Getting maximum energy out of biomass

Biomass is loaded with energy. According to the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), the amount of biomass that accumulates in Switzerland every year could offer around 97 petajoules of usable primary energy. That is almost nine percent of Switzerland's annual primary energy consumption. But although the energy from biomass is renewable and climate-friendly, it remains largely unused. Only 53 petajoules, about half of this potential supply, are currently extracted through incineration or gasification, with the rest ending up in the garbage.

This is due not least to the fact that biomass comes mostly in wet forms, as liquid manure, sewage sludge, biowaste, or leftovers. Unlike dry wood or harvest residues, these cannot simply be burned in order to use the resulting heat. Aqueous organic waste has to be dried in a laborious manner beforehand. It's hardly worth it. Methane, which is the main component of natural gas, can also be obtained from biomass. However, the way conventional biogas plants work is comparatively inefficient. At best, they gain only around 30 percent of the net energy that is in the biomass.

Now at the Paul Scherrer Institute PSI, a newly developed system is going into operation that was able, in its first test runs, to utilise 60 to 75 percent of the energy contained in wet biomass – more than doubling the yield. This increase in efficiency is the result of a lengthy development process. Researchers at PSI have been working for the past 20 years to lay the groundwork for the new technology. Six years ago, with a small laboratory system called Konti-C, they demonstrated that it is possible to process one kilogram of biomass per hour with this technology. Since then, they have designed a larger pilot system that can handle 100 kilograms per hour. This is scheduled to start working in March 2021.

From pilot plant to industrial operation

"With this pilot plant we will test everything that an even larger industrial plant, one that will process two to five tons of biomass per hour, should be able to do later on", says chemical engineer Frédéric Vogel, head of the Catalytic Process Engineering Group in the Bioenergy and Catalysis Laboratory at PSI. The researchers want to show that the plant can cope with the various forms of wet biomass and that no undesired byproducts are created. They are conducting test runs with water and nitrogen to check for leaks. They are testing to see if corrosion problems arise anywhere and to determine how quickly the system can heat up and cool down without components suffering from thermal expansion. And they are testing whether the heat exchanger works as desired. This element, crucial for achieving high efficiency, was not yet present in the laboratory system that processed one kilogram per hour.

The special quality of the novel system lies in its handling of the water from the biomass. This is not seen here as an obstacle to energy utilisation but actually contributes to it, as a reaction medium. In a process called hydrothermal gasification, the sludge is put under a pressure of 280 to 300 bar and heated to 400 degrees Celsius. "Under these circumstances, the water remains liquid despite the high temperature and eventually reaches a supercritical state", Vogel explains. "In this form, it has particularly good properties for breaking down the biomass – in other words, making large molecules into small molecules that are especially reactive." This hydrothermal breakdown prepares the biomass for the next step, in which a special catalyst comes into play as a reaction accelerator.

The conversion into biogas is also promoted by thorough mixing in the system's pipes before the molecules encounter the catalyst. "In this way we make sure that the solid particles are completely surrounded by water with which they can then react, with the help of the catalyst", says Vogel. At this stage, the biomass is like petroleum. It is then passed through an activated carbon filter; within its fine pores the active catalytic material, in this case ruthenium, awaits the small biomass molecules to generate methane from them.

The right balance of temperature, pressure, and flow rate

In recent years fundamental research has focused not only on finding the right balance among temperature, pressure, flow rate, and the type of mixing, but also on identifying the ideal catalyst. "Since biomass, like crude oil, consists of hundreds of different substances, the reactions of which are impossible to calculate in detail, we had to experiment a lot", says Vogel. PSI offered ideal conditions for this, because with the Swiss Light Source SLS, materials and their reactions can be investigated with atomic resolution. "In this way we were able to follow exactly how and why one catalyst works better than the other."

Another advantage of supercritical water is that no more salts can dissolve in it. This means that valuable nutrients such as phosphates and minerals contained in the biomass can be easily separated out with a salt precipitator and reused, for example in fertilisers. At the same time, this protects the catalytic component, which these substances would otherwise clog. To prevent other harmful substances from contaminating the tiny pores of the activated carbon filter, an additional filter has been installed upstream from the system that is now being put into operation: granules that react with sulphur and prevent it from causing problems.

At the end of this complex process, the HydroPilot system, like biogas plants, produces a mixture of methane, carbon dioxide, and hydrogen, from which the latter two are largely separated so that the methane can be fed into the natural gas network. Beyond that, only the recovered nutrients and pure water are produced. Remnants of minerals and heavy metals are processed in cement works or dumped. In terms of energy, the system is largely self-sufficient – it only needs power for the electrical operation of the pump. It generates the needed pressure with the same pump that transports the sludge. Some of the gas produced is branched off to fuel a gas burner that generates the needed heat. Independent of this, the yield is 60 to 75 percent.

Besides the usual types of biomass, HydroPilot can also process fermentation residues from biogas plants and extract their remaining energy content. The system could also be fed with high-energy algae, which, unlike maize, can be produced very efficiently without competing with food production.

Initially, the pilot plant will be operated at PSI itself – primarily with sewage sludge, since this is the most complex form of biomass. "If we can do it with sewage sludge", Vogel says, "we can also do it with the other types." There are already concepts for much larger plants on an industrial scale, and the researchers are in close contact with companies such as KASAG Swiss AG and TreaTech Sàrl.

The HydroPilot project is funded as part of the Swiss Federal Office of Energy SFOE's pilot and demonstration program, with additional significant contributions from TreaTech sàrl, KASAG Swiss AG, ExerGo sàrl, and Afry Schweiz AG. It is part of the Swiss Competence Centre for Energy Research BIOSWEET, funded by Innosuisse, and the Energy System Integration Platform at PSI.