Boilers combustion efficiency

Coal is a raw material for many chemical syntheses as weU as fuel. Coal is dried to increase its calorific value and simplify loading, unloading, transport, and to improve boiler combustion efficiency. It is also dried for processes like briquetting, coking, gasification, carbonization, and liquid fuel synthesis. Coke oven efficiency can increase 30%-50% in preheating and 10%-15% in drying. 

Table 18.1 shows the heat input required to produce 11b of saturated steam at different operating pressures and varying feedwater temperatures, while Table 18.2 lists the typical energy content and boiler combustion efficiency for several common... 

Another furnace that does not require fuel preparation is the stoker boiler, which was used by New York State Electric Gas Corporation (NYSEG) in its TDE tests. At NYSEG, the stoker boiler, which has a 1649°C (3000°E) flame temperature (as does the cyclone boiler), has routinely blended low quaUty coal, and more recently, wood chips with its standard coal to reduce fuel costs and improve combustion efficiency. In the tire-chip tests, NYSEG burned approximately 1100 t of tire chips (smaller than 5x5 cm) mixed with coal and monitored the emissions. The company determined that the emissions were similar to those from burning coal alone. In a second test-bum of 1900 t of TDE, magnetic separation equipment removed metal from the resulting ash, so that it could be recycled as a winter traction agent for roadways. 

FIG, 5-22 Thermal perfo rmance of weU-stirred fiirnace chambers reduced efficiency as a function of reduced firing density D and reduced sink temperature Tj. (a) Radiant section, oil tube stills, cracking cods, (h) Domestic boiler combustion chambers, (c) Open-hearth furnaces, (d) Soaking pits. 

In AFBC units, heat is removed from the flue gas by a convection-pass tube bank. The particulates leaving the boiler with the flue gas consist of unreacted and spent sorbent, unburned carbon, and ash. Multiclones after the convection pass remove much of the particulate matter and recvcle it to the combustor, increasing the in-furnace residence time an improving combustion efficiency and sulfur retention performance. Bubbling PFBC units do not have convection-pass tube banks and do not recycle solids to the boiler. 

Give an example of how opacity monitoring of a coal-fired boiler could be used to improve combustion efficiency. 

Boiler plants are a major user of energy. The combustion efficiency of a boiler plant can easily be set at the optimum, and Table 30.2 suggests the parameters for this for various fossil fuels ... 

Boilers may be direct fired or indirect fired. Energy supply designs account for various combustion methods using fossil fuels, municipal waste, process residues, waste heat, and by-products. Special boiler combustion systems to reduce pollution or improve efficiency include fluidized-bed and combined cycle. 

The modem FB boiler is a compact economical design (typically more compact than SM boilers) and provides minimum combustion efficiencies of 80% for gas-fired and 83% for oil-fired models. They usually are small units, seldom exceeding 150 hp, and operate on a variety of fuels at typically 4 to 5 sq ft of surface area per hp. 

Typically, online cleaning must be carried out fairly slowly and carefully, so that the programs may take 3 to 6 months, perhaps longer, before particularly satisfactory results are achieved. If the boiler is particularly dirty before an online clean is considered necessary, then the combustion efficiency will be lower than desirable. It consequently will take some time before this reduced efficiency improves significantly, and therefore there is an additional fuel cost that must be considered, as well as the cost of the online cleaning program. 

One of several different types of flue-gas analysis equipment (such as electronic, Fyrite, or Or sat types). They are used to determine boiler fuel combustion efficiency. 

Economizers improve boiler thermal efficiency by recovering heat from the combustion flue gases exhausted from the steam boiler section. The recovered heat is used to heat colder streams (heat sinks), before ultimate discharge of the waste gas to atmosphere. This recovered heat displaces the need to bum additional fuel to heat these same streams. 

FIG. 24-59 Characteristic curves for boiler thermal efficiency as a function of flue gas effluent temperature and flue gas water dew points. Based on the LHV of a fuel, and stoichiometric reaction, 100 percent efficiency would be achieved if sufficient combustion heat were recovered and removed, so that the temperature of the effluent flue gas was reduced to 25°C. For a flue gas with a 55°C dew point, recovering additional heat via condensation by cooling from 175 to 35°C (as shown) would increase the overall efficiency by more than 13 percent. Courtesy Combustion b- Energy Systems, Ltd. www.condexenergy.com.)... 

The three-step model was developed as a consequence of the extreme complexity of a PBC system. This author had a wish to describe the PBC-process as simple as possible and to define the main objectives of a PBC system. The main objectives of a PBC system are indicated by the efficiencies of each unit operation, that is, the conversion efficiency, the combustion efficiency, and the boiler efficiency. The advantage of the three-step model, as with any steady-state system theory, is that it presents a clear overview of the major objectives and relationships between main process flows of a PBC system. The disadvantage of a system theory is the low resolution, that is, the physical quantity of interest cannot be differentiated with respect to time and space. A partial differential theory of each subsystem is required to obtain higher resolution. However, a steady-state approach is often good enough. 

The models are a set of equations that relate key performance measurements to the major control variables influencing combustion efficiency. Owing to the complexity and uncertainty of the analytical models that are derived from physical principles, empirical models, based entirely upon the plant data, are typically used for practical boiler optimization control. 

The Consolidated Edison test results, as shown in Table VI, indicated complete suitability of SRC-II coal liquids as a high quality boiler fuel. No operational problems were encountered and no deposits were observed. Combustion efficiency was comparable to that for the low-sulfur No. 6 fuel oil, as were the levels of carbon monoxide and hydrocarbon emissions. Modifications to burner equipment required to handle the SRC-II fuel oil are considered to be no more extensive than those required for similar variations in petroleum fuels. Particulate emissions for the SRC-II fuel oil were generally lower than for the No. 6 fuel oil, and were in all cases below the new source performance standards proposed by EPA (0.03 lbs/MM Btu). 

A plug flow char combustion model was used to predict the combustion efficiencies of SRC under simulated commercial boiler operating conditions. Inputs were based on the volatile yields and char characteristics measured in the CMHF. 

Based on the experience of these 75 t/h boilers, a 220 t/h CFBC boiler has been designed and is now being fabricated. A two-stage channel separator, as shown in Fig. 39, is used on these CFBC boilers. This is followed by multi-cyclones. By using this combination of gas-solid separators, and with fly ash reburn, the combustion efficiency has reached 97.5% for a 12,000-kJ/kg low-grade coal. 

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