Olive oil residue gasification and syngas integrated clean up system

Fuel 158 (2015) 705–710

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Olive oil residue gasification and syngas integrated clean up system B. de Caprariis ⇑, M. Scarsella, A. Petrullo, P. De Filippis Department of Chemical Engineering, Sapienza University of Rome, Via Eudossiana 18, 00184 Rome, Italy

h i g h l i g h t s  Innovative gasifier design to produce high quality syngas maximizing tar removal.  Two steps of tar removal: a char moving bed followed by a catalytic reforming unit.  Ni–Co/CeO2–Al2O3 catalyst is used in the reforming to avoid catalyst deactivation.  The obtained syngas is composed by 47% CO and 30% H2.  The syngas has a low tar amount and can be directly used in many applications.

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Article history: Received 4 March 2015 Received in revised form 1 June 2015 Accepted 3 June 2015 Available online 13 June 2015 Keywords: Gasification Tar reforming Biomass Syngas

a b s t r a c t Gasification is one of the most promising technologies to convert low quality fuels into more valuable ones. The main problem in the use of biomass in gasification processes is the high amount of tar released in the pyrolysis step. It is thus necessary to recover the tar and to transform it in lighter combustible gas species such as CH4, CO and H2 by means of catalytic processes. In this work the gasification of olive husk is performed in order to produce a high quality syngas, composed mainly of carbon monoxide and hydrogen, using an innovative laboratory scale plant composed of a unique reactor divided into three sections. The first section is dedicated to the pyrolysis, the second to the gasification of char produced during pyrolysis and the third to the catalytic reforming of tar. In the reformer two catalysts were tested: a CeO2 promoted bimetallic Ni–Co catalyst and a Ni catalyst, both supported on c-Al2O3. This plant design allows one to minimize the heat dispersion enhancing the energy efficiency of the unit. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The interest on the exploitation of biomass as source of energy has continuously increased in the last decade. Biomass has been recognized as one of the most attractive alternatives to fossil fuel, even if it is not yet competitive with it. The main problem in using biomass as feedstock in energy conversion processes relies in its low energy density and high water content, making it not convenient from an energetic point of view. Gasification is a promising method to produce energy with biomass. Gasification converts biomass into a gaseous product that is easier to treat than the biomass itself. The combustion of this gas allows one to obtain high rates of heat release and high burning efficiencies without particle emission. Another advantage is that burners with simple geometries are required [1,2]. Even though gasification is a well-established technology for coal, its adaptation to biomass feed is not straightforward. ⇑ Corresponding author. Tel.: +39 0644585173. E-mail address: [email protected] (B. de Caprariis). http://dx.doi.org/10.1016/j.fuel.2015.06.012 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

During gasification, biomass undergoes a partial oxidation and produces a gaseous mixture called ‘‘syngas’’ that consists mainly in H2, CO, CO2, CH4 and N2, in proportions that depend on the adopted gasifying agent and on the gasification system used. Additionally, a high amount of tar is produced. Tar is a complex mixture of condensable hydrocarbons, comprising single-ring to 5-ring aromatic compounds and other oxygen-containing organic molecules [3,4]. The presence of tar can cause operative problems to the gasification plants, such as obstruction of pipes and filters and reduction of the heat exchange efficiency due to its condensation in the downstream section. The tar tolerance limit varies depending on the application: it is 500 mg/Nm3, 100 mg/Nm3 and 5 mg/Nm3 for compressors, internal combustion systems, and direct-fired industrial gas turbines, respectively [5]. Therefore the most difficult challenge in designing a reactor for biomass gasification consists in minimizing the tar contamination within the syngas. To this aim different solutions have been studied. Primary methods provide tar removal directly into the gasifier itself, while secondary methods provide tar removal in a separate section [6]. The secondary methods include physical removal, for example by

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means of filters [7], and/or chemical conversion processes, which are more preferable since they allow to recover all the tar energy and enrich the syngas in valuable species [8–10]. The processes that occur during chemical conversion are cracking and reforming, where tar decomposes in lighter species such as methane, hydrogen and carbon monoxide. The principal reactions taking place in these processes are:

Cn Hm ! pCn Hx þ rH2

cracking reaction

 m H2 Cn Hm þ nH2 O ! nCO þ n þ 2 Cn Hm þ nCO2 ! 2nCO þ

m H2 2

steam reforming reaction

dry reforming reaction

CO2 þ H2 () CO þ H2 O reverse water gas shift reaction All these reactions are endothermic and occur at high temperatures (T > 650 °C). The use of a catalyst reduces the operative temperature and enhances the rates of both steam and dry reforming reactions. In order to maximize the efficiency of the process it is necessary that syngas from the gasifier is sent to the reforming at the highest available temperature. The design of the gasifier and its thermal integration, thus, has a key role in the minimization of the heat dispersion of the syngas [11]. In this work an innovative air gasifier is proposed and tested for olive husk gasification. The gasification system is composed of a unique reactor divided into three sections; this design allows one to minimize the heat dispersion and to enhance the energy efficiency and the syngas quality. In the first section of the plant the pyrolysis of the biomass takes place. In the second section the gasification reactions occur while the third section is dedicated to syngas clean-up by means of tar catalytic reforming. In particular, steam and dry reforming are supposed to occur in order to convert tar and at the same time to lower the CO2 dilution of syngas. In the proposed system the tar abatement occurs into two steps. In the first step the pyrolysis volatile products are forced to pass through a char bed where part of the tar is converted into lighter species. It is known from literature, in fact, that char is a very active catalyst for tar cracking. Its high porosity allows increasing the residence time of the gas in the reactor, thus favouring cracking reactions; furthermore the alkali content of the char ash acts as catalysts promoting tar conversion [12,13]. The second step of tar abatement consists in a catalytic reformer where the activity of Ni catalyst, promoted with Co and CeO2, was studied. 2. Experimental section 2.1. Experimental set-up A laboratory scale, semi-continuous, gasification plant was used for the tests. The plant, which capacity is of 500 g/h, is divided in three zones: a feeding system constituted by a hopper and screw conveyor, a reaction unit and a collection system.

The reaction unit can be further divided into three sections: a first section, where pyrolysis occurs, a second section for tar cracking and gasification and a third section for tar catalytic reforming. In Fig. 1 is reported the conceptual flow scheme of the plant. The entire plant is built in AISI 310L stainless steel. The pyrolysis section is constituted by a tube of 40 mm i.d. and 700 mm length with a tilt angle of 70° with respect to the ground. The tube, externally heated by means of an electrical heater, contains a screw that allows one to control the residence time of the biomass into the reactor. Four K-type thermocouples are placed at different reactor heights to measure the inner temperature. The upper part of this reactor is filled with the char produced by the pyrolysis before it falls into the gasification section. The bed of char at the top of the pyrolyzer is about 100 mm high and it constitutes the first step of tar abatement. The tar, in fact, passing through the char bed, undergoes cracking reactions, being in part converted into lighter species. At the end of this section the char falls in the connected vertical gasification reactor (stainless steel tube, i.d. = 40 mm; L = 400 mm). Then the gas and the remaining tar flow through the reforming reactor (i.d. = 40 mm; L = 500 mm) and finally reach the collection system. The reforming reactor is equipped with a cable heater to minimize the heat loss from the reactor walls and to sustain the endothermic reactions, and with a steel net to support the catalyst bed and a thermocouple to measure the temperature of the bed. Two different configurations of the reaction section were tested, which differ for the position of the reforming unit, as shown in Fig. 2. In the configuration ‘‘a’’ the gasification reactor can be considered as an up-draft gasifier while in the configuration ‘‘b’’ it can be considered as a down-draft [14]. During the experimentation in the ‘‘a’’ configuration the gasifying agent (air in the performed tests) is injected from the bottom of the gasifier 30 min after the beginning of the test to allow the formation of the char bed. The advantage of this configuration, with respect to the traditional up-draft gasifier, is that tar and gas are immediately sent to the reforming reactor without cooling them, so avoiding tar condensation [15]. In traditional up-draft gasifiers, in fact, the solid fuel and the gas flow in counter-current, so that the hot gases, produced during gasification, give the majority of their heat to the fresh feedstock that is heated up, dried and then pyrolyzed [16]. In this way the produced gases exiting from the reactor are cooled down even up to 300 °C. This temperature is too low to allow the occurring of the reforming reactions without heating up again the gas. In the adopted experimental set-up, however, the char bed is cooled down by the injected air and so the temperature in the gasifier became too low to ignite the endothermic gasification reactions. To overcome this problem a rod heater is used to trigger the combustion reaction. The heater is activated when the char bed is formed on the gasifier and it is kept switched on only for the time needed for the initiation of the char combustion. After that time the process proceeds in an autothermal regime. In the second configuration, as already said, the gasifier behaves like a down-draft gasifier, with the air and the char fed from the top of the gasifier and flowing in the same direction. In this way

Fig. 1. Conceptual flow scheme of the experimental plant.

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Fig. 2. Two configurations of the experimental laboratory scale plant.

pyrolysis gas and tar are mixed at high temperature with air forming a flame that allows the achievement of high temperatures in the gasifier and thus the initiation of the char combustion reactions. Furthermore the tar produced in the pyrolysis is partly burnt leading to a reduction of the heavy organic species that can cause a rapid fouling of the catalyst in the reforming reactor. Gas and tar leaving the gasification reactor pass through the reforming reactor. In both the configurations, at the exit of the reforming reactor iced-cooled traps in series are used to collect the residual tar, according to the method CEN/BT/TF 143. A gas chromatograph is used to measure the concentration of H2, CO, CH4 and CO2 into the produced syngas. The operative conditions used for the experimental tests are reported in Table 1. The temperature of the pyrolysis reactor was fixed to 600 °C, i.e. the minimum temperature needed to obtain a complete conversion of biomass into char in the reactor zone

where the char filter for tar removal is present. The reforming temperature was set to 700 °C according to a previous study [17], considering that at least 700 °C are needed to avoid the inverse Boudouard reaction, responsible for the deactivation of the catalyst for carbon deposition. The choice of both these temperatures is a compromise between efficiency and energy consumption of the process. The space velocity (GHSV) used in the experimental tests is about 5300 h1 and 8000 h1 for the tests performed with a E.R. of 0.3 and 0.4 respectively. These values of GHSVs are the typical ones used in the industry for nickel catalysts [18]. 2.2. Catalyst preparation Two catalysts were used in the experimentation, a Ni catalyst and a Ni–Co/CeO2 catalyst, both supported on c-alumina. The catalysts were prepared by wet co-impregnation of commercial

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Table 1 Experimental operative conditions. Operative conditions Pyrolysis T (°C) Reforming T (°C) E.R.a GHSV in the catalyst bed (h1) (ER = 0.3) GHSV in the catalyst bed (h1) (ER = 0.4) Catalyst amount (g) Residence time during pyrolysis (h) Biomass feeding rate (g/h) Particle diameter (mm)

600 700 0.3–0.4 5400 8000 50 0.25 500 5

a E.R. = Equivalence Ratio: ratio between the actual air and the air for a complete combustion.

c-Al2O3 (SBET = 230 m2/g, pore volume = 0.66 cm3/g), using a weighted amount of aqueous solution of the precursor salts in order to obtain a loading equal to 10 wt% Ni (Ni Al2O3), and 10 wt%. Ni, 3.33 wt% Co, 3.33 wt% Ce (Ni–Co/CeO2 Al2O3), respectively. The used salts were Ni(NO3)26H2O, Co(NO3)26H2O and Ce(NO3)36H2O. The obtained wet solids were dried at 105 °C for 24 h under vacuum and calcinated for 6 h (2 h at 500 °C, 2 h at 600 °C and 2 h at 700 °C). Before use the catalysts were activated in situ in 0.5 NL/min flow of 50% H2/N2 at 550 °C for 1 h and then held at 750 °C for 2 h. 2.3. Materials All the tests were conducted using as biomass the residue of olive oil production, the main properties of which are reported in Table 2. Proximate composition was determined using a thermogravimetric (TG) method according to the ASTM D5142/02. The ultimate analysis was carried out with a EA3000 (Eurovector) elemental analyzer. As it can be noticed this kind of biomass has a high amount of volatiles comprising condensable species (tar + water) and gaseous compounds. 3. Results and discussion To study the efficiencies of each stage of the tar abatement system and their effect on the global performances of the plant, three different sets of experimental tests were performed. In the first set of experiments, the plant was used as a simple pyrolyzer. The amount of tar released during the pyrolysis process was measured before and after the formation of the char bed in the upper part of the pyrolysis reactor, in order to study the influence of the char bed on the tar removal. In the second set the pyrolyzer was coupled with the reforming unit and the effect of the two prepared catalysts on the pyrolysis products was studied in order to evaluate the improvement achieved when Co and Ce are added to the traditional Ni catalyst. Ni was extensively used as catalyst

for steam reforming applications and it is well known that it suffers from carbon deposition [8,19,20]. Co is added to enhance the reforming of oxygenated species such as those present in the biomass tar [21], while the CeO2 was added into limit deactivation of the catalyst by means of carbon deposition [22,23]; Chen et al. [22], indeed, demonstrated that when CeO2 is added in Ni–Al2O3 catalysts, it exists as a CeAl2O3 phase that has the function of greatly enhancing the catalyst carbon-resistance without having any evident impact on the reactivity. Finally in the third set, gasification tests in both configuration schemes were performed using the Ni–Co/CeO2 catalyst in the reforming unit.

3.1. Pyrolysis and reforming experimental tests In Fig. 3 the amounts of tar and water recovered during the first two sets of experimental tests are reported. In the pyrolysis of the olive husk the tar released is about 16% by weight of the former biomass. The addition of the char bed allows obtaining a tar abatement of about 45%; the tar amount decreases, indeed, from 16 to 9% by weight, confirming that it constitutes the first stage of tar removal. Char is the carbon rich residue of the pyrolysis of the olive husk biomass and being a very porous material, tar and gas, passing through the char bed, increase their residence time allowing the occurring of the cracking reactions. Furthermore the alkali metals, present in the ashes, act as catalyst for tar reforming increasing the rate of the reactions [24]. Tar is so decomposed in lighter species and in gaseous species that contribute to the increase of gas amount (Table 3). The tar cracking forms mainly methane and light hydrocarbons such as ethane and propane; the methane concentration, thus, increases from 16.2% to about 19%. The obtained decrease of tar is important in this phase of the process since it ensures a longer activity of the catalytic stage limiting the carbon deposition phenomenon caused by the high carbon to hydrogen ratio of the tar. In the second set of experimental tests, the catalytic bed allows a great reduction of tar at the exit of the plant that reaches values of about 5% and 3% by weight with respect to the fed biomass for Ni and Ni–Co/CeO2 catalyst, respectively, corresponding to a reduction of more than 80% of the tar obtained by the simple pyrolysis. As a result the gas yield increases, doubling its value when the Ni–Co/Ce catalyst is used. Another interesting result is the decrease of the water amount that leads to an increase of the hydrogen in the syngas. This behaviour was expected because of the occurring of the steam reforming reaction. It can be noticed from the results reported in Fig. 3 and in Table 3 that the water decrease is more pronounced when the Ni–Co/CeO2 catalyst is used: the produced water is 21% for

Table 2 Olive husk biomass properties. Proximate analysis (%wt) Moisture Volatiles (%dry) Fixed carbon (%dry) Ashes (%dry)

10–20 65 29.6 5.4

Ultimate analysis (%wt daf) C H N S O (diff)

44.2 5.8 1.8 0 48.2

LHV (MJ/kg)

17.6

Fig. 3. Amount of tar and water obtained by the four tests performed in pyrolysis conditions.

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B. de Caprariis et al. / Fuel 158 (2015) 705–710 Table 3 Gas composition expressed on N2 free basis and normal conditions for the pyrolysis tests. % vol.

Pyrolysis

Pyrolysis + char

Pyrolysis Ni

Pyrolysis Ni–Co/Ce

CO CO2 CH4 H2

20.9 39.5 16.2 23.4

19.1 40.0 18.7 22.2

18.3 36.5 16.5 28.7

15.2 35.5 17.5 31.8

Total gas yield (N l/g biomass)

0.25

0.29

0.34

0.47

the Ni catalyst and 16% for the Ni–Co/CeO2 one, meaning that this last catalyst is more active for the steam reforming reaction. To study the catalyst activity, the experimental tests lasted 4 h. The Ni–Co/CeO2 catalyst is more resistant to deactivation as it can be seen in Fig. 4, where the temperature profiles at the center of the catalyst bed are reported. The cracking and reforming reactions are endothermic so the decrease of the temperature in the catalyst bed with respect to the external temperature is representative of the occurring of the reactions. The deactivation of the Ni catalyst takes place after only about one hour and a half from the beginning of the process, leading to a progressive increase of the catalyst bed temperature that at the end of the process reaches the value imposed by the external heater. In fact, the tar amount recovered in the iced-cooled traps, measured after 1.30 h, results to be the same released with the Ni–Co/CeO2 catalyst, while in the last part of the test, when the Ni catalyst begins its deactivation, gradually less tar is converted and its amount increases. On the contrary, the Ni–Co/CeO2 catalyst is still active after the 4 h test even if the temperature begins to increase slowly. This result was expected due to the presence of Ce into the catalyst that prevents the carbon deposition. 3.2. Gasification tests The gasification tests were performed on both configurations using only the Ni–Co/CeO2 catalyst. Different amounts of air were used in the tests: for the configuration ‘‘a’’ only a E.R. of 0.3 was used, while for the configuration ‘‘b’’ two E.R. values of 0.3 and 0.4 were tested. In Fig. 5 the temperature profiles in the gasifier measured by the four thermocouples placed at different height of the reactor are shown. It has to be noticed that in the configuration ‘‘a’’ the air is injected at a height of 0 cm while in the configuration ‘‘b’’ the air is injected at the maximum height corresponding to 40 cm. In the up-draft set-up, as the combustion reactions are triggered, the temperature increases instantaneously: the maximum value of

Fig. 5. Temperature profiles as a function of the char bed height for the two configurations.

1022 °C is reached at the bottom of the reactor. The combustion zone of the reactor is approximately 7 cm height and the heat developed by this exothermic reaction is sufficient to keep the reactor in an autothermal regime. When the air is consumed, the CO2, produced by the combustion reaction, reacts with the char beginning the endothermic gasification and therefore the temperature decreases. In this configuration the main reaction that takes place is the Boudouard reaction that is promoted for temperature higher than 700 °C. The minimum temperature achieved in the reactor is 800 °C in the zone where the fresh char arrives from the pyrolysis reactor at a temperature of about 600 °C. The gas is then mixed with the pyrolysis volatiles and sent to the reformer unit. In the down-draft set-up, the air, injected from the top of the reactor, is mixed with the pyrolysis products forming a flame that leads to an immediate increase of temperature that achieves a maximum value of 935 and 1042 °C for an E.R. of 0.3 and 0.4, respectively. As expected, by increasing the amount of air injected, the reaction rates of the combustion are higher leading to a major increase of the temperature in the reactor. The temperatures then gradually decreases as the gasification takes place due to the endothermicity of the reactions. As it is shown in Table 4 where the results are reported on N2 free basis, the syngas obtained in both the configurations is composed for the 70% of hydrogen and carbon monoxide, the dilution in CO2 is low as well as the amount of methane. The configuration ‘‘b’’ allows to obtain a syngas with higher amount of hydrogen, due to the steam gasification reactions occurring in this case, and with a lower amount of tar exiting from the plant. The tar, indeed, is partly burnt as it exits from the pyrolysis step and its content is partly reduced. Therefore the lower content of heavy organic species, which are the principal causes of catalyst deactivation for carbon deposition, dragged with the gas contributes to delay this phenomenon in the reforming reactor. The calorific value of the syngas produced ranges between 3.9 and 4.3 MJ/Nm3, these values

Table 4 Gas composition expressed on N2 free basis and normal conditions for the gasification tests.

Fig. 4. Temperature profiles as a function of time obtained with the two catalysts in the reforming reactor for a four hours test.

% vol.

Configuration ‘‘a’’ Configuration ‘‘b’’ Configuration ‘‘b’’ (E.R. = 0.3) (E.R. = 0.4)

CO CO2 CH4 H2

49.3 22.4 2.1 26.2

44.5 19.4 4.1 32.0

47.4 20.1 2.3 30.2

Total gas yield 1.11 (N l/g biomass) Tar (%wt) 2.9 Calorific value 4.0 (MJ/Nm3)

0.95

1.05

1.9 4.3

1.5 3.9

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The calorific value of the obtained syngas ranges between 3.9 and 4.3 MJ/Nm3 that is a typical value for fixed bed air gasifier. The relatively low tar content exiting within the syngas in the down-draft configuration, about 450 mg/Nm3, allows its direct use in many syngas applications without the necessity of expensive and energy consuming further purification treatments. References

Fig. 6. Gas composition as a function of time for the test performed with the second configuration using a E.R. = 0.3.

are in accordance with the values obtained for air gasification in literature and in the existing plants [25,26]. The gasification tests lasted 4 h, from Fig. 6 it can be noticed that after the first 30 min, that is the time needed for the formation of the char bed in the gasifier, the concentration of the measured gas are constant for the whole test duration. This means that the catalyst does not deactivate and that the process is stable. The small fluctuation of the concentration ±5% are due to the non-uniformity of the biomass composition. The plant efficiency, calculated as the ratio between the calorific value of the syngas and the calorific value of the feedstock, was measured to be about 48% for all the configurations. The fraction of carbon not converted into syngas during the gasification was 20% with an E.R. of 0.3 and 16% with an E.R. of 0.4. The majority of the carbon is lost as unreacted char within the ashes and the rest is lost as unrecovered tar and as unreacted light char particles that are dragged along with the syngas. 4. Conclusions In this paper an innovative gasification/tar reforming laboratory plant, divided into three sections, is studied in order to produce a high quality syngas with low tar content and to maximize the energy efficiency. In the first section of the reactor the pyrolysis and the first step of tar abatement through a char bed occurs; the tar removal amounts here to about 45%. In the second section the gasification takes place, the produced syngas is further cleaned in a reforming step. Two gasification-reforming configurations were tested: in a first one the gasifier behaves as an up-draft reactor, in the second one as a down-draft. In both cases the gasification step proceeds in autothermal regime and the heat losses are minimized. In view of a possible scale-up of the plant this configuration presents a better arrangement due to the easy way of the ash discharge. However the amount of tar in the syngas is still too high to be used as it is, so a further stage of removal must be considered after the reforming unit. The third section is dedicated to the tar reforming with Ni/Al2O3 and Ni–Co/CeO2 Al2O3 catalysts. The addition of Ce and Co to the catalyst allows to increase the resistance of the catalyst and to obtain a reduction of the tar content by up to 85%, with a great improvement in the syngas quality, increasing the H2 content of 150%. The studied innovative gasification/tar reforming laboratory plant allows producing a syngas composed, on N2 free basis, for the 70% of hydrogen and carbon monoxide with a low CO2 dilution.

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