By fluid-bed combustion

Econ-Abator A process for oxidizing hydrogen sulfide and other sulfur compounds in waste gases by fluid bed combustion in the presence of an oxide catalyst. Licensed by ARI Technologies. In 1992 there were 90 installations. 

The advantages of fluid bed combustion over the more traditional technology arise from the increased turbulence provided by the bed particle action. This fluidization increases the interaction of the fuel particles with the combustion air and creates a veiy accelerated combustion environment for the incoming fuel. Additionally, the sand, initially heated to an ignition temperature for the incoming fuel, provides a... 

The fluid-bed combustion method (2) has been chosen, however, for process development in the regeneration of spent melts from the hydrocracking of coal. In this method, from one to two parts by weight of spent melt is generated for each part of coal fed to the hydrocracking process. The carbonaceous residue, sulfur, and ammonia retained in the melt are burned out with air in a fluidized bed of inert solids. The zinc chloride is simultaneously vaporized, the ash separated from the overhead vapors, and the zinc chloride vapor is condensed as pure liquid for return to the process. 

We believe, based on the results just presented and on other results, that fluid-bed combustion provides a workable process for regenerating zinc chloride from direct hydrogenation of western sub-bituminous coals. Other work not presented here indicates that the process can also be applied successfully to melts from direct hydrogenation of eastern bituminous coals. The process is restricted, however, to coals having relatively low sodium and potassium contents so that economically prohibitive amounts of chlorine are not lost to these alkali metals. Lignites are the major type of coal that would be ruled out by the above restriction. 

A system particularly suited K> provide both steam and power to a coal-to-methanol complex is the Circulating Fluid Bed Combustion Process together with conventional heat recovery and power generating systems. This technology, developed by LURGI in the seventies, combines high efficiency with low SO2 and NOx emissions. More than 90% of the sulphur contained in the coal is captured in die combustion zone already, and due to low combustion temperature, less than 200 mg of nitrous oxides per m of flue gas can be analysed. Coal burnout is between 98 and 99%, depending on the fuel. 

The United States Department of Energy sponsors many research projects, particularly into next-generation pressurized fluid bed combustion combined-cycle power plants. The goal is to design plants with a net system efficiency of more than 50 percent, extremely low sulfur and nitrogen oxide emissions well below 2010 emission limits, and at a power-generation cost of three-quarters by a conventional coal-fired power plant. The European Union similarly sponsors research in this area, as does Japan and other developed or developing countries. 

Hydrocarbon fumes evolved during anode baking are generally disposed of by burning. Additional fuel is required to support combustion because the hydrocarbon concentration is low. In some plants these products are now absorbed in a fluid bed of alurnina for burning in a concentrated form. This treatment also catches the fluoride evolved during anode baking. 

While the rotary dryer shown is commonly used for grains and minerals, this system has been successfully applied to fluid-bed diying of plastic pellets, air-hft diying of wood fibers, and spray drying of milk solids. The air may be steam-heated as shown or heated By direct combustion of fuel, provided that a representative measurement of inlet air temperature can be made. If it cannot, then evaporative load can be inferred from a measurement of fuel flow, replacing AT in the set point calculation. 

A fluid-bed incinerator uses hot sand as a heat reservoir for dewatering the sludge and combusting the organics. The turbulence created By the incoming air and the sand suspension requires the effluent gases to be treated in a wet scrubber prior to final discharge. The ash is removed from the scrubber water by a cyclone separator. The scrubber water is normally returned to the treatment process and diluted with the total plant effluent. The ash is normally buried. 

Fluid bed boilers have also been applied as a cure to sulfur dioxide air pollution from power plants. Various schemes have been developed in which combustion of a sulfur containing fuel takes place in a fluidized bed of particles which absorb or react with sulfur dioxide. The particles are usually regenerated to recover sulfur, which often has enough by-product value to make a significant contribution to process economics. 

The chamber is externally insulated and clad. Combustion equipment for solid fuel may be spreader or traveling-grate stokers or by pulverized fuel or fluid bed. Oil and gas burners may be fitted either as main or auxiliary firing equipment. The boilers will incorporate superheaters, economizers and, where necessary, air preheaters, grit arresters, and gas-cleaning equipment to meet clean air legislation. 

To its advantage, the fluid bed may utilize fuels with high ash contents, which affect the availability of other systems. It is also possible to control the acid emissions by additions to the bed during combustion. They are also less selective in fuels and can cope with a wide range of solid-fuel characteristics. 

Sulfur dioxide is manufactured mostly by combustion of sulfur or its iron sulfide mineral, pyrite, FeS2, in air. The flame temperatures for such combustion of sulfur in the air are usually in the range 1,200 to 1,600°C. Many types of sulfur burners are available and are used to produce sulfur dioxide. They include rotary-kiln, spray, spinning-cup and air-atomizing sulfur burners. Selection and design of burners depend on quality of sulfur to be burned, and rate and concentration of sulfur dioxide to be generated. Pyrites or other metal sulfides may be burned in air in fluid-bed roasters to form sulfur dioxide. 

Municipal solid wastes (MSW) gasification unit which is under development in the project consists of two fluid bed reactors (Figure 2). The first reactor is a gasifier, the second reactor is a combustion chamber for charcoal. To obtain producer gas of middle calorific value water steam is applied as a blowing. Fluid bed is organized by supplying water steam to gasifier (inert material is sand) and air to combustion chamber. The installation is equipped with all necessary devices to measure rate, temperature, and pressure. 

The second major reactor type that requires much further quantification is the fluid-bed chemical reactor, which is of tremendous industrial importance, as indicated by Table 1. A related reactor is the fluid-bed combustor that is employed for combustion of relatively coarse solids with reduced emissions. 

Cracking is carried out in a fluid bed process as shown in Fig. 7.9. Catalyst particles are mixed with feed and fluidized with steam up-flow in a riser reactor where the reactions occur at around 500°C. The active life of the catalyst is only a few seconds because of deactivation caused by coke formation. The deactivated catalyst particles are separated from the product in a cyclone separator and injected into a separate reactor where they are regenerated with a limited amount of injected air. The regenerated catalyst is mixed with the incoming feed which is preheated by the heat of combustion of the coke. 

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