Soletair demo plant produces renewable hydrocarbon fuel
Finland's Lappeenranta University of Technology and VTT Technical Research Centre of Finland have commenced testing of the Soletair demo plant, a facility that uses air-captured (DAC) carbon dioxide to make renewable chemicals and fuels. The plant is comprised of four units: a solar power plant, a DAC carbon dioxide and water separator, hydrogen-producing electrolysis equipment and a crude-oil substitute synthesis component which combines carbon dioxide and hydrogen to produce a range of petrochemicals including light olefins.
Phase 1: Renewable energy. Solar photovoltaic (PV) electricity is used as a renewable energy source in the Soletair system to produce electricity especially for the hydrogen production unit—the most energy intensive part in the system. The renewable energy plant consists of flat roof, carport, wall, 2-axis tracking, and manual tracking solar PV installations. The total installed power is 206.5 kW.

Phase 2: Hydrogen production. Proton exchange membrane (PEM) water electrolysis is used for hydrogen production. The system produces high-purity hydrogen gas at elevated pressures. Hydrogen is used with recycled carbon dioxide to produce renewable fuels, raw materials, and chemicals. The hydrogen gas can also be used as a chemical energy storage and can later be reconverted into electricity in a fuel cell, albeit with an additional penalty in terms of losses in conversion. The hydrogen production system is built in a standard shipping container and virtually connected to the 206.5 kW solar PV power plant at LUT. Due to the compact design of the proton exchange membrane electrolyzer, the cathode compartment pressure can be adjusted to a much higher pressure compared to the anode compartment pressure. The PEM electrolyzer at LUT can produce hydrogen at a maximum outlet pressure of 50 bar, while the oxygen outlet pressure is kept at 2 bar. Operating temperature of the electrolysis unit is controlled to 70 °C by water cooling. The hydrogen production unit is produced by a Danish company EWII, but modified to enable the adjustment of both gas outlet pressures by back-pressure valves. The produced dry hydrogen gas is stored into two 350 l composite cylinders. From the gas storage, the hydrogen can be supplied to the mobile synthesis unit (MOBSU) or a PEM fuel cell located in the hydrogen production container.

Phase 3: Direct air capture. Direct air capture (DAC) is the carbon source of the SOLETAIR project. DAC falls under the class of carbon sequestration technologies. However, direct air capture is the only carbon capture technology that can directly capture CO 2 previously emitted in the atmosphere. When surplus renewable energy drives the unit, DAC has the potential of being 100% negative carbon emission technology. The current DAC unit is a modified version of air-scrubbing units for civil shelters. The main principle for collecting carbon dioxide is adsorption/desorption process using solid amine sorbents. The sorbents used in the direct air capture unit are amine-functionalized polystyrene spherical beads.

Phase 4: Mobile synthesis (MOBSU). The MOBSU uses Fischer-Tropsch synthesis to combine carbon and hydrogen and produce valuable gas, liquid and solid products for various uses. The Soletair team is currently working on two different production lines which are tailored for either natural gases or liquid and wax component production. These units are positioned side by side inside the Mobile Synthesis Unit. The Fischer-Tropsch production line has two major steps. First, CO 2 is converted to CO using a reverse water gas shift reaction (rWGS) in which a gas mixture of CO, CO 2 , H 2 and H 2 O is balanced at 800 °C with the help of a precious metal catalyst. The reactor operates at the same pressure as the following Fischer-Tropsch synthesis in order to avoid compression between process steps. The design of rWGS reactor and the catalyst is VTT in-house know-how. Secondly, carbon monoxide and hydrogen are reacting to hydrocarbons in the Fischer-Tropsch reactor. Project partner IneraTec GmbH designed and manufactured this ultra-compact and efficient reactor. The Fischer-Tropsch reaction produces a wide range of products from light hydrocarbon gases such as methane, to liquid components such as diesel and up to more solid wax components. The main parts of Fischer-Tropsch module are the intensified reactor, a hot trap to condense the wax products, and a cold trap to condense the liquid products. The Soletair Fischer-Tropsch unit has a cobalt catalyst in a novel compact reactor with integrated water evaporation cooling cycle. The MOBSU is positioned in a 2.5 x 9.1 x 3 m sea container and easily transportable. It contains both Fischer-Tropsch and methanation reactors together with a control room.

Phase 5: Refining. The share each type of product from the MOBSU varies depending on the reaction conditions and the catalyst used in the Fischer-Tropsch reaction. It is essential to utilize all of these products fully to make an economically feasible process. The renewable product that is in gaseous form at room temperature consists of methane, the main component of natural gas, and other light hydrocarbons. It is easy to separate the gaseous fraction from the liquid and solid products. In larger refineries light olefins—ethylene, propylene and butenes—are separated from this fraction. These basic petrochemicals form the basis for the manufacture of a wide range of plastics and other products. On the other hand, the light paraffins generally known as Liquefied Petroleum Gas, are sold to customers to be used for instance in stoves, grills and refrigerators. In smaller scale such as in the case of the Mobile Synthesis Unit, the preferred use of the gaseous fraction is for energy. The liquid product can be fractioned by distillation to renewable gasoline and middle-distillate hydrocarbons. The gasoline fraction is further hydrotreated and reformed over a platinum catalyst in order to increase its octane number and to improve other characteristics for motor use. The middle-distillate fraction is also hydrotreated and thereafter distilled to renewable jet-fuel and/or diesel.