Abstract
Industrial combustion processes are unique compared to the other chapter subjects in this book by virtue of their scale and diversity. For example, a large flame produced by a flare in a petrochemical plant could exceed 100 m in length, and individual burners in large power boilers could be over 100 MW firing capacity each. There are numerous types of combustors that have many names such as heaters, furnaces, ovens, kilns, dryers, boilers, thermal oxidizers, crackers, reformers, and smelters. The processes range in temperatures from as low as 300K for some food heating and drying processes to as high as over 2,000 K for some glass and metals production processes. Fuels can be solid (e.g., coal), liquid (e.g., oil), or gaseous (e.g., natural gas). Some processes are batch (e.g., rotary aluminum melting furnaces) or continuous (e.g., float glass furnaces). The combustors come in a wide range of shapes and sizes including both horizontal (e.g., cement kiln) and vertical cylinders (e.g., vertical cylindrical process heaters), approximately spheres (e.g., electrical arc furnaces with supplement burners), and rectangular boxes (e.g., cabin heaters used in the petrochemical industry). Some include heat recuperation that may be recuperative (e.g., air preheaters used on process heaters) or regenerative (e.g., air preheaters used in glass melting furnaces). Some processes use oxygen-enhanced combustion ranging from low-level enrichment (e.g., to enhance scrap aluminum melting) up to complete replacement of the combustion air with pure oxygen (e.g., supplemental burners used to enhance electric arc furnaces).
Introduction
Industrial combustion processes are unique compared to the other chapter subjects in this book by virtue of their scale and diversity. For example, a large flame produced by a flare in a petrochemical plant could exceed 100 m in length, and individual burners in large power boilers could be over 100 MW firing capacity each. There are numerous types of combustors that have many names such as heaters, furnaces, ovens, kilns, dryers, boilers, thermal oxidizers, crackers, reformers, and smelters. The processes range in temperatures from as low as 300K for some food heating and drying processes to as high as over 2,000 K for some glass and metals production processes. Fuels can be solid (e.g., coal), liquid (e.g., oil), or gaseous (e.g., natural gas). Some processes are batch (e.g., rotary aluminum melting furnaces) or continuous (e.g., float glass furnaces). The combustors come in a wide range of shapes and sizes including both horizontal (e.g., cement kiln) and vertical cylinders (e.g., vertical cylindrical process heaters), approximately spheres (e.g., electrical arc furnaces with supplement burners), and rectangular boxes (e.g., cabin heaters used in the petrochemical industry). Some include heat recuperation that may be recuperative (e.g., air preheaters used on process heaters) or regenerative (e.g., air preheaters used in glass melting furnaces). Some processes use oxygen-enhanced combustion ranging from low-level enrichment (e.g., to enhance scrap aluminum melting) up to complete replacement of the combustion air with pure oxygen (e.g., supplemental burners used to enhance electric arc furnaces).
This chapter has been divided into six sections: metals industry, process heating, power generation, infrared heating and drying, flares, and oxygen-enhanced flames. Some of those sections are further broken down into subsections. The purpose of this chapter is not to attempt to be comprehensive, as that would be beyond the scope of the book, but rather to be representative of the many types of industrial combustion processes. This should give the interested reader a flavor for what combustion is used for in industry. As might be expected, heat transfer, thermal efficiency, pollution emissions, and reliability are important in most of these processes.
There are some good reference books on the subject of industrial combustion the interested reader could consult for further details on many of the processes pictured in this chapter. Griswold’s (1946) book is very practically oriented and includes chapters on gas burners, oil burners, stokers and pulverized-coal burners, heat transfer, furnace refractories, tube heaters, process furnaces, and kilns [1]. Stambuleanu’s (1976) book on industrial combustion has information on furnaces [2]. British Gas sponsored a very useful book on industrial combustion processes that specifically use natural gas as the fuel [3]. Trinks et al. (2004) have written what many consider to be the bible of industrial furnaces [4]. Deshmukh (2005) discusses a range of topics of interest in industrial combustion including fuels, burners, and refractories, where the emphasis is more on metal treatment [5]. Mullinger and Jenkins (2008) discuss a wide range of industrial furnaces and processes [6]. There is a series of books on industrial combustion that includes much information on a wide range of aspects including heat transfer [7], computational fluid dynamics [8], pollution [9], burners [10], testing [11], oxygen enhancement [12], and specific types of process in the petrochemical industry [13, 14].
5.1 Metals Industry
5.1.1 Ladle Preheating
Figure 5.1 shows a schematic of a burner firing into a vessel called a ladle that is used to transport molten metal from one station to another in a metals production plant. Burners are used to keep the refractory lining of the ladle hot so the molten metal will not freeze and solidify inside the ladle and the refractory will not be thermally shocked. Figure 5.2 shows a burner mounted in an end wall firing into a ladle. Figure 5.3 shows a burner firing on an end wall of a ladle preheating station without a ladle in place so the flames can be seen. This burner uses both air and oxygen as the oxidizer. The amount of oxygen used depends on the size of the ladle, the temperature of the ladle, the cost of the oxygen and the fuel, and the turnaround time needed for the ladle. Figure 5.3 shows a 2 MW natural gas flame firing with (a) minimum oxygen participation, (b) medium oxygen participation, and (c) maximum oxygen participation.
Figure 5.1 Drawing of a ladle used to transport molten metal which is being heated by a burner to keep the liquid metal from solidifying.
Figure 5.2 Actual ladle on a stand with a burner mounted on an endwall on the left with the burner firing from left to right.
Figure 5.3 2 MW natural gas flame with (a) minimum oxygen enrichment of the combustion air, (b) medium oxygen enrichment, and (c) high-purity oxygen.
5.1.2 Experimental Investigation of a Concept of Low-Scale Reheating with Fuel-Rich Combustion
Industrial furnaces for reheating semifinished metal products are often direct fired with natural gas and air. To ensure a complete combustion, the furnaces are fired fuel lean. Oxidation of the metals exposed to the furnace atmosphere causes significant material losses and additional work in further processing. A reheating concept, which reduces scale formation, was developed. It involves fuel-rich combustion, post-combustion of the unburned off-gas, and efficient preheating of the combustion air. Experiments show a significant reduction of scale formation for copper and copper-nickel alloys in the off-gas of a fuel-rich combustion with an air ratio of λ = 0.96 and a maximum temperature of 950°C, but also for steel, alloy 1.2367 in off-gas of a combustion with an air ratio of λ = 0.95 and a maximum temperature of 1,152°C.
As part of the project, the primary combustion is investigated with different measurement techniques. To characterize the combustion at different operating conditions, especially the dimension of the flame, a camera system with an intensified CCD-sensor and a band pass filter (peak transmission at 308 nm) for the detection of OH*-chemiluminescence is used and compared to photos of the flame with an exposure time of 1/60 s. A conventional recuperative burner with a maximum capacity of 40 kW is used, which is fired with natural gas and cold air. The combustion chamber has a height of 0.94 m and a diameter of 0.6 m.
Figure 5.4 shows a photo of the flame compared to the measured OH*-intensity at a temperature of 900°C and 1,100°C, a burner capacity of 30.5 kW, and an air ratio of λ = 1.15. The figure of the OH*-intensity is obtained by calculating the average of 300 exposures that were recorded in 30 seconds. Figure 5.5 shows the distribution of the OH*-radicals for an increasing burner capacity from 10.4 kW to 40.5 kW at an air ratio of λ = 1.15 and a furnace temperature of 900°C. Figure 5.6 shows the distribution of the OH*-chemiluminescence at different air ratios at a furnace temperature of 1,050°C.
Figure 5.4 Photographs (top) and OH* intensity measurements for a recuperative burner used in metal reheating at furnace temperatures of 900°C and 1100°C.
Figure 5.5 OH* intensity measurements for a recuperative burner used in metal reheating at 10.4 kW and 40.5 kW firing rates in a furnace at 900°C.
Figure 5.6 OH* measurements at different air/fuel ratios for a recuperative burner used in metal reheating in a furnace at 1050°C.
The results show a stable combustion for a burner capacity of 50–100% with an air ratio of 1.15–0.7, respectively. The burner capacity of 50% was limited due to soot formation. For the fuel-rich combustion the measurements of the OH*-distribution show a decreasing length of the reaction zone with increasing air ratio, whereas the position of the maximum intensity remains constant.
The project of the Research Association for the Mechanical Engineering Industry e. V. (FKM) proposed by the Research Association for Industrial Thermoprocessing Equipment e. V. (FOGI) was promoted via the German Federation of Industrial Research Associations e. V. (AiF, AiF-No. 17810 N) as part of the program for the promotion of Industrial Collective Research (IGF) by the Federal Ministry for Economic Affairs and Energy on the basis of a decision by the German Bundestag.
5.1.3 Flame Impingement
Direct flame impingement is used in the metals industry for reheating metals before they are shaped and formed. An example application is reheating the edge of a continuous metal sheet because the edges cool off more quickly than the center. The more nonuniform the temperature of the sheet, the more difficult it is to handle and process the metal, which often means either lower quality or more waste if the edges are cut off. Figure 5.7 shows a schematic of a flame impinging normal to a plane surface. A series of experiments were conducted using a commercial burner shown in Figure 5.8 made by Nordsea designed to using natural gas and high-purity oxygen. Figure 5.9 shows the effects of the oxidant composition with 5 kW natural gas flames using the following oxidants: (a) air and (b) high-purity oxygen. Figure 5.10 shows the effect of firing rate with oxygen/natural gas flames at firing rates of (a) 15 kW and (b) 35 kW. Figure 5.11 shows the effect of the distance between the burner and the target where (a) shows a close spacing and (b) shows a longer spacing.
Figure 5.7 2D schematic of a flame jet impinging normally onto a plane surface.
Figure 5.8 Outlet of a Nordsea burner designed to fire high-purity oxygen with natural gas for use in metal heating.
Figure 5.9 Burner in Figure 5.8 firing natural gas with (a) air and (b) high-purity oxygen.
Figure 5.10 Burner in Figure 5.8 firing natural gas with high-purity oxygen at firing rates of (a) 15 kW and (b) 35 kW.
Figure 5.11 Burner in Figure 5.8 firing natural gas with high-purity oxygen with (a) long and (b) short spacing between the burner and the plane target surface.
This research was partially funded by the Gas Research Institute (Chicago, IL) for the project entitled “Development of Rapid-Heating Furnaces for Metal Reheating.”
5.1.4 Development of an Energy-Efficient Burner for Heat Treatment Furnaces with a Reducing Gas Atmosphere
A main reason for metal loss of semifinished metal products during heating in reheating and heat treatment furnaces is scale formation. In this project, a burner was developed that produces a low oxidizing (reducing) atmosphere in the furnace. A recuperative burner generates a reducing furnace atmosphere due to fuel-rich combustion of natural gas and air. Complete combustion of the furnace atmosphere is ensured by injection of additional air and takes places in an open radiant tube resulting in high process energy efficiency.
Experimental and numerical methods were used to quantify operating conditions for the prototype burner design. The experimental setup consisted of a concept-burner (see Figure 5.12), which is installed in a combustion chamber. In the experiments, off-gas composition and temperature in the annular gap and in the off-gas after the recuperator were measured to determine the maximum temperature in the annular gap, length of the post-combustion zone, off-gas emissions, and energy efficiency of the burner.
Figure 5.12 Drawing of a concept burner installed in a heat treating furnace.
The burner operates in a flame and a flameless combustion mode. Figure 5.13 shows the burner in the flameless combustion mode, which is fired with natural gas and air. The secondary air for the post-combustion of the off-gas is injected through small metallic tubes at the off-gas inlet of the open radiant tube. The burner is designed for a maximum capacity of 40 kW. The primary, fuel-rich combustion was investigated for different air–fuel ratios in the range of 0.7 ≤ λprimary < 1. The total air ratio is less λtotal < 1.05.
Figure 5.13 Burner shown in Figure 5.12 operating in flameless mode.
Figure 5.14 shows the burner firing in the flame mode. Secondary air is injected inside the annular gap. Therefore, there is no need for metallic tubes outside of the open radiant tube. The nominal burner capacity was 80 kW. The burner operated with a primary air ratio of λprimary = 0.75 and a total air ratio of λtotal ≈ 1.15, depending on the fuel and oxidizer. Due to the combination of fuel-rich combustion and post-combustion, the total NOX emissions were low. For further reduction and temperature homogenization, the burner can operate in the flameless combustion mode (see Figure 5.15).
Figure 5.14 Burner shown in Figure 5.12 operating in the visible flame mode with secondary air injection.
Figure 5.15 Burner shown in Figure 5.14 operating in the flameless mode.
This project is supported in the Central Innovation Programme for SMEs (ZIM) by the Federal Ministry for Economic Affairs and Energy on the basis of a decision by the German Bundestag.