Open-Path Fourier Transform Infrared Analysis of Pyrolysis of Sagebrush

Introduction 

Many prescribed and wildland fires circulate through live rather than dead fuels. However, wildfire ignition from live fuels is not well understood. This work is an effort to collect gaseous preignition composition from live fuels to inform chemical kinetic models used for fire prediction and prevention. More specifically, Open-path Fourier Transform (OP-FTIR) spectroscopy is used to study pre-combustion gasification of live sagebrush (Artemisia Tridentate). Pyrolysis typically occurs between 400-800℃ and produces biochar, bio-oil and various gases. This study focuses on the gaseous emissions from the pyrolysis of sagebrush between 400-500℃.  

Materials and Methods 

Prior to experimentation, we looked at research that had been conducted on the pyrolysis of various plant species. We examined the documented light gas compositions resulting from slow pyrolysis of fourteen various live plant samples from this research. We took the average of these composition mixtures to estimate the expected chemical composition (%Wt.) of pyrolysis of sagebrush to be the following – 52.1% CO, 38.4% CO₂, 8.04% CH₄, 1.46% H₂. Our study neglected the analysis of H₂ as it does not absorb infrared radiation, making it invisible to OP-FTIR measurements. We simulated the expected absorption spectrum using the HITRAN database. The simulated spectra assumed a 0.1 cm pathlength, a temperature of 296 K, and a pressure of 1 atm. We conducted our experiment using the Thermo Scientific Nicolet iS10 FTIR Spectrometer. A background measurement was taken prior to experimentation and was automatically subtracted from the measurements. The sagebrush was heated using a Hemispherical 50 mL Glas-Col Heating Mantle, in a glass 3.0 mL vial with an inner diameter of 1.5 cm. We estimated the pathlength to be ~1 cm based on the diameter of the vial. The experiment was conducted directly on the spectrometer stage with no smart accessory connected. The temperature of the sagebrush was monitored using the VWR Traceable Infrared Thermometer Gun (12777-846). Prior to, and post pyrolysis, the mass of the sagebrush was measured using the Sartorius ENTRIS224-1SUS Analytical Balance. FTIR absorption and temperature measurements were taken in ~20 second intervals for an overall time of six to seven minutes to ensure the pyrolysis process was completed.   

Results and Discussion 

The experimental OP-FTIR spectra exhibited noticeable similarities to the simulated spectra and appear to confirm the presence of the expected molecules. CO₂ and CH₄ peaks were evident in the experimental spectra, however, there was a significant amount of noise in many areas making the spectra difficult to analyze. We believe the noise was due to an insufficient pathlength. There was also an expectation that we would be able to observe the absorbance of the present molecules increase, then decrease with time. However, our experimental data exhibited random increases and decreases of the visible molecules. We believe this was due to inconsistent heating throughout the sagebrush sample. The spectral data was analyzed using OriginLab Software. Baseline correction was performed using the first and second derivative method. Signal smoothing techniques were attempted to combat noise, but were not successful due to spectral peak warping. The mass of the sagebrush prior to, and post pyrolysis were 0.5042 g and 0.1200 g, respectively. This indicates a mass loss of 0.3842 g or 76.2% of the initial mass. Temperature measurements of the sagebrush throughout the pyrolysis process ranged from 415-490℃.      

Conclusion                  

The experimental data appears to confirm the presence of the expected molecules, but also exhibits noise and unexpected randomness making it difficult to analyze. We plan to continue this research and improve upon the experimental methods by using crushed or chopped sagebrush (i.e., homogenous) so that it can be heated consistently to allow for time lapse absorbance data. We also plan to research possible means to increase the pathlength.