Technical field

The present invention relates to a method for refining biomass. More precisely, the method according to the present invention comprises activation milling of the biomass, optionally modifying with minerals and combusting of the homogeneous mass to obtain valuable ash products. In case of clean and well defined biomass raw materials containing material from which valuable fractions can be produced, these fractions can be recovered by screening after milling.

Background of the invention

Biomass or solid biofuel is biological (plant-based) material from living or recently living organisms. In other words, biomass is material originating from wood and agricultural crops, wood, straw, organic residues, forest residues or aquatic biomass. As renewable resource its utilization for different purposes is of constant interest. Biomass refining aims at obtaining value-added products from these materials.

Biomass, especially woody biomass, is typically used as an energy source either directly or as converted into other energy products. Typically annual biomass consump- tion in a large scale bio power plant has a composition of 60% - 90% of wood chips and 10% - 30% peat, the rest being municipal waste.

Biofuel fly ash is formed when biomass i.e. forest or field biomass, peat or combinations thereof are burned or during their biogasification. The amount of formed biofuel fly ash is increasing as the use of renewable energy is increasing. Biofuel fly ash can be used e.g. as a substitute for lime. This is highly recommended at least from the ecological point of view as the use of lime enhances greenhouse effect in several stages, i.e. mining, crushing and transportation. The main oxides occurring in major types of biofuel fly ashes are presented in tables la and lb. The general composition of regular Portland cement is also given in table la. The composition of the fly ash is completely dependent on the combustible material in question. In the case of biofuel ashes, a few generalizations can be made, but these generalizations should be dealt with using extreme care since there is a wide variability between the compositions of the fly ashes, even between fly ashes of similar or even the same biofuels. Generally speaking, biofuel fly ashes tend to be classified mostly as class C fly ash - that is to say they are rich in CaO and MgO with varying concentrations of Si0 ₂ . Another fact that can be clearly seen from the different biomass fuel types is the relatively high content of alkalis in the ashes. Grass and grass-comparable material seems to produce fly ashes that have higher concentrations of Si0 ₂ .

The different elements i.e. oxides are present in the fly ash either as a crystalline min- eralogical phase or as an amorphous glass phase. The most common mineral types present in the fly ashes are: Si0 ₂ (Quartz), CaS0 ₄ (Anhydrite), Fe ₂ 0 ₃ (Hematite), CaC0 ₃ (Calcite), CaAl ₂ Si0 ₈ , (Feldspar), CaO (lime), MgO (Periclase), Ca(OH) ₂ (Portlandite) and (Ca ₂ Al)(AlSi)0 ₇ (Gehlenite). The determination between the crystalline (mineralogical) phases and the amorphous phases in the fly ash is complicated. The standard technique for determination of crystalline phases is x-ray diffractometry. When the signal or signals of the crystalline phase is/are too weak, it is difficult to determine whether the substances are present in a crystalline or amorphous state. The reactivity of fly ashes and the elements within them is related to their physical states, i.e. crystalline or amorphous.

Table la. Typical compositions (wt%) of biofuel ashes with references to Portland cement and coal ash

* LOI = Loss on ignition

Table lb. Typical compositions (wt%) of biofuel ashes

* LOI = Loss on ignition

Biofuel fly ashes are an inhomogeneous group of materials. Thus, it is clear that biofuel ashes are not suitable for large quantity production in their unmodified state. However, it has been shown through scientific cutting-edge research that the proper- ties of ashes and the combustion behavior of the biofuel can be modified. A huge proportion of the ongoing biomass ash research is concentrated on optimizing the combustion behavior of different types of fuels.

The addition of an aluminium and silicon containing chemical compounds to biomass before combustion is used as a potassium absorbent during combustion of biofuel. Potassium is efficiently incorporated in newly formed mineral formations like potassium-feldspar.

The prior art discloses an attempt to modify the sintering properties of biomass ashes. Several additives were tested to reduce the sintering of biomass in combustion boilers. It was found that these additives, consisting of different mineral compounds lessened the sintering behavior of the ash in the boilers and heating tubes. The focus of this research was purely in the combustion issues, e.g. fouling and sintering.

Especially for biomass combustion related ashes it is important to know the exact composition of the material to be combusted. It has been shown in the prior art that biomass material even from the same source can produce different ash types. For example in forestry residuals, various alkali metals are much more enriched in the bark than in other parts of the wood. In US patent 5,888,256, a method is disclosed for producing a fuel for kilns from at least two waste materials having different compositions. The chemical composition information for the respective materials is obtained and the required chemical composition for the resulting ash is specified, since the ash becomes part of the clinker product from the kiln. The fuel is prepared by mixing the waste materials in proportions selected to yield a given minimum energy value as well as the required chemical composition in the ash. Inorganic supplemental compounds can be added to the fuel to obtain the desired ash composition.

The aim of the invention of US 5,888,256 is predicting equilibrium conditions in and optimizing the operation of a cement kiln. In contrast, according to one aspect of the present invention, the hardening properties of the product can be predicted.

According to the present invention, the focus of the subject matter has been changed completely. It was surprisingly figured out that controlled and predetermined valuable ash products are obtained by selecting the starting biomass materials with precision, subjecting the material to activation milling and, when required, by modifying the composition of the starting material with minerals and/or chemicals to end up with the desired ash product. Brief description of the figures

Fig. 1 A flow chart of the process according to the present invention

Fig. 2 Particle size distribution of pine bark residue after a mechanical activation process

Fig. 3 Differences in water uptake of pure and activated cellulose

Fig. 4 The w/c ratio of untreated and treated fly ash

Fig. 5 Effect of milling on heat treated wood

Fig. 6 Separation of fractions from birch bark

Fig. 7 Ternary diagram of Si0 ₂ , A1 ₂ 0 ₃ and CaO

Detailed description of the invention

The biomass refining method according to the present invention is now described in more detail with reference to the accompanied figures. A flow chart of a process according to the present invention is shown in figure 1. Process steps shown in boxes with dashed lines are optional.

According to the present invention, information on the individual chemical composi- tion of ashes resulting from the combustion of individual biomass types is used to provide, when necessary by mixing, a selected biomass which upon combustion should result in ash having a target chemical composition. The thus selected biomass of a determined composition is activation milled using a disintegrator mill comprising at least two counter-rotating discs. The homogeneous, activated biomass, optionally in form of pellets, is then combusted to produce cementitious ash having a CaO/Si0 ₂ molar ratio of 0.5 - 3.

Optionally, the selected biomass may be crushed to a particle size suitable for the activation milling.

Optionally, the selected biomass is modified using minerals and/or chemicals . Exam- pies of useful minerals are aluminum-containing silicates, magnesium-containing silicates, iron-containing silicates, glassy wastes like recovered glass or mineral wool or precipitated wastes from the chemical industry. Ashes or finely ground slags can also be used to modify the biomass. The modifying step is carried out in a disintegrator mill as described below to ensure fine homogenisation and activation of both the mineral and the biomass components.

According to an embodiment of the invention, the milled and activated biomass is screened to separate different fractions, as shown in figure 1.

In the context of the present invention "biomass" means wood chips, peat, bark, straw, plastic, municipal waste, biocoal, torrefaction or other waste material (side streams) e.g. from the paper and pulping industry or combinations of these different biomass types.

As discussed in the background part of this application, biomass has a complicated element composition, reflected in the resulting ash. In case of the first embodiments of the present invention, selection of the biomass to produce valuable ash must be made. In this case selection of the biomass is based on the accurate knowledge of the composition of the biomass, in particular its specific ash-producing composition. The selection criteria also include knowledge of the quantity of the ash it is capable to produce and the combustion behavior of the biomass. The knowledge of the composition and combustion behavior of the selected biomass or biomasses is of extreme importance. Biomass or biomasses is/are selected with accurate proportions in order to produce valuable ash. Fraction IV from the second embodiment can also be used as a raw material in this ash producing embodiment. The biomass or biomasses and optionally modification minerals and chemicals are carefully selected and mixed in right proportions in order to obtain a desirable

CaO/Si0 ₂ molar ratio for the ash product; the desirable ration being 0.5 - 3.0. The correct raw materials and their right proportions are selected by using the knowledge of the composition of the ash producing elements making up the ash resulting from the relevant raw materials. The composition of the ash in terms of oxides and other compounds can be obtained from the literature or from databases, or by analyzing the raw materials in question. Examples of this selection are given below. According to an embodiment of the present invention, the selected biomass raw material to be used has to be clean and well defined and contain valuable material from which some valuable fractions can be obtained. Fraction I may be classified as a fraction which can be utilized without further modifications or processing. For example the outer bark fraction is used for water purification applications and the stem wood fraction can be used for pulping applications e.g. for cooking. Fraction II may be classified as a fraction which is used in different composite products. For example heat treated i.e. torrefied wood can be used in wood-polymer composites or light cement composites. Fraction III may be classified as specialty products, meaning that this fraction is further processed to produce e.g. medical extractives. For example different parts of the bark can be utilized in this way. Fraction IV is the milled biomass material not included in fractions I to III and is further processed.

In case the initial particle size of the biomass is too large, meaning that the size of the "particles" is so large that the biomass cannot be effectively fed to the activation milling step, it has to be crushed to obtain a suitable particle size. The crushing step is thus optional and therefore shown as a box with dashed line in Fig. l. The person skilled in the art can easily determine from the appearance of the biomass whether crushing is needed or not. Crushing is used either to enable the feeding of the biomass to the activation milling or to maximize the amount of fed biomass per time unit to maximize the yield of obtained valuable end products.

During the activation milling process, the selected biomass materials are mixed and milled using the disintegrator technique described in further detail below, optionally together with certain chemical compounds in order to achieve an optimal composition of ash-producing mass. For controlled combustion it is necessary to ensure that the kinetics of combustion for all substances involved in the process are on the same level. This can be achieved by the milling, which ensures a truly homogenous mix of the mass and a very close proximity of the particles of the mixed mass.

To further enhance certain properties, minerals or other chemical can be added to the mass during the milling step. This is shown as "modification with minerals" is figure 1. By adding chemicals, minerals or other fuels containing a specified ash forming composition, the amount and the quality of the formed ash can be substantially increased. The chemicals or minerals can be waste materials from chemical, mining or metallurgical industry. The activation milling can be applied to both organic and inorganic materials. It is even possible to process liquids. Mainly wet material or sludges can be dried or blended with other biofuels.

The mass is fed to the milling chamber either with the aid of a feeding screw with controllable speed or it is fed straight to the milling chamber with the aid of air flow generated by the high speed rotating discs. For example, the commercially available DESI 15/16C -disintegrator or DESI 21 - disintegrator made by Desintegraator Tootmise θϋ, Estonia, can be used in the activation milling process according to the present invention. In addition to the particle size reduction and surface activation, the milling process simultaneously produces a highly homogenous mass of milled parti- cles.

After the activation milling step the homogeneous mass can be pelletized or briquetted if desired. The transportation and storing of the homogeneous mass is easier when it is in pelletized form. Also the feeding of the briquettes or pellets is more efficient and the handling of the material is more convenient.

The homogenized mass, possibly in the form of briquettes or pellets, is fed to a burning furnace, where the ash-producing reactions occur. The combustion process produces heat, which can be used in energy production. The combustion conditions are controlled with precision; for example, the combustion temperature is kept within the range of 800 to 1200 °C. The novel idea in the control of combustion is that rather than focusing on the minimizing of slag and sinter formation as in the prior art, the focus is clearly on the formation of the valuable, cementitious ash and its properties. This novel cementitious ash can be used for example as a substitute for Portland cement or as a strength- providing filler in concrete products.

The properties of the cementitious ash are further enhanced by the activation milling and/or modifying with minerals or chemicals to be used in cement products possessing extremely high strength properties. The activation of the fly ash can be monitored by measuring the amount of water the fly ash is capable to bind. In cement industry the w/c- i.e. water to cement ratio is an important parameter of cement quality. The cement can become more dense and has greater strength the less the water- to-cement ratio is. From a theoretical point of view, the hardening reactions of cement require a given ratio of water, but any excess water leads to weakening of the whole system. It is shown in figure 4 that activation milling significantly reduces the w/c ratio of fly ash.

During the activation milling, the material is activated and milled into small particles, i.e to a fine powder with a narrow size distribution. The disintegrator comprises at least two counter-rotating rotors. The activation proceeds in two steps so that the material first hits the edges of the blades of the inner rotor and then are carried by the airflow to the blades of the second rotor. High velocity impacts occur between the cutting elements and the material but also between the material particles themselves. In addition, material also collides with the outer surface of the milling chamber. The speed of these collisions is extremely high and can reach up to 300 m/s, typically the speed is between 100-150 m/s. The rotational speeds of the rotors are adjustable; therefore the kinetic energy of the collisions is also controllable. The geometry of the milling chamber and the cutting elements also affects the activation process. Especial- ly the placement and the shape of the cutting elements have a significant effect on the efficiency of the activation process. The spacing and geometry of the elements can be optimized to obtain desirable results. The reason behind this is that the angle of collision and therefore the direction of the kinetic energy of the collisions can be adjusted. Additionally, the cutting elements themselves can be coated with a catalytic material or materials to further increase the desirable activation of particle surfaces.

The collisions produce kinetic energy in the material, which causes physico-chemical changes, e.g

- the creation of free radicals,

- the creation of point dislocations,

- structural modifications,

- in relation to the creation of free radicals, creation of highly reactive chemical surface groups. The specific physico-chemical changes mentioned above in addition to the substantial increase in the surface area by size reduction leads to highly active surfaces in the milled particles. The aim is to transfer collision energy as efficiently as possible to the surfaces of the particles.

The effect of activation milling can be seen in Table 2. The compression strengths of wood - fly ashes mixed with water have been measured before and after activation milling. The beneficiary effect of the milling is clearly proven. The measured compression strengths is shown to increase up to 128 %.

Table 2. Compression strength in unmilled and activation milled material

Definitive proof of the activation milling can also be seen in Table 2 where the pH and electric conductivities of water solutions of milled and unmilled ashes are compared. A clear increase in both pH and conductivity is apparent for the milled ashes. This is proof that the active soluble solid phases of the ashes show further increase in activity towards water solutions due to the effect of activation milling.

Table 3. pH and electrical conductivity of unmilled and activation-milled ashes.

Traditional mechanical treatment, such as ball milling, planetary milling or attrition milling cannot achieve the same results. All types of milling effect the setting and hardening of water - ash mixes. However, the precise control of the mechanical ener- gy injected to the material is of utmost importance. The disintegrator possesses an unique design in that specific area. The magnitude and direction of the mechanical energy can be controlled by the rotation speeds and shapes and placement of the cutting elements. Especially for treatment of heterogeneous mixes of differently reacting phases such as biomass fly ashes, this control is important.

According to another embodiment of the present invention, the optionally crushed biomass is fractionized . Fractionization is carried out using similar equipment as is used for activation milling. During fractionization, the biomass particles are activated as described in connection with activation milling. The fractionization is controllable. Different particle sizes of starting material can be obtained using different intensities of milling. An example of the control of particle sizes is shown in figure 5 where the particle size distribution of heat treated wood resulting from different milling intensities is shown.

It is also possible to separate stem wood from bark (e.g. pine bark), because the particles with a size of 1mm (diameter) or over are composed mainly of stem wood, whereas the particles with the size of 500 μιη or less are composed entirely of bark. The separated stem wood fraction can be classified as fraction I (see upper part of figure 1). Optionally, with more intense milling more of the material can be milled to finer particles. Fractions with particle size of less than 1 mm in diameter can be separated from the milled mass. This fraction has a possible use in purification applications. It can be for example used to separate oil from water. The fraction separated for this purpose can still be classified as fraction I (see upper part of figure 1). In figure 2 it is shown a particle size distribution of a pine bark cutting residue wherein the initial feed material has a size of 4-6 mm (diameter).

Some materials are resistant to milling; for example the outer region of the birch bark is resistant to milling. This is beneficial as it enables the separation of the outer region of the bark. Particle size distribution for a different intensity of milling is shown in figure 6. The outer region of the bark having a size over 1 mm in diameter, can be used for a variety of applications, for example in the medical field where extracts from birch bark can be used (fraction III). Alternatively the outer bark region of birch can be used for example as a substitute for cork, as it has comparable properties with cork (fraction I). It is shown in Fig. 6 that even with increasing milling efficiency a certain fraction of the material stays intact, while the material susceptible to milling becomes smaller in particle size with increased milling efficiency.

In case the biomass raw material is cellulose, activation milling can be used to enhance the properties of cellulose without any specific need for fractionation. For example the milling increases the water uptake capacity of the cellulose fibres. In figure 3 is shown a diagram of the water uptake of an unprocessed (i.e. without activation milling) cellulose fibres and the water uptake of the activated fibres. The starting material is pine-derived cellulose fibre, which is cut to pieces of about 5 x 70 mm. When activated during the milling, this material changes into a puffy, airy material consisting of tiny semi-microscopic fibres having increased water uptake capacity. Fraction II is used as a starting material for different types of composites. For different types of composites different properties of the starting materials are required. For example fraction milled and thereby activated heat treated wood can be used as a starting material in wood plastic composites. With the activation effect it is possible to obtain up to 40 wt of wood in a wood-polymer- composites without the need of a compatibilizer. This is a striking proof of the highly active surfaces.

Examples

For the following examples all of the biomass samples were collected from local bio- mass-utilizing energy producers. The compositions of the ash-producing elements (i.e. oxides) were obtained from the literature and the accuracy of the literature values was verified by comparing the literature values to laboratory analyses when available. The compositions are shown in table 4. The moisture levels were measured as collected from different facilities using a common laboratory oven and scale. For every example biomasses and optionally minerals were carefully weighed before the homogenization milling. The homogenous mass produced containing the exact weight partitions of desired components were fed into the combustion chamber where the mass transformed to a cementitious ash. The temperature of the combustion chamber was kept in each example in a range of 800 - 1200°C. The person skilled in the art can easily determine the time needed to ensure the complete combustion of the homogeneous material. The required combustion time depends e.g. on the material, amount of the material and the combustion conditions.

The compositions of the ashes produced in the examples are given in table 5. Addi- tionally a ternary diagram of the system Si0 ₂ , CaO, A1 ₂ 0 ₃ with the exemplary ashes included is shown in figure 7.

Table 4. Main ash-making elements of biomasses and minerals (wt%)

Main oxSiO Al ₂ Fe ₂ Ca Mg Na ₂ K ₂ 0 so ₃ P ₂ 0 TiO Moisides pre2 o ₃ o ₃ O O O 5 2 ture sent

Wheat 30 straw 50 2 1 8 3 4 25 4 0

Pine Bark 9 7 3 57 6 2 8 3 5 0 64

Birch Bark 4 1 2 69 6 2 9 3 4 0 40

Spruce 47 Bark 6 1 2 72 5 2 7 2 3 0

RDF 39 15 6 27 6 1 0 3 1 2 5

Paper mill 58 sludge 38 22 1 28 5 0 1 0 1 1

Peat 38 20 14 10 2 0 1 12 3 0 15

Forest 27 residue 21 3 1 48 7 2 10 3 5 0

Soda boi75 ler sludge 47 14 8 18 2 1 1 8 1

Pine 64 Wood 33 5 4 49 0 0 3 3 0 0

Magnesite 16 - 14 - 69 - - 0 - - 1

Estonian

Oil Shale 31 9 4 24 3 0 0 0 1 Table 5. Compositions of the reactive ashes from examples.

Example 1.

Birch bark and wheat straw were combined in the ratio of 9: 1 (bark:wheat) by weight by activation milling to give a homogenous biofuel blend. The combustion process followed exactly the guidelines given above. Once removed from the combustion chamber, the ash was allowed to cool. The composition of the ash is given in table 5. The molar CaO/Si0 ₂ ratio for this ash is 2.99.

Example 2.

Spruce bark, birch bark and wheat straw in a ratio of 4 parts spruce, 4 parts birch and 1 part of wheat by weight were activation milled to give a homogenous biofuel blend. The combustion process followed exactly the guidelines given above. Once removed from the combustion chamber, the ash was allowed to cool. The composition the ash is given in table 5. The molar CaO/Si0 ₂ ratio for this ash is 2.95. Example 3.

Birch bark and RDF (short for refuse derived fuel) were combined in a ratio of 97 % Birch and 3 % of RDF by weight were activation milled to give a homogenous biofuel blend. The combustion process followed exactly the guidelines given above. Once removed from the combustion chamber the ash was allowed to cool. The composition the ash is given in table 5. The molar CaO/Si0 ₂ ratio for this ash is 3.09

Example 4.

Pine bark and paper mill sludge were combined in a ratio of 99 % bark and 1 % of Sludge by weight were activation milled to give a homogenous fuel blend. The combustion process followed exactly the guidelines given above. Once removed from the combustion chamber the ash was allowed to cool. The composition the ash is given in table 5. The molar CaO/Si0 ₂ ratio for this ash is 2.80 Example 5.

Peat and Forest cutting residue 1 part peat and 5 part forest residue were activation milled to give a homogenous fuel blend. The combustion process followed exactly the guidelines given above. Once removed from the combustion chamber the ash was allowed to cool. The composition the ash is given in table 5. The molar CaO/Si0 ₂ ratio for this ash is 1.43.

Example 6.

Forest cutting residue and sludge collected from a soda boiler 5 parts FR and 1 part of Soda sludge by weight were activation milled to give a homogenous fuel blend. The combustion process followed exactly the guidelines given above. Once removed from the combustion chamber the ash was allowed to cool. The composition the ash is given in table 5. The molar CaO/Si0 ₂ ratio for this ash is 1.04.

Example 7.

Pine wood residue and magnesite having of 99 % pine and 1 % of magnesite by weight were activation milled to give a homogenous fuel blend. The combustion process followed exactly the guidelines given above. Once removed from the combustion chamber the ash was allowed to cool. The composition the ash is given in table 5. The molar CaO/Si0 ₂ ratio for this ash is 1.30.

Example 8.

Birch bark and Estonian oil shale were combined in a ratio of 98 % birch bark and 2 % of oil shale by weight were activation milled to give a homogenous fuel blend. The combustion process followed exactly the guidelines given above. Once removed from the combustion chamber the ash was allowed to cool. The composition the ash is given in table 5. The molar CaO/Si0 ₂ ratio for this ash is 2.81.

The selection of different biomasses used in the examples follows the criteria described earlier. The main target was to obtain the desired molar ratio of CaO/Si0 ₂ between 0.5 - 3. Additionally in examples 4 and 5 the combustion behavior of the mass was considered. The object was to considerably increase the amount of ash and simultaneously minimize the formation of harmful fouling and sintering.

It is shown in figure 7 that the compositions of all the exemplary ashes match roughly the composition of Portland cement in a Si0 ₂ -CaO-Al ₂ 0 ₃ system and the CaO/Si0 ₂ ratio is within the target range. The ashes showed high reactivity with water and were successfully used to produce high strength cement products

It can be concluded that by determining exactly the compositions of the biomasses and by calculating the exact proportions of the different raw materials, high quality cementitious ashes can be produced. It should be noted that this high quality product can essentially be produced from waste or cheap side stream materials

Additionally, if beneficial and economically sensible, valuable fractions from waste materials can be separated (e.g. outer bark for cork substitute, outer bark for infiltration).