Captured CO
Renewable electricity
Raw methanol
Electrolyser H O = H + O
e-methanol production
Methanol
Distillation
H O
Hydrogen
O
H O
Figure 1 Methanol production route beginning with renewable electricity
progress in employing copper-based and oxide catalyst systems. Also, a novel technique for converting CO 2 to methanol has recently been created at TU Wien (Vienna). Liquid methanol is formed from CO 2 with the aid of a unique catalyst material consisting of sulphur and molybdenum. Increasingly abundant low-cost renewable electricity enabled electrochemical 3 processes to compete with tra- ditional thermocatalysis methods. The availability of elec- trolysis at the scale needed to supply hydrogen to methanol plants is a key challenge, and significant efforts are being made to scale up electrolysers. Also, the high-temperature electrolysis to produce CO and syngas using solid oxide electrolyser cell (SOEC) systems could be advantageous if coupled with thermochemical processes to reduce heat- ing cycles. However, unlike low-temperature electrolysis, SOECs are not able to reduce CO 2 directly to other hydro- carbons and oxygenates. Methanol synthesis from CO 2 over heterogeneous cata- lysts suffers from several shortcomings, such as harsh operating conditions like high pressure and temperature. Also, CO 2 thermocatalytic hydrogenation is limited by ther- modynamics and continuous separation of methanol from CO 2 and byproducts necessary in the recirculating process. Several alternatives for CO 2 reduction to methanol have emerged in recent years involving homogeneous, enzy- matic catalysis, photocatalysis, and electrocatalysis. The advantages of the emerging processes include tempera- tures lower than in heterogeneous catalysis, alternative sources of energy (light or electricity) use, and potentially higher methanol selectivity. In some alternatives, water is used for CO 2 reduction instead of costly green hydrogen. Green/renewable methanol synthesis The simplest and most mature method is to make hydrogen through the electrolysis of water using renewable electric- ity, followed by catalytic reaction with CO 2 to form metha- nol (see Figure 1 ). In the presence of catalysts like Cu/ZnO/Al 2 O 3 , CO 2 reacts with hydrogen to form methanol at a pressure of 5-10 MPa and temperature of 210-270°C. Produced methanol is sep- arated from water and residual gases and purified through distillation. To produce 1,000 kg of methanol, about 1,400 kg of CO 2 , ~200 kg of hydrogen, and ~1,700 kg of water are needed. Around 10-11 MWh of renewable electricity is required to produce 1,000 kg of methanol, a predominant part of which is used for the electrolysis of water. Methanol synthesis starting from pure CO 2 and hydro- gen greatly simplifies the reaction products. In terms of chemistry, it is reduced to the following three reactions:
CO 2 hydrogenation (1), reverse water gas shift (2), and CO hydrogenation (3): CO 2 + 3H 2 ⇌ CH 3 OH + H 2O ΔH = -49.16 kJmol −1 (1) CO 2 + H 2 ⇌ CO+H 2O ΔH = 41.22 kJmol −1 (2) CO + 2H 2 ⇌ CH 3OH ΔH = -90.8 kJmol −1 (3) The composition of the crude methanol stream from the reactor effluent contains virtually no CO2 or other contami- nants besides water and CO 2 . Therefore, a single fraction- ation column is required for purifying the desired methanol product. Further, recycling the removed CO 2 to the reactor generates a continuous loop that may ultimately convert all CO 2 to methanol. A disadvantage is that CO 2 -to-methanol is typically less reactive than CO syngas, which leads to larger reactors being needed. In addition, more water is typically produced in this reaction due to the RWGS because of the higher CO 2 partial pressure and typically more CO 2 in the crude metha- nol. Largely for the same reason, there is typically more CO 2 in the crude methanol. Among the advantages of this pathway are that the CO 2 - to-methanol reaction is more selective and results in fewer byproducts, and the reaction conditions are milder due to less exothermic reaction. Also, better carbon utilisation is achieved compared with conventional syngas. From the previous discussion, it may be seen that pro- ducing methanol directly from separate sources of CO 2 and hydrogen is advantageous, less energy intensive, cleaner, and more environmentally friendly than conventional fossil fuel-based processes. One key operational difference between traditional fossil and green methanol production is the significant produc - tion of water in the exit stream, with ~30-40% water by mass in crude methanol, which reduces the activity and lifetime of the catalyst. Therefore, for green methanol syn- thesis, catalysts (compared to traditional) with higher sta- bility and activity in the presence of water are required for CO 2 hydrogenation. Pilot/demonstration and commercial development projects E-methanol plants are in the pilot as well as demonstration- scale production around the world. Though not fully com- prehensive, some examples are: • George Olah plant, Iceland, was first commissioned in 2012 with a capacity of 1,300 t/y renewable methanol and
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