Method for measuring the efficiency of a combustion and an apparatus for carrying out the method

Abstract: A method and an apparatus for measuring the efficiency of a combustion whereby ash samples (2) are drawn at predetermined time intervals from a region of a combustion plant (3), each drawn sample is set in an exhausted reaction cell (4) wherein comburent containing reaction gas is introduced under controlled pressure and a superficial layer of said sample (2) is heated to the carbon combustion temperature by a CO₂ laser beam, the reaction gas is drawn from said cell (4) and the amount of carbon dioxide as produced by the carbon combustion is measured in a calibrated detector (16) in order to measure the amount of unburnt carbon contained in the ashes on the basis of a preceeding colibration carried out on ashes of known carbon content.

Description: This 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 the ashes; such methods involve intricate operational sequences and long time periods which makes them not suitable to control a combustion in real time. However, a method for the combustion control in real time allows to optimize the combustion and to get the consequent advantages as to energetic saving, high quality ash production and environmental pollution. Obviously, such a method has the additional advantage of allowing to control the combustion in transient state also or, anyway, in non standard operation conditions. Through the techniques practiced so far 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. In particular, document FR-A-2-539-230 is known. It discloses a method and an apparatus for measuring the amount of unburnt carbon in a combustion whereby a sample of ash is taken and introduced in a region heated by an electric device upto the unburnt inflammation temperature in order to estimate the unburnt amount from the developped heat, the temperature in said region being kept constant along the unburnt combustion and the power spent for maintaining said constant temperature is measured taking into account that said power is proportional to the heat amount developped during the combustion. All the above techniques are affected by 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 under test 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 this invented method as characterized in the amended claims, the measurement is carried out of the developped carbon dioxide and/or of the decrease of the oxygen in a hermetically sealed 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 CO₂ laser maximum gain line falls, which makes such laser suitable to this purpose; 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 upto 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 CO₂ 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, anables to state a relation between the CO₂ 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 CO₂ is conditioned by the oxidative environment (gas pressure and kind). Obviously, the oxygen available in the cell shall be enough for completely burning the carbon as contained into the reaction ash volume. As an alternative or addition to the CO₂ 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 few grams of ashes, also two or three grams only. According to known methods, said detector is 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 CO₂; no longer requiring an exact measurement of the amount of the ashes as drawn since the laser radiation is absorbed in a layer of few tenths millimeter thickness; rapidly measuring the amount of the unburnt carbon thanks to the kind of the heat source and to the small amount of material to be drawn and analyzed; supplying a method and an apparatus for measuring the combustion efficiency in real time. The invention, as defined in independent claims 1 and 7, will be described in detail herebelow 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 hermetically sealed 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 to said filter-support 5 and closed with a plate 9 made of zinc selenide allowing the CO₂ 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 CO₂ 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 CO₂ 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 upto 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 micro-processor 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 cm³. The heat absorption due to laser radiation (λ = 10,6 µm) 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 oxidise the ash volume as heated by the laser (2,5 x 10⁻² - 9,0 x 10⁻² cm³) with a radiation time period ranging from 30" to 2'. 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.

Claims: A method for measuring the efficiency of coal combustion by serially measuring the unburnt carbon amount contained in analysis ash samples (2) located in a heating region (4) wherein the temperature is raised to the carbon combustion temperature, characterized in that the said method comprises the the steps of: a) drawing each analysis ash sample (2) from a region of a coal combustion plant (3) and placing said sample (2) on a filter-support (5) in a reaction cell (4) which is hermetically sealed under control; b) exhausting said reaction cell (4); c) supplying a controlled amount of reaction gas under a controlled pressure into said reaction cell (4); d) raising the temperature of a superficial layer of said ash sample (2) to the carbon combustion temperature by projecting onto a surface (S) of said ash sample (2) a laser beam (12) of such kind and power as to be absorbed by said ash sample; e) drawing the reaction gas from the said chamber togheter with CO₂ as produced in said cell (4) by the combustion of the carbon contained in said part of said ash sample (2) which is heated by the laser beam (12) and measuring the amount of said CO₂ in a first calibrated detector (16) in order to estimate the amount of unburnt carbon in the ashes on the basis of a preliminary calibration. A method according to claim 1 characterized in that it comprises the additional step of drawing from said cell (4) the reaction gas together with the O₂ as residual of the carbon combustion and measuring the amount of said O₂ in a second calibrated detector (16) in order to estimate the oxygen consumption in said combustion and evaluating from that consumption the amount of unburnt carbon in the ashes on the basis of a calibration already carried out. A method according to claims 1 or 2 characterized in that said laser beam is a beam of a CO₂ laser. A method according to claims 1 to 3 characterized in that the time period for rising the temperature of said ashes to at least the carbon combustion temperature may be varied by varying the laser beam power. A method according to claims 1 to 4 characterized in that the time period for rising the temperature of said ashes to at least the carbon combustion temperature may be varied by varying the laser beam cross section. A method according to claims 1 to 4 characterized in that the time period for raising the temperature of said ashes to at least the carbon combustion temperature may be varied by varying the laser bean power and cross section. An apparatus for measuring the efficiency of coal combustion by serially measuring the unburnt carbon amount contained in analysis ash samples comprising; drawing means (1) for drawing ash samples (2) from a first region in a combustion plant (3) into a second region (4) associated with a heating means (12) the said heating means being able to raise the temperature of said ash samples (2) to the carbon combustion temperature, characterized in that said second region is a reaction cell (4) hermetically sealed under control and provided with a filter-support (5) for supporting each analysis ash sample (2) supplied from said drawing means (1) and in that the said apparatus further comprises: means (15) for exhausting said reaction cell (4); a reaction gas source (6) in communication with the inside of said reaction cell (4) in order to supply therein a controlled amount of reaction gas under controlled pressure; said heating means being in the form of a laser source (11); an aperture (8) positioned opposite the said filter-support (5) being provided for passing the beam (12) of said laser (11) and being closed by a plate (9) of a material which allows the laser beam (12) to pass through; means (13, 14) being provided for directing said laser beam (12) onto a surface (S) of the analysis ash sample (2) set on said filter-support (5) in order to burn the carbon contained in a superficial layer of said analysis sample; a detector (16) being provided in the apparatus and being calibrated for measuring the carbon dioxide amount contained in the gas drawn from the said reaction cell; an exhauster (15) being provided for drawing the said gas from the inside of said reaction cell (4) in order to deliver it to said detector (16); an ejector (18) being provided for removing the ashes of said ash sample (2) from the reaction cell (4) after combustion; a baffle plate (10) movable between a closing and an opening position being provided between said aperture (8) and said filter-support (5), the said baffle plate being in a closed position while ashes enter said reaction cell (4) in order to protect said plate (9); means (17) being provided for heating the reaction cell (4) in order to remove the possible humidity contained in the ash analysis sample (2). An apparatus according to claim 7 characterized in that the detector (16) is calibrated in said apparatus for measuring the amount of CO₂ in the said gas. An apparatus according to claims 7 or 8 characterized in that it also comprises a second detector (16) calibrated for measuring the amount of oxygen in the said gas. An apparatus according to any of the claims 7-9 characterized in that the laser source (11) is a CO₂ laser source. An apparatus according to claim 10 characterized in that the power of said laser source (11) ranges from 20 to 30 watts. An apparatus according to claims 7 to 11 characterized in that a programmed controller (C) is associated with the combustion plant (3) and with at least one calibrated detector (16) and with all operative means of the apparatus in order to activate at predetermined time intervals the analysis steps according to claims 1 to 6 and optimize the combustion of the combustion plant (3) according to the analysis results as provided from the at least one calibrated detector (16) and a predetermined program.

Inventors: CIONI MARIO, CURCIO FRANCO, DE MICHELE GENNARO, MUSCI MIRELLA, CIONI, MARIO, CURCIO, FRANCO, DE MICHELE, GENNARO, MUSCI, MIRELLA
Applicants: ENTE NAZ ENERGIA ELETTRICA, ENTE NAZIONALE PER L'ENERGIA ELETTRICA - (ENEL)
Application year: 1990
Main classification: F23N5
IPC codes: F23N5