Patent Publication Number: US-5155047-A

Title: Method and apparatus for measuring and controlling efficiency of a combustion

Description:
The present invention refers to a method for measuring the efficiency of a combustion, in particular a method for measuring in real time the content of unburnt carbon in the coal ashes and an apparatus for carrying out the method. 
     There are known chemical methods used in a laboratory for measuring the unburnt carbon amount in ashes. Such methods involve intricate operational sequences and long time periods which make them unsuitable for controlling a combustion in real time. 
     However, a method for the combustion control in real time allows one to optimize the combustion and to get the consequent advantages of energy saving, high quality ash production and less environmental pollution. Obviously, such a method has the additional advantage of allowing control of the combustion in a transient state or, anyway, in non standard operation conditions. 
     By the techniques practiced previously for measuring unburnt material amounts in real time, ash samples are drawn through suitable flues in communication with a boiler and a property related to the unburnt carbon content is detected in the shortest possible time. 
     Examples of such known techniques are those that: are based on the optical analysis of samples wherein the heat depends on the elementary carbon content; measure the sample weight variation before and after heating in air since carbon develops by combustion; measure the reflection factor of a microwave signal since the dielectric constant of the ashes depends on their chemical composition. 
     All the above techniques have great inaccuracy since the measured properties are related to the unburnt carbon content in an indirect and often non univocal way. Moreover, these techniques require that the amount of the ashes tested be known exactly and often require that considerable amount of material be drawn (tens of grams) which means extending the time necessary for measurement. 
     According to the invented method, as characterized in the appended claims, the measurement is carried out on the developed carbon dioxide and/or on the decrease of the oxygen in a reaction cell during a superficial and localized combustion caused by a laser beam in a small analysis ash sample. The coal ashes substantially consist of aluminium silicates presenting a strong absorption band in the mean infrared region wherein the CO 2  laser maximum gain line falls, which makes such laser suitable to this purpose, i.e., the laser beam is so well absorbed by said aluminium silicates that its radiation is absorbed in a superficial layer of a few tenths millimeter thickness in said analysis sample and is converted into heat. It will be appreciated that the thickness of said layer depends on the ratio, w/s, between the laser beam power and the surface as hit by the same beam. Conveniently, said analysis sample will be some millimeters thick to prevent the heat produced by the laser from dispersing through the support whereon said sample is placed. The object of the laser beam is to heat a very small layer of ashes in the sample surface S rapidly (typically from 10 to 30 seconds) and locally up to high temperatures (700° C.-1200° C.), depending on laser power. In an oxidative environment caused by introduction of air or oxygen as reaction gas the unburnt carbon reacts with oxygen and produces carbon dioxide. The reaction gas is drawn from the inside of the reaction cell and the CO 2  amount is measured by means of a detector suitable to such gas. An adequate preliminary calibration, carried out in the invented apparatus on calibration ash samples having known carbon content, enables the establishment of a relation between the CO 2  amount, as produced in said cell, and the percentage content of unburnt carbon as contained in the analysis ash samples. In connection with predetermined laser beam specifications, the amount of the produced CO 2  is conditioned by the oxidative environment, i.e., the type of oxidation gas used and the pressure thereof. Obviously, the oxygen available in the cell shall be enough for completely burning the carbon contained in the reaction ash volume. As an alternative in addition to the CO 2  analysis, the oxygen consumption during combustion in said cell is measured in order to measure the carbon amount burnt and contained as unburnt carbon in an analysis sample. Moreover, attention is drawn to the fact that the necessary analysis sample contains only a few grams of ashes, for example two or three grams. 
     According to known methods, said detector may be associated with a programmer adapted at least: a) to drive the above described step sequence sequentially, i.e. at prescribed time intervals; b) to adjust the combustion plant operation according to a predetermined memorized program using the results of the analysis in said detector. 
     At least the following main advantages are afforded by this invention: directly detecting unburnt carbon amount through its transformation into CO 2  ; no longer requiring an exact measurement of the amount of the ashes as drawn since the laser radiation is absorbed in a layer of a few tenths of a millimeter thickness; rapidly measuring the amount of the unburnt carbon due to the kind of the heat source and to the small amount of material drawn and analyzed; supplying a method and an apparatus for measuring the combustion efficiency in real time. 
    
    
     Brief Description of the Drawing 
     The figure illustrates the elements of the invention including the analyzer and controller. 
    
    
     The invention will be described below in detail with reference to the accompanying drawing which illustrates only one specific embodiment. 
     The apparatus comprises: a device 1 for sequentially drawing an analysis ash sample 2 from a region in a combustion plant 3 located between the ash precipitator and the air-preheater, both not shown in the drawing; a reaction cell 4 bearing a filter-support 5 to support said analysis sample 2; an oxygen source 6 in communication with the inside of said reaction cell 4 through a duct 7 to supply said cell with a controlled amount of oxygen under controlled pressure; a port 8 opposite said filter-support 5 and closed with a plate 9 made of zinc selenide allowing the CO 2  laser beam to pass through; a baffle plate 10, located between said filter-support 5 and port 8, moved by motor means M between a closing position and the opening position shown in the drawing to protect said plate 9 from ash dust when analysis samples are introduced into the reaction cell; a CO 2  laser source 11 which directs the laser beam 12, through a lens 13 and a mirror 14, on a surface S of the analysis sample 2 set on the filter-support 5 in order to burn the carbon contained in a small layer of said surface S; an exhauster 15 which draws the gas from said reaction cell and delivers it in a calibrated detector 16 able to measure the amount of CO 2  in the reaction gas (the detector is of the NDIR type, non-dispersive infrared photometer); a further object of said exhauster 15 is to exhaust the reaction cell up to about 0.1 torr; an electric resistance heater 17 to remove possible humidity contained in the analysis sample 2; an ejector 18 to remove from the filter-support 5 and consequently from the reaction cell 4 the ash of the analysis sample at the end of the operation. All ducts D in the apparatus are controlled by solenoid valves V. 
     The operative means of the combustion plant 3 (fuel and air feeding, air and gas locks, registers, etc.), calibrated detector 16, motor means for the device 1, oxygen source 6, exhauster 15, ejector 18, baffle plate 10, laser source 11, solenoid valves V and electric resistance 17 are all associated in a conventional manner with a microprocessor controller C adapted to drive at predetermined time intervals the described analysis cycle and to adjust the working of the operative means of the combustion plant 3 depending on the analysis result as supplied from the detector 16 according to a predetermined optimized combustion program. Wires w connect said controller C with all controlled parts. 
     The laser power ranges from 20 to 30 watts; the diameter of laser beam on said surface S ranges from 8 to 15 mm; the analysis sample 2 has 4 mm thickness and 28 mm diameter; the reaction cell volume is 300 cm 3 . The heat absorption due to laser radiation (=10,6 m) causes in the concerned material a temperature rise ranging from 900° C. and 1100° C. in a time period ranging from 10 to 15 seconds. The reaction gas in the reaction cell may be air or oxygen under a pressure ranging from 200 to 600 torr. Under said operative conditions and apparatus specifications, the amount of oxygen in said cell is enough to completely oxidize the ash volume as heated by the laser (2,5×10 -2  -9,0×10 -2  cm 3 ) with a radiation time period ranging from 30 seconds to 2 minutes. The range of the unburnt carbon percentages which may be analyzed by means of this apparatus is from 1% to 40%. 
     After laser radiation, the carbon development from said sample is evidenced by a clear spot on said surface S.