Coal burners

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). 

Coal burners Type Size range (MW) Fuels Grate thermal loading (mW/m ) Bed thickness Turndown ratio Ashing system Main applications... 

Coal burners demand design consideration in all the aspects mentioned under gas and oil burners, but, in addition, need attention to the aspect of ash removal. The extent to which ash removal plays a part in the combustion system design often determines the ability of the burner to burn specific coals, particularly those with a high ash content. Principal types are as follows. 

Figure 24.13 Fixed-grate coal burner with front feed...

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. 

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. 

Fluidized bed combustion is a newer technology that burns coal in an efficient manner and can produce both electricity and heat. A mixture of finely crushed coal and limestone rides on a stream of air, which allows the coal to be burned at temperatures lower than conventional coal burners. This reduces the nitrogen oxide produced. The limestone absorbs sulfur from the coal, which reduces the sulfur dioxide. 

Fuel may constitute only about 1 per cent of the total volume of the solids in the bed and the particle size needs to be reduced to about 10 mm, compared with 100 im in pulverised coal burners. 

Solely on the basis of volatility profiles, fossil fuel burning is expected preferentially to transfer As, Hg, Cd, Sn, Sb, Pb, Zn, Tl, Ag, and Bi to the atmosphere (1). In a study designed to detect fallout from a major coal burner equipped with a precipitator, Klein and Russell (27) showed that Ag, Cd, Co, Cr, Fe, Hg, Ni, Ti, and Zn were deposited in the surrounding soil (115 sq mi), and with the exception of mercury, enrichment correlated with the respective metal concentrations in the coal. Mercury was more widely disseminated to the environment. Previous work has indicated that mercury exists primarily in the volatile phase of the flue gas and consequently as much as 90% bypasses the electrostatic precipitation control device (2). Bolton and co-workers have evidence that selenium and arsenic may present a similar problem (see Chapter 13). 

Particulate emissions from the synthetic fuels were very low, on the order of 0.01 lb per million BTU, and approximately one order of magnitude less than particulate emissions from No. 6 fuel oil. Data on particulate morphology and submicron particle size distributions Indicate a unlmodal size distribution for the No. 6 fuel oil and the SRC-II fuels while Indicating a blmodal size distribution for the H-Coals. Burners out of service and low excess air (LEA) operation did not significantly contribute added particulate emissions although 1t did tend to shift the unlmodal distributions toward a larger number of smaller particles. 

Midwest utilities are coal burners. They have the know-how and facilities to utilize solid fuels. Solvent refined coal, which has the potential of being the lowest cost coal liquefaction product because of its low hydrogen content, is of interest to this group. 

Figure 2. Molecular beam mass spectrometer. Key A, nozzle-skimmer chamber B, middle chamber C, mass spectrometer chamber D, sampling nozzles E, skimmers F, choppers G, gate valve assembly H, mass spectrometer and I, nylon flanges permitting electrical biasing for detection of ions from coal burner.

The stability of the main boiler steam cycle has been excellent. The large openings that were made for the low Btu gas burners have not caused any disturbances in the water/steam circulation system. Furthermore, as regards the operation of the product gas burners, the product gas combustion has been stable even though the moisture content of the solid fuel has been mostly high and the heating value of the gas very low. The stability of the main boiler coal burners has been normal despite the fact that the product gas burners were integrated very close to the... 

Probably the rotary horizontal kiln is the most versatile, since it allows a feed of lumps or fines of limestone or marble, or wet or dry calcium carbonate sludges (Fig. 7.1). The main component of this calcination system is a 2.5- to 3.5-m diameter by 45- to 130-m long firebrick-lined inclined steel tube. Heat is applied to the lower end of this via oil, gas, or coal burners [7]. The feed to be calcined is fed in at the top end. Slow rotation of the tube on its axis gradually moves the feed down the tube, as it tumbles countercurrent to the hot combustion gases. In this way, wet feed is dried in the first few meters of travel. Further down the tube, carbon dioxide loss begins as the temperature of the feed rises. By the time the solid charge reaches the lower, fired end of the kiln it reaches temperatures of 900-1,000°C and carbon dioxide evolution is virtually complete. Normally the temperature of the lower end of the kiln is not allowed to go much above this as it reduces the life of the kiln lining. It also adversely affects the crystal structure of the lime product since it produces a dead-burned or overburned lime. Overburned lime is difficult to slake to convert it to calcium hydroxide and raises... 

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