Renewable Energy 81 (2015) 251e261 Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene An experimental study on catalytic bed materials in a biomass dual fluidised bed gasifier € ransson*, U So €derlind, P Engstrand, W Zhang K Go Department of Chemical Engineering, Mid Sweden University, Sundsvall SE-85170, Sweden a r t i c l e i n f o a b s t r a c t Article history: Received April 2014 Accepted 10 March 2015 Available online April 2015 A study on in-bed material catalytic reforming of tar/CH4 has been performed in the 150 kW allothermal gasifier at Mid Sweden University (MIUN) The major challenge in biomass fluidised-bed gasification to produce high-quality syngas, is the reforming of tars and CH4 The MIUN gasifier has a unique design suitable for in-bed tar/CH4 catalytic reforming and continuously internal regeneration of the reactive bed material This paper evaluates the catalytic effects of olivine and Fe-impregnated olivine (10%wtFe/ olivine Catalyst) with reference to silica sand in the MIUN dual fluidised bed (DFB) gasifier Furthermore, a comparative experimental test is carried out with the same operation condition and bed-materials when the gasifier is operated in the mode of single bubbling fluidised bed (BFB), in order to detect the internal regeneration of the catalytic bed materials in the DFB operation The behaviour of catalytic and non-catalytic bed materials differs when they are used in the DFB and the BFB Fe/olivine and olivine in the BFB mode give lower tar and CH4 content together with higher H2 þ CO concentration, and higher H2/CO ratio, compared to DFB mode It is hard to show a clear advantage of Fe/olivine over olivine regarding tar/CH4 catalytic reforming © 2015 Elsevier Ltd All rights reserved Keywords: Biomass gasification Tar reforming Catalytic bed material Dual fluidised bed Introduction Bio-automotive fuels and chemicals can be produced from highquality syngas (mainly hydrogen and carbon monoxide) [1] Efficient cleaning of raw syngas from biomass gasification is important for commercialization of the technology for applications such as electricity generation and synthetic fuel production The syngas from a typical indirect gasifier contains H2, CO, CO2, CH4, H2O, trace amounts of higher hydrocarbons, possible inert gases from biomass, gasification agent and various contaminants There has been much experience gained from gas cleaning related to engine and turbine applications Product gas for synthesis normally has a much stricter specification of impurities than these applications [2] The major challenge in biomass fluidised-bed gasification to produce high-quality syngas, is the reforming of tars and CH4 (except for methanation application) to a minimum allowable limit Reduction of tars and CH4 to an acceptable low level is usually achieved by high temperature thermal cracking, low temperature catalytic cracking, or physical tar treatment like water scrubbing * Corresponding author € ransson) E-mail address: kristina.goransson@miun.se (K Go http://dx.doi.org/10.1016/j.renene.2015.03.020 0960-1481/© 2015 Elsevier Ltd All rights reserved and sedimentation or oil scrubbing and combustion [2e10] Catalytic cracking efficiency can reach 90e95 % at reaction temperatures about 800e900 C [11], whereas thermal cracking requires temperatures above 1200 C to reach the same efficiency at expense of energy losses and big investments on high temperature materials The catalysts can be used in downstream catalytic reactors [12,13], such as catalytic beds, monoliths and filters, or added directly in the fluidised-bed gasifier as the bed material Use of bed materials as catalyst for tar reduction is simple, reliable and can be reactivated by the combination of combustion and gasification in the dual fluidised bed gasifier (DFBG) The main function of the bed material in a DFBG is to transfer heat from the combustor reactor to supply energy for the endothermic gasification of biomass in the gasification reactor In addition, the bed material makes in-situ gas conditioning possible Reactive bed materials can be applied to improve agglomeration behaviour, to enhance tar cracking and to increase H2 content, and perform catalytic activity, CO2 capture, oxygen transportation, etc The use of catalytically active bed materials promote char gasification, water-gas-shift (WGS) and steam reforming reactions, which enhance tar/CH4 reforming and increase the H2 content in 252 €ransson et al / Renewable Energy 81 (2015) 251e261 K Go the syngas In the same time, the bed material behaviour can be improved with respect to a reduced risk of bed agglomeration [2] One potential catalytic bed-material is olivine ((Mg, Fe)2 SiO4), a natural mineral containing magnesium, iron oxide and silica The oxygen transport capacity of olivine can be 0.5wt% [14] Hence, the produced gas in the gasifier will be partially oxidized by olivine as an oxygen carrier in DFB operation Reduction of bed material in the steam gasifier with the following oxidation in the air combustor achieves a catalyst recovery cycle, similar to the chemical looping combustion (CLC) [15] Catalytic activity of olivine in cracking and reforming of tars and enhanced steam and dry reforming of hydrocarbons are reported in a number of articles [15e20] The catalytic activity of iron species is considered to be related to their oxidation state Some researchers suggest that the efficiency of olivine in tar cracking relies on free iron (III) oxides present at the surface, while others have the opposite opinion Nordgreen et al [21] studied the decomposition of tars on metallic iron and iron oxides in the temperature range of 700e900 C In this study, only iron in the metallic state showed considerable activity for tar decomposition At 900 C the tar decomposition activity was similar to calcined dolomite [21] Metallic iron is known to be an active species for aromatic hydrocarbon decomposition and iron oxides are known to be a good catalyst for the WGS reaction [22] Compounds of iron and oxygen occurring in nature, include Fe1ÀxO (wustite), a-Fe2O3 (haematite), g-Fe2O3 (maghemite), and Fe3O4 (magnetite) The ideal and stoichiometric FeO consists of Fe2þ -ions, the a-Fe2O3 and g-Fe2O3 of Fe3þ -ions, and the Fe3O4 of Fe2þ and Fe3þ -ions [23] In the air combustor, the olivine decomposes to binary iron oxide, silica oxide and magnesium silicate, reaction (1): (Fe0.1, Mg0.9)2 SiO4 þ 0.05O2 / 0.1Fe2O3 þ 0.1SiO2 þ 0.9Mg2SiO4(1) The binary iron oxide diffuses to the surface of the bed material Fe2O3 enters the steam gasifier, where it reacts with hydrocarbons and is reduced to FeO, reaction (2): 5Fe2O3 þ 2CxHy / 10FeO þ 2xCO2 þ yH2O (2) Reduced iron oxide (FeO) is transported back to the air combustor where it reacts with air and is oxidized to Fe2O3 [15], reaction (3): 2FeO þ 0.5O2 / Fe2O3 (3) The catalytic bed material can be pre-treated by calcination [17] to increase the free iron (III) concentration on the olivine surface for better catalytic activity Besides, olivine is a very flexible structure that can be a host for transition metal [24] A better conversion can be achieved by the use of modified olivine, such as Ni-supported olivine or Fe-supported olivine Ni-supported olivine is highly effective in reduction of tars and CH4 [25], but an important drawback is the toxicity of nickel and the volatiles particles that occurs in FB gasifiers Fe/olivine, however, is a relatively harmless and cheap catalyst [26] Many investigations, e.g the research project UNIQUE [27], have shown that Fe/olivine is efficient in tar reforming and also active in CH4 steam reforming [22,24,28e30] Biomass ash can be treated as a catalyst which may significantly improve the performance of biomass gasification In the ash, the calcium-rich compounds interact with the bed material, and build calcium-rich layers around the particles The catalytic effect could be dominated by the calcium-rich layer [18,31] The catalyst can be deactivated due to carbon deposition, chloride, sulphur poisoning, oxidation, and sintering However, the lifetime of the catalyst can be prolonged by the oxygen balance in a DFBG [13,25] This can be seen as continuously internal regeneration of the catalytic bed-material, where the carbon deposit is burned away CH4 is the most recalcitrant hydrocarbon to reform The steam reforming of CH4 consists of three reversible reactions: the strongly endothermic reforming reactions (4) and (6) and the moderately exothermic WGS reaction (5) It is found that the WGS reaction is very fast at reforming conditions, and hence, the WGS equilibrium is always established during steam reforming [32] CH4 þ H2 O4CO þ 3H2 CO þ H2 O4CO2 þ H2 DH298 ¼ þ206 kJ=mol DH298 ¼ À41 kJ=mol CH4 þ 2H2 O4CO2 þ 4H2 DH298 ¼ þ165 kJ=mol (4) (5) (6) Steam reforming is favoured by high temperature and low pressure; in contrast the exothermic shift reaction is favoured by low temperature, while unaffected by changes in pressure The amount of steam will enhance the CH4 conversion A 150 kW DFBG was built at Mid Sweden University (MIUN) in 2007 [33] which has a unique design suitable for in-bed tar/CH4 catalytic reforming and continuously internal regeneration of the reactive bed material This paper evaluates the catalytic effects of calcined olivine and Fe-doped olivine (10%wt Fe/olivine Catalyst) with reference to non-catalytic silica sand in the MIUN gasifier when it is operated in the mode of dual fluidised beds (DFB) Furthermore, a comparative experimental test is carried out with the same operation condition and bed-materials when the gasifier is operated in the mode of single bubbling fluidised bed (BFB), in order to detect the internal regeneration of the bed materials in the DFB operation The measurement results have been evaluated by comparing the syngas composition and tar/CH4 content in the syngas from the gasifier operated in the two modes under different operation conditions Experimental 2.1 Gasification test in the MIUN gasifier The MIUN gasifier is a DFBG (see Fig 1) and consists of an endothermic steam BFB gasifier and an exothermal circulating fluidised bed (CFB) riser combustor, and has the biomass treatment capacity of 150 kWth, i.e approx 25 kg biomass feed per hour The heat carrier between the reactors is the bed-material The biomass is fed into the dense bed in the gasifier The fluidisation agent in the gasifier is steam and the syngas exits from the top of the gasifier The residual biomass char is then transferred by bed-material into the combustor through the lower pressure lock In the combustor, the fluidisation agent is air, which results in an oxidation of the char that produces heat at the temperature of 950e1050 C The hot bed-material separates from the flue gas in the particle separator to be recycled into the gasifier through the upper pressure lock, which prevents gas leakage between the separate environments in the gasifier and the combustor The gasifier is supported by electrical heaters and is heavily insulated The electrical heaters allow separate operation of the steam gasifier as a BFB gasifier At BFB operation, the interconnections between the gasifier and the riser are blocked, and hence are the vessels divided The only heat source for BFB operation is the electrical heaters A large part of heat for DFB gasification comes from electricity energy as well The gasifier and the combustor have a height of 2.5 and 3.1 m, and inner diameters (i.d.) of 300 and 90 mm, respectively The MIUN gasifier has been described in detail in a previous article [33] €ransson et al / Renewable Energy 81 (2015) 251e261 K Go The tar content in raw untreated syngas from the MIUN gasifier with silica sand is approx 20 g/Nm3 or more The concentration of CH4 is about 10% corresponding to one third of syngas energy, which cannot join the downstream synthesis reaction for liquid fuels The content of tars and CH4 in the syngas from the MIUN gasifier needs to be reduced to an acceptable low level Hence, internal tar/CH4 reforming with catalytic bed materials needs to be investigated in detail These tests are carried out in both BFB mode and DFB mode to compare the bed materials under different gasification conditions The tar/CH4 reforming test runs in three cases: 1) basic condition with silica sand (no catalytic activity), 2) calcined olivine, 3) Fe-impregnated olivine (10%wtFe/olivine Catalyst), at the temperatures of 750, 800, 850 and 900 C, and at the steam-to-carbon (S/C) ratios of 0.6, 1.2 and 1.8 in weight (kg/kg) The S/C ratio is calculated according to equation (7) S=C ¼ m_ steam þ nH2 O Â m_ biomass nC Â m_ biomass 253 (7) where m_ steam represents the mass flow of steam (kg/s) m_ biomass represents the flow of biomass (kg/s) nC represents the carbon mass fraction in the biomass nH2 O represents the water mass fraction in the biomass Silica sand is the reference case for comparing the activity of the catalytic bed materials The biomass feedstock is wood pellets (see Table 1) In the experiment, the gasifier is fluidised with steam and the riser with air at atmospheric pressure Default, air or argon is used for fluidisation of the upper pressure lock, and for aeration of the Fig The 150 kWth MIUN biomass DFBG gasifier 850 1.8 10 0.30 1.7e2.2 The space time of total gas in gasifier bed (bed height 0.50e0.65 m) a Fe/Ol 850 0.6 14 0.29 1.7e2.3 900 1.2 10 0.26 1.9e2.5 Fe/Ol Fe/Ol 850 1.2 10 0.25 2.0e2.6 800 1.2 10 0.24 2.1e2.7 Fe/Ol Fe/Ol 750 1.2 10 0.23 2.2e2.9 850 1.8 10 0.30 1.7e2.2 Olivine Olivine 850 0.6 14 0.29 1.7e2.3 900 1.2 10 0.26 1.9e2.5 Olivine Olivine 850 1.2 10 0.25 2.0e2.6 800 1.2 10 0.24 2.1e2.7 Olivine Olivine 750 1.2 10 0.23 2.2e2.9 850 1.8 10 0.30 1.7e2.2 Sand Sand 850 0.6 14 0.29 1.7e2.3 900 1.2 10 0.26 1.9e2.5 26 25 24 23 22 21 20 750 1.2 10 0.23 2.2e2.9 Steam at bed temperature 750e900 C Olivine 223 3300 0.03 2.8 7e10 Sand a Silica sand 200 2650 0.02 2.2 11e16 Temperature ( C) S/C (kg/kg-) Biomass feed rate (kg/h) Steam flow rate (kg/h) Gasifier Superficial Velocity (m/s) The space time in bed (s)a Material Mean particle size (mm) Density (kg/m3) Minimum fluidisation velocitya Umf (m/s) Terminal velocity Ut (m/s) Gasifier superficial velocity (U/Umf) Bed Material Table Bed material (Fe/olivine is here considered to have equal properties as olivine) 19 The main syngas stream from the gasifier is led to an incinerator for complete combustion A slip stream of the syngas passes Test BFB Test DFB 2.3 Analysis of gas composition and tars Table Gasifier operation conditions for BFB and DFB mode, atmospheric pressure Two catalytic bed materials are used in this test, olivine and 10% wt Fe/olivine A sufficient high content of free iron oxide in olivine requires a high calcination temperature and a long calcination time Calcination at temperature below 900 C causes reduction of the surface iron oxide at low temperature, which is not convenient for steam reforming of hydrocarbons due to sintering and carbon deposition [22] A very high temperature to improve iron reduction is unnecessary and not gainful The olivine in these tests is calcined inside the DFB reactor at 900 C for 10 h, with air at slightly elevated pressure The 10 wt%Feolivine catalyst is synthesized by impregnation of an aqueous iron nitrate solution The iron nitrate is received as an aqueous solution (9.3e9.7 wt% Fe; 40.3e42.0 wt% Fe(NO3)39H2O) The natural olivine ((Mg,Fe)2SiO4) is added (1.0 kg olivine to 1.1 kg aqueous iron nitrate solution) and stirred vigorously in the aqueous solution of iron nitrate (for approx 15 min) The next step is solvent evaporation and drying in a vertically revolving kiln with a propane flare as heat source (for approx h), until all liquid is evaporated and the olivine particles are rust-coloured In the rotating vessel, the bed material has a steep temperature gradient; the estimated average temperature for the Fe/olivine is 250e300 C Finally the Fe/olivine is calcined at 900 C for 10 h as the natural olivine described above 27 2.2 The catalytic bed materials Sand 16 34 15 33 14 32 10 28 11 29 12 30 13 31 lower pressure lock The bed material and operation conditions for the tests are described in Tables and Each test started after stabilization of the gasification temperature, which ran for approximately h The same batch of each bed material (silica sand, olivine or Fe/olivine) was used during the whole test series with each bed material This means that the biomass ash may accumulate over tests for each bed material The gas and tar sampling were carried out when the gasifier has reached steady state condition 850 1.2 10 0.25 2.0e2.6 42.7 6.5% 98.1% 667 kg/m3 18.849 MJ/kg 20.159 MJ/kg Sand Oxygen O (calc.) Moisture Durability of pellets Bulk density Net calorific value as rec Net calorific value db 800 1.2 10 0.24 2.1e2.7 0.4