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Analysis on the by-pass ventilation technology of cement kil

  Abstract: the technology of coordinated disposal of municipal solid waste (MSW) in cement kilns can not only take MSW as raw material and fuel, but also reduce the consumption of resources.

  However, alkali, chlorine and sulfur compounds brought by waste will circulate and accumulate in the kiln system, causing crust blockage.

  In order to solve this problem, the bypass ventilation technology should be adopted.

  Based on a plant bypass snort technology as an example, analyzes its bypass snort collect primary secondary dedusting system kiln ash composition, puts forward the primary problems of dust removal must be controlled in 500 ℃ or so, can prevent the chloride ion in the kiln ash back to kiln system.

  Finally, the above viewpoints are proved by pilot test.

  0 overview

  At present, China's urban per capita household garbage production is about 1~2kg/d, and the annual rate of 8%~10% is increasing rapidly year by year.

  Therefore, China has a large amount of waste, so it is urgent to take reasonable and effective measures to deal with it.

  China's cement kilns are characterized by high temperature, high heat capacity and thermal inertia, long residence time in the high-temperature logistics area, and thorough decomposition of harmful components, which are suitable for the technical requirements of harmless disposal of wastes.

  Therefore, the coordinated disposal of waste by cement kilns is an important way to harmless, reduce and recycle hazardous waste and municipal solid waste [1].

  According to incomplete statistics, by the end of 2018, about 160 cement kiln collaborative disposal production lines have been put into operation, with an annual waste disposal capacity of 15.66 million tons.

  Among them, cement kiln co-disposes more than 57 lines of household garbage, with an annual consumption of about 6.77 million tons of household garbage.

  More than 41 lines were put into operation for sludge disposal in cement kilns, with an annual consumption of about 3.57 million tons of sludge.

  1 impact analysis of collaborative disposal of household waste by cement kilns

  Using cement kiln collaborative disposal of urban living garbage is garbage can be as the original, fuel, reduce the consumption of resources, but also make full use of the cement rotary kiln in alkaline fine thick solid phase of the advantages of high temperature combustion environment, will completely get rid of harmful substances, truly realize garbage disposal "harmless and resource recovery, intensive" the diversity of goals and objectives, but the collaborative disposal of urban living garbage can produce hydrogen chloride, sulfur dioxide, nitrogen oxides, dioxins and heavy metals, etc., which is the most important and serious alkali, chlorine, sulfur content increase [2].

  Alkali, chlorine and sulfur compounds brought by waste will be circulated and enriched in the kiln system (rotary kiln and preheater), which is prone to clogging in the kiln tail smoke chamber, decomposing furnace outlet and C2 preheater. In the kiln, the long and thick kiln skin and rear ring will be easy to form core material and anise-ball, and grate cooler makes snowman, etc. [3].

  We selected 5000t/d scale cement production line with 260 tons of daily domestic garbage treatment to analyze and analyze the skinning composition, and found that the skinning is the easiest at the calciner outlet, and the main skinning problem is that SO3 and Cl- are concentrated here.

  Bypass venting technique

  From the above analysis, it can be seen that the main factors affecting the stable operation of cement kilns are the concentration of chlorine and sulfur. In order to avoid the cyclic concentration of chlorine and sulfur in the kiln system, destroy their circulation in the system, and effectively solve the clogging of crust in the cement kiln system, cement enterprises have adopted the kiln tail bypass ventilation technology to alleviate this problem [4].

  In the by-pass ventilation system, the flue gas passing through the dedusting system after cooling with an admixture of cool air is generally discharged to the standard after being incorporated into the inlet of the kiln tail dust remover.

  Kiln ashes collected through the dust removal system have a high concentration of chloride ions and cannot be returned to the kiln.

  However, if the chlorine ion brought by household garbage is too high and the kiln ash is mixed with the mixed material, the chlorine ion in cement will also exceed the standard and affect the cement quality.

  Therefore, the Japanese bypass ventilation technology adopts two-stage dust removal treatment. The first stage cyclone collects about 70% of the dust in the bypass ventilation. At this time, chloride ions are in the gaseous state and not attached to the dust.

  Therefore, the kiln ash collected by the first stage cyclone can be used back in the kiln to reduce the loss of materials and energy.

  At this time, with the decrease of temperature, most of the chloride ions have been attached to the kiln ashes. The secondary cyclone traps about 30% of the remaining dust and 80% of the chlorine in the by-pass ventilation.

  This part of the kiln ash is less, and contains a large number of chloride ions, dioxins and heavy metals, Japanese enterprises will treat it as hazardous waste.

  The specific flow chart is shown in figure 1.

   A cement enterprise with a daily output of 5000 tons of cement kiln and coordinated disposal of 260t/d household garbage adopted this technology.

  We analyzed the kiln ash of the first-stage cyclone barrel and second-stage cyclone barrel in this system, and the specific data are shown in table 3.

  As can be seen from table 3, the content of chloride ions collected from the first-stage cyclone tube is still very high, accounting for about 70% of the chloride ions in the bypass vent. If this part of kiln ashes enters into the kiln system, the chloride ions released just now will be brought into the original system for enrichment, which will seriously affect the firing of the kiln system.

  Therefore, the first stage cyclone kiln ash can only be mixed as cement.

  Kiln ash system level of cyclone tube to collect down above high chloride ion content is the main problem of cyclone tube inlet temperature below 300 ℃, most of the chloride ion has been attached in the dust, so the high content of chlorine ion of the kiln ash.

  In order to solve this problem, we established a set of pilot equipment and studied it.

  Pilot test of bypass ventilation system

  The pilot test of main technological process is: bypass snort (about 1000-1100 ℃) by the high temperature flue gas quenching machine cooled to 450-500 ℃, into the primary cyclone cylinder preliminary to remove particulate matter, and then mixed with cold air flue, after the flue gas temperature dropped below 250 ℃, into the synthesis reactor, removal of dust and other harmful substances, the preheater precipitator of induced draft fan into the entrance.

  Chlorine ion in 450 ℃ when the bypass flue gas is not stained with dust, so the level of cyclone tube to collect the dust under the chlorine content is less, can return to kiln system.

  After the flue gas temperature drops to 250 degrees Celsius, a large number of chloride ions are completely attached to the dust and collected by the comprehensive reactor. The dust in this part contains not only a large number of chloride ions, but also dioxins and a large number of heavy metals. Therefore, the dust in this part is treated as hazardous waste.

  We analyzed the first-stage cyclone in the pilot test and the kiln ash in the comprehensive reactor, and the data were shown in table 4.

  As can be seen from the above table, the content of chlorine ion collected by the first-stage cyclone is only 1.87%. The part of the kiln ash back to the kiln system will bring 27.64% of the released air back to the kiln, which has little impact on the kiln.

  When the integrated processor is used as the dust collector, the removed kiln ash contains 19.58% chlorine and 2.77ngTED/kg dioxin, which must be treated as hazardous waste.

  4 conclusion

  Most of the production lines for collaborative disposal of domestic garbage in cement kilns are equipped with by-pass ventilation system. If the by-pass ventilation system is properly designed, the disposal capacity of domestic garbage can be greatly increased.

  Chlorine ions from domestic garbage have a great influence on the kiln condition of cement kiln system. It is necessary to prevent the kiln ash in the by-pass ventilation system from bringing chlorine ions back to the kiln system.

  The kiln ash collected by the secondary dust removal system contains a large number of chloride ions and dioxins, so it can only be considered as a mixed material after inspection and evaluation.

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