WO2016115804A1 - 一种锅炉燃烧火焰中的气相碱金属浓度的在线检测方法 - Google Patents

一种锅炉燃烧火焰中的气相碱金属浓度的在线检测方法 Download PDF

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WO2016115804A1
WO2016115804A1 PCT/CN2015/080739 CN2015080739W WO2016115804A1 WO 2016115804 A1 WO2016115804 A1 WO 2016115804A1 CN 2015080739 W CN2015080739 W CN 2015080739W WO 2016115804 A1 WO2016115804 A1 WO 2016115804A1
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alkali metal
flame
gas phase
concentration
temperature
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娄春
田艳飞
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Huazhong University of Science and Technology
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/0004—Gaseous mixtures, e.g. polluted air
    • G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
    • G01N33/0027—General constructional details of gas analysers, e.g. portable test equipment concerning the detector
    • G01N33/0036—General constructional details of gas analysers, e.g. portable test equipment concerning the detector specially adapted to detect a particular component
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/71—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light thermally excited
    • G01N21/72—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light thermally excited using flame burners
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/30—Measuring the intensity of spectral lines directly on the spectrum itself

Definitions

  • the invention belongs to the technical field of flame emission spectrum detection, and more particularly relates to an on-line detection method for gas phase alkali metal concentration in a boiler combustion flame.
  • the fiber optic spectrometer with CCD array as the spectral measuring component can detect the spectral radiation information of the combustion flame at different wavelengths.
  • the atomic characteristic line of the main alkali metal is: lithium. Li (670.776 nm, 670.791 nm), sodium Na (588.995 nm, 589.592 nm), potassium K (766.490 nm, 769.896 nm) and ruthenium Rb (780.027 nm, 794.760 nm), which provide for qualitative and quantitative analysis of alkali metals. condition.
  • LIBS online detection technology also has obvious shortcomings: the equipment structure is complicated, the operation is cumbersome, the cost is high, and the LIBS detection does not consider the temperature change. It is necessary to maintain the temperature constant, the measurement conditions are demanding, and it is not convenient for industrial promotion.
  • the present invention provides an on-line detection method for gas phase alkali metal concentration in a combustion flame of a boiler, wherein the flame emission spectrum analysis is directly used according to the characteristics of the combustion flame in the boiler furnace.
  • the detection of the concentration of the gas phase alkali metal in the boiler flame can effectively solve the problem that the existing detection method is difficult to be directly applied to the detection of the gas phase alkali metal concentration in a large industrial boiler, and has the advantages of simple and accurate detection, simple equipment, low cost, and the like.
  • the present invention provides an on-line detection method for gas phase alkali metal concentration in a combustion flame of a boiler, which comprises the following steps:
  • the detection spectrometer is subjected to absolute radiation intensity calibration, and then the flame object of the known alkali metal concentration is measured by the calibration spectrometer to obtain the alkali metal characteristic line radiation intensity and flame temperature, and the measurement is performed.
  • the corresponding characteristic line radiation intensity and flame temperature of a series of different alkali metal concentrations are brought into the fitted model The specific value of the polynomial fitting coefficient a mn of the model is obtained;
  • step (1) the fitting model described in step (1)
  • the steps to build are as follows:
  • g i and g 0 are the statistical weights of the excited state and the ground state, respectively, E i is the excitation energy, k is the Boltzmann constant, and T is the excitation temperature.
  • I ij N i ⁇ A ij ⁇ h ⁇ v ij (2);
  • the atomic characteristic line radiation intensity is related to the ground state atom number N 0 and the excitation temperature T, and the ground state atom number N 0 is related to the concentration, so the alkali metal characteristic line radiation intensity I Alkali can Expressed as a function of the alkali metal concentration C Alkali and the flame temperature T, the fitted model is obtained by least squares fitting:
  • the detection spectrometer is calibrated on a black body furnace.
  • the invention is based on the analysis of the emission spectrum of the alkali-containing metal flame in the visible light range, and constructs a fitting model of the alkali metal characteristic line radiation intensity with respect to the gas phase alkali metal concentration and the flame temperature in the flame, and establishes a fitting model.
  • the detection method of the invention can conveniently and accurately detect the concentration of gas phase alkali metal in the combustion flame in the furnace, and can simultaneously detect the content of various alkali metals on the line, the device is simple, the cost is low, and the industrial field measurement is convenient, and the method can be directly It is applied to the detection of gas phase alkali metal concentration in large industrial boilers to realize on-line measurement of gas phase alkali metal concentration in the combustion process of alkali metal fuel in furnace.
  • Figure 1 is a flow chart of the present invention
  • Figure 2 is a diagram of an experimental calibration processing system
  • Figure 3 is the emission line of the waste combustion flame in the visible light band.
  • FIG. 1 shows a block diagram of the present invention.
  • the on-line detection method of the gas phase alkali metal concentration in the combustion flame comprises the following steps:
  • the element Under normal conditions, the element is in the ground state. When the atom or ion of the element absorbs the energy in the high temperature flame, it is activated into the excited state (high energy state). Because the excited state is extremely unstable, it will transition to the low energy state or the ground state, and release the excess.
  • each alkali metal has its own atomic characteristic line, and the strongest characteristic lines in the first resonance line are: Li (670.776 nm, 670.791 nm), Na (588.995 nm). , 589.592 nm), K (766.490 nm, 769.896 nm) and Rb (780.027 nm, 794.760 nm), which are the alkali metal characteristic lines to be selected in the theory.
  • the characteristic line should be selected in combination with the minimum wavelength that can be resolved by the spectrometer used.
  • the invention uses the AvaSpec-2048-USB2 spectrometer to measure the radiation spectrum of the flame in the waste incinerator. Since the resolution of the spectrometer is 0.8 nm, the characteristic line of the alkali metal atom actually detected is lithium Li (670.451 nm), sodium. Na (589.139 nm), potassium K (766.538 nm, 769.886 nm) and ruthenium Rb (779.920 nm, 794.388 nm), wherein the lines of Na (589.139 nm) and K (766.538 nm, 769.886 nm) are most apparent.
  • the above-mentioned characteristic lines can be used for quantitative measurement, but considering the characteristic line interference of other elements, such as argon Ar will appear at 772.376nm and 772.421nm. Spectral line, so the selection of the characteristic lines of the K and Rb elements should try to avoid the interference. Therefore, the alkali metal characteristic lines of the present invention are selected as lithium Li (670.451 nm), sodium Na (589.139 nm), potassium K (766.538 nm), and ruthenium Rb (794.388 nm).
  • I Alkali is a fitting model between the gas phase alkali metal concentration C Alkali and the flame temperature T:
  • g i and g 0 are the statistical weights of the excited state and the ground state, respectively;
  • E i is the excitation energy;
  • k is the Boltzmann constant;
  • T is the excitation temperature.
  • the atomic characteristic line radiation intensity is related to the ground state atom number N 0 and the excitation temperature T, and the ground state atom number N 0 is related to the concentration, so the alkali metal characteristic line radiation intensity I Alkali can Expressed as a function of the alkali metal concentration C Alkali and the flame temperature T, a least squares method is fitted to obtain a binary polynomial function of the formula (4):
  • a mn is a polynomial fitting coefficient, and the orders m and n are selected as needed, so that the fitting error is sufficiently small (1%-5%), and the binary polynomial function is the constructed fitting model.
  • the unit of the alkali metal characteristic line radiation intensity I Alkali is W/m 3 /sr
  • the unit of the temperature T is K
  • the unit of the gas phase alkali metal concentration C Alkali is ppm.
  • a collimating lens is installed at the end of the fiber detection; the original output signal of the spectrometer is a voltage value obtained by photoelectric conversion, and cannot The radiation energy is completely reflected.
  • the spectrometer system is calibrated on a black body furnace, and the flame object with the known alkali metal concentration is measured by a spectrometer system that has been subjected to absolute radiation intensity calibration to obtain an alkali metal characteristic spectrum.
  • the line radiation intensity and flame temperature are experimentally calibrated, and the corresponding characteristic line radiation intensity and flame temperature of a series of different alkali metal concentrations are taken into equation (5) to obtain the polynomial fitting coefficient a mn of the model.
  • the model polynomial fitting coefficient a mn is determined on the experimental calibration processing system shown in Figure 2.
  • the main equipment includes laminar premixed flame multi-jet burner, fuel and oxidant supply, air supply, mass flow control. , spectrum detection system, etc.
  • the mass flow meter carries the alkali metal salt mist generated in the ultrasonic atomizer 4, enters the gas mixer 3, and the four gases are uniformly mixed with the alkali metal liquid mist, and then enter the jet line of the burner 5, accompanied by the air flow. It is supplied by the air compressor 6, enters the mass flow controller, and finally enters the accompanying gas flow line of the jet burner 5.
  • the spectrum detection system mainly comprises a collimator lens 7, an optical fiber 8, a spectrometer 9, a USB cable 10 and a notebook computer 11. Table 1 shows the specific conditions of the experimental calibration:
  • C s is the concentration (%) of the alkali metal solution
  • ⁇ s is the consumption rate (g/s) of the alkali metal solution, which can be obtained by letting the atomizer work for a long time, and statistically averaged
  • a m is a mole fraction (%) of the alkali metal salt
  • M is a molar mass (g/mol) of the alkali metal salt
  • the total gas flow rate v f (L/s) in the flame zone can be expressed as:
  • T f is the average flame temperature K
  • T r is the laboratory room temperature K
  • R is the ideal gas constant and P is the pressure
  • the volume concentration of the gas phase alkali metal in the flame can be calculated by the formula (11);
  • the absolute radiant intensity I d ( ⁇ ) calibrated by the black body furnace is selected, and the two wavelengths ⁇ , ⁇ + ⁇ with the interval ⁇ are selected, and the flame temperature T ⁇ is calculated:
  • Planck constant C 2 1.4388E-2; I d ( ⁇ ), I d ( ⁇ + ⁇ ) are the spectral radiance at wavelength ⁇ , ⁇ + ⁇ , respectively.
  • the calibrated spectrometer system is used to measure the combustion flame radiation spectrum in the furnace, and the flame temperature is calculated by using the law of heat radiation, and the alkali metal characteristic line radiation intensity value to be measured in the flame radiation spectrum is extracted, and the calculated flame temperature and characteristic spectrum are calculated.
  • the line radiation intensity value is substituted into the formula (5) of the known fitting coefficient to determine the gas phase alkali metal concentration in the combustion flame.
  • Figure 3 is a flame radiation spectrum of the AvaSpec-2048-USB2 portable spectrometer detected on a waste incineration boiler.
  • the spectrometer detector is a 2048 pixel CCD array with a measurement wavelength range of 200 to 1100 nm, a resolution of 0.8 nm, and a signal-to-noise ratio of 200:1.
  • the flame temperature T is calculated according to the flame radiation spectrum actually measured in the furnace.
  • the alkali metal characteristic line radiation intensity I Alkali and the flame temperature T to be measured are substituted into the formula (5) of the known fitting coefficient a mn to obtain a specific concentration of the gas phase alkali metal in the measured flame.

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Abstract

一种直接采用火焰发射光谱分析在线检测锅炉火焰中气相碱金属浓度的方法，首先选择要测定碱金属的特征谱线，然后构建特征谱线辐射强度关于火焰温度、气相碱金属浓度的拟合模型，再次通过实验标定确定拟合系数，最后由便携式光纤光谱仪测量火焰对象的辐射光谱，得到火焰温度和碱金属特征谱线辐射强度，代入建立的已知拟合系数的拟合模型计算得到火焰中气相碱金属的浓度。该方法能够简便、准确地在线检测炉内燃烧火焰中气相碱金属的浓度，可以同时检测多种碱金属含量，设备简单、成本低廉，便于工业现场测量。

Description

一种锅炉燃烧火焰中的气相碱金属浓度的在线检测方法 【技术领域】
本发明属于火焰发射光谱检测技术领域，更具体地，涉及一种锅炉燃烧火焰中的气相碱金属浓度的在线检测方法。
【背景技术】
随着国家发展，用电量逐渐增加，能源消耗大幅增长，同时环境问题日益突出，使得电厂不得不燃用新煤种，并开发新型燃烧方式来应对能源与环境的双重压力。近年发现的准东煤储量大、煤质较好、灰分含量低，是良好的动力煤，但因其碱金属含量较高，导致炉膛结渣及受热面积灰等问题，成为研究利用的难点。林木废弃物、农业废弃物、水生植物、油料植物、有机物加工废料、人畜粪便及城市生活垃圾等生物质能被认为是一种具有应用前景的可再生能源，然而与常规化石燃料相比，生物质燃料中碱金属含量普遍较高，燃烧过程中极易进入气相，引起高温腐蚀、受热面结垢、炉内结渣等问题。因此，准确测量燃烧火焰中气相碱金属浓度对燃高碱煤和生物质锅炉的安全运行有着十分重要的意义，同时对研究碱金属释放机理及模型建立也有重要的指导作用。
以CCD阵列作为光谱测量元件的光纤光谱仪可以检测到燃烧火焰在不同波长下的光谱辐射信息，根据National Institute of Standards and Technology(NIST)Atomic Spectra Database可知，主要碱金属的原子特征谱线为：锂Li(670.776nm、670.791nm)，钠Na(588.995nm、589.592nm)，钾K(766.490nm、769.896nm)和铷Rb(780.027nm、794.760nm)，这为碱金属的定性和定量分析提供了条件。传统碱金属检测方法主要是离线检测，如火焰原子吸收光谱法、火焰光度法、电感耦合等离子体原子发射光谱法、石墨炉原子吸收光谱法等，但这些方法都需要对采集的样品进行前 处理，以达到实验条件，操作复杂，而且这种离线检测方法无法获取燃烧过程中炉内气相碱金属分布规律。随着激光技术的发展，研究者利用激光诱导击穿光谱技术Laser-Induced Breakdown Spectroscopy(LIBS)成功测量了褐煤和松木燃烧时气体中碱金属Na、K含量。但对于大型工业炉来说，LIBS在线检测技术也有明显的缺点：设备结构复杂，操作繁琐，成本高昂，并且LIBS检测没有考虑温度变化，要维持温度恒定，测量条件要求苛刻，不便于工业推广。
【发明内容】
针对现有技术的上述缺点和/或改进需求，本发明提供了一种锅炉燃烧火焰中的气相碱金属浓度的在线检测方法，其中根据锅炉炉膛中燃烧火焰的特点，直接采用火焰发射光谱分析在线检测锅炉火焰中气相碱金属的浓度，相应的可有效解决现有检测方法难以直接应用于大型工业锅炉中气相碱金属浓度检测的问题，具有检测简便、准确、设备简单、成本低廉等优点，因而尤其适用于大型工业锅炉中气相碱金属浓度的在线检测。
为实现上述目的，本发明提出了一种锅炉燃烧火焰中的气相碱金属浓度的在线检测方法，其特征在于，包括如下步骤：
(1)基于特征谱线构建拟合模型：选取要测量的碱金属特征谱线，基于上述选取的碱金属特征谱线，构建燃烧火焰中碱金属特征谱线辐射强度IAlkali关于气相碱金属浓度CAlkali与火焰温度T之间的拟合模型：
Figure PCTCN2015080739-appb-000001
其中amn是多项式拟合系数，阶数m、n为正整数，T表示作为测量对象的火焰的温度，其单位为K；
(2)执行标定处理：将检测光谱仪进行绝对辐射强度标定，然后用该标定后的检测光谱仪测量已知碱金属浓度的火焰对象，得到其碱金属特征谱线辐射强度和火焰温度，把测量的一系列不同碱金属浓度下对应的特征 谱线辐射强度和火焰温度带入所述拟合模型
Figure PCTCN2015080739-appb-000002
中，得到模型的多项式拟合系数amn的具体数值；
(3)在线检测：采用上述标定后的检测光谱仪实时测量锅炉燃烧火焰辐射光谱，获得相对应的碱金属特征谱线辐射强度，同时获得火焰温度，将所述碱金属特征谱线辐射强度、所述火焰温度以及步骤(2)获得的多项式拟合系数amn的具体数值代入步骤(1)中所构建的拟合模型中，计算获得锅炉燃烧火焰中气相碱金属的实际浓度，以此实现锅炉燃烧火焰中气相碱金属浓度的在线检测。
作为进一步优选的，步骤(1)中所述拟合模型
Figure PCTCN2015080739-appb-000003
的构建步骤如下：
(1.1)根据火焰光谱分析，在热力学平衡时，元素的原子或离子吸收高温火焰中的能量由基态变为激发态，单位体积的基态原子数N0与激发态原子数Ni之间的分布满足统计力学中麦克斯韦—玻尔兹曼分布定律，即：
Figure PCTCN2015080739-appb-000004
其中，gi、g0分别为激发态与基态的统计权重，Ei为激发能，k为玻尔兹曼常数，T为激发温度。
(1.2)两能级i、j之间跃迁所产生的谱线强度Iij与激发态原子数目的关系表示如下：
Iij＝Ni·Aij·h·vij   (2)；
其中，Aij为两个能级间的跃迁几率，h为普朗克常数，vij为发射谱线的频率；将上述式(1)带入该式(2)中得到下式：
Figure PCTCN2015080739-appb-000005
(1.3)根据式(3)可知，原子特征谱线辐射强度与基态原子数N0和激发温度T有关，而基态原子数N0又与浓度有关，因此碱金属特征谱线辐射强度IAlkali可以表示为碱金属浓度CAlkali与火焰温度T的函数，采用最小二乘法拟合得到拟合模型：
Figure PCTCN2015080739-appb-000006
作为进一步优选的，所述检测光谱仪在黑体炉上进行标定。
总体而言，通过本发明所构思的以上技术方案与现有技术相比，主要具备以下的技术优点：
1.本发明基于对含碱金属火焰在可见光范围内发射光谱的分析，构建了碱金属特征谱线辐射强度关于火焰中气相碱金属浓度、火焰温度三者之间的拟合模型，建立了一种简单有效的燃烧火焰中气相碱金属浓度检测方法，从而获得炉内气相碱金属分布规律，对于分析预防碱金属腐蚀和结渣具有重要的意义，且本发明属于非接触式测量方法，对测量火焰对象无干扰。
2.本发明的检测方法能够简便、准确的在线检测炉内燃烧火焰中气相碱金属的浓度，并可以同时在线检测多种碱金属的含量，设备简单、成本低廉，便于工业现场测量，可直接应用于大型工业锅炉中气相碱金属浓度的检测，实现炉内含碱金属燃料燃烧过程中气相碱金属浓度的在线测量。
【附图说明】
图1是本发明的流程框图；
图2是实验标定处理系统图；
图3是垃圾燃烧火焰在可见光波段的发射谱线。
【具体实施方式】
为了使本发明的目的、技术方案及优点更加清楚明白，以下结合附图及实施例，对本发明进行进一步详细说明。应当理解，此处所描述的具体实施例仅仅用以解释本发明，并不用于限定本发明。此外，下面所描述的本发明各个实施方式中所涉及到的技术特征只要彼此之间未构成冲突就可以相互组合。
图1给出了本发明的流程框图，所述燃烧火焰中气相碱金属浓度的在线检测方法包括如下步骤：
(1)选取特征谱线并构建拟合模型步骤
正常状态下，元素处于基态，当元素的原子或离子吸收高温火焰中的能量时，被激活成激发态(高能态)，由于激发态极不稳定，会跃迁到低能态或基态，同时释放多余能量，从而发射出一系列特征光谱线，由最低激发态返回到基态所发射的谱线称为第一共振线，它是众多光谱线中最强的，也是元素分析中最灵敏的谱线。根据NIST Atomic Spectra Database数据库可知，每种碱金属都有自己的原子特征谱线，其第一共振线中强度较强的特征谱线分别为：Li(670.776nm、670.791nm)，Na(588.995nm、589.592nm)，K(766.490nm、769.896nm)和Rb(780.027nm、794.760nm)，这就是理论中要选取的碱金属特征谱线。实际测量时，由于光谱仪的分辨率不同，选择特征谱线时还要结合所用光谱仪能分辨的最小波长选取。
本发明用AvaSpec-2048-USB2光谱仪去测量垃圾焚烧炉中火焰的辐射光谱，由于该光谱仪的分辨率为0.8nm，实际检测到的碱金属原子特征谱线分别为锂Li(670.451nm)，钠Na(589.139nm)，钾K(766.538nm、769.886nm)和铷Rb(779.920nm、794.388nm)，其中Na(589.139nm)和K(766.538nm、769.886nm)的谱线最明显。
理论上上述检测到的特征谱线都可以用于定量测量，但考虑到会有其他元素的特征谱线干扰时，如氩Ar会在772.376nm和772.421nm出现特征 谱线，因此K、Rb元素特征谱线的选取应尽量避开其干扰。因此，针对本发明碱金属特征谱线选取为：锂Li(670.451nm)、钠Na(589.139nm)、钾K(766.538nm)和铷Rb(794.388nm)。
基于所选择的特征谱线构建火焰中碱金属特征谱线辐射强度IAlkali关于气相碱金属浓度CAlkali与火焰温度T之间的拟合模型：
(1.1)根据火焰光谱分析，在热力学平衡时，元素的原子或离子吸收高温火焰中的能量，由基态变为激发态，单位体积的基态原子数N0与激发态原子数Ni之间的分布满足统计力学中麦克斯韦—玻尔兹曼分布定律，即：
Figure PCTCN2015080739-appb-000007
其中，gi、g0分别为激发态与基态的统计权重；Ei为激发能；k为玻尔兹曼常数；T为激发温度。
(1.2)而i、j两能级之间跃迁所产生的谱线强度Iij与激发态原子数目成正比，可以表示为：
Iij＝Ni·Aij·h·vij   (2)
其中，h为普朗克常数；Aij为两个能级间的跃迁几率；vij为发射谱线的频率；式(1)带入式(2)得：
Figure PCTCN2015080739-appb-000008
(1.3)根据式(3)可知，原子特征谱线辐射强度与基态原子数N0和激发温度T有关，而基态原子数N0又与浓度有关，因此碱金属特征谱线辐射强度IAlkali可以表示为碱金属浓度CAlkali与火焰温度T的函数，采用最小二乘法拟合得到形如式(4)的二元多项式函数：
Figure PCTCN2015080739-appb-000009
其中，amn是多项式拟合系数，阶数m和n根据需要选取，使得拟合误差足够小(1％-5％)即可，此二元多项式函数即为所构建的拟合模型。
在本实施例中，所述阶数m、n最大为4，将上述数值代入式(4)中，得到拟合关系式为：
IAlkali＝a00+a01*T+a02*T2+a03*T3+a04*T4+a10*CAlkali+a11*CAlkali*T
+a12*CAlkali*T2+a13*CAlkali*T3+a20*CAlkali 2+a21*CAlkali 2*T   (5)
+a22*CAlkali 2*T2+a30*CAlkali 3+a31*CAlkali 3*T+a40*CAlkali 4
其中，碱金属特征谱线辐射强度IAlkali的单位为W/m3/sr，温度T的单位为K，气相碱金属浓度CAlkali的单位为ppm。
(2)标定处理步骤
为了保证进入光纤，并被光谱仪接收的信号全部是来自测量位置视线累积方向上的火焰辐射信号，在光纤探测一端安装准直透镜；光谱仪的原始输出信号是一个通过光电转换得到的电压值，不能完全反映相应辐射能量的大小，为了得到光谱绝对辐射强度，光谱仪系统在黑体炉上进行标定，用进行过绝对辐射强度标定的光谱仪系统测量已知碱金属浓度的火焰对象，得到其碱金属特征谱线辐射强度和火焰温度，通过实验标定，把测量的一系列不同碱金属浓度下对应的特征谱线辐射强度和火焰温度带入式(5)中，得到模型的多项式拟合系数amn。
模型多项式拟合系数amn的确定是在图2所示的实验标定处理系统上进行的，主要设备包括层流预混火焰多射流燃烧器、燃料及氧化剂供给装置、空气供给装置、质量流量控制器、光谱检测系统等。乙烯、氧气、氮气分别从钢瓶中流出，通过调压阀调整压力，然后进入质量流量控制器1，通过四通阀2进入气体混合器3；氩气从钢瓶中流出后依次经过调压阀和质量流量计，并且携带超声波雾化器4中生成的碱金属盐液雾，进入气体混合器3，四种气体与碱金属液雾混合均匀后一并进入燃烧器5的射流管路，伴随气 流由空气压缩机6供给，进入质量流量控制器，最后进入射流燃烧器5的伴随气流管路。光谱检测系统主要有准直透镜7、光纤8、光谱仪9、USB连接线10及笔记本电脑11组成。表1给出了实验标定的具体工况：
表1实验工况设定
Figure PCTCN2015080739-appb-000010
实验过程具体如下：
(2.1)计算气相碱金属浓度CAlkali和碱金属特征谱线辐射强度IAlkali：
在一个工况下，保持燃料与氧化剂流量恒定即维持一个恒定的火焰温度，改变碱金属溶液的浓度Cs，得到一系列火焰中气相碱金属浓度CAlkali(0ppm，2ppm，4ppm，6ppm，8ppm，10ppm)以及其对应的碱金属特征谱线辐射强度值IAlkali；
对于碱金属在气相中浓度的计算，由于雾化碱金属溶液是与燃气充分混合后再进入射流管，因此可以假设火焰中碱金属分布是均匀的。碱金属溶液的溶质质量流量νm(g/s)通过式(6)计算得到：
νm＝Cs·νs   (6)
其中，Cs为碱金属溶液的浓度(％)，νs为碱金属溶液的消耗速率(g/s)，可以通过让雾化器长时间工作，统计平均得到；
火焰中碱金属的摩尔流量νms(mol/s)通过式(7)计算得到：
Figure PCTCN2015080739-appb-000011
其中，am为碱金属盐的摩尔分数(％)；M为碱金属盐的摩尔质量(g/mol)；
火焰区域总气体流量vf(L/s)可以表示为：
Figure PCTCN2015080739-appb-000012
其中，
Figure PCTCN2015080739-appb-000013
和
Figure PCTCN2015080739-appb-000014
分别为氧气、氮气和乙烯的体积流量，通过流量控制器控制得到；Tf为火焰平均温度K；Tr为实验室房室温K；
根据理想气体的状态方程，火焰区域总气体的摩尔流量vmf(mol/s)可以表示为：
Figure PCTCN2015080739-appb-000015
其中，R为理想气体常数，P为压力；
将式(8)带入式(9)整理得：
Figure PCTCN2015080739-appb-000016
将式(7)与式(10)做比值，即可得到火焰区域气相碱金属的体积浓度CAlkali(ppm)：
Figure PCTCN2015080739-appb-000017
至此，在已知碱金属溶液浓度、设定气体流量、已知室温的前提下，可以通过式(11)计算得到火焰中气相碱金属的体积浓度；
然后，改变工况，即营造不同温度环境，重复上述过程，从标定过的光谱仪中测量得到不同火焰温度下的碱金属特征谱线辐射强度值IAlkali；
(2.2)计算火焰温度：
对于火焰温度的计算，利用热辐射定律，对经过黑体炉标定的绝对辐射强度Id(λ)，选取间隔为Δλ的两波长λ、λ+Δλ，计算得到火焰温度Tλ：
Figure PCTCN2015080739-appb-000018
其中，Planck常数C2＝1.4388E-2；Id(λ)、Id(λ+Δλ)分别为波长λ、λ+Δλ下的光谱辐射强度。
(2.3)计算拟合系数amn：
把表1中五个实验工况下的不同火焰温度和不同气相碱金属浓度及对应的碱金属特征谱线辐射强度值，代入式(5)中建立的拟合关系式，计算得到拟合系数amn，至此，确立碱金属特征谱线辐射强度关于气相碱金属浓度和火焰温度之间的函数模型。
(3)在线检测步骤
用标定过的光谱仪系统去测量炉内燃烧火焰辐射光谱，利用热辐射定律，计算得到火焰温度，提取火焰辐射光谱中要测量的碱金属特征谱线辐射强度值，将计算的火焰温度和特征谱线辐射强度值代入已知拟合系数的式(5)中，求解出燃烧火焰中气相碱金属浓度。
图3是AvaSpec-2048-USB2便携式光谱仪在垃圾焚烧锅炉上检测得到的火焰辐射光谱图。
该光谱仪探测器是2048像素的CCD阵列，测量波长范围为200～1100nm，分辨率0.8nm，信噪比为200：1，根据炉内实际测量得到的火焰辐射光谱，计算得到火焰温度T,把要测量的碱金属特征谱线辐射强度IAlkali和火焰温度T，代入已知拟合系数amn的式(5)中，即可得到该测量火焰中气相碱金属的具体浓度。
本领域的技术人员容易理解，以上所述仅为本发明的较佳实施例而已，并不用以限制本发明，凡在本发明的精神和原则之内所作的任何修改、等同替换和改进等，均应包含在本发明的保护范围之内。

Claims (3)

  1. 一种锅炉燃烧火焰中的气相碱金属浓度的在线检测方法，其特征在于，包括如下步骤：
    (1)基于特征谱线构建拟合模型：选取要测量的碱金属特征谱线，基于上述选取的碱金属特征谱线，构建燃烧火焰中碱金属特征谱线辐射强度IAlkali关于气相碱金属浓度CAlkali与火焰温度T之间的拟合模型：
    Figure PCTCN2015080739-appb-100001
    其中amn是多项式拟合系数，阶数m、n为正整数，T表示作为测量对象的火焰的温度，其单位为K；
    (2)执行标定处理：将检测光谱仪进行绝对辐射强度标定，然后用该标定后的检测光谱仪测量已知碱金属浓度的火焰对象，得到其碱金属特征谱线辐射强度和火焰温度，把测量的一系列不同碱金属浓度下对应的特征谱线辐射强度和火焰温度带入所述拟合模型
    Figure PCTCN2015080739-appb-100002
    中，得到模型的多项式拟合系数amn的具体数值；
    (3)在线检测：采用上述标定后的检测光谱仪实时测量锅炉燃烧火焰辐射光谱，获得相对应的碱金属特征谱线辐射强度，同时获得火焰温度，将所述碱金属特征谱线辐射强度、所述火焰温度以及步骤(2)获得的多项式拟合系数amn的具体数值代入步骤(1)中所构建的拟合模型中，计算获得锅炉燃烧火焰中气相碱金属的实际浓度，以此实现锅炉燃烧火焰中气相碱金属浓度的在线检测。
  2. 如权利要求1所述的一种锅炉燃烧火焰中的气相碱金属浓度的在线检测方法，其特征在于，步骤(1)中所述拟合模型
    Figure PCTCN2015080739-appb-100003
    的构建步骤如下：
    (1.1)根据火焰光谱分析，在热力学平衡时，元素的原子或离子吸收高温火焰中的能量由基态变为激发态，单位体积的基态原子数N0与激发态原子数Ni之间的分布满足统计力学中麦克斯韦—玻尔兹曼分布定律，即：
    Figure PCTCN2015080739-appb-100004
    其中，gi、g0分别为激发态与基态的统计权重，Ei为激发能，k为玻尔兹曼常数，T为激发温度。
    (1.2)两能级i、j之间跃迁所产生的谱线强度Iij与激发态原子数目的关系表示如下：
    Iij＝Ni·Aij·h·vij        (2)；
    其中，Aij为两个能级间的跃迁几率，h为普朗克常数，vij为发射谱线的频率；将上述式(1)带入该式(2)中得到下式：
    Figure PCTCN2015080739-appb-100005
    (1.3)根据式(3)可知，原子特征谱线辐射强度与基态原子数N0和激发温度T有关，而基态原子数N0又与浓度有关，因此碱金属特征谱线辐射强度IAlkali可以表示为碱金属浓度CAlkali与火焰温度T的函数，采用最小二乘法拟合得到拟合模型：
    Figure PCTCN2015080739-appb-100006
  3. 如权利要求1或2所述的一种锅炉燃烧火焰中的气相碱金属浓度的在线检测方法，其特征在于，所述检测光谱仪在黑体炉上进行标定。
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