Characterization of Tar Deposits, Extraction and Sorption Properties

Abstract The main goal of this paper was to characterize and find a useful solution for the decomposition of tar deposits. For the experimental part, tar deposits, formed by polymerization and condensation reactions, were chosen from a storage tank for tars. At first the initial analyses of tar deposits (elemental, thermogravimetric, and calorimetric analyses) were performed. After the characterization, the tar deposits were extracted in the Soxhlet extractor by acetone, toluene, and quinolone and activated with potassium hydroxide. As the final step of this work, the sorption characterization on the 3Flex Surface Characterization Analyzer (Micromeritics) was performed. The specific surface area of the samples was evaluated using two methods - a single point measurement at p/p0=0.2 and BET method. Micropore and external surface areas were calculated based on a t-plot analysis (carbon black model).


INTRODUCTION
Tar properties depend on the source of their origin. Most of tar is formed from a thermal conversion of coal (coal tar); the second part of tar is formed during pyrolysis or gasification of biomass. Due to efforts to use renewable sources of energy instead of fossil fuels, new technologies for pyrolysis and gasification are developed, especially for thermal processing of biomass [1][2][3][4].
Removal or conversion of tar is considered a technologically very challenging problem, because tar can condense or polymerize into more complex structures, which can lead to clogging pipes or heat exchangers. Tar elimination from the gasification product must be done before additional usage [5,6]. Around 2.5 to 4.0 mass % of tar is obtained from the process of coal carbonization. Coal tar contains more high molecular compounds, than tar generated from biomass and it is an important source of aromatic compounds. From coal tar 15 to 25 % of BTX (benzene, toluene, and xylene) is obtained, 95 % of multi-ring hydrocarbon can only be obtained, and more than 90 % of anthracene, acenaphthene, pyrene, phenol, and most of heterocyclic aromatic compounds are gained. Nearly 100 % of carbazole, quinolone, and thiophene are from coal tar [7,8].
Tar can polymerize, condensate, and form tar deposits. Nowadays tar deposits are characterized as a waste material and are mostly incinerated, therefore the main goal of this paper was to use tar deposits for preparation of activated carbon. Activated carbons are produced from different organic precursors. The traditional production of activated carbon consists of two steps, carbonization and activation. The properties of the final product depend on the nature of raw material used and the activating agent [9][10][11][12].

CHARACTERIZATION OF TAR DEPOSITS
Tar deposits, formed by polymerization and condensation reactions, from a storage tank for tars were used for the experiment. At the start elemental, thermogravimetric, and calorimetric analyses were performed (with the use of the units LECO CHNS 628, TGA 701, and AC 600) to determine basic properties of the tar deposits (see Table 1). From the results it can be seen that tar deposits are high carbon based material with around 88 wt. % of carbon in dry sample and with a higher heating value of around 31 MJ•Kg -1 . Around 6 wt. % of ash was in dry sample; therefore the next step consisted in determining the composition of ash from the tar deposits (see Table 2) with X-ray fluorescence (ARL QUANT'X EDXRF Spectrometer -Thermo Scientific).

GeoScience Engineering
Volume LXII (2016), No. 2 http://gse.vsb.cz p. 1-4, ISSN 1802-5420  In the ash from the tar deposits, Si, Al, and Fe are the most abundant elements. The elements S, Ca, Zn and K are there in a lesser amount. The sulphur content of 3.21 wt. % is quite surprising as this means that sulphur is present in a form which will not burn at 900 °C.

EXPERIMENTAL PART
After the characterization, the tar deposits were extracted in the Soxhlet extractor, with acetone, toluene, and quinolone. The extraction was made to extract lighter hydrocarbons from the tar deposits. Such prepared extracted residues were activated with KOH. Approx. 15 g of sample was activated in a ratio of sample: KOH: distilled water 1: 3: 10, then the mixture was thermally treated in an inert atmosphere of nitrogen at 850 °C for 1 h. After cooling, the activated sample was neutralized with HCl, filtered and finally washed with water. The sorption characteristics were measured in the flow reactor and with the sorption of N 2 at 77 K.

RESULTS AND DISCUSSION
Two adsorption tests were made to measure sorption properties of the sorbent prepared from the tar deposits. First the sorption of selected VOCs (methyl ethyl ketone, isopropanol, toluene, and heptane) in the flow reactor was performed. The sample was first degassed at 350 °C and then approximately 0.1 g of sample was put into the reactor. Then the sorption and desorption properties were measured (see Table 3). The temperatures used for the VOCs sorption and desorption were: for methyl ethyl ketone -41 °C, isopropanol -50 °C, heptane -56 °C, and toluene -68 °C. For each VOC, the temperature corresponds to the relative pressure p/p0 -0.4. Each experiment was repeated 3 times.

Figure 1: Adsorption isotherms of activated samples
The measured data from BET and t-plot confirm that the surface areas of the activated sorbents are similar, and they have sorption properties close to higher quality activated carbon (see table 4); the surface area of commercial activated carbons is in the range from 500 to 1500 m 2 •g -1 . The non-activated samples have almost zero sorption properties.

CONCLUSION
From the laboratory results it can be concluded that tar deposits are high carbon based materials with around 88 wt. % of carbon in dry sample, with a higher heating value of around 31 MJ•Kg -1 . The second part of the experiment was focused on the extraction and activation of tar deposits to create sorbents. From the comparison of the sorption properties for non-activated and activated sorbents, prepared by the extraction from tar deposits, it can be stated that the activated sorbents have a lot better sorption properties. The sorption properties depend mostly on well carried out activation; the selection of extraction solvent (acetone, toluene, quinoline) has negligible effect on the sorption properties. From the results it could be concluded that tar deposits, which are a waste material that is mostly incinerated, could be used for preparation of sorbents with a surface area of around 800 m 2 •g -1 .