2.1. System Dynamic Structure and Functional Unit
The first step in calculating CO
2 flux is to determine the system boundary of wood-based carbon flow, which uses two major system boundaries [
26]. One is the cradle-to-grave system boundary, which consists of five phases, namely, raw material extraction, product production, packaging and transportation, product use, and waste disposal [
27]. As illustrated by the flow diagram in
Figure 1, the cradle-to-grave analysis applied in the life cycle of harvested wood product (HWP) assesses various environmental impacts over the entire product life cycle, from raw material extraction (the cradle), via production, transportation and use, to waste management (the grave) [
28]. Wood-based carbon stocks are divided into many sub-fluxes passing through various pools and processes before being released into the atmosphere [
29]. The other is the gate-to-gate system boundary, which consists of two phases, namely, product manufacture and use [
20,
30].
For our analysis, the gate-to-gate system boundary was used (
Figure 2). Forest trimmings and industrial waste wood from sawmills, such as shavings, sawdust, ply trim, veneer, and chips, are sources of raw wood materials. Plywood mostly uses large-diameter timbers as raw materials. Fiberboard and particleboard mostly use small-diameter timbers and wood residues (i.e., harvesting, building, and processing residues) from sawmills and other wood industries as major raw materials. Moreover, bark from logs and sawdust may be used to produce wood-based panels [
31].
Plywood production involves raw material preparation; peeling, drying, and veneer finishing; gluing and hot processing; sawing and sanding; and packaging. Carbon emission primarily occurs during gluing [
32]. Fiberboard production includes raw material preparation, pulping, forming, hot processing, and after-treatment. Pulping and forming consume much energy during production [
33]. Particleboard production comprises raw material preparation, particle preparation, particle drying, particle sizing, slab paving, hot pressing, and after-treatment. The energy is primarily consumed through particle preparation, drying, slab paving, and hot pressing, which account for 87.46% of the total energy consumption [
34]. The three panels are produced by coordinating various procedures, which also produce pollutants and GHGs.
Using the atmospheric carbon flow as the evaluation objective, IPCC [
35] stipulates that direct and indirect CO
2 emissions are positive GHG contributors and that carbon stock and substitution emission reduction are negative GHG contributors. The equation for CO
2 flux is given by:
where
,
and
represent the values of CO
2 flux, CO
2 emission, and CO
2 stock, respectively. If the value is above zero, then the product is a net emitter. Otherwise, the product is a net sink. CO
2 emission reduction is stipulated as a negative value [
36]. The capability of carbon sinking depends on the absolute value. A high absolute value indicates increased contribution to CO
2 removal from the atmosphere [
25].
The functional unit describes the quantitative measure of the functions that a product or service provides [
37]. In this study, the investigated products were wood-based panels. The 2013 Revised Supplementary Methods and Good Practice Guidance Arising from the Kyoto Protocol (The 2013 IPCC Guidelines) [
11] stipulates that wood-based panels are reported in cubic meters (m
3) solid volume, a functional unit used in previous wood-based panel studies [
10,
38,
39]. Therefore, the volumetric unit (m
3) was adopted as the reference in the current work to compare the gate-to-gate life cycle CO
2 flux among different panels.
2.2. CO2 Emission and Energy Consumption Standard
This work calculated the CO
2 emissions from the production process in accordance with energy consumption standards. The overall energy consumption standards of wood-based panel production are China’s forestry industry standards issued by State Forestry Administration of the People’s Republic of China. These standards are recommended standards and define the relevant terms of on-site manufacture of wood-based panels and the indexes of energy consumption for producing 1 m
3 of panels. q
1 represents the specific value of the indexes and is given in kgce/m
3. Through the investigation of typical wood-based panel enterprises in China, the energy consumption of different types of enterprises was obtained. Surveyed enterprises accounted for more than 20% of the entire industry, and more than 50% of the total output was investigated. The energy consumptions per unit production of enterprises with an annual production of 50,000 m
3 of plywood and 0.2 million m
3 of particleboard were taken as the basic values [
40,
41,
42], on which the different levels of indexes were based. The index values of different grades can represent the actual energy consumption of Chinese wood-based panel enterprises. The amounts of energy consumption in the entire production system were summed up and converted into standard coal [
32,
33,
34]. Energy consumptions are divided into three levels in accordance with the standards, and the specific meaning of each level is as follows.
The third and qualified grades represent the limit values of energy consumption per unit product and are mandatory indicators. The purpose of these grades is to eliminate the backward production capacity of 20%–30%. The second and good grades represent the threshold values of energy consumption per unit product and are mandatory indicators. They are admittance values for production of per unit value of energy consumption for new construction, reconstruction, and expansion of enterprises. The first and excellent grades represent the advanced values of energy consumption per unit product and are recommended indicators. The numerical values indicated by the indexes are in line with the international advanced levels or the leading domestic levels for production of wood-based panels and are the goals of enterprises for on-site manufacture. Energy consumption standards are the principles that enterprises should follow in producing wood-based panels. The enterprises should calculate the energy consumption of the actual production process and match them to the grade in the standards. The comprehensive energy consumption per unit output of an enterprise in actual production refers to the ratio of total energy consumption to qualified output in the same statistical period.
Table 1 and
Table 2 show the different indexes according to the energy consumption standards between 1990 and 2015. The different levels of indexes were used to judge the actual production level of wood-based panel industry and service as a reference for the government to supervise the consumption of energy for production. The quantitative management of resource consumption by the state may urge enterprises to examine the energy consumption in each production process in accordance with the energy consumption levels and to reduce waste of resources. The backward production capacity that fails to meet the third and qualified grades will be eliminated [
43]. The implementation of the total energy consumption standards of the wood-based panel industry in China occurs in two stages. The first stage includes LY/T 1529–1999 Total Energy Consumption in Plywood Production (LY/T 1529–1999), LY/T 1451–1999 Comprehensive Energy Consumption for Hard Fiberboard Production on the Wet Process (LY/T 1451–1999), and LY/T 1530–1999 Total Energy Consumption in Particleboard Production (LY/T 1530–1999), which were released in 1989 and implemented in 1990 [
32,
33,
34].
In the first stage, the four major producers of Chinese wood-based panels are Linyi, Shandong Province; Pizhou, Jiangsu Province; Jiashan, Zhejian Province; and Wen’an, Hebei Province. Among these producers, three are located in the northern provinces (The Technical Requirements of Cleaner Production for the Wood-based Panel Industry in China defines the southern and northern provinces as the area with heating facilities and the area without heating facilities, respectively.) (i.e., Linyi, Pizhou, and Wen’an), and the total output and market share of wood-based panels of Shandong, Jiangsu, and Hebei provinces account for more than 80% of the national share. This work selected the northern provinces as regional indicators.
In terms of first-level indexes, the average energy consumptions of plywood, fiberboard, and particleboard were 510, 750, and 375 kgce/m
3, respectively (
Table 1)
The second stage (2008–2015) includes three current national standards: LY/T 1529–2012 Comprehensive Energy Consumption of Plywood Production (LY/T 1529–2012), LY/T 1451–2008 Overall Energy Consumption for Fiberboard Production (LY/T 1451–2008), and LY/T 1530–2011 Comprehensive Energy Consumption of Particleboard Production (LY/T 1530–2011) [
40,
41,
42]. Replacing the previous energy consumption standards with the current ones includes the implementation of Excellent, Good, and Qualified index levels. The north and south areas are also unified in the same index. The reduction in energy consumptions for wood-based panel production is attributed to the application of cleaner production technologies. China’s enterprises have been urged to adopt cleaner production technologies through the improvement of technologies and utilization of clean materials [
44]. Thus, the calculations based on the “first grade” (
Table 1) and “excellent grade” (
Table 2) met the requirements for cleaner production in China’s wood-based panel industry. According to the statistics, the energy consumption of China’s advanced enterprises is 163–182 kgce/m
3, and that of most small- and medium-scale enterprises is 250 kgce/m
3 [
45]. In addition, in the research of 20 representative enterprises, the energy consumption per unit of particleboard is 113.03 kgce/m
3 [
46]. The values of actual practice conform to the first-level indexes. The results of related foreign literature showed that the energy consumptions for plywood production in the United States and Canada are 158.46 and 80.15 kgce/m
3 [
47] and that the domestic practical conditions of China are close to advanced levels. Therefore, the index selection conforms to the current situation of the industry and the actual situation.
In the second stage, the average energy consumptions for on-site manufacturing of plywood, fiberboard, and particleboard in terms of excellent-grade indexes were 200, 320, and 120 kgce/m3, respectively.