The demand for sustainable growth has increasingly led to use of alternative and renewable energies. Even in some countries where access to other energy sources is widespread, charcoal is an important technological utility, such as in the case of the production of certain iron castings in Brazil that require pig iron as a sulfur-free feedstock, a chemical element that is present in the coal.
The use of charcoal as an energy input is secular and prevalent, and it is the only source of energy for a significant portion of the world’s population, such as in many African countries accounting for more than 60% of world production [1
]. Of the total charcoal produced in Brazil, more than 90% is consumed in industrial processes, mainly in the metallurgy sector [2
]. However, the use of archaic and inefficient kilns for its production prevails, even in Brazil, the world’s largest producer of this input (around 6.2 Mt in 2015) [1
]. The use of traditional systems such as Kenya’s earth mound kiln and unsustainable harvesting of wood, for example, lead to the waste of 85–91% of biomass and cause negative environmental impacts [3
The quest for sustainability is important for the charcoal value chain to ensure that wood procurement, the carbonization stage, input/product distribution logistics, and charcoal use comply with sustainability criteria. Regarding wood harvesting, the Brazilian charcoal production sector uses wood from eucalypt forest plantations (87% of the total charcoal produced in 2015). This practice lowers the pressure on natural forests [4
], but, in sub-Saharan Africa (SSA), the harvesting of wood from planted forests does not reach 5% [5
The reduction in the volume of carbon emissions varies according to the carbonization technology chosen [6
]. However, it is common practice to release the wood pyrolysis gases into the atmosphere [7
]. Large Brazilian companies with processes dependent on charcoal have invested in research and development in the search for technologies that are able to obtain higher yield of wood, greater homogeneity of the charcoal, less time in the production process, and scalability, besides the use of the pyrolysis gases for energy cogeneration [8
], as these are considered emerging technologies. These technologies, being of industrial nature in most cases, are still in consolidation and are responsible for a small portion of Brazilian charcoal production.
In the same way, Brazilian researchers coupled furnaces to the circular and rectangular masonry kilns for the combustion of carbonization gases, in aim of the following: reducing emissions of greenhouse gases (GHG) [9
]; identifying the potential of using the heat generated for drying the wood [11
]; evaluating the influence of this process on the quality and gravimetric yield of charcoal [12
]; and obtaining carbon credits [15
]. This combined furnace system also has the characteristic of affordable cost [16
]. It is also considered to be emerging in the process of consolidation, has the potential to reach most Brazilian producers, and has application anywhere in the world.
The emphasis on the differences between the technologies used in the studies presented in this paper and the emerging technologies reviews the use of the pyrolysis gases in the cycle itself for drying the wood and the cogeneration of electricity on an industrial scale, as shown in Bailis et al. [7
] and Vilela et al. [8
]. Considering the historical annual volume of 6–9 Mt charcoal production in Brazil, there is a potential of electric generation of 3–5 TWh per year [17
In a recent and thorough review of the literature on the state of the art of the technologies used to burn the carbonization gases, Pereira et al. [10
] mention studies that have presented the reduction in emissions of carbon monoxide (CO) and methane (CH4
) for combined kiln and burner systems. Adoption of these systems is compatible with thousands of producers and can be used in any operation producing charcoal, as a way of mitigating GHG and improving working conditions. However, there is no evidence of LCA research on the charcoal production with burning in combined furnace systems.
One way of verifying whether charcoal production is being conducted sustainably is by assessing its impact on climate change [1
]. In this sense, Ugaya and Walter [18
] showed that the use of charcoal for steel production in Brazil contributed to the reduction of waste generation in the study of the car’s life cycle. In this sense, this article aims to evaluate and compare the environmental impacts on charcoal production with combustion of the pyrolysis gases through LCA of three processes: without combustion of gases in a rectangular masonry kiln (RK1 Process); with combustion of gases in a rectangular masonry kiln coupled to a masonry furnace (RK2 Process); and with the combustion of the gases in a circular masonry kiln coupled to a metal furnace (CK Process). Thus, the technologies and processes involved were identified: three wood carbonization processes; identification of the relevant impact categories; and evaluation and comparison of the potential environmental impacts of these technologies, using LCA.
This study contributes to development of a technology capable of improving the environmental impact of charcoal production for a large portion of the world population, evaluated by the assessment of the life cycle presented. The search for clean and renewable energy sources has retained the attention of researchers, industries, and society. This study results present the advantages and disadvantages of each process and can stimulate the adoption or public policies on charcoal production technologies that have lower environmental impacts.
The article presents the following structure: Section 2
identifies previous studies on LCA in the production of charcoal and the methods used; Section 3
presents the LCA methodology used to analyze the present study; Section 4
compares the results of the analyzed processes, emphasizing those related to the impacts in the wood carbonization phase found in previous studies; and Section 5
evaluates the limitations of the study and presents suggestions for future studies.
2. Charcoal Production LCA
Standard guidelines for LCA were established by the International Organization for Standardization (ISO), currently standardized by ISO 14040: 2006 and ISO 14044: 2006, in a framework that encompasses the following: goal and scope definition (functional unit/reference flow and impact categories); inventory analysis (technology description, period, region, limitations, validation, data quality criteria, and data collection); impact assessment (characterization); and interpretation (process contribution, elementary flow contribution, and sensibility).
The LCA methodology encompasses a cradle-to-grave (C2Gv) analysis, given the environmental impacts subject to a product or process from the collection or extraction of the inputs, transportation, production, and use or final destination. However, the analysis can be performed in certain parts of the life cycle: cradle-to-gate (C2G) includes resource extraction to manufacturing/service operations, excluding subsequent phases; Gate-to-gate (G2G) is restricted to the manufacturing stage; and gate-to-gate (G2G) includes processes of distribution, use and final disposal of the product [19
Piekarski et al. [20
] consider that among the wide range of methodologies used to analyze the environmental profile of products, the most extensive method is the LCA. Khoo et al. [21
] also consider the application of LCA to assess the performance of a technology of interest in economic and social terms.
The LCA has been widely used in the evaluation of the impacts caused in the construction industry [22
]; in the production of biofuels [24
]; in the generation of energy [25
] from different biomasses; in teaching and research [26
]; and in food systems [27
], among many applications. Studies demonstrate the joint use of LCA with other methodologies.
As examples, Theodosiou et al. [28
] adopted the LCA methodology with the multicriteria analysis model for the development of an optimization model applied in the analysis of energy systems projects; and Koroneos and Stylos [29
] used the Exergy Analysis methodology combined with LCA in a sustainability analysis of energy production in photovoltaic systems.
The first study on the amount of air pollution during wood carbonization in an earth mound kiln occurred in 1994 in Côte d’Ivoire [30
]. Emission factor calculations in the production of charcoal were done by other authors [31
]. As an example, other LCA studies have evaluated the use of forest residues in charcoal production as a way to quantify sequestered carbon [36
] and to evaluate the environmental performance of different carbonizers in the production of biochar [37
A LCA of the charcoal value chain encompasses stages from the wood harvest (originated from natural forest and planted forest), carbonization (kiln efficiency and process energy recovery), transport (logistics), and charcoal utilization (domestic and industrial). It also includes the reuse of process products until final disposal (charcoal fine products and cogeneration of electricity) [1
]. Stages of the life cycle in the charcoal value chain presented in previous studies are shown in Table 1
The authors’ LCA studies presented in Table 1
do not individually consider all stages of the life cycle in the charcoal value chain. The LCA of Bailis et al. [38
] is one of the few to contemplate all stages of this value chain and compares emissions from the use of charcoal with emissions from some alternative cooking fuels.
Khoo et al. [21
] evaluated the carbonization of wood residues and briquetted tree prunings. They established mass Functional Unit (FU) for 1 t of the charcoal for the G2G border and analyzed the impact on the global warming potential (GWP), acidification, human toxicity, and photochemical oxidant potential categories. The result of the comparison between the technologies evaluated shows a reduction of GHG emissions (54–85%) and a 90% improvement in human health when using the Japanese carbonizer. The other impact categories did not present significant values in any of the technologies.
The study by Afrane and Ntiamoah [39
] analyzed the impacts on charcoal production in earth mound kilns. The use of charcoal was compared to the use of biogas and liquefied petroleum gas (LPG) as alternative sources of energy for cooking. The FU used was the production of 1 MJ of useful energy from each fuel system. The cradle-to-grave boundary (C2Gv) was established and analyzed impacts in the following categories: acidification, eutrophication, freshwater aquatic ecotoxicity, GWP, human toxicity, photochemical ozone creation (smog), and terrestrial ecotoxicity potentials. The result shows that, of the total impact of the GWP assigned to the charcoal, 61% occurred during the production and 39% occurred during the use for cooking.
Bailis et al. [7
] compared the production of charcoal in hot-tail kiln—the most common in Brazil—with Rima container kilns (RCK) production, using the gases generated in the wood carbonization process in a Brazilian company producing this input. The carbonizations were performed with eucalyptus from planted forests. The adopted FU was 1 kg of charcoal, and C2G was used as the product system. GHG emissions were analyzed by the 100-year GWP. Other empathy categories were analyzed: energy return on investment; ozone depletion potential; photochemical oxidation; acidification potential; eutrophication potential; and water use. The study verified the possibility of reduction of more than 50% in the carbon footprint in the scenario with charcoal production in RCK and use of all the gases for cogeneration of energy.
Ekeh et al. [40
], in Kampala, Uganda, used the GHG emission factors found in the study by Pennise et al. [33
] to compare with emissions from charcoal production in a CH4
-free process technology (“PYREG methane-free charcoal production equipment”), using wood from a sustainably managed plantation. The established FU was 1 kg of charcoal at the C2G boundary and analyzed the GWP. This comparison showed that GHG emissions in the carbonization phase decreased by approximately 28% when the PYREG process was used.
Partey et al. [41
], in Ghana, compared the impacts related to the carbonization of three biomass species of sustainable origin, using circular masonry kilns. The established FU was 1 MJ energy produced from the three species, at the C2G boundary.
In relation to the LCA methods used in previous studies, GaBi [21
] and SimaPro [7
] software packages were used in the previous studies using GaBi [21
] and SEMCo [7
]. The life cycle impact assessment methods used were EDIP 1997 [21
], CML 2001 [39
], Ecoinvent data v2.1 [7
], and Ecoinvent v3 and Idemat 2015 [41
]. The analysis of data quality in these studies was not evidenced. The sensitivity analysis was presented by Ekeh et al. [40
] and Bailis et al. [7
4. Results and Discussion
In this work, the relative environmental impacts of the three technologies mentioned in the scope for the production of charcoal were evaluated. When evaluating only the environmental impacts of the carbonization process and the furnace (G2G), it can be observed that the burning of the gases is significantly better when compared to the direct emissions to the atmosphere. The RK2 Process data are the relatively least impacting G2G boundary, as shown in Figure 4
. In general, it is evident that the inclusion of furnaces for the gas combustion reduces all categories of potential environmental impacts by at least 90% for both the CK Process and the RK2 Process.
In a second analysis, the calculations were carried out to verify the influence of NO2 and SO2 emissions on the G2G analysis for the three technologies analyzed. The emission impacts of particulate matter, photochemical oxidant formation, and terrestrial acidification have been altered due to NO2 and SO2 emissions.
For the categories of particulate matter emission, photochemical oxidant formation, and terrestrial acidification, it can be observed that the CK process presents better results, as shown in Figure 5
Regarding climate change, it is observed that the impact reduction with the combustion of gases is 90% for the CK process in the G2G system. However, in the charcoal production process (G2G), the best technology in terms of climate change was the RK2 Process, having approximately 63% less impact than the CK process.
The search for carbonization alternatives with gas combustion to generate benefits for environmental protection and use of renewable energies is also observed in the study by Khoo et al. [21
]. The authors found that, in a Japanese carbonization model, approximately 85% less GHG were produced than in the conventional carbonization system, and the combustion furnaces (RK2 Process and CK Process) presented more than 90% reduction, in terms of impacts on climate change. The study by Khoo et al. [21
] also demonstrated reduction in impacts related to acidic gas emissions in the gas combustion process. This corroborates the results of this study, where gas combustion technologies reduce the impacts of acidification, photochemical oxidant potential, and human toxicity.
The study by Ekeh et al. [40
], which quantified GHGs from the production, transport, and distribution of charcoal in an earth mound kiln of Uganda, found that approximately 53.3% of the impacts related to climate change (828,316 tCO2
eq.) are related to the charcoal production phase. The authors verified the potential of reducing GHG emission through a reduction of “free-methane” in the charcoal production process and the use of non-sustainable wood in the process. In turn, Partey et al. [41
] found that the pyrolysis and transport phases did not exceed 10% of the total ecological impact cost in the GWP categories, human health, and ecosystems when analyzing the impacts of production (from nursery of seedlings) and the use of three forest species for charcoal production in C2G boundary.
In this sense, by expanding the boundaries of the study and connecting the technological system of wood production with the ecoinvent database, as presented previously in Figure 1
, the results of the C2G impacts can be obtained. The results are presented by category in Figure 6
When considering the life cycle of charcoal production at the C2G boundaries, it can be observed that environmental impacts have a major dependence on the efficiency of each type of technological process in charcoal production. It was observed, for example, that, for the climate change impact category, the gas-fired circular kiln technology (CK Process) had the highest impact (1.38 × 10−5
DALY/41.84 MJ of charcoal), while the best environmental performance for this category was presented by the RK2 Process (1.10 × 10−6
DALY/41.84 MJ of charcoal). In Figure 5
, it can be seen that the CK Process has 90% better performance for the impact of climate change compared to the RK1 Process, at the G2G boundary. The fact that the CK process consumes 15% more wood (on a dry basis) when compared to the RK1 Process (see Table 3
) results in a higher C2G impact. The G2G boundary represents about 6% of the total impacts of the climate change categories for the RK1 Process. As for the processes with gas combustion, the impact drops to 0.58% and 0.52% (CK Process and RK2 Process, respectively). That is to say, it can be observed that when the impacts of the G2G boundary is low, the behavior of the environmental impacts will be associated with the efficiency of the charcoal production kilns. The efficiency of the analyzed processes is related to the data collected from LCI, and they can be analyzed as points of generation of innovation in the industry, as suggested by Luz et al. [52
The categories of environmental impacts that presented a low percentage impacts (<10%) for the G2G boundary in the life cycle of charcoal production were: agricultural land occupation, climate change ecosystems, climate change human health, fossil depletion, freshwater eutrophication, ionizing radiation, metal depletion, natural land transformation, ozone depletion, and urban land occupation. For all of these categories, the CK Process had a higher environmental impact (Figure 6
) because it had higher wood consumption (on a dry basis) to generate the same calorific value from the charcoal (41.84 MJ) when compared to the rectangular kilns. However, the categories that have a significant impact for the G2G boundary in the charcoal life cycle (C2G) have been shown to be dependent on the impacts generated by the non-combustion of the gases in the processes. In other words, the RK1 Process (which does not burn the combustion gases) had the highest relative environmental impact for the other categories.
In this sense, analyses on the influence of NO2
emissions during carbonization in C2G analysis were performed. It can be affirmed that the C2G results undergo variations when including NO2
emissions during carbonization. This is because the carbonization process is one of the most impactful in some categories of the charcoal life cycle. To analyze the relative contribution of the carbonization process in the charcoal production for the three different technologies listed in this study, all impact categories in which the carbonization processes presented contributions above 1% were identified (Table 6
Only the categories of photochemical oxidation formation, particulate matter formation, and terrestrial acidification presented contributions above 1% for the three technologies analyzed in this study. Among them, the terrestrial acidification that presents percentage above 95% in all the technologies is highlighted, especially due to the emissions of NO2 and SO2.
According to Table 6
, the RK1 Process has impact greater than 1% in eight impact categories due to non-combustion of the gases. The greater contributions are from the terrestrial ecotoxicity impact (98%), especially due to the emission of acetic acid. Additionally, to identify the substances that contribute most to the impact categories displayed in Table 6
, Table 7
lists the ones whose values surpass 1% of impact contribution in the three technologies.
When comparing the RK1 Process and the RK2 Process, the contribution of NO2 and SO2 emissions is approximately 97% for terrestrial acidification. In the formation of photochemical oxidation, NO2 has a change in contribution, from 76% in RK2 to 25% in RK1. Acetic acid contributes to photochemical oxidation (33%), terrestrial ecotoxicity (79%), fresh water ecotoxicity (39%), and marine ecotoxicity (14%) in the RK1 Process. Phenol, 2,6-dimethyl has a contribution of 55% for marine ecotoxicity when carbonization is performed in the RK1 Process.
A sensitivity analysis of the allocation factors was performed, and the result is presented in Figure 7
shows that, in the economic allocation for the CK process, 99% was allocated to the charcoal relative to the base, and 1% was allocated to the fines. For RK1 and RK2, 98% and 97% were allocated to charcoal and the other 2% and 3% to charcoal fines, respectively. Considering the same basis, the mass allocation shows that 90% was allocated to charcoal and 10% to charcoal fines. For RK1 and RK2, 90% and 87% were allocated to the charcoal and the other 10% and 13% to charcoal fines, respectively.
The sensitivity analysis also considered the energy factor. However, the values were the same as those for the mass factor.
5. Conclusions and Limitations
The search for clean and renewable energy sources is of great importance for researchers, society, industries, and public policies. This study allowed for conclusion that different technologies of charcoal production have different environmental performances, which are linked to two major aspects: equipment efficiency and impacts on the G2G boundary.
The substances and impact categories, which do not have high large contribution in the total environmental impact on the cradle-to-gate boundary of the charcoal, suggest that the processes with greater technical efficiency in the conversion of wood to the calorific value for the produced charcoal are the ones that have better environmental performance. In other words, the potential environmental impacts for agricultural land occupation, climate change ecosystems, climate change human health, fossil depletion, freshwater eutrophication, ionizing radiation, metal depletion, natural land transformation, ozone depletion, and urban land occupation tend to be smaller for processes with better efficiency in the charcoal production process. In this study, the best performance for these categories was the RK2 Process. On the other hand, the environmental performance was also better for processes with the combustion of gases in the furnace. There are impact categories that have more impact (>80%) for the G2G boundary in C2G (photochemical oxidant formation, terrestrial acidification, terrestrial ecotoxicity, freshwater ecotoxicity, and marine ecotoxicity). For these categories, the results indicated that the technologies evaluated have a better environmental performance when the furnace gases burn.
In the G2G analysis, the RK2 Process presented the best performance when not considering NO2 and SO2. When considering NO2 and SO2, there were changes in the particulate matter and terrestrial acidification emission impacts, and, for these two categories, the CK process presented the best performance (C2G). However, it did not present the best performance in terms of consumption of wood vs. calorific value produced from the charcoal. For the other categories (photochemical oxidant formation, terrestrial ecotoxicity, freshwater ecotoxicity, and marine ecotoxicity), the RK2 Process maintained the best environmental performance. The study suggests that the rectangular masonry kiln coupled with the masonry furnace (RK2 Process) presented the best environmental performance in general. Actions that result in reduction in NO2 and SO2 emissions can be developed for this process to improve environmental performance in the impact categories of particulate matter and terrestrial acidification, where the circular masonry kiln coupled with the metal furnace (CK Process) presented better performance.
According to Bailis et al. [7
], theoretically, the environmental impacts on charcoal production can be reduced with the implementation of pollution controls, as is the case of the gas combustion furnaces analyzed in this work. However, there is little regulatory and fiscal incentive for the deployment, development, and investment of such technologies in rural Brazilian areas. Pereira et al. [10
] state that the combustion practice of carbonization gases will reach the entire charcoal production chain in a solid and cohesive way when society’s knowledge is disseminated and all technical difficulties are overcome by the productive sector.
Theoretical and Practical Implications
This case study has theoretical and practical value for researchers. It is necessary to advance technological development to increase the efficiency of equipment for the production of charcoal. The most efficient equipment for wood-to-charcoal conversion is one that has the best environmental performance.
Another practical implication highlighted in the study is technological development in the gate-to-gate frontier of charcoal production. Even though the environmental impacts of carbonization are relatively low considering for the entire life cycle, emissions can have significant impacts, both locally and regionally. Although the carbonization gas combustion technologies presented in this study are in development, it is of great importance that new studies evaluate the following: the synchrony between kilns coupled to the same furnace; obtaining the optimum temperature and flow rate of the gases in order to favor their combustion; and the residence time of these gases inside the combustion chamber. These efforts aim to decrease the use of auxiliary fuel and reduce CO and CH4 emissions. For the gate-to-gate boundaries, gas-fired furnaces showed the best environmental performance.
In addition, process residues that are used for fertilization or other purposes should be evaluated in view of the need to analyze other chemical compounds, such as heavy metals.
Finally, the results presented in this work contribute to the development of life cycle inventories for charcoal production in Brazil. These results can be useful in understanding the advantages and disadvantages of different technologies for the production of charcoal, besides stimulating public policies to encourage the installation of charcoal production technologies with lower environmental impacts and the development of new clean technologies.