ITI Energy Limited, Advanced Manufacturing Park, Brunel Way, Rotherham S60 5WG, UK [ Manuscript received November 30, 2012; revised March 14, 2013 ] Abstract Sulphonated nano-structured
Trang 1Syngas cleaning with nano-structured micro-porous ion exchange polymers in biomass gasification using a novel downdraft gasifier
Galip Akaya∗, C Andrea Jordanb, Abdulaziz H Mohameda
a Process Intensif ication and Miniaturization Centre, School of Chemical Engineering and Advanced Materials, Newcastle University,
Newcastle upon Tyne, NE1 7RU, UK; b ITI Energy Limited, Advanced Manufacturing Park, Brunel Way, Rotherham S60 5WG, UK
[ Manuscript received November 30, 2012; revised March 14, 2013 ]
Abstract
Sulphonated nano-structured micro-porous ion exchange polymers, known as sulphonated PolyHIPE Polymers (s-PHPs) were used in syngas cleaning to investigate their impact on tar composition, concentration and dew point depression during the gasification of fuel cane bagasse
as a model biomass The results showed that the s-PHPs used as a secondary syngas treatment system, was highly effective at adsorbing and reducing the concentration of all class of tars in syngas by 95%–80% which resulted in tar dew point depression from 90◦C to 73◦C It was
shown that tars underwent chemical reactions within s-PHPs, indicating that tar diffusion from syngas was driven by chemical potential It was also observed that s-PHPs also captured ash forming elements from syngas The use of s-PHPs in gasification as well as in an integrated thermochemical biorefinery technology is discussed since the tar loaded s-PHPs can be used as natural herbicides in the form of soil additives
to enhance the biomass growth and crop yield
Key words
biorefinery; biomass; downdraft gasifier; gasification; PolyHIPE Polymer; syngas cleaning; tar removal
1 Introduction
1.1 Integrated bioref inery
Biomass (including biomass waste) offers a unique
op-portunity in the prevention and abatement of global warming
since it is the only renewable source which can
simultane-ously provide chemicals, energy and fuel (CEF) In order to
establish CEF generation technologies based on biomass (i.e.,
biorefinery), the potential energy value, availability,
conver-sion characteristics of feedstock and their environmental
im-pact must be examined to ensure sustainability
The availability of energy from biomass waste is some 8
times that of current global energy demand whereas the
avail-ability for solar and wind is 100 and 10 fold, respectively
However, the physical distribution of these energy sources
be-comes broader as their energy potential increases
Further-more, unlike solar and wind, unused/waste biomass is
envi-ronmentally detrimental and a net contributor to global
warm-ing due to the generation of methane durwarm-ing its decay While
solar and wind resources generate only power, biomass can generate valuable chemicals through the production of syn-gas, all the essential chemicals can be produced through the establishment of a biorefinery technology to replace existing oil-refineries
One of the impacts of global warming and reduced fos-sil fuel resources is the emergence of food, energy and water shortages (FEWs) Solar and wind energy generation does not have any direct impact on food and water, but any biomass based CEF generation technologies must take into account its impact on food and water production and land availabil-ity Therefore, it is necessary to ensure that the biomass based CEF technologies are holistic and integrated with agriculture which consume 80% of water resources and energy used by industry
Integration of chemical or biochemical processes within
a chemical/biochemical plant is well known as it provides energy efficiency as well as reduction in capital and op-erating costs However, process integration on a wider scale is not practiced partly because of the nature of chem-ical/biochemical plants for which the ‘sustainability’ is
pro-∗Corresponding author Tel: +44-191-2227269; E-mail: Galip.Akay@ncl.ac.uk
This work was supported by the EU FP7 Integrated Project (COPIRIDE) Andrea Jordan was supported for her PhD studies by a National Development Scholarship from the Government of Barbados and a research grant from the Barbados Light and Power Company Limited which also supplied fuel cane bagasse for the experiments Abdulaziz Mohamed was supported for his PhD studies by the Libyan Ministry of Higher Education and Scientific Research.
Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences All rights reserved.
Trang 2vided through the ‘economies of scale’ As a result, oil
refineries, power generators and petrochemical plants are very
large centralized production platforms consistent with the
cen-tralized nature of the feedstock chemicals and intermediates
(i.e., fossil fuels) Costs and environmental impact
associ-ated with feedstock transport and product distribution from
centralized production facilities can be avoided by localized
small scale production facilities operating near the feedstock
source and product consumption which can be achieved with
biomass based CEF technologies However, such production
facilities will not benefit from ‘economies of scale’
There-fore, it is necessary to develop a technology which does not
have the burden of ‘economies of scale’ This technology is
already available in the form of Process Intensification (PI)
[1] and is highly suitable for small scale CEF generation
The full benefits of PI can only be realized if intensified
unit operations are integrated to obtain chemical-biochemical
plants Process integration on a global scale can further
en-hance the sustainability of biorefineries This approach has
been recently proposed and exemplified in the form of
am-monia production from biomass through integrated intensified
processes [2,3] and subsequently extended to biofuel
produc-tion and power generaproduc-tion [4,5] The critical intensified unit
operations of this ammonia technology include gasification,
syngas cleaning, gas separation and various reactors for
am-monia conversion and steam reforming of syngas The
in-tegration of the unit operations and global scale inin-tegration
with water and food generation technologies require novel
processes, catalysts and material particularly polymers, in
or-der to enhance biomass and crop yields and water and fertiliser
uptake efficiency by plants [6,7] which can be achieved by
nano-structured micro-porous (NSMP) polymers [8,9] These
materials are a special class of micro-porous polymers known
as PolyHIPE Polymers (PHPs) [8,10]
1.2 Nano-structured micro-porous polymers in process
inten-sif ication and energy
Based on the so called ‘confinement phenomenon’
us-ing NSMP-polymers [1], various intensified processes have
been discovered in agriculture (A-PI) [6,7,11], biology
(B-PI) [12,13] Intensified processes have also been observed
in chemicals sector (C-PI) [14,15] and energy conversion
(E-PI) [3] with intensification factor reaching well over 200 fold
compared with the current technology In these intensified
processes, various types of PHPs have been used and the
un-derlying mechanism of intensification is similar More
re-cently, these materials have been used as ‘intermediate
chemi-cals’ in the manufacture of ammonia from syngas in a catalytic
low temperature plasma induced process which operates at
at-mospheric pressures Here, sulphonated PHPs (s-PHPs) are
used as solid acid for ammonia adsorption in a multi-stage
process since at each stage only ca 10% conversion takes
place This method thus creates ammonium sulphonate within
the pores of s-PHP as slow release fertiliser However,
am-monia loaded s-PHP is also used as soil additive functioning
as synthetic rhizosphere (SRS) to enhance interactions among plant roots, nutrients, water, bacteria and root exudates which result in the enhancement of biomass generation and crop yield; i.e., Agro-Process Intensification [6,7,10] These inter-actions occur because the plant roots penetrate into the s-PHPs which are also highly hydrophilic As a result of hydrotropism and chemotropism, root penetration into s-PHPs is not random [6,10]
1.3 Tars as natural herbicides
Low molecular weight polycyclic aromatic hydrocarbons (PAHs) are found to show some phytoxicity in soil [16–18] However, tars are also used as protection against weeds, harm-ful insects and rodents and it was suggested that tars such as birch tar oil should be preferable to organic herbicides [18] Therefore, the profile of tars recovered from syngas and pro-cess water cleaning can be modified and low molecular tars can be oxidised to achieve non-phytoxicity and herbicidal properties This suggests that some of the PAHs captured by s-PHP during syngas cleaning can therefore be used as herbi-cides
1.4 Gasif ication and syngas cleaning
Gasification has emerged as one of the primary energy conversion technologies for sustainable energy production, particularly from low heating value biomass residues, due to the higher efficiencies of fuel conversion to energy as com-pared with combustion [19−21] It represents a key tech-nology in the establishment of a thermochemical biorefinery [3] having the lowest environmental impact in energy conver-sion technologies [21] as concluded through exergy analysis [22,23] During gasification condensable organic compounds known as tars are produced and become entrained in the syn-gas [24−28] Tars can cause widespread fouling of operating equipment as they can condense on downstream users causing extensive coking of heat exchangers and plugging of valves [29] Consequently in most post gasification applications, in-cluding power generation and gas to liquid conversion, tars
as well as particulates must be removed to varying degrees [29−31]
Biomass fuels which are high in lignin such as fuel cane bagasse (FCB), yield higher concentrations of tar compared with other fuels such as bone meal or municipal solid waste which contain catalytic contaminants useful in tar cracking Tars can be classified into five classes on the basis of their condensation behaviour and water solubility [32] Various tar compounds in each class are available (Tables 3 and 5 in ref-erence [28] Also see refref-erence [32]) In this classification system, the potential for condensation of a given composition
of tars is determined by calculating the tar dew point that is defined as the temperature at which the real total partial pres-sure of tar equals the saturation prespres-sure of tar [25,30] The
Trang 3classification system (Class 1−5) [32] indicates that at very
low concentrations of Class 4 and 5 tars, these compounds
will condense even at high temperatures while Class 2 and 3
tars condense only at extremely high concentrations and
tem-peratures below 0◦C
Tars not only cause fouling of downstream process
equip-ment but can represent as much as 10% of the low heating
value of the syngas produced [28,30] which is lost to the
syn-gas if not converted to H2, CO and CH4 The production of tar
therefore lowers the overall conversion efficiency of biomass
to syngas and increases the capital operating costs
Conse-quently the reduction and/or removal of tar from syngas is
critical to the wide spread development of commercial
small-scale biomass gasifier systems
Currently, the preferred option for tar reduction is in the
gasifier itself through process control and the use of primary
measures such as additives (i.e., CaO) and catalysts which
modify gasification conditions to produce less tar and more
hydrogen [4,30,33,34]
However, even by the addition of catalysts, tars are still
formed and in certain applications such as syngas-to-liquid
conversion or syngas-to-power conversion through fuel cells,
tars as well as degraded catalysts need to be captured from
syngas Several techniques for this secondary tar removal
have been developed [4,35,36]
A third method of tar removal is catalytic tar cracking
af-ter syngas generation This method combines the advantages
of the above methods [26] In this study we investigated the
use of nano-structured microporous and highly hydrophilic
ion-exchange polymers (s-PHPs) as a secondary measure to
remove tars from syngas produced through the gasification of
fuel cane bagasse (FCB) waste residues in a novel downdraft
gasifier
2 Materials and methods
2.1 Fuel cane bagasse (FCB)
Fuel cane, grown at various locations in Barbados (13o
10N, 59o 32 W) and at elevations ranging from 60–90 m
was harvested by mechanical harvesters in February during
the dry season They were cut approximately 15 cm above
ground and the stalk, cane tops and trash were immediately
loaded unto trailers and delivered to the Portvale sugar factory
where sugar was extracted within 48 h of harvesting To avoid
changes in biomass structure caused by complete drying, the
produced bagasse was then air dried outdoors in covered
ar-eas under ambient conditions (32◦C) to a moisture content
of 20−25 wt% After drying, the bagasse was sealed in
polypropylene bags and shipped over a two week period to the
United Kingdom for use in this study On arrival at the
labo-ratory in the UK, FCB was air dried indoors under labolabo-ratory-
laboratory-ambient conditions During this time the heaps were mixed
every two days to ensure even drying and the moisture content
monitored periodically until equilibrium with the ambient
at-mosphere (9.4−10 wt%, dry basis) was obtained It was then shredded in a hammer mill and pelletised into 8 mm diameter pellets using a Swedish Power Chippers AB commercial pellet press PP300, the final moisture content of the pellets ranged from 6.0−7.4 wt% (dry basis) The proximate and ultimate analyses of the FCB are presented in Table 1
Table 1 Proximate and ultimate analysis of fuel cane bagasse
Ultimate analysis
High heating value (HHV) (MJ ·kg −1) 18.9±0.3 Low heating value (LHV) (MJ ·kg −1) 17.6±0.2 Proximate analysis
Volatile matter (wt% db) 65 ±5 Fixed carbon (wt% db) 31 ±4
Size and bulk density Bagasse type size (mm) density (kg ·m −3)
Fibrous bagasse 0.09 −4.0 68 ±5 Pelletised bagasse D = 8 mm 727 ±3
* Calculated by difference db—dry basis
2.2 50 kWe air-blown downdraft gasif ier
A schematic of the downdraft gasifier system used in this work is shown in Figure 1 Gasification of FCB was carried out at atmospheric pressure in an intensified autothermal air-blown 50 kWe throated downdraft gasifier (Figure 1) The re-actor has a double wall and heat loss is further reduced by fibreglass lagging which covers the outer shell The basic gasification system was described in reference [4]
Briefly, fuel is batch fed manually into the reactor through the hopper at the top; after loading the gasifier the induced draft fan was switched on and the reactor started by manually lighting the air inlet ports with a butane torch Air, the gasify-ing agent, was sucked into the gasifier through the main air in-let valve at a controlled flowed rate and into the chamber sur-rounding the throat by the induced draft fan From there the air then flowed into the oxidation zone through a plane of air noz-zles The syngas generated in the gasifier was then extracted from the reactor by the suction effect of the induced draft fan
As the solid fuel was converted to syngas, this caused the re-maining fuel to flow down through the reactor under gravity The ash and char produced during gasification were manually emptied into the ash box by turning the ash box handle peri-odically during the gasification Thermocouples located at T1 (drying zone), T2 (pyrolysis zone), T3 (oxidation zone) and T4 (reduction zone) continuously monitored the gasification zone temperatures Online syngas sampling and analysis was
Trang 4carried out by Agilent HP 6890 gas chromatograph (GC)
con-nected at sampling point S3 whilst syngas samples were
col-lected manually in gas tight Tedlar sample bags at sampling Points S2 and S3 These samples were also analysed by GC
Figure 1 Newcastle university 50 kWe gasification system
2.3 Gasif ication of fuel cane bagasse
Gasification was carried out under optimal operating
conditions for FCB [19] Approximately 150 kg FCB was
gasified in 6 experimental run The equivalence ratio (ER)
was 0.26 and the gasification of pelletised FCB could be
sustained without bridging, thus yielding consistent syngas
quality FCB with a moisture content of 11.5 wt% was
used in these experiments; the typical operating
tempera-tures in the pyrolysis zone was 716±66◦C and in the
oxi-dation zone was 1040±130◦C The typical molar percentage
syngas composition (dry basis) was: H2= 12.1, CO = 17.2,
CH4= 3.6, CO2= 15.9, O2= 1.0 with N2representing the
bal-ance and trace amounts of C2H4and C2H6 The low heating
value (LHV) was 5.7±0.6 MJ·m−3
n (dry basis) with cold gas efficiency of 82%
2.4 Tar collection and storage
The filter box located immediately after the water
scrub-ber was filled with 1 kg s-PHP discs and the discs exposed
to a constant flow of 150 Nm3of syngas from gasification of
fuel cane bagasse over a 5 h period The effect of s-PHP on
tar concentration and composition in syngas was evaluated by
comparison of these parameters before and after the filter box
Tar sampling was carried out according to the draft Tar
Proto-col [37] with some modification Immediately after
stabilisa-tion of syngas producstabilisa-tion, samples of syngas were collected
simultaneously at sampling Points S2 and S3 under isothermal
conditions and at a constant flow rate for 3.5 h To prevent
condensation and/or thermal decomposition of target analytes
in the sample line, the line was trace heated to 300◦C for the
duration of sample collection The syngas was then bubbled
through a heated glass fibre thimble filter at a flow rate of
0.6 Nm3·h−1into a series of three impingers heated to 40◦C
and another three contained in a salt and ice bath at−12◦C (standard conditions are defined here according to NIST as 293.15 K and 101.325 kPa) All the impingers contained iso-propanol (99.9%) and after flow through the impingers the syngas stream was discharged to the atmosphere
On completion of sample collection, the isopropanol in the impingers was mixed, the impingers and tubing were rinsed with additional isopropanol and the rinsate was added into the impinger solutions and stored in an air tight brown bottle at 4◦C until the sample could be analysed
2.5 Tar recovery for analysis
The tars contained in the glass fibre thimble filters were extracted by Soxhlet extraction over a period of 5 h using iso-propanol After each extraction, 100 mL of the extract was re-moved and the remainder was added to the stored solution col-lected from the impingers After Soxhlet extraction the glass fibre filter thimbles were dried in an oven at 105◦C overnight and then cooled in a desiccator The difference in mass be-tween the initial filter used and the extracted filter represents the mass of particulate matter contained in the sampled syn-gas To determine the mass of gravimetric tar contained in the filters, the 100 mL extract was evaporated at 55◦C and
180 mbar using a rotary evaporator and the mass of the residue was recorded Determination of GC-detectable tar content of the gravimetric tar was carried out by re-dissolving the residue
in 25 mL isopropanol which was then stored in a sealed brown bottle at 4◦C until it could be analysed by GC/MS
2.6 Supercritical f luid extraction of tar from sulphonated PolyHIPE Polymers
PHP discs which were exposed to syngas containing tars were subjected to extraction in an SFT-100 supercritical fluid
Trang 5extractor using liquid CO2at 4000 psi and 85◦C Each
sam-ple was soaked in supercritical CO2for 15 min; the extract
produced was then released and collected in a separator This
was followed by another soaking for 10 min, release of the
extract and a further soaking for another 10 min after which
the extract was released The process of soaking with CO2
was repeated until no further increase in tar extraction was
observed The extraction vessel was then vented and the
col-lected tar extract was stored in a brown bottle at 4◦C prior to
analysis using GC/MS
2.7 Sulphonated PolyHIPE Polymer
Sulphonated PolyHIPE Polymer (s-PHP) was used for the
collection and removal of tars from syngas after flow through
the water scrubber It was prepared in batches and carried out
as described in reference [8] A synopsis of the methodology
used is described below
2.7.1 Emulsion preparation
Continuous phase (100 mL) and aqueous phase (1 L) were
prepared separately 25 mL of continuous phase was poured
into a stainless steel mixing vessel and 225 mL of aqueous
phase was pumped continuously into the continuous phase
for exactly 5 min with constant stirring Mixing was done
using a 9 cm diameter double blade impeller in which the
two blades are positioned 1 cm apart at right angles to each
other The base of the impeller was positioned 1cm above
the bottom of the vessel and the rotational speed of the
im-peller was 300 rpm On completion of the 5 min dosing period
for the aqueous phase, stirring continued for another minute
The resulting high internal phase emulsion (HIPE) was then
poured into 50 mL plastic tubes (26 mm diameter) and placed
in the oven overnight at 60◦C to allow polymerisation to
oc-cur After the polymerisation of HIPE, solid PolyHIPE
Poly-mer (PHP) cylinders were removed from the tubes and sliced
into 4 mm thick discs The void volume in PHP was 90% The
polymerisation-crosslinking reactions and chemical structure
of PHP are shown in Figure 2
2.7.2 Washing and drying of PolyHIPE Polymer
The 4 mm thick PHP discs were washed for 30 min with deionised water, then rinsed, and the process of washing and rinsing was repeated twice On completion, the discs were air dried overnight in a fume cupboard Dryness was deter-mined qualitatively by visual inspection of the tissue surface
on which the discs had been placed, and the dried discs were then sulphonated
Figure 2 Polymerisation-crosslinking reactions between styrene and divinyl
benzene and the chemical structure of PolyHIPE Polymer (PHP)
2.7.3 Sulphonation of PolyHIPE Polymer
To increase the absorptive capacity of PHP, discs were soaked in concentrated H2SO4 (97%) for 2.5 h During this time the containers were agitated periodically to ensure that both surfaces of the discs remained in contact with the acid After soaking, the discs were removed from the acid, placed
on the microwave turntable and microwaved at 850 W, 180◦C for 5×30 s periods with four alternating 1 min cooling peri-ods They were inverted after the third heating interval so as
to reduce the occurrence of uneven heating On cooling, the discs were washed in deionised water for 10 min, rinsed and the process repeated once more They were then left to dry
in a fume cupboard, once dried the discs were then ready to
be used in gas cleaning The sulphonation reaction of Poly-HIPE Polymer with sulphuric acid and the chemical structure
of sulphonated PolyHIPE Polymer (s-PHP) are shown in Fig-ure 3
Figure 3 Sulphonation reaction of PolyHIPE Polymer with concentrated sulphuric acid and production of sulphonated PolyHIPE Polymer (s-PHP)
Trang 62.8 Tar analysis
The sample extracts collected were analysed for tars
by gas chromatography/mass spectroscopy (GC/MS) using
an HP 5971A GC/MS The column used was an HP 5MS
30 m×0.25 mm i.d, 0.25 μm film thickness The carrier
gas was high purity helium (99.999%) at a flow rate of
1.0 mL·min−1. The temperature programme was: initial
50◦C where it was held for 5 min, then to 325◦C at a rate
of 8◦C·min−1, where it was held for another 5 min The
in-jector temperature was set at 250◦C and 2 μL of each sample
was injected in the split mode with a split ratio of 50 : 1 MS
was operated in the electron ionization (EI) mode at the
elec-tron energy of 70 eV The transfer line and ion source
temper-atures were 280◦C and 160◦C, respectively Identification
of the tars was done using the NIST spectral library and the
MassBank high resolution mass spectral database;
quantita-tive analysis was carried out in full scan mode in the range of
50–500 u using internal and external standards
2.9 Scanning electron microscopy-energy dispersive X-ray
(SEM-EDX) analysis
Investigation of the microstructure of sulphonated PHP
was carried out using Environmental Scanning Electron
Mi-croscopy (ESEM) Identification of the substances deposited
on the fractured surfaces of PHP was done using SEM-EDX
2.10 Tar dew point calculation
The tar dew point of each mixture of tars produced was
calculated using the tar dew point model developed by the
Energy Research Centre of the Netherlands (ECN) [38] This
model has an accuracy of±3◦C in the temperature range of
20–170◦C and is the sum of all the dew point values of each
tar species present It is based on the behaviour of ideal gases
and Raoult’s law and Antoine equation are applied for
calcu-lation of the dew point of a mixture of hydrocarbons, using the
vapour pressure data of individual compounds Only tars with
molecular weights between toluene and coronene are
consid-ered, the heavier Class1 tars are not included in the calcula-tion Since Class 1 tars have high tar dew points at low con-centration, this means that the actual tar dew point for syngas containing these tars is higher than the calculated value
3 Results and discussion
3.1 Tar scavenging from syngas using sulphonated PolyHIPE Polymer (s-PHP)
The performance of s-PHP in scavenging tar from syngas was investigated using a flow-through system in which syn-gas from the syn-gasifier flowed through a packed bed of s-PHP particles ( = 25 mm, thickness = 5 mm) The s-PHP parti-cles were loaded into the filter box (Figure 1) and retained by
a stainless steel mesh to ensure that carryover of PHP from the filter box to the induced draft fan did not occur The s-PHP was exposed over a period of 5 h to 150 m3of syngas containing tar Simultaneous tar collection was carried out at sampling Points S2 and S3 immediately before and after the s-PHP packed bed (Figure 1) GCMS chromatograms of the tars in syngas collected before and after passing through the s-PHP bed are shown in Figure 4 and Figure 5, respectively The identification of various peaks together with
mass-to-charge ratio (m/z) is shown in Table 2 It can be seen from
Figures 4 and 5 that a large number of components have been
reduced except for m-xylene/p-xylene and ace napthylene As
discussed below, this may be due to tar reaction with s-PHP Nevertheless, there is substantial reduction in the overall tar content in syngas following extraction with s-PHP
Quantitative analysis of tar removal was also carried out
in order to determine the tar extraction capacity of s-PHP The tar removed by the s-PHP was extracted by supercritical fluid extraction using liquid CO2as described in Section 2.6 Quantitative analysis of the tar extracted from the s-PHP was conducted and it was found that s-PHP captured 0.11 g total tar/g s-PHP used The compounds found in the extracted tar are categorised into their respective tar class in Table 3 which also shows the calculated dew point of the tar decreased from
90◦C to 73◦C
Figure 4 GCMS chromatogram of tar in syngas before tar extraction (sampling location S2 before the filter box shown in Figure 1) Full scale of the
abundance is 65000 units
Trang 7Figure 5 GCMS chromatogram of tar in syngas after tar extraction with
s-PHP (sampling location S3 after the filter box in Figure 1) Note that the
full scale of abundance is 25000 units
Table 2 Identification of tar components from the tar
collected from syngas
12 204 9,10-bis(chloromethyl) anthracene
Table 3 Tar compounds extracted by supercritical CO 2
from sulphonated PolyHIPE Polymer which
was exposed to tar laden syngas
2 3-phenoxy-1,2-propanediol
2,4-dibenzyl-3,4-dimethylpyridine 3-phenylpropanal 2-pyridinecarbonitrile 4-nitrobenzaldehyde 3-formyl-2-methyl indole
3 3-hydroxy-4-methoxybenzaldehyde
fluorene benzophenone oxime benz[anthracene]
phenanthrene anthracene 4,5-methylene phenanthrene 9,10-bis(chloromethyl) anthracene
6 H-dibenzo[b,d] pyran pyrene
Tar dew point before extraction 90 ±6 ◦C
Several of the tar compounds extracted from the PHP were not present in the syngas sampled before the PHP bed This suggests the possibility of tar conversion promoted by active –SO3H sites on PHP This view is further strengthened
by the fact that the –SO3H sites have been used for the selec-tive removal of surface acselec-tive species in the demulsification
of crude oil-water emulsions [14,15] and it was also used as a solid-state acid for ammonia removal from the reaction mix-ture following ammonia synthesis Therefore, PHP does not only simply act as a filter adsorbing the tars but also interacts with these compounds
The tar compounds contained in the syngas after flow through the s-PHP are listed in Table 4 It is evident that many of the Classes 2 and 3 compounds were removed by the s-PHP and no longer present in the syngas However, the appearance of some previously undetected compounds such
as styrene in syngas may be due to leaching of the unreacted styrene dissolved in PHP On the other hand, other previously undetected compounds, 1,2-benzenediol and 3-phenoxy-1,2-propanediol are likely to be due to tar reaction in the s-PHP Figure 6 shows the changes in concentration and composition
of the tar classes after syngas flow through the s-PHP
Table 4 Tar compounds in syngas after flow through
sulphonated PolyHIPE Polymer
1,2-benzenediol 3-phenoxy-1,2-propanediol
dibenzofuran
phenanthrene
pyrene
It can be seen from Table 3 that use of the sulphonated PHP decreased the tar dew point to 72.6◦C, however since Class 1 tars are also present, this value is not the true tar dew point Use of this material as the sole syngas clean up system would require the true tar dew point be determined experi-mentally so as to prevent the deposition of tar on equipment downstream of the gas clean up system Overall, s-PHP was 83% efficient in the removal of tar from the syngas With re-spect to the individual tar classes the efficiency of s-PHP for removal of Classes 1, 2, 3, 4 and 5 tars was 95%, 81%, 83%, 80% and 85% respectively, as shown in Figure 6
The high removal efficiency of Classes 2 and 5 tars exhib-ited by PHP provides strong evidence for the potential use of a combination of primary tar removal within the gasifier, which
is followed by use of a secondary treatment system consisting
of a packed bed of s-PHP as means of syngas polishing Syngas tar analysis before and after s-PHP extraction as well as analysis of tars extracted from PHP indicate that both polar tars (Class 2) and non-polar tars (Classes 1,3,4,5) have been absorbed by s-PHP
Trang 8Figure 6 Concentration of tars in each class before and after PHP
3.2 Interactions between tars and sulphonated PolyHIPE
Polymer
The interactions between tar and sulphonated PHP were
investigated by examining s-PHPs used in tar extraction
Fig-ure 7(a) shows the presence of tar deposits within the pores
of s-PHP after exposure to tar loaden syngas Through use
of SEM-EDX in Figures 7(b), 7(c) and 7(d), droplets of tar
associated with fragments of char embedded in the
micro-porous network were identified, providing clear evidence that
tar scavenging is not limited to adsorption on the external
sur-faces of the polymer but that it also occurs across the
inter-connected pore network Although tars consist primarily of
C, H and O, during gasification of FCB ash forming elements
and elemental sulphur released to the syngas will also adsorb
onto droplets of tar [28] The SEM-EDX spectra produced of
Points 0, 1 and 2 in Figure 7(a) are shown in Figures 7(b),
7(c) and 7(d) Both Figures 7(b) and 7(c) show the presence
of C, H, O and S (which comes from the PHP itself) as well
as the main ash forming elements in FCB In Figure 7(d) the
SEM-EDX of sulphonated PHP alone is shown
It was noted during the collection of PHP after removal
of the tar species from the syngas, that the PHP species no
longer had a spongy texture but had become extremely brittle
and readily broke into small pieces when compressed
Al-though mechanical characteristics of s-PHP changed upon
re-action with tars, we have not determined their tar absorption
capacity as a function of time on-stream Our on going
stud-ies indicate that once s-PHPs become saturated, it is possible
to treat them chemically with acid to reverse the elasticity of
s-PHPs and to oxidise the tar compounds Investigation of the
sulphonated PHP morphology after exposure to the syngas
us-ing ESEM showed that:
(i) adsorption of tar by sulphonated PHP occurred not
only on the surface of the particles but also inside the PHP
particles as well;
(ii) associations of tar droplets and char particles ranging
from 20−80 μm were captured in the microporous network as
the syngas flowed through the PHP monoliths
Figure 7 ESEM and EDX examinations of PolyHIPE Polymers after tar
de-position (a) ESEM image of tar droplets and char captured in sulphonated PHP (fractured surface) after 3 h exposure to syngas ( ×1000) Points 0, 1 and
2 indicate the location at which EDX spectra shown below were taken (b,
c, d) EDX spectra of Points 0, 1 and 2 from Figure 7(a) The EDX spectra
of Point 2 shows the spectrum of PHP only Note the difference in elemental composition as compared with the spectra of the char and tar at Points 0 and 1
3.3 Mechanism of tar removal
The mechanism of tar removal from syngas can be ex-plained by the confinement phenomenon [1,39] which has
Trang 9been utilised in process intensification including bioprocess
intensification [13], tissue engineering [12], separation
pro-cesses [14,15], agro-process intensification [6,7] and more
re-cently in chemical catalysis [39] In general terms,
accord-ing to the confinement phenomenon, the behaviour of matter
(including cells/bacteria and reactive chemical species) is
dic-tated by the size and biochemical/chemical structure of the
confinement media in which the matter is present Clearly,
the size of the confinement media must be comparable with
the size of the matter that is confined Although the size of
the individual molecules are small compared with that of the
pores of PHP, nevertheless, surface active or polar molecules
can grow as clusters through aggregation within the
Poly-HIPE Polymers as shown previously [40,41] These
struc-tures are highly stable (low entropy) especially in the
pres-ence of confinement media and hpres-ence, there is a driving force
for such molecules to diffuse from the bulk fluid (liquid or
gas) into the confinement media where they are stabilised In
liquid systems, this phenomenon was successfully applied to
oil-water demulsification [14,15], chemical catalysis [39] and
surfactant separations [40,41]
In the case of tar removal from syngas, there is an
addi-tional driving force for tar diffusion into s-PHP from syngas
due to the chemical reactivity of tars As shown before
(Sec-tion 3.1) tars appeared to undergo chemical changes upon
ad-sorption by s-PHP, therefore tar diffusion enhancement based
on chemical potential can be expected
It is clear that there is a chemical potential driven diffusion of tar molecules from the bulk of the syngas Once within the pores, some of these molecules undergo chemical reaction which are stabilised It may be possible to use such tar loaden s-PHPs from gasification of biomass as slow re-lease natural herbicides so that when herbicidal effectiveness disappear, these s-PHPs can then act as soil additives in agro-process intensification [6,7] Although such applica-tions require further research, the current method of tar re-moval illustrates the potential of s-PHP in an integrated holis-tic biorefinery technology
Tar removal can be further intensified by process inten-sification fields such as electric and plasma fields with or without PolyHIPE Polymer We have recently shown that tar removal efficiency can be increased over 98% [5] using such hybrid methods which also crack tars, thus increasing its calorific value while enabling syngas for catalytic conversion
to fuels and chemicals such as ammonia
This tar removal method was also applied to syngas clean-ing in a 1 MWe capacity gasifier (scaled up version of the cur-rent gasifier) and produced clean gas as indeed observed by the change in the flared syngas as shown in Figure 8 Fig-ure 8(a) shows the orange colour of the flared syngas be-fore tar extraction with s-PHP while Figure 8(b) illustrates the colour of the flared syngas after tar extraction The descrip-tion of this 1 MWe gasifier was disclosed previously [42,43]
Figure 8 Visual demonstration of the tar cleaning effectiveness by the method carried out with 1 MWe scaled-up gasifier Colour of the flare (a) before syngas
cleaning, (b) after syngas cleaning
4 Conclusions
We have shown that tar extraction from syngas using
sulphonated PolyHIPE Polymers can result in 83% tar re-moval and substantial depression of tar dew point from ca
90◦C to 73◦C with un-optimised tar removal capacity ca 0.1 g tar/g s-PHP This method is especially useful as a
Trang 10secondary measure of tar removal The reactivity of the tars
after captured by s-PHP has been demonstrated The
sutainability of the technique can be further enhanced by
s-PHP as a chemical intermediate (high temperature solid acid)
in the reactive removal of ammonia from product stream
in ammonia synthesis as described recently [3] followed by
tar/hydrocarbon removal from gasification process water
re-mediation [42] This ammonia containing s-PHP can then be
used as soil additive for the multiple roles of [6,7] (a) slow
release fertiliser, (b) natural herbicide, (c) water and nutrient
in the soil, (d) synthetic rhizosphere (SRS) [6,7] in order to
enhance water and nutrient uptake through the plant root
sys-tem, and (e) support for nitrogen fixing bacteria as part of the
SRS function Our estimates indicate that, these functions can
be carried out sustainably at a production cost of ca 10 £/kg
s-PHP
Acknowledgements
This work was supported by the EU FP7 Integrated Project
(COPIRIDE) Andrea Jordan was supported for her PhD studies by a
National Development Scholarship from the Government of
Barba-dos and a research grant from the BarbaBarba-dos Light and Power
Com-pany Limited which also supplied fuel cane bagasse for the
experi-ments Abdulaziz Mohamed was supported for his PhD studies by
the Libyan Ministry of Higher Education and Scientific Research
We are grateful for all the support received
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