WO2013163998A1

WO2013163998A1 - Method for continuous treatment of biological material

Info

Publication number: WO2013163998A1
Application number: PCT/DK2013/000032
Authority: WO
Inventors: Peder FOSBØL; Troels HILSTRØM
Original assignee: Haarslev Industries A/S
Priority date: 2012-05-03
Filing date: 2013-05-03
Publication date: 2013-11-07

Abstract

The present invention relates to a method for continuous treatment of biological material at an elevated pressure before anaerobic digestion, said method comprising at least the steps of feeding the biological material to an infeed device serving as a sluice system and increasing the pressure in the infeed device; and transferring the biological material from the infeed device to a reactor by means of pressure differences or gravity; and optionally further increasing the pressure in the reactor.

Description

METHOD FOR CONTINUOUS TREATMENT OF BIOLOGICAL MATERIAL

The present invention relates to a method for continuous treat- ment of biological material at an elevated pressure before anaerobic digestion.

Thermal hydrolysis has several advantages in connection with anaerobic digestion. Some of the advantages are: the biological material is hygienized/sterilized, the viscosity is lowered for easy mixing, and the yield of biogas is increased.

Apparatus for the thermal hydrolysis process by both continuous and batch processes has been described.

International patent application WO 2011006854 Al discloses a method and device for thermal hydrolysis and steam explosion of biomass. The disclosed method is a batch process. The material is transferred from a pulper to a reactor at ambient pressure by a pump. Then the reactor is closed and heated.

International patent application WO 2009121873 Al discloses an apparatus and a method for continuous thermal hydrolysis of biological material, and international patent application WO 07/3221 Al discloses a method and an arrangement for continuous hydrolysis of organic material. Both patent applications describe a continuous process for the treatment of sludge at an elevated temperature. In both methods, pumps are used for transfering the material from the ambient pressure to the elevated pressure inside the reactor. An object of the present invention is to provide a method by which biological material may be prepared for the manufacture of biogas in an uncomplicated and cost-effective manner. Thus, the present invention relates to a method for continuous treatment of biological material at an elevated pressure before anaerobic digestion, said method comprising at least the steps of: feeding the biological material to an infeed device serving as a sluice system and increasing the pressure in the infeed device; and transferring the biological material from the infeed device to a reactor by means of pressure differences or gravity; and optionally further increasing the pressure in the reactor.

Biological material in respect of the present application is one of the following materials or combinations of them.

Agricultural products: grass, switch grass, rye grass, miscanthus, sugar beet pulp, sugarcane bagasse, rice straw, rice hulls, barley straw, wheat straw, com cobs, corns stover, empty fruit bunches, agricultural waste/byproducts.

Forest products: bark, wood chips, paper sludge, sawdust, hardwood, softwood, forest waste/byproducts. Waste: organic household waste, park and garden waste.

Biological industrial waste/byproducts: food and feed processing, breweries and distilleries, greenhouse. Animal products: meat waste/byproducts from slaughterhouses, dead animals.

Biological sludge: wastewater treatment, industrial wastewater treatment, sediments.

Farm waste/byproducts: Cattle manure, pig manure, deep bedding. Effluent from anaerobic digestion of one or more of the above mentioned biological materials.

According to the method, an infeed device functioning as a sluice system is used. None of the known continouos systems for ther- mal hydrolysis of biological material for anaerobic digestion disclose use of a sluice system to transfer the biological material from the ambient pressure to the elevated pressure inside the treatment reactor. Consequently, the biological material is fed to the infeed device, and valves at both ends of the infeed device are closed, and the material is subjected to steam injection which will increase the pressure to a pressure which corresponds to or is slightly above the pressure in the reactor. When the elevated pressure has been reached, the valve which is positioned between the infeed device and the reactor is opened. The material in the infeed device will then be transferred to the reactor due to gravity and/or pressure difference. The possibility of transferring the biological material by means of gravity and/or pressure difference is unexpected, as it is normally considered necessary to use pumps or other mechanical means to transfer the biological material into a pressurized reactor or system.

As it may be desirable to remove water/steam and other volatile material from the biological material which has been treated in the reactor, the method may comprise the further step of treating the biological material in an economizer or a flash vessel.

For the purpose of obtaining a method which is highly cost- effective at least a portion of the flash steam in the economizer or flash vessel is re-circulated and reused in the infeed device.

The biological material may be transferred from the reactor to the economizer or flash vessel by means of differential pressure. In such an embodiment, the use of pumps and other mechanical means, such as scrapers, screws or propellers may be avoided which also contributes to making the method uncomplicated and cost-effective. In an embodiment of the method, the biological material is transferred from the infeed device to the reactor via a transfer pipe. This makes it possible to heat the biological material and optionally increase the pressure before the material is fed to the reactor. The temperature of the biological material and the pressure may be increased by subjecting the material to steam injection in the transfer pipe.

To obtain the best possible properties of the treated biological material, the reactor is operated at an elevated pressure in the range of 2 to 25 barg, preferably in the range of 3 to 12 barg. Moreover, the reactor is operated at a temperature in the range of 110 to 240 °C, preferably at a temperature in the range of 130 to 190 °C. The specific conditions in respect of pressure and temperature in the reactor depend on the nature of the biological material and may be optimized by the skilled person operating the reactor.

To optimize the properties of the treated biological material, e.g. in respect of content of dry material, the method according to the invention comprises an embodiment in which the biological material is transported through a series of vessels by means of differential pressure. The vessels may e.g. be economizers or flash vessels which may serve to remove water/steam and other volatile components from the biological material.

Moreover, the method may comprise an embodiment in which the biological material is subjected to vacuum in the final vessel. Treatment by vacuum is a very efficient way of removing volatile material such as water and gases, and also of lowering the temperature.

In a preferred embodiment of the method according to the invention, the transport of biological material inside the reactor is mainly obtained by means of gravity. By using substantially gravity for the transport inside the reactor, e.g. combined with pressure, a very economical method is achieved.

In an alternative embodiment of the method according to the invention, the transport of biological material inside the reactor is mainly effected by mechanical means, such as screws, scrapers, propellers or pumps. The embodiment is particularly suitable for biological material with a high content of dry material, i.e. material with a relatively high viscosity. The embodiment is also suitable for biological material that appears sticky.

For the purpose of optimizing the method, it may be desirable to use two or more economizers or flash vessels in parallel.

The treated biological material is highly suitable for use in the production of biogas.

The invention will now be described in further detail with reference to drawings, in which : Fig. 1 shows an embodiment of the invention; and

Fig. 2 shows an alternative embodiment of the invention.

For reasons of simplicity, the same reference numbers are used for the same parts in the figures.

The biological material is transferred to the thermal hydrolysis reactor 1 via an infeed device 2. The reactor is at an elevated temperature and/or pressure.

To transfer the biological material from the ambient pressure to the reactor at an elevated pressure, the infeed device in the form of a sluice may be used. The sluice operates by means of differential pressure or gravity to move the biological material from the ambient pressure to the elevated pressure inside the reactor.

In figure 1, the sluice is a vessel 2 with an inlet valve 8 and an outlet valve 7 and a steam/gas inlet valve 14.

The infeed device 2 with the sluice system has the following advantages: The biological material does not need to be pumpable. Therefore, there is no limit to the dry material content of the biological material. By increasing the dry material, the energy for heating (if needed) is significantly reduced. No high pressure pump is required.

The reactor may be operated in a mode close to constant pressure and may be operated continuously. This significantly increases the capacity of the high pressure reactor compared to a batch system having the same reactor volume because filling, emptying and heating the reactor are done continuously.

The pressure cycles for the reactor are significantly reduced compared to a batch process. This will reduce the construction and maintenance cost of the reactor

The continuous process is also an advantage for the dimensions of the steam generator (if steam is used in the process), because the steam consumption is at a more constant level. Heat and mass integration with other processes is also simplified in a continuously operated system.

In the embodiment in figure 1, the inlet valve 8 is at the top of the sluice 2, and the outlet valve 7 is at the bottom of the sluice 2.

Preferably at least one of the inlet valve 8 or outlet valve 7 is closed at all times to form a pressure tight seal from the reactor 1 to the surrounding environment.

The biological material is fed to the sluice vessel 2 through the open inlet valve 8. As an example, feeding of material may be effected by a pump or a screw conveyor. The inlet valve 8 is sub- sequently closed, and the steam/gas valve 15 is opened to increase the pressure. The pressure in the sluice vessel 2 is increased to a pressure equal to or higher than the pressure inside the reactor 1. Then the outlet valve 7 is opened, and the material inside the sluice vessel 2 is transferred to the reactor 1 by the pressure difference and/or gravity. The outlet valve 7 is then closed, and the inlet valve 8 is opened again to feed a new batch of biological material.

The sluice vessel 2 is equipped with a pressure reduction valve 13. The pressure reduction valve 13 is opened to reduce the pressure inside the sluice vessel 2 before the inlet valve 8 is opened.

This will reduce the amount of steam escaping into the transport system for the biological material. The steam may be used to pre- heat the biological material before feeding into the sluice. In the embodiment shown in figure 1, there is an economizer 4 connected to the reactor 1 operating at a lower pressure than the reactor 1 and with an elevated pressure compared to the sur- roundings.

The economizer 4 is connected to the sluice 2 by a pipe 11 and a valve 14. If installed the valve 14 is opened after the biological material is added to the sluice vessel 2 and the valve 8 is closed, but before the steam/gas valve 15 is opened. After a period of time when the pressure in the two vessels 2 and 4 is approximately the same, the valve 14 is closed again and the steam valve 15 is opened and the cycle continues as described previously.

The reuse of steam from the flash vessel 4 reduces the amount of high pressure steam required for the process.

In figure 2, the outlet valve 7 is connected to the reactor 1 via a pipe 30. This makes it possible to place the sluice 2 on the side of the reactor to reduce the building height.

In figure 2, steam is added through a steam valve 31 in the pipe 30. The addition of steam in the pipe 30 will improve the head transfer to the biological material compared to heading the material in the reactor.

Moreover, a chemical dosing valve 38 is mounted on the pipe 30. The valve 38 may be used for optional addition of chemicals. In the embodiments shown in the figures 1 and 2, the biological material is added to the highest point of the reactor 1. This will facilitate plug flow in the reactor, and therefore the retention time may be controlled.

The reactor 1 is oriented in such a manner that the material is transported through the reactor 1 substantially by means of gravity. The flash valve 3 is placed at the lowest or close to the lowest point of the reactor. No mechanical parts are used in this configuration, which reduces the cost of equipment and for maintenance.

In an alternative embodiment, the transport through the reactor 1 is effected by means of a mechanical transport or mechanical transportation in combination with gravitation transport. The mechanical transportation device may e.g. be a screw or a paddle mixer.

The alternative configuration may be used where some mixing is needed inside the reactor, or where height limitations prevent the construction of a gravity reactor, or if the biological material is sticky or has a high viscosity.

In figure 2, an air relief valve 23 is installed at the highest point or close to the highest point of the reactor 1. This valve 23 will make it possible to remove non-condensable gases inside the reactor. This will reduce the pressure needed to gain a given temperature if steam is used for heating, because only steam is present inside the reactor. One or more steam valves 5 may be added on the reactor 1 to obtain better temperature and pressure control.

There are several options to control the flash valve 3.

The flash valve 3 may be connected to the economizer through a pipe 9.

The flash valve 3 may open and close in intervals.

The flash valve 3 may be regulated.

The above-mentioned options may be used alone or in combinations.

The economizer 4 is equipped with an outlet valve 17 at the lowest point or close to the lowest point.

In one embodiment of the method, the economizer 4 is operated at ambient pressure or close to ambient pressure.

In figure 2, the flash vessel 32 is connected to the economizer outlet valve 17. The vessel 32 operates at a lower pressure than the economizer 4, and the vessel 32 has a steam/gas pipe 33 mounted at the top.

A valve 34 is placed on the steam/gas pipe 33. The flash steam/gas may be used for preheating the biological material in a preheater 37. The preheater 37 may use indirect heat, which makes it possible to remove the condensates from the flash steam/gas from the process. The dry material of the biological material is not reduced due to condensation of flash steam/gas.

The second flash vessel 32 may be operated at a pressure lower than the ambient pressure. A possible vacuum system may consist of a heat exchanger and a vacuum pump or a steam ejector (system not shown).

This will reduce the temperature in the treated biological material and increase the dry material content in the treated biological material by increased evaporation. It will also reduce the amount of volatile compounds in the treated biological material.

In the embodiment shown in figure 2, a mixer 35 and a liquid addition valve 36 are installed in the flash vessel 32. It is clear that the number of flash vessels may vary in the system depending on the requirements in respect of the treated biological material. The mixer 35 makes it possible to mix liquid with the biological material, and in that way it is easier to pump the material from the vessel 32.

The reactor 1 may be connected in parallel with two or more economizers 4. Alternatively, the economizer may be connected in parallel with two or more flash vessels.

If there is a need for a guaranteed and documented specified retention time at a specified temperature and pressure, this may be obtained by the above described solution. In a further alternative embodiment, any one of the flash vessels may be designed as a cyclone to separate the biological material from the steam/gas. Such an embodiment may increase the ef- ficiency of the process.

Claims

PATENT CLAIMS

1. A method for continuous treatment of biological material at anelevated pressure before anaerobic digestion, said method comprising at least the steps of:

feeding the biological material to an infeed device serving as a sluice system and increasing the pressure in the infeed device; and transferring the biological material from the infeed device to a reactor by means of pressure differences or gravity; and optionally further increasing the pressure in the reactor.

2. A method according to claim 1, comprising the further step of treating the biological material in an economizer or a flash vessel.

3. A method according to claims 1 to 3, wherein the biological material is transferred from the reactor to the economizer by means of differential pressure.

4. A method according to claim 1, wherein the biological material is transferred from the infeed device to the reactor via a transfer pipe.

5. A method according to claim 4, wherein the biological material is subjected to steam injection in the transfer pipe.

6. A method according to any one of the preceding claims, wherein the reactor is operated at an elevated pressure in the range of 2 to 25 barg, preferably in the range of 3 to 12 barg.

7. A method according to any one of the preceding claims, wherein the reactor is operated at a temperature in the range of 110 to 240 °C, preferably at a temperature in the range of 130 to 190 °C.

8. A method according to any one of the preceding claims, wherein at least a portion of the flash steam in the economizer is re-circulated and reused in the infeed device.

9. A method according to any one of the preceding claims, wherein the biological material is transported through a series of vessels by means of differential pressure.

10. A method according to claim 9, wherein the biological ma- terial is subjected to vacuum in the final vessel.

11. A method according to any one of the preceding claims, wherein the transport of biological material inside the reactor is mainly by means of gravity.

12. A method according to any one of the preceding claims, wherein the transport of biological material inside the reactor is mainly by mechanical means.

13. A method according to any one of claims 2-12, wherein two or more economizers or flash vessels are used in parallel.

14. A method according to any one of the preceding claims, wherein the treated biological material is used for the production of biogas.