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Pilot plant initial results for the methanation process using CO2 from amine scrubbing at the Łaziska power plant in Poland
Fuel 263 (2020) 116804
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Full Length Article
Pilot plant initial results for the methanation process using CO2 from amine scrubbing at the Łaziska power plant in Poland
T
⁎
Tadeusz Chwołaa, , Tomasz Spietza, Lucyna Więcław-Solnya, Adam Tatarczuka, Aleksander Krótkia, Szymon Dobrasa, Andrzej Wilka, Janusz Tchórzb, Marcin Steca, Janusz Zdebb a b
Institute for Chemical Processing of Coal, Zamkowa 1, 41-803 Zabrze, Poland TAURON Wytwarzanie S.A., Promienna 51, 43-603 Jaworzno, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: CNG CO2 utilization Energy storage Methanation SNG
This article presents preliminary results of the methanation process using CO2 from amine scrubbing. The studies were carried out at the CO2-SNG pilot plant. The installation was built by TAURON Wytwarzanie S.A. at the Łaziska Power Plant in Poland. After the commissioning, the Institute for Chemical Processing of Coal (IChPW) was responsible for conducting research. Synthetic methane (SNG) is produced by the reaction of CO2 captured from flue gas (using amine absorption) with H2 obtained from water electrolysis. The methanation reaction takes place in a two-stage catalytic reactor. After the compression the SNG could be used as a fuel for internal combustion engines (CNG). This paper describes in detail the construction of the pilot plant as well as the guidelines for the methanation process. Furthermore, the impact of process gas flow, reactor temperature and system pressure on the conversion of CO2 to methane is presented. The tests were carried out at process gas flows in the range of 9.9–23.0 m3N/h, at pressures of 1.5–3.0 bara and temperatures of 280–350 °C. The maximum obtained CO2 conversion was 98%. The produced SNG consisted of about 82% of methane, 13% of hydrogen and 5% of CO2.
1. Introduction The abundant use of fossil fuels has become a cause of concern due to their adverse effects on the environment, particularly related to the emission of carbon dioxide, a major anthropogenic greenhouse gas. In order to meet the European Union’s CO2 targets [1], CO2 capture and storage (CCS) as well as CO2 capture and utilization (CCU) techniques are implemented [2–4]. Increasing the share of renewable energy requires a more flexible of the energy system which can be achieved, for example, by converting excess renewable electricity into fuel. The use, storage and processing of these fuels is common and technically advanced. A power to gas (PtG) process converting hydrogen and CO2 into SNG is receiving increasing attention. There are many PtG research projects in Europe and in the world: ETOGAS, HELMETH, BioPower2Gas, Underground Sun Storage and many others [5–9]. Fuel produced in PtG processes can be ecological when hydrogen is produced from RES and CO2 is taken from flue gas or other process gases (blast furnace gas, gas from cement industry etc.) [10].
The production of fuels is one of the most prospective directions for carbon dioxide utilization both in terms of the size of reduced CO2 emission and the size of the potential market. However, it is important to note that without the implementation of a product development strategy, the synthesis of fuels from CO2 will constitute only a small share of the CO2 utilization technologies. In addition to reducing CO2 emissions, the conversion of carbon dioxide to fuels is also a good way to solve other issues of current EU energy policy, such as the management of surplus electricity production and the renewable energy storage [11]. Depending on the catalyst used and the reaction conditions, reacting CO2 with H2 produces either methane or methanol (Fig. 1) [12]. The methanation process is based on a chemical reaction in which carbon monoxide and/or carbon dioxide is converted to methane. The production of methane in the Sabatier reaction (1) is a well-known process of transforming CO2 into useful chemical products.
CO2 (g ) + 4H2 (g) ↔ CH 4 (g) + 2H2 O(g); ΔH°298K = −165 kJ mol−1
(1)
Abbreviations: bara, absolute pressure; CCS, carbon capture and storage; CCU, carbon capture and utilization; CNG, compressed natural gas; EU, European Union; m3N, cubic meter at standard temperature and pressure, 273.15 K and 100 kPa; PtG, the power to gas; RES, renewable energy sources; SNG, synthetic natural gas ⁎ Corresponding author. E-mail address: [email protected] (T. Chwoła). https://doi.org/10.1016/j.fuel.2019.116804 Received 31 May 2019; Received in revised form 23 September 2019; Accepted 30 November 2019 Available online 09 December 2019 0016-2361/ © 2019 Institute for Chemical Processing of Coal. 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/).
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become necessary when the share of renewable energy exceeds 50%. To study the conversion of CO2 to methane, the pilot plant was designed and constructed within the project CO2-SNG of InnoEnergy [16]. The TAURON Wytwarzanie S.A. (belongs to Tauron Group, the largest distributor of electricity in Poland) being a project leader was responsible for building and testing the CO2-SNG pilot plant. The AGH University of Science and Technology (Kraków, Poland) was responsible for the kinetic study of methanation reaction over various catalysts. ATMOSTAT (a subsidiary of the Alcen group, France) was responsible for the development and delivery of a modular structured methanation microchannel reactor for the CO2-SNG pilot plant. CEA (French Alternatives Energies and Atomic Energy Commission) was responsible for pilot research of the lab-scale reactor at a gas flow of 3–4 m3N/h and conceptual engineering for a demonstration plant (also, being the part of the project). Exergon Sp. z o.o. (Poland) was responsible for thermodynamic modelling of the process, and contributed to preparing the feasibility study and business case. IChPW (Institute for Chemical Processing of Coal) was responsible for CO2-SNG pilot plant research and the carbon capture process scale-up. RAFAKO S.A. (belonging to PBG Group, the largest European boiler manufacturer) was responsible for co-designing of the demonstration plant, supervising construction works on the plant and commercialization of the project results. The WTT (West Technology & Trading Polska Sp. z o.o.) was responsible for preparing the process documentation required for the plant design and for purchasing devices and equipment and their integration. This paper describes the results obtained during research campaigns carried out at the CO2-SNG pilot plant at the Łaziska Power Plant in Poland in 2018 (Fig. 3). The aim of these campaigns was to verify the correct operation of all devices in the newly-built CO2-SNG pilot plant. The influence of basic process parameters, such as temperature, pressure or gas flow rate on the CO2 conversion and the composition of produced SNG was also investigated. This paper focuses on the SNG section, where the methanation process occurs. An integral part of the CO2-SNG installation is the carbon capture plant. The description of this section and its efficiency have been described in detail in our previous papers [17,18].
Fig. 1. Diagram illustrating the reaction of carbon dioxide with hydrogen.
However, methane production using this method has never been implemented on a large scale due to good availability and a low price of natural gas. This technology has gained renewed interest in recent years due to the EU’s regulations and the need to develop effective methods of utilizing carbon dioxide and energy storage from RES. The Sabatier reaction occurs at elevated temperature and pressure in the presence of metallic catalysts, leading to the formation of methane and water. The hydrogenation reactions of carbon monoxide and carbon dioxide are strongly exothermic. Therefore, a high temperature limits the CO conversion, and especially the conversion of CO2. To achieve a CO2 conversion of 98%, temperatures below 230 °C (at 1 bar) or 325 °C (at 20 bar) are required [13]. In order to be able to use SNG in the gas grid, it must meet natural gas standards. A typical natural gas contains more than 80% methane [14]. The other components are ethane, propane and butane which increase the calorific value of the gas compared to pure methane. However, natural gas may also contain nitrogen and carbon dioxide which reduce the calorific value of the gas. The methanation process conducted over a nickel catalyst has a selectivity of almost 100%. In the case of 98% CO2 conversion, the methane content in the produced gas is slightly above 90% vol. For 99% conversion, the CH4 content is 95% vol. The presence of inert components or over-stoichiometric H2/CO2 ratio decreases methane content in the produced gas. Therefore, the purity requirements for feed gas (directed to the methanation reactors) are very important [7]. The overall efficiency of energy storage and reuse (power-to-power) is quite high. In the case of SNG synthesis, the energy stored in methane accounts for 53.2% of the input energy. The maximum recoverable energy in the form of heat and electricity is 37.3% [15]. The process chain for energy storage in the form of methane is presented in Fig. 2. Literature data indicate that large-scale chemical energy storage will
2. Experimental 2.1. Chemicals Hydrogen (the purity > 99.5% vol.) was obtained by water electrolysis in electrolyzer manufactured by IDROENERGY S.p.A. (Italy). Carbon dioxide was withdrawn from the carbon capture plant (aminebased post-combustion capture) and its purity was > 96.7% vol. Technical Nitrogen 4.0 was obtained from Messer Polska Sp. z o.o. Potassium hydroxide (the purity > 90%) was provided by the electrolyzer manufacturer. Thermal Oil (Therminol 68) was supplied by KRAHN Chemie Polska Sp. z o.o. 2.2. CO2-SNG pilot plant The CO2-SNG plant is one of the first pilot plants in the world using a microchannel reactor. The target of the plant is a conversion of CO2 to 4.5 m3N/h of SNG. The CO2 is captured from the flue gas withdrawn from the coal-fired boiler of Łaziska Power Plant.[19] To be compatible with the use of renewable electricity, the pilot plant has been designed to work in the range from 20% to 100% of the nominal capacity. It has been designed and constructed using standard dimension containers. The pilot plant consists of a five containers: the supervision, power control, gas analyzer, electrolyzer and methanation (length: 40 feet). The methanation container is the heart of the system, it includes gas pressure and flow controllers of gases, the two-stage reactor and cooling circuits. So far, the methanation systems used in the industry consist of
Fig. 2. Process chain for power storage systems methane. 2
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Fig. 3. CO2-SNG pilot plant at the Łaziska power Plant.
b) Deep CO2 purification from sulfur compounds
cascaded adiabatic reactors or tubular reactors. In these types of reactors, process temperature control is difficult and operating and maintenance costs are high. The methanation reactors developed by CEA and ATMOSTAT are based on a unique design that provides better control of the methanation process temperature. In the structural microchannel reactor concept, the methanation reaction is carried out in “reaction channels” filled with fine-grained catalyst, while the heat is transferred from the reaction zone by the thermal oil circulating in the “cooling channels”. The production of SNG in the pilot plant comprises the following unit operations:
The carbon dioxide captured from the CO2 capture plant is at a slight overpressure of about 10 kPa and therefore it is necessary to compress the gas in a compressor. The CO2 compression system consists of a compressor, cooling system, the CO2 dryer, a buffer tank and filters. If CO2 cannot be obtained from the CO2 capture plant, carbon dioxide is supplied from a 50 L gas cylinders. The catalyst for the methanation reaction is sensitive to sulfur poisoning. Therefore, it is necessary to use a deep desulphurization system for the inlet CO2 stream. The deep desulphurization system consists of an adsorption filter with a 3-stage bed. The system removes of CS2, SO2 and SO3 compounds to a concentration below 1 ppm.
• purifying CO before compressing and directing to the methanation process, • hydrogen production through water electrolysis, • feeding of CO and H in a suitable flow ratio to the gas mixer, • pumping the process gas to I methanation stage, • pumping the process gas to II methanation stage, • cooling methane with the condensation of water vapor • SNG drying, • compression of SNG to CNG or combustion of SNG in the flare. 2
2
c) First and second methanation stage
2
This system consists of the following units:
• First and second preheater – are shell-and-tube heat exchangers wherein the heating medium is thermal oil (Therminol 68). • Methanation reactors –the first reactor stage consists of three stan-
dalone sections connected in parallel. Each section comprises reaction channels filled with a catalyst and heating/cooling channels containing thermal oil. The first reactor stage has been designed so that it is possible to receive the heat of reaction using thermal oil. Thermal oil is used in the start-up phase as a heating medium, while during normal operation of the reactors it receives heat generated during the methanation process. In order to adjust the oil temperature, a cooler was installed on the oil pipeline. Additionally, the oil tank was equipped with a heater. The cooling of the reactor is controlled via the flow rate of thermal oil. The second reactor stage consists of only one element but its principle of operation is the same as the first reactor. The second reactor is fed with thermal oil from another tank than the first reactor. As a result, the second stage of the reactor can operate at a different temperature than the first one. The temperature regulating system is the same as in the first reactor. The most important parameters of the methanation reactors are presented in Table 2. Condensers and water separators – to condense water formed in the reaction, the water saturated gas is directed to the condensers (shelland-tube heat exchangers). Thermal oil and water are cooling media for first and second condenser, respectively. The cooling media flow in tubes, while gas flows in the inter-pipe space. The thermal oil of the first condensate system flows in a closed loop system between the buffer tank and the air cooler. The gas after the condensation is directed to the water separators. After cooling, the condensate flows through the carbon filters and then is collected in a tank placed on a balance. d) SNG
The scheme of the CO2-SNG installation is shown in Fig. 4. The SNG section of the pilot plant consists of the following systems: a) Hydrogen production by the electrolyzer This system consists of the following units:
• electrolyzer with hydrogen refiner and gas dryer, • hydrogen compressor – compresses the hydrogen from inlet pressure (3–4 bar ) to outlet pressure max. (18 bar ), • hydrogen buffer tank – it is a high-pressure tank with a volume of a
a
0.5 m3 from which hydrogen is directed to the process.
The electrolyzer was delivered and commissioned by the IDROENERGY S.p.A. The electrolyzer uses electric current to produce oxygen and hydrogen through electrolysis of an alkaline solution of KOH. Oxygen is released into the atmosphere and hydrogen is directed to the hydrogen refiner and the dryer and finally is pressurized in the compressor. The electrolyzer operating parameters are presented in Table 1. The hydrogen refiner removes the trace oxygen. The refiner is based on the exothermic reaction between hydrogen and oxygen over a platinum catalyst (catalytic oxidation). As a consequence, the oxygen content in the produced H2 stream is in the range 1 to 5 ppm. The dryer cools down and dries the gas using a Freon circuit. In the case of the electrolyzer failure, hydrogen is supplied from a 50 L gas cylinders.
•
3
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Fig. 4. Scheme of CO2-SNG pilot plant.
Two continuous gas analyzers (X-STREAM XEXF) supplied by Emerson Automation Solutions (Poland) are used to measure the gas composition. The first is responsible for continuous SO2 measurement in the gas inlet to the installation. The second measures following gas components: CO2 (0–100% vol.), CO (0–40% vol.), H2 (0–100% vol.), O2 (0–25% vol.) and CH4 (0–100% vol.). The X-STREAM analyzers use non-dispersive infrared photometry (NDIR), paramagnetic oxygen, and thermal conductivity (TCD) sensor technologies. The accuracy of the gas analyzers is 1% of the measuring range. The gas composition can be determined individually at four different points: after the gas mixer, after the first and the second stage of the reactor and before the CNG compressor. Gas from each sampling line enters the gas analyzer one at a time by means of a multiplexer. The analyzer is automatically calibrated (once a day) using certified calibration standard gas mixtures.
Table 1 The electrolyzer operating parameters. Electrolyzer
Model: 33.0 PLC
Max hydrogen production Max oxygen production Max working pressure Gas purity Electric power
22 m3N/h 11 m3N /h 5–7 bara > 99.5% 122 kW
SNG obtained from the reactors is flared or pumped to the compressor in order to produce CNG. The CNG is directed to the CNG storage and the loading station (and can be further used to fuel a car). The CNG compressor is equipped with pre-and final filters. The capacity of the compressor is 5 m3N, operating pressure: 200–250 bar. The CNG filling station is comprised of four gas storage tanks (total storage capacity is 600 m3N / 300 bar of gas), buffer tank, reduction valve (10 bar / 0.2 bar), and a compressor control system.
2.3. Experimental program Every research campaign lasted approximately 100 h of continuous operation. Campaigns were divided into shorter periods during which the influence of different process parameters on the process was investigated. The following numbering scheme for tests is used in this
e) Gas analyzer
4
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Table 2 The most important parameters of the methanation reactors. Parameter
I stage
II stage
Gas side
Oil side
Gas side
Oil side
Max working pressure (abs.) Max working temperature Working temperature Flow rate
19 bara 400 °C 250–350 °C 4.5–22.5 m3N /h
7 bara 400 °C 250–350 °C 160–200 l/min
19 bara 400 °C 250–380 °C
7 bara 400 °C 250–400 °C 50–70 l/min
Mass of the catalyst H2 (%vol.) CO2 (%vol.)
< 5 kg 80% 20%
– – –
2.4–12 m3n /h < 2 kg 10–35% 3–4%
– – –
Fig. 5. Primary process parameters during the steady state.
results are given in Fig. 6. An increase in inert gas flow rate at pressure of 2.5 bara and 5.0 bara causes the pressure drop across the reactor bed to increase. Moreover, the pressure drop across the second stage is higher than in the first stage. This dependence is more noticeable at the pressure of 2.5 bara than at 5.0 bara. The results can be used for further modelling, design and upscaling of the installation or reactor.
paper: C3.1 is the first test of campaign 3. The results presented in this paper were carefully selected from the database of trends registered in ASIX software (by ASKOM). ASIX is responsible for visualization and controlling of the plant. Data was collected every minute. The tests were considered valid when the plant reached steady state, namely the parameters (such as pressure, temperature and gas composition) were constant. The steady-state usually lasted from 1 h to2 h. The average of every parameter during the steady-state was used for further calculations. The primary process parameters during the steady-state are shown in Fig. 5.
3.2. Influence of process gas flow rate The plot in Fig. 7 shows the effect of the inlet gas flow rate on the CO2 conversion in both reactor stages. The temperature in the first reactor was maintained at 300 °C. The pressure was constant and was 3 bara. The tests were carried out in two series at two different temperatures in the second stage of the methanation reactor. In Fig. 7 a) it can be seen that the CO2 conversion decreases when the inlet gas flow rate increases. The H2/CO2 flow ratio was approx. 3.9/1. In test C3.8, the gas flow ratio was slightly higher (4.16/1). The CO2 conversion in the first stage was lower than in the second stage. In Fig. 7 b) it can be seen that an increase in the inlet gas flow rate resulted in a decline of CO2 conversion. Via comparison of the results shown in Fig. 7 a) and Fig. 7 b) one can observe that increasing the temperature in the second reactor stage from 280 °C to 350 °C resulted in an increase in the overall CO2 conversion. This dependence was observed for an inlet gas flow rate in the range of 4.7–10.4 kg/h. The highest CO2 conversion was obtained at the gas flow rate of 5.1 and 10.1 kg/h respectively. The overall CO2 conversion was 95% (after two stages of the reactor). A high conversion obtained at the gas flow rate of 10.1 kg/h could result from the stoichiometric ratio of H2/CO2 (3.99/1). Moreover, a lower H2/CO2 ratio in tests C4.2-C4.5 than in test C3.9-C3.12 could be responsible for a lower CO2 conversion after first methanation stage. The second stage of the reactor increased the CO2 conversion by
3. Results and discussion This chapter describes the results of the conducted research campaigns R1-C4. The most important parameters of the conducted tests are presented in Table 3. CO2 conversion is calculated as follows:
CO2 conversion =
nCO2in − nCO2out ∙100% nCO2in
(2)
where nCO2in, nCO2out are mole flow rates of CO2 in the inlet and the outlet gas (kmol/h), respectively. 3.1. Pressure drop in I and II stage reactors The reactor pressure drop in both stages was measured after commissioning of the CO2-SNG installation. For pressure drop measurement, nitrogen 4.0 was used. Tests were performed at ambient temperature (15–20 °C) and system pressures of 2.5 bara and 5 bara. During the tests, the inert gas flow rate was varied from 3 to 16 kg/h. When the system pressure was leveled off, the pressure drop was measured. The 5
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Table 3 The most important process parameters of the tests conducted in the pilot plant. Test no
CO2, m3N/h
H2, m3N/h
CO2/H2 ratio
Gas total flow rate, kg/h
I stage pressure drop, kPa
I stage CO2 conversion %
I stage temp. ˚C
II stage pressure drop, kPa
II stage overall CO2 conversion%
II stage temp. ˚C
R1.21 R1.22 R1.23 R1.24 R1.25 C3.1 C3.2 C3.3 C3.4 C3.5 C3.6 C3.7 C3.8 C3.9 C3.10 C3.11 C3.12 C4.1 C4.2 C4.3 C4.4 C4.5 C4.11 C4.10 C4.12 C4.13
4.3 4.3 4.3 4.2 4.4 4.3 4.4 4.3 4.3 4.3 4.3 4.6 4.5 4.0 3.4 2.7 2.0 4.3 3.8 3.3 2.8 2.2 4.4 4.5 4.4 4.4
17.5 17.5 17.4 17.4 18.1 18.0 18.3 18.0 17.8 18.5 18.2 18.3 18.5 15.6 13.1 10.5 7.9 17.3 14.1 12.4 10.4 8.3 16.7 16.6 16.6 16.6
1:4.07 1:4.07 1:4.07 1:4.13 1:4.07 1:4.2 1:4.19 1:4.15 1:4.11 1:4.33 1:4.24 1:4.02 1:4.16 1:3.87 1:3.88 1:3.87 1:3.86 1:3.99 1:3.74 1:3.72 1:3.72 1:3.74 1:3.77 1:3.72 1:3.75 1:3.75
10.1 10.0 10.0 9.9 10.4 10.1 10.2 10.2 10.1 10.1 10.1 10.6 10.4 9.3 7.8 6.3 4.7 10.1 8.7 7.7 6.4 5.1 10.2 10.3 10.2 10.2
37 37 37 38 36 37 37 37 37 37 38 38 38 34 29 24 19 34 30 27 23 19 38 41 44 46
60 69 82 89 89 92 91 91 91 88 90 87 85 87 88 90 92 83 85 85 87 89 80 80 79 78
279 284 290 294 299 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300
– – – – – 56 55 55 54 55 56 53 55 43 35 26 17 57 48 39 31 23 – – – –
– – – – – 98 97 96 95 91 96 90 88 90 91 92 94 94 93 93 94 95 – – – –
– – – – – 350 340 329 320 311 300 290 279 279 279 279 279 348 350 350 350 350 – – – –
Combined standard uncertainty for CO2 conversion is ± 2.8%.
3.3. Influence of temperature During these tests, the system pressure was maintained at 3.0 bara and the inlet gas mass flow rate was 10 kg/h. The temperature of the oil system of the first methanation stage was changed in the range 280–300 °C, by 5 °C steps. Fig. 8a) shows the effect of heating medium temperature on the CO2 conversion. During the tests, growth in CO2 conversion from 60% at 280 °C to 89% at 300 °C was observed. It should be taken into account that the indicated temperature is the temperature of the oil inside the reactor. The actual temperature of the gas directed to the reactor is slightly lower than the indicated temperature due to heat loss. The temperature of 300 °C was considered the best operating temperature in the first reactor stage because the CO2 conversion rate levelled off and the temperature in each reaction channel reached up to 570 °C. Further raising the oil temperature could cause an adverse effects in the form of sintering the catalyst which might lead to its faster deactivation. Fig. 8 b) shows the effect of heating medium temperature on the CO2 conversion in the second reactor (temperature in the first stage reactor was maintained at 300 °C). The increase in oil temperature resulted in a higher CO2 conversion rate. The tests results showed an increase in the CO2 conversion from 88% at 279 °C to 98% at 350 °C. The temperature of 350 °C was the limit for the second reactor stage. The increased temperature is favorable because of improved activity of the Ni-based catalyst at higher temperatures. Although a high-temperature operation limits the CO2 conversion (due to exothermic nature of CO2 methanation), chemical reactions are kinetically limited at low temperature.
Fig. 6. The impact of inert gas flow rate on pressure drop in both reactor stages at 2.5 and 5.0 bar (absolute pressure).
about 2–3 percentage points (pp) in C4.2-C4.5 tests and by about 7 pp in C 4.1 test in relation to the tests (C3.8-C3.12) carried out at the second reactor temperature of 280 °C. The studies have shown that a temperature of 350 °C is sufficient for the efficient work of the second stage of the reactor. An increase in temperature in the second stage of the reactor causes a significant growth in CO2 conversion. The composition of produced gas in the methanation reactor for selected test is given in Table 4. Comparing the results from Fig. 6, Table 4 and Table 3 it can be seen that lower gas total flow and stoichiometric H2/CO2 ratio favor a high CH4 content in produced gas. The best results were obtained in test C4.5 where methane content was 58.1% and 82.4% in first and second reactor stage, respectively. The other components in produced gas are unreacted hydrogen and CO2. Large quantities of these gases are particularly presented in first methanation stage and at lower methanation temperature.
3.4. Influence of system pressure In addition to the effect of temperature on CO2 conversion, the effect of system pressure was investigated. The results are shown in Fig. 9. During the test, the pressure was changed in the range 1.5–3.0 bara. The pressure of 3.0 bara was the operating limit. The plot shows that increasing the system pressure increases CO2 conversion. However, the increase in CO2 conversion is not as significant as during the 6
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Fig. 7. The influence of inlet gas flow rate on the CO2 conversion in both reactor stages: a) 3.0 bara, 300 °C – I stage and 280 °C – II stage, b) 3.0 bara, 300 °C – I stage and 350 °C – II stage.
temperature change experiments. The pressure increases CO2 conversion according to Le Chatelier’s principle. Increasing pressure shifts the equilibrium of the CO2 methanation reaction (reaction (1)) to the products side where a lower number of moles is formed.
•
3.5. Issues encountered during the operation of the pilot plant
•
During the commissioning and operation of the CO2-SNG pilot plant some difficulties have been encountered. Hydrogen and CO2 compressors work periodically to fill a buffer tank. When the pressure in the buffer tank reaches a set value, the compressor switches off automatically. Periodic operation of compressors resulted in large fluctuations in the gas flow entered the methanation reactor. These fluctuations were lower when H2 and CO2 were supplied from gas cylinders. During the long tests (over 24 h), a decrease in CO2 conversion was observed. It was probably caused by accumulation of water on the catalyst. Unfortunately, the authors could not experimentally confirm this statement. Furthermore, even small variations in feed gas composition caused by normal operation of the automatic control valves (for H2 and CO2) can result in a decrease in CO2 conversion (and thus reducing CH4 content in SNG).
• •
4. Conclusions
respectively. The lowest conversion was observed for inlet gas flow rate 8.7 kg/h and it was 93% (at process parameters: pressure 3.0 bara, I reactor temperature: 300 °C, II reactor temperature 350 °C). The produced SNG consisted of about 82% of methane, 13% of hydrogen and 5% of CO2. However, when the process parameters were suboptimal, CH4 concentration was about 47% vol., the rest of the gases were H2 (45% vol.) and CO2 (8% vol.). The CO2 conversion increases significantly with the increase of reactor temperature and it is related to better catalyst performance at higher temperatures. Raising the temperature in the I reactor from 279 to 300 °C resulted in an increase in the CO2 conversion from 60 to 89%. Raising the temperature in the II reactor from 279 to 350 °C resulted in an increase in overall CO2 conversion from 88 to 98%. The temperature of 300 °C and 350 °C in the first and second reactor respectively, is sufficient for the efficient work of the methanation reactor. In this case, CO2 conversion was approximately 95%. Increasing the system pressure causes growth in CO2 conversion according to Le Chatelier's principle. However, the CO2 conversion growth is not as significant as during the temperature change experiments. The CO2 conversion increased from 78% at 1.5 bara to 80% at 3.0 bara.
Finally, it must be stressed that the results presented in this paper comprise only the initial studies. More complex tests, including tests at higher system pressures, repeatability will be the subject of the future work.
As a part of the CO2-SNG project, the consortium designed, built and commissioned the CO2-SNG pilot plant that converts carbon dioxide to methane in the modular structured reactor over a catalyst. Four initial research campaigns were described and the conclusions are as follows:
Acknowledgements
• The CO
2 conversion decreases when the inlet gas flow rate increases. The highest conversion and methane content was observed for inlet gas flow rate of 5.1 kg/h and it was 95% and 82.4% vol.,
This activity has received funding from the European Institute of Innovation and Technology (EIT). This body of the European Union’s Horizon 2020 research and innovation programme. Project: CO2 METHANATION SYSTEM FOR ELECTRICITY STORAGE THROUGH
Table 4 The composition of the produced gas in first (I) and second (II) methanation reactor stage. C4.5
C4.3
C4.1
C3.12
C3.10
C3.8
component
I
II
I
II
I
II
I
II
I
II
I
II
CH4 % vol. CO % vol. CO2% vol. H2 % vol.
58.1 0.0 9.3 32.5
82.4 0.0 4.4 13.2
51.8 0.0 11.3 36.7
75.4 0.1 6.6 17.8
46.3 0.0 10.0 43.5
70.9 0.1 1.9 27.0
67.3 0.0 6.5 26.1
73.5 0.1 5.2 21.1
55.7 0.1 8.8 35.4
59.4 0.0 7.7 32.7
44.2 0.1 8.5 47.1
46.8 0.1 7.9 45.1
Combined standard uncertainty for H2, CO2 and CH4 is ± 0.8% vol. 7
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Fig. 8. The effect of temperature on CO2 conversion rate a) – first reactor stage, b) – second reactor stage at the gas flow of 10 kg/h and pressure of 3.0 bara, temperature of first stage 300 °C. 10.1016/j.jcat.2016.04.003. [5] Audi AG. World premiere: Audi opens power-to-gas facility, https://www.audiworld.com/articles/world-premiere-audi-opens-power-to-gas-facility/. Published 25.01.2013 [accessed 16.09.2019]. [6] Rönsch S, Schneider J, Matthischke S, et al. Review on methanation – from fundamentals to current projects. Fuel 2016;166:276–96. https://doi.org/10.1016/j. fuel.2015.10.111. [7] Wilk A, Więcław-Solny L, Tatarczuk A, et al. Energy storage in methane as a form of CO2 utilization in the energy sector. Przemysł Chemiczny 2017;96(5):1146–51. https://doi.org/10.15199/62.2017.5.34. [8] Power-to-Gas Demonstration Projects, http://www.europeanpowertogas.com/demonstrations. [accessed 24.01. 2016]. [9] Bailera M, Lisbona P, Romeo LM, Espatolero S. Power to gas projects review: lab, pilot and demo plants for storing renewable energy and CO2. Renew Sustain Energy Rev 2017;69:292–312. https://doi.org/10.1016/j.rser.2016.11.130. [10] Dobras S, Więcław-Solny L, Wilk A, Tatarczuk A. Methane from Power to Gas processes – ecological fuel for powering combustion engines. Zeszyty Naukowe Instytutu Gospodarki Surowcami Mineralnymi i Energią PAN. 2018;(104):97–106. doi: 10.24425/124359. [11] Ampelli C, Perathoner S, Centi G. CO2 utilization: an enabling element to move to a resource- and energy-efficient chemical and fuel production. Phil Trans R Soc A. 2015;373(2037):20140177. https://doi.org/10.1098/rsta.2014.0177. [12] Więcław-Solny L, Wilk A, Chwoła T, Krótki A, Tatarczuk A, Zdeb J. Catalytic carbon dioxide hydrogenation as a prospective method for energy storage and utilization of captured CO2. J Power Technol 2016;96(4):213–8. [13] Götz M, Lefebvre J, Mörs F, et al. Renewable power-to-gas: a technological and economic review. Renewable Energy 2016;85:1371–90. https://doi.org/10.1016/j. renene.2015.07.066. [14] Faramawy S, Zaki T, Sakr AA-E. Natural gas origin, composition, and processing: a review. J Nat Gas Sci Eng 2016;34:34–54. https://doi.org/10.1016/j.jngse.2016. 06.030. [15] Bertau M, Offermanns H, Plass L, Schmidt F, Wernicke H-J. Methanol: The Basic Chemical and Energy Feedstock of the Future: Asinger’s Vision Today. Springer Science & Business Media; 2014. [16] CO2-SNG, http://www.innoenergy.com/innovationproject/our-innovation-projects/co2-sng/ [accessed 27.02.2019]. [17] Stec M, Tatarczuk A, Więcław-Solny L, et al. Demonstration of a post-combustion carbon capture pilot plant using amine-based solvents at the Łaziska Power Plant in Poland. Clean Techn Environ Policy 2016;18(1):151–60. https://doi.org/10.1007/ s10098-015-1001-2. [18] Stec M, Tatarczuk A, Więcław-Solny L, Krótki A, Ściążko M, Tokarski S. Pilot plant results for advanced CO2 capture process using amine scrubbing at the Jaworzno II Power Plant in Poland. Fuel 2015;151:50–6. https://doi.org/10.1016/j.fuel.2015. 01.014. [19] CO2 Methanation System for Electricity Storage Through SNG Production. 2019. Project information folder, https://www.tauron-wytwarzanie.pl/-/media/wytwarzanie/innowacje/co2-sng/tw-folder-co2-sng.ashx [accessed 16.09.2019].
Fig. 9. The effect of system pressure on CO2 conversion in the first reactor stage (300 °C, 10 kg/h).
SNG PRODUCTION (CO2-SNG). Reference: 30_2014_IP108_CO2-SNG. References [1] Climate strategies & targets. Climate Action – European Commission. https://ec. europa.eu/clima/policies/strategies_en. Published 23.11. 2016. [accessed 16.09. 2019]. [2] Cormos A-M, Dinca C, Petrescu L, Andreea Chisalita D, Szima S, Cormos C-C. Carbon capture and utilisation technologies applied to energy conversion systems and other energy-intensive industrial applications. Fuel 2018;211:883–90. https:// doi.org/10.1016/j.fuel.2017.09.104. [3] Rafiee A, Rajab Khalilpour K, Milani D, Panahi M. Trends in CO2 conversion and utilization: a review from process systems perspective. J Environ Chem Eng 2018;6(5):5771–94. https://doi.org/10.1016/j.jece.2018.08.065. [4] Aresta M, Dibenedetto A, Quaranta E. State of the art and perspectives in catalytic processes for CO2 conversion into chemicals and fuels: the distinctive contribution of chemical catalysis and biotechnology. J Catal 2016;343:2–45. https://doi.org/
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Full Length Article
Pilot plant initial results for the methanation process using CO2 from amine scrubbing at the Łaziska power plant in Poland
T
⁎
Tadeusz Chwołaa, , Tomasz Spietza, Lucyna Więcław-Solnya, Adam Tatarczuka, Aleksander Krótkia, Szymon Dobrasa, Andrzej Wilka, Janusz Tchórzb, Marcin Steca, Janusz Zdebb a b
Institute for Chemical Processing of Coal, Zamkowa 1, 41-803 Zabrze, Poland TAURON Wytwarzanie S.A., Promienna 51, 43-603 Jaworzno, Poland
A R T I C LE I N FO
A B S T R A C T
Keywords: CNG CO2 utilization Energy storage Methanation SNG
This article presents preliminary results of the methanation process using CO2 from amine scrubbing. The studies were carried out at the CO2-SNG pilot plant. The installation was built by TAURON Wytwarzanie S.A. at the Łaziska Power Plant in Poland. After the commissioning, the Institute for Chemical Processing of Coal (IChPW) was responsible for conducting research. Synthetic methane (SNG) is produced by the reaction of CO2 captured from flue gas (using amine absorption) with H2 obtained from water electrolysis. The methanation reaction takes place in a two-stage catalytic reactor. After the compression the SNG could be used as a fuel for internal combustion engines (CNG). This paper describes in detail the construction of the pilot plant as well as the guidelines for the methanation process. Furthermore, the impact of process gas flow, reactor temperature and system pressure on the conversion of CO2 to methane is presented. The tests were carried out at process gas flows in the range of 9.9–23.0 m3N/h, at pressures of 1.5–3.0 bara and temperatures of 280–350 °C. The maximum obtained CO2 conversion was 98%. The produced SNG consisted of about 82% of methane, 13% of hydrogen and 5% of CO2.
1. Introduction The abundant use of fossil fuels has become a cause of concern due to their adverse effects on the environment, particularly related to the emission of carbon dioxide, a major anthropogenic greenhouse gas. In order to meet the European Union’s CO2 targets [1], CO2 capture and storage (CCS) as well as CO2 capture and utilization (CCU) techniques are implemented [2–4]. Increasing the share of renewable energy requires a more flexible of the energy system which can be achieved, for example, by converting excess renewable electricity into fuel. The use, storage and processing of these fuels is common and technically advanced. A power to gas (PtG) process converting hydrogen and CO2 into SNG is receiving increasing attention. There are many PtG research projects in Europe and in the world: ETOGAS, HELMETH, BioPower2Gas, Underground Sun Storage and many others [5–9]. Fuel produced in PtG processes can be ecological when hydrogen is produced from RES and CO2 is taken from flue gas or other process gases (blast furnace gas, gas from cement industry etc.) [10].
The production of fuels is one of the most prospective directions for carbon dioxide utilization both in terms of the size of reduced CO2 emission and the size of the potential market. However, it is important to note that without the implementation of a product development strategy, the synthesis of fuels from CO2 will constitute only a small share of the CO2 utilization technologies. In addition to reducing CO2 emissions, the conversion of carbon dioxide to fuels is also a good way to solve other issues of current EU energy policy, such as the management of surplus electricity production and the renewable energy storage [11]. Depending on the catalyst used and the reaction conditions, reacting CO2 with H2 produces either methane or methanol (Fig. 1) [12]. The methanation process is based on a chemical reaction in which carbon monoxide and/or carbon dioxide is converted to methane. The production of methane in the Sabatier reaction (1) is a well-known process of transforming CO2 into useful chemical products.
CO2 (g ) + 4H2 (g) ↔ CH 4 (g) + 2H2 O(g); ΔH°298K = −165 kJ mol−1
(1)
Abbreviations: bara, absolute pressure; CCS, carbon capture and storage; CCU, carbon capture and utilization; CNG, compressed natural gas; EU, European Union; m3N, cubic meter at standard temperature and pressure, 273.15 K and 100 kPa; PtG, the power to gas; RES, renewable energy sources; SNG, synthetic natural gas ⁎ Corresponding author. E-mail address: [email protected] (T. Chwoła). https://doi.org/10.1016/j.fuel.2019.116804 Received 31 May 2019; Received in revised form 23 September 2019; Accepted 30 November 2019 Available online 09 December 2019 0016-2361/ © 2019 Institute for Chemical Processing of Coal. 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/).
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become necessary when the share of renewable energy exceeds 50%. To study the conversion of CO2 to methane, the pilot plant was designed and constructed within the project CO2-SNG of InnoEnergy [16]. The TAURON Wytwarzanie S.A. (belongs to Tauron Group, the largest distributor of electricity in Poland) being a project leader was responsible for building and testing the CO2-SNG pilot plant. The AGH University of Science and Technology (Kraków, Poland) was responsible for the kinetic study of methanation reaction over various catalysts. ATMOSTAT (a subsidiary of the Alcen group, France) was responsible for the development and delivery of a modular structured methanation microchannel reactor for the CO2-SNG pilot plant. CEA (French Alternatives Energies and Atomic Energy Commission) was responsible for pilot research of the lab-scale reactor at a gas flow of 3–4 m3N/h and conceptual engineering for a demonstration plant (also, being the part of the project). Exergon Sp. z o.o. (Poland) was responsible for thermodynamic modelling of the process, and contributed to preparing the feasibility study and business case. IChPW (Institute for Chemical Processing of Coal) was responsible for CO2-SNG pilot plant research and the carbon capture process scale-up. RAFAKO S.A. (belonging to PBG Group, the largest European boiler manufacturer) was responsible for co-designing of the demonstration plant, supervising construction works on the plant and commercialization of the project results. The WTT (West Technology & Trading Polska Sp. z o.o.) was responsible for preparing the process documentation required for the plant design and for purchasing devices and equipment and their integration. This paper describes the results obtained during research campaigns carried out at the CO2-SNG pilot plant at the Łaziska Power Plant in Poland in 2018 (Fig. 3). The aim of these campaigns was to verify the correct operation of all devices in the newly-built CO2-SNG pilot plant. The influence of basic process parameters, such as temperature, pressure or gas flow rate on the CO2 conversion and the composition of produced SNG was also investigated. This paper focuses on the SNG section, where the methanation process occurs. An integral part of the CO2-SNG installation is the carbon capture plant. The description of this section and its efficiency have been described in detail in our previous papers [17,18].
Fig. 1. Diagram illustrating the reaction of carbon dioxide with hydrogen.
However, methane production using this method has never been implemented on a large scale due to good availability and a low price of natural gas. This technology has gained renewed interest in recent years due to the EU’s regulations and the need to develop effective methods of utilizing carbon dioxide and energy storage from RES. The Sabatier reaction occurs at elevated temperature and pressure in the presence of metallic catalysts, leading to the formation of methane and water. The hydrogenation reactions of carbon monoxide and carbon dioxide are strongly exothermic. Therefore, a high temperature limits the CO conversion, and especially the conversion of CO2. To achieve a CO2 conversion of 98%, temperatures below 230 °C (at 1 bar) or 325 °C (at 20 bar) are required [13]. In order to be able to use SNG in the gas grid, it must meet natural gas standards. A typical natural gas contains more than 80% methane [14]. The other components are ethane, propane and butane which increase the calorific value of the gas compared to pure methane. However, natural gas may also contain nitrogen and carbon dioxide which reduce the calorific value of the gas. The methanation process conducted over a nickel catalyst has a selectivity of almost 100%. In the case of 98% CO2 conversion, the methane content in the produced gas is slightly above 90% vol. For 99% conversion, the CH4 content is 95% vol. The presence of inert components or over-stoichiometric H2/CO2 ratio decreases methane content in the produced gas. Therefore, the purity requirements for feed gas (directed to the methanation reactors) are very important [7]. The overall efficiency of energy storage and reuse (power-to-power) is quite high. In the case of SNG synthesis, the energy stored in methane accounts for 53.2% of the input energy. The maximum recoverable energy in the form of heat and electricity is 37.3% [15]. The process chain for energy storage in the form of methane is presented in Fig. 2. Literature data indicate that large-scale chemical energy storage will
2. Experimental 2.1. Chemicals Hydrogen (the purity > 99.5% vol.) was obtained by water electrolysis in electrolyzer manufactured by IDROENERGY S.p.A. (Italy). Carbon dioxide was withdrawn from the carbon capture plant (aminebased post-combustion capture) and its purity was > 96.7% vol. Technical Nitrogen 4.0 was obtained from Messer Polska Sp. z o.o. Potassium hydroxide (the purity > 90%) was provided by the electrolyzer manufacturer. Thermal Oil (Therminol 68) was supplied by KRAHN Chemie Polska Sp. z o.o. 2.2. CO2-SNG pilot plant The CO2-SNG plant is one of the first pilot plants in the world using a microchannel reactor. The target of the plant is a conversion of CO2 to 4.5 m3N/h of SNG. The CO2 is captured from the flue gas withdrawn from the coal-fired boiler of Łaziska Power Plant.[19] To be compatible with the use of renewable electricity, the pilot plant has been designed to work in the range from 20% to 100% of the nominal capacity. It has been designed and constructed using standard dimension containers. The pilot plant consists of a five containers: the supervision, power control, gas analyzer, electrolyzer and methanation (length: 40 feet). The methanation container is the heart of the system, it includes gas pressure and flow controllers of gases, the two-stage reactor and cooling circuits. So far, the methanation systems used in the industry consist of
Fig. 2. Process chain for power storage systems methane. 2
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Fig. 3. CO2-SNG pilot plant at the Łaziska power Plant.
b) Deep CO2 purification from sulfur compounds
cascaded adiabatic reactors or tubular reactors. In these types of reactors, process temperature control is difficult and operating and maintenance costs are high. The methanation reactors developed by CEA and ATMOSTAT are based on a unique design that provides better control of the methanation process temperature. In the structural microchannel reactor concept, the methanation reaction is carried out in “reaction channels” filled with fine-grained catalyst, while the heat is transferred from the reaction zone by the thermal oil circulating in the “cooling channels”. The production of SNG in the pilot plant comprises the following unit operations:
The carbon dioxide captured from the CO2 capture plant is at a slight overpressure of about 10 kPa and therefore it is necessary to compress the gas in a compressor. The CO2 compression system consists of a compressor, cooling system, the CO2 dryer, a buffer tank and filters. If CO2 cannot be obtained from the CO2 capture plant, carbon dioxide is supplied from a 50 L gas cylinders. The catalyst for the methanation reaction is sensitive to sulfur poisoning. Therefore, it is necessary to use a deep desulphurization system for the inlet CO2 stream. The deep desulphurization system consists of an adsorption filter with a 3-stage bed. The system removes of CS2, SO2 and SO3 compounds to a concentration below 1 ppm.
• purifying CO before compressing and directing to the methanation process, • hydrogen production through water electrolysis, • feeding of CO and H in a suitable flow ratio to the gas mixer, • pumping the process gas to I methanation stage, • pumping the process gas to II methanation stage, • cooling methane with the condensation of water vapor • SNG drying, • compression of SNG to CNG or combustion of SNG in the flare. 2
2
c) First and second methanation stage
2
This system consists of the following units:
• First and second preheater – are shell-and-tube heat exchangers wherein the heating medium is thermal oil (Therminol 68). • Methanation reactors –the first reactor stage consists of three stan-
dalone sections connected in parallel. Each section comprises reaction channels filled with a catalyst and heating/cooling channels containing thermal oil. The first reactor stage has been designed so that it is possible to receive the heat of reaction using thermal oil. Thermal oil is used in the start-up phase as a heating medium, while during normal operation of the reactors it receives heat generated during the methanation process. In order to adjust the oil temperature, a cooler was installed on the oil pipeline. Additionally, the oil tank was equipped with a heater. The cooling of the reactor is controlled via the flow rate of thermal oil. The second reactor stage consists of only one element but its principle of operation is the same as the first reactor. The second reactor is fed with thermal oil from another tank than the first reactor. As a result, the second stage of the reactor can operate at a different temperature than the first one. The temperature regulating system is the same as in the first reactor. The most important parameters of the methanation reactors are presented in Table 2. Condensers and water separators – to condense water formed in the reaction, the water saturated gas is directed to the condensers (shelland-tube heat exchangers). Thermal oil and water are cooling media for first and second condenser, respectively. The cooling media flow in tubes, while gas flows in the inter-pipe space. The thermal oil of the first condensate system flows in a closed loop system between the buffer tank and the air cooler. The gas after the condensation is directed to the water separators. After cooling, the condensate flows through the carbon filters and then is collected in a tank placed on a balance. d) SNG
The scheme of the CO2-SNG installation is shown in Fig. 4. The SNG section of the pilot plant consists of the following systems: a) Hydrogen production by the electrolyzer This system consists of the following units:
• electrolyzer with hydrogen refiner and gas dryer, • hydrogen compressor – compresses the hydrogen from inlet pressure (3–4 bar ) to outlet pressure max. (18 bar ), • hydrogen buffer tank – it is a high-pressure tank with a volume of a
a
0.5 m3 from which hydrogen is directed to the process.
The electrolyzer was delivered and commissioned by the IDROENERGY S.p.A. The electrolyzer uses electric current to produce oxygen and hydrogen through electrolysis of an alkaline solution of KOH. Oxygen is released into the atmosphere and hydrogen is directed to the hydrogen refiner and the dryer and finally is pressurized in the compressor. The electrolyzer operating parameters are presented in Table 1. The hydrogen refiner removes the trace oxygen. The refiner is based on the exothermic reaction between hydrogen and oxygen over a platinum catalyst (catalytic oxidation). As a consequence, the oxygen content in the produced H2 stream is in the range 1 to 5 ppm. The dryer cools down and dries the gas using a Freon circuit. In the case of the electrolyzer failure, hydrogen is supplied from a 50 L gas cylinders.
•
3
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Fig. 4. Scheme of CO2-SNG pilot plant.
Two continuous gas analyzers (X-STREAM XEXF) supplied by Emerson Automation Solutions (Poland) are used to measure the gas composition. The first is responsible for continuous SO2 measurement in the gas inlet to the installation. The second measures following gas components: CO2 (0–100% vol.), CO (0–40% vol.), H2 (0–100% vol.), O2 (0–25% vol.) and CH4 (0–100% vol.). The X-STREAM analyzers use non-dispersive infrared photometry (NDIR), paramagnetic oxygen, and thermal conductivity (TCD) sensor technologies. The accuracy of the gas analyzers is 1% of the measuring range. The gas composition can be determined individually at four different points: after the gas mixer, after the first and the second stage of the reactor and before the CNG compressor. Gas from each sampling line enters the gas analyzer one at a time by means of a multiplexer. The analyzer is automatically calibrated (once a day) using certified calibration standard gas mixtures.
Table 1 The electrolyzer operating parameters. Electrolyzer
Model: 33.0 PLC
Max hydrogen production Max oxygen production Max working pressure Gas purity Electric power
22 m3N/h 11 m3N /h 5–7 bara > 99.5% 122 kW
SNG obtained from the reactors is flared or pumped to the compressor in order to produce CNG. The CNG is directed to the CNG storage and the loading station (and can be further used to fuel a car). The CNG compressor is equipped with pre-and final filters. The capacity of the compressor is 5 m3N, operating pressure: 200–250 bar. The CNG filling station is comprised of four gas storage tanks (total storage capacity is 600 m3N / 300 bar of gas), buffer tank, reduction valve (10 bar / 0.2 bar), and a compressor control system.
2.3. Experimental program Every research campaign lasted approximately 100 h of continuous operation. Campaigns were divided into shorter periods during which the influence of different process parameters on the process was investigated. The following numbering scheme for tests is used in this
e) Gas analyzer
4
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Table 2 The most important parameters of the methanation reactors. Parameter
I stage
II stage
Gas side
Oil side
Gas side
Oil side
Max working pressure (abs.) Max working temperature Working temperature Flow rate
19 bara 400 °C 250–350 °C 4.5–22.5 m3N /h
7 bara 400 °C 250–350 °C 160–200 l/min
19 bara 400 °C 250–380 °C
7 bara 400 °C 250–400 °C 50–70 l/min
Mass of the catalyst H2 (%vol.) CO2 (%vol.)
< 5 kg 80% 20%
– – –
2.4–12 m3n /h < 2 kg 10–35% 3–4%
– – –
Fig. 5. Primary process parameters during the steady state.
results are given in Fig. 6. An increase in inert gas flow rate at pressure of 2.5 bara and 5.0 bara causes the pressure drop across the reactor bed to increase. Moreover, the pressure drop across the second stage is higher than in the first stage. This dependence is more noticeable at the pressure of 2.5 bara than at 5.0 bara. The results can be used for further modelling, design and upscaling of the installation or reactor.
paper: C3.1 is the first test of campaign 3. The results presented in this paper were carefully selected from the database of trends registered in ASIX software (by ASKOM). ASIX is responsible for visualization and controlling of the plant. Data was collected every minute. The tests were considered valid when the plant reached steady state, namely the parameters (such as pressure, temperature and gas composition) were constant. The steady-state usually lasted from 1 h to2 h. The average of every parameter during the steady-state was used for further calculations. The primary process parameters during the steady-state are shown in Fig. 5.
3.2. Influence of process gas flow rate The plot in Fig. 7 shows the effect of the inlet gas flow rate on the CO2 conversion in both reactor stages. The temperature in the first reactor was maintained at 300 °C. The pressure was constant and was 3 bara. The tests were carried out in two series at two different temperatures in the second stage of the methanation reactor. In Fig. 7 a) it can be seen that the CO2 conversion decreases when the inlet gas flow rate increases. The H2/CO2 flow ratio was approx. 3.9/1. In test C3.8, the gas flow ratio was slightly higher (4.16/1). The CO2 conversion in the first stage was lower than in the second stage. In Fig. 7 b) it can be seen that an increase in the inlet gas flow rate resulted in a decline of CO2 conversion. Via comparison of the results shown in Fig. 7 a) and Fig. 7 b) one can observe that increasing the temperature in the second reactor stage from 280 °C to 350 °C resulted in an increase in the overall CO2 conversion. This dependence was observed for an inlet gas flow rate in the range of 4.7–10.4 kg/h. The highest CO2 conversion was obtained at the gas flow rate of 5.1 and 10.1 kg/h respectively. The overall CO2 conversion was 95% (after two stages of the reactor). A high conversion obtained at the gas flow rate of 10.1 kg/h could result from the stoichiometric ratio of H2/CO2 (3.99/1). Moreover, a lower H2/CO2 ratio in tests C4.2-C4.5 than in test C3.9-C3.12 could be responsible for a lower CO2 conversion after first methanation stage. The second stage of the reactor increased the CO2 conversion by
3. Results and discussion This chapter describes the results of the conducted research campaigns R1-C4. The most important parameters of the conducted tests are presented in Table 3. CO2 conversion is calculated as follows:
CO2 conversion =
nCO2in − nCO2out ∙100% nCO2in
(2)
where nCO2in, nCO2out are mole flow rates of CO2 in the inlet and the outlet gas (kmol/h), respectively. 3.1. Pressure drop in I and II stage reactors The reactor pressure drop in both stages was measured after commissioning of the CO2-SNG installation. For pressure drop measurement, nitrogen 4.0 was used. Tests were performed at ambient temperature (15–20 °C) and system pressures of 2.5 bara and 5 bara. During the tests, the inert gas flow rate was varied from 3 to 16 kg/h. When the system pressure was leveled off, the pressure drop was measured. The 5
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Table 3 The most important process parameters of the tests conducted in the pilot plant. Test no
CO2, m3N/h
H2, m3N/h
CO2/H2 ratio
Gas total flow rate, kg/h
I stage pressure drop, kPa
I stage CO2 conversion %
I stage temp. ˚C
II stage pressure drop, kPa
II stage overall CO2 conversion%
II stage temp. ˚C
R1.21 R1.22 R1.23 R1.24 R1.25 C3.1 C3.2 C3.3 C3.4 C3.5 C3.6 C3.7 C3.8 C3.9 C3.10 C3.11 C3.12 C4.1 C4.2 C4.3 C4.4 C4.5 C4.11 C4.10 C4.12 C4.13
4.3 4.3 4.3 4.2 4.4 4.3 4.4 4.3 4.3 4.3 4.3 4.6 4.5 4.0 3.4 2.7 2.0 4.3 3.8 3.3 2.8 2.2 4.4 4.5 4.4 4.4
17.5 17.5 17.4 17.4 18.1 18.0 18.3 18.0 17.8 18.5 18.2 18.3 18.5 15.6 13.1 10.5 7.9 17.3 14.1 12.4 10.4 8.3 16.7 16.6 16.6 16.6
1:4.07 1:4.07 1:4.07 1:4.13 1:4.07 1:4.2 1:4.19 1:4.15 1:4.11 1:4.33 1:4.24 1:4.02 1:4.16 1:3.87 1:3.88 1:3.87 1:3.86 1:3.99 1:3.74 1:3.72 1:3.72 1:3.74 1:3.77 1:3.72 1:3.75 1:3.75
10.1 10.0 10.0 9.9 10.4 10.1 10.2 10.2 10.1 10.1 10.1 10.6 10.4 9.3 7.8 6.3 4.7 10.1 8.7 7.7 6.4 5.1 10.2 10.3 10.2 10.2
37 37 37 38 36 37 37 37 37 37 38 38 38 34 29 24 19 34 30 27 23 19 38 41 44 46
60 69 82 89 89 92 91 91 91 88 90 87 85 87 88 90 92 83 85 85 87 89 80 80 79 78
279 284 290 294 299 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300
– – – – – 56 55 55 54 55 56 53 55 43 35 26 17 57 48 39 31 23 – – – –
– – – – – 98 97 96 95 91 96 90 88 90 91 92 94 94 93 93 94 95 – – – –
– – – – – 350 340 329 320 311 300 290 279 279 279 279 279 348 350 350 350 350 – – – –
Combined standard uncertainty for CO2 conversion is ± 2.8%.
3.3. Influence of temperature During these tests, the system pressure was maintained at 3.0 bara and the inlet gas mass flow rate was 10 kg/h. The temperature of the oil system of the first methanation stage was changed in the range 280–300 °C, by 5 °C steps. Fig. 8a) shows the effect of heating medium temperature on the CO2 conversion. During the tests, growth in CO2 conversion from 60% at 280 °C to 89% at 300 °C was observed. It should be taken into account that the indicated temperature is the temperature of the oil inside the reactor. The actual temperature of the gas directed to the reactor is slightly lower than the indicated temperature due to heat loss. The temperature of 300 °C was considered the best operating temperature in the first reactor stage because the CO2 conversion rate levelled off and the temperature in each reaction channel reached up to 570 °C. Further raising the oil temperature could cause an adverse effects in the form of sintering the catalyst which might lead to its faster deactivation. Fig. 8 b) shows the effect of heating medium temperature on the CO2 conversion in the second reactor (temperature in the first stage reactor was maintained at 300 °C). The increase in oil temperature resulted in a higher CO2 conversion rate. The tests results showed an increase in the CO2 conversion from 88% at 279 °C to 98% at 350 °C. The temperature of 350 °C was the limit for the second reactor stage. The increased temperature is favorable because of improved activity of the Ni-based catalyst at higher temperatures. Although a high-temperature operation limits the CO2 conversion (due to exothermic nature of CO2 methanation), chemical reactions are kinetically limited at low temperature.
Fig. 6. The impact of inert gas flow rate on pressure drop in both reactor stages at 2.5 and 5.0 bar (absolute pressure).
about 2–3 percentage points (pp) in C4.2-C4.5 tests and by about 7 pp in C 4.1 test in relation to the tests (C3.8-C3.12) carried out at the second reactor temperature of 280 °C. The studies have shown that a temperature of 350 °C is sufficient for the efficient work of the second stage of the reactor. An increase in temperature in the second stage of the reactor causes a significant growth in CO2 conversion. The composition of produced gas in the methanation reactor for selected test is given in Table 4. Comparing the results from Fig. 6, Table 4 and Table 3 it can be seen that lower gas total flow and stoichiometric H2/CO2 ratio favor a high CH4 content in produced gas. The best results were obtained in test C4.5 where methane content was 58.1% and 82.4% in first and second reactor stage, respectively. The other components in produced gas are unreacted hydrogen and CO2. Large quantities of these gases are particularly presented in first methanation stage and at lower methanation temperature.
3.4. Influence of system pressure In addition to the effect of temperature on CO2 conversion, the effect of system pressure was investigated. The results are shown in Fig. 9. During the test, the pressure was changed in the range 1.5–3.0 bara. The pressure of 3.0 bara was the operating limit. The plot shows that increasing the system pressure increases CO2 conversion. However, the increase in CO2 conversion is not as significant as during the 6
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Fig. 7. The influence of inlet gas flow rate on the CO2 conversion in both reactor stages: a) 3.0 bara, 300 °C – I stage and 280 °C – II stage, b) 3.0 bara, 300 °C – I stage and 350 °C – II stage.
temperature change experiments. The pressure increases CO2 conversion according to Le Chatelier’s principle. Increasing pressure shifts the equilibrium of the CO2 methanation reaction (reaction (1)) to the products side where a lower number of moles is formed.
•
3.5. Issues encountered during the operation of the pilot plant
•
During the commissioning and operation of the CO2-SNG pilot plant some difficulties have been encountered. Hydrogen and CO2 compressors work periodically to fill a buffer tank. When the pressure in the buffer tank reaches a set value, the compressor switches off automatically. Periodic operation of compressors resulted in large fluctuations in the gas flow entered the methanation reactor. These fluctuations were lower when H2 and CO2 were supplied from gas cylinders. During the long tests (over 24 h), a decrease in CO2 conversion was observed. It was probably caused by accumulation of water on the catalyst. Unfortunately, the authors could not experimentally confirm this statement. Furthermore, even small variations in feed gas composition caused by normal operation of the automatic control valves (for H2 and CO2) can result in a decrease in CO2 conversion (and thus reducing CH4 content in SNG).
• •
4. Conclusions
respectively. The lowest conversion was observed for inlet gas flow rate 8.7 kg/h and it was 93% (at process parameters: pressure 3.0 bara, I reactor temperature: 300 °C, II reactor temperature 350 °C). The produced SNG consisted of about 82% of methane, 13% of hydrogen and 5% of CO2. However, when the process parameters were suboptimal, CH4 concentration was about 47% vol., the rest of the gases were H2 (45% vol.) and CO2 (8% vol.). The CO2 conversion increases significantly with the increase of reactor temperature and it is related to better catalyst performance at higher temperatures. Raising the temperature in the I reactor from 279 to 300 °C resulted in an increase in the CO2 conversion from 60 to 89%. Raising the temperature in the II reactor from 279 to 350 °C resulted in an increase in overall CO2 conversion from 88 to 98%. The temperature of 300 °C and 350 °C in the first and second reactor respectively, is sufficient for the efficient work of the methanation reactor. In this case, CO2 conversion was approximately 95%. Increasing the system pressure causes growth in CO2 conversion according to Le Chatelier's principle. However, the CO2 conversion growth is not as significant as during the temperature change experiments. The CO2 conversion increased from 78% at 1.5 bara to 80% at 3.0 bara.
Finally, it must be stressed that the results presented in this paper comprise only the initial studies. More complex tests, including tests at higher system pressures, repeatability will be the subject of the future work.
As a part of the CO2-SNG project, the consortium designed, built and commissioned the CO2-SNG pilot plant that converts carbon dioxide to methane in the modular structured reactor over a catalyst. Four initial research campaigns were described and the conclusions are as follows:
Acknowledgements
• The CO
2 conversion decreases when the inlet gas flow rate increases. The highest conversion and methane content was observed for inlet gas flow rate of 5.1 kg/h and it was 95% and 82.4% vol.,
This activity has received funding from the European Institute of Innovation and Technology (EIT). This body of the European Union’s Horizon 2020 research and innovation programme. Project: CO2 METHANATION SYSTEM FOR ELECTRICITY STORAGE THROUGH
Table 4 The composition of the produced gas in first (I) and second (II) methanation reactor stage. C4.5
C4.3
C4.1
C3.12
C3.10
C3.8
component
I
II
I
II
I
II
I
II
I
II
I
II
CH4 % vol. CO % vol. CO2% vol. H2 % vol.
58.1 0.0 9.3 32.5
82.4 0.0 4.4 13.2
51.8 0.0 11.3 36.7
75.4 0.1 6.6 17.8
46.3 0.0 10.0 43.5
70.9 0.1 1.9 27.0
67.3 0.0 6.5 26.1
73.5 0.1 5.2 21.1
55.7 0.1 8.8 35.4
59.4 0.0 7.7 32.7
44.2 0.1 8.5 47.1
46.8 0.1 7.9 45.1
Combined standard uncertainty for H2, CO2 and CH4 is ± 0.8% vol. 7
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Fig. 9. The effect of system pressure on CO2 conversion in the first reactor stage (300 °C, 10 kg/h).
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