JOINT MEETING THE GERMAN AND ITALIAN SECTIONS OF THE COMBUSTION INSTITUTE SORRENTO, ITALY 2018

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1 COMPARISON BEHAVIOUR OF COMMERCIAL ACTIVATED CARBON AND COAL-DERIVERED CHAR IN HOT SYNGAS CLEANING F. PARRILLO 1, G. RUOPPOLO 2, U. ARENA 1 1 Department of Environmental, Biological and Pharmaceutical Sciences and Technologies, University of Campania Luigi Vanvitelli, via A. Vivaldi 43, Caserta, Italy. 2 Combustion Research Institute, National Research Council-CNR, P.le Tecchio 80, Naples, Italy. Abstract This study aims at investigating and comparing the performances provided by a coalderived char and a commercial activated carbon as catalysts for tar conversion. The char has been specifically produced in our facilities by steam activation of a Colombian coal s granulates, carried out at 900 C for 100 minutes. The activated carbon, named Norit RB4W, has been selected among those available on the market for utilization at temperature higher than 700 C. Naphthalene has been used as tar model compound. The influence of temperature (in the range C) on the initial naphthalene conversion and the evolution of the naphthalene conversion with time at 750 C have been investigated. A correlation between the changes in the internal structure and the observed activity has been observed. In particular, for both the adsorbents the reduction of activity is associated to a decrease of the total specific pore surface. Introduction The gasification of a solid fuel (biomass, waste, coal) produces a syngas consisting of some major compounds (CO, CO 2, H 2, H 2O, CH 4, and N 2). The produced fuel gas can be burned to produce electricity or further processed to manufacture chemicals and liquid or gaseous fuels. The obtained syngas generally contains a not negligible amount of tars, a mixture of heavy hydrocarbons condensing at temperatures below 400 C, which can strongly limit the number of possible final applications [1]. One of the most promising options for removal of tars is their adsorption on the surface of activated carbons or chars [2-8]. These adsorbents appear attractive for their low cost and the extraordinary physical adsorption capability, which is sometime coupled with a catalytic action for the tar cracking reactions. The activated chars generally show high resistance to poisoning and can be produced within the gasification process [3]. On the other hand, there is a lack of knowledge about the parameters involved in their catalytic activity. Some authors report that tar conversion depends on the amount and the nature of the char surface available for tar cracking reaction [9]. Other researchers attribute the high activity of char to the presence of oxygenated surface groups and alkali and alkaline earth metallic (AAEM) species distributed over its surface [8-10].

2 The physico-chemical properties of activated chars depend on different parameters, such as the composition of the parent material, activating agent and reaction temperature [11]. Steam activation produces chars with higher mesopore volume while CO 2 activation produces higher micropore volume [10-12]. It has been also reported that their activity changes during the tar conversion [5, 9] due to the evolution of the pore size distribution and the concentration of active groups on the surface, as consequence of a balance between the rate of carbon conversion and that of soot deposition. This study investigated the decomposition of naphthalene (model tar compound) over a char and a commercial activated carbon, with a special attention to the evolution of the pore size distribution and the surface during the conversion of naphthalene. Materials and characterization A commercial activate carbon (Norit RB4W) and a homemade char (Char 100) have been used. The char has been prepared by devolatising a Colombian coal at 900 C in a bubbling fluidized bed, and a successive steam activation at 900 C for 100 min. The adsorbents have the characteristics reported in Table 1. Table 1. Proximate and ultimate analyses of Norit RB4W and Char 100 Norit RB4W Char 100 Proximate Analysis (d.b.) Volatile matter Fixed Carbon Ash Ultimate Analysis (as received) Carbon Hydrogen Nitrogen Oxygen Ash Experimental apparatus and test procedure Naphthalene conversion tests have been carried out in the experimental apparatus reported in Figure 2, having four different sections: feeding system, reactor, sampling/cleaning device and gas analyser. Further details can be found elsewhere [13]

3 Figure 2. Photo and scheme of the experimental apparatus used for the naphthalene conversion tests The gas at the exit of the cleaning section is analysed by means of a micro-gc with a TCD for the online determination of short chain hydrocarbons (from acetylene to benzene) and molecular hydrogen. The off-line determination of the naphthalene concentration in the isopropanol sample has been performed by using a gas chromatography coupled with mass analysis. Two kinds of tests have been carried out. In the first set of experiences, the initial conversion of naphthalene has been measured. A known amount of solid naphthalene has been deposited within two glass impingers (about 20 g each) immersed in oil at 65 C. The reactor, with the sample inside, has been heated at 750 C, 800 C or 900 C in pure nitrogen stream. When system reaches the steady state, the nitrogen flow has been driven through the naphthalene saturator to the reactor (naphthalene concentration 22 mg/nl). After five minutes of stabilization, naphthalene has been sampled for two minutes. Then, the reactor has been purged and cooled down with pure nitrogen. A nitrogen flow has been used to obtain a residence time of 0.15 s, using a bed of 3 cm height: the related flow rates were 0.49 NL/min at 750 C, 0.47 NL/min at 800 C, and 0.43 NL/min at 900 C. In the second kind of experiences, the evolution with time of the naphthalene conversion and the internal structure of the char samples have been studied. To achieve this, the same conditions as first set of tests have been used, but the test duration has been varied between 5 and 310 minutes. Various gas samples have been taken during the tests to measure the naphthalene concentration in the exit gas at different times. Results and discussion Figure 3 presents the naphthalene conversion as measured after five minutes of test (i.e. initial conversion), the two different materials at different temperature for a gas residence time of 0.15 s.

4 Conversion, % Conversion, % 100% 80% 60% 40% 20% 0% Norit RB4W Char 100 Figure 3. Naphthalene conversion obtained with Norit and Char-100 at 750 C, 800 C and 900 C for a gas residence time of 0.15 s. The results show that in the range 750 C-800 C the temperature does not have a significant influence on the naphthalene conversion. A further increase of temperature improves the naphthalene conversion up to the 100%, in agreement with results obtained in a previous study [13]. The initial activity of the Char 100 is slight lower than that obtained for the commercial activated carbon whatever the reaction temperature adopted. Figure 4 reports the evolution of the naphthalene conversion for Norit RB4W and Char 100 obtained at 750 C for a gas residence time of 0.15 s. 100% 80% 60% 40% 20% 0% Time, min Norit RB4W Char 100 Figure 4. Evolution of the naphthalene conversion for Norit and Char 100 at 750 C and gas residence time of 0.15 s. The conversion efficiency of the commercial activated carbon is higher than that of char whatever is the adopted length of the reaction; moreover a longer deactivation time is observed for the commercial sample. According to a high naphthalene conversion, a higher amount of hydrogen (not here reported for brevity) is produced using the Norit RB4W.

5 Table 2. Naphthalene conversion obtained on Norit and Char-100 at 750 C, 800 C and 900 C for a gas residence time of 0.15 s. Sample Char 100 Norit RB4W Time, min Specific Surface Area, m 2 /g Micropore Area, m 2 /g Mesopore Area, m 2 /g The porosimetric analysis of the samples (fresh, after five minutes and at the end of the test) are reported in Table 2. It is possible to observe that the amount of mesopores initially present in the Norit RB4W is enhanced in the first five minute. During the test, a progressive reduction of the specific surface area occurs up to the complete deactivation of the sample. This can be explained with the mechanism of soot deposition from tar cracking. Further studies are required to assess the role of the presence of oxygenated surface groups and alkali and alkaline earth metallic (AAEM) species since the deactivation occurs for different value of the final surface area for the two samples. Conclusions The decomposition of naphthalene (model tar compound) under the same experimental conditions over a char produced in laboratory (char 100) from coal and a commercial activated carbon (Norit RB4W) has been investigated. The adsorbents have been characterized by ultimate, proximate and porosimetric analysis. The increasing of temperature influences the activity especially for temperature higher than 800 C. The Norit RB4W shows better performance than Char 100 in terms of both higher initial conversion and slower deactivation. The activity decrease for both samples is associated to the reduction of the amount of mesopore even if further studies are required to elucidate the role of oxygenated surface groups and alkali and alkaline earth metallic (AAEM) species. References [1] Li, C., Suzuki, K., Tar property, analysis, reforming mechanism and model for biomass gasification, An overview, Reew. Sust. Ener. Rev. 13: (2009). [2] Abu el-rub Z., Bramer E.A., Brem G., Experimental comparison of biomass chars with other catalysts for tar reduction, Fuel 87: (2008).

6 [3] Hosokai S., Norinaga K., Kimura T., Nakano M., Li C.Z., Hayashi J.I., Reforming of volatiles from the biomass pyrolysis over charcoal in a sequence of coke deposition and steam gasification of coke, Energ. Fuel. 25: (2011). [4] Fuentes-Cano D., Gómez-Barea A., Nilsson S., Ollero P., Decomposition kinetics of model tar compounds over chars with different internal structure to model hot tar removal in biomass gasification, Chem. Eng. J. 228: (2013). [5] Di Gregorio F., Parrillo F., Cammarota F., Salzano E. and Arena U., Removal of naphthalene by activated carbons from hot gas, Chem. Eng. J. 291: (2016). [6] Nestler F., Burhenne L., Amtenbrink M.J., Aicher T., Catalytic decomposition of biomass tars: The impact of wood char surface characteristics on the catalytic performance for naphthalene removal, Fuel Process. Technol. 145:31-41 (2016). [7] Morgalla M., Lin L., Strand M., Decomposition of benzene using char aerosol particles dispersed in a high-temperature filter, Energ. 118: (2017). [8] Feng D., Zhao Y., Zhang Y., Sun S., Meng S., Guo Y., Huang Y., Effects of K and Ca on reforming of model tar compounds with pyrolysis biochars under H 2O or CO 2, Chem. Eng. J. 306: (2016). [9] Moliner R., Suelves I., Lazaro M., Moreno O., Thermocatalytic decomposition of methane over activated carbons: influence of textural properties and surface chemistry, Int. J. Hydrogen. Energ. 30: (2005). [10] Wang Y., Chen X., Yang S., He X., Chen Z., Zhang S., Effect of steam concentration on char reactivity and structure in the presence/absence of oxygen using Shengli brown coal, Fuel Process. Technol. 135: (2015). [11] Guizani C., Jeguirim M., Gadiou R., Escudero Sanz F.J., Salvador S., Biomass char gasification by H 2O, CO 2 and their mixture: Evolution of chemical, textural and structural properties of the chars, Energ. 112: (2016). [12] Rodríguez-Reinoso F., Molina-Sabio M., González M.T. The use of steam and carbon dioxide as activating agents in the preparation of activated carbons, Carbon 33:15-23 (1995). [13] Cano D.F., Parrillo F., Ruoppolo G., Gomez Barea A., Arena U., The influence of the char internal structure and composition during heterogeneous naphthalene conversion, Fuel Process. Technol. 172: (2018).