1. Basic concept of hazardous waste
Hazardous waste is a kind of special solid waste, which has some characteristics of corrosiveness, toxicity, reactivity, flammability and radioactivity. According to the solid waste law of the people’s Republic of China, hazardous waste is a substance listed in the list of hazardous waste implemented by the state on August 1, 2016, or determined by relevant national departments to meet the relevant standards of hazardous waste.
1) Pollution of the atmospheric environment. There may be fine particles and dust in the hazardous waste piled up for a long time, which will drift along with the wind direction and enter the atmosphere, or drift towards the more hazardous waste in the atmosphere, and chemical reactions may occur, causing secondary pollutants and endangering human health. In addition, different degrees of odor pollution may be produced in the accumulated waste.
2) Impact on soil environment. First of all, the random stacking of hazardous waste will occupy a lot of land, destroy vegetation and change the structure of soil properties. In addition, the soil is the place where the micro organism lives, and the environment around the microbial tablet constitutes a complete ecosystem. When the hazardous waste is not treated properly, the heavy metal ions contained in the waste and some of the refractory soil will cause the physiological activities of microorganisms to be affected to death, and eventually destroy the ecological purification capacity of the soil, and the soil will be polluted, with a huge cost and a long time for recovery. Ring. After the hazardous wastes are piled up in disorder, the hazardous substances in the wastes will migrate into rivers and lakes with the precipitation and wind, resulting in no survival of aquatic organisms, and return to the source, endangering human health. During unreasonable storage and filling, the leachate will permeate the soil and enter the groundwater, causing groundwater pollution.
2. Hazardous waste incineration technology
Incineration is a high-temperature treatment technology, which can reduce the volume of hazardous waste after high-temperature incineration, and reduce or remove harmful substances at the same time. The high-temperature flue gas generated by incineration is used for heating and power generation and energy recovery through waste heat boiler steam. The process of hazardous waste incineration and the structure of incinerator are mainly related to the types, properties and combustion characteristics of waste. Furnace type mainly includes grate incinerator, fluidized bed incinerator, vertical incinerator and rotary kiln. Grate incinerator is mainly used for the incineration of domestic waste. The fluidized bed incinerator is mainly used in the vertical incinerator, which is mainly used in the direct combustion of liquid and gas. Rotary kiln is mainly used for the incineration of hazardous wastes and medical wastes, and can handle solid wastes, liquid wastes and gas wastes. The raw materials of hazardous waste in this project include waste liquid, viscous waste liquid and solid waste. It is suitable to select the rotary kiln + second combustion chamber incineration process waste with wide adaptability, mature technology and stable operation for incineration. Hazardous waste incineration process includes pretreatment and feeding system, incineration system, waste heat utilization system, flue gas quench system and flue gas purification system.
3 Existing problems and treatment in the design of hazardous waste treatment
3.1 pretreatment system
Because some inflammable substances are contained in hazardous waste, in order to prevent fire, explosion and other conditions in the process of hazardous waste crushing, the crusher system is equipped with the oxygen content detection function before and after the N2 injection oxygen control system. Once the oxygen content is higher than 6%, the system will automatically close the upper and lower hydraulic gates, and start the button system to complete the functions of N2 injection, oxygen detection, discharge, etc. Reliable operation, crusher system inlet and outlet set hydraulic gate, double protection.
3.2 feed sealing and treatment
The feeding system of kiln head is easy to cause the tempering of kiln head, so it is necessary to design double hydraulic flap door to ensure the safety of operation. When feeding, when the level of the two-stage hydraulic valve does not reach the set value, the upper hydraulic valve will open automatically. After the blanking, the upper hydraulic valve will close, the lower hydraulic valve will open, and the solid waste will enter the rotary kiln through the chute. The intermediate hopper between the feed chute and the two hydraulic doors is cooled by soft water. The whole feeding process operates in sealed and cooled state to ensure safe operation.
3.3 coking problem and treatment of rotary kiln
In the past project operation, coking often occurs at the end of rotary kiln. This is because the combustion machine of the rotary kiln is installed at the kiln head, and the combustion temperature is very high. When the feed contains waste materials with low melting point salts, the ash is easy to become molten state. When these ash are close to the kiln end, the combustion temperature is relatively low, and the molten ash is easy to bond on the refractory. The length of the rotary kiln designed in this project is 11m, which is also prone to coking. This design is directly optimized. The tail of rotary kiln is inserted into the second combustion chamber, sharing a slag outlet with the second combustion chamber, and the bottom of the second combustion chamber is provided with a large opening for slag (3300x800mm), which is convenient for large particle slag to fall. At the same time, a multi-component burner is set at the bottom of the second combustion chamber to ensure that the combustion temperature of the second combustion chamber is higher than 1100 ℃, and the temperature of the rotary kiln tail is raised at the same time. According to the operation parameters, the temperature of the kiln tail can reach more than 1000 ℃, so as to ensure that the molten slag directly falls into the bottom of the water sealed slag extractor. Before the waste is put into the furnace for incineration, the waste into the furnace shall be properly matched. In addition to the calorific value compatibility mentioned above, various waste materials shall be tested. Waste containing low melting point salts must be mixed before entering the furnace. During the operation, it is necessary for the operator to ensure the stable combustion of the rotary kiln and the second combustion chamber by the incineration temperature of the second combustion chamber of the rotary kiln and the reasonable ratio of the primary air and the secondary air. At the same time, operators should regularly patrol. Once coking phenomenon is observed from the kiln end sight glass, waste with high calorific value and without low melting point salt should be replaced for incineration, the temperature of kiln end should be increased, and coking and melting should be carried out slowly as far as possible. Both sides of the second combustion chamber shall be provided with openable observation holes. At present, after the successful commissioning of the project, there is no coking event at the kiln end within half a year of operation guarantee.
3.4 boiler ash corrosion problems and treatment
Ash deposition and corrosion chamber of heating surface of waste heat boiler are common problems in hazardous waste incineration. The common layout of waste heat boiler includes horizontal waste heat boiler and vertical waste heat boiler. Horizontal waste heat boiler is rarely used in hazardous waste combustion, mainly because the horizontal waste heat boiler is easy to accumulate ash and corrosion. This project adopts single drum natural circulation vertical water tube boiler. The boiler is composed of two radiation vertical flue pipes, surrounded by membrane walls, without convection evaporation surface and gas economizer, so as to prevent ash accumulation in the boiler. The way of soot blowing is steam soot blowing, which can effectively remove the accumulated dust. In the design, the flue gas temperature entering the superheater shall not exceed 650 ℃, and the wall temperature of the superheater shall be ensured to be lower than 450 ℃, so as to avoid high temperature chlorine corrosion of the superheater. At the same time, in the long-term operation, the temperature of the second combustion chamber should be controlled below 1200 ℃, so as to avoid the superheater corrosion caused by over temperature operation and the problem of nitrogen oxide exceeding the standard caused by high-temperature combustion.
3.5 corrosion of two stage wet deacidification system
In order to increase the contact area between the flue gas and the reaction alkali and prolong the reaction time of the flue gas and the alkali, the washing tower and the adsorption tower are all equipped with the multi-faceted hollow Bauer ring packing, which is made of polypropylene and has good acid corrosion resistance and alkali corrosion resistance. The washing tower and adsorption tower are made of corrosion-resistant FRP. As the adsorption tower of the washing tower is set before the induced draft fan, in order to ensure the strength of the washing tower and the adsorption tower, the carbon steel lined FRP is selected. The quench section of the upper part of the washing tower adopts regular ceramic packing, and the quench section adopts special high temperature resistant FRP material. All circulation pipelines are made of carbon steel lined FRP to ensure the stable operation of the system.