Page 25 - Impiantistica Italiana Maggio Giugno 2014

Table 2 - Tar removal
technology comparison

in the fuel as well as the main limestone reactions into “light tar” and “heavy tar”, according to their
[2]. molecular weight being lower or greater than about
The 3D CFB model has been further developed 200 g/mol. Some examples of tar are: pyridine,
from only solving combustion reactions to handle toluene, anthracene, naphthalene and pyrene.
the most important gasifcation reactions as well: The presence of tar in the syngas exiting the gas-
the water gas, the Boudouard and the shift con- ifer is one of the problems of primary concern for
version reactions. The 3D CFB gasifcation model the gasifcation of biomass, regardless of whether
is applicable for both air and oxygen-steam blown the purpose is power generation or chemical pro-
applications both in atmospheric and pressurized duction. In fact, the presence of organic com-
conditions. The model is able to estimate for ex- pounds entails signifcant problems associated
ample the concentrations of the main gas compo- with the fouling of some equipment (heat exchange
nents, bottom and flter ash compositions, carbon surfaces, flters, adsorbent etc.) and tcondensate
conversion, temperature levels and reaction rates contamination during the synthesis gas cooling
in different stages of the gasifer. process.
In the following case study the model has been The formation of tar appears to be signifcant for
applied into calculation of a 50 MW air blown ga- temperatures below 1000 °C and the quantity and
th
sifer. Fuel and secondary air are introduced into composition mainly depends on the feedstock
the system from both sides of the reactor, whereas characteristics, the conditions of pyrolysis in the
limestone and sand injections are located on oppo- gasifer, and the secondary reactions in the gas
site sides at fuel and air feeds. The bed material is phase.
fuidized by blowing air through the grid. Foster Wheeler derived from its experience in fu-
In this case the Cartesian, structured mesh of the idized beds, including nearly 10,000 fuel samples
model consists of 41,200 hexahedral control volu- and over 1000 tests in about 150 CFB units, semi-
mes as shown in fgure 6. empirical computer tools (probability models) ca-
The numeric results calculated by the model are pable of generating different probability indexes for
converted to 3D fgures, where the colour gradients agglomeration, fouling, and corrosion. Although the
describe the profles of the gasifcation conditions. models are based on some simplifying assump-
Some examples of the results (50 MW modeling tions, the results have been proven to match quite
th
case) are presented here, such as the temperature well the behavior of commercially operating CFBs.
profle of the air blown gasifer (fgure 6). The fuel Tar can be removed through specifc processes
feeds and secondary air inlets are clearly visible as downstream of the gasifer, such as: water scrub-
cooler areas on the sides of the reactor. bing, oil scrubbing, thermal cracking, catalytic
Two of the key needs for the model are to utilize it cracking. Table 2 summarizes all the main advan-
in the scaling-up of results from the pilot scale tests tages and disadvantages of these tar removal tech-
and secondly to use it actively in the design of the nologies.
large, commercial scale pressurized circulating fu-
idized bed oxygen-steam blown gasifcation units.
3.1. Water scrubbing
3. Tar removal The water scrubbing process is fnalized to cool the
synthesis gas by contact with water and to remove
The defnition of tar is unfortunately not univocal. tar content by condensation. The output tempera-
According to the designation used during the IEA ture from this process is usually in the range of 35-
Gasifcation Task meeting in 1999, for example, 75 °C, which corresponds to the dew point tem-
tars are “organic compounds with boiling temper- perature of the tar downstream of this treatment.
ature higher than that of benzene” (80 °C). As a The main advantages of this process include good
reference, the syngas from a fluidized bed gasifier effciency in terms of tar removal and good operability.
can contain tar with a boiling temperature as high On the other hand, the tar is transferred from the
as 350 °C (heavy tar). gaseous to the liquid phase and a considerable
On top of this categorization, tar can also be split quantity of contaminated waste water is gener-

Impiantistica Italiana - Maggio-Giugno 2014 21