JPS6357120B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating sludge in an anaerobic digestion tank, which is often installed in sewage and human waste treatment plants, and sludge equipment having dewatered sludge drying means, and more specifically, a method for heating an anaerobic digestion tank. The present invention relates to a method and related method for treating dry exhaust gas.

Recently, sewage and human waste have been treated with sludge under both aerobic conditions and anaerobic conditions, resulting in methane fermentation,
Denitrification is often carried out. Further, the final sludge after these treatments is generally filtered and dehydrated, and the solid content is dried and incinerated.

In order to actively digest sludge in an anaerobic digestion tank, it is necessary to maintain the temperature of the sludge liquid at 30 to 40°C. For this reason, anaerobic digestion tanks have conventionally been heated using external heat. The following various methods have been implemented as conventional heating methods for anaerobic digestion tanks. One of them is to install a heating pipe inside the digestion tank.
A method of indirectly heating water by circulating hot water of 60° C. or lower from a heating tube installed in a boiler that burns the digestion gas (methane-containing gas) generated from the tank to the heating tube is used. In addition, a heat exchanger is installed outside the digestion tank, and the sludge liquid or desorbed liquid is circulated through the heat exchanger, and the heat exchanger exchanges heat with the hot water obtained from the digestion gas combustion boiler. A method of heating the liquid in the tank has also been used. Furthermore, steam obtained from a digestion gas combustion boiler is directly blown into the digestion tank to heat the inside of the digestion tank.

However, all of these methods use digestion gas as a heat source, and the amount of digestion gas used for other purposes is reduced accordingly. Also, if there is a shortage of digestion gas, other auxiliary fuels are required. Furthermore, since the hot water or steam used for heating is generated using a boiler, the thermal efficiency is reduced by the efficiency of the boiler.
In the indirect heating method using heating tubes or heat exchangers, sludge adheres to the heat transfer surface.
The disadvantage is that the heat transfer efficiency deteriorates and the device becomes large. In addition, the method of heating by directly blowing steam has a good heat transfer effect, but because the steam temperature is high, there is a lot of heat loss and thermal efficiency is poor.

On the other hand, the exhaust gas from the sludge dryer contains a large amount of water vapor and odor components, so in order to release it into the atmosphere, it is necessary to perform deodorization treatment and to remove the water vapor to prevent white smoke. When using high-temperature air for drying, the dry exhaust air is generally guided into a combustion type deodorizing furnace to burn and decompose odor components, but since the exhaust air contains a large amount of water vapor, the thermal efficiency of the combustion furnace is reduced. However, there are disadvantages in that the amount of exhaust gas from the incinerator increases and the exhaust gas treatment equipment must be enlarged.

An object of the present invention is to heat the anaerobic digestion tank in a sludge treatment facility having an anaerobic digestion tank and dewatered sludge drying and incineration means by effectively and efficiently recovering waste heat from the drying equipment, and to heat the anaerobic digestion tank by effectively and efficiently recovering waste heat from the drying equipment. An object of the present invention is to provide a sludge treatment method that can effectively realize heat utilization and air and gas flow.

The sludge treatment method of the present invention is a method carried out in a sludge treatment facility having an anaerobic digestion tank and dewatered sludge drying and incineration means.

A feature of the present invention is that in a sludge treatment apparatus consisting of an anaerobic digestion tank, a hot air direct dryer for dehydrated sludge (cake), a high temperature gas-to-air heat exchanger, and a sludge incinerator, a gas-liquid direct contact means is provided to incinerate the sludge. The high-temperature exhaust gas of the furnace and air are heat exchanged in the heat exchanger to heat the air within a range of 700°C or less, and the heated high-temperature air is used to dry the dehydrated sludge (cake) in a hot air direct dryer. Introducing air below 200°C, which has become medium-temperature and humid during the drying process, into a gas-liquid direct contact means and bringing it into direct gas-liquid contact with the liquid sludge to be fed into the anaerobic digestion tank, reducing the temperature and humidity of the air to below 50°C. The liquid sludge is heated to over 50℃ under aerobic conditions and supplied to the digestion tank.The low-temperature, low-humidity air is used as combustion air in the sludge incinerator, while fresh air is supplied as a heat medium. By doing so, the inside of the gas-liquid contact device is maintained in an aerobic state, and the dried sludge obtained in the hot air direct dryer is charged into a sludge incinerator and incinerated.

In the method of the present invention, medium-temperature, high-humidity air discharged from a dehydrated sludge drying means is brought into gas-liquid contact with digested sludge liquid or heat transfer water in a newly added gas-liquid direct contact means, and is cooled and dehumidified. The low-temperature, low-humidity odor-containing air is used for incineration or incineration of dehydrated sludge. The digested sludge liquid that comes into gas-liquid contact with the drying means exhaust air in the first gas-liquid direct contact means, that is, the gas-liquid contact device, is circulated between the anaerobic digestion tank and the first gas-liquid contact device. The heated digested sludge liquid returned from the tank raises the temperature inside the anaerobic digestion tank and stirs it. The heat transfer water that comes into gas-liquid contact with the exhaust air of the drying means in the first gas-liquid contact device is heated in the device and then sent to the second gas-liquid direct contact means, that is, the gas-liquid contact device. After being brought into gas-liquid contact with the digestion gas drawn from the anaerobic digestion tank in the device and cooled down,
It is returned to the previous gas-liquid contact device. The digested gas with increased temperature and humidity is blown into the digested sludge liquid in the digestion tank, raising the temperature of the liquid and stirring it.

Next, the method of the present invention will be explained in detail based on drawings of examples. 1 and 2 are system diagrams of a first embodiment and a second embodiment of the method of the present invention, respectively. Third
The figure is a system diagram of a reference example of the method of the present invention.

In the embodiment, the means for incinerating the dehydrated sludge S is illustrated as a multi-stage incinerator 2, but the present invention is not limited to this. As is well known, the multistage incinerator 2 is a vertical furnace partitioned into multiple stages, with a central shaft 4 inserted through the center, and an arm 5 having teeth (not shown) for each stage. is installed. The hearth of each stage is provided with incineration sludge drop ports alternately in the center and periphery. Dehydrated sludge S (or dried sludge) is supplied from the top of the furnace,
As the arm rotates, it is lowered to the lower stage. The sludge self-combusts with the combustion air sent into the lower stage of the furnace by the fan 12, and incinerated ash A is taken out from the bottom of the furnace. Flue gas is taken out from the top of the furnace. However, the second embodiment shown in FIG. 2 is slightly different, which will be described later. Cooling air is sent to the central shaft 4 and the arm 5 by a fan 3, and the air whose temperature has increased by cooling the shaft 4 etc. is effectively used as described later.

As a drying means for the dewatered sludge S, a dryer 1 is shown in FIGS. 1 and 3 of the first embodiment and the reference example assuming a cocurrent rotary dryer. In FIG. 2 of the second embodiment, the upper stage 1A of the multistage incinerator 2 is used as a drying means, and the lower stage 2A is used as an incineration means. Therefore, dehydrated sludge S is supplied from the top of the furnace and gradually falls while drying and burning, and combustion ash A is taken out from the bottom of the furnace, but air and exhaust gas for each means are divided into upper stage 1A and lower stage 2A. It is taken in and taken out.
Note that the arrows in the incinerator 2 in FIG. 2 indicate the flow direction of air and exhaust gas. Of course, the means for drying the dewatered sludge S is not limited to the illustrated embodiment.

9 is an anaerobic digestion tank. Reference numeral 7 denotes a first gas-liquid contacting device that brings the dehydrated sludge S into direct gas-liquid contact with medium-temperature, high-humidity air from the dryer 1 or 1A and the digested sludge liquid or heat transfer water. Reference numeral 8 in FIG. 3 is a second gas-liquid contact device that is separately provided when the heat transfer water is fed into the first gas-liquid contact device 7.

After the dehydrated sludge S is dried in the dryer 1 or the drying stage 1A, it is incinerated in the multistage incinerator 2 or the incineration stage 2A of the furnace, and is discharged as incinerated ash A. After cooling the central shaft 4 and arm 5 of the incinerator 2, the air blown by the shaft cooling fan 3 enters the heat exchanger 6, where it is heated by exchanging heat with the incinerator exhaust gas and becomes heated high-temperature air. and enters the dryer 1 or drying stage section 1A. The incinerator exhaust gas cooled by the heat exchanger 6 is attracted by the exhaust gas induction fan 11 and sent to the exhaust gas treatment device 14.

The exhaust air that has become medium temperature and high humidity in the dryer enters the first gas-liquid contacting device 7, contacts the digested sludge liquid or heat transfer water, and is cooled and dehumidified as low-temperature, low-humidity air by the fan 12. It is supplied to the bottom stage of the incinerator 2 and used as combustion air for sludge incineration.

In the first and second embodiments shown in FIGS. 1 and 2,
The digested sludge liquid in the anaerobic digestion tank 9 is sent to the first gas-liquid contact device 7 by the pump 10, where it comes into gas-liquid contact with dry exhaust air at medium temperature and high humidity, removes its sensible heat, and condenses water vapor. The heat of condensation increases its own temperature and is returned to the digestion tank 9 by the pump 13. At this time, the pressure of the pump 13, piping, etc. are appropriately set so that the digestive fluid in the digestion tank 9 is stirred by the returned digestive fluid. In this way, the temperature of the digestive fluid in the digestion tank 9 is uniformly increased. It goes without saying that instead of feeding the sludge liquid from the digestion tank 9 to the gas-liquid contacting device 7, the sludge to be introduced into the digestion tank 9 may be directly fed to the gas-liquid device 7.

In the reference example shown in FIG. 3, heat transfer water is supplied to the first gas-liquid contact device 7 instead of the sludge liquid.
The temperature of the heat transfer water is increased in the same manner as the sludge liquid in the previous example. This heat transfer water is circulated by pumps 15 and 16 between it and a second gas-liquid contact device 8 provided separately. Digestion gas in the digestion tank 9 is fed into the gas-liquid contact device 8 by a circulating gas blower 17.
The gas comes into contact with the heat transfer water to increase its temperature and humidity, and is returned to the sludge in the digestion tank 9, where it bubbles into the sludge to agitate the sludge and raise its temperature.

Next, some specific operating values when the method of the present invention is implemented according to the above examples and reference examples will be described.

The temperature of the air that cools the central shaft 4 and arms 5 of the multistage incinerator 2 is 150 to 200°C. Meanwhile, the exhaust gas temperature reaches 700-800℃. The cooled air heat-exchanged in the heat exchanger 6 reaches 500 to 700°C, and is introduced into the dryer 1 or the drying stage section 1A, where the dehydrated sludge is dried to a moisture content of 20 to 30%. During this drying process, the exhaust air discharged from the drying section becomes medium-temperature and humid, with a temperature of 100 to 200 degrees Celsius, and its sensible heat is not small, but since it contains a large amount of water vapor, the amount of sensible heat is about 3
Contains twice as much latent heat. Therefore, the first gas-liquid contact device 7 can effectively increase the temperature of the contact liquid. In this gas-liquid contact device 7, circulating sludge at 30 to 40°C is heated to 60 to 80°C, and heat medium water at 40 to 50°C is heated to 60 to 80°C. The air is cooled to 40 to 50°C in the gas-liquid contactor 7, dehumidified, and used as combustion air for the furnace 2.

When heat transfer water is used in the first gas-liquid contact device 7, the heat transfer water whose temperature has risen to 60 to 80°C comes into gas-liquid contact with the digestion gas in the second gas-liquid contact device 8, and the digestion gas This digestion gas contacts the sludge liquid and raises the temperature of the sludge liquid due to its sensible and latent heat.

Incidentally, the means for incinerating the dehydrated sludge can be replaced with a simple hot air generating furnace, and in this case, the dried sludge obtained in the dryer and having a water content of, for example, about 30% can be taken out of the apparatus as a product as it is. The hot air generating furnace naturally requires fuel, but it is possible to use digestion gas as this fuel, and depending on the properties of the sludge such as the water content of the dehydrated cake, this fuel can be 100% covered by digestion gas. be able to. That is, the apparatus as a whole can produce dried sludge without using external heat or external fuel.

The features and effects of the method of the present invention are summarized as follows.

(1) Effectively utilizes the latent heat of water vapor contained in dry exhaust air to heat the sludge liquid in the anaerobic digestion tank, and
By lowering the humidity of dry exhaust air and using it as combustion air, the thermal efficiency of the incinerator is improved and the amount of exhaust gas is reduced.
Can be reduced by ~50%.

(2) The waste heat of the sludge treatment equipment, especially the latent heat of water vapor, is effectively used to heat the sludge liquid in the anaerobic digestion tank using humidified air from the drying equipment.
Digestion gas can be used effectively and used for power generation, etc.

(3) Since the heat exchange is carried out directly by the gas-liquid contact device at a relatively low temperature level (80-0°C), the heat loss is not small.

(4) By appropriately selecting the pressure and piping for feeding the returned sludge liquid or digestion gas from the gas-liquid contactor into the digestion tank, the liquid or gas can equalize the temperature inside the digestion tank and activate digestion. be able to.

(5) Since the exhaust air of the dryer is used as a heat source, even if the sludge liquid comes into contact with it, the digestibility of the sludge liquid will decrease and anaerobic digestive bacteria will die due to harmful substances in the combustion exhaust gas. There is no risk of such.

[Brief explanation of the drawing]

1 and 2 are process diagrams of different embodiments of the method of the present invention, and FIG. 3 is a process diagram of a reference example of the method of the present invention. 1... Dryer, 2... Multi-stage incinerator, 3... Shaft cooling fan, 6... Heat exchanger, 7, 8... Gas-liquid contact device, 9... Anaerobic digestion tank, 14... Exhaust gas treatment equipment, S... dehydrated sludge, A... incineration ash.

Claims (1)

[Claims] 1. In a sludge treatment equipment consisting of an anaerobic digestion tank, a hot air direct dryer for dehydrated sludge, a high-temperature gas-to-air heat exchanger, and a sludge incinerator, a gas-liquid direct contact means is installed to connect the high-temperature exhaust gas of the sludge incinerator to the air. Heat exchange is performed in the heat exchanger to heat the air to a temperature below 700°C, and the heated high-temperature air is used to dry the dehydrated sludge in a hot air direct dryer. of air is introduced into the gas-liquid direct contact means to bring it into direct gas-liquid contact with the liquid sludge to be charged into the anaerobic digestion tank.
The liquid sludge is heated to 50℃ or higher under aerobic conditions by lowering the temperature and humidity to below ℃ and is supplied to the digestion tank, and the low-temperature and low-humidity air is used as combustion air in the sludge incinerator. On the other hand, the inside of the gas-liquid contact device is maintained in an aerobic state by supplying fresh air as a heating medium, and the dried sludge obtained in the hot air direct dryer is charged into a sludge incinerator and incinerated. Law.