The promise of hydrothermal gasification

After anaerobic digestion, pyrogasification and gas generation, a fourth sector of biomethane production is emerging: hydrothermal gasification. Report on Switzerland, one of the most advanced countries in this technology.

Nestled in the depression of the Aargau hills, halfway between Basel and Zurich, the Paul-Scherrer Institute (PSI) hosts, since 2020, the largest hydrothermal gasification plant with catalysis in Europe. Developed with start-up TreaTech, this pilot plant is capable of processing 110 kg/h of raw material. In general terms, the system consists of transforming humid organic residues into gases rich in methane, via a closed process capable not only of recovering mineral salts, nitrogen and water, but also of eliminating bacteria, viruses and other micropollutants, all without the slightest emission into the atmosphere.

What a waste?

L o la anaerobic digestion and the pyrogasification value solid inputs, quite dry, and at the same time energy for gas uses excess renewable energy production that cannot be stored, hydrothermal gasification (GH) aims to produce a low-carbon renewable gas from liquid or wet inputs, or solid inputs mixed with water. For example, agricultural residues (manure, slurry), methanization digestates, agro-food residues (dairy, sugar, cereals, etc.), bio-waste, sludge from treatment plants (stage), knowing that those from the stage are more rich in carbon than those of urban steplands, but also contain more pollutants, metals and other inorganic substances In total, more than a hundred potential biogenic residues have already been identifiedannounces Robert Muhlke, Hydrothermal Gasification Manager at GRTgaz, who leads the GH working group launched in 2021 in France.

More than a hundred potential biogenic wastes have already been identified

Interest also goes to waste of fossil origin, in particular plastic, but also chemicals and petrochemicals, which often end up by incineration, adds Mr. Muhlke. Because this garbage contains a lot of carbon, andcarbon free, [il n’y a] no methane🇧🇷

supercritical water

Another prerequisite regarding inputs: they must contain as few components as possible that could corrode stainless steel installations (halogens, sulphur). Above all, they must not be too viscous. Inlet must be pumpable, summarizes Gal Peng, co-founder and technical director of TreaTech. After homogenizing its composition, the raw material starts its journey through a pump where the pressure level is increased to 250-300 bar and around twenty degrees. And then preheated thanks to the heat exchanger heat and injected into a salt separator: in the supercritical phase (over 374C and over 221bar), the characteristics of the water present in the mixture change completely, it acquires new properties that allow it to separate the elements present.

Specifically, the solids initially dissolved in the flow are precipitated: everything that is inorganic in the dry matter, metals and minerals in particular (phosphorus, potassium, calcium), falls by gravity inside the reactor and is recovered for later recovery. Above all, this inorganic fraction no longer interferes with the rest of the process, which continues in the gasifier where the organic matter is transformed into a gas-liquid mixture. At the exit, this mixture is separated: the liquid residue essentially contains water saturated with COtwo and, depending on the composition of the input, nitrogen in the form of ammonium; Synthesis gas can be used as it is for self-consumption (cogeneration, boiler) or treated (purification, methanation) before injection into the network.

The composition of the gaseous mixture obtained at the gasifier outlet varies according to the process used: with catalysis or high temperature. At the PSI site in Villigen, the researchers work with a catalyst (nickel, ruthenium), which makes it possible to lower the reaction temperature to 400 C (but not less) and generate a synthesis gas very rich in methane up to 70% without artificial addition of hydrogen. The remainder of the mixture is composed of carbon dioxide (between 20 and 30%), hydrogen (less than 10%) and, on very rare occasions, carbon monoxide (trace).

In the so-called high temperature process (between 550 and 700 C), which is very advanced Netherlandsthe composition of the synthesis gas obtained is different: 25 to 40% methane, 30 to 50% hydrogen, about 30% carbon dioxide and alkanes (ethane, butane, propane) with a higher calorific value than methane.

What performance?

The Swiss team tested their process with catalysis with about fifteen inputs, including digested sludge, molasses residues and used solvents, with respective PCIs of 13, 19 and 31 MJ/kg. O results showed a carbon-in-gas recovery rate of 75% for digested sludge, 86% for molasses residues and 95% for used solvents, with a net production Yearly of gas injected into the network of 15, 20 and 50GWh respectively.

synthesis gas purification included, cost of a 3t/hat unit estimated between 7.6 and 9.8 million euros at most by TreaTech, which intends to install, during 2023, its first mobile demonstration unit in a customer, to reach the 115 modules implemented in 2030, with capacities between 3 and 4 or 5 t/h. All waste producing entities most frequently sent for incineration are targeted, which is mandatory for sewage sludge in Switzerland and the Netherlands. Used solvents, chemical production waste and contaminated water are also potentially involved.





Article published on December 20, 2022

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