OA19576A - A Process for the Conversion of Biomass of Plant Origin and a Combustion Process - Google Patents

Abstract

The invention provides a process for the conversion of biomass into a biomass product which is suitable for use as a fuel. The biomass is of plant origin and comprises microorganisms naturally occurring in the biomass. The process comprises - preparing a slurry by dispersing the biomass comprising the naturally occurring microorganisms in an aqueous liquid, maintaining the slurry at conditions suitable for aerobic digestion by the microorganisms to obtain a slurry comprising the biomass product as a dispersed solid phase, and - recovering the biomass product. The recovering comprises washing and drying the biomass product. The invention also provides a combustion process.

Description

A PROCESS FOR THE CONVERSION OF BIOMASS OF PLANT ORIGIN, AND A COMBUSTION PROCESS

The présent invention relates to a process for the conversion of biomass wherein the biomass is of plant origin. The invention also relates to a combustion process which combustion 5 process comprises obtaining a biomass product from a biomass of plant origin, and combusting the biomass product.

The production of renewable energy has been the subject of mimerons studies during past décades. Renewable energy may corne in various forms, for example in the form of biomass, in particular biomass of plant origin. biomass of plant origin may be combusted, directly yielding 10 energy in the form of heat, or it may be converted into convenient energy carriers, for example combustible liquids, such as hydrocarbons or alcohols, and combustible gasses, such as methane.

The handling and conversion of biomass of plant origin, however, is difficult because of its physical characteristics, in particular morphology, and because the biomass comprises components which disturb or are harmful in combustion or conversion processes. Undesirable 15 components of biomass of plant origin comprise, for example, carbohydrates, chloiides, alkali métal and alkaline earth métal salts, in particular potassium, calcium and magnésium salts, ammonium salts, nitrogen and sulfur in a form bound to peptides or proteins, ash piecursors, and water. Unpleasant odours may affect the environment when handling the biomass, in particular when drying or when combusting the biomass.

Many attempts hâve been made to treat biomass in order to bring it into a form suitable for the production of energy or energy carriers. Such treatments included steps, such as - grinding, cuttmg, milling, or other mechanical treatment aimmg at particle size réduction, - Extrusion, pressing or heat treatment aiming at destruction or opening of the biomass cell structure, or removal of water;

- Extensive heating or roasting, aiming at melting or pyrolysis, yielding fuels, such as charcoal, tar or gas; and

- Treatment with strong acid or strong base, or oxidizing agents, aiming at conversion of the biomass’ lignocellulosic components and making them digestible by enzymes.

A process for the conversion of mown grass is known from ECN report ECN-C-1-050, 30 titled “Cascadering van maaisel”, by J. van Doorn, E.R.P. Keijser and H.W.Elbersen (Energy Centrum Nederland (E.C.N), Westduinweg 3, NL-1755ZG Petten, The Netherlands). The known process comprises the steps of:

- Treating the mown grass by extrusion, or by steam treatment and extrusion,

- Soaking the mown grass so treated in water, and

- Recovering from the resulting mixture a solid phase, yielding a grass product.

According to the ECN report, the purpose of treating the mown grass by extrusion, or by steam treatment and extrusion, is destroying the grass’ cell structure, so that components présent within the cell structure are made accessible to extraction during the soaking.

The known process alleviates only some of the difficulties associated with the handling 5 and conversion of biomass of plant origin. In particular, it is stated that the removal of nitrogen is at most 50% complété, while sometimes hardly any nitrogen is removed, and that insufficient removal of nitrogen is likely caused by the fact that mown grass species tested were not processed while fresh, but only after ageing. Removal of moisture from the grass product is stated to be insufficient, likely necessitating an additional drying step. A problem associated with 10 the known process is that steps of extrusion and steam treatment are energy intensive.

The présent invention provides a process for the conversion of biomass into a biomass product which is suitable for use as a fuel, wherein the biomass is of plant origin and comprises microorganisms naturally occurring in the biomass, which process comprises:

- preparing a slurry by dispersing the biomass comprising the naturally occurring 15 microorganisms in an aqueous liquid,

- maintaining the slurry at conditions suitable for aérobic digestion by the microorganisms to obtain a slurry comprising the biomass product as a dispersed solid phase, and

- recovering the biomass product.

The process of the invention comprises recovering from the slurry a solid phase, yielding 20 the biomass product. The process of the invention may additionally comprise recovering from the slurry a liquid phase, yielding a biomass extract.

In an embodiment of the process of the invention, the aqueous liquid may be water. In another embodiment of the process of the invention, the aqueous liquid may comprise at least a portion ofthe biomass extract which was obtained previously in the process ofthe invention.

The présent invention also provides a combustion process, which combustion process comprises the steps of

-Preparing the slurry by dispersing a biomass in an aqueous liquid, wherein the biomass is of plant origin and comprises microorganisms naturally occurring in the biomass,

-Maintaining the slurry at conditions suitable for aérobic digestion by the microorganisms to 30 obtain a slurry comprising a biomass product as a dispersed solid phase, and

-Combusting the biomass product.

It has been found that the invented process is energy efficient and environmentally friendly in that it employs mild conditions, and it allows the use of recycle streams. Ihe invented process is efficient in the removal of undesirable components from the biomass. Foi example, 35 carbohydrates, such as mono- or disaccharides, alkah métal and alkaline earth métal salts, in particular potassium, calcium and magnésium salts, ammonium salts, chlorides, other salts, sulfur and nitrogen, and combinations thereof can be removed to a high degree. Further, the morphology of the biomass product is such that liquids, for example biomass extract or water, can be removed relatively easily from the biomass product, which also adds to the efficiency of 5 the process, in particular energy efficiency.

Not wishing to be bound by theory, it is believed that the exertion of strong mechanical forces and heat on the biomass by extrusion and stream treatment, as taught by the prior art cited hereinbefore, has a disadvantageous effect on the extractability of the biomass. For example, it may cause denaturing of proteins présent in the biomass so that the proteins become virtually 10 insoluble in the aqueous phase and the proteins hâve, therefore, a strong tendency to stay in the solid phase. It is also thought that the exertion of strong mechanical forces and heat on the biomass may cause particles of the biomass to become too small in size foi efficient handling. For example, during the removal of liquid from the biomass by pressing, clogging of particles within the biomass may hinder or prevent flow of the liquid to leave the biomass. Further, small 15 particles may clog filters and membranes of the equipment which may be used in the recovery of the biomass and liquid products. In respect of these problems, the présent invention provides an improvement.

The skilled reader will appreciate that, together with the destruction of the cell structure of the biomass, the exertion of strong mechanical forces and heat on the biomass, as taught by 20 the prior art cited hereinbefore, also causes the destruction of the micioorganisms natuially occurring in the biomass, yielding a stérile biomass.

Thus, it is surprising that, while the prior art teaches the destruction of the cell structure of the biomass and the naturally occurring microorganisms of the biomass, the présent invention shows that advantageous results can be obtained by using in the process biomass that comprises 25 intact cell structures, living microorganisms and enzymes. Not wishing to be bound by theory, it is believed that microorganisms and /or enzymes associated with the microorganisms are capable of digesting plant components, such as membranes and cell walls, making components of the biomass présent within the cell structure accessible to extraction. The extraction involves extraction of components from the biomass into liquid phase of the slurry. Microorganisms may 30 also be extracted from the biomass into the liquid phase. The extracted components may or may not be subject to further digestion.

The process of the invention employs biomass of plant origin. In general, such biomass comprises lignocellulosic materials. The biomass may comprise aquatic biomass, such as whose, alga, seaweed and duckweed. Preferably, the biomass my comprise land biomass, in particular, 35 forestry waste or agricultural waste. The biomass may be plant parts, such as wood parts, leaves and roots. Examples of suitabie land biomass may be wood chips, leaves, reed, straw, corn plants or parts thereof, cabbage plants or parts thereof, sugar beet leaves, line seed plants or parts thereof, bagasse from sugarcane, tomato plants after harvest, rice straw, and, in particular, mown grass, and mixture thereof. The biomass for use in the invention may or may not hâve been dried, 5 or the biomass may or may not hâve been ensilaged.

The biomass for use in the invention may be used as such. However, for easy handling and in view of the spécifie process equipment in use, it may be désirable to reduce the size of particles of the biomass, for example, by grinding, milling or cutting. Suitably, the largest dimension of the particles may be in the range of from 0.1 cm to 5 cm, more suitably from 0.5 10 cm to 3 cm. Suitably, the biomass may be mixed with a second biomass, which second biomass may be of plant origin, as well. Such second biomass may not comprise microorganisms naturally présent in the biomass. Suitabie second biomass may comprise, for example, filter cakes or other secondary product streams obtained in agriculture and/or food processing, such as remnants from vegetable oil extraction processes, and brewing dregs.

The biomass may comprise the microorganisms within its particles, or on the particles surface. Generally, the microorganisms naturally présent in the biomass comprise microorganisms which are capable of converting saccharides into lactic acid, lactic acid salts, éthanol or other organic acids, salts or alcohols, typically under mesophylic or psychrophylic growth conditions. Examples of such microorganisms may be bacteria, yeasts and moulds, and 20 combinations. Exemplified species may be lactic acid bacteria such as Lactobacillus, Lactococcus and Streptococcus species, yeasts such as Sacchoromyces species and moulds such as Aspergillus species.

The process of maintaining the dispersion at conditions suitabie for digestion may be a batch process, or it may be a continuons process. In a batch process, the process may be carried 25 out in a tank reactor, for example a vessel or an open tub or container. In a continuons process, the process may be carried out in a tank reactor, or in a plug flow reactor in the form of, foi example a tube or a trough. A plurality of reactors may be used, in sériés, or parallel. The biomass mav be dispersed in the aqueous liquid, for example, by adding the aqueous liquid to the biomass. Alternatively, the biomass may be added to the aqueous liquid, or the biomass and the 30 aqueous liquid may be fed simultaneously. Mechanical action, such as gentle stirring, may facilitate forming the slurry. In a continuons process, the process may be carried out by guiding a continuons bed of the biomass through one vessel or through a plurality of vessels comprising the aqueous liquid. Suitably, the weight ratio of the aqueous liquid to the biomass may be in the range of from 1:1 to 50:1, more suitably 2:1 to 20:1, wherein the weight of biomass is taken as 35 the weight of solid matter of the biomass and the weight of aqueous liquid is taken as the total oi the weight of aqueous liquid fed and the weight of moisture présent in the biomass. The weight of solid matter of the biomass and the weight of moisture présent in the biomass are calculated from the moisture content of the biomass.

In n embodiment, the aqueous liquid may be water. The water may be tap water, or it 5 may be distilled water or demineralised water. Water may be selected as the aqueous liquid when the process is carried out for the first time. On the other hand, the water may also comprise at least a portion of water recovered in the process of the invention which has been carried out previously, in which case the aqueous liquid comprises water as a recycle stream. The application of recycle streams improves the process efficiency.

In another embodiment, the aqueous liquid comprises at least a portion of the biomass extract which obtained in the process of the invention carried out previously. In this embodiment, the aqueous liquid comprises biomass extract as a recycle stream, which improves the process efficiency, as will be explained in more detail hereinafter.

Suitable conditions for digestion by the microorganisms may comprise a température of 15 at least 2°C, suitably at least 5°C, more suitably at least 10°C. Typically, the température is at most 65°C, more typically at most 40°C, in particular at most 30°C. The pressure is not essential to the process. Suitably the pressure may be in the range of from 0.08 MPa to 0.2 MPa absolute, more suitably in the range of from 0.09 MPa to 0.15 MPa absolute, preferably in the range of from 0.095 MPa to 0.12 MPa absolute. In particular, the pressure may be atmospheric. Suffîcient 20 oxygen may naturally be présent in the biomass and/or in the aqueous liquid applied in the process supporting aérobic digestion by the microorganisms, and counteracting anaérobie digestion. It may be désirable to gently aerate the slurry. The skilled person will be able to décidé on whether or not apply aération, and on suitable methods and conditions of aération. For example, aération may be effected by stirring using a mechanical stirrer, or by circulating liquid 25 phase pr applying a flow of air or another gas.

The digestion by the microorganisms comprises conversion of mono- and/or disaccharides extracted from the biomass into the liquid phase into lactic acid and/or lactic acids salts. This conversion may resuit in a decrease of the pH of the liquid phase. Also, during the course of the digestion some species of the microorganisms may increase in number, as a resuit 30 of natural growth, and other species may die because of the decrease in pH. As a resuit of natural sélection, the composition of the microorganisms’ population may change during the digestion. In the general practice ofthe invention, the total population may increase in size. Changes in the composition of the liquid phase during digestion may be monitored by measuring pH, refractive index, and/or electrical conductivity or resistivity. Throughout this patent document, pH is 35 defined as pH as measured at 20°C. Digestion may corne to an end by déplétion of digestible components, for example the mono- and/or disaccharides. Digestion of mono- and/or disaccharides may become slow as a resuit of the decrease in pH. Although the digestion may become slow, extraction of components from the biomass into the aqueous liquid may continue. If desired, the rate of digestion may be increased by increasing the pH by adding a suitable 5 quantity of base to the slurry, for example aqueous potassium hydroxide, aqueous sodium hydroxide or ammonia. The slurry may be maintained at conditions suitable for digestion by the microorganisms until the digestion has been corne to an end, or, more suitably, until the rate at which the composition of the liquid phase changes has become low.

In the embodiment of the invention that the aqueous liquid is water, the pH of the water employed may be essentially neutral, such as in the range of from above 6 to 8.5. More frequently the pH is in the range of from 6.5 to 8.5 or even in the range of from 7 to 8. As explained hereinbefore, in the course of digestion, the pH of the liquid phase will decrease. The pH may decrease until it is in the range of from 5.5 to 7, at which point digestible components may be depleted. In the practice of this embodiment, the slurry may be maintained at conditions suitable for digestion by the microorganisms for a time in the range of from 100 hours to 500 hours, more suitably in the range of from 150 hours to 300 hours.

In the embodiment that the aqueous liquid comprises at least a portion of the biomass extract which was obtained previously, the pH of the aqueous liquid employed may generally be at most 6.7, suitably in the range of from 3 to 6, more suitably in the range of from 4 to 6.

In the course of digestion, the pH of the liquid phase may decrease. The pH may decrease until it is in the range of from 3 to 5, more suitably in the range of from 3.2 to 4.5. In the practice of this embodiment, the slurry may be maintained at conditions suitable for digestion by the microorganisms for a time of at least 0.5 hours, more suitable at least 1 hour, in particular in the range of from 0.5 hours to 150 hours, more in particular in the range of from 1 hour to 100 hours.

In general, the time may preferably be in the range of from 10 hours to 150 hours, more suitably in the range of from 20 hours to 100 hours. In a continuous process comprising guiding a continuous bed of the biomass through one vessel or through a plurality of vessels comprising the aqueous liquid, the time may preferably in the range of from 0.5 hours to 20 hours, more preferably in the range of from 1 hour to 10 hours, per vessel, for example 2 hours.

The application of biomass extract as a recycle stream, in accordance with the latter embodiment, has several advantages. The application of recycle stream reduces the quantity of liquid used in the course of multiple batch operations of the process. In the course of such multiple operations, the quantity of microorganisms présent in the subséquent recycle streams increases as a result of natural growth and as a resuit of feeding multiple batches of biomass.

Hence, during subséquent operations of the process an increased rate of digestion may be observed. Also, in the course of multiple operations, there may be an increase in the concentration of digestion products, for example lactic acid and lactates, in the subséquent biomass extracts. Further, in the course of such multiple operations, there may be an increase in the concentration of materials which heave been extracted from the biomass but hâve escaped 5 digestion, for example saccharides, alkali metals salts, alkaline earth salts, peptides, proteins and aminoacids. The concentration of extracted components and digestion products may increase to such an extent that the biomass extract becomes an economically attractive starting material for further processing. The skilled person will appreciate that the same advantages apply in an analogous manner when the process is carried out as a continuons process.

In the batch operation of the process, the biomass extract obtained in a first operation of the process may be used in from 1 to 6 times (inclusive) subséquent operation of the process. Preferably, the biomass extract obtained in the first operation of the process may be used in from 3 to 5 times (inclusive) subséquent operations of the process. Alternatively, a steady State multiple operations may be reached by bleeding a portion of recycled biomass extract, and 15 replacing that portion by water. In an analogues manner a steady State may be reached in a continuous process.

Any method for solid-liquid séparation may be used for recovery of the biomass product and the biomass extract from the slurry, for example sédimentation, filtration or centrifugation. In a typical method, a filter plate or a screen may be employed for this purpose. To this end, the 20 slurry may rest on top of a filter plate positioned near the bottom of a vessel which holds the slurry, of which filter plate the openings are small enough t substantially resist solids of the slurry to pass the filter plate, while biomass extract may pass the filter, in particular under the influence of gravitational force or pumping, leaving the biomass product at the side of the filter plate which is adjacent to the slurry. As an alternative, a screen, in particular an in-line screen, 25 may be employed of which the openings are small enough to substantially resist solids of the slurry to pass the screen and of which screen the openings are small enough to substantially resist solids of the slurry to pass the filter plate, while biomass extract may pass the filter, m particular under the influence of gravitational force or pumping, leaving the biomass product at the side ofthe screen which is adjacent to the slurry. As yet another alternative, the bottom of the 30 vessel may hâve one or more drainage slits or drainage channels of suitable dimensions such that biomass extract may pass the drainage slits or drainage channels and leave the vessel, while solids ofthe slurry substantially remain in the vessel. This process may be facilitated by exerted pressure onto the slurry, for example by pumping, by means of a piston or by pressurizing the atmosphère above the slurry. In general, the pressure exerted onto the slurry will be kept low, for 35 example in the range of from 0.0001 MPa to 0.05 MPa, more suitably in the range of from0.0005

MPa to 0.02 MPa, preferably in the range of from 0.001 MPa to 0.015 MPa. A preferred method of gently exerting pressure onto the slurry is by having floating on top of the slurry a bag containing water, which bag has a size and a shape and is made of a flexible material, for example a plastic or rubber, such that the bag is capable of adapting its shape to the shape of the 5 vessel to completely cover the slurry. If desired, a stack comprising two, three of four bags may be applied. The total height of the one or more bags may typically be in the range of from 0.1 m to 2 m, more typically from 0.2 m to 1.5 m, for example 0.5 m to 1 m. Typically, the vessel is of a cylindrical shape, preferably a circular, elliptic, rectangular or squared cylindrical shape. The vessel may be positioned such that the axis of the cylindrically shaped vessel is in the horizontal 10 or vertical direction.

In the normal practice of the recovery method described in the previous paragraph, it may appear that a portion of the biomass extract tends to remain in the biomass product. In such a case, it may be désirable to remove more of the biomass extract from the biomass product, increasing the solids content of the biomass product. It has been found that this can be achieved 15 effectively and in an energy efficient manner by slowly building up mechanical force onto the biomass product. For example, the biomass product may be charged to a process and the pressure in the press is slowly increased. By slowly increasing the pressure, plugging or clogging within the biomass product may be diminished or prevented, which plugging or clogging would otherwise hinder or prevent biomass extract leaving the biomass product.

independent of whether or not the biomass product has been subjected to the treatment described in the previous paragraph, the biomass product may be washed to further remove biomass extract. The washing may be carried out as a single washing step. It may be preferred to apply a plurality of washing steps, for example, up to 5 (inclusive) washing steps. Any liquid which is miscible with the biomass extract may be suitable as a washing liquid. Water is a very suitable washing liquid. It is preferred to apply a plurality of washing step in a counter current process. In particular, in the counter current process water is applied as the washing liquid in the last washing step. In a continuons process comprising guiding a continuons bed of the biomass through a plurality of vessels comprising the aqueous liquid, the vessels may be fluidly connected to one another to form a sériés arrangement of vessels, an a flow of the aqueous liquid may be maintained countercurrently to the movement of the bed of the biomass. In this case, water may be fed to the first vessel in the sériés arrangement and biomass extract may be withdraw from the last vessel in the arrangement; the number of the vessels in the sériés arrangement may suitable be from 3 to 6 (inclusive).

Associated with each washing step, the effluent may be separated from the biomass 35 product in a manner similar as the recovery of the biomass product and the biomass extract fiom the slurry, as described hereinbefore, including the methods of removing biomass extract from the biomass product, as described hereinbefore. Effluents of the one or more washing steps, in particular the effluent of the first washing of multiple countercurrent washing steps, may be added to the biomass extract. Alternatively, effluents may be applied as the aqueous liquid, or as 5 a portion of the aqueous liquid.

The washing biomass product may be dried to reach a low moisture content. A wide range of dryers may suitably be applied, operating at a high température or at a low température; operating in continuous mode or batch-wise; applying vacuum or operating at overpressure, such as a stream dryer; or with the biomass présent as a stacked bed or as a fluidized bed. Drying may 10 be effected, typically, at a température of at most 600°C, more typically at most 400°C. Preferably, drying may be effected at a température in the range of from 25°C to 600 C, more typically 30°C to 400°C. For energy efficiency, the dryer equipment may comprise the dryer, a heating system for heating the biomass product entering the dryer, a condenser and a heat pump system recycling energy set free in the condenser to the heating system. Condensate water 15 obtained from the drying step may be applied elsewhere in the process, for example as washing liquid or a portion thereof, or as the aqueous liquid or a portion thereof. It has been found that the off-gasses of the drying step are low in badly smelling components and low in dust, so that they can be handled easily and they are relatively environmentally friendly. Thus, alternatively, drying may be accomplished by spreading the biomass product on a field for exposure to outside 20 weather conditions, in particular for sun drying. It is an advantage of this invention that, for example for weeks or even for months, spread-out or piled-up, without showing noticeable signs of rotting or génération of heat. This represents a convenient method of outdooi stoiage, as an attractive alternative to ensilage.

The solids content of the biomass product obtained in accordance with this invention may 25 be at least 25% by weight, typically at least 50% by weight, more typically at least 60% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, in particular at least 90% by weight. In the normal practice of this invention, the moisture content of the biomass product may be at most 99% by weight, more frequently at most 95% by weight. The moisture content of the biomass product may be at most 75% by weight, typically at most 50% 30 by weight, more typically at most 40% by weight, preferably at most 30% by weight, more preferably at most 20% by weight, in particular at most 10% by weight. In the normal practice of this invention, the moisture content of the biomass product may be at least 1% by weight, more frequently at least 5% by weight. Throughout this patent document, moisture content is defined as moisture content in % by weight as measured by using ISO 11722, and the content of solid 35 matter is defined as 100% by weight minus the moisture content.

The biomass product obtained in accordance with the invention has a high content of lignocellulosic materials. The biomass product has a low content of alkali métal, alkaline earth metals, nitrogen, phosphates, sulphate, chloride, proteins and saccharides. Typically the biomass product may hâve a content of potassium of at most 0.1% by weight, more typically ai most 5 0.05% by weight, in particular at most 0.03% by weight, relative to the weight of solid matter. In the normal practice of this invention, the content potassium is frequently at least 0.001% by weight, more frequently at least 0.005% by weight, relative to the weight of solid matter. Typically the biomass may hâve a content of chloride of at most 0.1% by weight, more typically at most 0.05% by weight, in particular at most 0.03% by weight, relative to the weight of solid 10 matter. In the normal practice of this invention, the content of chloride is frequently at least 0.001% by weight, more frequently at least 0.005% by weight, relative to the weight of solid matter.

The biomass product is excellently suited as a fuel or starting material for combustion or conversion processes, such as gasification and pyrolysis, because it has a low ash content, a high 15 ash fusion température, a low tendency to cause corrosion, and a low tendency to cause émissions of nitrogen oxides and sulfur oxides. The biomass product may be employed as such, or together with another fuel or starting material.

It has been found that the biomass product has a high calorie value. For example, on the basis of the weight of mown grass, more heat can be generated by burning the biomass product 20 obtained from the mown grass by using the invention, compared with the heat which can be generated by burning the methane obtained by anaérobie digestion (fermentation) of the mown grass, even including the heat which can additionally be obtained by burning the digestate coproduced in the fermentation.

The biomass product may be used as a fuel, irrespective of its moisture content and 25 irrespective of whether the biomass product has been dried, or not. Namely, water présent in the biomass product may evaporate during the drying process or during the combustion; this is a choice an operator may make. An environmentally friendly option may be outside drying, in particular sun drying, of the biomass product prior to its use as a fuel, as in this case the heat of évaporation of water is supplied by the sun, and will not be at the expense of the heat obtained in 30 the combustion process.

The biomass product obtained in accordance with the invention is also excellently suitable for use as a starting material in an anaérobie digestion for the production of methane gas. The biomass product obtained in accordance with the invention is also excellently suitable for use, for example as a filler material for plastics, concrète or bitumen, or for making board 35 material for building purposes.

The biomass extract obtained in accordance with the invention may hâve a high content of salts, such as alkali métal salts, alkaline earth salts, lactates and chlorides, and lactic acid and neutral organic compounds, such as saccharides. It also has a relatively high content of microorganisms. The biomass extract has a remarkable stability and may be stored for extended 5 periods of time, such as weeks or months. If desired, the biomass extract may be concentrated by removing water. Water may be removed from the biomass extract, for example, by évaporation or by applying reverse osmosis. The water obtained in this manner may be recycled, for example, for use as (a portion) the aqueous liquid, or for use as a washing liquid.

With or without prior removal of water, as described in the previous paragraph, the 10 biomass extract may be subjected to a séparation process, such as membrane filtration. The biomass extract may be separated into, on the one hand, an aqueous effluent comprising salts, such as alkali métal salts, alkaline earth salts, lactates and chlorides, and lactic acid and, other hand, an aqueous concentrate comprising neutral organic compounds, such as mono- and disaccharides, and proteins. The biomass extract obtained in accordance with this invention 15 behaves favourably in membrane séparation techniques, in terms of low membrane fouling and long filtration run times. The aqueous effluent or a portion thereof may advantageously be recycled as a portion of the aqueous liquid. If desired, water may be removed from the aqueous affluent and/or the aqueous concentrate by methods described in the previous paragraph, and, optionally be recycled, for example for use as (a portion of) the aqueous liquid, or for use as a 20 washing liquid.

The invention will be further illustration by means of the following working examples.

EXAMPLE 1 (in accordance with the invention)

A container was provided having inner dimensions of 3m x 2m x 1.7m (length x width x height), a fiat, rectangular, horizontal, concrète bottom and four fiat, rectangular, vertical, 25 concrète walls, and having placed therein a solid wooden plank supported by wooden blocks placed on the bottom at regular distances, the plank having been placed such that an open drainage channel (20 cm width and 20 cm depth) was formed at one side of the bottom in the longitudinal direction of the container. The container was charged with 540 kg of mown grass (37% by weight moisture content, representing 200 kg water) and 940 kg of water (a mixture of 30 rain water and tap water) having pH 6.75, electric conductivity 0.62 mS/cm, refraction 0°Brix and température 9°C. in order to assist in obtaining and maintaining a slurry of the grass in the liquid phase, liquid phase was circulated by pumping at a rate of 5000 kg/h from the bottom of the drainage channel in one corner of the container onto the upper surface of the mixture of grass and water in opposite corner of the container. After 1 hour the pH of the liquid phase had 35 reached 6.6, refraction l°Brix, electric conductivity 4.6 mS/cm, température 9.5°C. (Throughout this patent document, values of pH, refraction and electric conductivity are as measured at 20°C). The circulation of liquid phase was continued.

The next day, this procedure was repeated, yielding a second batch of slurry. After circulating liquid phase for 168 hours the two batches were combined, resulting in the following: 5 a total of about 2960 kg slurry consisting of about 2280 kg of liquid phase and about 680 kg of solid phase, the liquid phase being a slightly foaming, clear, dark brown coloured liquid, and having a fresh sour odour and the following characteristics: electric conductivity 4.8 mS/cm, pH 6.6, refraction l°Brix, température 9°C.

Then circulation of liquid phase was stopped and, instead, liquid phase was pumped into 10 a separate container of similar size. In this manner about 45% by weight of the liquid phase could be separate from the slurry, yielding about 1350 kg of a first lot of biomass extract.

A rubber and nylon-fibre lined bag having dimensions such that the bag fits the horizontal inner dimensions of the container was placed in the container, covering the slurry. Then the bag was filled with tap water and closed. The height of the column of water inside the 15 bag was 50 cm. a second bag of similar dimensions was placed on top of the first bag and filled with tap water until the height of the column of water inside the second bag was 50 cm. additional liquid phase was drained by pumping from beneath the drainage channel. In this manner about 500 kg of a second lot of biomass extract was obtained.

The biomass product remaining in the container was then washed. To this end, the 20 biomass product was re-dispersed in 1000 kg of tap water and the liquid phase so obtained (1000 kg) was drained by pumping from beneath the drainage channel with the two water filled bags still in place. Washing was repeated two times. The washing effluents were combined and stored for use in Example 2, hereinafter.

10-kg samples of the washed biomass product were transferred into a hydraulic juice 25 press filling the cylinder of the press and subjected to pressing. For each pressing, the pressure inside the press was increased in 2 min from 0.1 MPa to 5 MPa, while pressing juice was drained and collected. After pressing the biomass product had a moisture content of 47% by weight. The weight of each of the blocks of biomass product obtained was about 5 kg.

The biomass product blocks where loaded into open containers (1.5m length, 1m width, 30 1m height), each container having a wooden pallet as a bottom and four wire nettings supported by a métal framework as side walls. The wire nettings possessed 10cm x 10cm openings for passing drying air. The open containers had no cover. The open containers filled with biomass product blocks were placed in a drying kiln as a stack of three layers of two by three open containers each. The drying kiln was operated as a condenser dryer at températures of 45-65°C, 35 for 96 hours. After drying, the biomass product had a moisture content of 10% by weight.

The first lot of biomass extract was processed by means of a membrane filter installation provided with commercially available spirally wound polymer nano-filtration membranes and protective filter candies positioned up-stream relative to the membranes. The membrane filter installation was operated at 2.5 Mpa (25 bar) and at a rate of 200 kg/h, producing 70-100 kg/h of 5 permeate and 100-130 kg/h of concentrate. During the operation the concentrate was recycled into the container comprising the biomass extract, and permeate was collected in another, similar container Accordingly, the concentrate became progressively more concentrated in mainly proteins, mono- and disaccharides, and it became more dark brown and cloudy. The permeate, comprising chlorides, nitrates and other salts and lactic acid, had a water clear appearance.

During the nano-filtration process, the température of the concentrate increased slowly from about 10°C to about 40°C, caused by the supply of energy to the pumping equipment of the membrane filter installation. The higher température resulted in 200% more filtration capacity of the process. The membrane filtration appeared to be a very efficient process for the removal of salts and lactic acid from the biomass extract and for concentrating the biomass extract. It is 15 remarkable that the nano-filtration process could proceed with long run times and -with no prefiltering other than by applying the filter candies- without signifïcant fouling of the filter candies and the membranes.

The permeate was concentrated by removing water by means of reverse osmosis applying a pressure of 7 MPa.

The first lot of biomass extract, the concentrate obtained at the end of nano-filtration process, and permeates obtained at the start and at the end of the nano-filtration process were analyzed (see Table I, below; “COD” means Chemical oxygen demand). The results depicted in Table I are indicative for a concentration factor of 15-20 for the concentrate, relative to the biomass extract, and that potassium chloride is présent in the permeate in a high concentration.

Table I

	Electric conductivity (mS/cm)	pH	Density d2o (g/l)	Refraction (°Brix)	KCI content (g/i)	COD (g/D	Dry matter (% by weight)
Biomass extract	6.52	3.83	1.002	1	-	6.5-	-
Concentrate	104	4.4	1.072	17	-	140- 150	19.5
Permeate at start	2.3	3.9	-	0	1.0-1.25	-	-
Permeate at end	11	3.7	-	1	15.0- 20.0	-	-
- : not analysed __________________

EXAMPLE 2 (in accordance with the invention)

Example l was repeated with the différence that the container was charged with 1000 kg of the mown grass and then filled with 2000 kg of washing effluent obtained in Example l, instead of water. After circulating liquid phase for 96 hours, the liquid phase had reached pH 6.0. 5 1800 kg of biomass extract and 1200 kg of wed biomass product were obtained. After pressing the biomass product had a moisture content of 50% by weight and after drying, the biomass product had a moisture content of 10% by weight. The pressing juice obtained from pressing in the hydraulic juice press (about 200 kg) was collected.

EXAMPLE 3 (in accordance with the invention)

Example 2 was repeated with the différence that, after charging with 1000 kg of the mown grass, the container was charged with 1800 kg of biomass extract obtained in Example 2 and 200 kg of the permeate obtained in Example 1, instead of the washing effluent obtained in Example 1. After circulating liquid phase for 48 hours, the liquid phase had reached pH 4.2. 1800 kg of biomass extract and 1200 kg of wed biomass product were obtained. After pressing 15 the biomass product had a moisture content of 50% by weight and after drying, the biomass product had a moisture content of 10% by weight.

EXAMPLE 4(in accordance with the invention)

Example 3 was repeated with the différence that, after charging with 1000 kg of the mown grass, the container was charged with 1800 kg of biomass extract obtained in Example 3, 20 and 200 kg of the permeate obtained in Example 2, instead of the biomass extract obtained in Example 2, and the permeate obtained in Example 1. After circulating liquid phase for 48 hours, the liquid phase had reached pH 3.8. 1800 kg of biomass extract and 1200 kg of wed biomass product were obtained. After pressing the biomass product had a moisture content of 50% by weight and after drying, the biomass product had a moisture content of 10% by weight.

Claims (17)

1. A Process for the conversion of biomass of into a biomass product which is suitable for use as a fuel, wherein the biomass is of plant origin and comprises microorganisms naturally occurring in the biomass, which process comprises

5 - preparing a slurry by dispersing the biomass comprising the naturally occurring microorganisms in an aqueous liquid,

- maintaining the slurry at conditions suitable for aérobic digestion by the microorganisms to obtain a slurry comprising the biomass product as a dispersed solid phase, and

- recovering the biomass product, which recovering comprising washing using water as a 10 washing liquid and drying the biomass product.

2. A Process as claimed in claim 1, wherein the process comprises in addition recovering from the slurry a liquid phase, yielding a biomass product.

3. A Process as claimed in claim 1 or 2, wherein the aqueous liquid is water.

4. A Process as claimed in claim 3, wherein the pH of the water, as measured at 20°C, is in 15 the range of from 6.5 to 8.5, in particular in the range of from 7 to 8, and wherein the slurry is maintained at conditions suitable for digestion by the microorganisms for a time in the range of from 100 hours to 500 hours, in particular in the range of from 150 hours to 300 hours.

5. A Process as claimed in claim 1 or 2, wherein the aqueous liquid comprises at least a portion of the biomass extract which was obtained in the process of claim 2.

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6. A Process as claimed in claim 5, wherein the pH of the aqueous liquid, as measured at 20°C, is in the range of from 3 to 6, in particular in the range of from 4 to 6, and wherein the slurry is maintained at conditions suitable for digestion by the microorganisms for a time in the range of from 0.5 hours to 150 hours, in particular in the range of from 1 hours to 100 hours.

7. A Process as claimed in any of claims 1-6, wherein the biomass comprises forestry waste 25 or agricultural waste, in particular mown grass.

8. A Process as claimed in any of claims 1-7, wherein the naturally occurring microorganisms comprise microorganisms which are capable of converting saccharides into lactic acid or lactic acid salts.

9. A Process as claimed in any of claims 1-8, wherein the weight ratio of the aqueous liquid 30 to the biomass is in the range of from 1:1 to 50:0, in particular in the range of from 2:1 to 20:1, and wherein the conditions suitable for digestion by the microorganisms comprise a température is in the range of from 5°C to 40°C, in particular in the range of from 10°C to 30°C.

10. A Process as claimed in any of claims 1-9, wherein recovering of the biomass product from the slurry comprises employing a filter plate or a screen, while exerting a pressure onto the 35 slurry, which pressure is in the range of from 0.0005 MPa to 0.02 MPa.

11. A Process as claimed in any of claims 1-10, wherein recovering the biomass product from the slurry comprises a plurality of washing steps in a counter current process, and wherein water is applied as the washing liquid in the last washing step.

12. A Process as claimed in any of claims 1-11, wherein the recovering comprises drying to 5 achieve moisture content of the biomass product of at most 20% by weight, in particular at most 10% by weight.

13. A combustion process, which combustion process comprises the steps of

- preparing a slurry by dispersing a biomass in an aqueous liquid, wherein the biomass is of plant origin and comprises microorganisms naturally occurring in the biomass,

10 - maintaining the slurry at conditions suitable for aérobic digestion by the microorganisms to obtain a slurry comprising a biomass product as a dispersed solid phase,

- recovering the biomass product from the slurry, which recovering comprises washing using water as a washing liquid, and

- combusting the biomass product.

15

14. A combustion process as claimed in claim 13, wherein the combustion process comprises drying the biomass product recovered from the slurry by exposing the biomass product to outside weather conditions.

15. A Process for the conversion of biomass of into a biomass product which is suitable for use as a fuel, wherein the biomass is of plant origin and comprises microorganisms naturally 20 occurring in the biomass, which process comprises

- preparing a slurry by dispersing the biomass comprising the naturally occurring microorganisms in an aqueous liquid,

- maintaining the slurry at conditions suitable for aérobic digestion by the microorganisms to obtain a slurry comprising the biomass product as a dispersed solid phase,

25 - recovering the biomass product,

- recovering the slurry a liquid phase, yielding a biomass extract, and

- separating the biomass extract into an aqueous effluent comprising salts, and an aqueous concentrate comprising neutral organic compounds.

16. A method of use of a biomass product as a raw material in a process for making a board 30 material for building purposes, wherein the biomass product is obtained in a process as claimed in any of claims 1-12.

17. A board material for building purposes, which board material, is obtainable by the method of claim 16.