Modified dolomite in biomass gasification with simultaneous tar reformation and CO 2. capture: effect of metal loading

Transcription

1 IOP Conference Series: Materials Science and Engineering Modified dolomite in biomass gasification with simultaneous tar reformation and CO capture: effect of metal loading To cite this article: L Di Felice et al 0 IOP Conf. Ser.: Mater. Sci. Eng View the article online for updates and enhancements. Related content - Plasma Treatments and Biomass Gasification J Luche, Q Falcoz, T Bastien et al. - Air plasma gasification of RDF as a prospective method for reduction of carbon dioxide emission A N Bratsev, I I Kumkova, V A Kuznetsov et al. - Hydrogen Production by Low-temperature Steam Reforming of Bio-oil over Ni/HZSM- 5 Catalyst Song-bai Qiu, Lu Gong, Lu Liu et al. Recent citations - Gasification of Woody Biomass Jianjun Dai et al This content was downloaded from IP address on /0/08 at 09:4

2 Modified dolomite in biomass gasification with simultaneous tar reformation and CO capture: effect of metal loading L Di Felice,, C Courson, P U Foscolo, A Kiennemann Chemical Engineering Department, University of L Aquila, Via Campo di Pile - Zona industriale di Pile, 6700 L'Aquila Laboratoire des Matériaux, Surface et Procédés pour la Catalyse, ECPM, UMR755, 5 rue Becquerel, 67087, Strasbourg Cedex, France difeluca@gmail.com Abstract. The CO absorption capacity of calcined dolomite [a (CaMg)O solid solution] doped with iron and nickel transition metals has been investigated in a fixed bed reactor operating at temperature conditions (650 C) at which the sorption process is thermodynamically favoured at ambient pressure. The presence of metals (catalytic sites) between CaO grains (CO absorption sites) may improve the potential of dolomite for the simultaneous process of catalytic tar reforming and CO capture in biomass gasification, with the aim of developing an effective combined catalyst and sorbent particle. It was found that iron and nickel may be optimised in the substrate reducing critical limitations on CO capture capacity. A Sorption Enhanced Reforming test is proposed, at 650 C, for both iron and nickel doped calcined dolomite, using toluene as model tar compound: iron has been found to be not active in such conditions, whereas the 4% Ni/(CaMg)O has been indicated as the most suitable combined catalyst and sorbent particle.. Introduction The global emissions of anthropogenic CO and other greenhouse gases (GHG) have increased significantly in the last years enhancing the natural greenhouse effect and producing the so-called global warming [,]; fossil fuel power plants contribute approximately 40% of total CO emissions into the atmosphere [3]. For these reasons, a wide range of technologies for carbon-emission-free energy production are currently under development. Recently, CO capture and storage (CCS) has attracted significant research efforts, deemed as a promising means to reduce CO emissions. One of the solutions is to store the captured CO in secure geological locations, although its use in a new CO - based chemistry can also be considered. The main problem of this solution relies on the technology employed to capture CO : the major barrier is that capture of CO is very expensive, which makes up of 75% of the overall CCS cost [4]. Currently the most adopted approach in use commercially, albeit on a very small scale, is the use of solvents like aqueous methanolamine, but some evident drawback is to be considered: the corrosive nature of the solvent, amine loss, the formation of solvent byproducts (salts), the low operational temperature (less than 40 C) that prevent the use of such a technology for CO removal from hot gas streams. By contrast, membranes and solid sorbents based processes offer the potential of higher temperature capture. Published under licence by Ltd

3 Processes based on sorption are mainly of two types, according to the method for adsorbent regeneration: by changing the pressure or the temperature. Among them, pressure, vacuum, temperature and electrothermal swing adsorption (PSA, VSA, TSA and ESA, respectively) are considered as promising methods. Carbon dioxide capture can be incorporated within a gasification process using three different techniques: pre-combustion, post-combustion [5] and oxyfuel combustion decarbonization [6]. The socalled precombustion capture approaches offer a number of advantages over the end of stack postcombustion capture. The primary advantage is the significantly higher available carbon dioxide partial pressure, that implies lower volume of gases and facilities. Moreover, from syngas several interesting synthetic substances can be produced [the so-called Integrated Gasification Combined Cycle (IGCC) process], but a specific gas composition (e.g. hydrogen to carbon monoxide ratio) is often required, as reported in Table ; pre-combustion CO capture offer an innovative way for reaching the high stoichiometric H /CO ratio involved among these polygeneration applications. Table. Synthetic substances produced by syngas. H /CO Synthesis Reactions F-T CO+H -CH - +H O Methanol CO+H CH 3 OH DME.5 CO+CO +5H CH 3 OCH 3 +H O Methanation 3 CO+3H CH 4 +H O In order to prevent the cooling and reheating steps of pre-combustion CO capture required if liquid adsorbents are used, which increase the capital cost and reduce the efficiency of the power cycle [7], an interesting opportunity is to integrate pre-combustion CO sorption by solid sorbents within the gasifier or the water-gas shift reactor (Sorption Enhanced Reforming or WGS) [8]. At low temperatures where CO sorption is allowed ( C, depending on sorbent used), some limitations can derive from thermodynamics (for the endothermic reforming reactions) or kinetics (for the exothermic water-gas shift); however, by capturing CO at this step, the concentration driving force involved in reforming and WGS reactions will increase, and an increase of reaction rate is in turn expected. Recently, in the literature new developments have been reported to run a gasification process including CO capture[9]. It has been proposed to add a CO sorbent (a natural mineral substance, such as limestone or dolomite) to the FICFB (Fast Internally Circulating Fluidised Bed[0]) reactor bed inventory. The sorbent circulates between a gasifier - CO capture bubbling bed, and a combustor calciner riser, in order to run the whole process continuously. In the reactor chamber devoted to biomass gasification and CO capture, the endothermic gasification and the exothermic solid carbonation processes combine well together and their coupling reduces the amount of the solid circulation rate required to sustain thermally the devolatilization and gasification reactions. On the other hand, the riser provides the calcined solid sorbent and the thermal loading, by combustion of residual char (and/or additional fuel). When this is performed utilizing pure oxygen, a CO stream is easily obtained (by steam condensation), available for storage and sequestration. The final goal of our research project is to optimise the granular, mineral solid material for a dual bed system with a CO sorbent performing a calcination carbonation loop. The present contribution is only the first part of a more complex study including the cyclic behaviour on CO capture of adopted materials. Iron and nickel dosed on dolomite, a well known high temperature CO absorbent [,], are proposed and characterized, by examining a wide range of sorbent formulations calcined dolomite doped with varying amounts of nickel (4-0% by weight) and iron (from to 0% by weight) and so providing a comprehensive picture of CO capture properties of this natural mineral substance

4 modified with transition metal oxides. The role of metal consists in improving the catalytic activity of dolomite for reforming and water-gas shift reactions in order it to perform the double function of catalyst as well as CO sorbent. A Sorption Enhanced Reforming test is therefore proposed, using a single solid phase combined catalyst and sorbent, 4% Ni/(CaMg)O, by processing toluene as model tar compound.. Experimental.. Catalysts preparation The catalytic combinations used in this work were as follows: pre-calcined dolomite (CaMg)O impregnated by % by weight of iron; pre-calcined dolomite (CaMg)O impregnated by 4-0% by weight of nickel. A natural dolomite [(CaMg)CO 3 ] (see [3] for chemical and physical properties) was used as substrate, pre-calcined at 900 C for 4h at a 3 C/min heating rate to give (CaMg)O. An iron (3+) nitrate and nickel (+) nitrate salts (Acros Organics) was used as metal precursors. The preparation procedure has been previously described [3,4] and may be summarized as follow: the salt precursor (iron or nickel nitrate) is dissolved in water to which the substrate is then added and stirred to obtain a suspension. The water is evaporated off at 0 C, and the solid recovered, dried (0 C, 5 h), and crushed (80 < dp < 300 µm). The material is then thermally treated in air at 900 C for 4 h after being raised to that temperature at a heating rate of 3 C/min... CO capture and Sorption Enhanced Reforming (SER) tests The runs for testing the prepared sorbents for plain CO capture and Sorption Enhanced Reforming (SER) tests were carried out in a fixed bed reactor under atmospheric pressure (Figure ). A quartz reactor (8 mm I.D.), labelled in Figure, was charged with a quantity ( mg) of catalyst (80 < d p < 300µm), inserted between two plugs of quartz wool, and placed in the centre of an electric furnace (labelled in Figure ). The instantaneous catalyst bed temperature was monitored by a thermocouple located near to the catalyst bed. The flow rate of the gaseous compounds was controlled by mass flow meters (5); nitrogen was always added to the reactant gases at a fixed and known molar flow rate so as to provide a reference flow for gas-chromatographic calibration and hence the evaluation of absolute yields of the reaction products. The outlet gas passing through a condensation unit (3) was analyzed using two gas-chromatograph columns equipped with TCD: the first, packed with a 5Å molecular sieve, for H, N, CH 4, CO; and the second, a Hayesep Q column, for Ar, CH 4 and CO. Out heated lines Evaporator 4 Water Line Line 5 N Ar Model tar compound CO Catalytic bed 3 Ar N, H, CH 4, CO Gas-chromatographs Ar, CH 4, CO Figure. Schematic diagram of the experimental apparatus (fixed bed reactor):, quartz reactor;, furnace; 3, condenser; 4, motorised syringes for feeding liquids; 5, mass flows controllers. 3

5 The feeding system consisted of two main lines (Lines and in Figure ). Line, feeding directly to the reactor, was used to maintain the catalyst bed in a neutral (argon) atmosphere during heattreatment, and also for purging the gas-chromatograph units downstream of the reactor. Line (Ar and N mixture) provided the carrier gas for the steam and tar model compounds during SER test; it connects with the evaporation chamber where water and model tar compounds are introduced as liquids by motorised syringes (labelled 4 in Figure ). The temperature of the evaporation chamber and all feeding tubes was kept above the dew point of toluene (50 C) to prevent condensation. For the CO capture tests a CO mass flow meter was connected to Line. The temperature was first raised to 850 C at 0 C/min and the flue gas monitored to ensure complete calcination of the bed material (pre-calcined dolomite may absorb some CO from the atmosphere). The temperature was then adjusted to 650 C, and the test was started (experimental conditions given in Table ). For the Sorption Enhanced Reforming tests the nickel based catalyst was first activated by carrying out a conventional toluene steam reforming test at 850 C for about 30 minutes to reduce and activate the metal phase. Then the gas inlet was switched to inert flow (from Line to Line ) and the temperature reduced to 650 C; the Sorption Enhanced Reforming test was then started by switching back the inlet gas from Line to Line. The CO capture tests were carried out with raw calcined dolomite, -5-0% Fe/(CaMg)O, 4-0% Ni/(CaMg)O. Sorption Enhanced Reforming tests were carried out with 4%Ni/(CaMg)O. Iron was found to be inactive in toluene steam reforming at the low temperatures at which CO capture is thermodynamically favoured at ambient pressure (650 C in our tests). Table. Feeding composition and test conditions for CO capture and Sorption Enhanced Reforming (SER) tests. CO absorption SER Ar (NL/h).0.0 H O (NL/h of steam) N (NL/h) Toluene (NL/h vapour) CO (NL/h) Reactor temperature ( C) Particle size (µm) Bed mass (mg) Results and Discussion 3..Catalysts characterization The 4%Ni/(CaMg)O and 0%Fe/[CaO, MgO, (CaMg)O] were previously characterized by X-ray diffraction (XRD) and Mössbauer spectroscopy [3,4] and the different phases present are summarized in Table 3. For all quantities of metals added to calcined dolomite in this work (-0% by weight), no modifications result in the detected phases (by XRD), therefore enabling these results to be generalized. 4

6 Table 3. Interactions of iron and nickel transition metals with CaO, MgO and (CaMg)O. Metal Substrate MgO CaO (CaMg)O Iron (III) Magnesioferrite Brownmillerite-like Brownmillerite-like MgFe O 4 Ca Fe O 5 Ca Fe O 5 solid solution Brownmillerite-like Brownmillerite-like Iron (II) (Fe,Mg)O Ca Fe O 5 Ca Fe O 5 + Fe 3 O 4 -Fe (3-x) Mg x O 4 Nickel (II) - NiO, CaO (Ni,Mg)O solid solution 3.. CO sorption tests The equilibrium equation for the reaction between CO and CaO, at ambient pressure, is expressed by Equation () [5,6]: = P CO ( ) exp eq atm () T ( K) where P CO eq (atm) is the equilibrium CO pressure expressed in atmospheres, and T(K) the system temperature expressed in K. This equation predicts the value of 0.96% for CO composition exiting from the reactor at ambient pressure when absorption takes place without kinetic limitations at 650 C. Figure shows CO absorption curves for calcined dolomite. The knowledge of the CO feeding rate, the bed overall sorption capacity and the corresponding thermodynamic equilibrium concentration under test conditions results in an estimate of the breakthrough time (i.e., the time needed for sorbent saturation) of about 46 minutes (marked with a dotted vertical line), in good agreement with the experimental curve of Figure % CO Time (min) thermodynamic value Figure. CO volume % composition in the reactor exit stream, as a function of time, for CO capture tests using calcined dolomite ( ). 5

7 The MgO in the calcined dolomite does not react with CO at the test operating temperature, thereby providing a continuous free passage for CO diffusion, which renders virtually the entire active particle accessible, provided that its size is reasonably limited [7]. The conversion of calcined dolomite and CaO may be defined by Eq. (), where C t represents the CO moles absorbed at any time instant, and C max the theoretical value for 00% CaO conversion: Ct X CaO = () resulting in a final value of C max 3... Metal/(CaMg)O: CO sorption The attachment of metal oxides to the surface of calcined dolomite was investigated in order to check the presence of additional gas-solid kinetic limitations and/or sorption capacity decay. It has been shown that iron interacts mainly with CaO, as Ca Fe O 5, and nickel with MgO, as NiO-MgO solid solution. Therefore, a general behaviour of Metal/(CaMg)O sorbents can be described as schematized in Figure 3, when the metal interacts either with MgO or CaO. Ca Fe O 5 Fe/(CaMg)O Fe/dol Ni/(CaMg)O Ni/dol NiO-MgO solid solution CaO MgO CaO MgO Fe/dol: Fe O 3 -CaO interaction Ni/dol: NiO-MgO interaction O 3 -CaO interaction NiO-MgO interaction Figure 3. Metal/(CaMg)O interaction for Fe/(CaMg)O and Ni/(CaMg)O Fe/(CaMg)O: CO sorption The experimental conditions adopted for CO capture tests are summarized in Table. The effect of iron addition on calcined dolomite carbonation is shown in Figure 4, indicating the conversion of CaO (X CaO ) sites. It is clear that CO absorption capacity decays by increasing the iron content of the sorbent, showing that Ca Fe O 5 partially inhibits CO capture. The release of free Fe O 3, by reaction (R), Ca FeO5 + CO CaCO3 + FeO (R) 3 does not therefore occur, as confirmed by after-test X-ray diffraction analysis of these samples. However, the kinetics of CO capture, i.e. the slope of X CaO curves before reaching the final sorption capacity values, remains quite unaffected by iron loading of the substrate Ni/(CaMg)O: CO sorption A preliminary study of the reactivity of Ni/(CaMg)O was carried out in order to see how the CO sorption capacity and kinetics depended on the amount of metal dosed on calcined dolomite. Table shows experimental conditions and Figure 5 the conversion of CaO (X CaO ) sites. 6

8 0,8 X CaO 0,6 0,4 (CaMg)O % Fe/(CaMg)O 5% Fe/(CaMg)O 0% Fe/(CaMg)O 0, time (min) Figure 4. CaO conversion as a function of time, for CO capture tests using (CaMg)O ( ), % Fe/(CaMg)O ( ), 5% Fe/(CaMg)O ( ) and 0% Fe/(CaMg)O ( ). X CaO 0,8 0,6 0,4 (CaMg)O 4% Ni/(CaMg)O 0% Ni/(CaMg)O 0, Time (min) Figure 5. CaO conversion (X CaO ) as a function of time for CO capture tests using (CaMg)O ( ), 4% Ni/(CaMg)O ( ) and 0% Ni/(CaMg)O ( ). It is clear that nickel has the effect of limiting the gas-solid reaction between CO and the sorbent particles for both sorption capacity and kinetics, even though the metal does not interact directly with CaO. This means that nickel oxide, in solid solution with MgO (the nickel reduction to the metallic form is strongly limited by this kind of interaction) causes some changes in the inert porous matrix of MgO, leading to a partial pore blockage. This result is confirmed by the fact that an increase in the nickel loading of the substrate from 4% to 0% gives rise to progressive inhibition of the CO absorption process so much so that the higher loadings become quite inadequate for the purpose of combined steam reforming and CO capture. 3.3 Toluene steam reforming and CO capture test: Sorption Enhanced Reforming 7

9 Fe/(CaMg)O, as well as raw calcined dolomite, were found to be inactive in toluene steam reforming at the low temperatures at which CO capture is thermodynamically favoured at ambient pressure (650 C in our tests). For this reason, the tests of CO capture suggest the 4%Ni/(CaMg)O combination to be a practical proposition for toluene steam reforming with simultaneous CO capture: the so-called Sorption Enhanced Reforming process. Table sets out the experimental conditions used to test this process in the fixed bed reactor and Figure 6 summarises the experimental results obtained. The kinetic limitations of CO capture due to the NiO-MgO interaction, detected previously in the plain CO capture tests, are again evident if compared with observations reported in the literature [3, 8]: a slight steady increase of CO concentration in the outlet gas, together with a decrease in H production is observed during test up until complete saturation of available sites for CO capture is reached. This state corresponds to about 55% conversion of CaO in the bed; after that the concentrations of CO and H keep quite constant values until the end of the test. The value of saturation is slightly higher than those reported for the plain CO capture test; it may be explained on the basis that part of the nickel is extracted from the NiO-MgO solid solution in the bulk of dolomite due to partial reduction of nickel in the reactive atmosphere. 0,9 0,8 0,7 H Molar Fraction 0,6 0,5 0,4 0,3 0, 0, 0 CO CO CH Time (h) Figure 6. Sorption enhanced toluene steam reforming test: experimental gas concentrations (dry and nitrogen-free basis) as a function of time for the catalyst and sorbent 4% Ni/(CaMg)O. The reforming conversion may be obtained from values of the hydrogen concentration in the gas exiting from the reactor, Eq. 3: X t [ H ] out [ C7H 8 ] in = (3) 8 where water-gas shift reaction is considered complete due to the conversion improvement promoted by CO capture. When CO absorption takes place simultaneously with the reforming reactions, an instantaneous carbon balance in the gaseous phase is no longer possible; however, the hydrogen balance is assumed here to be reliable enough for significant conclusions to be drawn. On this basis, toluene conversion was found to be near total in the first part of the test, afterwards decaying to about 85% following saturation of the dolomite. 8

10 An additional measure of performance for Sorption Enhanced Reforming is the P CO /P CO ratio. In these tests this ratio reduces from a value from.65 at the beginning of the test, when the capture of CO proceeds at the highest rate, to.06 at the end, when CO is no longer being absorbed. 4. Conclusions In this work, effective low cost sorbent materials with catalytic properties, based on iron and nickel oxides in interaction with calcined dolomite for tar reforming and CO capture have been developed. For all combination of metals/(camg)o it was found that lower metal content causes lower kinetic limitations and sorption capacity decay. It has been also demonstrated that with metal-cao interactions (as detected for iron) kinetic limitations are less important than is the case for metal-mgo interactions (as reported for nickel), bringing to light the critical role of free MgO in promoting CO diffusion through the porous sorbent particle. In conclusion, a good compromise between catalytic activity and CO capture properties has been found for nickel doped calcined dolomite, demonstrating the feasibility of Sorption Enhanced Reforming using a combined catalyst and sorbent for example 4% Ni/(CaMg)O. References [] Stern N, in: U.H.M. Treasury (Ed.), Review on the Economics of Climate Change, Cambridge University Press, 006. [] Metz B, Ogunlade, D, Connink H, Loos M, Meyer L 005 Special Report on Carbon Dioxide Capture and Storage. Special Report of the Intergovernmental Panel on Climate Change. [3] Carapellucci R, Milazzo A 003 J. Power Eng [4] Feron PHM, Hendriks CA 005 Oil Gas Sci. Technol. Rev [5] Rao AB, Rubin ES 00 Environmental Science and Technol [6] Buhre BJP, Elliott LK, Sheng CD, Gupta RP, Wall TF 005 Progress in Energy and Combustion Science 3 83 [7] Ciferno J, Chen S, Yang W 008 AIChE Spring National Meeting April 6-0, 008. [8] Cobden PD, van Beurden P, Reijers HTJ, Elzinga GD, Kluiters SCA, Dijkstra JW, Jansen D, van den Brink RW 007 Int. J. Greenhouse Gas Control 70 [9] Marquard-Möllenstedt T, Zuberbuehler U, Specht M, Proceedings of 6th European Biomass Conference and Exhibition From Research to Industry and Markets, -6 June 008, Valencia, Spain, pp [0] Marquard-Möllenstedt T, Sichler P, Specht M, Michel M, Berger R, Hein KRG, Höftberger E, Rauch R, Hofbauer H, Proceedings of nd World Conference on Biomass for Energy, Industry and Climate Protection, 0-4 May 004, Rome, Italy, pp [] Harrison DP 008 Ind Eng Chem Res [] Florin NH, Harris AT 008 Chem Eng Sci [3] Di Felice L, Courson C, Jand N, Gallucci K, Foscolo PU, Kiennemann A 009 Chem Eng J [4] Di Felice L, Courson C, Niznansky D, Foscolo PU, Kiennemann A 00 Energy Fuels [5] Garcia-Labiano F, Abad A, de Diego LF, Gayan P, Adanez J 00 Chem Eng Sci [6] Barin I 989 Weinheim VCH [7] Stendardo S, Di Felice L, Gallucci K, Foscolo PU 00 Chem Eng Sci, in press [8] Johnsen K, Ryu HJ, Grace JR, Limb CJ 006 Chem Eng Sci