Researchers from the Sapienza University of Rome jointly with the Italian government agency ENEA have estimated the efficiency and economic viability of producing synthetic jet fuel from carbon dioxide and water. This technology uses renewable energy to first convert water into hydrogen, which then reacts with CO₂ to form hydrocarbon fuel. The key feature of the proposed technology is that the two main stages of converting CO₂ into hydrocarbons take place simultaneously in the same reactor, as opposed to sequentially. This helps streamline the production chain and reduce the amount of equipment required. The Italian scientists calculate that kerosene has become the biggest product of the process, accounting for 33% of the total mass of the resulting products. The synthetic fuel’s key energy characteristics proved to be close to those of conventional jet fuel: its calorific value stood at 43.57 MJ/kg versus approximately 43.1 MJ/kg for conventional jet fuel.
The proposed method of synthetic kerosene production comprises several stages. First, hydrogen is produced via electrolysis of water using renewable energy. Next, it is mixed with CO₂ and fed into a tubular reactor packed with an iron-based catalyst with added potassium. At a temperature of 300°C and a pressure 25 times that of the atmosphere, two chemical reactions occur simultaneously. CO₂ reacts with hydrogen to form carbon monoxide and water; the carbon monoxide and hydrogen then combine via the Fischer–Tropsch process to form longer hydrocarbon chains. The result is a form of synthetic crude oil, a hydrocarbon mixture which is then separated in a distillation column into several fractions, including kerosene and diesel.
The kerosene fraction consists primarily of molecules with chains of some 8 to 15 carbon atoms, with an especially high proportion of compounds containing 9 or 10 atoms. In the total product mass, kerosene accounts for 33%, the gas fraction for 28.3%, the diesel fraction for 13.2%, heavy naphtha for 12.5%, heavy residue for 9.7% and light naphtha for 3.8%. A single pass through the reactor converts 57.5% of the input CO₂. In order to improve feedstock utilization efficiency, unreacted carbon dioxide and hydrogen are recycled back into the process, while the water produced is purified and reused for electrolysis.
The scientists also included the use of by-products in their calculations. Oxygen that gets released alongside hydrogen during water electrolysis is utilized for the combustion of exhaust gases. The recovered heat along with the heat produced by the chemical process is used to produce high-pressure steam that activates the turbine and generates electricity for the facility. However, hydrogen production remains the primary energy consumer: the electrolyzer accounts for 97.7% of the total electricity required by the unit. Calculations show that producing 1 kg of synthetic kerosene requires 173.5 kWh, of which the unit can offset 25.1 kWh by generating power on its own. Thus, the net electricity consumption is 148.4 kWh per kilogram of fuel, with the calculated efficiency of the entire process reaching 27.3%.
Hydrogen output is largely key to the cost of the fuel. The electrolyzer accounts for some 35% of a given facility’s capital expenditures and 49% of its operating costs. The researchers calculated the economics of units with capacities ranging from 472 to 100,000 tons of kerosene per year, demonstrating that a scale-up significantly improves performance. Based on the prices assumed in the study, revenue begins to exceed production costs when the output reaches 60,000 tons of kerosene per year. For the largest-scale options, the probability of a positive net present value for the project stood at about 85–95%. For an uncertainty evaluation, the authors conducted 1,000 simulations with varying feedstock costs, capital expenditures and final product prices. Since the baseline scenario assumes electricity costs of $60 per MWh and a synthetic kerosene price of $6 per kg, the resulting economic estimates are highly dependent on electricity costs and market fuel prices.
The synthetic kerosene proved to be similar to conventional jet fuel in many of its properties. In addition to having a comparable calorific value, it has no sulfur or aromatic compounds, which are associated with the formation of sulfur oxides and soot during combustion. However, this product cannot be used in its current form. Its calculated freezing point is –37.8°C, while the required freezing point is –40°C or lower for Jet A fuel, and –47°C or lower for the more common Jet A-1. It will require further processing, the addition of special additives, or blending with conventional jet fuel.
According to the scientists’ calculations, the most favorable conditions for synthetic fuel production exist in Norway and Sweden: both countries scored above 0.85 on the authors’ producibility index. The index takes into account the affordability of electricity and water, as well as CO₂ emission levels in a given country. The high indicators of the Northern European countries are primarily attributable to the large volume of affordable low-carbon electricity coupled with relatively low CO₂ emissions. This is especially important for the production of synthetic kerosene, as hydrogen generation accounts for nearly all the electricity consumed by the unit.
