Biogas plant: Build your own model in class
The film credits are rolling. The names of Marvin, Cord and Lisa appear. The class gives thunderous applause. "That was great! I really understood a lot," is the first feedback.
The three young film producers stand in front of the class, somewhat embarrassed, but also proud. They have just shown their video about their excursion to a biogas plant. The idea for the biogas film project came from a student who lives in the village. On the outskirts of the village there has been a biogas plant for a few years.
Agriculture versus environmental protection
We have just started to talk about the new lesson topic "Field of tension: agriculture – nutrition – nature conservation". Cord reports that he read in the newspaper that the Nature and Biodiversity Conservation Union (NABU) considers biogas plants to be an ecological catastrophe.
He had always thought that these plants would provide environmentally friendly energy. "It's true, corn is grown everywhere. Nothing else grows at all. When do you ever see a butterfly there? And all because of this alleged energy transition!" says Lisa. "How do you imagine it without renewable energies? Keep it up with nuclear power and coal?", Marvin counters. This discussion, briefly outlined here, is very emotional. The young people use their half-knowledge from the media and probably also reflect expressions of opinion that they have picked up at home. I can see that some of the other students are listening to this heated discussion with great interest, but cannot really follow the argumentation.
The situation is not easy for me right now. If I were to let the discussion about biogas plants continue, preconceived opinions would be expressed further and the dispute would threaten to escalate. On the other hand, this is a hot topic right now. A good opportunity to have this researched profitably in class.
I ask the students if they would like to take a closer look at the topic of "biogas plants"in order to then report to the class. For example, the schematic representation of a biogas plant is suitable for this purpose. Cord suggests that they could visit the biogas plant in their village and make a video about it. I think that's good, because then the topic becomes very concrete and authentic.
But first of all, it is important to me that the students inform themselves thoroughly. I suggest that we formulate our research question as follows:
"Biogas plants – sustainable energy generation or ecological catastrophe?"
To do this, we collect all the questions we want to clarify in this context on the board:
- Why have so many biogas plants been built in recent years?
- What gas do biogas plants produce?
- Can the gas be used for gas heating?
- Do biogas plants also produce electricity?
- Are they only "fed" with corn or is it also possible with manure?
- How does such a system work?
- What are the effects of increasing maize cultivation?
- Are biogas plants economically viable?
- What does the life cycle assessment look like?
- Is more energy generated than spent?
- Can we produce biogas ourselves in an experiment?
I suggest to the class that they work in pairs to create a portfolio on our research question. In it, they are supposed to answer the questions as thoroughly as possible. In addition, current newspaper reports are also to be collected and commented on in an extra chapter.
In the next planning step, we compile criteria for the later assessment of the portfolios.
The portfolio is intended to ...
- provide technically sound and understandable answers to the questions
- be clearly designed, i.e. contain graphics and images
- Illustrations are to be analysed in it
- do not contain any texts taken from the Internet, but only your own formulations
- Contain current newspaper reports with your own comments
- contain their own reasoned opinions on disputed issues
- Be well structured
- be equipped with a table of contents and sources
- contain as few spelling mistakes as possible
- be written neatly by hand or on the computer.
The team of Cord, Marvin and Lisa gets the order from me to inform themselves about the topic on the basis of the questions on the Internet. However, you don't need to create a portfolio. They are to produce a video about the visit to the biogas plant, including an interview with the farmer.
Experiment on biohydrogen production
Since I know that Tom, Helena and Jorge are not experts in written tasks, but are very fond of experimenting, I suggest that they be allowed to build and operate a small biogas plant for the production of hydrogen (H2). I will provide them with an experiment kit for this. You can photograph and film the construction and operation of the system (see Info 1).
The small "bioreactor" is filled with desugared beet pulp and a mixture of bacteria and lime and mixed with warm water. After several hours, hydrogen in particular is to be produced. So no methane, as in the professional biogas plants. The hydrogen is then fed into a fuel cell. There, the gas reacts with the oxygen in the air to form water. This produces electrical energy that is used to operate a propeller motor.
Since the three young researchers know hardly anything about the production of hydrogen and its technical use, I will give them a suggestion sheet in the next Na-Wi lesson, with the help of which they can explore this topic before they start the experiment.
The start of the bioreactor at school proves to be difficult. The room temperature is lowered at night. The experiment cannot be regularly supervised by the group either. So I allow Tom to take the suitcase home with him. There, after several hours of lead time, they manage to get the engine running (see Fig. 2). Helena films this with her smartphone.
In class, I use the biohydrogen set from Hedinger (see Fig. 3). The materials are in a sturdy hard-shell case. In addition to the "bioreactor" made of glass with a fixed screw lid with four gas openings, the set also includes a fuel cell, a vane motor, a large disposable syringe, a container each with beet pulp and with a bacterial mixture including hoses, cables and small parts. The price is about 400 €. The set is the result of a Jugend forscht project of the Kreisgymnasium Riedlingen. I usually use this set as part of a NaWi lesson on fuel cell technology. Depending on the framework topic, it can be used in physics, chemistry and biology lessons in grades 9/10. The structure is relatively simple and learners at this age usually succeed well, even if the enclosed instructions are very long and not necessarily easy to understand. The illustration of the assembled set can therefore be a very good orientation aid. Substrate I consists of desugared and dried beet pulp. Substrate II contains the bacterial mixture of saccharolytic clostridia, enriched with minerals and lime to adjust the pH value.
Clostridia are anaerobic bacteria that are found in the soil and also in the digestive tract of higher organisms. There are species that cause serious illnesses, such as tetanus. The saccharolytic clostridia used in this substrate are harmless to health from a bacteriological point of view. Under the fermentation conditions, no other organisms can grow. Therefore, no other substrates may be used. They feed on the residual sugar from the beet pulp and operate butyric acid fermentation. Almost 80% hydrogen is produced in the process. The rest is carbon dioxide. Hydrogen sulfide (H2S) and, of course, somewhat foul-smelling butyric acid are also produced in very small quantities. Therefore, the reaction vessel should not be opened in the living room or classroom after operation. The remaining substrate can be disposed of in organic or residual waste. Unlike the large biogas plants, no methane is produced. This gas would only be produced if the reactor were allowed to run for several days and more beet substrate was added.
Researchers' Conference
Three weeks later, the students' products are ready. I collect the portfolios and select groups to present their findings on a question orally at a research conference. I put together some of the graphics I used from the Internet into my own PowerPoint presentation, so that I can show them if necessary. Max and Rico have found an animation about the function of biogas plants at "Planet Schule". They show them via beamer. They provide the explanations themselves.
While some students read their texts aloud, others can also freely report on their newly gained insights. On suitable occasions, I also ask for their personal opinions. This sometimes leads to short but interesting discussions.
Marvin, Cord and Lisa finally show us their film. They interviewed the biogas plant operator in their village. A graphic of a biogas plant is displayed, on which the film authors explain the structure and function to us. This gives us viewers a good overview, so that we can understand the subsequent cinematic tour of the facility well.
At the various stations, Cord interviews the farmer. And he explains what he uses to operate the system, how it works and how he can control it on the PC. He also answers critical questions from the young researchers with clear words. This is particularly well received by the audience.
Producing hydrogen gas
The last group tells us about the production of hydrogen gas with the help of renewable energies and their possible uses.
They show us the biohydrogen set mounted on a trolley. In the meantime, the engine no longer runs. All the better that they can now show us their short video.
Some students are surprised that hydrogen and not methane is produced in this small plant. They want to know whether hydrogen is also produced in the large plants. Since I had already pointed out to the group that they had to expect this question, they prepared well for it.
In fact, hydrogen and carbon dioxide are also produced in the large plants, but microorganisms convert these two substances into methane. The only problem is that too little hydrogen is produced for the rather large amounts of CO2. Therefore, the CO2 must be captured.
For a few years now, researchers have been testing a new process in which two electrodes – cathode and anode – are dipped directly into the fermentation substrate and electrified. They break down the water contained in the substrate. Hydrogen is produced at the cathode, oxygen at the anode. The oxidizing oxygen must be discharged. The microorganisms use the resulting hydrogen directly to produce methane. The researchers hope to be able to increase the methane yield from 50 to 95 % in the future with this power-to-gas technology (see Info 2). The electricity for electrolysis should then come from wind or solar energy. For the final discussion, I write our big research question on the board again. Now the exchange of arguments is much more factual.
The benefits of biogas production are recognised, but the environmental impact of maize cultivation is still viewed very critically. Some young people suggest using more manure for the biogas plants, even if the effectiveness is lower than with a plant with corn. After all, manure would be available in large quantities.
Research to increase yields is also welcomed.
A fresh breeze is blowing across the country. But half of the wind turbines are not in operation. At the moment, there is apparently an oversupply of renewable energy on the electricity market. Solar and wind energy are subject to natural fluctuations that are not based on our demand for electrical energy. This energy would have to be able to be stored on a larger scale and for a longer period of time.
An initiative of various companies is working to put this idea into practice. Electrical energy from wind and solar plants is to be used for gas production. Hydrogen and methane can be produced in the process. These gases can be stored and can be used for heat supply, for the propulsion of vehicles and also for the generation of electricity. It makes sense to build power-to-gas plants where a lot of green electricity is produced. In biogas plants, electrical energy can be used for electrolysis. In this process, water from the biogas reactor is split into oxygen and hydrogen. Special microorganisms then convert hydrogen and carbon dioxide into methane.
In this way, the yield of methane in biogas plants can be significantly increased and at the same time the emission of the greenhouse gas CO2 is reduced. In this way, green electricity becomes primary energy and can be used nationwide for heat generation or converted back into electricity as needed via combined heat and power. This linking of electricity and natural gas grids is an important prerequisite for the success of the energy transition.
https://www.energie-tipp.de/neue-energie/speicher/power-to-gas-so-funktionierts/
Hint: In biogas plants, microorganisms take over the task of catalyst.
Further suggestions
While researching this topic, I came across an interesting biogas project: In Kenya and India, there are development projects in which biogas is obtained from faeces. This results in an interesting research task for high-performing and, above all, interested young people. Using the worksheet (see material download), they can work on this topic and present the results to the class.
Another idea for implementing it in the classroom was evident in the parallel class: There, the topic was dealt with as part of a project work. The result is, among other things, a self-made model (see tip) and a test setup for gas extraction from liquid manure (see material download).
To obtain the intermediate school leaving certificate, pupils in Schleswig-Holstein present a project work - with a written and practical component. Tjorben Tollschnibbe from the Ida-Ehre-Schule Bad Oldesloe and a friend have chosen the project topic "Biogas plants": "Based on the key question: Are biogas plants useful as a method of renewable energy generation?" we considered subtopics and clarified questions through Internet research. We visited a biogas plant in Leezen to interview the operator, of which we recorded a video. For better illustration, we built a model (Fig. 4) and carried out an experiment (Fig. 5). We filled a plastic bottle with cattle manure and corn silage and sealed it airtight with a balloon. The bottle was then placed in a warm and dark place. After a few days, the balloon was filled with biogas. We presented our video, our experiment and our findings orally with a PowerPoint presentation."
Worksheets & material for download
All worksheets on the biogas plant can be downloaded here in PDF format: