VGB POWERTECH 7 (2020) - International Journal for Generation and Storage of Electricity and Heat

The Biofficiency Project | Part 1 VGB PowerTech 7 l 2020
was designed from the ground up, using
the results of the project. The results of this
will be summarised in Part 2 of this series
of technical papers.
2.3.2 Pulverised fuel combustion
The pulverised fuel combustion technology
offers the advantage of relatively high load
flexibility combined with the highest available
electrical efficiencies [9]. Therefore,
most big industrial boilers are pulverised
fuel boilers. During the Biofficiency project
combustion properties of pulverised biomass
material, the influence of pre-treatment
technologies and the use of additives
were investigated in in lab-, pilot- and fullscale
measurements.
F i g u r e 4 shows the Biofficiency test facilities.
The project approach uses test facilities
at all sclaes leading to proves comparability
of results gained. The conducted
tests, the investigated fuels and additives
are shown in Ta b l e 4 .
Four different pulverised biomass fuels
were utilised in lab-scale experiments including
SE bark, HTC leaves, milled wood
pellets and milled wheat straw. Deposit
tests showed that the use of kaolin and coal
fly ash as additives showed a pronounced
effect on deposit propensity, the additive
amount being of greater importance than
the type of additive.
Both additives showed positive effects on
the deposition behaviour, reaching reduction
of deposition propensity from ~ 80 %
to 20 % when fired with Danish wheat
straw. This is due to the capture of K by the
additive, thereby preventing K from making
deposits and fly ash particles as sticky
as when fired without an additive. Wheat
straw as an agricultural biomass fuel represents
a difficult fuel containing high concentrations
of K and Cl in the ash. The
other tested biomass fuels did not show
sinilar high tendencies to form deposits.
Nevertheless, the use of additives is changing
the deposit chemistry in a positive way,
decreasing the chlorine content significantly
and, thus lowering the chlorine-induced
corrosion potential in boilers and
abating fast deactivation of denox catalysts
due to potassium poisoning.
Pilot-scale tests combusting beech wood,
bark and SE bark with and without kaolin
and coal fly ash as additives showed positive
effects of additives on the combustion
properties as well. Confirming the obtained
results in the lab-scale test rig that
the amount and not the type of additive is
the main parameter affecting the fine particle
and deposit formation phenomena
during PF combustion.
The quantity and the chemical composition
of formed combustion aerosols from pulverised
biomass fuels is directly linked to
the ash amount and the ash composition of
the fuel. Alkali species, predominately K
(and Na), in the biomass fuel cause fine
particle formation during combustion. K is
5 kW th electrically heated
Entrained Flow Reactor
200 kW th pilot-scale
combustion test rig
From lab- to full-scale
800 MW th CHP plant
Avedøre Unit 2
Fig. 4. The used pulverised fuel combustion facilities used in the Biofficiency project:
a) lab-scale combustion test rig at DTU, b) pilot-scale combustion test rig at TUM,
c) full-scale combustion unit at Avedøre power plant (AVV2 Ørsted).
Tab. 4. Performed Tests in the PF combustion facilities in the Biofficiency project.
Combustion
facility
Approx. Thermal
Input:
Fired Fuels
Tested Additives
Additive/Fuel ratio
w/w:
Performed Tests
Lab-scale entrained
flow reactor
Pilot-scale biomass
combustion test rig
Full-scale CHP boilers
(AVV2/SSV3)
~ 5 kW ~ 200 kW ~ 800 - 900 MW
Wood pellets
Wheat straw
HTC leaves
SE bark
CFA Asnæs
CFA Hofor
Kaolin
Beech wood
Bark
SE Bark
CFA Asnæs
Kaolin
Wood pellets
CFA Asnæs
0 - 6.3 0 - 7.8 1.0 - 3.0
Depositon propensity
Analysis of deposits
Analysis of fly ash
Fine particle formation
Deposition tests
Flue gas analysis
Fine particle formation
Deposition tests
Soot blowing optimisation
Mill wear tests
PF transportation tests
Corrosion investigations
mostly present in a water-soluble phase
such as KCl in the fuel. During combustion
the K from the fuel is released to the flue
gas as KCl(g) and KOH(g). Those gaseous
K-species react with SO 2 in the flue gas
forming K 2 SO 4 . Once the flue gas reaches
cooler sections in the boiler, super saturation
of the K-species leads to the homogeneous
nucleation and formation of submicron
aerosol particles. The use of suitable
additives prevents the formation of submicron
particles in large quantities by capturing
the K in the gas phase.
Two use of additives was also investigated
for the reduction of aerosol formation. The
results showed the fine particle concentration
for submicron particles (PM 1 ) for pulverised
beech wood combustion can be
decreased by 33 % using about 1 wt.-%
kaolin. The fine particle reduction capacity
is strongly dependent on the used amount
of additive. By adding 2.4 wt.-% kaolin
PM 1 decreased by approx. 75 %.
Full-scale fine particle measurements at
two industrial wood-fired boilers (Avedøre
Unit 2 and Studstrup Unit 3) showed that
an increased coal fly ash-to-fuel ratio decreased
the formation of submicron aerosol
particles composed of K, Ca, S, P and Cl
confirming results obtained in the pilotscale
experiments. Operation at different
boiler loads at Studstrup Unit 3 revealed a
slightly reduced amount of fine particles by
increasing the boiler load.
Deposit measurements at both wood-fired
boilers showed only a very limited amount
of formed deposits during the measurement
periods. Neither a reduced coal fly
ash addition (change from 3 to 2 wt.-%
coal fly ash) nor increased probes surface
temperature (550 to 650 °C) lead to an increase
in deposit formation. The deposit
formation on the downstream side is mainly
due to deposition of submicron K-Ca-S
containing particles, while on the upstream
side impaction of larger Si-Al-K rich ash
particles are more important. The content
of chlorine was very low in all deposits.
The use of coal fly ash as additive in pulverised
fuel combustion of wood pellets provides
low deposit formation rates in the
boiler, but does not eliminate the risk of
formation of sintered K-Ca-S deposits consequently
efficient soot blowing for deposit
removal should be maintained.
Material tests of superheater tube samples
(TP347H, TP347HFG, SUPER 304H, Esshete
1250) installed at Avedøre Unit 2 and
Studstrup Unit 3 (5,000 – 13,000 operation
hours) showed that the use of 2-3 wt.-%
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