Abstract
In the coming years, a range of technological solutions aimed at reducing CO2 emissions should be developed. The objective of this paper is to study the technical feasibility of CO2 recycling into ethanol using solid oxide electrolysis cells (SOEC) technology, simulate the whole process using Aspen Plus software and make an economic assessment and a carbon footprint.
The results of the economic analysis demonstrated that the ethanol production plant can deliver ethanol at a cost of $1.1/kg, assuming an internal rate of return on investment of 8%. Main challenges for this process are improvement of ethanol synthesis reaction catalysts and the SOEC performance.
1. Introduction
With the objective of limiting average temperature increase in the long term, it becomes necessary to reduce by 2050 the global emissions of carbon dioxide (CO2), which is the main greenhouse gas (GHG). These emissions should be reduced by 50 to 85 % compared to the measured level in 2000. It is therefore essential to develop in the upcoming years, a range of technological solutions for the limitation and reduction of CO2 emissions. One of these solutions is to capture and recycle the CO2 emitted. The main challenge of this industry is to find new applications, to improve and develop existing ones, while ensuring a neutral impact on the environment. The CO2 could be used as raw material in industrial processes, through access to non-fossil energy, it is transformed chemically or biologically to produce biofuels, chemicals or pharmaceuticals.
There are several possible pathways of CO2 recycling, and these pathways can be classified into three groups. The first group concerns the using of CO2 without transformation, meaning that dioxide carbon is used for its physical and thermodynamic proprieties as a solvent or refrigerant for example. The majority of these pathways are well known and already deployed on an industrial scale, such as Assisted Hydrocarbon Recovery (AHR), which is widely used with CO2 from natural storage. The second group consists of biological transformation of carbon dioxide: CO2 is used, through photosynthesis in biological organisms such as algae, to synthesize products of interest (carbohydrates, fats and cellulose compounds). Several pilot plants exist in different countries. The industrialization of this technology is expected in the next five years [1]. The third group regroups CO2 chemical transformation pathways: The carbon dioxide reacts with other highly reactive components, in order to complete the synthesis of basic chemicals or products with high energetic value. Among these pathways: The hydrogenation of CO2 which produces methane, methanol, or synthetic fuels. There are already pilot plants for the hydrogenation of CO2, and is expected to become industrialized in few years [1]. The recycling of CO2 has the advantage of recycling large volumes of CO2 with an average duration of CO2 sequestration [2]. Many technological challenges, and very few of feedback are the main challenges of this pathway. It should be noted that these pathways differ with respect to their degree of maturity, their potential for emergence, duration of CO2 sequestration, energy consumption, volume of CO2 recovery and energy efficiency [1].
The main objective of this paper is to deepen the CO2 electrochemical recycling, by studying the technical feasibility, simulating the whole process using Aspen Plus software and making an economic assessment and a carbon footprint. The proposed process consists of two main steps. The first one is the high temperature co-electrolysis of CO2 and water vapor, using solid oxide electrolysis cells (SOEC), to produce the syngas (H2+CO). The second step consists of converting syngas into synthetic fuel (gasoline, diesel, ethanol...) via Fisher Tropsch reactions synthesis, or convert it to ethanol or methanol via catalytic and exothermic reactions whose products depend on the operating parameters and catalysts used [3].
Fig. 1: presents the overall scheme of the studied process.
2. CO2 and H2O Co-electrolysis mechanism
Electrochemical recycling of carbon dioxide consists of using carbon-free energy (renewable or nuclear) as a source of heat and / or electricity, to allow the dissociation of CO2 and H2O. This separation can be carried out via thermolysis, photolysis or electrolysis of water vapor and carbon dioxide [2]. In this study we chose to use electrolysis to dissociate carbon dioxide and water vapor. There two ways to produce syngas via electrolysis of CO2 and H2O: dissociate CO2 and H2O separately in two different electrolysis cells or simultaneously in the same electrolysis cell [4]. Recent research has shown that co- electrolysis H2O and CO2 is more interesting in terms of energy consumption and conversion rate than separate electrolysis of water vapour and carbon dioxide. [2, 4]
The co-electrolysis of water vapor and carbon dioxide can be summarized by the following reaction:
H2O + CO → H2 + CO + O2 (1)
The first reaction to be considered is the reverse reaction of water gas shift:
CO2 + H2 ↔ CO + H2O (2)
It is slightly endothermic reaction, since △H = 41 kJ/mol
Another side reaction that can also affect the electrolysis operation is the formation of coke (carbon
deposition on the surface of the electrolysis cell) as follows:
CO → C + 1/2 O2 (3)
At a temperature below 700 ° C, and in the presence of Ni as catalyst methane is formed according to the following reaction:
CO + 3 H2 → H2O + CH4 (4)
Therefore it is preferable to use temperatures above 700 ° C to avoid the formation of methane [5].
In order to determine the composition (mole fractions of the chemical species) at the outlet of the electrolysis cell according to the operating parameters including the current density i, T the temperature and gas flow rates. H2O and CO2 co-electrolysis model is much more complicated than simple electrolysis of water or carbon dioxide. The reaction mechanism of co-electrolysis of H2O and CO2 is complicated and not fully understood because it includes three simultaneous reactions: electrolysis of CO2, H2O and the reverse reaction of water gas shift (RWGS). Until now it is not precisely known if the CO produced is formed by electrolysis of CO2, or via the reverse reaction of water gas [6]. Many researchers believe that Water Gas Shift Reaction (WGS) is the source of CO produced during co- electrolysis, because it is thermodynamically favorable to these conditions [7]. It should be noted that the electrolysis reactions are not equilibrium reactions, because of SOEC tightness that allows the separation of products and reactants, the only equilibrium reaction is the RWGS.
In our model we consider that the CO2 and H2O co-electrolysis involves three stages: the first is the RWGS reaction that takes place when gas at the inlet of the electrolysis cell are preheated at a temperature between 200 ° C and 300 ° C. The second step is the electrolytic reduction of CO2 and H2O. The last one is RWGS reaction at high temperature, typically 800 ° C, which takes place at the output of the electrolysis cell. Similarly to the model developed in [7], by using a mass balance for the four reactions mentioned above we determine the outlet composition of the electrolysis cell. Once the gas outlet composition is determined, using an energy balance of the electrolysis cell we determine the energy consumption of the electrolysis operation.
3. Ethanol synthesis
Ethanol is a 2-carbon alcohol with a molecular formula CH3-CH2-OH. It is a colourless versatile solvent miscible with water. During the last decade world production of ethanol has doubled and it is expected to triple by 2015. This constant increase can be explained by the high demand for biofuels.
As presented in Fig. 2, syngas could be converted to ethanol in three different pathways. The first pathway consists of direct synthesis of ethanol form syngas. The second pathway is syngas conversion to methanol which is converted to ethanol via methanol homologation reaction. The last pathway is known as the ENSOL process. Syngas is first converted to methanol over a methanol commercial synthesis catalyst followed by methanol carbonylation to acetic acid in the second step and, then, subsequent hydrogenation of acetic acid to ethanol. All these pathways differ in term of catalysts used, operation conditions, selectivity for ethanol and C2 oxygenates [3]. The pathway which adopted in this paper is the direct synthesis of ethanol from syngas.
Fig. 2: Possible pathways for syngas conversion to ethanol
The direct conversion of syngas to ethanol is the most studied way in terms of used catalysts and operating conditions. Thus tens of catalysts have been developed in this perspective [3]. Considerable research has been conducted on this topic in order to understand more the reaction mechanism which remains currently poorly understood, and in order to characterize the kinetics of the reaction [8]. We can distinguish several types of catalysts, which can be used for the direct conversion of ethanol. We will limit ourselves in this study to two types (groups) catalysts most used and most effective, these two groups are: Rh based catalysts and Fischer-Tropsch modified catalysts [9].
The Rhodium based catalysts, which is a noble metal, are known to have the best selectivity to ethanol (to C2+ oxygenates in general) with a good carbon monoxide conversion [3, 9, 10]. Rhodium occupies an interesting position in the periodic table. It is located between the metals (eg, Fe and Co), which dissociate CO easily to form higher hydrocarbons, and atoms which do not dissociate CO and can produce methanol (for example, Pd, Pt and Ir). The catalysts with small amount of Rh can form methane, alcohol, or other oxygenated compounds, it also allow the hydrogenation of CO according to the media, promoter and reaction conditions [10].
These catalysts despite their performance are more expensive than the other catalysts. The Rhodium price, as a raw material, exceeds 200 $ per g. Velu Subramani et al (2008) suggested that the amount of Rh in the catalyst should be very small (less than 0.1 wt %) to develop a catalyst at a commercial level [3]. We should note that the latest researches (2010 and 2011) on this subject have led to a significant reduction of Rh weight percentage in the Rh-based catalysts, which decreased from 7% to 1% currently, keeping very good performance. This promises a further reduction of Rh amount in Rh-based catalysts. We noticed also that the price of Rhodium dropped from €220/g in 2008 to €43 /g beginning of this year, which means that the price of Rh-based catalysts must be re-evaluated according to this variation.
The Fischer-Tropsch modified catalysts are based in general on Co, Ru and Fe. These catalysts have been reported to form higher alcohols when properly modified with additions [10]. Some researchers have reported the synthesis of higher alcohols using Ir/Ru-SiO2 or Ir/Co-SiO2 is due to the interaction between metals that easily dissociate CO (Ru and Co) and Ir which does not dissociate CO. The combination of these two metals would provide a catalyst which combines the separation and insertion of CO on the catalyst support [3]. The modified Fischer-Tropsch catalysts have a moderate selectivity to ethanol, a high selectivity to methanol and exhibit a high methane yield which is thermodynamically favorable [10].
The selectivity to ethanol is low for most of the catalysts which is due to the carbon chain growth mechanism for the formation of higher alcohols. While ethanol is formed from methanol by complicated and slow reaction, ethanol is rapidly converted to higher alcohols via a rapid carbon chain growth mechanism [3, 11]. We used the Rh-based catalysts, which allows you to have good selectivity to ethanol and minimize the selectivity to other products (methane, methanol, hydrocarbons...). This is very important because it will allow us to reduce the cost of investment operations thus reducing separation and purification costs. The catalyst used is Rh/SiO2 which has high selectivity to ethanol. The addition of vanadium as a promoter increases the catalyst activity and selectivity to ethanol [9].
4. Simulation results
4.1.Process description
The process flow-sheet simulated in Aspen PlusTM is presented in Fig 3. As mentioned earlier the studied process is composed of two main steps: The first step is the production of synthesis gas via the high- temperature electrolysis. The second one is converting syngas through a catalytic reaction to ethanol.
The gas composition supplied to the SOEC is: carbon dioxide (40%: molar fraction), steam (40%) and hydrogen (10%). Gas outlet of the electrolysis cell is composed of syngas (which is the major product), CO2 and steam. 90% of gas produced enters the reactor after conditioning. The remaining 10% are recycled to the SOEC. Since almost all of the CO2 is converted to CO, the influence of CO2 on the ethanol synthesis reaction is very limited. As explained earlier by-products of synthesis reaction and secondary reaction are diverse. The only reactions that are taken into account in this simulation are: conversion reactions of ethanol, methanol and methane. The reactor outlet gas composition consists of ethanol, methanol, methane, carbon dioxide, carbon monoxide, hydrogen, and steam.
Fig. 3: Overall process flow-sheet simulated in Aspen Plus TM
4.2. Process energy consumption
Energy consumption of the process was calculated for an ethanol production rate of 500 tonnes per day, which corresponds to an annual production of 165 000 tonnes. Table 1 presents energy consumption per Kg of ethanol for different process units. As shown in this table, the total thermal energy consumption to produce one kg of ethanol is 4.383 MJ, while the electrical energy consumption is 36.9 MJ. Assuming a conversion factor of primary energy into electrical energy equal to 33% (as for a nuclear power plant) we find that the total primary energy consumption is 109.8 MJ / kg ethanol. We also note that the operation of the electrolysis is the largest consumer of energy terms. Improving the energy efficiency of the electrolysis cell, which is a function of current density and cell voltage [2], can significantly reduce total energy consumption. The distillation unit as it is one the important energy consumers in this process, could be improved to reduce the energy consumption. New methods of distillation exist, for example the extractive distillation can reduce significantly energy consumption [12]. To compare this process with usual ethanol production processes in term of energy consumption, all the costs should be integrated from of raw materials transport to ethanol production.
Table 1. Total process energy consumption per kg of ethanol
In Table 2 we present a global mass balance of the studied process. We note that effectively this process recycles important amount of CO2. To produce one ton of ethanol, 3.5 tons of CO2 and 5 tonnes of H2O are consumed. 95% of CO2 produced is recycled to the process and the remaining 5% are purged.
As presented in the table 2, amount of hydrogen produced is five times of ethanol amount. Given the industrial uses of hydrogen and its energy potential, it would be appropriate to add a hydrogen recovery unit. We can also add another separation unit for methane production. By integrating all of these improvements, which will certainly complicate the process more, the overall efficiency of the process will increase and energy efficiency will be much higher.
Table 2: global mass balance of the process
4.3. Economic assessment
Using economic evaluation method established by Horacio Perez [13] and some of the production costs presented in the same reference, we present below main economic assessment assumptions.
As showed (Table 3), this process deliver ethanol at a cost price of 1.1 $ per kg. Usual ethanol production processes deliver ethanol at a cost price ranging from $0.4/kg to €0.6/kg. This important difference in ethanol price is due essentially to the difference of industrialisation degree for the considered processes. While the process proposed in this study is not even realised at pilot plant level, the other processes are already industrialized and then have been optimized at all levels to produce ethanol at a lower price. This difference in cost price does not mean that this way of ethanol sustainable production should be ignored. On the contrary, more research effort should be put to optimize energy and operational parameters which will make this process more competitive. For more precise economic assessment of this process, we must take into account some important externalities aspects. The most important externality is reducing dependence on energy imports, which is today in France about 50% and is expected to increase to 70% in 2030 [14]. We can therefore predict a significant reduction of the country's energy dependence due to the industrialization of this process on a large scale. For example, production of bioethanol generates more than process. Another important aspect that should be taken into account is the carbon dioxide consumption, which allows the possibility to sell carbon dioxide quotas (allowances) and reduce consequently ethanol price cost.
Table 3: Main economic assessment assumptions
5. Conclusion
The main objective of this study is to explore one of the possible pathways of carbon dioxide recycling to produce chemical compounds. The studied process consists of two main steps. The first one is the high temperature co-electrolysis of CO2 and water vapor, using solid oxide electrolysis cells (SOEC), to produce the syngas (H2 + CO). To determine the gas outlet conditions of the SOEC we used a simplified mathematical model of the co-electrolysis equilibrium. The second step is the ethanol synthesis from syngas in a catalytic reaction. The considered catalyst is Rh based catalyst promoted with vanadium, which have the best selectivity for ethanol and oxygenates.
Aspen Plus was used to simulate the process and to optimize the operational parameters. The process was designed for ethanol production rate of 60 000 t/year. Simulation results showed that we need 3.5t of CO2 and 5 t of H2O to produce 1 ton of ethanol, which means that more than 21 000 t of CO2 and 300 000 t of water vapor are consumed each year. The total primary energy consumption rises to 109.8 MJ per 1 kg of ethanol, which is more than the energy consumption of the usual ethanol production processes.
Due to lack of experience feedback, the economic analysis of the whole process was carried out using cost estimating assumptions based on pilot plant costs and similar processes costs. The results of the economic analysis demonstrated that the ethanol production plant driven by a high-temperature nuclear power plant can deliver ethanol at a cost of $1.1/kg, assuming an internal rate of return on investment of 8%. This final cost could be lowered if we consider the co-production of methane and hydrogen. Also, if we take into account oil price increase and the subsidies it is estimated that this process can be competitive with other conventional methods of producing ethanol in the next 10 to 15 years.
Source: Youness El Fouih, Chakib Bouallou
Advertisement
The 10 largest coal producers and exporters in Indonesia:
