3.1. Fuel properties
3.1.1. Proximate analysis
Results of the proximate and ultimate analyses of all feedstock and biochar samples are summarized in Table 1. The VM content in raw materials was much greater than the FC and ash contents. The VM content ranged between 71.0–88.7% and was greatest in the Po, JL, and Mi samples. Among biochar samples, the VM content varied 22.3–43.4%, depending on the feedstock, and was significantly lower than in the raw materials. For example, VM content was the greatest in woody biochars (JLB, PoB), followed by MiB and DMB, while RHB and BGB contained the lowest amount. The FC content in raw samples was lowest in DM (10.5%) and greatest in Mi (16.9%). In biochar samples, the FC content varied significantly (27.5–61.4%) among the feedstocks and followed the trend: DMB < RSB < RHB < JLB < PoB < MiB < BGB. Ash content of the biomass also varied greatly between 0.0–17.6%, depending on the biomass type. The ash content was greatest in agricultural residues with DM (17.6%), RH (14.3%), and RS (12.2%), while the ash content of JL, Po, Mi, and BG samples was relatively low (0.0–6.7%). According to Vassilev et al., ash content of biomass was in the order: animal and human-derived biomass > herbaceous and agricultural biomass > woody biomass [13]. Consistent with the literature, ash content based on biomass category was the same in the present study. In biochar samples, ash content had a wide range (1.3–41.2%) and increased in the order: JL < Po < Mi < BG < RS < RH < DM. Due to pyrolysis, ash content of the biochars produced from high-ash raw materials (RH, RS, DM) increased significantly, whereas ash content in the rest of the samples increased slightly.
Table 1
Proximate and ultimate analysis of raw and carbonized biomass
Feedstock | Ultimate analysis (%db) | Proximate analysis (%db) |
| C | H | N | O | VM | FC | Ash |
RH | 40.6 | 5.7 | 0.8 | 38.6 | 71.0 | 14.7 | 14.3 |
RS | 40.1 | 5.4 | 0.6 | 41.7 | 73.6 | 14.3 | 12.2 |
DM | 43.9 | 5.6 | 1.5 | 31.4 | 71.9 | 10.5 | 17.6 |
Mi | 45.5 | 5.7 | 0.3 | 45.4 | 79.9 | 16.9 | 3.1 |
Po | 47.5 | 6 | 0.3 | 45.7 | 88.7 | 10.8 | 0.5 |
JL | 48.6 | 5.9 | 0.3 | 45.2 | 86.4 | 13.6 | 0.0 |
BG | 43.8 | 5.6 | 0.9 | 43.0 | 78.8 | 14.5 | 6.7 |
RHB | 48.9 | 2.7 | 0.8 | 13.9 | 23.4 | 42.9 | 33.7 |
RSB | 55.7 | 2.7 | 0.7 | 11.9 | 28.9 | 42.1 | 29.0 |
DMB | 46.2 | 2.7 | 2.4 | 7.5 | 31.3 | 27.7 | 41.2 |
MiB | 68.6 | 3.1 | 0.3 | 20.2 | 32.9 | 59.4 | 7.8 |
PoB | 72.3 | 2.9 | 0.3 | 20.2 | 39.1 | 56.6 | 4.2 |
JLB | 75.3 | 3.1 | 0.3 | 20.1 | 43.4 | 55.3 | 1.3 |
BGB | 61.7 | 3.1 | 1.3 | 17.6 | 22.3 | 61.4 | 16.3 |
Overall, an increase in FC and ash content in biochar samples was due to VM release caused by carbonization. The VM reduction rate due to carbonization was greatest in BG (71.70%) followed by RH (67.04%), RS (60.73%), Mi (58.82%), DM (56.47%), Po (55.92%), and JL (49.77%). Itoh et al. (2020) suggested that woody biomass that contains less than 50% VM can be used to minimize PM emissions [8]. In the present study, after carbonization, all biochars contained less VM than 50%, suggesting the potential for carbonization to reduce PM emissions.
3.1.2. Ultimate analyses
The ultimate analysis revealed that the C content of the raw materials ranged between 40.1–48.6%, while that of biochar samples was 46.2–75.3%. The least and greatest C-containing biomass samples were RS and JL, respectively. Recovery of C from each feedstock was calculated based on the mass yield of the biochar. The C recovery values for RH, RS, DM, Mi, Po, JL, and BG were 47.0, 58.2, 31.0, 49.9, 55.8, 52.9, and 51.1%, respectively. All materials underwent C enrichment due to dehydration and decarboxylation during the pyrolysis process. Biomass with a high lignin content are reported to yield more C in the charred materials, whereas animal manure- and sewage sludge-derived biochars recover less C than the lignocellulosic biomass [14]. In this study, C recovery from rice straw was greatest and was similar to the high-lignin biomass (Po, JL). In contrast, DM biochar had the lowest carbon recovery, which is consistent with the previous study.
The H content of raw biomass was between 5.4–6.0% while that of biochar was 2.7–3.1%. The N content in raw and biochar samples ranged between 0.3–1.5% and 0.3–2.4%, respectively. The O content in raw material was lowest in RH (38.6%) and greatest in Po (45.7%), whereas that of char samples was 7.5–20.2%. Volatile organic compounds are released during carbonization, therefore, the char samples had less O and H than the raw biomass, resulting in lower emissions of primary pollutants such as carbon monoxide and hydrocarbons [15, 16]. In addition, the calorific value of fuel increases with the reduction of O in the fuel. Since N content in biochars remained stable or increased slightly in DMB, BGB, and RSB, the N in the biomass studied was not volatilized significantly under the pyrolysis conditions used. The increase in the N proportion of the biomass could be explained by degradation of amino acids and protein, which are further adsorbed in the char [17]. The N in the fuels could contribute to the emission of NOx and other nitrogenous species during combustion of biomass [18]. However, torrefied biomass (biochars) with increased N do not increase the emission of NOx, which instead is reduced in most cases [16, 19]. Therefore, the expected reduction in volatile matter or gas emission (CO, HCs, NOx, etc.) and C enrichment in the biochar indicate that fuel quality was improved significantly.
3.2. PM emission behavior
The PM EF of all biomass and biochar samples at different heating temperatures (650°C, 750°C, and 850°C) are presented in Fig. 2. To simply compare PM emissions among the feedstocks, mean PM EF of each biomass or biochar sample was calculated by averaging the emission values at three combustion temperatures. The data indicate that PM EF of raw biomass increased in the order: RH < Po < RS < Mi < BG < JL < DM (9.67, 12.93, 13.22, 13.74, 14.58, 15.21, and 19.54 mg/g-fuel, respectively). Meanwhile, the PM EF of some biomass varied significantly depending on combustion temperature. For example, for the RS, DM, and Po samples, the EF increased with combustion temperature, while combustion temperature did not have a significant effect on PM emissions from RH, BG, Mi, and JL.
The average PM EF of biochars occurred in the order: PoB < MiB < JLB < RHB < BGB < RSB < DMB (1.00, 1.26, 1.30, 1.75, 3.15, 7.84, and 12.99 mg/g-fuel, respectively). Similar to biomass samples, PM emissions from some biochars were influenced by the combustion temperature. As the combustion temperature increased, more PM was emitted from RS and DM biochar, while the rest of the samples did not show any specific temperature dependency.
Pyrolysis significantly reduced PM emission from all biochar samples, and the magnitude of reduction varied considerably depending on the combustion temperature of the specific biochar. For example, PM emission was reduced by 75.5% (DM)–92.66% (Mi) at a combustion temperature of 650°C, 14.24% (DM)–90.74% (Po) at 750°C, and 9.64% (DM)–95.45% (Po) at 850°C. The data indicate that pyrolysis of biomass is effective in reducing PM emission, yet the efficiency depends on feedstock and combustion temperature.
The types of biomass containing the greatest amount of VM were Po, JL, and MI. Consequently, the average PM emission from JL, Po, and Mi was the greatest except for DM. A reduction in PM can be achieved by reducing the VM content. In the present study, the reduction in VM from biomass due to carbonization ranged from 49.77–71.70%, which suggests that VM was efficiently reduced by pyrolysis. The results also indicate that carbonization efficiently reduced PM emissions from low-ash biomass (≤ 6.7%) due to the reduction in VM regardless of combustion temperature (Fig. 2d-g). In contrast, combustion temperature had a significant effect on the emission of PM from high-ash biochars. For example, DM, which was one of the lowest VM-containing biomasses, had the greatest PM emissions, likely due to the emission of PM originating from the ash fraction.
As hypothesized, PM emission reduction was most effective at low combustion temperatures (650°C) in high-ash biochar samples. The higher combustion temperature caused more PM emissions from high-ash chars except for RH char (Fig. 2a). The PM emissions from RH char was not changed with combustion temperature. This phenomenon was somewhat counter-intuitive. The average EF of the RH raw sample was 9.67 mg/g, which is similar to that reported by Abah et al. (2020) [20]. In that study, the PM EF of rice husk biomass samples was 5.5–13.6 mg/g at combustion temperatures between 600–900°C. However, no comparable study of RH char EF has been done. Surprisingly low PM emission from high-ash RH char, even at high combustion temperatures, could be attributed to compositional differences in the ash.
A reduction in alkali metals in biomass and coal could be beneficial for PM emission reduction.[21] To confirm that, the K and Na content of RH, RS, and DM char was compared with their PM EF at three different temperatures. As shown in Fig. 3, at combustion temperatures of 750°C and 850°C, the PM EF of high-ash biochars strongly correlated with their K + Na content. Higher combustion temperature significantly promoted the emission of PM due to the greater alkali metal (K and Na) content. A previous report indicated that the dominant release of alkali and alkali earth metal occurs either below 500°C or above 800°C [14]. Sublimation of the KCl could be the main path of K release [22]. The current study demonstrated that biochar fuel containing high alkali emitted more PM under high burning temperature conditions. Therefore, burning those chars at low temperatures could be a reasonable option for PM emission reduction. Further technological studies are required to determine how to reduce PM emissions and ash content from high-ash releasing biomass.
3.3. Ash composition analysis of high-ash biomass
Due to high ash content and subsequently greater PM emissions, RH RS, and DM samples underwent ash fractionation analysis to elucidate the mechanism of PM emission from ash. The insoluble and soluble (in acid) portions of the ash (600°C) and heated ash samples (at 650, 750, and 850°C) are shown in Fig. 4. Results showed that 87.9%, 67.4%, and 57.2% of the RH, RS, and DM ash, respectively, were non-digestible by the HNO3 acid system. Reduction in the soluble part of the ash samples was observed when increasing the heating temperature of RS and DM samples. This implies that at high heating temperatures, soluble (volatile at high temperatures) minerals such as K and Na in the ash can be emitted as particulate matter, causing a reduction in soluble alkali metals.
The ash of rice straw and rice husk are both dominated by Si (94.7% and 73.2%, respectively), but rice straw ash is more abundant in alkaline and alkaline earth metals [23]. Total content of Al, Ca, K, Mg, Na, P, and Fe in the RH leachates was significantly lower than in the RS and DM samples (Figs. S1-S3). The dominant species in the ash sample of RH were K, Fe, and P (5.8; 1.9; 1.3 mg/g, respectively), whereas K, Ca, Fe, and Mg (51.6; 6.1; 3.3; 1.2 mg/g, respectively) content in RS ash was much greater. The other elements contained less than 1 mg/g in both the RH and RS ash samples. The DM contained even greater amounts of K, Ca, P, Na, Mg, Fe, and Al in the ash (100.0, 96.4, 35.5,17.3, 10.4, 4.2, and 2.3 mg/g, respectively).
The water-soluble alkali metal fraction is likely to be released during combustion, forming PM in the flue gas while the water-insoluble alkali metals will likely contribute to ash transformation and ash melting [24]. Therefore, the acid-soluble (including the water-soluble fraction) alkali content of the three biomass ash samples was determined and compared to the K and Na content remaining after heating the ash samples at higher temperatures (Table 2).
The alkali metal content was significantly different among the feedstocks. The K content was 5.86, 51.61, and 96.84 mg/g, while the Na content was 0.14, 0.52, and 17.33 mg/g for RH, RS, and DM ash, respectively (600°C). With an increase in heating temperature, the alkali metal content in RS and DM ash was reduced, but not for RH ash. This implies that the vaporization of alkali metals from the ash of DM and RS increased with heating temperature, which may have been released as PM. The same phenomenon was observed in a previously study [8].
Table 2
K and Na content of ash and heated ash samples
| K (mg/g) | Na (mg/g) |
| RH | RS | DM | RH | RS | DM |
Ash | 5.86 ± 0.04a | 51.61 ± 8.88a | 96.84 ± 4.73a | 0.14 ± 0.00c | 0.52 ± 0.01a | 17.33 ± 0.79a |
Ash heated at 650°C | 5.40 ± 0.08b | 61.11 ± 1.51a | 94.64 ± 4.32a | 0.14 ± 0.01c | 0.53 ± 0.01a | 18.62 ± 0.88a |
Ash heated at 750°C | 4.92 ± 0.11c | 19.16 ± 1.81b | 62.26 ± 3.57b | 0.16 ± 0.01b | 0.38 ± 0.00b | 14.10 ± 0.72b |
Ash heated at 850°C | 5.89 ± 0.07a | 1.29 ± 0.18c | 19.36 ± 1.07c | 0.25 ± 0.01a | 0.15 ± 0.01c | 9.52 ± 0.53c |
Values in the table represent mean of three replicates ± standard deviation. Superscripted letters indicate significant differences within the same column as determined by Tukey’s test (p < 0.05).
The reduction in minerals in the heated ash samples was due to vaporization of the alkali metals because metal content decreased when the heating temperature was close to the boiling point of the metals. However, in RH samples the alkali content was significantly less than that of the other two high-ash biomass types. Therefore, the emission of those metals from RH may be negligible, which caused lower PM emissions from RH biochar. Thus, both the ash content and composition of the alkali metals appears to strongly influence PM emissions from biochar combustion.