Tannins have shown antifungal effects and have been considered a potential natural compound for wood preservation. Extracts produced from softwood bark contain both tannins and non-tannin compounds, which may reduce the effectiveness of tannin used as a wood preservative. The purpose of this research is to study the environmental impact of hot water extraction, identify the hot spots within the tannin cradle-to-gate life cycle and give suggestions to optimize its environmental profile. Different extraction and post-extraction scenarios of tannin production are compared using the life-cycle assessment method. Experiments were designed to study the tannin yield under different extraction scenarios; the post-extraction scenario analysis was based on literature review. The results show that the extract drying process is the primary contributor to the environmental impact of tannin production. Both preliminary cold water extraction and ultrafiltration after extraction are beneficial as they have fewer non-tannin compounds in the final products; however, preliminary cold water extraction had a considerably lower environmental performance. Successive extractions using fresh water at each cycle increased the total tannin yield, but increased the environmental burden. Using only evaporation to obtain a desired tannin concentration is not environmentally efficient. This paper provides a quantified environmental analysis for the development of tannin-treated wood products and discusses the different tannin extraction scenarios from an environmental point of view.
Preservation of wood using antifungal agents usually results in increased environmental impact (
To some extent, recent wood preservation research focused on the exploration of bio-based chemicals,
Tannins are a versatile group of polymeric phenolic compounds resulting from the plant’s secondary metabolism. In trees, tannins provide protection from herbivore, insect, fungi and bacteria attacks (
The advantages of extracting tannins from bark are obvious: abundancy and availability of bark, possibility of simple hot-water extraction processes without chemicals (
The use of condensed tannins extracted from European softwood is currently a niche market and product solution. One obvious drawback is the high amount of co-extracted non-tannin compounds, particularly carbohydrates (
This paper aims to compare the environmental impact of alternative extractions and post-extraction tannin processes from the Norway spruce (
The pilot-scale model was based on the PHWE process for the recovery of tannins from Norway spruce bark. The PHWE batch system used is similar to the system described by
Laboratory-scale extraction from spruce bark was carried out to gather information about the extraction yields at different temperatures and process sequences. The experiments were done using bark removed from spruce logs of about 40 cm diameter at breast height, felled one week before the bark collection and then left exposed to natural weathering. The collected bark flakes, containing all layers of bark and trace amounts of woody tissue, were milled to a fine-powder (< 50 µm) with a vibrating disk mill (Herzog HSM 100H, Osnabruck, Germany). All ground bark samples were kept in the dark and in deep-freeze (-20 °C) until extraction.
The extractions were performed using an accelerated solvent extraction system (the ASE200 Dionex®, Sunnyvale CA, USA). Each extraction was then dried to a fine powder using a freeze dryer (Christ Alpha 1-4 LSC, Osterode, Germany). The dry extracts were analysed following the Folin-Ciocalteu method (
Two separate extraction sequences were conducted. Sequence 1 simulates the extraction process with only hot water. Approximately 2 g of milled bark powder was mixed with 1 g diatomaceous earth (diatomite) and loaded into a 22 ml ASE extraction cell. About 18 ml of tap water was added then heated to 90 °C within five minutes. After 20 minutes at constant temperature and pressure (10 MPa), the extract was flushed into the collection vessel. The process was repeated four times on the same bark sample, and extracts were collected in parallel vessels after each process. Sequence 2 was designed to study the extraction process with a preliminary step of cold-water extraction. The bark, prepared following the same procedure as Sequence 1, was initially extracted for 25 minutes with cold water (approximately 10 °C) and 10 MPa. The extraction then was continued using the same procedure as Sequence 1. Both Sequence 1 and Sequence 2 were repeated twice.
The aims of this assessment were: (1) to quantify the key environmental impact of pilot-scale dried tannin production to be used as raw material for environmentally friendly wood preserving agents; (2) to identify critical process stage(s) contributing to environmental load; and (3) to compare the impact of different extraction and post extraction process scenarios during the cradle-to-gate life cycle of bark extract ready to be applied as a preservative.
This LCA follows the guidance of
The system under study is described in
Allocation of environmental impact between by-products can be critical in LCA. The bark appears twice as a by-product in this system. The first allocation occurs after debarking at sawmill. Based on its economic allocation factor provided by
The environmental impact of extract production was analysed based on combining one step of hot water extraction with evaporation and spray drying. To simulate possible industrial tannin productions, we have defined four extraction (E1, E2, E3, E4) and three post-extraction (P1, P2, P3) scenarios. Extraction scenarios refer to designed experiments, and the resulting solutions are treated after extraction, according to P1, P2 or P3 scenarios as listed in
This scenario follows the method used in mimosa tannin production, a commercialized tannin production (
This scenario is based on the idea that environmental and economic benefits might be obtained if the extract is concentrated just to the level that is assumed to be optimal in tannin content as a preservative. Thus, energy consumption, because of excess drying in P1, is avoided for preservative use since tannin needs to be mixed with water and other agents. Based on a previous study by
This scenario simulates the use of ultrafiltration equipment in industry to separate tannins from lower molecular weight compounds with lower energy requirements and greater efficiency (
The functional unit (FU) is 1 kg tannin yield after post extraction treatment. The equivalent masses of dried extracts used in P1 scenario were: 2.045 kg, 1.684 kg, 2.041 kg and 1.792 kg, corresponding to E1 to E4 scenarios. The equivalent mass of liquid extracts is 20 kg with the same tannin concentration (5%) for both P2 and P3, achieved by removing different amount of water in different E scenarios.
Tannin yield through the PHWE process with different extraction method configurations were predicted using results from laboratory experiments. Ecoinvent v3.0 (
To manage the ecoinvent’s global context and match the conditions of Finland geographically and temporally, all the electricity data were modified using the Finnish electricity grid mix according to the national statistics (
The forest establishment and Norway spruce harvesting schemes are assumed to be like Sweden’s, since the forest management system and technology used in both countries are very similar (
The impact assessment is based primarily on the CML-IA baseline method developed by the Center of Environmental Science of Leiden University (
The results of the laboratory-scale extractions are presented in
Because the 4th step of extraction only produced 4.1 g extracts per kg of bark in Sequence 1 and 7.5 g per kg of bark in Sequence 2, only the first three steps were further considered. The results show the impact of adding one step of cold water extraction and of repeating the extractions on the obtained tannin concentration. Preliminary cold water extraction (E2 and E4) resulted in higher cumulative tannin concentration but lower yields (<34 g kg-1 dry bark) due to tannin loss in cold water. Repeated extractions (E3 and E4) resulted in higher cumulative dried extracts yields (≥24 g kg-1 dry bark) and higher cumulative tannin yields (≥12 g kg-1 dry bark) compared to single-step hot water extraction (E1 and E2). However, only minor variations in the tannin concentration were observed between one-step and three-step extractions (E2 and E4).
The amount of water to be removed after each post extraction scenario (
The environmental impacts and resource use to produce the FU are shown in
High energy intensity of evaporation and extraction is indicated as their relative high contributions in each impact category (
The environmental impact of the proposed extract post-treatments (P scenarios) was compared first. While evaporation made the largest contribution to the impact in P1,
The P3 scenario has roughly half of the impact of P1. The extract volume reduces remarkably, and the energy needed for ultrafiltration is low (
The GWP of dried extracts production through the four proposed extraction scenarios are compared in
A sensitivity test was carried out by examining the results if a change of 10% input was simulated. The results are sensitive to the electricity mix (7.3% to 9.8% in all categories for a 10% input change) because electricity will influence the stages of evaporation, hot water extraction, spray drying and ultrafiltration. The evaporation stage is the largest contributor to the environmental profile in the production of the FU. The results are not sensitive (less than 1% for a 10% input change) to the other upstream processes, including bark chips preparation, internal transportation and facilities preparation, as well as downstream processes (including waste water treatment). The evaporation stage has clearly the most influence on the modelled profile. And a 10% of increase would lead to a 6.4% to 8.5% increase in the results. Therefore, more accurate results can be attained if more practical data can be acquired from industry.
The purpose of this study was to examine the environmental impact of hot water extraction of tannins from spruce bark and to illustrate how the LCA tool can be used to support this kind of process under development. The extraction experiments and the literature based inventories should both be considered preliminary, since scaling up the processes has not been thoroughly assessed. The main limitation of this study is that tannin yields at a pilot-scale plant might be different from the laboratory experiment (
However, the evidence found on the critical importance of evaporation as a post-extraction treatment and the resulting low environmental performance of multi-extraction processes is rather evident in the batch extraction case. The key to improve environmental performance appears to be a lower amount of water use for the extraction process.
Ultrafiltration also appeared to be a promising post-extraction technology from an environmental point of view. It not only reduces the extract volume but also filters away some of the undesired, smaller-sized molecules like monosaccharides and other monomers, thus improving the quality of the extract for use as a preservative. It should be also noticed that tannin retention after ultrafiltration process contributes significantly to the result. Therefore, it is crucial to select the proper membrane type and the processing conditions. Because the related literature on ultrafiltration is more focused on the phenolic compound concentration than on sugars, further studies on compounds (molar mass and their chemical characterization) in permeate and rejected fractions is needed.
High energy intensity of evaporation rests partly on the selected system boundary. In this study, we assumed that the bark is taken from the sawmill and returned after extraction to its original use as energy for kiln drying. Alternatively, we could have assumed that the bark residue is used as an energy source for the extraction. If the bark residue is incinerated and the generated energy is used within the system, the global warming potential would be approximately 65% of the one found here. However, if no excess bark is available at the mill, fossil fuels are likely used, at least partly cancelling the gain in extraction. Even if this might be more realistic, the selected system boundaries serve the process development better.
This study establishes a cradle-to-gate environmental analysis of a tannins pilot-scale extraction from Norway spruce bark through PHWE. The focus is to compare different extractions and post-extraction scenarios. The evaporation process is the largest contributor to all environmental impacts and resource use categories. The use of ultrafiltration can halve the environmental burdens if all tannins are recovered. Although tannin yields are higher, preliminary cold water extraction and multiple extractions have a higher environmental impact for 1 kg of tannin than a single hot water extraction. Preliminary cold water extraction and ultrafiltration might be beneficial processes in terms of obtaining less non-tannin compounds, including sugars, which might be used in metabolizing the fungi in dried extracts.
The utilization of spruce bark tannins as an antifungal agent is still in development. Some other issues than tannin purity are also arising,
The authors acknowledge COST Action FP1407 and STSM- 33649 for support.
System boundary for the tannin production. Detailed flow chart of the simulated tannin production unit operations from cradle to gate (CW: cold water extraction; HW: hot water extraction). The dash dot line describes the system boundary.
Contribution of process stages on impact and resource categories. Figure 2a scenario P1 and E1, Figure 2b scenarios E1 and P3. Abbreviations: (ADPE): Abiotic Depletion Potential Elements (kg Sb eq); (ADPF): Abiotic Depletion Potential Fossil (MJ); (GWP): Global Warming Potential (kg CO2 eq); (ODP): Ozone Depletion Potential (kg CFC-11 eq); (POCP): Photochemical Ozone Creation Potential (kg C2H4 eq); (AP): Acidification Potential (kg SO2 eq); (EP): Eutrophication (kg PO4-3 eq) ); (CEDNR): Cumulative Energy Demand (MJeq), Non-renewable; (CEDR): Cumulative Energy Demand Renewable (MJeq). Raw material includes bark extraction and water; Facilities includes PWHE, evaporator and spray dryer).
Environmental impacts for the production of one functional unit. Comparison of post extraction scenarios P1, P2 and P3 after hot water extraction (E1). (ADPE): Abiotic Depletion Potential Elements (kg Sb eq); (ADPF): Abiotic Depletion Potential Fossil (MJ); (GWP): Global Warming Potential (kg CO2 eq); (ODP): Ozone Depletion Potential (kg CFC-11 eq); (POCP): Photochemical Ozone Creation Potential (kg C2H4 eq); (AP): Acidification Potential (kg SO2 eq); (EP): Eutrophication (kg PO4-3 eq); (CEDNR): Cumulative Energy Demand (MJeq), non-renewable; (CEDR): Cumulative Energy Demand Renewable (MJeq).
Comparison of (a) spray drying (P1) and (b) ultrafiltration (P3). Global warming potential of producing 1 FU. Other stages represent the aggregated stages of raw material, facilities, transportation and waste water treatment.
Tannin extraction (E) and post extraction (P) scenarios (30% is weight percentage (w/w) of all compound in extracts solution; 5% is weight percentage (w/w) of tannin in extracts solutions).
Scenarios | Compound | Description |
---|---|---|
Extraction | E1(1HW) | One step of hot-water extraction |
E2 (1CW+1HW) | One step of cold-water extraction and one step of hot water extraction | |
E3 (3HW) | Three steps of hot water extraction | |
E4 (1CW+3HW) | One step of cold-water extraction and three steps of hot water extraction | |
Post extraction | P1 | Extractives → evaporation to 30% → spray drying → dried extract1 |
P2 | Extractives → evaporation to 5% → liquid extract 2 | |
P3 | Extractives → ultrafiltration → evaporation to 5 % → liquid extract 3 |
Collected data of energy consumption for each unit operation included to the model.
Operation | Unit | Quantity | Source |
---|---|---|---|
Milling bark chips (air-dry spruce bark with hammer milling) | kWh kg-1 | 0.014 |
|
Hot water extraction (from room temperature to 90 °C) | kWh kg-1 | 0.13 | Laboratory data |
Evaporation | kWh kg-1 | 1.4 |
|
Spray drying | kWh kg-1 | 1.6 |
|
Ultrafiltration | kWh L-1 | 0.0052-0.006 |
|
Dried extracts and tannin yield results from two laboratory extraction sequences; sequence 1 with four successive hot water extractions (steps 1-4) and sequence 2 with preliminary cold water extraction (step 0). The table presents the average results of two repetitions. (T): temperature; (d.b.): dry bark; (d.e.): dried extracts.
Sequence | Step | T (°C) | d.e. yieldin step(g kg-1 d.b.) | Cumulative d.e. yield(g kg-1 d.b.) | Tannin yieldin step(g kg-1 d.b.) | Cumulative tannin yield(g kg-1 d.b.) | Cumulative tannin concentration(% d.e.) |
---|---|---|---|---|---|---|---|
1 | 1 | 90 | 78.5 | 78.5 | 38.4 | 38.4 | 48.9 |
2 | 90 | 18.7 | 97.2 | 9.5 | 47.9 | 49.3 | |
3 | 90 | 5.1 | 102.3 | 2.2 | 50.1 | 49.0 | |
4 | 90 | 4.1 | 106.4 | 1.5 | 51.6 | 48.5 | |
2 | 0 | 25 | 30.8 | - | 12.6 | - | - |
1 | 90 | 44.3 | 44.3 | 26.3 | 26.3 | 59.4 | |
2 | 90 | 14.5 | 58.8 | 7.5 | 33.8 | 57.8 | |
3 | 90 | 9.6 | 68.4 | 4.3 | 38.1 | 55.8 | |
4 | 90 | 7.5 | 75.9 | 3.1 | 41.2 | 54.3 |
Removed amount of water in different extraction (E1, E2, E3, E4) and post extraction (P1, P2, P3) scenarios.
Post extractionscenario | Extractionscenario | Ultrafiltration(kg) |
---|---|---|
P1 | E1 | - |
E2 | - | |
E3 | - | |
E4 | - | |
P2 | E1 | - |
E2 | - | |
E3 | - | |
E4 | - | |
P3 | E1 | 68.52 |
E2 | 103.79 | |
E3 | 156.53 | |
E4 | 210.56 |
Inventories for the FU (P1 P2 P3 scenarios). (*): Recovered energy is to present the possible benefit of burning bark residual in sawmill, as additional information, as it is not included in the system boundary. The values were calculated based on fresh bark heat capacity value.
Kind | FU | Step | Unit | E1 | E2 | E3 | E4 |
---|---|---|---|---|---|---|---|
Input | Milled bark (dry) | kg | 26.1 | 38.0 | 19.9 | 26.2 | |
Water | kg | 220.7 | 530.4 | 406.5 | 685.3 | ||
Hot water extraction | kWh | 22.8 | 34.4 | 41.9 | 56.61 | ||
P1 scenario | Evaporation | kWh | 211.1 | 326.3 | 494.4 | 669.5 | |
Spray drying | kWh | 7.6 | 6.3 | 7.6 | 6.7 | ||
P2 scenario | Evaporation | kWh | 192.8 | 306.3 | 476.1 | 650.0 | |
P3 scenario | Ultrafiltration | kWh | 0.41 | 0.62 | 0.94 | 1.26 | |
Evaporation | kWh | 97.5 | 162.0 | 258.5 | 357.3 | ||
Output | Co-product (dry bark residue) | kg | 25.1 | 37.0 | 18.9 | 25.2 | |
Tannin | FU | 1 | 1 | 1 | 1 | ||
Waste water (P1) | m3 | 0.16 | 0.44 | 0.36 | 0.62 | ||
Waste water (P2) | m3 | 0.14 | 0.42 | 0.34 | 0.60 | ||
Waste water (P3) | m3 | 0.14 | 0.42 | 0.34 | 0.60 | ||
Recovered energy* | kWh | -43.0 | -63.6 | -30.4 | -42.3 |
Life cycle impact assessment results of the FU, P1, E1 scenario.
Kind | Impact category | Unit | Value |
---|---|---|---|
Impact | Abiotic Depletion Potential Elements (ADPE) | kg Sbeq | 1.27E-05 |
Abiotic Depletion Potential Fossil (ADPF) | MJeq | 4.55E+02 | |
Global Warming Potential (GWP) | kg CO2eq | 4.11E+01 | |
Ozone Depletion Potential (ODP) | kg CFC-11eq | 4.22E-06 | |
Photochemical Ozone Creation Potential (POCP) | kg C2H4 eq | 6.89E-03 | |
Acidification Potential (AP) | kg SO2 eq | 1.20E-01 | |
Eutrophication (EP) | kg PO4-3eq | 5.22E-02 | |
Resource use | Cumulative Energy Demand, Non-renewable (CEDNR) | MJeq | 1.29E+03 |
Cumulative Energy Demand , Renewable (CEDR) | MJeq | 6.81E+02 |