Flue gas conditions and selection of waste heat boilers

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01 Characteristics of flue gas conditions and the corresponding suitable furnace types: “Clean” flue gas. Here, “clean” refers to exhaust gases from furnaces that use gaseous, liquid, or high-quality solid fuels, or from various internal combustion engines, which are not significantly contaminated by the main process. When an inert gas is used as the circulating coolant in the main process, the circulating gas remains clean flue gas, provided it is not severely contaminated by the main process. Applicable boiler types include: (1) bare-tube heat pipe or water-tube waste heat boilers, with natural or forced circulation, and vertical or horizontal configuration ； (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 tube boiler drum. Dusty flue gas: Flue gas containing dust can cause wear on the heated surfaces, and it can 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 aim primarily at preventing wear of the heating surfaces as well as ash accumulation, blockage, and bridging in the flues. Depending on the level of dust-laden 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) Horizontally arranged forced-circulation heat pipe or water-tube waste heat boiler ； (3) Double-drum heat pipe or water-tube waste heat boiler. Stickiness of flue gas: The stickiness of flue gas refers to the property whereby, at operating flue gas temperatures, the entrained soot and dust, as well as sublimed and vaporized substances, adhere to the boiler heating surfaces or other components under certain conditions. Water-tube boiler types suitable for such flue gas conditions include: (1) forced-circulation heat pipe water tube waste heat boilers with a radial cooling chamber, typically without superheaters, featuring suspended serpentine tube convective heating surfaces, plain tubes or tubes with longitudinal straight fins; dust removal or soot blowing is achieved through shaking or vibration, and the flue gas flows horizontally through the boiler in a single pass ； (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. Corrosive flue gases are very common in industries such as acid production, non-ferrous metallurgy, and petrochemical processing. The flue gases from the main processes of these industries contain corrosive substances such as SOX, NOx, H2S, and S. For 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 components of the boiler still need to be replaced periodically. Therefore, the structural design of the boiler should take this characteristic fully into account. High-pressure flue gas: To prevent any leakage of flue gas, for high-pressure flue gas (p = 3–4 MPa), duct-type boilers are generally not used. This is because it is not economically feasible to reinforce flat expansion plates sufficiently 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) For tubular water-tube boilers, the flue gas is sealed using a cylindrical shell and end caps. For ultra-high-pressure flue gas (p=32–500 MPa), currently only serpentine tube water-tube boilers are available, with various specific structural designs in which the flue gas is enclosed by cylindrical shells and end caps. 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 shell-and-tube structures to seal flue gas. Flue gas that requires rapid cooling: In chemical processes, flue gas often needs to be cooled to a specified temperature within a set time frame. Due to the strict time constraints on heat exchange in the waste heat boiler, this determines the flow rate of the flue gas under the actual structural conditions of the flue gas passage length. 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 accumulation and coking on the heated surfaces, smoke 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 headers 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. For flue gases with a high dew point, in order to prevent operational issues such as corrosion, scaling, and ash buildup, the basic requirement for the design of waste heat boilers is to avoid dew formation on the heating surfaces. The common practice nowadays 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. In addition to the pressure parameters on the soda side, high-dewpoint flue gas is suitable for various types of waste heat boilers. Flue gas containing ultra-fine dust. There are two problems associated with ultra-fine dust: (a) the loose dust has strong adhesiveness on heat surfaces, forming a loose, soot-like ash layer ； (b) Under appropriate 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 blocked, in order to minimize uneven scouring by the flue gas; in particular, it is necessary to avoid areas where the flue gas flow stagnates or forms vortices. Hence, whenever possible, boiler designs with direct flow of flue gas should be utilized ； (2) The panel-type forced water circulation heating surface subjected to longitudinal flue gas scouring has the advantages of reducing dust adhesion and facilitating the use of vibration-based ash removal methods. It has been applied in waste heat boilers within processes such as oxygen-enriched open hearth furnaces and fluidized-bed roasting for acid production, achieving certain results. For flue gases with unstable parameters, fluctuations in aspects such as flow rate, temperature, chemical composition, and dust content are characteristic features. These fluctuations can be regular and periodic, or irregular and aperiodic ； The characteristics of fluctuations can be gradual or shock-like ； 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. Consequently, corresponding 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 criterion for selecting a waste heat boiler. 02 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 multifactorial 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 secondary factors has no relation to that of primary factors, then they should be given due consideration when selecting the furnace type and parameters. In any case, the \"primary factor\" is the main basis for selecting the furnace type. 2) “Multiple factors” flue gas: When there are two or more factors, each of which plays a non-negligible role in the selection of the furnace type, all these factors that have a decisive influence on the furnace type selection are referred to as “main 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 selection are opposite to each other; in such cases, the choice of furnace 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 regarding the boiler, and to \"screen\" the furnace types based on these factors. 03 Classification of flue gases and selection principles according to Ministry of Machinery standards: Article 3.10 of JB/T 7603-1994 \"Guidelines for the Design of Flue-type Waste Heat Boilers\" specifies that waste heat boilers are divided into five categories. Category 1: Boilers in which the dust content in the flue gas is not more than 20 g/m3. For selection, either water-tube or smoke-tube types of waste heat boilers can be used. Category 2: Waste heat boilers whose dust content in the flue gas is greater than 20 g/m3 but not more than 70 g/m3. Selection: In case of damage to the heating surface, use finned tubes or bare tubes. When using bare tubes, the flue gas can flow across or diagonally; however, the transverse and longitudinal pitches between the tubes should be appropriately increased to prevent dust bridging. For flue gas with good ash flow properties and minimal wear on pipes, the flue gas velocity can be increased to enhance its self-cleaning ability. Third category: Waste heat boilers with dust content in flue gas less than 70 g/m³ ； Selection: A multi-flue furnace type equipped with a dust settling chamber or a radiant cooling chamber, or a straight-through furnace type, should be used. The flue should be equipped with a cold ash hopper. For the heating surfaces of such waste heat boilers, finned tubes or membrane walls are preferred. Anti-wear measures should also be taken at the areas of the heated surface where wear is likely to occur. Category 4: Waste heat boilers with sticky soot in the flue gases ； Selection: Same as Category 3 and Category 5. For waste heat boilers with strongly corrosive components in the flue gas or toxic fumes, the selection should involve using a boiler structure designed for high dust concentrations, in addition to a steel plate enclosure for the furnace walls. Note: “m3” refers to the volume of the smoke at 0.1013 MPa and 0°C; the same applies hereafter.