Analysis of Energy-saving Status of Kilns in the Ceramic Industry

Release Time:

2015-01-27 16:50

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　　Modern architectural sanitary ceramics encompass the production of sanitary ceramics, architectural ceramics, arts and crafts ceramics, glazed products, and auxiliary materials for architectural sanitary ceramics such as colored glazes and frits. When discussing energy saving in architectural sanitary ceramics, we usually refer to architectural sanitary ceramics and their related industries. Of course, within these industrial systems, architectural sanitary ceramics, especially architectural ceramics, are the main products. Therefore, the discussion on energy saving should focus on the primary contradictions. Currently, the kilns used in the architectural sanitary ceramics and related industries mainly include tunnel kilns, track kilns, shuttle kilns, push-plate kilns, and frit kilns. Some industries also involve rotary kilns, bell kilns, electric kilns, and other types, but their proportion in the overall architectural sanitary ceramics sector is relatively small and can be considered supplementary rather than the main focus. However, regardless of the kiln type, energy saving and consumption reduction are issues, especially given China's current challenges with air pollution and mandatory pollution emission standards in the architectural sanitary ceramics industry.

　　1. Stakeholders in Ceramic Kiln Energy Saving

　　Within the industry, there is a unanimous call for energy saving and consumption reduction in ceramic kilns, without the intense and endless debates seen with other new technologies, new processes, or coal-to-gas conversions. Since the inception of ceramic kilns, the industry has continuously renovated, discussed, designed, and improved ceramic kilns. Currently, there remains significant potential for energy saving in ceramic kilns. Why has there been little progress after decades of improvements? Is it a technical issue, a materials issue, or a cost-benefit ratio problem? All these factors play a role. To truly and voluntarily solve the energy saving problem of kilns, it is necessary to realistically present the relationship between energy saving and consumption reduction and enterprise development, local economy, environmental protection requirements, material resources, and product cost performance. Analyzing and effectively addressing the obstacles and difficulties can clear the way for a positive cycle of voluntary energy saving and emission reduction. Relying solely on high-pressure policies is neither sustainable nor reliable.

　　Energy saving in ceramic kilns is closely related to local environmental policies and enterprise economic benefits. To achieve effective energy saving in ceramic kilns, enterprises must be willing to invest the necessary funds for renovation. For newly built kilns with excellent energy-saving performance, the materials used, kiln design, utilization of waste heat, and new technologies and processes should all be up to date, which generally requires significant investment. Therefore, local governments should provide certain support or compensation. Assessment and compensation based on emission levels, heat utilization rates, or energy consumption per unit product are good approaches. Fortunately, the architectural sanitary ceramics industry has already established relevant energy-saving standards, and the energy consumption standards per unit product have been completed. It is hoped that all parties will work hard to raise the energy-saving effectiveness of ceramic kilns to a new level.

　　2. Ways to Save Energy in Ceramic Kilns

　　There has been considerable discussion on energy-saving technologies for kilns in architectural sanitary ceramics. In summary, the main solutions include:

　　① Kilns should be reasonably designed according to the characteristics of the fired products.

　　② Reasonable selection of refractory materials for kilns.

　　③ Effective utilization of kiln waste heat.

　　④ Adoption of new combustion technologies.

　　⑤ Use of clean energy.

　　2.1 Reasonable Design of Kilns

　　Kilns are the most critical thermal equipment in ceramic enterprises and also the most energy-consuming. However, the energy consumption level of kilns mainly depends on the kiln structure and firing technology. The kiln structure is fundamental; without a good design, improving firing technology is very difficult or even impossible. Conversely, a good kiln structure requires advanced firing technology to ensure quality. These two aspects are interdependent and indispensable. Only by reasonably coordinating both can the firing quality of new kilns be improved while reducing energy consumption.

　　Currently, China's architectural sanitary ceramic kilns are mainly tunnel kilns and roller kilns, both based on the same design principles. Where space permits, ceramic kilns should be reasonably sized in length, width, and height. What is reasonable? Different kiln experts have different opinions depending on the situation; some say kilns should be as long as possible, others disagree. The kiln width has increased from just over 1 meter to more than 3 meters. Height is less discussed and is designed based on the maximum height of ceramic pieces (including kiln cars and kiln furniture). Regarding kiln length, personally, I believe it is better to have relatively longer kilns, but unlimited length increases operating costs. For example, transferring waste heat from the cooling section to the preheating section becomes more difficult, multi-pipe transfer increases costs, and longer kilns face greater maintenance pressure, especially when tunnel kilns experience backfiring, which is more troublesome. Similarly, kiln width should be as wide as possible within the reasonable combustion range of burners. Temperature difference is a major consideration in kiln width design. Current kilns have burners on both sides, so width must be considered in conjunction with combustion technology. There is no definitive answer on the best kiln width; it depends entirely on product characteristics and production capacity. As long as the internal temperature difference is not too large, the width should be fine. Additionally, sufficiently long preheating and cooling zones benefit the product, especially when firing products change frequently. This makes the kiln more adaptable, like buying a large coat that fits both tall and short people, whereas a small coat is less adaptable.

　　2.2 Reasonable Selection of Refractory Materials for Kilns

　　The main performance of the kiln body is determined by the technical and economic performance of refractory insulation materials, which directly affect the kiln's investment cost, working performance, thermal efficiency, and operating energy consumption costs. General principles for selecting refractory insulation materials include:

　　① Kiln performance and thermal characteristics.

　　② Material safe use temperature, thermal conductivity system, high-temperature strength, and chemical stability.

　　③ Service life.

　　④ Investment cost and operating maintenance cost.

　　Generally, heavy refractory materials focus more on certain technical performance indicators such as high-temperature stability and chemical stability; lightweight insulation materials emphasize comprehensive technical and economic indicators of investment and operation.

　　When selecting lightweight insulation materials, the product of the thermal conductivity coefficient (λ) and the cost per unit volume of insulation material (Ρ) is used to determine suitability; the smaller the "λΡ" product, the better. A smaller thermal conductivity coefficient indicates better insulation and lower energy costs during operation. A lower cost per unit volume indicates lower investment costs and better economic investment effects.

　　Refractory materials for ceramic sanitary ware kilns include the kiln walls and roof, generally divided into three layers inside and outside. The inner layer consists of lightweight insulation cotton, felt, boards, etc.; the middle layer is made of heavy or lightweight insulating bricks; the outer layer mainly uses red bricks or low-temperature insulating bricks. The lightweight insulation layer of the inner layer has developed rapidly in recent years, mainly using low-temperature mullite fibers, also including high-temperature mullite fibers, high-alumina mullite fibers, zirconia-containing mullite fibers, with temperature ranges from 1200°C to 1600°C across various systems. Fiber manufacturing processes include drawing, spinning, and blowing. The inner layer also uses lightweight mullite, high-alumina lightweight bricks or boards, mainly composed of lightweight alumina, mullite, and hollow spherical bricks. The middle layer's heavy materials still mainly use heavy materials made from low-alumina high-silica, high-alumina low-silica, mullite and cordierite, spinel and mullite, etc. The outer layer's low-temperature refractory materials are mainly made from bauxite, spinel, clay, etc., with relatively low aluminum content, poorer temperature resistance but lower cost.

　　Refractory materials for kiln cars include the kiln car surface, kiln car supports, firing plates, crucibles, etc. Except for the basic structural surface materials of the kiln car, the other materials are collectively called kiln furniture materials. The refractory materials for kiln cars are basically the same as the kiln wall materials. Kiln furniture materials vary depending on the situation and include mullite, high-alumina mullite, cordierite, mullite-cordierite, SiC, oxide-bonded silicon carbide, silicon nitride-bonded silicon carbide, etc.

　　2.3 Effective Utilization of Kiln Waste Heat

　　An important process in ceramic production is firing, which takes place in kilns. Ceramic production kilns include continuous tunnel kilns, roller kilns, and intermittent down-draft kilns. Currently, most ceramic enterprises use the first two types, while down-draft kilns were more commonly used previously. Thermal efficiency is one of the key indicators to evaluate a kiln's quality and an important metric for energy saving and emission reduction. The thermal efficiency of ceramic kilns is closely related to combustion loss, heat dissipation loss, and flue gas loss. According to data, the heat loss carried away by exhaust gas from firing tunnel kilns accounts for about 20% to 40% of total heat, while for down-draft kilns, the heat loss carried away by exhaust gas accounts for about 30% to 50% of fuel consumption. Therefore, recovering and utilizing the heat from kiln tail exhaust gas is key to improving kiln efficiency. Domestic tunnel kiln exhaust temperatures generally range from 200°C to 300°C, with some reaching as high as 400°C, and individual down-draft kiln exhaust temperatures can reach up to 560°C. Thus, effective utilization of waste heat is an effective way for enterprises to increase benefits and contributes to energy saving and emission reduction.

　　Currently, waste heat utilization mainly involves heating air for drying green bodies. Waste heat recovery from tunnel kilns and roller kilns is mainly used to heat air as a heat source for drying green bodies, and can also be used as combustion-supporting air to improve the kiln's thermal efficiency. The choice between these depends on the specific conditions of each factory.

　　It is undeniable that although waste heat utilization in ceramic sanitary ware kilns has been greatly improved, many problems still exist, the most serious being the low waste heat utilization rate, manifested in heat loss during waste heat transmission and secondary waste heat utilization after initial use. Some factories also install regenerator chambers behind roller kilns for waste heat utilization, which is a good method. Overall, waste heat utilization is a comprehensive project for any ceramic enterprise. If done well with a high utilization rate, the product's unit consumption will be significantly reduced, and the overall comprehensive cost will also decrease accordingly.

　　2.4 Adoption of New Combustion Technologies

　　The combustion efficiency of ceramic kilns is one of the key factors to evaluate kiln quality. Effectively utilizing the energy of the fuel itself is the main direction for energy saving in ceramic kilns. Measures to improve kiln combustion efficiency mainly include adopting low air coefficient combustion methods, oxygen-enriched combustion, and increasing the temperature of combustion-supporting air. Meanwhile, microwave firing technology has also been continuously developing in recent years, achieving promising results.

　　Under actual production conditions, to ensure complete combustion of fuel, air supply is generally higher than the theoretical air requirement. The ratio of actual air supplied to theoretical air required is called the excess air coefficient or air coefficient, usually denoted as n. Low excess air coefficient combustion supplies the minimum air amount to ensure complete combustion by adjusting combustion air distribution, making the combustion process as close as possible to the theoretical air amount. With reduced excess oxygen in flue gas, NOx formation can be suppressed, generally reducing NOx emissions by 15% to 20%. Effective methods to achieve low excess air coefficient include oxygen-enriched combustion, pure oxygen combustion to increase oxygen concentration, and methods to reduce nitrogen content such as desulfurization and denitrification technologies. The goal is to minimize nitrogen in the air carrying away excess oxygen and heat during combustion.

　　Microwave firing technology is a new technology and has not yet been successfully fully applied in ceramic sanitary ware kilns in China. According to reports from Taocheng News, foreign research on microwave firing ceramics is making progress. Using microwave firing can greatly shorten firing time and product production cycles, and significantly reduce energy consumption. The working principle of microwave firing ceramics is that the ceramic green body absorbs microwaves at a frequency of 2.45 GHz, then the ceramic body itself emits heat, heating the ceramic semi-finished product to the predetermined sintering temperature. Currently, microwave-fired ceramic kilns have been successfully developed in the United States and Japan. These kilns have a double-layer structure of microwave-absorbing ceramics and insulation materials. The microwave firing technology used differs from conventional methods that rely on raising the surface temperature of the green body. Instead, it uses internal heating by microwaves, allowing uniform temperature rise during firing, reducing ceramic deformation and uneven coloration defects, and reducing emissions of CO2, SO2, and other exhaust gases during firing, thus lessening environmental pollution. In China, the Dehua ceramic kiln microwave-assisted firing energy-saving system has been put into use. It is reported that this technology overcame the adaptability of high-power microwaves and solved the common thermal runaway problem in microwave applications. Developed by Shunmei Company with an investment of over 4 million yuan, it uses microwave heating to preheat green bodies before firing, enabling rapid moisture removal and organic decomposition, significantly reducing cracking rates. On-site measurements show that the system can increase ceramic product yield to 99%.

　　Overall, in terms of combustion, ceramic kilns should focus on combustion technologies with low excess air coefficients and low-energy microwave technologies to achieve better thermal utilization.

　　2.5 Adoption of Clean Energy

　　Clean energy refers to environmentally friendly energy, meaning it is eco-friendly, has low emissions, and causes little pollution. The precise definition of clean energy should be: a technological system for the clean, efficient, and systematic application of energy. There are three points in its meaning: first, clean energy is not a simple classification of energy but refers to the technological system of energy utilization; second, clean energy emphasizes not only cleanliness but also economy; third, the cleanliness of clean energy refers to meeting certain emission standards. From the above explanation, it is not difficult to see that previously we simply considered natural gas as clean energy, while coal, heavy oil, coke oven gas, and other fuels were not clean energy. This understanding is somewhat biased or one-sided.

　　For a long time, the kiln combustion of building sanitary ceramics mostly used coal-to-gas conversion. During this process, a large amount of harmful gases such as sulfides, nitrides, and carbides were produced. Although coal slag has been effectively utilized, these harmful gases have caused significant adverse effects on the ceramic concentration areas. The local environment and atmosphere have been harmed, and residents have occasionally complained. For building sanitary ceramics to progress further, it is necessary to tighten the environmental protection and energy-saving constraints, forcing enterprises to use energy cleanly.

　　From the explanation and definition of clean energy, no matter what fuel is used, it can be made into clean energy, not just referring to natural gas alone. Natural gas is just the most convenient clean energy, but it may not be the most economical for enterprises. Now, new technologies are continuously emerging. Technologies such as desulfurization, denitrification, and automatic pollutant testing have basically made great progress. While technology continues to develop, fuels like coal and heavy oil, previously considered non-clean energy, will eventually become clean energy through technological transformation. Of course, this process is somewhat painful and difficult.

　　In principle, using clean energy definitely increases firing costs; this is an undeniable fact. However, with the tightening of environmental protection policies across regions, adopting clean energy has become an inevitable trend.

　　3. Conditions for Deepening Energy Saving in Ceramic Kilns

　　From a macro perspective, there are only three conditions for deepening energy saving in ceramic kilns: attention in ideology, concern from local governments, and support from banks.

　　Attention in ideology means that building sanitary ceramic enterprises, especially the top leaders, must fully recognize the importance and urgency of energy saving and emission reduction. This is not only an economic benefit issue but also a major matter concerning the health and well-being of future generations. No matter how large an enterprise is or how wealthy an individual is, without the continuation and development of human history, those things have no value. Currently, building sanitary ceramic enterprises have production departments, technical departments, quality control departments, procurement departments, etc., but there is currently no energy resources department, which is inappropriate. This shows that we have not truly recognized the importance of energy utilization and energy saving and emission reduction to enterprises ideologically, indicating that the emphasis is still only verbal.

　　The current operating situation of building sanitary ceramic enterprises is not optimistic. Although we have nearly 10 billion square meters of building ceramic output and nearly 200 million pieces of sanitary ceramic output, these only indicate that our output is large. But what about demand? It only reaches 50%-60% of our current output. What is building sanitary ceramics competing on? Competing on price and cost. The result is a lack of emphasis on environmental protection and energy saving, which is why building sanitary ceramics are disliked in various places. But the fact exists, so local governments should show concern and care, strive to solve the difficulties and problems of enterprises, guide according to circumstances, find solutions, provide support and help, and avoid expansion and new additions, so that existing building sanitary ceramic enterprises can develop healthily and steadily.

　　In the process of energy saving and emission reduction in building sanitary ceramics, we also call on banks to provide certain support. Only when these enterprises develop healthily and steadily will the local finance, taxation, bank loans, and deposits avoid major fluctuations. Under the new normal of enterprise transformation and environmental friendliness, banks should also adopt new normal thinking. Effectively supporting the energy-saving transformation of building sanitary ceramic enterprises is like digging a deep well. The process is long and difficult, but the water drawn will definitely be uncontaminated healthy water, and it will be sweet.

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