| Rapid economic development is bound to drive the huge consumption of energy,so far fossil fuel occupies an important position in the energy structure of our country,while the consumption of fossil fuels has caused great damage to the environment.since the reform and opening up to the outside world,with the rapid development of economy China has been confronted serious air pollution problems,for example: the frequent haze swept through most of the provinces in northern China.Although the country has increased management in recent years,the road is still a long way to go.Nowadays,one of the important causes of these air pollution phenomena is respirable particulate matter.Coal-fired power plants are one of the most important air pollutant emission industries in China,previrously the use of dust removal means mainly based on electrostatic precipitator,however,a large number of studies have shown that the efficiency of electrostatic precipitator filtration,especially for small particles,is much smaller than that of bag filters.Due to the imminent environmental control,the country has extended the use of electrostatic precipitator and bag dust collection,or bag filter as an effective means of dust removal from 2014.However,it is found that the filter bags are often damaged and ineffective due to mechanical,chemical and other reasons.The service life of filter materials is reduced,and the operation cost of the enterprise is increased,so that the technology can not be extended in a large scale.As the core of bag filter,filter bag is used to filter dust,exhaust clean gas,currently the products were mainly composed of two layers of web and a layer of woven scrim,then were consolidated by needle-punch and completed by a series of finishing processes,in which the web and the woven scrim play the role of filtration and reinforcement respectively.When the filter material is used,the weight of the filter cake is not only to be taken,but also the pressure drop during the operation and the terrible twitter cauced by high pressure airflow in the process of pulse jet cleaning,so the strength of filter media is the most basic premise to guarantee the service life of filter material,however,when consolidating the filter,the needle will inevitably damage the woven scrims,also for the single fiber,which posses the performance of low strength,excellent high temperature and corrosion resistance,for example,polytetrafluoroethylene fiber,this kind of damage is particularly fatal;secondly,the filtration performance of the filter material is very important,and PM2.5 is much more harmful to human health,atmosphere and traffic,it is more important to improve the filtration efficiency,especially to improve the filtration efficiency of fine particles;in the end,this paper hopes to make the filter material keep good performance in the high temperature environment with corrosive gas and water vapor when taking into account the above two kinds of performance.The research content and main conclusions of this work are listed as follows:Developed a new type of needle which has a cross-section like a cross star,and the needle is compared with the traditional triangular needle.By comparing the strength of woven scrim and filter material consolidated by two kinds of needles under same process conditions,it was found that new type needle will significantly reduce the damage and improve the strength of filter material,at the same time,this kind of needle is used to consolidate the filter without woven scrim under same process conditions,through comparison,the tensile fracture mechanism of filter material containing woven scrim is analyzed,it was found that the interaction between fabric and fibers has some influence on the strength of filter media,and can bear part of the load generated during stretching,the damage of woven scrim is lower,the strength provided by woven scrim is bigger.At last the tensile strength at 5 different off-axial angle in the range of 0°-90° for the fabric filter consolidated by new kind of needle was analyzed,it was found that the weak area appeared at the angle of 22.5° with the Y axis,by fitting the dimensionless curve,the cubic polynomial of off-axial angle and strength under different needling density.was obtained,so the experimental basis was got in order to optimize the strength distribution of fabric filter,solve the short boardeffect of filter and improve the service life of fabric filter.Comparison with the triangular needle,the effect of new kind of needle on thickness,air permeability,pore size and filtration efficiency were continued to study,it was found that needle type had no significant effect on weight,thickness.the air permeability,mean pore size of fabric filter consolidated by new kind of needle are smaller than that of triangular needle,meanwhile,static filtration efficiency was excellent than that of triangular needle;by studying the relationship of initial resistance and filtration velocity,it was found that the tendency of curve met Darcy’s law,illustrating the fabric filter consolidated by new type needle met typical filtration behavior of filter material;by comparing the relationship curve of pressure drop and filtration time,it was found that both the new type needle and triangular needle had same filtration mechanism,that the filtration mechanism in the early stage is deep filtration,while the filtration mechanismin the later stage is surface filtration,meanwhile the filtration efficieny for the new type is superior to the triangular needle.Compared with the fabric filters consolidated by needle-punching process,the hydroentanglement process was used as the finishing process to further consolidate the PTFE/PPS filter media of different components,the following conclusions are drawn: the hydroentanglement process can make the PTFE fiber fibrillation and reduce the diameter of the fiber;Analysed by two-way ANOVA method,it is considered that the fiber blend ratio and consolidating process have significant influence on the strength,air permeability and average pore size of the fabric filter;the needle-punch/hydroentanglement composite process was used to consolidate PTFE/PPS fabric filter with 4 different blend ration,the growth rate of tensile strength increases with the increase of PTFE fiber content,while needlepunched fabric filter had a reverse trend,which shows that the composite process is more conducive to the consolidation of PTFE fiber;the air permeability and mean pore size of the filter media consolidated by composite process decreased with the increase of PTFE fiber content;Compared with needle-punched fabric fitler,for the fabric filter consolidated by composite process,the filtration efficiency of the particles in the range of 0.2-5μm is greatly improved,meanwhile,with the increasement of PTFE content,the filtration efficiency of particles less than 2.5 μm is improved significantly.At last,the Micro CT method was used to analyse the difference of three-dimensional structure of fabric filters consolidated by two different consolidating process,illustrating that hydroentanglement process don’t form through holes in the filter media,the distribution of fibers in the thickness direction is more uniform.After making sure that needlepunch/hydroentanglement composite process can improve the tensile strength and filtration efficiency of PPS/PTFE fabric filter,the microstructural changes of 100% PPS fabric filter in 95℃ nitric acid solution were analyzed by infrared spectroscopy,elemental analysis,DSC and XRD,by studying the mechanism of acid aging,it is considered that the PPS fiber has many disordered substituent groups on the regular molecular chain due to nitration and oxidation reaction,so that the single fiber strength and water repellency of PPS fiber declined;the strength retention and water repellency of the filter material after acid aging can be greatly improved by adding PTFE fiber,for the filtration efficiency,although the higher the content of PPS fiber,the more obvious the average pore diameter decrease,but it can be seen from the filtration test for the cleanable filters,When the nitric acid action time exceeded 75 h,the dynamic filtration efficiency changed from the original upward trend to the downward trend,the larger the content of PTFE fiber is,the smaller the degree of decrease.It shows that addition of PTFE fiber can decrease the negative effect of acid on the filtration efficiency,improve the corrosion resistance of filter material,and prolong the service life of filter material. |
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