RU2644615C1 - Method for determining the thermal resistance of coals to their cyclic freezing and thawing - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to metrology, in particular to methods for determining the thermal stability of coals during their cyclic freezing and thawing. Essence: cyclical freezing and thawing of the same type of coal samples are performed at M number of cycles equal to the serial number of the corresponding sample in the series. Further, in parallel recording the parameters of acoustic emission, each of the samples is slowly uniformly heated to a temperature in the range of (80–90) ± 5 °C and held at it for at least 4 hours. Boundaries of successive time intervals are then determined, the first of which begins at the time of heating the sample to 30 °C and ends with stabilization of its temperature at a constant value, and the second – of the same duration, begins with an increase in the level of activity of acoustic emission to a value of not less than one and a half times higher than the background noise level. In each of these intervals, average values of acoustic emission activity are calculated. During the slow heating-up of the sample to the holdup temperature, the initially weak structural bonds are destroyed and the source of emission becomes weak, and when the critical stresses are formed in the sample under prolonged thermal loading, the remaining, initially strong bonds become sources of emission. Therefore, the coefficient K, equal to the ratio between the acoustic emission activity in the second and the first of the indicated time intervals, reflects the residual thermal stability of the coal after freezing and thawing. Value of the thermal stability of the coal with respect to cyclic freezing and thawing is defined as the dependence reduction point K(M), which shows the number of cycles after which there are practically no strong structural bonds in the coal under study.

EFFECT: technical result: possibility of determining the thermal stability of coal during its cyclic freezing and thawing.

1 cl, 5 dwg

Description

The invention relates to the field of research of coal resistance to cyclic thermal effects, each cycle of which covers a negative and positive temperature range. It can also be used to determine the current degree of cryogenic disintegration of coal products.

A known method of testing materials for heat resistance, namely, that the surface of the test material sample is subjected to cyclic heat exposure, including heating and subsequent cooling, the surface of the test material sample is controlled, and the heat resistance of the material is judged by the number of heat exposure cycles until cracks appear on the surface in this case, after cooling, the material layer with a thickness corresponding to the intensity is periodically removed from the surface of the material sample the evidence of material wear during operation, and the surface of the test material sample is controlled after removing the material layer from the sample surface after a specified number of heat exposure cycles or after each cycle (RF Patent No. 2117274, CL G01N 3/60, G01N 3/56. Publisher . 08/10/1998).

The disadvantage of this method is that it is not applicable to the study of initially fractured and significantly structurally heterogeneous materials, which, in particular, include coal.

The closest in technical essence to the present invention relates to a method for determining the heat resistance of coal, which consists in applying to the test samples a cyclic alternating temperature effect, in predetermined time intervals which determine the average activity of acoustic emission, which is used to judge the heat resistance of coal (RF Patent No. 2593441, G01N 3 / 60. Published on 08/10/2016).

The disadvantage of this method is that it does not allow to determine the thermal resistance of coals to their cryogenic disintegration under the influence of cyclic freezing and thawing.

The noted drawback is due to the fact that the known method involves the study of the acoustic emission response of coal to a series of thermal shock increasing in intensity, each of which represents a short-term high-amplitude (carried out at high speed) thermal effect lying in the temperature range from 100 ° C to 190 ° C. Disintegration processes occurring under such an action differ from the corresponding processes under the influence of negative temperatures, and even more so when the latter change cyclically, passing from the negative to the positive and vice versa. In this case, the main mechanism of coal disintegration is associated with a cyclic change in the phase state of pore moisture, that is, its transition to ice and then ice to liquid, and not to intensification of surface oxidation and rapid volume expansion, different for individual structural elements (as is the case with thermal shock exposure - in the prototype). Moreover, in real climatic conditions of transportation and storage of coal, the temperature effect on it changes relatively smoothly and slowly with an insignificant temperature gradient over time, that is, it does not have a thermal shock.

This application solves the problem of developing a method for determining the heat resistance of coals to their cryogenic disintegration under the influence of cyclic freezing and thawing by taking into account the features and modes of such an effect on the parameters of thermally stimulated acoustic emission in coals.

The technical result of the invention is to determine the heat resistance of coal to their cryogenic disintegration under the influence of cyclic freezing and thawing.

The technical result is achieved by taking into account the features and modes of influence of cyclic freezing and thawing on the parameters of thermally stimulated acoustic emission in coals.

To solve the problem in a method for determining the heat resistance of coals to their cyclic freezing and thawing, which consists in applying to the test samples a cyclic alternating temperature effect, in the given time intervals which determine the average acoustic emission activity, which is used to judge the heat resistance of coal, a series of similar samples is subjected to cyclic freezing and natural thawing to positive temperatures with the number M of such cycles for each sample is equal to m its serial number in the series, after which all samples of the series are uniformly heated at a low speed to the same temperature in the range (80-90) ± 5 ° C, at which they are held for at least four hours, while the measurement of acoustic emission activity is carried out in the process heating and aging, determine the boundaries of consecutive time intervals, the first of which begins after heating the sample to 30 ° C and ends when it reaches a constant temperature, and the second interval of the same duration begins with increasing ur the acoustic emission activity to a value not less than one and a half times higher than the corresponding background noise level, a coefficient K is calculated for each sample, equal to the ratio of the average acoustic emission activities in the second and first intervals, the dependence of the coefficient K change in function of the number M of freezing cycles is built and natural thawing and the number of these cycles, during which the indicated dependence K (M) is flattened, is used to judge the heat resistance of coal to the influence of cyclically changing negative flaxen and positive temperatures.

термостимулированной акустической эмиссии образцов бурого угля первой выборки (марка 3Б) в функции от времени с начала эксперимента. На фиг. 3. представлена характерная экспериментально полученная зависимость активности

термостимулированной акустической эмиссии образцов бурого угля второй выборки (марка 2БР) в функции от времени с начала эксперимента. На фиг. 2 и фиг. 3: 7 - временной интервал неустановившегося температурного режима продолжительностью Т от момента прогрева образца до 30°С и до момента достижения им постоянной рабочей температуры Т_р, на которой производится длительная выдержка образца; 8 - временной интервал той же продолжительности Т, начинающийся в момент возрастания уровня активности акустической эмиссии до значения, не менее чем в полтора раза превышающего соответствующий уровень фоновых шумов. На фиг. 4 и фиг. 5 представлены зависимости коэффициента К (равного отношению среднего значения

активности акустической эмиссии в интервале 8 к среднему значению

активности акустической эмиссии в интервале 7), от числа М циклов замораживания и оттаивания образцов бурого угля первой и второй выборок соответственно.The method for determining the heat resistance of coals to their cyclic freezing and thawing is illustrated in FIG. 1, FIG. 2, FIG. 3 and FIG. 4. In FIG. 1 is a diagram of a laboratory setup for implementing the method, where 1 is a coal sample, 2 is a circular platform, 3 is a tubular heating furnace, 4 is a quartz waveguide, 5 is a receiving acoustic transducer, 6 is an acoustic emission measuring system. In FIG. 2 shows a characteristic experimentally obtained activity dependence

thermostimulated acoustic emission of brown coal samples of the first sample (grade 3B) as a function of time from the beginning of the experiment. In FIG. 3. The characteristic experimentally obtained dependence of activity is presented.

thermostimulated acoustic emission of brown coal samples of the second sample (brand 2BR) as a function of time from the beginning of the experiment. In FIG. 2 and FIG. 3: 7 - the time interval of an unsteady temperature regime of duration T from the moment the sample is heated to 30 ° C and until it reaches a constant working temperature T _p , at which the sample is held for a long time; 8 - a time interval of the same duration T, starting at the moment of increase in the level of activity of acoustic emission to a value not less than one and a half times higher than the corresponding level of background noise. In FIG. 4 and FIG. 5 shows the dependence of the coefficient K (equal to the ratio of the average value

acoustic emission activity in the range of 8 to the average value

acoustic emission activity in the interval 7), from the number of M cycles of freezing and thawing of brown coal samples of the first and second samples, respectively.

акустической эмиссии. Полученные в результате для каждого образца зависимости

, представленные на фиг. 2 и фиг. 3, состояли из двух областей I (начальной по времени) и II с повышенными значениями

, между которыми располагалась область с пониженным значением

. Очевидно, что областям I и II соответствует повышенное дефектообразование. Причем в области I, где действуют относительно малые термонапряжения, дефектообразование связано с разрушением имеющихся в образце исходно слабых (нарушенных) структурных связей. В то же время в начале области II (сдвинутой по времени относительно области I) терморапряжения достигают уровня, сопоставимого с пределом прочности исследуемого угля. В результате в этой области интенсифицируется процесс дефектообразования, в ходе которого термическая стойкость и связанные с ней свойства объекта испытания снижаются практически по экспоненциальному закону вплоть до полного разрушения образца. При этом у изначально менее нарушенного угля уровень

в области II выше за счет большего количества нереализованных источников акустической эмиссии, т.е. сохранившихся до этого момента структурных связей. Поэтому, сравнивая у образцов одной выборки уровни

в области II, принципиально можно судить о том, какая часть их структурных связей была изначально устойчива настолько, что они разрушились только под действием критических термонапряжений. Чтобы исключить влияние на величину

размеров и индивидуальных структурных особенностей конкретного образца, эта величина была пронормирована к безразмерному численному показателю К, характеризующему степень устойчивости угля к циклическому замораживанию и оттаиванию.

, где

- средняя активность акустической эмиссии в области I на временном интервале 7 (на фиг. 2 и фиг. 3), который начинается после прогрева образца до 30°С и заканчивается достижением им постоянной температуры, a

- средняя активность акустической эмиссии в области II на временном интервале 8 (на фиг. 2 и фиг. 3) той же длительности, который начинается при возрастании уровня активности акустической эмиссии до значения, не менее чем в полтора раза превышающего соответствующий уровень фоновых шумов.The proposed method is based on experimentally established laws of thermally stimulated acoustic emission established by the authors during prolonged exposure at a certain temperature (lying in the range (80-90) ± 5 ° С) of brown coal samples of two samples consisting of 10 samples each. The first sample was represented by coal grade 3B of the Azeyskoye deposit, the average value of which equilibrium moisture W was W ~ 10%. The second sample was represented by coal grade 2BR of the Kansk-Achinsk deposit, the average value of the equilibrium moisture of which was W ~ 24%. Each of the samples of both samples with the current number M was subjected to M freeze / thaw cycles, including cooling the sample to a minimum temperature T _min = -30 ° C, holding at this temperature for 90 minutes. and subsequent natural thawing to a temperature T _max = + 10 ° C. Further, all samples were heated to a temperature of 90 ° C and subsequent exposure at this temperature for 4 hours with simultaneous measurement and recording of activity

acoustic emission. The resulting dependencies for each sample

shown in FIG. 2 and FIG. 3, consisted of two regions I (initial in time) and II with increased values

between which there was an area with a lower value

. Obviously, areas I and II correspond to increased defect formation. Moreover, in region I, where relatively small thermal stresses act, defect formation is associated with the destruction of initially weak (broken) structural bonds existing in the sample. At the same time, at the beginning of region II (shifted in time relative to region I), thermal stresses reach a level comparable to the tensile strength of the investigated coal. As a result, the process of defect formation is intensified in this area, during which the thermal resistance and the properties of the test object associated with it decrease almost exponentially until the complete destruction of the sample. Moreover, the level of initially less disturbed coal is

in region II above due to a larger number of unrealized sources of acoustic emission, i.e. surviving to this point structural ties. Therefore, comparing the levels of samples of one sample

in region II, it is in principle possible to judge how much of their structural bonds were initially so stable that they collapsed only under the influence of critical thermal stresses. To exclude the effect on the value

sizes and individual structural features of a particular sample, this value was normalized to a dimensionless numerical indicator K, which characterizes the degree of coal resistance to cyclic freezing and thawing.

where

- the average activity of acoustic emission in region I at a time interval of 7 (in Fig. 2 and Fig. 3), which begins after heating of the sample to 30 ° C and ends when it reaches a constant temperature, a

- the average activity of acoustic emission in region II at a time interval of 8 (in Fig. 2 and Fig. 3) of the same duration that begins when the level of activity of acoustic emission increases to a value not less than one and a half times higher than the corresponding level of background noise.

The dependences constructed according to the results of acoustic emission measurements (Fig. 4 - brand 2BR; Fig. 5 - brand 3B) changes in the coefficient K as a function of the number M of freezing and natural thawing cycles showed the following. A flattening of the K (M) dependence for less water-saturated (W ~ 10%) and, accordingly, more resistant to cyclic freezing and thawing (frost-resistant) of brown coal of grade 3B of the Azeisk deposit occurred after the 7th cycle. In turn, brown coal of grade 2BR of the Kansk-Achinsk deposit with W ~ 24% withstood only 5 cycles. It follows that a decrease in K (M) shows a relative increase in the share of structural bonds affected by the phenomenon of cryogenic disintegration, and stabilization (flattening) of K (M) shows that the structural bonds in the control object that are not affected (not weakened) by frost exposure are already almost completely absent (conditional “frost resistance resource” worked out).

Comparing the values obtained for different coals for the corresponding flattening points of the K (M) dependence, which shows the number of cycles after which practically no strong structural bonds remain in the investigated coal, these coals can be ranked by their relative frost resistance.

To determine the degree of cryogenic disintegration in coals of one type, take K _min (flat point) for 0, and K _max (initial coal that has not experienced frost exposure) for 1, then we can calculate the relative degree S of the development of cryogenic disintegration in coal, for which K = X , according to the formula:

S = (XK _min ) / (K _max -K _min )

Considering that during cryogenic disintegration, the initial structural bonds of the geomaterial can both break down and pass into the category of “weak” ones (for breaking short enough low-amplitude heating is enough), a microscopic study of the surface and relief of polished sections of test coals was carried out on an optical video installation consisting of an OLYMPUS 51BX microscope combined with a video capture and image processing system. It was established that it is the mechanism of weakening of structural bonds that prevails, and not their destruction, because significant changes in the specific fracturing of coal (before and after each cycle of their freezing / thawing) were not detected. Accordingly, bonds that have experienced cryothermal effects are destroyed and become sources of acoustic emission precisely during the thermoacoustic emission test (heating stage), and not at the sample preparation stage (freezing / thawing stage).

The implementation of the method

A method for determining the heat resistance of coals to their cyclic freezing and thawing is implemented as follows.

In accordance with the requirements of current regulatory documents (now GOST 9815) coal samples are taken, from which a series of samples is made in the form of prisms of approximately the same volume with each other with a rib length of 25-35 mm. Each of a series of samples with the current number M is subjected to M cycles of freezing to a temperature of T _min = -30 ° C, holding at this temperature for 90 minutes and subsequent natural thawing to a temperature of T _max ≈ + 10 ° C.

акустической эмиссии в функции от времени (фиг. 2 и фиг. 3).Next, each of the investigated coal samples is subjected to thermoacoustic emission tests. For this, coal sample 1 is placed on a circular platform 2 located in a tubular heating furnace 3, for example, of the type Nabertherm RT 50/250/11 (Fig. 1). Sample 1 is pressed against the platform 2 using a quartz waveguide 4, at the free end of which a receiving acoustic transducer 5 is fixed, which is connected to an acoustic emission measuring system 6, for example, A-Line 32D. The temperature regime of the tests, including heating the sample 1 at a speed of no more than 3 ° C / min to a temperature lying in the range (80-90) ± 5 ° C, and subsequent exposure at maximum temperature for at least 4 hours, is established with using the controller of the heating furnace 3. The signals of thermostimulated acoustic emission arising in the sample 1 are fed through a quartz waveguide 4 to the receiving acoustic transducer 5 and then to the measuring system 6, with which the activity is measured and recorded

acoustic emission as a function of time (FIG. 2 and FIG. 3).

термостимулированной акустической эмиссии образца 1 угля выделяют временной интервал 7 неустановившегося температурного режима продолжительностью Т от момента прогрева образца до 30°С и до момента достижения им постоянной рабочей температуры Т_р, на которой производится длительная выдержка образца. Затем на указанной зависимости выделяют временной интервал 8 той же продолжительности Т, начинающийся в момент возрастания уровня активности акустической эмиссии до значения, не менее чем в полтора раза, превышающего соответствующий уровень фоновых шумов. В каждом из интервалов 7 и 8 рассчитывают средние значения активности акустической эмиссии,

и

соответственно. Далее для испытанного образца находят значение коэффициента

.On the experimentally obtained activity dependence

of the thermally stimulated acoustic emission of coal sample 1, a time interval 7 of an unsteady temperature regime of duration T from the moment of heating the sample to 30 ° C until it reaches a constant operating temperature T _p , at which the sample is held for a long time, is isolated. Then, a time interval 8 of the same duration T, which starts at the moment of increasing the level of activity of acoustic emission to a value not less than one and a half times higher than the corresponding level of background noise, is isolated on the indicated dependence. In each of the intervals 7 and 8 calculate the average values of the activity of acoustic emission,

and

respectively. Next, for the tested sample find the value of the coefficient

.

Based on the totality of the results of determining the coefficients K of all tested samples, the dependence of the coefficient K as a function of the number M of freezing and natural thawing cycles is built. The number of these cycles, during which the K (M) dependence is flattened, determines the number of freezing and thawing cycles that are held by coal until the transition to the final stage of disintegration. So, for example, in FIG. 4, corresponding to brown coal 2BR of the Kansk-Achinsk deposit with an average value of W of equilibrium moisture W ~ 24%, is 5 cycles, and for an example in FIG. 5, corresponding to brown coal 3B of the Azeisk deposit with W ~ 10%, - 7 cycles. Comparing the number of these cycles obtained for different batches of coal, the relative thermal stability of each of them to the effect of freezing and thawing is judged. Comparing the K values for samples of coal of the same brand, the degree of development of cryogenic disintegration in each of these samples is determined. Based on this information, knowing the sampling points, it is possible to draw up a map of the development of the cryogenic disintegration process in the places of its storage - to identify priority areas for the implementation of relevant preventive measures.

An example implementation of the method

акустической эмиссии.We studied two samples of brown coals of grades 3B and 2BR with an average value of equilibrium moisture W ~ 10% and W ~ 24%, respectively. Each of the two samples consisted of 10 samples. The samples were subjected to 10 cycles of freezing / thawing, including cooling the samples to a temperature of T = -30 ° C, holding at this temperature for 90 minutes and subsequent thawing to a temperature of T = + 10 ° C, then the samples were heated to a temperature of T = 90 ° C and kept at this temperature for 4 hours, simultaneously recorded activity

acoustic emission.

On the recorded thermoacoustogram (Fig. 2) corresponding to sample 3B, a time interval 7 of an unsteady temperature regime of duration T from the moment of heating the sample to 30 ° C until it reaches a constant working temperature T _p at which the sample is held for a long time was isolated.

On the recorded thermoacoustogram (Fig. 2) corresponding to sample 3B, a time interval of 8, duration T, starting at the moment of increasing the level of activity of acoustic emission to a value not less than one and a half times the corresponding level of background noise, was allocated.

On the recorded thermoacoustogram (Fig. 3) corresponding to sample 2BR, a time interval 7 of an unsteady temperature regime with a duration T from the moment of heating the sample to 30 ° C until it reaches a constant operating temperature T _p at which the sample is held for a long time was allocated.

On the recorded thermoacoustogram (Fig. 3), corresponding to sample 2BR, a time interval of 8, duration T, starting at the moment of increasing the level of activity of acoustic emission to a value not less than one and a half times higher than the corresponding level of background noise, was allocated.

и

соответственно. Далее рассчитывали значение коэффициента

и строили зависимость коэффициента К в функции от числа циклов замораживания/оттаивания для угля марки 2БР (фиг. 4) и для угля марки 3Б (фиг. 5).In each of the intervals 7 and 8 (Fig. 2 and Fig. 3), the average values of the acoustic emission activity were calculated,

and

respectively. Next, the coefficient value was calculated

and built the dependence of the coefficient K in function of the number of freeze / thaw cycles for coal grade 2BR (Fig. 4) and for coal grade 3B (Fig. 5).

Brown coal of grade 3B (W ~ 10%) withstood 7 cycles until the dependence K (M) was flat, and coal of grade 2B (W ~ 24%) lasted 5 cycles until the dependence K (M) was flat. This indicates a greater heat resistance of coal grade 3B compared with coal grade 2BR.

Thus, the proposed method provides a determination of the heat resistance of coal to cyclic freezing and thawing by taking into account the features and modes of such an effect on the parameters of thermally stimulated acoustic emission in coal.

Claims (1)

A method for determining the heat resistance of coals to their cyclic freezing and thawing, which consists in applying to the test samples a cyclic alternating temperature effect, in predetermined time intervals of which the average acoustic emission activity is determined, which is used to judge the heat resistance of coal, characterized in that a series of similar samples is subjected to cyclic freezing and natural thawing to positive temperatures with the number M of such cycles for each sample, equal to its order number in the series, after which all samples of the series are uniformly heated at a low speed to the same temperature in the range (80-90) ± 5 ° C, at which they are held for at least four hours, while the measurement of acoustic emission activity is carried out during heating and exposure, determine the boundaries of consecutive time intervals, the first of which begins after heating the sample to 30 ° C and ends when it reaches a constant temperature, and the second interval of the same duration begins with increasing activity level acoustic emission to a value not less than one and a half times higher than the corresponding level of background noise, a coefficient K is calculated for each sample, equal to the ratio between the acoustic emission activity in the second and first intervals, the dependence of the coefficient K in function of the number M of freezing and natural cycles is built thawing and the number of these cycles, during which the indicated dependence K (M) is flattened, is judged on the heat resistance of coal to the influence of cyclically changing negative and put nyh temperatures.

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