Pumping Coal Slurry Mixtures

Methods for burning mixtures of pulverized coal in oil (variously called colloidal fuel, coal-in-oil slurry, or coal-oil suspension) have been studied for nearly a century and require the production of coal-in-oil suspensions. Stable short-term suspensions of coal in residual fuel oil are easily attained if the coal is pulverized to 200 mesh (75 am) and, by adding surfactants, long-term stability can be obtained so that the coal will not settle out of the mixture even over periods as long as a few months. The interaction between the coal and the hydrocarbon allows the apparent viscosity of the coal-oil mixture (COM) to be 10 times greater than the fuel oil base and special precautions may be taken to provide adequate pump capacity, heating systems for the slurry, and properly sized burner nozzles. 

Kreusing, H., and F. H. Franke. 1979. Investigations on the Flow and Pumping Behavior of Coal-Oil Mixtures with Particular Reference to the Injection of Coal-Oil Slurry in the Blast Furnace. Working paper C-2, BHRA Group, Hydrotransport 6, BHRA. 

A schematic of the experimental unit is shown in Figure 1. This unit primarily consisted of a feed tank, charge pump, preheater, reactor and product receivers. A separate hydroclone unit was used to prepare recycle solvent. The reactor was 6 cm I.D. by 122 cm long. The unit was operated with a cocurrent upflow coal slurry rate of 1.8 kg/hr, the feed slurry consisted of mixture of 40% coal and 60% hydroclone overflow. Big Horn (WY) subbituminous coal was used for the experiments. It was pulverized and screened to minus 20 mesh, but was not predried therefore, the moisture level of the feed coal was typically 22 wt%. The ash content was 3.4 wt%. On a moisture-free basis, carbon, hydrogen and oxygen contents of the coal were 69.34, 4.6 and 19.9 wt%, respectively. All runs were made at 24.1 MPa pressure and a gas rate of 2575 dm /hr. A Gulf developed catalyst was used. 

In short process lines slurries are readily handled by centrifugal pumps with large clearances. When there is a distribution of sizes, the fine particles effectively form a homogeneous mixture of high density in which the settling velocities of larger particles are less than in clear liquid. Turbulence in the line also helps to keep particles in suspension. It is essential, however, to avoid dead spaces in which solids could accumulate and also to make provisions for periodic cleaning of the line. A coal-oil slurry used as fuel and add waste neutralization with lime slurry are two examples of process applications. 

The H-Coal process (Stotler and Schuller, 1979) (Figure 19.14) is an extension of the ebullated-bed technology that is used to convert heavy oils and residua into lighter liquid products. Crushed (-60 mesh) coal is slurried with recycle oil and pumped to a pressure of up to 3000 psi after which the mixture is preheated and introduced into the bottom of the ebullated-bed reactor. The overhead products are fractionated into gases, light distillate, and heavy distillate. 

A slurry is a mixture of a liquid and solid particles. The term sludge typically refers to a highly concentrated slurry containing fine particulate material. Each year, vast tonnages of slurries are pumped. Slurries are often used to transport coal, phosphates and minerals. Dredging of sand and silt in the maintenance of waterways is another example of sohds handled in slurry form. In most slurries, the liquid phase is water. However, coal-oil and coal-methanol fuels are examples of slurries made up with liquids other than water. 

Wear is an important cost to be paid for transporting soUds by liquids. This will be discussed later in the book when exploring slurry pumps and pipelines. The modern slurry engineer can serve the mining and power industries by making possible the transportation of minerals, coal, coal-crude oil mixtures over very long distances, and also play a major role in dredging sediments behind dams to avoid dam failure and to provide arid lands with much needed silt. 

Vanderpan (1982) recommended the use of Ni-hard as a material to cast the impellers and liners of coal handling slurry pumps. For certain high pH applications due to acidic water, or in the case of high-salt mixtures, special high-alloy irons may be used instead of... 

Recent work by the Coal Utilization Research Laboratory in Leatherhead, England, has shown that a slurry of 68% coal in water burned very well in a pressurized fluidized bed reactor. However, much more work remains to be done before such mixtures can be pumped through pipelines and burned directly in specially designed reactors. 

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