The invention relates to a method and a system for production of fuel pellets or briquettes from surplus lignocellulose containing biomass from tree material used in a Kraft pulping process.

BACKGROUND

In wood processing industries, such as chemical pulping mills (e.g. a Kraft pulp mill), incoming tree material is debarked, chipped and screened to remove material not suitable for Kraft pulping the process. The tree reject material, in the following denoted, “surplus biomass” material comprises a mixture of wet bark flakes and wet wood chip screening rejects. The surplus biomass material may be burnt at the mill site in a dedicated biomass boiler to produce steam which can be used in the pulping process and for production of electrical power. This energy is normally used in the chemical pulping process. Kraft mills have however over recent years become increasingly energy efficient and all the energy required for the pulping process is produced from black liquor (evaporated spent cooking liquor) burnt in the soda recovery boiler. It is no longer needed to burn surplus biomass for internal energy uses in the mill.

It is common practice to distribute any surplus electricity of a Kraft mill 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 surplus biomass (mostly wet 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 low energy density (high moisture), high content of ash and difficulties involved in storing and transporting of wet raw bark.

An interesting option is to upgrade the surplus biomass 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). Kraft mill integrated production of fuel pellets with lignin as a binder is disclosed in Swedish patent SE541263. Potential users of bark pellets are, for example, existing power plant operators who considers replacing pulverized fossil coal with a green-house gas neutral fuel, e.g. biomass-based pellets or briquettes.

The wood pulping process is furnished with highly effective systems for treatment of gaseous, liquid and solid effluent streams such that the harmful environmental impact of the mill is reduced. Liquid effluent(s) are lead to waste water system(s) which treat them for example by sedimentation, filtration, aeration or digestion. Organic and inorganic contaminants are separated as sludge(s) from the waste water which exits the mill to a lake, sea or river. The share of inorganic material in mill sludge(s) is increasing because the mills will use (flocculation) chemicals in their waste water treatment systems to be able to reach new cleaned water standards. An urgent sludge disposal problem arises since landfilling of such sludge(s) is/are not allowed by environmental legislation. The sludge(s) is/are too moist to be combusted as such, but methods are available of mixing “bio” sludge also called “secondary sludge” from waste water treatment with black liquor for combustion in the soda recovery boiler. But, co burning black liquor and flocculant enriched sludge in the soda recovery boiler is not a preferred solution as the flocculants increase the concentration of harmful non-process elements in the Kraft recovery cycle. It is highly desirable to identify new methods of effective disposal of mill sludge(s) with low environmental impact.

SUMMARY

An object of the invention is to provide a method and system for production of fuel pellets or briquettes from surplus lignocellulose biomass of tree material used in a Kraft pulping process which solves or at least improves on one or more of the problems mentioned above.

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 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 or process for production of fuel pellets or briquettes from surplus lignocellulose biomass of tree material used in a Kraft pulping process. The method comprises separating the tree material into wood material and surplus lignocellulose biomass, processing the wood material in the Kraft pulping process to produce pulp, withdrawing at least one effluent from the Kraft pulping process to a waste water treatment plant, treating at least one of the effluents to obtain at least one sludge, reducing the water content of the at least one sludge to obtain dewatered sludge, adding at least a portion of the dewatered sludge to at least a portion of the surplus lignocellulose containing biomass to obtain a biomass and sludge mixture, and densifying the mixture to form fuel pellets or briquettes.

In other words, pellets or briquettes are produced from surplus lignocellulose biomass and dewatered sludge originating from the same tree material.

The step of treating may comprise treating at least one of the effluents with at least one flocculant, which may comprise an aluminous and/or a ferrous settling chemical. The flocculant may in other words comprise at least one aluminum and/or iron compound.

Consequently, a Kraft pulping mill integrated method and system is disclosed which provides fuel pellets or briquettes of high quality and reduces the environmental impact of the Kraft pulp mill since the need for landfilling of sludge from the waste water treatment plant is reduced or even eliminated.

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 suitable for Kraft pulping remaining after removal of bark and optionally other surplus biomass material 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 notably Kraft mills are well known for the person skilled in the art and will not be described in further detail herein.

In embodiments, the step of treating effluent(s) from a Kraft pulp mill may comprise a primary treatment using sedimentation, a secondary treatment using aerated lagoon(s) or anaerobic digester where organic material is decomposed by biological processes, and a tertiary treatment where aluminum or iron flocculants (settling chemicals) are added. The tertiary sludge obtained contains a high amount of flocculants and it is difficult to dewater it mechanically. Tertiary sludge typically has a moisture content around 85 %. Such primary, secondary and tertiary treatment steps per se are well known in the art and will not be described in further detail here.

In embodiments, the surplus lignocellulose containing biomass comprises bark separated from said wood material. The surplus lignocellulose containing biomass may additionally or alternatively comprise tree or wood reject. With wood reject is here meant reject from screening of pulp-wood chips, i.e.” fines” and oversize wood material not suitable for Kraft pulping.

In embodiments, the step of reducing water content of the sludge comprises thermal drying and optionally mechanical dewatering prior to said thermal drying. The mechanical dewatering may be carried out using a screw press, filter-press or alike. The thermal drying may be carried out using a dryer, such as low temperature belt dryer using surplus low-grade heat from the pulp mill. Other suitable dryer types include for example indirectly (steam) heated disc dryers or rotating tube dryers.

In embodiments, the step of reducing water content further comprises, prior to the thermal drying, pressure heating/thermal treatment of the sludge at elevated pressure and temperature followed by steam explosion discharge. The steam exploded sludge may thereafter be mechanically dewatered. Such an embodiment is advantageous since pressure heating and steam explosion destroys the sludge structure such that more water can be separated from the sludge mechanically by pressing and filtering. The dry solids content of the pressed sludge then increases. Also, the improved mechanical dewatering has a washing effect such that water-soluble unwanted salts are better separated from the sludge. For example, the concentrations of potassium, sodium and chlorine which may cause fouling and corrosion in a steam boiler, are reduced. It is understood that flocculants are not well washed from the sludge with this treatment since they are bound to the organic compounds with electrostatic forces and forming the gelatinous sludge substance. On the contrary, sodium and potassium salts are water soluble and will to a large extent be dewatered from the sludge. In embodiments, the method further comprises reducing the particle size of the surplus biomass prior to the adding of sludge. The size reduction may be carried out using, for example, a hammermill.

In embodiments, the method further comprises thermal drying of the surplus biomass prior to the reducing of the particle size. The drying may be carried out using for example a low temperature belt dryer using surplus low-grade heat from the pulp mill.

In embodiments, the method further comprises pressure heating of the surplus biomass at elevated pressure and temperature followed by steam explosion discharge. In embodiments comprising both thermal drying and pressure heating, the pressure heating may be performed before or after thermal drying.

The adding of dewatered and dried sludge to the surplus biomass is performed preferably after pressure heating and steam explosion of surplus biomass, albeit possible to do it before as well.

According to a second aspect of the invention there is provided a system for production of fuel pellets or briquettes from surplus lignocellulose containing biomass of tree material used in a Kraft pulping process, wherein the production of pellets is integrated into the Kraft pulp mill. The system comprises separating device(s) (such as a debarking device and a wood chip screening device) arranged to separate the tree material into wood material and surplus lignocellulose containing biomass, a Kraft pulping line configured to process the wood material, the pulping line comprising means for withdrawing at least one effluent, a waste water treatment plant for treating the at least one effluent to obtain at least one sludge, a dewatering apparatus configured to reduce the water content of the at least one sludge to obtain dewatered sludge, mixing means configured to add at least a portion of the dewatered sludge to at least a portion of the surplus lignocellulose containing biomass to obtain a biomass and sludge mixture, and a pressing apparatus configured to densify the mixture to form pellets or briquettes. The above-mentioned means for withdrawing at least one effluent is understood as, for instance, a conduit leading from one or more process apparatuses (such as a washing apparatus) in the Kraft pulping line.

The wastewater treatment plant may be configured to treat the at least one effluent with a flocculant, which may comprise an aluminous and/or a ferrous settling chemical. The surplus lignocellulose containing biomass may comprise bark separated from said wood material. The surplus lignocellulose containing biomass may additionally or alternatively comprise tree or wood reject.

In embodiments of the system, the wastewater treatment plant comprises a primary treatment device using sedimentation, a secondary treatment device using aerated lagoon(s) or an anaerobic digester where organic material is decomposed by biological processes, and a tertiary treatment device where the flocculants (settling chemicals) are added. Such primary, secondary and tertiary treatment devices per se are well known in the art and will not be described in further detail here.

In embodiments of the system, the sludge dewatering apparatus may comprise a thermal dryer and optionally a mechanical dewatering device prior to the thermal dryer. The mechanical dewatering device may be a screw press, filter-press or alike. The thermal dryer may be a low temperature belt dryer using surplus low-grade heat from the pulp mill.

In embodiments of the system, the dewatering apparatus may comprise a pressure heating device configured to continuously treat the sludge at elevated pressure and temperature. The pressurized vessel of the pressure heating device comprises an adjustable blow valve or a constant diameter discharge orifice to achieve a steam explosion discharge of sludge. The steam exploded sludge is thereafter fed to the mechanical dewatering apparatus.

In embodiments of the system, it further comprises a thermal dryer configured to reduce the water content of the surplus biomass.

In embodiments of the system, it further comprises a pressure heating device configured to continuously treat the surplus biomass at elevated pressure and temperature. The pressurized vessel of the pressure heating device comprises an adjustable blow valve or a constant diameter discharge orifice to achieve a steam explosion discharge of biomass.

In embodiments comprising both a thermal dryer and a pressure heating device, the pressure heating device may be arranged before or after thermal dryer. In embodiments of the system, it further comprises a size reduction device configured to reduce the particle size of the surplus biomass prior to the adding of sludge. The size reduction device may be arranged before the thermal dryer, after the thermal dryer, or between the thermal dryer and the pressure heating device.

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 first embodiment of a system according to the second aspect of the invention, fig. 2 shows a schematic illustration of a second embodiment of a system according to the second aspect of the invention, and fig. 3 shows a flowchart illustrating an embodiment of a method according to the first aspect of the invention.

DETAILED DESCRIPTION

Figure 1 shows a schematic illustration of a first embodiment of a system according to the second aspect of the invention. A separating device 1 is arranged to separate incoming tree material into wood chips/material and surplus lignocellulose containing biomass, for instance bark and/or wood waste. The woods chips are conveyed to a Kraft pulping line 2 configured to process the wood material 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 pulping line comprises means for withdrawing one or more effluents, which effluent(s) is/are fed to a wastewater treatment plant 3 for treating the effluent. The wastewater treatment plant may comprise primary, secondary and tertiary treatment units, each outputting a sludge. Primary sludge is obtained from the primary treatment unit which involves sedimentation (settling) of the effluent. The settled primary sludge mostly contains wood fibres. Secondary sludge is obtained from the secondary treatment unit which comprises aerated lagoon(s) or an anaerobic digester where organic material is decomposed by biological processes. The residual organic material settles, and it is recovered as secondary sludge sometimes also called “bio sludge”. Secondary sludge contains more inorganic material than primary sludge, flocculants (see below) may be added to improve sludge settling in secondary treatment. Tertiary sludge is obtained from the tertiary treatment unit which comprises addition of an aluminum or iron flocculant for settling of the residual organic content in the effluent. Thus, tertiary sludge obtained contains a high amount of the flocculation chemical. The sludges of the wastewater plant are normally dewatered mechanically, separately or mixed. Sludges from wastewater treatment plants typically have a moisture content in the range 60-90%. Primary sludges normally dewater mechanically to lower moisture content than secondary and tertiary sludges.

At least tertiary sludge, enriched in flocculants added, is fed for dewatering with a thermal dryer 4 for example a low temperature belt dryer configured to reduce the water content to obtain dried (tertiary) sludge. Optionally, also the primary and/or secondary sludges (dotted lines in the figure) is/are fed to the thermal dryer 4. As schematically indicated in the figure, the dryer 4 is heated with low-pressure steam which preferably is low-grade steam from the chemical recovery island of the Kraft mill.

Surplus biomass separated from the wood material in separating device 1 is fed to a thermal dryer 9, for example a low temperature belt dryer configured to reduce the water content of the surplus biomass. As shown in Figure 1 , the dryer 9 is provided with air and low-pressure steam, which low pressure steam is preferably steam from the recovery island of the Kraft pulp mill.

At least a portion of the dewatered and dried sludge is mixed with at least a portion of the surplus lignocellulose biomass using mixing means 5 illustrated schematically as a mixer to obtain a biomass and sludge mixture.

The biomass and sludge mixture is then fed to the pressing apparatus 6, which is configured to densify the mixture to form pellets or briquettes. Pressing apparatuses such as pelleting machines and briquette presses are known in the art and are therefore not explained in further detail here.

Figure 2 shows a schematic illustration of a second embodiment of a system according to the second aspect of the invention. This embodiment corresponds to the first embodiment in that it comprises corresponding devices as follows: separating device 11 , Kraft pulping line 12, wastewater treatment plant 13, thermal drier 14, mixing means 15, dryer 19 and pressing apparatus 16. The second embodiment differs from the first embodiment in that it comprises several additional devices, as will be explained in the following.

Tertiary sludge (and optionally the primary and/or secondary sludges) is fed to an optional pressure heating device 17 being a pressurized vessel designed for continuous thermal treatment of the sludge(s) at elevated pressure and temperature. The pressure heating device is preferably heated with medium pressure steam from the chemical recovery island of the Kraft pulp mill (not shown). The sludge is rapidly heated close to the saturation temperature of the steam used. The heated sludge is then discharged through an adjustable blow valve or a constant diameter discharge orifice to achieve a steam explosion discharge of the sludge. The rapid pressure heating followed by steam explosion discharge destroys the cellular structure of the sludge such that more water can be drained from it by mechanical pressing. The pressure in the pressurized vessel is around 12 bar, when using medium pressure steam (preferred) and 4-6 bar, if low pressure steam is used for heating. The heating and treatment time is short from 30 s up to 5 min. The discharged thermally treated sludge is fed to a screw press, filter press or like, 18 to mechanically dewater the sludge.

After the screw press, the sludge is fed to the thermal dryer 14, being of the same type described above with reference to Figure 1. In other embodiments, the pressure heating 17 and/or the screw press 18 is/are omitted, and the sludge is fed directly from the wastewater treatment plant to the dryer 14 or to the screw press 18.

The dried surplus biomass is fed to a pressure heating device 111 being a pressurized vessel driven to provide continuous thermal treatment of the surplus biomass at elevated pressure and temperature. The pressure heating device is heated by medium pressure steam from the recovery island of the Kraft pulp mill (not shown). The thermally treated biomass is discharged through an adjustable blow valve or a constant diameter discharge orifice to achieve a steam explosion discharge of the biomass. The pressure heating/thermal treatment is performed at a pressure of 15-25 bars, and at a temperature of 150-240 °C for less than 15 minutes.

At least a portion of the sludge from the thermal dryer 14 is mixed with at least a portion of the thermally treated surplus lignocellulose biomass with a mixer, illustrated schematically as 15 in Figure 2, to obtain a biomass and sludge mixture. Alternatively, or additionally, at least a portion of the sludge is mixed with the surplus biomass prior to being fed to the pressure heating device 111 using mixer 15’ and/or is mixed with the biomass during the pressure heating/thermal treatment in the pressure heating device 111 by being injected therein using mixing means in the form of an injector 15”.

The biomass and sludge mixture is thereafter fed to the pressing apparatus/plant 16, which is configured to densify the mixture to form pellets or briquettes. Pelletizers and briquette presses are known in the art and are therefore not explained in further detail here.

Optionally, in other embodiments, the pressure heating device 111 is omitted.

Optionally, a size reduction device 110 in the form of a hammermill is arranged between the thermal dryer 19 and the pressure heating device 111. In embodiments where the pressure heating device 111 is omitted, the optional size reduction device 110 is arranged between the thermal dryer 19 and the pressing apparatus 16.

Optionally, it is possible to install the dryer 19 after the pressure heating device 111 , albeit the location of Figure 2 is preferred.

In other embodiments, which correspond to the first or second embodiment, separated lignin is added to the bark/surplus biomass as a binder in the same way as described in SE541263. In other embodiments, which correspond to the first or second embodiment, mixing of dried sludge to surplus biomass is advantageously accomplished in the pressing apparatus (pelletizing or briquetting plant) 6/16 which may be furnished with a small container and a dosing screw which adds the dry sludge to surplus biomass before the pelleting (or briquetting) presses.

Figures 1 and 2 could also be considered illustrating corresponding embodiments of a method according to the first aspect of the invention, where the shown devices correspond to method steps.

Figure 3 shows a flowchart illustrating an embodiment of a method according to the first aspect of the invention. The method comprises separating 21 the tree material into wood material and surplus lignocellulose containing biomass, processing 22 the wood material according to the Kraft pulping process to produce pulp, withdrawing 23 at least one effluent from the Kraft pulping process to a waste water treatment plant, treating 24 the at least one effluent with a flocculant comprising at least one aluminum and/or iron settling chemical to obtain at least one sludge, reducing 25 the water content of the at least one sludge to obtain dewatered sludge, optionally drying 26 the surplus biomass, optionally reducing 27 the particle size of the dried surplus biomass (for instance using a hammermill), optionally thermally treating/pressure heating 28 of the biomass, adding 29 at least a portion of the dewatered sludge to the pressure/thermal treated biomass and finally densifying 210 the biomass and sludge mixture to form pellets or briquettes.

The step of reducing 25 water content comprises thermal drying of the sludge and optionally mechanically dewatering the sludge prior to said thermal drying. The step of reducing 25 water content may optionally further comprise, prior to the thermal drying, pressure heating/thermal treatment of the sludge at elevated pressure and temperature followed by steam explosion discharge.

In other embodiments, the adding 29 may additionally or alternatively be performed before or simultaneously with the pressure heating 28. 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, although a single effluent from the Kraft pulping line is shown in the embodiments above, it is understood that two or more effluents may be withdrawn in other embodiments. Furthermore, the sludge(s) and surplus biomass may be dried, dewatered and/or thermally treated using other equivalent devices/techniques known to the person skilled in the art. The scope of protection is determined by the appended patent claims.