The basic principles of biogas

The basic principles of biogas

By DaLuSt | Science for non-scientists | 18 Jul 2020


Biogas is a gas or gas mixture that is created as a result of biological enzymatic or chemical processes. On average, biogas consists of 60% methane and 35% carbon dioxide (Plugge, 2017), but there are wide variations. In addition to methane (CH4) and carbon dioxide (CO2), water vapor and residual fractions of ammonia, organic fatty acids and siloxanes, hydrogen gas (H2), and hydrogen sulfide (H2S) are also variations of biogas. The hydrogen sulfide often gives biogas the unpleasant odour smelling of rotten eggs. Most of the gas is created as a result of the fermentation of organic material such as manure, sewage sludge, organic waste, grass, maize, and so on. The end product of biogas production is also known as the digested remains as a residual product. An example of biogas that forms naturally is swamp gas. (Plugge, 2017)

Because of its biological origin, biogas can be a sustainable energy source. By using biogas, the release of methane which is a strong greenhouse gas can be limited. Methane can also be produced through a power-to-gas process. Especially in developed countries, biogas is extracted from wastewater treatment plants, waste landfills, and digestion plants. (Patterson, Esteves, Dinsdale, Guwy, & Maddy, 2013) This is generally accepted in the Netherlands and Belgium. Fermentation plants use energy other than, for example, solar panels from wind turbines. Heat to keep the fermentation tank at 38-40°C and electricity to pump and stir the biomass. Cleaning and purifying the biogas, in particular by removing water and hydrogen sulfide, improves the quality of set biogas, as the purer the biogas is the better it can be used for generating power because the less the biogas is polluted the less extra measures need to be taken into account. For this article, we will look into the working of two gasses, namely, bio-methane and hydrogen.

Bio-methane

One of the main focuses of present-day biogas research has its eyes on bio-methane production. Just as the name might suggest bio-methane is methane made from biological processes. In contradiction to fossil methane, which has taken thousands or even millions of years to generate and stockpile its current levels in the ground and atmosphere, bio-methane can be produced in a matter of months to a wide-scale usable level. Without depleting the planet of this source and thus without adding to the number of strong greenhouse gases in the atmosphere. The production of biomethane is as old as the time of when the planet was still covert in swamps, were anaerobic bacteria were transforming and using rotten plant debris to feed off on and to release them to them unusable methane into the environment.

Methane consists of just 5 molecules, namely one carbon and 4 hydrogens. This relatively small molecule does pack a whole lot of energy. Methane is important for electricity generation by burning it as a fuel in a gas turbine or steam generator. Compared to other hydrocarbon fuels, methane produces less carbon dioxide for each unit of heat released. At about 891 kJ/mol, methane's heat of combustion is lower than that of any other hydrocarbon. However, it produces more heat per mass than any other organic molecule due to its relatively large content of hydrogen in its structure, this hydrogen accounts for 55% of the heat in combustion (Schmidt-Rohr, 2015). but contributes only 25% of the molecular mass of methane. In many countries, methane is piped into homes for heating and cooking. The production of methane is and can be done in many shapes and sizes.

Geological routes

The two main routes for geological methane generation are organic, which are thermally generated, and inorganic, which is generated abiotically (Etiope & Sherwood Lollar, 2013). Thermogenic methane occurs due to the breakup of organic matter at elevated temperatures and pressures in deep sedimentary strata. Most methane in sedimentary layers is thermogenic; therefore, thermogenic methane is the most important source of natural gas production. Thermogenic methane components are typically considered to be a relic from an earlier time. Generally, the formation of thermogenic methane can occur through organic matter breakup or organic synthesis. Both ways can involve microorganisms, but may also occur inorganically. The processes involved can also consume methane, with or without microorganisms.

The more important source of methane at depth is abiotic. Abiotic means that methane is created from inorganic compounds, without biological activity, either through magmatic processes or via water-rock reactions that occur at low temperatures and pressures, like serpentinization, which is a process where whereby rock is changing, with the addition of water into the crystal structure of the minerals found within the rock. Metamorphic processes usually involve the addition of heat and pressure for example a rock is buried, heats up and is squeezed, and the minerals change in an attempt to regain equilibrium with the new environment. With this formation, the pressure has to be released to contain a balance, here is where methane is created and released to reduce the overall pressure of the surroundings. (Holm, Oze, Mousis, Waite, & Guilbert-Lepoutre, 2015)

Biological routes for biomethane production

Most of Earth's methane is biogenic and is produced by methanogenesis,(Muñoz-Velasco et al., 2018) a form of anaerobic respiration only known to be conducted by some members of the domain Archaea. (Dean et al., 2018) Methanogens occupy landfills and other soils,(Whiticar, 1999) ruminants such as cows and other cattle(Sirohi, 2010), the guts of termites, and the anoxic sediments below the seafloor and the bottom of lakes. This multistep process is used by these microorganisms for energy. The net reaction of methanogenesis is: CO2 + 4 H2→ CH4 + 2 H2O

Metagenesis involves four steps, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. To start with the hydrolysis, in this step, large complex molecules are broken down into simpler molecules. For example, a complex molecule such as lignin, which is a large part of lignocellulosic plants(Xu et al., 2019), is broken down into monosaccharides using hydrolysis enzymes. The next step is acidogenesis also known as fermentation. Here the monosaccharides are reduced into volatile fatty acids and alcohols, and some by-products such as ammonia, carbon dioxide, hydrogen, and sulfide. Acetogenesis, the third step in methane production is where higher organic acids and alcohols produced during acidogenesis are converted largely to acetic acid with some by-products including hydrogen and carbon dioxide. The last step involves methanogenesis, which usually operates in two groups of methanogenic bacteria. The first group split the acetate into methane and carbon dioxide while the second group uses the carbon dioxide produced and hydrogen to make methane. (Sirohi et al., 2010)

Methane production form livestock

Ruminants, such as cattle, belch methane, accounts for about 22% of the U.S. annual methane emissions to the atmosphere. One study reported that the livestock sector in general produces 37% of all human-induced methane. A 2013 study estimated that livestock accounted for 44% of human-induced methane and 15% of human-induced greenhouse gas emissions (Sirohi et al., 2010). The production itself follows a similar path Metagenesis. For example, a cow eats grass, the grass gets digested, the microorganism produces methane form that digested material, and the cow release that gas into the environment.

Aquatic methane production

Most of the subseafloor is an anoxic environment because oxygen is removed by aerobic microorganisms within the first few centimetres of the sediment on the seafloor. Below this anoxic seafloor, methanogens produce methane that is either used by other organisms or becomes trapped in gas hydrates. These other organisms which utilize methane for energy are known as methanotrophs or simply said methane eating, and are the main reason why little methane generated at depth reaches the sea surface. groups of Archaea and Bacteria have been found to oxidize methane via Anaerobic Oxidation of Methane shorted to AOM, the organisms responsible for this are Anaerobic Methanotrophic Archaea and Sulphate-Reducing Bacteria. (Knittel, Wegener, & Boetius, 2019)

Industrial routes

There is little incentive to produce methane industrially. Methane is produced by hydrogenating carbon dioxide through the Sabatier process. Methane is also a side product of the hydrogenation of carbon monoxide in the Fischer–Tropic process, which is practiced on a large scale to produce longer-chain molecules than methane. Power to methane is a technology that uses electrical power to produce hydrogen from water by electrolysis and uses the Sabatier reaction It involves the reaction of hydrogen with carbon dioxide at elevated temperatures (optimally 300–400 °C) and pressures in the presence of a nickel catalyst to produce methane and water. As of 2016, this is mostly under development and not in large-scale use. Theoretically, the process could be used as a buffer for excess and off-peak power generated by highly fluctuating wind generators and solar arrays. However, as currently very large amounts of natural gas are used in power plants to produce electric energy, the losses in efficiency are not acceptable. (Adnan, Ong, Nomanbhay, Chew, & Show, 2019)

Hydrogen

A new and upcoming source of renewable energy is hydrogen, if you have not seen at least one article in a newspaper or any other source of information about the potential of hydrogen in the last 2 years, well then you might have been living under a literal rock. Many researcher swear that hydrogen is the future of energy. And that it is one of the most abundant recourses in our universe is also a big bonus. Hydrogen is considered as the simplest element in existence. As a gas it is not found naturally on earth and thus must be made. This mainly is because hydrogen gas is lighter than air and rises towards the atmosphere, because of this hydrogen is always linked with elements such as water, coal, or petroleum.

Hydrogen is considered to have the highest energy content of any other fuel type by weight, but on the other hand, it has the lowest energy content by volume. one thing is sure it is the lightest element known and is a gas at normal temperature and pressure. A single hydrogen mole only weighs about two grams but it can produce a whopping 286 Kilojoules of energy per mole. This is a mere 32% of the energy produced by methane but hydrogen is also only 25% of the weight of methane. So if you would burn the same weight of hydrogen as 1 mole of methane, you would get 8 times more energy. (M.C Akanyıldırım, 2015) but unfortunately, since hydrogen is a rare sight in the form of gas on our planet's surroundings, we must obtain it by utilizing breaking other chemical substances. Luckily we have quite a few ways of doing this. Two of the most commonly used methods of hydrogen production are electrolysis of water and steam reforming, but also newer methods are used.

Steam reforming

Steam reforming is currently one of the most used methods of hydrogen production, Steam reforming of natural gas produces 95% of the world's hydrogen(Ogden, 2004), mainly because it is the cheapest way of producing it. Unfortunately, the building of the power plants to perform this process is rather expensive. This method is in the industry used to separate hydrogen atoms from carbon mainly from methane. In an equilibrium reaction of CH4 + H2O ⇌ CO + 3 H2. (K. Liu, Song, & Subramani, 2009)

The reaction is performed in a reformer reactor where a high-pressure mixture of steam and methane are put into contact with a nickel catalyst. A catalyst with a high surface to volume ratio is handy because of the diffusion limitations due to high operating temperature. Such shapes of catalyst have a low overall pressure drop which is beneficial for a reaction like these (Reimert et al., 2011). Via the so-called water gas shift reaction, additional hydrogen can be obtained by using the carbon monoxide generated in the steam reforming process with water to gain the reaction of CO + H2O ⇌ CO2 + H2. Thus increasing the yield of hydrogen in the process.

cd9cd3a0c3c4ffa964a34991844e17796bae63f5efca7095d7a0b79416eebf49.jpegFlow diagram of steam-reforming process (Elshout, 2010).

For every ton of hydrogen produced in this method, nine tonnes of CO2 is also produced. So because methane is a fossil fuel and in the process of steam reforming produces a lot of carbon dioxide, it does mean that the process results in an enormous release of greenhouse gasses, which is linked to global warming. So steam reforming might be a rather cheap and easy hydrogen producer, it is not a clean method of production. (Shirasaki & Yasuda, 2013)

Hydrogen from water

Another industrial method of hydrogen production is by ways of electrolysis. Electrolysis is the passing of electrical current through water to separate water into hydrogen and oxygen. Hydrogen will then move to a negatively charges cathode and oxygen will move towards a positively charged anode, The oxygen can be released into the environment or used for other processes and the hydrogen can be collected and stored in tanks for later use or it can be directly pumped into pipes to deliver to households or used in other energy plants. (LEROY, 1983) Since electrolysis only involves the use of water, electricity, and no other chemical components, the hydrogen produced is extremely pure. Unfortunately, the process of electrolysis is currently very expensive because you need to use electricity to produce potential power. But the price may be lowered over time because of new ways of producing energy for cheaper, and with the price of electricity lowering the cost of water, electrolysis will also be lower.

ce910de0e0ab35f4c62c7fbd0ddd4388a33e0c3e7a360f2d4c1e831c9fb7c8be.pngbasic overview of hydrogen production using electrolysis  https://www.energy.gov/eere/fuelcells/hydrogen-production-electrolysis)

Renewable energy hydrogen production

With the ever-increasing efficiency and cost reduction of energy production by the means of solar, wind, and other renewable sources, these sources of energy can be coupled to the production of hydrogen through electrolysis. You generate electricity by the use of for example solar energy, and then you use that electricity to power the production of hydrogen by water electrolysis. This way the method becomes a lot more sustainable because you do not only produce the hydrogen itself from a renewable source, the overcharge of electricity can also be stored in batteries, and in the case of solar power, this can then at night be used to continue the production of hydrogen. The overcharge of energy by solar power can in turn also be directly linked to the electricity network for consumers to use. And at the end of the process, the only thing that should be released into the atmosphere is the oxygen left from the water electrolysis. (K. Liu et al., 2009)

9982dc3000b0a352882cc1139aca8c492d0650d47b6de56f05443a2b663adf41.jpegHydrogen production using electrolysis from renewable energy  (image: NIMS (https://www.nims.go.jp/eng/))

Most of these methods of hydrogen, methane, and power production do sound too good to be true but there are a couple of downsides to some of these ways of production. I will go into more detail about these downsides in later articles.


DaLuSt
DaLuSt

This is a story about a young dutch adolescent who studies Biomedical Research. Join him in his epic quest to find the one thing that everyone seeks, the ever-growing need for entertainment.


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