EP1564515A2

EP1564515A2 - Method for drying wet waste materials

Info

Publication number: EP1564515A2
Application number: EP05397006A
Authority: EP
Inventors: Ilkka Ilmavirta
Original assignee: Vapo Oy
Current assignee: Vapo Oy
Priority date: 2004-02-17
Filing date: 2005-02-17
Publication date: 2005-08-17
Also published as: FIU20050396U0; FI7014U1; FI20045042A0; EP1564515A3

Abstract

The invention relates to a method in the drying of wet materials, such as sludges and bark mass, in which the materials to be dried are spread as a thin layer (16) on a base (12) and solar energy is used in the drying, by exposing the spread material to the effect of solar radiation and the wind. The drying is performed in rapid cycles, in periods of at most 3 days, on the basis of the forecast evaporation.

Description

The present invention relates to a method in drying wet waste materials, such as sludges and bark mass, in which the materials to be dried are spread on an essentially waterproof base and solar energy is exploited in the drying, in which the initial moisture content of the materials is 60 - 90 % and the target moisture content 30 - 55 %, and in which the drying takes place as batch drying, by spreading one batch of the material to be dried at a time on the base, and removing it after drying for further use. For a bark mass, the target moisture content is usually 45 - 50 % and for industrial sludges 35 - 40 %.
In industry creates enormous amounts of wet byproducts and masses classified as waste, the disposal of which is a financial problem. Especially the sludges and bark masses of the forest-products industry are waste masses that are difficult to recycle. It is very expensive to dry them thermally using external energy, as water cannot be easily removed from the cellular structure of a biomass. These masses are difficult to dry effectively using solar energy, because the drying effect scarcely penetrates deeper than the surface. This has even led to the disposal of wet waste fractions by burning them in a boiler with the aid of a considerable amount of support fuel. It is quite normal for a pulp mill to produce the said byproducts in the order of 100 000 m³. An industrial plant of this kind also produces huge amounts of low-temperature waste heat, which has not been suitable for drying masses.
Several solutions are known for exploiting solar energy for drying waste sludges in a greenhouse-like drying plant. Such solutions are disclosed, for example, in the publications WO 2004/020922, WO 2004020921, and EP 1416 241. All of these typically use air blown from beneath. This allows the layer thickness to be increased to a considerable height. As blowing begins to form channels as the drying proceeds, the layer must be mixed from time to time. This kind of mechanical drying is, however, a difficult and expensive solution.
Japanese application publication JP5261707 additionally discloses the exploitation of waste heat accumulated under the glass roof of a plant, in addition to solar energy, in the drying of bark and timber waste, which is conveyed under this glass roof. The solution requires quite complex conveyor solutions.
Attempts have been made to dry bark mass in thick (50 - 100 cm) layers outdoors. Once the surface of the layer has dried, a thin layer can be removed. The results are quite poor and yields for an entire season have remained low. Though in summer evaporation is exceptionally great at times, on the other hand rain frequently wets the entire layer.
The invention is intended to create a new type of method for drying wet waste materials, with the aid of which an industrial plant can solve its waste problem more economically than before. The characteristic features of the invention are stated in the accompanying Claims. A quite surprising observation is that in variable weather conditions, such as those in Finland, the best result is achieved using very short-cycle drying, particularly when it takes account of the expected evaporation, based on weather forecasts. This is due to the fact that using this method the material can be dried even on days on which it rains heavily for part of the day. For example, the forecast may predict great evaporation during the morning, but heavy rain in the afternoon. The length of the drying period is then set to that of the period of great evaporation and the spreading thickness is defined to be thin enough for the target moisture content to be achieved during this period. The material is then removed before the rain. Such days are numerous. In this method, a rainy day is not allowed to break the drying period, which must be attempted to be defined beforehand, according to the weather forecast.
Using the method according to the invention, the spreading thickness can vary within wide limits. Normally, the spreading thickness in the daily cycle is 2 - 4 cm (the average evaporation on a sunny summer day is 4,5 mm /day), though the spreading thickness can be exceptionally as much as 10 cm, when the evaporation is very great. The combined effect of the heated base and an exceptional drying day can raise evaporation to even 20 mm/day, in which case the layer thickness can be even greater than the above.
Similarly, the layer thickness can be greater, if the drying period is extended to two or three days in good conditions (continued high pressure - reasonable evaporation).
Another important factor is that the mass spread on the base should be coarse. Often the waste material (sludge) is sufficiently coarse naturally, but if necessary a sufficient amount of bark mass or, for example, peat is mixed with it.
According to one preferred embodiment, soaking wet waste material is pre-dried mechanically by pressing it to remove free water. Bark mass is sometimes extremely wet, due to either log-floating transportation or stockpile wetting. In this case, it has been observed that it is not worth using the heavy presses generally used in the industry, substantially lighter and faster pre-drying being instead sufficient in connection with field drying. Thus, a bark mass with, for example, a water content of 75 % by weight is pressed to, for example, 66 %, which reduces the duration of the actual field drying to half, or else the layer thickness can be doubled.
In the following, the invention is examined with reference to the accompanying schematic drawings showing one embodiment of the invention.

Figure 1: shows a drying field according to the invention, in connection with an industrial plant
Figure 2: shows a cross-section of a roofed field according to the invention
Figure 3: shows a drum press, used in the pre-drying of bark mass.

In this case, the asphalted base is marked with the reference number 12 and the heating piping located under the base with the reference number 10. In the drying area, ditches or drains are arranged for rainwater while the surface of the area is cambered to lead the water into them (not shown). In Figure 1, low-temperature (40 - 70 °C) waste process heat from the industrial plant 8 heats the circulating water of the ground piping 10 to the range 20 - 50 °C. However, the method can also be used when heating from beneath is not available, though heating will substantially improve the method.
Wet masses, such as purification-plant and other sludges and bark waste arise continually at the industrial plant (pulp mill). The crushing bucket of a wheeled loader can be used to grind and mix these into a mixture for drying, which is spread as an essentially even and bulky layer 16 on of the asphalt base 12. When drying, the layer thickness of the material is set in the range 1 - 10 cm, preferably 2 - 5 cm on a non-heated base and 4 - 10 cm on a heated base. Preferably, the drying period and layer thickness are defined from the evaporation assumed from the current weather forecast. The drying target (typically 45 - 50 % by weight for bark) should be achieved in the set drying period, i.e. the layer thickness for each period is defined on the basis of the drying requirements of the mass and the weather conditions.
The length of the drying period is usually 1 day or shorter, for example, 3 - 10 hours in good conditions, in which case 2 - 4 harvests per day will be obtained. The thickness of the layer being spread is determined according to the evaporation conditions. Using bark mass or other auxiliary masses, the coarseness of the material being dried is set to be large enough that its particle size is 10 - 30 % over a 50 % share. Drying will then take place through the entire layer.
The base is partly roofed, but is completely open to the wind. The canopy 18 is therefore set on pillars 20, 22. The canopy 18 is sufficiently high for a machine to fit easily to work under it. The sloping roof is transparent over at least a large area, to allow solar radiation to heat through it. The effective drying area is preferably even considerably larger than the roofed area, as the roofed space is only required as protection during periods of rain.
Heating from beneath substantially improves the drying efficiency. Effective drying will now take place at all seasons, even when the sun is not shining directly. Even in winter, there are often good conditions for drying, because the air is very dry. Only periods of torrential rain, when rainwater driven by a high wind can wet the mass, cannot be included in the drying periods. The wetting damage of even these periods can be reduced, if the mass is gathered under protection on the basis of weather forecasts. The heating from beneath rapidly dries and heats the asphalt after rain. The heating of the field prevents the bark mass from freezing into a solid lump during winter storage.
The harvesting equipment used is the simplest possible, such as a wheeled loader 13 equipped with a crushing sieve bucket 14 (mesh size 40 mm, generally 25 - 60 mm), a manure-spreader device, collector trailers, a bucket, and, for example, a ridge windrower, with the aid of which the dry mass is collected into stockpile ridges. With the aid of the invention it is calculated that a production of at least 30 000 - 120 000 cubic metres per hectare per annum (m²/ha/a) can be achieved.
The use of the method and equipment according to the invention achieves the following advantages, among others:

maximum utilization of the waste heat of the industrial plant and of solar energy,
maximization of drying effect, because:
- the production season is lengthened, as there are no frost-heave, frost-damage, base moisture, and coldness problems.
- when the asphalt is heated, it is possible to start early in spring and continue until late in the autumn, or even continue the whole year round, to start quickly after rain, and to improve the drying of the mass even at night,
- drying becomes more efficient, when drying energy is obtained both as radiation from above and as heat from beneath,
- short spells of dry weather can be exploited, as the base does not absorb moisture, and
- weather conditions can be exploited more precisely, because the mass can be rapidly harvested from small surface areas,
environmental-friendliness,
reduction of weather risk/storage requirement.

From the point of view of environmental friendliness in particular, it is important that operations take place in one small surface area at a time, thus reducing environmental and other detriments. The most important aspect is the conversion of difficult waste to a usable form, with the aid of a fairly small amount of primary energy.
Various tests can be made to determine the rate of drying of a mass on asphalt, or on other surfaces in different situations. Though costs arise from the equipment and drying fields, the increased and particularly more certain production than before creates a competitive alternative for the treatment of wet byproducts.
It is possible that, in the case of biomasses, effective drying is affected by a small degree of composting, which opens the cell structure. On the other hand, it may be advantageous when utilizing waste heat to raise the heating temperature slightly with the aid of a heat pump, for which a very high thermal coefficient is obtained in this case. On the other hand, fans can be used to create an artificial wind, which will significantly improve the result on a calm day. The heating power can be dependent on the state of the weather, for example, a greater heating power appears to be useful in winter.
Bark mass coming from a de-barking plant is extremely wet, so that it is worth pre-drying it mechanically prior to field drying. In Figure 3, the free water is effectively removed from the bark mass using a drum dryer. In the figure, the drum is marked with the reference number 26 and the chain-mesh conveyor rotating against it with the reference number 28. This rotates through the effect of friction created by the rotation of the drum 26 and the bark mass along with it. The bark mass C is brought by the conveyor 24 to the chain-mesh conveyor 28, which consists of a chain mesh 31 set on idlers 29 and 30, as well as spring-loaded counter drums 33 arranged inside the mesh loop. These press the mass against the drum 26, so that water is pressed out of the mass and the mechanically dried mass drops onto a lower conveyor 35. The speed of rotation of the drum can be used to vary the pressing time. By setting different speeds in the drum and in the chain mesh, rubbing is also created, which further increases water removal from the bark mass.

In the above, the spreading layers have been referred to nominally in centimetres. This is, however, often an imprecise definition, as the layer thickness cannot be measured precisely. Instead, the spreading layer can be defined precisely with the aid of square masses, from which it is also possible to calculate the amount of water to be evaporated. Thus, the 5-cm spreading layer referred to above, of a bark mass with a water content of 75 %, refers to a 3,6 kg/m² layer of solids, which will dry to a 50 %-target moisture content in 10 hours.

evaporation 4,5 mm/day	drying time (h) - target 50 %
amount spread	without base heating	on a heated base
75 % bark cm	solids kg/m²	75 %	66 %	75 %	66 %
2,5	1,8	10	5	5	2,5
5	3,6	-	10	10	5
10	7,2	-	-	-	10

In the above, the effect of heating from beneath is from calculation. Though its output would only be 2 - 10 % of maximum solar radiation power (800 W/m²), its continuous operating time makes it a significant addition, as it is 10 - 50 % of the 150 W/m² daily mean power of the production season. For example, using even a modest level of heating from beneath, the drying days can increase from 100 to 200 annually.

Claims

A method in the drying of wet materials, such as sludges and bark mass, in which the materials to be dried are spread on an essentially waterproof base (12) and solar energy is exploited in the drying, in which the initial moisture content of the materials is 60 - 90 % and the target moisture content is 30 - 55 %, and in which drying takes place as batch drying, by spreading a batch at a time of the material to be dried as a layer (16) on the base (12) and removing it for further use from the base (12) after drying, characterized in that

the drying is carried out on an essentially open base (12) by exposing the spread material directly to the effect of solar radiation and the wind,

the coarseness and spreading thickness of the mass to be dried are set in such a way that drying can be performed in short cycles, in periods of at most 3 days, more preferably of 4 - 24 h.
A method according to Claim 1, characterized in that the materials are ground and spread using loader (13) equipped with a crushing sieve bucket (14) to form an even layer on the base (12).
A method according to Claim 1 or 2, characterized in that the waste process heat from an industrial plant (8) is led, using the closed circulation (10) of a medium to the base (12), in order to heat the material from beneath as well.
A method according to any of Claims 1 - 3, characterized in that the layer thickness (16) of the material when drying is set in the range 1 - 10 cm, preferably 2 - 5 cm on a non-heated base and 4 - 10 cm on a heated base (12).
A method according to any of Claims 1 - 3, characterized in that the layer thickness (16) of the material when drying is set essentially according to the evaporation according to the weather forecast achieves the set drying target using this set layer thickness.
A method according to any of Claims 1 - 5, characterized in that the base (12) is an asphalt base.
A method according to any of Claims 1 - 6, characterized in that at least part of the base (12) is roofed (18), in order to reduce losses due to rain.
A method according to any of Claim 3, characterized in that the heating of the base (12) takes place using low-temperature, i.e. app. 20 - 50 °C circulating water.
A method, according to any of Claims 1 - 8, in the drying of a bark mass, characterized in that the bark mass is pre-dried in a special drum dryer, in which water is pressed out of the bark mass by leading it between a rotating drum (26) and a chain (31).
A method according to any of Claims 1 - 9, characterized in that the base (12) is allowed to heat through the effect of solar radiation for 1 - 3 h, between the previous batch being removed and a new batch being spread.