Method and apparatus for estimation of heat value using dual energy x-ray transmission and fluorescence measurements

Abstract

A method and apparatus for estimating a heating value of a biological material. The method includes irradiating of the biological material with X-ray radiation of at least two different energy levels, measuring of an amount of radiation transmitted through the biological material at these energy levels, and measuring fluorescent radiation emitted by the biological material when irradiated at these energy levels. A final estimate of the heating value is then determined based on a preliminary estimate of the heating value of the biological material based on the measured transmitted radiation and a correction value based on the fluorescent radiation.

Claims

1. A method for estimating a heating value of a biological material, comprising: irradiating the biological material with X-ray radiation of at least two different energy levels; measuring an amount of X-ray radiation transmitted through said biological material at said at least two different energy levels; measuring fluorescent X-ray radiation emitted by the biological material when irradiated at said at least two different energy levels; and determining a final estimate of the heating value of said biological material, based on a preliminary estimate of the heating value of said biological material, wherein said preliminary estimate is based on said measured transmitted X-ray radiation, and a correction value, based on said measured fluorescent X-ray radiation.

2. The method of claim 1, wherein the preliminary estimate of the heating value is based on said measured transmitted X-ray radiation in correlation with correlation values obtained by X-ray radiation transmitted through a number of different reference materials with known heating values.

3. The method of claim 1, wherein the fluorescent X-ray radiation is used to determine the amount of at least one element having an atomic number greater than 10 in the biological sample, and to determine a correction value based on the determined amount of the at least one element.

4. The method of claim 1, wherein the determination of the final estimated heating value comprises the step of subtracting the correlation value from the preliminary estimate of the heating value or adding the correlation value to the preliminary estimate of the heating value.

5. The method of claim 1, wherein the correction value is determined by estimation of a spectrum integration over at least one energy band of the spectrum of the fluorescent X-ray radiation emitted by the biological material.

6. The method of claim 5, wherein the spectrum integration is made in at least one energy band comprising at least one of: 1.5-10 keV and 25- 90 keV.

7. The method of claim 5, wherein the spectrum integration is made over at least one energy corresponding to a spectrum peak characteristic of at least one of a heavy metal, calcium, potassium, silicon, magnesium and phosphorus.

8. The method of claim 7, wherein the correction value is estimated as an addition of weighted spectrum integrations over specific energies or bands of energies of the fluorescence spectrum.

9. The method of claim 5, wherein the spectrum integration is correlated to a corresponding reduction in heating value by means of measurements of known reference materials.

10. The method of claim 2, wherein the determination of the preliminary estimate of the heating value comprises the steps of: determining a quotient between transmission estimates based on said transmission values of two of said at least two energy levels, for each combination of said at least two energy levels; multiplying each quotient with a coefficient for each quotient; and adding said quotients multiplied by said coefficients, wherein said coefficients are determined by said correlation.

11. The method of claim 10, wherein the transmission estimates in said quotients between transmission elements are logarithmic quotients of a calibrated reference value for the transmission at the energy level and the measured transmission values through the biological material at the same energy level.

12. The method of claim 11, wherein the quotients between said transmission estimates are K-values, said K-values being calculated as: $K_{\mathrm{AB}}=\frac{\ln \left(N_{0A}\text{/}{N}_A\right)}{\ln \left(N_{0B}\text{/}{N}_B\right)}$ wherein N0A, N0B are the calibrated reference values for the transmission at the two energy levels A and B, and NA, NB are the transmission values through the biological material at said energy levels.

13. The method of claim 1, wherein the biological material is transported on a conveyor line, and wherein the biological material is irradiated with X-ray radiation, of at least two different energy levels, in a plane substantially perpendicular to a direction of advancement of said conveyor line.

14. The method of claim 1, wherein the X-ray radiation of both said energy levels are emitted from a single X-ray radiation source operating in the energy range of 20-150 kVp.

15. An apparatus for estimating a heating value of a biological material, comprising: an X-ray radiation source for irradiation of a biological material with X-ray radiation of at least two different energy levels; a detector for receiving X-ray radiation transmitted through said biological material, said detector determining, for each energy level, the amount of X-ray radiation transmitted through the biological material; a fluorescence detector for measuring fluorescent X-ray radiation emitted by the biological material when irradiated at said at least two different energy levels; and a controller arranged to determine a final estimate of the heating value of said biological material based on a preliminary estimate of the heating value of said biological material, wherein said preliminary estimate is based on said measured transmitted X-ray radiation, and a correction value, based on said measured fluorescent X-ray radiation.

16. A method for estimating a heating value of a biological material, comprising: irradiating the biological material with X-ray radiation of at least two different energy levels; measuring an amount of X-ray radiation transmitted through said biological material at said at least two different energy levels; measuring fluorescent X-ray radiation emitted by the biological material when irradiated at said at least two different energy levels; and determining a final estimate of the heating value of said biological material, based on a preliminary estimate of the heating value of said biological material, wherein said preliminary estimate is based on said measured transmitted X-ray radiation, and a correction value, based on said measured fluorescent X-ray radiation, wherein the determination of the final estimated heating value comprises the step of subtracting the correlation value from the preliminary estimate of the heating value or adding the correlation value to the preliminary estimate of the heating value.

17. An apparatus for estimating a heating value of a biological material, comprising: an X-ray radiation source for X-ray irradiation of a biological material with X-ray radiation of at least two different energy levels; a detector for receiving X-ray radiation transmitted through said biological material, said detector determining, for each energy level, the amount of X-ray radiation transmitted through the biological material; a fluorescence detector for measuring fluorescent X-ray radiation emitted by the biological material when irradiated at said at least two different energy levels; and a controller arranged to determine a final estimate of the heating value of said biological material based on a preliminary estimate of the heating value of said biological material, wherein said preliminary estimate is based on said measured transmitted X-ray radiation, and a correction value, based on said measured fluorescent X-ray radiation, wherein the determination of the final estimated heating value comprises the step of subtracting the correlation value from the preliminary estimate of the heating value or adding the correlation value to the preliminary estimate of the heating value.

18. The method of claim 5, wherein the spectrum integration is made in an energy band of 1.5-10 keV.

19. The method of claim 5, wherein the spectrum integration is made in an energy band of 25- 90 keV.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For exemplifying purposes, the invention will be described in closer detail in the following with reference to embodiments thereof illustrated in the attached drawings, wherein:

(2) FIG. 1 schematically illustrates a measurement device for estimating a heating value in a biological material transported on a conveyor line; and

(3) FIG. 2a-b schematically illustrates an embodiment of the invention where the material to be measured is arranged in a sample container, wherein FIG. 2a is a schematic top view of the measurement apparatus, and FIG. 2b is a simplified side view of the apparatus of FIG. 2a, where some of the components of the apparatus as shown in FIG. 2a have been excluded for increased clarity; and

(4) FIG. 3 is a graph showing the correlation between heating values estimated based on the method of the present invention, and heating values measured in an adiabatic bomb calorimeter, for a number of different biological materials.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) FIG. 1 schematically illustrates an embodiment of a measurement device 100 for estimating a heating value of a biological material 102 transported on a conveyor line 103. The biological material 102 may typically be wood chips, or other biomass fuels.

(6) If the height and properties of the material varies, it is preferred to scan essentially all of the material moved past the measurement device. If there is no significant variation in height and material properties over time, it may suffice to measure in a single point or target area.

(7) In order to scan essentially all of the material, the measurement device comprises a radiation source 104 adapted to irradiate a target area 105 that spans across the width of the conveyor line. The radiation source 104 is adapted to provide radiation of at least two different energy levels/wavelengths. Preferably, the radiation source is an X-ray tube for provision of X-ray radiation of two or more different energies. Preferably, the X-ray tube operates in the range 20-150 kVp. The output radiation from the radiation source is preferably directed towards the target area through a collimator and a lens (not shown). The radiation source 104 is controlled by means of a controller 106.

(8) Alternatively, the radiation source 104 may comprise two or several separate juxtaposed radiation tubes, wherein the juxtaposed radiation sources radiate the different energies either simultaneously or sequentially. However, preferably the radiation having different energies traverses the material to be measured along essentially the same path. When radiation of two (or more) energies is emitted simultaneously from the radiation source the intensity of the two signals should preferably be measured individually. This may be implemented directly by making provisions such that certain portions of the detector by filtration only measure radiation having a certain energy level while others measure other energy levels. It may also be implemented by subsequent treatment of signals, allowing superimposed signals to be separated.

(9) On the opposite side of the target area 105, a transmission detector 107 is arranged to receive radiation transmitted through material located in the target area 105. The detector is preferably a semiconductor detector comprising a linear array of semiconductor detector areas 107a-c distributed across the width of the conveyor line. The number of detector areas may vary due to the expected variations of ash content in the material, etc. The detector 107 is connected to a control unit 108 with a processor, e.g. an ordinary personal computer. The control unit receives detection data from the detector through a suitable interface, such as through a USB port.

(10) On the side of the target area 105, a fluorescence detector 110 is arranged to receive fluorescence radiation emitted by the radiated material located in the target area 105. This detector may also be a semiconductor detector comprising a linear array of semiconductor detector areas distributed across the width of the conveyor line. The number of detector areas may vary due to the expected variations of ash content in the material, etc. The detector 110 is also connected to the control unit 108, and data from the detector may be received through a suitable interface. The fluorescence detector may also be situated in other positions than the one shown in the illustrated example, as long as the detector is outside the direct radiation path of the radiation emitted by the X-ray source.

(11) In operation, the radiation source 104 irradiates the material in the target area 105 with electromagnetic radiation of at least two different energy levels. This may be achieved by sequentially irradiating the material with radiation of a first energy, and radiation of a second energy, i.e. the radiation source initially emits rays having one energy and then, by altering the voltage across the radiation tube, a different energy.

(12) For each energy level, the amount of radiation transmitted through the material located in the target area 105 is measured on the opposite side of the target area 105 by the transmission detector areas 107a-c of the transmission detector, wherein each detector area 107a-c receives radiation that has penetrated the material 102 along a different radiation path 109a-c. Simultaneously, the fluorescence detector 110 measures the fluorescence radiation emitted by the material due to this irradiation.

(13) In order to get a reference value for calibration, it is preferred to measure a calibration material. This can be achieved, for example, by measuring without any biological material present. Thus, in this case, a calibration measurement is obtained with air as a calibration material. Alternatively, the biological material may be replaced with a calibration material with known properties, such as aluminum. The calibration measurements may be obtained before measuring of the biological material, during initialization, or repeatedly during the process. Alternatively, calibration measurements may be obtained by letting the radiation source 104 and the detectors 107 and 110 measure an empty conveyor line such that the radiation passes through air and belt only on its way from the radiation source to the detector. It is also possible to use additional radiation sources and detectors situated on one or both sides of the conveyor belt.

(14) Based on these calibration measurements, calibration values for the transmitted radiation are determined as:
N.sub.01,02=N.sub.Air1,2 exp(x)
where N.sub.01 and N.sub.02 are the calibration values for energy level 1 and 2, respectively, N.sub.Air1 and N.sub.Air2 are the detected transmission values after passage through the known distance of air, is the known attenuation coefficient for air (cm.sup.1) and x is the known distance of air (cm) that separates the radiation source and the detector.

(15) A K-value for the material is determined for the radiation received by each detector area 107a-c. The K-value is calculated as:

(16) $K=\frac{\ln \left(N_{01}\text{/}{N}_1\right)}{\ln \left(N_{02}\text{/}{N}_2\right)}$
wherein N.sub.01, N.sub.02 are the calibrated reference values for the transmission at the two energy levels and N.sub.1, N.sub.2 are the transmission values through the biological material at the energy levels.

(17) A correlation between heating values and the amount of radiation transmitted through the biological material is then determined. This is determined based on reference measurements of a number of different reference materials.

(18) The reference measurements are preferably made with a calorimeter measurement of standard type, and preferably an adiabatic bomb calorimeter measurement is used. Most preferably, the adiabatic bomb calorimeter measurement is made in accordance with international standard ISO 1928:1995.

(19) The correlation between the heating values of the reference measurements and the transmission values is preferably made by correlation to the above-discussed K-values. Preferably, the heating value is calculated based on the quotient between two or more measurements of different energy levels, as:
W.sub.prel=a.sub.1*K1+b.sub.1*K2+c.sub.1*K3+ . . .
where K is the quotients between each and every combination of measurements at different energy levels. Hereby, if two energy levels are used, one K is obtained. If three energy levels are used, three K:s are obtained. If four energy levels are used, six K:s are obtained, etc. If three energy levels are used, the three K:s would be: K1=R1R2, K2=R2/R3 and K3=R1/R3. Thus, for only two energy levels, the heating value may be estimated as W=a*K1, and if three energy levels are used, as W.sub.prel=a.sub.1*K1+b.sub.1*K2+c.sub.1*K3, and if four energy levels are used, as W.sub.prel=a.sub.1*K1+b.sub.1*K2+c.sub.1*K3+d.sub.1*K4+e.sub.1*K5+f.sub.1*K6. The coefficients, denominated a.sub.1-f.sub.1 above, are determined and optimized mathematically to provide a correlation between the reference measurements and the heat energy as estimated based on the transmission measurements. Thus, the K-values may be used in a linear or polynomial representation of the correspondence between the K-value and the heating value, and this function may then be used for an estimate of the heating value based on the measured and calculated K-values of the sample material.

(20) The correction value is further calculated, based on the detected fluorescence radiation. This may e.g. be correlated to previously measured reference materials with known heating values, in a similar way as in respect of the transmission radiation.

(21) The correction value D may be defined as:
D=W.sub.prelW.sub.final
where D is the error between the preliminary estimate obtained by the K-model, and the true heating value.

(22) The correction value may be determined based on the flourescence measurements, e.g. as:
D=a.sub.2*xrf1+b.sub.2*xrf2+c.sub.2*xrf3+ . . . +z.sub.2*xrfn
where xrf1-xrfn are integrated energy counts obtained at certain energy leves and/or energy bands over the fluorescence spectrum. The coefficients, denominated a.sub.2-z.sub.2 above, are determined and optimized mathematically to provide a correlation between the reference measurements and the heat energy as estimated based on the fluorescence measurements. The coefficients may be determined with a suitable algorithm, such as least squares, partial least squares, or similar methods.

(23) A final estimate of the heating value may then be calculated as:
W.sub.final=W.sub.prelD=a.sub.1*K1+b.sub.1*K2+c.sub.1*K3+ . . . (a.sub.2*xrf1+b.sub.2*xrf2+c.sub.2*xrf3+ . . . +z.sub.2*xrfn)

(24) It has been found by the present inventors that a good approximation of the heating values, and a good correlation between the reference measurements and the estimation based on the transmission and fluorescence measurements, can be achieved. In FIG. 3, a graph is provided showing heating values estimated based on the above-discussed method on one axis, and heating values measured by reference measurements in an adiabatic bomb calorimeter on the other axis, for a number of different biological materials. Radiation having two different energy levels were used, in accordance with the previous discussion, and the content of non-combustible material in the samples varied significantly.

(25) As can be determined from FIG. 3, the estimates of the heating values are a very good approximation of the real heating value, which enables fast, accurate and cost-efficient estimation of the heating values, which can e.g. be used in continuous in-line measurements and similar.

(26) The estimated heating values may be used by the control unit 108, or by other control units, to control e.g. a burning or combustion process effectively.

(27) FIG. 2a-b schematically illustrates an alternative embodiment of a measurement device according the invention. The measurement device 100 comprises a radiation source 104 for irradiating a target area with at least two energy levels/wavelengths. Preferably, the radiation source is an X-ray tube for provision of X-ray radiation of two or more different energies. Preferably, the X-ray tube operates in the range 20-150 kVp. The output radiation from the radiation source is preferably directed towards a target area through a collimator. The radiation source is controlled by means of a controller 106. A transmission detector 107 is arranged on the opposite side of the target area. Further a fluorescence detector 110 is provided. The fluorescence detector 110 is preferably situated where it can have a free line of sight to the material which is measured and which is illuminated with the x-ray source e.g. at some angle beside the x-ray source. A suitable location for the fluorescence detector 110 is illustrated schematically in FIGS. 2a-b, but other positions are also feasible.

(28) The detectors are connected to a control unit 108 that receives detection data from the detector. In this embodiment, the material to be measured is arranged in a sample container 301. The sample container is then arranged on a carrier 302, which is movable in such a way that the sample container is moved through the target area, and thus through the radiation path 109. The carrier may e.g. be moved by means of a conveyor 103. However, other means for moving the carrier are also feasible, such as linear motors, screw arrangements, rail arrangements and the like.

(29) During operation, the sample container is moved through the target area such that preferably all of the material in the sample container is scanned. At the first passage, the material sample is irradiated with radiation of a first energy, and in the second passage, during the return movement, with radiation of a second energy. In order to get a reference value for calibration, it is preferred to measure a calibration material, preferably a predetermined amount of aluminum, at the beginning and end of the passage of the sample container.

(30) Based on these calibration measurements, calibration reference values may be determined in the same way as discussed above, and further, K-value correction values and heating values for the biological material may be calculated as discussed above.

(31) Specific embodiments of the invention have now been described. However, several alternatives are possible, as would be apparent for someone skilled in the art. For example, the number of detectors and/or detector areas may be varied, and the detectors may be arranged at different positions relative the biological sample.

(32) Further, the radiation paths through the material may be arranged in various ways. For example, the paths may travel essentially along a single line, between a radiation source and a detector, or several detectors arranged overlapping or close to each other. However, the radiation paths may also be arranged along parallel lines, to form a curtain like measurement zone. It is also possible to use a plurality of non-parallel paths, e.g. extending from a single radiation source to a plurality of spread out detectors, to form a fan shaped measurement zone. Similarly, it would also be possible to use a plurality of separated radiation emerging points, and a single detection point, or the like. Many other types of geometries for the paths are also feasible.

(33) Such and other obvious modifications must be considered to be within the scope of the present invention, as it is defined by the appended claims. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting to the claim. The word comprising does not exclude the presence of other elements or steps than those listed in the claim. The word a or an preceding an element does not exclude the presence of a plurality of such elements. Further, a single unit may perform the functions of several means recited in the claims.