Simplified description of the status quo and development trend of sewage sludge thermal chemical treatment methods. The sludge incineration treatment and sludge pyrolysis treatment were analyzed and compared.
With the popularization of urban sewage treatment, sludge from urban sewage plants will increase significantly. If these sludges are directly discharged without disposal, serious pollution will occur. At present, there are many kinds of sludge treatment methods, but most of them have some disadvantages, such as incomplete treatment, easy to produce new secondary pollution and so on. Sludge thermochemical treatment method (sludge heating and chemical reaction to stabilize, reduce the volume of the method) has a good sterilization effect, rapid processing, relatively small footprint, after the disposal of sludge is stable and can be used to implement its contained organic matter Energy recycling and other advantages. It can achieve the purpose of reducing, harmless, and recycling sludge disposal. Therefore, it is considered to be a promising sludge treatment method and has received extensive attention.
Sludge thermochemical methods can be divided into: incineration; low temperature pyrolysis.
1 Sludge incineration
Sludge incineration involves placing the sludge in an incinerator and incinerating it with excess air added. After the incineration, the moisture content of the final sludge is 0, in which PAHs are no longer present, and the content of other organic pollutants is almost 0 (heavy metal ions cannot be effectively removed and deposited in coal ash), and their volume is greatly reduced. The ultimate disposal of sludge is extremely convenient.
1.1 Incineration process and elements
a. The moisture content of the sludge is a key factor in sludge incineration. It directly affects the sludge incineration equipment and disposal costs (when the sludge volatile content is high and the moisture content is low, it usually maintains spontaneous combustion). Therefore, reducing the moisture content of the sludge is crucial for reducing the sludge incineration equipment and disposal costs. Generally, if the ratio of the sludge moisture content to the volatile matter content is less than 3.5, spontaneous combustion can be formed and the fuel can be saved. b. Temperature, time, oxygen content, and volatile content are also basic conditions for the success of incineration. Organics can be burned when the temperature exceeds 800°C. However, the sludge combustion will generate a lot of odorous stimulating gases. To eliminate such gases, the temperature is generally raised to 1000 °C or additional secondary combustion equipment. The longer the burning time, the more thorough the burning. But it will increase energy consumption, so the general sludge burning time is 0.5 ~ 1.5h. Oxygen is the driving force for incineration and is usually supplied by air (insufficient air, inadequate combustion, excessive air, heated air can consume too much heat, and is not suitable). Generally use 50% to 100% excess air. The volatile content of sludge generally reflects the amount of potential heat contained in the sludge. This part of the heat can be used for sludge incineration, but it is often not enough to support combustion, so it needs to be supplemented with heat from the outside world. c. Sludge pretreatment. Sludge is pre-treated before it enters the incinerator, allowing the incineration to proceed more efficiently. The sludge can be crushed and the sludge in the furnace can be evenly distributed to ensure sufficient combustion. Sludge preheating can reduce the moisture content and reduce the energy consumption when the sludge is burned. This procedure should be based on incinerator type, setting or exemption.
1.2 Equipment
a. There are various models of sludge incineration equipment, the most common of which is a vertical multi-stage furnace. This furnace operates in counter-current mode and has high thermal efficiency. The reason why it is not like grate burning in general urban garbage is mainly because the sludge wastes are generally very viscous, easy to form into cakes after ignition or surface ash cover on the appearance of the burning material, so that the flame is extinguished. It is necessary to constantly stir and repeatedly update the combustion surface so that the sludge can be fully oxidized. In each section of the furnace, a stirring surface is provided, and the residence time of the material in the furnace is also long, so that the sludge can be completely burned [4]. In order to ensure the smooth progress of the process, in addition to incinerators need to add a sludge device (with a grinder), multi-point blast system, heat recovery device plus (when setting up secondary combustion equipment, especially pay attention to this point), auxiliary heat source (Start burners) and auxiliary equipment such as ash removal equipment. The problems with multi-stage incinerators are mainly mechanical equipment, which requires more repairs and maintenance. The energy consumption is relatively high and the thermal efficiency is low. In order to reduce the smoke pollution caused by combustion, additional secondary combustion equipment is required.
In addition, there are more rotary kiln (rotary) furnaces and fluidized bed furnaces. The processing effect is not much different from that of the multi-stage furnace.
b. Flue gas treatment system. Sludge incineration generates a large amount of flue gas with fly ash. These flue gases contain a variety of toxic substances, such as dioxins, methyl mercaptan, SOx, etc., which cause secondary pollution. The flue gas treatment process is complex, technically difficult, and expensive to handle. In addition, some potential hazards of exhaust gas are still being further recognized by environmental protection workers, such as dioxins. Therefore, the treatment of smoke must not be taken lightly.
According to available data, the vast majority of foreign sludge incineration equipment has been transformed from a combination of electrostatic precipitators and dry scrubbing methods in the past into a combination of high-performance electrostatic precipitators and wet scrubbing equipment and denitrification equipment. method. A few new plants have adopted bag filter-type dust removal equipment combined with other equipment to remove toxic substances such as dioxins and furans. For example, a set of sludge incineration equipment built in the United States in 1991 used dry scrubbers, slaked lime spray, and bag filter dust collectors to effectively remove dioxins. Kawasaki Heavy Industries of Japan uses a wet scrubber, a bag filter, and a denitration reaction tower. At present, in addition to the use of bag filter type dust collectors in the world, it is also widely used to solve the problem by improving the combustion state of incinerators. That is, keeping the high temperature, keeping the burning time, and completely burning the sludge. The waste gas treatment process of sludge incineration needs to be further improved, and there is still great potential to be tapped. After sludge was incinerated, the volume was greatly reduced. For example, Aypernon Wastewater Treatment Plant had 9443m3 digester sludge per day, 7930m3 after digestion, 1160m3 after dehydration, and a volume of 99m3 after incineration, which was 1.05% of the mud volume. The burden of final disposal of sludge is greatly reduced. Slag sludge incineration ash can be disposed of using a safe landfill method. The main disadvantage of sludge incineration is that it consumes too much energy. For example, Japanese sludge incineration consumes 70% of the energy consumed by sludge disposal. Each year, it consumes 3.9×105m3 of heavy oil, and the incineration equipment is complicated. The construction and operation costs are higher than the normal sludge treatment methods. Therefore, in addition to a few specialized systems in China, no sludge incineration facility has been put into use.
2 sludge thermal decomposition
Sludge thermal decomposition is an emerging sludge heat treatment process. Sludge is heated to a certain temperature (high temperature: 500-1000°C, low temperature: <500°C) under conditions of no oxygen or less than theoretical oxygen, to decompose solid matter into oil, non-condensable gas and charcoal. Combustibles. Some of the products are used as energy for pre-drying and pyrolysis, and the remaining energy is recovered. Due to high energy consumption due to high-temperature thermal decomposition, the current research focuses on low-temperature thermal decomposition.
2.1 Process and Products
Researchers at Goettingen University in Germany have created a batch-type reaction process that converts sludge into liquid and solid fuels [9]. In this process, dry sludge is heated to 300-350° C. for approximately 30 minutes in an oxygen-free environment. Canadian environmental researchers made further studies. After testing the sludge from 18 different wastewater plants in the United States, Canada, and the United Kingdom, HW Campbell and TR Brille obtained the data.
Under the same conditions, the pyrolysis of different types of sludge results in different product rates. The yield of raw sludge oil was significantly higher than that of digested sludge. In the pyrolysis product, the calorific value of the non-condensable gas is very low and the yield is not high, but it has a very strong odor. Including CO, H2S, CH4, methyl mercaptan, dimethyl sulfide, methyl sulfide, ammonia and so on. This type of gas is a flammable gas that can be deodorized by combustion. The heat generated is used as a supplemental energy source, but additional equipment is needed.
The high calorific value of oil production can be stored as energy after it is collected. According to the author's test, the oil appears in the vapor phase during the pyrolysis process, and is separated by oil-water condensation. After cooling, the oil appears tan, tar-like, sticky, smelly, and can be ignited by naked flames. The properties are stable. According to Shanghai Tongji University and HW Campbell, when the sludge temperature is below 450°C, the temperature rises and the oil production rate increases. In addition, sludge with a lighter degree of microbial treatment has a higher content of organic matter and a higher oil production rate. The product calorific value is basically inversely proportional to the reaction temperature. The char produced by the pyrolysis of sludge is a matt porous black block. The volume of carbon is approximately 1/3 of the original sludge volume. The sludge carbon yield decreased with increasing temperature. In order to obtain a higher yield of charcoal, the pyrolysis temperature is controlled below 300°C, and sludge carbon with better combustion performance can be obtained. Sludge and charcoal are stable in nature and can be used for coal blending or direct pyrolysis to supplement energy sources [10]. Sludge pyrolysis, condensation of water, contains a lot of impurities, need to be re-treated before discharge.
Titanium Dioxide For Coating,Titanium Dioxide Used For,Titanium Dioxide Used As,Titanium Dioxide Uses For Skin
Guangdong Jintai Titanium Industry Co.,ltd , https://www.pzhtio2.com