1. Background and Purpose
The Paris Agreement, a framework for climate change control, came into effect in November 2016, and measures to significantly reduce greenhouse gas (GHG) emissions are required in all aspects of society. CO2
is a major GHG. In Japan, CO2
emissions from the construction sector account for approximately one-third of the total emissions [1
], and it is recognized as an area where countermeasures are highly important. Therefore, comprehensive evaluation of the environmental efficiency of buildings and significant reductions of GHG emissions are urgently needed.
LEED (Leadership in Energy and Environmental Design), BREEAM (BRE Environmental Assessment. Method), and GB tool (Green Building Tool) are well-known systems for evaluating the environmental performance of buildings. Comparative studies of these evaluation systems are underway [2
]. In this, for example, an evaluation factor included in LEED is the use of building materials that quantify GHG emissions based on the life cycle assessment (LCA) method. CASBEE, the Japanese version of the environmental efficiency evaluation system for buildings, has been displaying life cycle CO2
emissions in building evaluation results since the 2008 version, indicating the environmental efficiency of the building quantitatively [4
]. LCA is an effective method for quantitative analysis of GHG emissions, and the importance of LCA for buildings is growing.
Many previous studies have quantified GHG emissions by performing LCA of buildings and have been systematically summarized in several reviews [5
]. In addition, comparative studies have been conducted on the effects of reducing GHG emissions by replacing interior walls and windows [7
] and exterior walls [8
] with environment friendly products. Further, it has been pointed out that GHG emissions resulting from the building usage processes account for a large proportion of overall GHG emissions from buildings [5
]. The introduction of high-performance energy-saving equipment to buildings will minimize emissions from the use and maintenance processes [11
]. In addition, the use of renewable energy to power building use and maintenance processes can significantly reduce GHG emissions. There have been some advanced studies on the introduction of renewable energy having good price parity, suggesting the potential for future wind and solar deployments [12
]. If emissions reductions from the use and maintenance steps are realized, the rate of emissions from the other processes, that is, the manufacturing and procurement steps for building materials and the disposal and recycling processes for building materials, becomes relatively large, and the importance of countermeasures increases. An LCA review of building materials points out that there is little data on building materials in the LCA database, so it is necessary to accumulate basic knowledge [15
Wood-plastic recycled composite (WPRC) is a building material that uses certain amounts of recycled wood and plastic materials contained in wood-plastic composite (WPC) [16
]. These materials are mainly used as exterior deck materials. They are characterized by multiple recycling processes in which products that become post-consumer materials are technically able to be recycled to produce WPRC products [18
]. This high environmental consideration differentiates these materials from others.
JIS A 5741 in the Japanese Industrial Standard (JIS) [19
] stipulates the types and blending rates of recycled materials used as raw materials for WPRC, basic physical properties required in WPRC, and their testing methods. Additionally, WPRC manufactured at JIS-certified factories are also marketed. Moreover, WPRC products that have obtained JIS certification are designated under the “Basic Policy on Promoting Green Procurement” as specified procurement items under the “Green Purchasing Law”. In this manner, the marketing of environmentally friendly products is becoming a popular trend.
Previous studies on LCA of WPC include that of Bolin and Smith [20
], who described a case where WPC using HDPE (High Density Polyethylene) as a plastic raw material was used as a deck material and compared it with a solid wood deck treated with alkaline copper quaternary (ACQ). For this LCA, WPC was modeled as if manufactured from 50% recycled wood flour, 25% post-consumer recycled HDPE, and 25% virgin HDPE. In addition, a sensitivity analysis was performed where all HDPE used was virgin HDPE. Sensitivity analysis showed that WPC made with post-consumer recycled HDPE was less impactful than those made with virgin HDPE. The results showed that WPC made with consumer HDPE was less impactful than those made with virgin HDPE. In addition, Philipp et al. [21
] performed LCAs on WPCs in their own research and found that in WPC made of virgin materials, the more the amount of wood used, the lower the potential environmental impacts. In WPC made with a high amount of secondary wood, processing of secondary wood particles contributes to the overall environmental impacts because secondary plastic granulates are directly useable in the context of an established market for high-quality secondary plastic granulates. These studies are important findings that suggest that the use of recycled materials in plastic-based materials will reduce the environmental impact. However, foreground data are rarely collected at the production step for important processes.
GHG emissions during the WPRC life cycle were quantified in our 2009 study based on production-level foreground data provided by eight WPRC manufacturing companies [22
]. In addition, the reduction in GHG emissions in comparison to WPCs was also revealed. However, the number of companies that acquired the JIS A 5741 certification increased to the current four in 2011, and GHG emissions from WPRC based on JIS standards have not been quantified. In addition, there is no research case quantifying the effect of reducing GHG emissions through multiple recycling.
Therefore, in this study, we quantified GHG emissions during the life cycle of WPRC manufactured by JIS A 5741-certified companies. In addition, we conducted a simulation showing changes in GHG emissions when the mixing rate of recycled materials was changed and verified the effects of the mixing rate of recycled materials on GHG emissions. Furthermore, to clarify the GHG emission reduction effect of multiple recycling, multiple scenarios were simulated and GHG emissions in each scenario were compared.
2. Evaluation Target and Methods
2.1. Evaluation of GHG Emissions from the Life Cycle per Ton of WPRC Products
GHG emissions from the WPRC life cycle were calculated using the LCA method. The evaluation targets were WPRC products manufactured by four companies that acquired the JIS A 5741 certification, and the functional unit was one ton of these products. This is because the shape of the product varied depending on the manufacturing companies, so setting the basic unit as weight was most appropriate. The system boundary was from the cradle to the grave. Figure 1
shows the life cycle flow diagram. The shapes of wood-based and plastic-based raw materials upon arrival at the factory during the raw material procurement step differed depending on each company. Therefore, in Figure 1
, each company is denoted as Pattern A (1 company), Pattern B (2 companies), and Pattern C (1 company).
This section explains data collection during each process. Foreground data were collected as much as possible through interviews with the four companies being evaluated. For those processes where foreground data could not be collected, conditions were set based on the scenario and background data that seemed reasonable. The data were collected for one year from April 2016 to March 2017.
2.2. Evaluation of GHG Emissions When the Recycling Rate of Plastic-Based Raw Materials Is Changed
JIS A 5741 defines the recycling rate of raw materials as 40% or more, and WPRC available in the market has various recycling rates. However, in most cases, recycled materials such as building demolition materials, wood shavings generated at sawmill, and timber from forest thinning are used as wood-based raw materials, and virgin materials are not included. Therefore, in this study, all wood-based raw materials were considered recycled materials, and the GHG emissions were estimated by changing the recycling rate of plastic-based raw materials. All other raw materials such as pigments and additives were virgin materials.
2.3. GHG Emission Reduction Effect by Multiple Recycling of Used WPRC
WPRC has the advantage of “multiple recycling” in that we can collect used products such as deck materials and repeatedly use them as raw materials for WPRC. In evaluating the GHG emission reduction effect resulting from this multiple recycling, we referred to the evaluation scope proposed by Wada et al. [23
]. Then, the evaluation range was set as shown in Figure 2
, and the GHG emissions from WPRC were calculated. The life cycle flow of product A in the multiple recycling case presented in Figure 2
was assumed to occur as shown in Figure 1
; the life cycle flow of product B was set as shown in Figure 3
, and data were collected. The data collection method for each process was the same as that explained in Section 2.1
In the multiple recycling cases, scenario 1 was first established in which all the used WPRC materials generated in the “disposal and recycling step” in the life cycle of product A were recycled as raw materials, except for those that were landfilled. Next, it was assumed that some of the used WPRC had degraded quality and some were difficult to recover owing to problems in the construction environment. Therefore, scenario 2 was established in which 50% of the used WPRC were recovered and recycled. In addition, for comparison, scenario 3 was set considering the case without multiple recycling, representing the current situation.
By comparing the GHG emissions between these scenarios, we examined the effects of multiple recycling on GHG emission reduction.
This study targeted WPRC products manufactured by companies that have acquired the JIS A 5741 certification. It quantified the GHG emissions from the life cycle of these products, evaluated the effect of the mixing rate of recycled materials on GHG emissions, and analyzed the GHG emission reduction effect resulting from multiple recycling. The following conclusions could be drawn from the findings of this study:
The GHG emission during the life cycle of the targeted WPRC materials was 3489 kg-CO2e/t, and the ratio of the emission from the product manufacturing process in the product manufacturing step and the combustion of waste process in the disposal and ready for recycling step was found to be particularly high.
For the raw materials used in the preparation of WPRC, GHG emissions were calculated assuming that the recycling rate of wood-based materials was 100%, but the recycling rate of plastic-based materials fluctuated between 0% and 100%. It was found that setting the recycled material rate of plastic materials to 100% would reduce GHG emissions by 28% (1316 kg-CO2e/t) compared to when the recycled material rate was 0%.
We found that GHG emissions can be reduced by up to approximately 28% through multiple recycling of WPRC. In the multiple recycling cases, GHG emissions from the disposal of used WPRC could be significantly reduced, while emissions from the transportation process of the used WPRC were relatively large.
Through this study, we were able to quantify the environmental performance of WPRC whose quality was standardized by the JIS from the viewpoint of GHG emissions. The results of this study are based on data provided by all Japanese companies that have acquired JIS certification for WPRC. This is a benchmark for GHG emissions from WPRC products made in Japan and is an important achievement for determining the direction of GHG emission reduction measures for the entire industry.
Finally, in the future, if a WPRC manufacturing company considers expanding product sales based on research results, it will be necessary to obtain a type II environmental label, such as EPD (Environmental Product Declaration). Furthermore, to implement multiple recycling of WPRC in society, it is necessary to construct a system for collecting and recycling raw WPRC. This is a future issue to be solved socially.