The invention relates to a method for thermal digestion of pumpable biomass, comprising the steps of supplying fresh biomass, preheating the supplied fresh biomass, hydrolyzing the preheated biomass, cooling the hydrolyzed biomass and discharging the cooled biomass. Such a method is already known in different variants.

"Pumpable biomass" is understood in this application to mean particularly sewage sludge, a paste -like material, although other biodegradable materials of similar consistency and viscosity can also be envisaged.

Applicant already markets under the name TurboTec ^® an installation in which biomass, particularly sludge from the purification of waste water, can be hydrolyzed in a continuous process. The installation consists of a reactor, a steam supply, a mixing/separating unit and a number of heat exchangers. Fresh biomass, which can for instance come from a mechanical pre-concentration, is supplied and brought into contact with a partially cooled hydrolyzed flow. The temperature of the fresh biomass is hereby increased by several tens of degrees in one operation. The heated fresh biomass is then pumped through a heat exchanger. In this heat exchanger the supplied biomass is preheated to a temperature in the order of 100° Celsius, this being the temperature at which the preheated biomass enters the reactor. In the reactor the temperature is increased to more than 100° Celsius by supplying steam, while a high pressure is effected by pumps and restrictions such that the biomass in the reactor does not boil. Cell structures in the biomass which are difficult to break down are "cracked" at the high temperature and pressure, and more easily degradable components are released. In the TurboTec ^® process this "cracking" takes place at a pressure in the order of 2-8 bar and a temperature in the order of 120-170° Celsius. When the hydrolyzed biomass leaves the reactor at this high temperature, it is guided through a heat exchanger so as to be cooled before the cooled hydrolyzed biomass is guided via the mixing/separating unit to a fermenting installation. The heat extracted from the hydrolyzed biomass during cooling can be used here to preheat the fresh biomass.

Known from WO 03/043939 A2 are a method and an installation for treating

biodegradable organic waste supplied in particulate form. This is a batch-type process in which biodegradable domestic refuse, particularly kitchen and garden waste, is first reduced to parts of a size in the order of 6 to 50 mm. The waste is then preheated by a liquid from a hydrolysis reactor. This heating can take place by mixing or by contactless heat exchange. In the case of mixing the mixture of the waste and the liquid is then separated, after which the solid waste is carried to a steam chamber where it is preheated with steam. From here the waste is carried to the reactor and hydrolyzed at a temperature of up to 130-170°C and a pressure of 300 kPa to 2.5 mPa (3-25 bar). The mass is then carried from the hydrolysis reactor to a flash tank, where steam is drawn off for reuse in the steam chamber. The hydrolyzed mass is then separated in a separator into a fraction which is composted and the liquid fraction for preheating the supplied waste. Finally, after mixing and separating the liquid is further treated in an anaerobic reactor to form methane gas, purified effluent and solids. As stated, this is a non-continuous or batch-type process.

A general drawback of the known processes is that the technology is deemed commercially to be relatively complex. In addition, traditional fermentation in digestion tanks produces only biogas, which is a relatively low-grade residual product.

The invention now has for its object to improve a method of the above described type such that the complexity of the system is reduced and that high-grade components are recovered as raw material from the hydrolyzed biomass. This is achieved according to a first aspect of the invention in that the supplied biomass is preheated and the hydrolyzed biomass is cooled by mixing at least a part of the supplied biomass with at least a part of the hydrolyzed biomass immediately after this biomass has been hydrolyzed.

Mixing the supplied biomass with (a part of) the hydrolyzed biomass results in a very direct heat transfer which cannot be achieved in a heat exchanger. The installation to be used can moreover take a much simpler form since expensive heat exchangers are not necessary. The hydrolyzed biomass is not processed or cooled before it is mixed with the supplied biomass, so that it still contains a large amount of residual heat. As a result of the mixing the supplied fresh biomass is preheated to an extent sufficient to limit as far as possible the steam supply necessary in or upstream of the reactor vessel. Another advantage is that the viscosity of the supplied fresh biomass is decreased after mixing such that it can be pumped more easily, thereby reducing the required pump capacity of the installation.

In order to preheat the supplied fresh biomass as quickly and thoroughly as possible, at least 25 percent, more preferably at least 75 percent and most preferably substantially 100 percent of the hydrolyzed biomass can be mixed with the supplied fresh biomass.

According to another aspect of the invention, the hydrolyzed biomass is separated at high temperature into a solid fraction and a liquid fraction, in particular by centrifugation. The final dewatering is thus included in the TurboTec ^® process and the liquid fraction is separated from the solid fraction at an early stage of the process, immediately after the hydrolysis. This solid fraction can be further dried and is optionally usable as fertilizer or fuel. By separating the hydrolyzed biomass at high temperature the use of flocculants can further be wholly avoided or largely limited, this resulting in a large reduction in cost.

Chemicals and/or nutrients are then preferably recovered from the liquid fraction. The liquid fraction can thus be converted to high-grade residual products. Biogas can in addition or instead be produced from the liquid fraction by means of an anaerobic treatment with retention of solids (anaerobic biomass). Use can for instance be made for this purpose of an Upflow Anaerobic Sludge Bed (UASB) reactor or an Anaerobic Membrane Bio Reactor (AnMBR), wherein sludge is retained.

In an advantageous variant of this method at least a part of the liquid fraction is mixed with the supplied biomass. The liquid fraction can thus be cooled before being further recycled, and the residual heat present therein can be usefully applied for preheating the supplied fresh biomass.

In order to achieve the desired hydrolysis temperature the preheated biomass is preferably further heated by mixing steam therewith prior to or during the hydrolysis.

The supplied fresh biomass can have a viscosity which is one or more orders greater than the viscosity of the hydrolyzed biomass and which can for instance amount to several thousand or tens of thousands of mPaDs, while the viscosity of the hydrolyzed biomass amounts to several hundred mPaDs. The mass is however in both cases highly uniform, without discrete particles.

The high viscosity of the supplied fresh biomass can be achieved in simple manner when prior to the mixing a flocculant is added to the fresh biomass and water is extracted therefrom. A polyelectrolyte can be used as flocculant.

The mixture formed during the preheating is preferably substantially

separated/concentrated again to form preheated, fresh biomass and partially cooled, hydrolyzed biomass. The hydrolyzed biomass which has been sufficiently cooled by the mixing can thus be further processed properly.

During separation a part of the hydrolyzed biomass is preferably entrained by the preheated fresh biomass. Relatively large flakes or agglomerates in the already hydrolyzed biomass which have been cracked insufficiently can thus be subjected once again to a hydrolysis process. Substantially wholly hydrolyzed biomass is hereby discharged to the fermentation installation, so increasing the efficiency of the fermentation compared to conventional methods.

The fresh biomass and the hydrolyzed biomass are preferably mixed such that the fresh biomass is heated by several tens of degrees. A considerable rise in temperature of for instance 40- 80°C is thus already obtained, whereby the desired entry temperature in the hydrolysis reactor, in the order of 60-100°C, can be achieved with relatively little effort.

The biomass mixture is preferably separated by being screened. Because the fresh biomass will have a considerably higher viscosity (in otherwise similar conditions) than the hydrolyzed biomass, a highly effective separation can be achieved in simple manner by screening, for instance with a vibrating screen or a rotating screen. Other separating techniques such as filtering, centrifugation or cyclone separation can however also be envisaged.

Following the step of mixing and optional separation the hydrolyzed biomass can be further cooled so as to be brought to a temperature suitable for further processing.

The hydrolyzed biomass can be further cooled by mixing a cooling medium, in particular cold water, therewith. In order to enable further processing of the hydrolyzed biomass in relatively dry form it is preferably de watered, in particular by centrifugation at high temperature, prior to the further cooling.

The invention further relates to an installation for thermal digestion of pumpable biomass. A conventional thermal digestion installation for pumpable biomass, for instance applicant's own TurboTec ^® , comprises means for supplying fresh biomass, means connected to the supply means for preheating the fresh biomass, a reactor connected to the preheating means for hydrolyzing the preheated biomass, means connected to a discharge side of the reactor for cooling the hydrolyzed biomass and means connected to the cooling means for discharging the cooled biomass.

The installation according to a first aspect of the present invention is now distinguished from this conventional installation in that the preheating means and the cooling means are combined with each other and comprise a mixing device connected to the supply means and to the discharge side of the reactor for the purpose of mixing the supplied fresh biomass and the hydrolyzed biomass.

According to another aspect of the invention, the installation is characterized by means connected to the discharge side of the reactor for separating the hydrolyzed biomass into a solid fraction and a liquid fraction at high temperature.

Preferred embodiments of the thermal digestion installation according to the invention are described in the dependent claims 18 and 20-29.

The invention will now be elucidated on the basis of a number of embodiments, wherein reference is made to the accompanying drawing in which corresponding components are designated with reference numerals increased in each case by 100, and in which:

Figure 1 is a schematic representation of an installation according to a first embodiment of the invention,

Figure 2 is a view corresponding to figure 1 of an alternative embodiment, wherein a considerably larger buffer on the outlet side provides for a longer retention time of the hydrolyzed biomass after the mixing and separating,

Figure 3 is a view corresponding to figures 1 and 2 of an embodiment wherein the hydrolyzed biomass is dewatered at high temperature after the mixing and separating, and

Figure 4 is a view corresponding to the foregoing figures of yet another embodiment wherein the hydrolyzed biomass is dewatered at high temperature immediately following the hydrolysis.

An installation 1 for thermal digestion of pumpable biomass, such as sewage sludge, comprises means 2 for supplying fresh biomass FB, means 3 connected to supply means 2 for preheating the fresh biomass FB and a reactor 4 connected to preheating means 3 for hydrolyzing the preheated biomass PHB. Connected to reactor 4 is a steam supply 5. The installation further comprises means 6 for cooling the hydrolyzed biomass HYB which are connected to a discharge side 7 of reactor 4, and means 8 for discharging the cooled biomass CB which are connected to cooling means 6.

According to the invention preheating means 3 and (a part of) cooling means 6 are combined with each other in the form of a mixing device 9 for mixing the supplied fresh biomass

FB and the hydrolyzed biomass HYB. This mixing device 9 is connected on one side to supply means 2 and connected on the other to discharge side 7 of reactor 4.

Mixing device 9 thus serves on the one hand to bring the supplied fresh biomass to a suitable entry temperature for reactor 4, and serves on the other to cool the hydrolyzed biomass to a temperature at which it can be further processed, or at least a temperature not too far thereabove.

Installation 1 according to the invention also has a device 10 for separating the biomass mixture M formed in mixing device 9. This separating device 10 can comprise one or more screens, for instance vibrating screens or rotating screens.

In the shown embodiment a buffer 11 is also placed between separating device 10 and reactor 4, whereby differences in the supply speed of the fresh biomass and the speed at which the biomass is hydrolyzed can be compensated.

Cooling means 6 comprise two separate stages in the shown embodiment. The first and most important stage is formed by mixing device 9. Another part of cooling means 6 is located between separating device 10 and discharge means 8 for the cooled biomass CB and is configured to mix a cooling medium with the cooled biomass CB. This part of cooling means 6, which forms a further cooling stage, comprises a cold water conduit 13 in the shown embodiment. Another buffer

14 is here also placed between separating means 10 and cold water conduit 13 of the further cooling stage.

The operation of the above described thermal digestion installation 1 is now described on the basis of a numerical example.

It is assumed here is that supply means 2 supply a quantity Q ₀ of fresh biomass FB having a starting temperature T ₀ of 10-30° Celsius. This fresh biomass FB will normally have a dry substance content (DS) of 5-20 percent. The fresh biomass FB has already been pre-screened upstream of supply means 2, whereby all parts larger than a determined size, here 2 mm, have already been removed. A flocculant, for instance a polyelectrolyte, has also already been added to the pre-screened fresh biomass, and water extracted, whereby the fresh biomass FB has a uniform, paste-like consistency.

The fresh biomass is mixed in mixing device 9 with hydrolyzed biomass HYB which, following hydrolysis in reactor 4, is been processed prior to the mixing. The fresh biomass and the hydrolyzed biomass HYB have greatly differing viscosities prior to mixing. The viscosity of the hydrolyzed biomass HYB lies in the order of magnitude of 100 to several hundred mPaDs. The viscosity of the supplied fresh biomass can on the other hand amount to several thousand or even tens of thousands or hundreds of thousands of mPaDs. The mixture of these biomass flows of such widely divergent viscosity forms an emulsion.

In the shown embodiment the whole mass flow Q ₃ of hydrolyzed biomass HYB is fed to mixing device 9. It is however also possible to mix only a part of the hydrolyzed biomass HYB with the fresh biomass, wherein the positive effects of the invention are particularly manifest when 25 percent or more of the hydrolyzed biomass HYB is admixed. "A part" is here otherwise not understood to mean a specific fraction (for instance the liquid fraction) from the hydrolyzed biomass HYB, but only a part of the overall mass flow. The mass flow Q ₃ is greater than the supplied quantity of fresh biomass Q ₀ because a part of hydrolyzed biomass HYB is recirculated in reactor 4, and a determined quantity of steam Q _st is in addition continuously supplied. The increase in the temperature of the supplied biomass will hereby be greater than the decrease in the temperature of the hydrolyzed biomass. In other words: the temperature of the mixture M which is formed in mixing device 9 will be higher than the average of the temperatures of the supplied biomass and of the hydrolyzed biomass. The numerical example assumes that the fresh biomass FB has a temperature of 20° Celsius and is supplied in a mass flow Q ₀ = 8 m ³ /h. Due to the addition of steam the volume flow of the hydrolyzed biomass HYB after leaving reactor 4 is greater than the volume flow of the fresh biomass: Q ₃ = Qi + Q _st > Qo- Use is made in this example of a volume flow Q ₃ = 9 m ³ /h having a temperature T ₃ of about 140° Celsius, whereby the mixture M with a volume flow Q _M = 17 m ³ /h finally reaches a temperature T _M of 83.5° Celsius. A considerable rise in the temperature of the supplied fresh biomass FB is thus achieved. The volume flows of the fresh biomass and the steam supply are adapted to each other in practice such that the mixture M will reach a temperature T _M of 75-90° Celsius.

The mixture M is supplied in a quantity Q _M to separating device 10, where the supplied fresh biomass is again separated from the hydrolyzed biomass by screening of the emulsion. Surprisingly, it has been found here that it is indeed possible in principle to completely separate the emulsion again into the starting components, the supplied very high-viscosity fresh biomass - in fact a paste -like material - and the very low-viscosity hydrolyzed biomass - in fact a thin liquid. The viscosities of the part-flows leaving separating device 10 once again differ greatly from each other. When leaving separating device 10, the viscosity of the preheated biomass PHB can be at least twice as high as the viscosity of the hydrolyzed biomass.

In the shown embodiment separating device 10 is configured such that, along with the fresh biomass, a part of the biomass which comes from reactor 4 but which has not yet been fully hydrolyzed is also separated from the fully hydrolyzed biomass. The biomass which is not fully hydrolyzed will comprise larger flakes or agglomerates than the fully hydrolyzed biomass, while the flakes or agglomerates in the fresh biomass FB will be even larger. In the shown embodiment a quantity Qj of the mixture M is in this way separated in the form of preheated fresh biomass - with a small fraction of incompletely hydrolyzed biomass therein - from a mass flow Q ₂ consisting substantially of fully hydrolyzed biomass. This latter flow Q ₂ is carried via buffer 14 to cold water conduit 13 of the further cooling means and there cooled to a temperature T ₄ in the order of 40-60° Celsius by being mixed with cooling water of for instance 20° Celsius.

In order to keep the conditions in reactor 4 substantially constant the mass flow Qj will have to be roughly constant. In the shown embodiment it is the case that the flow of preheated biomass PHB is roughly equal to the flow of fresh biomass FB: Qj _ Q ₀ . The additional mass flow resulting from the steam injection is thus entrained here in the outgoing mass flow during separation: Q ₂ ~ Qo + Qs _t -

The flow of preheated biomass PHB and the fraction of incompletely hydrolyzed biomass entrained therein is guided via buffer 11 to reactor 4. A quantity Q _st of steam is mixed with the flow of biomass in this reactor 4 or shortly upstream thereof. The biomass is heated in reactor 4 by this admixture of the steam to a temperature T _reactor of 120-170° Celsius, in this example about 140° Celsius. At these temperatures a pressure of 2-8 bar, in the shown embodiment a pressure in the order of 4 bar, is maintained in reactor 4. As a result of the increased temperature and pressure the cell walls of the bacteria in the biomass are broken open so that the degradable component of the biomass enclosed therein is released. More biogas can hereby be produced in a later fermenting step, while the decomposition of the dry substance is also improved.

In an alternative embodiment of the thermal digestion installation 101 (fig. 2) the buffer 114 in the discharge branch takes a considerably larger form than in the first embodiment. The retention time of the hydrolyzed biomass in buffer 114 is hereby considerably longer than in the first embodiment. This is important because the hydrolyzed biomass hereby has sufficient time after being mixed and separated again to comply with the temperature requirements laid down in international, and particularly American, standards for so-called "EPA Class A biosolids". Biomass which complies with this EPA standard can be used without further precautionary measures, for instance as fertilizer, and is thereby considerably more valuable than biomass which does not comply with the standard.

In yet another embodiment of installation 201 (fig. 3) the hydrolyzed biomass is first dewatered at high temperature after leaving buffer 214, before being further cooled by admixing cold water from conduit 213. The moisture content of the hydrolyzed biomass hereby decreases, whereby the yield of the subsequent processes will be higher. A liquid fraction L, which can be converted into relatively high-grade residual products by suitable further processing, is moreover thus recovered from the hydrolyzed biomass. Placed for this purpose between buffer 214 and further cooling stage 213 is a high-temperature centrifuge 220 in which the hydrolyzed biomass is separated at relatively high temperature of 60°-100°C into a solid fraction S and a liquid fraction L. In order to control the temperature at the location of the separation it is possible to include line TQ, through which a part of the hydrolyzed biomass HYB with a high temperature (in the order of 140°C) can be fed from the reactor to separating device 220. A part of the cold water can also be used herefor, for which purpose a second temperature control line TC ₂ can be branched from cold water conduit 213.

The solid fraction S is carried by discharge means 208 to a further processing unit, for instance a dryer.

The liquid fraction L is carried to the further cooling stage 213 and there cooled by admixing of cold water before being converted to high-grade products in a recycling unit 221.

Recycling unit 221 can for instance be an anaerobic reactor with which the liquid fraction can be converted to biogas BG. Use can for instance be made for this purpose of an Upflow Anaerobic Sludge Bed (UASB) reactor or an Anaerobic Membrane Bio Reactor (AnMBR), wherein sludge is retained. Other recycling techniques can however also be envisaged. Chemicals and nutrients C/N can thus be recovered from the liquid fraction, while the liquid fraction also comprises components which could be used as raw material for 'bioplastics' .

In a further embodiment (fig. 4) the hydrolyzed biomass HYB is separated into a liquid fraction L and a solid fraction S immediately after leaving reactor 4, when it still has a very high temperature in the order of 100°-160° (depending on the use of a cooling exchanger or admixing of cold water). Use is again made for this purpose of a high-temperature centrifuge 320. The solid fraction S can be carried directly to a dryer here by discharge means 308, while the liquid fraction L is carried to mixing device 309 and there mixed with the supplied fresh biomass FB. The biomass still to be hydrolyzed PHB, which has been considerably preheated by the mixing, is subsequently separated in separating device 310 from the liquid fraction L of the hydrolyzed biomass. Use can optionally also be made of a (second) centrifuge for this separation. This liquid fraction is again carried via buffer 314 to further cooling stage 313, where cold water is admixed. The cooled liquid fraction CB-L of the biomass can then be recycled in a recycling unit 321, for instance by extracting chemicals and nutrients C/N therefrom. The liquid fraction CB-L can be treated in a UASB-reactor or AnMBR, or be processed into raw material for bioplastics.

The invention thus makes it possible, making use of a relatively simple installation without heat exchangers, for a flow of supplied fresh biomass to nevertheless undergo a relatively great temperature increase.

Although the invention has been elucidated above on the basis of a number of

embodiments, it will be apparent that it is not limited thereto but can be varied in many ways. Situations can thus be envisaged in which it is possible to dispense with a separation after the mixing. Other options can also be envisaged for heating of the reactor than the supply of steam, for instance by making use of a heating spiral in which thermal oil circulates.

The scope of the invention is therefore defined solely by the following claims.