The 2020 national annual report on the prevention and control of environmental pollution by solid waste in large and medium-sized cities shows that in 2019, the output of general industrial solid waste in 196 large and medium-sized cities in China was 1.38 billion tons, the output of industrial hazardous waste was 44.989 million tons, and the output of urban domestic waste was 235.602 million tons. The pressure on the disposal and utilization of solid waste is increasing. The CPC Central Committee and the State Council attach great importance to the prevention, treatment and utilization of environmental pollution by solid waste. With the implementation of the law of the people's Republic of China on the prevention and control of environmental pollution by solid waste and the national list of hazardous wastes revised in 2020 and the promotion of the pilot construction of "waste free city", the collaborative disposal and utilization of solid waste industry has developed rapidly, forming a new development pattern with multiple ways and high added value.
1. Collaborative disposal and utilization of cement industry
The technology of cement kiln collaborative disposal of solid waste is to use the unique high-temperature environment and alkaline atmosphere of cement kiln, which can effectively prevent the dissolution of harmful substances in incineration waste, neutralize hydrogen fluoride, hydrogen chloride and sulfur dioxide, avoid the re synthesis of dioxin and furan, and use solid waste to replace some raw materials or fuels, so that cement production enterprises can save raw material demand, reduce energy consumption and completely realize resource utilization, At the same time, the carbon dioxide released by the decomposition of calcium carbonate is reduced, and the waste landfill is greatly saved, which meets the requirements of energy conservation, consumption reduction and emission reduction, and has obvious economic and environmental benefits. At present, the solid waste for collaborative disposal and utilization in China's cement industry mainly includes domestic waste, hazardous waste, sludge, etc. the types of disposal and utilization are mainly alternative raw materials, alternative fuels and waste disposal.
As for domestic waste, as of October 2018, China has 57 cement kiln collaborative domestic waste disposal projects, with a disposal capacity of 7.7 million T / A. Based on the basic principle, the collaborative disposal technology of cement kiln can be divided into two categories. The first category is to prepare waste derived fuel (RDF) into cement kiln for disposal and utilization, that is, the combustible components in domestic waste are made into RDF through dehydration, crushing, screening and other processes to replace some coal and incinerate in the high-temperature area of cement kiln; In the second category, after dehydration, crushing and other pretreatment, the domestic waste will be incinerated with newly added supporting incineration equipment, and the waste gas and waste residue will enter the cement kiln. Huaxin Cement Co., Ltd. and Sinoma International Engineering Co., Ltd. adopt class I process, while Beijing Jinyu Group Co., Ltd., Anhui Conch Cement Co., Ltd. and China Resources Cement Holding Co., Ltd. adopt class II process. The cement kiln collaborative disposal technology adopted by the above five enterprises has its own characteristics in terms of investment, operation cost and applicability: in terms of investment, the investment cost of Huaxin, Sinoma International, conch and China Resources is high, and the investment in 500t / D domestic waste project is more than 100 million yuan, while Jinyu is relatively low. In terms of operating costs, the operating costs of the five enterprises are between 120-220 yuan / T, of which Jinyu and China resources are relatively low, with costs of 130-150 yuan / T and 120-140 yuan / T respectively. From the perspective of applicability, Huaxin technology is applicable to domestic waste projects with high moisture content, which requires collaborative disposal of cement kilns and joint operation of landfill. The construction outside the pretreatment plant will increase the land occupation and transfer cost; Sinoma International's technology aims at high-quality domestic waste, and the screened waste enters the raw material batching system; Jinyu's technology is applicable to all kinds of domestic waste, but the thermal efficiency is low due to the use of gasifier outside the kiln, which reduces the utilization efficiency of waste calorific value; Conch technology is applicable to all kinds of quality waste. However, due to the great impact on the cement kiln system, the collaborative disposal of waste in the cement kiln is limited and has a great impact on the clinker production capacity. Therefore, it is not suitable for projects with large disposal and utilization. The pretreatment is simple, mainly disposal, and the utilization of alternative fuels is lacking; CR's technology is applicable to all kinds of domestic waste, especially the domestic waste with high moisture content, high calorific value, flammability and good quality.
As for hazardous waste, as of 2018, there were 60 cement kiln collaborative disposal projects with hazardous waste business licenses in China, with a disposal scale of about 3.68 million T / A. from the perspective of enterprises, the top three are Hongshi Holding Group Co., Ltd., Anhui Conch Cement Co., Ltd. and Beijing Jinyu Group Co., Ltd. respectively. The collaborative disposal capacity of hazardous waste of the three enterprises accounts for 46% of the whole industry. In terms of regional distribution, there are 20 provinces and municipalities directly under the central government in China with the capacity of cement kiln collaborative disposal of hazardous waste, among which the top five provinces are Zhejiang (660000 T / a), Henan (300000 t / a), Guangxi (220000 T / a), Jiangsu (220000 T / a) and Fujian (190000 T / a), accounting for 56% of the country. In terms of cost, cement kiln collaborative disposal has the advantages of less investment, low cost, short construction period and disposal price far lower than incineration and landfill. According to the data, the investment of cement kiln collaborative disposal of hazardous waste is about 1 / 3 ~ 1 / 5 of that of professional incinerator, and the profit of 5 million yuan can be obtained from the disposal of 5000t / a hazardous waste. In terms of environmental protection, the unique structure and incineration process of cement kiln can completely decompose all kinds of refractory organics and stably sinter all kinds of heavy metals into clinker. At the same time, it has unique advantages in reducing incineration pollution, especially in curbing the generation of dioxins. In addition, the kiln ash produced by flue gas disposal in this process can be returned to the raw material feeding system, which can effectively avoid the secondary storage and disposal problems caused by new hazardous waste. In terms of capacity, the collaborative disposal capacity of cement kilns is basically more than 10000 tons, which is greater than the incineration and landfill of hazardous waste. However, there are some disadvantages in the collaborative disposal and utilization of hazardous wastes in the cement industry. Firstly, cement production is affected by industrial policies, market supply and demand, industry fluctuations, especially in areas that mainly rely on the collaborative disposal and utilization of hazardous wastes by cement kilns. Once the production of cement is reduced or stopped, it will have an impact on the timely and safe disposal and utilization of hazardous wastes; Secondly, China's relevant laws, regulations and technical specifications are relatively weak. The existing cement kiln collaborative enterprises have not yet formed a scale system, showing problems such as poor R & D ability, lack of practical experience and lack of professional and technical personnel, which is difficult to effectively ensure the quality of collaboration; Finally, because the collaborative disposal and utilization of cement industry needs to consider the impact of collaborative hazardous waste on cement quality, output and equipment, it can only be limited to hazardous waste with stable source and composition.
As for sludge, by 2019, China has 24 production lines for collaborative disposal of sludge in cement kilns, with a disposal capacity of about 1.95 million T / A. Relying on the equipment of cement kiln, the technology of collaborative disposal and utilization of sludge has the advantages of complete decomposition of organic matter, no residual heavy metals, no secondary pollution, high resource efficiency, large amount of disposal and utilization, stable process, less investment and low operation cost. China's relevant standards and specifications point out that the collaborative disposal of sludge in cement kiln should be carried out on the new dry process cement clinker production line with a scale of 2000t / D and above. At present, the main methods of disposal and utilization of sludge in China's cement enterprises include raw material batching, direct smoke delivery chamber or calciner incineration, the use of chemicals and plate and frame pressure filtration collaborative technology, direct drying incineration, indirect drying incineration, biological drying technology, etc. these methods have their own advantages and disadvantages: the advantages of raw material batching method are less investment and simple method. When the amount of sludge is appropriate, it has no adverse impact on the quality of clinker, Conducive to energy conservation; The disadvantage is that if the dioxin and other harmful substances are discharged without disposal, it will cause air pollution. The advantages of direct flue gas delivery room or calciner incineration method are no pollution to the environment and less investment. The disadvantage of using the Synergistic Technology of reagent and plate and frame pressure filtration is that the addition of ferric chloride makes the sludge have a large amount of chloride ions, and the disposal and utilization of sludge is limited, which has a great impact on the output of cement kiln and clinker. The advantage of direct drying incineration method is that it can improve the amount of sludge disposal and utilization; The disadvantage is that the amount of odor is relatively large, so the equipment investment and operation cost of odor disposal are also large. The advantage of indirect post drying incineration method is the large amount of sludge disposal and utilization. The advantage of biological drying technology is that the aerobic fermentation of microorganisms is the source of energy required for drying. It is a very economical and energy-saving drying technology, and forced air blowing is carried out on the materials, which promotes the whole drying process and shortens the drying cycle; The disadvantage is that there are still various technical problems, and the disposal and utilization effect is difficult to achieve the expectation.
In addition, the types and scale of cement kiln collaborative disposal of general solid waste are also increasing. For example, Guangyuan Haichuang cement kiln collaborative disposal project, which was officially put into operation on December 27, 2019, has a daily disposal utilization of 330t and an annual collaborative disposal utilization of 70000 T, which can dispose of ash generated by waste incineration, industrial waste, industrial sludge, municipal sludge and industrial tailings produced by industries such as automobile production, electronics, cable, electroplating, printing and dyeing, papermaking and chemical industry. In the process of collaborative disposal of cement kiln, low moisture combustibles are directly burned in the high temperature zone of calciner through pretreatment, crushing and transportation to replace some fuels; Semi solid and solid wastes are crushed, mixed and stored, and then transported, crushed, stirred and measured to the calciner for incineration; Fly ash waste is stored, measured and pumped to the kiln head for incineration. At present, China has carried out a lot of work and achieved some results in the collaborative disposal of solid waste in cement kilns, but there are still many problems. For example, China mainly takes the collaborative disposal of cement kilns as a means of harmless waste. As long as the environmental emission and product heavy metal content standards are met, the collaborative disposal of solid waste containing heavy metals in cement kilns can be carried out, resulting in the transfer of a large number of heavy metals to cement products.
Although no environmental risk has been detected based on the current detection methods and equipment, it will still have potential ecological and health risks. China's solid waste pretreatment procedure is only to meet the requirements of kiln incineration, and the calorific value of solid waste is not adjusted before entering the kiln, which increases the negative impact on the kiln condition and reduces the collaborative disposal capacity of cement kiln and cement production capacity. The construction of solid waste classification system in China has just started and is not perfect enough, which makes it difficult for cement enterprises to collect combustible waste. China has not yet issued the construction plan for collaborative disposal of cement kilns, which is likely to lead to vicious competition between collaborative disposal of cement kilns and disposal methods with higher prices such as incineration and landfill in some areas of China, and the operation of collaborative disposal of cement kilns is greatly affected by market demand such as cement. Therefore, collaborative disposal of cement kilns should be used as a supplementary method and cannot replace disposal methods such as incineration and landfill. The public's lack of understanding of pollutants and risk control in the process of cement kiln collaborative disposal of solid waste leads to their resistance to the technology of cement kiln collaborative disposal and the cement products produced, which affects the market share of cement products prepared by cement kiln collaborative disposal of solid waste. In addition, China's current solid waste emission reduction system, such as carbon dioxide trading and quota, is lack of incentive system.
In order to deal with the above problems, the top-level design should be improved from the development planning of cement kiln collaborative disposal and carbon emission reduction incentive policies. Relevant departments should speed up research from the aspects of technology, equipment, materials and risks according to the actual situation of collaborative disposal of solid waste by cement kilns in China, so as to improve the level of environment, human health and safety, the amount of solid waste disposal and utilization and the quality of cement. Through the establishment of demonstration projects and media publicity, we can actively promote the collaborative disposal of solid waste in cement kilns, improve the public's awareness of it and promote its healthy development.
2. Collaborative disposal and utilization of concrete industry
At present, more than 80% of the structural volume of materials used in buildings, roads, pipelines, bridges, high-speed railways, subways and airports are concrete. The concrete industry should mainly utilize solid waste from three aspects: aggregate, admixture and cement containing about 30% of industrial solid waste. Taking ready mixed concrete as an example, every 1m3 of ready mixed concrete weighs about 2400KG, most of which contain about 500kg of construction waste and 80kg of fly ash. At present, waste glass, waste rubber powder, steel slag, some tailings, slag, incineration ash and waste rock have been proved to be applicable to concrete. China produces about 10.4 million tons of waste glass every year, accounting for about 5% of the total solid waste. Waste glass is mainly mixed into concrete in two ways: the first is to replace some coarse or fine aggregates, and the second is to grind the glass into powder to replace cement. No matter which method is used to mix concrete, it may face alkali silica reaction (ASR) expansion. The problem of ASR expansion is the main research direction at present. The research shows that the strength of rubber coating and sound insulation layer is not expected to exceed 10% of the requirements for road deck. At the same time, adding waste rubber powder into concrete may greatly improve shrinkage resistance, toughness, impact resistance and frost resistance. At present, the key of applying waste rubber powder to concrete lies in its modification technology and cost control. The main applications of steel slag are: coarse steel slag instead of coarse aggregate, coarse grinding steel slag instead of fine aggregate, using steel slag to produce cement and steel slag as admixture. Compared with ordinary concrete, steel slag concrete has the advantages of high splitting tensile strength and elastic modulus, but its dry shrinkage performance is low. The test shows that the better mechanical properties of steel slag concrete enable it to achieve the expected effect in the application of road engineering. However, there are still some problems to be further studied in the application of steel slag in concrete, such as the ratio of steel slag to replace coarse aggregate, fine aggregate and cement, which example should be used to give full play to the maximum efficiency of steel slag and make the concrete have the best performance. In addition, other industrial solid wastes used in concrete include waste rock, coarse-grained tailings, waste concrete slag, etc. as aggregates. Fly ash, slag, phosphorus slag, metakaolin slag, silica fume, diatomite, zeolite powder, coal gangue, lithium slag, manganese slag, copper slag, waste incineration ash, sludge, rice husk ash, red mud, coal slag, tuff, pumice powder, etc. containing a large amount of amorphous silica and aluminum oxide, or high activity calcium oxide can be used as active mineral admixture to reduce the use of Portland cement in concrete and increase plasticity Reduce the peak value of hydration heat of cement and improve the compactness and durability of concrete. Prepare special concrete, such as preparing light concrete with pumice powder, tuff powder, fly ash ceramsite, coal gangue powder and slag, preparing concrete with sound insulation, heat insulation, shock absorption and crack resistance with waste rubber tire as one of the aggregates, and preparing concrete with crack resistance and bending resistance with waste plastics, polyester, steel and other fibers. In the future, relevant departments will strengthen research, formulate relevant standards and other technical documents for the preparation of concrete by various solid wastes, so as to guide and standardize the application of solid wastes in the concrete industry, and constantly explore solid waste concrete materials with wider application types of solid wastes, excellent product performance and appropriate price.
3. Collaborative disposal and utilization of ceramsite industry
The main components of ceramsite raw materials are silicon dioxide and aluminum oxide. The main components of some solid wastes such as fly ash and sludge are consistent with them, and the particle size is small. Using them as ceramsite raw materials can greatly reduce the crushing and grinding cost, absorb them at the same time, and obtain considerable economic and social benefits. Ceramsite raw materials are composed of two parts, namely, ceramic matrix and admixture. Ceramic matrix gradually develops from clay, shale and other non renewable resources to solid waste. Fly ash, sludge, red mud, tailings and waste rocks, construction waste and so on can be used as ceramic matrix. In general, the composition of solid waste that can be used as ceramsite raw materials is different, but it is mainly silica and aluminum oxide, and contains calcium, iron, potassium, sodium, magnesium and a small amount of heavy metals. Different solid wastes need to be mixed with other powders to achieve appropriate composition in order to prepare ceramsite with excellent performance. The composition of fly ash is very consistent with that of ceramsite raw materials, including 52% ~ 65% silicon dioxide, 16% ~ 36% aluminum oxide, iron oxide, calcium oxide, sodium oxide, magnesium oxide, titanium oxide, etc. Therefore, only a small amount of fly ash and waste residue need to be added in the preparation process of fly ash ceramsite; However, for other types of solid waste, due to the poor consistency between the composition and ceramsite raw materials, clay, shale, fly ash and other non renewable or expensive raw materials are mostly added to make ceramsite. Even the addition of clay in some solid waste ceramsite exceeds 50%, which greatly increases the consumption of non renewable resources such as clay and shale, which violates the requirements of green industry and sustainable development to some extent. At present, scholars only use a variety of solid wastes to obtain ceramsite with good performance. In addition, the density of ceramsite prepared by solid waste is generally as high as 600 ~ 700 or even higher, and there are few ultra-light ceramsites less than 500. Therefore, the application and sales channels of ceramsite in the direction of light materials are limited. At the same time, the price of high-density ceramsite is cheap and the added value of products is low. In recent years, the development of ceramsite raw materials in China has shown the following trend. The proportion of solid waste ceramsite in ceramsite raw materials has increased significantly, among which the market share of construction waste clay ceramsite and sludge ceramsite has increased significantly. The survey data show that 72% of the enterprises in the survey produce ceramsite with construction waste soil as the main raw material, and 52% of the enterprises in the survey have added 25% ~ 30% of the sludge into the ceramsite raw materials. In addition, some enterprises began to try to add organic matter such as crop straw, rice husk and bran to make ceramsite, which not only reduced energy consumption and cost, but also consumed agricultural waste. At the same time, there are only a few ceramsite enterprises with single clay as raw material. Although shale ceramsite has been developed, the proportion of shale ceramsite in the national ceramsite production has been reduced due to the addition of construction waste clay ceramsite and sludge ceramsite. The performance limitations of fly ash ceramsite have led to its production reduction, and coal gangue ceramsite has begun to emerge. In the future, the raw material structure of ceramsite will be more inclined to solid waste, and sludge ceramsite, coal gangue ceramsite, tailings and waste rock ceramsite will gradually become the mainstream products; Production lines and equipment with accurate design, high production efficiency, friendly environment, low energy consumption and diversified products will be gradually completed, put into operation and used; The driving force of the policy should be to strengthen the quality inspection of ceramsite products and reasonably optimize the market scheduling, encourage and support the use of ceramsite in different industries, such as the use of ceramsite in the construction industry to replace or partially replace the traditional aggregate, and the application of ceramsite in the fields of petrochemical industry, food safety, environmental greening and so on; Develop high value-added applications of ceramsite, such as surface modification, chemical treatment, coating and other secondary processing, so that it has unique surface structure, special adsorption and other new properties, so as to increase the sales price of ceramsite.
4. Collaborative disposal and utilization of iron and steel industry
China's iron and steel industry covers major processes such as coking, sintering (pelletizing), blast furnace and converter. It has the characteristics of long process and large amount of solid waste and pollutants. In 2018, China's crude steel output accounted for 53.3% of the world, reaching 996.3 million tons, which means that half of the left and right iron and steel products, by-products and solid wastes of global iron and steel enterprises will be produced in China. China's iron and steel enterprises practice the concept of clean production and green development. By combining the solid waste generated in the enterprise with raw materials and using their own smelting kilns to incinerate and pyrolyze the solid waste, they have realized the "non delivery of solid waste" and the reduction, non harm and resource utilization of solid waste. Compared with solid waste incinerators and cement kilns, smelting kilns in iron and steel enterprises also have the process characteristics of collaborative disposal and utilization of solid waste, such as high temperature of 1200 ~ 1700 ℃, reduction (blast furnace) oxidation (converter) atmosphere, high alkaline environment and complete environmental protection facilities. Compared with the outsourcing disposal and utilization, the cost of collaborative disposal and utilization of iron and steel enterprises is more than 1 / 2 lower than the market price, and the disposal and utilization has many types, large scale, strong applicability, safe production, controllable environmental risks and up to standard pollutant discharge. For example, coke ash is suitable for blast furnace injection, converter dust removal to prepare iron pigment, and iron containing dust is recycled to sintering ore preparation for re sintering. At present, many projects for collaborative disposal of solid waste in steel smelting furnaces and kilns have been approved or implemented. For example, the project of industrial solid waste collaborative disposal in steel smelting furnaces and kilns of Shaoguan Iron and Steel Co., Ltd. approved in 2020 has a total investment of 88.8317 million yuan, and the construction scale is 180000 T / a of hazardous waste collection, storage, collaborative disposal and utilization, including 20000 t / a of waste activated carbon, 110000 T / a of steel plant soot and 50000 T / a of waste iron ladle. Although China has carried out a lot of work and achieved some results in the collaborative disposal of solid waste in iron and steel smelting furnaces and kilns, there are still the following problems. There is a lack of standards, technical specifications and detailed rules such as the standard for pollutant control of solid waste collaborative disposal in cement kilns (GB 30485-2013), the technical code for environmental protection of solid waste collaborative disposal in cement kilns (HJ 662-2013) and the guide for business license review of hazardous waste collaborative disposal in cement kilns, which makes it impossible for iron and steel enterprises in various regions to rely on how to legally and legally dispose and utilize internally generated solid waste, It also leads to different progress in the reduction, harmlessness and recycling of internal solid waste by iron and steel enterprises in various provinces and cities. In order to deal with the above problems, relevant government departments should issue and implement relevant policies, laws and regulations on the coordinated disposal of solid waste in iron and steel smelting furnaces and kilns, so as to standardize and guide the legal and compliant disposal and utilization of solid waste by iron and steel enterprises, and promote enterprises to take the road of green, circular and low-carbon development. In addition, government departments should carry out pilot demonstration of collaborative disposal of solid waste in steel smelting kilns and gradually promote it in the industry.
5 conclusions and Prospects
As a typical problem, pollution and resource waste caused by solid waste have attracted great attention from all walks of life. At present, cement, concrete, ceramsite, steel and other industries have been widely studied and applied in the collaborative disposal and utilization of various solid wastes by using the characteristics of their respective equipment, processes and materials. However, various industries may face problems such as lack of top-level design such as construction planning and incentive policies for collaborative disposal and utilization of solid waste, lack of relevant standards and other technical documents for collaborative disposal and utilization of solid waste in different industries, the addition of solid waste affects the output and quality of equipment and products, uncertain product environmental risks, and the market share of products made of solid waste needs to be improved. In the future, it is necessary to improve the relevant laws, regulations, policies and standards for the collaborative disposal and utilization of solid waste by various industries to standardize, guide and encourage enterprises to carry out the collaborative disposal and utilization of solid waste; Increase research efforts to improve the quality of solid waste preparation products, reduce product preparation costs and increase product market share; Carry out pilot demonstration of collaborative disposal and utilization of solid waste in various industries and gradually promote it.
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