Summary of waste treatment technology in 1 cement kiln system
1. 1 Significance of waste treatment technology in cement kiln system
Co-disposal technology of waste in cement kiln is actually one of waste incineration technologies (mechanical grate furnace technology, fluidized bed technology, pyrolysis gasification technology and co-disposal technology in cement kiln), which combines the process of producing cement clinker in cement kiln with the process of solid waste treatment, so that their advantages complement each other and improve cement production efficiency and waste treatment effect. Different from the traditional incineration treatment, the cement kiln co-disposal technology can also fix the refractory substances in the waste into the produced cement on the basis of utilizing the calorific value of the waste, so as to reduce the possibility of these substances leaking out of the environment [1].
1.2 Advantages of cement kiln system in waste treatment
1.2. 1 The cement kiln has a high production temperature, which can completely decompose the harmful components in the waste. When waste is treated in cement kiln, the solid temperature in the kiln is about 1450℃, and the temperature of waste gas in the kiln can be raised to about 1800℃ through combustion. The residence time of high temperature gas in cement kiln is longer than 8s, and the time when the temperature is higher than 1 100℃ is about 4s. Incinerated garbage stays in cement kiln for a long time, which can basically be maintained at about 20~30min. Due to the input of combustion-supporting gas, the whole combustion garbage is in a fluidized state, and oxygen is sufficient, which is helpful for the full reaction of harmful substances. According to calculation, the removal rate of harmful substances can reach 99.999%[2].
1.2.2 There is an alkaline atmosphere in the kiln, which can inhibit the generation of dioxins. In the process of production and operation, cement kiln will produce more calcium oxide with smaller particle size and uniform distribution. These calcium oxides can not only adsorb some acidic substances, but also fully react with them, forming a good solid alkaline environment in the kiln. This characteristic of cement kiln can effectively inhibit the production of "dioxins". According to statistics, the cement kiln co-processing waste system has a large heat capacity and a stable high-temperature environment. The subsequent devices can effectively reduce the waste temperature, so that the dioxin content in the waste gas is far below the relevant standards.
1.2.3 cement kiln incineration adopts closed-circuit production measures, and no new wastes are generated. The cement kiln collaborative waste disposal system can effectively reduce the production of solid waste residue, and at the same time, the content of pollutants in the waste gas is low, which makes the secondary pollutants generated in the treatment process less and cannot be leaked to the environment. In addition, the waste gas generated by the waste treatment system of cement kiln currently used needs to be treated at high temperature after dust is collected by the dust removal device, which reduces the discharge of waste gas, leachate, waste residue and other substances and will not generate new waste [3].
1.2.4 absorption of heavy metals by clinker in cement production According to the above contents, it can be seen that the production process of cement clinker can effectively solidify heavy metals in waste, and at the same time, cement has a certain barrier effect, which can effectively reduce the pollution caused by heavy metals and organic substances entering the environment.
1.3 Applicability of waste treatment technology in cement kiln system in China Although China is gradually strengthening urbanization, there are a large number of towns and villages at present, so the reasonable design of solid waste treatment methods should be considered from the perspective of this kind of things. The waste treatment capacity of cement kiln system is about 100t/d, and the types of waste treated are mostly domestic garbage, sludge, kitchen waste and even hazardous waste with high calorific value. The construction area is small, the required investment amount is low, and the equipment used does not need to be transported or transported for a long distance, which is friendly to villages and towns and convenient for formal use.
Engineering application of garbage disposal technology in cement kiln system
2. 1 Pretreatment of waste disposal
In order to improve the efficiency and safety of garbage disposal in cement kiln system as much as possible, primary domestic garbage needs pretreatment and pre-incineration before entering the kiln for incineration. 2. 1. 1 Garbage pretreatment is mainly divided into two ways: making RDF fuel, crushing+biological drying+mechanical dehydration. RDF is a kind of garbage-derived fuel, and its main source is domestic garbage. After extracting its high calorific value, it can be used as fuel. The treatment process includes crushing, reducing the moisture in the garbage, and making the garbage produce liquid, gas or liquid with burning ability through pyrolysis or composting. The RDF mentioned in this paper is usually solid, and its calorific value is high, generally reaching 2000~4000kcal/kg. Its particle size is uniform, it can be stored with preservatives for a long time, and it is suitable for incineration in garbage incinerator. However, manufacturing RDF also has some disadvantages, such as complicated production process and incomplete disposal of waste. The pretreatment process of crushing+biological drying+mechanical dehydration is a pretreatment method of crushing the primary domestic garbage to a particle size below 200mm, then reducing the water content of the garbage to 40% through biological drying, and finally squeezing and dehydrating the garbage through a mechanical extruder to ensure the drying effect again, reduce the garbage volume and increase the garbage density. This pretreatment method is specially designed for co-processing in cement kiln, which combines the advantages of RDF and abandons the disadvantages of its preparation process. 2. 1.2 waste pre-incineration is mainly divided into two ways: gasifier pre-incineration and hot plate pre-incineration. Domestic garbage often contains many macromolecular organic compounds, which can be cracked under the condition of isolating oxygen and applying high temperature to generate small molecular gaseous organic compounds or other combustible gases. If you want to crack the domestic waste, you need to ensure that the organic matter in the domestic waste has strong thermal instability, and secondly, you need to ensure that the domestic waste has high calorific value. The pyrolysis gasification technology of co-disposal in cement kiln is to convert garbage into combustible gas and ash by pyrolysis gasifier, and let combustible gas enter cement kiln for incineration to replace part of pulverized coal fuel; Cement raw meal is partially mixed with ash as raw material for cement production. This method can effectively reduce the treatment efficiency of some high-ignition substances in domestic garbage and reduce the treatment load of cement kiln system. The main technical feature of the hot plate furnace is that it is in an "online" relationship with the cement kiln, and it is a waste combustion device directly connected with the lower cone of the calciner. The equipment structure and process arrangement are characterized in that a large-diameter turntable rotating slowly and horizontally is embedded in the cone of the calciner above the smoke chamber at the kiln end, and the high-temperature tertiary air enters the hot plate furnace in whole or in part, so that the gas with the temperature above 1000℃ generated by garbage combustion in it enters the calciner.
2.2 Dry cement kiln incineration process
The new dry cement kiln co-disposal is to use cement rotary kiln equipment to burn waste and produce clinker at the same time. While inheriting the advantages of the traditional garbage incinerator, it organically exerts its own advantages such as high temperature and circulation. It can not only make full use of the calorific value of organic components in waste to realize energy saving, but also make full use of inorganic components in waste as raw materials to produce clinker; It can not only completely burn the toxic and harmful organic matter in the garbage under the high temperature environment of the new dry rotary kiln, but also fix the toxic and harmful heavy metals in the garbage into the clinker. At the same time, it can also treat the odor and sewage concentrated solution produced in the process of garbage disposal, which can effectively prevent secondary pollution. By adopting the pre-incineration technology of hot plate furnace, the whole combustion process of solid waste takes place in the hot plate furnace, which minimizes the influence of waste combustion on the decomposing furnace, increases the residence time of solid waste in the furnace for 5 ~ 45 minutes, and enables the solid waste to be completely decomposed at the high temperature of 1045℃. There are three air lock valves on the feed pipeline of the hot plate furnace to prevent air leakage. When the system is powered off or manually operated, the safety gate valve below the gas gate valve will be closed immediately. The angles of all discharge valves are greater than 55 to prevent garbage accumulation. All distribution valves and gate valves are equipped with refractory materials. Waste needs to enter the hot plate furnace first and be burned in the furnace. Small particles such as waste residue and waste gas produced by combustion will be sent to the calciner together, and will be decomposed by the heat carried in the waste gas and the high temperature applied by the calciner, while large particles will be sent to the rotary kiln for high temperature calcination to form cement clinker. In addition to the incineration system, the cement kiln system needs to be equipped with a bypass windproof system for waste treatment. Bypass ventilation system aims to remove harmful components such as potassium, sodium, chlorine and sulfur that enter the cement production system in the process of treating urban and rural domestic garbage through raw materials, fuels or co-disposal. The high-temperature gas is pumped out of the system, cooled and separated into coarse particles, which are sent back to the cyclone for further treatment to reduce the emission of metal ion particles and prevent excessive dust from entering the atmosphere.
3 Conclusion
Based on the process technology and engineering scheme of cooperative disposal of domestic waste in new dry-process cement kiln, the comparison between cooperative disposal of domestic waste in cement kiln and traditional disposal methods, the comparison and selection of various cooperative disposal process schemes, and the overall process design were carried out, which provided technical support and reference for the process technology optimization of cooperative disposal of domestic waste in new dry-process cement kiln and the application and popularization of demonstration projects.
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