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 tar- content can influenced by carefully controlling the combus- tion 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.