3.1. Environmental Impacts Assessment of Pellets Biofuel
Environmental impacts for 1 kg wood pellets, manufactured from the sawdust of three different species, were evaluated as shown in the
Figure 1. Among the environmental impacts, fossil fuel was the major contributor to the environmental impacts with the value of (5.116 MJ surplus), followed by ecotoxicity (1.188 PAF.m
2yr (Potential of Affected Fraction of species, in terms of ecotoxicity, this is measured as the percentage of all species present in the environment living under toxic stress)), mineral depletion (0.114 MJ surplus), and acidification/eutrophication potential (0.073 PDF.m
2yr (Potential of Disappeared Fraction of plant species)). The major environmental impacts in fossil fuels and ecotoxicity were caused due to Kikar wood pellets (1.87 MJ surplus, 0.48 PAF.m
2yr) followed by Oak wood pellets (1.72 MJ surplus, 0.361 PAF.m
2yr) and lowest was in Mesquite wood pellets (1.52 MJ surplus, 0.347 PAF.m
2yr). Mineral depletion and Acidification/eutrophication potential was highest in Kikar wood pellets, followed by Mesquite and Oak wood pellets, respectively, as shown in the
Figure 1 below. Mineral depletion and Acidification/eutrophication, compared to fossil fuel and ecotoxicity, were negligible. Carcinogens and respiratory inorganics were almost the same and negligible. They showed lower environmental impacts.
The highest impact on the environment came from fossil fuels 1.875 MJ surplus used during wood pellets production from Kikar saw dust, as shown in
Figure 2. The major hotspot source for fossil fuel depletion was lubricating oil 0.875 MJ surplus followed by UF resin 0.805 MJ surplus, Bio binder 0.165, and saw dust 0.0259 MJ surplus, respectively. The major environmental impacts were caused by wood pellets produced from Mesquite saw dust responsible for fossil fuel 1.526 MJ surplus, and the leading responsible factor for fossil fuel was lubricating oil 0.831 MJ surplus, proceeded by UF resin 0.568, bio binder 0.099, and saw dust 0.0228 MJ surplus, respectively, as shown in
Figure 3. Environmental impacts caused by wood pellets produced from Oak saw dust were shown in
Figure 4, which showed that the hotspot source for environmental impacts caused by fossil fuels was 1.715 MJ surplus. The most prominent factors responsible for fossil fuels depletion were lubricating oil, UF resin, and bio-binder with values of 1.03, 0.532, and 0.1212 MJ surplus, respectively.
The second highest environmental impact category was ecotoxicity caused by wood pellets from Kikar saw dust. The hotspot sources of ecotoxicity were caused due to UF resin, Bio-binder, lubricating oil, and saw dust, as shown in the
Figure 2. In Mesquite wood pellets main hotspot sources for ecotoxicity were UF resin, lubricating oil, bio-binder, and saw dust, respectively. The ecotoxicity caused due to wood pellets from Oak saw dust was due to UF resin, lubricating oil, and bio-binder, with values, of 0.198, 0.077 and 0.074 PAF.m
2yr, respectively, while the other two values were minor.
Mineral depletion in Kikar wood pellets were caused due to UF resin, lubricating oil, and bio-binder, respectively, while the other material such as saw dust, water, and electricity used for production had minor values, as shown in
Figure 4. In Mesquite, mineral depletion is due to UF resin and lubricating oil, while the other values were minor in the same way as Kikar. UF resin, Bio-binder, and lubricating oil were the major contributors in mineral depletion from wood pellets produced Oak saw dust. UF resin, bio-binder, and lubricating oil were responsible for acidification potential in wood pellets produced from Kikar saw dust. Here, UF resin and bio-binder had the same impact on acidification potential, while being slightly higher than lubricating oil. The major contributors in acidification/eutrophication were UF resin, Bio-binder, lubricating oil, and saw dust with values of 0.009, 0.008, 0.003, and 0.001 PDF.m
2yr, respectively, in wood pellets produced from Mesquite saw dust. In Oak wood pellets, the acidification potential was maximum due to bio-binder, lubricating oil, UF resin, lubricating oil, and saw dust, respectively, as shown in the
Figure 4. The other impacts, such as carcinogens, respiratory organics, respiratory inorganics, climate change, and ozone layer were minor, with values of 9.47 × 10
−7, 2.91 × 10
−9, 1 × 10
−6, 1.93 × 10
−7, and 2.33 × 10
−10 DALY, respectively, as compared to the above impacts.
3.3. Single Score Damage Assessment
The damage to human health was very low from Kikar saw dust used during pellets production. UF, bio-binder, and lubricating were the hotspot sources for human health damage with values of 45.27, 33.70, and 15.86 (milli-point) mPt, respectively. Human health was also disturbed very little from the saw dust used in the manufacturing of wood pellets but UF resin, Bio-binder, and lubricating oil were the major factors for deterioration of human health from Mesquite tree species. Once again UF resin, bio-binder and lubricating oil were the major factors responsible for damage to human health with values of 29.93, 24.71, and 15.06 mPt, respectively. Wood pellets, manufactured form Oak saw dust as shown in the
Figure 6, had minor or negligible damage to human health. The damage of wood pellets, produced from Kikar saw dust, on ecosystem quality was the highest impact from saw dust, lagged by UF resin, bio-binder, lubricating oil, and electricity, respectively.
Figure 7 showed the damages created due to wood pellets from Mesquite tree species. Saw dust, UF resin, lubricating oil, and bio-binder had the highest damages to ecosystem quality, respectively, with values of 0.24, 0.033, 0.014, and 0.010 PAF.m
2yr.
Figure 8 showed human health, ecosystem quality, and resources depleted due to pellets manufactured from Oak tree saw dust. Ecosystem quality was depleted mostly due to saw dust, UF resin, lubricating oil, bio-binder, and electricity, respectively. The ecosystem quality depleted due to electricity was negligible. The damage was highest in resources depletion, which was mainly due to burnt lubricating oil and UF resin, while the other factors (bio-binder, saw dust, and electricity) had the least impact as compared to the former. In Mesquite wood pellets, the lubricating oil, UF resin, and bio-binder had impacts on our resources, while saw dust and electricity had very low impacts on resources. In Oak wood pellets, the resources were depleted and disturbed mostly due to lubricating oil, UF resin, Bio-binder, and saw dust, respectively, as shown in the
Figure 8.
3.5. Single Score Exergy
Single score exergy was calculated for 1 kg wood pellets biofuel through sima-pro v9.1 software Eco-indicator 99 (E) V2.10 baseline methodology. The highest non-renewable fossil exergy for Kikar wood pellets was from lubricating oil (11.351 MJ) followed by UF resin (9.989), bio-binder (2.20), and saw dust (0.341 MJ), respectively (
Figure 10). Non-renewable fossil exergy from Mesquite wood pellets was highest for lubricating oil, UF resin, bio-binder, and saw dust, with values of 10.784, 7.04, 1.33, and 0.30 MJ, respectively (
Figure 11). The exergy of wood pellets (Oak saw dust) for non-renewable fossil was highest due to lubricating oil, followed by UF resin, bio-binder, and saw dust, as shown in the
Figure 12. Renewable biomass exergy, when observed in Kikar wood pellets, was highest in saw dust (5.83 MJ), proceeded by bio-binder (4.856) and UF resin (0.131 MJ). Similarly, renewable biomass, when observed in Mesquite wood pellets, showed that saw dust was the highest contributor, followed by bio-binder and UF resin, as shown in the
Figure 11. Saw dust, bio-binder and UF resin were the top contributors to renewable biomass with values of 4.52, 3.56 and 0.08 MJ, respectively, in wood pellets produced from Oak saw dust as shown in the
Figure 12. UF resin and bio-binder were the top contributors to renewable water in wood pellets from Kikar tree species. The other factors—saw dust, lubricating oil, and electricity—showed minor values. In wood pellets from Mesquite saw dust, impacts on renewable water were mostly due to saw dust (0.577 MJ) and bio-binder (0.115 MJ), as shown in
Figure 11. In Oak saw dust, bio-binder and electricity were the main factors in renewable water. Saw dust and lubricating oil showed minor/negligible values in renewable water.
3.7. Discussion
In the study, the moisture content of wood pellets was 8.8, 7.9, and 8.8% for Mesquite, Kikar, and Oak wood pellets, respectively. The moisture of wood pellets manufactured in Finland was 7–12% [
39]. The pellet had moisture content (MC) between 6.0 (E) and 7.8% (D). According to the Swedish standard SS-187120, the MC of pellets should be less than 10% or less than 12%, depending on which standard group it belongs to [
40]. Moisture content (MC) decreases heating values of wood pellets [
41]. Higher MC in the wastes results in higher durability and bulk densities [
42]. Opposite relationship was observed amongst MC and dimensions of wood pellets in a study of around eight different species [
43]. According to [
42], no such differences were observed for diameter and length, and they were according to the norms. While in the present study, differences were observed in length 29.3, 31.16, and 47.71 mm and diameter 8.71 mm with moisture variation of 8.8, 7.9 and 7.8% for Mesquite, Kikar and Oak, respectively.
The ash content was found to be 0.5% for wood pellets manufactured in Finland [
39]. Quality of the wood pellets could be further improved by reducing the ash content limit instead of changing other parameters [
44]. In several studies, it was illustrated that ashes increases mineral nutrition, soil fertility, and forest productivity [
45,
46], while in our study, the ash content was within the limit as mentioned in Italian standards. Concerning bulk density, most tests showed values higher than 600 kg/m
3, which was higher than the lower limit [
42]. In one more study, in which the burning of four different types of wood pellets were analyzed, it was found out that the bulk densities exceeded the lower recommended values [
47]. The bulk and energy density were directly related to one another. The species
B. tuldoides had the highest and significant bulk density value (0.35 g/cm
3) [
48]. The results obtained for bulk density in our study showed lower values than the lower limit when compared to different studies, as well as recommended Italian standard.
Higher heating values for white coir was 18.5 ± 0.3 MJ/kg and for brown coir was 19.0 ± 0.2 MJ/kg. Slight increase was observed in the results of HHV during the coconut maturation process. This could be due to degradation of hemicellulose [
47]. The HHV is directly linked to lignin content [
14]. In a study on the quality of wheat straw pellets, when wood residue and other types of biomass were used as a binder, the higher calorific value (17.98–18.77 MJ/kg) increased significantly [
49]. The pellets produced presented very low values, considering nitrogen and sulphur content (0.32 ± 0.01 and 0.04 ± 0.002%, respectively). In other words, roughly 80% less than the upper limit of the criteria consulted. Hence, it is estimated that both sulphur and nitrogen oxide emission limits would be negligible [
40]. Nitrogen and Sulphur contents were low in all the three types of wood pellets, as required by DIN 51731 (1996) [
41]. Nitrogen (0.28%) and sulphur (0.02%) content for sugarcane bagasse pellets found in our work was in accordance with all international standards [
50].
Our findings are consistent with previous research in that environmental burdens are mainly associated with the production of adhesives and the burning of fossil fuels [
51]. Study evaluated the results that natural gas-generated heat has a higher effect (6.74 mPt) than wood pellet heat (3.19 mPt), primarily due to the loss of fossil resources [
9]. In another study, it was concluded that the use of wood bioethanol, as a partial replacement for petrol, decreases the fuel’s global warming impact [
52]. The primary reason for initiating global warming potential in the life cycles of the production of wood pellets is fossil fuel consumption [
53]. The results showed a strong resemblance to the impacts of climate change, suggesting that the combustion of fossil fuels in the production chains of the WP dominates GHG emissions [
54].
More than 50% of the total Ecotoxicity Potential was given by the underlying electricity mix used to produce the pellets in each of the WP production scenarios, lagged by the ignition of biomass (more than 30% of total) for drying the wood pellets [
55]. Literature shows that UF resin, transportation, and electricity had the largest contribution to ecotoxicity: 37.60%, 31.42%, and 16.76%, respectively [
35]. The second highest environmental impact category was ecotoxicity caused by wood pellets in our study. The major sources of ecotoxicity were: UF resin, bio-binder, and lubricating oil.
The main causes of abiotic depletion (AD) were natural gas (36.87%), UF resin (34.17%), and electricity (14.91%) [
35]. On the other hand, in the Brazilian and Portuguese particleboard development processes, UF resin (30%) and HFO (35%) were responsible for most of the impacts in the AD effect group [
56,
57]. The AD effects of the production of HFO are mainly related to the extraction of minerals, coal, crude oils, and other non-renewable resources needed for its manufacture [
58]. The hotspot source for mineral depletion was Kikar wood pellets, while the leading factor was UF resin.
The burning of wood pellets is dominated by acidification and eutrophication, due to ammonia, nitrogen oxide, and other pollutants during pellet combustion [
52]. The energy mix accounts for more than 60% of the total acidification potential for the wood pellet scenarios, while sawmilling operations account for an additional 20% of the total acidification potential. The transport of wood pellets in the roundwood is the next major contributing factor at 13% of the overall acidification capacity [
55]. The greatest contribution to acidification was from energy, UF resin, and transport; 41.6%, 25.49%, and 21.94%, respectively [
55]. Heavy fuel oil (HFO) and UF resin are an important hotspot in the acidification potential effect group of Brazilian particleboard production due to the production of sulphur, methanol, and urea [
56,
57]. UF resin, bio-binder, and lubricating oil were responsible for acidification potential in wood pellets produced from Kikar, Mesquite, and Oak saw dust.
The environmental impact on human health is much more significant than the impact on the quality of the ecosystem and resources [
9]. The most damage affected resource depletion and human health [
59]. In damage assessment, the leading factor for damage was from Kikar wood pellets (190.68 mPt), lagged by Oak wood pellets (157.19 mPt) and Mesquite wood pellets (146.22 mPt). The highest damage was posed to resources depletion with the contribution of (5.35 MJ surplus), followed by ecosystem quality and human health with values of (0.927 PAF.m
2yr) and (5.35 × 10
−6 DALY).
Among the cumulative exergy impact categories, the highest contribution was made by non-renewable fossil sources (80%), while the combustion of renewable biomass in dryers was identified as the second largest contributor (9%), followed by renewable water (3%), among the different manufacturing processes, the production of UF resins, the consumption of fossil fuels, transportation, and elective activities [
32], which is in accordance with other studies [
35]. In cumulative exergy demand, the highest impact was from Kikar saw dust wood pellets (37.19 MJ), followed by Oak wood pellets (31.99 MJ) and Mesquite wood pellets (29.41 MJ). Among the production process, the highest exergy was obtained for non-renewable, fossil for wood pellets obtained from the saw dust of Kikar, followed by Oak, and the lowest value obtained was for Mesquite, with the values of 23.93, 21.99, and 19.51 MJ, respectively. Renewable biomass and exergy attained was highest for wood pellets obtained from Kikar (10.859 MJ) saw dust, proceeded by Oak (8.226 MJ) and Mesquite (8.202 MJ), respectively. Exergy obtained from renewable water was highest in Kikar wood pellets, followed by Mesquite and Oak wood pellets, respectively, with values of (0.94 MJ), (0.62 MJ), and (0.59 MJ), respectively.
3.8. Sensitivity Analysis for Wood Pellets
Sensitivity analysis determines how different values of an independent variable affect a particular dependent variable under a given set of assumptions [
60]. Sensitivity analysis focused on lubricating oil and UF resin to find impacts caused due to the above hotspot source.
Table 4 showed the comparative environmental impacts from base line study with data in which 20% reduction in lubricating oil and UF resin, while in second case 0% of usage of UF resin, were observed.
Table 5 showed the reduction in fossil fuel depletion, ecotoxicity, mineral depletion, and acidification/eutrophication potential. When 20% reduction was carried out in lubricating oil, similarly, 19% reduction was observed in fossil fuels, 20% in ecotoxicity, 20% in mineral depletion, and 21% in acidification/eutrophication. Furthermore, when 20% reduction in UF resin was done and 16% reduction was observed in fossil fuel, 27.25, 26.73, and 25% reduction were observed in ecotoxicity, mineral depletion and acidification/eutrophication, respectively. In case 3, when no UF resin was used, a huge decrease was observed in environmental impacts with values of 42.67, 66, 63.37, and 51.67% for fossil fuel, ecotoxicity, mineral depletion, and acidification or eutrophication potential, respectively. The comparative damage assessment to human health, ecosystem quality, and resources form the baseline results, with 20% reduction in lubricating oil, 20% UF resin, and without usage of UF resin, which had been shown in
Table 5. A 20% reduction in lubricating oil brought 20.47, 13.32, and 19% decreases in human health, ecosystem quality, and resources, respectively. In case 2, 20% reduction in UF resin carried out reduction in human health, ecosystem quality, and resources by 24, 15, and 16%, respectively. Similarly, in case 3, when 0% of UF resin was used for wood pellets production, 53% decrease was observed in human health, 2% in ecosystem quality, and 43.19% in resources.
Table 6 showed the scenario analysis of wood pellets for exergy, in which 20% reduction in lubricating oil caused 19% reduction in non-renewable fossil, 16% in renewable biomass, and 21% in renewable water. Similarly, 20% decrease in UF resin caused 15% decrease in non-renewable fossil, 16% in renewable biomass, and 37% decrease in renewable water. Furthermore, without usage of UF, resin caused 42% decrease in non-renewable fossil, 2.5% reduction in renewable biomass, and 92% decrease in renewable water.