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CONVERTING LOW-VALUE FEEDSTOCK INTO ENERGY: RECENT DEVELOPMENTS IN GASIFYING
PAPER REJECTS, RDF AND MBM AT 5 KWTH, 25 KWTH AND 80 MWTH SCALE
A.J. Grootjes1, G. Aranda Almansa2, C.M. van der Meijden3, W. Willeboer4, M. Spanjers5, H.F. de Kant6 & R. Spit7
1 Corresponding author: Energy research Centre of the Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands,
Phone: +31 224-564983, Fax: +31 224-568487, E-mail: grootjes@ecn.nl
2 ECN, Phone: + 31 88 515 4209, E-mail: aranda@ecn.nl
3 ECN, Phone: +31 224-564582, E-mail: vandermeijden@ecn.nl
4 RWE Essent, E-mail: wim.willeboer@essent.nl
5 RWE Essent, E-mail: martijn.spanjers@essent.nl
6 HoSt, E-mail: dekant@host.nl
7 NEM, E-mail: rspit@nem.nl
ABSTRACT: The aim of decreasing the production cost of renewable electricity is expected to reduce the
governmental subsidies for (clean, relatively expensive) biomass supply in the future. This makes necessary to find
alternative (cheaper, yet troublesome) low-value feedstock. The main technical challenges associated to these fuels
are fouling, deposition and corrosion in the gas cooling sections of the plant. The adaptation and optimization of
indirect co-firing technology for low-value, difficult feedstock can reduce costs, thus widening the application
market.
In this work, several low-value, troublesome feedstock (paper rejects, refuse-derived fuel RDF, and meat and
bone meal MBM) have been tested both at laboratory scale (5 kWth BFB and 25 kWth MILENA facilities at ECN) and
commercial scale (80 MWth CFB gasifier at the Essent’s Amer 9 power plant). The tests have given insight on the
required adaptation of the gasifier, the gas cleaning section and the boiler when operating with these specific
feedstock. Focus has been placed on the fate and distribution of troublesome compounds (alkalis, chlorine, sulfur,
heavy metals).
The lab-scale results have shown that a decrease in gasification temperature from 850°C to 750°C leads to a
trade-off between fuel conversion and release of contaminants to the gas phase. Furthermore, a different quantitative
distribution of fouling elements in the solid- and gas phase has been observed. On the other hand, the concentration of
Cl and NH4+ in producer gas is similar from either MILENA indirect gasification or direct BFB gasification. Lastly,
results of wood/RDF co-gasification at the 80 MWth CFB gasifier have shown that the gas cooler fouling, the main
concern issue, can be reduced by decreasing the gasification temperature, thus leading to higher plant availability.
KEYWORDS: co-gasification, corrosion, deposition, fouling, cofiring, low-value feedstock, meat and bone meal,
paper rejects, RDF, residues, waste.
1 INTRODUCTION
Power, an essential part of modern society, is currently
generated mainly from fossil fuels like oil, natural gas and
coal. However, an increasing number of countries are
setting objectives and obligations to replace part of their
fossil fuel consumption with (low-value) biomass and
waste to reduce CO2 emissions. The aim of decreasing the
production cost of renewable electricity is expected to
reduce the governmental subsidies for (clean, relatively
expensive) biomass supply in the future. This makes
necessary to find alternative (cheaper, yet troublesome)
low-value feedstock. The main technical challenges
associated to these fuels are fouling and deposition in the
gas cooling sections of the plant.
A combination of a gasifier and a boiler (i.e. indirect
co-firing) can be used to efficiently convert difficult
feedstock into heat and power. The gasifier firstly
transforms the solid feedstock into a relatively easier
gaseous fuel, thus acting as a fuel pre-treatment. The
adaptation and optimization of the technology for
low-value, difficult feedstock can reduce costs, thus
widening the application market.
Within the framework of a joint project between ECN
(research institute), RWE Essent (utilities company), HoSt
(gasifier manufacturer) and NEM (boiler manufacturer),
several low-value, troublesome feedstock (paper rejects,
refuse-derived fuel RDF, and meat and bone meal MBM)
have been tested both at laboratory scale (5 kWth BFB and
25 kWth MILENA facilities at ECN) and commercial
scale (80 MWth CFB gasifier at the Essent’s Amer 9
power plant). The experiments have given insight on the
required adaptation of the gasifier, the gas cleaning
section and the boiler when operating with these specific
feedstock.
With this background, the main objective of this work
is the assessment of the effect of the temperature (main
operating parameter) on the gasification of low-value
fuels (paper rejects, meat and bone meal, and RDF) at
lab-scale (5 kWth BFB, 25 kWth MILENA) and the
evaluation of the effect of replacing part of the fuel
(wood) with a low-value feedstock (RDF) in a
commercial-scale gasifier (80 MWth). This paper, which
focuses on the fate and distribution of troublesome
compounds (alkalis, chlorine, sulfur, heavy metals), will
present the most relevant results obtained during the
project.
2 EXPERIMENTAL SECTION
2.1 Feedstock tested
Based on a prior selection taking into account
different factors (cost, availability and logistics, tendency
to fouling, agglomeration and corrosion, emissions, ash
quality and required pre-treatment), three different low-
value fuels have been selected for the project:
Paper rejects supplied by ESKA (cardboard
manufacturer). The raw feedstock consists of
shredded material (mainly plastic, paper, fabrics,
and metals), and has a large moisture content
(approximately 40-60% wt. wet basis).
Refuse-derived fuel (RDF) supplied by RWE
Essent (energy company). This feedstock was
tested both at lab- and commercial-scale, and is
largely composed of plastics, paper and metals,
similarly to paper rejects.
Meat and bone meal (MBM): classified as
category 3 (low-risk material), the tested MBM
is composed of slaughterhouse waste from
animals allocated for human consumption. It is
received as a dry, powder-like material.
The characterization of the tested feedstock is
summarized in Table I and Figure 1. As can be seen, all
the feedstock contain a large content of ash. Meat and
bone meal has a high content in calcium and phosphorus,
whereas RDF and paper rejects contain significant
concentrations of chlorine, aluminum, calcium, iron,
copper and silica.
Table I: Characterization of the low-value feedstock
tested.
MBM
Paper
rejects
RDF
Ash 550
(% wt. dry)
26
14
24
Volatile
matter
(% wt. dry)
65
75
67
HHV
(MJ/kg, dry)
18.8
26.3
22.6
C
(% wt. dry)
42
57
49
H
(% wt. dry)
6
8.1
6.7
N
(% wt. dry)
8.6
0.3
0.95
O
(% wt. dry)
19
23
23
S
(mg/kg dry)
3800
894
2933
Cl
(mg/kg dry)
5000
18014
14381
Figure 1: ICP analysis of the low-value feedstock tested.
Paper rejects and RDF are supplied as bulky, fluffy
materials with a high moisture content and containing
pieces (metals, stones) that can risk the operation of the
different laboratory equipment (pelletization, grinding,
and feeding systems). Therefore, a pretreatment step of
the fuels was necessary in order to allow reliable and safe
operating conditions. Firstly, manual sorting of the
material was carried out by removing some hard plastic
particles and (almost) all metals. Then, the material was
pelletized twice with a 8 mm dye. During this process,
the material was largely dried. The first pellets were air
dried and pelletized again for six times using a 4-mm
dye. The moisture content of the 4-mm pellets produced
was ~ 35%. The pellets were air dried again, and grinded
to 2-3 mm particles which were suitable for feeding in
the laboratory. Representative samples of the grinded
material was taken for analysis.
2.2 Experimental facilities
2.2.1 5 kWth BFB, ECN
An experimental plan for the study of the effect of
gasification temperature on the performance of the
selected low-value feedstock was carried out at the
5 kWth BFB facility located at the ECN laboratories
(Figure 2). As can be seen, the outlet gas line includes a
cyclone for bulk dust capture, a bypass for the sampling
of dust/soot and gas phase, and sampling points for SPA
tar measurement and micro-GC analysis. Dust/soot was
collected in a Soxhlet filter heated up at 400°C.
Furthermore, a deposition probe (stainless steel, 20 mm x
20 mm x 6 mm) was implemented on the gasifier
freeboard for the study of fouling and deposition
processes. Via water cooling, the temperature of the
deposition probe was kept at 250-350°C. The metal
probes collected after each experiment were subjected to
SEM/EDX analyses.
The lab-scale gasification tests were performed using
air/steam mixtures as gasification agent. The steam added
to the gasifier aims at simulating the initial moisture
content of the different fuels. For each fuel, tests at
750°C and 850°C (also at 700°C for paper rejects) were
carried out. Quartz sand (particle size above 0.25 mm)
was used in all cases as bed material. In all tests,
approximately 0.3 kg/h feedstock was fed to the gasifier.
Figure 2: 5 kWth BFB facility at ECN.
Dry producer gas composition (CO, CO2, H2, CH4,
C2H2, C2H4, C2H6, H2S, COS, and N2) was determined
online by micro-GC analysis. Tar content and
composition was measured using Solid Phase Adsorption
(SPA) analysis. This paper focuses on the distribution of
troublesome compounds (alkalis, chlorine, sulfur, heavy
metals). Therefore, the dust filter, the deposition probe,
the cyclone ash and the final bed material were weighed
before and after the test and sampled for further chemical
analysis. A slipstream of producer gas was bubbled
through a 1 M HNO3 solvent for sampling and further
determination of the gas phase composition. From the
resulting liquid solution, the NH3 content was determined
by Ammonia Flow Injection Analysis, and the HCl
content was analyzed via Ion Chromatography. The
solution, was also analyzed via Inductively-coupled
Plasma Optical Emission Spectroscopy (ICP-OES), as
well as the cyclone ash, the dust collected in the filter and
the final bed material, for the determination of the
distribution of the elements (including alkali and heavy
metal content) in each product fraction.
2.2.2 25 kWth MILENA gasifier, ECN
MILENA is an indirect gasification technology
developed by ECN. The MILENA gasifier is composed
of a riser, where the fast devolatilization/gasification of
biomass takes place, and a BFB combustor, where the
remaining char is oxidized. In the settling chamber, solids
(char and bed material) are separated from the producer
gas and recirculated to the combustor via the downcomer.
Heat is transferred between the combustor and the riser
through the circulation of bed material. The main
advantages of MILENA include the total conversion of
the fuel and the production of a N2-free gas without the
need for an air separation unit, in an integrated design
[1]. The performance of direct BFB gasification and
indirect gasification in terms of formation of
contaminants has been compared. With this purpose, an
experimental test using paper rejects as feedstock has
been performed at the 25 kWth MILENA gasifier located
at the ECN laboratories. A schematic layout of the unit is
shown in Figure 3.
Figure 3: 25 kWth MILENA gasifier at ECN.
The gasification test was carried out using 3.5 kg/h
paper rejects, and Austrian olivine as bed material. The
riser temperature was kept at ~730°C. 100 NL/min
primary air was fed to the BFB combustor. Producer gas
composition, tar concentration/composition, and gas
phase composition have been measured and analyzed
using a similar procedure as described in Section 2.2.1.
However, no deposition probe was inserted in the
gasifier.
2.2.3 80 MWth CFB gasifier at the Amer 9 plant,
Geertruidenberg
The Amer 9 power plant (Figure 4), owned by RWE
Essent, has an electricity output of 640 MWe, mainly
produced by coal. In the boiler, producer gas from
biomass/waste gasification is co-fired. Approximately
33 MWe is produced from biomass. The standard
feedstock used in the gasifier is demolition wood.
Figure 4: 80 MWth CFB gasifier in Amercentrale 9
Essent plant, Geertruidenberg, Netherlands.
3. 5 KWth BFB GASIFICATION OF MBM, PAPER
REJECTS AND RDF
3.1 Tar production and composition
Tars produced during gasification have a strong
influence on the fouling properties of the producer gas
and thus on the downstream equipment like gas coolers.
In this work, the ECN classification, based on the
structure and solubility of tars, was used for the
determination of tar composition. The results are
summarized in Figure 5. As can be seen, class 2 and 3
tars decrease at temperatures above 750°C, whereas class
4 and 5 (PAHs, most troublesome tars) increase with
gasification temperature regardless of the feedstock
tested. Moreover, in the case of paper rejects, it can be
observed that by decreasing the temperature from 750°C
to 700°C, the concentration of total tar is dramatically
increased. Therefore, high gasification temperatures leads
to a lower total tar production (directly related to higher
fuel conversion levels), but with a higher fraction of
heavy, troublesome tars. On the contrary, low-
temperature gasification boosts the total tar production
(thus indicating a lower fuel conversion degree), but with
a lower fraction of (heavy) class 4 and 5 tars. Thus,
gasification temperatures ~750°C lead to a trade-off
between tar production (i.e. fuel conversion) and tar
composition (i.e. tar dew point). Both RDF and paper
rejects contain a large fraction of plastics that partially
breakdown during gasification into aromatic compounds
that can mature into PAHs. The large fraction of plastics
might be the responsible for the significantly higher tar
concentration of RDF and paper rejects compared with
meat and bone meal. Figure 6 shows as an example the
detailed composition of tars produced from paper rejects
gasification. As can be seen, naphthalene and (o-xylene +
styrene) are the most significant compounds detected in
the tars. RDF gasification (not shown) leads to a similar
tar composition.
Figure 5: Effect of temperature on the distribution of tars
for the three feedstock tested.
Figure 6: Composition of tars produced from paper
rejects gasification.
3.2 Distribution of elements in gas phase, cyclone ash
and filter dust
Figure 7 shows the gas phase composition of
producer gas from MBM, paper rejects and RDF
gasification. The composition was determined by
ICP-OES analysis performed to the liquid solution
resulting from the sampling of the gas.
(a)
(b)
(c)
Figure 7: Effect of temperature on the gas phase
composition of producer gas: (a) MBM gasification; (b)
Paper rejects gasification; (c) RDF gasification.
As can be observed in Figure 7, the most abundant
compounds contained in the gas phase are NH4+ (which is
due to the decomposition of NH3 in the solution),
chlorine and sulfur. Producer gas from MBM gasification
contains mainly nitrogen (20-26 g/Nm3), with small
amounts of sulfur (0.15-0.35 g/Nm3) and traces of
chlorine (0.01 – 0.08 g/Nm3). Producer gas from paper
rejects gasification has the highest concentration of
chlorine (1.8-2.7 g/Nm3) in the gas phase, with low
concentrations of nitrogen (~0.5 g/Nm3) and sulfur
(0.01 – 0.025 g/Nm3). RDF gasification produces a gas
with mainly nitrogen (3.2-3.6 g NH4+/Nm3), minor
concentrations of Cl (0.02– 0.1 g/Nm3), Ca
(0.01 – 0.04 g/Nm3) and S (~ 0.03 g/Nm3), besides traces
of Al, Fe, K, Mg, Na and Si (< 0.01 g/Nm3). In general, it
can be seen that lower temperatures lead to decreased
concentrations of chlorine released to the gas phase.
Figure 8: Effect of temperature and feedstock on NH3,
HCl and H2S concentration in the dry-basis producer gas.
Figure 8 compares the concentration of H2S
(measured by micro-GC), NH3 (analyzed via Ammonia
Flow Injection Analysis applied to the liquid solution
obtained from sampling of the slipstream of producer
gas), and HCl (determined by ion chromatography
performed on the same liquid solution sample). In all
cases, the concentrations refer to dry gas. As can be seen,
producer gas from MBM gasification contains by far the
highest concentration of NH3 (2.5-3.3% vol. dry) among
the low-value feedstock tested, whereas the concentration
of HCl (~1700 ppmv dry) is dramatically higher in
producer gas from paper rejects gasification. On the
contrary, paper rejects gasification leads to the lowest
H2S concentration among the feedstock tested. On the
other hand, it can be seen that lower gasification
temperatures promotes the production of NH3 (this effect
being particularly significant in the case of MBM),
whereas lower temperatures decrease the HCl
concentration of the producer gas. No clear trends have
been found in the case of H2S concentration. It is
interesting to observe the dramatic difference in HCl
concentration in the producer gas from paper rejects and
RDF gasification despite the similar Cl concentration in
both feedstock (Table I). The difference behind the
different behavior of both feedstock might be on the
different moisture content of the feedstock. As presented
in Section 2.2.1, different steam flow rates were added to
the gasifying air in order to simulate the inlet moisture
content of the fuels. For this reason, 300 g/h steam was
added in the paper rejects tests, compared to 70 g/h steam
in RDF gasification. Steam promotes the hydrogen
content of producer gas during gasification, and H2 in
turn is reported to react with chlorides to form HCl [2].
This implies that not only the gasification temperature,
but also the inlet moisture content of the fuel can
significantly influence the HCl formation in the process,
and therefore the associated problems (deposition,
corrosion, etc.). Thus, a smart process design (fuel
pretreatment, gasification operating conditions, gas
cleaning section) can significantly limit the technical
problems associated to low-value feedstock.
Figure 9: Effect of temperature on the cyclone ash
composition for the three feedstock tested.
Figure 9 and Figure 10 display the results of
concentration of elements in the cyclone ash and the dust
collected in the filter, respectively. It must be emphasized
that during the test performed at 750°C with paper
rejects, clogging of the cyclone took place, which is
responsible for the abnormal distribution of elements
between the cyclone ash and the filter dust observed (no
dust is retained in the cyclone, and thus very high dust
concentrations are retained in the filter). As can be seen
in Figure 9, lower gasification temperatures lead to
higher concentration of elements retained in the cyclone
ash. Fly ash from MBM gasification is rich mainly in Ca
and P, with traces of Cl, K, Mg and Na. On the other
hand, the composition of the cyclone ash from
gasification of paper rejects and RDF is similar, with
significant contents of Ca, Al, Si, Cl, Fe, and minor
concentrations of K, Mg, Na and Ti.
Figure 10: Effect of temperature on the filter dust
composition for the three feedstock tested.
In Figure 10 it can be observed that the filter dust
produced from MBM gasification contains mainly Ca
(2.2 – 4.7 g/Nm3 dry) and P (1.1 – 2.4 g/Nm3 dry), with
less significant concentrations of chlorine, potassium and
sodium (< 0.3 g/Nm3 dry). The filter dust produced in
paper rejects- and RDF gasification has a similar
composition, calcium being the most abundant
compound, followed by Si, Al, Cl, Fe, and lower
concentrations of K, Mg and Na. In all cases, lower
gasification temperatures lead to lower concentrations of
inorganic elements retained in the dust captured in the
filter.
To sum up this section, it has been shown that
gasification temperature shifts the distribution of the
elements retained in each product fraction. In particular,
lower temperatures lead to a decrease of the chlorine
released to the gas phase, an increase of retention in the
cyclone ash and a decrease in the fraction retained in the
filter dust.
3.3 Deposition probe analysis
In order to make a consistent study of the fouling
tendency of the fuels tested, a deposition probe was
designed and implemented during the lab-scale
experiments. The probe consists of two concentric steel
pipes through which cooling water flows. Water at
ambient temperature (~ 100 L/h) enters though the inner
pipe and after reaching the bottom, circulates through the
outer pipe. At the bottom there is a stainless steel probe
(20 mm x 20 mm x 6 mm) attached, with an internal
thermocouple for temperature control. The whole piece
is surrounded by ceramic insulating material and is
flanged at the top of the gasifier, so that the probe is
located below the outlet pipe of the producer gas (i.e. all
the gas flows along the probe). The probe temperature,
kept around 250-350°C during the tests in order to
simulate the fouling behavior on the gas cooler surface,
is online monitored and logged during the tests.
The probe was weighed before and after the tests, a
visual inspection was carried out, and was stored for
further SEM/EDX analysis. Figure 11, Figure 12 and
Figure 13 display some examples of the SEM/EDX
analyses performed on the deposits of the collected
probes, including examples of composition of selected
spectra.
Figure 11: SEM/EDX analysis of deposits from MBM
gasification: 750°C (left), 850°C (right).
Figure 12: SEM/EDX analysis of deposits from paper
rejects gasification: 750°C (left), 850°C (right).
Figure 13: SEM/EDX analysis of deposits from RDF
gasification: 750°C (left), 850°C (right).
The SEM/EDX measurements have revealed that
deposits produced during gasification of paper rejects
(Figure 12) contain a significant amount of salts like
CaCl2, NaCl, KCl, or AlCl3. Deposits from RDF
gasification (Figure 13) are similar to those of paper
rejects, but it is noteworthy remarking their larger content
of PbCl2. On the contrary, deposits produced from MBM
are rich in Ca and P, but contain less chlorine and
therefore less salts in deposits. In general, it has been
found that gasification at high temperature (850°C)
results in more dense, partially molten and sintered
deposits, with significantly higher concentration of
chlorine. On the contrary, lower gasification temperatures
result in more porous and fluffy deposits.
4 PAPER REJECTS GASIFICATION IN 25 KWth
MILENA GASIFIER
Low-temperature gasification of paper rejects was
tested in the lab-scale MILENA. The gasification
temperature (approximately 730°C) was selected on the
basis of the conclusions obtained in the 5 kWth BFB tests
(see Section 3).
Unlikely direct gasification, two different gaseous
streams are generated in indirect gasification: producer
gas and flue gas. Each of these gases contain dust with a
different composition. Figure 14 displays the gas phase
composition of producer gas and flue gas generated in
MILENA gasification. As can be observed, the most
significant contribution to the producer gas phase is Cl
(~1.7 g/Nm3 dry gas, which is equivalent to 1098 ppmv
dry), followed by NH4+ (0.3 g/Nm3 dry, i.e. 386 ppmv
dry), and traces of S, Si, Sn and Pb (< 0.025 g/Nm3 dry).
The concentration of the rest of elements is below
detection limits. When comparing with results from
direct BFB gasification of paper rejects at low
temperatures (Figure 7 (b)), it can be checked that the
gas-phase concentration of Cl and NH4+ in producer gas
from MILENA indirect gasification is about the same
order as that from direct gasification despite the different
residence time of the gasifiers. On the other hand, the
only significant contribution to the MILENA flue gas
phase is Cl (~0.5 g/Nm3 dry or ~341 ppmv dry).
Figure 14: Composition of producer gas phase and flue
gas phase from MILENA gasification.
Figure 15 plots the composition of the dust contained
in the producer gas and the flue gas streams. It can be
seen that the dust entrained in the producer gas is mainly
composed of Ca, Cl, Si, Mg, Al, and Fe. On the contrary,
the dust collected in the cyclone of the flue gas side is
mainly composed of Mg and Si, with small amounts of
Al, Fe, Ca and Cl. When comparing with results of direct
BFB gasification at low temperature (Figure 9), it can be
checked that, despite a similar distribution of elements,
MILENA gasification produces roughly twice as much
dust fly ash than direct BFB gasification. This might be
due to the higher gas velocity in the MILENA riser,
which favors the entrainment of particles in the gas.
Figure 15: Composition of producer gas dust and flue
gas cyclone ash generated in MILENA indirect
gasification.
5 LARGE-SCALE (80 MWth) RDF/WOOD
CO-GASIFICATION TESTS
The effect of replacing wood with a fraction of
low-value feedstock (RDF) has been assessed at the
80 MWth CFB gasifier located at the Amer 9 power plant.
In preliminary co-gasification tests with 20% wt. RDF
and 80% wt. demolition wood at 840°C, it was found that
fast fouling of the gas cooler took place. The fouling
appeared to be caused by CaCl2. After that, it was
suggested that the gasifier temperature should be reduced
in order to decrease the fouling rate of the gas cooler.
After several adjustments in the gasifier operation
(fuel/air ratio, removal rate of bed material), the
gasification temperature was progressively decreased.
The temperature range between 770-800°C was found to
be a critical margin for the vaporization and condensation
of salts. During January-March 2014 a long-duration
co-gasification test was carried out in the CFB gasifier at
~750°C using RDF/wood mixtures as feedstock. During
the test, the fraction of RDF fed to the gasifier was
increased from 0% to 25% and finally 40% wt. A total of
5685 ton fuel (3717 ton demolition wood + 1968 ton
RDF) was gasified during 302 hours, with reduced gas
cooler fouling appreciated. An overview of the test can
be seen in Figure 16.
Figure 16: Wood and RDF supply and consumption in
the 80 MWth CFB gasifier co-gasification test.
During the test it was found that, with respect to
100% wood gasification, RDF/wood co-gasification leads
to an increase by 50% in the bottom ash production due
to the higher ash content of RDF. This implies that the
removal rate of bottom ash had to be increased, and this
is the limiting factor of the maximum feasible amount of
RDF than can be fed to the gasifier. Thus, the bottom ash
removal system will probably have to be adapted if
higher fractions of RDF are to be used. Even though the
higher build-up of ash from the fuel would lead to
decrease of the required make-up sand supply, issues like
different fluidization behavior or accumulation of certain
ash compounds in the bed might occur. The amount of fly
ash is also higher when replacing part of the wood with
RDF. The removal efficiency of the double cyclone
system was found to be lower during RDF/wood
co-gasification than in pure wood gasification.
With respect to producer gas composition, 40% wt.
RDF/wood co-gasification leads to the production of a
gas with a heating value of 5.6 MJ/Nm3 dry, i.e. 10%
higher than producer gas from pure wood gasification.
The normalized concentration of heavy metals in the
producer gas, (As+Co+Cr+Cu+Mn+Ni+Pb+Sb+V)/fuel
heating value, was found to be 19.3 mg/MJ, that is, below
emission limits (30 mg/GJ).
Therefore the large-scale tests have shown that the
gas cooler fouling, the main concern issue in the plant,
can be limited by decreasing the gasification temperature,
thus increasing plant availability, in consistency with the
results obtained in the laboratory. By applying this
measure, up to 15-20% wt. RDF can be co-gasified
without remarkable operating problems. However, higher
fractions of RDF in the fuel cause a fast fouling even at
lower temperatures, mainly due to the fly-ash content in
the producer gas. Therefore, additional modification of
the gas cooler will be necessary.
6 CONCLUSIONS
With the aim of the adaptation and optimization of
the gasifier + boiler technology for the efficient
conversion of low-value feedstock in order to reduce the
production costs of renewable electricity, experimental
tests have been carried out both at lab-scale (5 kWth BFB,
25 kWth MILENA gasification) and commercial-scale
(80 MWth) using different troublesome, low-value
feedstock.
In the 5 kWth BFB tests it has been found that a
decrease in gasification temperature from 850°C to
750°C leads to a trade-off between fuel conversion and
release of contaminants to the gas phase. Furthermore, a
different quantitative distribution of fouling elements in
the solid- and gas phase has been observed. Even though
lower temperatures lead to a higher production of tars
(i.e. lower fuel conversion to producer gas), the tars
produced contain a lower fraction of heavy tars, and most
importantly, the concentration of HCl in the gas phase is
decreased. Moreover, at lower temperatures, there is an
increase of the elements retained in the cyclone ash, and a
lower concentration of elements retained in the filter dust.
Producer gas from MBM gasification contains the highest
concentration of NH3 (2.5-3.3% vol. dry), whereas paper
rejects gasification produces the highest amount of HCl
in the gas (~1700 ppmv dry). The large moisture content
of paper rejects might be responsible for the dramatically
higher HCl content in the producer gas with respect to the
(similar in composition) RDF. The SEM/EDX analyses
performed to the produced deposits have revealed that
high-temperature gasification results in more dense,
partially molten and sintered deposits with higher Cl
concentration (thus, prone to fouling and corrosion),
whereas low-temperature gasification results in more
porous and fluffy deposits. Paper rejects gasification
produces deposits with a significant amount of salts like
CaCl2, NaCl, KCl, or AlCl3. Deposits from RDF
gasification are similar to those of paper rejects, but
contain a larger amount of PbCl2. Deposits from MBM
gasification are rich in Ca and P, but contain less chlorine
and therefore less salts.
Low-temperature paper rejects gasification carried
out at the 25 kWth MILENA has shown that the gas-phase
concentration of Cl and NH4+ in the producer gas from
MILENA gasification is similar to that from direct
gasification despite the different residence time of the
gasifiers.
Lastly, results of wood/RDF co-gasification at the
80 MWth CFB gasifier of the Amercentrale 9 plant have
shown that the gas cooler fouling, the main concern issue
in the plant caused by the replacement of wood by RDF,
can be reduced by decreasing the gasification
temperature, in consistency with the results obtained in
the laboratory. By applying this measure, up to
15-20% wt. RDF can be co-gasified without remarkable
operating problems. Therefore, lower gasification
temperatures lead to higher plant availability. However,
higher fractions of RDF in the fuel cause a fast fouling
even at lower temperatures, mainly due to the fly-ash
content in the producer gas. Additional modification of
the gas cooler and the bottom-ash removal system will be
necessary if higher fractions of RDF are to be fed to the
gasifier.
7 REFERENCES
[1] C.M. van der Meijden. Development of the MILENA
gasification technology for the production of Bio-SNG.
Doctoral Thesis. Eindhoven University of Technology
(2010). ISBN: 978-90-386-2363-4.
http://www.ecn.nl/docs/library/report/2010/b10016.pdf
[2] C. Higman, M. van der Burgt. Gasification, 2nd ed.,
GPP, ISBN-13: 978-0-7506-8528-3 (2008).
8 ACKNOWLEDGEMENT
The Dutch Ministry of Economic Affairs is gratefully
acknowledged for its support through the TKI Project
“Vergassen laagwaardige brandstoffen” (reference
TEBE213002). The authors are also indebted to M.
Geusebroek, P. Heere, D. Slort and A. Toonen for their
invaluable assistance during the experiments.
