5:15 PM - 6:45 PM
[HCG26-P01] Quantitative analysis of low molecular weight gases produced from pyrolysis of peat soil
Keywords:Peatland fire, Global warming, Haze, Gas Chromatograph
1. Introduction
Peat is a type of biomass soil formed by the partial decomposition and deposition of dead plants. It is composed mainly of woody biomass such as lignin and cellulose, and also contains trace amounts of metal components. In Indonesia, which has particularly large peatlands, large-scale peatland fires frequently occur when peatlands are dried out by the El Nino phenomenon and ignited by illegal open burning for the purpose of agricultural land development. Air quality risk assessment of peatland fires is important because the peatland fires release large amounts of greenhouse gases (such as CO2 and methane) and volatile organic compounds (such as ethylene and phenol) into the atmosphere, causing global warming and haze, which is an international environmental problem.
Peatland fires progress in two forms: surface fires and ground fires. The former burns with flames on the ground surface, and the latter smolders without flames in the underground. While the ground fires progress at relatively low temperatures (200-600 °C), they are considered a significant environmental risk because they can spread into deep underground and smolder for long periods. The ground fire temperatures are distributed such that the greater the distance from the ground surface, the lower the temperature. Therefore, understanding the amount of peat pyrolysis products at each temperature is important for environmental risk assessment. Low molecular gases such as methane, which has a large global warming potential, and ethylene, which contributes to haze, have not been quantitatively analyzed in previous studies, so it is necessary to determine their emission factors.
In this study, quantitative analysis of peat pyrolysis gases was performed by pyrolysis gas chromatography (Py-GC) at 255-740 °C in the absence of oxygen, assuming the ground fire. This will clarify changes in the amount of gas produced at different pyrolysis temperatures and aim to elucidate the generation mechanism of low molecular weight gases and determine their emission factors at each pyrolysis temperature.
2. Methodology
The composition of peat soil used in this study is shown in Table 1. The peat was crushed in a small grinder, measured for moisture content, and dried overnight at 50 °Cin a small electric furnace. Samples were pyrolyzed in a Curie point injector (JCI-22, Japan Analytical Industry), and the product gas was analyzed using a gas chromatograph-mass spectrometer (GCMS-QP2010PLUS, SHIMADZU) and hydrogen flame ionization detection (FID) of the same instrument. The peat sample volume was 2.00±0.10 mg, and the pyrolysis temperature was set at 9 points in the range of 255-740 °C. Three measurements were made at each temperature. The carrier gas for the Curie point injector and GC was helium, with a flow rate of 9.0 mL/min for GCMS and 50.0 mL/min for GCFID.
3. Results and discussion
The temperature variation of the peak areas of 2-furan methanol and phenol and the emission factors of methane and ethylene from the Py-GC measurements are shown in Figure 1. 2-Furan methanol showed the largest peak area at 358 °C, and other furans showed similar trends. Phenol showed two production peaks at 445 °C and 500 °C. Similar trends were observed for other phenols. This trend is due to the thermal decomposition of cellulose in peat. On the other hand, methane and ethylene production increased at higher pyrolysis temperatures, and other low molecular weight gases showed similar trends. From these results, it can be inferred that in the low temperature range below 500 °C, primary pyrolysis of peat dominates to produce furans and phenols, and as the pyrolysis temperature increases, secondary pyrolysis of materials produced by primary pyrolysis dominates to produce methane and ethylene with smaller molecular weight. This result is consistent with the pyrolysis mechanism of woody biomass such as cellulose and lignin, which are components of peat. Table 2 shows the emission factors for low molecular weight gases estimated from the experimental results. The amount of low molecular weight gas formed at 255 °C and 315 °C was minute and difficult to quantify. Based on the results of the present study, methane emissions from ground fires in peatland fires in Indonesia in 2015 were estimated. The emission amount was calculated to be 59.5-97.8 Mt-CO2 assuming that the ground fire was uniform at 358 °C and spread to a depth of 1 m and using the area of fire spread (1 million ha), the density of typical peat (1.4-2.3 g/cm3), and the methane emission factor obtained in this study. This value exceeds the anthropogenic methane emissions (28.4 Mt-CO2) in Japan in FY 2019.
Peat is a type of biomass soil formed by the partial decomposition and deposition of dead plants. It is composed mainly of woody biomass such as lignin and cellulose, and also contains trace amounts of metal components. In Indonesia, which has particularly large peatlands, large-scale peatland fires frequently occur when peatlands are dried out by the El Nino phenomenon and ignited by illegal open burning for the purpose of agricultural land development. Air quality risk assessment of peatland fires is important because the peatland fires release large amounts of greenhouse gases (such as CO2 and methane) and volatile organic compounds (such as ethylene and phenol) into the atmosphere, causing global warming and haze, which is an international environmental problem.
Peatland fires progress in two forms: surface fires and ground fires. The former burns with flames on the ground surface, and the latter smolders without flames in the underground. While the ground fires progress at relatively low temperatures (200-600 °C), they are considered a significant environmental risk because they can spread into deep underground and smolder for long periods. The ground fire temperatures are distributed such that the greater the distance from the ground surface, the lower the temperature. Therefore, understanding the amount of peat pyrolysis products at each temperature is important for environmental risk assessment. Low molecular gases such as methane, which has a large global warming potential, and ethylene, which contributes to haze, have not been quantitatively analyzed in previous studies, so it is necessary to determine their emission factors.
In this study, quantitative analysis of peat pyrolysis gases was performed by pyrolysis gas chromatography (Py-GC) at 255-740 °C in the absence of oxygen, assuming the ground fire. This will clarify changes in the amount of gas produced at different pyrolysis temperatures and aim to elucidate the generation mechanism of low molecular weight gases and determine their emission factors at each pyrolysis temperature.
2. Methodology
The composition of peat soil used in this study is shown in Table 1. The peat was crushed in a small grinder, measured for moisture content, and dried overnight at 50 °Cin a small electric furnace. Samples were pyrolyzed in a Curie point injector (JCI-22, Japan Analytical Industry), and the product gas was analyzed using a gas chromatograph-mass spectrometer (GCMS-QP2010PLUS, SHIMADZU) and hydrogen flame ionization detection (FID) of the same instrument. The peat sample volume was 2.00±0.10 mg, and the pyrolysis temperature was set at 9 points in the range of 255-740 °C. Three measurements were made at each temperature. The carrier gas for the Curie point injector and GC was helium, with a flow rate of 9.0 mL/min for GCMS and 50.0 mL/min for GCFID.
3. Results and discussion
The temperature variation of the peak areas of 2-furan methanol and phenol and the emission factors of methane and ethylene from the Py-GC measurements are shown in Figure 1. 2-Furan methanol showed the largest peak area at 358 °C, and other furans showed similar trends. Phenol showed two production peaks at 445 °C and 500 °C. Similar trends were observed for other phenols. This trend is due to the thermal decomposition of cellulose in peat. On the other hand, methane and ethylene production increased at higher pyrolysis temperatures, and other low molecular weight gases showed similar trends. From these results, it can be inferred that in the low temperature range below 500 °C, primary pyrolysis of peat dominates to produce furans and phenols, and as the pyrolysis temperature increases, secondary pyrolysis of materials produced by primary pyrolysis dominates to produce methane and ethylene with smaller molecular weight. This result is consistent with the pyrolysis mechanism of woody biomass such as cellulose and lignin, which are components of peat. Table 2 shows the emission factors for low molecular weight gases estimated from the experimental results. The amount of low molecular weight gas formed at 255 °C and 315 °C was minute and difficult to quantify. Based on the results of the present study, methane emissions from ground fires in peatland fires in Indonesia in 2015 were estimated. The emission amount was calculated to be 59.5-97.8 Mt-CO2 assuming that the ground fire was uniform at 358 °C and spread to a depth of 1 m and using the area of fire spread (1 million ha), the density of typical peat (1.4-2.3 g/cm3), and the methane emission factor obtained in this study. This value exceeds the anthropogenic methane emissions (28.4 Mt-CO2) in Japan in FY 2019.