Life cycle inventory of polyethylene glycol-modified lignin and greenhouse gas emission reduction by replacing existing resins

This study investigated the greenhouse gas (GHG) emissions of polyethylene glycol-modified lignin (PEG-modified lignin) from the raw material procurement stage to the production stage based on the production process of PEG-modified lignin in a commercial-scale plant and analyzed the GHG emission reduction by replacing resins derived from fossil resources. The GHG emissions of the PEG-modified lignin were 0.85 kg-CO₂e/kg. The establishment of a process to recover and recycle used PEG in the product manufacturing process and reduce the input of virgin PEG has a significant impact on reducing GHG emissions. Furthermore, in the production of a masterbatch of PEG-modified lignin mixed with nylon (PA6), GHG emissions were reduced with an increase in PEG-modified lignin content, with reductions of approximately 14 and 36% for the PEG-modified lignin content of 20 and 50%, respectively. This indicates that PEG-modified lignin has the potential to contribute to GHG reduction as an alternative to fossil fuel resources.

Lignin is one of the major components of wood; however, its extremely diverse chemical structure limits its use as an industrial material. Therefore, the waste liquid containing lignin (black liquor) discharged in the process of making wood pulp has been used as fuel alternatively. Therefore, recent research has focused on the derivatization of lignin to develop more effective ways of using lignin [1,2,3,4,5,6,7,8]. In recent years, a system has been developed to produce a substance in which cedar (Cryptomeria japonica), which contains lignin with a relatively homogeneous chemical structure, is mixed with polyethylene glycol (PEG) and heated to modify and extract lignin ("PEG-modified lignin") [9].

Several studies have shown that mixing the developed PEG-modified lignin with other resins improves the performance of the resin [10, 11].

PEG-modified lignin is derived from woody biomass and is expected to reduce greenhouse gas (GHG) emissions by replacing resin and other products derived from fossil resources. However, PEG-modified lignin is a new material that has recently been developed, and the amount of resources and energy used in its production and associated GHG emissions are still unclear. It is crucial to quantify the life cycle GHG emissions of modified lignin and understand the magnitude of the reduction effect.

In this study, a life cycle inventory of PEG-modified lignin from the raw material procurement stage to the production stage was created by setting up production equipment and operating conditions in a commercial-scale plant based on the production equipment and operating conditions implemented in bench and pilot plants for PEG-modified lignin operating in Japan. Based on this inventory, GHG emissions were calculated through life cycle assessment, and the effect of reducing GHG emissions by replacing existing fossil-fuel-derived resins was clarified.

Functional units and system boundaries

A life cycle inventory per unit of PEG-modified lignin and calculation of GHG emissions were performed using the life cycle assessment methodology. One kilogram of PEG-modified lignin was used as a functional unit. A life cycle flow diagram was created, as shown in Fig. 1, and the system boundaries were established.

Inventory of production stage

To develop an inventory of the product manufacturing process in a commercial-scale PEG-modified lignin production plant, an inventory of raw materials and chemical inputs and products, co-products, and waste generation was obtained from the material balance data generated during plant design for a PEG-modified lignin bench plant. The production of PEG-modified lignin per cycle in the bench plant was 42 kg (61.5% W.B). The equipment conditions of the bench plant are shown in Fig. 2. Wood powder and PEG were fed into the reactor (1) and decomposed using a solvent. The liquid separation is carried out by the filter press in (2), at which time the co-product pulp is produced as the solid content. The mixed solution containing PEG-modified lignin was coagulated and precipitated by pH adjustment in step (3), and then solid–liquid separation occurred in the washing and filtering process in step (4) to produce PEG-modified lignin. Due to the high water content at this stage, a drying process is required after this stage. In contrast, the regenerated PEG is recovered from the solution (supernatant) and separated from the solids in (4) through the regeneration treatment process in (5). The recovered regenerated PEG is fed into the process (1) of the plant. We developed an inventory for commercial plants based on the facility conditions and inventories. The annual production capacity of a commercial-scale plant was assumed to be approximately 2500 tons. The bench plant did not include the drying process, so it was added to the new facility. The facility conditions are listed in Table 1.

This section describes the energy input for each process, including electricity, during product manufacturing. The manufacturing equipment installed at the bench plant was a small test machine, and its energy efficiency was not sufficiently high enough for commercial use. Therefore, we assumed a commercial scale-up based on the facilities of a pilot plant in operation in Japan, and calculated the amount of electricity required to operate the facilities (Table 1). The maximum production capacity of the pilot plant was 100 tons per year. For natural gas, the amount of heat required to raise the temperature of the mixture (from 80 to 140 °C) was calculated from data on the raw materials, chemicals, and water input to the process, and the amount of natural gas required to obtain this amount of heat was determined.

Inventory of raw material procurement stage

The raw material procurement conditions were established based on raw material data obtained from the product manufacturing process inventory created in "Inventory of production stage" section. The primary raw material, wood flour, was assumed to be the by-product of the sawmills as its raw material, and the process after the fine powder processing of the by-product was assumed to be within the boundary of the system. The energy consumption of the wood flour fine powder process was obtained from Ref. [3] for a wood flour mill with a power of 30–45 kW and a crushing capacity of 115 kg/h. Since the purpose of developing PEG-modified lignin is local production for local consumption of forest resources, the transportation distance of wood powder was assumed to be 100 km by a 10-ton truck (100% loading rate, return trip included in the evaluation) within the same prefecture. Since all transportation conditions involved domestic procurement, a uniform transportation distance of 500 km by a 4-ton truck was assumed.

Determination of GHG emissions

The life cycle inventories of the assessment targets were multiplied by the GHG emission intensity, and the GHG emissions from the assessment scope were calculated using the stacking method. The emission factor for electricity was the 2023 national average of the emission factors for electricity published by Japan’s Ministry of the Environment [12], and the emission factor for gas was the emission factor for natural gas combustion published by the Ministry of the Environment in Japan [13]. All other data were taken from AIST-IDEA v 3.4, an LCA database developed by the National Institute of Advanced Industrial Science and Technology (AIST) that boasts the world’s largest number of data sets. AIST-IDEA v 3.4 [14] contains several GHG emission intensities calculated under different conditions, of which the "IPCC 2021 GWP 100a with LULUC" value was used in this study. Incidentally, the GHG emission intensity for truck transport was quoted assuming an average loading rate, from AIST-IDEA v3.4.

Allocation

Since pulp is produced as a co-product in the manufacturing process of PEG-modified lignin, an allocation between the PEG-modified lignin and pulp was made. Among various processes during product manufacturing shown in Fig. 1, the processes directly related to pulp production are "reaction," "solid–liquid separation," and "regeneration treatment," which were considered for allocation. The "pH adjustment" and "washing and filtration" processes are not directly related to pulp production, but are necessary to obtain the supernatant that is fed to the "regeneration treatment" process of PEG, so they were included in the allocation. Only the "drying" process was excluded from the allocation.

GHG emissions from these processes were allocated based on the ratio of PEG-modified lignin to pulp by dry weight (32:68).

Creation of master batches by blending with existing resins

A realistic distribution method for PEG-modified lignin involves blending masterbatches with existing resins. Here, as one of the distribution patterns of PEG-modified lignin as a raw material, we assumed blending with nylon (PA6) and calculated the GHG emissions per kilogram of the masterbatch when the PEG-modified lignin contents were 20% and 50%. The 20% content rate is the rate currently considered possible. The 50% content rate is a future target value. The masterbatch processing involved twin-screw blended extrusion with underwater cutting after extrusion. To determine the energy consumption required for these processes, foreground data measured on an actual twin-screw extruder with a 2500 kg/h production capacity were used. The twin-screw extruder consisted of a twin-screw extruder body, belt-type weight feeder, screen changer, and underwater cutting device. The partial change in raw material to PEG-modified lignin was neglected as it had little effect on the equipment conditions or the production capacity. The GHG emission intensity quoted for the calculation of GHG emissions for the masterbatch is the same as that in "Determination of GHG emissions" section.

Life cycle inventory of PEG-modified lignin

The life cycle inventory of PEG-modified lignin from the raw material procurement to the production stage is shown in Table 2. Water and PEG, which did not react with lignin, were recovered and circulated. Table 2 assumes that the material balance after a certain duration has elapsed after plant operation when PEG, water, and other materials are circulated in a stable manner.

Wood flour (8% W.B.; 2.98 kg) and PEG (dry; 3.76 kg) were used as raw materials, resulting in the production of 1.01 kg (5% W.B.) of PEG-modified lignin. In a previous study [9], the input ratio of wood flour to PEG during the reaction was used as 1:5; however, owing to technological innovations in the reaction process and increased efficiency through larger scale plants, the input ratio was modified to 1:1.3 in this study. The amount of PEG recovered without reaction was 3.56 kg dry weight, and the amount of pulp produced as a by-product was 1.56 kg dry weight. The difference between the input, production, and recovery was mixed with the pulp or residue in the wastewater.

GHG emissions from the life cycle of PEG-modified lignin

The GHG emissions were calculated by multiplying the life cycle inventory created in "Life cycle inventory of PEG-modified lignin" section, by the GHG emission intensity of each process and allocated between PEG-modified lignin and pulp. The resulting GHG emissions per kilogram of PEG-modified lignin in the commercial plant were 0.85 kg-CO₂e/kg. The breakdown of this process is shown in Fig. 3.

Raw material procurement and production stages emissions accounted for approximately 54% of the total emissions. In the raw material procurement stage, the procurement of wood powder and virgin PEG (including transportation) each accounted for 18% of total emissions. The amount of virgin PEG input was approximately 5% of the total PEG input, and most of the PEG used was recycled PEG recovered from the plant. If all the PEG used were virgin PEG, the GHG emissions per kg of PEG-modified lignin would be 3.45 kg-CO₂e/kg, or about four times higher, suggesting that the in-plant regeneration process has a significant impact on reducing the GHG emissions of PEG-modified lignin. During the production stage, the largest GHG emissions were from electricity consumption at the plant. This accounted for 25% of total emissions.

GHG emissions from masterbatches blended with existing resins

From the GHG emission intensity of PEG-modified lignin calculated in "GHG emissions from the life cycle of PEG-modified lignin" section and the inventory in Table 3, the GHG emissions per kilogram of masterbatch blended with PEG-modified lignin and nylon (PA6) were determined, as shown in Fig. 4. The GHG emission intensity of PEG-modified lignin was lower than that of nylon (PA6), and the GHG emissions decreased as the content of PEG-modified lignin increased. Compared to the nylon (PA6)-only masterbatch, the masterbatch containing 20% PEG-modified lignin was found to reduce GHG emissions by approximately 14% and the masterbatch containing 50% PEG-modified lignin by approximately 36%. Emissions from electricity consumption in the twin-screw extruder in the masterbatch production process accounted for only a small percentage of the total emissions (less than 5%).

Thus, it is clear that the PEG-modified lignin content significantly impacts the reduction of GHG emissions from masterbatches.

Comparison of GHG emissions with PEG-modified lignin produced in a bench-scale plant

The GHG emissions of PEG-modified lignin calculated in "Life cycle inventory of PEG-modified lignin" section were compared to the GHG emissions of PEG-modified lignin produced in a bench-scale plant to examine the change in GHG emissions due to the larger scale. Conditions of the bench plant are presented in Table 4, whereas inventory data for each kg of PEG-modified lignin are listed in Table 5. As in "Determination of GHG emissions" section, GHG emissions were first multiplied by the life cycle inventory to be evaluated multiplied by the GHG emissions intensity, and then the results were summed using the stacking method. The results were compared to the GHG emissions of PEG-modified lignin produced in a commercial-scale plant (Fig. 5). The drying process is excluded in Fig. 5 because the bench plant does not have a drying process.

The GHG emissions per kg of the PEG-modified lignin produced by the bench plant were 5.64 kg-CO₂e/kg. Our findings suggest that GHG emissions could be reduced by 86% by increasing plant scale and efficiency.

When considered by process, the reduction effect was the greatest for reclaimed PEG. This is because bench plants use natural gas for hydrolysis and dehydration processes. In contrast, commercial-scale plants no longer use natural gas, as it has been replaced by activated carbon-based adsorption and desorption processes.

In addition, the amounts of PEG, H₂SO₄, NaOH, and other raw materials used in the reaction were reduced to approximately one-fifth of the original input, contributing to GHG emission reduction. This is the effect of the reactor in a commercial-scale plant, which was not observed in a bench-scale plant. The new reactor in the commercial-scale plant made it possible to reduce the amount of PEG required for the reaction with wood flour to the values listed in Table 2. The amounts of chemicals and water fed into the reaction process were reduced, resulting in a reduction in the energy input required to increase the temperature.

Biogenic carbon stored by PEG-modified lignin

The biogenic carbon stored in the PEG-modified lignin was calculated from the perspective of the contribution of woody biomass materials to climate change mitigation. As the lignin carbon content is estimated to be approximately 60–65% [15, 16], a value of 60% was assumed for the calculations in this study. The resulting biogenic carbon per kg of PEG-modified lignin was 1.6 kg-CO₂, well above the amount of GHGs emitted in the system boundaries. In the present study, the system boundary is defined as the process from the raw material procurement stage to the production stage. However, if the system boundary is defined as Cradle to Grave, all biogenic carbon in the PEG-modified lignin is released into the atmosphere during the disposal stage after product use, resulting in a biogenic carbon balance of zero over the life cycle. In addition, to evaluate biogenic carbon for carbon removal from the atmosphere, forest sustainability from which woody biomass is sourced must be guaranteed. Both forest sustainability and long-term product use made with PEG-modified lignin are necessary for achieving carbon neutrality.

Data uncertainty and study limitations

In the present study, facility conditions and inventory data from a bench-scale plant, along with facility conditions from a pilot plant, were used to simulate facility conditions and inventory data for a commercial-scale plant. Based on these results, the GHG emissions of the PEG-modified lignin produced in a commercial-scale plant were calculated. Since no commercial-scale plant existed in operation, all the data were theoretical and subject to uncertainties. For the equipment conditions, power consumption was calculated based on the rated output of each facility. However, since the power consumption is likely to fluctuate depending on the changes in load during operation, there may be a slight overestimation. In addition, seasonal variations in energy input were expected for processes that involve raising or maintaining high temperatures; however, they were not considered in the present study. Additional equipment and raw materials may also be required depending on the PEG-modified lignin properties.

"GHG emissions from masterbatches blended with existing resins" section clarified the GHG emissions per kg of masterbatch containing PEG-modified lignin. The set conditions are reasonable because products have already been developed in which PA6 is replaced partially by PEG-modified lignin. However, not all products using PA6, which are prevalent in society, can use PEG-modified lignin. For some product applications, it is necessary to verify whether the physical and chemical properties of the masterbatch are compromised by the inclusion of PEG-modified lignin.

In this study, the scope of assessment ranged from the raw material procurement stage to the production stage of PEG-modified lignin, and the GHG emissions per kilogram of product were calculated using life cycle assessment. In addition, the GHG emissions per kilogram of product for a master batch in which PEG-modified lignin was substituted for a portion of the existing nylon (PA6) were calculated to determine the reduction in GHG emissions compared with the existing resin. The following conclusions were drawn:

1. 1.

   The GHG emissions from PEG-modified lignin were 0.85 kg-CO₂e/kg. Establishing a process to recover and recycle the used PEG in the product manufacturing process and reduce the input of virgin PEG significantly impacts the reduction of GHG emissions.

2. 2.

   The GHG emissions of masterbatches containing 20% and 50% PEG-modified lignin were 4.77 kg-CO₂e/kg and 3.59 kg-CO₂e/kg, respectively. These reduced GHG emissions by approximately 14% and 36%, respectively, compared to those without PEG-modified lignin. The process of blending and extruding nylon (PA6) and PEG-modified lignin had no significant impact on the overall GHG emissions.

As noted in this study, the theoretical GHG emission calculations are subject to several uncertainties. Nevertheless, it is critical to understand its potential contribution to carbon neutrality when the material is adopted in society. Our findings show that it is possible to estimate areas where PEG-modified lignin could provide significant reductions if substituted, as well as areas where it would not have a substantial impact.

In addition, the study discloses as many detailed facility conditions and inventory data as possible to allow future verification of the validity of the evaluation.

The theoretical GHG emission quantification in parallel with the development of new materials could facilitate the production of PEG-modified lignin with even lower GHG emissions.

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

GHG:

Greenhouse gas

PEG:

Polyethylene glycol

We would like to thank Editage (www.editage.jp) for English language editing.

This research was supported by JPJ008722, "Development of Production and Utilization Technologies for Next-Generation Materials Derived from Wood Lignin," a project research commissioned by the Ministry of Agriculture, Forestry and Fisheries of Japan.

Authors and Affiliations

1. Graduate School of Bioresources, Mie University, Tsu, Mie, 5148507, Japan

   Yuki Fuchigami

2. Forestry and Forest Products Research Institute, Tsukuba, Ibaraki, 3058687, Japan

   Yasunori Ohashi, Eri Takata & Tatsuhiko Yamada

Contributions

Yuki Fuchigami: conceptualization, investigation, writing—original draft, investigation, analysis. Yasunori Ohashi: investigation, resources. Eri Takata: investigation, resources. Tatsuhiko Yamada: conceptualization, investigation, resources, supervision. All the authors read and approved the final manuscript.

Corresponding author

Correspondence to Yuki Fuchigami.

Competing interests

The authors declare that they have no competing interests.

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