulfur components, e.g., thiophens cannot be removed by ZnO
adsorbers.

Almost all synthesis reactions are under elevated pressure and the synthesis gas has to be
compressed during the gas treatment. By the choice of a pressurized gasifier, the electricity

consumption of the compressor can be reduced or avoided. Until now only oxygen or oxygen-
steam blown gasifiers were operated under elevated pressure, in which case the investment
and operation costs for an air separation unit have to be taken into account. For this reason in