Fuel Processing Technology 137 (2015) 259–268

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Fuel Processing Technology

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Reforming of tars and organic sulphur compounds in a plasma-assistedprocess for waste gasification

Massimiliano Materazzi a,b, Paola Lettieri a,⁎, Luca Mazzei a, Richard Taylor b, Chris Chapman b,⁎⁎a Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UKb Advanced Plasma Power, South Marston Business Park, Swindon, SN3 4DE, UK

⁎ Corresponding author. Tel.: +44 207 6707867; fax: +⁎⁎ Corresponding author. Tel.: +44 1793 238523; fax: +

E-mail addresses: p.lettieri@ucl.ac.uk (P. Lettieri), chri(C. Chapman).

http://dx.doi.org/10.1016/j.fuproc.2015.03.0070378-3820/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 November 2014Received in revised form 9 March 2015Accepted 10 March 2015Available online 7 April 2015

Keywords:Waste gasificationFluidized bedPlasmaTarSulphur

Waste gasification is considered a valuable and sustainable solution to the production of clean energy (via gasturbines or gas engines) and bio-fuels, such as synthetic natural gas and bio-hydrogen, provided that the syngasproduced in the gasifier is free of condensable tars and organic sulphur contaminants that cause equipment foul-ing and deactivation of catalytic stages downstream. In particular, catalytic reaction stages are highly sensitive tospecific trace contaminants (e.g. PAHs, thiophenes, etc.), necessitating the use of additional cleaning operationsto remove these residues to levels where the catalyst degradation is acceptable. In this work, the use of thermalplasma (coupled with primary waste treatment) to completely reform tars and organic sulphur compounds tosimple gaseous products (predominantly H2 and CO) is assessed. To this end, a 20-hour waste gasification runwas performed on a two-stage fluid bed-plasma demonstration plant to investigate the tar evolution in thesyngas, with special attention on the chemistry of generic and sulphur-substituted aromatics within the plasmastage. The organic fraction in the gas phase was found to be completely reformed under plasma conditions,leaving essentially CO, H2 and H2S as ultimate products. In particular, reduction efficiencies typically exceeded96%v/v for complex organics (e.g. PAH) and thiophenes were observed. The syngas, after a tertiary simplifiedgas cleaning process, is suitable for high efficiency power generation, or conversion to a fuel gas capable of injec-tion into national or industrial supply grids.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

The major technical obstacle faced by conventional waste gasifiers/pyrolysers is the high level of tars and organosulphur contaminationin the syngas, which can cause fouling and corrosion problems in thepower island, as well as the inactivation of the catalysts used in conver-sion processes by poisoning and carbon deposition. This has been theprimary cause of waste gasification developments failing over the last30 years and has prevented their commercial adoption with efficienttechnologies for power generation (e.g. gas engines, fuel cells, etc.)and catalytic transformation to generate either hydrogen or alternativefuels [1–5].

The level of tars and contaminants in the syngas is equallydetermined by the gasifier design, feedstock nature and process condi-tions [5]. Fluid bed systems are currently the most employed in wastegasification [6]. They include a range of different designs, such as bub-bling fluid bed (BFB), circulated fluid bed (CFB) and dual fluid bed [6].Fluid bed gasifiers (FBG) have been shown to be very flexible in their

44 207 3832348.44 1793 828 287.

s.chapman@app-uk.com

ability to handle a range of wastes, without the need for additional fossilfuels orwoody biomass to be added to the process. The expanded bed ofinert particles creates a large mass of hot material that is able to absorband mitigate fluctuations in fuel conditions with little-to-no change inperformance. This “flywheel effect” is better suited to minimise spikesin emissions due to the wide fuel variability when working withwaste-based materials, where small differences in fractions of certain‘key components’, plastics in particular, may cause disproportionatechanges in the gasification product yields. However, with high volatility(more than 60%) and low ignition temperature (250–350 °C), wastematerial is prone to devolatise immediately after the injection into thereactor [7], making it more susceptible to tar formation and sulphurcontaminants release when compared to other fuels, such as biomassor coal. The assumption is oftenmade that tars and organosulphur com-pounds are progressively converted to CO, H2, and H2S with increasingreaction severity, which is a function of oxygen content, temperatureand time. This is truewith primary tar products, characterised by prima-ry vapours andmixed oxygenates, such as phenols, acetaldehyde, cresoland sulfones, which are very reactive in high temperature conditions.However, the relative proportion of aromatic and longer chain polycy-clic aromatic hydrocarbons (PAH) increases with increasing tempera-ture, and a more refractory tar is left in the producer gas [8].Naphthalene and thiophene, for example, do not thermally react in

260 M. Materazzi et al. / Fuel Processing Technology 137 (2015) 259–268

the gas phase up to temperatures as high as 950 °C. So, once formed, it isunlikely that they will decompose at the normal gasification conditionsused [8].

Furthermore, waste materials are likely to contain substances invarying quantities in the ash-forming fraction (i.e. soda, potash, sul-phates, phosphates, chlorides etc.) which can form low melting pointeutectics in the fluid bed. The maximum temperature of operation ofthe FBG is therefore limited to around 800-850 °C, to avoid the meltingdanger of the various mineral phases in the system, which would pro-mote coalescence of solid particles and defluidisation of the bed [7]. Atthese temperatures, a certain range of problematic components is stillstably present, or newly generated. The literature reports an average tarloading of FBG reactors of about 10 g/Nm3 (mainly PAHs) [8], while thetotal sulphur content is in the order of 100 ppm (142 mg S/m3) [9]. Cuiet al. [10] analysed theproducer gas of afluid bed gasifier at 800 °C fuelledwith wood chips, and identified principally tar species with an averageconcentration of 15.5 gm−3 (dry gas basis) and several organosulphurcontaminants such as thiophene (5.58 mg S/m3), benzo[b]thiophene(0.85 mg S/m3) and an unknown (0.9 mg S/m3) compound. AlthoughPAHs and thiophenic sulphur compounds in low concentration areoften not adequately evaluated, they are more persistent than othercontaminants in gas cleaning processes, and require specialised unit op-erations. There has therefore been a significant effort to develop technol-ogies that can remove these contaminants in a technically robust andeconomical way.

One possible approach is to condense the tars from the gas streamusing either water or oil washing, and attempt to recover the significantenergy contained in the tar condensate by recycling it to the thermaltreatment stage [11]. An example of such approach has been developedat the Energy Research Centre of the Netherlands (ECN)with amultiplestage scrubber in which the gas is cleaned by special scrubbing oil [12].However, particularly in the treatment of wastes, contaminants includ-ing sulphur, chlorine, heavy metals and particulates are then inevitablyconcentrated in the recycle stream. This may limit or even preclude thepossibility of tar recycling, leaving a problematic waste for disposal and/or clean-up which is not only highly toxic but typically embodiesaround 10% of the energy value of the input fuel [13]. This streamcould be incinerated or combusted separately for energy recovery, butthis introduces further technical and economic challenges given thehigh concentration of pollutants (primarily sulphur and chlorinebased) both in the solid residue and in the emissions from the combus-tor. The removed organics may also be partially water-soluble and somay be present at elevated levels in the water scrubbing/effluentclean-up system. Moreover, although such removal systems have beendesigned and operated on fuel gases generated from low tar/sulphurbiomass feedstocks (such as hard wood), their application to wastefuels, where the complex spectrum of contaminants generated is verydifferent, has not been reliably tested.

The other approach reported in the literature is the addition ofcatalysts and transition metal oxides within the gasifier to reduce thetar loading and promote hydrogenation and/or steam reforming of theorganics [8,14]. Analogously, organic sulphur compounds can be con-verted into hydrogen sulphide and similar compounds [15]. However,the results have not been promising due to a combination of cokingand friability of the added catalyst [8]. Furthermore, waste streams con-tain high levels of ash and inerts, which dilute and reduce the efficacy ofthe added materials, so that the extent of tar and organosulphur reduc-tion that can be attained by this method is limited. These catalysts alsoadd to the operating cost of the plant, especially if we consider thatthey are often used in separate hydrodesulphurisation units down-streamof the gasifier [16]. The ongoing research is thenmoving towardsdeveloping inexpensive catalysts which are active in the presence ofboth hydrocarbons and sulphur compounds, and can be commerciallyoperated at relatively mild conditions [16].

A further technical issue that is frequently associated with conven-tional waste gasifiers/pyrolysers is that they may produce a solid ash/

deposit residue that can contain high levels of carbon, residual tarsand sulphates. This not only represents a loss in energy conversion inthe process but, more importantly, represents a large volume of waste(typically 10–60% of the ash mass) that must be sent to a hazardouslandfill facility [17].

To avoid all these problems, the use of thermal plasma (coupledwithprimary waste treatment) has recently been applied for its ability tocompletely reform/crack the tars to simple gaseous products (predom-inantly H2 and CO), which contribute to the net calorific value (NCV) ofthe cleaned gas. Simultaneously, inorganic particulate and ash-typecomponents are converted into a stable vitrified product. The hydrogenrich syngas from the two-stage process contains very low residual levelsof tars and, in addition to potential use for high efficiency power gener-ation, is suitable for the production of liquid fuels, renewable hydrogenand/or substitute natural gas [18–20].

Thermal plasma arcs are characterised by their high temperatureand intense UV radiation. The energy density of the plasma arc is typi-cally two orders of magnitude higher than a combustion flame [21].The energy input from plasma is readily controllable and (unlike com-bustion systems) is independent of the process chemistry, generatinglow off-gas volumes, reducing the size, complexity and the associatedcapital and operating cost of the downstream gas cleaning equipmentand rendering the process flexible to changes in the fuel characteristics,typical of waste materials.

The main advantage of coupling the two technologies is that theoxidant addition rate and power input in the two-stage process can becontrolled independently while, unlike single stage processes, thegasification stability is not dependent on the gas evolved from the fuelitself. Critically, the heat input to the units and emission control arenot dependent on the ability to combust residues such as char andtars that are generated in the process andwhose composition and quan-tity cannot be readily controlled [22]. This is an especially importantconsideration in treating waste feedstocks, which are heterogeneousin nature and contain much higher levels of ash and volatile matterthan conventional biomass.

This study is concerned principally with the thermal treatment ofsulphur-rich refuse derived fuel (RDF) prepared from municipal solidwaste (MSW), commercial and industrial (C&I) and automotiveshredded residue (ASR) wastes. A 20-hour waste gasification run wasperformed on a moderately large scale demonstration plant whichincorporates a plasma processing stage (i.e. plasma converter, or PC)for conditioning the gas generated from a fluid bed gasifier (FBG). Anexperimental program was defined to investigate tar evolution in thehot syngas, focusing on the chemistry of generic and sulphur-substituted aromatics within the plasma stage. This provides a clearerunderstanding of the mechanism as to how these contaminants evolveand provides a basis for assessing the role of plasma chemistry on thereforming process.

2. Experimental

2.1. Materials

ThepreparedRDF comes fromanumber ofwaste treatment facilitiesin floc form of size ranging from 5 to 40 mm. Key rawmaterials arisingin the original waste were food (3–7%), paper and card (15–30%), plas-tics (10–15%), wood (5–10%) and textiles (5–10%). Other combustibles(~2-5%) and other non-combustibles (5–10%) were also prominent.Although not specified, anecdotal evidence would suggest that theother combustibles would include materials such as rubber, underlayand foam (used in upholstery and bedding, for example). Similarly,the non-combustible fractionwould containmaterials such as construc-tion and demolition waste (C&D), rubble and ceramics. Once received,the RDF was bagged into individual flexible intermediate bulk con-tainers (FIBC) containing ca. 70–150 kg each. These bags were thensampled, using a cone and quartering technique, and a weighted

Cellulose47%

Hemicellulose23%

Lignin20%

Polyethylene4%

Styrene1%

Other (PET, LDPE, PVC,

etc.)5%

0

2

4

6

8

10

12

14

16

18

Lignin Hemicellulose Cellulose Mixed plastic

S/ m

g g

-1

Organically bound S (-SH,-S, -S-S)

Inorganic S (SO4)

Elemental S

Fig. 1. (Top) Estimated RDF organic matrix composition in its principal basic constituents(values given inweight %, dry-ash free); (Bottom) Rough estimation of the relative contri-bution from sulphur species in RDF constituents derived from literature [24–27].

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average samplewas prepared. This samplewas then sent to an indepen-dent, United Kingdom Accreditation Service (UKAS)-certified testinglaboratory for ultimate and proximate analysis, along with calorificvalue (CV) measurement. The results of this analysis are summarisedin Table 1.

The wide range of componentmaterials associated with the originalwastes, and sulphur content therein, makes the exact organic groupsspeciation very challenging. Nonetheless, the organic RDF matrix canbe viewed as a combination of different complexmacromolecular struc-tures (e.g. cellulose, hemicellulose, lignin, polyethylene, etc.) reflectingthe parent materials composition. Organic sulphur is bound in thesefunctional groups, and can be categorised into one of the functionalitiessuch as thiols (−SH), sulphide bonds (−S), thiophenes, thiopyrones,etc. [16,23]. Sulphur is also present in elemental form (S) and two inor-ganic forms, being pyritic sulphur (FeS2), and sulphates (Na2SO4, CaSO4,FeSO4), largely found in car-tyre scrap and demolition wood [23].

The relative distribution of the different forms of sulphur in themainfour RDF organic structures (i.e. lignin, hemicelluloses, cellulose andplastic polymers) is reported in the literature [24–27] and shown inFig. 1. Thus, in the studied RDF sample the inorganic sulphate is about30–35% of the total sulphur, the elemental sulphur is about 1–2% andthe rest of the sulphur, approximately 60–70%, is either tightly adsorbedpolysulphide and/or covalently bounded to the organic matrix as thiolsor sulphides. However, given the high variability of RDF composition,this apportioning of sulphur between organic and inorganic is notcertain, and remains only a reasonable approximation.

2.2. Process equipment

Data reported and discussed in this paper are generated from a spe-cific set of test work performed with the Advanced Plasma Power plantin Swindon (UK), which is a reduced capacity version of a commercialfluid bed-plasma plant. It comprises a FBG designed for steam/oxygengasification closely coupled to a single carbon electrode plasma convert-er, as shown in Fig. 2.

The waste feed is metered into the FBG under controlled conditions,using a variable speed screw feeder at rates of up to 100 kg/h, and acrude syngas is produced. The FBG is specifically configured to handlea broad range of feeds, from the relatively simple (e.g. biomass) tothose usually consideredmore problematic (such as RDF). The FBG con-tains a bed of nominally 1 mmmullite particles, whose particle size hasbeen selected tomatch the hydrodynamic conditionswithin the bed en-abling efficient fluidisation whilst avoiding high rate of elutriation.Mullite is also chosen on account of its low cost, high availability, andgood resistance to thermal shock and attrition. The fluid bed is main-tained at a temperature of between 700 and 850 °C, with the actual

Table 1Proximate and ultimate analysis of the RDF used.

RDF (as received)

Mechanical characteristicsMean particle size, mm 30Bulk density, kg/m3 70

Proximate analysis, % (w/w)Fixed carbon 6.4Volatile matter 59.6Ash 16.18Moisture 13.08

Ultimate analysis, % (w/w)C 41.84H 5.01O 21.65N 1.58S 0.4Cl 0.25NCV, MJ/kg (dry basis) 19.47

operating condition depending on fuel characteristics and desired reac-tion profiles. The flows of steam and oxygen are finely controlled tomaintain the bed temperature, ensure good fluidisation of the gasifierbed and achieve the required syngas quality. Bottom ash is drawndown to the bottom of the FBG and then screened at 2 mm to separateout the oversize material. The undersized material is returned to thefluid bed unit, while the rest is fed into the PC. The crude syngas pro-duced exits from the top of the gasifier and is then passed by way of arefractory lined duct to the PC. The PC is a transferred arc plasma furnacewith amolten pool of slag andmetals at the base, formed from themelt-ing of the inorganic fraction of the fuel feed. The slag melt also ensuresthat an effective electrical path is maintained with the conductivehearth/anode return electrodes which are built into the converterhearth. The syngas enters the side of the converter chamber above theslag level and circulates around the periphery of the cylindrical chamberallowing the gas to increase in temperature (~1200 °C) while receivingmaximumexposure to the intense ultra violet light from the plasma arc,aiding cracking of tar substances. Separation of small particulates (flyash, entrained bed particles, char fragments) from the syngas is promot-ed by the cyclonic action, which forces particles in the range of 10–1000micron to impact thewall, losemomentumand fall in the slagmelt. Thisvortex produced by the swirling gas ensures efficient recovery of parti-cles in the PC and converts these into slag, which is tapped periodicallyfrom the base of the furnace. Downstreamof the PC, the syngas is cooleddown to below200 °C in a steamboiler prior to a very basic treatment toremove any residual particulates and acid gas contaminants (mostly,HCl and H2S). This includes a dry filter (incorporating a ceramic filterunit with sodium bicarbonate dosing and activated carbon) followedby a wet scrubber. Water condenses from the syngas as it is cooledbelow the dew point when entering the scrubber. The water mixedwith the scrubbing alkaline liquor dissolves almost all the nitrogenouscompounds, chloride, fluoride, and sulphur gases present as well as

Fuel addition system

Wet scrubber

unit

Fig. 2. Schematic of the Gasplasma process and gas sampling points (Gi).

262 M. Materazzi et al. / Fuel Processing Technology 137 (2015) 259–268

lesser amounts of CO2. Before the cleaned gas leaves the top of thescrubber, it passes through a demister to reduce any entrained waterdroplets and is then discharged through the gas outlet at the top ofthe scrubber. The liquor collects in the sump of the scrubber fromwhere it is pumped by the centrifugal pump back to the top of thetower. Slightly negative pressure (5–10 mbar) is maintained withinthe process using an induced draft (ID) fan located after the wet scrub-ber. In the demonstration plant, the refined gas (LHV=10-14MJ/Nm3)is directed to a gas engine for production of power before the oxidisedgases are released to atmosphere.

Themajor difference that affects gas composition between this plantand the full-scale plant is themuchgreater relative quantities of inertinggas (i.e. Argon) that are used to provide both purge and gas seals in theSwindon plant as compared to those used in the full-scale plant. Thereason for this is that the requirement for sealing and purging gasdoes not scale proportionally with the size of the plant. Nevertheless,the information from the demonstration plant is sufficiently reliable tounderstand the behaviour of organics in a two-stage process and assessthe capacity of plasma to crack and reform problematic components,such as tars and thiophenes, without excessive boundary layer effects.A full and more detailed description of the apparatus and plant opera-tion is available in the literature and not reported in this paper [28,29].

2.3. System monitoring and analysis

The plant was specifically instrumented with direct and continuousmeasurements of flows, gas composition, temperatures and pressures,to study the reforming of tars at high temperatures [29]. The on-linesyngas composition was monitored at different locations of the plantusing a Gasmet Fourier Transform Infrared (FTIR) Spectroscopy gasanalyser. The FTIR samples the gas stream ten times per secondand can be set to average these samples over varying time periods(10–60 seconds). Specific gas species analysed included: CO, CO2, H2O,NH3, NOx, SOx, COS, HF, HCl and a number of key volatile organic species(i.e. CH4, C2H2, C2H4, C6H6, C7H8, C6H5OH and C10H8). A K1550R Hydro-gen Gas Analyser supplied by HiTech Instruments (with compensationfrom an IR600 CO2 analyser) is also used for the direct monitoring ofhydrogen. Additional CO/CO2 IR monitoring using is undertakenutilising a XEntra 4210 analyser. The calorific value of the gas and itsWobbe index are monitored using a CWD 2005 Calorimeter.

A Biogas 5000 electro-chemical analyser is used to allow for the con-tinuousmonitoring of hydrogen sulphide (0–500 ppm). The unit simul-taneously allows the measurement of CH4, CO2, O2 and H2S. For safety

reasons, the oxygen concentration of the syngas is also monitoredusing a paramagnetic sensor which is incorporated in the Xentra 4210Servomex analyser. A Michell XTP-601 paramagnetic oxygen sensor isalso used as a redundant system to the Servomex unit. If, when gasify-ing, the concentration of O2 in the syngas exceeds 1%, then the systemis automatically shut down.

Tar species and organosulphur compounds are identified using gasbag sampling. Gas samples from different locations are drawn intoTedlar bags and sent for off-line measurement. An aliquot of sample iswithdrawn from each bag using a glass syringe and subsequentlyinjected into amodified purge & trap (P/T) concentrator. The volatile or-ganic species are thermally desorbed from the trap, separated byGC andanalysed by a very sensitive positive ion electron impact MS.

Before starting the plant, a total of 100 kg (50:50 w/w) of iron pigand stimulant-flushing slag (33.7% SiO2, 28.8% CaO, 3.2% Al2O3, 34.3%Fe2O3) was charged to the PC to act as a return path for the DC plasmacircuit. In normal, steady state operation, the slag slowly accumulatesin the base of the converter over time, giving a more uniform tempera-ture, avoiding excessive temperatures in the plasma arc impingementzone and more rapid melting of the ashes from the FBG. At the comple-tion of the trials, a slag samplewas extracted from the plasma converterand prepared to less than 4 mm grain-size by crushing and sieving.Chemical analysis was carried out by an independent, UKAS accreditedlaboratory (Environmental Scientifics Groups Ltd.) with the requiredsuite of tests for detecting sulphates and the organic content in thesolid residue, including total dissolved solids (TDS), dissolved organiccarbon (DOC), total organic carbon (TOC), loss on ignition (LOI) andtrace organic parameters including benzene, toluene, ethylbenzene,and xylenes (BTEX), polychlorinated biphenyls (PCBs) and PAHs.X-ray fluorescence (XRF) analysis was also employed to obtain quanti-tative compositional data on the bulk oxides after sample fusion into aglass bead using lithium tetraborate. Trace element analysis was carriedout by inductively coupled plasma optical emission spectroscopy (ICP-OES).

3. Results and discussion

The most important ranges of the operational parameters used inthe demonstration plant for the test to be discussed here are listed inTable 2.

The RDF was fed with an addition rate of 35–50 kg/h. The oxidantsrates were adjusted to maintain a bed fluidisation velocity of between0.6 and 0.8 m/s. The plasma power was controlled to provide a syngas

Table 2Test conditions in the APP demonstration plant.

Operating parameters Values

RDF feed rate (kg/h) 35–50FBG temperature (C) 750FBG superficial gas velocity (m/s) 0.75Oxygen to fuel ratio (w/w) 0.28–0.33Steam to O2 (mole) ratio 2.5–3Plasma converter exit temperature (C) 1100–1200Energy at the electrode (kW) 100–130

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temperature of 1100–1200 °C from the plasma converter and keep theslag in liquid form. Gas measurements were performed as discussed inthe previous section.

3.1. Syngas composition

The average data for the most prominent syngas constituents re-corded during the FTIR monitoring periods at several locations of theplant are presented in Table 3. To allow for comparison, the change ofsyngas GHV is also given.

The syngas composition varies considerably depending upon the lo-cation of the sampling point. The cleaned syngas (G3)mainly consists ofH2 andCO (40.4% and26.8% respectively), but also contains CO2 (15.7%),small amounts of N2 and residual water vapor. Note that all values arefor a gas generated in the pilot plant unit which contains relativelyhigher levels of inerting gas (Argon) than would occur in a commercialsystem. The level of nitrogen in all samples is of uncertain origin - themost likely explanation for most of this N2 is the ingress of some quan-tities of air into the fuel addition system (i.e. FBG feeder), due to the lowpressure at which the whole pilot plant is operated. When the effect ofthe inert gases is accounted for, the projected GHV of the syngas isabove 10 MJ/Nm3.

The rawproducer gas exiting the FBG (G1) is still shown to be highlyaffected by the devolatilised RDF input. In addition to CO and H2, it con-tains a myriad of hydrocarbon species from methane through to longchain and aromatic tar components (e.g. naphthalene), resulting in arelatively poor quality of the syngas. Of relevance, the steam, CO2 andSO2 content is also very high.

The reason for this can be reasonably linked with the feedstock na-ture and composition, and the complex gasification dynamic within

Table 3Average gas composition measured by FTIR at different location of the plant. *Diatomicmolecules are derived by K1550RHydrogen Gas Analyser and cross-checking calculations.

Description: DownstreamFBG

DownstreamPC

Downstream gascleaning

Sampling point Units G1 G2 G3

Syngas:CO vol % 8.1 21.4 26.8H2* vol % 20.8 32.1 40.4CO2 vol % 21.4 11.4 15.7H2O vol % 35.6 25.1 3.4Ar vol % 3 6.8 8.7N2* vol % 2.1 2.2 4.0NH3 ppmv 45 45 3SO2 ppmv 171 75.4 2.91

Hydrocarbons:Methane vol % 9 1 1.2Acetylene ppmv 1759 56.8 46.7Ethylene ppmv 38480 914 878Benzene ppmv 8446 37.7 36Toluene ppmv 2646 27.6 25.4Naphthalene ppmv 1243 42 25Phenol ppmv 252 10.4 5.3GHV (tar free) MJ/Nm3 7.25 7.19 8.98

the fluid bed gasifier. The FBG system is autothermal, in the sense thatit usually relies on a fraction of the input char, being ‘fixed carbon’,that reacts with the oxygen through the bed and provides sufficientheat to sustain the endothermic reactions that produce H2 and CO.The strong turbulence and mixing in the bed distribute the char andthe devolatilising fuel particles throughout the bed, establishing a gasenvironment where these solids are converted [30]. However, reactoroperations resulting in lack of contact between char/oxygen and anunfavorable consumption of oxygen by the devolatilisation gases arefrequently observed in large scale FBGs [30]. These issues are exacerbat-ed when operating on RDF, due to themuch lower fixed carbon content(ca 5%) and much higher volatile content (50-60%) compared to otherfuels, such as biomass or coal. Crucially, the higher levels of volatiles re-duce the delivery of ‘fuel’ char into the bed, and more devolatilisedproducts, in which tar is the less reactive, react with oxygen producingCO2, H2O and SO2. This effect is evenmore evidentwhen high quantitiesof moisture and ash are present in the feedstock material and the oxi-dant supply ratemust be enhanced to generate sufficient heat to sustainthe gasification reaction. Eventually, this leads to a syngas with highercontent of combustion gases and lower GHV.

In comparison, the syngas from the plasma converter (G2) appearsto be much richer in the primary syngas constituents (i.e. H2 and CO),with the CO/CO2 andH2/H2O ratios considerably higher than thosemea-sured in G1. Furthermore, an examination of the gasifier exit gas com-position revealed that up to ~10% methane and other volatile organicswere present in the measured data, while almost no organic carbon(NC1) presence was detected downstream of the PC (methane isabout 1%). Such shift in gas composition is strongly indicative of increas-ingly favourable conditions for hydrocarbons cracking and reformationroutes, both catalysed by the plasma action [29,31], where the residualcarbon monoxide and hydrogen contents of the syngas are strongly en-hanced by the destruction of organic species, such as:

CnHx þ nH2O→ nþ x=2ð ÞH2 þ nCO ð1Þ

CnHx þ nCO2 → x=2ð ÞH2 þ 2nCO ð2Þ

At the high temperatures observed in the plasma converter, thewater gas shift (WGS) equilibrium also plays a crucial role. The equilib-rium describes the relative concentration of the main syngas constitu-ents in the following:

CO þ H2O↔CO2 þ H2 ð3Þ

The reaction is reversible and themain reacting direction is evident-ly dictated by the process temperature. Since the WGS is equilibriumcontrolled, it is not surprising that WGS dominates the gas-phasecomposition at the plasma converter, where equilibrium conditionsare always achieved [22]. This is due to the combined effect of highoperating temperature - given by the high arcing frequency - and longresidence time of gas in the reaction chamber (N2 seconds). These re-sults can be very interesting for applications of synthesis gas in secondgeneration biofuel that require tar concentration below 0.1 mg m−3

and a specific H2/CO ratio. In particular, the hydrogen to carbonmonox-ide ratio of the syngas can be controlled via a modification of theexternal energy into the plasma converter, with or without the use ofadditives such as steam or CO2 [32].

Fig. 3 gives the changes of the WGS constituents ratios measureddownstream the PC over time. It can be noted that the concentrationswere fairly stable during the trial, although a general upward trend inCO/CO2 ratio was observed.

The increase in CO/CO2 ratio cannot be attributed to the WGS reac-tion, which depends only upon the temperature of the system. In fact,the WGS reaction should also result in an increase in H2/CO ratio,

00.5

11.5

22.5

33.5

44.5

00:00:00 02:00:00 04:00:00 06:00:00 08:00:00 10:00:00 12:00:00 14:00:00 16:00:00

CO

/CO

2 ra

tio

Test duration [hh:mm:ss]

0

0.5

1

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2.5

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H2/

CO

rat

io

Test duration [hh:mm:ss]

Fig. 3. Dynamic CO/CO2 (top) and H2/CO (bottom) ratio profiles downstream the plasma converter.

264 M. Materazzi et al. / Fuel Processing Technology 137 (2015) 259–268

which instead remained stable over the duration of the run. This shiftin simultaneous H2 and CO production could possibly have beenaccelerated during the test due to the accumulation of catalyticallyactive elements in the slag. Several studies proved that, at the same tem-perature, the gasification reaction rate with slag is higher than thatwithout slag, so the furnace slag can accelerate slow reaction of tarand carbon with steam and CO2 to produce more H2 and CO [32,33].The slag accumulationwas significantwith a rate of ca. 4.5 kg/h in a fur-nace of 100 kg of starting material. The high contents of Mg and Ca ofmolten ashes (see Table 6, Section 4.2.1) could be, therefore, the reason

Table 4Composition and concentration for main tar components by gas chromatographer analysis of o

Description: Downstream FBG

Sampling point - G1

Volatile organic carbons:Benzene mg/m3 2000Toluene mg/m3 1489Ethylbenzene mg/m3 1165m + p Xylene mg/m3 221o-Xylene mg/m3 115Styrene mg/m3 290Indane mg/m3 250Indene mg/m3 100

Tars:Naphthalene μg/m3 13501-Methyl-naphtalene μg/m3 5002-Methyl-naphtalene μg/m3 278Acenaphthylene μg/m3 700Acenaphtene μg/m3 60Fluorene μg/m3 180Phenanthrene μg/m3 120Anthracene μg/m3 82Fluoranthene μg/m3 22Pyrene μg/m3 12

thatmade slag act as a catalyst for hydrocarbons gasification. Analogousobservations are reported in the literature [33–35].

3.2. Tar and organosulphur conversion

The chromatographic analyses of the raw FBG syngas extracts con-ducted herein confirmed their contaminationwith condensable organiccompounds, PAHs in particular. It was found that, due to the high levelsof tarry/sooty products prior to plasma treatment, it was only possibleto sample the gas stream exiting the FBG for a relatively short period

ff-gas bag samples. Boiling points (b.p.) are given at 25 °C and 1 bar.

Downstream PC Downstream gas cleaning

G2 G3 b.p (°C)

222 15.66 80.015 4.4 110.60.21 - 136.60.22 - 138.40.15 - 1443 1.1 1458 - 176.50.5 - 182.4

182 18 21840 - 24012 - 24160 - 2795 - 2791 - 2955 - 3401 - 3401 - 3751 - 404

265M. Materazzi et al. / Fuel Processing Technology 137 (2015) 259–268

(maximum 15 minutes) before blocking of the sampling line occurred.This also resulted in a quite relevant analysis error (±20%). By contrast,sampling of the off-gas downstream of the plasma converter could beconducted indefinitely.

Table 4 shows the concentrations of hydrocarbons compoundsassayed in the syngas before and after plasma treatment, along withthe respective boiling points.

As mentioned above, the groups of organic contaminants assayed inthe syngas directly from the FBG contained primarily VOCs and PAHs.Gas samples revealed eight different VOC species (boiling point below200 °C): toluene, benzene, ethylbenzene, m + p xylene, o-xylene, sty-rene, indene, indane, at high concentrations (e.g. toluene above1000 mg/Nm3). Of interest, the plasma downstream product spectrumshows the complete destruction of these compounds except for a traceof benzene (~200 mg/Nm3) (Fig. 4).

Polynuclear aromatics are also present in the FBG sample, with highlevels of naphthalene, acenaphthylene, 1-methyl-naphtalene,acenaphtene, fluorine, phenanthrene, anthracene, fluoranthene andpyrene, with cumulative levels of up to ca. 3000 μg/Nm3. These are re-duced to below 250 μg/Nm3 after plasma treatment. Key compoundsin the ultimate products are the olefins, such as ethylene (~900 ppmv,Table 3), which are likely intermediates in aromatic hydrocarbonsreforming, and are often used for tracing the tar reforming andevolution chemistry [29,36]. Under a high temperature environmentwith a gas composition as indicated in Table 3, naphthalene reductionwas observed to be 96.6%, which was higher than that of genericsteam/dry-reforming conditions, where the presence of H2 can act as in-hibitor during the reforming of tars [37–39]. As shown later inSection 4.2.2, the reason for the higher conversion may be due to thepresence of CO, O and OH radicals in the gas mixture produced by the

0

500

1000

1500

2000

2500

VO

C c

on

cen

trat

ion

in g

as s

amp

le(m

g/m

3 )

Pre plasma

Post plasma

0

200

400

600

800

1000

1200

1400

Tar

co

nce

ntr

atio

n in

gas

sam

ple

(µg

/m3 )

Pre plasma

Post plasma

Fig. 4. Tars and VOC distribution measured pre and post plasma.

plasma arc. These species tend to oxidise aromatic and aliphatic inter-mediates, preventing them from re-combining with other radicals toform tertiary tars and soot [29].

From the examined samples, it is evident how the complex chemicalstructure of the parent RDF substrate is seemingly reflected in the FBGproducts, which maintain a large portion of the original organic func-tional groups. For example, lignin in RDF represents a potential precur-sor for PAH formation, due to the aromatic nature of this polymer [40].Analogously, sulphur is released as hydrogen sulphide (H2S) and amyr-iad of compact and relatively stable molecules such as methanthiole,COS, carbon disulphide (CS2), thiophene, and benzothiophene(Table 5). As for the other tars, the organosulphur decomposition inthe plasma converter occurs due to destabilisation of the hydrocarbon,which leads to fragmentation of the molecule by breakage of S-Cbonds. These fragments undergo different reactions forming gaseousproducts, such as H2S and COS (Table 4).

As a result, the level of organic sulphur in the gas phase after theplasma treatment alone (i.e. before the gas cleaning) is less than 500ppbv, i.e. 93% less than that of a single stage FBG. Fig. 5 shows the effectof thermal cracking on thiols and thiophenes. It was observed that theconcentration of sulphur-substituted aromatics in the FBG sample wasvery high with peak levels of up to 3000 ppb; this was reduced toextremely low levels (average of 30 ppb) after plasma treatment.There was also a marked reduction in the level of methanthiole froman average of 750 ppb, before, to around 70 ppb, after the plasma con-verter. These very low levels of organic sulphur in the gas streamwould support the operation of the downstream gas cleaning unitsand catalytic stages in a commercial plant.

Unlike other solid fuels, interaction between gas sulphur species andchar, to form char-bound sulphur in the FBG bed, was not recorded inthis test, due to low quantity of fixed carbon in RDF (residual carbonin FBG sand sampled at the end of the trial is considerably below 1% inweight). Thus, almost the totality of the organic sulphur, quantified asthe 60–65% of the initial sulphur measured in the feedstock, reportedto the FBG gas phase, and subsequently reformed in the plasma convert-er (Fig. 6). Further release of gaseous sulphur, initially as SO2, may occurin the plasma converter as a result of evaporation and/or dissociation ofinorganic sulphur, i.e. sulphates, in the ashes [41]. In fact, althoughsulphates (e.g. CaSO4, K2SO4) are stable at up to more than 1000 °C intheir pure form, thermodynamic calculations have indicated that thepresence of silicon and aluminium in the ashes lowers the thermal sta-bility, and K and Ca are preferably incorporated into alumina-silicatestructures above 800–900 °C [41]. These conditions are largely met inthe PC, were the ash fraction, which is relatively rich in silica and alumi-na, is captured and melted in the slag pool at 1500–1600 °C. As a result,the sulphur release in the PC increases from 65% to nearly 91%, whichsuggests that the majority of inorganic sulphur initially present in the

Table 5Composition and concentration for main sulphur component in off-gas bag samples. Boil-ing points (b.p.) are given at 25 °C and 1 bar.

Description: DownstreamFBG

DownstreamPC

Downstreamgas cleaning

Sampling point - G1 G2 G3 b.p. (°C)

Gas sulphur:H2S ppmv 210 356 53 - 60SO2 ppmv 170 71 5 - 10

Organic sulphur:COS ppmv 16 32 8 - 50.2CS2 ppbv 1170 360 74 46.24Methanthiole ppbv 750 70 25 5.95Thiophene ppbv 2800 34 18 84.22-Methylthiophene ppbv 300 b5 - 113.33-Methythiophene ppbv 350 b5 - 114Benzothiophene ppbv 610 b3 - 221

0

500

1000

1500

2000

2500

3000

3500O

rgan

ic s

ulf

ur

con

cen

trat

ion

in g

assa

mp

le (

pp

bv)

Pre plasma

Post plasma

Fig. 5. Thiols and thiophenes distribution measured pre and post plasma.

Table 6Slag bulk and trace analysis.

Trace Metals by ICP (dry) Oxides in RDF slag

Metal mg/kg Metal mg/kg Oxide % w/w

Aluminum 218438 Chromium 124.7 SiO2 32.5Calcium 122073 Copper 62.2 Al2O3 35.6Iron 49357 Nickel 15.2 Fe2O3 5.9Magnesium 14165 Vanadium 48.2 TiO2 1.4Phosphorus 446.6 Zinc 46.4 CaO 19Silicon 141825 Cobalt 3.3 MgO 2.1Potassium 2593 Manganese 12754 Na2O 0.7Sodium 5291 Lead 10.0 K2O 0.3Titanium 12385 Antimony 4.1 Mn3O4 2.1Thallium 0.1 Arsenic 1.7 P2O5 0.1SO4 126 SO3 0.3

266 M. Materazzi et al. / Fuel Processing Technology 137 (2015) 259–268

feedstock (~35% w/w) has dissociated in the slag phase. The remainingamount of sulphur (less than 10%) was not measured and was possiblybound in the fly ashes as sulphate or sulphide species.

There also appears to be a direct relationship in the PC (samplingpoint G2), with H2S and COS trending upwards as the SO2 contentdecreases. This suggests that the SO2 produced in the FBG and newlygenerated in the PC is subsequently converted to reduced sulphurspecies (i.e. H2S, COS), indicating that the high temperature and longresidence time in the PC are such that equilibrium conditions are alwaysattained.

3.2.1. Slag analysisThe analytical results for the slag sample are summarised in Table 6,

where the primary components are silica and alumina.Table 6 shows also a reasonable retention of more volatile compo-

nents such as calcium and magnesium, which are known to promoteslow carbon-steam reactions.

As anticipated in the previous section, the very low level of measur-able sulphur concentrations in the slag indicates that sulphur is notretained as simple sulfide salts. Themajority of the inorganic and tar sul-phur initially present in RDF is recovered in the gas cleaning section(third column in Table 5), indicating that it is released during plasmareforming as a volatile component, mainly H2S.

Testing of the slag samples against other organic parameters showedalso near zero values (Table 7).

Total Organic Carbon (TOC) was below 1% in weight, and levels ofPCBs and PAHs were cumulatively less than 7 μg/kg and 2.0 μg/kg, re-spectively. Levels of BTEX were also less than 0.6 mg/kg. These very

0

100

200

300

400

500

600

700

H2S COS SO2 Total gas S

H2S

, CO

S a

nd

SO

2 co

nce

ntr

atio

n

(pp

mv)

G1 G2 G3

100

90

80

70

60

50

40

30

20

10

0

S in

gas p

hase / in

itial S (%

)

Fig. 6. Gas sulphur species distribution after FBG (G1), PC (G2), and gas cleaning units(G3).

low values are due to the combination of high temperature, turbulenceand residence time attained in the plasma converter, which results inthe conversion of residual carbon and tars to gas-phase carbon-containing species, e.g. CO and CO2.

Testing of different samples against inorganic (i.e. heavy metals)parameters has also showed compliance with waste acceptance criteriaWAC for inert wastes [42]. As non-hazardous wastes (European WasteCatalogue/List of Waste Regulations code 19 04 01 ‘vitrified wastes’)this material is suitable for landfilling at an inert waste landfill, or capa-ble of further materials processing to fabricate enhanced quality ceram-ic glass products.

3.2.2. The effect of plasmaIt is well known that the plasma arc formed at a high temperature to

a stream of gas and dust is effective due to the large amount of thermalenergy involved leading to tar and char reforming. At the same time,many studies report that the plasma generates a large fraction of radi-cals, ions, electrons and excited molecules that together with thehigh-energy radiationmake the environmentwithin the PC highly reac-tive [18–22,44]. All these factors coexist and interact with each other,making the exact understanding of chemical role of the plasmachallenging.

In order to better assess the role of plasma in tar conversion againstthermal effects, the plasma arc was subject to a short planned disrup-tion. The duration of the disruption (less than five minutes) was shortenough to not significantly affect the reactor temperature, whichremained above 1000 °C. This was possible thanks to the high mitiga-tion effect produced by the refractory walls and the large hot mass ofslag at the base of the converterworking as ‘thermal reservoir’. As a con-sequence, the test had minimal-to-no impact on syngas characteristics,which maintain a stable trend over the whole duration of the test (seeFig. 3). Nevertheless, this operational decision led to sudden rises inthe presence of benzene and naphthalene, both known to be highlyrefractory compounds, as shown in Fig. 7.

Naphthalene levels are seemingly the most sensitive to plasmapower input rate. The disruption quickly leads to a stepwise rise in re-sidual naphthalene levels to values above 350 ppm. However, whennormal operation is restored, this additional load is quickly reducedbelow 15 ppm.

A similar observation is made for benzene levels. During normaloperation, the levels observed are below 1000 ppm, and progressivelyreduce as the PC refractory reaches thermal stability and more slag

Table 7Residual organics analysis for slag sample.

Parameter DOC TDS PCB PAH BTEX TOC LOI

mg/kg mg/kg μg/kg μg/kg mg/kg w/w% %

Slag sample 3.7 45 b7.0 b2.0 b0.6 b0.96 b0.2

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0

50

100

150

200

250

300

350

400

00:00:00 02:00:00 04:00:00 06:00:00 08:00:00 10:00:00 12:00:00 14:00:00 16:00:00

Ben

zen

e [p

pm

v]

Nap

hta

len

e [p

pm

v]

Test duration [hh:mm:ss]

Naphtalene

Benzene

Mean Power-120 kW Outlet Temperature-1150 C Feed rate-102 kg/h

Disrupted Power-6 kW Outlet Temperature-1005 CFeed rate -98 kg/h

Fig. 7. Variation of plasma conditions during benzene and naphthalene monitoring (G2 sampling).

267M. Materazzi et al. / Fuel Processing Technology 137 (2015) 259–268

accumulates in the system. However, the disruption in plasma applica-tion leads to a short term spike as observed with naphthalene. Further-more, the benzene levels were seemingly slower to restore undercommon values when compared to naphthalene. The reason for thismay be attributed to the different nature of the contaminants and tothe mechanism of tar conversion in a plasma reactive environment, asalready discussed by the authors in a previous paper [26]. In a highthermal environment (above 900 °C), the tar decomposition proceedsby thermal cleavage and hydrogenation of double bonds that breakthe aromatic structure into smaller intermediates (e.g. phenyl radicals,acetylene, etc.), which become the main decomposition products [43].In absence of plasma activated species, the primary tar radicals wouldnaturally tend to combine together into aromatic (benzene) and poly-cyclic aromatic (naphthalene) hydrocarbons, eventually leading tosoot formation. This would explain the sudden increase in these con-taminants in the gas phase when the plasma is interrupted, even atabove 1000 °C. The intermediate step of phenyl hydrogenation duringnaphthalene reforming, however, tends to have a ‘buffering effect’ onbenzene content, since, if more phenyl radicals are produced, morephenyl hydrogenation is likely to occur, releasing benzene as a product.This results in a slower decrease in total benzene concentration overtime. These effects could be offset by injecting small quantities of oxi-dants (e.g. O2 or H2O) into the PC, assisting in the oxidative degradationof aromatic intermediates [26,44].

A very similar behavior can be assumed for sulphur substituted aro-matics. Thiophene, for example, is a very stable compound and does notthermally decompose in gas up to temperatures as high as 1100 °C. So,once formed, it is unlikely that it will decompose at the thermalconditions that were achieved in the PC. These properties have been at-tributed to its aromatic nature which allows the free electron to belargely delocalised [45]. In the presence of plasma, however, catalyticdestabilisation of the aromatic ring may occur even at temperatureslower than 1100 °C, as also reported by Mohammedi at al. [46]. Thehigh activity of plasma causes the thiophene ring to fragment throughthe impact of energetic electrons, and form radicals such as R·, SH·,S · and so on. The interaction of these radicalswith O, OH andH can pro-duce some small hydrocarbons (mainly acetylene), H2S, H2 and oxygenbearing compounds such as CO and COS [45,46].

As a result, most of the organosulphur constituents consisting large-ly of thiophenes are reformed under active plasma conditions, leavingessentially CO, H2 and H2S as ultimate products.

4. Conclusions

In the present work, the evolution of tars and sulphur containingcompounds in the syngas generated from a waste fluid bed gasifierand subsequently reformed in a plasma converter was investigated.An experimental test mimicking the normal commercial operation of a

fluid bed-plasma plant was conducted over 20 hours of operationwith sulphur-rich RDF prepared from municipal solid waste (MSW),commercial and industrial (C&I) and automotive shredded residue(ASR) wastes. The efficacy of the plasma treatment is well illustratedin the investigative testwork campaign which shows measurementsfrom sampling the syngas before and after the plasma converter froma demonstration plant for the main organic, volatile and condensablecompounds. It is seen that both tars and organic sulphur species arepresent in the crude syngas exiting from the FBG stage at elevatedlevels. When exposed to the influence of the plasma arc converterthey are reduced to very low levels. Reduction efficiencies typicallyexceeded 96%v/v for complex organics (e.g. PAH) and thiopheneswere observed.

The data suggests that the percentage of sulphur in the RDF convert-ed to H2S and organosulphur compounds in the FBG ranged between50% and 60%, which is similar to that initially measured for the organi-cally bound sulphur in RDF. The organic fraction in the gas phase isthen reformed under plasma conditions, leaving essentially CO, H2 andH2S as ultimate products. The second dominant sulphur species underthe plasma experimental condition was SO2, which equated to around90% conversion of the initial RDF sulphates content, and is subsequentlyconverted to H2S and COS. There was no detectable organic sulphurobserved in the outlet gas, nor in the slag by-product off the PC.

The test has also highlighted the importance of the plasma chemistryas opposed to thermal activationmechanism in contaminants reforming.It was observed that aromatics behaviour is much more affected byplasma generation than by the actual thermal regime. An abruptdecrease of the plasma power input quickly leads to a stepwise rise inresidual naphthalene and benzene levels to values above 350 and3500 ppm, respectively. This occurs despite the temperature beingmaintained above 1000 °C. Only when normal operation is restored,does this additional load quickly reduce back to normal values.

In summary, it was evident that the plasma converter stage was es-sential to the overall process in terms of generating a syngas of predict-able and consistent composition which was free from tars and organicsulphur contaminants. For commercial applications the syngas wouldbe suitable for use in catalytic stages for bio-fuel production, as a sourceof hydrogen for use in energy systems and in SOFC for power genera-tion, after removal of H2S and SO2 in conventional gas cleaning units.

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