Flue gas streams

Some PFBC boiler designs incorporate high-temperature, high-pressure (HTHP) filter devices in the flue-gas stream. These are installed primarily to protec t the gas turbine from erosion damage by the fine particles that escape the cyclones, but as the filters remove virtually all the suspended particulates, they also eliminate the need for back-end removal. The commonest HTHP filter elements used are rigid ceramic candles. 

The above equation has been charted in Figure 2. F(t) for commonly found flue gas streams has been computed and plotted in the same figure. 

Operating conditions are important determinants of the choice of fabric. Some fabrics (e.g., polyolefins, nylons, acrylics, polyesters) are useful only at relatively low temperatures of 95 to 150°C (200 to 300°F). For high-temperature flue gas streams, more thermally stable fabrics such as fiberglass. Teflon, or Nomex must be used. 

For PM control from combustion sources, the tlue gas enters a coagulation area (e.g., ductwork, a chamber, or a cyclone) to reduce the number of ultrafine particles, and then a gas conditioner to cool the gas to a suitable temperature and saturation state. This is generally accomplished by means of a waste heat recovery heat exchanger to reduce the temperature of the flue gas or by spraying water directly into the hot flue gas stream. 

Fabric filters (baghouscs) represent a second accepted method for separating particles from a flue gas stream. In a baghouse, the dusty gas flows into and through a number of filter bags, and the particles are retained on the fabric. Different types are available to collect various kinds of dust with high efficiency. 

The tlow rates of each component in the flue gas stream are ... 

Additionally, the vanadium acts as a catalyst for the oxidation of sulfur dioxide (S02) to sulfur trioxide (S03). Consequently, the S03 content of the furnace gases increases from perhaps 10 ppm to 30 ppm or more. At 30 ppm S03, the dew point of the sulfuric acid generated is high enough to saturate the air heater, and this unit rapidly corrodes (typically within 1-2 years). To prevent this corrosion, the S03 content in the flue gas stream must be below 7 ppm. 

Rather than selective non-catalytic reduction, the reduction can be carried out over a catalyst (e.g. zeolite) at 150 to 450 C. This is known as selective catalytic reduction. Figure 25.31 shows a typical selective catalytic reduction arrangement10. Either anhydrous or aqueous ammonia can be used. This is mixed with air and injected into the flue gas stream upstream of the catalyst. Removal efficiency of up to 95% is possible. Again, slippage of excess ammonia needs to be controlled. 

Limited data is available on the concentration of volatile organic compounds, semi-volatile organic compounds (SVOCs), and polycyclic aromatic hydrocarbons (PAHs) from gasification processes. The data that is available indicate that VOCs, SVOCs, and PAHs are either non-detectable in flue gas streams from IGCC process or, in some cases where they were detected, they are at extremely low levels (on the order of parts per billion and lower). The analysis of syngas also indicates greater than 99.99 percent chlorobenzene and hexachlo-robenzene destruction and removal efficiencies and part per billion or less concentration of selected PAHs and VOCs.9-14... 

Fly ash is a light material, which is carried out of the combustion chamber in the flue gas stream. The ash from fuel oil combustion is usually in the form of fly ash. The many elements that may appear in oil ash deposits include vanadium, sodium, and sulfur. 

This effect becomes especially pronounced at low temperatures where the generation of active centers by the N-H-O system slows down and the hydrogen accelerates the radical initiation process. The injection of hydrogen can reduce the NO reduction temperature window enough so it could be applied on some conventional flue gas streams without COBs. Others have documented use of sodium salts as another additive that can generate OH radicals and has the same effect as hydrogen addition. However, these additives have not been commercialized. Figure 17.5 shows the overall reactions for SNCR. 

ABS formation on a FCCU SCR is a major concern because it is a sticky foulant. Once formed, it traps catalyst fines to its surface. Once these particles are no longer moving within the flue gas stream, it is very difficult to reentrain them even with the use of soot blowers. 

Based on these results, it becomes necessary to install guide vanes to prevent flue gas flow stagnation at the corner as well as erosion of the far wall. The goal of this work is to deliver a well-mixed homogeneous flue gas stream to the inlet of the SCR catalyst. Angular entry of the flue gas into the SCR catalyst is avoided to protect the catalyst from erosion. 

The NORIT Porta-Powdered Activated Carbon (Porta-PAC) dry injection system pneumatically conveys an adjustable amount of powdered activated carbon (PAC) from bulk bags into the flue gas streams of incinerators for mercury and dioxin emission reductions. PAC is metered using a volumetric feeder into a pneumatic eductor where moving air transfers the carbon to the injection point. A series of interlocks control the operation of the unit and allow local or remote operation and monitoring of the unit. This technology is commercially available. All information is from the vendor and has not been independently verified. 

When coal is combusted a number of ash products are produced. In a conventional coal-fired power station the ash that enters the flue gas stream is referred to as the fly ash or pulverized fuel ash. This is volumetrically the most important fraction and although considerable progress has been made in utilizing this material, nevertheless there is an excess production, much of which ends in lagoons or, on a longer term basis, in landfill sites. The potential impacts of fly ash on surface water and groundwater therefore have to be considered both in the short and long term. The annual European production of fly ash in 2000 was 38.96 x 106 t, of which... 

The heat balance on the differential element of the flue gas stream yields... 

The steam production allows for the computation of the exergy flow at various points (Table 9.7). For example, the exergy flow at point (1) is simply 2.73 x 10-4 x 1402.8 = 0.38kj/s. The exergy value of the flue gas stream (5) is computed by using the first law efficiency and realizing that the flue gas stream is a heat stream being discarded ... 

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