Agglomeration of Coal

The roll-type briquet machine , roller press, or Belgian press was developed in Europe during the second half of the last century. The objective of its invention was to find an economic method for the agglomeration of coal fines. 

Fig. 6.10-2 Manual agglomeration of coal fines in Germany in about 1900 (Cewerkschaft Susanna, [B.l])...

When certain bituminous coals are heated, they soften and become fluid commensurate with the evolution of gas and tar (Van Krevelen et al., 1956 Waters, 1962). The plastic behavior is transient and the mass eventually thickens, swells, and fuses to form a porous solid or coke. This phenomenon is of the utmost importance to the production of metallurgical coke, and in other processes sensitive to caking and agglomeration of coal. Because of the impact of plastic behavior on industrial processes, the solvent extraction of coal has been studied in the past in an attempt to isolate the coking principle from the coal (Burgess and Wheeler, 1911 Illingworth, 1922 Dryden and Pankhurst, 1955). 

The work at Battelle s Columbus Laboratories (62) also demonstrated that impregnation of CaO into coal before gasification can prevent agglomeration of coal and greatly increase the reactivity and hydrocarbon yields in the gasifier even for large coal particles. The reaction rate for production of methane from devolatilized chars in hydrogen can be expressed as ... 

Fig. 16. Flow sheet for oil agglomeration of fine-coal slurries (96).

Several laboratory tests (3,6) are used to determine the desirabiHty of a coal or blend of coals for making coke. These are empirical and are carried out under conditions that approach the coking process. The three properties that have been studied are swelling, plasticity, and agglomeration. 

Free-swelling tests are commonly used to measure a coal s caking characteristics. A sample of coal is packed in a cmcible or tube, without compaction, and heated at a fixed rate to about 800°C. Infusible coals distill without changing appearance or state of agglomeration. The fusible coals soften, fuse, and usually sweU. The profile of the resultant coke is compared to a series of reference profiles so that a swelling index can be assigned. The profiles represent indexes between 0 and 9. The best cokes come from coals having indexes between 4 and 9. 

It is well known that during liquefaction there is always some amount of material which appears as insoluble, residual solids (65,71). These materials are composed of mixtures of coal-related minerals, unreacted (or partially reacted) macerals and a diverse range of solids that are formed during processing. Practical experience obtained in liquefaction pilot plant operations has frequently shown that these materials are not completely eluted out of reaction vessels. Thus, there is a net accumulation of solids within vessels and fluid transfer lines in the form of agglomerated masses and wall deposits. These materials are often referred to as reactor solids. It is important to understand the phenomena involved in reactor solids retention for several reasons. Firstly, they can be detrimental to the successful operation of a plant because extensive accumulation can lead to reduced conversion, enhanced abrasion rates, poor heat transfer and, in severe cases, reactor plugging. Secondly, some retention of minerals, especially pyrrhotites, may be desirable because of their potential catalytic activity. 

This method was similar to that used by Hiteshue et al (3). In this method sand (50 g, mesh 0.42 - 0.15 mm) was mixed with the coal (25 g, mesh 0.5 - 0.25 mm). The addition of sand to the coal helped to prevent agglomeration (4). All the experiments used an aqueous solution of stannous chloride impregnated on the coal as a catalyst. The amount of catalyst added on a tin basis was 1% of the mass of the coal. These mixtures were placed in a hot-rod reactor and heated to 500°C at a heating rate of 200°C per minute. Residence time at temperature was 15 minutes. Hydrogen at a flow rate of 22 liters/minute and a pressure of 25 MPa was continously passed through the fixed bed of coal/sand/catalyst. The volatile products were collected in high-pressure cold traps. A schematic of the apparatus used is shown in Figure 2. 

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