Tangential-Fired System

tangential firing is a method of firing a fuel to heat air in thermal power stations in which the flame envelope rotates ensuring thorough mixing within the furnace, providing complete combustion and uniform heat distribution. 

With tangential firing, the furnace is essentially the burner consequently, air and coal quantities need not be accurately proportional to the individual fuel nozzle assemblies. Turbulence produced in the furnace cavity is sufficient to combine all the fuel and air. This continuously insures uniform and complete combustion so that test performance can be maintained throughout daily operation. With other types of firing, the fuel and air must be accurately proportioned to individual burners making it difficult to always equal test results. 

With this type of firing, combustion is extremely rapid, and short flame length results. The mixing is so intense that combustion rates exceeding 35,000 Btu/(ft h) are practical under certain conditions. However, since there is considerable impingement of flame over the furnace walls it is absolutely necessary that they be fully water cooled. This sweeping of the water-cooled surfaces, in the furnace, by the gas increases the evaporation rate. 

in addition to absorption by radiation from the flame envelope, there is transfer by convection, and the resulting furnace temperatures are lower than with other types of burners, even though the heat liberation rates may be somewhat higher. Tangentially fired furnaces are usually clean in the upper zone and, as a result, both the furnace and the boiler are comparatively free from objectionable slag deposits. 

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Plant description. Two nearly identical 430-Mw(e), western, conventional pulverized-coal-utility boilers (referred to as plants A and D) were tested. Both units use tangentially fired burners and burn low-sulfur 200-mesh coal of heat content approximately 27 000 J/g. Both units are equipped with cold-side electrostatic precipitators (ESP) of design efficiency of 99.5% or greater, and a modern flue-gas desulfurization (FGD) system consisting of four verticle spray towers. 

Conversion of the RDF into a low calorific value,fuel gas is accomplished in a fluidized bed realtor. Both air and solid fuel are fed into the reactor under positive pressure. The RDF is immediately decomposed into gas, char and small amounts of tar. The char is subsequently oxidized by air to provide the heat necessary to sustain reaction conditions. Ash from the system is removed by cyclone collectors for subsequent disposal. The cleaned off-gas is then burned in a tangentially fired combustor. 

Zolo reports an interesting example that illustrates the potential benefit of measuring temperatures in a coal-fired power plant boiler (Zolo 2009c). In this case, a nine-path ZoloBOSS system was installed on an 830 MW tangentially fired plant. As shown in Figure 14.23, six beams were placed in a 3 x 3 matrix below the boiler nose, and three parallel beams were installed at stations in the superheater sechon in an orientation perpendicular to the flow. 

FIGURE 1 Tangentially fired boiler. [Reproduced with permission from Singer, J. G. (ed.), (1981). Combustion Fossil Fuel Systems, 3rd Ed., Combustion Engineering, Windsor, CT. 

Pulverized coal (PC) boilers are the most common large combustion systems for the generation of electricity in the US and the industrialized economies of the world. PC boilers include both wall-fired boilers and tangentially-fired (T-fired) boilers wall-fired boilers include both front wall and opposed wall configurations. Like cyclone boilers, they have the potential to use petroleum coke as a fuel provided that a sulfur dioxide scrubber has been installed on the unit. The vast majority of the petroleum coke fired in the USA is burned in PC boilers due to their dominance of the industry. Again the low volatility in the petroleum coke limits its use in PC boilers the typical cofiring percentage is on the order of 20 - 30 percent (calorific value basis). 

Battista, J.J., and R.A. Ashworth. 1998. Co-firing Coal Water slurry Fuel on a Tangentially-Fired Boiler. Proc. 23" International Technical Conference on Coal Utilization and Fuel Systems. Coal Technology Association. Clearwater, FL. March 9 -13. pp. 361 - 370. 

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