Dipl. Ing. Matthias Mayerhofer Technische Universität München ...

10 Biomass Gasification
poses. It is important to state the requirement to maintain the gas temperature above acid dew
points. Hence, hot gas cleaning is the most appropriate method to maintain the desired temperatures.
This system could improve energy efficiency and lower operational costs for high temperature
applications of the product gas such as H2 production (by the shift reaction), use in combined
cycle systems and power generation by a fuel cell (Mondal,2011), (Wang,2008).
2.3 Hot Gas Cleaning
The temperature at which the gas cleaning takes place depends on the technology that is being
used and of course the requirements of the downstream application of the product gas.
Principal Gas Impurities (Kohl,1997):
1) Hydrogen Sulfide
2) Carbon Dioxide
3) Water Vapor
4) Sulfur dioxide
5) Nitrogen oxides
6) Volatile Organic Compounds (VOCs)
7) Volatile Chlorine Compounds (e.g. HCl, Cl2)
8) Volatile fluorine compounds (e.g. HF, SiF4)
9) Basic Nitrogen compounds
10) Carbon Monoxide
11) Carbonyl Sulfide
12) Carbon Disulfide
13) Organic sulfur compounds
14) Hydrogen cyanide
Generally, to achieve removal of vapor-phase impurities from hot gas streams five main procedures
exist. Mostly used procedures are the absorption into a liquid and the adsorption on a solid.
The firsts consist of the transfer of a gas phase component to a liquid phase, which is soluble. The
latter involves the selective concentration of one or more components of a gas at the surface of a
micro porous solid. In this case the adsorbent can be released (desorbed) by raising the temperature
or by reducing the partial pressure of the component in the gas phase. Alternative methods are
permeation through a membrane, chemical conversion to other compounds and condensation
(Kohl,1997).
The possible procedures which are being used in order to achieve clean gas are various. The order
in which the impurities are being removed plays an important role. For biomass gasification the
basic traces that should be removed are sulfur compounds, alkali species, halogens, nitrogen species,
metal traces, particulates and tar. In order to minimize the number of separation stages the
following procedure is suggested (Sharma,2010). First the halides and alkalis are being separated
from the raw gas via a sorbent then desulfurization occurs, particulates removal is followed and
finally catalytic tar decomposition which leads to a clean syngas. In most experimental facilities
though, the particulates removal is prior to the separation of the various traces from the syngas and
tar removal is the final stage. In the case physical removal of tar can be achieved as the tar condenses
on the particulates surface. It is essential to operate this procedure at high temperatures
(800-900 °C), otherwise tar condensation on particulate surfaces can result into gas cleaning
equipment plugging and fouling problems. The acceptable availability is 95-99% (Sharma,2010).
The most widely used mechanical technologies are cyclones, bag filters, and electrostatic precipitators
and baffle filter (Wang,2008).
However, if this order is applied it is possible that coke, which is formed due to thermal tar cracking,
plugs the filter and decreases its efficiency. For this reason the use of monolith catalysts, honeycomb
structures is an appealing alternative as they can operate with particulate containing gas.
Hence, tar is firstly removed and then the particulates are being filtered (Corella,2004). These