Kağıt sanayinde hava kirleticileri tür ve miktarları ve kontrol teknolojileri

Abstract

Dünyanın atmosferi kendi kendini temizleme kapasitesinin üst sınırına gelmiştir. Bir başka değişle kirletici konsantrasyonu yönünden doygun haldedir. Bunun, volkanik patlamalar, orman yangınları, kum fırtınaları, vb gibi doğal kaynaklı nedenleri olmakla beraber insan aktivitelerinin etkisi daha ağırlıktadır. Gerek atmosferde, şu anda var olan kirliliğin yapısal karmaşıklığı, gerekse bu kirliliğin giderilmesinde teknolojinin yetersiz kalması (özellikle Türkiye için) bugün çözümü doğrudan, kirlilik kaynaklarında getirilecek önlemlerde aramamıza yöneltmektedir. Endüstriyel veya evsel olsun herhangi bir nokta kaynaktan çıkan baca gazı emisyonlarının halk sağlığı ve doğal hayat üzerindeki olumsuz yönlerini görebilmek gerekmektedir. Bu amaçla ilk kıstas olarak baca gazının yayımladığı emisyonların bileşimi ve konsantrasyon miktarı akla gelmektedir. Şayet, kaynağın oluşturduğu gerekimisyon ve gerekse emisyon konsantrasyonlarının tesbiti ve düzenli kontrolü şeklinde bir sistem oluşturabilirsek çevre havasının ve çevre sağlığının kontrolünü sağlayabiliriz. Bu da, yeterli dataları, uzun ve düzenli çalışmaları içeren hava kirliliği modellemesiyle mümkündür. Çalışmada hava kalitesi modellemesi kapsamında baca gazlarının kaynaktan belirli uzaklıklarda ve yer seviyesindeki konsantrasyon hesabına ilişkin bilgisayar programına da yer verilmiştir. Çalışmada endüstriyel hava kirliliği kaynakları arasında önemli bir noktada olan kağıt endüstrisi proseslerinin hava kirliliği ve kirleticileri açısından incelenmesi ve kirletici emisyon tür ve miktarlarının değişim aralıkları ve kontrol teknolojileri belirlenmiştir. Daha önce de söz edildiği gibi çalışmanın sonunda nokta kaynaklardan atmosfere verilen kirletici emisyonlarının meteorolojik etkiler de göze alınarak kaynaktan belirli uzaklıklardaki huzme yükseklikleri bir tablo halinde verilmektedir. Kaynaktan yayılma yönünde seçilen belirli bir uzaklıktaki huzme yüksekliği, efektif baca yüksekliği ve yer seviyesindeki kirletici konsantrasyonu da tesbit edilmektedir.

The atmosphere's capacity has reached to a critical level for humanity and environment. Because it can't assimilate pollutants emitted into it. In spite of air pollution based on the natural sources, volcanic erup­tions, send storms etc., man’s activities are important and basic sources of it.Weber dedicated as "The presence of substances in the ambient atmosphere, resulting from the activity man or natural processes, causing adverse effects to man and the environment. " [25] To prevent this advers effects on environment and human. Determination of pollutants of their sources and control techniques them has to be done.In this study, different processes of the pulp and paper industry such as kraft pulping, sulfite pulping and sulfite semi chemical pulping processes are researched natural. The major sources of atmspheric emissions from pulp and paper industry is given in the study. What kind of pollutants is given from the industry and how these can be removed from processes, what the control technologies are.As in the control of pollutants the emission of it in the pulp and paper industry can be controlled by internal process.The major sources of atmospheric emission is indicated from a kraft mill. The particulate emissions are:Sodium compound from smelt tank Sodium sulfate from the recovery tank Sodium carbonate from the recovery tank Calcium compounds from the lime kiln Fly ash from wood and coal fired boilers Organic sulfides are extremely odours. Sulfur dioxide is chemically active has hazardous effects on wild life. Fly ash can be settle.Particulate control devices are employed on kraft recovery boilers.electrostatic precipitators with cyclones or contact evaporators venturi type evaporator scrubbers The recovery boilers process has generated. Sodium salts Na3SCU, Na=CO- , some of these salts evaporate because of temperature inside of the recovery furnace and condense as from of particles because of cooling. The removal efficiency is about %97 .The smelt tank has generated some particulates as composed of sodium sulfide, and sodium carbonate, and sodium hydroxide. These particulates can be caught with mist eliminator pads which consist of fine wire mesh screen. Pad efficiency is between %71 and 93.The lime kiln has generated calcium oxide or carbonate and sodium salts. Mechanical cyclone can be easily remove the particulates flow down via the scrubber.The major gaseous emissions are reduced hydrogen sulfide and dimethyl disulfide. These kinds of emissions are odor sources. That's why A Kraft Pulp Mill Industry needs odor control.The main source of odor comes from the recovery furnace as reduced sulfur compounds. Black liquor oxidation is a kind of odor control mechanisms. Because this process oxidized Na2S to innocuous salts to prevent the releas of H3S. The black liquor oxidation reaction is ;2Na=S + 203 + HsC) > Na2S03 + 2NaOH This reaction can occur in either weak or strong black liquor. The oxidation exists within air tray towers, porous diffusers, agitated air spargers and packed absorption towers for weak black liquor.The disadvantages of this mechanism is excessive foaming. The general types of oxidation system for strong black liquor are single and double stage unagitated spargers.Advantages of black liquor oxidation are the following:1- More effective system to reduce emmissions which in the Table 1 and 2 shown.Increase yield of tall oil Reduce corrosion of evaporator tubes Lower chemical make up requirement Reduced scaling Higher evaporation rates. Disadvantages of it as the following:Remain methyle sulfides an volatile and odorousRemain sulfide ion without oxidation Forming sulfur or polysulfide through partial oxidation and reverting to hydrosulfide ion Loss in heating value The effluent air poses problems of foaming.Table 1. Effect Of Black Liquor Oxidation On Sulfur GasEmissions During Direct Contact Evaporation [23] *Kg sulfur per metric ton of air dried pulp Table 2. Effect Of Weak Black Liquor Oxidation OnMalodorous Sulfur Gas Emissions From Evaporator Noncondensable Gases [23] * Kg sulfur per metric ton of air dried pulp Another kind of odor source is digestion process. Batch digesters are commanly used but generate more odor then continious system. Odor Controlling systems of them are condensation and containment of relief gases.Either noncondensable gases or the condansate from the weak liquor evoparators are the other odor sources.The major techniques to treat melodoroussulfur gas emission are thermal oxidation and liquid absorption. The main mechanisms of them are the lime kiln and catalytic furnaces Thermal Oxidation in lime kilns is a pretreatment technique of sulfur recovery.Another Atmospheric Emission Sources are Neutral Sulfite Semichemical (NSSC) and Neutral Sulfite Chemi- mechanical (NSSCM) Pulping Processes. The main difference between two process is the emphasis; semichemical uses more chemical action. The neutral sulfide pulping process has fewer emissions problems than the kraft mill process. Neutral Sulfide Semichemical process produces odorous reduced sulfur compounds. The primary of it S03 from absorption tower blow pit, spent liquor evaporation and fluidized bed reactors. Some chemical plants have converted to " sulfur free " soda because of lower emission of odorous compound with the low consentration of sulfur in the cooking chemicals; high cost of neutral sulfite cooking chemicals, poor markets for spent liquor.The neutral sulfite liquor process is two-step process, sulfur is burned to from SO* S + 0* > £z?L);32 and the SO* is passed through an absorber Na=CCU + SO* > Na2S0a + C02 Na*C03 + C03 + H*0 > 2 NaHCO»If it can be done proper design SO* absorption has completely been finished. The N0„ emission from recovery boilers are low. The NO« emission from ammonium sulfite liquors are higher.The air pollution problems of the Sulfite Mill Industry are quite different from Kraft Mill Industry .The main particulates comes from that kind of Industry are SO* and only ammonia base NO«. Related to economical perspection The Sulfite Mills has been prefer to Kraft pulping process. Because it needs inexpensive chemicals during operation of it's units.Evoparation and burning of calsium based spent liquors are normally used as recovering process. Recovery processes for magnesium and sodium involve evaporation of spent liquor, combustion of the organics and heat. Particulates are removed the flue gas by mechanical dust collectors and srubbing in absorption towers. Dust collected is usually recycled to the process. The efficiency of collection is about %99.SO* as ga3esous pollutant released in the acid preparation digester, blow pit, washers,evaporetors, and recovery units. The continous digester unit hasn't major air pollution problem because it is closed process SO* gases after evaporators are returned to acid preparetion system for SO* recovery.The SO* from the blow pit of batch digester can be scrubbed with an alkaline solution and returned to acid preparation. SO* recovery efficiency of this method i3 approximately 97 percent.Power Boilers are main parts of Pulp and Paper Mill. During fuel combustion in power boilers; Ash, sulfur and nitrogen appear as air pollution component. SO* emissions from fuel combustion are directly related to sulfur content. To minimize SO* emissions from power boiler, low sulfur oil coal or natural gas has to be selected.The other important factor affecting the combustion process and related to particulate emissions is air-to- fuel ratio. Variation of this ratio influence the combustion efficiency decreases, particulate emissions increases.To minimize pollutant emissions the following factors are also effective -method of fuel mixing and firing -combustion chamber size and configuration.Air pollution modeling provides a means for relating changes in the rate of emission of a pollutant from a given source or sources to changes in the concentration of the pollutant in the ambient air. In general, the objective in air pollution modeling is to calculate the pollutant concentration C(x,y,z) at a given point (x,y,z) downwind from a pollutant source, the plume emitted from a single stack. The x coordinate is measured downwind from the center of the stack in the mean wind direction, y is the crosswind direction, and z is the vertical direction measured from ground level. The stack height is denoted by h, and h is the plume rise, i.e., the vertical distance above the top of the stack to which the plume rises due to its buoyancy and vertical momentum. The effective stack height, H. is defined to be h + h. As the plume is carried downwind by the mean wind, ambient air is entrained in the plume by the action of turbulent eddies.Thus, the plume spreads out and is diluted as it travels downwind. This process is called turbulent diffusion or dispersion. At the edge of plume,the concentration should approach the background concentration (usually assumed to be zero), while at the center of the plume, the concentration should be a maximum.The purpose of a model is to provide a quantitative description of the concentrâtion of field.There is a Gaussian dispersion equation for an elevated point source. This equation is obtained by assuming that the pollutant concentration profile at any downwind position,x. Since the profiles become flatter and wider with increasing distance downwind, the standard deviations must also increase with x.We consider the plume from a single stack and make the folowing assumtions:-Concentration profiles in plume are Gaussian in both y and z directions -Constant mean wind speed, u(m&sn), and direction -Continuos steady pollutant emission rate,Q(kg/s) -Dispersion in the x-direction is negligible compared with bulk transport of pollutant by the mean wind -The pollutant is a stable gas or aerosol which does not react chemically or settle out.We can then write that ; C (x , y , s) = {Q/2itua1,oI} {exp ( -1/2 (y/av) = } {exp( -1/2 ( (z-HJ/a«:)3) + exp {-1/2 ( (z+HJ/a*)38) >(1)The Eq.(l) is often used to calculate pollutant concentrations that will occur at ground level (z=0).The larges ground level concentrations will occur directly beneath the plume center line (y=0). For this special case, the equation simplifies to C(x,0,0) = {Q/-n:uavCt,> {exp< 1/2 (H/a* ) = )