BRIQUETTES

TECHNICAL FIELD

The invention relates to a method and a pulp mill integrated system for production of fuel pellets or briquettes from tree material.

BACKGROUND

In wood processing industries, such as chemical pulping mills (e.g. a Kraft pulp mill), incoming tree material is de-barked and chipped prior to the actual chemical pulping process. The bark may be burnt in a bark boiler to produce steam which can be used in the pulping process and for production of electrical power. This energy is consequently used in the chemical pulping process. Kraft mills have however over recent years become increasingly more energy efficient, and all the energy produced from the bark may not be needed for internal use in the mill.

It is common practice to distribute any surplus electricity and sell it to external consumers. In some cases this may however not be possible due to limited capacity of the external power grid at the mill site. In other cases, a low market price of electricity may render such external distribution uneconomical. This means that surplus energy in the form of bark may remain unused and it will pile up somewhere close to the mill. This is an unfortunate situation as bark is a renewable energy source of increasing interest. It is possible to sell the bark as-is, for use at other sites for producing heat and/or electricity. Bark however attracts a low market price due to the limited energy density and difficulties involved in storing and transporting of raw bark.

An interesting option is to upgrade bark to fuel pellets or briquettes, which are, in comparison to wet raw bark, easier to handle, have a higher energy density and potentially can be sold for a higher price (than un-treated bark). Processes for producing fuel pellets from tree bark are known for example from EP1776440, wherein a process is described which comprises steam treatment at elevated temperature and pressure followed by pressure reduction in order to defibrate the material prior to pelletizing. Such a process, based on the steam explosion principle, may however, depending on the raw material used, not result in adequate pellet quality in terms of mechanical rigidity and wet strength.

SUMMARY

An object of the invention is to provide a method and system for production of fuel pellets or briquettes from tree material which is economical and/or results in pellets or briquettes of improved quality.

These and other objects are achieved by the present invention by means of a method and a system for production of fuel pellets or briquettes from tree material as defined by the independent claims. Preferred embodiments are defined in the dependent claims.

According to a first aspect of the invention, there is provided a method for production of fuel pellets or briquettes from tree material, which method comprises de-barking the tree material to separate bark and wood material, processing the wood material in a chemical pulping process, separating lignin from spent cooking liquor from the chemical pulping process and pelletizing or briquetting bark to form pellets or briquettes, wherein the pelletizing or briquetting comprises adding at least some of the separated lignin as a binder. The chemical pulping process is preferably a Kraft process.

In other words, pellets or briquettes are produced using bark and lignin originating from one and the same tree material. Thus, the production of fuel pellets or briquettes may be integrated into a chemical pulping mill, which preferably is already provided with means for separating lignin from spent cooking liquor. The invention is based on the insight that, since surplus bark and lignin are readily available in the mill, it is possible to achieve a highly integrated and economical production process, where transports are kept at a minimum, and where surplus materials are upgraded to high quality pellets or briquettes. The invention is furthermore based on the insight that bark pellets or briquettes may be effectively strengthened using lignin separated from spent cooking liquor. The invention is furthermore based on the insight that the quality of bark pellets varies greatly depending the source (tree) material. It is understood that tree material refers to raw material such as whole tree logs, and that wood material refers to the cellulose containing material remaining after removal of bark from the tree material. The whole tree logs may comprise one single wood species or a combination of several wood species. Chemical pulping mills in general and Kraft mills are well known for the person skilled in the art, and will not be described in further detail herein.

In embodiments, the added lignin corresponds to less than 50 %, or less than 25%, or preferably 10% or less of the dry weight of the pellets. The added lignin may furthermore correspond to 1 % or more of the dry weight of the pellets.

In embodiments, the method comprises continuous thermal treatment of the bark at elevated pressure and temperature using steam prior to pelletizing. The thermal treatment may be performed at a pressure of 5-30 bars, or preferably 15-25 bars, and may be performed at a temperature of 150-240 °C. The thermal treatment may be performed for less than 25 minutes, or more preferably less than 15 minutes.

The thermally treated bark is preferably discharged using steam explosion discharge of the thermally treated bark. Thus, the hot and softened biomass from the thermal treatment is released or blown from the reactor through a blow valve or orifice, while the pressure drops to atmospheric pressure. The structure of the biomass breaks, partly due to the expanding steam, and also by the shear forces and impact during the blow through the orifice or valve. Steam explosion may simplify subsequent pelletizing due to the (still) soft substrate being easier to densify. Steam explosion may furthermore improve the strength of the resulting pellet due to various

substances such as lignin and sugars being released during the steam explosion. For some tree materials (pine tree bark for example), the resulting pellet quality may still be inadequate, and the addition of lignin may be of importance to achieve the desired quality in terms of strength.

Alternatively, the thermally treated bark may be extruded to form pellets or briquettes. The thermally treated bark is preferably cooled to a temperature below 120°C prior to extrusion in order to avoid boiling of the water content in the bark. In embodiments, the bark is dried prior to pelletizing, preferably to a moisture content of 7-13 % by weight. The step of drying may be performed prior to the thermal treatment. In other embodiments, the method further comprises a step of washing and dewatering the bark after the thermal treatment, resulting in a solid residue and a liquid pressate. The (optional) step of drying is performed after the step of washing and dewatering, and prior to pelletizing. Thus, the pellets are produced from dried solid residue, with the addition of lignin as a binder. In this embodiment, the lignin addition may be particularly advantageous and effective since the bark has undergone a further step of washing and dewatering prior to pelletizing. The liquid pressate resulting from the step of washing and dewatering may be further treated to recover chemicals such as sugars, tannins, furans, alcohols and organic acids. The processes used for recovery of these chemicals are known for the person skilled in the art is will not be described further herein. This embodiment is thus particularly advantageous in mills where it is desired to produce such chemicals. Although the pellet yield may, depending on the implementation, be lower compared to other embodiments, the overall process may still be more advantageous, especially at market conditions where the above-mentioned chemicals may be sold at

comparatively high prices, and/or where the pellets attract a comparatively lower market price.

In embodiments, the chemical pulping process is a Kraft process, and wherein the step of separating comprises precipitation of lignin by acidifying black liquor from the process, dewatering the precipitated lignin to obtain a lignin filter cake, suspending the lignin filter cake to obtain a lignin suspension, dewatering of the lignin suspension and washing of the dewatered lignin. Such a separation process is described in more detail in for example EP1794363B2. According to a second aspect of the invention there is provided a system for production of fuel pellets or briquettes from tree material, comprising a de-barking apparatus, a chemical pulping line and a pelletizing apparatus. The de-barking apparatus is configured to de-bark said tree material to separate bark and wood material. The chemical pulping line is configured to process the wood material in a chemical pulping process, and comprises means for separating lignin from spent cooking liquor. The pelletizing or briquetting apparatus is configured to pelletize the bark to form pellets or briquettes, and comprises means for adding at least some of the separated lignin as a binder. The system is preferably integrated into a chemical pulping mill, which preferably is a Kraft mill.

In embodiments of the system according to the second aspect of the invention, the system comprises a pressurized vessel (for example a vertical cylindrical vessel) provided with means for addition of medium pressure steam to provide continuous thermal treatment of the bark at elevated pressure and temperature prior to

pelletizing. The thermally treated bark is preferably discharged from the reactor through an adjustable blow valve or a constant diameter discharge orifice to achieve a steam explosion discharge of the thermally treated bark. The system furthermore preferably comprises a cyclone configured to separate steam from the biomass.

In embodiments of the system according to the second aspect of the invention, the system comprises a dryer, for example a low temperature belt dryer using surplus low-grade heat from the pulp mill, to dry the biomass. In embodiments comprising a pressurized vessel as described above, the dryer may be placed either before or after the reactor.

The features of the embodiments described above are combinable in any practically realizable way to form embodiments having combinations of these features. Further, all features and advantages of embodiments described in connection with the method according to the first aspect of the invention may be applied in corresponding embodiments of the system according to the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail with reference to the appended drawings, which show presently preferred

embodiments of the invention, wherein:

fig. 1 shows a schematic illustration of a prior art chemical pulping process, fig. 2 shows a schematic illustration of a prior art chemical pulping process comprising means for lignin separation,

fig. 3 shows a schematic illustration of a method per an embodiment of the first aspect of the invention, and

fig. 4 shows a schematic illustration of a method per another embodiment of the first aspect of the invention.

DETAILED DESCRIPTION

Figure 1 shows a schematic illustration of a prior art chemical pulping process. As a first step, raw tree material is de-barked and chipped 1 , i.e. the tree logs are debarked and thereafter reduced in size to form wood chips. The wood chips are conveyed to the chemical pulping process 2, where the wood chips are treated according to the Kraft process in order to produce wood pulp. The Kraft process is well known for the person skilled in the art, and will not be described in detail here. The bark left over after de-barking is conveyed to a bark boiler, where the bark is combusted 3 to produce energy in the form of steam and electricity. This energy is used in the chemical pulping process, as shown in the figure. By-products of the chemical pulping process, i.e. black liquor (containing lignin, hemicellulose and spent liquor), is recovered 4 using a recovery boiler, which is well known in the art, to produce energy in the form of steam for use in the pulping process and for production of electricity and recover chemicals, which are returned to the pulping process 2.

Figure 2 shows a schematic illustration of a prior art chemical pulping process which is identical to that in figure 1 , except that the process involves a step 5 for separating lignin from the black liquor. Several methods for separation of lignin is known in the art, for example LignoBoost as provided by the applicant, and will therefore not be described in detail herein. Separated lignin is typically sold as a biofuel or as a raw material for various applications. Figure 3 shows a schematic illustration of a method for producing pellets per an embodiment of the first aspect of the invention. This embodiment is based on the process shown in figure 2, with the main difference being that the step of combusting the bark 3 is replaced with steps for producing pellets comprising drying 106, thermal treatment 107 and pelletizing 108. The raw (wet) bark resulting from the step of de- barking and chipping 101 is dried 106, where after the dried bark is optionally thermally treated and subjected to steam explosion 107. The thermal treatment is performed at a pressure of 15-25 bars, and at a temperature of 150-240 °C for less than 15 minutes.

The dried and thermally treated bark is finally pelletized (or briquetted) 108 using separated lignin from the lignin separation 105 as a binder. In another embodiment, the step of drying is omitted, or is performed after the thermal treatment and steam explosion 107. In still another embodiment, the thermal treatment step 107 is omitted and the dry bark is co-pelletized with precipitated lignin from the pulping process.

An embodiment of a system for producing pellets per an embodiment of the second aspect of the invention may also be described with reference to figure 3. A de- barking and chipping apparatus (corresponds to 101 ) de-barks the tree logs and thereafter reduces the wood material in size to form wood chips. The de-barking and chipping apparatus is connected to a chemical pulping line (corresponds to 102), where the wood chips are treated according to the Kraft process in order to produce wood pulp. By-products from the chemical pulping line, i.e. black liquor (containing lignin, hemicellulose and spent liquor), is conveyed to a recovery boiler (corresponds to 104), which is well known in the art, to produce energy in the form of steam for process uses and electricity production and recover pulping chemicals, which are returned to the pulping line. Part of the black liquor from the chemical pulping line is fed to lignin separation means (corresponds to 105) to separate lignin from the black liquor. The de-barking and chipping apparatus is furthermore connected to a bark dryer (corresponds to 106) which is driven by waste heat from the pulp mill. Dried bark from the dryer undergoes continuous thermal treatment in a pressurized vertical cylindrical vessel (corresponds to 107) driven by medium pressure steam from the recovery boiler. The thermally treated bark is discharged from the reactor through an adjustable blow valve or a constant diameter discharge orifice to achieve steam explosion discharge of the thermally treated bark. The system furthermore preferably comprises a cyclone (not shown) configured to separate steam from the steam exploded bark. The cyclone is connected to recovery and/or treatment means (not shown) for contaminated blow vapor, and to a pelletizing apparatus (corresponds to 108) where the thermally treated bark is pelletized using separated lignin as a binder.

Figure 4 shows a schematic illustration of a method for producing pellets according to another embodiment of the first aspect of the invention. This embodiment differs mainly from the embodiment in figure 3 in that drying of the bark is performed after the thermal treatment, and further in that washing and dewatering is performed of the thermally treated bark in order to produce a liquid pressate from which chemicals can be recovered.

The raw (wet) bark resulting from the de-barking 201 is subjected directly to thermal treatment and steam explosion 209, which is driven by steam from the recovery step 204. The thermally treated bark is thereafter washed and dewatered 210 to produce a solid wet residue and a liquid pressate. The solid wet residue is thereafter dried 21 1 . The dried solid residue is finally pelletized 208 using separated lignin from the lignin separation 205 as a binder. The liquid pressate is further treated (not shown) in order to recover valuable chemicals such sugars, tannins, furans, alcohols and organic acids. Unlike the embodiment in figure 3, this embodiment is specifically designed to produce a liquid pressate containing these chemicals.

An embodiment of a system for producing pellets according to another embodiment of the second aspect of the invention may also be described with reference to figure 4. The system differs from the above described system with reference to figure 3 in that the de-barking and chipping apparatus (corresponds to 201 ) is not connected to a dryer, but instead to a pressurized vertical cylindrical vessel (corresponds to 209) driven by medium pressure steam from the recovery boiler to provide continuous thermal treatment of the (wet) bark. The thermally treated bark is discharged from the reactor through an adjustable blow valve or a constant diameter discharge orifice to achieve a steam explosion discharge of the thermally treated bark. It is understood that the thermal treatment in fig. 4 requires more energy than in fig. 3 since the bark has a higher moisture content. The vessel is connected to a washing and dewatering apparatus (corresponds to 210) which produces a solid wet residue and a liquid pressate from the thermally treated bark. The washing and dewatering apparatus is connected to a drying apparatus (corresponds to 21 1 ) in order to dry the solid wet residue, and thereafter to a pelletizing or briquetting apparatus (corresponds to 208) which comprises means for adding separated lignin as a binder. The washing and dewatering apparatus is furthermore connected to recovery means (not shown) configured to recover valuable chemicals such sugars, tannins, furans, alcohols and organic acids from the liquid pressate.

The description above and the appended drawings are to be considered as non- limiting examples of the invention. The person skilled in the art realizes that several changes and modifications may be made within the scope of the invention. For example, the energy or steam used for various steps may have different origin, and the described reactors and driers may be replaced with equivalents known to the person skilled in the art. The scope of protection is determined by the appended patent claims.