Coal conversion process fluidized beds

Although there has long been talk of coal conversion processes, fluidized bed combustion, and other advanced technologies, flue gas desulfurization (FGD) is still the only commercially significant technology for abatement of sulfur dioxide from fossil-fired power plants. FGD has been used since the early 1970 s, but is still not a mature technology. The chapters in this book present recent advances in the science and technology of FGD processes. 

Whereas Geldart s classification relates fluidized-bed behavior to the average particle size in a bed, particle feed sizes maybe quite different. For example, in fluidized-bed coal (qv) combustion, large coal particles are fed to a bed made up mostly of smaller limestone particles (see Coal conversion processes). 

Kurkela E. Stahlberg P, (1992) Air Gasification of Peat, Wood and Brown Coal in Pressurised Fluidized Bed Reactor. I. Carbon Conversion, Gas Yield, and Tar Formation. Fuel Process. Technol, 31, 1-21. 

One physical property of significant importance in many coal-processing schemes is particle size. For any given process, there usually exists an optimum particle size range that is desired for the feed material. This desired size range may be a function of the conversion technique employed (i.e., moving bed, fixed bed, fluidized bed, etc.), or it may be determined by the solids handling systems used (i.e., dry lock, hopper, slurry, etc.). Some current coal conversion processes require feed to be pulverized to less than -200 mesh while other processes can handle feed up to 1/4 in. (6 mm). 

Exxon was the first to investigate the suitabiUty of a wide range of different U.S. coals for conversion. Operation of the EDS process was demonstrated in a 230 t/d unit in Baytown, Texas that had a start-up in May of 1980. Data on the response of a variety of coals to once-through and bottoms recycle operations are shown in Eigure 4. Eigure 5 presents typical Hquefaction product distributions for the system operated both with and without the Elexicoking (fluidized-bed coking) option. 

A computer model has been developed to provide numerical simulations of fluidized bed coal gasification reactors and to yield detailed descriptions, in space and time, of the coupled chemistry, particle dynamics and gas flows within the reactor vessels. Time histories and spatial distributions of the important process variables are explicitly described by the model. With this simulation one is able to predict the formation and rise of gas bubbles, the transient and quasi-steady temperature and gas composition, and the conversion of carbon throughout the reactor. 

The important factors in coal conversion are the physical and chemical properties of the coal, heat supply (autothermal or allothermal), reactor type (fixed bed, moving bed, fluidized bed, or entrained bed), gasification agent (air, oxygen, steam, or a combination thereof), and process conditions. Typically, coal conversion is carried out at high temperatures (900—1000 C) be-... 

Figures 1 and 2, respectively, show the old and new processes. The major innovations are use of (1) a spray dryer absorber in place of the wet venturi, absorber, centrifuge, rotary dryer combination (2) a cyclic hot-water reheat system interconnecting thermally the calciner product solids and the effluent gas from the spray dryer absorber and (3) a coal-fired, fluidized-bed reactor for conversion of magnesium sulfite (MgSO ) and sulfate (MgSO ) to MgO and SO gas. Otherwise, the two systems are very similar, utilizing a regenerable absorbent to recover the sulfur material as a usable commercial grade of concentrated sulfuric acid.

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