With the rolling bed dryer, Allgaier has developed and patented a new type of dryer, which has now been launched on the market. It combines the advantages of various familiar dryers, at the same time as excluding many of the disadvantages associated with individual dryer designs. The rolling bed dryer is suitable for drying wood chips, green waste and landscape management wood as well as sugar beet pulp, bagasse, fruit marc and other irregularly shaped organic residues and waste such as domestic waste or paper industry residues. The rolling bed dryer can use the waste heat or hot exhaust gases which arise in many recycling processes, putting them to optimum use in the drying process.
Fuels from biowaste offer a range of advantages compared to conventional energy sources, both economically and ecologically. Usually, they occur locally, and thus reduce transport costs as well as the associated emissions, permitting CO2-neutral combustion. The existing combustion technology can be used if the secondary fuels are prepared in such a way that they can be used similarly to conventional fuels. In this regard, drying of the residues to be processed represents an important issue. The rolling bed dryer from Allgaier Process Technology GmbH meets all the requirements in this regard. (Fig. 1).
Drying organic residues
There are numerous examples of preparing organic residues to make replacement fuels, such as the manufacture of pellets from woodchips, production of briquettes from waste wood, green waste and bark as well as using waste from the paper industry. Other organic residues can range from digested residues, sugar beet pulp, bagasse, domestic waste, fruit marc and grape marc through to horse manure. The various residues are very easy to store in dry condition. Biological degradation processes are suppressed and the calorific value is increased.
(click on picture to enlarge)
Fig. 1:
Allgaier rolling bed dryer at the
"Interpellets 2011" show in Stuttgart
Depending on the substance, the values for dried biomass can be between 10 MJ/kg and 18 MJ/kg. This means 2 to 3 kg of secondary fuel is enough to replace one litre of heating oil. Biofuels are environmentally friendly and represent a valuable source of energy because they can be burned as required, at the same time as achieving high process temperatures. Many of them arise as bulky and awkward materials containing impurities and, usually, a high level of moisture. They also have irregular shapes and sizes, are insensitive to temperature and liberate volatile constituents. Last but not least, it is necessary to comply with the requirements of fire and explosion protection. All of these properties place particular requirements on the plant technology used for drying.
Requirements on the ideal biomass dryer
Drying represents an important process step on the path from the waste material to the secondary fuel. Various classic dryer types already exist for this purpose, such as drum dryers, fluidised bed dryers or belt dryers, and each offers advantages and disadvantages.
Drum dryers or rotary drum dryers are large, heavy apparatus for drying the product with the drying gases in the in-flow principle. The interior of drum dryers is difficult to access if multi-stage drum dryers or crosswise installations are used in order to make the drying process more effective. In order to operate drum dryers with good energy efficiency, they have to be operated at very high drying air temperatures, thus increasing the fire risk.
Fluidised bed dryers or vibration fluidised bed dryers operate with a crossways flow of air and the material being dried. The product moves through the apparatus in the form of a flat fluidised bed. This means fluidised bed dryers are only suitable for homogenous bulk materials with regular particle shapes and sizes. Interlocking products form "mats" which cannot be fluidised. Large bulk heights of irregularly shaped materials cannot be used in vibration fluidised bed dryers, because the products can then no longer be fluidised. This means fluidised bed dryers can only achieve relatively short holding times, thus reducing the effectiveness of drying. The air distributor plates of fluidised bed dryers must be configured for a significant air-side pressure loss, involving the use of high pressure fans.
Belt dryers are characterised by having layers of product resting on moving belts, as a result of which the bulk material on the belt takes on different product moisture levels, leading to the risk of individual product layers overheating. Where lumpy or particularly irregular products are involved, it is difficult to achieve a consistent and even distribution of the material over the belt. Only relatively low drying temperatures can be used, in order to avoid local overheating of the solid material or the emergence of volatile components such as lignin from wood. This results in very large waste air volumes, meaning that the dryers have to be particularly large. If separate waste air treatment by cyclones or bag filter systems is not used then the large waste air flow rate means that significant total volumes of dust are emitted even if the specific residual dust loads are low. Due to having many rotating parts, belt dryers require intensive maintenance.
When considering the advantages and disadvantages of various traditionally available dryer systems, the following wish list arises for an optimum biomass dryer:
•Long solid material holding times
•High bulk heights of the drying material in the process space
•Good mixture of the bulk material during drying
•Use of residual energy on various temperature levels
•Suitability for a very wide range of products
•Low pressure loss in the air flow
•Variability of the air flow in the dryer
•Dedusting and cleaning effect on the drying material
•Simple design and small dimensions
•Few moving parts in the dryer
•Good accessibility
• Minimum maintenance complexity
The concept of the rolling bed dryer (WB-T)
The rolling bed dryer, WB-T for short, is Allgaier Process Technology GmbH's response to the requirements listed above.
The WB-T combines the advantages of conventional drying processes in one apparatus. The dryer offers a compact product bed for optimum heat transfer. The bulk product is comparatively high and is moved evenly by slowly rotating agitator elements. The drying air flows through the solid material to be dried from the bottom to the top, with a relatively long contact time between the air and the drying material. This means homogenous drying is achieved, even with the most difficult products, with a high heat transition rate. The high level of bulk material can be adapted to the solid material moisture levels, which are usually very high at up to 70% in some cases, thereby achieving very long solid material holding times.
The solid material is transported in the dryer at right angles to the air flow from the infeed to the outfeed by a combination of the air flowing through it and movement of the agitator elements; at the end of the dryer, it emerges at a weir. There is a rolling motion of the drying material, especially on one side of the inside space of the dryer. This, in conjunction with ventilation of the solid material, makes it possible to achieve low drive power levels for the agitator elements. The recirculation of the solid material is responsible for the main "rolling bed dryer". It was modelled on the manual work which people carry out when forking over green waste (Figure 2).
(click on picture to enlarge)
Fig. 2:
Principle of the rolling bed dryer WB-T
Also, solid materials with highly inhomogeneous formats or bulky materials such as branches, twigs, bark and leaves in green waste or irregularly sized wood chippings can be reliably transported through the dryer. In spite of a slight pressure loss in the airflow, the mechanical mixture and distribution of the solid material means that the airflow is very evenly distributed. The mechanical mixing and associated transport of the solid material means that the rolling bed dryer does not rely on fluidisation by the drying air. The air distributor plate can be designed for a very low pressure loss.
The continuous mixing and movement of the dry material means that localised product overheating is avoided, and so higher air temperatures can be used for drying than in the case of belt dryers, for example. Also, because of the crossways flow between the air and the material as well as the long solid material holding times, it is possible to achieve very effective process management with low waste air and product temperatures, even if the temperatures available in the drying gases are relatively low.
As a result, the new dryer makes effective use of available residual energies even at a low temperature level, such as frequently occur in large amounts as part of many recycling operations. The low waste air and product temperatures prevent evaporation of volatile constituents and their emission, even at relatively high air inlet temperatures. This effect has been proven, for example, when drying sugar beet pulp.
The variably adjustable amount of air and the long solid material holding times guarantee a dedusted end product with consistent residual moisture. The continuous mixture of the drying material removes mineral impurities such as sand and soil from the materials being dried, allowing them to drop through the air distributor plate and be transported out by means of the equipment under the dryer and a separate auger conveyor. For example, if wood chips are to be pelletised in subsequent process steps, the fact that the chips have been purified significantly reduces wear on the pellet presses.
The Allgaier rolling bed dryer WB-T solves the problems of conventional dryer types when dealing with organic residues. Worldwide patents have been registered. With the rolling bed dryer, Allgaier has extended its already wide range of available products to include a further interesting technology.
Application bandwidth
Other difficult products such as refuse-derived fuel (RDF), fruit and grape marc through to lumpy mixtures of straw and horse manure can also be dried in the rolling bed dryer. Even sticky products with very high input moisture levels of 70% and more can be brought to the required final moisture level using residual energy, and leave the dryer as an evenly pourable bulk material.
Test plant at the Allgaier Technical Centre
There is an apparatus with a 3x1 metre dryer base surface available for experimental tests at the AllgaierTechnical Centre.This has allowed a large variety of different products to be tested. The test plant makes it possible to obtain product-specific drying properties which can be used for the reliable configuration of production systems. Figure 3 shows a view into the drying chamber of the test plant. The plant is available to all potential customers for investigations on their own products or for demonstration purposes.
(click on picture to enlarge)
Fig. 3:
View into the drying chamber of the test plant
Reference Systems
Figure 4 shows the construction site at Topell Energy in The Netherlands. There, two parallel rotary bed dryers are installed for drying green waste, wood chips and other organic residues. Topell uses the predried materials from the Allgaier dryers to produce fuel pellets using the company's own process. This involves pyrolysis or "torrefaction", resulting in pellets that are ideal to transport and store and can be ground for use in conventional coal-fired power stations for CO2-neutral power generation.
(click on picture to enlarge)
Fig. 4:
The Topell Energy construction site with
two large Allgaier rolling bed dryers
Further applications have been installed for pre-drying of wood chips for wood gasification and electricity generation in a gas engine. The rolling bed dryers used for this purpose are supplied in the form of a compact unit which can be installed extremely quickly (Figure 5). At the present time, they are the smallest size of rolling bed dryer that can be rationally installed.
Drying Organic Residues
Drying organic residues in the rolling bed dryer
Hermann Weiß, Mathias Trojosky, Markus Klein,
Allgaier Process Technology GmbH, Uhingen
With the rolling bed dryer, Allgaier has developed and patented a new type of dryer, which has now been launched on the market. It combines the advantages of various familiar dryers, at the same time as excluding many of the disadvantages associated with individual dryer designs. The rolling bed dryer is suitable for drying wood chips, green waste and landscape management wood as well as sugar beet pulp, bagasse, fruit marc and other irregularly shaped organic residues and waste such as domestic waste or paper industry residues. The rolling bed dryer can use the waste heat or hot exhaust gases which arise in many recycling processes, putting them to optimum use in the drying process.
Fuels from biowaste offer a range of advantages compared to conventional energy sources, both economically and ecologically. Usually, they occur locally, and thus reduce transport costs as well as the associated emissions, permitting CO2-neutral combustion. The existing combustion technology can be used if the secondary fuels are prepared in such a way that they can be used similarly to conventional fuels. In this regard, drying of the residues to be processed represents an important issue. The rolling bed dryer from Allgaier Process Technology GmbH meets all the requirements in this regard. (Fig. 1).
Drying organic residues
There are numerous examples of preparing organic residues to make replacement fuels, such as the manufacture of pellets from woodchips, production of briquettes from waste wood, green waste and bark as well as using waste from the paper industry. Other organic residues can range from digested residues, sugar beet pulp, bagasse, domestic waste, fruit marc and grape marc through to horse manure. The various residues are very easy to store in dry condition. Biological degradation processes are suppressed and the calorific value is increased.
(click on picture to enlarge)
Fig. 1:
Allgaier rolling bed dryer at the
"Interpellets 2011" show in Stuttgart
Depending on the substance, the values for dried biomass can be between 10 MJ/kg and 18 MJ/kg. This means 2 to 3 kg of secondary fuel is enough to replace one litre of heating oil. Biofuels are environmentally friendly and represent a valuable source of energy because they can be burned as required, at the same time as achieving high process temperatures. Many of them arise as bulky and awkward materials containing impurities and, usually, a high level of moisture. They also have irregular shapes and sizes, are insensitive to temperature and liberate volatile constituents. Last but not least, it is necessary to comply with the requirements of fire and explosion protection. All of these properties place particular requirements on the plant technology used for drying.
Requirements on the ideal biomass dryer
Drying represents an important process step on the path from the waste material to the secondary fuel. Various classic dryer types already exist for this purpose, such as drum dryers, fluidised bed dryers or belt dryers, and each offers advantages and disadvantages.
Drum dryers or rotary drum dryers are large, heavy apparatus for drying the product with the drying gases in the in-flow principle. The interior of drum dryers is difficult to access if multi-stage drum dryers or crosswise installations are used in order to make the drying process more effective. In order to operate drum dryers with good energy efficiency, they have to be operated at very high drying air temperatures, thus increasing the fire risk.
Fluidised bed dryers or vibration fluidised bed dryers operate with a crossways flow of air and the material being dried. The product moves through the apparatus in the form of a flat fluidised bed. This means fluidised bed dryers are only suitable for homogenous bulk materials with regular particle shapes and sizes. Interlocking products form "mats" which cannot be fluidised. Large bulk heights of irregularly shaped materials cannot be used in vibration fluidised bed dryers, because the products can then no longer be fluidised. This means fluidised bed dryers can only achieve relatively short holding times, thus reducing the effectiveness of drying. The air distributor plates of fluidised bed dryers must be configured for a significant air-side pressure loss, involving the use of high pressure fans.
Belt dryers are characterised by having layers of product resting on moving belts, as a result of which the bulk material on the belt takes on different product moisture levels, leading to the risk of individual product layers overheating. Where lumpy or particularly irregular products are involved, it is difficult to achieve a consistent and even distribution of the material over the belt. Only relatively low drying temperatures can be used, in order to avoid local overheating of the solid material or the emergence of volatile components such as lignin from wood. This results in very large waste air volumes, meaning that the dryers have to be particularly large. If separate waste air treatment by cyclones or bag filter systems is not used then the large waste air flow rate means that significant total volumes of dust are emitted even if the specific residual dust loads are low. Due to having many rotating parts, belt dryers require intensive maintenance.
When considering the advantages and disadvantages of various traditionally available dryer systems, the following wish list arises for an optimum biomass dryer:
•Long solid material holding times
•High bulk heights of the drying material in the process space
•Good mixture of the bulk material during drying
•Use of residual energy on various temperature levels
•Suitability for a very wide range of products
•Low pressure loss in the air flow
•Variability of the air flow in the dryer
•Dedusting and cleaning effect on the drying material
•Simple design and small dimensions
•Few moving parts in the dryer
•Good accessibility
• Minimum maintenance complexity
The concept of the rolling bed dryer (WB-T)
The rolling bed dryer, WB-T for short, is Allgaier Process Technology GmbH's response to the requirements listed above.
The WB-T combines the advantages of conventional drying processes in one apparatus. The dryer offers a compact product bed for optimum heat transfer. The bulk product is comparatively high and is moved evenly by slowly rotating agitator elements. The drying air flows through the solid material to be dried from the bottom to the top, with a relatively long contact time between the air and the drying material. This means homogenous drying is achieved, even with the most difficult products, with a high heat transition rate. The high level of bulk material can be adapted to the solid material moisture levels, which are usually very high at up to 70% in some cases, thereby achieving very long solid material holding times.
The solid material is transported in the dryer at right angles to the air flow from the infeed to the outfeed by a combination of the air flowing through it and movement of the agitator elements; at the end of the dryer, it emerges at a weir. There is a rolling motion of the drying material, especially on one side of the inside space of the dryer. This, in conjunction with ventilation of the solid material, makes it possible to achieve low drive power levels for the agitator elements. The recirculation of the solid material is responsible for the main "rolling bed dryer". It was modelled on the manual work which people carry out when forking over green waste (Figure 2).
(click on picture to enlarge)
Fig. 2:
Principle of the rolling bed dryer WB-T
Also, solid materials with highly inhomogeneous formats or bulky materials such as branches, twigs, bark and leaves in green waste or irregularly sized wood chippings can be reliably transported through the dryer. In spite of a slight pressure loss in the airflow, the mechanical mixture and distribution of the solid material means that the airflow is very evenly distributed. The mechanical mixing and associated transport of the solid material means that the rolling bed dryer does not rely on fluidisation by the drying air. The air distributor plate can be designed for a very low pressure loss.
The continuous mixing and movement of the dry material means that localised product overheating is avoided, and so higher air temperatures can be used for drying than in the case of belt dryers, for example. Also, because of the crossways flow between the air and the material as well as the long solid material holding times, it is possible to achieve very effective process management with low waste air and product temperatures, even if the temperatures available in the drying gases are relatively low.
As a result, the new dryer makes effective use of available residual energies even at a low temperature level, such as frequently occur in large amounts as part of many recycling operations. The low waste air and product temperatures prevent evaporation of volatile constituents and their emission, even at relatively high air inlet temperatures. This effect has been proven, for example, when drying sugar beet pulp.
The variably adjustable amount of air and the long solid material holding times guarantee a dedusted end product with consistent residual moisture. The continuous mixture of the drying material removes mineral impurities such as sand and soil from the materials being dried, allowing them to drop through the air distributor plate and be transported out by means of the equipment under the dryer and a separate auger conveyor. For example, if wood chips are to be pelletised in subsequent process steps, the fact that the chips have been purified significantly reduces wear on the pellet presses.
The Allgaier rolling bed dryer WB-T solves the problems of conventional dryer types when dealing with organic residues. Worldwide patents have been registered. With the rolling bed dryer, Allgaier has extended its already wide range of available products to include a further interesting technology.
Application bandwidth
Other difficult products such as refuse-derived fuel (RDF), fruit and grape marc through to lumpy mixtures of straw and horse manure can also be dried in the rolling bed dryer. Even sticky products with very high input moisture levels of 70% and more can be brought to the required final moisture level using residual energy, and leave the dryer as an evenly pourable bulk material.
Test plant at the Allgaier Technical Centre
There is an apparatus with a 3x1 metre dryer base surface available for experimental tests at the AllgaierTechnical Centre.This has allowed a large variety of different products to be tested. The test plant makes it possible to obtain product-specific drying properties which can be used for the reliable configuration of production systems. Figure 3 shows a view into the drying chamber of the test plant. The plant is available to all potential customers for investigations on their own products or for demonstration purposes.
(click on picture to enlarge)
Fig. 3:
View into the drying chamber of the test plant
Reference Systems
Figure 4 shows the construction site at Topell Energy in The Netherlands. There, two parallel rotary bed dryers are installed for drying green waste, wood chips and other organic residues. Topell uses the predried materials from the Allgaier dryers to produce fuel pellets using the company's own process. This involves pyrolysis or "torrefaction", resulting in pellets that are ideal to transport and store and can be ground for use in conventional coal-fired power stations for CO2-neutral power generation.
(click on picture to enlarge)
Fig. 4:
The Topell Energy construction site with
two large Allgaier rolling bed dryers
Further applications have been installed for pre-drying of wood chips for wood gasification and electricity generation in a gas engine. The rolling bed dryers used for this purpose are supplied in the form of a compact unit which can be installed extremely quickly (Figure 5). At the present time, they are the smallest size of rolling bed dryer that can be rationally installed.
(click on picture to enlarge)
Fig. 5:
Compact rolling bed dryer for small
and medium-sized applications
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