Received 26 August 2014; revised 16 October 2014; accepted 12 November 2014

ABSTRACT

Contents of fly ash are important factors for the operation of coal-fired plants. Real-time monitoring of coal and fly ash such as unburned carbon in fly ash can be an indicator of the combustion conditions. Because of the strong signal intensity and the relative simplicity of the LIBS (Laser- Induced Breakdown Spectroscopy) technique, LIBS can be applicable for real-time composition measurement of coal and fly ash. This research presented here focused on the clarification of the effects of plasma temperature and coexisting materials on quantitative measurement of fly ash contents. Quantitative capability of LIBS was improved using the proposed plasma temperature correction method. The CO₂ effect was also discussed to accurately evaluate unburned carbon in fly ash in exhausts. Using the results shown in this study, quantitative measurement of fly ash contents has been improved for wider applications of LIBS to practical fields.

Keywords:

Laser-Induced Breakdown Spectroscopy, Fly Ash, Temperature Correction, CO₂ Effect

1. Introduction

The coal provides the largest share of world electricity generation and the thermal power plant share is dominant in total electric power supply in the world. Operating characteristics of coal-fired boilers are heavily influenced by factors such as differences in fuel properties and the combustion conditions. Accordingly, in order to achieve optimal operation of multiple coal-fired boilers, it is necessary to accurately understand the state of combustion and to adjust the control parameters. The research presented here focused on the quantity of unburned carbon in fly ash as an indicator of the combustion state, considering the measured value as a basis for optimal and stable boiler combustion control, as well as fly ash quality control.

Recently, Laser-Induced Breakdown Spectroscopy (LIBS) technique has been applied to various industrial fields [1] - [7] because of the fast response, high sensitivity, real-time and non-contact features. For the purpose of improving the economy and security of power plant, the content measurement of unburned carbon and other materials is essential for adjusting the boiler combustion. LIBS is suitable for composition measurement in gas, liquid, and solid materials [8] - [16] . This method is also suited to the analysis of heterogeneous materials such as fly ash and coal [17] - [20] and has been widely applied to analyze the compositional characterization of coal. For example, inorganic species in coal were analyzed quantitatively under different conditions [21] [22] . The solid sample features higher homogeneity than powdered counterparts. LIBS has also been applied for detection of unburned carbon in fly ash, char and pulverized coal under high-pressure and high-temperature conditions. The calibration difficulty of aerosol sample was surpassed by the correction factors. This automated LIBS apparatus was also applied in a boiler-control system of a power plant with the objective of achieving optimal and stable combustion [23] [24] , which enabled real-time measurement of unburned carbon in fly ash. This apparatus featured a detection time capability of less than 1 minute, which showed better real-time capability compared to most of the studies with the preparation of samples and analysis procedures [18] [25] . The boiler control in the real power plant was also demonstrated to achieve an optimized operation without time consumption. The contents of size-segregated particles depending on particle diameter have been measured using LIBS. The plasma temperature correction method was introduced to the size-segregated fly ash and pulverized coal to detect their quantitative contents [26] .

In this study, improvement on quantification of LIBS has been demonstrated in terms of plasma temperature and surrounding gas effects to further applications. These factors are important for practical applications; however, they have not been fully discussed in previous studies [23] [24] . Dependence of elemental signals of LIBS on the plasma temperature attributes to a very complex process in plasma. Several studies have reported the LTE (Local Thermodynamic Equilibrium) condition of plasma in several types of plasmas [27] . The plasma temperature is a very important factor for the quantification of the LIBS measurement [1] [24] . The correction factor depends on the experimental parameters such as the optical setups, pressure, buffer gas composition, and so on. Because the plasma induced by the LIBS process is not uniform and complex, LTE can be assumed in the limited range of these conditions. The effects of non-equilibrium were discussed using the intensity ratio of the emission pair of magnesium atom as a plasma temperature indicator and the appropriate range of the plasma temperature indicator was shown for quantification. The CO₂ effects were also shown and the requirement for quantification was discussed for the real-time measurement of unburned carbon in fly ash for the application of coal-fired power plants.

2. Theory

A calibration of the LIBS signal is necessary for quantitative analysis. Despite the fact that the LIBS processes involved are complex, the emission intensity from the atomized species can be described by the following equation with the assumption of uniform plasma temperature [1] [23] [24] [28] :

(1)

In the above expression, I_i is the emission intensity of species i, n_i is the concentration of species i, K_i,j is a variable that includes the Einstein A coefficient from the upper energy level j, g_i,j is the statistical weight of species i at the upper energy level j, E_i.j is the upper level energy of species i, k is the Boltzmann constant and T is the plasma temperature. In Equation (1), there are several factors that affect the emission intensity I_i, including plasma temperature, plasma non-uniformity, and matrix effects, etc. The appropriate correction factors must be contained in K_i,j to obtain quantitative results.

The main components of fly ash are Si, Al, Fe, Ca and C (carbon). The carbon content in fly ash can be calculated using the emission intensity ratios of Al, Fe, Ca and C to Si that is the main element in fly ash, from the following relation:

(2)

α_i is a variable factor related to species i, which contains the plasma temperature correction factor. These parameters have to be determined in the experimental operating conditions. Other species contents besides carbon, such as Al, Fe and Ca, can also be calculated using the similar relation.

In Equation (1), a uniform plasma temperature is assumed. Actually, emission intensity I_i is a function of concentration of species i and plasma temperature. The emission intensity for each element also fluctuates with plasma temperature, which introduces error into quantitative measurements. There are several methods of plasma temperature correction. The following equation is used in this study. α_i is defined by use of the intensity ratio of I_m,j1/I_m,j2:

(3)

D_i,j is the emission intensity correction factor from the upper energy level j of species i. I_m,j1 is the emission intensity from the upper energy level j1 of species m, I_m,j2 is the emission intensity from the upper energy level j2 of species m, and b_i,m is the temperature correction factor of the emission pair of I_m,j1 and I_m,j2 for species i.

is dependent on plasma temperature and is used for plasma temperature correction. Different

spectral lines from the same species are selected to correct the plasma temperature dependence of the emission intensity, and the intensity ratio of the emission pair is determined from Equation (1):

(4)

Applying the intensity ratio to Equation (3), α_i becomes:

(5)

Taking C (carbon) from the upper energy level jC for example, the concentration ratio is as follows:

(6)

This ratio becomes a function of species concentration and does not depend on the plasma temperature on condition that b_C,m is given as follows:

(7)

From this analytical result, the correction factor can be calculated using the above equation. However, the correction factor depends on the experimental parameters such as the optical setups, pressure, buffer gas composition, and so on. The plasma induced by the LIBS process is not uniform and complex and LTE can be assumed in the limited range of these experimental conditions. In the case of ion emission intensities it is also necessary to consider the degree of ionization which can be calculated by the Saha equation as a function of plasma temperature and electron number density in a LTE condition. Therefore the correction factor cannot be determined by the theory and should be evaluated under actual experimental conditions. In this study the ranges of LTE conditions were categorized using the intensity ratio of the emission pair of magnesium atom as a plasma temperature indicator and Equation (3) was used for the plasma temperature correction.

3. Experimental Apparatus

Figure 1 shows the experimental apparatus used in this study. A Q-switched Nd:YAG laser (LOTIS TII, LS-2137U, 6 - 8 ns, energy stability: 2.5%, beam diameter: 8 mm, 10 Hz) was operated at 1064 nm with pulse energy of 30 - 90 mJ/p and laser beam was focused into the measurement area to make a plasma using a f = 200 mm lens. The samples were introduced to the measurement area by the 1.8 mm (inner diameter) stainless steel pipe and the breakdown of the particles was occurred along the laser path around the focusing point. The emission signals were separated from the laser path using a splitter and focused into abundled optical fiber with 24 fibers. The spectra were dispersed using a spectrometer (JASCOCT-10S) and detected by an ICCD camera (iStar 334T Series, Andor) with the wavelength resolution of 0.072 nm/pixel, FWHM of 0.5 nm and spectral bandwidth of 65 nm. The ICCD camera consisted of 1024 × 1024 active pixels with an effective active area of 13.3 mm × 13.3 mm and was cooled to −30˚C. The effective CCD pixel size was 13 μm × 13 μm. The frame rate was 4 image/sec. The row of the ICCD camera was used as wavelength axis and the column as the position axis of 24 fibers. Each spectrum was evaluated by summing the signal intensity of each column. The temperature correction factors were detected using laser power of 30 mJ/p and different delay time from 800 ns to 3000 ns by the measurement of a fly ash sample. Temperature correction factors and LTE conditions were determined using a fly ash sample by the intensity ratio of the emission pair of magnesium atom as a plasma temperature indicator. The data was acquired by setting the accumulation of 10 s each time. Pulverized coal (Newlands Coal) was added to combustion-supporting gas of methane and air using a feeder (Nisshin Engineering Inc. Feedcon-μ Mtype) and then introduced to the Bunsen-type burner. The flow rates of methane, air and coal were 3.9 l/min, 15.6 l/min and 0.41 g/min, respectively. A sampling pipe above the burner was employed to connect with the vacuum chamber of LIBS analyzer. The compositions of coal and fly ash used in this study are summarized in Table 1.

4. Results and Discussion

4.1. Temperature Correction Factors

In LIBS applications the plasma generation process contains several physical phenomena such as electron impact ionization and ion recombination, and its plasma temperature intrinsically fluctuates causing the fluctuation of evaluated concentrations from the LIBS signal intensities. Therefore the plasma temperature correction becomes important to assure the quantitative measurement, especially for the particle measurement in gases. The fly ash sample was used to determine the plasma temperature correction factors under different experimental conditions. Figure 2 shows LIBS spectra of fly ash in delay time of 800 ns, 1500 ns and 3000 ns with the laser power of 30 mJ/p. The intensity concerning species concentration varied with different delay time, which means the alteration of plasma temperature. The emission lines listed in Table 2 were used for the analysis of species concentration [29] . The ratio of I_Mg1/I_Mg2 was used for the plasma temperature correction factor as shown in Equation (3). I_Mg1/I_Mg2 was altered between 4 - 24 by changing the delay time between 800 ns - 3000 ns. I_Mg1/I_Mg2

became bigger according to the decrease of the delay time. The typical temperature correction curves between I_Mg1/I_Mg2: 4 - 24 and temperature corrected results of I_C/I_Si and I_Fe/I_Si are shown in Figure 3. It is clear from Figure 3(a) and Figure 3(b) that the ratios are influenced by the plasma temperature, which is directly related to the ratio of I_Mg1/I_Mg2. By applying the temperature correction scheme described in Equations (2)-(7), the fluctuation of I_C/I_Si and I_Fe/I_Si became smaller at different plasma temperatures, i.e. I_Mg1/I_Mg2. However, the plasma temperature dependencies of I_Mg1/I_Mg2 are apparently different between I_Mg1/I_Mg2: 4 - 8, which means the low plasma temperature condition, and I_Mg1/I_Mg2: 12 - 24, i.e. high plasma temperature condition. The high temperature condition corresponds to the initial process of the plasma generation from the fly ash particles and its expansion process to the surrounding air. While the low temperature condition is formed after the plasma expansion to the surrounding air. This result means that the plasma induced by the LIBS process is not uniform and complex and LTE can be assumed in the limited range of plasma temperature, i.e. I_Mg1/I_Mg2. The limitation of LTE assumption has been also discussed in the glow discharge plasma [27] , and the understanding of the phenomena is important for the quantitative measurement, which requires the accurate plasma correction.

The temperature correction curves between I_Mg1/I_Mg2: 4 - 8 and temperature corrected results of I_C/I_Si and I_Fe/I_Si are shown in Figure 4. Figure 5 shows those between I_Mg1/I_Mg2: 12 - 24. It is clear from Figure 4(a) and Figure 4(b) that the correction curves are much more accurate than those in Figure 3(a) and Figure 3(b) and Figure 5(a) and Figure 5(b), and b_i,m in Equation (3) takes the different values depending on the range of I_Mg1/I_Mg2, which means the limitation of LTE assumption. In the high temperature plasma, i.e. I_Mg1/I_Mg2: 12 - 24, the plasma is not uniform due to the initial process of the plasma generation from the fly ash particles and its expansion process to the surrounding air. With the increase of delay time, the plasma becomes uniform compared to the initial process of the plasma generation and tends to follow the LTE condition. The fluctuations of the concentration ratios of C/SiO₂ and Fe₂O₃/SiO₂ corrected by the correction factor b_i,m in the range of I_Mg1/I_Mg2: 4 - 24 are 17% and 8.9%, respectively. The fluctuations of the concentration ratios do not improve by limiting the range of I_Mg1/I_Mg2: 12 - 24. On the other hand the fluctuations of these ratios corrected by the correction factor b_i,m in the range of I_Mg1/I_Mg2: 4 - 8 reduce to be 11% and 3.7%. This phenomenon also appears in other species

such as I_Al/I_Si and I_Ca/I_Si. The results for C/SiO₂, Al₂O₃/SiO₂, Fe₂O₃/SiO₂ and CaO/SiO₂ are summarized in Table 3. The correction characteristics for these concentration ratios show the same tendency as described above and it is important to set measurement condition within an appropriate I_Mg1/I_Mg2 range. The cause of fluctuation in the temperature corrected results is also attributed to the sample uniformity. Because the fly ash contents are not uniform in the microscopic sense. Using this set of experimentally determined correction factors by choosing the appropriate I_Mg1/I_Mg2 range, the fluctuations of the concentration ratios such as Fe₂O₃/SiO₂, Al₂O₃/SiO₂, CaO/SiO₂, C/SiO₂, were greatly reduced.

There are several plasma temperature correction methods such as employing the Boltzmann plot method which uses several emission lines from the same atom from the wide-wavelength LIBS spectra. The calibration method using the Boltzmann, which often called “CF (calibration free) LIBS”, is a systematic approach for the temperature correction. However it requires wide-wavelength LIBS spectra and causes the loss of the signal intensity and the complication of the systems [30] [31] . The method employed in this paper can be applied using the LIBS spectra with the wavelength range of 240 - 320 nm and keeps the sufficient signal intensity with sufficient plasma temperature correction capability for fly ash measurements. Considering the system simplicity and real-time measurement capability, the temperature correction method employing emission pair of I_Mg1 and I_Mg2 is preferable compared to others, which demonstrated that the method is workable and satisfactory for practical applications by choosing the appropriate plasma temperature correction range.

4.2. Influence of Coexisting Gases

In the case of fly ash measurement at actual conditions such as the real-time measurement of fly ash contents in exhausts of coal-fired boilers, influence of coexisting gases such as CO₂ must be considered to maintain the quantitative measurement of this method. In the case of particle contents measurement in gases using LIBS, the effects of coexisting gases become important because the gas phase materials also produce plasma caused by the particle breakdowns. Figure 6 shows the LIBS signals from the burned gas of the burner without coal addition using different laser power. It is clear from Figure 6(a), the burned gas was broken by the laser input of 90 mJ/p and the strong C signal caused by the CO₂ breakdown was detected at 247.9 nm. When reducing the laser power, the C signal became small and the C signal disappeared at 37 mJ/p as shown in Figure 6(b). On the other hand, when the pulverized coal was added in the fuel and the fly ash in the burned gas was measured. The signals of

fly ash contents such as Si and Fe were measured along with the strong C signal as Figure 7(a) shows. This C signal is mainly produced by CO₂ in burned gas and it interferes with the measurement of carbon content in fly ash. Figure 7(b) shows that the C signal was also detected using the laser power of 37 mJ/p in the case of the fuel with pulverized coal, though it was not detected without pulverized coal as shown in Figure 6(b). This is because the particles such as fly ash became the core of the plasma and the plasma expanded to the surrounding gases. The influence of coexisting gases in LIBS depends on the measurement conditions, especially the particle conditions in gases, and its effects must be evaluated in the actual measurement applications.

Figure 8 shows the LIBS spectra of fly ash with different CO₂ concentration. In this experiment the fly ash was introduced into the measurement area with the addition of CO₂ in air. As CO₂ concentration increases, the C signal increases compared to other signals while other signal ratios such as Fe and Si remain the same. This is because the breakdown of CO₂ added in air was induced by the fly ash plasma. As shown in Figure 6(b) the C signal does not appear without pulverized coal addition. Figure 9 shows the dependence of I_C/I_Si on CO₂ concentration that was measured two times at each CO₂ concentration. The CO₂ effect is rather sensitive to the concentration in surrounding gas and less than 1% CO₂ causes the spurious C signal in LIBS spectra of fly ash. The measured results also fluctuated under the conditions with the same CO₂ concentration because of the unstable breakdown phenomena of CO₂ induced by fly ash. Therefore it is important to reduce the CO₂ content in burned gas when applying LIBS for unburned carbon content in fly ash under burned gas conditions. The threshold

level of CO₂ depends on the measurement conditions such as the range of carbon contents and particle density. CO₂ effect is highly laser power-dependent. An appropriate laser power setting is essential to minimize this spurious carbon signal. Increased pressure aggravates the problem due to the breakdown of CO₂ molecules. Careful attention is needed under high-pressure conditions [23] . The experimental parameters must be determined for the actual operation conditions. The sampling of fly ash using a cyclone employed in the coal-fired power plant is one of the effective methods to reduce this CO₂ effect [24] .

5. Conclusions

Contents of fly ash are important factors for the operation of coal-fired plants. This research has specified the effects of plasma temperature and coexisting materials to the quantitative measurement of contents of fly ash. Quantitative capability of LIBS was improved using the plasma temperature correction method by choosing the appropriate temperature correction range. The CO₂ effect was also clarified to accurately evaluate unburned carbon in fly ash in exhausts.

1) LTE was assumed in a limited range of plasma temperature in laser-induced plasma and the quantitative measurement was improved by choosing the appropriate plasma temperature range. In the case of fly ash measurement, the emission intensity ratio of Mg: I_Mg1/I_Mg2 became a plasma temperature indicator and the range of I_Mg1/I_Mg2: 4 - 8 showed better LTE condition resulting in the accurate plasma correction compared to the range of I_Mg1/I_Mg2: 12 - 24.

2) In the measurement of fly ash contents, CO₂ caused the spurious C signal in LIBS spectra of fly ash. The CO₂ effect was rather sensitive to the concentration in surrounding gas and less than 1% CO₂ caused the spurious C signal. It became important to reduce the CO₂ content in burned gas when applying LIBS for the measurement of unburned carbon content in fly ash under the burned gas conditions.

References

Nomenclatures

D_i,j: emission intensity correction factor from the upper energy level j of species i;

E_i,j: upper level energy of species i/J;

I_i: emission intensity of species i/J∙s⁻¹;

I_m,j1: emission intensity from the upper energy level j₁ of species m;

I_m,j2: emission intensity from the upper energy level j₂ of species m;

K_i,j: a variable that includes the Einstein A coefficient from the upper energy level j/J∙m³∙s⁻¹;

T: plasma temperature/K;

b_i,m: temperature correction factor of the emission pair of I_m,j1 and I_m,j2 for species i;

g_i,j: statistical weight of species i at the upper energy level j;

k: the Boltzmann constant/1.38 × 10⁻²³ J∙K⁻¹;

n_i: concentration of species i/m⁻³;

α_i: a variable factor related to species i, which contains the plasma temperature correction factor.