Pulverized fuel burners

Prior to being fed to a pulverized fuel burner, coal is ground to a size generally specified such that at least 70% passes a 200 mesh screen (75 Tm) and less than 2% is retained on a 52 mesh screen (300 Tm). The top size is deterrnined by the classifying component of the cmshing mill, oversize material being retained for further grinding (54,100,101). 

Pulverized fuel burners 1-50 All types including anthracite hgnite and bituminous <15 1 None Limited use in shell boilers mainly kilns and power stations... 

Initial preheating of the combustion chamber by gas or oil is normally required in order to provide the necessary temperature environment to release the volatiles that provide the stabilization in the base of the flame. Some small PF systems have used another fuel for flame support, but this compromises the economics. A typical pulverized fuel burner is shown in Figure 24.17. 

CFD Evaluation of a Rotary Kiln Pulverized Fuel Burner... 

Figure 6.13 CFD results for the velocity distribution of the flow field in the combustion zone of a rotary kiln with a pulverized fuel burner showing interaction between primary air jet and the entrained secondary air.

As for oil and gas, the burner is the principal device required to successfully fire pulverized coal. The two primary types of pulverized-coal burners are circular concentric and vertical jet-nozzle array burners. Circular concentric burners are the most modem and employ swid flow to promote mixing and to improve flame stabiUty. Circular burners can be single or dual register. The latter type was designed and developed for NO reduction. Either one of these burner types can be equipped to fire any combination of the three principal fuels, ie, coal, oil and gas. However, firing pulverized coal with oil in the same burner should be restricted to short emergency periods because of possible coke formation on the pulverized-coal element (71,72). 

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. 

Pulverized fuel coal burners (typically turbulent air burners, vertical burners, or nozzle burners) receive hot primary air containing the PF and introduce the mixture to secondary air in such a way that it provides a stable flame. The flow rates of both primary and secondary air are controlled by dampers. An ignitor is required to initiate combustion, and the flame front is maintained close to the burner, with the heat of combustion used to ignite incoming PF. A flame safety device electronically scans the flame and initiates corrective action if required. 

This formulation is proposed as a complete treatment, comprising powdered and crystalline combustion catalysts and slag modifiers. It is designed for pulverized fuel (PF), brown coal, lignite, peat moss, and bark, and for application with cyclone burners, chain grates, underfed, and spreader stokers. 

FIGURE 7.7 Schematic of a low-NOx/SOx pulverized coal burner. The addition of oxygen is staged to produee an initial fuel-rich zone in the burner that results in reduced emissions of nitrogen oxides. Courtesy, M. P. Heap, Energy and Environmental Research Corporation. 

The simultaneous splitting and distribution of solids-gas mixtures for applications requiring multipoint injection, where the mixtures are transported usually under positive-pressure conditions. Some common examples include tuyere injection for blast furnaces, large burner nozzles for pulverized coal-fired boilers, small coal-fired plasma torches providing startup and support energy for boilers, injection of pulverized fuel into calciners, etc. 

Fuel preparation for a pc boiler occurs in a mill, where coal is typically reduced to 70% through a 200-mesh (JA-fim) screen. Figure 2 shows a particle-size distribution from a bowl-mill grinder typically used in power plants. A typical utility specification for particle size distribution of the coal is that 70% of the mass should pass through a 200-mesh screen, which is equivalent to 70% less than 74 /tm. Sometimes the top size is limited to about 120 /rm. The pulverized fuel is pneumatically transported to burner levels (commonly one level fed by each mill) with each level made up of a series of burners. 

Figure 6.12 Burner nozzle geometry for pulverized fuel combustion in a rotary kiln. Left swirl vanes Right center insert for flame stabilization.

When coal is burned, the amount of excess air required may be as high as 50%. With oil, gas, or pulverized coal fuel (PF), excess air requirements drop to only 10 to 30%, and often much less (perhaps < 5%) with modem burner and furnace designs. 

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... 

---