Suspension firing pulverizers

Ptilverizers The pulverizer is the heart of any solid-fuel suspension-firing system. Air is used to dry the coal, transport it through the pulverizer, classify it, and transport it to the burner, where the transport air provides part of the air for combustion. The pulverizers themselves... 

The suspension-fired boilers are similar to the pulverized coal firing technology and involve combusting the fuel in the form of small particles as they are fed into the boiler. A great deal of pretreatment is required of the fuel, which is a potential disadvantage. However, the higher boiler efficiency is an advantage. 

A demonstration unit sponsored by the Environemtal Protection Agency (EPA) to produce RDF at St. Louis proved the basic feasibility of mechanical separation processes, transport and storage techniques, and combustion of fluff RDF to replace 5 to 27 percent of the pulverized coal used in suspension-fired utility boilers. However, the refinement of equipment components and the technical and economic optimization of the basic technology still require a great deal of work. 

Multichannel burners In the 1970s when the kilns became shorter with the introduction of dry process with suspension preheater and precal-ciner fired on solid fuels, they were developed in order to reduce the primary air supply and to obtain a short, divergent with strong swirl flame suitable for firing pulverized fossil coal first and petcoke. 

Pneumatic conveying dryers have been developed for simultaneous pulverization and drying of bark or hog fuel, for instance. The resulting powder with a moisture content of 10%-15% can be used in a suspension-fired boiler or a limekiln in a pulp mill. In Figure 36.7, a bark dryer that delivers pulverized fuel to a limekiln is shown. Wet biomass is metered into the system via a rotary valve. It is conveyed by the flue gases, which in this case are delivered from a recovery boiler, up and down in two drying towers. Predrying of... 

Fluidized bed combustion of coal is used to reduce the emission of sulfur dioxide and NO into the enviromnent. In this technology the pulverized fuel is burnt in a fluidized bed combustor after being blended with limestone or dolomite, which act as soibents for SO2. The burning temperature is significantly lower than is common in the production of fly ashes in suspension-fired furnace chambers, at around 850°C. The process may take place at atmospheric pressure (atmospheric fluidized bed combustion, AFBC) or elevated pressure (pressurized fluidized bed combustion, PFBC). 

Conventional TDF cannot easily pass through a pulverizer and must therefore be injected separately from the coal. Further, reduction of TDF to particle sizes appropriate for suspension firing is prohibitively expensive. 

The first suspension-fired furnace in the United States was designed like the one shown for vertical firing in Fig 17.24. Pulverized coal (about 70% through a 200-mesh screen) is transported to the burner with primary air, the amount of this air being about 20 percent of that needed for complete combustion. The... 

The WP L cyclone boiler will bum I DE continuously with coal, as about 5% of its fuel mix, with htfle or no modification. By contrast, pulverized-coal boilers, which account for about 80% of the coal-fired capacity in the United States, probably caimot bum tire chips without significant modifications. In these boilers, which bum very fine coal particles in suspension, the heavy chips will fall from the area where best combustion occurs. 

CycUme Furnaces In cyclone firing (Fig. 24-l4d) the coal is not pulverized but is crushed to 4-mesh (4.76-mm) size and admitted tangentially with primary air to a horizontal cylindrical chamber, called a cydone furnace, which is connected peripherally to a boiler furnace. Secondary air also is admitted, so that most all of the coal bums within the chamber. The combustion gas then flows into the boiler furnace. In the cyclone furnace, finer coal particles burn in suspension and the... 

In a pulverized boiler, the coal is ground to the consistency of talcum powder in a mill, and then entrained in an air stream that is fed through the burners to the boiler combustion chamber.6 Firing, therefore, occurs in suspension. Pulverized boilers can be wet-bottom, which means that coals with low ash fusion temperatures are used, and molten ash is drained from the bottom of the furnace, or can be dry bottom, which means that coals with high ash fusion temperatures are used, and dry ash removal techniques can occur.6... 

---