CC BY-NC-ND
0CrossMark
Available online at www.sciencedirect.com
ScienceDirect
Energy Procedía 95 (2016) 475 - 481
International Scientific Conference "Environmental and Climate Technologies", CONECT 2015,
14-16 October 2015, Riga, Latvia
Mathematical model for the simulation of the syngas methanation process
Inese Tilla*, Elina Dace
Institute of Energy Systems and Environment, Riga Technical University, Azenes iela 12/1, Riga, LV-1048, Latvia
Abstract
With rapid population growth there is an increased trend of global energy consumption. On a global scale most of the energy supply is currently generated by fossil fuels with all the subsequent consequences including depletion of non-renewable energy resources (e.g. oil, coal and natural gas), greenhouse gas emissions and climate change. With respect to the increasing global demand for cheap, reliable and environmentally safe energy, there is a need for innovative technologies and an increase in renewable energy share.
Biomass is a unique renewable resource with a wide range of uses and a huge potential. However its relevance for food industry poses an ethical dilemma. Therefore, finding the possible solutions for more efficient biomass use are increasingly important. In this paper, results of a study identifying parameters and conditions necessary for increasing the quality of outgoing gases from biomass gasification and digestion processes are presented. A methanation process is simulated for upgrading the quality of the gases. It is a reaction where carbon oxides present in the gases react with hydrogen to produce methane. The reaction is condition-dependent, therefore a mathematical model is developed for simulating parametric values that influence the process. In the paper, the simulation results are presented.
©2016 The Authors.Published byElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment. Keywords: biomass; methanation; syngas; hydrogen
1. Introduction
Increase of the renewable energy share is a great challenge on a global scale. Biomass is a resource with high potential, however more and more efficient biomass conversion methods are being sought and studied, anaerobic
* Corresponding author: E-mail address: inese.tilla@gmail.com
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment. doi:10.1016/j.egypro.2016.09.070
digestion and gasification among them. The whole system of biomass-to-methane is complex and consists of several facilities and processes. Each process requires certain conditions in order to obtain a favourable result. One of the issues related to biomass conversion into a gaseous product is an efficient end use. By supplementing the inlet gas with hydrogen by a methanation process, a methane content in the outlet gas composition is enhanced. Thus a higher energy amount is delivered to the end user providing efficient energy production and end use. The gas can also be introduced into the existing natural gas grid, thus substituting the extensively used natural gas which is a nonrenewable resource.
Methanation (also known as hydrogénation) is a reaction where carbon oxides present in a gas react with hydrogen to produce methane [1]. Reactions are present in the methanation reactor under controlled conditions and parameters such as pressure, temperature, reactant ratios, gas composition, flow rate, mass, etc. [2]. Although both biogas and syngas are applicable for methanation, gas composition has to be considered while setting out the appropriate conditions and parameters, e.g. amount of H2added for the methanation reactions [3]. Furthermore for biogas/syngas methanation an external hydrogen source is required. H2 is produced by hydrolysis using irregular energy sources such as solar and wind power thus integrating biomass and irregular renewables into a single system. Also, the composition of input gas has an influence on the methane yield. The main concern of methanation is high temperatures due to methane formation processes that cause carbon formation [4]. To avoid this issue an optimization ofparameters and conditions are required [4]. To initiate the reactions, an external addition ofa catalyst is needed.
Methane is a valuable energy carrier that can be used with integration of irregular renewable sources via methanation for production of synthetic natural gas (SNG). It must be noted that SNG production is complex due to high concentrations of CO and C02that have to be removed [4]. In this study, an integrated system of methanation, gasification and irregular renewable energy sources is presented and preliminary results of mathematical process modeling are discussed. Although calculations and the mathematical model in this case have been made only for the syngas, the same methodology can be applied for biogas methanation.
2. Methodology
A mathematical model is developed to simulate the methanation processes and conditions that allow obtaining the highest quality of the outgoing gas. Mathematical modeling is used to explain the processes of a system and understand the relations between components to forecast the possible results. A model consists of different relations and variables. Relations are described by mathematical equations, and variables representing the value ofparameters that are assumed to vary while obtaining the desirable result [4]. Considering methanation reaction's condition-dependent nature, simulation of processes with differing parametric values is conducted. For this research a Ni/MgAl204 catalyst have been chosen according to the reviewed study of Xu and Froment [5]. In this specific case study, syngas has been chosen as an inlet gas. An integrated system of methanation, gasification and irregular renewable energy sources is presented in this study (Fig. 1).
| Syngas
Mixing t ank
Catalyst C^iy
) Synthetic Natural Gas
Hydrogen
Electrolysis
Wind Solar power power
Fig. 1. Principal scheme ofan integrated system ofmethanation, gasification and irregular renewable energy sources.
Scenario modeling was conducted in this study to consider the feasible alternatives of future outcomes and the range of parameter optimum for the best result. Besides several parameters (e.g., pressure, reactant ratios, gas composition and flow rate) the influence of the temperature change in the mixing tank is discussed as one of the main issues ofmethanation process due to catalyst sensitivity to highly exothermal methane formation nature. In this study a mathematical model is based on the base case scenario and temperature change scenario analysis.The base case scenario supplies a platform for construction of possible situations which may develop. Basic assumptions for the base case scenario are set according to the scientific literature review and investigation of related experimental studies [5-7]. A catalyst chosen for this mathematical modelling is Ni/MgA1204 as it has demonstrated good results during experiments made by scientists Xu and Froment [5]. Scenario analysis is done by using mathematical modeling and simulating methanation processes expressed by mathematical equations. With this method it is possible to make a theoretical estimation ofthe most efficient outcome by setting various parameters.
Since the processes and reactions of gas methanation are complex and condition dependent, the analyzed system of methanation was separated into two parts - processes in the mixing and pre-heating tank and processes in the methanation reactor (Fig. 2).
Syngas/ biogas
Composit ,,
External heating
Composit, .
th„,
Composit,
catalyst fn„st
Composite
Fig. 2. Principle scheme ofsystem boundaries (a) and flow charts ofthe mixing tank (b) I and methanation reactor (c).
Both parts (b and c of Fig. 2) were analyzed separately - in the mixing tank the gas is being prepared for the methanation - mixed and pre-heated. Several variables, e.g. dimensions of the mixing tank, gas residence time, volumetric flow rate, amount of H2 added, temperature etc., are defined for the inputs and outputs of the mixing tank having high influence on overall process activity. The inlet for the mixing tank consists of two flows - syngas or biogas and additional hydrogen. Despite the fact that syngas or biogas already contains some amount of H2, according to the literature [9] additional hydrogen contributes in higher methanation outlet yield. The mixing tank is a pre-stage of methanation reactor ensuring that the input for the reactor is at the right conditions - well mixed, at the right temperature and composition. For initial process modeling some basic assumptions are set and listed below in Table 1.
Table 1. List of constant modeling parameters
Parameter Value
Pressure, p 1.01325 bar = 1 atm Volumetric flow rate in the reactor/mixing tank 0.0998 m3/h
Diameter ofthe mixer 0.0297 m
Radius of themixer 0.01485 m
Height ofthe mixer 0.4 m
Diameter ofthereactor 0.0297 m
Radiusofthereactor 0.01485 m
Heightofthereactor 0.206 m
Mass ofNi/Al203 catalyst in the reactor 0.21 kg
The methanation reactor is the main part of the system. It is a unit in which reactions of methane formation are present under controlled conditions. The reactions corresponding to the methanation process are presented in Eqs.
(l),(2)and(3).
CO + 3H2 ^ CH4 + H2O (1)
CO2 + H2 ^ CO + H2O (2)
CO2 + 4H2 CH4 + 2H2O (3)
As the initial syngas composition includes all components presented in Eqs. (1) - (3), in the methanation reactor, all three reactions occur simultaneously. In order to understand the processes happening in the reactor and estimate the final gas composition, reaction rates have to be determined. The reaction rates were calculated according to the study presented by Xu and Froment [4]. The reaction rates determine consumption of reactants for formation of products. The formation and disappearance rates ofchemical components were calculated according to Eqs. (4) - (8).
reo = -ri + r2 (4)
rc02 = —T2 ~ T3 (5)
rH2 = -3ri - r2 - 4r3 (6)
TH20 = ri + r2 + 2r3 (7)
rcH4 = ri + r3 (8)
ri reaction rate ofreaction I, kmol/(kgcat h);
r2 reaction rate ofreaction II, kmol/(kgcat h);
r3 reaction rate ofreaction III, kmol/(kgcat h);
rco, rC02, rH2, rH20, rcH4 reaction rates of gas components, kmol/(kgcat h).
The formation and disappearance rates of chemical compounds are further used for determining the final composition of the inlet gas mixture. The syngas that is used in this study as an inlet gas for the methanation is prepared in a gasifier from second generation biomass. It is assumed that the volumetric gas composition of inlet gas consists of 23 % H2, 8 % CO, 11 % C02, 4 % CH4, 14 % N2, 40 % H20 [7]. All ofthe basic assumptions for the base case scenario, for example, mass flow rate, volumetric flow rate, mass of catalyst and gas mix temperature are set according to the scientific literature review and investigation ofrelated experimental studies.
3. Results
The calculation results show that, although all of reactions in the methanation reactor are present at the same time, each reaction has a different reaction rate. Reaction rates are highly dependent on temperature change. With temperature increase, the difference between reaction rates grows (see Fig. 3).
300 325 350 375
Temperature, °C
rl r2 r3
Fig. 3. Relation between reaction rates and temperature.
From Figure 3 it can be observed that reaction rates ri, r2 and r3 increase with temperature growth. The temperature range selected for the calculations is 300 - 400 °C with a step of 25 °C. According to the results presented in Figure 3, the first reaction has the highest reaction rate (ri). The second reaction (r2) is much slower than the first but faster than the third (r3). Summarizing the results, it can be concluded that ri>> r2> r3.This contributes to the consumption ofreaction raw materials and affects end production.
r_H2 r_H20 r_CH4 •r_CO r C02
Temperature, °C
Fig. 4. Rates of component formation and disappearance depending on temperature.
The results in Figure 4 show the ability and pace of the gas components to disappear (react) or form during the methanation reaction. As it can be seen, hydrogen has the highest rate of disappearance as a result of transformation into methane and water during the chemical reactions. Thus, the amount of H2 in the reactor is the limiting factor for CO and C02 removal. A similar trend is observed in case of carbon dioxide - a disappearance of C02 can be observed during the chemical reactions in the methanation reactor, however with a lower rate. CH4 and H20 have a trend of formation. Whereas, CO will form or disappear depending on the prevailing reaction, i.e. CO will react with H2 to form CH4 and H20 in reaction I (Eq. (1)), but it will also be formed in reaction II (Eq.(2)), though at a slower rate.
The outlet gas composition was determined as an indicator for the assessment ofthe final result (see Fig. 5).
In Out
Out without H20
CO C02 H2 CH4 N2 H20 C2H6 C2H4
Fig. 5. Allocation ofmass fraction to inlet and outlet gas composition.
In Figure 5, the reactor's inlet gas composition is compared to composition of the outgoing gas. In this study, the main gas component that determines the quality of the outflowing gas is methane. As it can be seen from the graph, there is a significant increase in the mass fraction of methane and yet, high fractions of CO and CO2 are still present in the outflowing gas. In addition, mass fraction of CO has increased as a result of reaction II. As all H2 has been consumed by reactions I and III, there is no further possibility for CO and CO2 removal. Therefore, injection of excess amount of H2for further CO and CO2 removal should be assessed in further studies. It can also be noted, that a large amount of H2 is consumed for formation of H2O as the rate of H2O formation is higher than the rate of CH4 formation (see Fig. 4). By removing water from the outflowing gas the methane's mass fraction can be further increased to 22 % (see Fig. 5). To increase the methane content of the outflowing gas, secondary and even tertiary methanation reactors have to be considered for enclosure into the system [9, 10]. In that case, extra hydrogen supply for the additional reactors would be required. Also, other methods and equipment for improving the final gas composition could be implemented such as CO2 removal, H2O condensation, etc.
4. Conclusion
Mathematical modeling results have approved that, besides the catalyst, temperature is the main driving force for reactions in mixing tank and methanation reactor. Reaction rates for reactions I, II and III increase with temperature growth. Reaction rate for the first reaction (CO+3H2 ^ CH4+H2O) is much higher than reaction rate for the second reaction (CO2+H2 ^ CO+H2O) and reaction rate for the third reaction (C02+4H2^ CH4+2H20) (ri»r2>r3). Molar flow rate of gas components and amount of catalyst added to reactions have a crucial influence on the conversion rate of reactants.
Hydrogen has the highest rate of disappearance. Hydrogen will disappear as a result of transformation into methane and water during the chemical reactions. A similar trend is observed in case of carbon dioxide - a disappearance of CO2 can be observed during the chemical reactions in the methanation reactor however with lower reaction rate.CH4 and H2O have a trend of formation. Whereas, CO will form or disappear depending on the prevailing reaction, i.e. CO will react with H2 to formCH4 and H2O in reaction I, but it will also be formed in reaction II, though at a lower rate.
The results of mathematical modeling in this study hereafter ought to be validated in lab-experiments and furthermore will supply a valuable platform for further research and lab-experiments.
Acknowledgements
The work has been supported by the National Research Program "Energy efficient and low-carbon solutions for a secure, sustainable and climate variability reducing energy supply (LATENERGI)".
References
[1] Gao J, Wang Y, Ping Y, Hu D, Xu G, Gu F, Su F. A thermodynamic analysis of methanation reactions of carbon oxides for the production of synthetic natural gas. An internationaljournal to further the chemical sciences 2012;6.
[2] Dace E, Rusanova J, Gusca J, Blumberga D. Selecting a Catalyst for Methanation Process: Technical and Economic Performance Based TOPSIS Analysis. Proceedings of the 27th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact ofEnergy Systems (ECOS 2014). Turku: AboAkademi University; 2014.
[3] Dace E, Blumberga D, Veidenbergs I. Modeling syngas composition in an integrated system of biomass gasification, electrolysis and methanation. Energy Proc 2015;75:801-806.
[4] Er-Rbib H, Bouallou C. Modelling and simulation of methanation catalytic reactor for renewable electricity storage. Chemical Engineering Transactions 2013;541-546.
[5] Xu J, Froment GF. Methane Steam Reforming, Methanation and Water -Gas Shift: I. Intrinsic Kinetics. Gent; 1989.
[6] Sankaranarayanan, Sivashunmugam, Srinivasan, Kannan. Carbon dioxide - A potential raw material for the production of fuel, fuel additives and bio-derived chemicals. Indian Journal of Chemistry - Section A Inorganic, Physical, Theoretical and Analytical Chemistry 2012;1252-1262.
[7] Kienberger T, Zuber C, Novosel K, Baumhakl C, Karl J. Desulfurization and in situ tar reduction within catalytic methanation of biogenous synthesis gas. Fuel 2013;107:102-112.
[8] Green WH. Chemical and Biological Reaction Engineering. Spring; 2007.
[9] Kopyscinski J. Production of synthetic natural gas in a fluidized bed reactor. Germany; 2010.
[10] Jurascik M, Sues A, Ptasinski J. Exergetic evaluation and improvement ofbiomass-to-synthetic natural gas conversion. The Royal Society of Chemistry; 2009.
