Investigation On Transportable Moisture Limit (TML) Of Coal

An Investigation On Transportable Moisture Limit (TML) Of Coal Using Proctorfagerberg Method


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.