Power and Utilities
Power to gas, liquids, chemicals: An approach to industrial decarbonization with H2
S. Sakthivel, Tata Consulting Engineers Ltd., Mumbai, India
Around the world, electricity is being generated from renewable resources like sun, wind, hydro, tidal and geothermal. These renewable energy technologies have been advanced in recent decades, and prices for renewable energy are dropping, allowing renewable energy to be used as a substitute for some fossil fuel applications. However, some energy systems still require clean fuels or chemicals instead of electricity. The electricity produced from renewables can be converted into gas, synthetic fuels, chemicals and industrial/residential heating, which are referred to as “power-to-X” or “power-to-gas” technologies.
Fig. 1 depicts renewable electrical power to produce a gaseous fuel (H2 and methane), a liquid fuel (methanol, dimethyl ether and synthetic gasoline, diesel or kerosene), an industrial feedstock (e.g., ammonia) and industrial/residential heat by using various technologies. This article describes available technologies for the production of clean fuels and chemical and industrial feedstocks from renewable sources, along with their levels of maturity and market opportunities and challenges.
Fig. 1. A look at power to gas/liquid/chemical feedstock.
The intention of power-to-gas technology is to replace fossil fuels usage in the industrial sector, which could be a key element to long-term decarbonization. Almost all energy sectors are undergoing substantial changes in light of the energy transition to promote better efficiency, reduce emissions and increase the use of renewable energy. The use of power-to-gas technology can help achieve these goals. The high energy content of two gases like H2 and synthetic methane are produced from the power-to-gas process by the water electrolysis and methanation processes, respectively.
Water electrolysis technology splits the water into hydrogen (H2) and oxygen (O2) gas molecules by using a direct-current (DC) power supply from preferably renewable sources. A simple water electrolysis unit consists of an anode and a cathode immersed in the electrolyte solution [commonly potassium hydroxide (KOH) and sodium hydroxide (NaOH)]. The electrodes are connected through an external DC power supply. At the cathode, the electrons combine with the H2 protons to produce H2. Then, H2 ions move toward the cathode, whereas hydroxide ions move toward the anode. H2 and O2 gases develop at the cathode and anode, respectively.
Water electrolysis is categorized into four types based on their electrolyte and ionic agents (OH–, H+, O2–):
At present, AWE and PEM electrolysis are the prominent technologies being commercialized worldwide. Typically, AWE and PEM cell stacks consume power in the range of 3.8 kWh/Nm3 and 4.53 kWh/Nm3, respectively.1 These two technologies are dominating the market over SOE.2 However, AEM water electrolysis has several advantages that can replace conventional noble metal electrocatalysts. AEM electrolysis is still under research and development at laboratory scale and is not yet commercialized.
Several regions are developing green energy initiatives focused on H2, including the U.S., Canada, Saudi Arabia, India, Denmark, Austria, New Zealand, Australia, Singapore, Germany, Chile, Spain, China, Portugal and Japan.
The methanation process is the conversion of carbon monoxide (CO) or carbon dioxide (CO2) to methane (CH4) in the presence of H2 at the catalyst surface (e.g., nickel or Cu–Zn) through the hydrogenation reaction, as shown in Fig. 2. Earlier, energy producers presumed that the methanation process was not economically feasible due to the costs of H2 and CO2 capture and purification. Today, however, two factors are pushing the technology: (1) the imposition of carbon taxes on carbon emitters and (2) the need to reduce the cost of low-carbon H2. The methanation process has been gaining attention for its utilization of CO2, which helps mitigate some CO2 release into the atmosphere.
Fig. 2. Methane from green H2 and CO2.
Low-cost renewable electricity can be stored as fuels, chemicals and energy carriers. The best example is ammonia as an energy carrier, which is produced using H2 via renewable energy.
Ammonia production involves the catalytic reaction of H2 and nitrogen (N2) at high temperature and pressure, which is based on the Haber-Bosch (H-B) process. The natural gas reforming process for H2 production is the major energy consumer, accounting for 75% of the total energy demand. The balance 25% energy is consumed during ammonia synthesis, gas compression and ammonia separation. The H2 generation process is also highly carbon emissions-intensive, accounting for 90% of the total process emissions. The only pathway to achieve deep decarbonization is through the use of green H2.
Green ammonia refers to the process of making ammonia using 100% renewable and carbon-free resources, without use of hydrocarbons, as shown in Fig. 3. A typical green ammonia production unit comprises three segments:
Fig. 3. Process flow diagram for green ammonia production.
A few projects are under construction to produce green ammonia:
Further research and development are required to reduce the cost and improve the efficiencies of green ammonia technologies.
The power-to-liquid process can be used to produce liquid hydrocarbons from water and CO2 by using electricity. The power-to-liquid process contains three steps:
Two principal pathways are available to produce liquid hydrocarbons from renewable energy via (a) the Fischer-Tropsch (F-T) process and (b) methanol synthesis from the hydrogenation of CO2 and electrochemical reduction of CO2. Furthermore, dimethyl ether (DME) is produced from the dehydration of methanol. Both the F-T process and methanol from the hydrogenation of CO2 provide a high technology readiness level (TRL) of between 8 and 9. The following section describes the significance of value-added liquid fuels like methanol and synthetic hydrocarbon fuels.
Methanol can be produced through two new routes: (a) electrochemical reduction and (b) hydrogenation of CO2. The CO2 can be converted into value-added chemicals like formic acid, methane, ethylene and ethanol by using electrochemical reduction or electrocatalytic process. This process can be considered a viable strategy for mitigating greenhouse gas emissions and can potentially reduce dependence on fossil fuels. It has gained significant attention in connection with renewable energy and due to its viable controllability, modularity and simple scale-up. The process also operates at room temperature and ambient pressure. However, this process has not yet been commercialized because the yields do not meet industrial needs. In 2021, a research agreement was signed by Shell and the National University of Singapore to produce cleaner fuels (ethanol) and useful chemicals (n-propanol) from CO2.
An alternative route for methanol production is the hydrogenation of H2 and CO2.3 Fig. 4 shows the process flow diagram for methanol production using renewable power and CO2 as feedstock.
Fig. 4. An alternative route for methanol production from renewable sources.
Generally, stationary sources of CO2 are available in the cement industry, iron and steel plants, power plants and oil refineries, where fossil fuels are used as feedstock. Typically, 1 metric ton of methanol can be produced from 1.38 metric tons of CO2, as per the stoichiometric calculation. The viability of this process will increase with H2 produced from renewable routes and greater availability of low-cost CO2.
Dimethyl ether (DME). DME is produced via the dehydration of methanol. DME can be used as an alternative fuel in diesel motors, and it emits low emissions of particulate matter (PM) and NOX; therefore, it can be considered a sustainable fuel.
Synthetic fuels. Generally, F-T synthesis requires CO and H2 to produce hydrocarbon fuels. The captured CO2 is converted to CO through an inverse CO shift reactor, using the reverse water-gas shift (RWGS) reaction. Fig. 5 depicts the process flow diagram for producing synthetic gasoline, diesel and kerosene, using upgraded hydrocracking, isomerization and distillation processes.
Fig. 5. Process flow diagram for the production of synthetic hydrocarbon fuels from renewable sources.
Renewable power can also be used to enhance heat and steam. Heat pumps and passive thermal storage are available for the utilization of renewable energy as heat energy. This uses large-scale heat pumps or electric boilers for industrial processes such as heating, drying, distillation, etc. Power to heat has great potential to reduce energy consumption and greenhouse gas emissions and to replace fossil fuel usage.
A number of specific opportunities are available for the utilization of power-to-gas technology:4
Several challenges to power-to-gas technology also exist:
Power-to-gas technology is a long-term strategy to decrease emissions from fossil fuels and help achieve the industrial ambitions of the energy transition. It derives a great advantage from renewable energy due to the decreasing cost of renewable energy and its low carbon footprint. Technologies are being commercialized around the world in the emerging areas of power to gas (e.g., H2 and methane), power to liquid (e.g., methanol, DME, synthetic H2 fuels) and power to chemicals (e.g., ammonia).
1 Sakthivel, S., “Way forward to carbon-free electricity for e-mobility,” Chemical Industry Digest, June 2019.
2 Sakthivel, S., S. S. Swami and A. Choudhari, “Green hydrogen: A perspective,” Proceedings of the 35th Indian Engineering Congress on Engineering for Self-Reliance and Sustainable Goals, December 18–20, 2020, India.
3 Sakthivel, S., “Pathways and industrial approaches for utilization of carbon dioxide,” TCE Tech Speak, March 14, 2018, online: https://www.tce.co.in/blogs/pathways-industrial-approaches-for-utilization-of-co2
4 Lewandowska-Bernata, A. and U. Desideri, “Opportunities of power-to-gas technology,” 8th International Conference on Applied Energy, Energy Procedia 105, 2017.
Sakthivel is a Senior Technologist at the Technology Group of Tata Consulting Engineers Ltd. in Mumbai, India. His areas of focus include green chemicals, green fuels, the energy transition and decarbonization, with responsibility for the evaluation of emerging technologies and commercialization. Dr. Sakthivel has experience in process engineering; technology analysis, screening and selection; techno-economic analysis; pilot setups; process hazard analysis; basic, applied and market research; and powder and science technology. He holds a PhD and has published several articles in national and peer-reviewed international journals.