FR3095656A1 - Process for the treatment of solid biomass by steam cracking integrating the energy of the co-products - Google Patents

Abstract

The present invention relates to the field of processes for transforming solid biomass by steam cracking into a substrate for green chemistry and biotechnologies. More specifically, the invention relates to a process for the treatment of solid biomass by steam cracking in which the energy contained in the evaporates and in the water vapor generated is kept within the system or used for other types of industries such as than electricity production.

Description

Solid biomass treatment process by steam cracking integrating the energy of co-products

The present invention relates to the field of processes for transforming solid biomass by steam cracking into a substrate for green chemistry and biotechnology. More specifically, the invention relates to a method for treating solid biomass by steam cracking in which the energy contained in the evaporates and in the water vapor generated is kept within the system or used for other types of industries such as than the production of electricity.

The production of energy (electric and thermal) of renewable origin can be done from the environment (sun, wind, tide, swell, geothermal, hydraulic) or from biomass. River or dam hydraulics, and geothermal energy are currently intermittent renewable energies, due to a lack of energy storage means. In this context, only biomass constitutes a primary energy that can be transported to its place of transformation into heat and/or electricity. However, biomass is a sparse, variable and perishable energy.

Indeed, one way to exploit biomass is to use woodchips, related sawmill products (sawdust) or wood pellets without modification. These renewable materials are very sparse (it takes 5 to 8 volumes of chips to produce the equivalent of energy from a single volume of coal), which implies an oversized logistics chain and tools, not to mention the variability of content of supplies that impacts the operation of the boiler (moisture, ash, heavy metals, organics, nitrogen, chlorine, sulphur, dust). Wood pellets (or “white pellets”) are more standardized, but they are expensive, and do not withstand exposure to water and outdoor conditions. Finally, the production of biogas requires the management of diluted liquid effluents, or wet solids that are not very transportable, and of a localized use, often of small size.

To counteract these problems, ligno-cellulosic biomass (wood, agricultural residues, co-products from agriculture and agro-industry) can be transformed into an energy-dense, transportable and easily storable compound that would make it possible to develop and consolidate a industrial stationary energy sector (biofuel used at a fixed point, the home, unlike biofuels) and to reduce environmental impacts (emission of fossil CO2, with biomass without fertilizers or phytosanitary products). This solid biofuel, also called “black pellets” (or “black pellet”) can advantageously replace coal, even fuel oil and gas in boilers.

There are several ways to densify and stabilize lignocellulosic biomass, including carbonization, roasting, and steam cracking. The first, which makes it possible to manufacture charcoal, involves a high price of the material due to a significant loss of yield (of the order of 70 to 80%), and is reserved for uses at high selling prices such as than the BBQ. Roasting is a technology with an acceptable yield (10% to 20% loss), but the price remains prohibitive, and the technology is not mature. To date, only steam cracking seems technically and economically viable, provided production costs are contained. This technology makes it possible to manufacture “black pellets” (or “black pellets”).

The process for preparing black granules schematically comprises 3 steps: 1) preparation of the raw material (grinding, drying, etc.), 2) steam cracking and 3) granulation, all of which consume energy.

In the case of a plant manufacturing black pellet of steam-cracked biomass, the steam cracker enters a dried biomass and clean pressurized hot water steam. The two streams leaving the steam cracking are:

• a solid flow of steam-cracked biomass which will be granulated, and

• a gas stream of lower pressure steam containing volatile organic compounds (VOCs), called evaporates, from biomass processing.

One of the problems associated with the combustion of biomass by steam cracking is the production of these evaporates charged with VOCs which can be toxic for the environment. It is therefore necessary to treat the evaporates to remove the VOCs before releasing clean water vapor out of the system. To do this, it is usual to burn the evaporated so that the VOCs decompose into CO ₂ and CO, molecules which can then be released into the atmosphere without danger to the environment, in the form of white fumes (steam of water).

It is desirable to reduce the production costs of black granules to allow their industrial use.

State-of-the-art processes report the use of co-product energy in biomass condensation processes. By way of example, the document US9447326 describes a process for treating biomass by steam cracking followed by gasification of the biomass. The gas produced is treated by a cyclone separator which separates gas and water vapour, the water vapor being reinjected into the final product.

The inventors propose to optimize the use of co-products from the transformation process to reduce the production costs of black pellets. More specifically, it is proposed to burn the evaporates in a biomass boiler generating compressed steam used for steam cracking (recycling), and which can also be used by another local consumer (industrial using steam, or electrician turbines the steam to manufacture electricity).

It is more generally proposed to use the energy associated with evaporates and water vapor by combustion of volatile organic compounds and condensation of water vapour.

The other types of production of combustible material from biomass require dry heat (white pellets or roasting) which often uses fossil gas. The environmental balance is therefore degraded for these last two types of solid biofuel production. These processes do not generate water vapour.

Thus, the invention consists of a process for the production of solid fuel material integrating the energy of the co-products comprising a stage of steam cracking of biomass characterized in that:

• the volatile organic compounds produced in the steam cracking enclosure are injected, with the steam cracking, directly into the boiler to produce clean steam intended for steam cracking and possibly for another local energy consumer;

• the energy for the production of steam cracking steam is produced at least in part by the combustion of said recycled volatile organic compounds.

The yield of this process can also be increased by condensing the combustion water vapors from the evaporates to dry the biomass before steam cracking.

In addition, it is possible to bark the wood used as raw material, or to recover bark from an external source and burn this bark in the boiler.

Advantages of the invention

The process according to the invention makes it possible to use available and usually unused energy sources to reduce the energy balance of the overall process and make it more ecological. Thus, the reduction in material yield of the biomass entering the steam cracking (generally between 5% and 15%) is not totally lost because it generates energy.

This process includes the optimized recovery of the energy available within the biomass transformation device by steam cracking, namely:

• the energy contained in the VOCs of the evaporates generated by steam cracking; this energy is released by combustion of volatile organic compounds;
• the energy contained in the water vapor accompanying the evaporates (steam cracking), and recovered by condensation;
• the energy contained in the water vapor produced by the combustion of evaporated water, and recovered by condensation;
• and possibly, in the same way, the energy contained in the bark and released by combustion, as well as the water vapor produced during this combustion and recovered by condensation.

The originality of the process according to the invention is therefore to use the energy contained in the evaporates and in the water vapor to conserve/save the overall energy of the process, instead of letting it escape.

This energy recycling loop is particularly suitable for steam cracking because this combustion mode produces a large quantity of water vapor unlike other methods such as roasting, pyrolysis, etc. which operate in a dry atmosphere.

The fact of using the energy internal to the system in particular for the drying of the granules and in the form of water vapor introduced for the steam cracking increases the yield of the process and makes it more ecological.

This process is particularly advantageous when applied to a biomass treatment producing large amounts of VOCs, for example when the severity of the steam cracking conditions is high. It should be noted that the severity is generally controlled to avoid the production of large quantities of VOCs which reduce the yield of the process since the evaporates are co-products not used in conventional processes. Here, the energy balance integrates the energy associated with evaporates and water vapor rather than counting it in losses. This process therefore allows the implementation of severe conditions since the losses of combustible materials are in fact compensated since they are used to produce energy.

Advantageously, when the wood introduced into the system is previously treated to remove the bark, these barks are introduced into the boiler. The bark, which is the least interesting fraction for the targeted applications (often less caloric, more mineralized and potentially contaminated by pollutants), then becomes a residual co-product that it is possible to exploit by burning it in the boiler serving in particular to produce steam for steam cracking, or intended for other industrial needs.

Such a solid biomass treatment process including the recycling of evaporates and water vapour, or even bark, is therefore particularly advantageous from an industrial, economic and ecological point of view since it provides an energy gain which can go up to 30% compared to a process allowing the energy of the water vapor and the potential of the evaporates to escape.

The invention relates to a method for treating solid biomass integrating the energy of the co-products comprising a biomass steam cracking step, characterized in that:

• the volatile organic compounds produced in the steam cracking chamber are injected, with the steam cracking, directly into the boiler to produce clean steam intended for steam cracking and possibly for another local energy consumer;
• the energy for the production of steam cracking steam being produced at least in part by the combustion of said recycled volatile organic compounds.

In a particular embodiment, the biomass thus treated is used for the production of combustible material with a high calorific value, in particular in black granules. In another particular embodiment, the biomass thus treated is used as a pulverulent substrate for applications in green chemistry and in biotechnology.

Thus, the invention relates to the use of the biomass treated according to the process of the invention either to produce combustible material with high calorific value, or as a pulverulent substrate for applications in green chemistry and biotechnology.

In a particular embodiment, the energy for the production of steam cracking steam is produced in a mixed mode comprising the combustion of the recycled VOCs and the combustion of a main external energy source. In a particular alternative embodiment, the energy for the production of steam cracking steam is produced by the combustion of the VOCs as well as by condensation of the water vapor available in the boiler, and possibly an external energy source.

By “solid combustible material” within the meaning of the invention, is meant a powder (black powder) or black granules (black pellet) obtained by compression of the black powder, this powder and these granules being made up of steam-cracked biomass.

Black pellets with high calorific value can be obtained thanks to a process of transformation of biomass by steam cracking under severe conditions.

In a particular embodiment, such a method is implemented as follows:

- obtaining, from wood chips, wood fragments whose size is between 0.5 and 14 mm and with a humidity level between 5 and 27%;

- continuous introduction of a predetermined volume per minute of said wood fragments into a pressurized reactor, said reactor being supplied with substantially saturated steam, the pressure of which is between 10 and 25 bars and the temperature is between 180 and 220°C;

- exposure of the fragments of wood introduced into said reactor to said steam for a time sufficient to obtain steam cracking of between 5 and 30 minutes, the value of said exposure time and the value of the temperature of said substantially saturated steam being selected so that the severity factor is between 3 and 5, preferably between 3.5 and 4;

- continuous extraction from said reactor of the same predetermined volume of wood fragments per minute, through a plurality of orifices opening into a conduit substantially at atmospheric pressure, so as to cause explosive decompression of said extracted wood fragments of said reactor in said conduit;

- separation of said decompressed fragments of wood and of the residual steam extracted from said reactor, said fragments of wood obtained after separation forming said combustible material in powder form.

The wood fragments harvested in the form of powder at the outlet of the separator are then dried until their humidity level reaches 10%, before possibly being compacted in the form of pellets intended to be used as fuel. It is possible to moisten the wood fragments obtained after separation in order to facilitate granulation.

The Treatment Severity Factor is defined by the formula: FS=Log10(time(min)*exp((T°C-100)/14.75)). The higher the temperature and the longer the treatment time, the more the severity increases, the more transformation is observed in the product, the more carbonaceous matter is lost in the evaporates

The solid fuel material with high calorific value (black pellets) obtained by implementing the method as defined above has a calorific value greater than or equal to 5 MWh/t, preferably greater than or equal to 5.6 MWh/t. It shows an increase in mass less than or equal to 10% after complete immersion in a container of water for one hour and draining for 30 minutes. This combustible material can be used in industrial ovens and domestic boilers and stoves.

The condensation of a ligno-cellulosic biomass by steam cracking produces an evaporate which must be cleaned of VOCs and this requires energy and specific washing equipment. The advantage of the process according to the invention is that the energy of the co-products is recycled, namely that the evaporates are burned and that the energy produced by the combustion of the VOCs in the form of water vapor is reused as well as the other vapors clean water available, in particular that resulting from steam cracking.

It should be noted that the nature of the effluents depends on the method used to condense the biomass. Steam cracking produces energy-rich VOC-laden effluents as well as a significant amount of water vapor (compared to alternative biomass condensation methods). This specificity fully justifies an interest in recycling this available energy. This is all the more true when the conditions applied for steam cracking are severe.

On the other hand, in a controlled boiler, the parameters will be adjusted according to the pressure and/or the chemical characteristics of the effluent produced. A boiler attached to a steam cracker will therefore have a specific mode of operation.

In a preferred embodiment of the invention, the method further comprises a step of condensing at least part of the combustion water vapors from the evaporates for drying the biomass before steam cracking.

In fact, the water vapors from the combustion of the evaporates can be condensed: by giving up their heat through hot loops to forced air, they supply a current for drying the raw material, which allows a reduction in humidity. Thus the biomass enters drier during steam cracking. The amount of energy consumed to heat the biomass is then lower because there is much less water to heat (5 to 10% instead of 40 to 50% water). The steam-cracked product comes out between 5 and 10%, therefore granulable and delivering a black granule that is denser in energy.

In another preferred embodiment, the method further comprises a step of condensing at least part of the steam cracking water vapors for drying the biomass before steam cracking.

If the energy generated in the form of steam in the boiler, in particular by combustion of VOCs, is not fully mobilized for the production of steam cracking steam or for the drying of the biomass, it is possible to make it available to another local energy consumer such as an industrialist using steam or an electrician using steam turbines to produce energy.

By “biomass” within the meaning of the invention, is meant lignocellulosic biomass originating both from agricultural and forestry residues, from wood processing by-products and from dedicated crops. It may be in particular wooden chips, with or without bark. The quality of this raw material will directly influence the quality and energy characteristics of the solid combustible material. For example, the most used woods are hardwoods and softwoods.

Further energy integration is possible since by definition the production sites of black pellets by steam cracking are sites for the production of steam from biomass: it is therefore possible to select the best wood or parts of wood for the manufacture of pellets, and the less noble ones are burned for endogenous energy.

In an advantageous embodiment, the bark from the debarking of the biomass is recovered as fuel to supply the boiler for the production of steam. The purpose of debarking is to reduce the minerals in the black pellets (mainly located in the bark) and to extract the least calorific fraction of the wood, the bark, to generate more energetic black pellets. In addition, the fact of burning the bark residues in the boiler makes it possible to recover the energy they contain and to increase the overall yield of the process by reducing the losses of raw material.

In a particular embodiment, the steam-cracked biomass consists of wood and the method according to the invention further comprises:

1. a step of separating the bark from said biomass before steam cracking and
2. a step of burning said bark in said boiler.

It is also possible to introduce into the boiler residual bark and other biomasses and co-products that are not very recoverable in black pellets or from another process as combustible raw material for the production of energy and condensable water vapour.

Thus, in a particular embodiment, the biomass introduced into the boiler comprises bark.

Figure 1: Diagram representing a mode of implementation of the process according to the invention integrating the recycling of the energy of the co-products.

EXAMPLE

The manufacture of black pellets (or black pellets) replaces the grinding-refining used during the manufacture of white pellets (or white pellets) by steam cracking, which consumes electrical energy and mainly thermal energy from steam.

The black granule manufacturing process produces energy not previously used and which the present process according to the invention makes it possible to recover. Indeed, the outgoing product sees the energy of the evaporates (approximately 180 kWh / ton of black pellet) and the bark (approximately 260 kWh / ton of black pellet) recovered in the form of thermal energy, i.e. 440 kWh / ton of pellet black. In addition, the condensation of water, brought by the humidity of the bark, as well as that resulting from the combustion of bark and evaporates, makes it possible to generate 80 kWh/tonne of black pellet. Finally, the steam cracking steam, which is a carrier fluid that releases its sensible heat and retains its latent heat, also generates 290 kWh/tonne of black granulate.

In the end, 810 kWh/tonne of black pellets of energy are recovered during the recovery of evaporates and bark (and 500 kWh/tonnes of black pellets for evaporates alone, specific to black pellets, without taking into account the recovery of the bark, which can also take place with the white pellet).

The energy requirements for the production of black pellets are 830 kWh/tonne of black pellets for drying and 430 kWh/tonne of black pellets for steam cracking, i.e. 1260 kWh/tonne of black pellets.

Thus, the thermal energy needs for the production of black pellets are covered at 64% by renewable resources (evaporate and bark co-products) and by thermal integration (combustion and condensation energy, recycling of steam flows).

In the case of white pellets, drying alone (also around 830 kWh) is covered by only 310 kWh of bark recovery, i.e. 37% of thermal energy coverage.

The process for preparing the black granule therefore has an energy gain of 27% compared to that of the white granule which must use additional exogenous energy resources.

Claims (8)

Process for the treatment of solid biomass integrating the energy of the co-products comprising a biomass steam cracking step characterized in that:
• the volatile organic compounds produced in the steam cracking chamber are injected, with the steam cracking, directly into the boiler to produce clean steam intended for steam cracking and possibly for another local energy consumer;
• the energy for the production of steam cracking steam being produced at least in part by the combustion of said recycled volatile organic compounds. Process according to Claim 1, in which the energy for the production of steam cracking steam is also produced by the combustion of an external energy source. Process according to one of Claims 1 to 2, further comprising a step of condensing at least part of the water vapors originating from the steam cracking and resulting from the combustion of the evaporates for drying of the said biomass before steam cracking. Method according to one of Claims 1 to 3, in which the said other local energy consumer is chosen from among an industrialist using steam or an electrician using steam turbines to produce energy. Process according to one of the preceding claims, in which the biomass is a ligno-cellulosic raw material such as whole wood or wood without bark. Process according to one of the preceding claims, in which the said biomass consists of wood and further comprises:

1. a step of separating the bark from said biomass before steam cracking and
2. a step of burning said bark in said boiler.

Method according to one of the preceding claims, in which the said biomass introduced into the said boiler comprises bark. Use of the biomass treated according to the process defined in one of claims 1 to 7 either to produce combustible material with a high calorific value, or as a pulverulent substrate for applications in green chemistry and biotechnology.