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The Principle of Glass Melting Furnace Regenerators and the Pros and Cons of Different Structures

Original Paper | Published:27thMarch 2018
Author: Kakugy Guo
12k Accesses | 114 Citations | 689 Altmetric | Metrics

Abstract

Introduction to glass melting furnaces

The temperature of flue gas (waste gas) discharged from glass melting furnaces is typically very high, around 1400-1500°C. As a result, the waste gas contains a significant amount of thermal energy. Therefore, the utilization of waste heat from the flue gas in glass melting furnaces is of great importance.

In glass melting furnaces, a flame temperature of approximately 1700°C is required. Besides providing heat energy through fuel combustion, preheating combustion air and gas (in the case of oil-fired furnaces, only combustion air) using the high-temperature waste gas is a critical condition to ensure the flame reaches a high temperature. Hence, regenerator chambers are used as waste heat recovery equipment to preheat air and gas in float glass melting furnaces.

The common structure of a vertical regenerator chamber in glass melting furnaces is illustrated in the diagram below. The lower part serves as the air and gas flue, with a refractory checkerwork on top for air and gas storage. Grid bricks are stacked above the checkerwork. The partition wall between the air and gas storage compartments is known as the air and fire partition wall. Due to the high temperature and erosion from flying materials, it tends to experience spalling and requires a greater thickness and strict construction. To reduce heat loss from the outer wall of the regenerator chamber, it is typically lined with insulating bricks. Since regenerator chambers require periodic maintenance, they are equipped with access doors for grid brick replacement and ash removal openings in the flue.

Using regenerator chambers as waste heat recovery equipment can improve the thermal efficiency of the furnace, increase the preheating temperature of combustion air and gas, thereby raising the flame temperature and reducing fuel consumption. This, in turn, helps to lower production costs. Therefore, researching and adopting efficient waste heat recovery devices is a common requirement for the melting process, thermal operations, and economic calculations.

Requirements for glass melting furnaces Performance
  1. Ensure that the combustion air and gas are preheated to a certain temperature, and maintain a stable preheating temperature.
  2. Efficiently utilize the waste heat from the flue gas.
  3. Ensure even distribution of airflow across the cross-section of the regenerator chamber with minimal airflow resistance.
  4. Keep the structure simple, compact, and robust, allowing for easy inspection, cleaning, and maintenance.

The heating action of the regenerator chamber on the gases is intermittent, while the production in the glass melting furnace is continuous. Therefore, two sets of equipment must work in rotation. Regenerator chambers are always used in pairs, and the smaller furnaces that correspond to them are also used in pairs.

Working Principle and Function of Glass Furnace Regenerator Chambers

Regenerator chambers serve as intermittent heat exchange devices and fall under the category of periodic and unstable temperature fields, where heat transfer occurs in an unsteady state. Their operation is similar to that of counterflow heat exchangers, which is why regenerator chambers are often analyzed as counterflow heat exchangers to simplify the heat transfer process over an entire cycle.

During the heat transfer process, flue gas primarily transfers heat to the checkerwork’s surface through radiation and, to a lesser extent, through convection, before conducting heat into the checkerwork’s interior. When heating the combustion air, heat is mainly transferred to the air through convection and radiation, as oxygen (O2) and nitrogen (N2) in the air are symmetric diatomic molecules that neither emit nor absorb radiation, even at high temperatures. However, when heating the gas, the role of radiation becomes more pronounced. The duration of the flow reversal (switching) time significantly affects the heat transfer efficiency of the regenerator chamber, and the analysis suggests that the most suitable time is approximately 20 to 25 minutes. Additionally, factors such as the density, specific heat capacity, arrangement of checker bricks, and gas flow inside the channels also influence the effectiveness of the heat exchange process. The uniformity of gas flow, particularly across the cross-section of the chamber, is vital for improving heat transfer and enhancing thermal efficiency. The gas flow should adhere to the principle of vertical gas movement, with flue gas moving from top to bottom and air or gas moving from bottom to top. Compared to vertical ascending regenerator chambers, box-type regenerator chambers improve gas flow distribution, increase the volume of the checkerwork, enhance thermal efficiency, and reduce heat dissipation surface area. A larger cross-sectional area of the regenerator chamber leads to less uniform gas flow, especially around the bends, which is why box-type regenerator chambers require sufficient height in the upper part.

The working principle is as follows: when high-temperature waste gas from the furnace enters the regenerator chamber from the top through the small furnace opening and the air and gas passages, it heats the checkerwork inside the regenerator chamber. The temperature of the checkerwork gradually increases, accumulating a certain amount of heat. After the fuel changes, combustion air and gas, serving as the combustion support, enter the regenerator chamber from the bottom through the lower flue passage. At this point, the checkerwork inside the regenerator chamber preheats the combustion air and gas using the heat it had previously stored, causing the temperature of the checkerwork to gradually decrease. The working process is illustrated in the diagram provided.

The advantages and disadvantages of combined and separate regenerator chambers are listed in the table below.

After practical comparisons, it is generally considered that separate regenerator chambers offer more advantages. Therefore, most factories use separate regenerator chambers when constructing new furnaces. Some older factories, constrained by their existing facilities, still use combined regenerator chambers. However, switching to separate regenerator chambers is possible when switching to heavy oil as a fuel source. Whether it’s combined or separate regenerator chambers, their structures consist of three main parts: the lower flue passage, the checkerwork, and the upper space. The lower parts of the air and gas chambers serve as air and gas flue passages, with each regenerator chamber having an ash removal door. At the top of the flue, the checkerwork supports the weight of the regenerator chamber, and, to enhance its strength, it is usually reinforced in the air gap between the two checkerworks. The checkerworks are laid with clinker bricks for leveling, and then checker bricks are stacked. The checkerworks are typically built using first-class clay bricks.

The top of the regenerator chamber is often equipped with a half-round crown. Since the upper part of the regenerator chamber has a higher temperature, high-alumina bricks are used for the half-round crown. The air regenerator chamber operates at a higher temperature and has a larger span, so its half-round crown is thicker than that of the gas regenerator chamber. The air half-round crown is generally about 350 mm thick, while the gas half-round crown is about 250 mm thick.

The partition wall between the air and gas regenerator chambers is called the air and fire partition wall. Since the upper part of the air and fire partition wall has a high temperature and is subject to severe erosion from flying materials, and because it can develop leaks after burning damage, the partition wall must be thick and built tightly. High-alumina bricks are commonly used for the upper part of the air and fire partition wall, while clay bricks are used for the lower part, where the temperature is lower.

To reduce heat loss from the outer walls of the regenerator chamber, insulating bricks are generally added outside the first layer of clay bricks. Since regenerator chambers require periodic maintenance, they are equipped with heat repair doors (sealed tightly during normal operation) on the outer walls and ash removal doors at bends in the flue.

Structure of Regenerator Chamber for Heavy Oil-Fired Glass Melting Furnace

In heavy oil-fired glass melting furnaces, separate regenerator chambers are commonly used to facilitate control over the combustion air supply to individual furnace burners and to enhance control over temperature and atmosphere distribution.

Because heavy oil is a high-energy fuel, the regenerator chamber omits the gas section, retaining only the air regenerator chamber to preheat the combustion air. Its structure closely resembles the air section of the regenerator chamber in a furnace where gas is used. It consists of a bottom flue passage, checkerwork, and upper space. The structural forms of various components and the refractory materials used for the linings in different parts are essentially the same as those in the air section of the regenerator chamber in a gas-fired combustion furnace.

About the Authors . . .

kakugy Guo Senior Engineer

CEO & Author Introduction
Advanced Refractory Materials and Furnace Technology
Leader in Oxygen Combustion Technology
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