Introduction

Underground fires in forests are smouldering combustion in humus and peat layers, with frequency of occurrence lower than above-ground forest fires, the damage of which is considerable (Huang and Rein 2017). Researchers have found that particulate emissions from smouldering combustion are much higher than from other forest combustions (Akagi et al. 2011). Suspended in the air for a long time in small sizes, particulate emissions accumulate and lead to large-scale haze (Rein 2013; Black et al. 2016).

During 1997–1998, combustion from smouldering fires in Indonesia led to large-scale hazy conditions in Southeast Asia, with millions of people affected with respiratory diseases, millions of hectares of forests destroyed and billions of dollars lost (Tacconi 2003). In 2010, forest fires in Russia resulted in approximately 50 smouldering combustions, and weeks-long air pollution (Cancellieri et al. 2012). In 2015, smouldering combustion caused economic losses of $ 16 billion in South Sumatra, Indonesia, 1.9% of GDP. Air pollution in Indonesia and surrounding countries was critical, with PM2.5, fine inhalable particles, up more than 1500, causing serious damage to biodiversity and ecosystems (Atwood et al. 2016).

In addition to particulate emissions, smouldering combustion also leads to emission of various gases, with upwards of more than 100 types of gases and aerosols being released during the combustion process, such as carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), ammonia (NH3) and alkanes (Stockwell et al. 2014; Hatch et al. 2015), with CO2 and CO being the highest emissions (Hu et al. 2018a). In addition, because the smouldering combustion is an incomplete process with an oxygen deficit, CO emissions are much higher than from ordinary forest combustion (Rein 2015).

Smouldering combustion spreads slowly without flames, making difficult to discover by standard forest fire monitoring technologies (Huang and Rein 2017). However, the high concentrations of gas emissions might be used as fire monitoring indexes (Hu et al. 2018b) to monitor the occurrence and spread of smouldering combustion.

However, research on gas emissions from smouldering combustion is still in preliminary stages, and there are only a few studies on gas varieties and their concentration that can be used for smouldering combustion monitoring (Hatch et al. 2015; Wilson et al. 2015). The purpose of this study is to explore the marker gases in the smouldering combustion process and to analyze the influence of concentrations.

Materials and method

Study area

The study area is the eastern slope of the Daxing’an Mountains, 15 km south of the Jiagedaqi region in the forest management technology extension station of Jiagedaqi (50°20′–50°23′ E, 123°57′–124°0′ N). Annual average temperatures range from −1 to 2 °C, with the annual effective accumulative temperature 1800–2000 °C, a frost-free period of 90–120 days, and annual rainfall 450–500 mm. The main tree species are Quercus mongolica (Fisch.) Turcz., Larix gmelinii (Rupr.) Rupr., Betula platyphylla Suks., B. davurica Pall. and Populus davidiana Dode.

Sample collection

Larix gmelinii plantations were the sample fields selected (Table 1), and 15 soil samples of 50 cm × 50 cm were collected. According to the classification of smoldering (Usup et al. 2004), we divided the sample depth into two groups. The depth in the shallow smoldering furnace was less than 20 cm, and the depth in the deep smoldering furnace was between 20 and 50 cm.

Table 1 Sample plot information
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Sample processing

The soil samples were placed into a smoldering furnace 1.5 m long × 0.5 m wide × 0.5 m deep (Fig. 1). A far-infrared heating plate (700 °C) was used as the ignition device; a k-type thermocouple and data acquisition module were used to record the surface soil temperatures during the smouldering combustion process. The concentration of gas emissions was detected by an emission analytical instrument (ecom-J2KN) every two hours.

Fig. 1
figure 1

Schematic diagram of the smouldering combustion experiment

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Statistical analysis

Statistical analysis was performed by SPSS 19.0, and two-way ANOVA used to assess the influence factors of gas emissions. Least significant difference (LSD) was used in the multiple comparisons and correlation analysis and multiple linear regression used to establish the regression equations. Statistical significance was accepted at * = P < 0.05, ** = P < 0.01. Figures were furnished by OriginPro 9.1.

Results

Smouldering combustion process

The process was carried out in the combustion furnace and the soil surface temperature measured every two hours. The results showed that the variations in temperature and combustion kinetics at different times were inconsistent (Fig. 2) (Hu et al. 2018a) and the process was divided into four stages: (1) Ignition Stage (0–2 h): soil temperature rose rapidly to more than 100 °C; (2) Combustion Stage (2–6 h): soil temperature rose gradually to about 400 °C; (3) Steady Stage (6–12 h): soil temperatures stayed around 400 °C to 500 °C; (4) Extinguishing Stage (after 12 h): soil temperatures decreased slowly by degrees, and no CO, NOx and SO2 could be detected after 12 h.

Fig. 2
figure 2

Surface temperature of the soil in the smouldering combustion process

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Gas emissions detection

According to the depth of the combustion, smouldering combustion could be classified as shallow (< 20 cm) and deep (> 20 cm) (Usup et al. 2004). In this study, the soil surface temperature ranged from 81.0 °C to 486.0 °C, and the average combustion temperature was 312.2 ± 19.6 °C. Carbon monoxide, NOx and SO2 were detected during the smouldering but surface temperatures had no significant influence on emissions (P > 0.05).

High concentrations of CO were detected and declined gradually as the combustion time increased; the rate of decline in the steady stage was higher than that in the combustion stage. The release of NOx was mainly concentrated in the ignition and combustion stages but could also be detected in the steady stage. Small quantities of SO2 could be detected in the combustion and steady stages (Fig. 3).

Fig. 3
figure 3

Gas emissions of the smouldering combustion process: a CO emissions; b SO2 emission; c NOx emission of the shallow smoldering furnace; d NOx emission of the deep smoldering furnace

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The highest emission of CO was 187.5 mg/m3 after 2 h, gradually dropping and by 16 h could not be detected in some experiments. The average CO concentration was 66.2 ± 8.8 mg/m3.

The emission characteristics of NOx in various smoldering furnaces were different. Concentrations in the deep smoldering furnace ranged from zero to 3.4 mg/m3, with an average concentration of 0.9 ± 1.1 mg/m3. Levels consistently decreased and could not be detected after 6 h.

NOx concentrations in the shallow smoldering furnace ranged from zero to 4.7 mg/m3, with an average of 2.0 ± 1.1 mg/m3. Unlike the deep smoldering furnace, the concentration continued upward after 6 h until 12 h.

SO2 concentrations ranged from zero to 8.6 mg/m3, with the average being 0.8 ± 1.8 mg/m3. A peak was detected at 6 h and then rapidly decreased. However, the data samples of SO2 were not abundant in this study and further research is required.

Effect of combustion time and smoldering furnace depth on gas emissions

This study analyzed the effect of combustion time and smoldering furnace depth on CO and NOx, with SO2 not further studied for a lack of data.

The results of the analysis by two-way ANOVA showed that combustion time had a significant impact on the concentration of CO (P < 0.01), while the smoldering furnace depth had no significant impact (P > 0.05) (Table 2). As the result of difference analysis, the concentration of CO had no significant difference between ignition stage (0–2 h), combustion stage (2–6 h), and the early time of the steady stage (6–8 h) (Fig. 4).

Table 2 The effect of combustion time and smoldering furnace depth on CO emissions by two-way ANOVA
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Fig. 4
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Effect of combustion time on CO emissions by multiple comparison (no significant difference between values that have at least one identical letter; there is a significant difference between values that have no identical letter)

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The combustion time and depth of the smoldering furnace had a significant impact on the concentration of NOx (P < 0.01) based on the results of two-way ANOVA which was different from CO factors (Table 3). This was consistent with the smoldering experiments, i.e., the NOx emissions in the deep smoldering furnace and shallow smoldering furnace was not the same (Fig. 3).

Table 3 Effect of combustion time and smoldering furnace depth on NOx emissions by two-way ANOVA
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Through further analysis, this study examined the effect of combustion time on NOx emissions when the smoldering furnace depth was the same. Combustion time had no effect on NOx emissions in the shallow smoldering furnace (P > 0.05), but did affect emissions in the deep smoldering furnace (P < 0.05); high concentrations of NOx occurred at 2 h, 4 h and 6 h, with a significant difference between 2 and 6 h and the other combustion times (P < 0.05, Fig. 5). The main reason for the significant difference was the sharply decreased concentration of NOx after 6 h which could not be detected after 8 h.

Fig. 5
figure 5

Effect of combustion time on NOx emissions in the deep smoldering furnace by multiple comparison; (no significant difference between values that have at least one identical letter; there is a significant difference between values that have no identical letter)

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This study evaluated the effect of smoldering furnace depth on NOx emissions when combustion times were the same. As the results show (Table 4), depth had no significant effect on the NOx emissions at 2 h, 4 h, 6 h, 8 h and 18 h (P > 0.05); depth had a significant effect on NOx emissions at 10 h, 12 h, 14 h, 16 h (P < 0.05). This was because NOx could still be detected after 10 h of the shallow smoldering furnace but the concentration tended to zero and could not be detected after 10 h of the deep smoldering furnace.

Table 4 ANOVA for the effect of smoldering furnace depth on NOx emissions
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Regression analysis of the CO and NOx emissions

Concentrations of CO had high negative correlation with combustion time as shown by multivariate linear regression, and with the increase of combustion time, concentration of CO gradually decreased. However, there was no correlation between the concentration of CO and the smoldering furnace depth (Table 5). The regression equation for the concentration of CO was Cco = 156.989−16.626 t (R2 = 0.606), where t is the combustion time.

Table 5 Significance test of regression equations (P value)
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The concentration of NOx was highly negatively correlated with both combustion time and smoldering furnace depth (Table 5) and the regression equation for the concentration of NOx was CNOx = 3.637 − 0.252 t − 0.039 h (R2 = 0.458), where t is the combustion time; h is the smoldering furnace depth.

Discussion

Combustion times and smoldering temperatures

This study divided the smouldering combustion process into the ignition stage, combustion stage, steady stage, and extinguishing stage, according to temperature variations. This is consistent with the conclusion of Hu et al. (2018a). In the ignition stage, oxygen in the soil taking part in the reaction was an important factor for accelerated combustion (Ohlemiller 1985) so temperatures rose rapidly in a short time. In the combustion stage, heat energy diffused into the surrounding areas and sustained the combustion without the ignition source (Hu et al. 2018a). In the steady stage, the burning areas extended to the maximum, combustion heat accumulated continually, and the smouldering combustion process reached the peak temperature (Rein et al. 2009). In the extinguishing stage, the smouldering combustion did not continue because of loss of heat energy, reduced combustibles, and shortage of oxygen, and the combustion temperatures fell until the smouldering was extinguished.

Marker gases of the smoldering combustion

There are considerable CO2 and CO gases in the combustion for the oxidation of carbon particles and shortage of oxygen (Rein et al. 2009). High concentrations of CO were continuously detected during the process, above the ambient air quality standards of China and the background value of CO in the general forest environment (Table 6) (Su et al. 2013). CO emissions were also the predominant emissions in this study, consistent with the results of Hu et al. (2018a). Nitrogen and sulfur in the semi-decomposed and humus layers were also released as NOx and SO2 and the concentrations were much higher than the ambient air quality standards of China and the background value of NOx and SO2 in the general forest environment (Su et al. 2013).

Table 6 Concentration of marker gases in different combustion stages (mg/m3)
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CO, NOx and SO2 can be detected by a handheld device or online monitoring instrument, and therefore easier monitored in the field. Compared to other gas emissions during the smouldering combustion such as methane (CH4), particulate matter (PM), and other organic gases (VOCs), the detection of CO, NOx and SO2 is relatively simple. These gases can therefore be used as marker gases to detect the occurrence of smouldering combustion. As noted earlier, smouldering combustion is difficult to monitor because it is flameless (Huang and Rein 2017), and thus marker gases could help to solve the difficulty in locating and monitoring smouldering combustion.

Carbon monoxide may be regarded as the key marker gas for large emissions, NOx as the periodical marker gas for different emission trends in different smoldering depths. Although there was only a little SO2 released during the smouldering combustion process, SO2 emissions were much higher than general forest emissions. Therefore, SO2 could be regarded as a secondary marker gas.

Influence factors of gas emissions

Heat energy spreads continuously into the deeper soil layer as the combustion time increases, so this layer also burns (Svensen et al. 2003). Soil carbon is continuously converted into CO gas and released due to a lack of oxygen. Nitrogen in the semi-decomposed and humus layers is also be transformed into NOx and released.

The concentration of CO in the ignition stage was the highest in this study. Because smouldering combustion mainly focused on the soil surface layer which is rich in humus and is soft and high in organic carbon, the rate of combustion increases rapidly (Hu et al. 2018a). Although combustion spreads downward with the increase in temperature, the concentration of CO in the combustion and steady stages was lower than in the ignition stage because of the reduced carbon and oxygen contents.

As noted previously, both combustion time and smoldering furnace depth had a significant effect on NOx emissions. NOx could only be continuously detected in the shallow smoldering furnace and was undetected after 6 h in the deep smoldering furnace. The main reason for this is that, as the smouldering combustion spreads downward, humus and total nitrogen (TN) contents decline (Knops and Bradley 2009; Yao et al. 2020), so the concentration of NOx is extremely low after 6 h in the deep smoldering furnace. In addition, as the smouldering combustion proceeds, a large mass of combustion particles are released and leads to plugging of soil pores (Hu et al. 2018b). Gas emissions of the deep smoldering furnace are unable to spread to the surface smoothly, and NOx was not detected after 10 h.

Conclusions

Gas emissions from smouldering combustion were investigated in a smoldering furnace. The main conclusions are as follows:

  • High concentrations of CO were continuously released during the smouldering combustion process. Combustion time had a significant impact on CO emissions with a peak detected in the early stage of the process. In order to improve the efficiency of monitoring smouldering combustion, CO may be considered as a key marker gas as it was highly concentrated and easily monitored.

  • Both combustion time and furnace depth had a significant impact on NOx emissions. NOx were continuously detected during the entire combustion process in the shallow smoldering furnace, and only detected during the ignition and combustion stages in the deep smoldering furnace.

  • The concentration of SO2 was higher than the ambient air quality standards of China and the background value of SO2 in forest. But SO2 emissions were difficult to analyse in depth in this study because the sample was inadequate. Further research will be carried out in the future.