Bio-gas and the greys of green solutions Mechanical Engineering
· 13 min read
My relocation to Asia has generated discussions with EU and US companies providing green energy solutions. While we are used to talking about renewables as a binary domain where energy sources are either bad or good to the environment (e.g. oil and coal, compared to wind and solar), real-world implementations involve also less clear scenarios. Cases like nuclear power encourage being inquisitive considering the intuitive issues related to radioactive waste. However, almost any implementation presents similar problems to solve. That is true also for one of the latest projects I have been working on. The initiative was about natural gas and its green alternative, bio-gas. Inspired by that specific experience, this post could provide useful inputs for the many unclear and debatable scenarios we will increasingly face in the foreseeable future. We will critically cover energy production from fossil fuels and the benefits of using bio-alternatives like bio-gas and bio-methane. The result may sound surprising to some but, gas extracted from underground and bio-gas from livestock and grass will indeed turn out to be similarly polluting. However, we will also identify the opportunities related to bio-alternatives, possibly providing interested readers with a useful and general reference for critical decision-making.
The three paragraphs below will answer, with increasing quantitative character, the three questions below:
In a nutshell, we are dealing with only two types of gas rather than four. Methane and bio-methane are just refined forms of respectively natural gas and bio-gas. The difference between the last two is just in their origins: similar to oil in the former case, similar to renewable sources in the latter. Still, they have very similar chemical compositions, being all in major part CH4, which is methane. Let us go over a few additional details.
Methane is the major constituent of natural gas (say from 75 to 95% depending on the provenience), which in turn is a gas which we could think of as being a close relative of oil, indeed extracted similarly or in conjunction with oil. The isolation of methane from natural gas is usually required to eliminate CO2, water vapor, and non-hydrocarbon substances which would either hurt the end processes or not serve their purposes. Note, the elimination of the CO2 does not imply we will not emit CO2 while using the gas to produce energy. That “new” CO2 will come from the breakdown of the methane (CH4) and the combination of its carbon (C) with the oxygen used as comburent.
Important distinction between CO and CO2: while in insufficiently oxygenated machines like domestic stoves we may have a big part of the carbon combined in carbon monoxide (CO) because of incomplete combustion, within industrial energy-generating processes we usually have a prevalent production of carbon dioxide (CO2). They could be both considered greenhouse gas contributing to global warming by trapping energy from the sun. However, CO2 has probably more direct and heavy effects on the latter, while CO could be more directly related to human health issues.
The same argument we just made above applies to bio-gas and bio-methane. The difference is that compared to natural gas, bio-gas has a more renewable origin. Rather than from underground, bio-gas comes from animal digestion (e.g. cattle), anaerobic digestion (e.g. waste & water treatment), other bio-mass, etc. The concentration of the methane in the original bio-gas blend could be as low as 40%.
For sake of this post, and considering that differences are mainly about filtration, the terms natural gas and methane will be used here interchangeably even when not specified. The same will apply to bio-gas and bio-methane.
Let’s now look at how all that gas produces energy. Regardless of the type of gas we used, the process to produce energy would be very similar. We can picture two main options - though not the only ones. The first one is an engine similar to the one of a car but of industrial size, and where we would substitute gasoline with one of those four slightly different types of gas. This is usually deployed in modular applications, where small units and intermittent use are among the main specs of the project. The second option is a process where we burn gas to heat a separate circulating fluid which in turn then cools down while propelling a turbine – the separate circulating fluid could be dropped, letting the gas directly do the job. This is usually deployed when bigger and more constant flows characterize the project. Both the engine and the turbine of the two options would produce energy through their spinning shafts being connected to a generator. Disregarding for a moment the slightly different polluting emissions related to the slightly different compositions, any one of those four types of gas would similarly pollute by releasing CO2 while burning. Even in the case of bio-gas, we would always be talking mostly about burning molecules of carbon and hydrogen (CH4 - methane) and emitting CO2.
Among the ones we are discussing, the gas with fossil origins (i.e. natural gas, therefore methane) is very common in the production of energy. We can see in figure 1 how natural gas makes for a big share of the sources for the production of electricity in the USA. The picture for bio-gas (i.e. bio-methane) is completely different. While in figure 1 some bio-gas participates to the 4% of the 12% share of renewable energy (<1% of the total) the majority of the whole biomass’ share is stuff directly burned into waste-to-energy plants. It is often more convenient to just burn what we have rather than extract bio-gas from it through fermentation. The transformation into bio-gas is preferred when it occurs almost naturally or when energy extraction through incineration is not possible - respective examples are methane emission from livestock’s digestion and bio-gas obtained from wastewater treatment. At the moment the numbers on bio-methane are still very small, and specific details can be found here.
Even though bio-gas, therefore bio-methane, does not seem to bring many benefits so far, here is a big catch: while bio-gas would come almost for free since most of the bio-mass we would use for its production would exist anyway as scrap and waste from other processes, fossil fuels like oil, coal, or natural gas – remember, this is a fossil fuel - must be specifically extracted from the ground. The latter is a polluting factor by itself and could be related to the negative “fracking” argument in the USA. Moreover, if not used for extracting bio-methane, existing biomass would pollute, either directly or by emitting spontaneously methane going unused into the atmosphere. Therefore, there is indeed an argument for considering bio-gas as a greener fuel and even a CO2 reducer when used to replace fossil fuel, and when utilized rather than left spreading into the atmosphere because unused – think about the often mentioned big impact of livestock’s emissions, estimated to account for a share of CO2 emissions between a few percentage points and tens of points worldwide.
In a few lines, we will summarize the opportunity above with a graphic returning us a strategic view. Before that, let us refresh a couple of useful principles that will help us through the logic:
The amount of energy we can extract from a fuel can be approximated with the amount of heat we obtain with its combustion. Note for the interested reader: thermodynamic laws put a limit then to the conversion into work of that heat, but we can ignore it here since we are comparing the energy that could be extracted. Moreover, note that we are saying “heath from combustion”, which is different from the total internal energy, part of which could be used directly by the gas for its expansion depending on the volumetric constraints, and not released then through the combustion.
Therefore, although different quantities would be needed, the same amount of energy could be extracted from any fuel discussed here by feeding a combustion chamber with it until a specific amount of calories (i.e. heat) was obtained. Note for the interested reader: different would be the energy per unit of mass, say the “specific energy”, as well as different would be the energy per unit of time, say the “power”.
Finally, while the two approximations above are fairly honest and we will hold them true till the end of the article, let us make a temporary simplification which we will then correct: using different amounts of different fuels to produce the same amount of energy (point 2, above) would also produce and emit the same amounts of CO2. This is not true, not even as a first approximation. However, we will correct this assumption only in the following last paragraph. In this second paragraph, this assumption can help us with the logical steps.
We can now pass to figure 2 and the following bullet points summarizing the opportunity related to bio-gas. We will compare solutions made constant the amount of energy produced (point b, above).
Point 3 above is critical because unused methane – or bio-methane – once released into the atmosphere becomes a potent pollutant. There is a whole discussion debating whether methane is worse than CO2, with strong evidence in that direction - methane is often discussed as being "front-loaded", meaning having short life but much stronger greenhouse character than CO2. In any case, methane is indeed a pollutant, and in the scenario on the left of figure 2 it is released in parallel to the CO2 coming out of the process producing energy. Conversely, in the scenario on the right, that same bio-methane is used to produce energy. Still, the reader must keep in mind the following: while in the non-renewable scenario we are emitting bio-methane into the atmosphere as it is naturally released by the biomass left to rotten or untreated, in the renewable scenario we would try to expedite and maximize the extraction through processes like membrane filtration, etc. Therefore, rate of emission and quantity emitted may differ.
The whole argument summed up in figure 2 could be customized for any of the sources included in the category “biomass” of figure 1. Any energy source in that group could replace oil, coal, and natural gas by being directly burned into furnaces, and without being first transformed into bio-gas. However, we are focusing here on bio-gas (i.e. bio-methane) because, as said above, sometimes bio-methane is the only solution when solid stuff cannot easily be burned into furnaces. Moreover, while among the most dangerous situations there is the one where bio-methane is spontaneously produced and left spreading into the atmosphere, we will also see immediately below that biomass has issues of its own when directly burned.
The fuels involved in figure 2 – fossil and “green” ones - would not release the same content of CO2 for the same amount of heat extracted (i.e. energy produced). The table in figure 3 is from the U.S Energy Information Administration, and it shows the different amounts of CO2 released for the same amount of heat produced (i.e. energy).
Even though the picture above is not comprehensive, by looking at the values in the last column we can have a rough idea of the different emissions for the same quantity of heat produced (i.e. energy):
Note: while we have focused on CO2 and a couple of other factors, those are not the only dimensions characterizing a complete comparison. Ashes and other residuals exist, though they could be considered not to be greenhouse gas (i.e. meaning, contributing to rising temperatures by trapping solar energy). For example, both natural and bio-gas release in general fewer particles dangerous to human health (i.e. ashes), which is another advantage over other fossil fuels. However, even just that alone makes of them a solution not completely green.
This last consideration on the additional emissions that we have not treated in this article (e.g. ashes) serves us well in highlighting how the ultimate decision should be made on comprehensive analyses on a case-by-case basis. How we handle the greys of green solutions can make or break an initiative.
We could somehow relate to bio-gas (i.e. bio-methane) the “net-zero CO2” argument often associated with biomass. Pellet-stoves for example are sometimes discussed as net-zero CO2 emitters because the carbon dioxide that the lignite emits while burning is believed to have been previously “fixed” during the life of that same fuel - see photosynthesis. However, we are personally not completely sold on that. Lignite, being a close parent of coal, is a strong CO2 emitter with emissions double those of natural gas (figure 3). Pellet would still release a lot of that CO2 (and CO) in urban environments where stoves are heavily utilized and where we would need much cleaner solutions ... Let us say that there are at least some greys.
To conclude. Focusing on the real advantages while not covering the disadvantages and dangers of grey solutions like gas could help us deploy the best engineering we have currently at our disposal. At the same time, that would help us move the focus away from solutions that are not completely green, and it would allow us to focus on the need to implement as soon as possible new and truly green solutions.
Tags of the two original images used for the following customization of the main image of the article:
https://unsplash.com/@flashdantz?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText
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