The synthesis gas obtained carries with it, under the effect of high temperatures and in an uncontrolled way, heavy organic compounds generated by thermal decomposition and called tars. These tars condense easily on cold spots, causing pipe fouling and loss of heat exchange efficiency. In addition, tars can form coke by coking or soot by polymerization.
Although tars are not the only source of poisoning (particles, metal salts, inorganic sulphur, chlorine and nitrogen compounds), they are still the only source of poisoning. most difficult pollutant to remove.
There are different definitions of tars. In general, the tars or « Tars » refer to a complex mixture of aliphatic or aromatic hydrocarbons from one to several rings which may or may not contain a heteroatom.
The tar composition in a synthesis gas depends directly on the operating conditions (temperature, pressure, oxidizing or non-oxidizing atmosphere, residence time), the type of reactor used and the nature of the biomass.
However , tar analysis and more specifically their extreme environment sampling (high temperatures and pressures, high humidity) are proving to be a major factor in difficult task. The difficulty is to have a sample that, under laboratory conditions, either representative as much as possible of the « synthetic gas ».
The reference method for tar sampling is called the « Tar Protocol »
This technique is based on discontinuous sampling of synthesis gases containing particles and tars under isokinetic conditions.
The sampling system consists of several trapping devices A heated particle filter, a condenser and a series of bubblers containing solvents. A heating device for the sampling lines partially prevents the condensation of tars before the devices provided for this purpose.
The sampling process consists of 4 modules and sub-modules.
The module 1 consists of a sampling valve system and a heated isokinetic probe that allows the gas sampling, whether in a pressurized reactor or not. Sampling lines are generally maintained at temperatures above 350°C to minimize the condensation of heavy tars.
The module 2 is composed of a gravimetric filter that allows to recovering particles at the exit of the reactor. Indeed, these particles could lead to clogging of the sampling supports located downstream (condenser and bubblers).
In the module 3, the bubblers of tar collection are placed in different baths with temperatures ranging from 20 to -20 °C. The baths absorb the heat generated by the cooling of the gases and their condensation. Direct condensation of liquid effluents without dilution medium (only by cold trap) can cause reactions between the trapped compounds, so a solvent is placed in each of the bubblers to absorb these compounds and thus avoid any parasitic reaction.
Isopropanol has been determined as the solvent most appropriate for the collection of tars due to its low toxicity and its ability to solubilize tars for their collection.
Immediately after sampling, the contents of the bubblers are stored in a bottle which is kept hermetically sealed at a temperature below 5°C for further analysis.
The module 4, as for him, allows to measure and settle the gas parameters.
The sampled gas flow rate is kept constant by means of a vacuum pump. It is important to ensure that the sampling rate is not too high in relation to the cooling and adsorption capacity of the bubblers.
