Page 54 - Impiantistica Italiana 5/2016

Fig. 8 - Fabric filter, dry and wet ESP arrangement
Fabric Filter (FF) and Electrostatic Precipitator (ESP)
technologies (figure 8) are used in most of PC boi-
ler applications, while wet scrubbers are used far
less due to the high power consumptions and the
lower particulate removal efficiencies, especially of
fine ash particles. In particular, fabric filter acts as a compliance with the current IED emission limits for
physical barrier for the dust and its porous surface
removes dust from flue gas while electrostatic pre-
cipitator induces an electrostatic charge that remo-
ves dust from flue gas.
Fabric filtration was used worldwide to remove
particles (especially fly ash) from the flue-gas of in-
dustrial and smaller combustion plants. However,
its application in larger scale combustion plants is
nowadays increasing.
FF can achieve very high removal efficiency, up to
99.95%, down to sub-micron size, lowering the dust
content in the flue gas exiting the unit below 5 mg/
Nm3. The product built up in the filter bags results in
additional flue gas pressure drop, thus affecting the
plant auxiliary consumptions together with the addi-
tional power required for filter pulse cleaning.
The ESP is used extensively in LCPs and is capa-
ble of operating over a wide range of temperatu-
re, pressure and dust content. It is not particularly
sensitive to particle size, and collects dust in both
wet and dry conditions, while is sensitive to the re-
sistivity of the gas.
The ESP can achieve a very high removal efficiency
even for smaller particles, up to 99.9% depending
on applied charge and gas characteristics, and can
handle very large gas volumes with low pressure
drops due to the flue gas open path structure. As
for that, the energy penalty is mainly related to the
power required for charging the flue gas.
Fabric filter is a proper solution for boiler retrofit due with respect to the solution with ESP. The following
to the shop-assembled and modular design and its
high degree of customization, which allows mee-
ting project and customer needs.
As de-SOx and de-NOx technologies, main key
factors to be considered when retrofitting an exi-
sting boiler with FF (or ESP) are the constructability
factors, the existing equipment design factors (e.g.
fan capacity and allowable pressure drop) and exi-
sting plant operation factors, as well as economic
factors (mainly the required investment cost).
52 Impiantistica Italiana - Settembre-Ottobre 2016
Study Case on Lignite and
Coal
A case study is developed to evaluate the impact
of retrofitting an existing subcritical PC boiler for
existing power stations. Considering as base case
a subcritical PC power station, the following avai-
lable emission abatement technologies have been
investigated as retrofit:
• Case 1: addition of SCR;
• Case 2: addition of SNCR;
• Case 3: addition of fabric filter;
• Case 4: addition of wet FGD;
• Case 5: addition of wet FGD + fabric filter;
• Case 6: addition of wet FGD + fabric filter +
SCR.
In each case the impact of the retrofit has been eva-
luated for both lignite and hard coal as feedstock.
SCR is the preferred solution for hard coal fired
plant, due to higher NOx generation in the boiler,
while most of lignite fired power plants require the
installation of the SNCR, as the high ash content in
the feedstock negatively impacts the catalyst ope-
ration and performance.
Being the most widely used of all the FGD systems,
accounting for about 80% of all the installed capa-
city, wet FGD is selected as reference desulphuri-
zation technology for the case study development.
Fabric filter is considered for dust abatement due
the high removal efficiency (up to 99.95%) combi-
ned with low specific cost, high degree of custo-
mization and lower impact on plant performance
main technical and economic assumptions are
considered for the analysis:
• site location: Turkey (lignite) / Poland (hard
coal) in order to exploit indigenous resources
with no constructability constraints for retrofit
modifications;
• net power output of the base case: 150 MWe;
• lignite LHV: 4.6 MJ/kg; sulphur content: 1%wt AR;
• hard coal LHV: 25.8 MJ/kg; sulphur content:
0.86%wt AR;