FR3075074A1 - PROCESS FOR THE MECHANICAL TREATMENT OF WOOD WASTE, WOOD-TYPE B, FOR THEIR COMBUSTION, ESPECIALLY IN BOILER ROOM - Google Patents

Abstract

 The invention relates to a process for the mechanical treatment of wood waste, of wood type B, comprising:  - a step of grinding said waste,  - a step of screening crushed waste.

Description

PROCESS FOR THE MECHANICAL TREATMENT OF WOOD WASTE, OF THE TYPE

WOOD B, FOR THEIR COMBUSTION, ESPECIALLY IN BOILER

Technical area

The present invention relates to the field of use of wood waste for heating purposes, in particular in industrial boilers.

It relates in particular to wood type B waste.

The invention aims to allow the use of wood waste B as fuel, in industrial wood boilers (boiler rooms) for district heating or any other type of heating.

Prior art

In a context of energy transition, the valorization of biomass as an alternative energy, in particular by combustion to produce heat, is brought to occupy an increasingly important place.

Currently, French industrialists mainly use raw wood in the form of forest chips and wood verifying the criteria for leaving waste status (SSD) according to the decree of July 29, 2014 (pallets, packaging ...).

However, tensions have been felt in recent years over the availability of this resource.

It is therefore interesting for manufacturers to be able to broaden their spectrum of resources, in particular by using wood waste from construction or furniture. This wood with low additive content, designated wood B, is nevertheless likely to contain various pollutants and cannot be accepted as is in a boiler room.

In France, wood waste is classified into three categories, as follows:

- wood A, which is a used but clean wood, which can already be used in the boiler room (boxes, crates, pallets, packaging, etc.). It corresponds to the SSD wood of the decree of July 29, 2014;

- wood B, which is a lightly treated wood and which is currently sent directly to incineration plants or is recycled in the form of panels (demolition wood, furniture, particle board, etc.);

- wood C, which is wood treated at heart, considered as hazardous waste and sent to incineration plants or to landfills (railway sleepers, telegraph poles, palisades treated at heart, etc. ).

The wood resource B has been estimated at 0.5 Mt / year on average in the Rhône-Alpes region and at 5 Mt / year in France. In Europe, the resource would be 40 Mt / year. However, this figure is questionable since the very definition of B wood is not the same in different countries.

Wood B therefore represents significant volumes, but is by nature very heterogeneous. It contains products glued with resins or adhesives (panel family), surface treated wood (varnished, painted wood) or elements treated against humidity or insects. This leads to a great diversity of pollutants potentially present in this resource: nitrogen, metals, halogens ...

Also, if we want this raw material to be authorized to be used in existing boiler rooms without modification of the latter, it is necessary to make it sufficiently clean.

Studies on the subject aim to design, optimize and validate, on a laboratory then pilot scale, pretreatment chains making it possible to make wood B suitable for exiting waste status, that is to say respecting limits in fixed pollutants for the input solid and for polluting emissions in biomass boiler rooms.

Before defining a suitable treatment for cleaning wood B, its composition should be finely characterized.

The numerous studies that can be found in the literature, carried out with the aim of using wood B for combustion purposes, demonstrate the variability of the constituents of wood B, their composition in polluting elements and therefore, treatment means to be put in place to eliminate these pollutants.

In the types of treatment implemented, liquid treatments can be cited to eliminate certain pollutants: [1],

We can also cite thermal pretreatment studies by pyrolysis to remove nitrogen or other components from certain polluted woods: [2],

Studies carried out on different samples collected (see study [3]) have shown that certain characteristics of B wood were not very variable from one sample to another, with differences that can be explained by the presence of pollution. external (presence of earth, stones, nails ...). This pollution can be eliminated from the start, that is to say during the selection of B wood at the recycling center.

It also emerges from these studies that the common characteristics of wood waste B are:

- carbon, hydrogen and oxygen content,

- lower calorific value (PCI, in English "Lower Heating Value", acronym LHV),

- humidity,

- the content of non-metallic inorganics (Ca, K, Mg, Mn, Na, P, Si).

On the contrary, other properties are directly affected by the treatment undergone by wood B. This is the case for the contents:

- nitrogen, which comes from the resins used in particle boards,

- heavy metals, especially lead and zinc, often associated with paints and coatings,

- copper and chromium, which are also very strongly present in wood waste C,

- chlorine, especially present in coatings,

- boron, mainly from wood treated with fungicides, mainly Coriphene.

In study [3], the concentration of certain elements in medium batches was compared with target values obtained mainly based on the specifications of French legislation concerning biomass boiler rooms.

It appeared that the specifications were clearly exceeded for several of the elements, namely for:

-nitrogen, with values between 1.3 and 2.7% m (% m: mass percentage) for a target value of 1.5% m,

- metals, the concentration of which reaches 7300 mg / kg in a batch N ° 5 for a target value of 1000 mg / kg. These metals can have two distinct origins. They can come from external elements (nails, staples), and can then be eliminated quite easily upstream. This is particularly the case for iron and aluminum. They can also be linked to the treatment undergone by the wood during implementation. This is the case for metal oxides used in paints and coatings (Ba, Cd, Pb, Ti, Zn). The lead and zinc specifications are exceeded by almost all medium lots,

- copper and chromium, which mainly come from samples of wood waste C.

There is therefore a need to find a solution which makes it possible to eliminate at least reduce the content of several elements present in wood waste B, in this case nitrogen, contained especially in the structure of particle boards, metals, in particular zinc and lead, present mainly on the surface in paints and coatings, and copper and chromium, coming almost exclusively from impregnated wood, in order to be able to use wood B as heating fuel, more particularly in biomass boiler rooms.

The object of the invention is to meet this need at least in part.

Statement of the invention

To do this, the invention relates, in one of its aspects, to a process for the mechanical treatment of wood waste, of wood type B, comprising a step of grinding said waste, in particular with a view to their combustion, say of their burning.

According to an advantageous embodiment, the method further comprises a step of screening the ground waste.

The invention also relates in another of its aspects, to a process for the mechanical treatment of wood waste, of wood type B, comprising:

- a step of grinding said waste,

- a step of screening crushed waste.

By analyzing the processes which had been implemented to treat wood waste B in order to make it compatible with use in a boiler room, the inventors realized that no purely mechanical treatment had been tested.

The inventors have demonstrated that a treatment by grinding, preferably with additional screening, provides very positive results, whatever the grid used for screening.

The treatment solution according to the invention consists in splitting the surface of wood B by grinding while retaining a particle size compatible with use in the boiler room, and thus concentrating the pollutants in the fines.

Grinding thus considerably reduces the levels of pollutants in the fraction usable in the boiler room and is situated below, or at least approaches, as the case may be, the targets targeted from a regulatory point of view.

The cost associated with mechanical treatment by crushing / screening wood waste B is low. Also, substantial savings can be expected through the use of this abundantly available resource.

Mechanical treatment by grinding with preferably screening according to the invention therefore now appears as a key step to be considered in the context of a wood pre-treatment chain B, before use in the boiler room, so as to greatly reduce the fuel content. in metallic pollutants.

The mechanical treatment according to the invention is therefore a pretreatment, in the sense that it is prior to the burning of wood B for heating.

Advantageously, the grinding step is carried out by a knife type grinder, preferably provided with screening grids.

The screening grids can have a screening mesh of 30 mm, 50 mm or 80 mm.

The invention also relates to the use as fuels, of wood waste B ground according to the process described above, in a boiler room installation, in particular for district heating.

detailed description

Other advantages and characteristics of the invention will emerge more clearly on reading the detailed description of examples of implementation of the invention made by way of illustration and not limitation with reference to the following figures, among which:

- Figure 1 is a graphical representation by bar diagram of the analysis of carbon, hydrogen, oxygen, nitrogen, chlorine and volatile matter composition of wood waste B following their treatment by grinding and screening according to a first example of l 'invention;

- Figure 2 is a bar diagram of the analysis of composition in inorganic minor elements of wood waste B following their treatment by grinding and screening according to the first example of the invention;

- Figure 3 is a bar chart of the concentrations of certain elements in the different fractions of wood waste B compared to the target values of the regulatory specifications, following their treatment by grinding and screening according to the first example of the invention;

- Figure 4 is a bar diagram of the mass distribution of the crushed wood fractions B separated after screening for different grids, according to a second example of the invention;

- Figure 5 is a bar chart of the metal content before treatment and in the wood waste fractions B, crushed separated after screening for the different grids, compared to the target values of the regulatory specifications, according to the second example of l 'invention;

- Figures 6 and 7 are bar graphs of the lead and zinc content respectively, before treatment and in the wood waste fractions B, crushed separated after screening for the different grids, compared to the target values of the regulatory specifications , according to the second example of the invention.

In order to be able to use wood B, as firewood in particular in biomass boiler rooms, the inventors have thought of carrying out a purely mechanical treatment of this wood waste B which consists of grinding followed by screening of the particles of crushed wood.

To validate this process, the inventors carried out an experimental study by carrying out grinding tests using industrial shredders with knives, equipped with different grids, namely with a mesh size respectively equal to 30 mm, 50 mm and 80 mm.

Two separate crushing and screening test campaigns were carried out. For each of these campaigns, the results were compared with wood B before crushing, referenced in the figures as pre-crushed.

The first campaign was carried out on a sample of B wood. The crushed wood was sieved into four separate fractions before being analyzed in terms of elementary composition.

The four wood fractions were sieved according to their particle size as follows:

- size less than 0.45 mm;

- size between 0.45 and 1.4 mm;

- size between 1.4 and 2.5 mm;

- size greater than 2.5 mm.

The distribution of the initial mass between the different fractions is given in Table 1 below.

TABLE 1

Size of wood particles B crushed and sifted Mass percentage (% m) less than 0.45 mm 6 between 0.45 and 1.4 mm 18 between 1.4 and 2.5 mm 20 greater than 2.5 mm 56

It appears from this table 1 that more than half of the mass, more precisely

56% m, is found in the fraction of crushed particles larger than 2.5mm, which is the fraction generally most often used in boiler rooms.

The mechanical pretreatment according to the invention therefore appears to be economically compatible with the use of wood in the boiler room, especially as the intermediate fraction, between 1.4 and 2.5 mm, could possibly also be.

In this case, the mass loss linked to the pretreatment would be less than 25%.

The analysis of elementary composition shows that certain characteristics of elements (C, H, O, N, Cl, volatile matter) are rather homogeneous between the different fractions (Figure 1).

On the other hand, it has been observed that the inorganic elements tend to concentrate appreciably in the finest fraction of the particles, ie for a size less than 0.45 mm (FIG. 2).

If we observe more specifically the different metals, we see that the mechanical treatment according to the invention makes it possible to significantly decrease the metals, in particular Pb with a reduction factor 5 and Zn with a factor 3, in the fraction particles larger than 2.5 mm, but with little impact on the Ti concentration.

Table 2 below summarizes the target values established according to regulatory specifications.

TABLE 2

Concentration of elements in different fractions in wood B according to the state of the art Size of wood particles B crushed and sifted Total Regulatory standard less than 0.45 mm between 0.45 and 1.4 mm between 1.4 and 2.5 mm greater than 2.5 mm Nitrogen (% m) 2.9 2.4 1.9 2.2 2.2 1.5 Lead (mg / kg) 1879 831 180 69 344 50 Zinc (mg / kg) 3832 2294 993 432 1096 200 Sum of metals (mg / kg) 7890 4808 2190 1603 2701 1000

It appears from this table 2 that the regulatory target values are exceeded for nitrogen, lead, zinc and the sum of metals.

After treatment by grinding and screening according to the invention, the concentrations remained above these target values, but nevertheless considerably approached them for the fraction whose size is greater than 2.5 mm (FIG. 3).

Following this first campaign, a second test campaign was carried out by the inventors, in order to confirm the results and to optimize the treatment according to the invention.

For this second campaign, the ground wood B was sieved into three separate fractions before being analyzed in terms of elementary composition.

The three wood fractions were sieved according to their particle size as follows:

- size less than 0.45 mm;

- size between 0.45 and 1.5 mm;

- size greater than 1.5 mm.

For this second test campaign, the inventors have found that:

- the pollutant contents in the batch of wood B before treatment according to the invention were already below the standards expected at the input of a biomass boiler;

- the values measured on the batch of crushed wood B are not exactly the same as those obtained by adding the contributions of the different fractions, probably due to imperfect sampling. Only the trend results should therefore be considered without dwelling on overly precise comparisons.

As can be seen in Figure 4, the material balances show that the mass proportion of fines, including fractions of size less than 0.45 mm and those less than 1.5 mm, represents a small part of the total, included between 5 and a little more than 15% of the total fuel, ie values lower than that obtained during the first test campaign.

This therefore confirms the economic viability of the process by crushing with screening according to the invention with a view to the use of wood B in biomass heating. As for the differences observed according to the sizes of grids used, it was difficult to explain them and they were not considered significant.

FIG. 5 shows the metal content before treatment according to the invention and in the crushed fractions separated after screening according to this second test campaign, for the different grids with comparison with the target according to the regulatory standard symbolized by the horizontal line.

As seen in Figures 6 and 7, the results confirmed the marked increase in the concentration of metals, as well as Pb and Zn in the fines. It should be noted that in these Figures 5 to 7, the term "reconst" refers to the reconstituted data corresponding to an artificial recomposition of the crushed wood. In fact, the wood was crushed with different grids, namely with a mesh size respectively equal to 30 mm, 50 mm and 80 mm, then it was sieved into three different fractions. The composition of the wood before sieving was obtained by multiplying the contents of each fraction by the mass percentage of each fraction.

The two results in Figures 5 to 7 therefore validate the interest of the pretreatment studied to clean wood B, with marked elimination of pollutants and limited fuel loss. Again, no clear influence of the screening grid was observed, which leaves the free choice of which one to favor.

The results obtained during the two test campaigns validate the advantage of mechanical pretreatment by grinding and screening to clean wood B before it is burned, that is to say its combustion, with a marked elimination of the metal pollutants present on the surface. and limited fuel loss.

No clear influence of the grid was observed, which leaves the choice of which one to privilege free.

Other variants and advantages of the invention can be achieved without departing from the scope of the invention.

For example, if in the context of the tests, the mill used was an industrial knife mill, it is possible to consider using any other type of mill, preferably already equipped with one or more screening grids.

The invention is not limited to the examples which have just been described; one can in particular combine together characteristics of the examples illustrated within variants not illustrated.

REFERENCES CITED [1]: Elbariji, S., Elamine, M., Eljazouli, H., Kabli, H., Lâcherai, A., Albourine, A., “Treatment and valorization of the wood by-products. Application to the elimination of the industrial slains. CHEMISTRY REPORT, Volume: 9 Issue: 10 Pages: 1314-

1321DOI: 10.1016 / j.crci.2006.05.006 Published: OCT 2006.

[2]: Girods P, Dufour A, Rogaume Y, Rogaume C, Zoulalian A. “Thermal removal of nitrogen species from wood waste containing urea formaldehyde and melamine formaldehyde resins. J Hazard Mater 2008; 159: 210-21.

[3]: Maïté Huron, Sophia Oukala, Jérôme Lardière, Nicolas Giraud, Capucine Dupont, “An extensive characterization of various treated waste woodfor assessment of suitability with combustionprocess. Fuel, 202 (2017) 118/128.

Claims (4)

1. Process for the mechanical treatment of wood waste, of wood type B, comprising: 5 - a step of grinding said waste, - a step of screening crushed waste. 2. Treatment method according to claim 1, the grinding step being carried out by a knife type grinder, preferably provided with grids of 10 screening. 3. Treatment method according to claim 2, the screening grids having a screening mesh equal to 30 mm, 50 mm or 80 mm. 15 4. Use as fuel, wood waste B crushed according to the method according to one of the preceding claims, in a boiler plant, especially for district heating.