Apparatus and method for direct measurement of coal ash sintering and fusion properties at elevated temperatures and pressures

A high-pressure microdilatometer is provided for measuring the sintering and fusion properties of various coal ashes under the influence of elevated pressures and temperatures in various atmospheres. Electrical resistivity measurements across a sample of coal ash provide a measurement of the onset of the sintering and fusion of the ash particulates while the contraction of the sample during sintering is measured with a linear variable displacement transducer for detecting the initiation of sintering. These measurements of sintering in coal ash at different pressures provide a mechanism by which deleterious problems due to the sintering and fusion of ash in various combustion systems can be minimized or obviated.

BACKGROUND OF THE INVENTION 
The present invention relates to apparatus and method for directly 
measuring ash fusion properties at elevated temperatures and pressures, 
and more specifically to a high-pressure microdilatometer (HPMD) which 
measures ash fusion and sintering behavior by independent but simultaneous 
measurement of expansion/contraction characteristics and electrical 
resistivity of ash samples at elevated temperature and pressures in 
oxidizing or reducing atmospheres. 
In coal combustion and gasification systems such as fluidized beds, 
slagging fixed-beds and entrained-flow systems the fusion, sintering, and 
deposition of ash impose serious operating problems which have been 
difficult to cope with or overcome. These problems are becoming even more 
difficult to understand so that suitable corrections may be made due to 
the trend in using coal conversion systems which operate at relatively 
high temperatures and pressures. Coal ash has different characteristics 
when subjected to high temperatures and pressures. For example, in a 
slagging fixed-bed gasifier coal becomes devolatilized in a highly 
reducing atmosphere, but the coal ash undergoes fusion at elevated 
temperature and pressure in an oxidizing atmosphere. The fusibility and 
sinterability of coal ash critically affect slagging and, hence, fouling 
in combustors and gasifiers. 
Coal ash fusibility has been previously determined by an ASTM test used for 
evaluating the slagging tendency of coal ash by measuring gross changes in 
shape of a conical compact of coal particles heated at 80.degree. C. per 
minute (at 1 atm pressure) in a specified atmosphere. Four characteristic 
temperatures defining ash fusibility were based on the deformation of the 
cone with rising temperature: (1) initial deformation temperature where 
the apex of the cone first becomes rounded; (2) softening temperature 
where the cone fuses and the height is equal to the base; (3) 
hemispherical temperature where the height of the cone is equal to half of 
the base width; and (4) fluid temperature where ash flows into a fluid 
layer. 
It was found that the ASTM technique yielded only the gross tendencies of 
bulk samples with data likely applying to large ash particles in the 
conical compact. The "low melting" components providing minor 
concentrations in the ash may lead to particle-to-particle bonding of fly 
ash below the melting point temperature of bulk ash so as to prevent the 
ASTM technique from revealing the fusion and melting behavior of minor, 
e.g. alkali, components in the ash. Also, the ash melting and fusion 
process may occur at temperatures differing form those observed by the 
ASTM technique. 
It has been shown that ash resistivity drops suddenly when the temperature 
of the ash reaches a certain transition temperature, (T.sub.r) as 
discussed in Cumming et al, "An Electrical Resistance Method for Detecting 
the Onset of Fusion in Coal Ash, "Fouling and Slagging Resulting from 
Impurities in Combustion Gases, R. W. Bryers, ed., New York: Engineering 
Foundation, 1983, pp. 329-341. This transition temperature which indicates 
the first presence of a trace liquid phase is invariable below the 
temperature where initial deformation of the conical compact occurred when 
using the ASTM technique. 
The comparison of sintering point data from both volume change and 
electrical resistivity measurements has also been used. The sintering 
points of several coal ashes, except for high sodium North Dakota lignite, 
have been found to agree closely. Electrical resistivity measurements for 
high-sodium coal indicates a much lower sintering temperature which was 
possibly due to an Na.sub.2 O-induced liquid phase. This technique 
provided a valuable tool for determining sintering effects due to addition 
or removal of mineral constituents such as Na.sub.2 O, and provided for 
the assessment of models describing vitrification. However, it has been 
found that no presently available techniques or equipment can provide 
accurate measurement of the behavior of coal ash at elevated temperatures 
and pressures 
SUMMARY OF THE INVENTION 
While the previous techniques provided some information relating to various 
properties of coal at elevated temperatures there is presently no 
apparatus or method for accurately measuring the thermophysical properties 
of coal ash at elevated temperatures and pressures such as would be 
present in advanced gasifiers and combustors. 
Accordingly, it is an object of the present invention to provide an 
apparatus and method for directly measuring ash fusion behavior at 
elevated temperatures and pressures in oxidizing and reducing atmospheres. 
Generally, the apparatus of the present invention is a microdilatometer 
for measuring thermophysical properties of ash of fossil fuel at elevated 
temperatures and pressures and comprises a pressure vessel means having an 
enclosable volume therein. Heating means are disposed in said volume and 
have a vertically oriented cavity therein. Sample holding means are 
positionable in said cavity and are adapted to contain a sample of ash of 
a fossil fuel. First and second electrode means are positionable in said 
sample holding means for respectively contacting vertically spaced apart 
first and second surface portions of said sample of ash when the sample of 
ash is contained in said sample holding means. Circuit means are coupled 
to said first and second electrode means for determining the resistivity 
of said sample of ash when subjected to said elevated pressures and 
temperatures. Transducer means are adapted to contact a surface of said 
sample of ash when contained in said sample holding means for detecting 
volume changes in said sample of ash when subjected to said elevated 
pressures and temperatures which are provided by pressurizing said volume 
in the pressure vessel means and actuating said heating means. 
Further scope of the applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description.

DETAILED DESCRIPTION OF THE INVENTION 
The apparatus of the present invention is utilized for determining the 
sintering and fusion properties of various types of coal ash at elevated 
temperatures and pressures under the influence of an inert, reducing or 
oxidizing atmosphere. As shown in FIGS. 1 through 3, the apparatus is a 
microdilatometer generally indicated at 10 which comprises a pressure 
vessel 12 capable of being pressurized with a suitable gas to a pressure 
up to about 1250 psi. The pressure vessel 12 may be provided with water 
cooling coils (not shown). The pressure vessel 12 is provided with a 
removable cover 14 which permits access to an enclosable volume 16 within 
the pressure vessel 12. Inlet lines 18 and 20 are provided for charging 
the pressure vessel volume 16 with a suitable pressurized gaseous 
atmosphere such as one formed of air, oxygen, hydrogen, carbon monoxide, 
carbon dioxide, nitrogen, helium, or steam as well as mixtures thereof. 
The gaseous charge within this volume 16 can be exhausted or vented 
through either of these lines 18 or 20 or through a suitable exhaust line 
22 shown in the base 24 of the pressure vessel 12. The location and number 
of these lines 18, 20 and 22 are not critical and can be suitable varied. 
The pressure vessel volume 16 is shown provided with a horizontally 
disposed plate 26 carried by vertically oriented supports 28 and 30, which 
extend between the cover 14 and the base 24 of the pressure vessel 12. 
This plate 26 is vertically positionable along the supports 28 and 30 and 
is used to support a heating mechanism such as a resistance furnace 32 for 
heating the sample of coal ash to the desired elevated temperature needed 
for determining sintering and fusion properties of the ash. This furnace 
32 may be of any suitable construction and is shown comprising a 
cup-shaped body of thermal insulation such as alumina with an centrally 
located cavity 36. Resistance heating wires 38 for the furnace are shown 
wound about an alumina or quartz tube 40 and disposed within the cavity 36 
along essentially the entire vertical length thereof for uniformly heating 
the cavity volume and the material placed therein. The tube 40 supports 
the heating wires and also defines the working volume within the furnace 
cavity 36. Suitable thermocouples 42 and 43 as best shown in FIG. 2 
respectively project through the insulation 34 and the alunina or quartz 
tube 40 and into the cavity 36 from the open end thereof for monitoring 
and providing a signal of the furnace temperature to a suitable control 
mechanism (not shown) which is utilized to regulate the temperature within 
the furnace cavity 36. The furnace 32 may be resistance heated to provide 
heat within the first cavity at a maximum rate of about 400.degree. C. per 
minute to a maximum temperature of about 1,200.degree. C. which is 
sufficiently high for determining the sintering and fusion properties of 
any coal ash. 
In order to measure the sintering and fusion temperature of coal ash a 
sample of the ash is heated in the furnace cavity while the pressure 
vessel volume 16 is at a selected elevated pressure. While the ash sample 
is being heated, resistivity and conductivity measurements as well as 
expansion and contraction measurements are made on the ash sample. These 
measurements determine the influence of pressure with various atmospheres 
on the sintering and fusion properties of coal ash. 
To provide for the measurements of the sintering and fusion temperature of 
ash of a particular coal, a closed-bottom tubular sample holder 44 is 
positionable within the furnace cavity 36 and vertically extends there 
from to a location adjacent to the cover 14 of the pressure vessel 12 
where the upper end of the sample holder 44 bears against a surface of an 
ash expansion and contraction measuring mechanism as provided by a linear 
variable differential transducer (LVDT) to be described in detail below. 
A vial 46 of a suitable material such as quartz is utilized for containing 
the ash sample generally shown at 48 for the sintering and fusion 
temperature measurements. This vial 46 is positionable within the sample 
holder 44 against the base or closed end thereof. This ash sample should 
contain an adequate quantity of discrete ash particulates so that 
sufficient physical changes in the ash will occur during sintering and 
fusion thereof so as to permit taking the measurements required for 
providing the data indicative of sintering and fusion temperatures of the 
particular coal ash at different elevated pressures and atmospheres. 
Normally a quantity of coal ash in the range of about 100 to 400 
milligrams is adequate in the apparatus of the present invention for 
obtaining the desired measurements. The ash sample is preferably compacted 
to a density of about 1.2 to 2.0 of theoretical density for the particular 
ash. Packing the ash particulates to a density of about 1.46 grams per 
cubic centimeter is usually sufficient. This compaction is achieved by 
using any convenient mechanism such as a tamping rod. 
To provide the measurements of changes in resistivity/conductivity of the 
coal ash during the heating thereof to determine the influence of pressure 
on ash sintering and fusion, the ash sample is positioned between and 
contacted by two disk-shaped electrodes 50 and 52. Electrode 50 can be 
attached to and carried on the base of the sample vial 46. Electrode 52 on 
the other hand is placed in contact with the uppermost side of the coal 
ash sample which is opposite the side contacted by the electrode 50. If 
desired the base of the sample vial 46 can be formed of a material 
suitable to provide the electrode 50. Satisfactory results have been 
achieved by using platinum as the electrode material for both electrodes 
50 and 52. 
Wire leads 54 and 56 are coupled to the electrodes 50 and 52 and extend to 
a resistivity meter 57 as schematically shown in FIG. 3. These wire leads 
54 and 56 and leads to the thermocouples 42 and 43 may pass through 
suitable openings in the sidewalls of the sample holder 44 and are coupled 
to terminals which permit these leads to pass through the walls of the 
pressure vessel. For example, as shown in FIG. 1 terminals 58, 60 and 62 
at the base of the pressure vessel may be used for providing the 
connections to the internal leads for transmitting signals to the external 
circuitry used for controlling and measuring the events within the 
pressure vessel. The thermocouples 42 and 43 are connected to a suitable 
conventional furnace control mechanism (not shown) while the electrode 
lead wires 54 and 56 are connected to the resistivity meter 57 which is 
utilized for determining the sintering temperature of the ash by 
resistivity measurements as will be discussed in detail below. The ash 
sample 48 acts as a resister during the resistivity measurements and 
completes the circuit between the electrodes 54 and 56. The resistance of 
the ash sample 48 varies during heating under the influence of pressure 
with this resistance decreasing as sintering and fusion occurs. A variable 
resister 64 is connected in series with the ash sample 48 and a constant 
voltage "V" from the voltage from the source shown in FIG. 3 is applied to 
this series circuit. The voltage drop (V.sub.r) across the variable 
resistor 64 is measured and from the Ohm's law the current "I" through the 
circuit can be calculated: 
EQU I=V.sub.r /R.sub.res 
where R.sub.res is the resistivity of the variable resistor 64. 
After the current through the circuit has been determined Ohm's Law can 
then be used to calculate the total resistivity of the circuit (R.sub.T) 
since the resistors are in series R.sub.ash =R.sub.T -R.sub.res. With the 
variable resistor 64 adjusted to provide a small voltage drop thereacross 
the R.sub.ash &gt;R.sub.res can be simply calculated by the equation: 
EQU R.sub.ash =V/I 
where "V" is the constant voltage applied to the circuit. The voltage drop 
across resistor 64 is indicative of the resistivity of the ash sample 48 
at the selected temperature and pressure. 
In addition to the resistivity measurements the volume changes of the ash 
sample 48 are measured to provide a further determination as to the 
temperature at which sintering and fusion of the ash occurs. As the 
sintering of the ash is initiated, the density of the ash compact 
increases and the volume thereof decreases. The changes in the volume of 
the ash sample 48 can be accurately measured on a 
time-temperature-pressure basis by employing a linear variable 
differential transformer (LVDT) generally shown at 66. The LVDT 66 is 
supported by and extends through a suitable opening the pressure vessel 
cover 14. The LVDT is shown comprising a vertically extending cylindrical 
housing 68 which is threaded attached to the cover 14 and is vertically 
adjustable with respect thereto. The cylindrical housing 68 is provide 
with longitudinal passageway 69 and is vertically adjusted so that the 
lower most portion or end thereof will bear against the upper end of the 
sample holder 44 when the coal the ash sample 48 is in place so that any 
movement detected by the LVDT 66 occurring in the vessel is due to volume 
changes in the ash sample 48. The coil utilized to receive the signal 
indicative of vertical movement within the LVDT is shown generally at 70 
and it selectively positionable on the cylindrical housing 68. This coil 
70 detects and provides a signal indicative of any vertical displacement 
of a ferrous core 72 disposed within the passageway 69. This core 72 is 
attached to one end of a vertically movable probe or rod 74 which extends 
into the passageway 69 of the cylindrical housing 68 and into a 
significant length of the sample holder 44 to contact the top of the ash 
sample 48. The upper electrode 52 of the resistivity measuring circuit may 
be affixed to the distal or lower most end of the probe 74 contacting the 
top surface of the ash sample 48 to assure contact is maintained between 
the electrode 52 and the ash sample 48 during any volume changes occurring 
therein. 
Displacement of the probe and the ferrous core 72 attached thereto due to 
any changes in volume of the coal ash sample 48 is detected by the coil 
72. As with conventional linear variable displacement transducers a 2.5 
kHz drive signal may be provided by conventional circuitry (not shown). 
The LVDT is preferably provided with a linear range of about.+-.0.5 inches 
for assuring accurate measurement of the volume changes in the ash sample 
during sintering. 
In order to provide a more facile understanding of the present invention, 
Examples are set forth below and directed to the measurement of the 
sintering and fusion properties of different coal ashes at various 
temperatures and pressures to illustrate the changes in sintering 
occurring due to the influence of pressure and atmospheres at elevated 
temperatures. The coal ash utilized to illustrate the effect of various 
atmospheres such as air and helium and mixtures thereof at pressures at 
ambient to 500 psig were prepared from North Dakota Lignite (PSOC 1507) 
and Illinois No. 6 (PSOC 1493). A quantity of each of these coals were 
prepared by grinding the coal and grinding it to a size of-74 microns. 
These coal particulates were then converted to ash in a muffle furnace in 
an air atmosphere at 700.degree. C. The compositions of the coal expressed 
as percent oxides are presented in the Table below. 
TABLE 
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Oxide Compositions of High Temperature 
Coal Ash Used In Examples l an II 
(Ash Derived from PSOC Coals) 
PSOC Coal No. 
Oxide 1943 1507 
______________________________________ 
Sio.sub.2 41.2 19.7 
Al.sub.2 O.sub.3 
15.7 9.34 
TiO.sub.2 0.76 0.37 
Fe.sub.2 O.sub.3 
23.9 12.9 
MgO 0.90 5.33 
CaO 7.39 23.2 
Na.sub.2 O 0.40 5.87 
K.sub.2 O 1.70 0.69 
P.sub.2 O.sub.5 
0.26 0.34 
SO.sub.3 7.90 20.0 
______________________________________ 
EXAMPLE I 
The sintering temperature/pressure relationship for the North Dakota 
lignite ash is illustrated in FIG. 4. The curves in this FIG. represent 
sintering temperature as determined by both resistivity and shrinkage in 
gaseous environments provided by air and helium. The sintering temperature 
as determined by electrical resistivity measurements increased with 
increasing pressure in both air and helium. The sintering temperature in 
air was found to be about 80.degree. C. greater at any given pressure than 
that in helium. This difference in sintering temperature was expected 
since alkalis present in the ash would be oxidized in air so as to yield 
higher melting components which result in a higher sintering temperature. 
In a typical measurement utilizing the present invention the sintering 
temperature of the North Dakota lignite ash at ambient inert pressure 
occurred at 600.degree. C. whereas at elevated pressures of 200 and 500 
psig the sintering temperature was determined to be 640.degree. C. and 
740.degree. C. These measurements show significant increases in the 
sintering temperature of the coal ash occurs with increases in pressure. 
Further, by changing the atmosphere of the sintering environment to air 
the sintering temperature is also changed, often by as much as about 
1.degree. C. to 80.degree. C. when sintering occurs at the above mentioned 
pressure ranges. 
EXAMPLE II 
The sintering/pressure relationship for Illionis No. 6 (PSOC 1493) is 
illustrated in FIG. 5 for a helium atmosphere and in FIG. 6 for an air 
atmosphere. As shown by the curves in FIG. 5 the sintering temperature as 
determined by resistivity measurements and by shrinkage appear to be very 
close at any given pressure which is expected to be due to the lower 
concentration of the calcium and sodium in this coal ash as compared to 
the North Dakota lignite ash of Example 1 and also as shown in the above 
Table. As indicated by the curves in FIG. 5 an essentially linear increase 
occurs in the sintering temperature with increasing pressure up to a 
pressure of about 200 psig. There was also a linear increase in the 
sintering temperature with an increase in pressure in an air atmosphere as 
shown in FIG. 6 and as determined by resistivity measurements. 
In FIG. 7 the North Dakota lignite ash is utilized to provide two curves 
which are a plot of the log of the sampled resistance versus the 
reciprocal of the absolute temperature. This plot provides an illustration 
of how sintering temperature is determined by measuring the electrical 
resistivity. 
It will be seen that the present invention provides a high pressure 
apparatus capable of determining the sintering and fusion behavior of 
various coal ash at elevated temperatures and pressures. The apparatus of 
the present invention can be utilized to clearly show that the oxidizing 
atmosphere provides a higher sintering temperature than in an inert 
atmosphere and that increases in pressure increases the sintering 
temperatures in both inert and oxidizing atmospheres. The sintering 
temperature determined by shrinkage measurements was found to decrease 
slightly with increased pressure due to compaction of a sample during heat 
treatment while the resistivity measurements appear to be more sensitive 
than the shrinkage measurements and perhaps may provide a better 
indication of the sintering temperature of coal ash. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims. For example, the subject 
invention could be used for determining the sintering behavior of pure 
minerals or ceramics. Further, electrical resistivity measurements of 
various materials including superconductors can be made at low temperature 
by injectng liquid helium or nitrogen into the pressure vessel.