Flue gas conditions and selection of waste heat boilers
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A waste heat boiler is a boiler that uses the waste heat from industrial processes to generate steam. An important feature of such boilers is that the conditions of the flue gases depend on the industrial process, and it is not possible to modify these conditions in a way that is favorable for the boiler. Therefore, flue gas conditions have a significant impact on the design and operation of boilers. 1. Characteristics of flue gas conditions and the corresponding types of furnaces suitable for them1.1 “Clean” flue gas
By “clean” here, we mean exhaust gases from furnaces or various internal combustion engines that burn gaseous, liquid, or high-quality solid fuels, and which are not significantly contaminated by the main process. When inert gas is used for circulating cooling in the main process, the circulating gas remains clean flue gas as long as it is not severely contaminated by the main process. Applicable furnace types include: (1) radiant tube, heat pipe, or water tube waste heat boilers, with natural circulation or forced circulation, and vertical or horizontal layout ； (2) Heat pipe or water-tube waste heat boilers with partial or full helical fins or longitudinal fins and other extended heating surfaces; featuring natural or forced circulation, and vertical or horizontal configuration ； (3) Straight flue drum boiler. 1.2 Dusty flue gas: Flue gas containing dust can cause wear on the heated surfaces, and it may also lead to the accumulation of ash, resulting in issues such as ash blockage and bridging. Often, these two mechanisms with opposite effects may exist simultaneously in a single boiler. The selection of such flue gas waste heat boilers should primarily aim to prevent wear of the heating surfaces as well as ash accumulation, clogging, and bridging in the flue ducts. Based on the level of dusty flue gas and the characteristics of the dust, the suitable boiler types are: (1) Vertically arranged forced-circulation heat pipe or water-tube waste heat boilers ； (2) Horizontal-mounted forced-circulation heat pipe or water tube waste heat boiler ； (3) Double-drum heat pipe or water-tube waste heat boiler. 1.3 Adhesive flue gas: The adhesiveness of flue gas refers to the property whereby, at the operating temperature of the flue gas, the dust and sublimated or vaporized substances it contains adhere to the heating surfaces of the boiler or other components under certain conditions. The types of water-tube boilers suitable for such flue gas conditions are: (1) Horizontally arranged forced-circulation heat-pipe water-tube waste heat boilers with a radiant cooling chamber; they generally have no superheater. They feature a suspended serpentine tube convective heating surface made of plain tubes or tubes with longitudinal straight fins. Devices for soot blowing or dust removal via vibration or shaking are employed. The flue gas flows transversely across the boiler and passes through it only once ； (2) A vertical multi-chamber arrangement of forced-circulation heat pipe or water tube waste heat boilers with one or two radiative cooling flues, in which the convective heating surfaces typically consist of tubes with longitudinal straight fins and rapping ash removal devices. 1.4 Corrosive flue gases Corrosive flue gases are extremely common in the acid production industry, non-ferrous metallurgy industry, and petrochemical industry, among others. The flue gases from the main processes of these industries contain corrosive substances such as SOX, NOX, H2S, and S. Regarding these components, the corresponding boilers require thorough consideration in terms of selection, parameter choice, and certain structural features. However, for corrosive flue gases, even when theoretically and practically effective measures are taken, certain boiler components still need to be replaced periodically. Therefore, the structural design of the boiler should take this characteristic fully into account. 1.5 High-pressure flue gas To prevent any leakage of flue gas, for high-pressure flue gas (p = 3–4 MPa), flue-type boilers are generally not used, as it is not economically feasible to reinforce flat expansion plates enough to withstand such high pressures. The commonly used types at present include: (1) shell-and-tube straight-flue boiler ； (2) Double-tube straight-flue boiler ； (3) Smoke-tube boiler combining double tubes and spiral tubes ； (4) U-tube smoke tube boiler ； (5) Insert a tubular water-tube boiler; the flue gas is sealed by a cylindrical shell and end heads. For ultra-high pressure flue gases (p = 32–500 MPa), currently only serpentine-tube water tube boilers are available, with various specific structural forms in which the flue gas is sealed by a cylindrical shell and end heads. The higher the flue gas pressure, the higher the overall heat transfer coefficient of the heating surfaces, allowing the boiler to be designed to be relatively compact. This is very advantageous for using cylindrical shells to seal flue gas. 1.6 Flue gases requiring rapid cooling In chemical processes, flue gases often need to be cooled to a specified temperature within a given time period. Due to the strict temporal constraints on heat exchange in the waste heat boiler, this determines the flue gas velocity under actual structural conditions regarding the length of the flue gas passage. For this type of flue gas, both smoke-tube boilers and water-tube boilers are currently in use. However, when there is a high likelihood of soot and coking forming on the heated surfaces, tube boilers are often preferred. To this end, modern waste heat boiler technology has developed various types of smoke tube boilers that have achieved considerable success, such as: (1) double-tube straight smoke tube boilers, which use oval-shaped collectors to absorb the thermal expansion differences of the straight smoke tubes and to withstand the direct impact of high-temperature flue gases ； (2) Smoke tube boilers that combine double tubes with spiral tubes feature a compact structure and the ability to use charring operations. 1.7 Flue gases with high dew point: For flue gases with high dew point, in order to prevent operational issues such as corrosion, scaling, and ash deposition, a basic requirement in the design of waste heat boilers is to avoid dew formation on the heated surfaces. The common approach currently used is to ensure that the metal wall temperature of the boiler’s heating surfaces remains above the dew point temperature under all operating conditions. Specific measures include: (1) selecting an appropriate working pressure on the steam and water sides of the boiler, so that the corresponding saturation temperature of the boiler water is higher than the dew point temperature of the flue gases ； (2) To prevent boiler water at a temperature below the saturation temperature from coming into contact with the heating surfaces, it is necessary to thoroughly mix and preheat the boiler water. Additionally, low-temperature sections similar to those found in economizers used in combustion boilers should be avoided ； (3) In terms of operation, to prevent dew formation in the boiler during low load conditions and when it is shut down, appropriate insulation measures are often required. Apart from the steam-side pressure parameters, flue gas with a high dew point is suitable for various types of waste heat boilers. 1.8 Flue gas containing ultra-fine dust. There are two problems related to ultra-fine dust: (a) The loose dust has strong adhesiveness on heated surfaces, forming a loose, soot-like ash layer ； (b) Under suitable conditions, dust may melt or coagulate with other sticky components in the flue gas to form dense deposits that are difficult to remove. Therefore, for boilers handling flue gas containing extremely fine dust, the following measures are often considered: (1) Using a design that makes it difficult for ash to accumulate or get stuck, in order to minimize uneven scouring by the flue gas; in particular, stagnation or vortex areas in the flue gas flow should be avoided, so boiler designs that allow for direct flow of the flue gas should be used whenever possible ； (2) The forced water circulation heating surface with screen samples to resist longitudinal flue gas erosion has the advantages of reducing dust adhesion and facilitating the use of rapping ash removal methods; it has been applied in waste heat boilers used in processes such as oxygen-enriched blast furnaces and bubbling roasting for acid production, achieving satisfactory results. 1.9 Flue gas with unstable parameters: One of the characteristics of flue gas from waste heat boilers is the fluctuations in aspects such as flow rate, temperature, composition, and dust content. The nature of these fluctuations can be regular and periodic, or irregular and aperiodic ； The characteristics of fluctuations can be gradual or abrupt ； It can be a single-parameter fluctuation or a complex fluctuation with multiple parameters. The selection of a waste heat boiler must begin with an analysis of the entire cycle of the main process flow, in order to determine the characteristics of the flue gas at each time interval within the cycle. Accordingly, appropriate measures can be implemented one by one in the design of the waste heat boiler. The most prominent feature among them is often the main basis for selecting a waste heat boiler. 2. The multi-“factor” characteristics of flue gas conditions: The nine categories mentioned above basically encompass the waste heat flue gases from various common process flows. However, in actual practice, the various “factors” related to flue gas conditions rarely exist in isolation; instead, flue gases with multiple factors are quite common. There are several types of multi-factor flue gas conditions as follows ； 1) The distinction between “primary” and “secondary” factors: a factor that is decisive in determining the choice of furnace type is called a “primary factor”” ； All other factors are referred to as “secondary factors”. This type of flue gas is called “multi-factor” flue gas. If the secondary factor has a similar effect on furnace selection as the primary factor, its influence can often be ignored, or it can serve as a reference when selecting specific parameters. If the effect of the secondary factor is not correlated with that of the primary factor, appropriate consideration should be given when selecting the furnace type and parameters. Under any circumstances, the \"primary factor\" is the main basis for selecting the furnace type. 2) “Multiple-factor” flue gas refers to a situation where two or more factors each play a significant role in the selection of furnace types; those factors that have a decisive impact on the choice of furnace type are known as “primary factors”. The various combinations of these “key factors” are as follows: (a) The effects of each factor operate independently of one another; in this case, the choice of furnace type is similar to that in the single-factor flue gas scenario ； (b) The effects of various factors on furnace type selection are opposite to each other; in such cases, the choice of furnace type is generally based on the factor with the greatest influence, in order to ensure the normal operation of the boiler under the most unfavorable conditions ； (c) The various factors influence the choice of furnace type without interfering with one another; at this point, it is necessary to consider the requirements of each factor for the boiler, and use these factors to \"screen\" possible furnace types.