Infrared Spectroscopy for Online Measurement of Tars, Water, and Permanent Gases in Biomass Gasification

Online measurements of the raw gas composition, including tars and water, during biomass gasification provide valuable information in fundamental investigations and for process control. Mainly consisting of hydrocarbons, tars can, in principle, be measured using Fourier transform infrared (FT-IR) spectroscopy. However, an instrument subjected to raw gas runs the risk of condensation of tars on optical components and subsequent malfunction. Therefore, an external cell, heated to at least  400 ℃, has been designed to ensure that tars remain in the gas phase during FT-IR measurements. The cell was used for on-line FT-IR measurements of permanent gases (CO, CO2, CH4), water, and tars during the operation of a lab-scale downdraft gasifier using wood pellets, bark pellets, and char chips. Based on calibration, the measurement error of permanent gases was estimated to be 0.2%. Concentrations evaluated from spectral signatures of hydrocarbons in tar are in good agreement with results from solid-phase adsorption measurements and correlated well with operational changes in the gasifier.

Measurements at different concentrations were made by dilution of the gas mix with nitrogen.
Results from analyses of CO, CO2, and CH4 are presented in Figure S1a, where the evaluated mole fractions are plotted versus the set mole fractions of the mixes. The dashed line represents points where evaluated and set concentrations would be equal. Up to mole fractions of 10 % for CO and 5% for CO2 and CH4, the absolute error in the evaluated concentration is less than 1%. While the error for CH4 remains below 1% up to a mole fraction of 10%, the error for CO and CO2 increases. At the maximum mole fractions for CO and CO2, 29.9 and 10.0%, the error is 3 and 2%, respectively. The increasing error may be due to partially saturated absorption for which the Beer-Lambert law is no longer directly valid. The uncertainties of the calibration mixture given in Table S1 represent percentages of the specified fractions. Uncertainties for the set mole fractions in the experiments have been estimated from the values of Table S1 combined with uncertainties in mass flow controller readings assumed to be 1% of full scale. The uncertainties in set mole fractions are thus estimated to be 0.2-0.7% for CO and 0.1-0.2% for CO2 and CH4. Figure S1b-d show measured and fitted spectra of CO, CO2, and CH4 at the lowest mole fractions with good agreement between experimental data and simulations. Figure S1. Calibration of the FT-IR setup with the external cell at temperature 400 °C using a mixture of permanent gases CO, CO 2, and CH 4 with known concentrations at levels similar to the product gas from gasification. a) Graph of evaluated concentrations versus mixture composition. Experimental FT-IR spectra (blue) and fitted HITRAN spectra (red) spectra for calibration gas b) CO (2.9%), c) CO 2 (0.9%), and d) CH 4 (1.0%). Residual plots below each graph shows the difference (ΔA) between the experimental and the fitted spectrum.  Figure S2 shows concentrations of CO2 and CH4 obtained during gasification from the separate infrared instrument and FT-IR measurements. Sharp drops in the CO2 concentration obtained from the FT-IR appear because the cell is directly connected to the gasifier, and intermittent disturbances in gasifier operation due to the stirring of the bed are captured. Figure S2. Comparison of CH 4 and CO 2 concentrations obtained from the IR (dry gas) and FT-IR instruments (wet gas) measured during the gasification of wood pellets.

Comparison with IR instrument
Mole fractions of CO2 and CH4 measured by the IR instrument are higher than results obtained with FT-IR. In contrast to FT-IR, which measures the wet gas, the IR instrument measured the dry gas, which had been cooled to 20 °C. Measurements on dry gas result in marginally higher readings for the IR instrument compared with FT-IR and does probably not fully account for the observed differences. The gas monitored by the IR device was cleaned using a particle filter to avoid damage to the instrument, whereas the particle-laden raw gas was passed through the heated cell. The presence of particles may affect the FT-IR measurements via extinction, i.e., absorption and scattering of light by the particles. However, extinction should mainly affect the spectrum baseline, which is compensated for in the data evaluation. Nevertheless, uncertainties in baseline compensation and some impact of extinction cannot be ruled out completely.
However, the comparison suggests that evaluated FT-IR data from the gasification could be underestimated. Figure S3 and S4 present results obtained during gasifier operation with a closed cell for the FT-IR measurements.  time of addition of fuel has been removed intentionally from Figure S4 and S5, as the gas flow was disturbed. Increased concentrations in CO, CH4, H2O, and tars can be seen after the addition of fresh fuel, which then comes in contact with the heated bed and undergoes devolatilisation.

Measurements with closed cell
Concentrations of the species mentioned above later reduce with time as the fuel dried and was converted into char.