Investigation on transportable moisture limit (tml) of coal

Introduction:

Coal is an organic material which has been subjected to heat and pressure over time. It is primarily composed of carbon with variable quantities of other components and it changes in the proportions of carbon to the other components which describe the coal rank. As the rank increases, which is as a result of the effects of increasing pressure and temperature over millions of years, so too does the calorific value. Coal is usually shipped in the form of lumps, but some are very fine grained (called ‘fines’) and may exhibit liquefaction characteristics, hence these require further loading controls in the form of a Transportable Moisture Limit Certificate (TML) and a Moisture Content (MC) Certificate. The International Maritime Organization (IMO) has put guidelines in place on how shippers can monitor their moisture control methods, and the Competent Authority of the Port of Loading is now required to supply an additional certificate which details their approval of these methods. Another coal-related cargo is listed as Coal Slurry, and this also consists of fine coal particles, often washed off larger lumps. Since this too can liquefy, it requires a TML Certificate and MC certificate prior to loading. After the mandatory implementation of the International Maritime Solid Bulk Cargoes Code (IMSBC Code) from 1stJanuary 2011 along with the recent incidents involving bulk carriers have encouraged research institutions and industry partners to perform extensive studies into the test methods used to determine the Transportable Moisture Limit (TML) of ‘Group A’ or liquefiable cargoes.

In the present work, an effort has been made to determine the TML values of different types of coal. The modified Proctor Fagerberg method has been adopted to determine this property. The test method is based upon use of the Proctor apparatus developed in soil mechanics. The standard Proctor/Fagerberg test was adopted by the International Maritime Organization, for use in the IMSBC Code, between 1991 and 1998 for ores. However, in order to have a concrete understanding about the process further research work has been carried out and this work still continues.

In general, TML is the moisture content corresponding to the intersection of the 70%degree saturation curve and the test sample compaction curve. In the case of coal sample where moisture freely drains from the sample such that the test sample compaction curve does not extend to or beyond 70% saturation, the test is taken to indicate a cargo where water passes through the spaces between particles and there is no increase in pore water pressure. Therefore, the cargo is not liable to liquefy. The following definitions are helpful to interpret the results of the TML determination of coal:

Degree of Saturation: Percentage of voids of the test portion occupied by water.
Gross Moisture Content: The mass of moisture divided by the wet mass of the sample.

Optimum Moisture Content: The moisture content corresponding to the maximum compaction under the specified compaction condition.

True Density: Mass in air of a unit volume of particles of coal excluding the volume of the voids between and within the particles.

Transportable Moisture Limit: Maximum moisture content allowed for a safe maritime transportation of a cargo of material. For coal, the TML is the moisture content at 70% degree of saturation.

Experimental:

At least 20 kg was obtained for this test for a particular specimen. To minimize changes to the flow characteristics of the sample, it shall not be completely dried during its preparation. The sample selected for this study was below -6 mm size and the moisture of the sample reduced to 4-6%. The test sample was homogenized thoroughly and split it into 10 test portions of approximately 1.5kg each. Each test portion was stored in a closed recipient to avoid any extra moisture intake from the atmosphere. The material preparation and testing procedure was accomplished within a reasonable time to minimize moisture losses. All suitable sample containers, including plastic sample bags used for the test were carefully sealed.

a) Apparatus

1. Proctor/Fagerberg Mould: A cylindrical iron mould, having capacity of 1000 cm3 with an internal diameter of 105mm ± 0.5mm and height 115.5mm ± 1mm or with an internal diameter of 100mm ±0.4mm and height of 127.3mm ± 0.3mm. The mould is fitted at its upper end with a removable extension collar of the same internal diameter and having approximately 50mm height

2. Proctor/Fagerberg hammers: Metal made having 50mm diameter and weighing 350g equipped with a pipe open at its lower end and a suitable arrangement for controlling the hammer drop height to 200 mm
3. Drying Oven: Equipped with a temperature indicator and control apparatus of regulating the temperature at any point in the oven at 105°C ± 5°C and so designed as to maintain the temperature
4. Pycnometer: Preferably water or kerosene oil pycnometry equipment to determine the density of solid material
5. Tray for hand mixing
6. Weighing device
7. Spray bottle

b) Procedure
1.5kg in each set of test portion was taken and known amount of water was added into it. The mixture was packed very tightly in airtight condition .After 24hr of saturation, the sample was equally divided into 5 parts. One of these parts was placed into the mould, levelled and then tamped uniformly over its surface by dropping the hammer 25 times vertically through the full height of the guide pipe, moving the guide pipe to a new position after each drop. The procedure was repeated four more times so that there are 5 tamped layer of materials in the mould.

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