Estimation of parameters for endogenous hotspot emergence during coal cargo transportation by bulk carriers

. The article briefly describes the fire and explosion properties of coal raw materials (hard coal of different grades). The relevance of the problem is shown on the example of incidents occurring during transportation of coal raw materials by different modes of transport. The issues of bulk coal transportation are considered in more detail. The paper aims at determining kinetic parameters of low-temperature oxidation processes leading to self-heating and/or spontaneous combustion of coal, and at determining optimal forms of combustion. On the basis of the data obtained, it is suggested to use bulk cargo holds with strictly defined geometric characteristics. The methodological approach of Prof. J.S. Kiselev and the method of synchronous thermal analysis were chosen as the main method of research. The method proposed by J.S. Kiselev is based on the application of a dry-air thermostat. It permits the study of dependence of the occurrence of spontaneous combustion processes in the coal mass on the thermal-physical parameters and the ambient parameters by obtaining the curves of the heating and cooling rates. The method of synchronous thermal analysis applied by the authors made it possible to prove the need for more careful control of fine coal fractions. Thus, the authors have established the dependence of the rate of oxidative thermochemical processes of pyrolysis on the fractional composition of the sample by applying a highly scientific method. As a result of experimental studies, the authors have calculated the main kinetic parameters of autoignition, namely, activation energy and preexponential multiplier. They suggested geometric characteristics of the shape of the coal accumulation and the bulkhead hold that would minimise the probability of spontaneous combustion within coal accumulation.


Introduction
Coal is currently the most widely used fuel. The Russian Federation is one of the leaders in exporting coal to the world market. The Russian coal deposits are accessible and their development with the application of new technologies is not very restrictive. Today, the Kuznetsk coal basin is the main supplier of coal in many regions of the Russian Federation and abroad [1,2].
The advantage of coal is its low cost. At the same time, one of the main drawbacks is the increased fire and explosion risk due to the high tendency to spontaneous combustion. Analysis of recent statistical data has shown that more than 100 transport fires occur every year because of spontaneous combustion. Therefore, the problem of studying the causes and conditions of spontaneous ignition of bulk cargoes transported primarily by sea transport has not ceased to be topical. It should be noted that over 50% of goods in the world are transported by sea. Such a high demand for this type of cargo transportation is conditioned by the possibility of intercontinental transportation of goods with minimum expenses. The cost of sea freight is significantly lower than that of air freight. Large volumes of coal are shipped through seaports. Bulk transportation by sea is used for transportation of bulk materials. This type of transport by sea is used when a vessel is fully loaded. The safety of human life on a vessel engaged in bulk transportation can only be achieved if its determinants, such as weather conditions along the transportation route, properties of the vessel, and the cargo being carried, are taken into account. The

Problem statement
The properties of the cargo are determined by its characteristics and the voyage load (specific fire load). Raw coal must be heat treated and cooled before shipment. According to the UN International Classification, bulk coal is classified as dangerous, prone to spontaneous combustion, and has a common identification number UN1361.
In addition, methane, an explosive gas, can be released during transport. Low-temperature methane and carbon monoxide pyrolysis gases are produced as a result of a hot spot or an externally introduced ignition source. These explosive gases can form an explosive mixture with oxygen from the air at which the resulting gas mixture ignites / explodes [5,6].
An initial methane explosion, very localised and insignificant, can stir up dust. The initial ignition produces a moving flame front. This produces pressure pulses which, in turn, lead to the formation of a shock wave. It usually travels with higher velocity than the flame front and carries a large overpressure.
When the shock wave is reflected from obstacles, such as walls, in the opposite direction to the direct wave, the propagation of reflected shock waves with much greater destructive power begins.
The shock wave interacts with methane concentrated in the upper areas of rooms and with coal dust deposits, resulting in more or less complete transfer of coal dust deposits into the suspended state and their uniform distribution throughout the hold volume.
At that time, the initial explosion (flash) of methane can shake the coal dust, deposited in the hold loading process, and lead to a stronger explosion of the dust-methane-air mixture. In this case the consequences can be more severe [7,8].
At present, the endogenous hazards of coal transportation processes in bulk carriers have not been investigated sufficiently.
Analysis of literature sources [1,4,6] has shown that: -The indicators of dust explosiveness vary in many countries because the coal available in sufficient quantity in the given territory is chosen as a basic sample. So, for example, in the United States, it is Pittsburgh coal, in the Russian Federation lignite and some varieties (grades) of hard coal. They have different properties and fire characteristics [1,9,10]; -qualitative analytical procedures for determining the degree of fire explosiveness of coal dust (criteria of evaluation, methods) are absent.
We believe that just for that reason, it is necessary to conduct research related to studies of low-temperature self-heating processes resulting in the creation of self-heated spots of coal and its dust, contributing to the origin of spontaneous combustion and/or explosion of flammable gases, such as methane, carbon monoxide, etc., generated during the lowtemperature pyrolysis of coal [8,9,11].

Materials
As samples for the study, in terms of calorific value and amount of freight were selected samples of the following grades of hard coal: lean-burning (LB), weak-burning (WB), lean (L) and fat (F) coal, as they have a high calorific value (about 6500 kcal/kg).
For simultaneous thermal analysis, samples were dispersed to fractions from 0 to 300 µm, and samples were ground to grade commercial fractions of 5-50 mm for dry-air thermostat testing.

Methods
In accordance with the set purpose and objectives of the study, two complex methods were applied. The first method known as the Y.S. Kiselev method allows to determine kinetic indices of the process of self-heating / spontaneous combustion on the basis of data of cooling and heating rates of the studied samples. The method is based on a dry-air thermostat and the improved, by D.A. Frank-Kamenetsky, A.G. Merzhanov and J.S. Kiselev, theory of thermal spontaneous combustion by N.N. Semenov. Prediction of spontaneous combustion and determination of the geometric shape of the material accumulation under study and the method of its stacking are based on thermal-physical characteristics of the material and on activation energy and the pre-exponential multiplier determined in the course of experimental data processing.
The synchronous thermogravimetry (STA) method has been applied as a method for investigating thermochemical transformations in hard coal samples. The method allows, depending on the task, to obtain simultaneously results of thermogravimetric (TG+DTG) and/or results of differential scanning thermogravimetry (TG+DSK).
Unlike method of J.S. Kisilev STA method is highly scientific, i.e. for performance of experimental work it is required to pass a special training or as a rule the laboratory results are received together with the scientific employee (operator) owning skills of work on such kind of equipment.

Results
The results of the simultaneous thermal analysis suggested that the reactivity of hard coal depends on the particle size in the fractional composition. The less the finer impurities (dust: particle size less than 800 μm) and the larger the fraction of the grade coal, the safer the transported hard coal will be. The obtained results do not contradict the data of other researchers. The thermogram of the investigated sample of fatty coal of different fractions is shown in the Figure 1.  Figure 1 shows the results of the thermal synchronous analysis of hard coal samples of J grade. The obtained results prove the necessity of careful control of parameters of the shipped cargo (hard coal) in terms of the presence of small fractions and its grindability in the process of transportation. Because the presence and / or formation of fractions of a dispersion greater than 1 mm will intensify the process of oxidation of the coal mass and thus contribute to the emergence of the focus self-heating.
Larger samples were worked with according to the method developed by J.S. Kiselev. In accordance with this method, in accordance with the method of experimental work, the investigated samples of bulk density were placed in baskets of size K-15, K-30 and K-50 [1,12,13].
As an example of the results obtained, namely the direct cooling of the investigated samples for better visualization and further work, the cooling rate curves were presented in logarithmic coordinates. The results are shown in Figure 2. From the experimental values, using Arrhenius formulae (expression 1) and Newton's formulae (2), by equating (3) and logarithmizing expression 1 (4), we calculated the values E (activation energy) and C (pre-exponential multiplier): .
here: R is a universal gas constant 8.314, J/(molK) We proceeded to logarithm the expression (1) and finally obtained: By denoting as: Equation (4) has taken the following form: The solution of this expression (6) makes it possible to determine the kinetic characteristics of spontaneous combustion of coals (using J grade as an example): E = 65.93 kJ/mol C = 7.1K/c Thus, the kinetic characteristics of spontaneous combustion of coals (by the example of J grade) were determined in accordance with the method of J.S. Kiselev: activation energy was E = 65.93 kJ/mol and the pre-exponential multiplier C = 7.1K/c [1,14,15].
Computational prediction of thermal spontaneous combustion was carried out for cylindrical, plane-parallel and cubic forms of cargo accumulation.
Using the method developed by J. S. Kiselev, prognostication for ambient temperatures in a range from 0°C to 100°C (with an interval of +10°C), the shape and critical size of the accumulation, the criterion of spontaneous combustion of hard coal of CC (slightly meltable hard coal) and J (fat hard coal) in terms of its storage and transportation were carried out.
According to the experimental data obtained, it has been established that regardless of the coal grade the safest form of coal accumulation is the cubic form.
Therefore, we consider it safer to transport coal in bulk carriers having cubic shape of holds or close to it. The most appropriate shapes of bulk carriers' holds are shown in Figure  3.  The hold shapes shown in Figure 4 are OBO and OBC (Ore Bulk Oil, Ore Bulk Cotainers) type bulk carriers. It is most expedient to transport coal in Handsize and Handymax bulk carriers. However, in the fast-developing world, we cannot rule out the use of larger deadweight bulk carriers such as Panamax and Capesize, in which case we believe that the mandatory use of cubic shape holds will minimize the risk of cargo fire before it is shipped to the consignee [3,16,17].

Conclusion
Several conclusions can be drawn from the experiments conducted and the experimental data obtained. 1. The fire hazard of hard coal lies in its tendency to spontaneous combustion. 2. Experimental results obtained by the authors show that the main characteristic affecting the handling (storage, storage, transport) of coals is the activation energy of spontaneous combustion. The less it is, the more dangerous the coal is to fire, the more dangerous it is to transport. 3. The economically expedient and less endogenous fire hazardous form of coal accumulations formed in the holds of bulk carriers having a cubic or close to it form. 4. Coal that is more oxygen-absorbing, i.e., has developed pores, is more prone to spontaneous combustion (e.g., J grade). CC coals are more resistant to spontaneous ignition. 5. At ambient temperatures much different from spontaneous combustion temperature (Tsp.comb.), The decisive factor for the spontaneous combustion of coals is the parameter E (activation energy). The lower the activation energy, the higher the risk of spontaneous combustion. In the case of equal values of the activation energy (parameter E), the possibility of spontaneous combustion of coals will be higher in the case of fewer Tsp.comb. As the ambient temperature approaches Tsp.comb., the spontaneous combustion process is determined only by this temperature (Tsp.comb.), rather than by the E parameter.