In the same reactor set-up at 800°C, fresh olivine was comparatively less efficient than feldspar in reducing tar levels.. Other groups have obtained values one order of magnitude lower;
Trang 1Use of alkali-feldspar as bed material for upgrading a biomass-derived
producer gas from a gasifier
Division of Energy Technology, Department of Energy and Environment, Chalmers University of Technology, SE-41296 Gothenburg, Sweden
h i g h l i g h t s
The alkali-feldspar showed high potential for WGS even at low temperature
No reduction of the methane content in the cleaned gas
The elimination of most of the C2H2and C3H6already at 800°C
Tar selectivity: exclusively pure ring-compounds in the reformed gas
Fresh olivine was less efficient than feldspar in reducing tars
a r t i c l e i n f o
Article history:
Received 17 September 2015
Received in revised form 22 December 2015
Accepted 18 February 2016
Available online 4 March 2016
Keywords:
Alkali-feldspar
Biomass gasification
Catalytic gas cleaning
Tars
Bed material
a b s t r a c t
Gasification of biomass has strong potential for biofuels production However, one challenge to its competitiveness is the efficient elimination of the tars present in the raw gas The use of active bed materials in tar reforming is an appealing approach and can be employed as part of primary and/or secondary measures Here, the activity of an alkali-feldspar [(K, Na)AlSi3O8] ore was assessed in relation
to upgrading the producer gas from the Chalmers 2–4 MW indirect biomass gasifier This material was tested in a single bubbling bed reactor previously developed for studies involving catalytic tar reforming The bed of feldspar was fluidized with raw gas The material was tested at three temperatures: 700°C,
800°C, and 900 °C The results indicate that alkali-feldspar shows: (1) a high potential for the Water–Gas Shift reaction even at low temperature (H2/CO 3); (2) no reduction of the methane content
in the cleaned gas and net formation of methane at 900°C; (3) the elimination of most C2H2and C3H6
already at 800°C; and (4) a striking tar selectivity, which resulted in the reformed gas having exclusively pure ring-compounds In the same reactor set-up at 800°C, fresh olivine was comparatively less efficient than feldspar in reducing tar levels At 900°C, the feldspar decomposed 47% excepting benzene Moreover, it retained its mechanical integrity, withstood longer reducing periods (3 h), and displayed neither signs of agglomeration nor loss of activity, despite the formation of a carbon deposit The deposits were readily removed by introducing an oxidizing atmosphere Furthermore, the raw feldspar showed a low capacity for oxygen transport, advantageous in this application Taken together, our results indicate that this material is clearly promising and warrants further investigations
Ó 2016 Elsevier B.V All rights reserved
1 Introduction
The thermochemical transformation of biomass via gasification
results in the formation of a so-called ‘producer gas’ or raw gas that
can be used subsequently to synthesize a variety of biofuels and
value-added chemicals and/or for the polygeneration of combined
heat and power (CHP)[1–9] The use of biomass in such
applica-tions is attracting increasing interest as a means for reducing the
demand for fossil fuels and paving the way towards a more sustainable society The gasification products include permanent gas components, such as H2, CO, CO2, CH4, and light hydrocarbons,
as well as undesirable condensable hydrocarbons (tars), which are formed during the primary step of biomass conversion While the definition of a tar varies slightly between different studies, in this work, a tar is defined as an aromatic hydrocarbon with a molecular mass greater than that of benzene [10–13] These molecules have a high dew-point, and the larger tars, e.g., pyrene, start to condensate already at 400°C Thus, they are often prone to deposit on cooler surfaces e.g on gas ducts/piping or
http://dx.doi.org/10.1016/j.cej.2016.02.060
1385-8947/Ó 2016 Elsevier B.V All rights reserved.
⇑Corresponding author.
E-mail address: nicoberg@chalmers.se (N Berguerand).
Contents lists available atScienceDirect Chemical Engineering Journal
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c e j
Trang 2heat exchanger surfaces leading to fouling or precluding the
instruments efficiency If reaching the downstream fuel synthesis
equipment, these can even contribute to deactivate catalysts
There are two principal approaches to treating the raw gas
generated by gasification: hot gas cleaning; and wet cleaning
with solvents Hot gas methods include thermal cracking at
high temperatures (>1100°C) and catalytic tar destruction using
catalyst particles at moderate to high temperatures (700–900°C)
The latter can be implemented in the gasifier bed as a primary
measure and/or in downstream reactors as secondary measures,
most often in a fixed-bed manner[14–16] Since tars can contain
up to 10% of the total energy value of the fed biomass, their
elimination should ideally involve recovery of some of the
energy to the raw gas, so as to promote the efficiency of
the resulting cold gas [17,18] In this context, catalytic gas
conditioning is a particularly interesting alternative, as the heavy
hydrocarbons are degraded into smaller and more-desirable
molecules, often having higher energy contents, while the
temperatures required for the reactions enable thermal
integra-tion with the gasificaintegra-tion step Moreover, the process does not
create an additional waste stream of costly solvent, as is the case
with scrubbing techniques The reactions involved also often
permit an adjustment of the cleaned gas composition, which in
turn facilitates the downstream gas treatment and/or fuel
synthesis steps
An application of catalytic gas cleaning that is currently
attract-ing attention is a concept that exploits the advantages of the
circu-lating fluidized bed (CFB) principle, referred to as chemical-looping
reforming (CLR) The process originated from the petrochemical
industry and was based on the development of fluid catalytic
cracking (FCC) to convert high-molecular-weight hydrocarbons in
oil refineries[19] The idea was reintroduced by Mattisson et al
[20,21] for hydrogen production in the context of CO2 capture
and climate change mitigation The principle involves a
dual-fluidized bed configuration in which catalytic hydrocarbon
reforming is achieved in one vessel and the deactivated catalyst
is subsequently regenerated in another vessel Regeneration
involves the removal of principally carbon deposits formed during
hydrocarbon reforming, as well as other impurities (e.g., sulfur)
carried over by the particles to the regenerator[10] In previous
related research conducted at Chalmers University of Technology
with laboratory-scale reactors, a dual fluidized bed reactor with a
reformer and a regenerator was convincingly applied to the
upgrading of a biomass-derived producer gas, with the emphasis
being placed on tar destruction[22–27] This technique can be
deployed as a secondary gas cleaning measure or can be
intro-duced as a primary measure in existing processes, whereby the
catalyst will undergo cyclical exposure to raw gas and to a
regen-erating agent, e.g., air or oxygen-depleted air This is, for instance,
the case with indirect gasification using a gasifier-boiler loop
[3,8,28]
2 Theory
In fluidized bed gasification, the main focus of studies to enhance the conversion of tars has been the use of metal oxide cat-alysts In practical terms, these catalysts can be used either in a pri-mary or in a secondary measure For instance, oxides of iron, aluminum, nickel, manganese, and magnesium have been proven
to be efficient for tar degradation[10,16,22,25,29–36] In particu-lar, the iron-magnesium silicate olivine has been successfully employed in indirect gasifiers[9,36,37] Furthermore, in a recently submitted paper, Marinkovic and colleagues used bauxite – an aluminum ore – in the Chalmers indirect gasifier and showed a clear effect on tar reduction[35] Despite having proven catalytic activity, such materials contain sufficient concentrations of metal (a major contributor to the catalytic effect) to enable the carry-over of oxygen from the oxidizing vessel to the reducing vessel, wherein fractions of the valuable gases are inevitably burned off For example, despite its positive catalytic efficiency, olivine can transport oxygen Koppatz et al showed that the olivine used in their work had an oxygen transport capacity of 2.2%, resulting in a lower heating value for the producer gas[37] Other groups have obtained values one order of magnitude lower; for example, an olivine material used in the Chalmers gasifier resulted in an oxygen transport capacity of around 0.15% [36] Studies conducted with the iron titanium oxide ilmenite have reported that although the tar levels are reduced, a considerable part of the gas is combusted in the gasifier For activated ilmenite, the oxygen transport capacity varies slightly with the activation procedure; for instance a value of 2.1% is stated in[38] Larsson
et al reported a decrease in the heating value of the gas from
17 MJ/m3to 12.5 MJ/m3using only 12% ilmenite in an inventory
of silica sand, as compared to using silica sand alone[17] One can conclude that this drawback related to a lower heating value of the gas through oxygen transport is to some extent counterbalanced by the increase of heating value associated with the catalytic reforming of hydrocarbons One way to address the oxygen transport problem would be to reduce the catalyst before
it comes in contact with the reforming gases An alternative strategy would be to find a substitute material that has negligible oxygen carrier capacity but that still displays an active phase for tar elimination
Finally, it has been shown that the effects of the natural con-stituents of the bed materials described above are augmented by the introduction along with the bed material of low-cost sub-stances, such as alkali salts[39,40] Alkali salts should be added
in a controlled and well-defined manner, as they are associated with corrosion of the heat exchanger tubing and bed agglomera-tion[41,42] For both olivine and bauxite, the uptake and release
of alkali through interactions of the bed material with the biomass ash have been shown to have significant impacts on the observed gas upgrading performance[35,36]
Nomenclature
CFB circulating fluidized bed
CLC chemical-looping combustion
CLR chemical-looping reforming
DAF dry ash-free (fuel)
FCC fluid catalytic cracking
PSD particle size distribution SEM scanning electron microscopy SNG substitute natural gas SPA solid-phase adsorption SPE solid-phase extraction WGS Water–Gas Shift (equilibrium)
Trang 32.1 Alkali-feldspars
The present work is very much in line with previous studies on
catalytic bed materials for producer gas upgrading performed at
Chalmers, except that it introduces a material new for the
applica-tion[17,24,36] In the present work, an alkali-feldspar is assessed
for secondary gas cleaning Feldspar is a naturally occurring ore
that makes up to 60% of the Earth’s crust[43] The geographical
distribution and large reserves on the five continents mean that
it has ubiquitous availability
Within the feldspar family, alkali-feldspar is a sub-group
that comprises the tectosilicates with the general formula (K, Na)
AlSi3O8 The silicate family includes minerals that have been
proven to be active bed materials for upgrading the gases from
bio-mass gasifiers This is the case for olivine (Mg, Fe)2SiO4[8,36,37]
Moreover, the zeolites in FCC catalysts present a tectosilicate
struc-ture that is suitable for reforming higher hydrocarbons Some of
these are actually prepared from feldspars, as detailed in a patent
issued to Cox and Elston in the early 1950’s[44] Furthermore,
FCC catalysts are often doped with alkali metals to enhance
hydro-carbon decomposition while concomitantly reducing the levels of
carbon deposits on the particles [45] Thus, alkali-feldspars are
likely to express activity for reforming raw gas tars
Despite the large amounts of K and Na, the melting point of
alkali-feldspar is rather high At temperatures <1000°C, no melting
should be expected[46] These temperatures are higher than the
typical temperature range for catalytic gas cleaning, 700–900°C,
within which range the experiments in the present study were
conducted
Since alkali-feldspar is already saturated with alkali, the alkali
released from biomass ash should not induce melting leading to
agglomeration Furthermore, the high contents of K and Na could
benefit the reactions, as alkali is known to enhance the catalytic
destruction of hydrocarbons For instance and in the same context,
continuous addition of K salt to the GoBiGas indirect gasifier[9]
fluidized with olivine has been shown to improve the efficiency
of the operation by decreasing the levels of tars In contrast, the
low level of metallic compounds in alkali-feldspar should entail a
limited oxygen transport capacity In an indirect gasifier, this is
ben-eficial in terms of preventing combustion of the producer gas, as
discussed above For the material assessed in the present study,
the detailed composition is given in the experimental section
Alkali-feldspar has certain advantages over, for example, olivine
or bauxite, including its much lower price (roughly similar in price
to silica-sand) and the absence of hazardous impurities (see the
Experimental section), whereas olivine usually possesses high
levels of Ni and Cr, which are problematic with respect to the
reg-ulations for disposal of spent inventories[36] This means, that for
comparable catalytic activity, the alkali-feldspar would outperform
most of the other materials
The goal of the present study is to validate the use of this
alkali-feldspar as a catalyst for upgrading a biomass-derived producer gas
in an application new for this material To achieve this, the
mate-rial must be:
– Physically stable: being able to withstand fluidization under
severe raw gas conditions and high temperature for a longer
period of time;
– Chemically active and stable: having activity towards raw gas
tars and possibly being able to modulate the permanent gas
composition in a favorable manner; this must occur without
any decay, e.g., through poisoning; and
– Its interactions with the biomass ash should not have a negative
impact on the performance, but rather contribute to enhance
the catalytic activity
This study aims at validating the first two points above using the experimental system detailed in the Experimental section This configuration is representative of a secondary gas upgrading mea-sure with a filter upstream of the catalytic reactor The third point listed above, which includes ash interactions, corresponds to a pri-mary measure and will be the focus of upcoming studies, provided that the first two conditions can be established
2.2 Tar decomposition and gas upgrading The general reaction associated with the reforming of tars with general formula of CnHmOpin the presence of a reduced catalyst and reforming agents (e.g., steam, carbon dioxide) produces lighter hydrocarbons of the form CiHjOk, as well as H2and CO, with carbon deposition (C) This can be simplified as Reaction(1):
CnHmOp þ H2Oþ CO
2 ! CiHjOkþ H2þ CO þ C ðreaction1Þ
Furthermore, during the reducing phase when the catalyst is exposed to the raw gas, the gas-phase reactions that occur are a result of the complex raw gas composition itself and the interme-diary products formed from the contact with the catalyst at high temperature.Table 1lists the potential tar-decomposing reactions, andTable 2gives the equilibrium reactions that the permanent gases may undergo[23]
2.3 Experiences of the Chalmers biomass gasification group The experiments referred to in this section were performed at Chalmers using the same gasifier and bench-scale reactor as in the present work and serve as the basis to interpret the results The Chalmers indirect gasifier is fluidized with steam, resulting
in a total steam content of 50–60% in the raw gas[17,18] Thus, the reactions that involve the decomposition of hydrocarbons in the presence of steam are favored and the catalyst enhances this effect, provided that the temperature is suitable This is the case for reac-tions (2) and (3), and even for the Water–Gas Shift (WGS) (9) In relation to this topic, Nguyen et al conducted experiments in a bench-scale reactor fluidizing ilmenite with synthetic gases that mimicked the Chalmers raw gas[47] The incentive to use ilmenite arose from a previous work by Larsson et al.[17]which showed that, depending on the quantity introduced into the gasifier and the level of steam fluidization, ilmenite could both catalyze tar reforming and potentially enhance hydrocarbon polymerization Both directions of tar evolution are thus possible Eventually, if it
is not optimized, the use of this material as a primary tar catalyst can result in higher tar levels, which is a key outcome with respect
to understanding tar decomposition mechanisms It seems plausi-ble that similar results would be obtained with other materials
3 Experimental The experiments reported in the present study were conducted using a tar-laden slipstream of raw gas from the Chalmers 2–4 MW indirect biomass gasifier The Chalmers gasifier is detailed elsewhere, e.g in [48] Operation involved a given batch of alkali-feldspar fluidized with this stream and using three different reactor temperatures
3.1 Experimental setup The reactor system utilized in the experiments is shown in schematic form inFig 1 It was constructed at Chalmers within the scope of previous investigations, in particular those with ilme-nite, where different gas-catalyst contact times were obtained and
Trang 4served as the basis for the development of kinetics and
tar-degradation models[49] The reactor depicted in Fig 1contains
the fluidized bed and its body consists of a tube with inner
diam-eter of a 55 mm and height of 379 mm (measured from the porous
plate, which serves as a gas distributor, to the reactor outlet) It is
constructed using a chromium-rich austenitic stainless steel, called
RA 253 MA Along the height of the reactor, there are five pressure
taps (three in the bed and two in the freeboard) and three
thermo-couples (one in the bed, one in the lower part of the freeboard and
one close to the reactor roof and exit duct) To monitor fluidization,
the pressures are assessed as the pressure differences between two
taps or between one tap and the atmosphere
The reactor is inserted in an oven built with a two-door
config-uration for ease of access The oven ensures the heat requirement
for the reactions and can be set at temperatures up to 900°C
Fluidization of the catalyst batch is achieved using either a mixture
of air and nitrogen or raw gas (Fig 1) A slipstream of raw gas is extracted from the gasifier and flows through the high-temperature valve to supply the reactor (Fig 1) [49] Helium is used as a tracer gas The flow rate of He is used to calculate the tar yields in g/kg dry ash-free fuel (DAF) and the molar yields of permanent gases in mol/kg daf fuel[17,18,50] These units were chosen to relate the yields to the actual fuel conversion and remove the gas dilution effect Here, ‘‘fuel” refers to the wood pel-lets fed to the gasifier
Downstream of the reactor, a tar sampling port for solid phase adsorption (SPA) analysis is available In the present study, the SPA method used to quantify the tars was adapted by Israelsson
et al [51] from the original method developed by Brage et al [52] Israelsson et al.[51]improved the method so as to minimize errors associated with the sampling, eluation, and analysis of the SPA samples A Gas Chromatograph – Flame Ionization Detector (GC–FID) was used to quantify the tars in the eluted samples
A parallel sampling line was used to determine the raw gas composition and the tar levels for comparisons with the reactor outlet (Fig 1) In the current work, the SPA method was performed using dual-layer solid phase extraction (SPE) columns with Supel-clean ENVI-Carb/NH2SPE tubes (Sigma–Aldrich)
After gas conditioning, which included the removal of water and remaining tars from the CLR, the permanent gas composition was analyzed online using a Rosemount NGA 2000 Multi-component gas analyzer, which sampled the gas every second
Table 1
Potential tar-decomposing reactions.
Steam dealkylation C n H m O p þ ðk þ n i pÞH 2 O ! C i H j O k þ 1 ðm j þ 2ðk þ n i pÞÞH 2 þ ðn iÞCO (3)
Table 2
Equilibrium reactions.
Water–Gas Shift CO þ H 2 O $ H 2 þ CO 2 (9)
Methanation CO þ 3H 2 $ CH 4 þ H 2 O (10)
CO 2 þ 2H 2 $ 2H 2 O þ C (13)
Nitrogen
Oven
Mixing chamber Reactor
Gas Mi
SPA
Air
Gas condioning
He
Gas condioning
analyzer cro-GC
Micro - GC
High temp valve
Gasifier
SPA Raw gas Raw gas
Mass flow controller
Trang 5and the micro–GC Varian 4900 Micro Gas Chromatograph
(micro-GC), with a sampling rate of three minutes For O2quantification,
the paramagnetic principle was employed, and CO2, CO, and CH4
were measured using infrared (IR) techniques The micro–GC used
Ar and He as carrier gases and measured the concentrations of H2,
He, CO, CO2, CH4, C2H2, C2H4, C2H6, C3Hx, and N2 Similar equipment
was used to measure the levels of permanent gases in the raw gas
3.2 Bed material
The bed material was an alkali-feldspar procured from
SIBELCO-Finland and produced from pegmatite ore using flotation
separa-tion, from which the mica was removed to produce feldspar and
quartz After removal of most of the quartz, the feldspar was dried
and cleaned by magnetic separation, to remove the iron-containing
metals This procedure was achieved by SIBELCO-Finland The
batch of alkali-feldspar as received had a grain size of <1 mm with
7.7% of the grains being <63lm From this starting material,
batches of 200 g of feldspar in the size range of 125–180lm were
prepared by sieving for the experiments The mean particle
diameter was 150lm The specific gravity was approximately
2.6 g/cm3, while the bulk density of the material was 1.4 ton/m3
Its hardness was 6 on the Mohs scale, similar to that of silica sand
The mineralogical and chemical compositions of the material are
given inTables 3 and 4, respectively
Note the particularly high content of potassium (Table 4)
3.3 Experimental conditions and procedure
3.3.1 Experimental conditions
Three experiments were performed at 700°C, 800 °C, and
900°C, the operating conditions for which are given in Table 5
‘‘Gas RT – bed” corresponds to the average gas-bed catalyst contact
time and is calculated according to Eq.(1)originally presented in
[24]:
GasRT bed ¼ Vbedebed 2
V0Raw gasþ V0
Reformed gas
ð1Þ
in which Vbedis the bed volume,ebedis the bed voidage, and V0
Raw gas
and V0
Reformed gasare respectively the raw and reformed gas flows
‘‘Gas RT – reactor” represents the average gas residence time in
the reactor body, i.e., including the freeboard The solids inventory
was the same for the three tests For the experiments conducted at
700°C and 800 °C, a He-tracing flow of 0.24 Ln/min was introduced
directly upstream of the reactor (seeFig 1), although this is not
included in the ‘‘raw gas flow rate” inTable 5 For the 900°C case,
a different approach was used, with a total flow feed of He to the
gasifier of 20 Ln/min, resulting in a much reduced He stream to
the CLR This discrepancy explains the fairly small difference in
the calculated ‘‘Gas RT – bed” values with respect to the presented
flows, despite the significant difference in temperature It is
evi-dent that the gas–solids contact times are rather short at <1 s In
a fully-deployed system, the times allowed for the reactions would
be longer, which most likely would benefit the reforming reactions
[18] It must also be stressed that the temperature range for the
investigation, 700–900°C, is representative of applications of this material in primary and secondary measures, even though the ash interaction in a real system, not present here, would most likely have an impact on the results Under real biomass gasifica-tion condigasifica-tions and with longer contact times, the reacgasifica-tions would
be kinetically promoted
3.3.2 Experimental procedure
An experiment was initiated with air fluidization of the bed material at 1 Ln/min, while the temperature was slowly increased
to the set point for the oven During this phase, the oxygen concen-tration in the exiting gas stream was measured In the absence of combustion products, a decrease in the oxygen concentration to less than the level of oxygen in the air (21%) was attributed to con-sumption via bed material oxidation A mass balance of oxygen gives the oxygen transport capacity of the feldspar Rothrough Eq (2):
Ro¼Vm MO 2
mbed
Z
ox
where _FoutðtÞ is the time-dependent normal volumetric flow of oxygen-depleted air leaving the reactor, O2(t) is the time-dependent oxygen concentration in this stream, Vmis the normal molar volume, MO2is the molar mass of oxygen, and mbedis the mass of feldspar in the reactor The designation ‘‘ox” indicates that the integral is calculated throughout the period of oxygen consumption
The air flow was then decreased in a stepwise manner and pro-gressively replaced with inert nitrogen until no oxygen was detected by the gas analyzer The high-temperature valve (see Fig 1) was then opened, the nitrogen flow was decreased in a step-wise fashion, and the raw gas flow was increased progressively until the nitrogen was completely replaced
Table 3
Mineralogical composition of the alkali-feldspar.
Table 4 Chemical composition of the alkali-feldspar.
Table 5 Operating conditions.
Operating properties Experiments
Gasifier
Steam flow rate (kg/h) 180 Experiments
Raw gas flow rate (wet) (Ln/min) 1.73 1.57 1.92
Trang 6The raw gas operation continued for 2–3 h, after which it was
discontinued due to the restricted time available to the gasifier
operating team The raw gas sequence was followed by an inert
period, during which the nitrogen was increased in a stepwise
manner until no combustible gases were detected The
shut-down procedure, in which the oven was turned off, was executed
in two different ways – in oxidizing and inert environments – to
investigate the carbon deposits Under inert conditions, the idea
was to provide samples of the bed material with deposits, while
shut-down in air permitted the burning off and quantification of
the deposits by measuring the carbon as CO2plus CO in the
com-bustion gases
At the end of each of the three experimental runs, samples of
spent bed materials were collected and examined to determine
the physicochemical properties of the feldspar and any evidence
of changes in the elemental composition, as compared with the
fresh particles Scanning Electron Microscopy coupled with Energy
Dispersive X-ray spectroscopy (SEM-EDX) analysis was conducted
on the samples to provide information on the distribution of
ele-ments within cross-sections of the catalyst particles The particles
were initially mounted in epoxy, polished, and then characterized
For the analysis, the FEI Quanta 200 Field Emission Gun ESEM
elec-tron microscope, which was equipped with an Oxford Inca EDX
system for chemical analysis, was used From both fresh and used
feldspar samples, several individual particles were randomly
cho-sen and analyzed, to ensure homogeneity of the composition
profiles
3.3.3 Reference experiment
As a reference experiment for the present study, previous
inves-tigations were used[26]in which tests were conducted with raw
gas from the Chalmers gasifier with a bed of silica sand using a
dual-fluidized bed reactor of the same construction material as
that used here This was to assess whether the reactor material
and/or the sand could have some catalytic effect that would yield
a change in gas composition and thereby influence the comparison
with actual feldspar operation The results showed that the
combi-nation of fresh silica-sand bed and a reactor composed of RA 253
MA material was inert with respect to the raw gas composition
4 Results
4.1 Oxygen-carrying capacity
The oxygen concentrations in the product gas stream during air
fluidization are shown in Fig 2 From these measurements, the
oxygen-transport capacity R0was calculated using Eq.(2)as being
0.2% A similar R0value was obtained for olivine used in the
Chal-mers indirect gasification unit[36] While this value is low, it lies
within the range expected for the composition of the feldspar This can also be compared with the oxygen capacities of other materials used in the same reactor, e.g., ilmenite with a value of 2.1% for used particles[38]
4.2 Tar yields The SPA results (in g/kg daf fuel) for the three temperature cases (700°C, 800 °C, and 900 °C) are summarized inFig 3 The unknown compounds depicted in Fig 3group together tar molecules with molecular masses between those of benzene and chrysene and for which the GC-FID was not calibrated They are plotted on the far-right of the figure for clarity The first bar corresponds to the average tar composition of the inlet raw gas during the three tests and the error bars indicate the standard deviations of the three measurements
While a slight increase in the tar yields was observed for almost all the species at the lower temperature,Fig 3reveals an explicit decreases in the amounts of branched tars in the stream passing through the bed of feldspar at 800°C and above In addition, from
at least 800°C, all the phenols are reformed, as well as most of the branched aromatic compounds, leaving a tar mixture characterized
by compounds with pure aromatic rings, e.g., benzene, naph-thalene, phenanthrene, fluoranthene, and pyrene, for which the throughputs were slightly increased Even the ‘‘unknowns” see their yields reduced as the temperature is raised and half of these compounds are removed at 800°C These results accord with the trends observed by Lind, Berguerand and Marinkovic [22–25] when using manganese, nickel, and ilmenite in a secondary reactor with raw gas from the Chalmers gasifier
As shown in Fig 3, there are considerably fewer tar species remaining after reforming with feldspar, which in itself is a posi-tive result When the temperature was increased from 800°C to
900°C, the yields of some of the heavier tars with pure ring struc-tures also started to decrease This was the case for naphthalene, acenaphthylene, phenanthrene, and anthracene Overall, only ben-zene showed an increase in yield concomitant with the increases in temperature, most likely because it is a breakdown product of lar-ger tars Benzene is one of the most challenging components to reform owing to its resonance stability, which makes the ring structure difficult to open and at such short contact times, the rate
of formation is higher than the rate of destruction If this com-pound is excluded from the results shown inFig 3, the tar removal efficiency reaches 27% at 800°C and is up to 47% at 900 °C In com-parison, Nguyen et al achieved 62% conversion of tars when they used ilmenite at 800°C with a stream of raw gas from the Chalmers gasifier for the same contact time[49] It is noteworthy that longer gas-catalyst contact times, such as those that prevail in deployed processes, are beneficial for tar-reforming performance [10,49] Interestingly, if benzene and naphthalene – compounds for which there exist established separation methods – are excluded from the calculation, the tar conversion rate reaches 45% and up to 70% at
800°C and 900 °C, respectively
Within the framework of olivine experiments in the Chalmers gasifier [36], Marinkovic employed fresh olivine (from SIBELCO)
in the same reactor setup as inFig 1and at 800°C During this test for which the results are still unpublished[53], the gasifier was operated under conditions similar to those used in this study Feld-spar was found to be more active than olivine in stripping off the branches from the aromatic rings[53] Additionally, slightly higher yields of benzene and naphthalene are noticed for the olivine case While the feldspar efficiently removed all the phenol, indene, and fluorene and almost all the styrene at 800°C, fresh olivine left some of these species and left 50% more toluene Thus, the feldspar yielded a more favorable tar composition in the reformed gas than did the fresh olivine
Trang 7Overall, the alkali-feldspar profoundly modified the tar
spec-trum and displaced the species distribution towards the
elimina-tion or diminuelimina-tion of most components and the formaelimina-tion of the
more stable pure ring structures Higher temperatures produced
a reformed gas that had fewer tars, as expected by the enhanced
kinetics associated
4.3 Effects on the gas composition Fig 4shows the molar yields of the four most abundant perma-nent gas species in the raw gas (H2, CH4, CO, CO2) and those of the exiting reformed gas streams in the three temperature cases The resulting impact is a function of the quantity and quality of the tar reforming discussed above, but also equilibrium reactions, such
as those listed inTable 2, which can be catalyzed by the bed mate-rial The values shown are in mol/kg daf fuel, to enable compar-isons of the actual consumption and production rates of the species The error bars indicate the standard deviations of the four measurements For the three days of experiments, the gas compo-sition from the gasifier was fairly constant and the values pre-sented inFig 4are averaged over these three days
InFig 4, an appreciable trend is seen for the levels of H2and
CO2, both of which increase with temperature, while the level of
CO decreases accordingly This can be attributed in part to the WGS [see reaction (9)] However, at the higher temperature of
900°C, CO production starts to compensate for its consumption,
as witnessed at lower temperatures This is also the case for methane The stripping off of branched tars described in the previ-ous section likely contributes to these observations
Fig 5presents the molar yields of C2H2, C2H4, C2H6, and C3H6 There is an important decrease in the level of ethylene as a func-tion of temperature This is also the case for the other hydrocar-bons presented in Fig 5 Interestingly, the most problematic compounds in fuel synthesis, C2H2 and C3H6, are completely decomposed from 800°C C2H2is, for instance, known to be a pre-cursor to soot formation In the study conducted by Marinkovic
et al [53], the same permanent gas composition was obtained with fresh olivine at 800°C as in the feldspar case (Figs 4 and 5)
Fig 3 Comparison of tar yields between the inlet and outlet of the CLR reactor (g/kg daf fuel).
Fig 4 Molar yields of permanent gases for the dry raw and reformed gases at three
different temperatures.
Trang 8However, the feldspar converted all the C2H2 and C3H6 at this
temperature, while olivine left residual amounts, i.e., 0.01 and
0.012 mol/kg daf fuel, respectively These results, together with
the tar analysis results presented in the previous section indicate
the superiority of fresh feldspar over fresh olivine for this
application
The balance of C, H and O between the raw and reformed gases
and based on the permanent gas species was calculated and
expressed in mol/kg daf fuel The results are detailed inFig 6
As shown inFig 6, the carbon balance is seemingly achieved,
even though slightly more carbon can be found in the reformed
gas at 900°C The somewhat lower carbon yields detected at the
temperatures of 700°C and 800 °C can partially be explained by
the formation of a carbon deposit on the particles, which is favored
at lower temperatures, as confirmed by the trend shown inFig 6 This is eventually compensated for by the decomposition of unde-tected carbon-containing species into measured molecules at
900°C The balance of H and O for the three temperatures shown
inFig 6is highly consistent with the stoichiometry of water con-sumption through the WGS or any steam-reforming reaction, pro-ducing two H molecules for one O molecule in the dry reformed gas [see Reaction (10)]
Lastly, the raw gas produced in the Chalmers indirect gasifier has a high sulfur content of 100–300 ppm of H2S Although these levels are high, there was no indication of a decay in the catalytic activity during the raw gas sequence, which lasted several hours Indeed, the permanent gas composition in the exiting stream remained stable, which means that hydrocarbon reforming was not markedly altered
4.4 Material characterization The SEM/EDX images for the fresh particles are presented in Fig 7 The elemental distribution across the cross-section of a par-ticle is shown inTable 6 Considering the nature of the material, namely a mix of principally potash-feldspar (K-potash) and sodium-feldspar (Na-potash), the lines for the analysis were drawn over two types of the particles, so as to allow conclusions to be drawn regarding the differences in chemical composition Both
Na and K were distributed evenly over the particles
The same analysis was carried out on the material that had been exposed to the raw gas at 800°C The results are shown inFig 8 Comparing Figs 7 and 8, there are no noticeable differences between the fresh and used particles with regards to general shape and aspect ratio, and the physical integrity seems to be main-tained For instance, used feldspar does not display the character-istic smoother edges of the fresh material, as is often the case for catalysts in fluidized bed applications and is the result of friction between single particles[54] The particle size distribution (PSD) was unchanged after exposing the feldspar batch to the fluidization treatment detailed in the Experimental section The exposed bed particles had diameters in the range 125–180lm and the mean diameter was 150lm, i.e similar to the fresh material Moreover, the totality of the original bed inventory was retrieved after oper-ation, confirming that no erosion or elutriation had taken place
Fig 5 Molar yields of permanent gases for the dry raw and reformed gases at the
three different temperatures.
Fig 6 Yields of elemental carbon, hydrogen and oxygen for the dry raw and
Trang 9Table 7features the results of the line scans performed for the
particles presented inFig 8
Comparing the results inTables 6 and 7, no significant
differ-ences are noticeable A thermochemical equilibrium simulation,
carried out with the Factsage 6.2 software and using the feldspar
and raw gas compositions for the three tested temperatures,
indicated that alkali-K and -Na should not be released from the
particles The results inTable 7 confirm this simulation, as the levels of these elements are the same as those listed inTable 6 The alkali content contributed to reaching an equilibrium between hydrocarbon reforming and coke formation (see the next section)
At this stage of the study, the observed effect of the feldspar on the tar spectrum might be explained by the actual composition and structure of the material, which have compositional similarities with active silicate-based materials, such as olivine or silica-aluminum FCC catalyst, as presented in the theory
Aside from the consistency of composition, the feldspar showed
no indication of melting, which confirms that it can sustain tem-peratures of at least 900°C This finding is valid for the conditions prevailing in the batch reactor and throughout the test duration Other parameters, such as ash exposure, would probably change the result obtained for a biomass gasification unit with feldspar
as a primary measure
4.5 Carbon deposition For the 700°C and 800 °C cases with shut-down under inert conditions, the samples collected at the end of the experiments had a very different color compared to the virgin feldspar: while the fresh material had a yellow sand-like tone, the used material was dark-grey/black and had a metallic tone.Fig 9 shows pho-tographs of fresh and used particles The used batch on the right was collected for the 800°C case
Shut-down in air in the 900°C experiment revealed peaks of CO and CO2in the outgoing gas flow, allowing quantification of the carbon deposits Calculation from the integrated concentrations indicated that nearly 1 g of the carbon fed in the raw gas had formed deposits on the particles Based on the permanent gas and tar species in the former sections, this value represents roughly 1.5% of the total carbon fed This indicates that low levels
of carbon deposits are sufficient to give the particles a totally dif-ferent coloration Carbon deposition or coking on hydrocarbon-reforming metal catalysts is often synonymous with deactivation [55–57] In the present study, in contrast, the low metal content
of the feldspar to some extent explains how the catalytic integrity was maintained during these 2–3 h of operation In any case, the presence of carbon proves that reforming of hydrocarbons had occurred, although since the catalytic activity was not affected,
an equilibrium between the competing processes of formation and removal must have been reached, so that a sufficient number
of active sites still remained After elimination of the carbon deposit using air fluidization, the batch of spent feldspar retrieved its original sandy tone and resembled the fresh material shown in Fig 9
5 Discussion From the results obtained, it is clear that the alkali-feldspar has the properties required to upgrade a biomass-derived raw gas Overall, the feldspar functions as a tar-reforming catalyst despite its unconventional elemental composition, as compared with uti-lized and/or investigated metal-based catalysts The silicate struc-ture of feldspar is shared by olivine, which is successfully used in biomass gasifiers, and it also resembles the structures of the zeo-lites used in FCC for cracking heavy hydrocarbons This may explain the effects on the tar molecules observed in the present study As a consequence of its metal-deficient composition, feld-spar has the distinct and favorable property of being a poor oxygen-carrier, which means that in a deployed system, the upgraded gas quality should not be significantly penalized by partial combustion
Table 6
Elemental analysis of fresh feldspar particles.
K-feldspar
Na-feldspar
Fig 8 SEM/EDX image of a used feldspar particles.
Table 7
Elemental analysis of the used feldspar particles.
K-feldspar
Na-feldspar
Trang 10Overall, it seems that even at the rather low temperature of
800°C, all the phenolic compounds and most of the branched tars
were suppressed, an effect that was even more pronounced at
900°C A general feature of using this material is the shift in the
tar spectrum from numerous species to mainly pure ring
struc-tures This trade-off between the extent of desired tar removal
capacity and the co-production of certain valuable tar molecules,
e.g., benzene or naphthalene, is of interest to the chemical
indus-try In the present study, 70% of the tars (excluding benzene and
naphthalene) were converted at 900°C For the short residence
times (<1 s) used in these experiments, the rate of formation of
benzene exceeded that of its destruction If proper separation is
implemented, through for example distillation processes or the
use of activated carbon, one might not necessarily aim at reforming
all the tar compounds, but instead recover these pure ring species
Furthermore, we show that alkali-feldspar is active in the
decomposition of alkenes, which is usually beneficial for
down-stream fuel synthesis For instance, ethylene and propene are
effi-ciently converted, even at the low temperature of 700°C Acetylene
is fully converted starting from at least 800°C, while the olefins are
more difficult to eliminate The material exhibits appealing
promising capacity to produce methane in the higher temperature
case, and yields a H2/CO ratio of almost 3 at 800°C and 900 °C,
which is favorable for a downstream methanation reaction, as is
achieved in the GoBiGas plant[9] Indeed, the alkali-feldspar seems
to be appreciably active in the WGS reaction, resulting in the
pro-duction of H2and CO2and the consumption of CO
As the contents of K and Na of the particles were unchanged at
the ends of the experiments, the catalytic activity of the material
cannot be correlated to the amount of alkali present in the virgin
material Further studies with this feldspar are required to
scruti-nize the effects of structural and compositional changes and the
possible formation on the particles of an outer coating with lower
melting point or the potential to transport alkali species This could
involve longer-term experiments, looking at biomass-ash
interac-tions and the effects of repeated alternating exposure to oxidizing
and reducing gases on the activity of the catalyst The increased
porosity often seen in ageing particles could, for instance, benefit
the catalytic reactions
Interestingly, we found that the catalyst was capable of
with-standing raw gas conditions and fluidization over a long period
of time This period might even be longer if the tests had not been
aborted – despite the clear presence of a carbon deposit In a dual
fluidized bed, continuous catalyst regeneration is inherent to the
looping process This is the case in a dual-bed CLR and in an
indirect gasifier where the boiler serves as the catalyst regenerator There were no signs of deteriorated activity during these periods, which facilitates the definition of adequate circulation levels in
an up-scaled system
In any case, this is a resilient, inexpensive, and environmentally friendly material These properties permit higher rates of regener-ation, even if the activity decreases quicker than expected from the results of the present study Furthermore, screening of materials with other alkali contents or K/Na ratios can be conducted to elu-cidate catalytic efficiency trends and to identify the most suitable alternatives
Finally, one must bear in mind that the experimental configura-tion used in the present study is representative of the condiconfigura-tions that would prevail in a reactor system downstream of the gasifier,
at which point the biomass-ash has already been removed The ash interactions will be the focus of an upcoming investigation using bench-scale controlled experiments with ash addition and a full-scale gasifier in which feldspar will form the bed inventory
6 Conclusions The possibility to use an alkali-feldspar ore as a catalytic bed material to upgrade biomass-derived producer gas was investi-gated Tests involved a bubbling bed batch reactor fluidized with
a slipstream of raw gas generated by the Chalmers 2–4-MW indi-rect gasifier Experiments were carried out at 700°C, 800 °C, and
900°C, with focus on the extent of tar reforming and the change
in permanent gas composition The results show that the alkali-feldspar is robust and holds promise for use as a tar-abatement catalyst This conclusion is supported in the present study by the following lines of evidence:
– The material has mechanical properties that are suitable for flu-idized bed applications, including a lack of attrition, preserva-tion of the physical integrity, and no signs of agglomerapreserva-tion, even for longer exposure periods (up to 3 h) under harsh reduc-ing conditions of the raw gas
– The material has a low oxygen transport capacity (0.2%), which means that none of the larger fractions of the valuable producer gas is burned off in a circulating fluidized bed system and that the cold gas efficiency is not penalized as much as it is with the use of metal oxide catalysts
– The high sulfur content of the raw gas (>100 ppm) does not affect the catalytic activity towards hydrocarbon reforming Fig 9 Pictures of fresh (left) and used (right) feldspar particles.