Supercritical water gasification of industrial organic wastes

J. of Supercritical Fluids 46 (2008) 329–334 Contents lists available at ScienceDirect The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu Supercritical water gasification of industrial organic wastes M.B. Garcı́a Jarana ∗ , J. Sánchez-Oneto, J.R. Portela, E. Nebot Sanz, E.J. Martı́nez de la Ossa Department of Chemical Engineering, Food Technology and Environmental Technologies, Faculty of Sciences, University of Cádiz, 11510 Puerto Real (Cádiz), Spain a r t i c l e i n f o Article history: Received 15 October 2007 Received in revised form 19 February 2008 Accepted 1 March 2008 Keywords: Supercritical water gasification Hydrothermal gasification Cutting oil wastes Cutting fluids Vinasses a b s t r a c t Supercritical Water Gasification (SCWG), in which supercritical water is not only a solvent for organic materials but also a reactant, is one of the applications for producing fuel from organic resources. In this way, besides the destruction of wastewaters, it is aimed to harness their energy potential by burning the gas effluent generated in the process, which contains a high level of heating power due to its high content in hydrogen and light hydrocarbons. In the work described here, SCWG has been tested on a laboratory scale continuous-flow system with two different industrial wastewaters that contain a high concentration of organics, with both wastes having a high energy potential: cutting oil wastes, oleaginous wastewater from metalworking industries, and vinasses, alcohol distillery wastewater. Reports on SCWG processes on these types of wastewaters have not previously been published in the literature. The influence of the temperature, amount of oxidant and catalyst addition on the yield and composition of the gas phase generated were studied. Experiments were carried out in the temperature range 450–550 ◦ C, the amount of oxidant ranged from the absence of oxygen (oxygen coefficient, n = 0) to 20% of stoichiometric oxygen (n = 0.2), and 250 bar of pressure in all cases. A maximum of 0.19 mol H2 per initial CODm (CODm is given as mol O2 consumed for total oxidation) was obtained in the gas phase under the best conditions. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Hydrothermal processes have attracted worldwide attention because of the fascinating characteristics of water as a reaction medium at elevated temperatures and pressures [1]. Hydrothermal processes have shown great potential and effectiveness for the treatment of industrial wastewaters with high organic concentrations. Among the hydrothermal processes it is possible to highlight Supercritical Water Gasification (SCWG), a process that occurs in the partial or total absence of dissolved oxygen. In this work, SCWG has been applied to different industrial wastewaters that contain a high concentration of organics and have a high energy potential. In this way, besides the purification of industrial wastewaters, it is sought to harness their energy potential by burning the gas effluent generated in the hydrothermal gasification—a gas that should have great heating power due to its high content in hydrogen and light hydrocarbons. Hydrogen is an obvious alternative to hydrocarbon fuels. This gas has many potential uses, it is safe to manufacture and it is environmentally friendly. Hydrogen is expected to play a key role in ∗ Corresponding author. Tel.: +34 956 016458; fax: +34 956 016411. E-mail address: belen.garcia@uca.es (M.B. Garcı́a Jarana). 0896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2008.03.002 the world’s energy future by replacing fossil fuels and, as such, it is gaining increasing attention as an encouraging future energy source [2]. In SCWG the water contained in wastewaters is used as a solvent as well as a reactant, meaning that a drying procedure is not required. Amin et al. [3] studied the SCWG of glucose and the composition of the gas phase produced consisted mainly of carbon dioxide, hydrogen and carbon monoxide, along with small amounts of methane and gases with C2 (two carbons). Gasification of glucose in supercritical water can be considered as a good model for the gasification of more complex wastewaters in supercritical water. However, recent studies [4] have suggested that glucose with salt seems to be a much better model system for salts containing biomass. Yu et al. [5] found that glucose at low concentrations (0.1 M) can be completely gasified in supercritical water at 600 ◦ C and 34.5 MPa. The major products were hydrogen and carbon dioxide and char or tar products were not detected. More recently, Williams and Onwudili [6] studied glucose as biomass model compound, under sub-critical and supercritical water, giving the composition of obtained products in SCW experiments. The results showed that an increase in the concentration of the oxidant, hydrogen peroxide, decreases the amounts of char, oil, and water-soluble products obtained. The yield and product 330 M.B. Garcı́a Jarana et al. / J. of Supercritical Fluids 46 (2008) 329–334 composition at the sub-critical water temperatures did not significantly change as the temperature (and pressure) increased into the supercritical water region. At 330 ◦ C the conversion of the glucose hydrogen to hydrogen gas was 19% conversion with a 4.5 wt% hydrogen peroxide. At 380 ◦ C the hydrogen yield increase to 34% conversion. A reaction mechanism was proposed to describe a possible reaction route for the formation of characteristic compounds found in the oils. More specific details regarding the evaluation of SCWG technology of biomass can be found in two recent review papers [7,8]. In the case of real wastewaters [9], not all of the organic molecules are transformed into hydrogen or carbon dioxide. As a result, tars and chars can be formed during the supercritical water gasification. As a result of sedimentation, these chars and tars usually plug reactors after several hours of running. Moreover, they limit the amount of hydrogen formed. Chars come from non-converted organics, while tars are unwanted reaction products, usually formed by pyrolysis of organic molecules. The amounts of these materials can be effectively reduced by the use of a catalyst in the SCWG process to obtain high conversion yields. For example, Schmieder et al. [10] showed that the addition of KOH increases the production of H2 . Matsumura et al. [11] concentrated on the use of activated carbons as catalysts in supercritical water. Their experiments showed that extraordinary yields of gas with a high content of hydrogen were obtained in the carbon-catalyzed gasification of real wastes such as wood sawdust or potato starch, while the amount of residual chars and tars was significantly decreased. Watanabe et al. [12] studied catalytic effects of NaOH and ZrO2 in the partial oxidative gasification of n-hexadecane and lignin in supercritical water (40 MPa, 400 ◦ C). They showed that the yield of H2 from n-C16 and lignin with zirconia was twice the obtained without catalyst under the same conditions for both compounds with and without O2 . The H2 yield with NaOH was almost 4 times higher than that without catalyst (with and without O2 ). Thus, a base catalyst has a positive effect to produce gaseous product such as H2 . The addition of alkali salts increases the reaction rate and suppresses the formation of soot and tar. According to the literature [2] this effect could decrease the CO yield and increase the CO2 yield in the gasification. Schmieder et al. [10] found that the best effect obtained on comparing alkali salts in their screening experiments was achieved by the addition of potassium. They showed that at 600 ◦ C and 250 bar all compounds are completely gasified by the addition of KOH or K2 CO3 , with an H2 -rich gas formed that contains CO2 as the main carbon compound. In a similar way, we studied the effect of the addition of alkali to the feedstock solution. Sinag et al. [13] summarized the catalytic mechanism of K2 CO3 for biomass gasification in SCW. In that mechanism it could be observed that KOH plays a very important role in the production of hydrogen. In the paper of Kritzer [14], both carbonate (CO3 − ) and hydroxide (OH− ) are present as low-soluble salts in the medium on reaction. In this way, it seems that both alkali salts can lead to similar results. At present there are very few reports in the literature on the SCWG of real wastes. Schmieder et al. [10] carried out experiments in two tubular flow reactors and in two batch autoclaves with carbohydrates, aromatic compounds, glycine as a model compound for proteins and with real biomass (straw, wood and sewage sludge). The results obtained are reported for different residence times, temperatures and pressures. Yoshida et al. [15] studied mixtures of hardwood and grass lignin, which were gasified with a nickel catalyst in supercritical water at 400 ◦ C and 250 bar. The gasification ratios were lower than expected from their components, which indicated that even in real biomass, the interaction between each component occurred. Antal and co-workers [16] investigated the coke-catalyzed gasification at temperature range of 600–650 ◦ C, 280 or 345 bar of pressure and reaction times of ∼30 s in a tubular flow reactor of model substances, biomass (aquatic plants, sugar cane bagasse, sewage sludge and glycerol) and wastes of the Department of Energy. At 600 ◦ C and 345 bar, most feeds at concentrations up to ∼0.2 M were gasified completely to a hydrogen-rich gas. Coke or tar formation was not observed. Guo et al. [17] showed the results of different real biomass gasification (wood sawdust, rice straw, rice shell, wheat stalk, peanut shell, corn stalk, corn cob and sorghum stalk) under the same conditions (923 K, 25 MPa and residence time, 27 s) in a miniature plant. In general, the molar fraction of hydrogen is about 40% while the molar fraction of CO is less than 1%. The K2 CO3 content in biomass influenced the low CO content. The different behaviour of biomass gasification is that the cellulose, lignin and hemi-cellulose contents in the studied feedstocks are different. The unconverted total organic carbon (TOC) in liquid effluent is high. In this way, a portion of biomass is converted to liquid products instead of gaseous products. In a previous study carried out in our research group [18], we investigated the hydrothermal gasification of lixiviates and vinasses at temperatures and pressures below the critical point of pure water (sub-critical region). Appreciable amounts of hydrogen were not obtained in the gas phase. Due to the relatively high ion product for sub-critical water, ionic reactions can be dominant while freeradical reactions, which are necessary to generate gases, are less pronounced [19–21]. In the work described here, we studied the hydrothermal gasification at supercritical conditions, in which the density of water is significantly lower. This means that the ionic product for water is much lower and ionic reactions are inhibited because of the low relative dielectric constant of water. The lower density favours free-radical reactions, which are necessary to generate gases such as H2 or CH4 . In this work we present the results obtained in the supercritical water gasification of two different industrial wastewaters containing a high concentration of organics, both with a high energy potential: cutting oil wastes, oleaginous wastewater from metalworking industries, and vinasses, wastewater generated in alcohol distilleries. The influence of temperature, addition of oxidant and catalyst in relation to the yield and the composition of the gas phase generated were examined. In this case, KOH was selected as a catalyst on the basis of the good results presented in the literature [10]. 2. Experimental 2.1. Equipment The supercritical water gasification experiments were carried out in a laboratory scale continuous-flow system. A schematic diagram of the experimental set up is shown in Fig. 1. All wetted parts, from the pumps to the back-pressure regulator, were made of stainless steel 316. The reactor was constructed from a 2.5 m length of 1/4 in.-o.d. tubing. The oxidant feed stream was prepared by dissolving hydrogen peroxide with deionized water in a feed tank. Another feed tank equipped with a magnetic stirrer was loaded with an aqueous solution of the corresponding waste. The two feed streams were pressurised in two different lines by two highpressure metering pumps and then separately preheated. In order to ensure that all H2 O2 decomposed to give H2 O and O2 , two in series preheating systems were used for the oxidant feed stream: M.B. Garcı́a Jarana et al. / J. of Supercritical Fluids 46 (2008) 329–334 331 Fig. 1. Schematic diagram of the continuous-flow reactor system. (1) by flowing through 3 m coiled 1/8-in.-o.d. tubing electrically heated and (2) by flowing through 5 m of coiled 1/8-in.-o.d. tubing submerged in the fluidized sand bath (Techne Model SBL-2). Based on the studies of Croiset et al. [22], it has been shown that H2 O2 was completely decomposed in the preheaters. A similar preheating system was used for the waste feed stream. Moreover, the organic feed stream was preheated by flowing through 3 m coiled 1/8-in.-o.d. tubing submerged in the fluidized sand bath. After preheating, the two lines were mixed at the reactor inlet. On exiting the reactor, the effluent was cooled rapidly in a counter current heat exchanger and the pressure was subsequently reduced using a back-pressure regulator. The product stream was then separated into liquid and vapour phases. 2.2. Materials and analytical methods Vinasses are organic wastewaters generated during the alcohol production process by wine distillation in alcohol distillery industries, whose main problems are its acidic state and its high concentration of organics. The vinasses contain phosphates (which are in quantities superiors to commonly found in urban wastewaters, it implies a certain risk of eutrophication), nitrogen (found in 90% organically and comes mainly from proteins of wine and the yeast cell) and organics acids (mainly tartaric acid and fewer malic acid and lactic acid). Other constituents of vinasses are glycerin (from the lipids of the grape), polyphenols and little amount of sugTable 1 Main characteristics of the wastewaters studied in this work ars. At the same time, it may contain certain amount of potassium, which comes from the grapes and from the addition of potassium bicarbonate or tartrate, employed in the stabilization of wines [23]. Cutting oils are emulsionable fluids widely used in metalworking processes. Their composition is normally oil, water, and additives (fatty acids, surfactants, biocides, etc.) leading to a toxic waste after a long use. Generally, such a waste is too dilute to be incinerated and is difficult to treat biologically. This type of waste has been widely studied by our research group. More specific details of this waste are described in our previous publications [24–26]. The main characteristics of the two wastewaters studied in this work are shown in Table 1. Hydrogen peroxide (Panreac, 30%, w/v aqueous solution) was used as a source of oxygen. Diluted feed solutions of the required concentration were made using deionizated water. Gas samples were analyzed using an HP 6890 Series gas chromatograph with a Thermal Conductivity Detector (TCD). Two in-series columns were used to separate CO from CO2 . The first column was a Porapack-Q column and the second a molecular sieve Carvosieve column (Supelco). A temperature gradient from 55 to 160 ◦ C (at 15 ◦ C/min) was used. The system was calibrated with a standard gas mixture containing H2 , O2 , N2 , CO2 , CO and CH4 . The COD measurements were obtained according to the dichromate standard method [27]. 3. Results and discussion 3.1. Effect of oxidant and catalyst addition Waste Vinasses Cutting oil wastes pH Conductivity (mS/cm) COD (g O2 /l) 3.73 3.62 27 8.6 3.5 160 Experimental conditions for a series of experiments conducted in order to evaluate the effect of oxidant and catalyst addition are presented in Table 2. All experiments were carried out at 450 ◦ C, 250 bar and the initial concentration of the wastes (expressed as 332 M.B. Garcı́a Jarana et al. / J. of Supercritical Fluids 46 (2008) 329–334 Table 2 Summary of operating conditions for SCWG experiments Waste Temperature (◦ C) Residence time (s) Oxygen coefficient (n)a Catalyst Vinasses 450 450 450 450 20 18 18 19 0 0.1 0 0.1 – – KOH (4 × 10−3 M) KOH (4 × 10−3 M) Cutting oil 450 450 450 450 17 19 17 18 0 0.1 0 0.1 – – KOH (4 × 10−3 M) KOH (4 × 10−3 M) a n = oxygen/stoichiometric oxygen. The oxygen coefficient is unity (n = 1) when the quantity of initial oxygen corresponds to the stochiometric amount needed to completely oxidize all initial COD. COD) was around 12 g O2 /l in all cases. The efficiency of the SCWG process was followed in terms of the reduction in chemical oxygen demand (COD) and the composition of the gas phase generated (H2 , CH4 , CO2 , and CO). Half of the experiments were carried out in the total absence of oxygen (n = 0) and the other half with 10% of the stoichiometric amount of oxygen (n = 0.1) estimated from the initial COD. Moreover, half of the experiments were carried out with catalyst addition and the other half without the addition of catalyst. In the experiments with catalyst addition, the amount of KOH added was based on the work of Schmieder et al. [10]. 3.1.1. COD removal Experimental results for COD disappearance in both cases are shown in Fig. 2. In the case of vinasses, the addition of oxygen and catalyst has a dramatic effect on the results obtained, while in the case of cutting oil wastes no appreciable effect was observed with 10–15% COD removal in all cases. It can seen that for vinasses better results were obtained at conditions of n = 0.1 in the presence of catalyst. This difference between vinasses and cutting oil wastes could be due to the different compositions. Vinasses are a desirable feedstock for SCWG because the presence of alkali in biomass may have an effect on the gasification process, as demonstrated Schmieder et al. [10] and Hao et al. [2]. Alkali decreases coke formation and increases hydrogen yield. However, the addition of KOH does not seem to affect cutting oil gasification. These results may suggest that the presence of KOH is not the only factor that affects the gasification process, because there are many components that may produce a wide number of unknown interactions affecting the process. In this work we have studied the possibility of producing H2 but we have not studied the mechanisms. 3.1.2. Composition of the gas phase produced The compositions of the gas phases generated in the experiments are shown in Fig. 3. In this and subsequent figures, the product yield is defined as moles of product divided by the initial COD (kg O2 ). As can be seen, the highest amount of H2 is obtained at conditions of n = 0.1 and in presence of KOH in both cases. The influence of catalyst is shown in Fig. 3 by comparison of the experiments at 450 ◦ C with and without added KOH. The experiments show that the addition of KOH leads to a decrease in the CO concentration in the product gas. It can be concluded that the addition of alkali salts, probably as an acid–base catalyst, increases the rate of the water-gas shift reaction (CO + H2 O ↔ CO2 + H2 ). Lee et al. [28] summarized the major reaction pathways associated with hydrogen production from glucose gasification in supercritical water. Glucose was first converted to give large amounts of water-soluble intermediates and most of these then contributed to produce CO. Meanwhile, some of the intermediates are converted to CO2 and H2 by a steam reforming reaction (Cn Hm Oy + (2n − y) H2 O → nCO2 + (2n − y + m/2)H2 ). The carbon monoxide formed is finally converted to carbon dioxide and hydrogen through the water-gas shift reaction. Moreover, the presence of the oxidant promoted the steam reforming reaction to give hydrogen. The amount of hydrogen increased on increasing the oxidant concentration; however, the data from the experiment carried out in our previous studies [18] showed that, at oxidant concentrations of n = 0.25, the hydrogen levels dropped. This means that the oxidant concentration is an important parameter for the optimization of the hydrogen production process. The influence of oxidant concentration will be analyzed in Section 3.3. 3.2. Effect of temperature The effect of reaction temperature on the gasification process was studied through the cutting oil waste gasification experiments. The pressure was maintained at 250 bar, the initial concentration of the wastes expressed as COD was around 12 g O2 /l, the oxygen coefficient was n = 0.1 and KOH was present in all experiments, since these conditions led to better results in the previous experiments. The other operation conditions are shown in Table 3. The yields of the major identified products from the cutting oil waste gasification experiments are shown in Fig. 4. As can be seen, the COD removal, hydrogen yield and carbon dioxide yield increased markedly as the temperature increased. At the same time the CH4 yield increased and the CO yield is practically negligible. The enhancement in hydrogen and carbon dioxide and the low carbon monoxide composition with increasing temperature has been highlighted previously by other authors for supercritical water environments [5,28–30]. Although, the water-gas shift reaction rate is slow and its equilibrium constant decreases with increasing temperature in the gas phase. These authors observed a fast-type water-gas shift reaction in their studies of glucose gasification in supercritical water. In terms of COD removal, it seems necessary to work at 550 ◦ C because 80% COD removal is obtained at this temperature. Table 3 Summary of operating conditions for SCWG experiments in order to study effect of temperature Fig. 2. Experimental results for COD removal under different operating conditions. All experiments were carried out at 450 ◦ C and 250 bar, and the initial concentration of the wastes expressed as COD was around 12 g O2 /l. Waste Temperature (◦ C) Residence time (s) Oxygen coefficient (n) Cutting oil 450 500 550 20 18 8 0.1 0.1 0.1 [KOH] = 4 × 10−3 M; P = 250 bar; CODo = 12 g O2 /l. M.B. Garcı́a Jarana et al. / J. of Supercritical Fluids 46 (2008) 329–334 333 Fig. 3. Composition of the gaseous phase in all cases. All experiments were carried out at 450 ◦ C and 250 bar, and the initial concentration of the wastes expressed as COD was around 12 g O2 /l. According to the literature, the best results are obtained at 600 ◦ C but our equipment is not suitable for work at this temperature. 3.3. Effect of oxygen coefficient The results of the cutting oil SCWG at an oxygen coefficient range of 0.1–0.2, 250 bar of pressure and an initial concentration of waste (expressed as COD) around 12 g O2 /l are shown in Fig. 5. This study was carried out at two different temperatures, 500 ◦ C and 550 ◦ C. The main operation conditions for these experiments are shown in Table 4. The residence times could not be maintained at similar values in all experiments due to limitations of the equipment. It can be seen from Fig. 5 that at 550 ◦ C the hydrogen and carbon dioxide yields increase with increasing oxygen coefficient and COD removal is above 80% in all experiments. At 500 ◦ C the hydrogen and methane yields increase with increasing oxygen coefficient but the carbon dioxide yield decreases. In this case only 60% COD removal is obtained. Fig. 5. Effect of oxygen coefficient on the yields of gaseous products from cutting oil waste gasification in supercritical water at 250 bar, 500 ◦ C and 550 ◦ C. In SCWG in which a small amount of oxygen is present, there are two competing pathways: gasification and oxidation reactions [18]. For low levels of oxygen (around n < 0.25) the steam reforming reaction to hydrogen is more important than oxidation. However, when the oxygen coefficient values were above 0.25 approximately, the Table 4 Summary of operating conditions for cutting oil SCWG at 250 bar, CODo = 12 g O2 /l, n = 0.1 and in the presence of catalyst Fig. 4. Effect of temperature on the composition of the gaseous phase and %COD removal from cutting oil gasification at 250 bar, CODo = 12 g O2 /l, n = 0.1 and the presence of catalyst ([KOH] = 4 × 10−3 M). Waste Temperature (◦ C) Residence time (s) Oxygen coefficient (n) %COD removal Cutting oil 500 500 500 500 550 550 550 18.5 20.2 21.0 20.6 7.7 7.9 7.5 0.07 0.10 0.15 0.19 0.10 0.13 0.19 58.7 60.2 62.9 65.8 80.5 87.7 87.2 334 M.B. Garcı́a Jarana et al. / J. of Supercritical Fluids 46 (2008) 329–334 hydrogen levels dropped because oxidation becomes predominant. Although the amount of oxygen is not sufficient to produce complete oxidation of organics to carbon dioxide, CO levels were very low in all cases. Similar results were obtained by other authors. For example, Lee et al. [28] studied glucose gasification and most of the carbon monoxide produced was expected to come from water-soluble organic compounds that were the first products from gasification. Holgate et al. [29] detected about 26 organic compounds in the liquid effluent from glucose hydrolysis in supercritical water at 500 ◦ C. These authors found that the stability of the organic compounds produced in the process decreased sharply with increasing temperature, with the exception of several species, including 5-hydroxymethylfurfural, acetic acid and acetaldehyde. It was found that these temperature-resistant compounds were finally converted to methane or carbon dioxide but not to carbon monoxide. In general, comparison of the two temperatures studied shows that the highest hydrogen, methane and carbon dioxide yields were obtained at 550 ◦ C and these conditions led to the lowest carbon monoxide yield. 4. Conclusions SCWG of cutting oil wastes and vinasses has been tested on a laboratory scale continuous-flow system. In the case of vinasses, the addition of oxygen and catalyst (KOH) has a dramatic effect on the results obtained, while in the case of cutting oil wastes no appreciable effect was observed. This difference between vinasses and cutting oil wastes could be due to the different compositions. In the case of the vinasses, the composition is similar to biomass wastewaters, where the effect of KOH was observed. The highest amount of H2 is obtained at conditions of n = 0.1 and in presence of KOH in all cases. It can be concluded that the addition of alkali salts, probably as an acid–base catalyst, increases the rate of the water-gas shift reaction. The amount of hydrogen produced in the gas phase increased on increasing the oxidant concentration but only in a limited ranged, since oxidation and gasification are competing reactions. For low levels of oxygen (around n < 0.25) the steam reforming reaction to hydrogen is more important than oxidation. However, when the oxygen coefficient values were above 0.25 approximately, the hydrogen levels dropped because oxidation becomes predominant. 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