Tar measurement in synthetic gas produced by plasma gasification by solid phase microextraction (SPME) M. Hlina, M. Hrabovsky, T. Kavka, M. Konrad
[email protected] Institute of Plasma Physics, ASCR, Za Slovankou 3, Prague 8, Czech Republic Abstract: There are many sampling methods for tar analysis. Solid phase microextraction (SPME) is one of them. SPME method might be used as a sampling technique for tar measurements in synthetic gas (syngas) produced in plasma gasification process. Gasification is carried out in the reactor equipped with hybrid gas-water stabilized DC torch. The plasma torch features relatively low mass flow rate of plasma and very high enthalpy. The previous results revealed low tar content that is in good accordance with high reaction temperature and also with the possibility of using SPME sampling method. Keywords: tar, sampling, gasification, solid phase microextraction, SPME
1. Introduction The plasma gasification of biomass is studied very intensively nowadays. The concentration of major gas components such as hydrogen and carbon monoxide are always carefully monitored; however, further utilization of produced gas is also strongly influenced by tar content. What is tar? Webster dictionary gives us a general answer: “a dark brown or black bituminous usually odorous viscous liquid obtained by destructive distillation of organic material (as wood, coal, or peat)” [1] that is definitely correct, but scientific approach needs more accurate definition. Nevertheless, surprisingly there is not just one right definition, but more than 30 in 1999 [2]! Tar is the sum of organic compounds with boiling point above the boiling point of benzene (80.1 °C) accordingly to Tar Protocol [3, 4] and many other related sources. Tar can be classified with respect to its physical properties [5] which are also closely related to GC measurements aspects (Table. 1). The lower tar content the better but some applications assume relatively high tar concentration and possible usage of almost “tar-free” syngas would economically handicap them. For example for engines the maximum tar content of 100 mg/Nm3 is
acceptable [6] but for Fischer-Tropsch synthesis tar concentration has to be less than 0.1 mg/Nm3 [7]. Tar yield in gasification process might be also plotted as the function of the temperature of gasification, the higher temperature the lower tar yield. Minimum is achieved approximately at temperature of 1000 °C and higher [8].
2. Tar sampling & separation Sampling and separation are due to the huge range of the physical properties of tar a key-step at tar measurement. While upstream pointed heated probe with heated particle filter is an ordinary setup, sample preparation (condensation, concentrating) and sample modifying for further analysis gives us wider choice of possibilities. There are two techniques preferably used for tar sampling: -
cold trapping
-
solid phase extraction (SPE)
Cold trapping is the most used method but it requires a lot of manual work and is also very time consuming. Frequently used setup is in Fig. 1 [9] syngas is forced to flow through several impinger bottles filled with isopropanol in series connection.
Table 1. Tar classification
Tar trapped in solvent is after decanting and concentration analyzed.
after sorption, solvent with dissolved tar concentrated and then analyzed. It is possible to do very rough estimation of tar content (especially of heavier fraction) by simple weighing of unheated filter before and after the passing of syngas.
3. Solid phase microextraction (SPME) SPME is widely used as a sample preparation technique in other fields but there is also possibility to use it for tar sampling and extracting.
Figure 1. Possible setup for tar collection – impinger bottles.
Solid phase extraction (SPE) uses unheated sorbent with high affinity to tar connected to a sampling duct as a filter. Sorbents have to be sometimes activated by rinsing with a proper solvent. Sorbent is washed
SPME involves the use of a fibre coated with an extracting phase. When the fibre gets to contact with sample the quantity of extracted analyte is proportional to its concentration in the sample as long as equilibrium is reached. SPME fibre is transferred to the injection port of separating instruments, such as a gas chromatograph, where desorption of the analyte takes place and analysis is carried out. The attraction of SPME is that the
extraction is fast and simple and can be done without solvents, and detection limits can reach parts per trillion (ppt) levels for certain compounds [10, 11]. Several polymer coatings were tested for tar measurements at French Atomic Energy Commission and 100 μm polydimethylsiloxane (PDMS) fibre was the most suitable. It allows extraction of the wide range of compounds and it features also a good repeatability. Possible setup for SPME tar extraction is in Fig. 2.
stabilization of arc. This torch features very high plasma temperature together with relatively low mass flow rate [12]. Inner volume of the reactor is 0.22 m3 and it is insulated by 400 mm of ceramics from water-cooled steel walls. Gasified material is supplied to the reactor by a screw feeder that enters in the proximity of the nozzle of the torch [13]. Total input power of the torch during the measurement of presented results was 95 and 138 kW. Saw dust from spruce with humidity of 20% was used as a gasified material. The reactor was operated at a wall temperature from 1100 to 1400 °C. Syngas was a gas mixture that comprises mainly hydrogen and carbon monoxide. Tar content was very low (Table 2) [14]. The measurements of the content of tar were based on solid phase extraction method (SPE) using DSCNH2 phase or silica gel as an adsorbent. The samples were analyzed by means of gas chromatography. Table 2. Content of polyaromatic hydrocarbons in syngas
Figure 2. Possible setup for SPME tar extraction from syngas (sampling line).
A glass bulb is vacuumed before syngas enters. When the glass bulb is filled with syngas (pressure equals to reactor pressure and is roughly constant during all experiments and calibration) and thermal equilibrium (at a given temperature) is reached, SPME fibres are inserted and let in the vessel for certain time (till adsorption equilibrium is reached). Then the fibres are desorbed in GC injector and tar content analyzed. Distribution coefficients between fibre and syngas in the glass bulb can be calculated, however calibration for concrete compounds and conditions is believed to be more accurate.
4. Plasma gasification Gasification was carried out in the reactor equipped with the thermal plasma torch with gas-water
Results revealed low polyaromatic hydrocarbon concentrations (that is in good accordance with high reaction temperature) which might meet detection limits of SPE method. Simple increasing of the volume of the syngas passing the absorbent is not usually possible due to low porosity of the absorbent and fast sticking of it.
5. Conclusions SPME is a new promising method for tar measurement from this point of view apart from the fact that SPME is much simpler, faster and no hazardous solvents are needed.
Acknoledgement The authors gratefully acknowledge the support by the Grant Agency of the Czech Republic under the project No. P 205/11/2070.
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