In practice, the initial moisture of biofuels varies with time, mainly depending on the season, weather and the origin of the fuel. For example, ac- cording to moisture analyses by a Finnish pulp and pa- per factory, the moisture of biofuel varied between 53 - 62 % in the course of a year. This means in practice that when the moisture content of the fuel is highest, it contains 45 % more water than in its driest state.

Actually, the range of variation is probably even larger because moisture analyses are not undertaken very regularly.

If the fuel is to be dried to a given final moisture level, e. g. to about 20 w-%, then it is obvi- ous that the final moisture will vary depending on the initial moisture unless the operation mode of the drier is regulated on the basis of the initial mois- ture. Above all, in respect of boiler space utiliza- tion, but also in respect of energy consumption in the factory as a whole, it would be desirable to keep the moisture of the fuel supplied into the boiler continu- ously about the same regardless of the various change- able factors affecting the initial moisture of the biofuel.

At present, most biofuel is burned as undried bark waste in industry, the main purpose of combustion being merely to destroy the waste. The moisture of bark waste varies greatly. In hot summer weather, it may be very dry, but when still fresh directly after the debarking or during the rainy period in fall, it is dripping wet. and in winter often icy and covered

with snow. Some of the biofuel may consist of munici- pal waste, chipped construction waste, chipped cut- tings and stumps from forest felling areas or chipped wood mass saved in connection with improvement fell- ing. Thus, it is understandable that the treatment and burning of biofuel supplied to a power plant requires accurate planning and regulation if it is to be used to energy economical advantage instead of merely de- stroying it. Above all in respect of boiler utiliza- tion and combustion emissions, but also in respect of overall energy utilization in the factory, it would be desirable to keep the moisture of the fuel supplied into the boiler continuously about the same, regard- less of the various factors affecting the moisture.

The object of the invention is to eliminate the above-mentioned drawbacks. A specific object of the invention is to disclose a new type of control method that can be used for drying biofuel so that the fuel supplied to a power plant will be as homogeneous as possible in respect of its moisture level.

As for the features of the invention, refer- ence is made to the claims.

In the method of the invention for regulating an at least two-stage drying method for the drying of biofuel in different drying stages, the biofuel is dried using different heat sources derived alterna- tively from an industrial process and/or from a power plant, e. g. using secondary heat generated in the in- dustrial process and power plant or back-pressure steam and bled steam of the power plant. According to the invention, the drying of the fuel is measured sub- stantially continuously in at least one drying stage, and the heat sources to be used in different consecu- tive drying stages are selected and controlled on the basis of the measurement results. Thus, the essential point in the method of the invention is that the mois- ture of biofuel is measured in a substantially con-

tinuous process, via on-line measurement. The measure- ments can be taken at suitable intervals or as a fully continuous measuring process, yet without taking any samples to be analyzed from the fuel in the tradi- tional manner but by measuring the moisture of a mov- ing biofuel mass flowing towards a combustion stage.

The regulation method of the invention aims at regu- lating the temperature of drying gases supplied into different drying stages, i. e. selecting the heat sources so that the final moisture of the fuel to be dried will be about the same within a given range of variation. Another aim is to select the heat sources for the drying process in a manner as advantageous as possible in respect of energy economy. In other words, secondary heat is used whenever possible and when it brings an advantage. Likewise, bleed steam is only used when the power or available supply of back- pressure steam is insufficient.

The drying of the biofuel is preferably meas- ured in a first drying stage and, based on the drying occurring in this stage, the entire drying process is adjusted, in other words, the heat sources for all drying stages are selected. Another possibility is to measure the drying in other drying stages as well.

The drying of the biofuel is measured by measuring the temperature and/or humidity of the air that has passed through the fuel layer. It is also possible to measure some other fuel property that changes in correlation to the moisture of the fuel.

Depending on the fuel, such other properties may in- clude e. g. radiation emitted by the fuel, its color changes, weight or some other property that can be measured substantially continuously and preferably without touching the fuel.

The degree of drying of the fuel is prefera- bly measured at several points in the same drying stage in order to measure the change in the degree of

drying and the rate of change. Measurements may be taken at regular intervals or throughout the drying stage. In practice, it has been established that only 4-6 measuring points can give a sufficiently accurate indication of the progress of drying in a given drying stage. Another possibility is to use a continuous measuring method that produces a continuous curve de- scribing the drying.

In the first drying stage, preferably secon- dary heat is used as heat source. In the other drying stages too, the primary heat source is always secon- dary heat; in other words, secondary heat is always used in each drying stage when its use results in a sufficient drying result in the stage in question. The heat source to be used in the second drying stage or in one of the subsequent drying stages is selected from among the heat sources available at the power plant in each situation.

In an embodiment of the invention, comparison curves describing the progress of drying of fuels hav- ing different starting values are generated for dif- ferent fuels. After this, a drying curve corresponding to the comparison curves is generated for the fuel to be dried, said drying curve describing the drying of this fuel. When the drying curve is compared to the comparison curves, a comparison curve is found that is closest to the drying curve and that corresponds to a given initial fuel moisture value. When the initial moisture is known, the heat sources to be used in dif- ferent stages can be determined on the basis of it.

The starting values of a fuel may consist of one or more of the following, among others: moisture, tem- perature, grain size, material, ice content.

Preferably a given comparison curve corre- sponds to a given combination of heat sources to be used in different drying stages. In other words, a given comparison curve determines the heat sources

that are to be used in different drying stages in a drying process according to this comparison curve. Af- ter the drying curve of a fuel has been measured e. g. in the first drying stage, this curve is compared to the comparison curves of the corresponding fuel. The comparison curve that best corresponds to the drying curve is selected and the drying process is controlled using heat sources determined by this comparison curve. Thus, the drying process is not affected by different fuel batches having different properties or by small changes and variations in fuel properties be- cause small variations in moisture or other properties do not change the drying curve of the fuel, so if the drying curve changes somewhat, the same comparison curve still bears the closest correlation to it.

The method of the invention has significant advantages as compared with prior art. The drying tem- peratures, i. e. heat sources used in the method can be changed according to the moisture content of the fuel so that in each case the most advantageous heat source regarding energy economy is in use. Thus, sources of secondary heat generally wasted are always primarily used in the method and back-pressure heat or bleed steam is only used if necessary. When the fuel is drier, a larger proportion of its energy can be util- ized. Another significant advantage of the method is that an even fuel moisture level results in a more even combustion process in the boiler, allowing sim- pler and easier boiler control. This also leads to smoother and easier control of energy flows in the en- tire industrial process associated with the power plant.

In the following, the invention will be de- scribed in detail with reference to the attached draw- ings, wherein

Fig. 1 presents a diagrammatic representation of a drying process employing the method of the inven- tion, and Fig. 2 presents an example of curves used to de- termine the moisture of biofuel.

Fig. 1 represents a four-stage drier 1 and the energy and drying air flows associated with it, which make it possible to implement the method. The drying energy for the drier can be taken alternatively from secondary heat 2 produced by the industrial proc- ess or power plant, from the back-pressure steam 3 of the power plant or from the bleed steam 4 of the power plant.

When only secondary heat is used in the drier, drying air 5 is supplied either through one or through both of the heat exchangers 6 and 7 into all drying stages 8,9, 10 and 11 of the drier 1. When only secondary heat is used, the flows 12,13, 14 and 15 of exhaust air from different stages of the drier can no longer be used to energy-economical advantage, so they can be let out into the environment.

When the heat energy of secondary heat 2 alone is not sufficient in the drier 1, the back- pressure steam 3 is resorted to, and its heat energy is transferred into the drying air in a heat exchanger 16. This same air is preferably first heated in heat exchanger 6 using secondary heat 2. After heat ex- changers 6 and 16, the drying air is passed into one or more of the latter drying stages 10 and 11 of the drier 1 because it is always the most energy- economical alternative to use drying air heated by secondary heat in the first drying stage 8 or stages 9.

If the heat energy of the back-pressure steam is still insufficient for the need or if there is not enough of it available, then it is possible to use the bleed steam 4 of the power plant, by passing it

through a heat exchanger 17. The hot drying air ob- tained from this heat exchanger is then passed suita- bly into drying stage 11 and possibly into drying stage 10 as well.

If drying air heated using back-pressure steam or bleed steam is used in one of the drying stages, then it is probable that, regardless of its high moisture content, the gas flowing out of that stage is warm enough to be worth recovering and pass- ing either into an intermediate heating stage or into the boiler.

Fig. 2 shows the change of outlet moisture of drying air in a drying stage as a function of time during the first 10 minutes for four'different fuels having different initial moisture levels and one fuel that is icy but otherwise identical with the other fu- els, in fully identical drying conditions.

The curves are presented in the order of ini- tial moisture of the fuel to be dried; in other words, the topmost curve corresponds to an initial moisture value of 61.4 %. Thus, the outlet moisture of drying air in the case of the wettest fuel is highest and in the case of the driest fuel lowest, as is natural.

However, the curve for the icy fuel slowly rises, so the moisture content of the drying air increases slowly as the ice is melting and evaporating. Since other conditions except the initial moisture of the particles to be dried are identical, the difference between the curves must be due to the different ini- tial moisture levels.

In the method of the invention, the determi- nation of the moisture of the fuel may be based on producing, either by experimenting or by calculating, comparison curves of the type described above for dif- ferent moisture content values of the fuel to be used, showing the value e. g. in steps of 5 percentage units.

In this way, a corresponding curve describing the fuel

to be treated is generated on the basis of measuring in the drier the outlet moisture or temperature of the drying air. After this, the measured curve is compared to the comparison curves, each of which corresponds to a given fuel moisture value. The comparison curve that most closely corresponds to the measured curve gives the moisture of the fuel. Next, by choosing heat sources and other adjustments corresponding to this selected comparison curve for different drying stages, the fuel can be dried to the desired even moisture level. Thus, it is not even necessary the know the moisture value of the fuel to be treated, but when the comparison curve closest correlated with it is found, the drying process can be controlled directly by a control scheme corresponding to this curve.

In practice, it is advisable to determine the moisture or comparison curve on the basis of several consecutive measurement curves. In this way, the ef- fect of various other changeable factors on the final outlet moisture value can be diminished.

In the foregoing, the invention has been de- scribed by way of example with reference to the at- tached drawings while different embodiments of the in- vention are possible in the scope defined in the claims.