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This paper was accepted for presentation at the
5TH INTERNATIONAL SYMPOSIUM ON GAS ANALYSIS BY TUNABLE DIODE LASERS.
Held in Freiburg, Germany, February 1998.

Simultaneous in-situ measurement of
O2 , HCl, HF, CO and dust
in gas from a waste incinerator
using diode laser spectroscopy

O. Bjorøy, I. Linnerud, V. Avetisov and K. H. Haugholt
Norsk Elektro Optikk A/S
Solheimv. 62A
P.O.box 384
N-1471 Skårer, Norway.

Abstract

Diode laser based monitors for measuring O2 , HCl, HF, CO and dust have been installed in a waste incinerator plant for continuous measurement of emissions and optimisation of the running conditions of the furnace. The measurement principle for the gas monitors is wavelength modulation spectroscopy (WMS) using harmonic detection. Long and short time field measurements are presented and discussed. It was found that the gas concentrations, in particular the concentration of CO, show large variations on a time scale less than 15 seconds which reflects fast changes in the combustion process. The monitors have been running reliably for more than a year showing performance characteristics that fulfil requirements for commercial monitors.

1 Introduction

The gases output from combustion can be divided into two groups; those which depend on the operating conditions of the process, and those which depend on the type of fuel. O2, CO, CO2, NOx and uncombusted hydrocarbons (HC) will typically depend on combustion temperature, air to fuel ratio, turbulence, time spent in the furnace and similar parameters. The concentration of HCl , HF and SO2 on the other hand, depend mostly on the content of Cl, F and S in the fuel. Similarly with metals. The metals are mostly carried out by the dust particles which avoid flue gas cleaning. The amount of dust will largely be determined by the amount of incombustible parts in the fuel and the efficiency of the flue gas cleaning. For inefficient combustion it will also contain some carbon. The methods for removing dust are not efficient for removal of NOx , SO2 , HCl and HF. Removal of NOx is often done by Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR) in which NH3 is added to the flue gas. This is also efficient for reducing emissions of HCl , HF and SO2. For such installations monitoring of NH3 is useful for optimising the consumption of the gas, as well as reducing corrosion and environmental impact from excessive use.

In situ monitoring of the gases mentioned above is advantageous over extractive methods since it allows the process gas to be measured directly with fast response and eliminates problems associated with gas extraction. Norsk Elektro Optikk (NEO) has developed a series of monitors based on tuneable diode laser absorption spectroscopy (TDLAS) for continuous in situ measurements of O2 , HCl, HF, CO, NH3 and dust. Monitors for other gases are being developed. The monitors are capable of measuring process gases with large variations in temperature, pressure and composition. The measurements work in atmospheres where other measurement techniques tend to fail, e.g. at low pressures, very high temperatures and very corrosive gas mixtures. Rugged industrial design allows the instruments to withstand hash environments. This paper demonstrates simultaneous in situ measurements of several gas monitors installed on the stack of an incinerator plant.

2 NEO LaserGas monitors

The NEO LaserGas monitors are in situ single-component gas analysers utilising TDLAS with second-harmonic (2f) detection [1,2]. The monitors use near-infrared diode lasers designed to operate at specific wavelengths. The wavelengths are carefully selected to match absorption lines that give maximum sensitivity and minimum cross interference with absorption lines from other gases.

Low concentrations of the measured gases and relatively weak transition line strengths in the near-infrared region necessitate the use of a modulation technique to improve the sensitivity. WMS with second-harmonic detection efficiently discriminates against the sloping laser intensity baseline and makes it possible to detect absorbances as low as 10-5 - 10-6 . This imposes special requirements on the optical design of in situ gas monitors. In practice the detection sensitivity is limited by optical noise such as etalon effects and laser feedback. To minimise these effects the monitors have been designed with a minimum of optical components between laser and detector.

The mechanical layout of a monitor installed on a stack is shown in Fig. 1. The laser and detector are located in the transmitter and receiver units respectively. Both units are mounted on the stack using standard flanges purged with dry air to keep the optical windows clean. The detected signal is transmitted through a cable to the electronics unit which may be mounted up to 80 meter from the stack. The electronics unit contains signal processing electronics controlled by an embedded microprocessor. The measured concentration is displayed on an LCD display, and for data recording and logging it is sent through an RS232 digital output and through a standard 4-20 mA analogue output .

Figure showing Installation

Figure 1: Schematic drawing of the NEO gas and dust monitors.

3 The waste incinerator

A typical application for NEO’s monitors is combustion and emission control systems for boilers and waste incinerators. Monitors for O2 , CO, HCl, HF and dust have simultaneously measured in-situ in the stack of a 27 MW circulating fluidized bed (CFB) combined boiler and incinerator at a paper mill in Norway. The boiler produces a maximum of 40 tons of steam per hour at 210 °C and 20 bar. The steam is used in the paper mill production and may vary rapidly from 20 -100% capacity. The boiler is designed to burn municipal waste, plastic, wood, paper, waste oil and coal. These fuels have greatly different heating values and this puts high demands on process control and flue gas cleaning.

After the combustion in the CFB reactor the flue gas passes through two cyclones and the boiler before it is cleaned in a multi cyclone and an electro scrubber. The flue gas is then let out through a 40 meter high stack where the gas and dust monitors are located. This plant has no SCR or SNCR cleaning system, therefore an NH3 monitor has not been installed.

The monitors have been running reliably with little maintenance for more than a year. During this period the gas monitors were only used for monitoring and not for (automatic) process control. Data from the dust monitor have not been logged continuously and are not presented in this paper.

4 Measurements

O2 and CO concentrations in the flue gas are the most important gases for monitoring of combustion efficiency. Complete oxidation cannot be obtained with a stoichiometric amount of O2, therefore an excessive amount of air is used. Too much air, however, will cool down the combustion and increase the amount of CO in the flue gas. There exist an optimal amount of air. Figure 2-a and b show the O2 and CO concentrations respectively, for varying efficiency of the combustion. When the O2 concentration drops below 5.5 %vol the CO concentration peaks sharply to values as high as 4000 mg/Nm3, and when the O2 concentration is 6.2 - 7.0 %vol the CO concentration is approximately 50 mg/Nm3. We may further notice that the fluctuations in CO concentrations are extremely fast and large. In Figure 2 the concentrations have been averaged over 1 minute and we see changes from 50 - 2300 mg/Nm3 from one sample to the next. The fastest response of the CO monitor is 15 seconds, and we have seen cases where the concentration has changed from 100 - 9000 - 100 mg/Nm3 in three successive samples at this sampling rate. It is difficult to believe that conditions can change this fast in such a large furnace, but considering a gas flow of 20 m3/s at a speed of 20 m/s, we realise that all the gas in the plant has been replaced in less than 15 seconds. Such fast response measurements can therefore give valuable information about the nature of the combustion of waste.

In Figure 3 we see data collected over 9 days where each sample has been averaged over 30 minutes. The furnace has been stopped and started several times during this period. This can be seen from the O2 levels. On day 304 the furnace was closed with large amounts of combustible material still in it. This produced large amounts of CO for several hours in the cooling down period. The CO concentration peaks again during start-up due to the low initial temperature of the furnace. This pattern repeats every time the furnace is shut down.

Experience from incinerators [3] has shown that approximately 90% of the Cl in the fuel will end up as HCl in the flue gas, while only 10 % of the F ends up as HF. Also there is much more Cl than F in most organic materials and household waste. We may therefore expect significantly more HCl than HF in the flue gas. This is confirmed by the measurements displayed in Figure 2 and 3. For HCl there seems to be no correlation with the O2 or CO concentrations when we disregard the stop periods of the furnace. During the days 308 and 309 we see an increased concentration of HCl which is probably due to an increased amount of Cl in the fuel. A close registration and analysis of the fuel has not been done in this study, and it is very difficult to trace back the original constituents in the pre-processed municipal waste.

Even when the furnace is stopped there is a significant reading (approximately 20 mg/Nm3) of HCl. This is significantly over the detection limit of the instrument which is approximately 0.5 mg/Nm3 for this installation. This concentration may be explained by the very low ventilation of flue gas in the stack when the furnace is stopped, combined with diffusion of HCl off surfaces and dust particles in the stack. This HCl has previously been adsorbed during the operating periods of the furnace. The HF concentrations are generally very low and rarely above the detection limit which is approximately 0.05 mg/Nm3 for this installation.

Fig. 2a, O2 Plot

Fig. 2b, CO Plot

Fig. 2c, HCl Plot

Fig. 2d, HF Plot

Figure 2: Gas concentrations measured over a period of 5 hours where each sample is averaged over 60 seconds. The CO concentration peaks abruptly when the O2 concentration goes below approximately 5.5 %vol. The HCl and HF concentrations do not show a similar correlation.

Fig. 3a, O2 Plot

Fig. 3b, CO Plot

Fig. 3c, HCl Plot

Fig. 3d, HF Plot

Figure 3: Gas concentration measured over a period of 9 days during which the furnace has been shut down several times. Each sample is averaged over 30 minutes. Note especially the large concentrations of CO at shutdown and start-up of the furnace.

5 Conclusions

In-situ measurements of O2 , CO, HCl and HF in flue gas from a waste incinerator using diode laser spectroscopy have proven to be well suited for continuous emission monitoring. The measurements have also given valuable information about optimisation of the running conditions of the furnace. For example, the measurements have shown that the CO concentration increased abruptly when the O2 concentration in the flue gas went below approximately 5.5 %vol. It was also found that the CO monitor needs a response time faster than 15 seconds if rapid changes in the combustion process are to be time resolved. The CO needs a large dynamic range since under good running conditions the concentration was approximately 50 - 100 mg/Nm3, but sometimes peaked rapidly above 9000 mg/Nm3. O2 , HCl and HF showed equally fast fluctuations, but generally had a much smaller dynamic range. HCl concentrations were typically in the range 50 - 200 mg/Nm3 and HF less than 0.1 mg/Nm3.

The monitors have been running continuously for more than one year with little maintenance. Although the monitors have not been used for automatic process control in this study, the experience with respect to reliability and response time has convinced us they are well suited for such an application.

References

[1] J. Reid and D. Labrie, "Second-harmonic detection with tuneable diode lasers -comparison of experiment and theory," Appl. Phys. B 26, 203-210 (1981).

[2] J. A. Silver, "Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods," Appl. Opt. 31, 707-717 (1992).

[3] Forbrenningsanlegg - veiledning for saksbehandlere (Combustion plants - guidelines for environmental inspectors), State Pollution Control Authority, Norway, (1995). (In Norwegian). General background literature cited in the above reference:

i) EPA Handbook: Vol. II of the hazardous waste incineration guidance series (EPA/625/6-89/019) (1989)

ii) D. A. Tillman: The combustion of solid fuels and wastes. Academic Press (1991)

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