Muthusamy Karthikeyan

Low-Rank Coal Drying Technologies—Current Status and New Developments

Despite their vast reserves, low-rank coals are considered undesirable because their high moisture content entails high transportation costs, potential safety hazards in transportation and storage, and the low thermal efficiency obtained in combustion of such coals. Their high moisture content, greater tendency to combust spontaneously, high degree of weathering, and the dusting characteristics restrict widespread use of such coals. The price of coal sold to utilities depends upon the heating value of the coal. Thus, removal of moisture from low-rank coals (LRC) is an important operation. Furthermore, LRC can be used cost effectively for pyrolysis, gasification, and liquefaction processes. This article provides an overview the diverse processes—both those that utilize conventional drying technologies and those that is not yet commercialized and hence in need of R&D. Relative merits and limitations of the various technologies and the current state of their development are presented. Drying characteristics of low-rank coal as well as factors affecting drying characteristics of coal samples are also discussed.

A Critical Assessment of Industrial Coal Drying Technologies: Role of Energy, Emissions, Risk and Sustainability

Low-rank coals (LRCs) constitute about 45% of the total coal reserves and hence will soon be the fossil fuel of choice in many countries despite their high moisture content on mining, which varies from 30% to as high as 66%. It is important to reduce their water content to enhance the heating value and reduce transportation costs while enhancing combustion efficiency, safety, and reduction of emissions on combustion. The level of moisture to be achieved upon drying LRCs depends on the end application; it varies from as low as 0% for hydrogenation processes to 15% for briquetting and gasification processes. Numerous drying technologies have been proposed for drying coal; they include pulse combustion, vacuum, fluid bed, rotary, flash, microwave, and superheated steam drying. Each technology has some pros and cons, which are not always clearly spelled out in the literature. In addition, it is necessary to develop sustainable rather than just cost-effective drying systems for LRC. In this article we assess various coal drying techniques critically and identify their strengths and weaknesses. Some theoretical comparisons of different dryer types are carried out based on energy utilization and carbon footprints. The jury is still out on optimal drying technology for LRC and innovative design concepts should be evaluated before finalizing the selection.

Factors Affecting Quality of Dried Low-Rank Coals

The chemical and physical properties of coal are strongly affected by the upgrading process employed. For high-moisture coals, upgrading involves thermal dehydration to improve the calorific value of the coal on mass basis. This study evaluates the feasibility of upgrading a low-rank/grade coal using the oven drying method. The objective of this research work is to study the drying characteristics of low-rank coals and to understand the factors affecting the quality of dried low-rank coals. This article describes laboratory experiments conducted on the characterization of the low-rank coals before and after the drying process. The results on drying kinetics, re-absorption of coal samples, and proximate analysis of coal samples before and after drying are discussed. It was found that the upgrading process produced coal with better heating value and combustion characteristics than those of the raw coal samples.

Minimization of Moisture Readsorption in Dried Coal Samples

Low-rank coals csonstitute a major energy source for the future as reserves of such high-moisture coals around the world are vast. Currently they are considered undesirable since high moisture content entails high transportation costs, potential safety hazards in transportation and storage, and the low thermal efficiency obtained in combustion of such coals. Furthermore, low-moisture-content coal is needed for the various coal pyrolysis, gasification developed. Hence, various upgrading processes have been developed to reduce the moisture content. Moisture readsorption and spontaneous combustion are important issues in coal upgrading processes. This article discusses results of laboratory experiments conducted to study the options for minimization of readsorption of moisture after drying of selected coal samples. Results suggest that there is little benefit in drying low-rank coal at high temperatures. It was found that the higher the amount of bitumen used for coating, the lower is the readsorption of moisture by dried coal. Also, mixing high-temperature-dried coal with wet coal in appropriate proportion can yield reduced moisture content as the sensible heat in the hot coal is utilized for evaporation.

In Situ Evaluation of Radioisotope Cone Penetrometers in Clays

A comprehensive site investigation was performed at a 12-year-old man-made island in Singapore, which was reclaimed using dredged clay lumps. The objective of the site investigation was to evalute the performance of the radioisotope (RI) cone penetrometers to measure in situ wet density and water content, which are the two important parameters in the characterization of soils formed from clay lumps. The site investigation program included RI cone penetration tests, high quality soil sampling, and laboratory testing. The in situ water content and wet density profiles measured with RI cone penetrometers were compared with data obtained independently from the laboratory tests on undisturbed samples. The comparison shows very good agreement, and 92% of the RI cone measurements were found to be within ±5% of the laboratory measurements.

Profiling of Heterogeneous Soil Using the Nuclear-Density Cone Penetrometer

This paper describes a series of experimental investigations conducted to investigate and interpret the density profiles obtained by a nuclear-density cone penetrometer (ND-CP) used in the characterization of a highly heterogeneous lumpy fill. The key features involved in the interpretation of these wet density profiles, termed “signatures” in this paper, were discussed. First, it was established that the maximum radius of the influence zone for the ND-CP used in the present study is about 23.6 cm radius, and this parameter decreases with increasing density of the material. It was also established that the ND-CP measurement provides the average wet density of the composite soil within the measuring volume. The signatures of wet density profiles for layered soil and lumpy fills were also investigated so as to improve the interpretation of the actual soil profile. Backscatter and linear mixture theoretical models were adapted and applied to the ND-CP to provide a framework for this interpretation of the signatures of wet density profiles. A back analysis of a set of lumpy fill results obtained from the actual field tests was also conducted and the results demonstrated that the signature of wet density profiles obtained from the ND-CP is useful for identifying the zones of the soil occupying the big clay lumps and initial inter-lump voids, something that is very difficult to establish without the use of ND-CP and the technique to carry out the interpretation.