2.1. Framework for Estimating Carbon Emissions
LCA refers to an assessment product, process, or activity [
22]. It is a process covering the whole life cycle such as—raw material collection, production, transportation, sales, usage, reuse, maintenance, and final disposal [
23,
24]. LCA is widely used for calculation of carbon emission in energy research and the construction industry [
25]. There are two main approaches to LCA—process-based LCA (bottom-up) and LCA (top-down) based on input–output analysis [
26]. The former LCA involves a detailed understanding of construction information, such as material use and production efficiency. For this reason, this LCA can be adopted to analyze specific technologies that affect energy consumption during construction [
27,
28,
29]. In this article, the life cycle assessment method is used to divide a series of states of a building in the materialization stage into different stages. According to GB/T24040-2008 Environmental Management—Life Cycle Assessment—Principles and Framework National Standard Understanding, the research can be divided into four stages [
30]; shown in
Figure 1.
The first stage involves determination of purpose and scope. The system boundary of the construction stage should be determined and relevant data in each link of a production process should be collected. The second stage entails inventory analysis—classification, compilation, and quantification should be conducted on input energy and resources, and output pollution of the targeted products during the whole life cycle. The second stage entails inventory analysis—classification, compilation, and quantification should be conducted on input energy and resources, and output pollution of the targeted products during the whole life cycle. The fourth stage summarizes the results. Data pertaining to the inventory analysis and environmental influence should be summarized and a qualitative carbon price should be determined.
2.2. Modeling CO2 Emission in Each Subsystem
CE refers to emissions of some of the most harmful greenhouse gases, mainly CO
2, including CH
4 and N
2O. However, because each gas has a different potency, or “warming” effect on the atmosphere, a factor (CE factor) is applied to convert it to the carbon dioxide equivalent [
31,
32]. The CE factor refers to the generated amount of greenhouse gas accompanied by the consumption of unit mass material or energy. It is an important parameter representing the CE characteristics of a certain material or energy [
33,
34]. This article aims to compare the carbon emissions of different construction processes in the building materialization stage, so the measurement of carbon emissions ranges from the entry of building materials to the completion of construction. In this paper, according to the CECS 374:2014 Standard for Measuring, Accounting and Reporting of Carbon Emission from Buildings, the carbon emission factors of common energy are listed
Table 1.
According to the construction processes and contents at the materialization stage, systematic analysis was adopted to classify the construction system into six subsystems—decoration engineering, structural engineering, on-site transport, foundation engineering, installation engineering, and construction site facilities. The CO
2 emissions from each subsystem was studied, then synthesized into the CO
2 emissions at the materialization stage (
Figure 2).
2.3. Calculation of Carbon Emission at the Materialisation Stage
At the materialization stage, carbon emission units include transportation of construction materials, components, parts and equipment, operation of construction machines and tools, and the on-site office. At the materialized stage, measurements and calculations could be done according to regulations, as follows [
34].
The fuel consumption for the transportation of materials, components, parts and equipment was calculated according to Formula (1).
where
Dys is the total fuel consumption for the transportation of materials, components, parts, and equipment (t);
Gi represents the total consumption of materials, components, parts, and equipment of type
i (t);
ZGi is the average carrying capacity of the vehicle used to transport type
i materials, components, parts, and equipment (t);
Qsi is the unit fuel consumption of the vehicle used to transport type
i materials, components, parts, and equipment (t/km); and
Li is the distance of transportation of type
i materials, components, parts, and equipment (km).
Power consumption associated with the operation of the construction machines and tools calculated by Formula (2)
where
Djdx is the total power consumption of construction machines and tools (kW∙h);
Pdi is the electric power of type
i construction machines and tools (kW);
Tdi is the operating hours of type
i construction machines and tools (h); and
Ni is the number of type
i construction machines and tools (units).
Fuel consumption of the operation of the construction machines and tools was calculated by Formula (3).
where
Djxy is the total fuel consumption of construction machines and tools (t);
Pyi is the average fuel consumption of each type
i construction machines and tools per shift (t/shift); and
Tyi is the number (shifts) of type
i construction machines and tools under operation.
Water consumption associated with the operation of the construction machines and tools was calculated by Formula (4).
where
Djxs is the total water consumption of construction machines and tools (t);
Psi is the average water consumption of each type
i construction machines and tools per shift (t/shift); and
Tsi is the number (shifts) of type
i construction machines and tools (time).
Power consumption on the work site of the construction was calculated by Formula (5).
where
Dbgd is the total power consumption of on-site office work (kW h);
Pdi is the electric power of type
i electric equipment for on-site office work (kW); and
Tdi is the operating hours of type
i electric equipment for on-site office work (h).
During the building construction stage, the total carbon emission from buildings was calculated by Formula (6).
where
Esg is the carbon emission from buildings during building construction (tCO
2);
Dsgd is the power consumption of the unit processes during building construction (kW);
EFd is the carbon emission factor of electricity [tCO
2/(kW h)];
Dsgy is the fuel consumption of the unit processes during building construction (t);
EFy is the carbon emission factor of fuel (tCO
2/t);
Dsgm is the coal consumption of the unit processes during building construction (t);
EFm is the carbon emission factor of coal (tCO
2/t);
Dsgq is the water consumption of the unit progresses during building construction (t);
EFq is the carbon emission factor of coal (tCO
2/t);
Dsgsh is the water consumption of the unit progresses during building construction (t); and
EFsh is the water carbon emission factor (tCO
2/t).