样气伴热管线中CO的来源

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    Continuous emissions monitoring (CEM) systems traditionally use heated fluoropolymer sample lines because they are inexpensive and the fluoropolymer tubing is resistant to the acids that are present in most stack gas. The disadvantage with fluoropolymer sample lines is that fluoropolymer is a porous material. In fact fluoropolymers are often used to manufacture permeation membranes. One of the components measured in stack gas emission systems is CO. Since CEM samples are transported through the sample line under a slight vacuum the problem is that the heated sample line generates CO, which migrates into the sample tube giving a false reading for the stack emissions。

    连续排放监测 （CEM） 系统传统上使用加热的含氟聚合物样品管线，因为它们价格低廉，并且含氟聚合物管对大多数烟囱气体中存在的酸具有抵抗力。含氟聚合物样品线的缺点是含氟聚合物是一种多孔材料。事实上，含氟聚合物通常用于制造渗透膜。在烟囱气体排放系统中测量的成分之一是一氧化碳。由于 CEM 样品是在轻微真空下通过样品管路运输的，因此问题在于加热的样品管路会产生 CO，CO 会转移到样品管中，从而产生烟囱排放的错误读数。

    We designed a set of tests to determine the conditions under which background CO is generated, the source of the CO, and finally to test possible solutions. The testing of sample line contamination proved to offer a number of technical challenges.

   我们设计了一组测试来确定产生背景中CO的条件，CO的来源，最终得到测试可能的解决方案。样品管路污染测试证明，这提供了许多的技术挑战。

    A carbon monoxide analyzer was used for testing. A total of six separate test procedures were performed with a number of different sample line configurations. Each of the tests used a different approach in an attempt to isolate the mechanism of CO production and/ or migration. Theories were formulated using the results of previous tests, new tests were designed and run to validate or disprove the theory. It must be understood that the tests were performed to identify carbon monoxide only and the results may not apply to other gasses.

    使用一氧化碳分析仪进行测试。总共使用了六个单独的测试程序，使用了许多不同的样品管路配置。每个测试都使用不同的方法，试图分离一氧化碳产生或转移的机制。理论是使用先前测试的结果制定的，设计并运行新的测试以验证或反驳该理论。必须理解的是，这些测试只是为了识别一氧化碳CO，结果可能不适用于其他气体。

    Background CO, roughly 4 ppm on average, was found in ambient air at room temperature. When ambient air was heated the CO level was found to increase. Variable CO readings within the air created a challenge for test procedure and interpretation due to the fact that CO contamination from different sources had to be separated. Pure nitrogen was used in some tests to eliminate the background CO. Using nitrogen would isolate the CO source to only those generated by the heated sample line. However, the nitrogen source was pressurized, which impeded CO permeation into the sample tube.

    背景中的CO，在室温下在大气中被发现平均约为 4 ppm。当环境空气被加热时，发现一氧化碳浓度量级水平增加。空气中可变的一氧化碳读数给测试程序和解释带来了挑战，因为必须将来自不同来源的一氧化碳污染分开。在一些测试中使用纯氮气来消除背景气中一氧化碳，使用氮气可以将一氧化碳源隔离到仅由加热的样品管线产生的一氧化碳。然而，氮气被加压，这阻碍了一氧化碳渗透到样品管中。

带加热功能的采样管线

     The heated sample lines tested had a stainless steel braided fluoropolymer tube with stainless steel fittings. Fiberglass, silicone, and kapton were used for dielectric insulation. A nichrome element served as the series resistive heater circuit with fiberglass and polyester thermal insulation. An extruded polyurethane jacket covered the entire assembly. A thermocouple was installed on the core tube and a PID controller controlled the temperature of the test bundles.

    测试的加热样品管具有带有不锈钢配件的不锈钢接头含氟聚合物管。玻璃纤维、硅胶和卡普顿(聚酰亚胺薄膜材料)用于电子绝缘。镍铬合金元件用作串联电阻加热器电路，具有玻璃纤维和聚酯隔热材料。冒出的聚氨酯护套覆盖了整个组件。在芯管上安装了热电偶，PID控制器控制测试管线的温度。

   CO level was recorded at room temperatureand then the test bundles were heated. As the temperature of the bundle was increased the CO level increased. The major contributors of CO were the high temperature fiberglass insulation and fiberglass tape, with the kapton tape producing lower levels. We found no detectable levels emitted from the fluoropolymer tubing, metal sheath, or silicone. As the temperature is increased CO is formed from the materials mentioned above. The CO then travels through the silicone, the metal mesh, and the fluoropolymer tube into the process gas。

  在室温下记录CO浓度水平，然后加热测试管线。随着温度的升高，一氧化碳浓度增加。一氧化碳的主要贡献者是高温玻璃纤维绝缘材料和玻璃纤维胶带，而卡普顿胶带的含量较低。我们发现在含氟聚合物管、金属护套或硅胶根本没有检测到CO。随着温度的升高，上述材料会形成一氧化碳。然后，一氧化碳通过硅胶、金属网和含氟聚合物管进入工艺气体中.

CO浓度和加热时间以及温度曲线

  The amount of CO rose at a dramatic rate when the temperature reached 385°F. Taken and held at 400F the CO level continued to rise, peak, then begin a gradual decrease. This confirmed field observations that a “burn in” period reduces the background CO levels over time. “Burn in” refers to the procedure of heating the sample line for an extended period of time prior to service in an effort to deplete the contamination source. The testing did not show a significant reduction in contamination for a “burn in” time of 53 hours. Field reports indicate that a successful “burn in” period is from 5 to 7 days。

  当温度达到 385°F (196℃)时，一氧化碳的含量急剧上升。 继续并保持在 400°F(204℃) 时，一氧化碳水平继续上升，达到峰值，然后开始逐渐下降。这证实了现场观察结果，即“老化”期会随着时间的推移降低背景气中CO 水平。“老化”是指在维修前长时间加热样品管路以耗尽污染源的过程。测试没有显示 53 小时的“老化”时间的污染显著减少。现场报告表明，成功的“老化”期为 5 到 7 天。

    The results show that CO was generated from the construction materials of the heated sample line and migrate into the sample tube causing inflated readings. We found the CO levels were constant over a 48 – 72 hour period if the flow rate and temperature were held constant. We also found that CO levels were reproducible in any given hose when conditions are returned to previous levels.Considering that the contamination reading is repeatable and consistent calibration of the analyzer to zero out the background CO provides an accurate method to measure the sample content. For example, if the hose provides a reading of 12 ppm when sampling atmospheric gas and 18 ppm when reading sample gas the sample gas CO content would be determined to be (18ppm-12ppm) 6 ppm. The calibration offset would have to be periodically checked due to the possibility of a reduction in sample line contamination over time.

   结果表明，加热样品管线的制造材料会产生一氧化碳CO，并转移到样品管中，导致读数升高。我们发现，如果流速和温度保持不变，一氧化碳含量在 48 – 72 小时内是恒定的。我们还发现，当条件恢复到以前的水平时，任何给定软管中的一氧化碳水平都是可重复的。考虑到污染读数是可重复的，并且通过标定分析仪将背景中CO归零，为测量样品含量提供了一种准确的方法。例如，如果软管在对大气气体进行采样时提供的读数为 12 ppm，在读取样品气体时提供的读数为 18 ppm，则样品气体 CO 含量将被确定为6 ppm(18ppm-12ppm)。标定偏移量必须定期检查，因为随着时间的推移，样品管路污染可能会减少。

   Several methods were investigated in an attempt to reduce or eliminate the influence of background CO in the sample tube.Reducing the operating temperature below 300F has a dramatic affect as to the amount of CO introduced. Testing repeatedly showed a marked increase in CO levels as the temperature of the bundle was heated above 300F. There was a marked increase in CO above 385F. Increasing the wall thickness of the fluoropolymer tube reduces the amount of CO permeation. Doubling the wall thickness reduces the permeation by one half. Wrapping the core with a foil mylar was found to be ineffective. The applied barrier is not gas tight and only slowed the rate of change, not the rate of permeation. It took longer to reach the maximum CO content but the foil mylar did not reduce the maximum level. This would have the effect of increasing the needed ‘burn in’ time。

  研究了几种方法，试图减少或消除样品管中背景气中CO的影响。将工作温度降低到 300F(149℃) 以下会对引入的 CO 含量产生巨大影响。反复测试显示，当管线的温度加热到 300F 以上时，CO 含量显著增加。CO在385F(196℃)以上明显增加。增加含氟聚合物管的壁厚可减少 CO 渗透量。壁厚增加一倍可使渗透减少一半。用铝箔聚酯薄膜包裹核心部件也被发现是无效的。施加的屏障不是气密的，只是减缓了变化率，而不是渗透率。达到一氧化碳最大含量需要更长的时间，但铝箔聚酯薄膜并没有降低最大浓度。这将影响增加所需的“老化”时间。

    There are several varieties of fluoropolymers; PFA,PTFE, and FEP are the most common choices for tubing. Fluoropolymers are chosen because they are essentially chemically inert. They are an ideal transport medium for highly volatile chemical compounds and exotic fluids.There are very few chemicals, such as fluorine, chlorine trifluoride, and oxygen difluoride that are known to react with fluoropolymers. To a lesser degree, fluoropolymer tubing may absorb halogenated organic chemicals. This causes swelling and change in weigh.

   含氟聚合物有好几种PFA、PTFE 和 FEP 是最常见的管材选择。之所以选择含氟聚合物，是因为它们本质上是化学惰性的。它们是高挥发性化合物和稀有流体的理想运输介质。已知与含氟聚合物反应的化学物质很少，例如氟、三氟化氯和二氟化氧。在较小程度上，含氟聚合物管可以吸收卤代有机化学物质。这会导致肿胀和重量变化。

  Permeability is a major consideration with fluoropolymer tubing. Permeation depends upon several factors; the porosity of the tube material, the tube thickness, the molecular size of the permanent, and the relative concentration of the permanent inside and outside the tube. Increasing the temperature always raises the permeation rate. Increasing the wall thickness always reduces the permeation rate.Pressure rating at operating temperature must be considered. The allowable pressure rating for fluoropolymer tubes decreases very rapidly with increasing temperature. However, many analyzer applications using fluoropolymer tubing are under a slight vacuum.

   渗透性是含氟聚合物管的一个主要考虑因素。渗透取决于几个因素;管材的孔隙率、管材厚度、分子量大小以及管内外的相对浓度。提高温度总是会提高渗透率。增加壁厚总是会降低渗透率。必须考虑工作温度下的压力额定值。含氟聚合物管的允许压力额定值随着温度的升高而迅速降低。然而，许多使用含氟聚合物管的分析仪应用都处于轻微的真空状态。

    Fluoropolymer tubing is widely used for CEM applications because it is inert to most compounds found in stack gas. Permeation is a known problem with fluoropolymer tubing but specifying a thicker wall can reduce its effect.

   含氟聚合物管材广泛用于 CEM 应用，因为它对烟气中的大多数化合物呈惰性。渗透是含氟聚合物管的一个已知问题，但指定较厚的壁可以降低其影响。

注意：1.以上数据仅适用于AMETEK-OB的产品，其他产品具体情况视情况而定；，以测试数据结果为准；
2.72°F大约为22℃，200°F大约为93℃，400°F大约为204℃，1psi大约为6.9Kpa;