DE3534260A1 - Method for thermal drying of temperature-sensitive materials in rotary drums as well as devices for implementing this method - Google Patents

Abstract

The method is characterised by the cooling of the smoke gas from the combustion in a first indirect cooling stage and in a downstream second, direct cooling stage. The heat-flow content from the first cooling stage is transmitted to the material being dried by contact, the content from the second cooling stage by convection. The heat generation is regulated by a characteristic measured variable at the end of drying by adding a further characteristic measured variable of the contact heat transmission. By influencing the heat transmission conditions as well as by supplying colder gas, the heat flows are regulated and permitted temperatures are observed. The devices for implementing the method envisage both heat transmission systems flowed through by heat transfer medium and installation parts which are acted on directly by smoke gas and in good contact. The installations have one or two rotary drums. In the return vapour lines, preventive fire protection arrangements are installed. The heat storage capacity of hot installation parts is kept low.

Description

The invention relates to a method for thermal drying temperature-sensitive goods in rotary drums, such as. B. green fodder, Beet pulp, grain, wood chips, sawdust, wood bark, biomass, Straw, hard coal, lignite, oil meal, plastic granules, Plastic waste, combustible waste components or the like, and devices for performing the method.

In such processes, the required for drying Heat from burning fuels - especially from cheap, high-fiber waste fuels, such as B. wood waste, Barks, grinding dust or the like - in the form of a high-temperature Flue gas generated, but because of the temperature sensitivity of the drying good not directly with the good may be brought into contact.

The drying of temperature-sensitive bulk goods in rotary drums is used in previously known systems in two different ways Process operated.

In the first process, the flue gases are separated from one another Combustion chamber initially either with ambient air or - to reduce heat consumption - with returned vapors from the dryer exhaust air to the permissible dryer inlet temperature  mixed back and then passed through the drum, where they pass their usable heat through to the items to be dried give up direct contact with the goods. After that the Vapors are dusted and partly to the exhaust air chimney and partly as return vapors to a mixing chamber in front of the dryer.

The amount of back vapors is usually very large and often exceeds double the amount of flue gas from combustion. The in Dryer usable gas cooling is allowed because of the low Inlet temperature and the process-related, comparatively high outlet temperature only low. With high heat requirements drying results in high gas throughput due to the Drum and a high dedusting effort. The drying process is all the more efficient - better heat utilization, lower Temperature level, good thermal treatment, lower Final moisture, homogeneous dry goods etc. - the longer the Dwell time in the dryer is. As a result, the gas velocity is allowed do not get too high in the dryer, otherwise one undesirable high pneumatic transport effect occurs that a high dryer load prevents. As a result of this point Drum dryers with large diameters. The trend to ever larger units of performance leads to dimensions, the manufacturing, transportation and assembly very strong make it more difficult and more expensive.

In the second drying process with rotary drums, the required heat from hot, well-touched contact surfaces the good transferred by a heat transfer medium - z. B. Water, steam or thermal oil - are flowed through. Doing so the heat transfer medium through a pipe system between the dryer and a separately installed, stationary boiler in Circulation, with special sealing heads the stationary and connect the rotating pipe system in the dryer.  

These contact dryers are naturally more complex and expensive than the convective drum dryers previously described. In addition, the requirement to extract the exhaust air temperature from the Boilers keep the heat exchanging economically low Heating surfaces in the boiler and also in the dryer become large and complex.

For the usually heat-consuming drying of a good plays a role low heat price of the fuel plays an increasingly important role. Therefore, more and more on ballast fuels, mostly contaminated flammable waste or sanding dust from particle board production u. Ä. cheap fuels used.

The disadvantage is that these fuels contain residues which themselves have a low melting point Lower the melting point of the refractory material in the combustion chamber. In order to avoid destruction of the combustion chambers, therefore previous firings for these fuels by a cooled disproportionately high proportion of additional air or mixed air. The admixture of air, however, also provides here - as with everyone elsewhere of a dryer - because of the increase in exhaust air losses an economic disadvantage.

The invention has for its object to provide a method as well as devices for carrying out the method, which is the reduction of gas throughput at the convective Drying process and a reduction in the heating area with the indirect drying process. The one in the characteristic Part of claim 1 solution described this task is based on the idea of the two Drying processes previously used separately on targeted Way to combine with each other. This happens through a cooling of the flue gases from the combustion in two Stages. In a first indirect cooling stage, part of the Heat from the flue gases on heat transfer components over that this heat back in via well-touched heating surfaces pass the rotary drum on to the material to be dried.  

In the subsequent second convective cooling stage, there is partially cooled flue gas uses its remaining usable heat through direct Contact with the items to be dried.

The method according to the invention has the following advantages:

The first cooling stage also causes a reduction in the Furnace temperature, d. that is, the uneconomical admixture of Mixed air is no longer required.

The heat transfer component in the flue gas first indirect cooling stage can because of the extraordinary large usable temperature difference small and inexpensive will.

The gas speeds in the rotating drum can be considerable can be reduced because with the gas only a part of the required Heat must be introduced. The dryer filling and thus the residence time can be increased accordingly or the dryer performance can be improved.

The equipment and energy expenditure for vapor dedusting decreases according to the lower gas throughput.

While it is state of the art, a dryer produces flue gases to feed a separate boiler to make it relatively simple Way to exploit their residual heat, but it is always around two separately operated systems: the firing of the Boiler is only supplied according to the requirements of the Heating system driven, and the drying has its own Heat source, the fluctuating amount of boiler smoke can fully compensate. So that is the dryer to operate independently of the boiler.  

In contrast, the present invention provides a coupled one System with a common firing and a common Heat consumers. The indirectly and directly Total heat flow components transferred cover the total the respective heat requirement of drying, the well-conditioned and Equipment-related necessity, not permissible temperatures to exceed, however, make it necessary to use the two heat flow components quickly and effectively meet operational needs to divide and regulate accordingly.

Because of the mostly high heat output and the considerable fire risks the process places increased demands on the measurement, Control and regulating device, to the materials used and to the design of a drying system operating in this way, as explained below:

The total heat requirement for drying is characterized by a characteristic Measured variable at the end of the dryer, e.g. B. the vapor outlet temperature, measured. After this measured value, the Fuel supply and thus the smoke gas generation regulated. There with different levels of firing, also the heat emission directly influenced in the first indirect cooling stage is another characteristic parameter for contact heat transfer - e.g. B. the temperature of the well-contacted contact surface - required the appropriate first controller is activated and is suitable for short-term load fluctuations intercept. As a rule, the heat transfer components for the first indirect gas cooling stage for the sake of Efficiency only for an optimal operating case - such as B. maximum wet material moisture - designed. For other operating cases the heat emission can be smaller than it is for cooling the Flue gas required up to the permissible dryer inlet temperature is. In this case the flue gas is by admixing of a colder admixing gas, preferably from back vapors.  

The amount of this gas is determined by the dryer procedure Regulation constantly adjusted.

Depending on the nature and initial moisture of the goods, they are different Contact surface temperatures permitted or necessary. This adjustment is done by deliberately influencing the heat transfer conditions between the flue gas from the combustion and the heat transfer components for the first indirect Cooling level. Only when there is a strong deviation from normal operation - e.g. B. a sudden loss of the supply of wet goods - is the balance between the two heat transfer systems - directly and indirectly - so strongly shifted that cooling the heat transfer components by adding a colder Mixing gas - preferably back vapors - to the flue gases is required. After all, at all the essential points Safety temperature sensor installed when critical Shut down the system in a suitable manner or trigger additional protective measures.

The safe functioning of a method according to the invention working dryer system sets a quick adjustment of the Heat generation to the experience of fluctuating dryer loads ahead. Therefore, a conventional refractory brickwork for thermally highly stressed system components with your high heat storage capacity a stable dryer operation extraordinarily affect negatively or even completely impossible do. The embodiment according to the invention therefore sees use of thermally resilient mineral fiber material as well as externally insulated sheet metal constructions made of heat-resistant material. This keeps the heat storage capacity low and the controllability drying is flexible enough. At the same time it is due to heat build-up in the event of malfunctions such as B. power failure, Fire risk is reduced to a minimum.  

A particularly important point for the equipment execution a drying plant operating according to the inventive method are the heat transfer components for the indirect, by contact with the heat flow portion to be transferred to the goods. Because these are exposed to high-temperature flue gases and have to transmit high heat outputs must have reliable heat dissipation and high thermal stability to be guaranteed. The heating rate must be for an accident should be low enough for the measuring and control device can react before inadmissibly high temperatures can be achieved.

Measures for improvement play an essential role here the heat transfer conditions on the well-touched heating surfaces such as B. by enlarging the heating surface and by intensified Good movement.

The initiation of back vows, the cheapest in terms of heat economy Method for temperature reduction through colder admixtures, requires since these are usually flammable even after dedusting Contain dust, additional preventive protection against Fire and explosion. The combustible dusts tend to very often for the formation of deposits and deposits in the vapors Pipelines. All connection points of these pipelines Plant parts - such as B. fires - whose temperature is higher than the dust ignition temperature is, also in drying systems conventional danger is a constant danger.

The measures in conventional dryer systems with large Back vapor quantities - additional bricked-up mixing chambers and regular plant shutdowns to clean these dust deposits - are for the according to the inventive method working dryer systems unsuitable. Creates according to the invention here a preventive fire protection device to remove Deposits during operation Remedy.  

There are a number of devices for performing the Procedure into consideration. There are four in the schematic drawings Design variants shown in principle. The presentation is limited only focus on the essentials for better clarity constructive features, d. i.e., the MSR equipment are not shown in the drawing.

Fig. 1 shows a basic embodiment of an apparatus for performing the method.

The material to be dried passes through the wet material feed 2 into a rotating drum 1 and leaves the drying system via a drop housing 14 and a dry material discharge 3 . In a combustion chamber 10 of a stationary heating boiler 15 , hot flue gases are generated by combustion, which partially cool down on the heat transfer components exposed to the flue gas - shown here as a tube system 12 . At the same time, the combustion chamber is also cooled to a permissible temperature by heat radiation. The partially cooled flue gas reaches the rotary drum 1 via a heat-insulated connection 11 , where it releases its remaining usable heat convectively and thereby absorbs the evaporated moisture. The moist vapors are conveyed from the failure housing 14 by a vapor fan 4 into a dedusting device 5 and then into an exhaust gas line 7 or into a return vapor line 6 . To cool the flue gas-charged heat transfer components 12 - z. B. with irregular supply of wet material - is colder gas - such. B. back vapors - initiated by a cooling gas connection 9 .

A well-contacted contact surface system is installed in the rotating drum 1 - shown here as a heating tube system 13 - through which a heat transfer medium flows. The heat transfer medium is fed via a line 16 from a pump 17 to the boiler 15 . Special sealing heads 18 represent the connection between the stationary and rotating pipe system.

The heat transfer between the flue gas from the combustion and the heat transfer components 12 is regulated by changing the flow guide to the permissible temperature value dependent on the material to be dried by adjusting a flap 19 in a bypass 22 . Via a second cooling gas connection 8 , the partially cooled flue gas can enter a colder gas before entering the rotary drum - such as, for. B. Back vapors - are added if the flue gas temperature is impermissibly high.

Fig. 2 shows a second basic embodiment is an apparatus for performing the method.

Instead of a stationary boiler, an insulated heat chamber 20 is arranged next to the rotary drum 1 , in which a flue gas-charged heat exchanger 12 ' attached to the rotary drum rotates ' . The heat transfer medium flows into the directly connected well-contacted contact surface system 13 . Sealing heads can be omitted. The short heat-insulated connection 11 ' between the heat chamber 20 and rotary drum 1 is designed so that when colder gas is introduced - such as. B. back vapors - through the cooling gas connection 8, the gas mixing temperature can be measured reliably. All other parts of the system and their function correspond to the first embodiment.

Fig. 3 shows a third basic embodiment of an apparatus for carrying out the process. Instead of the heat transfer medium flowing through the heat transfer system of the indirect heat-current component is transmitted immediately over the drum to the material. In this case, part of the drum shell serves as a heat exchanger 12 ″ which is subjected to flue gas. In the drum 10 , the well-contacted contact surface system 13 ' causes an intensive circulation and transfer of heat. The combustion takes place in a separate combustion chamber 10 ' . The combustion chamber temperature is kept at a permissible value by supplying partially cooled flue gas 21 . The flue gases from the combustion chamber 10 ' enter an insulated heating chamber 20' which partially surrounds the rotary drum 1 . The regulation of the heat transfer between the flue gas and the drum shell is done in an analogous manner as in the previously described embodiments by adjusting the flap 19 ' in the bypass 22' or by introducing colder gas - such as. B. Back vapors - through the cooling gas connection 9 . About the heat-insulated connection 11 ″ is the partially cooled flue gas, possibly with the addition of colder gas - such as. B. back vapors - passed through the cooling gas connection 8 to adjust the permissible temperature in the rotary drum 1 and via the feed line 21 into the combustion chamber 10 ' . All other parts of the system and their functions correspond to the previously described embodiment.

Fig. 4 illustrates a fourth basic embodiment is an apparatus for performing the method.

The material to be dried passes through two rotary drums 1 ' and 25 connected in series on the good side. The first rotary drum 1 ' has - as described in the previous embodiment - the function of the flue gas-charged heat exchanger 12 ″ and has the well-contacted contact surface system 13' inside. About the wet material feed 2 , the material to be dried enters the drum 1 and leaves the downstream failure housing 14 ' in the partially dried state via the trigger 3' . A material conveyor 28 conveys the material for feeding 2 'of the second rotary drum 25th It leaves the drying system through the dry material discharge 3 on the failure housing 14 .

The indirect heat transfer system on the first rotary drum 1 ' contains the same system parts with the same functions as in the embodiment described above. Of the partially cooled and possibly by introducing colder gas - such. B. Back vapors - brought through the cooling gas port 8 to a permissible temperature, a part is passed through the connection 11 ″ in the first rotating drum 1 ' and further via the feed line 21 into the combustion chamber 10' . The rest of the part passes through the heat-insulated connection 11 '″ together with the vapors from the failure housing 14' of the first rotary drum 1 ' into the second rotary drum 25 . The gas inlet temperature of the second drum is lower than that of the first drum due to the lower moisture content. The drying system here has a vapor fan 4 and a dedusting device 5 .

Fig. 5 illustrates a basic embodiment illustrates a fire prevention device of the invention.

Such a fire protection device can be single or multiple in the vapor system of a drying plant - even in the past conventional drying systems - to be installed. In front the vapor lines should all immediately before high temperature Plant parts are kept free of combustible dust deposits to prevent backward smoldering fires.

In a back vapors 31 before connection to a combustion chamber 30 , a flushing device 32 is installed, the relevant line section with a washing liquid - such as. B. water - applied. With the system running, the dust deposits are discharged as sludge after a brief actuation of the flushing device 32 . In a separation zone 33 , sludge and vapors are separated from one another. The sludge is drawn off from the separation zone 33 via a sludge discharge 34 .

Claims (18)

1.Procedure for the thermal drying of temperature-sensitive goods in rotary drums, the heat required preferably being produced by burning cheap fuels - in particular those whose ash melting behavior allows comparatively low furnace temperatures - in the form of a high-temperature flue gas, which, however, is not directly related to that due to the temperature sensitivity of the material to be dried Good to be brought into contact, characterized in that the flue gas emits its heat to heat transfer components in a first, indirect cooling stage, which in turn emit this heat at lower permissible temperatures by contact with the good, and that the partially cooled flue gas subsequently in a second, convective cooling stage transmits its usable residual heat by direct contact to the goods, and the entire heat generation via a characteristic parameter at the end of drying - e.g. B. the vapor outlet temperature - is adjusted by automatic control to the total heat requirement of the drying and short-term load fluctuations via another characteristic parameter - z. B. the well-touched contact surface temperature - detected and this is switched to the automatic control, and the directly and indirectly transmitted heat flow components are divided and / or regulated so that no impermissibly high temperatures occur in the goods, on the indirect heat transfer components or in the combustion chamber. 2. The method according to claim 1, characterized in that
1. for uniform normal operation of the drying system, the heat transfer in the first, indirect heating stage via a characteristic measured variable - such as, for. B. the temperature of the well-contacted contact surface -
a) by influencing the heat transfer conditions between the flue gas from the combustion and the heat transfer components to a maximum permissible value dependent on the material to be dried, and
b) the smoke gas temperature permissible for the second, convective cooling stage may be regulated by admixing a colder gas or air, and that
2. for irregular operation - e.g. B. with low or irregular supply of wet goods - the heat transfer conditions for the first, indirect cooling stage can be deteriorated by introducing a colder gas or air to avoid impermissibly high temperatures on the heat transfer components. 3. The method according to claim 2, characterized in that as colder gas according to point 1b and point 2 according to Claim 2 vapors are used, and that the Influencing the heat transfer conditions accordingly Point 1a according to claim 2 by measures known per se - such as B. shielding from heat radiation, Change in gas and heat quantities and / or heat transfer coefficients  due to changed gas flow routing, Change of the effective gas temperature by gas mixture with an internal, already colder gas circuit or Like. - Is achieved, in such a way that when present an optimal application of wet goods to the cooling effect is completely dispensed with by back-steaming, and that at the same time these measures are adequate Cool the combustion chamber. 4. The method according to claim 2 or 3, characterized in that that influencing the heat transfer conditions for even normal operation of the drying system during operation on the drying goods dependent maximum permissible value happens. 5. Application of the method according to one or more of the Claims 1 to 4, for drying wood chips and Sawdust for the production of chipboard or for the Drying shredded wood, wood waste and bark for use as fuel or for drying of natural products of a similar type - e.g. B. straw - the be used for plate production, being used as fuel preferred wood waste, bark and / or in panel production by-products - such. B. grinding dust - be fired. 6. Device for performing the method according to one or more of claims 1 to 5, using an installed in a rotary drum, from a heat transfer medium - z. B. water, steam or thermal oil - flowed through, well-contacted contact surface system ( 13 ) for transferring the indirect heat flow through contact with the material, the heat transfer medium in the circuit is passed through a separate, stationary boiler, where it is in a first gas cooling stage Transmitting heat absorbs, characterized in that the flue gas-loaded heat transfer components ( 12 ) in the stationary boiler ( 15 ) are designed so that the flue gas from the combustion at rated power to a temperature of 200 ° C to 800 ° C, preferably to 250 ° C to 500 ° C is cooled, and at the same time the combustion chamber ( 10 ) is cooled to a temperature below 1000 ° C by radiation and / or convection, and the partially cooled flue gas is passed into the downstream rotary drum ( 1 ) while maintaining the permissible temperature. 7. The device according to claim 6, characterized in that a heat exchanger ( 12 ' ) is provided for heating the well-contacted contact surface system ( 13 ) flowing through the rotary drum in the rotary drum, which is attached to the rotary drum ( 1 ) and rotates with it, and this heat exchanger ( 12 ' ) is acted upon by the hot flue gas from the combustion, which is then passed into the rotary drum ( 1 ) while maintaining the permissible temperature. 8. An apparatus for performing the method according to one or more of claims 1 to 5 or claim 7, characterized in that instead of the rotating heat exchanger ( 12 ' ) through which a heat transfer medium flows or together with it the rotary drum ( 1 ) itself at least partially as Heat exchanger ( 12 ″ ) is acted upon by the hot flue gas from the combustion, and the heat transfer conditions between flue gas and heat transfer components and between material and contact surface system ( 13 ′ ) in a manner known per se through shielding plates, insulating coating, ribs, webs, pockets, channels through which flue gas flows or Like. Are adapted to each other so that inadmissibly high temperatures do not occur at any point with optimal heat transfer, and that the heat transfer inside, well-contacted contact surface system ( 13 ' ) - such. B. in the form of webs or ribs - at the same time constantly circulating the material to improve the heat transfer conditions. 9. An apparatus for performing the method according to one or more of claims 1 to 5 or claim 7 or 8, characterized in that the heat transfer medium-containing heat transfer system ( 12 ', 13 ) consists of several separate line systems, preferably of so-called "heat pipes" . 10. The device according to one or more of claims 1 to 5 or one or more of claims 7 to 9, characterized in that two separate rotary drums ( 1 ', 25 ) passing through one after the other are provided, and only the first rotary drum ( 1st ' ) - which is preferably operated at a higher speed - has the previously described components for the indirect transfer of heat from the hot flue gas from the combustion via the well-contacted contact surface system to the material to be dried, and at least part of the flue gas partially cooled on the first drum is introduced into the second drum while maintaining the permissible temperature, and the second drum ( 25 ) is designed for the transfer of the remaining usable heat of the flue gas. 11. Device according to one of claims 7 to 10, characterized in that the rotating drum ( 1 or 1 ' ) rotating, acted upon by the flue gas from the combustion heat transfer components ( 12' or 12 " ) within a heat chamber ( 20 or . 20 ' ) circulate, into which the hot flue gases are introduced from a separate combustion chamber ( 10' ), and the heat-insulating lining of the outer walls of the heat chamber for rapid adaptation of the heat requirement to the drying has only a low heat capacity. 12. The apparatus according to claim 11, characterized in that at least part of the furnace ( 10 ) for generating the hot flue gases is installed in the heat chamber itself, and the heat transfer to the heat transfer components ( 12 ' or 12 " ) of the rotary drum additionally by flame radiation he follows. 13. The apparatus according to claim 11 and / or 12, characterized in that the heat insulating lining of the heat chamber at least partially thermally resilient mineral fiber Has material, and / or the outer walls of the heat chamber fully or partially heat-resistant, heat-insulated on the outside Have sheets. 14. The apparatus according to claim 10, characterized in that for conveying the goods between the first rotating drum ( 1 ' ) and the second rotating drum ( 25 ) a material conveying member ( 28 ) is provided, which is preferably made of corrosion-resistant material. 15. The apparatus according to claim 14, characterized in that the conveying element ( 28 ) is designed as a pneumatic conveying path, the conveying gas of which is at least partially composed of flue gas and from back vapors. 16. The apparatus according to claim 10, characterized in that the device is designed such that a portion of the flue gas is introduced into the first rotating drum or behind it to lower the dew point of the rotating drum atmosphere and the vapors are discharged separately and, if necessary, dedusted, ie, not flow more through the second rotary drum ( 25 ) and its dedusting system. 17. The device, in particular for performing the method according to one or more of claims 1 to 5 or according to one of claims 6, 7, 8 or 10, characterized in that a preventive fire protection device ( 32, 33, 34 ) for removing dusty deposits combustible Components from the vapors from the vapor lines - at least from line parts which are connected to parts of the system whose temperature is higher than the dust ignition temperature - are intended for preferred use during operation. 18. The apparatus according to claim 17, characterized in that the preventive fire protection device has a wash-off device ( 32 ), a separation zone ( 33 ) for separating the sludge from the vapors and a sludge removal device ( 34 ).