Sorption-enhanced gasification (SEG) of agroforestry residues: Influence of feedstock and main operating variables on product gas quality

Sorption-enhanced gasification (SEG) is a promising route for the conversion of biomass into synthetic fuels. There are important aspects regarding syngas quality (e.g. tar composition and/or gaseous contaminants such as H 2 S) that may influence the scaling-up of the process and should be assessed. Experiments were performed in a 30 kW th bubbling fluidised gasifier to evaluate the effect of the feedstock used, temperature, sorbent-to-biomass (S/B) ratio and/or steam excess on such quality aspects. Larger amounts of tars were found for straw, which corresponded to highly stable tars (benzene, toluene, naphthalene and phenanthrene). Temperature, S/B ratio and steam excess favoured tar decomposition, causing the contribution of PAHs to total tar to increase. PAHs and phenol contents were found to be related to and dependant on the temperature and S/B ratio. Elutriated CaO particles reacted with the H 2 S formed until equilibrium conditions were reached, leading to syngas H 2 S contents of between 15 and 85 ppm. Sulphur in the syngas represented no more than 15% of the total sulphur introduced into the reactor, whereas 65 – 85% ended up as CaS with the partially converted sorbent particles leaving the


Introduction
The conversion of agricultural and forestry residues into synthetic fuels or chemicals (e.g.methane, Fischer Tropsch fuels, dimethyl ether, methanol, etc.) is a promising means of addressing their disposal given to their abundance and the fact that they do not compete with the food chain.By partially replacing fossil fuels with synthetic fuels obtained from agroforestry residues, the European Union will reduce its reliance on oil and gas-exporting countries, thus intensifying the use of locally generated waste, while reducing greenhouse gas (GHG) emissions.For instance, it has been estimated that approximately only 8% of the agricultural residues produced are currently being exploited [1], with straw being the most important lignocellulosic residue in this category with an estimated potential of 127 million tonnes per year for the EU-27 in 2020 [2].With regard to forestry residues, worldwide wood production amounts to 3.7 billion m 3 per year, of which 49 million m 3 was harvested in EU-27 in 2019 [3].Considering the magnitude of the agriculture and forestry residues produced, they are potentially a large and under-exploited source of resources for the bioenergy sector [4].
Gasification is the most suitable thermochemical conversion route for converting a solid fuel like biomass into a syngas that can be further converted into a final product through a catalytic process.Steam-oxygen gasification can be an option for this application since it enables the production of N 2 -free syngas with a high content in hydrogen and carbon monoxide.The partial oxidation of the fuel occurring in the gasifier supplies the energy for the endothermic gasification reactions in this case, which allows the process to be performed in a single fluidised bed reactor.However, the result is a syngas with a significant CO 2 content, making it necessary to include a CO 2 removal step before the synthesis process [5,6].Furthermore, a dedicated O 2 production unit would be also needed in this case, increasing capital and operating costs of the process and its complexity [7].In light of this, steam gasification through a sorption-enhanced gasification (SEG) process has emerged as a potential alternative for the conversion of biomass in a synthetic fuel production scenario [8][9][10].This gasification concept proposes performing steam gasification in the presence of a CaO-based material that reacts with the CO 2 produced through the carbonation of CaO (1) [11,12].
The energy required for the endothermic gasification reactions is supplied by the exothermic carbonation reaction of the CaO and by the sensible heat of the stream of CaO-rich solids that is fed in at high temperature from a secondary regeneration reactor.Owing to the presence of the carbonation reaction (1) in the gasifier, this reactor should be operated at certain temperature (i.e.600-750 • C) as defined by the CaO/CaCO 3 equilibrium [12,13].Despite the low temperatures used in the gasifier, the presence of CaO inside the gasifier allows low tar contents to be achieved in the resulting syngas compared to the conventional indirect gasification concept using olivine as bed material due to the catalytic effect of CaO on tar cracking [14].
Over the last decade, the feasibility of SEG technology has been demonstrated by different laboratory and pilot scale facilities [14][15][16][17][18][19][20], as well as by several demonstration scale plants throughout Europe [21,22], all of which were aimed at the production of H 2 -rich syngas in the gasifier [17,18].Wood and coal have so far been the preferred feedstocks for demonstrating SEG technology, although tests with alternative fuels have been also conducted [15,23,24].When focusing on synthetic fuels production, the H 2 , CO and CO 2 contents in the syngas produced should be adjusted according to the targeted final product, as well as the type of catalyst to be used.Typically, in conventional indirect gasification, the H 2 /CO ratio is adjusted downstream of the gasifier through several water-gas shift reactors, whereas the CO 2 content is tuned through an absorption (chemical or physical) unit.The main advantage of SEG technology is that a tailored syngas with the desired ratios of H 2 , CO and CO 2 can be produced directly in the gasifier [7,9,25,26], simplifying in this way the layout of the process compared to other gasification routes (i.e.completely removing the need for watergas shift reactors, CO 2 removal and O 2 production units).This flexibility regarding the resulting syngas composition can be achieved by modifying the temperature and/or sorbent circulation in the gasifier [25,27,28].In addition, the possibility of adjusting the H 2 -to-CO ratio in the gasifier would promote a synergy between SEG and renewables energies in a power-to-gas scenario, producing renewable hydrogen through water electrolysis to be mixed with the C-enriched syngas generated in a SEG process [25].In a CO 2 capture scenario, the SEG technology could be made carbon-negative operating the calciner/ combustor under oxy-fuel combustion conditions while using biomass as fuel in the gasifier.
Noteworthy research has been devoted to SEG technology in recent years targeting technical and process scale-up viabilities.However, research gaps have been detected in this sense.Given the importance being gained by the waste-to-fuel conversion route, dedicated tests with waste fuels are needed to demonstrate the technical feasibility of this gasification technology.Tar yield and composition need to be carefully evaluated for the specific design of the cleaning stages required when scaling up the process in order to avoid clogging problems in the cooler tubes located downstream of the gasifier.Furthermore, specific information will be needed on the role of pollutants such as Cl and S in the syngas when using waste fuels in the SEG process [7].Based on these premises, the purpose of this study was to evaluate the quality of the syngas produced in an SEG pilot plant using two different lignocellulosic residues as feedstocks.In addition to the syngas yield and analysis of the main gas compounds (i.e.H 2 , CO, CO 2 , CH 4 and C x H y ), comprehensive research was conducted on tar formation and tar composition in this work for the different fuels and operating conditions tested.Moreover, the fate of sulphur in this gasification plant was also evaluated for the operating conditions analysed when using a S-containing fuel.Overall, the information provided in this study would serve as a basis for the design of the conditioning and cleaning steps needed when scaling-up the SEG concept as part of a waste-to-fuel conversion plant.

Feedstocks characterization
In this work, agricultural and forestry residues, specifically wood and wheat straw, were the biomass feedstocks chosen as fuels for this study.
Both materials were in pellet form to facilitate their dosing into the bubbling fluidised bed (BFB) reactor.The wood pellets were 25-30 mm in length and 6 mm in width, and fulfilled A1 quality standards.The straw pellets were 10-15 mm in length and 9 mm in width, and were made from wheat straw from south-west Germany.Ultimate and proximate analyses, as well as the calorific value of these materials were determined, obtaining the results compiled in Table 1.The information shown in this table was determined according to the procedure and standards described in a previous work [24].Ash composition was also measured by inductively coupled plasma-optical emission spectrometry (ICP-OES) (UNE-EN 15290), producing the results shown in Table 1.According to the information in this table, the straw pellets used in this study had a higher ash content than the wood pellets, and this inorganic fraction mainly comprised potassium and silicon.
The CO 2 sorbent used as bed material in this work corresponds to that previously used in a recently published study [24].It consists of a high-purity limestone that was calcined in the BFB reactor used for the gasification experiments.The chosen calcination conditions were a temperature of 910 • C and the use of air, which allowed an almost fully calcined sorbent to be obtained (i.e. a content of 0.05 mol CaCO3 /mol CaO as determined by X-ray diffraction (XRD) analysis).The average particle size of this material was 277 μm.The Ca content of this sorbent determined through ICP-OES was 66.0 wt%, which corresponds to more than 92 wt% of CaO in the calcined sorbent.Sulphur content was 0.06 wt%, with CaSO 4 being the most likely form of this compound based on the operating conditions used in the BFB reactor during limestone calcination.The CO 2 carrying capacity of this calcined material was 0.49 mol CO2 /mol Ca when measured in a thermogravimetric analyser [24].

30 kW th BFB facility
Gasification tests were carried out in a 30 kW th (referring to the nominal thermal input of biomass in LHV-basis) BFB reactor plant located at ICB-CSIC.The scheme for this pilot plant is shown in Fig. 1.Details regarding the configuration of this plant have already been presented in previous works [23,24].Biomass and CO 2 sorbent flows were fed separately into the reactor using two screw feeders at the bottom part of the dense region of the fluidised bed.Steam was used as the fluidising agent in the experiments, which was fed into the reactor mixed with N 2 at 120-140 • C. Converted solids were pulled out from the reactor through a lateral overflow and collected in a hopper (as indicated schematically on the left-hand side of the reactor in Fig. 1).These solids comprised unconverted biomass pellets and partially carbonated sorbent particles.Permanent gases concentration (H 2 , CH 4 , CO and CO 2 ) during each gasification experiment was measured online using a SICK GMS810 analyser placed downstream from the condenser or downstream from the tar sampling system.Throughout the steady-state operation, Tedlar sampling bags were also used to take gas samples and measure composition offline in a Varian CP-3800 gas chromatograph (GC) equipped with both a thermal conductivity detector (TCD) and a flame ionisation detector (FID), enabling gas composition for permanent gases and light hydrocarbons (up to C 4 ) to be determined.The light hydrocarbons analysed included methane, ethane, ethylene, propane, propylene, isobutane, n-butane, trans-2-butene, 1-butene, isobutene, cis-2-butene and 1,3-butadiene.Table 2 is a summary of the gasification experiments performed for the two lignocellulosic feedstocks.The experimental routine followed for each experiment has already been explained in a previous work [23].Steady state operation was maintained for one hour, and gas/solid samples were taken every 15 min from the overflow, cyclones and the gas line connected to the analyser.Solid samples collected throughout the experiments were deeply characterised by different techniques to determine their composition.Proximate and ultimate analyses were performed for the unconverted char particles collected, whereas the XRD, ICP-OES, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) techniques were applied to the carbonated sorbent particles.An extensive description of the information extracted by each of these techniques when performed on the partially converted sorbent particles can be found in a previous work [24].Variables studied were the sorbent-to-biomass (S/B) ratio (on a weight basis), the steamto-carbon (S/C) ratio (that indicates the moles of steam introduced into the gasifier per mole of C introduced with the biomass) and the biomass thermal input that was modified in order to achieve different gasification temperatures.Table 2 shows the average solid bed temperature reached throughout the steady-state period in each experiment (corresponding to the average of T1 and T2 in Fig. 1), as well as the average amounts of partially converted solids collected in the overflow and of entrained solids collected during such steady-state operation.

Description of gas impurities measurement methods
Tar content in the syngas was determined by means of an offline method based on the protocol for biomass gasification published by Neeft [29].This method involved absorbing the tars by flowing the syngas through a train of seven impinger bottles, five of which were

Table 2
Operating parameters used for the SEG experiments performed in the 30 kW th BFB gasifier with agroforestry residues.Solid bed temperature, partially converted solids exiting the gasifier and entrained solids collected in the cyclones have been included for each experiment.filled with 100 ml of isopropanol with the first and seventh left empty.These bottles were placed in two different cooling baths as indicated in Fig. 1: four impingers (1, 2, 3 and 5) at room temperature and three (4, 6 and 7) at − 20 • C. The slipstream of hot syngas for tar sampling was extracted between the first and the second cyclone, being its flow rate regulated by a pump.The slipstream led from the facility through a metallic filter and a heated line (both at 400 • C) directly into the impinger system.Tar sampling was performed for 30 min for all the experiments, whereas the sampled gas volume varied between 0.05 and 0.15 Nm 3 of dry gas.
Once the experiment was finished, the isopropanol from the seven impingers was collected in an amber glass bottle together with the isopropanol used for rinsing the connection tubes.Each experiment generally produced between 0.7 and 0.8 l of isopropanol solution containing both tars and condensed water from the sampled syngas.Quantitative determination of the tars was performed in a Varian CP-3800 gas chromatograph connected to a Saturn 2200 Ion Trap Mass Spectrometer (GS-MS).An initial oven temperature of 60 • C was maintained for 3 min with a sustained heating rate of 7 • C/min until a final temperature of 300 • C was reached for about 16 min.The carrier gas was He (BIP quality) at a constant column flow of 1 ml/min.The injector, detector and transfer line temperatures were 300 • C, 200 • C and 300 • C, respectively.More than 20 compounds were identified and quantified by external calibration using the base peak as the quantification ion.The full list is included in Table 3.The quantification of each tar sample by GC-MS was determined in duplicate with a relative standard deviation (RSD) below 15%.
Gravimetric tar concentration was also determined for each experiment in Table 2.For this purpose, an aliquot of 100 ml of collected tar solution was concentrated in a rotary evaporator until dryness under controlled conditions of temperature and vacuum.The solvent-free solid residue was then kept in a desiccator for at least 12 h before being weighed in a microbalance with a precision of 0.1 mg.The solid residue was weighted in the microbalance three times until the weight value indicated was constant.Gravimetric tar was then calculated as the average weight value of such solid residue in proportion to the dry gas volume that passed through the sampling system during the analysis.Considering the different values of weight measured for each gravimetric tar determination, a maximum relative standard deviation of 1.5% was determined for gravimetric tar.Water content in the isopropanol solution was also calculated using the Karl-Fischer titration method (CRISON TitroMatic 1S kF apparatus).This measurement was used to estimate the steam content in the syngas sampled through the tar system, which was crossed checked with the steam content calculated when solving the atomic balance to hydrogen in the gasifier.
Sulphur compounds in the syngas were analysed by GC in the Tedlar bags sampled when passing the syngas through the tar sampling system shown in Fig. 1.The isopropanol solution collected for tar analysis was analysed by ICP-OES to determine whether any gas sulphur compounds could have been retained in the isopropanol solution during this analysis.However, no traces of sulphur were found in the isopropanol volume collected, indicating that all the sulphur remained in the syngas when passing through the tar sampling device.A Perkin Elmer Clarus 590 gas chromatograph equipped with a flame photometric detector (FPD) was used to determine the sulphur compounds in the gas sampled.
Separation was performed by using a 30 m Rt-Silica BOND capillary column.An initial oven temperature of 45 • C was maintained for 4 min.A heating rate of 15 • C/min was then implemented to reach a final oven temperature of 200 • C.This temperature was maintained for 2 min.The carrier gas was N 2 at a constant column flow of 2 Nml/min.The injector and FPD temperatures were 250 • C and 300 • C, respectively.Sulphur compounds analysed included carbonyl sulphide (COS), hydrogen sulphide (H 2 S), carbon disulphide (CS 2 ) and methyl mercaptan (CH 4 S).Certificated gas mixtures (Air Products) were used for identification and quantification purposes.

Product gas yield and composition
The influence of bed temperature in the gasifier on total gas yield was analysed by experiments performed with similar conditions of steam excess (i.e.S/C ratio of 1.4 for most of them except for tests 11 and 13) as those included in Table 2. Fig. 2 (left) shows the variation in total gas yield with bed temperature for SEG experiments performed with straw pellets (grey dots) and with wood pellets (dark dots).As expected, the product gas yield increased with gasification temperature due to the effect that this variable has on (i) gas production during the primary pyrolysis stage, (ii) steam cracking and reforming of heavier hydrocarbons and tars, and (iii) enhanced char gasification reactions [12,30,31].As shown in Fig. 2 (left), the product gas yield increased with the solid bed temperature from around 0.5 Nm 3 /kg at 645 • C to 0.7 Nm 3 /kg at 735 • C when using straw pellets, whereas it rose from 0.55 Nm 3 /kg at ca. 640 • C to 0.95 Nm 3 /kg at 711 • C when using wood pellets.In this figure, gas yield is expressed as Nm 3 of N 2 -free dry gas per kg of biomass (including moisture and ashes) fed to the gasifier.The gas yield from the wood pellets was significantly higher than that calculated for the straw pellets, particularly for gasification temperatures above 670 • C. Differences in biomass composition and in the conversion of char particles through H 2 O and CO 2 gasification reactions are the main reasons for such differences in the gas yield.Moreover, it can be also seen that the difference observed in the gas yield becomes more pronounced as gasification temperature increases.
To provide a better understanding for such differences in product gas yield, Fig. 2 (right) compares the individual gas yield obtained for wood and straw pellets under similar conditions of S/C and S/B ratios for two different gasification temperatures (i.e.665 • C and ca.700 • C).First of all, the variations in product gas yield can be explained by the different volatile content of both feedstocks.Biomass typically has a high volatile content, and char gasification contributes less to the distribution of gas products.In this respect, as evidenced in Table 1, the higher volatile content in wood pellets compared to that in straw pellets (79 wt% vs 70 wt%) contributes to the higher gas yield calculated for wood pellets.Another aspect influencing gas yield is char particles conversion.On the one hand, temperature and solid residence time in the gasifier influence the char conversion achieved in the gasifier.On the other hand, the reactivity of the char particles produced by primary pyrolysis reactions depends on the feedstock used.Certain authors have demonstrated that cellulose and lignin contents in the biomass noticeably influence the reactivity of the char produced by pyrolysis [31], whereas the metallic and alkaline contents in biomass ashes (particularly the K/Si ratio) can be also decisive for char reactivity [32,33].The fixed carbon conversion (X FC ) of the biomass particles was calculated for the experiments shown in this figure using the information from the characterization of solid samples taken from the overflow and the cyclones, as explained in [24].According to these calculations, X FC resulted in very similar values for both biomasses under the experimental conditions shown on the lefthand side of Fig. 2 (right) (i.e.25% and 19% for straw pellets and wood pellets, respectively).However, when the temperature was increased to 690-697 • C and the solid residence time of char particles reduced (i.e.increasing the S/B ratio), X FC turned out substantially higher for the wood pellets than for the straw pellets (63% and 25%, respectively).Such a substantial difference in the conversion of char particles as temperature is raised explains the steep increase in the gas yield with temperature for the wood pellets shown in Fig. 2 (left).The lower Si content of the wood pellet ashes may explain the greater reactivity of the char from wood pellets compared to that of straw (higher K/Si ratio) [33].
With regard to the individual gas yields shown in Fig. 2 (right), differences can be observed when the type of biomass used was changed.Regardless of the experimental conditions used, CH 4 and CO yields were higher for wood than for straw.This behaviour may be linked to the differences in the cellulose and lignin contents of both feedstocks [34].Pyrolysis of cellulose is known to produce larger amounts of CO due to the thermal cracking of carbonyl and carboxyl groups, whereas lignin generates more CH 4 (and also more H 2 ) due to its higher content of aromatic rings and methoxyl groups [35].Therefore, based on the results obtained, larger contents of cellulose and lignin in the wood pellets used may be the responsible of the higher CH 4 and CO yields observed when using this biomass in SEG tests [36].As for H 2 and CO 2 yields, differences observed between biomasses in Fig. 2 were due to the different conversion level fulfilled by char particles in both cases.X FC of wood pellets varied from 19% to 63% when the temperature was raised, resulting in higher H 2 and CO 2 yields in the syngas as a consequence of the char gasification reactions occurring in the reactor (reactions 2-5).Under high S/B conditions, the amount of CO 2 separated by the CaO increased, but the effect of the steep rise in X FC was dominant in the variation of the CO 2 yield observed for the wood pellets experiments.On the other hand, for the experiments with straw, X FC barely changed when raising the temperature (i.e.25% was calculated for both experiments), making the H 2 yield to slightly increase probably due to devolatilisation.Regarding the CO 2 yield, it was observed that it decreased from 0.09 Nm 3 /kg biomass to 0.08 Nm 3 /kg biomass when raising the S/ B proportion as a consequence of the larger amount of CO 2 separated by the CaO.With regard to the light hydrocarbons (C 2 -C 4 ), their yield increased with temperature due to the cracking reactions occurring in the gasifier [37].

Tar yield and composition
Operating conditions in the gasifier largely influence the quality of the syngas produced, and therefore tar content and tar composition.Hence, the first measures for reducing tar content in the syngas can be adopted directly in the gasifier, which would simplify downstream conditioning processes.In addition to the type of biomass used as feedstock, tar formation is influenced by the gasifier temperature, steam excess and S/B ratio used in the reactor.For the biomass feedstocks used in this work, the main compounds detected by GC-MS in the collected tar have been classified in phenolic compounds, polycyclic aromatic hydrocarbons (PAHs) and benzene, toluene and xylene compounds (BTX), as detailed in Table 3.The tar concentrations indicated in this section have been expressed on a dry syngas basis under normal conditions of temperature and pressure.
The influence of fuel type on the tars formed under SEG conditions was analysed first.Fig. 3 shows the tar concentration obtained (both gravimetric and GC-MS) for both fuels used under similar conditions of temperature and S/C and S/B ratios.Fig. 3 (left) compiles the information on tar formation for two experiments performed using relatively high temperatures and S/B ratios (i.e.690-697 • C and 1.7-1.8kg of sorbent/kg fuel), whereas Fig. 3 (right) shows the results for reversed operating conditions (lower temperatures and S/B ratios).The gravimetric and GC-MS analyses cannot be directly compared since they cover different but overlapping species.According to the experimental procedure explained above for each tar determination, gravimetric tar allows the determination of mainly heavy tar species, while most volatile compounds included in the BTX, especially benzene, could undergo losses due to the evaporation process.On the contrary, GC-MS determination allows the quantification of all light species (especially BTX and phenols) while some heavy compounds may be under the detection limit.Under the operating conditions shown in Fig. 3 (left), which are closer to those present in large-scale dual fluidized bed (DFB) systems in terms of S/B, more tars would be found in the syngas produced when using straw pellets as fuel than when using wood pellets.Gravimetric and GC-MS tars measured for straw pellets corresponded to 15 and 29 g/ Nm 3 respectively, whereas contents of around 10 and 20 g/Nm 3 were obtained when using wood pellets under the same operating conditions.Soukup et al. [15] also found this trend in a DFB SEG system for these two feedstocks, although the reported values for gravimetric tar were noticeably lower than those found in this work.DFB systems usually work with S/B ratios at the gasifier inlet ranging between 2.5 and 3.2 times higher than the S/B ratios used for this comparison and with higher gas-to-solid ratios throughout the freeboard region [24].Therefore, both factors contribute to an enhanced effect of the CaO on catalytic tar destruction in DFB systems, resulting in lower tar contents than in BFB reactors such as the one used in this work.Schmid et al. [38] also reported higher tar contents for straw pellets than for wood pellets in a BFB gasifier using CaO as bed material but under steam-oxygen blown conditions at temperatures of 850 • C. In any case, the tar contents when using any of the lignocellulosic biomasses in this work were lower than when using a municipal solid waste (MSW) material under similar operating conditions [24] due to the reduced volatile content in these feedstocks compared to that in MSW [8].
Regardless of the biomass and the operating conditions of temperature and S/B ratio used, GC-MS tars were found to be richer in low molecular weight compounds (i.e.BTX) than in PAHs and phenolic compounds (Fig. 3).In addition, the percentage of BTX in total GC-MS tars resulted in similar values, regardless of the biomass and operating conditions used.With regard to the GC-MS tar composition shown in Table 4, it can be seen that these low molecular weight compounds correspond to highly stable tars (benzene, toluene, naphthalene and phenanthrene), which are formed as a consequence of the favourable cracking conditions reached in the gasifier.Differences in tar composition can be appreciated in the PAHs and phenol contents.As the gasification temperature and the S/B ratio increased, the PAHs content in tars became larger for both biomass feedstocks [39].As appreciated in Fig. 3, the PAHs content changed from 6.5 and 10.8% to 31.3 and 21.2% for wood and straw, respectively, as the temperature and the S/B ratio increased.The increasing aromatization of tars would increase the differences between GC-MS tar and gravimetric tar, as seen in Fig. 3 (left).When the S/B ratio was reduced in the gasifier, the differences between gravimetric tar and GC-MS tar turned out to be smaller (see Fig. 3 (right)).Table 4 shows that phenols, which correspond to primary compounds formed from primary pyrolysis reactions [40], gained importance in the GC-MS collected tar, from which it can be elucidated that tar cracking into secondary and tertiary compounds did not occur as extensively as when operating at higher S/B ratios and temperatures.
Reactor temperature plays a crucial role in both tar formation and tar decomposition reactions, thus noticeably influencing tar content and tar composition.While the heating rate of biomass particles influences primary tar formation, the maximum temperature reached within the reactor explains the formation of secondary and tertiary tar species [41].First, biomass pyrolysis reactions occur and primary tars are formed.For such lignocellulosic biomasses as those used in this work, the presence of cellulose, hemicellulose and lignin determine primary tar composition.Cellulose and hemicellulose contain a lot of oxygen and mainly form alcohols, ketones, aldehydes and carbon acids; whereas substituted phenols occur mainly from lignin [40].Once these primary tar species are formed, increasing temperature as well as the presence of steam as an oxidant cause these oxygenated primary tars to be transformed into secondary tars, mainly, through dehydration, decarboxylation and decarbonylation reactions.Finally, due to the presence of temperatures over 800 • C or alternative reaction mechanisms, such as the Diels-Alder cycloaddition mechanism, tertiary tars are formed.Tertiary tars consist of benzene, naphthalene, phenanthrene, pyrene and benzo(a)pyrene [42].
For the experiments performed in this work, the temperature changed from 640 • C to 711 • C and 739 • C for wood pellets and straw pellets, respectively, when both the S/C and S/B ratios in the gasifier were modified as indicated in Table 2.However, in order to exclusively evaluate the effect of the gasification temperature, the experiments chosen for comparison were performed under similar S/C and S/B conditions.Fig. 4 shows the influence of solid bed temperature on tar Fig. 3. Tar contents (gravimetric and GC-MS tars) and composition for the different residues and operating conditions indicated on the X-axis.

Table 4
GC-MS tar composition (expressed in g/Nm 3 of dry gas) for wood and straw pellets under the conditions of temperature and S/B ratios used in Fig. 3. content (both GC-MS and gravimetric) for experiments with both lignocellulosic feedstocks.Specifically, Fig. 4 (left) shows the influence of solid bed temperature for tests 1 and 4 in Table 2 using straw pellets, which were done under the same S/C ratio (1.4) and a similar S/B ratio (0.6 and 0.4, respectively).Fig. 4 (right) shows this influence on tests 11 and 12 in Table 2 using wood pellets, carried out under an S/C ratio of 1.7 and a similar S/B ratio (1.6 and 1.9, respectively).For the two feedstocks tested, a noticeably decrease can be seen in the gravimetric tar content with increasing gasification temperature, as widely reported by many other authors in the literature [9,17,24,43].GC-MS collected tar was also observed to decrease with increasing gasification temperature [40,41], with such a decrease being more pronounced for the straw pellet experiments due to the operating conditions chosen (i.e.larger temperature increase and less variation in S/B ratio).With regard to the tar composition by groups listed in Table 3, the oxygen-containing species (i.e.phenols) showed a greater decrease with the increasing gasification temperature for both straw and wood pellets.In particular, the phenol content was found to decrease from 8.8 g/Nm 3 to 2.4 g/Nm 3 when the solid bed temperature rose from 645 • C to 736 • C using straw pellets as biomass, whereas it decreased from 2.6 g/Nm 3 to 0.9 g/Nm 3 when the temperature rose by 50 • C from 640 • C using wood pellets (Fig. 4).This sharp decrease in phenol content with temperature had previously been reported in the literature for lignocellulosic biomasses under a similar temperature range to that used in this work [41,44].Two possible mechanisms have been proposed for explaining such behaviour: the hydrodeoxygenation of phenol to benzene and/or phenol decarbonylation to form cyclopentadiene and its subsequent conversion to naphthalene following the Diels-Alder reaction [45].According to Fig. 4, the PAHs content also decreased with gasification temperature, but a different behaviour was observed for BTX.Table 5 provides the individual tar composition for the experiments represented in Fig. 4. As regards PAHs, most of the polyaromatic compounds included in this category decreased with the temperature, except acenapthylene, which remained almost constant, and phenanthrene, which slightly increased for both lignocellulosic residues.Naphthalene was the major compound included in PAHs category for both lignocellulosic residues.Contrary to what has been reported in the literature for the fluidised bed gasification of wood [40,41], naphthalene was observed to remain practically constant at 3.3-2.8g/Nm 3 when the temperature rose from 640 • C to 690 • C in this study for this feedstock.The influence of temperature on naphthalene content has been found in the literature at temperatures noticeably higher than those used in this work (i.e.above 750 • C) [40][41][42], which could explain the stability found for this compound at the moderate temperatures used in this comparison.Benzene was the major compound in the GC-MS collected tar for both residues, although it showed a different behaviour with gasification temperature, as observed in Table 5.For the operating conditions chosen using straw (i.e.S/C = 1.4 and S/B = 0.6), the benzene content remained practically constant at 11-13 g/Nm 3 when solid bed temperature increased by ca.90 • C.However, it increased from 7 to 11 g/Nm 3 when the temperature increased from 640 • C to 690 • C for the chosen wood pellet tests.The main reason behind this fact may be the noticeably high S/B ratio used for these experiments with wood pellets, Fig. 4. Influence of temperature on tar content in the syngas (GC-MS and gravimetric) using straw (left) and wood pellets (right) (S/C and S/B ratios used are indicated on the X-axis).

Table 5
GC-MS tar composition (expressed in g/Nm 3 of dry gas) for wood and straw pellets under the conditions of temperature, S/C and S/B ratios used in Fig. 4.  which may further promote the cracking of this tertiary tar compound into linear hydrocarbons and permanent gases.This behaviour observed for benzene explains the rise in BTX content shown in Fig. 4 (right).
As seen in the experiments shown in Table 5, the influence of temperature on tar composition cannot be understood independently of the S/B ratio in a SEG process since the CaO used as bed material and circulating solid has noticeable catalytic tar cracking properties.For this reason, Fig. 5 shows the evolution of phenol and PAHs content in the collected tar with solid bed temperature and S/B ratios for straw pellets.As noticed, the decomposition of phenols was enhanced by the increasing gasification temperatures as well as by the S/B ratio.As can be appreciated in this Figure, there were no phenols detected in the collected tar for experiment 6 in Table 2, despite the low gasification temperature reached in this case (i.e.640 • C) since the S/B ratio maintained throughout the experiment was significantly high (i.e.2.5 kg of sorbent per kg of fuel, in this case).On the contrary, working with high gasification temperatures of 735 • C caused the phenol content in the collected tar to be reduced to values close to 2.5 g/Nm 3 even when the S/B ratio was kept low (i.e.0.4 kg of sorbent per kg of biomass).Intermediate temperatures and S/B ratios led to a significant rise in the phenol content in collected tar, with a maximum content close to 12 g/ Nm 3 achieved at 652 • C and an S/B ratio of 1.1 (Fig. 5).Looking into these trends, it seems that the amount of CaO introduced into the gasifier could exert greater influence over the reduction in phenol content reduction than gasification temperature.With regard to the PAHs content in collected tar, it can be observed in Fig. 5 that this tar group demonstrates a behaviour contrary to that of phenols when temperature and S/B ratio are modified.The PAHs content in collected tar measured for experiments performed at high temperature and/or with large amounts of sorbent introduced into the gasifier showed higher values.Moreover, as the temperature and/or S/B ratio increased, the PAHs became richer in naphthalene, as well as in compounds with a larger number of benzene rings, such as pyrene and benzo(a)pyrene.More specifically, the naphthalene contents measured in the collected tar for experiments 4 and 6 in Table 2, performed at the highest temperature (i.e. 736 • C) and with the highest S/B ratio (i.e.2.5), corresponded to 3 and 3.7 g of naphthalene per Nm 3 of dry gas, respectively.This behaviour of PAHs and phenols had previously been observed by other authors in the literature [46].Therefore, based on these observations, phenols could be considered a precursor for PAHs (especially naphthalene) formation, decomposing through a Diels-Alder mechanism with a subsequent hydrogen separation into low molecular mass PAHs, as suggested by Morf et al. [47].
Finally, the influence of the steam excess used on tar formation was evaluated by comparing experiments 11 and 13 in Table 2, which were run under the same solid bed temperature (640 • C) and similar S/B ratios (1.6 and 1.8) but different S/C ratios (1.7 and 2.4).Fig. 6 shows the tar content and composition for the GC-MS collected tar for these two experiments performed under different S/C ratios.As can be appreciated, raising the steam excess in the gasifier of an SEG process causes the tar content in the syngas to be greatly reduced.Gravimetric tar values decreased from 19.9 g/Nm 3 to 10.6 g/Nm 3 when the steam excess was raised from S/C = 1.7 (i.e.1.3 kg of steam/kg of biomass) to S/C = 2.4 (i.e.1.8 kg of steam/kg of biomass) at the gasification temperature of 640 • C chosen for this comparison.Similarly, GC-MS collected tar was found to decrease with the steam excess used, although its reduction was not as pronounced as that observed for the gravimetric tar (i.e. from 21.6 to 16.6 g/Nm 3 ).This effect on tar content could be expected since steam introduced into the gasifier favoured hydrocarbon reforming, thus reducing tar content.It can be seen in Fig. 6 that all the tar species quantified by GC-MS decrease with the increasing S/C ratio for the operating conditions chosen of 640 • C and 1.6-1.8kg sorbent/kg biomass.Benzene, toluene and naphthalene are the dominant species in the GC-MS collected tar, and are therefore the tar compounds that decrease most with the steam excess in the gasifier.Although it cannot be appreciated in this figure, tar compounds heavier than pyrene (i.e.molecular weight > 202 kg/kmol) also decreased also with the S/C ratio from 0.7 g/Nm 3 to 0.2 g/Nm 3 .Based on this analysis, tar production in the gasifier of an SEG process can be reduced by increasing the amount of steam fed in together with the biomass.However, when choosing this tar reduction strategy, an appropriate heat integration scheme would be important for the process at commercial scale so as not to dramatically penalize the efficiency of the whole process.

Fate of sulphur in SEG tests using straw pellets
Finally, the fate of sulphur was analysed exclusively in the SEG tests using straw pellets due to the significant sulphur content of this biomass.Under steady state conditions in the gasifier, the sulphur introduced mainly with the biomass, and to a lesser extent with the CO 2 sorbent, ends up in the resulting syngas and in the solids collected through the overflow.Fig. 7 shows the sulphur distribution between the syngas, unconverted char and partially converted sorbent particles exiting the gasifier in experiments 5, 6 and 8 in Table 2 performed under different temperature and S/C conditions.The sulphur distribution among the different exiting streams were similar for the rest of the experiments performed with straw pellets, although the specific numbers differed.As seen in this figure, more than two-thirds of the sulphur introduced into the gasifier with the biomass and with the calcined sorbent was found as CaS in the partially carbonated solids exiting the gasifier.More specifically, between 65 and 85% of the inlet sulphur was found as CaS with the partially converted sorbent exiting the gasifier.CaO introduced with the calcined sorbent reacts with the H 2 S and COS formed during gasification according to reactions 6-7 [48].Characterization by ICP of the partially converted sorbent particles exiting the BFB reactor determined the amount of S in these samples, which resulted in CaS contents of ca.0.002 mol of CaS per mole of Ca in these solids for the experiments in Fig. 7.This reduced CaS content in the carbonated particles is due to the high Ca/S ratio used in these experiments (i.e.690-1000 mol of Ca introduced with the calcined limestone per mole of S introduced with the biomass).
As also depicted in Fig. 7, the fraction of sulphur that remained with the unconverted char pellets was practically constant at 14-15% for Fig. 5. Phenols and PAHs contents in GC-MS collected tar as well as total GC-MS collected tar using straw pellets as fuel for different S/B proportions and solid bed temperatures (S/C ratio 1.4).experiments 5 and 8, whereas it increased up to 25% for experiment 6. Fluctuations in the sulphur fraction recovered with the syngas are linked to the differences observed in the sulphur remaining with the unconverted char due to the different rate of char conversion achieved in each case.As char pellet conversion increased, both the C and S contents of the char pellets collected were reduced.Char gasification reactions (2-5) led to the conversion of C(s) into CO and CO 2 , whereas S was released as a gaseous sulphur species.Gas analysis performed on the sampling bags taken during the steady-state period determined that H 2 S was the major sulphur impurity in the syngas.COS and CH 4 S were also detected in the gas in rather similar amounts, but their concentration was one order of magnitude lower than that of H 2 S.However, no CS 2 was found in any of the experiments performed.
Since the ratio between the CaO introduced with the calcined sorbent and the sulphur introduced with the biomass was extremely high in the experiments performed, sulphur capture in the gasifier should have occurred until equilibrium of reactions ( 6) and ( 7) was reached.Equilibrium calculations for reactions ( 6)- (7) were made by means of Gibb's free energy minimisation, considering a product gas containing H 2 , CO, CO 2 and H 2 O, in addition to H 2 S and COS.Calculations were made at atmospheric pressure and for a given partial pressure of H 2 O and CO 2 (i.e. 60 vol% and 5 vol%, respectively, as an average of the results obtained for the experiments with straw pellets).Fig. 8 shows the concentration of H 2 S measured in the syngas for the experiments with straw pellets indicated in Table 2.The continuous line in this figure corresponds to the equilibrium calculation.The temperature used in this figure corresponds to the temperature in the first cyclone, which is the point where the slipstream of syngas for H 2 S measurement was located.At 650-700 • C, which is the temperature range covered for the solid bed in the gasifier, the minimum H 2 S concentration that could be reached within the gasifier would be 500-770 ppm (dry basis).However, as seen in this figure, the H 2 S concentration measured in the syngas was below 300 ppm in all the experiments performed.The reason for this behaviour is that the upper part of the reactor (corresponding to measurement T7 in Fig. 1) and the cyclones were at temperatures well below the temperature of the solid bed.In this way, sulphur capture still occurred at the outlet of the reactor by means of the CaO particles entrained out of the reactor, thus achieving the H 2 S equilibrium value for reaction (6) at the temperature of the first cyclone, as observed in Fig. 8.The COS content measured in the syngas was between 1 and 8 ppm in the experiments plotted in this figure.However, no CH 4 S was generally detected in the syngas, except in the experiments with either the lowest amount of CaO entrained out of the reactor or the lowest temperature in the cyclone, where 1 and 0.6 ppm of CH 4 S were measured, respectively.

Fig. 2 .
Fig. 2. (left) Influence of solid bed temperature on total gas yield (S/C ratio: 1.4-1.7)and (right) Individual gas yield for different operating conditions of temperature and S/B ratio (error bars included correspond to the standard deviation of the measurements and calculations made to the samples taken during the steadystate period (n = 4)).

Fig. 6 .
Fig. 6.Influence of S/C ratio on tar content in the syngas (collected and gravimetric) for experiments 11 and 13 in Table 2 using wood pellets (solid bed temperature = 640 • C and S/B proportion = 1.6-1.8).

Fig. 8 .
Fig. 8. H 2 S content in the syngas for experiments in Table2using straw pellets as fuel.Numbers included in the figure correspond to the g CaO /Nm 3 humid gas passing through the cyclones (continuous line represents the equilibrium for the sulphidation of CaO (6-7) for 60 vol% and 5 vol% of steam and CO 2 contents in the syngas, respectively).

Table 1
Composition and heating value of the biomass feedstocks used for SEG tests.

Table 3
Classification of detectable GC-MS compounds in collected tar.