Scholarly article on topic 'Development of the Experimental Scheme for Methanation Process'

Development of the Experimental Scheme for Methanation Process Academic research paper on "Materials engineering"

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{"CO2 recycling" / "Sabatier reaction" / "renewable methane" / "methanation system design"}

Abstract of research paper on Materials engineering, author of scientific article — Aleksandrs Lazdans, Elina Dace, Julija Gusca

Abstract The aim of this study is to develop an experimental scheme for realization of methanation reaction (CO2 + 4H2 → CH4 + 2H2O) realization. The existing experimental stands show that there are many factors and reaction parameters that may negatively act on CO2 conversion. The highest CO2 conversion rates are achieved at the temperature range of 300–400°C. Methane production efficiency also depends on reactor's space velocity, reaction's stoichiometry, catalyst's surface area and type. Nickel and Ruthenium are the most popular catalysts. Gas contaminants (like NO2, O2 and SO2) which may be in exhaust flow, act negatively on the reaction. According to this great amount of aspects acting on methanation process positive outcome special scheme was developed. It combines not only inlet gas properties, proper catalyst or reactor design selection, but also technical aspects such as preheating/mixing chamber and condensing unit design. This summarizing scheme will be used for real methnation reactor design in the future studies.

Academic research paper on topic "Development of the Experimental Scheme for Methanation Process"

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Energy Procedía 95 (2016) 540 - 545

International Scientific Conference "Environmental and Climate Technologies", CONECT 2015,

14-16 October 2015, Riga, Latvia

Development of the experimental scheme for methanation process

Aleksandrs Lazdans, Elina Dace*, Julija Gusca

aInstitute of Energy Systems and Environment, Riga Technical University, Azenes iela 12/1, Riga, LV-1048, Latvia

Abstract

The aim of this study is to develop an experimental scheme for realization of methanation reaction (CO2 + 4H2 ^ CH4 + 2H2O) realization. The existing experimental stands show that there are many factors and reaction parameters that may negatively act on CO2 conversion. The highest CO2 conversion rates are achieved at the temperature range of 300-400 °C. Methane production efficiency also depends on reactor's space velocity, reaction's stoichiometry, catalyst's surface area and type. Nickel and Ruthenium are the most popular catalysts. Gas contaminants (like NO2, O2 and SO2) which may be in exhaust flow, act negatively on the reaction. According to this great amount of aspects acting on methanation process positive outcome special scheme was developed. It combines not only inlet gas properties, proper catalyst or reactor design selection, but also technical aspects such as preheating/mixing chamber and condensing unit design. This summarizing scheme will be used for real methnation reactor design in the future studies.

© 2016 The Authors.PublishedbyElsevierLtd. Thisis 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: C02 recycling; Sabatier reaction; renewable methane; methanation system design

1. Introduction

Nowadays one of the most significant environmental problems is fossil fuel usage and, as a result, increasing carbon dioxide (CO2) concentration in the atmosphere [1, 2]. Also, technology progress and ever increasing economic activity has stimulated release of CO2 emissions inducing global warming. Today, large effort is made towards finding sustainable solutions for solving this problem without interrupting the economic development.

* Corresponding author. Tel.: +371-67089908 E-mail address: elina.dace@rtu.lv

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.2016.09.082

Most occurring activity is fossil fuel replacement with renewable energy sources (RES), such as wind, solar and biomass energy. The European Union (EU) has set the target to achieve 20 % of renewables in the primary energy supply by 2020. It means that installed capacities of RES will grow in the EU. The tendency of increasing RES share is observed also on the global scale. In 2012, the total global operating capacity of solar PVs reached 100 GW, while the total installed wind capacity was 283 GW. Between 2007 and 2012, the average annual growth rates of solar PV and wind energy capacities were 60 % and 25 %, respectively [3]. The rapid increase has been driven by price decrease, technology development, competition between manufacturers and governmental support.

Despite this positive tendency there is an existing storage problem of stochastic electricity from wind and solar PV energy sources: misbalance between generation and consumption can lead to severe consequences or even to blackout of power system [4]. At the present time, one of the options for RES energy storage is to accumulate it by producing hydrogen (H2) through water electrolysis. Due to the shortage of renewable H2 infrastructure, methanation where H2 is combined with CO2 to produce synthetic methane can be possible solution for recycling of increasing CO2 and accumulation of non-continuous electricity. Synthetic methane (CH4) production is convenient because of existing natural gas pipe system. CH4 can also be burned in an existing power generation stations recovering CO2 (which should be captured and used in the next cycle). Specific energy of CH4 is also higher than H2: 40.0 MJ/m3 versus 12.7 MJ/m3 [5]. Thus, methanizing H2 has several preferences.

The aim of this study is to develop an experimental scheme for testing the methanation reaction.

2. Methanation

Although, the methanation reaction was discovered in far 1910s, only in recent years it has attracted more attention, as the depletion of fossil fuels and air pollution problems have become more acute [6]. In the methanation reaction, H2 reacts with carbon oxides (CO and CO2) to produce methane (see reactions (1) and (2)). Both reactions are highly exothermic [7, 8].

CO2 + AH2 ^ CH4 + 2H2O AH = -165kJ / mol (1)

CO + 3H2 ^ CH4 + H2O AH = -206k/ / mol (2)

Although, the methanation reaction theoretically looks simple, in reality there are many factors and reaction parameters that may negatively act on H2 and CO2 conversion, and methane production. The most important aspects affecting the process are discussed below.

2.1. Catalysts

A catalyst is necessary to initiate the reactions (1) and (2). A wide variety of catalysts exist. For methanation, both noble and non-noble metals are used, e.g. Rhodium (Rh), Ruthenium (Ru), Cobalt (Co), Manganese (Mn) and Nickel (Ni) [9]. Some catalysts, especially noble metals, have high costs (e.g. Silver-Ag, platinum-Pt and gold-Au); hence it would not be economically feasible to utilise these metals in pure form. In such cases, it is suitable to use a support material - a less costly material than the catalyst itself, but with a high contact area relative to its size [10]. Various oxide supports exist, e.g. Ti02, Si02, AI2O3, Ce02, Zr02, CuO [11]. Despite the large variety, Nickel and Ruthenium catalysts are the most used and explored [12-16].

2.2. Temperature

Depending on the specific application and catalyst type, the methanation reaction is typically conducted at temperatures between 250-400 °C [17]. The reverse reaction is generally conducted at temperatures higher than 500 °C [7, 10].

2.3. Pressure

Pressure depends on the type of methanation reactor, but generally Sabatier reaction occurs at ambient pressure (latm). Yet, exceptions exist. Large scale reactors, e.g. CO2 removal and water production installations for International Space Station or submarines [18, 19], and "The Audi E-gas Project", use higher (even more than 1030 atm) pressure [20]. They operate with increased pressure because it positively acts on CO or CO2 conversion (10-15 % improvement) and CH4 selectivity (-10% improvement) [21].

2.4. Gas Hourly Space Velocity (GHSV)

Longer residence time of the feed gases in the reaction chamber, measured through space velocity, increases the conversion of CO2 to CH4 [9]. For the reactors filled with a catalyst, the volume of the packed catalyst bed is considered, not the volume of the entire reaction chamber. As stated by Hoekman & Broch [7], increasing the space velocity, residence time decreases, that, as expected, will stimulate the decrease ofC02 conversion.

Space velocity depends on CO/CO2 methanation system (both component input, reactor construction). Therefore, the value of space velocity ranges widely, i.e. between 4000 - 20000 h_1 [7]. Also, there are examples of gas hourly space velocity of 1200 h"1, whereas in high pressure methanation reactor applications even 31000 h"1 [21, 22].

2.5. Input gas purity

Sabatier reaction stoichiometry shows that the optimum molar ratio of H2/CO2 is 4/1. Thus, to convert 1 mole of CO2, 4 moles ofH2 are needed.

Generally, in laboratory experiments, pure H2 and CO2 diluted in a carrier gas (e.g. Argon or Nitrogen) are used. Pure renewable hydrogen is produced during water electrolysis reaction. Whereas, for CO2 source several options exist. Firstly, it is produced as a by-product in the industrial production of lime stone, ammonia and hydrogen. Secondly, CO2 can be absorbed and collected from air, brewery or sewage treatment. Yet, the most suitable source of CO2 is exhaust of biogas, natural gas, coal or other energy producing plant [23]. According to plant type, exhaust gas composition by volume (%) can differ. Generally, exhaust gases contain 65-75 % of N2, 5-20 % (e.g. wet biomass) of water vapour, 5-15 % ofC02, 1-8 % of O2. Some small percentage of other pollutants (NO2, SO2 and CO) may be present.

When, in the methanation reaction, exhaust gases are used for source of CO2, such chemical elements as O2, NO2 and SO2 negatively act on the performance of the methanation process. Nitrogen and sulphur dioxides are polluting and poisoning catalyst surface, thus, reducing CO2 conversion efficiency. Excess O2 reacts with H2 to produce water, accordingly decreasing the amount ofproduced CH4. 8 % of02 in the inlet gas can reduce CO2 conversion by approximately 20 % [5].

3. Experimental scheme

There are several examples of laboratory scale methanation reactor systems [23-29]. These laboratory installations have different sizes, some special technical solutions, yet the general concept is very similar. According to this concept, sources of CO/CO2 and H2 are necessary. Methanation reactor is a main system element. Before entering the methanation reactor where reaction occurs, inlet gas should be mixed and preheated. As was mentioned in the literature analysis, methanation reaction to occur a catalyst must be used. Further, produced gas should be condensed and collected. Entire methanation stand should be controlled through use of a computer control system including gas analysers, temperature sensors and controller.

Before methanation stand construction modelling and simulation must be done. Modelling and simulation work is an appropriate part of the analysis, optimization, and engineering of methanation reactors and concepts. Therefore this research presents a simple model displaying main aspects, which should be taken into account making methanation process design. Fig.l. depicts general scheme ofmethanation process.

The target is optimum methanation system design. In other words saying we have to produce outlet gas with most appropriate composition (best CO2 conversion and CH4 selectivity). To reach this target, firstly, proper reactor design should be selected. It includes mainly section of reactor type, calculation of construction and operational

parameters. For example, by temperature profile inside the reactor they are divided on three types: adiabatic fixed-bed reactors, isothermal, and polytropic reactors. Also are examples of fluidized-bed reactors, micro-channel reactors, honey-comb etc. However, reactor diameter, reactor volume, insulation thickness, and other are considered as construction parameter examples. Temperature, Pressure inside the reactor, inlet gas residence time, stoichiometric ratio (HVCOx) is main example of operational parameters.

Secondly, right/ proper catalyst should be selected. Selecting catalyst should be considered economical, ecological, technical, and conversion efficiency aspects. As was mentioned in literature analysis, some catalysts consist of noble high cost metals, (e.g. Silver-Ag, platinum-Pt and gold-Au). In demonstration or research scale reactors it may be feasible to use expensive catalysts, but in large-scale/ commercial applications careful analysis must be done. Also, selecting active compound and support for catalyst preparation, environmental impact analysis must be done. Selecting catalyst, its' preparation and pre-treatment difficulty should be taken into account.

Target for outlet gas composition

Inlet gas properties

and composition

§ .s?

Is pretreatment of gases required?

Selection of catalyst

Selection of pretreatment technology

Are gases sufficiently clean to avoid catalyst determination?

Is the right/ proper catalyst selected?

Selection of the reactor design

Is the target outlet gas composition reached?

Is the right/proper reactor design selected?

Design of outlet gas cooler and condensing unit

Is the right design selected

Assessment of economical aspects

Assessment of environmental aspects

Technical aspects of catalyst preparation

Conversion efficiency

Selection of reactor type

Calculus of construction parameters

Calculus of operational parameters

Design of mixing and heating chamber

Is the right design selected

Calculus of the final outlet gas composition and properties

Fig. 1. General concept of the methanation process.

The efficiency of a catalyst can be defined by COx conversion or CH4 selectivity. Higher catalyst activity and CH4 selectivity, higher demanded catalyst it is. Catalysts also should be high resistant to thermal degradation, fouling, crushing, and poisoning. For example, during methanation process at lower temperatures may form carbon. This carbon deposits on catalytic surface this way reducing catalyst performance.

Another aspect is catalyst poisoning with chemical compounds (e.g. sulphur, potassium). These undesired compounds may enter the reactor with inlet gas mixture. It depends on inlet gas source (e.g. biogas treatment, coal gasification). In this case, to avoid catalyst determination, pretreatment is required.

4. Results and conclusions

Methanation process has several advantages from the environmental point of view. This reaction kills two birds in one shot: it may reduce GHG emissions, recycle and converse carbon monoxide or carbon dioxide into high added value products and converse hydrogen into more easily exploitable source of energy - methane. In addition, the methane produced can replace natural gas, this way saving non-renewable resources. It is more feasible considering well-developed natural gas infrastructure. Moreover, this chemical energy can be obtained by using the excess energy produced by irregular energy sources (as wind and sun) solving energy storage problems.

Methanation reaction was discovered more than 100 years ago, but due to the debate about a sustainable energy supply, research on methanation have got a second wind during the last ten years. There are several examples of researches on methanation process. Many aspects are discussed. This research was aimed to combine all most important parameters and technical aspects into one scheme. It will be used for real methnation reactor design in the future studies.

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