Improved steam cracking method for producing combustible pellets using heterogeneous biomass

Abstract

The present invention relates to the production of steam cracked biomass in the form of black pellets in the field of solid biofuels. More particularly, the present invention relates to the production of black pellets from heterogeneous biomass. The objective of the present invention is to produce black pellets with good mechanical strength, good water resistance, and increased heating value.

Description

Detailed Description of the Invention

[0001] The present invention relates to the production of steam cracked biomass in the form of black pellets in the field of solid biofuels. More particularly, the present invention relates to the production of black pellets from heterogeneous biomass. The objective of the present invention is to produce black pellets with good mechanical strength, good water resistance, and increased heating value. [Technical field of the invention] The production of renewable energy (electricity and heat) can come from the environment (sun, wind, tides, waves, geothermal, hydro) or from biomass. Hydroelectric energy from rivers or dams and geothermal energy are currently intermittent renewables and will remain so unless energy storage methods are developed. Only biomass constitutes primary energy that can be transported to a site for conversion into heat and / or electricity. However, biomass is in fact a sparse, variable, and perishable energy.

[0002] There are several methods for densifying and stabilizing lignocellulosic biomass, including carbonization, roasting, and steam cracking. The first method allows for the production of charcoal, but involves a high material price due to significant yield losses (approximately 70-80%), limiting its use to high "sale" prices, such as for use in home barbecues. Roasting is a technique with acceptable yields (10-20% loss), but the price remains prohibitive and the technology is not yet mature. To date, only steam cracking is considered technically and economically viable when production costs are included. This technique allows for the production of "black pellets."

[0003] It should be noted that steam cracking cannot be compared to hydrothermal pretreatment or pyrolysis techniques.

[0004] Hydrothermal pretreatment, also known as aqueous fractionation, solvent cracking, hydrothermal decomposition, or hydrothermal treatment, involves the use of water at high temperatures and pressures to promote the disintegration and separation of the lignocellulosic matrix. This technique is not suitable for the production of black pellets because the resulting products are primarily liquid. Pyrolysis is the chemical decomposition of organic compounds by intense heating in the absence of oxygen. The compounds obtained after pyrolysis have different properties than those obtained by steam cracking. Steam cracking cannot be compared to pyrolysis techniques in that it is carried out in the presence of oxygen using a steam explosion.

[0005] The conversion of lignocellulosic biomass (wood, agricultural residues, agricultural and agro-industrial co-products) into energy-dense, transportable, and easily storable compounds allows the development and integration of a fixed-energy industrial sector (a fixed-point, domestically used biofuel, unlike other biofuels) with reduced environmental impact (biomass without fossil CO2 emissions, fertilizers, or phytosanitary products). Thermal treatment of biomass by steam cracking allows for the densification of energy by homogenizing the biomass into a steam-cracked powder according to precise parametric conditions, particularly particle size, temperature, and residence time. The powder is then granulated to facilitate its transport, storage, and use.

[0006] It is important to ensure that the granulation method ensures a constant and good quality product with high calorific value and suitable mechanical behavior for storage, transport and use in boilers, with the best properties in terms of mechanical and water resistance, reduced (fine) dust formation during handling, dryness to ensure good calorific value and good combustion, and stability over time and in storage (no self-heating fermentation, gas evolution, etc.).

[0007] Granulation of the steam-cracked powder through a pellet press produces dense, durable pellets. Traditionally, refined and compressed softwood sawdust (75% between 0.36 and 1 mm, 50% less than 0.5 mm) produces pellets with diameters between 6, 8, and 10 mm, lengths between 2 and 5 cm, and a bulk density of 600–650 g / L, a mechanical strength (durability) of 97.5%, and a PCI / total of 4.9 kWh / kg at moisture contents below 8% (usually 5–8%). However, they are not water-resistant (they disintegrate completely in water in less than 5 minutes). Hardwood black pellets have a bulk density of 650–750 g / L, a mechanical strength (durability) of 98.5%, and a PCI / total of over 5.0 kWh / kg at moisture contents below 10% (usually equal to or very close to 10%). It has an immersion resistance that allows it to reversibly capture less than 10% of its weight in water without a significant change in its durability after drying.

[0008] However, if steam cracked powder with too short fibers is pelletized, it will lead to poor durability, the presence of scale on the pellets, a low apparent density, and the production of more fines, especially during processing.

[0009] If the steam cracking process can be adjusted to suit the severity conditions to improve pelletization of black pellets, longer fibers can be produced. However, the lignin will lose cohesion and become less mobile at lower severities, with the expected increase in heat release and to the detriment of other properties such as water resistance or mechanical strength. Therefore, this approach is not satisfactory in itself.

[0010] Therefore, there is a need to control the pelletizing process by new means to improve the pelletizing quality of black pellets. [Prior art] Patent WO2013 / 191897 describes specialized equipment and methods for processing biomass using steam explosion as a pretreatment method prior to gasification or combustion. The moisture content of the residual biomass, left outside without further drying, is 15% to 60%. The method uses a steam explosion process in which biomass is introduced into a steam explosion unit, low-pressure steam is introduced, and then at least 10 times atmospheric pressure is finally introduced to heat and pressurize the biomass before rapid depressurization. The resulting fine, moist biomass particles are then fed into the feed section of a biomass gasifier, where they react with the biomass particles in a rapid biomass gasification reaction to produce syngas components. The product of this method is syngas.

[0011] Patent WO2019 / 054913A1 describes a method for producing wood pulp, enabling the production of fuel pellets or briquettes from wood. The method involves debarking logs, which are then converted into wood chips. Some of these chips are used to form a paste, while the remaining chips undergo a steam cracking process. The heat-treated bark is discharged from the reactor through an adjustable blow-off valve or a constant-diameter outlet, resulting in a steam explosion of the bark. After the steam explosion process, the resulting chips are granulated with lignin recovered during the wood pulp production process.

[0012] Patent WO2020 / 225504 relates to a powdered carbonaceous substrate for chemical or biochemical reactions, obtained by continuous steam cracking of lignocellulosic biomass at a humidity of 5% to 27%, without chemical auxiliaries, and by applying a severity factor of 3 to 5. The carbonaceous substrate may be in the form of slightly densified pellets. This form can be used to package the product for easier handling, but its properties are those of a powder when the substrate is impregnated with a solution (enzyme, acid, etc.). [DISCLOSURE OF THE INVENTION] The present inventors have unexpectedly shown that the quality of black pellets is improved when they are produced from steam cracked biomass of non-uniform particle size.

[0013] More generally, the inventors have shown that good pelletization can be obtained by mixing steam-cracked powders from different biomasses: for example, powders from soft hardwoods such as poplar, or certain hardwoods such as ash, give very fibrous powders that can be advantageously mixed with oak powder containing about 80% fines.

[0014] The inventors have also demonstrated that steam cracking powder from coarsely ground biomass (average diameter of around 30-40 mm instead of 10-20 mm) allows for obtaining powder with better calorific value (PCI) and black pellets of the required specifications, due to the presence of a sufficient amount of residual fiber (around 20%) that allows the powder to coexist with fine flocculent fibers, resulting in agglomeration and ensuring good pelletization.

[0015] Based on these observations, the inventors have therefore determined that to obtain good quality black pellets it is important to use a heterogeneous steam cracked biomass that has both fibres (approximately 20% greater than 1 mm) but also fines in the form of powder or very fine flocculent fibres (less than 0.5 mm).

[0016] The present invention therefore relates to a method for producing combustible black pellets by continuous steam cracking of biomass at a humidity of 5 to 27%, characterized in that the pelleting step is carried out on unwashed steam cracked biomass containing, on the one hand, at least 50% of powder compounds with a fraction of less than 0.5 mm and, on the other hand, at least 10% of fibrous compounds with a length of more than 1 mm, said powder and fibrous compounds being bound together, without additives, by the hemicelluloses and lignin present in the biomass. [Advantages of the invention] The method according to the invention has the advantage that it ensures the consistent production of good quality black pellets with good mechanical strength, good water resistance and increased calorific value.

[0017] Additionally, heterogeneous steam-cracked biomass allows for the production of high-quality pellets using steam-cracked lignin as a binder without the need for additives, which are expensive and generally undesirable due to their toxicity. Therefore, eliminating them is advantageous from both an economic and ecological perspective.

[0018] The heterogeneity of the powder can be obtained in different ways: - the use of an input biomass whose grinding results in platelet-like particles of non-uniform size, - the use of an input heterogeneous biomass consisting of a mixture of different plant species; -Adjustment of the severity of steam cracking conditions; - a mixture of biomasses obtained from the same input biomass but treated under different severity conditions during steam cracking, - A mixture of different steam-cracked biomasses obtained from various plant species.

[0019] Therefore, a method is proposed that allows for better management of biomass: the powder particle size can be measured after steam cracking to prepare a suitable heterogeneous mixture for the purpose of quality pelleting. [Detailed Description of the Invention] The present invention relates to a method for producing combustible pellets by continuous steam cracking of biomass at a humidity of 5 to 27%, characterized in that the pelleting step is carried out on a steam cracked biomass without additives and containing, on the one hand, at least 50% of powdery compounds with a fraction of less than 0.5 mm and, on the other hand, at least 10% of fibrous compounds with a length of more than 1 mm.

[0020] In this method, no additives are required because the powdery and fibrous compounds of wood, especially lignin and hemicellulose, are bound together in the pellets after chemical and physical modifications induced by steam cracking.

[0021] Within the meaning of the present invention, "additive" means a specific compound, such as a commercially available compound, of starch, flour or other vegetable or petrochemicals that provides a binder, introduced at between 0.5 and 10% by weight.

[0022] Within the meaning of the present invention, "biomass" refers to lignocellulosic biomass derived from both agricultural and forestry residues, by-products of wood conversion, and dedicated crops. It may in particular be wood chips with or without bark. The quality of this raw material directly affects the quality and energy properties of the solid combustible material. For example, the most commonly used woods are hardwoods and softwoods.

[0023] In a preferred embodiment, the humidity level is between 7% and 20%, and even more preferably between 8 and 14%.

[0024] In another preferred embodiment, the powdered compound having a fraction smaller than 0.5 mm represents at least 75% of the steam cracked biomass.

[0025] In another preferred embodiment, fibrous compounds having a length greater than 1 mm make up at least 20% of the steam cracked biomass.

[0026] Biomass also includes powdery compounds, fibrous compounds, and flocculent compounds.

[0027] In certain embodiments of the present invention, the steam cracked biomass is heterogeneous, as it is derived from a variety of plant species.

[0028] The composition of lignocellulosic biomass varies in its main components, namely cellulose, hemicellulose and lignin, depending on the plant species, which is why the powders obtained by steam cracking from different species are not of equal quality.

[0029] In a preferred embodiment, the steam cracking of various plant species is carried out in a single step.

[0030] In another particular embodiment, the steam cracked biomass is heterogeneous in that it is derived from multiple steam cracking operations under different severity conditions of the same initial biomass, and the resulting powders are mixed prior to pelletizing.

[0031] The different steam cracking operations under different severity conditions of the same initial biomass can be carried out at different times or in parallel, in a preferred embodiment they are carried out simultaneously in continuous reactors arranged in parallel.

[0032] For the implementation of the present invention, the particle size of the steam-cracked biomass powders must be known. To do this, the particle size of these powders can be measured before pelletization. This measurement can be performed by any means known to those skilled in the art, preferably by means that can be integrated into the production line, such as optical scattering (laser) measurement or measurement by a sieving system. The results of this analysis allow for external powder feeding, if necessary, taking into account the size ratio recommended for good pelletization.

[0033] Therefore, in certain embodiments, the method includes analyzing the particle size of the steam cracked biomass prior to pelletizing.

[0034] The density of the steam cracked powder is also an indicator of its non-uniformity and therefore its pelletizability.

[0035] In certain embodiments, the method further comprises adding lignin derived from steam cracked biomass prior to pelletizing.

[0036] The method can also include pre-pelletizing the various powders that make up the heterogeneous biomass separately and then mixing them together prior to pelleting. Pre-pelletizing the powders makes them easier to handle and transport.

[0037] The particle size of the biomass after steam cracking is also influenced by the particle size of the biomass before steam cracking, i.e., the size of the initial biomass in platelets or other forms. This parameter can be easily adjusted by adapting the grinding grid.

[0038] Therefore, the method may include a step in which the particle size of the comminuted biomass prior to steam cracking is non-uniform.

[0039] The present invention finds use with industrialists seeking to optimize biofuel production by standardizing the final product according to variable supply, making it desirable rather than tolerable; for the final pelletized product, the interest can be intermediate due to the mixture due to fluctuations in biomass availability and price.

Claims

1. 1. A method for producing combustible black pellets by continuous steam cracking of biomass at a moisture content of 5 to 27%, characterized in that the pelleting step is carried out on a steam cracked biomass without additives, which comprises, on the one hand, at least 50% by weight of powdery compounds having a fraction of less than 0.5 mm and, on the other hand, at least 20% by weight of fibrous compounds having a length of more than 1 mm.

2. 10. The method of claim 1, wherein the steam-cracked biomass is heterogeneous because it is derived from different plant species.

3. 3. The method of claim 2, wherein the steam cracking of the various plant species is carried out in a single step.

4. 10. The method of claim 1, wherein the steam cracked biomass is heterogeneous in that it is derived from multiple steam cracking operations of the same initial biomass under different severity conditions and mixing prior to pelleting.

5. 5. The method of claim 4, wherein the multiple steam cracking operations are carried out in parallel.

6. The method of any one of claims 1 to 5, wherein the particle size of the steam cracked biomass is analyzed prior to pelletizing.

7. 7. The method of any one of claims 1 to 6, further comprising the step of adding lignin derived from steam cracked biomass prior to pelletizing.

8. 6. The method according to any one of claims 2 to 5, wherein, prior to pelleting, the various powders constituting the heterogeneous biomass are pre-pelletized separately and then mixed.

9. The method according to any one of claims 1 to 8, wherein the particle size of the pulverized biomass before steam cracking is non-uniform.