Safir

The determination of toxic components from fire gases is difficult because the environment is hot, reactions are often temperature dependent, and a lot of soot may be produced. Due to the different properties of the gas components, a different time-consuming procedure for each species has traditionally been used. The use of FTIR (Fourier Transform InfraRed) spectrometers as a continuous monitoring technique overcomes many of the problems in smoke gas analyses. FTIR offers an opportunity to set up a calibration and prediction method for each gas showing a characteristic spectral band in the infra-red region of the spectrum.

The objective of this project was to further develop the FTIR gas analysis of smoke gases to be an applicable and reliable method for the determination of toxic components in combustion gases related to fire test conditions. The project included the following tasks: small scale and large scale sampling; analysis, calibration and software techniques; the verification of the method; and an interlaboratory trial.

The optimum probe design, filter parameters and the most suitable sampling lines in terms of flow rate, diameter, construction material and operating temperature have been specified. The gas adsorption onto the filter and the soot have been measured. In the large scale, special concern was given to the probe design and the effects of the probe location in relation to the fire source as well as practical considerations of the sampling line length.

Quantitative calibration and prediction methods have been constructed for different components present in smoke gases. Recommendations on how to deal with interferents, non-linearities and outliers have been provided and a verification method for the spectrometer for unexpected variations and for the different models have been described.

FTIR measurement procedures in different fire test scenarios have been studied using the recommendations of this project for measurement techniques and analysis, and real precision values for specific test scenarios have been estimated. Also a proposal for draft standard of the FTIR method for smoke gas analysis has been prepared.

An interlaboratory trial of the FTIR technique in smoke gas analysis was carried out to define the repeatability and reproducibility of the method in connection with a small scale fire test method, the cone calorimeter.

  

   
 Fire gas measurements in the door of the ISO 9705 room
conducted at SP in WP4 of the SAFIR project.  

 

Consortium

Four European research groups are being sponsored to carry out the SAFIR project. The project will begin in 1997 and last for two years. It will be performed by :

  • VTT Building Technology (Finland)
  • Scientific and Technical Centre of the Belgian Textile Industry (Belgium)
  • ELF ATOCHEM, Groupement de Recherche de Lacq (France)
  • Fire Research Station, Building Research Establishment (UK)
  • Laboratoire national d'essais (France)
  • L.S.F. SUD s.r.l., Laboratorio di Studi e ricerche sul Fuoco (Italy)
  • RAPRA Technology Ltd (UK)
  • Swedish National Testing and Research Institute (Sweden)
  • University of Ghent (Belgium)
  • University of Greenwich, School of Chemical & Life Sciences (UK)

Project description

The SAFIR project was divided into five work packages:

WP1 Sampling in small scale

The objectives of Work Package 1 were to define and optimise the FTIR sampling device in connection with different small scale fire tests. For this purpose, the different items of the sampling device were studied:

the probe to collect effluents from the fire effluent stream, 
the filter to retain particles, 
the transfer line to transport the effluents to the FTIR analysis,
the gas cell.

 

WP2 Sampling in large scale

WP2 aimed to test the correctness of the concentrations extracted from FTIR spectra measured in large scale tests with possibly high soot concentrations and high temperatures. Thus, the construction of probe, choice of filter, pumping regime, probe temperature, and inlet configuration needed to be optimised taking into consideration the special features of large scale testing. The work relied heavily on the results from WP1.

WP3 Data analysis, calibration and software

The objective of work package 3 was to study different techniques for the prediction of concentration of toxic gases, such as SO2, HCl, Acrolein, HCN, NO, NO2, HBr, CO and CO2, in smoke gases. 

WP4 Verification in different fire scenarios

WP4 was tasked with verifying the results of WP1 (sampling in the small-scale) and WP2 (sampling in the large-scale) by using the recommended parameters from those WP's during sampling and analysis from small and large-scale fire tests.

A key objective was to compare quantitatively the FTIR response in each case, with the response of a comparison method, which is an alternative method for a given species, known to be capable of giving reliable results in fire gas analysis. 

WP5 Interlaboratory trial

The objective of the interlaboratory trial (WP5) is to gain international acceptance for the FTIR analysis of smoke gases by determining the repeatability and reproducibility of the method according to the ISO 5725 principles.

Conclusions

FTIR spectrometers offer a continuous monitoring technique of many gases simultaneously in smoke gas analysis. Using FTIR, it is possible to set up a calibration and prediction method for each gas showing a characteristic spectral band in the infra-red region of the spectrum. The main problem in the measurements is to have a representative sample of fire gases analysed.

The principle in defining an optimum sampling system for FTIR was to transfer the fire effluents through the sampling device as quickly as possible and keep them unaltered during the passage. To ensure the representative smoke gas samples for the analysis, the following recommendations applicable to both small and large scale should be taken into account:

  • A multi-hole probe made of stainless steel with minimum hole diameter of 3 mm is recommended. The holes shall be oriented downstream from the fire gases.
  • Both circular (glass fibre except for HF) and cylindrical (e.g. ceramic) filters can be used. The initial amount of Cl and Br in the filters shall be checked.
  • A gas sampling line made of PTFE with inner diameter of 3 - 4 mm offers the best performance. The line should be as short as possible; lengths up to 4 m are found acceptable.
  • The flow rate shall be as high as possible; at least 3,5 l/min.
  • The temperature of the sampling device shall be as high as possible with minimum of 150°C and practical maximum of 190°C. To avoid cold points where condensation could appear, the temperatures of the different parts should be as close to each other as possible. If the same temperature cannot be maintained in all parts of the sampling system, the temperature shall preferable increase towards the end of the device.
  • When acidic gases are analysed by FTIR, it is recommended to wash the different parts of the sampling device especially when the concentrations determined are low. The washing solution can be analysed by an appropriate analytical method to evaluate the total amount of the gas produced by the combustion.
  • The pressure in the gas cell must be monitored during the experiments because the variation of the pressure inside the sampling configuration leads to a variation in the measured concentrations of gaseous products.

There are some features of the measuring system that are specific for large scale testing. Making measurements e.g. in the door of a fire room places extra constraints on the measuring system:

  • When measuring gases that are not well mixed the multi-hole probe sampling from the fire gases should have holes that are graded in size according to where they are on the probe.
  • Due to the size of the testing facilities it can sometimes be difficult to keep the sampling line short. In cases where the sampling line is very long and acid gases are being studied it is advisable to wash the line to check for adsorbed acids.
    The findings of this project concerning sampling techniques are most relevant for all other methods used for defining fire gases.

Quantitative calibration and prediction methods have been constructed for different components present in smoke gases. Recommendations on how to deal with interferents, non-linearities and outliers have been provided and a verification method for the spectrometer for unexpected variations and for the different models have been described. To circumvent the availability of software tools for analysing smoke, additional software has been written.

The following conclusions and recommendations are given related to data analysis, calibration and software:

  • Multivariate chemometrical techniques such as CLS, PLS, INLR and QTFA are needed for the prediction of toxic components in smoke gases generated during a fire test. For several gases, linear techniques are inadequate and non-linear techniques are needed.
  • For satisfactory analysis using multivariate or univariate methods, it is essential that good reference spectra are obtained for calibration. A sufficient number of calibration spectra should be obtained to allow modelling of non-linear behaviour with reasonable precision.
  • Extrapolation of concentrations outside the calibration range should be avoided.
  • It is strongly recommended to visually inspect the data output of the multivariate models. Unexpected behaviour (e.g. non-zero baseline) should be investigated.
    For the verification of the FTIR results, FTIR measurement procedures in different fire test scenarios have been studied using the recommendations of this project for measurement techniques and analysis, and real precision values for specific test scenarios have been estimated. A proposal for draft standard of the FTIR method for smoke gas analysis has been prepared.

As a results of the verification study, the following conclusions have been made: · FTIR is capable of making time resolved measurements on many species simultaneously. · Concentration/time profiles are realistic and follow the shape of the profiles from comparison methods and fire test characteristics. FTIR gives a good peak response. · Cell pressure shall be monitored due to its influence on the concentration results. · Concentrations will be influenced by water vapour, i.e. the FTIR results can be different from the comparison method results obtained with dried gas sample.

An interlaboratory trial was carried out using the cone calorimeter method testing three materials in three replicates. The FTIR method of measuring smoke gases was found repeatable and reproducible when analysed statistically according to the ISO 5725 principles and compared with corresponding data of well-known fire test methods.

A full report is available from the project co-ordinator.

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Per Blomqvist

Phone: +46 10 516 56 70

RISE Research Institutes of Sweden, Phone 010-516 50 00, E-mail info@ri.se

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