Thermodynamic Investigation of SNG Production Based on Dual Fluidized Bed Gasification of Biogenic Residues

Natural gas is an important commodity in the European energy market. A promising concept for the production of synthetic natural gas on a carbon neutral basis is presented by the gasification of biogenic residues and the further reaction to a methane-rich gas. This paper investigates the thermodynamics of methanation for different product gas compositions of the dual fluidized bed gasification technology. A complete methanation of the carbon oxides is possible by the utilization of a product gas from the sorption enhanced reforming process. For product gases from conventional or carbon dioxide gasification, only partial methanation of carbon monoxide occurs. Additionally, proper handling of carbon depositions through adjustments of feed gas composition and operational parameters in the methanation reactor are essential. Temperature and pressure variations allow a thermodynamically optimized operation, which can reduce energy costs for compression or lower the amount of gas upgrading for grid feed-in. Vice versa, it is shown that the feed gas can be optimally adjusted to the operational parameters of the methanation via the sorption enhanced reforming process.


Introduction
Increasing greenhouse gas emissions and the limited availability of primary energy carriers directed the energy policy of the European Union towards sustainable and innovative energy technologies [1]. Natural gas is one of the most important primary energy carriers in Europe, but its availability is heavily dependent on the non-European market. The production of synthetic natural gas (SNG) from biogenic residues offers a promising alternative to the utilization of fossil fuels and represents a novel concept to support the current energy strategy of the European Union [1,2]. One possible process route is the dual fluidized bed (DFB) gasification, which allows the utilization of locally available residual biogenic or waste resources and offers possibilities for the production of highly valuable secondary energy carriers on a carbon neutral basis [3,4]. In combination with sorption enhanced reforming (SER) this technology enables the production of a nitrogen-free product gas with adjustable hydrogen to carbon monoxide and hydrogen to carbon dioxide contents [5]. Before the product gas from the DFB process can be fed to the methanation unit, rigorous gas cleaning is required in order to protect the downstream equipment and the methanation catalyst [6][7][8].
For the methanation reactor itself, several concepts have been utilized. Adiabatic or cooled fixed bed reactors, fluidized bed reactors, three-phase reactors or microreactors. However, only adiabatic fixed bed methanation is commercially available as of today. This variety of reactor types also explains the wide range of operation conditions. Temperatures from 250 °C to 700° C and pressures from 1 bar a to 87 bar a have been applied. From a thermodynamic point of view the methanation is favored at low temperatures and high pressures [9,10]. In order to feed the generated gas into the Austrian gas grid, the feed-in regulations must be satisfied [11]. Alternatively, a mixture of CH 4 and H 2 , also referred to as hythane, can be generated as a substitute for natural gas in industrial applications [12]. The main chemical species, which are involved in the methanation reaction system, are CH 4 , H 2 , CO, CO 2 and H 2 O. The corresponding reaction equations are the CO-methanation, the reverse water gas shift reaction, and O (2) the CO 2 -methanation (combination of (2) and (1)). (3) Additionally, the reaction enthalpies at 300 °C ( ) are given. Besides these species, the product gas of the DFB gasifier also contains higher hydrocarbons. As the main component ethylene (C 2 H 4 ) is identified and is thus included here [13]. The hydrogenation to methane can be written as: (4) A deactivation mechanism of the catalyst, which cannot be prevented by gas cleaning steps, is the formation of solid carbon on the catalyst. While adsorbed carbon on the catalyst surface is a necessary reaction intermediate during methanation, the formation of stable deposits leads to catalyst fouling [14]. Thermodynamically, this deposition can be accounted for by the Boudouard reaction. (5) The deposited surface carbon can also be hydrogenated to methane, (6) or undergo gasification with steam [15].
These reactions show that increased amounts of H 2 , H 2 O or CO 2 in the product gas might prevent the carbon deposition. A different form of deposition can occur through the adsorption of higher hydrocarbons like C 2 H 4 on the catalyst surface. Between 500 and 600 °C, this can lead to coke deposits [16]. If kinetic models are considered, all of the above mentioned reaction pathways have to be taken into consideration. The catalytic methanation of syngas is, however, mostly limited by heat and mass transfer and not by kinetics [10]. For temperatures down to 320 °C the gas composition is close to the thermodynamic equilibrium [17,18]. A thermodynamic calculation thus provides a good estimation of the expected gas composition. Because of the broad variety of possible carbon species, deviations from thermodynamic equilibrium for carbon deposition have to be expected [10]. Nevertheless, graphitic carbon has previously been used to elucidate this issue, since kinetic models are often only valid for specific reaction conditions and catalysts [19]. Extensive studies have been performed on the thermodynamics of methanation [14,19,20]. However, for systems with multiple simultaneous reactions, the effect of different product gas mixtures as well as temperature and pressure is not straightforward. Thus, the thermodynamic calculations in this paper are applied to different product gas mixtures, which have been obtained by the gasification of different biogenic residues with the 100 kW th DFB gasifier at TU Wien. The chosen feed gas compositions for the methanation aim at covering the broad range of product gas compositions, which can be produced by the DFB gasifier.

Concept and methodology
In order to calculate the thermodynamic equilibrium, only four of the seven reaction equations (Eq. (1) to Eq. (7)) need to be considered. Otherwise, the system would be overdetermined, because only four equations are linearly independent of each other. For example, the CO 2methanation reaction can be seen as the reversed water gas shift reaction followed by the CO-methanation. Thermodynamic calculations are performed with HSC Chemistry and MATLAB. The main focus of this investigation is a low temperature methanation (300 °C) at ambient pressure. These parameter settings result from the current efforts in the design and construction of a lab-scale fluidized bed methanation test rig at TU Wien for the given parameters. Nevertheless, also a temperature variation from 200 °C to 500 °C and a pressure range from 1 bar a to 10 bar a are carried out. Graphite is chosen as the prevailing carbon species, since Frick et al. [19] found that the Gibbs free energy is lower than for amorphous carbon and is thus preferentially formed. In order to classify the feed gas composition the stoichiometric number ( ) is defined. (8) gives the ratio between the molar fraction of H 2 ( ) to the molar fractions of the carbonaceous species in the feed gas which react to CH 4 . If is equal to one, there is a stoichiometric amount of H 2 available according to Eqs. 1, 3 and 4. Because the regarded pressures in this study are relatively low, it is safe to assume ideal gas behavior. Molar fraction are thus equal to volume fractions. This definition of SN is not unambiguous, because the chemical equilibrium is influenced by all available species and therefore also by CH 4 and H 2 O. Nevertheless, it allows an approximate classification of the feed gas mixture. Typical product gases from the DFB gasification show similar CH 4 concentrations. Water concentrations in the feed gases are assumed zero. This is attributed to the required gas cleaning which is conventionally carried out at low temperatures [21]. If similar CH 4 concentrations and a water free feed gas are assumed, the implementation of SN is justified. Additionally, the CH 4 yield ( ), the carbon yield ( ), the CO conversion ( ), and the CO 2 conversion ( ) are defined.
Index refers to the carbonaceous species in the feed ( ).
Gas cleaning is not within the scope of this study. The feed gas mixture for the methanation is assumed free of impurities and other minor components. Besides, kinetics or heat and mass transfer phenomena are not considered.

Results and discussion
In Tab. 1 the investigated feed gas compositions for the methanation are shown. Feed gas no. 1 shows a typical SER gas with high hydrogen content. Limestone (L) is used as bed material at lower temperatures and bark (BA) is chosen as the feedstock. Feed gases no. 2no. 4 present product gases from conventional gasification. With feed gas no. 2 the same fuel and bed material is used but the gasification temperature is higher which results in lower H 2 and higher CO and CO 2 contents. For feed gas no. 3 lignin (LI) is used as fuel and olivine (O) as bed material. Sewage sludge (SS) and an olivine/limestone mixture (O/L) are the basis for feed gas no. 4, which results in low H 2 and high CO 2 contents. For feed gas no. 5, a CO 2 /H 2 O mixture is used as gasification agent and rapeseed cake (RSC) and O as fuel and bed materials, respectively. This results in even lower H 2 and high CO and CO 2 concentrations. Feed gas no. 6 shows a temperature variation for SER gasification. This is included to demonstrate the adaptability of the DFB gasifier to the requirements of the methanation process (also see Fig. 4). Data for this variation is only available for softwood (SW) as feedstock. In Fig. 1 results of chemical equilibrium calculations at 300 °C and 1 bar a are shown for feed gas nos. 1-5. The volume fractions of the dry gas components after methanation (referred to as raw-SNG) and the water content of the raw-SNG are depicted. C 2 H 4 is not displayed in any of the figures, because it is completely converted under all investigated conditions. CO is not shown in Fig. 1 4 . This is equal to a methane yield of 5.2 % and a CO 2 conversion of 2.2%, respectively. For feed gas nos. 2-5 the is below one. Since all feed gases are dry, this correlates with the carbon deposition. CO is almost completely converted for all feed gases. However, even small amounts of CO might exceed the allowed threshold level for grid feed-in on the one hand. On the other hand, CO 2 methanation is found to be kinetically hindered even for very low CO concentrations [26]. For feed gas no. 1 7 ppm v,db of CO remain in the raw-SNG in the thermodynamic equilibrium. At least 580-750 ppm v,db need to be expected for feed gas nos. 2-5.

Investigation of the sewage sludge product gas
In the following section a more in-depth discussion of the SS product gas follows (feed gas no. 4). Because of the expected carbon deposition for this feed gas composition, H 2 O should be added if a long catalyst lifetime and a high conversion efficiency are aimed at. Fig. 2 depicts the raw-SNG gas composition after the addition of H 2 O for temperatures from 200-500 °C and pressures of 1, 5 and 10 bar a (Fig. 2b). The amount of water added corresponds to the minimum amount needed to prevent carbon deposition. This minimum volume fraction of H 2 O in the feed gas ( ) as well as are also displayed (Fig. 2a). With increasing temperature, less CH 4 and CO 2 and more CO and H 2 are present. Accordingly, the CH 4 yield decreases from 41 % to 26 % with increasing temperature   Fig. 1 and Fig. 2 clarifies the influence of the added feed water at 300 °C and 1 bar a for the SS product gas. By the addition, the CH 4 content is elevated from 29 to 37 vol.-% db . At the same time, the CO 2 content is lowered from 66 to 56 vol.-% db . H 2 slightly increases from 5.6 to 6.6 vol.-%. The CO concentration is marginally lowered from 714 to 667 ppm v,db . Despite the addition of 48 vol.-% of H 2 O to the dry feed gas, the H 2 O concentration in the raw-SNG only increases slightly. This can be visualized by the steam gasification reaction (Eq. 7). Most of the steam is needed to gasify the carbon. Because no carbon is present after feed water addition, is negative under all displayed conditions. In other words, more CO 2 is formed during methanation than converted. For grid feed-in the raw-SNG needs to be freed of CO 2 . A maximum of only 2 vol.-% is allowed. A H 2 content below the allowed threshold level of 4 vol.-% after CO 2 separation could be achieved by increasing the pressure at 260 °C to 10 bar a . If the desired commodity is hythane only CO 2 separation is necessary.

Investigation of the SER product gas
Feed gas no. 1 is a typical SER product gas with a high H 2 content. is greater than one, which allows a practically complete methanation of the carbon oxides (CO+CO 2 ) at temperatures below 300 °C with a CH 4 yield of nearly 100 % (Fig. 3). Pressure again only has significant influence on the gas composition at higher temperatures. With pressurization, the decreasing trend of CH 4 and the increasing trends of H 2 , CO and CO 2 at higher temperatures can be counteracted. In addition, above 440 °C at 1 bar a carbon formation is thermodynamically possible. As is shown in Fig. 3a, H 2 O needs to be added. At higher pressures, this can be prevented. Below 300 °C there is practically no influence of pressure or temperature on the gas composition. Methanation around 300 °C and 1 bar a shows a favorable raw-SNG composition without the need of compression. Lower temperatures would not improve the gas composition, but increase the challenge of employing an active catalyst. For grid feed-in, only H 2 would need to be separated from the raw-SNG. For the application as hythane on the other hand, no further upgrading step is necessary except water condensation. Besides these advantages, the methanation of SER product gas also comes with drawbacks. The high H 2 content can only be reached by the increased transport of carbon from the fuel to the flue gas. However, the excess carbon (in the form of CO 2 ), which is still in the raw-SNG in case of conventional gasification, is already removed within the gasification process for the SER product gas. The overall process chain thus shows a low carbon utilization factor. But this lower carbon utilization factor lies primarily in the gasification process and not in the methanation process. Whereas the addition of H 2 from external sources (e.g. electrolysis) would allow the methanation of the leftover CO 2 and yield a high carbon utilization factor for conventional gasification. Another suggestion would be the installation of a water gas shift reactor prior to the methanation reactor, which was demonstrated successfully for hydrogen production from DFB derived wood gas [27].

Investigation of variable product gas compositions of the SER process
Fuchs et al. [25] already described the adaptability of the SER process with regard to the product gas composition. In Fig. 4 the evolution of the product gas components over the gasification temperature of the 100 kW th DFB gasifier at TU Wien is depicted. By temperature variation, the product gas can be adjusted from [25] to the required feed gas for methanation. However, this also adds an additional parameter to the modelling of the methanation reactions. . Temperature and pressure are again set to 300 °C and 1 bar a for the methanation process, respectively. In order to assess carbon formation is given. There is a decreasing trend for CO 2 , H 2 O and the amount of carbon formed for an increasing . CH 4 has a maximum at a slightly above one. At the same point carbon formation declines to zero and the small incline in H 2 turns into a sharp increase for higher . CO is only present in trace amounts (0.14-614 ppm v,db ) and is not displayed here. From a thermodynamic point of view, the feed gas with a of 1.09 generates a raw-SNG with the most favorable composition for the methanation at 300 °C and 1 bar a . A of 1.09 corresponds to a gasification temperature of about 680 °C. The associated compositions for the feed gas and the raw-SNG as well as the key figures are depicted in Tab

Conclusion and Outlook
In this work, the suitability of various product gases from the 100 kW th DFB gasifier for methanation has been evaluated from a thermodynamic point of view. It has been shown, that complete methanation is only possible for SER product gases. For all other gases, only the partial methanation of CO is possible, whereas CO 2 might even constitute the main raw-SNG component. Additionally, gases from conventional steam gasification or gasification with CO 2 admixture to the gasification agent (H 2 O+CO 2 ) are subject to carbon deposition. Therefore, H 2 O needs to be added for a stable operation. Furthermore, the influence of different operation conditions of the methanation on the raw-SNG composition have been visualized. By the careful choice of operation conditions, energy savings and/or less effort for further gas upgrading can be accomplished. A further investigation of the SER product gases has revealed that it is also possible to adapt the gasification process to suit certain methanation conditions optimally. However, if H 2 is available from an external source, conventional gasification can be beneficial because of a higher carbon utilization factor. It should be noted that all investigations in this paper are based on thermodynamic equilibrium calculations. There are many other parameters, which can influence the performance of the methanation. Catalyst poisoning due to insufficient gas cleaning, the choice of the reactor concept or kinetic limitations necessitate experimental investigations.

Acknowledgements
This work is part of the research project ReGas4Industry and receives financial support from the research program "Energieforschung" funded by the Austrian Climate and Energy Fund.