1.1 Environmental impacts through life cycle assessment (LCA)
Life cycle assessment (LCA) is the compilation and evaluation of inputs, outputs and potential environmental impacts during the life cycle of a product system. In this work, life cycle assessment method is used to analyze the life cycle environmental impacts of three main industrial gas products, i.e., oxygen, nitrogen and argon. According to the international standard ISO14040, the life cycle assessment method includes four stages: goal and scope determination, inventory analysis, impact assessment and interpretation. Based on the life cycle assessment method, this work establishes a research method model. The specific research method and process are shown in Fig. 1.
1.2 Definition of goal and scope
The goal of this study is to investigate, quantify and compare the effect on the environment of producing three industrial gas products, oxygen, nitrogen and argon from a 300,000 Nm3/hr external compression air-separation plant with Linde Double Column unit. The plant consists of a low pressure column (LPC) and a high pressure column (HPC) as shown in Fig. 2. The HPC has a condenser in the top and the LPC has a reboiler in the bottom. The condenser provides the heat needed by the reboiler, and the reboiler provides the cooling needed by the condenser. A two-stage intercooling compressor is used to compress air to around 6.2 bar for the case to produce 95 mol% oxygen purity. Water and other impurities are then removed by condenser and filter. The air stream is split into two stream, and further cooled in heat exchange 1 (HE1). A large fraction of air is fed into the bottom of HPC at the temperature just above the condensation temperature. Since nitrogen has the highest vapor pressure at same temperature compared with oxygen and argon, the pure nitrogen rises to the top of the HPC, and the oxygen rich liquid is obtained at the bottom of HPC with the purity of 35–38%. They are both further cooled in heat exchange 2 (HE2). Then the pure nitrogen is fed to the top of LPC as reflux to cool the raising gases. The preliminary purified liquid oxygen is fed to the middle of LPC after its pressure throttled from 5.5 to 1.35 bar. Another part of air is also further cooled in the HE1, and then expanded to the pressure closed to the situation of feed stage.
Figure 3 shows the boundaries for the system, which is defined as ‘cradle-to-gate’ boundary [11], [12]. For the production of oxygen, nitrogen and argon by air separation process, the system boundary includes air intake filters, air compressors, air-cooled towers, water-cooled towers, molecular sieve adsorbers, expanders, air separation towers (including upper towers, lower towers, argon towers, condensation evaporators, etc.), compressors, liquid storage, gas piping, filling and distribution, and other processes for the production of rare gases. The process boundary also includes energy and utility processes within the plant as well as transportation processes. In summary, it includes the entire production process from the acquisition, extraction, treatment and transportation of raw and auxiliary materials and energy, the production of air separation gases, until the gas products are metered and delivered to to the gas cylinders or tanks at the air-separation plant’s gate.
The boundary does not consider inventory data on distribution, use, disposal/end-of-life, or waste treatment. In addition, the midpoint LCIA technique is used in this study. The declared unit of this work is 1,000 Nm3 industrial gas product from the air-separation plant. This study normalized all the inputs to the declared unit.
1.3 Life cycle inventory analysis (LCIA)
This life cycle inventory analysis is carried out on the production of oxygen, nitrogen and argon in the external compression air-separation plant as shown in Fig. 2. A cradle-to-gate inventory involves all the processes/flows, raw materials and essential requirements needed to produce oxygen, nitrogen and argon from the plant available in the cylinders or tanks and ready for distribution. LCA can be performed from either an attributional or a consequential perspective. An attributional LCI attempts to describe the environmentally relevant physical flows from and to a life cycle product system [13] and can be used to assess a product’s environmental impact over time. Alternatively, consequential LCA describes how potential past or future decisions might have affected environmentally relevant physical flows [14], [15]. This study used the attributional method resulting in the environmental implication of 1,000 Nm3 of gas product (oxygen equivalent) produced from the air-separation plant. The inventory input/output data for the air-separation plant are included in Table 1.
Table 1
List of inputs/outputs of a 300,000 m3/hr external compression air-separation plant (Linde)
Inputs | Amount | Outputs | Amount |
Electricity | 1,648,443 MWh | Waste oil drum | 1.25 tons |
Steam | 115,293 tons | Used paint bucket | 3.4 tons |
Water | 2,719,600 m3 | Waste lead-acid battery | 3.35 tons |
Molecular sieve | 101.0 kg | Used oil | 42.8 tons |
Aluminum oxide | 69.7 kg | Oily rags | 4.8 tons |
Pearlite | 30.5 m3 | | |
Lubricant | 32.6 tons | | |
The principal environmental impacts, either by amount or by potential consequences from industrial gas production can be grouped into three categories, i.e., emissions, resource use and output flows/wastes [16]. The emission category include the emissions of green house gases (GHGs), nitrogen oxides (NOx) and sulfur oxides (SOx), volatile organic compounds (VOC), hydrochlorofluorocarbons (HCFC), chlorofluorocarbons (CFC), hydrofluorocarbons (HFC), etc., The resource use category includes the use of energy (electricity, and steam), the use of water for cooling, and the use of lubricant and auxiliary materials etc.. The output flows/wastes category include the discharges of waste materials from maintenance. Accordingly, this study focuses on 11 impact indicators as shown in Table 2 from the three impact categories. These indicators are the Global Warming Potential (GWP100), Acidifcation Potential (AP), Eutrophication Potential (EP), Ozone Depletion Potential (ODP), Photochemical Oxidant Creation Potential (POCP), Abiotic Depletion Potential (ADP), Use of non-renewable and renewable primary resources (NRPRE and RPRE), Use of net freshwater resources (FW) and Hazardous waste disposed (HWD).
SimaPro (version 9.5) software application was used as the LCIA tool in this study. Intergovernmental Panel on Climate Change (IPCC) 2021 method was adopted for characterizing green house gases indicator GWP100 as recommended in the Global Guidance for Life Cycle Impact Assessment Methods (GLAM) [17]; CML (Institute of Environmental Sciences, University of Leiden) method was used for AP, EP, ODP, POCP and ADP indicators [18]. Cumulative Exergy Demand (CExD) method was used for RPRE and NRPRE indicators [19]. WAVE Water Scarcity was used for quantifying the relative fresh water usage indicator [20]. All the 11 categories and the parameters & indicators are all taken from the ISO 14025 [21] and EN15804 (A1 + A2) standards [22], [23], except that China’s General Solid Waste Classification standards (GB/T 39198 − 2020) and National Hazardous Waste List was used to identify and quantify the HWD indicator [24], [25].
Table 2
Environmental impact category indicators using characterization factors based on the IPCC 2021, CML 2016 and CED methods.
Impact category | Indicators | Method | Unit |
Emissions | Global warming potential (GWP): indicator of potential global warming due to emissions of greenhouse gases to the air. Divided into 3 subcategories based on the emission source: (1) fossil resources, (2) bio-based resources, and (3) land use change. | IPCC 2021 | kg CO2 eq |
Emissions | Acidification Potential (AP): indicator of the potential acidification of soils and water due to the release of gases such as nitrogen oxides and sulfur oxides | CML 2016 | kg SO2 eq |
Emissions | Eutrophication Potential (EP): indicator of the enrichment of the terrestrial, marine and freshwater ecosystem with nutritional elements, due to the emission of nitrogen or phosphor-containing compounds | CML 2016 | kg PO43 − eq |
Emissions | Ozone Depletion Potential (ODP): Indicator of emissions to air that causes the destruction of the stratospheric ozone layer | CML 2016 | kg CFC−11 eq |
Emissions | Photochemical Oxidant Creation Potential (POCP): indicator of emissions of gases that affect the creation of photochemical ozone in the lower atmosphere (smog) catalyzed by sunlight | CML 2016 | kg C2H4 eq |
Resource use | Abiotic depletion potential (ADP-elements) for non-fossil resources: indicator of the depletion of natural non-fossil resources | CML 2016 | kg Sb eq |
Resource use | Abiotic depletion potential (ADP-fossil fuels) for fossil resources: indicator of the depletion of natural fossil fuel resources | CML 2016 | MJ |
Resource use | Renewable primary resources used as energy carrier (RPRE): Use of renewable primary energy, excluding renewable primary energy resources used as raw materials | CExD | MJ |
Resource use | Non-renewable primary resources used as an energy carrier (NRPRE): Use of non-renewable primary energy, excluding renewable primary energy resources used as raw materials | CExD | MJ |
Resource use | Use of net freshwater resources (FW): indicator of the relative amount of water used, based on regionalized water scarcity factors | WAVE Water Scarcity | m3 |
Output flows and wastes | Hazardous waste disposed (HWD): Hazardous waste has a certain degree of toxicity that necessitates special treatment | GB/T 39198 − 2020/National Hazardous Waste List, PR China | kg |