Application of waste gas in glass furnaces (continued)

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The application of waste gas in glass furnaces: Coke is an essential basic raw material for blast furnace smelting, non-ferrous metal smelting, and casting. Typically, coal is coked at high temperatures of 950–1050°C to produce coke and other chemical products. During the coking process, the gas produced by coal in the carbonization chamber is called raw gas (also known as crude gas). After the waste gas is processed to recover chemical products (that is, refined), many harmful substances are generated; therefore, the vast majority of coke ovens are located in remote areas far from residential zones. In such an industry, it is difficult to connect much of the coke oven gas to the city’s gas pipelines; as a result, a large amount of this gas cannot be refined and has to be burned away uselessly. Generally speaking, raw gas cannot be used as a gaseous fuel without being processed, or in other words, without being refined. And some coking plants, for certain reasons, do not have the facilities to refine coke oven gas. This not only wastes resources but also pollutes the environment. To this end, we tried using waste gas as fuel in glass furnaces. Let’s now make a summary. I. Characteristics of coke oven gas 1. The composition of raw coke oven gas, excluding coal gas, is approximately as follows (g/m3): Water vapor: 250–450; Cyanides: 1.0–2.5; Coke oil vapor: 80–120; Light pyridine bases: 0.4–0.6; Crude benzene: 30–45; Naphthalene: 10; Ammonia: 8–16; Others: 2–2.5; Hydrogen sulfide: 6–30. 2. The composition of pure coal gas is approximately as follows (g/m3): H2: 54–59; O2: 0.3–0.5; CH4: 23–28; N2: 3–5; CnHm: 2–3; CO: 5.5–7; CO2: 1.5–2.5. 3. The combustible and non-combustible components in coke oven gas are as follows: Combustible components (by volume percentage): H2: 55–60%; H2O (vapor): 4–5%; CO: 5–8%; CO2: 1.5–3%; CH4: 23–28%; N2: 3–5%; CmHn: 2–4%; O2: 0.4–0.8%. Benzene derivatives: 0.5–1%. 4. Characteristics of coke oven gas: a. Coke oven gas is a colorless (yellowish when no chemical products are recovered) gas that is odorous and toxic ； b. Coke oven gas has a high calorific value: 16.72—18.81 MJ/m3 (—3995.2—4494.6 kcal/m3) ； c. Coke oven gas contains little inert gas (about 4% nitrogen) and a high amount of hydrogen (near 60%), resulting in a fast burning rate and a short flame ； d. The explosive limit range of coke oven gas is wide: 5–30%, and when mixed with air it can form explosive gases ； e. Coke oven gas is highly flammable with a low ignition point: 600℃ ； f. Raw gas is dirtier, and pipes are prone to blockage by substances such as tar and naphthalene ； g. The condensate in coke oven gas can corrode pipes ； h. Density of coke oven gas: 0.48–0.52 g/m3. 5. Points to note during the combustion of coke oven gas: Since coke oven gas has a low density, it experiences significant buoyancy force upward, causing the flame to be unstable. Furthermore, coke oven gas mainly contains the volatiles of coal, with a hydrogen content of over 50%; as a result, it is more prone to leaking through the gaps in the refractory bricks. Special care must be taken to ensure that the bricks are properly sealed, especially where heat-resistant steel pipes meet the furnace, in order to prevent gas leakage and fire spread. If coke oven gas is burned in a furnace using the same type of gas generated by a furnace burner, deflagration is more likely to occur. II. Transportation of coke oven gas 1. Design of coke oven gas pipelines The products manufactured by these glass furnaces are medium-alkali glass balls, and the type of furnace used is a regenerative horseshoe-flame furnace. It is 1050m away from the raw gas connection point of the coking plant. The designed pipe has a nominal outer diameter of 519 mm and a wall thickness of 7 mm, taking into account a maximum flow rate of 4000 m3/h. A discharge hole with the same diameter as the main pipe is provided to drain impurities such as condensate, tar, and naphthalene. Along with the gas pipeline, a steam pipeline is installed as well; a steam purge hole is provided every 20 meters for purging the pipeline in case of blockages. Another function of the steam pipeline is to inject steam into the gas delivery pipeline in case it is necessary to shut off the gas due to an accident. This helps maintain sufficient positive pressure inside the pipeline, allowing the gas shut-off valves to be closed gradually. In this way, sudden shutdown of the gas prevents negative pressure from forming inside the pipeline, which could lead to the absorption of air and the creation of explosive gases. According to measurements, the gas escaping from the coke oven contains 5–9 g/m3 of naphthalene; after initial cooling, the naphthalene content in the gas drops to 1.5–2.0 g/m3. The presence of naphthalene in coke oven gas is detrimental to its long-distance transportation. When coke oven gas transported over long distances is cooled to a temperature below the saturation temperature corresponding to this naphthalene content, naphthalene vapor in the gas pipeline condenses (the melting point of naphthalene is 80°C) and deposits at the bottom of the pipeline, easily causing blockages. Even with such a low content of naphthalene in the gas, more than 50 T of naphthalene accumulates in the gas pipelines each year, and most of it settles in a certain section of the pipeline. To check for blockages in the gas pipelines, a pressure inspection port is installed at each end of the sections where blockages are likely to occur; a diaphragm pressure gauge can be used to monitor the degree of blockage in the gas pipelines at any time. 2. Pipe blockage 　　After a period of use, the pipe blockage is not severe, which may be due to the short duration of use. However, there is a significant amount of condensate water; more than 1 m3 of it accumulates at the lowest point of the pipes every 24 hours. At most, the condensate water needs to be drained three times a day. 3. Pressure testing of pipelines 　　Before the coke oven gas is put into use, a pressure test must be conducted on the newly installed gas pipelines. The acceptance criterion is: maintaining a pressure of 0.1 Mpa for 4 hours ； Maintain pressure at 0.2Mpa for 2 hours. The pipes have been made more airtight; firstly, this prevents leaks of coke oven gas when there is positive pressure in the pipes, thereby reducing waste and environmental pollution ； Secondly, it is necessary to prevent air outside the pipeline from entering the pipeline when there is negative pressure inside, as this could cause coke oven gas to become an explosive mixture and lead to danger. 4. Exhausting and filling the pipeline with gas 　　Since the coke oven gas pipeline is quite long, with a volume of around 200 m3, it is necessary to use coke oven gas to completely displace the air inside the pipeline before allowing coke oven gas to flow through it. With a supply pressure of 300 mmH2O, after venting through the release valve for about half an hour, an explosion test was conducted by taking gas samples; only after it was confirmed that the gas in the pipeline posed no explosion risk was gas supply to the furnace officially initiated. 5. Explosion test of gases in pipelines All gas pipelines should be emptied using coke oven gas during the gas supply process, and an explosion test should be conducted. The method for a simple explosion test of coke oven gas is as follows: First, take a larger plastic bottle and cut off the bottom of it ； Then open the bottle cap, place the bottle on top of the vent valve, and use the gas emitted by the vent valve to displace all the air from within the bottle ； Finally, tighten the bottle cap, making sure it remains on top throughout the sampling process. Since the specific gravity of coke oven gas is very low (0.5 kg/m3), after sampling, the bottle (with its opening facing upward) is moved to a distance away, so that the coke oven gas does not escape into the air from below. Then, a ignition test is conducted at the bottom of the bottle. Determine whether explosive gases are present in the pipeline based on the sound of the sample’s deflagration. 6. Pressurization of pipelines 　　For gas pipelines, gate valves or disc valves with good airtightness should be used as shut-off valves, to prevent leaks and accidents caused by poor sealing of these valves. Under normal circumstances, the delivery pressure of coke oven gas in a coking plant is no more than 500 mmH2O; after traveling 1000 meters, the pressure at the entrance to the furnace is still less than 200 mmH2O. In such cases, a blower must be installed on the coke oven gas pipeline at the furnace entrance to pump in coke oven gas. The blower can be a commonly used high-pressure blower; provided that the flow rate is maintained, its blowing pressure should be able to reach over 300 mmH2O. The blower is connected to the gas pipeline in front of the kiln; to ensure that production is not disrupted in case the blower fails and requires maintenance or replacement, a bypass should be installed on the coke oven gas pipeline in front of the kiln. When installing the blower, it is essential to ensure a tight seal; it is absolutely not allowed for the blower to draw air into the coking gas pipeline. III. Integration of coke oven gas with glass furnaces There are three ways to integrate coke oven gas with glass furnaces: 1. Install gas burners beneath the furnace, and use compressed air to inject coke oven gas into it, just as is done with natural gas and heavy oil. This design is safe and convenient, but it consumes a large amount of electricity for compressed air, resulting in increased costs. Furthermore, due to the low pressure of coke oven gas, its burners are very different from those of natural gas. 2. Install gas regenerative chambers and gas exchangers, and connect the coke oven gas pipelines to the gas exchangers. This setting method is the same as that for producing furnace gas. The advantages of this setup are, first, the ability to use waste gas, and second, the ability to use producer gas, or a combination of coke oven gas and producer gas. With this approach, there are no major changes to the structure of the furnace. However, since the main component of coke oven gas is hydrogen, if not handled properly during exchange, it is extremely prone to explosion. Therefore, special attention should be paid to the design and construction of the regenerator chamber, especially its cross partition. The thickness of the partition wall is generally 575 mm ； When a thickness of 460 mm is used, a seven-part joint or half tile strip should be added to each layer of bricks; every layer must be laid with three staggered joints, and the mortar should be filled fully and compacted with a hammer to ensure good airtightness. Walls constructed in this way can reduce the amount of bricks used, improve heat exchange efficiency, and decrease air leakage, thereby reducing the risk of explosions. We know that the explosive range of coke oven gas is quite wide; therefore, strict procedures must be followed during its transportation and use: a. Thoroughly inspect the gas pipelines as well as equipment such as exhaust fans, to prevent gas leaks or air from entering (the oxygen content in coke oven gas must not exceed 1%) ； b. For gas pipelines, especially those under negative pressure, it is necessary to clean them in accordance with technical specifications during maintenance and replacement, and to conduct gas explosion tests to prevent the presence of explosive gases within the pipelines. Use gas vapor or an inert gas (such as nitrogen) during cleaning ； c. When preparing an outlet on the pipeline for coke oven gas, the stop valve is installed in advance to prevent the use of open flames after gas is introduced. Strictly control sources of fire; whenever gas or electricity is used for equipment maintenance, fire prevention measures must be taken from time to time ； d. The walls at the flue and regenerator, as well as the base of the gas exchanger, should be sealed with sealing paint. Seal with coatings that offer good sealing properties to maintain heat retention while reducing air leakage ； e. The flue gate should also be sealed promptly after its position is adjusted ； f. The lid of the naturally ventilated air exchanger must be airtight; if it is not, air will leak into the flue, reducing its temperature and suction force. This in turn leads to a decrease in the amount of air available for combustion, resulting in incomplete burning. Combustible substances will be present in the smoke, and these substances can explode when they come into contact with the air that has leaked into the flue. If the flaps of the air exchanger are not repaired in a timely manner, it will disrupt the normal operation of the entire furnace, leading to a vicious cycle that may even damage the flue ducts and the exchanger itself. 3. Install air inlets on the furnace wall or the bottom plate of the furnace, and connect them to the furnace wall or bottom using heat-resistant steel pipes. For safety reasons, we opted for a system that supplies coke oven gas from a small furnace to the walls. Considering the occasional instability in the supply of coke oven gas during production, we retained the gas regenerator and gas exchanger, and installed a gas generator which is sealed with a water seal box, so as to ensure the melting temperature is maintained using gas from the generator when the supply of coke oven gas is insufficient. The material of the steel pipe connecting to the small furnace is 1Gr18Ni9Ti (code 321), and it can withstand temperatures up to 850°C. If the steel pipe is to be inserted into the small furnace, a heat-resistant steel with the material specification code 310S should be used; this type of steel can resist heat-induced corrosion at temperatures up to 1200°C, but it is quite expensive. When connecting the heat-resistant steel pipe to the small furnace, the joint area should be extended outward using durable refractory bricks in order to reduce the temperature of the heat-resistant steel pipe as much as possible. Finally, the joint should be treated with a sealing coating to prevent air leakage and fire spread at the joint. Even when the gas regenerator is retained and the coke oven gas does not pass through it, the flue gas should still flow through the gas regenerator; otherwise, the absence of flue gas flow through the gas regenerator will result in a reduction in the cross-sectional area for flue gas flow, thereby increasing the flue gas resistance and consequently raising the kiln pressure. The specific method is to lift the gas flue gate appropriately, allowing the gas exchanger to switch fires along with the air regenerator. To enable heat exchange in the gas regenerator, the inspection port of the gas exchanger is opened to allow combustion air to pass through the gas regenerator and into the kiln. With such a system, it is possible to reduce the load on the air heat storage chamber ； Second, it increases the heat storage area, thereby raising the heat recovery rate. It can also effectively reduce kiln pressure and improve the operating conditions of the furnace. It should be noted, however, that the amount of combustion air used at this time increases significantly, and it is necessary to find ways to control the air intake volume to the air heat storage chamber. If the gate valve of the gas duct is not raised, coke oven gas (due to reasons such as imperfect sealing of the valves) tends to accumulate in the gas regenerator. When the gas accumulated in the gas regenerator mixes with the excess oxygen in the waste gases and the oxygen that leaks from the air regenerator into the gas regenerator in a certain proportion, an explosion can occur. Therefore, the flue damper of the gas duct must be raised to create a certain degree of suction in the gas regenerator, so that a small amount of coke oven gas can be removed at any time and explosions can be prevented. IV. Kiln heating and fire replacement procedures Since coke oven gas is already available at the kiln, we use it to heat the kiln starting from the material feeding stage. We improved the original diesel hot-air baking furnace to make it suitable for burning coke oven gas. Practice has shown that using coke oven gas for kiln firing is both economical and convenient, especially as it is particularly easy to operate at high temperatures. In kilns that rely on natural ventilation, it is important to ensure that the suction force in the flue ducts is not too low during the heating process; if the suction force is insufficient, it will affect the introduction of air for combustion, thereby impacting the heating rate. If the suction force is insufficient, the temperature will not rise even if the gas flow is increased. At this time, the chimney will emit black smoke. If air is encountered outside the flue gate, a deflagration will occur. This is very dangerous. So, if the draft in the flue is insufficient before a large fire, the flue needs to be baked. It should also be noted that as the flue temperature rises, the seal at the flue gate may be compromised; therefore, the air leakage at the flue gate must be checked after each change of fire. Based on our experience at the flue gate, we have established procedures for changing the fire source: a. When changing the fire source, first close the shut-off valve of the pipeline on the side where the fire is currently burning. On the pipes connecting each side of the furnace to the smaller furnaces, there are two shut-off valves: one is a regular shut-off valve (i.e., a fine-tuning valve), and the other is a backup shut-off valve (i.e., a coarse-tuning valve). b. Switch the air exchanger and the gas exchanger after a 2–3 second interval. c. After an interval of about 5 seconds, slowly open the coke oven gas valve on the other side of the furnace. d. The next change of fire takes place after half an hour. When using an exhaust fan to draw coke oven gas, the bypass return flow can be utilized to regulate the flow rate of the coke oven gas. 　　Our experience of using waste gas as fuel in glass furnaces has proven that it is feasible to burn waste gas in glass furnaces equipped with gas regenerative chambers (and gas generators). This type of furnace can be used in two ways: it can burn producer gas alone or waste gas alone, and it is also possible to use a mixture of waste gas and producer gas. When burning waste gas, the gas regenerator can also be used as an air regenerator.