oxygen biomass gasification at pilot scale in an internally circulating bubbling fluidized bed reactor

FUPROC-04576; No of Pages 8 Fuel Processing Technology xxx (2015) xxx–xxx

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Steam/oxygen biomass gasification at pilot scale in an internally circulating bubbling fluidized bed reactor D. Barisano ⁎, G. Canneto ⁎, F. Nanna, E. Alvino, G. Pinto, A. Villone, M. Carnevale, V. Valerio, A. Battafarano, G. Braccio ENEA, Italian National Agency for New Technologies, Energy, and the Sustainable Development, Trisaia Research Center, S.S. 106 Jonica km 419 + 500, 75026, Rotondella, Matera, Italy

a r t i c l e

i n f o

Article history: Received 18 February 2015 Received in revised form 25 May 2015 Accepted 1 June 2015 Available online xxxx Keywords: Biomass gasification Internally circulating bubbling fluidized bed reactor Steam/oxygen Enriched air

a b s t r a c t An innovative 1000 kWth pilot plant based on a bubbling fluidized bed gasifier with internal recirculation was operated in experimental campaigns of biomass gasification. The evaluations were focused on the gasifier performances and quality of the product gas. To this aim, tests were carried out at atmospheric pressure using almond shells as a feedstock and three defined gasification mediums (i.e. steam/O2 mixture, 35wt.% and 50wt.% O2 enriched air); process temperature was in the range 820–880 °C. The result assessment allowed to evaluate the system flexibility to the gasifying agent and acquire data on the gas producible with this specific configuration. Based on the dry compositions, LHVs in the range 5.9–6.7 MJ/Nm3dry, 6.3–8.4 MJ/Nm3dry and 10.9–11.7 MJ/Nm3dry were respectively calculated for the three product gases. Correspondingly, an increase in the cold gas efficiency from 0.5 up to 0.7 was also estimated. Concerning the contaminant loads, in the case of the tests related to steam/O2 biomass gasification, particle and tar contents were found in the range 6–10 g/Nm3dry and 12–18 g/Nm3dry, respectively, while H2S, HCl and NH3 were at concentrations below 100 ppms (v). © 2015 Elsevier B.V. All rights reserved.

1. Introduction Lignocellulosic biomass is considered as an important source for the achievement of the goals the European Union has defined on energy, environment and sustainable development issues. As known in relation to these themes, two deadlines have been set for the year 2020 and 2050. The first one concerns the known “20–20–20” targets contained in the climate and energy package, which aims to ensure that the European Union meets by 2020 the three key objectives of a 20% reduction in greenhouse gas emissions from 1990 levels, raising the share of energy consumption produced from renewable resources to 20%, and finally a 20% improvement in the energy efficiency [1]. The second one instead concerns the roadmap the EU has set out in order to reach the objective of reducing Europe's greenhouse gas emissions by 80–95% compared to the 1990 levels and thus moving to a competitive lowcarbon economy in 2050 [2]. Though very ambitious, the achievement of such goals will represent the EU successful contribution to the emission reductions of GHG to hold the global warming below 2 °C, compared to the temperature in pre-industrial times. Many processes can be considered for the production of energy from biomass [3–12], among these, the thermochemical process of gasification is one of the most interesting due to the versatility of uses that ⁎ Corresponding authors. E-mail addresses: [email protected] (D. Barisano), [email protected] (G. Canneto).

the product gas can be addressed. Starting from a solid fuel, the gasification process allows to produce a very flexible gaseous energy carrier that can be used for combined heat and power (CHP) production by using the product gas in internal combustion engine (ICE), gas turbine (GT) and fuel cell (FC), or as intermediate and further conversion for production of derived energy carrier such as H2, SNG and biofuels [13–25]. In the short to medium terms, the technology of biomass gasification in fluidized bed reactors, compared to others, appears to be the most promising for many of the above-mentioned applications as it is suitable for continuous operation and process scalability. Bubbling fluidized bed (BFB) reactors have characteristics that make them advantageous with respect to fixed bed gasifiers. Such reactors have in fact larger tolerance to the particle sizes of the supplied feedstock, good control of the process temperature, uniformity of the reaction environment, and scalability at larger sizes. At the same time, higher simplicity in construction and operation, longer residence time of the fuel particles under the process condition and reduced particle entrainment, are some of the main aspects that can also give BFB gasifiers a certain advantage compared to reactors of fast circulating fluidized design. However when biomass is the considered feedstock, in the BFB reactors a certain tendency of the fuel to remain at the surface of the bed inventory can occur. Such tendency is strictly correlated to the particle density of the supplied feedstock, and it clearly represents a disadvantage as it can bring a lower conversion efficiency of the fuel in gaseous products and a higher entrainment in the product gas of particles from the feedstock and the

http://dx.doi.org/10.1016/j.fuproc.2015.06.008 0378-3820/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: D. Barisano, et al., Steam/oxygen biomass gasification at pilot scale in an internally circulating bubbling fluidized bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.008

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D. Barisano et al. / Fuel Processing Technology xxx (2015) xxx–xxx

generated char. The difficulty in maintaining the biomass particles inside the bed affects the performance of gasification. Process conditions at the surface of the bed (i.e. temperature, heat transfer and solid–gas contact) are less favorable to the reaction kinetics of char gasification and tar conversion than those inside the bed. The use of a reactor with interconnected chambers can overcome such disadvantages allowing for movement of the solid particles inside the bed, counteracting their tendency to segregate the bed surface and, at the same time, reducing the phenomenon of elutriation. Kuramoto et al. in their works [26,27] used an interconnected fluidized bed having the aim to improve the movement of the solid particles inside the bed, Snieders et al. [28] used similar devices to test the circulation of pellets and Abellon et al. [29] to determine the residence times of glass beads in a four-compartment interconnected fluidized bed. Foscolo et al. [30] in their tests with a cold model rig proved that a sufficient circulation of the bed inventory allows for a prolonged residence time of light fuel particles within the bed and avoids segregation to the bed surface. Experimental tests were conducted by Freda et al. [31] to study the fluidization and to measure the recirculation of glass spheres inside the solid material of the bed in a cold model apparatus. In a recent work numerical simulations were implemented by Canneto et al. [32] to study the fluidization quality and the circulation of the solid between the two chambers of a cold model reactor. Zhou et al. [33] studied the effect of different fluidization velocities on rice husk circulation rate. Zhi [34] reported good temperature homogeneity throughout the top and bottom zones of each bed of an interconnected fluidized reactor during sawdust gasification tests in a pilot plant. Xiao et al. [35], using animal-waste-derived, conducted experimental tests in an internally circulating fluidized-bed, with separated gasification and combustion reaction zones. A recent review [36] provides a comprehensive and up-to-date survey of the state of the art of the fluidized bed reactors, including the gasifiers with internal (and external) circulation of the solid material. Based on such concept, at Enea-Trisaia Research Centre (Italy) a gasification pilot plant of 1000 kWth rated power was built. In this paper the activity carried out at this pilot plant and the preliminary results achieved during the early experimental campaigns of biomass gasification are presented. These tests were aimed at evaluating the operability of the pilot reactor and the performance of the biomass gasification process. For the mentioned process, steam/oxygen is the gasification medium of reference, however during this experimental campaigns, tests with enriched air were also included. Such tests were considered in order to evaluate the flexibility of the process to gasification agents, collect data on the gas composition obtainable at the specific plant and operating conditions, and at the same time assess the beneficial effect of using steam/oxygen as gasification medium. Moreover, the use of oxygen-enriched air as gasifying agent may have some advantages compared to the gasification carried out with air. Keeping constant the thermal input of the gasification plant, the reduced amount of nitrogen in the gasifying agent, can in fact allow operating the gasification process with a reactor, and related equipment for full plant operation, of reduced sizes with consequent decrease in the required capital investment costs. The production of a gas with a higher LHV can also have an effect on the internal combustion engine. In fact, the availability of such a fuel gas can enable the use of smaller and cheaper internal combustion engines. Moreover, the availability of a technology based on enriched-air gasification can make it suitable for RDF gasification [37,38], thus expanding the fields of its applicability. Studies on low cost technology for oxygen-enriched air production are giving an important sustenance to the feasibility of such applications [39–42]. Soon after completing the stage of process evaluation in the abovedescribed configuration, the pilot plant will be implemented with an innovative cleaning system that will integrate the steps of gas filtration and conditioning directly into the reactor vessel. The ultimate result

will be a more compact and effective technology that will enable the achievement of better process performances and reductions of investment costs. 2. Experimental set-up 2.1. The 1000 kWth gasification pilot plant The working principle of this reactor is based on the concept developed by Kuramoto et al. [26,27] in relation to circulating fluidized particles within a single vessel. At each chamber, the gasifying agent is provided at different fluidizing velocities and this allows the particles of the bed material to move from one sector to the other, and recirculate around the baffle plate throughout the interconnecting orifice. The circulation of the bed particles is then sustained over time as long as the difference in the gas velocity of the fluidization medium at the two chambers is maintained [30]. A sketch of the gasifier design is presented in Fig. 1. Compared to a conventional bubbling fluidized bed configuration, the gasifier with internal recirculation (ICBFB) can favor the process of gasification in terms of the yield and quality of the product gas (e.g. reduced tar load). Experimental tests and numerical simulations of fluid dynamics made with a cold model have in fact shown that different ratios of fluidization velocity between the up-flowing and down-flowing chambers result in different circulation rates of the bed material between the two chambers. A higher circulation rate can give rise to a deeper and more effective sinking of the biomass particles in the bed material, as well as to a more uniform temperature of the bed itself [31,32]. Therefore, when in operation, the mechanism of internal recirculation is expected to counteract the tendency of the fed biomass to segregate over the surface of the bed. In the same way, also the elutriation of the produced fine carbon particles is reduced. These factors all favor the yield and quality of the product gas by providing a high temperature environment, a higher residence time of the fuel particles under the reaction conditions and therefore an overall improvement of the thermo-chemical reactions involved. Tests of steam/oxygen gasification were preliminarily carried out at a 10 kWth ICBFB bench scale facility and provided very promising results [43]. Thereafter, based on the same concept, a 1000 kWth ICBFB gasification pilot plant was also built. The facility is intended to demonstrate and validate the gasification process at a significant scale, thus collecting data useful for the further steps of scale-up and industrialization. In such perspective, the plant is fully equipped for on-line acquisition and monitoring of key process parameters, such as temperatures, pressures and flow rates. In Fig. 2 a sketch of the whole plant is shown. According to the figure, the plant was designed to maximize the efficiency of the process through integrated energy recovery, and to produce a cleaned gaseous stream ready to be used in CHP production. To this aim, downstream of the gasifier, the plant also consists of sections for heat recovery and gas cleaning where the product gas is treated to pre-heat the gasifying agent and to remove entrained particles and tar contaminants, respectively. For the process under development, the steam/oxygen mixture was selected as the gasifying agent of reference in order to have a product gas with a near-zero nitrogen content. Therefore, in addition to the cogeneration applications, after proper adjustment of the final composition and compression, the gas producible at such plant could also be considered for conversion into gaseous or liquid secondary energy carriers, such as H2, SNG, Fischer–Tropsch biofuels, methanol, and DME. 2.2. Gasification test campaigns As indicated above, the 1000 kWth pilot plant is designed to operate with steam/oxygen as the main gasifying agent, however the use of oxygen-enriched air is also possible. In order to evaluate the flexibility of the system to the gasification medium and acquire data on the gas

Please cite this article as: D. Barisano, et al., Steam/oxygen biomass gasification at pilot scale in an internally circulating bubbling fluidized bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.008

D. Barisano et al. / Fuel Processing Technology xxx (2015) xxx–xxx

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Fig. 1. A) Sketch of the internally circulating bubbling fluidized bed (ICBFB) gasifier; B) operation principle of the internal circulation of the bed inventory (UUFC: gas velocity at the upflowing chamber, UDFC: gas velocity at the down-flowing chamber).

composition producible from this specific configuration, besides tests with steam/oxygen mixture, gasification tests with enriched air were also carried out. In particular, taking into account the need to ensure the condition of bed fluidization and recirculation, oxygen enrichment up to a total content of 35wt.% and 50.% was considered. Table 1 presents a summary of the specific biomass gasification tests carried out and the corresponding operating conditions. In relation to the experimental set-up adopted, it has to be specified that the activity herein presented was also connected to the EU project named UNIfHY [44]. This project is aimed at the development of a new gas cleaning approach by integrating the steps of gas filtration and tar and light hydrocarbons reforming directly into the reactor vessel. As to assess the efficacy of the mentioned approach only

the characterization of the product gas at the exit of the gasifier is relevant, the decision to operate the plant in a simplified configuration by excluding the plant sections not strictly needed for the proper operation of the gasifier was taken. In accordance with this choice, during the gasification test campaigns the plant was operated including only the unit for heat recovery (EX-1), since it is relevant to the superheating of the gasifying agent, while the units for gas cooling (EX-2) and wet scrubbing (SCR-1) were by-passed. From the exit of the EX-1 unit, the product gas was then directly forwarded to the flare. In order to have indications about the yield of gas produced in each gasification test campaign, measurements of gas flow were made at the plant by means of a flow measuring calibrated flange inserted in the gas piping downstream of the heat exchanger EX-1.

Fig. 2. Sketch of the 1000 kWth ICBFB pilot plant.

Please cite this article as: D. Barisano, et al., Steam/oxygen biomass gasification at pilot scale in an internally circulating bubbling fluidized bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.008

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D. Barisano et al. / Fuel Processing Technology xxx (2015) xxx–xxx

Table 1 Gasification operating conditions.

Table 3 General characteristics of the commercial olivine Magnolithe GmbH.

Test

35wt.% O2

50wt.% O2

Steam/O2

Olivine

Biomass feeding rate (kgdry/h) Steam feeding rate (kg/h) O2 feeding rate (kg/h) O2 enrichment (%wt) Steam/biomass ER Gasification T (°C) Pressure (bara)

130 – 42 36 – 0.22 850–880 1.0–1.1

130 – 44 48 – 0.23 850–870 1.0–1.1

125 55 39 100 0.4 0.21 820–830 1.0–1.1

Origin: Density Bulk density Mean diametera Fusion point Thermal expansion Mohs hardness Specific heat

Austria 3400–3500 kg/m3 1900–2050 kg/m3 344 μm 1750°C 1.3% (@ 1100°C) 6.5–7.0 0.95–1.05 kJ/(kg °C)

Composition (wt.%)

2.3. Materials All gasification tests were carried out by using almond shells as biomass feedstock and an unmodified olivine commercially available (Magnolithe GmbH) for the fluidized-bed inventory. A summary of the main physical and chemical characteristics of the two materials is reported in Tables 2 and 3, respectively. In all gasification campaigns, a total amount of 500 kg of olivine was used. Based on the operating conditions adopted for biomass gasification tests (Table 1) and the characteristics of the selected olivine sand (Table 3), the conditions for internal recirculation of the bed material were obtained by supplying the gasification medium at the two reactor chambers, up-flowing and down-flowing, in accordance with the gas velocities (U) summarized in Table 4. In order to overcome the pressure drop exerted by the distributor plate and the bed material, the gasifying agent was supplied with an overpressure in the range 200–300 mbarg. 2.4. Sampling and analysis Several samplings were carried out at the plant in order to characterize the product gas and the process conversion efficiencies. Specifically, after reaching the steady-state conditions, measurements and samplings on the gaseous stream were carried out in order to acquire information on gas yield, dry gas composition, organic and inorganic contaminant loads, and particulate and humidity contents. Finally, in order to have an estimation of the carbon conversion efficiency, an evaluation on the amount of char residue was also carried out at the end of the test. 2.4.1. Gas analysis The dry gas composition was measured by sampling and cleaning the product gas throughout a train of steps constituted by a water condenser, a condensation separator and a final drying step obtained

Table 2 Biomass feedstock. Almond shells Bulk density (kg/m3) 410

Humidity (wt.%, wet basis) 10

Proximate analysis (wt.%, dry basis) Ash 1.2

Fixed carbona 18.2

Volatile matter 80.6

Ultimate analysis (wt.%, dry basis) C 47.9

H 6.3

N 0.32

Ob 44.27

Cl 0.012

Heating values (MJ/kgdry) HHV 19.5 a b c

LHVc 18.0

% FC = 100 − (% VM + % Ash). %O = 100 − (%C + %H + %N + %Cl + %S + %Ash). LHV (kJ/kg) = HHV (kJ/kg) − (24.43 × 8.936 × H(wt.%)) (kJ/kg).

S 0.015

SiO2 MgO Fe2O3 Al2O3 % H2O + % CO2b

41.9 49.5 7.1 1 0.5

Mineralogy (wt.%) Forsterite (Mg2SiO4) Fayalite (Fe2SiO4) a b

94 6

This value is obtained as a weighted arithmetic mean from a sieve analysis. Total percentage of H2O and CO2 included in the natural mineral during its formation.

with silica gel. The analyses were carried out on-line via a gaschromatography system Agilent Technologies (HP 6890 Series) equipped with a TCD detector. For the permanent gas analysis, the GC system was configured with chromatographic columns Molecular Sieve 5A and Poraplot Q, from Restek and Agilent respectively; an argon of 99.9995%v purity degree was used as gas carrier. In the course of the gasification tests, the gas composition was analyzed every 15 min. The identification and quantification of each component were based on retention time and multilevel external calibration.

2.4.2. Tar, water and particulate contents The tar load in the product gas was measured according to the CEN/TS 15439 procedure. In short, the product gas was sampled isokinetically using a glass fiber thimble (86R, Advantec) to collect the entrained particulates and 2-propanol as a tar absorbing solvent. After having sampled the gas for no less than 200 NL (dry basis) the sampling was stopped and the 2-propanol solutions contained in the six impinger bottles were all mixed in a single Erlenmeyer flask. To withdraw the sampled gas stream a Zambelli ZB1 volumetric pump was used. To recover the possible tar condensed on the particulate filter, the thimble was extracted with fresh solvent according to the standard procedure. The thimble was then dried and weighted for gravimetric particulate determination, while the extraction solution was mixed with that in the Erlenmeyer flask and properly treated for gas chromatography mass spectrometry (GCMS) analysis and gravimetric quantification. Chromatographic analyses were carried out with a GCMS system Agilent Technology-Mod. 5975 B equipped with an HP-5MS cross-linked 5% PhMe-siloxane 30 m × 0.25 mm × 0.25 μm film thickness column. Helium 99.9999%v was used as gas carrier. For tar molecule quantification and identification, a 6-level calibration curve and multi standard solutions containing up to 24 molecules were used.

Table 4 Gas velocity and overpressure of the fluidization medium at the gasifier. Test campaign

Umfa (cm/s @ 800 °C)

Udfcb/Umf

Uufcc/Umf

35wt.% O2 50wt.% O2 Steam/O2

6.1 6.0 6.3

2.6 1.9 2.6

10.4 7.3 9.9

a b c

Umf: gas velocity of minimum fluidization. Udfc: gas velocity at the down-flowing reactor chamber. Uufc: gas velocity at the up-flowing reactor chamber.

Please cite this article as: D. Barisano, et al., Steam/oxygen biomass gasification at pilot scale in an internally circulating bubbling fluidized bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.008

D. Barisano et al. / Fuel Processing Technology xxx (2015) xxx–xxx

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The gravimetric tar load was instead determined by complete removal of solvent with a rotary evaporator IKA Labortechnik RV06-LM. Finally, the water content in the wet product gas was measured on amounts of the 2-propanol tar sampling solution by Karl Fischer titration. 2.4.3. Inorganic contaminants: H2S, HCl and NH3 All gaseous contaminants were determined by absorption in a proper water solution. Specifically, H2S and HCl were absorbed in a solution of NaOH 2wt.% while NH3 by using an HCl 2wt.% solution. In each determination, the product gas was sampled, for no less than 200 NL, through a train of three impinger bottles, each containing 150 mL of absorbing solution and kept at 0 °C in a thermostatic bath, and a fourth one at room temperature filled with silica gel to preserve the gas sampling pump from humidity. For gas withdrawal, a volumetric pump Zambelli ZB1 was used. After proper dilution, the alkaline solution was analyzed for H2S and HCl determinations via High-Pressure Ion Chromatography (HPIC) at a Dionex DX 500 chromatographic system equipped with an electrochemical detector ED40. For H2S, the HPIC system was equipped with an Ionpac As7 column and the detector was operated in DC/0 V mode. 16 mM of Na2C2O4/400 mM of NaOH water solution was used as eluent. For HCl, the HPIC system was instead equipped with an Ionpac As12 column and the detector was operated in conductivity mode. 2.7 mM of Na2CO3/0.6 mM of NaHCO3 water solution was used as eluent. In both cases the contaminant quantification was obtained based on a 4-level calibration curve. Concerning the acid solution, ammonia was measured with a S47-K SevenMulti™ dual meter pH/conductivity (Mettler Toledo) equipped with an ion selective electrode DX218-NH4 Ammonium half-cell. The contaminant quantification was based on a 3-level calibration curve. 2.4.4. Char residue The char amount produced at the end of the gasification tests was evaluated by combustion of representative samples of the solid residue collected at the cyclone and from the bed inventory. Moreover, based on elemental analyses carried out at the analytical laboratory of the authors on gasification char residues from several biomass feedstocks, the average empirical formula CH0.64O0.12 was considered and used in the calculation of carbon conversion efficiency. 3. Results and discussion 3.1. Process performances at different gasification conditions Each gasification test campaign was started with the plant at room temperature; therefore, in order to bring the reactor at temperatures adequate for starting the process of biomass gasification, the reactor was preliminarily heated-up to about 600 °C by burning LPG at the reactor burner. The reactor heating was then completed by burning some biomass until the temperature of the reactor bed reached a value in the range 800–850 °C. The whole heating-up stage lasted from 5 to 6 h. After reaching the temperature, the operating conditions were changed from combustion to the specific gasification conditions, with enriched air or steam/oxygen, in accordance with the value reported in Table 1. In Fig. 3 the dry composition of the product gas obtained in the experiment carried out with enriched air, 50wt.% oxygen, is presented. The graph shows, by way of example, the gas trends recorded during the experimental transient stage, when moving from the conditions of feeding air only to those for biomass gasification. The trend of the permanent gases showed that the steady state conditions were achieved within 60 min. After that, the content of each gas component remained rather stable, showing restricted fluctuation around the corresponding average values. Similar trends were also

Fig. 3. Trends in the dry composition during the start-up stage moving from air feeding to biomass gasification with 50wt.% of O2 enriched air.

observed in the case of gasification carried out using enriched air with 35wt.% oxygen and steam/oxygen. In Table 5 a summary of the main process parameters and dry gas compositions reached at the steady state for the three gasification conditions is presented. By changing from enriched air to steam/oxygen mixture, the data confirmed the expected beneficial effect on the heating value of the product gas. The LHV was found to increase as a clear consequence of the reduction of the N2 content. However, the improvement in the gas quality was not only an effect of the decreased amount of the inert gas, but also a consequence of the gas upgrading due to the addition of steam. According to literature in fact, the use of steam in processes of biomass gasification, pure [45–51] or in combination with air, enriched air or oxygen [52–55] allows to foster the chemical reactions promoted by the H2O reactant, such as water gas shift, char gasification and hydrocarbon reforming, in which the ultimate effect is the enrichment of the product gas in its H2 content. Such effect is shown more clearly in Fig. 4 where a comparison among the compositions of the three product gases is presented by taking into account the mean value of each permanent gas component at the steady state, and calculated on dry and N2-free basis. The graph shows the significant increase of the H2 content achieved in the product gas when the biomass feedstock is gasified with steam/oxygen. The water mass balance further supports the effect of using steam in the gasifying agent on the H2 enrichment of the product gas. Based on the measurement of the water content in the product gas and the total amount of water supplied to the reactor as steam and humidity of the biomass feedstock, an average water conversion of 10–15wt.% can be estimated. Such result appears to be consistent with data of water conversion estimated by other authors. From the data presented by Gil et al. [56] in relation to tests of steam-oxygen biomass Table 5 Summary of the gasification tests results. Test

35%-wt O2

50%-wt O2

Steam/O2

Gas yield (Nm3/kgBiomass, dry) Char residue (g/kgBiomass, dry) Test duration (min)

1.46 n.a. 230

1.31 n.a. 280

1.04 80-100 310

Gas components

(%v, Dry gas)

CO H2 CH4 CO2 Light hydrocarbons N2 O2

18-23 7-9 6-7 17-24 b1 41-45 b1

20-25 11-13 6-9 19-25 b1 30-37 b0,5

28-32 30-33 9-11 22-27 1-2 2-5a b1

H2O (% v, wet gas)

na

na

40-50

LHV (MJ/Nm3)

5.9-6.7

6.3-8.4

10.9-11.7

a Some N2 was provided to the biomass feeding system in order to avoid leakage of syngas throughout the augers.

Please cite this article as: D. Barisano, et al., Steam/oxygen biomass gasification at pilot scale in an internally circulating bubbling fluidized bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.008

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D. Barisano et al. / Fuel Processing Technology xxx (2015) xxx–xxx Table 6 Contaminant contents in the gasification tests with steam/oxygen as gasifying agent.

Fig. 4. Dry and N2-free product gas composition at the three operating conditions.

gasification, a water conversion slightly higher than 10% can be calculated. Hofbauer and Rauch [57] in their study on the effect of steam-fuel ratio on the product gas composition, the tar content and the water conversion, showed that for S/B value from 0.3 to 0.5 water conversion from 20 to 15% was found. Finally, in their overview on variables effecting biomass gasification, Corella et al. [47] summarized that in gasification processes using steam, values of water conversion in the range 7–10% were often reported. The comparison among the gasification tests at the three operating conditions also allows an estimation of the cold gas efficiency (CGE) reached at the three different operating conditions. CGE is defined as the chemical energy of the gas at room temperature divided by the chemical energy of the biomass input.     Product gas flow rate Nm3dry =h  LHVgas MJ=Nm3dry    100   CGE ¼ Biomass feeding rate kgdry =h  LHVBiom MJ=kgdry ð1Þ By using Eq. (1), a CGE increasing from 0.5, when 35wt.% oxygen enriched air is used, up to 0.7, when using steam/oxygen gasification condition, is thus calculated. According to the stated aim of the work, being the ultimate scope of the experimental activity at the 1000 kWth pilot plant the development of a steam/oxygen gasification process, to characterize the performance of the gasifier, char residues from the bottom of the reactor were collected at the end of the run and characterized with respect to the carbon content. From the data reported in Table 4, taking into account the carbon content in the biomass feedstock and in the char residue, accounting to 48.5wt.% (daf) and up to 85.wt.% respectively, for the steam/oxygen gasification process a carbon conversion up to 86wt.% was estimated. In previous tests of steam/oxygen biomass gasification carried out at a 10 kWth ICBFB bench scale facility, a CGE of about 75% and a char residue of 40–50 g/kgBiomass were estimated [43]. Compared to these former results, the performances of the process carried out at pilot scale appear to be somehow lower than expected. However, taking into account that the results herein presented are referred to preliminary gasification campaigns, the authors believe that the estimated figures can be considered in good agreement with those obtained at bench scale and well promising toward the ultimate values, expected to be achieved at the optimized final conditions. 3.2. Gas contaminants In order to fully characterize the process under development, the product gas from the tests carried out with steam/oxygen gasifying mixture was also characterized with respect to the different contaminant

Contaminants

Steam/O2

Particles (g/Nm3dry) H2S (ppmv) HCl (ppmv) NH3 (ppmv) Gravimetric tar (g/Nm3dry) Tot chromatographic tar (g/Nm3dry)

6–10 40–50 40–50 70–80 7–10 12–18

loads. A summary about the range values observed for each of the main gas contaminants is presented in Table 6. The measurements indicated that in the raw gas the most abundant contamination were particles and tar, while the inorganic gaseous contaminants (i.e. HCl, H2S and NH3) were relatively limited being each at a concentration lower than 100 ppmv. The particle content was found to be in the range 6–10 g/Nm3 and was mainly due to entrainment of fine particles from the fluidized-bed inventory. Confirmation on that was provided by the proximate analysis carried out on the residue collected at the cyclone, at the end of the gasification run. Such residue in fact presented an inorganic content higher than 90wt.% Concerning tar, the GCMS analysis indicated the presence of many aromatic compounds, among which the most abundant were those with low molecular weight, such as benzene, toluene, phenol, indene and naphthalene. In Fig. 5 the chromatographic distribution of the main tar components identified and quantified is presented. The figure shows as the variety of molecules ranges over from aromatic compounds with single ring to polycyclic hydrocarbons with up to four condensed rings. The total amount of the compounds with low molecular weight accounts for 16 g/Nm3dry, corresponding to about 88wt.% of the total. The relative abundance of low molecular tar compounds is also confirmed by the gravimetric determination. In this measurement, in fact the tar content is found to be in the range 7–10 g/Nm3dry, thus lower than the total content quantified by GC–MS chromatography. The two results, gravimetric and chromatographic, are indeed consistent because in the procedure of reference (i.e. CEN/TS 15439), the gravimetric value is obtained by evaporating the solvent via heating and reduced pressure. Clearly, during such operation, compounds with high vapor pressure, such as benzene, toluene, naphthalene and phenol, to mention a few, are partially removed together with the solvent. Though analytically accurate, the results about tar were actually not completely expected, in terms of both total amount and distribution of tar molecules. In tests carried out at the 10 kWth ICBFB facility in fact, at comparable operating conditions and using same feedstock and olivine as bed material, the total chromatographic tar amount was found to be roughly 10 g/Nm3dry and naphthalene the major tar molecule, with a content amounting to 50wt.% of the total. In the case of steam/oxygen gasification carried out at the 1000 kWth pilot plant, on average naphthalene was only about 1.2 g/Nm3dry, while the most abundant molecules resulted to be single ring aromatic compounds. Benzene, toluene and phenol were found to reach concentrations of 3.8, 4.2 and 5.0 g/Nm3dry, respectively. Hence, the relative abundances for tar compounds appeared not very representative of a gasification process carried out at 820–830 °C; the distribution found seemed to be rather more distinctive of a process of lower temperature. The effect of temperature on content and distribution of tars is a well-known and consolidated correlation. In a process of biomass gasification, among other process variables, the tar product distribution is deeply influenced by temperature. On this latter aspect, it is well known that an increase in temperature produces a reduction of the total load in the product gas, as well as a conversion of the so-called primary tar, produced in the first gasification stages, to aromatic hydrocarbons [58–61]. More recently, Van der Meijden [62] reported an

Please cite this article as: D. Barisano, et al., Steam/oxygen biomass gasification at pilot scale in an internally circulating bubbling fluidized bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.008

D. Barisano et al. / Fuel Processing Technology xxx (2015) xxx–xxx

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Fig. 5. Chromatographic tar distribution in the experimental campaign of steam/oxygen biomass gasification.

increase in toluene yield with decreasing temperature in the range 770–880 °C, Wolfesberger et al. [63] studying the effect of temperature on tar distribution in the range 798–850 °C found a significant reduction of phenol and increase in naphthalene. According to the above reported references and results therein presented, the difference observed in tar content between tests carried out at the ICBFB bench scale facility and at the pilot plant can be explained by taking into account the process gasification temperature and the way the biomass feedstock is fed to the reactor. The bench scale and pilot scale gasification test campaigns were in fact carried out at average temperature of the bed inventory very close to each other, 855 °C and 825 °C respectively; on the other hand, it has to be specified that in the two reactors the biomass feeding occurred with a certain difference. At the 10 kWth reactor in fact, the biomass was introduced in the reactor through a vertical feeding piping which brought the biomass feedstock directly in the circulating bed material. In the case of the pilot plant, the biomass was actually fed somewhat above the bed surface. Due to this fact, though the temperature measured inside the bed was in the range 820–830 °C, the biomass most probably started to decompose before being taken up by the recirculating bed material and thus the collected tars were produced in an area of the reactor close to the bed inventory but at a temperature lower than the inside.

4. Conclusions Biomass gasification campaigns were carried out in order to test the performance of a 1000 kWth pilot plant based on a bubbling fluidized bed (BFB) reactor with internal recirculation (ICBFB). Such gasifier was designed to operate with steam/oxygen mixture as a gasifying agent of reference; however, in order to evaluate the process flexibility to other gasifying agents, tests with enriched air were also carried out. Taking into account the need to ensure both the fluidization and the recirculation of the bed, 35wt.% and 50wt.% oxygen enrichments were ultimately adopted. The collected experimental results provided positive answers to the reliability of the plant at the three selected conditions, and in particular about the possibility of producing a suitable product gas also when gasifying with enriched air at the lowest enrichment ratio (35wt.% O2). In relation to the steam/oxygen biomass gasification, though the test campaign was preliminary, the obtained product gas showed a dry composition that makes it of interest for many possible applications. Based on the heating value for instance (i.e. 10.9–11.7 MJ/Nm3dry) the gas appears useful for CHP application in conventional or advanced configurations (i.e. gas turbine and high temperature fuel cell). Meanwhile, thanks to the very low content of nitrogen (i.e. 2–5%v), the product gas also appears of interest as a starting stream that, after proper adjustment of the gas composition, can also be considered for conversion into advanced energy carriers, such as H2, SNG, FT biofuels, MeOH, DME, or chemicals.

However, the samplings carried out to evaluate the contaminant contents indicated that the product gas is not ready for immediate use. Particles and tar were found to be the most abundant, with values in the range 6–10 g/Nm3dry and 12–18 g/Nm3dry, respectively, while H2S, HCl and NH3 were all present at concentrations below the hundred ppms (v). On the other hand, though present at relevant concentrations, none of these contaminants can be considered a significant limitation to a further exploitation of the gas. Depending on the specific application of interest, there are technologies already at state of art that can be considered for proper gas cleaning, as well as also innovative solutions, possibly more effective and efficient. As a whole, the assessment of the results presented in this paper allowed to gain strong evidence about the reliability of the biomass gasification technology under development and on the ultimate goals that it should be possible to achieve at optimized operating conditions. Of course in that sense additional information still need to be collected in order to have a full characterization of the process, as for instance suggested by the results concerning the tar distribution observed in the preliminary test campaign. These results in fact provided indications on the fact that the combination between biomass feeding and internal recirculation of the bed material was not as well-matched as expected. Further experimental campaigns are already under preparation at the time of writing this paper. The next planned gasification tests will be devoted to the identification of the process parameter values at which the steam/oxygen biomass gasification will reach the best performances. On that occasion, higher attention will definitely be dedicated also to the phase of biomass feeding to the bed inventory, in order to gather additional insights on the subject and, if needed, take into account possible improvements of the system. Acknowledgments The activity described in the present work has received funding from the European Union's Seventh Framework Programme (FP7/2007– 2013). The authors gratefully acknowledge the financial support provided by the Fuel Cells and Hydrogen Joint Technology Initiative under the grant agreement no. 299732 (UNIfHY Project). References [1] The 2020 Climate and Energy PackageRetrieved from: http://ec.europa.eu/clima/ policies/package/index_en.htm May 8 2015. [2] Roadmap for Moving to a Low-carbon Economy in 2050Retrieved from: http://ec. europa.eu/clima/policies/roadmap/index_en.htm May 8 2015. [3] K. Srirangan, L. Akawi, M. Moo-Young, C.P. Chou, Towards sustainable production of clean energy carriers from biomass resources, Appl. Energy 100 (2012) 172–186. [4] S. Cornelissen, M. Koper, Y. Y. Deng, The role of bioenergy in a fully sustainable global energy system Biomass and Bioenergy 41 (2012) 21–33. [5] J.S. Lim, Z.A. Manan, S.R.W. Alwi, H. Hashim, A review on utilization of biomass from rice industry as a source of renewable energy, Renew. Sust. Energ. Rev. 16 (2012) 3084–3094. [6] A.V. Bridgwater, Review of fast pyrolysis of biomass and product upgrading, Biomass Bioenergy 38 (2012) 68–94.

Please cite this article as: D. Barisano, et al., Steam/oxygen biomass gasification at pilot scale in an internally circulating bubbling fluidized bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.008

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Please cite this article as: D. Barisano, et al., Steam/oxygen biomass gasification at pilot scale in an internally circulating bubbling fluidized bed reactor, Fuel Processing Technology (2015), http://dx.doi.org/10.1016/j.fuproc.2015.06.008