Hydrogen gas (H2) injection for extra methane production via “bio-methanation

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The storage of renewable energy is a problem. Bio-waste and biogas both need to be stored before they can efficiently be used.
The production of renewable energy from wind or sun to electricity has the downside that the electricity is produced intermittently (e.g. No production during the night or windless days). Peaks in electricity are also not good for the electricity grid because they can cause problems for the system like heating of the cables etc.
The production of renewable energy needs to be synchronized with the need. In other words, all renewable energy (also the peaks) needs to be valorized, i.e. used or stored.

There are different ways to do this:
Batteries are already present at a large scale in our society, but the problem here is the recycling of the lithium. The lifetime of a battery nowadays, still makes it not the most sustainable way to store energy.
Another way to store renewable energy before it is used, is hydraulic power. In countries with high reliefs, the energy can be used to pump water up to a water power plant (hydraulic dam) and the energy is stored in the form of the gravity working on the water. Downside is that not each country has the geographical specifications needed for hydraulic power plants.
A third option is the production of hydrogen gas from renewable energy. Systems that transform energy to hydrogen gas by means of hydrolysis are already available, mostly with a conversion efficiency of 60%. Nonetheless, hydrogen gas is difficult to store and this requires safety precautions.

The LIST and Université de Luxembourg as partner in the Perséphone project have been focusing on bio-methanation: in this process, H2 (from renewable energy:sun and wind) and CO2 are transformed into methane (CH4). One of the ways to perform this process is catalytic conversion at high temperatures. Yet, the project focusses on another way to perform this bio-methanation: by means of micro-organisms, which are less energy consuming.
The produced methane, is much easier and safer to store than H2: i.e. it can be injected in the natural gas grid, which is already present and it is calculated that the gas grid in Belgium has a storage capacity of 3-4 months. The only thing still needed are injection points for bio-methane.

There are already different concepts developed to perform this bio-methanation with micro-organisms. For example by creating modules that carry a layer of micro-organisms that are brought in contact with an H2 and CO2 flow.
The downside of this concept is that the produced biomethane, still contains trances of H2 and CO2. The natural gas grid in Belgium puts is limit of H2 on maximum 2%, so further upgrading of the gas would still be needed.

The Perséphone project (LIST), has developed their own concept of using membranes to create a more pure bio-methane with bio-methanation.
Only the injected H2 passes the membrane and goes to the compartment where the micro-organisms are present (anaerobic hydrogenotrophic Archaea) that directly consume it and produce methane.
These Archaea, are the same group of anaerobic micro-organisms that play an important role in the normal bio-methanisation process , only here their specific ability to produce methane from H2 and CO2 is used in a separate reactor.
So eventually there are 2 reactors: one normal digester doing anaerobic digestion (“bio-methanisation”) and producing CH4 and CO2, and one Hydrogen digester, consuming H2 and CO2, producing methane (“bio-methanation”).
Between the reactors, digestate is circulated, because it contains the micro-organisms and nutrients. In the hydrogen reactor, the conditions are favored in the advantage of the hydrogenotrophic anaerobic bacteria, so they would start the bio-methanation in this reactor.
For the project, a mobile installation is developed, including the 2 reactors. CH4 and CO2 and digestate is produced in the digester reactor and CO2 and digestate are transferred to the Hydrogen digester. In the Hydrogen digester also H2 (from renewable energy sources) is injected, and the methane is produced. This bio-methanation occurs at 37°C and the micro-organisms in the digestate need a week or 2 to favor the hydrogenotrophic anaerobic bacteria.
In the project, the mobile installation is tested and will be further upscaled. Different designs for the H2 injection will be tested and different parameters (like flow of the digestate, flow of the CO2 and H2) to get an optimal methane production in the hydrogen digester. All safety aspects have been taken into account.

The ultimate goal is to replace the normal anaerobic digester reactor, by a real full scale anaerobic digestion installation and create a hydrogen digester that is mobile and flexible that can be connected to any AD plant. The hydrogen digester will then be able to produce extra CH4 from H2 (i.e. converted renewable energy from wind or solar energy also available on the plant).
For the testing done in the project, H2 gas is supplied in gas containers and is not coming from renewable energy (solar power or wind power) that is transformed into H2 by a hydrolyse unit. The aspect of the supply of H2 gas, has not been included in this project, but as mentioned before, hydrolyse units are already available with conversion efficiencies of 60%.
This is still more than the energy that would be lost during the unused or unstored “peaks” in the renewable energy production.
The concept of bio-methanasation is not possible in 1 digester because it requires a group of bacteria being favored, which are also present in the cascade of steps of the bio-methanisation process. Because of this, in an anaerobic digester (bio-methanisation), too much H2 will inhibit the later reactions in the methane production.
By duplicating the bio-methanation process in a separate Hydrogen reactor and enhancing it, both 2 processes – bio-methanisation and bio-methanation- are used to the best extent.
Also, the injection of H2 in a normal digester would be too difficult to control and therefore it would also be too dangerous.

Refinery of digestate

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Amu Mundo explains that in the Perséphone project, they are working on separation of the digestate and hereby removing the water and lowering the transport and spreading costs.
Their system is a cascade of a screw press, which removes 5-20% of the volume as solid matter. The second step is a nanofiltration, where they work with ceramic disks. These allow the digestate to be separated in a filtrate and a NPK-concentrate (5-25% of the volume). The filtrate is further separated by a RO into water (50-80% of the volume) and an NK concentrate (5-15% of the volume).
The ceramic membranes appear to have less trouble with fouling because the rotation makes them self-cleaning. Nonetheless, a regular CIP is still needed.
This system is also been made mobile (in a container on a truck) to test in the framework of the project.
The ultimate goal is to make the system at full scale for an AD plant.
The dimensioning of the membrane filtration unit at full scale will be done after thorough analyses on the digestate to be treated.
The unit uses 10kW/ton digestate treated (on average), but this also depends on the type of digestate.
They claim, this can be the solution for the problem of costly storage of digestate, because water is removed. The system would cost 5-10EUR per ton digestate treated.

Field trials

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In this part of the project, the agronomic windows will be determined in each side of the Grande Region , comparing different fertilizers with mineral fertilizers.
There are different test sites that will provide different local climates and soil types.
The field trials are done on grass land and the majority of the field trials started at the end of the winter in 2017.
The site of La Bouzule, has 2 test fields:
One with permanent grassland and one with temporary grass land, which has been cultivated with corn the previous year.
The site of Grendel-Faascht, Emmels and Erpeldange are permanent grasslands and Steinborn is a temporary grass land.

The soils in the sites of La Bouzule are predominantly clay. This is also the case for Grendel-Faascht although a heterogeneity has been seen with sand-layers.
In Erpeldange, the soil profile varied, the top was loam clay-sand, the bottom was more clay, and local it had some gravel beds.
Steinborn had a lower layer of sandstone and was in general sandy.
The soils of Emmels had a lower layer of shale and sandstone, has loam on the surface and more clay in the depth.

In total 19 variants of fertilizers were tested (different sites and different fertilizers) at a dose of 230 kg N/ha:
– No fertilisation ( Blank)
– Raw digestate (reference)
– Local digestate
– Raw manure
– Ammonium nitrate

The results of the first year (2017) show differences in the N quantity present in the harvested grass.
The N amount in the grass fertilized with digestate is a bit better that this of manure (on average 15% and manure was 11%). Nonetheless, this is still twice as less efficient as ammonia nitrate (31%).

The potentially leachable nitrogen was also measured. This is the nitrogen present in the soil as NO3. For digestate this potential is estimated on 40kg of N-NO3 in comparison to 36 kg for the blank.
When mineral fertilier is used, the leaching potential goes up to 66kg, manure is in between: at 51 kg of N-NO3.

Odour emissions: different factors can contribute the perception of smell: like weather conditions, storage and spreading techniques.
The study in the Ecobiogaz project shows that odour emissions contributed by spreading are lower with digestate than with manure.

Ammonia emissions are more pronounced with digestate than with manure.
Part of the ammonia can be lost during spreading of organic fertilisers and can reduce the fertilisation to the grass. This risk is more pronounced with liquid fraction of digestate.

Conclusions

Digestate contributes to a basic fertilisation with other nutrients that mineral fertiliser does not supply. It also helps to maintain the pH and brings organic matter to the soil.
If applied with good practices, the leaching is non existing because N is principally present as ammonium and held by the soil matrix and can be absorbed by the plants. Nitrification from 6°C on makes that the plants can absorb it in growth stage

On the other side, there is a need to spread digestate only on cloudy weather, preferably followed by rainfall. The best results were obtained by means of injection. Otherwise, volatilisation can be significant.

The dry summer of 2017 gave some results that were not expected, f.e. nitrogen supply to the grass on the Blank fields, probably by means of nitrogen mineralisation or nitrogen deposits from the air. Therefore these conclusions are preliminary and should be looked at after more field trials are done (in the coming years).