Advanced Diagnostics in Oxy-Fuel Combustion Processes

This report sums up the findings in PSO-project 010069, “Advanced Diagnostics in Oxy- Fuel Combustion Processes”. Three areas of optic diagnostics are covered in this work: - FTIR measurements in a 30 kW swirl burner. - IR measurements in a 30 kW swirl burner. - IR measurements in a laboratory scale fixed bed reactor.

The results obtained in the swirl burner have proved the FTIR method as a valuable technique for gas phase temperature measurements. When its efficacy is evaluated against traditional thermocouple measurements, two cases, with and without probe beam stop, must however be treated separately. When the FTIR probe is operated with the purpose of gas phase concentration measurements the probe needs to operate with a beam stop mounted in front of it.

With this beam stop in place it was shown that the measured gas phase temperature was affected by cooling, induced by the cooled beam stop. Hence, for a more accurate determination of gas phase temperatures the probe needed to operate without the beam stop. When this was the case, the FTIR probe showed superior to traditional temperature measurements using a thermocouple as it could measure the fast temperature fluctuations.

With the beam stop in place the efficacy of the FTIR probe for gas temperature determination was comparable to the use of a traditional thermocouple. The evaluation of the FTIR technique regarding estimation of gas phase concentrations of H2O, CO2 and CO showed that the method is reliable though it cannot be stated as particularly accurate.

The accuracy of the method is dependent on the similarity of the reference emission spectra of the gases with those obtained in the experiments, as the transmittance intensity is not a linear function of concentration. The length of the optical path also affects the steadiness of the measurements. The length of the optical path is difficult to adjust on the small scales that are the focus of this work.

However, interpretation of the optic signal in terms of concentration, is done assuming a homogeneous mixture and so a small optic path length induces fluctuations in the measurements caused by flow phenomena (eddies and turbulent structures) as well as mixing limitations. The fluctuations, however , reflect the actual conditions in the reactor and so may considered a strength of the method compared to extractive methods where the signal is most often evened out due to mixing in the sampling equipment.

The use of the IR technique for determination of particle temperatures, particle sizes, and number density proved reliable in both the swirl burner and the laboratory scale fixed bed reactor. When the technique was used in the swirl burner the subsequent data treatment was sensitive to optical disturbances, such as very dense particle clouds and blurred areas, which were sometimes mistakenly interpreted as particles by the software.

This short come can however be avoided in future investigations by setting stricter identification criteria in the software. In the fixed bed reactor the use of the IR technique was an invaluable tool in the discussion of data obtained by gas analysis, and it allowed for estimation of combustion times in O2/CO2 where the high CO2 concentration prevents the use of the carbon mass balance for that purpose.

During the project the data have been presented at a conference, formed the basis of a publication and it is part of two PhD dissertations. The name of the conference the journal and the dissertations are listed below. - Joint Meeting of the Scandinavian-Nordic and French Sections of the Combustion Institute, Combustion of Char Particles under Oxy-Fuel Conditions: Formation of NO and Particle Temperature, Copenhagen, 9-10 November 2009. - Brix J, Navascués LG, Joachim, Nielsen JB, Bonnek PL, Larsen HE, Clausen S, Glarborg P, Jensen AD, Oxy-Fuel Combustion of Coal Char: Particle Temperature and NO Formation, Submitted to Fuel on the 19th of November 2010. - Brix J, Oxy-Fuel Combustion of Coal, Ph.D.

Thesis, CHEC Research Centre – Technical University of Denmark, 2011. - Toftegaard, MB, OxyFuel Combustion of Coal and Biomass, Ph.D. Thesis, CHEC Research Centre – Technical University of Denmark, 2011. In addition two students projects have been carried out in relation to the project: Bonnek, PL og Nielsen, JB, 2*20 point