b i o m a s s a n d b i o e n e r g y 5 ( ) 2 e2 3 Available online at www.sciencedirect.com http://www.elsevier.com/locate/biombioe Reactivity tests of the wateregas shift reaction on fresh and used fluidized bed materials from industrial DFB biomass gasifiers Stefan Kern*, Christoph Pfeifer, Hermann Hofbauer Vienna University of Technology, Institute of Chemical Engineering, Getreidemarkt 9/166, 1060 Vienna, Austria article info abstract Article history: The dual fluidized bed gasification process, offers various advantages for biomass gasifi- Received 29 May 2012 cation as well as the utilization of other solid feedstocks In order to improve the knowl- Received in revised form edge of the reactions in fluidized bed gasifier, different types of bed material used in the 28 January 2013 gasifier were tested in a micro-reactivity test rig It has been previously observed that Accepted February 2013 during long-term operation, the surface of the bed material used (calcined olivine) un- Available online 26 February 2013 dergoes a modification that improves catalytic activity The main reaction of interest is the wateregas shift reaction Olivine taken from long-term operation at the MW biomass Wateregas shift gasifier at Gu¨ssing (Austria), fresh olivine as a reference, and calcite, which is commonly used for enhancing in-bed catalytic tar reduction, were tested using the micro-reactivity Micro reactor test rig Tests were carried out at temperatures of 800, 850, and 900 C and space veloc- Gasification ities of 40,000 to 50,000 hÀ1 were applied CO conversions of up to 61.5% were achieved for Biomass calcite Used olivine showed a similar behavior, representing a large improvement Olivine compared to fresh olivine, which had CO conversion rates of less than 20% Keywords: ª 2013 Elsevier Ltd All rights reserved Calcite Introduction The worldwide demand for energy is constantly growing and the larger part of this growth is currently covered by oil and gas Developing countries in particular, exhibit high rates of growth, connected to their expanding economies [1] Consequently, finding new, more effective and wide-ranging applications for low grade and cheap biogenic and fossil fuels is essential Gasification of biomass or solid fuels multiplies the field of its conventional application The dual fluidized bed steam gasification system (DFB) is a key technology for the production of a high calorific product as pure steam is used as gasification agent [2,3] The heat for the allothermal gasification process is provided by a separate combustion reactor The DFB process has already proven its reliability at industrial sized plants [4] The utilization of the syngas produced by this process is not limited to heat and power production in a gas engine or a boiler There is also a huge potential for the production of liquid or gaseous fuels from syngas [5] by FischereTropsch synthesis, mixed alcohols synthesis or the production of synthetic natural gas These processes can use syngas made by the gasification of solid feedstock as a source, but each process requires a particular syngas composition, in terms of its H2:CO ratio, to sustain optimal operation However, the composition of the syngas, especially the H2:CO ratio, should be as close as possible to the required ratio to maximize product yield and process performance The possibilities for changing syngas * Corresponding author Tel.: þ43 58801 166382; fax: þ43 58801 16699 E-mail addresses: stefan.kern@tuwien.ac.at, st.kern@aon.at (S Kern) 0961-9534/$ e see front matter ª 2013 Elsevier Ltd All rights reserved http://dx.doi.org/10.1016/j.biombioe.2013.02.001 228 b i o m a s s a n d b i o e n e r g y 5 ( ) 2 e2 3 compositions are limited On the one hand, the process parameters, such as gasification temperature or steam-to-fuel ratio, can only be varied within a limited range, and the net effect on the gas composition is marginal [6] The more effective way is the utilization of a catalytic active bed material in the gasifier, which promotes reactions that force hydrogen production, since the H2:CO ratio is normally too low The main gasification reactions are shown in Table These reactions are considered as equilibrium reactions, with variable states of equilibrium depending on gas concentrations, temperature and pressure However, in a real fluidized bed gasifier a complete state of equilibrium will not be reached In the gasification reactor, these reactions can take place at the same time and location, and some reactions can be forced by operating parameters and by the utilization of catalytic bed material In the pilot and industrial DFB plants, the bed material used is calcined olivine [7,8], since it shows significant tar reduction compared to silica sand [6] and is perceived as a natural catalyst Kirnbauer and Hofbauer [9] reported modification of the olivine during long-term operation in industrial plants as two different calcium-rich layers are built around the olivine particle The inner layer consists mainly of calcium silicates, whereas the outer layer, which is in contact with the gas, char and fuel particles, has a similar composition to the ash from the feedstock in combination with calcium-rich additives like calcite In a previous study [10], used olivine from the MW demonstration plant at Gu¨ssing was employed in the 100 kW pilot plant at the Vienna University of Technology The tar contents measured in the used olivine were five times lower than those of fresh olivine Moreover, a significantly lower energy demand for gasification was observed during utilization of the used olivine particles in the gasifier As also a higher amount of H2 and CO2 was formed while a lower amount of CO was found in the product gas, it leads to the assumption that the enhancement of the slightly exothermic wateregas shift reaction was the main reason for this effect Consequently, the influence of the solids in the gasifier on the wateregas shift reaction has been studied in a microreactivity test rig in this study In the test rig reactor, a stoichiometric mixture of carbon monoxide and steam was passed through a fixed bed of the particles to be tested at temperatures of 800, 850 and 900 C These temperatures are comparatively high [12], as the wateregas shift reaction is favored at low temperatures (Figs and 2), but gasification conditions are usually set in this high temperature range, especially for fluidized bed gasifiers In wateregas shift reactors the common commercial catalysts are iron oxide based and copper based [13], whereas in gasifiers a natural, inexpensive bed material is favored The thermodynamic equilibrium composition of the wateregas shift reaction is plotted in Fig The CO conversion for equilibrium conditions and stoichiometric input of CO and H2O is shown in Fig Thermodynamic equilibrium of a reaction is reached when Gibb’s energy function (G) achieves a minimum G¼ XK n $DG0i i¼1 i XK XK þ RT i¼1 ni $ln yi þ RT i¼1 ni $ln P is the Gibb’s free energy for species i at In Equation (1), standard conditions, ni is the number of moles of species i, yi is the mole fraction of species i, R is the universal gas constant, T is the absolute temperature and P is the pressure Moe [15] indicated the equilibrium for the wateregas shift reaction with a suitable approach, as shown in Equation (2) 4577:8 À 4:33 Keq ¼ exp T Wateregas (i) Wateregas (ii) Boudouard Methanation Oxidation (i) Oxidation (ii) Wateregas shift Methane reforming Chemical equation DHR,850 (kJ molÀ1) Equation C þ H2O CO þ H2 C þ 2H2O CO2 þ 2H2 C þ CO2 2CO C þ 2H2 CH4 C þ O2 CO2 C þ 0.5O2 CO CO þ H2O CO2 þ H2 þ135.7 þ102.1 þ169.4 À89.8 À394.9 À112.7 À33.6 (1) (2) (3) (4) (5) (6) (7) CH4 þ H2O CO þ 3H2 þ225.5 (8) (2) The tests documented in this paper give an insight into the capability of inorganic bed material particles, fresh olivine, used olivine and calcite, in the dual fluidized bed steam gasifier for the promotion of the wateregas shift reaction and will be the basis of further research concerning gasesolid contact in dual fluidized bed gasifiers Materials and methods 2.1 Micro-reactivity test rig The flow sheet of the micro-reactivity test rig used in these investigations is shown in Fig The heart of the testing rig is a glass reactor where the catalyst can be placed, in this case the different types of olivine or the calcite The reactor has an inner diameter of 10 mm and is electrically heated Thus, the operating temperature of the reactor can be controlled, up to 900 C Gas mixtures from up to six different sources can be Table e Equilibrium reactions in biomass gasification [11] Name of reaction (1) G0i Fig e Thermodynamic equilibrium composition of the wateregas shift reaction [14] b i o m a s s a n d b i o e n e r g y 5 ( ) 2 e2 3 2.3 Fig e CO conversion for equilibrium conditions [14] dosed and mixed by thermal mass flow controllers A steam generator with a constant flow device is also installed for adding steam After mixing of the gaseous species at the inlet of the reactor, the gas mixture passes through the catalytic active material in the heated zone and leaves the reactor through a Liebig cooler, for water condensation, and into the gas analyzer 2.2 Analytics The measurement device for determining the permanent gas components at the reactor outlet is an extractive gas analyzer (model NGA2000) made by Rosemount 229 Particle characterization The materials tested in this experimental campaign are fresh olivine, used olivine from the Gu¨ssing CHP, and calcite Table shows characteristic values for the particle sizes of the materials used in this investigation The results of the X-ray fluorescence analyses (XRF) of these materials are given in Table As can be seen in Table 3, the main differences between fresh olivine and used olivine are significantly increased contents of calcium and potassium in the latter A detailed view on the structure of the olivine particles is provided by Figs and In Fig 5, the outer layer of the particle, which is created during long-term operation in the gasifier, is visible A more comprehensive study is given by Kirnbauer and Hofbauer [9] where also the results of the EDX (energy dispersive x-ray spectroscopy) analysis of the layers are listed There can be found that the calcium content increases massively in the layers compared to the particle inside Results and discussion The influence of the used materials on the wateregas shift reaction was investigated for each material at temperatures of 800, 850, and 900 C Apart from the temperature, each test was undertaken using the same operating conditions To provide stoichiometric conditions, an equimolar amount of carbon monoxide (CO) and steam (H2O) was fed into the reactor This ratio of CO to H2O was chosen as the focus in this work was only the wateregas shift reaction In the industrial sized DFB Fig e Flow sheet of the micro-reactivity test rig 230 b i o m a s s a n d b i o e n e r g y 5 ( ) 2 e2 3 Table e Characteristic values for the particle sizes of the used materials Unit dp10 dp50 dp90 mm mm mm Fresh olivine Used olivine Calcite 184 439 694 196 456 716 350 860 978 gasification plants as located in Gu¨ssing, there is no equimolar ratio of the educts for the wateregas shift reaction present as there the steam-to-carbon ratio varies between 1.2 and 1.6 kg kgÀ1 at a fuel input power of 8000 kW which yields a dry volume fraction of CO in the product gas of 20.1% [10] For the tests here, the chosen volumetric flow rate of CO was 30 dm3 hÀ1 (all gas volumes measured at standard conditions of 101.3 kPa and 273.15 K), which results in a corresponding mass flow rate of H2O of 24.1 ghÀ1 The desired carrier flow stream for the steam generator was nitrogen (N2), with a volumetric flow rate of 3$10À2 m3 hÀ1 These input flow rates supply an input concentration of CO of 50% based on dry gas volume (CO and N2) into the reactor The temperature was increased from 800 C to 900 C in 50 C steps For each test about 10 g of particles were used, which represented a fixed bed height of between and 10 cm This inventory resulted in a space velocity of 40,000 to 50,000 hÀ1 during the experiments The particles were present in the glass reactor as fixed bed and the gas flow was downward A test run with an empty reactor at each operating point was carried out for reference reasons The most important values in such tests are obviously the outlet gas composition, the CO conversion and the hydrogen selectivity Fig shows the gas composition at the reactor outlet following the empty reactor run for each tested temperature It can be seen that only a very small amount of the initial CO and H2O was converted to CO2 and H2 Nevertheless, the H2 content rose from 0.90 to 1.58% (based on dry gas volume) as the temperature in the reactor increased After filling the reactor with 10 g of fresh olivine, the wateregas shift reaction was obviously enhanced, compared to the empty reactor With these particles, H2 production yielded a dry volume concentration of 3.6% at 800 C, increasing to 5.7% at 900 C (Fig 7) Fig e SEM image of fresh olivine [9] The assumption that the calcium and potassium-rich layer around the used olivine particles forces the wateregas shift reaction is supported by Fig 8, which demonstrates that H2 production was much higher, compared to fresh olivine, where no such layer is available The H2 concentration in the gas after the reactor was more than four times higher than with fresh olivine Especially at lower temperatures, this gap increased The results for calcite are shown in Fig This material was tested for comparison reasons, since it is typically used in industrial plants as a tar reducing additive Given the high CaO content, the conversion of CO and H2O to CO2 and H2 reached the highest measured values The dry based H2 content ranged between 21.7% at 800 C and 22.26% at 900 C The complete (wet) composition of the gas streams after the reactor are summarized in Table It has to be kept in mind that, as mentioned before, nitrogen was used as a carrier gas, representing half of the volume of the dry gas flow into the reactor CH4 was also detected by the gas analyzer but in contrast to conventionally used Fe-based catalysts that are operated at stoichiometric CO to H2O values, there was no CH4 (