Method and apparatus for measuring and controlling efficiency of a combustion

A method and an apparatus for measuring and controlling the efficiency of a combustion whereby, ash samples are drawn at predetermined time intervals from a region of a combustion plant, each drawn sample is set in an exhausted reaction cell, combustion reaction gas is introduced under controlled pressure, a superficial layer of the sample is heated to the carbon combustion temperature by a CO.sub.2 laser beam, the reaction gas is drawn from the cell and the amount of carbon dioxide produced by the carbon combustion is measured in a calibrated detector. The amount of unburnt carbon contained in the ashes is determined based on a preceding calibration carried out on ashes of known carbon content.

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.sub.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.degree. C.-1200.degree. 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.sub.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.sub.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.sub.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.sub.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.sub.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.

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.sub.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.sub.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.sub.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.sup.3. 
The heat absorption due to laser radiation (=10,6 m) causes in the 
concerned material a temperature rise ranging from 900.degree. C. and 
1100.degree. 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.times.10.sup.-2 -9,0.times.10.sup.-2 cm.sup.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.