The life cycle greenhouse gas emission paths and emission factor database of Chinese tunnels
Tunnel is the traffic facility with high-density greenhouse gas (GHG) emission. With the booming development of Chinese tunnelling industry, a large number of energies, building materials and human sources are consumed. Various GHG and wastes are discharged into the natural environment. However, only few GHG emission researches about Chinese tunnels existed. No life cycle GHG emission paths or emission database applicable to Chinese tunnels are available. The accuracy and representativeness of the current studies can hardly be ensured which directly affects the general applicability of the research results. GHG emission activities go through the life cycle of tunnel. From the perspective of constituent element, the upstream products, construction activities and maintenance activities of the tunnel are divided into various unit processes. A series of raw materials, intermediate products and unit processes are included in the tunnel materialization and operation processes, involving abundant data of energy consumptions and GHG emissions. This paper studies the GHG emission boundary and paths in the life cycle of the tunnel system in China. The GHG emission path networks of the upstream, construction, operation and maintenance are established based on unit processes. The GHG emission data for the main upstream products and construction activities of China tunnels are collected according to the distribution characteristics of energy and resource. Two calculation examples about tunnel construction and transportation are given to help explain the GHG emissions of unit processes. Taking the regional differences of energy structure and production methods into consideration, when selecting the basic emission factor data, the first reference is the mature domestic emission database, followed by the research results in the domestic literature, and finally the foreign database and research results. Researchers follow the IPCC guidelines, national and international emission databases to quantify the GHG emission activities in China. The paper proposes a reference database of tunnel GHG emission factors in line with the actual situation of China which lays the cornerstone for the sustainable development of Chinese tunnel industry.