Flue loss

Figure 19.1 indicates the flue losses to be expected for different temperatures and excess air. It is seen that considerable savings can be made, particularly at higher temperatures, by reducing excess air levels to a practical minimum. It is also evident that a reduction in air/gas ratio to below stoichiometric will cause a rapid deterioration in efficiency caused by the energy remaining in the incomplete combustion of fuel. 

Figure 19.1 Gross flue losses versus excess air...

To minimize flue losses it is important to keep excess air to a minimum, but the practicalities of the burner must be considered and a safe operating margin incorporated. 

The CEN/TC 295 draft standard prEN 13240 [1] is based on measurements of efficiency and flue gas emissions at a nominal burning rate. The emission factors are based on concentration measurements of the pollutants in the due gas. The efficiency is calculated indirectly by the flue loss method taking into account the thermal due gas losses (sensible heat) and the chemical losses (combustible gases, here as carbon monoxide, CO). 

The slow heat release appliances are operated in an insulated and air cooled calorimeter room (Fig. 3), where the heat output can be measured directly. In parallel, the efficiency is determined indirectly with measurement of flue gas temperature and concentrations of carbon dioxide (CO3) and carbon monoxide (CO) in the flue gas, in accordance with the CEN/prEN flue loss method. 

Detemiination of efhciencies by flue loss method vs. calorimeter room. Measurements of emissions in the flue duct vs, dilution tunnel. 

The direct (with calorimeter room) and indirect (with flue loss method) efficiencies were simultaneously determined in the tests. 

Indirect measurement of efficiency with flue loss method. 

P.E. Denyer, D. Wilkins, Further investigations into the use of a flue loss method for measurement of the thermal efficiency of wood-burning roomheaters, CRE Group Ltd., Gloucestershire GB, F AT Report no. 70 (1994)... 

In the flue gas carbon monoxide and particulate shall be measured. CO-data can be used from the flue loss measurements. The data needs to be standardized to a given oxygen content (e.g. 13 vol %). If required also additional flue gas components can be analyzed such as nitrogen (NOx) and hydrocarbons (HC). The particulate measurements are done in the dilution tunnel. The particulate measurements shall not be interrupted during a bum cycle. 

Step 2. Predict the %available heat (which is 100% - %flue losses) by reading it from an available heat chart (figs. 5.1 or 5.2). Section 5.1 explains how to determine fiue gas exit temperature. 

In Fig. 6.27, the flue gas is cooled to pinch temperature before being released to the atmosphere. The heat releaised from the flue gas between pinch and ambient temperature is the stack loss. Thus, in Fig. 6.27, for a given grand composite curve and theoretical flcune temperature, the heat from fuel amd stack loss can be determined. 

A more obvious energy loss is the heat to the stack flue gases. The sensible heat losses can be minimized by reduced total air flow, ie, low excess air operation. Flue gas losses are also minimized by lowering the discharge temperature via increased heat recovery in economizers, air preheaters, etc. When fuels containing sulfur are burned, the final exit flue gas temperature is usually not permitted to go below about 100°C because of severe problems relating to sulfuric acid corrosion. Special economizers having Teflon-coated tubes permit lower temperatures but are not commonly used. 

As a general nile, the direct-heat units are the simplest and most economical in construction and are emploved when direct contact between the solids and flue gases or air can be tolerated. Because the total heat load must be introduced or removed in the gas stream, large gas volumes and high gas velocities are usually required. The latter will be rarely less than 0.5 m/s in an economical design. Therefore, employment of direct rotating equipment with solids containing extremely fine particles is likely to result in excessive entrainment losses in the exit-gas stream. 

Figure 27-13 shows the available heat in the products of combustion for various common fuels. The available heat is the total heat released during combustion minus the flue-gas heat loss (including the heat of vaporization of any water formed in the POC). 

The distance above the catalyst bed in which the flue gas velocity has stabilized is refened to as the transport disengaging height (TDH). At this distance, there is no further gravitation of catalyst. The center-line of the first-stage cyclone inlets should be at TDH or higher otherwise, excessive catalyst entrainment will cause extreme catalyst losses. 

The power train (Figure 8-10) was eommissioned in May 1989. Table 8-1 provides data on the maehine in question. Tables 8-2 and 8-3 show flue gas analysis from the regenerator to the gas expander turbine inlet and the relevant metallurgy, respeetively. There are many possible failure modes in gas expanders, whieh inelude erosion, eatalyst deposition, and exeessive meehanieal vibration. Obviously, these faetors may also eause power loss, and some power trains do indeed fall short of produeing the expeeted power. Nevertheless, in some eases operation at off-design expander system eonditions eould be the primary eause of performanee defieieneies. 

Flue gas recirculation (FGR) is the rerouting of some of the flue gases back to the furnace. By using the flue gas from the economizer outlet, both the furnace air temperature and the furnace oxygen concentration can be reduced. However, in retrofits FGR can be very expensive. Flue gas recirculation is typically applied to oil- and gas-fired boilers and reduces NO, emissions by 20 to 50%. Modifications to the boiler in the form of ducting and an energy efficiency loss due to the power requirements of the recirculation fans can make the cost of this option higher. 

The rotational operation of a CFB leads to a vortex motion in the freeboard which tends to inhibit particle loss by elutriation. Because of the relatively compact nature of the CFB and the operating flexibility provided by the rotational motion, the CFB has been proposed for a variety of applications including coal combustion, flue gas desulfurization, gas combustion, coal liquefaction and food drying. 

As another example of calculation and dimensioning of pneumatic conveying systems we consider an ejector shown in Fig. 14.20. In fluidized bed combus tion systems a part of the ash is circulated with the hot flue gas. The task of the ejector, is to increase the pressure of the circulating gas to compensate the pressure losses of the circulation flow. The motivation for using an ejector, rather than a compressor, is the high temperature of the flue gas. The energy... 

Figure 19.1 shows the heat carried away in flue gases, particularly for high-temperature processes. For example, for a furnace operating at 800° C with no excess, air losses are 40 per cent. Recovery of a proportion of these losses is possible by means of recuperation. 

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