Submerged Combustion Systems

There are several processes that have been proposed for the direct combustion (oxidation or gasification) of coal within a liquid medium, and three of these are described as follows. The Zimpro process is based on the oxidation of crushed coal suspended in an oxygen-saturated hot-pressurized aqueous medium and is an outgrowth of an attempt to produce oxidized chemical products from paper mill wastes and the development of wastewater reclamation systems. The process involves the injection of high-pressure air into a slurry of hot water and coal under high pressure. High rates of oxidation occur at temperatures between 200°C and 350°C (390°F and 660°F) the exact temperature required to complete oxidation with reasonable residence time is dependent on the coal. Since the energy released is used to vaporize water, there is a direct relationship between the liquid temperature and the reactor pressure to which the air used for oxidation must be compressed. 

After oxidation, the gases evolved from the slurry contain water vapor, carbon dioxide, nitrogen, and partially oxidized material unoxidized material is removed from the bottom of the reactor and inevitably is accompanied by some water. 

Although it appears that the Zimpro process may not be able to compete with FBC as a means of electricity generation (Table 14.7), there may be circumstances (e.g., cheap, low-grade fuel, and/or process heat) in which the process may be suitable for energy conversion. 

Comparison of the Fluidized-Bed Combustion Process with the Zimpro Process 

Heat release Fuel condition Combustion efficiency Problem areas 

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Gas-Fired water heaters are also made more efficient by a variety of designs that increase the recov-ei y efficiency. These can be better flue baffles multiple, smaller-diameter flues submerged combustion chambers and improved combustion chamber geometry. All of these methods increase the heat transfer from the flame and flue gases to the water in the tank. Because natural draft systems rely on the buoyancy of combustion products, there is a limit to the recovery efficiency. If too much heat is removed from the flue gases, the water heater won t vent properly. Another problem, if the flue gases are too cool, is that the water vapor in the combustion products will condense in the venting system. This will lead to corrosion in the chimney and possible safety problems. 

The specialty evaporators make up the second classification of evaporator types. These are generally much smaller and simpler than the tubular evaporation systems, and are often batch or multipurpose evaporators. The third group is a unique classification and the direct-fired, submerged combustion evaporator is the best example of this type. 

Hustad, J. E., and Smju, O. K. "Heat Transfer to Pipes Submerged in Turbulent Jet Diffusion Flames." In Heat Transfer in Radiating and Combusting Systems, 474-90. Berlin Springer-Verlag, 1991. 

Most submerged combustion units do not have heat transfer surfaces and are therefore used in applications where the lack of heat transfer surface can be put to advantage. Figure 11-34 illustrates such a system. Fuels can be either gaseous or liquid. 

Fluidized-bed combustors are operated at relatively low temperatures, namely 760°C-930°C (1400°F-1700°F). The reason lower temperatures are achieved in these systems compared to the pulverized coal system is that heat transfer surfaces are submerged in the bed and extract heat directly from the burning particles. In a pulverized coal combustor system, heat is not extracted until the coal is essentially burned up, resulting in higher gas temperatures. Because of the great amount of turbulence in a fluidized-bed boiler, the coal particles can be larger, which reduces the amount of grinding required prior to combustion. 

The industrial SCM design, illustrated in Fig. 9, is a result of extensive research efforts at the Gas Institute NASD, the highlights of which are discussed above. Its main components include melt bath, separation zone, recuperator, feeder, melt tap-hole, submerged burners, and stack. In addition, to ensure reliable and steady operation, it employs systems for tank cooling, natural gas and combustion air supply and process control, measurement, and safety. 

Both devices described above were developed in order to test the friability of FCC catalysts. Nowadays the application of these or similar tests is a common procedure in the development of fluidized bed catalysts. Contractor et al. (1989), for example, used a submerged-jet test to compare the attrition resistance of newly developed VPO catalysts. In fact, such tests can be applied to any type of fluidized bed processes. Sometimes they have to be slightly modified to adapt them to the process under consideration. The drilled plate may, for example, be substituted by a porous plate if only attrition in the bed is of interest. Even temperature and pressure can be adapted. Vaux and Fellers (1981) investigated for example the friability of limestone sorbent that is used for fluidized bed combustion. By surrounding a Gwyn-type test facility with a heating system, they took thermal shock and reaction into account. 

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