Selection Comparison -- Comparison between Corner Tube Boilers and Water-Coal Slurry Boilers

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Comparison between corner-tube boilers and water-coal slurry boilers I. Characteristics of corner-tube boilers and water-coal slurry boilers: (1) Characteristics of corner-tube boilers: Corner-tube boilers are a type of boiler with a new structural design. The downcomers of this boiler are arranged at the four corners of the boiler; they serve not only as channels for water circulation but also as a supporting framework for the boiler. The water circulation is unique and safe, with rapid startup, and a load regulation range of 30-110%. It is an energy-saving product that serves as a substitute for conventional coal-fired boilers. Comparison between corner tube boilers and ordinary bulk boilers: Comparison items – Corner tube boiler, Ordinary bulk boiler. Grate: 45° inclination; uses a constant-pressure air chamber for precise air distribution, which facilitates complete combustion of coal; features a scale-type grate that reduces coal leakage. With an inclination of 60°, combustion is incomplete, resulting in more dust and pollutants. Drum type: external, not exposed to heat, ensuring safe and reliable long-term use. The membrane wall structure eliminates air leakage, resulting in an excess air coefficient of only 1.3–1.5 for the heating surfaces; this reduces smoke exhaust losses and lowers fan power consumption as well. When the boiler drum is heated, improper operation can cause bulging. Heating surfaces: The convective heating surfaces have a flag-shaped structure, with an ample amount of such surfaces. In the case of convective heating surfaces, the flue gas flows through four paths; whereas in other types of convective heating surfaces, the flue gas flows through two paths. Steam quality: It is saturated steam, and pre-separation occurs before the steam and water enter the drum, with further separation taking place in a secondary separator after entry into the drum. Superheated steam, saturated steam. Flue gas temperature: 175°C; 170–180°C. Flue gas concentration: ≤1800 mg/m3; ≤2100 mg/m3. Flue gas volume: 47,500 m3/h (at standard conditions); greater than 50,000 m3/h (at standard conditions). In terms of environmental protection, an open furnace is used, resulting in a slow flow rate and low volume of flue gas; fine ash settles to some extent within the furnace, which enables the flue gas emission concentration to remain below 1800 mg/m3. The furnace is narrow and low, leading to a fast flow rate of flue gas and a higher volume of gas. In terms of operating costs and infrastructure installation: a 46MW corner-tube boiler can be installed in a 29MW bulk boiler room, resulting in savings of over 300,000 yuan in infrastructure investment. In terms of operational efficiency: thanks to the precise air distribution in the grate and the isobaric air chamber, combustion losses are reduced by about 3% compared to batch boilers ； Due to the sealing structure of the membrane wall, the heat loss from smoke exhaust is about 3% less than that of batch boilers. For these two reasons combined, corner-tube boilers are about 6% more efficient than bulk boilers of the same type. (II) Characteristics of water-coal slurry boilers: 1. The combustion of water-coal slurry is stable and thorough, with good furnace filling; the combustion efficiency reaches 96%–98% ； The thermal efficiency of the boiler is greater than 85%. The load is easy to adjust ； It can be adjusted down to 40% in automatic or manual mode. 2 The combustion of water-coal slurry requires light diesel (or 20# fuel oil) for ignition, but once ignition occurs and the furnace temperature reaches 800°C, the fuel supply is automatically stopped and the combustion switches to that of water-coal slurry. 3. The optimal ignition temperature for water-coal slurry is around 800°C; it can only burn after being atomized with compressed air, and it is not combustible at normal temperature and pressure. It can be seen that its storage, transportation, and handling are simpler and safer compared to fuel, making it easier to manage and suitable for various locations and environments. 4. The combustion temperature of water-coal slurry is generally 100°C–200°C lower than that of coal burning, and its low-pollution combustion technology helps to reduce the generation of NOx and increase the sulfur fixation rate. 5. The combustion characteristics of water-coal slurry are similar to those of fuel; it is burned by mixing with air after being atomized by compressed air through the nozzles of a burner, and it is a new type of fuel developed as an alternative to conventional fuels. It is a clean fuel. (III) Comparison of technical characteristics between water-coal slurry boilers and corner-tube boilers: Item, Water-coal slurry boiler, Corner-tube boiler; Operating conditions: Water-coal slurry is suitable for continuous operation; when not in use, it is necessary to clean the pipes and containers to prevent the slurry from solidifying. “The current production situation at Yashili’s Zhengzhou facility is that tower cleaning is carried out once every 1 to 2 days; during this time, steam supply has to be stopped, which allows the system to adapt to these operating conditions. Technical challenges: It is known that water-coal slurry faces issues such as easy coking and incomplete combustion; moreover, the pipelines and containers of the associated equipment need to be insulated and protected from freezing in winter. Dust contains about 35% water vapor, is highly hydrophilic, and can easily cause clogs in the end equipment. Currently, controlling such issues is quite complex and presents certain difficulties. With a grate inclination of 45°, the problem of powder leakage in bulk boilers is resolved; the use of equal-pressure air chambers ensures precise air distribution, which facilitates complete combustion of the coal. The technology is mature. Boiler sales: It is understood that the existing manufacturers of water-coal slurry boilers are generally small-scale, emerging companies. Sales of 25T boilers are low. It has mature technology, high sales volume, and wide applications. Smoke concentration: when water coal slurry is produced from finely washed coal, it is ≤300 mg/m3. Use second-class bituminous coal, ≤1800mg/m3. The flue gas exhaust temperature is around 180°C or 175°C. For desulfurization and dust removal, a water film wet method is used; alternatively, a bag-type dust removal system combined with secondary desulfurization can also be employed. As for the fuel, high-quality coal is used in the production of water-coal slurry – this coal has a high calorific value and low levels of ash and sulfur, thereby helping to control the concentration of polluting dust at the source. Class II bituminous coal. A better coal can also be chosen. Fuel supply: The sources of fuel for water-coal slurry are limited, and there are not many slurry production plants in Zhengzhou at present. Sources are diverse. II. Cost Analysis of Water-Coal Slurry Boilers and Practical Difficulties in Their Construction (I) Cost analysis of water-coal slurry boilers: 1. Loss in thermal efficiency: Water-coal slurry is essentially a powdered form of high-quality coal; its calorific value ranges from 19,000 KJ/kg to 20,000 KJ/kg (4,490–4,800 kcal/kg). Generally, refined coal that has been washed to have low ash content (less than 8%) and low sulfur content (less than 0.5%), as well as a certain volatilization value (not less than 17%), is used. This coal is ground into fine particles (with 80% of the particles having a size between 40–70 μm, and the maximum size not exceeding 300 μm), mixed with water, and then a small amount of stabilizer is added to create a slurry. The combustion process is similar to that of pulverized coal combustion, but due to the approximately 29% moisture content in water-coal, the output of the modified boiler is 30% lower compared to that of a boiler using pulverized coal, and its thermal efficiency is also about 3%–5% lower. 2. Comparison of the environmental benefits of water-coal slurry: Water-coal slurry is made from washed bituminous coal with low ash and sulfur content. It is more environmentally friendly than coal in terms of storage and transportation, and its environmental benefits are better than those of using ordinary raw coal. However, by converting coal boilers to use refined coal, it is also possible to meet environmental requirements. The environmental benefits of burning water-coal slurry are lower than those of burning oil and gas; it is the same as burning refined coal, and better than burning raw coal. 3. Economic comparison of water-coal slurry: Compared with fuel oil and gas, water-coal slurry is definitely more economical; however, when compared with refined coal, it is certainly less economical. (1) Operating costs: Project, Boiler type, Thermal efficiency (%), Combustion efficiency (%), Fuel name, Unit price per ton, Fuel consumption per ton of steam, Fuel cost per ton of steam, Electricity consumption per ton of steam, Operating cost per ton of steam, Cost ratio of water-coal slurry to other fuels. Water-coal slurry boiler: 83–84%, 98%; Grade 1 coal slurry: 920 yuan per ton, 150 Kg per ton of steam, 138 yuan per ton of steam, 24 yuan, 162 yuan; Grade 3 coal slurry: 650 yuan per ton, 160 Kg per ton of steam, 104 yuan per ton of steam, 24 yuan, 128 yuan. Tube boiler: 90%, 90%; Clean coal: 800 yuan per ton, 130 Kg per ton of steam, 104 yuan per ton of steam, 12 yuan, 116 yuan; Ratio: 0.72. Grade 2 bituminous coal: 500 yuan per ton, 155 Kg per ton of steam, 77.5 yuan per ton of steam, 12 yuan, 89.5 yuan; Ratio: 0.7. (2) Initial investment in equipment: For the two sets of 25-ton water-coal slurry steam boilers, the cost of the main units (including auxiliary equipment, dust removal systems, slurry storage tanks, etc.) is approximately 9–10 million yuan. The previous two sets of 25-ton coal-fired corner-tube steam boilers required an investment of around 3.78–4 million yuan for the boiler units themselves; adding other expenses such as auxiliary equipment and dust removal systems, which totaled nearly 3 million yuan, the overall investment came to about 6.8–7 million yuan. (II) Difficulties faced by *** (Zhengzhou) Company in constructing water-coal slurry boilers: 1. Supply of water-coal slurry: According to available information within the province, no large-scale commercial enterprises for producing water-coal slurry have been found. Small-scale production facilities are mainly used for internal needs or for small-scale sales, at high prices and with limited supply. Building a proprietary pulp production line is not appropriate; our group company is an enterprise whose main business is nutritional products, and it is not suitable or cost-effective from the perspectives of the environment, the nature of production, or the required investments. 2. Excessively high operating costs: Assuming our company constructs two 25-ton coal-water slurry boilers, the consumption amounts to the following: The two boilers will require 57,600 tons of coal slurry in one year (based on 300 days, not operating at full capacity), which translates to an annual cost of around 37.44 million yuan (assuming the use of third-grade coal slurry, at a cost of approximately 650 yuan per ton). Assuming our company constructs two 25-ton corner-tube boilers, the consumption will be as follows: The two boilers will require 55,600 tons of coal in one year (calculated based on 300 days, not operating at full load), which amounts to approximately 27.8 million yuan per year (at a cost of 500 yuan per ton of category II bituminous coal). Using water-coal slurry boilers results in an additional annual cost of 9.64 million yuan compared to using corner-tube boilers, placing excessive pressure on the company’s costs. 3. Initial capital investment for equipment: Two water-coal slurry boilers require an additional 3 million yuan compared to corner-tube boilers, **increasing the investment pressure on enterprises. In summary, our company faces significant challenges when using water-coal slurry boilers; the corner-tube boilers selected can meet environmental regulations. We will certainly strive to use better processes and equipment for dust and sulfur removal systems, in order to minimize pollutant emissions as much as possible. —————————— The main focus of this article is environmental impact assessment. Before writing it, our company had internally decided to choose angle-tube boilers, but the environmental protection agency insisted on coal slurry boilers. The environmental impact assessment for choosing an angular tube furnace has now been approved. Although this article was written with great effort, it isn’t very useful; what’s really crucial is dealing with the approval process by the environmental protection agency! This post was last edited by Liancheng on 2008-7-17 16:29.]