Pulverized Coal Boilers

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 

Because pulverized coal boilers are designed to bum fuel in suspension, small TDF are typically used.7 TDF is often a maximum of 1-inch in diameter, but can be as small as 1/4- 

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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. 

One significant advantage of pulverized coal boilers is the ability to use any kind of coal, including mn-of-mine or uncleaned coals. However, with the advent of continuous mining equipment, the ash content frequently is ca 25%, and some preparation is frequently practiced. There were 931 coal preparation plants in the United States in 1988, mainly in Kentucky, West Virginia, and Peimsylvania. 

More than 90% of the coal used by electric utilities is burned in pulverized coal boilers. In these boilers, 65-80% of the ash produced is in the form of fly ash. This fly ash is carried out of the combustion chamber in the flue gases and is separated from these gases by electrostatic precipitators and/or mechanical collectors. The remainder of the ash drops to the bottom of the furnace as bottom ash. While most of the fly ash is collected, a small quantity may pass through the collectors and be discharged to the atmosphere. The vapor is that part of the coal material that is volatilized during combustion. Some of these vapors are discharged into the atmosphere others are condensed onto the surface of fly ash particles and may be collected in one of the fly ash collectors. 

The wide dispersion of pulverized coal boilers (in number and capacity) translates into significant potential opportunities for biomass utilization [44]. 

Figure 12 presents the results of calculations on model (90)-(95) (curve 7) in comparison with those performed earlier (Gorban et al., 2001, 2006) and experimental data. As is seen from the figure the calculations on the new model were in good agreement with the earlier results and proved to be even closer to experimental volumes of NO emissions by pulverized-coal boilers. 

At the same time calculations on the modified MEIS are possible without additional kinetic models and do not require extra experimental data for calculations, which makes it possible to use less initial information and obviously reduces the time and labor spent for computing experiment. Furthermore, there arise principally new possibilities for the analysis of methods to mitigate emissions from pulverized-coal boilers, since at separate modeling of different mechanisms of NO formation the measures taken can result in different consequences for each in terms of efficiency. Consideration of kinetic constraints in MEIS will substantially expand the sphere of their application to study other methods of coal combustion (fluidized bed, fixed bed, etc.) and to model processes of forming other pollutants such as polyaromatic hydrocarbons, CO, soot, etc. 

Information on the velocity of the particle is found by measuring the transit time of the particle through the sample volume [104]. The instrument has been used in large scale pulverized coal boilers [105-108] char fragmentation and fly ash formation during pulverized coal combustion [109] and for coal slurries [110,111]. 

Coal-fired boilers for pelletized fuel Cofiring in pulverized coal boilers Direct and indirect... 

The combustion of pulverized coal particles can be broken down into two main processes (a) evolution of volatiles (pyrolysis) and their subsequent combustion and (b) heterogeneous combustion of the solid residue. Questions at issue in these two processes are (a) What is the effect on pyrolysis yield of the rate of heating, which in pulverized coal systems can reach 104°C/sec and (b) what are the reaction order and activation energy of the subsequent char burnout Answers to these questions can affect approach to design of both normal pulverized coal boilers and char... 

Bottom ash (BA) - The most common type of coal-burning furnace in the electric utility industry is the dry, bottom pulverized coal boiler... 

The temperature regimes in which this corrosion occurs are summarized in Figure 3.2. The data generally show that K2S2O7 will form from K2SO4 and SO3 at 400°C when SO3 concentration is at least 150 ppm as the temperature increases, the SO3 requirement increases, so that, at 500°C, at least 2000 ppm SO3 will be required to form liquid K2S2O7. Sodium pyrosulfate can form at 390°C with = 2500 ppm SO3, but at 485°C, 2% by volume SO3 will be required. Based on these results and the anticipated maximum level of 3500 ppm SO3 in a pulverized-coal boiler, Reid (1971) concluded that pyrosulfates can contribute to metal loss in the waterwall and economizer tubes but may not be a cause of corrosion in superheaters and reheaters in conventional systems (Natesan 2002). 

The most widely implemented coal-based technology for power generation is the pulverized-coal (pc) boiler. These boilers provide 88% of the total coal-based electric capacity in the United States and similar large fractions of electric capacity abroad. A typical pulverized coal boiler is shown in Fig. 1. Steps in the process are (1) mining of the coal from near-surface or deep mines, (2) transporting cmshed coal several centimeters in size to the power sta-... 

Table 13.1 whereas Table 13.2 reports the properties of significant streams in the plant. The size of the plant has been selected in order to fit with the large size, 50-Hz, heavy-duty gas turbine available on the market. The resulting performance is summarized in Table 13.3. Power output is 367.4 MW with a45.2% efficiency (LHV basis). This value poses the IGCC plant at about the same efficiency level of a pulverized coal boiler, ultra-supercritical steam cycle. Although the latter has higher sulfur oxide and parficu-late matter (PM) emissions, it currently represents the reference technology for large-scale power generation fi om coal given than its investment cost is at least 30% lower than an IGCC plant.

WEW Process (24,25). Vereingte Elektrizitatswerke Westfalen is a German electric utility that has developed a unique dry feed, entrained-flow gasification process. The VEW concept relies on partial gasification with preheated air to produce low-Btu-coal-derived gas and char. After waste heat recovery, steam generation, and sulfur removal, the coal gas is fired in a combustion turbine. The char is fired in a pulverized coal boiler with the hot flue gas from the combustion turbine serving as the preheated combustion oxidant. This design requires additional cleanup of the pulverized boiler flue gas to meet emission requirements. 

The data in Figure 2.6 demonstrate that the petroleum coke shows more of a tendency to an initial lag in volatile nitrogen evolution than the Black Thunder subbituminous coal. However the petroleum coke shows less of a tendency for an initial lag in volatile nitrogen evolution than the Pittsburgh 8 bituminous coal. Based upon these data, and the previous volatility data, it is readily apparent that NOx eimssions from petroleum coke firing are more likely to be jnoblematic in pulverized coal boilers with low residence times than in other types of firing systems. 

Cofiring Petroleum Coke in Pulverize Coal Boilers... 

CWS fuels developed for cofiring have been developed for both pulverized coal boiler firing and for cyclone boiler firing. The slurries... 

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