Discover more about this condensable species that are produced through combustion, gasification and pyrolysis of hydrocarbon-based fuels.
The goal of this article is to provide you information on the TAR, known also as condensable species that are produced in the treatment process (combustion, gasification, pyrolysis) of hydrocarbon-based fuels. In particular, the article is structured in three different areas:
Among all the discoveries made by man, probably one of the most significant is that of fire. The fire has allowed our ancestors to cook food, to extend their activities until late, to protect themselves from predators and to warm up in the freezing winter nights.
Today, as then, the combustion processes are essential. It is thanks to these processes that we see a cheery fire in the fireplace, we can move in the car and we can produce electricity in large power plants.
In the branch of production of electricity, a lot have done (and still do) fossil fuels. Alongside the fossil fuels are increasing the importance the renewable sources, always used and now more important to meet global energy needs reducing environmental impact of pollution.
In particular, are emerging increasingly efficient technologies for the treatment of biomass. When it comes to combustion or gasification of biomass (or other solid fuels), the efficiency is crucial. Indeed the reactions which take place are extremely complex, and can provide as output a series of chemical species that do not represent the typical products of partial or total combustion (CO, H2, CO2, H2O). A careful control of the operating parameters allows to considerably increase the efficiency of the system, in terms of energy performance and in terms of production of polluting species.
What are these “operating parameters”? An important parameter, which has important implications in the formation of pollutants, is the fuel / air ratio.
To analyze the impact of this important parameter, temporarily withdraw ourselves from the world of solid fuels and consider the simplest hydrocarbon species: methane. The methane, to give a complete combustion reaction, needs a well-defined amount of oxygen. In particular, two oxygen molecules for each molecule of methane will become necessary.
This defined proportion between quantity of methane burned and oxygen required is defined stoichiometric ratio of the complete combustion reaction.
The combustion reactions do not always occur at stoichiometric conditions: there may be an excess or a lack of air. To make a quantitative estimate of the combustion conditions it was introduced a parameter, which is called the equivalent ratio which is defined as follows.
Based on the operating conditions, we can distinguish three different regimes:
Now that we have defined the various possible operating conditions, it is imperative to make a clarification. If, for example, we feed a quantity of air corresponding to the stoichiometric one (Φ = 1), it is said that throughout the volume in which the combustion reaction occurs the fuel / air ratio is equal to the stoichiometric ratio. in fact there can be some areas with excess of fuel (Φ> 1) and areas with a shortage of fuel (Φ <1), on the basis of the mixing phenomena. Here is a basic condition: the proper mixing of fuel and combustion. The chemical aspect of combustion reactions is only one side of the coin: the other side corresponds to the complicated transport phenomena that occur.
On the basis of the temperature levels and local composition, they may in fact be formed polluting species. Let us examine this aspect taking into account the same molecule before (methane), and examining the complex nature of radicalic reactions that characterize its combustion process.
As we can see in the picture, the methane combustion mechanism involves a multitude of chemical species. In particular, we can see how the presence of oxygen leads to oxidation of the hydrocarbon species, while its absence leads to a pyrolytic nature mechanism. In the nature of the pyrolytic process we can notice that the reaction of two methyl radicals (CH3) leads to the formation of ethane (C2H6), which for subsequent reactions of de-hydrogenation form ethene (C2H4) and finally the acetylene (C2H2). The latter species contains a triple bond between the two carbon atoms, and behaves as a precursor of aromatic species (benzene and PAH – Polyaromatic Hydrocarbons).
It is in this process that takes the potential formation of the TAR. This term is to indicate broad classes of compounds, liquefiable and predominantly hydrocarbon, which develop in consequence to pyrolytic processes at high temperature.
We noted the complexity of the kinetic mechanisms of methane combustion / pyrolysis. Imagine how this complexity further increases when we consider the burning of hydrocarbon species less simple! If we consider the combustion of solid species (such as biomass) or liquid, the description of the combustion process is further makes more intricate because of the inter-phase transport mechanisms. In fact the non-efficiency of these processes can lead to the genesis of pyrolysis zones, resulting TAR formation and soot (particulates).
A solid biomass can be described on the basis of three main components:
To understand how it behaves in a biomass pyrolysis conditions, you can analyze how behaves each of its members under the same conditions. This analysis may be performed experimentally, and takes the name of thermo-gravimetric analysis. Imposing a certain rate of warming of the material, simply register varies the mass variation over time of the material in question. As the temperature increases, it can be noted how the mass of the solid matrix decreases, and instead tend to form liquid and gas substances, and in consequence to the depolymerization of the chains which characterize the species before described. It is precisely at this stage that there is the formation of the TAR, liquefiable species coming from the pyrolysis of the solid matrix.
Looking in more detail, the combustion of a solid matrix, one can recognize the following steps:
Quantify the effects of the TAR on human health is complex. One factor contributing to the complexity is definitely the high number of species that characterize the TAR: while some may be harmless, others can be potentially dangerous.
Benzene, for example, is carcinogenic for both men and animals. Other compounds that characterize the PAH, instead, have mutagenic action. For example benzo (a) pyrene, by action of cytochrome P-450, is epoxidized: then through subsequent reactions form a species that can react with DNA, introducing the mutations.
The question therefore arises spontaneously: it is possible to intervene to remove the tar from the process streams?
An answer to this question lies surely in a preventive action, which is to make an optimal design of boilers and reactors; This allows you to promote effective mixing between fuel and oxidizing.
There are also post-treatment methods, which we will analyze in a later article!
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[1] A. D’Alessio, A. D’Anna, T. Faravelli e E. Ranzi, «Particolato fine e ultrafine,» Chimica & Ambiente, pp. 34-42, 2005.
Discover more about this condensable species that are produced through combustion, gasification and pyrolysis of hydrocarbon-based fuels.
Discover more about this condensable species that are produced through combustion, gasification and pyrolysis of hydrocarbon-based fuels.
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