Tar Reduction Through Partial Combustion of Fuel Gas

Fuel 84 (2005) 817–824 www.fuelﬁrst.com

Tar reduction through partial combustion of fuel gas

M.P. Houben, H.C. de Lange*, A.A. van Steenhoven

Technische Universiteit Eindhoven, Department of Mechanical Engineering, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

Received 6 September 2004; received in revised form 8 December 2004; accepted 8 December 2004 Available online 28 January 2005

Abstract

A partial combustion burner is introduced as a cleaning system for the tar content of gaseous (bio) fuel. The results of experiments, using a synthetic low caloriﬁc gas mixture, demonstrate the effectivity of the proposed process. In these experiments naphthalene is added as a model tar component. The effect of partial combustion of the fuel gasmixture on the naphthalene is examined for different air/fuel ratios (l) and varying hydrogen-methane fuel concentrations. For a fuel gasmixture with high methane concentrations or for higher l-values the total tarcontent slightly decreases. In this case the naphthalene polymerises, i.e. forms higher ring components and sometimes even turn into soot. At lower l’s and higher hydrogen concentrations the tarcontent strongly decreases. Moreover, the naphthalene is now cracked, i.e. converted into lighter tars and permanent gases. It is found that, for fuel gases representative for biogasiﬁcation products and at a l of 0.2, the presented burner reduces the tar content of the gas with over 90% by cracking. The paper ends with a short discussion on the conditions that may determine the cracking/polymerisation mechanism. q 2005 Elsevier Ltd. All rights reserved.

Keywords: Tar; Biomass; Gascleaning; Partial oxidation; Tarcracking

1. Introduction combustion as a way to remove tars. Although, there have been studies in which air is added separately, i.e. combined For the introduction of small-scale biomass gasifiers the with external heating. In [2,3] external heating of pyrolysis production of tars in this process is one of the major gas is combined with air addition in a reactor at 800, 900 and problems. Apart from causing environmental hazards, tar is 1000 8C. Experiments are performed with an excess air ratio known to create process-related problems in the end use varying from 0 to 0.7. The minimum tar content was devices, such as fouling, corrosion, erosion and abrasion. measured at 900 8C together with an excess air ratio of 0.5. Before the gas can be introduced into the gas engine, the tar The results show that the temperature in the reactor only had content has to be reduced to low values. In literature, an influence at small excess air ratios. In [13] it was shown various overviews can be found of the existing types of that tar reduction is a function of temperature and oxygen gasifiers and cleaning methods (e.g. [5,22]). content. Tar-reduction seems to take place when raising the Several methods for tar removal are possible [24]: tar temperature from 500 to 900 8C. Furthermore, adding removal by physical processes (e.g. filters), thermal methods oxygen above 700 8C also results in a considerable and catalytic methods are the options that are most often reduction of the tar content. The tar reduction at 500 8Cis used. Most of these cleaning systems nowadays are too 88%. This increases to almost to 99% by raising the expensive or complex to be used in small-scale applications. temperature to 900 8C and adding oxygen. In [18] the same In this case thermal methods seem to be the most appropriate. tendency was found using partial oxidation of naphthalene Thermal treatment of the fuel gasmixtures can be realised in an artificial biomass producer gas. either by external heating or by partial combustion of In more practical studies different gasifier concepts have the fuel gasses. Until now little attention is paid to partial been developed. These concepts also show that the tarcontent can influenced by carefully controlling the combustion zone. For example an internal pyrolysis recycle loop * Corresponding author. Tel.: C31 402472129. E-mail address: [email protected] (H.C. de Lange). with a separate internal combustion chamber in a fixed bed

0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2004.12.013 818 M.P. Houben et al. / Fuel 84 (2005) 817–824 gasifier [25] leads to a synthesis gas with a very low tar Furthermore, the aim of the experiments is to determine content. This configuration resembles the Delacott gasifier whether the tar components will polymerise or crack into used for char production [16]. The use of a recycle loop (the lighter particles. Since naphthalene is a 2-ring aromatic ‘recycle gasifier’) is also tested in e.g. [8].Another hydrocarbon, it can show both reaction paths. comparable concept is the so-called ‘two stage’ gasifier (see e.g. [1,9]), which uses staged addition of air. A principle question which needs to be answered is whether the tars dissapear through polymerisation or 2. Experimental set-up cracking. In [11], it is shown that external heating of a tar containing gas (in a range of 900–1150 8C) results in In Fig. 1 a schematic overview of the whole set-up is polymerisation of small tar components, which produces shown. A saturator is used to saturate a small nitrogen flow heavier hydrocarbons. This polymerisation finally leads to with naphthalene. Solid naphthalene is placed inside the the formation of soot. In practice, this soot can be removed saturator. Hot air from an electrical blower is used to heat by means of a filter. Therefore, this process can be used for the wall of the saturator. The nitrogen flow is injected into gascleaning. It is, however, more desirable to reverse the the naphthalene, where it is saturated. The set point of the process: to crack the tars into lighter components instead of blower leads to a steady state temperature of 200 8C inside polymerise them. If this process could be optimised it could of the saturator. The piping downstream the saturator is also lead to complete cracking of the tars into permanent gasses. heated by the hot air from the blowers. The cold fuel There are indications (e.g. [6]) that the presence of gasmixture and the nitrogen/naphthalene flow are mixed in (H–)radicals in the heated zone indeed reverses the process. the mixing unit, also shown in Fig. 1 in the center of the A combustion chamber in which the fuel-gasmixture is figure. The fuel gasmixture is controlled by mass flow partially burned supplies both features (heating and radical controllers (MFC’s). production) at the same time. To prevent cold spots in the mixing unit, which might Therefore, this study focuses on the effect of a cause condensation during the mixing of fuelgas and combustion chamber where the tar-contamined gasses are naphthalene, the fuel gasmixture is preheated using copper partially burned. When compared to the thermal treatment, piping (depicted as the black left to right line around the unit). the temperature in this chamber is increased moderately (till Again, the unit is heated with hot air coming from an about 500 8C) by burning only a small amount of the low- electrical air blower. After leaving the mixing unit the calorific fuel gas from the gasifier. This small amount of the fuel/naphthalene mixture is fed into the burner where primary gas is burned by adding little air. The paper concentrates on air can be added through the injection nozzles. As shown in one main question: under which conditions does thermal the figure at the left side of the burner at the bottom of the treatment/partial oxidation (fuel-rich combustion) polym- glass bell, a small secondary air flow is added. This coflow is erise the tars into heavier hydrocarbons or crack them into used to stabilise the flow structures above the burner. It has no lighter components like carbon monoxide and hydrogen. effect on the combustion/cracking processes. First, the burner geometry and the experimental setup will be described. Two sets of experiments will be performed. In The burner geometry is described in detail in [10].Itis the first set, the air/fuel ratio is varied to determine the effect based on a central tube for the fuel gasflow with seven of partial combustion on the tars. In the second set of nozzles on the circumference through which air is injected. experiments, the behaviour of the tars will be studied as a On these nozzles separate local diffusion flames are function of the mole fraction of hydrogen. In this way the formed. As shown in Fig. 2, the burner consists of two effect of different concentration ratios on the tar can be concentric tubes. The fuel/tar gasmixture enters the central determined. The results of the experiments will proof that inner tube at the bottom of the set-up. The air enters the under certain conditions the desired cracking indeed occurs. outer tube at two sides symmetrically and then passes one The paper ends with a short discussion on the mechanism of the seven injection nozzles into the inner tube. The responsible for the occurence of polymerisation vs. cracking. flames stabilise at the injection nozzles in the inner tube. It is worthwile to notice that in the experiments a model Note that, the injection of air into the crossflow by means tar is chosen: naphthalene. There are a number of practical of swirling jets is not very common for air design systems. reasons for this choice. First, it is denoted as relatively The Babcock and Wilcox Company has currently adopted harmless, compared with more carcinogeneous components this type of air introduction system, based on numerical like benzene. Second, higher ring components compared to modeling studies. They state that uniform air distribution naphthalene are difficult to process due to their condensation by swirling jets allows the burner to achieve the lowest behaviour: the whole set-up should be heated to higher possible emissions [19]. The swirling air introduces temperatures, and inserting the component in the gas flow recirculation zones near the wall, whereas the inner jet would be more difficult. But there are also more funda- provides mixing in the core of the main flow. The system mental considerations. For example, in downdraft gassifica- allows thorough mixing in the core of the flow as well as in tion tertiary aromatics (a.o napthalene) are predominant. the near wall zone. M.P. Houben et al. / Fuel 84 (2005) 817–824 819

Fig. 1. A schematic overview of the set-up.

Downstream, the partly burnt gas mixture enters the before the burning and one for the sample taken downstream diverging part. The diverging outlet decreases the flow rate, of the burner. To measure the amount of naphthalene added, which helps to stabilise the combustion process. samples are also taken from the sampling point downstream In all experiments the base inlet gasmixture is: of the burner without igniting the flame. A gas chromatograph (Interscience) is used to analyse fuel gascomposition: [H2]: 22.4%, [CH4]: 5% and [N2]: the gases and tars. The permanent gases are measured using 72.6% the thermal conductivity detector (TCD). All hydrocarbons lower heating value (LHV): 4.2 MJ/Nm3; are measured using a flame ionisation detector (FID). For 3 added naphthalene (C10H8): 2.6 mg/Nm ; the analysis of the hydrocarbons a capillary column is K fuel flow: 65!10 3 Nm3 which is equivalent to 2.8 kW. placed in a programmable ultra-fast oven (UFO). Use of the UFO reduces the analysis time of the polyaromatic The air added is set to give a l of 0.2, unless mentioned hydrocarbons (PAHs) and the benzene, toluene and xylene otherwise. (BTX) to about three minutes. In our analysis the tars will be The condensation behaviour of the tar components is of seperated only by the amount of rings. When concentrations special concern in the design of the set-up; the tar in parts per million (ppm) are converted to grams per cubic components could clog the tubing, the burner or the metre the following (average) mole masses are used: for measuring system. Therefore, electrically traced tubing one-ring 78 g/mol, two-ring 128 g/mol, three-ring (heated at 200 8C) is used to transport the mixture of the 178 g/mol, four-ring 228 g/mol and five-ring 278 g/mol. gases and the naphthalene to the burner. Also for the transport of the gas-sample to the GC heated tubing is used (heated also at 210 8C). The inside diameter of this tubing is 3. Primary air 1/8 inch, which is small for traced tubing, therefore the tubing used is especially made for this application. For this In this section the effect of the air/fuel ratio on the sampling, two tubes are used: one for the sample taken naphthalene concentration is studied The air/fuel ratio is 820 M.P. Houben et al. / Fuel 84 (2005) 817–824

Fig. 2. The partial combustion burner in 2D and 3D.

expressed in l:

hm_ fuel m_ air l jexp jstoi m_ air m_ fuel in which m_ fuel and m_ air are the mass flow of fuel and air, respectively. The index exp indicates the present experimental conditions, while the index stoi indicates the flowratio in the stoichiometric case. Fig. 3 shows the tar components classified by the number of rings. Since benzene (C6H6) is generally not defined as a tar component, the amount of benzene is plotted separately, and excluded from the one-ring group. The increase in l results in an increase of the total tar concentration for the l range 0.2–0.65. Similar results (the increase of the tar concentration when increasing the l) can be found in literature ([15,17,21,28]). These studies state that there is an optimum in the addition of oxygen with regard to the reduction of the tar concentration: no oxygen Fig. 3. Ring-grouped tar components in the outlet gas as a function of l. M.P. Houben et al. / Fuel 84 (2005) 817–824 821 added leads to the formation of polyaromatic hydrocarbons and soot, but ‘too much’ oxygen does the same. In between, there is an optimum for the tar removal. The present results show that if there is indeed a minimum, it must be below l is 0.2. Note that the result presented in Fig. 3 means that the total mass of tar at the inlet differs from that at the outlet (in all cases this value is 2.6 mg/Nm3). Therefore, not all naphthalene is converted to measurable tar components in the outlet gases. It is turned either into permanent gases or into soot. To show whether partial combustion leads to cracking or polymerisation, it is interesting to see what happens to the naphthalene in more detail. The results show that the increase of l leads to the formation of higher ring aromatic components. As shown here, the benzene follows the same trend as the small tar components: when higher ring components evolve the benzene disap- pears. Another indication for polymerisation at higher l’s is Fig. 4. Ring-grouped tars components in the outlet gas as a function of hydrogen fraction. the flame colour. When increasing l a red/yellow flame appears, which generally indicates that there is soot formation in the flame. On the other hand, for very lean unmeasurable components strongly increases for gasmix- Z air combustion (at l 0.2), the tar concentration is low: only tures with a hydrogen fraction larger than 25%. 7.5% of the initial value. Furthermore, the remaining tars For increasing the hydrogen fraction the higher ring are mostly single ring. Clearly, in this situation the tars are components decrease. At high hydrogen fractions almost all indeed cracked. naphthalene is converted to smaller components; benzene In [28] the behaviour of tar components is studied for increases while the other aromatic components all decrease. different gasification conditions. One of the parameters For fuel mixtures with a higher methane concentration, studied is the equivalence ratio (ER); which is similar to higher ring components are observed. Apparently, a sooting varying the l as used in this paper. Their results show that tendency is present in these situations. As mentioned, the increasing the ER leads to heavier tar components (an naphthalene converts to unmeasurable components in all increasing amount of multi-ring tarcomponents). Similar situations. However, in the pure methane case these gasification results are found in [15], where it is shown that unmeasurable components are likely to be soot particles, the tar concentration decreases when adding some oxygen whereas in the pure hydrogen case these components are and increases again when the amount of oxygen is increased more likely to be small components like benzene and more. permanent gases. In literature similar results can be found in e.g. [14,23,26, 27].In[6] it is concluded that hydrogen, with its large 4. Inlet gas composition diffusivity, can be quite effective at suppressing soot inception, despite a corresponding increase in flame To gain more insight in the parameters that influence temperature. It is also known that once soot formation has the tar conversion process, the hydrogen-methane content started, there is an acceleration in the soot particle growth of the fuel is varied. To assure that the outlet temperture even at low hydrocarbon concentrations [26]. This accel- remains more or less constant both l (0.2) and the LHV of eration is caused by bonding of hydrocarbon radicals on the the fuel (4.2 MJ/Nm3) are kept constant in these growing surface [27]. experiments. Therefore, a decrease of the hydrogen The no-methane flame is of special interest, because the concentration is directly coupled to an increase of the naphthalene that is added is the only carbon source in this fuel methane content. flame. Therefore, it is possible to determine what happens to In Fig. 4 several grouped tar components are shown as a the naphthalene in detail. Table 1 shows the distribution of function of the hydrogen fraction in the inlet fuel. The the carbon containing components in the hydrogen- amount naphthalene at the inlet is about 2.6 mg/Nm3. naphthalene-nitrogen flame. The permanent gases are This figure shows a decrease in the tar concentration as shown in the first three rows of the table. As shown, the the hydrogen content of the fuelgas increases. Again, total amount at the outlet agrees well with the amount a difference is found between the total amount of tar at the measured at the inlet. Methane, carbon monoxide and inlet and outlet. So, in all experiments the naphthalene carbon dioxide are the components that are most formed in converts to unmeasurable components. This amount of the flame. This is probably due to the fact that the flame is 822 M.P. Houben et al. / Fuel 84 (2005) 817–824

Table 1 be similar to that of longer (more complex) tarcomponents The carbon balance in the no-methane experiment as they will be present in the productgas of a gasifier. C-input (mg/h) C-output (mg/h) At very low primary airrates (lZ0.2), the partial CO – 0.4361 combustion process reduces the total tarcontent with over R CO2 – 0.6853 90%. It is interesting that when more air (l 0.4) is added to CH4 – 0.7164 the burner, the same sooting tendency is found as in the case C2Hx – 0.1246 of thermal treatment only [11]. By changing the amount of Benzene (C6H6) – 0.3925 hydrogen in the inlet gas, the tar concentration in the outlet Toluene (C H ) – 0.0011 7 8 is considerably affected. For very low hydrogen concen- Xylene (C8H10) – 0.1221 Naphthalene (C10H8) 2.4919 0.0050 trations (methane-rich fuels) polymerisation/sooting is Higher-rings – - found. However, for a fuel gas without methane (with Total 2.4919 2.4914 40% hydrogen) almost no napthalene is found in the outlet gas; all product components are lighter hydrocarbons or a (very) fuel-rich flame. There is a lot of fuel (hydrogen) and even permanent gases. Therefore, in this case the naphtha- only little oxygen present. The gases are formed out of the lene is cracked. Hydrogen appears to be an inhibitor for soot naphthalene, after most of the oxygen has been consumed. formation: hydrogen in the inlet gas transforms the The 2-ring components are really low and even the 3-, 4- polymerisation/sooting process into cracking. This process and 5-ring components (not shown) are all zero. Clearly, the starts to take effect at low concentrations (at about 5%). For 3 naphthalene added at the inlet is converted to smaller a fuel gasmixture with a LHV of 4.2 MJ/Nm the optimal components in the outlet gas. hydrogen concentration seems to be about 20%. This type of Increasing the methane content of the gas leads to the fuel gas is representative for the product gas of biomass gasification. Therefore, the process created by the burner formation of larger polyaromatic hydrocarbons (for [H2] smaller than about 20%). In the no-hydrogen case the total geometry might be a promising method for tar-removal in tar content at the in- and outlet are almost equal. Almost (small-scale) biomass gasifiers. Testing this will be the next step in this research. 50% of the tars are now turned into 5-ring PAH’s. This indicates that at least part of the tars are converted into even higher ring-components. It is, therefore, clear that the 6. Discussion low/no-hydrogen combustion leads to polymerisation. Remarkebly, for the same experimental conditions without In biomass literature, little is known of the effect of naphthalene added to the inlet (described in [10]) the hydrogen on thermal cracking in combination with partial combustion process hardly produces any tars or soot. This oxidation From hydropyrolysis and gasification of coal, it resembles the results presented in [23]. They suggest that appears that methane might be formed by hydrogenation of methane is not an actual soot promoter in flame situations. one of the double bonds of an aromatic ring (e.g. [12,20]). However, methane does interact with other fuelcomponents This is probably the process that takes place in the cases to produce more polyaromatic hydrocarbons and soot than when the fuel is hydrogen rich. For the results presented, would otherwise have been expected. They state that the this methane forming process is evident for the pure synergy of methane with other hydrocarbons to produce hydrogen-nitrogen flame situation. polyaromatic hydrocarbons may be attributed to the ability For hydrogen concentrations lower than 20% a strong of methane to produce methyl radicals. These radicals will increase in the total tar concentration is found. Examin- then promote the production of aromatics. Benzene, ation of the composition of the tars shows that higher ring naphthalene and pyrene show the strongest sensitivity to compounds are formed. Therefore, the effect of the presence of methane: this synergy trickles down to soot methane and naphthalene might be dominant in this via enhanced inception and surface growth. This effect was situation. The hypothesis for these situations is that the found to be the strongest in fuel-rich diffusion flames, i.e. combination of methane and a little naphthalene results in conditions, which resemble the conditions used in the a sooting tendency. This result agrees with the findings experiments described in this paper. presented in [23]. They find that (in diffusion flames) methane together with a small amount of naphthalene interact synergistically to produce polyaromatic hydrocarbons and soot. 5. Conclusion There are a number of mechanisms available to explain the polymerisation of tars. In [7] a H-abstraction/C2H2- The effect of partial combustion on tar is studied in addition (HACA) reaction mechanism is proposed. Recent the burner geometry Naphthalene is used as a model literature shows that other species than acethylene can also component in these experiments. It is assumed that play an important role. For example, it has been shown [4] the polymerisation/cracking process of naphthalene will that for non-premixed flames, propargyl addition to benzyl M.P. Houben et al. / Fuel 84 (2005) 817–824 823 radicals is one of the key components in the formation of hydrocarbons can be broken into permanent gasses. This naphthalene. In [23] the recombination of cyclopentadienyl hydrogen inhibition process is confirmed by the results at radicals and the addition of benzyl and propargyl is stated to increased l’s. At higher l’s one would maybe expect an be the mechanism that leads to soot in methane-air diffusion increase of the cracking process through the increased flames doped with a small amount of hydrocarbons. They temperature. However, the experiments show a decreased conclude that the HACA mechanism is less important for cracking. This would then be due to the fact that the this flame type. increased l (increased air injection) induces a decreased For a basic understanding of the influence of the availability of H2 and thus stops the inhibition of the cyclic hydrogen concentration, it is interesting to take a closer hydrocarbons. look on the HACA mechanism, which states that aromatic It is noteworthy to recall that the presented results are rings grow by H abstraction, which activates the aromatic based on experiments using naphthalene (and not the molecules, and acetylene addition, which propagates combinations of tars as they are found in gasification molecular growth by cyclization. For the HACA mechanism productgas). However, it would seem that a reaction three regimes exist, in which hydrogen plays different roles: mechanism based on hydrogen addition will also work on % longer (more complex) tarcomponents. The next step in this I½C2H2 ½H2; research will, therefore, be the application of the partial [ combustion burner on real gasification productgasses. II½C2H2 ½H2;

/ : III½C2H2 ½H2 Because of the hydrogen variations performed, these References regimes are of special interest for the explanation of the results found. Note that, contrary to the present results, these [1] Bentzen JD, Hindsgaul C, Henriksen U, Sørensen LH. Straw regimes in [7] are connected to temperature regimes. The gasification in a two-stage gasifier. In: Palz W, Spitzer J, regimes II and III are of particular interest to the present Maniatis K, Kwant K, Helm P, Grassi A, editors. Proceedings of the paper. Roughly, regime (II) resembles the situations with a 12th European conference on biomass for energy, industry and climate protection, Amsterdam, 2002. p. 577–80. low hydrogen fraction (lower then 20%), while regime (III) [2] Brandt P, Henriksen U. Decomposition of tar in pyrolysis gas by resembles the higher hydrogen fractions. partial oxidation and thermal craking. In Proceedings of the ninth For regime (II), the growth of aromats to higher European bio-energy conference, pp. 1336–40, Kopenhagen; June molecular compounds is explained by a mechanism 1996. consisting of two reaction pathways: [3] Brandt P, Henriksen U. Decomposition of tar in pyrolysis gas by partial oxidation and thermal cracking. Part 2. In: Kopetz H, editor. (1) direct combination of intact aromatic rings, e.g. the Proceedings of the international conference: 10th European conference and technology exhibition, Wu¨rzburg, 1998. p. 1616–9. combination of two benzene rings leads to biphenyl, [4] D’Anna A, Kent JH. Aromatic formation pathways in non-premixed which reacts further towards PAH compounds methane flames. Combust Flame 2003;132:715–22. (2) a sequence of H-abstraction/C2H2 addition (HACA). [5] Devi L, Ptasinski KJ, Janssen FJJG. A review of the primary measures for tar elimination in biomass gasification processes. Biomass For regime (III), the growth rates of PAH vary with Bioenergy 2003;25:125–40. [6] Du DX, Axelbaum RL, Law CK. Soot formation in strained diffusion [H]/[H2]. The inverse dependence on [H2] is due to the flames with gaseous additives. Combust Flame 1995;102:11–20. reverse reaction AiCH20AiHCH. Here AiH denotes an [7] Frenklach M. On the driving force of PAH production. Twenty- aromatic molecule containing i aromatic rings and A i second symposium (International) on Combustion, The Combustion denotes an aromatic radical. In this way, chemical Institute, Pittsburgh; 1988. suppression of soot formation due to addition of hydrogen [8] Gaudemard S, Becker JJ. Pyrolysis and gasification, chapter Fixed bed to the fuel seems likely. The aromatic radicals are gasification of lignocellulosic biomass: CEMA GREF process. In: neutralised before they can combine together or with Ferrero GL, Maniatis K, Buekens A, Bridgwater AV, editors.. Luxemburg: Elsevier Applied Science. Commision of the European C2H2. Therefore, the reaction paths of regime II are closed. The present results appear to take this mechanism one Communities; 1989. [9] Gøbel B, Bentzen JD, Hindsgaul C, Henriksen U, Ahrenfeld J, step further. The growth of the aromats is not only stopped, Houbak N, Qvale B. High performance gasification with the two stage but even reversed. At high enough concentrations the H2 gasifier. In: Palz W, Spitzer J, Maniatis K, Kwant K, Helm P, and H appear to add to the hydrocarbon rings, while Grassi A, editors. Proceedings of the 12th European conference on acetylene addition no longer plays a part. Possibly, the thus biomass for energy, industry and climate protection, Amsterdam, induced cracking is a combination of the influence of the 2002. p. 389–95. moderately high temperature and the available hydrogen. It [10] Houben MP. Analysis of tar removal in a partial oxidation burner. PhD thesis, Technische Universiteit Eindhoven; 2004. seems that the moderate temperatures are sufficient for the [11] Houben MP, Verschuur K, de Lange HC, Neeft J, Daey Ouwens C. An hydrogen atoms to inhibit the naphthalene ringstructure analysis and experimental investigation of the cracking and (and form two benzene rings). Even more so, the cyclic polymerisation of tar. In: Palz W, Spitzer J, Maniatis K, Kwant K, 824 M.P. Houben et al. / Fuel 84 (2005) 817–824

Helm P, Grassi A, editors. Proceedings of the 12th European [20] Nelson PF, Huttinger KJ. The effect of hydrogen pressure on conference on biomass for energy, industry and climate protection, methane yields from hydropyrolysis of aromatics. Fuel 1986;65: 2002. p. 581–4. 354–61. [12] Howard JB. Chemistry of coal utilisation, chapter Fundamentals of [21] Pan YG, Roca X, Velo E, Puigjaner L. Removal of tar by secondary coal pyrolysis and hydropyrolysis, pages 665–784. Second supplemen- air in fluidised bed gasification of residual biomass and coal. Fuel tary volume, Elliot MA, John Wiley and Sons, Inc., New York; 1981. 1999;78:1703–9. [13] Jenssen PA, Larsen E, Jørgensen KH. Tar reduction by partial [22] Reed TB, Das A. Handbook of biomass downdraft gasifier engine oxidation. In: Chartier P, Ferrero GL, editors. Proceedings of the 9th systems. Solar Energy research Institute (SERI); 1988. European bioenergy conference, 1996. p. 1371–5. [23] Roesler JF, Martinot S, McEnally CS, Pfefferle LD, Delfau JL, [14] Jess A. Mechanisms and kinetics of thermal reactions of aromatic Vovelle C. Investigating the role of methane on the growth of hydrocarbons from pyrolysis of solid fuels. Fuel 1996;75(12):1441–8. aromatic hydrocarbons and soot in fundamental combustion. Combust [15] Jo¨nsson O. Thermal cracking of tars and hydrocarbons by addition of Flame 2003;134(3):249–60. steam and oxygen in the cracking zone. In: Overend RP, Milne TA, [24] Stassen HEM, Prins W, van Swaaij WPM. Thermal conversion of Mudge LK, editors.. London: Elsevier Applied science of publishers; biomass into secondary products the case of gasification and 1985. p. 733–47. pyrolysis. In: Palz W, Spitzer J, Maniatis K, Kwant K, Helm P, [16] Kaupp A, Gross JR. Small Scale Gass Producer Engine Systems. Grassi A, editors. Proceedings of the 12th European conference on Friedr.Vieweg und Sohn Verlag Gasellschaft mbH, first edition; 1984. biomass for energy, industry and climate protection, Amsterdam, [17] Kinoshita CM, Wang Y, Zhou J. Tar formation under different 2002. p. 38–44. gasification conditions. J Anal Appl Pyrolysis 1994;29:169–81. [18] Lammers G, Beenackers AACM. Effects of temperature and gas [25] Susanto H, Beenackers AACM. A moving-bed gasifier with internal composition on model tar compounds decomposition kinetics. In: recycle of pyrolysis gas. Fuel 1996;75(11):1339–47. Kyritsis S, Beenackers A, Helm P, Grassi A, Chiaramonti D, editors. [26] Tesner PA. In: Thrower PA, editor. Chemistry and physics of carbon. Proceedings of the 1st world conference on biomass for energy and New York: Marcel Dekker; 1984. p. 65–161. industry, Sevilla, 2000. p. 2052–5. [27] Tesner PA. Brief communication: growth rate of soot particles. [19] LaRose JA, Hopkins MW. Numerical flow modelling of power plant Combust Flame 1991;85:279–81. windboxes In BR-1603, editor, Proceedings of Power-Gen Americas, [28] Wang Y, Kinoshita M. Experimental analysis of biomass gasification pp. 1–5, Anaheim, California, USA; 1995. with steam and oxygen. J Solar Energy Eng 1992;49:153–8.