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