Evolution of Tar in Coal Pyrolysis in Conditions Relevant to Moving

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Evolution of tar in coal pyrolysis in conditions relevant to moving bed gasification Cesar Berrueco, Esther Lorente, Daniel Van Niekerk, and Marcos Gabriel Millan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef500633p • Publication Date (Web): 03 Jul 2014 Downloaded from http://pubs.acs.org on July 9, 2014

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Evolution of tar in coal pyrolysis in conditions relevant to moving bed gasification Cesar Berrueco1,3, Esther Lorente1, Daniel Van Niekerk2, Marcos Millan1*

1 Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK 2 Sasol Technology (Pty) Ltd., R&D Division, 1 Klasie Havenga Road, Sasolburg 1947, South Africa 3 Current Address: Bioenergy and Biofuels Area, Catalonia Institute for Energy Research, IREC, 43007 Tarragona, Spain

* Corresponding author: Tel. +44(0)2075941633; Fax: +44(0)2075945638; e-mail address: [email protected]

Abstract Most tars in gasification are produced in the pyrolysis step at lower temperatures than those required for gasification reactions. In updraft moving beds, the tars form through pyrolysis as the coal is relatively slowly heated. Tars are promptly carried away from the gasifier by the gas flow as part of the gas product. This study was designed in order to understand the effect of the coal bed on the tar cracking process during the pyrolytic step of a slowly heated South African inertinite-rich coal sample. A wire mesh reactor was employed to determine the intrinsic extent of tar release from the coal particles, while a fixed bed reactor enabled the measurement of the extent of tar cracking by interactions between tars and the heated coal bed. Experiments were carried out at atmospheric and high pressure and the effect of temperature and heating rate on tar quantity and quality was studied. A significant effect of the bed on tar reduction was observed even at the lower temperatures used in these experiments. Heating rate still showed a significant effect on tar yield despite slow heating being used in all cases. High pressure caused a decrease in tar yields due to intra-particle tar cracking and repolymerisation, which was more marked than the decrease in total volatiles. The effect of temperature and heating rate on tar quality was larger at atmospheric pressure. Primary tar cracking mainly took place at aliphatic bridges connecting aromatic rings. Subsequent condensation reactions led larger fused aromatic rings. Tars obtained in the fixed bed reactor were lighter than those from the wire mesh reactor.

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Keywords: Pyrolysis, coal, tar, wire mesh reactor, reactor design


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Introduction Coal gasification is a complex process, involving numerous interactions between physical and chemical phenomena occurring in a broad range of temperature. In moving bed gasification, heating rates are low and there is a clear distinction between pyrolysis and reduction zones within the gasifier. Tars evolved during pyrolysis deposit on hot particle surfaces, repolymerising to form char or cracking to produce lighter volatiles [1-5]. By contrast with downdraft gasification in which tars flow into the gasification and combustion zones, in updraft configurations the gas flow carries the remaining tars with the gas product, yielding a considerable amount of tar in the syngas. The yield and quality of the tar depend on the extent of cracking and repolymerisation reactions that take place in the pyrolysis zone. Tar conversion in the bed depends on various process variables, mainly temperature, residence time and type of contact solid/volatiles. An additional parameter to consider on coal devolatilisation processes is the effect of pressure [4,6]. The use of high pressure in gasifiers results in an increase in coal throughput, potential reductions in pollutant emissions and an enhancement of gasification reaction rates [7-9]. This study focuses on the comparison of tar yield and quality during coal pyrolysis obtained using a wire mesh reactor (WMR) and a fixed bed reactor (FBR) under slow heating rates relevant to moving bed gasification. The use of these two reactors enables the effect of the char bed to be investigated. The WMR operates with a monolayer of coal particles and therefore enables the characterization of the behaviour of the pyrolysing material itself, with no influence from reactions on a char bed [4, 10]. It has been mostly used to study coal and biomass behavior under the high heating rate conditions found in fluidized beds and entrained flow gasifiers. The FBR is capable of operating at high temperatures and pressures [11,12,13] and by contrast with the WMR, the outcome of its experiments is affected by reactions between evolving volatiles and heated solid particle surfaces [4,14]. The tar yields in pyrolysis experiments carried out at a range of conditions relevant to the pyrolysis zone of a moving bed gasifier were obtained with both reactors. Tar quality was evaluated using analytical techniques that provide information about the molecular weight distribution (size exclusion chromatography [15]) and the extent of fused aromatic rings (UVFluorescence spectroscopy [16]) in the sample. The comparison between the two different bench-scale reactors (WMR and FBR) has allowed the determination of the effect of the char bed on product yields and tar quality at different conditions.


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Experimental Raw materials and sample preparation A typical South African Highveld inertinite-rich coal was used in these experiments. South African Highveld coals, typically, has a random reflectance of 0.61 to 0.67 and is classified as high-volatile bituminous in rank. These coals are typically inertinite rich (>70 %, mineralmatter free basis) and contains significant amounts of mineral matter (~30 % dry basis ash in proximate analysis). These coals typically contain 22 % volatile matter and 48 % fixed carbon (dry basis from proximate analysis). The coal sample was ground to a size range of 106–150 µm. Sufficient sample was prepared in one batch for the whole test program. The bulk samples were stored under N2, in a freezer. Approximately 1 g and 5 mg of sample were used in each fixed bed and wire mesh reactor test respectively. Wire Mesh Reactor The high pressure wire mesh reactor used in this study has been described elsewhere [4,10,17,18,19]. Briefly, a folded wire-mesh is clamped between two water-cooled electrodes, one of which is spring-loaded. The mesh serves as resistance heater for the sample (typically 5-7 mg), spread as a monolayer between the two mesh layers. The heating rate is controlled by adjusting the current passed through the mesh. During an experiment, a stream of gas (He) is passed through the sample area of the mesh to quickly remove evolving volatiles and suppress their secondary reactions with the sample. The sample was heated up to the final temperature (450, 575 or 700 °C) at variable rates. The holding time applied at the final temperature was 30 s. Temperature, heating rate and pressure were varied in this study. A tar trap cooled by liquid N2 connected to the exit of the reactor collects any tars that are released. By careful weighing of the mesh, sample and tar trap, the amounts of char, tar and permanent gases are determined. After the tar trap was weighed, tars were recovered by washing the tar trap with a mixture of CHCl3:MeOH (4:1 v/v). The solvent was removed by purging with N2. The product was then dried in a circulating air oven at 50 ºC for 1 h. Fixed Bed Reactor For the purposes of this study, a single-stage fixed bed reactor, also described in the literature as “hot-rod reactor” [20,21,22], was used. The reactor consists of a vertical cylindrical tube,


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which serves as pressure vessel and electrical resistance heater. The body of the reactor has an internal diameter of 12mm, 2mm wall thickness, a length of 250mm and is made from AISI 316 grade stainless steel in the case of atmospheric runs and Incoloy 800 HT for high pressure experiments. It is fitted with a ‘T’ piece at the top to allow a thermocouple to be placed inside the bed and gas to be supplied to the reactor. The thermocouple is imbedded in the coal sample to ensure that the temperature reading represents the actual pyrolysis temperature. The bed is heated by passing a current along the reactor body, which is controlled to follow the required heating profile. The bottom of reactor has a welded flange, which connects it to a tar trap. A low flow (at a gas velocity of 0.1 m s-1) of inert gas (He) is used to sweep the released volatiles into the tar trap. The trap is placed in a liquid nitrogen bath, so that the volatiles released from the reactor can be condensed and trapped. Stainless steel mesh is packed in the outlet arm of the trap to ensure efficient trapping of the condensed material in the form of aerosol droplets. A standard suite of operating conditions was used in these experiments. 1 g of coal was placed in the reactor before each test. It was heated up to the final temperature at preestablished heating rates. Once the reactor reached the final temperature, it was held for 15 min before the test was ended and the reactor allowed to cool to ambient temperature. After each run and cooling period, the system was carefully taken apart and the reactor and tar trap were washed to determine the amount of tar collected during the run. A 4:1 (v/v) mixture of chloroform and methanol was used as the washing agent. The solution was then filtered using a preweighed Whatman number 1 filter paper and collected in a flask. A BUCHI Rotavapor R-3000 was used to evaporate most of the solvent. The rotating evaporator was operated at 90 °C (water bath temperature) and 40 rpm for 15 min. The remaining solution was washed, with a minimal amount of the solvent, into a small aluminum beaker and dried in a recirculating air oven, operated at 50 °C for 2 h, to evaporate the residual solvent and allow tar to be isolated. The tar was then weighed and its yield was determined as a percentage of the initial weight of coal, on dry basis. The char and preweighed wire mesh plug from each reactor were placed in a preweighed beaker. This beaker was then placed in the circulating air oven, operated at 50 °C for 1 h, to evaporate the residual solvent. After drying, the char yield was determined by weight.


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Size exclusion chromatography (SEC) The method and column calibration have been described elsewhere [23,24]. A 300 mm long, 7.5 mm i.d. polystyrene/polydivinylbenzene-packed Mixed-D column with 5µm particles has been used (Polymer Laboratories, Church Stretton, UK). The column was operated at 80 °C and a flow rate of 0.5 mL min-1 of N-methyl 2 pyrrolidinone (NMP) as the mobile phase. Detection was carried out using a Knauer Smartline diode array UV-absorbance detector. Detection of standard compounds and samples was performed at 270 and 300 nm respectively, where NMP is partially transparent. All sample solutions in NMP were prepared to a similar concentration, which was in the range of 0.1-0.3 mg mL-1. A total of 20 µL of solution was injected into the SEC system. UV-Fluorescence spectroscopy (UV-F) A Perkin Elmer LS55 luminescence spectrometer was used to obtain emission, excitation and synchronous spectra of the tar samples, using NMP as solvent in all cases (only synchronous spectra are shown in this work). The procedure has been described elsewhere [16, 25]. The spectrometer was set to scan at 500 nm min-1 with a slit width of 5 nm. Synchronous spectra were acquired at a constant wavelength difference of 20 nm. A quartz cell with 1 cm path length was used. Solutions were diluted with NMP to avoid self-absorption effects. Dilution was increased until the fluorescence signal intensity began to decrease in intensity while the relative intensities of the different maxima in the spectra ceased to change.

Results and discussion Evolution of tar yield during the pyrolysis stage has been studied under different experimental conditions of final temperature, heating rate and pressure and two reactor configurations. Table 1 presents the char and tar yields obtained in the wire mesh and fixed bed reactors as a function of experimental conditions. The table also shows the difference in tar and char yields between both reactors.


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Table 1. Experimental conditions and average yields from wire mesh reactor and fixed bed reactor runs. Heating Final Pressure Rate Temperature (bar) (˚C min-1) (˚C) 50 50 6 6 50 50 6 6 27.5 27.5 27.5

450 700 450 700 450 700 450 700 575 575 575

1 1 1 1 40 40 40 40 1 20 40

Average Yield (%wt.) WMR Tar Char 3.9±0.52 90.2±1.14 14.0±0.15 79.3±0.43 3.4±0.10 90.7±0.12 8.5±0.68 80.3±0.68 3.9±0.04 89.1±0.21 4.8±0.06 81.8±0.11 3.3±0.07 90.3±0.21 4.5±0.07 81.8±0.14 7.2±0.31 81.6±0.26 6.0±0.87 82.4±0.06 3.3±0.11 84.7±0.16

FBR Tar 1.7±0.07 4.3±0.06 1.1±0.01 3.8±0.07 1.7±0.05 2.6±0.07 1.3±0.06 2.5±0.01 2.9±0.14 2.8±0.15 2.4±0.01

Char 91.1±0.07 80.6±0.28 92.2±0.25 82.0±0.56 91.8±0.35 81.6±0.14 92.1±0.07 81.9±0.15 85.6±0.01 85.7±0.08 86.2±0.12

WMR-FBR ∆ Tar ∆ Char 2.20±0.52 -0.9±1.14 9.8±0.16 -1.3±0.51 2.3±0.10 -1.5±0.28 4.7±0.68 -1.7±0.88 2.2±0.06 -2.7±0.41 2.2±0.09 0.2±0.18 2.0±0.09 -1.8±0.22 2.0±0.07 -0.1±0.21 4.3±0.34 -4.0±0.26 3.2±0.88 -3.3±0.10 0.9±0.11 -1.5±0.20

Evolution of tar during pyrolysis experiments in the WMR The effect of final temperature on tar and char yields from pyrolysis experiments (He atmosphere) in the wire mesh reactor at two different pressures (1 and 40 bars) and heating rates (6 and 50 ºC min-1) can be observed in Table 1. These are typical conditions found in the pyrolysis zone of moving bed gasifiers. Heating rates are low in comparison with the usual rates applied in WMR experiments, which can reach up to 10,000 ºC s-1. As expected, tar yield increased as temperature was raised between 450 and 700 ºC, while the opposite effect was observed on the char yield. The heating rate showed a small effect on the char yield at atmospheric pressure, with a slightly smaller char yield for the higher heating rate studied (50 ºC min-1). The difference in the char yield when varying heating rates was even less significant at high pressure (40 bar). On the other hand, the tar yield was remarkably sensitive to heating rates, mainly at atmospheric pressure. Higher heating rates led to larger tar yields (14.0% wt. at 50 ºC min-1 vs 8.5% wt. at 6 ºC min-1; final temperature 700 ºC), at atmospheric pressure while the difference was very little at 40 bar. In summary, a lower heating rate caused tar to repolymerise to char and crack to produce greater gas yield. Although it has previously been reported in the literature that lower heating rates favour tar repolymerization reactions [4], these results show that heating rates can still have a significant effect when its variations are relatively small, mainly at high temperature and atmospheric pressure.


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Higher pressures favoured tar cracking, leading to a clear reduction in the tar yield, and charring reactions due to the physical suppression of volatile release. An increase in char yield of about 2.5% was observed when pressure varied from 1 to 40 bar at 700 °C and heating rate 50 °C min-1, and about 1.5% for 700 °C and 6 °C min-1. The data allow the observation of the combined effect of pressure and temperature on WMR tar yields during the pyrolysis experiments conducted at different final temperatures and heating rates. There was a decrease in tar yield with pressure in all the studied range. It can be noticed that the influence of pressure was more relevant at high final temperatures and high heating rates. The decrease in tar yield with pressure at 700 ºC and heating rate of 50 ºC min-1 (from 14.0% wt. at atmospheric pressure to 4.8% wt. at 40 bar) was much larger than the increase observed in char yield (from 79.3 to 81.8 % wt.), which is evidence that the extent of cracking induced by pressure is significant. Similar trends were observed comparing the results of the series of experiments obtained at 700 ºC and heating rate of 6 ºC min-1 at both pressures. In this case, the effect of a change in pressure from 1 to 40 bar on the tar yield of the experiments was much less relevant than the one observed at 50 ºC min-1. On the other hand, this effect was almost negligible at the lower final temperature (450 °C) and heating rate (6 °C min-1) studied. The decrease in tar yield with pressure can be explained, as mentioned above, by the greater resistance exerted by the sweeping gas at higher pressures on the escaping volatiles. This causes an increase in the material residence time within the particle, leading to cracking and repolymerization reactions and carbon reincorporation into the forming char [26,27,28]. This effect is more relevant at higher temperatures and heating rates as these conditions tend to favour tar release.

Evolution of tar during pyrolysis experiments in the FBR In order to understand the effect of the secondary reactions in the coal bed, a series of runs were conducted using a fixed bed reactor under the same experimental conditions as the ones used in the WMR experiments. Table 1 shows the effect of temperature and heating rate on the tar and char yields obtained in the pyrolysis experiments (under He) at two different pressures. Similarly to the behaviour observed in the WMR, tar yield increased as temperature rose in the range studied, while the opposite effect was observed in the char yield. The heating rate showed a smaller effect on the char yield, with slightly lower values for both final temperatures studied when the higher heating rate (50 ºC min-1) was applied.


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High pressure did not suppress tar release at 450 ºC (Table 1), in line with the observations from WMR experiments. However, at 575 ºC and 700 ºC, higher pressure resulted in a reduction in tar yield. The increase in char yield was smaller than the decrease in tar, showing that tar cracking was also enhanced at higher pressure. Overall, these results showed similar trends to those obtained using the WMR. However, the effect of the packed bed in the FBR was clearly observed. A greater extent of cracking and polymerization reactions [27] took place, leading to smaller tar yields than in the WMR. A full comparison between the outcomes of both reactors is given below.

Characterization of tar obtained in WMR and FBR experiments The information obtained through the characterization of the tars recovered from the pyrolysis experiments allowed a deeper insight into tar primary structure and the subsequent reactions undergone in the char bed. The tar characterization carried out in the present work involved the determination of molecular weight distributions by size exclusion chromatography and the comparison of structural features of the tar samples by UV-F spectroscopy. The results obtained by SEC showed in all cases bimodal signal distributions. The peak appearing at shorter elution times corresponds to material excluded from the pores of the column packing, due to either its large molecular weight or a rigid conformation that makes it unable to enter the pores [15,23]. The exclusion limit of the column, defined according to the behaviour of polystyrene standards, is about 200,000 u. The main peak appears at longer elution times and corresponds to material resolved by the column porosity. Molecular weight estimates can be calculated from a calibration based on the elution times of polystyrene and polycyclic aromatic hydrocarbon (PAH) standards [23].


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Figure 1. SEC of coal pyrolysis tars obtained at atmospheric pressure, showing the influence of temperature and heating rate: a) WMR b) FBR. The SEC of the coal tars obtained in the WMR (Figure 1a) at 6 and 50 ºC min-1 and 450 ºC showed very similar molecular weight distributions, indicating that the influence of heating rate at low temperatures is small. However, at 700 ºC the tar obtained at higher heating rates had a distribution clearly shifted towards larger molecular weights than that obtained under slow heating. This is consistent with slow heating favouring tar cracking within the coal particle, as also inferred from the larger tar yields and smaller gas yields obtained at 50 ºC min-1. The combination of low heating rate and high temperature caused tars obtained at 6 ºC min-1 and 700 ºC to have a distribution shifted to smaller values. By contrast, comparing the SEC of coal tars obtained in the FBR at different temperatures and heating rates at atmospheric pressure (Figure 1b), it can be observed that temperature was the predominant variable and heating rate effects were not as clear as in WMR tars. The tar obtained at the higher final temperature (700 ºC) was slightly heavier than that from 450 ºC runs for both heating rates. The interaction between the evolving tars and the char bed makes the effect of heating rate become blurry due to condensation and cracking reactions, which are favoured in this reactor configuration [4,28,29]. Synchronous UV-F spectroscopy shows differences in the size of aromatic chromophores, with signal from larger aromatic chromophores appearing at longer wavelengths [16,25]. However, there is a drop in the intensity of fluorescence as aromatic groups become larger, which imposes a ceiling to the size of observable aromatic chromophores. It has been reported that only PAH below approximately 3,000 g mol-1 can be detected [30]. Figure 2a shows that the spectrum of the coal tar obtained in the WMR at 50 ºC min-1, atmospheric pressure and higher temperature (700 ºC) appeared at shorter wavelengths in comparison to


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all other WMR tars. Taken together with the SEC analysis presented above, these results reflect that the tars with larger molecules have smaller polyaromatic ring sizes. This is consistent with the tar obtained at 700 ºC and 50 ºC min-1 having undergone less thermal degradation. The picture emerging from this analysis is that thermal reactions lead to cracking mostly at aliphatic bridges connecting aromatic structures, reducing molecular size but not necessarily the size of PAH groups. Condensation reactions in the released aromatic structures can subsequently increase their ring sizes. By contrast, the UV-F spectra obtained from the coal tar recovered during the FBR experiments presented little differences, which points at the levelling effect of the bed of char on tar structure, in good agreement with the SEC results previously shown. 700 ºC 700 ºC 450 ºC 450 ºC

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Figure 3. SEC of coal pyrolysis tars obtained at 40 bar, showing the influence of temperature and heating rate: a) WMR b) FBR. The SEC of tars obtained at 40 bar in the WMR are presented in Figure 3a. The differences in molecular weight distributions of the tars obtained at different conditions were much less


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relevant than at atmospheric pressure. All tars also showed very similar UV-F spectra (not shown). These results suggest that at higher pressure the effect of the rest of variables on tar composition diminishes, similarly to the influence of the bed in the FBR experiments. As discussed above, pressure exerts a resistance to the release of volatiles, promoting tar reactions within the particle. Likewise to the effects observed at atmospheric pressure, differences in the tars obtained in the FBR were even less marked at higher pressure (Figure 3b).

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Figure 4. SEC of coal pyrolysis tars, showing the influence of pressure at final temperature 700 ºC and 50 ºC min-1: a) WMR b) FBR. The effect of pressure on the tar from pyrolysis experiments at 700 ºC and 50 ºC min-1 is presented in Figure 4. Molecular weight distributions move slightly towards longer retention times (smaller molecular weight) with pressure regardless of the reactor used. Similar trends for the pressure influence were observed in experiments carried out at 450 ºC and 700 ºC at different heating rates. The differences increased with increasing final temperature and decreasing heating rates. These results confirm that at higher pressure tar cracking reactions are favoured.

Comparison of results from the FBR and WMR The results shown in Table 1 reveal a marked difference in performance between the two reactors. Tar (and total volatile) yields recovered from the fixed bed reactor were measurably lower than those obtained in the WMR at any given conditions of temperature, pressure and heating rate. The differences between the two sets of data reflect loss of volatiles through contact between evolving volatiles and the bed of pyrolyzing coal in the FBR [27,28]. This


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contact produces secondary reactions of the tar leading to repolymerization and charring [31]. The difference in tar yield between both reactors (∆Tar in Table 1) is larger at atmospheric pressure than at 40 bar. The influence of the bed is not only reflected in the tar and char yield obtained using the different reactor configurations, but also in tar characteristics. The WMR tars showed a heavier molecular weight distribution than the FBR tars obtained at the same conditions of final temperature, pressure and heating rate, as shown in Figure 5. The comparison of UV-F spectra obtained at 50 ºC min-1 and varying temperature and pressure is presented in Figure 6. Tars from the WMR tended to exhibit larger PAH systems than those from the FBR, as indicated by the shift in their spectra towards longer wavelengths. The exception is the experiment at 700 ºC and atmospheric pressure where the UV-F spectrum of FBR tars is marginally shifted to larger PAH groups. These results are determined by the relative contributions of cracking and repolymerisation to tar structure and PAH size.

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Figure 5. SEC of coal pyrolysis tars obtained in the WMR and FBR at 50 ºC min-1, final temperature (1) 450 ºC and (2) 700 ºC, and pressure a) 1 bar and b) 40 bar.


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0.6 0.4 0.2 0.0

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Figure 6. Height normalized UV-F spectra of coal pyrolysis tars obtained in the WMR and FBR at 50 ºC min-1, final temperature (1) 450 ºC and (2) 700 ºC, and pressure, a) 1 bar and b) 40 bar.

Conclusions A South African inertinite-rich coal was pyrolysed at rates and temperatures similar to those found in the pyrolysis zone of moving bed gasifiers. Experiments in a wire mesh reactor provided data on intrinsic particle behavior. As expected, tar yield increased and char yield diminished as temperature was increased. Although the heating rates used in this work were low in comparison with previously published WMR data, this variable did show a significant effect on the yields as lower heating rate caused tar to repolymerise to char and crack to produce larger gas yield. High pressure caused a marked decrease on the tar yields obtained, mainly at 700 °C, as a consequence of intra-particle cracking and repolymerisation of the


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tars. Tar yields diminished more sharply than total volatiles, indicating that higher pressure, in addition to producing an increase in char formation, led to an increase in the gas yield due to tar cracking reactions. The comparison between experiments carried out in the WMR and FBR allows the determination of the effect of secondary reactions on tar yields and quality. The results showed a decline in the tar yield from the FBR in comparison with the WMR, which became more relevant as temperature increased. The coal bed diminishes the effect of other variables, such as heating rate and pressure. In particular, the effect of pressure in FBR experiments was not as large as in the WMR. As high pressure was observed to enhance tar cracking and repolymerisation, the presence of the bed, which has a similar effect, hides the impact of increasing pressure. Tar quality was evaluated using analytical techniques that provide information about the molecular weight distribution (size exclusion chromatography) and key structural features of the samples (UV-Fluorescence spectroscopy). The main differences between tars obtained at varying conditions of temperature and heating rate appeared in atmospheric pressure runs in the WMR. The use of high pressure and the presence of a char bed had a levelling effect on tar structures, which became more independent from pyrolysis conditions. The opposite contributions of cracking and condensation reactions affected the nature of the tars. Characterisation of WMR tars showed that primary tar cracking predominantly takes place at aliphatic bridges connecting aromatic rings. Subsequent condensation reactions have led to increasing the size of aromatic fused ring structures. In general, the tars obtained in the FBR showed lighter molecular weight distributions in comparison to WMR. This is related to tar exposure to longer residence times of the tar in the heated zone and gas-solid interactions in the fixed bed.

Acknowledgments C. Berrueco is grateful to the Spanish Ministry of Economy and Competitivity for funding his Ramon y Cajal contract (RYC-2011-09202). Funding for this research from SASOL is gratefully acknowledged.


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Figure Captions Figure 1. SEC of coal pyrolysis tars obtained at atmospheric pressure, showing the influence of temperature and heating rate: a) WMR b) FBR. Figure 2. Height normalized UV-F spectra of coal pyrolysis tars obtained at atmospheric pressure, showing the influence of temperature and heating rate: a) WMR b) FBR. Figure 3. SEC of coal pyrolysis tars obtained at 40 bar, showing the influence of temperature and heating rate: a) WMR b) FBR. Figure 4. SEC of coal pyrolysis tars, showing the influence of pressure at final temperature 700 ºC and 50 ºC min-1: a) WMR b) FBR. Figure 5. SEC of coal pyrolysis tars obtained in the WMR and FBR at 50 ºC min-1, final temperature (1) 450 ºC and (2) 700 ºC, and pressure a) 1 bar and b) 40 bar. Figure 6. Height normalized UV-F spectra of coal pyrolysis tars obtained in the WMR and FBR at 50 ºC min-1, final temperature (1) 450 ºC and (2) 700 ºC, and pressure, a) 1 bar and b) 40 bar.


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Evolution of Tar in Coal Pyrolysis in Conditions Relevant to Moving

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