Chemicals and inventory data
The chlor-alkali industry produces chlorine gas, sodium hydroxide, and hydrogen gas simultaneously by the electrolysis of brine using 3 different technologies: Hg, diaphragm, and membrane cells (EIPPCB 2013). This study deals only with membrane technology as it is the most commonly used technology in Australia. From these membrane outputs, many other chemicals are produced including sodium hypochlorite and ferric chloride that are important for the water industry. The other relevant manufacturing process is the production of aluminum sulfate, which leaves no residuals and uses little electricity directly; however, the upstream environmental impact for this chemical is expected to be important due to the use of sulfuric acid and aluminum hydroxide that are produced in energy and resource-intensive processes (EIPPCB 2005). Additional details of the manufacturing processes of the chemicals included in this study are given in the Supplemental Data A.
Extensive data-gathering activities were undertaken to create the process models for the production of chlorine gas, ferric chloride, sodium hypochlorite and aluminum sulfate. First, we sent questionnaires to the different Australian chemical producers. Second, site visits were undertaken to support data collection activities and for data verification purposes. Finally, we performed informal interviews with chemical producers to discuss topics specific to each production process; this yielded detailed information regarding raw materials, energy and transport. A sample questionnaire is included in the Supplemental Data B.
Data for background systems was obtained from a life cycle database (PE International 2012). This included data for the regional Australian power grid mix and ancillary materials used in the chlor-alkali plant (e.g., carbon dioxide, sodium metabisulfite). Salt used in the chlor-alkali plant was considered to be rock salt (PE International 2012) due to the lack of data on the production of solar salt; therefore, we expect that the impacts associated with this input will be higher than those associated with the actual production of solar salt. Sulfuric acid, used as a feedstock in the production of aluminum sulfate and for chlorine gas drying, was considered as a production mix (80% from sulfur combustion and 20% as a coproduct in nonferrous metals production) and sourced from South Korea. Emissions to soil, water, and air associated with the chlor-alkali plant and aluminum sulfate production were taken from the Australia's National Pollution Inventory (NPI) (DSEWPC 2011). The NPI is a publicly available internet database that keeps annually updated information of the amounts of 93 toxic substances released to the environment by more than 4000 Australian industrial facilities. The reference year for the data gathered was 2008 maintaining consistency with other related work based on input-output analysis (IOA) (Alvarez-Gaitan et al. 2013).
Solving allocation by system expansion (substitution)
Applying system expansion to solve the multifunctionality problem associated with the LCI modeling of chlorine gas requires the identification of the products and processes that are potentially suitable substitutes for chlor-alkali-produced hydrogen gas and sodium hydroxide in the Australian market. In the case of hydrogen gas, the principal commercial route for its production is steam reforming of natural gas, so initially this seemed an appropriate form of system expansion for this output. Further investigations, however, revealed that the rate of H production in the chlor-alkali industry is small and in practice does not represent a suitable opportunity for further downstream processing outside the facility. In some chlor-alkali plants, H is used for the production of hydrochloric acid and as a fuel to produce steam that is used in sodium hydroxide evaporation (Orica Watercare, personal communication, August 19 2011). Therefore, the most suitable use of hydrogen gas in this study is as a fuel for combustion and the ratio of substitution is 1 to 2.84 H to natural gas that is calculated from their higher heating values (Demirel 2012).
Sodium hydroxide is considered a coproduct of chlorine production that is sold mainly to the pulp and paper, alumina, and general chemical industries (Orica Watercare 2013). Although sodium hydroxide is crucial in the production of alumina (Santen 1998), in pulp and paper applications it may be substituted for sodium carbonate (Na2CO3) (FMC Corporation 2004). Outside these 2 main markets, sodium hydroxide might be also substituted with sodium carbonate in pH adjustment, acid neutralization, and flue gas desulfurization. Therefore, the function of the output of sodium hydroxide in this study is considered as an alkali suitable for acid neutralization, and the ratio of substitution is 1.325:1 sodium carbonate/sodium hydroxide based on their molecular weights and Equation (1) (FMC Corporation 2000):
An overview of the approach taken to model chlorine gas production using system expansion is presented in Figure 1. Once chlorine gas was modeled, the environmental burden of sodium hydroxide could be calculated by solving ENaOH from Equation (2):
Where E represents the environmental burden and the subscripts are for the whole electrolysis process, chlorine gas, sodium hydroxide, and hydrogen gas, respectively. The small contribution of hydrogen gas is estimated by using natural gas as a surrogate. After obtaining the figures for chlorine gas and sodium hydroxide using system expansion, it is possible to model the production of liquefied chlorine gas, sodium hypochlorite, and ferric chloride in GaBi 5 software (PE International 2012). It is important to clarify that the environmental burden of chlorine gas calculated earlier is “wet” chlorine that requires further drying and compression before being sent to liquefaction or sodium hypochlorite/ferric chloride production.
Ferric chloride is produced using chlorine gas, water, steel scrap, and spent pickle liquor (SPL), where steel scrap represents 93% of the Fe source. Chlorine gas was modeled as previously explained and the environmental burden of postconsumer Fe scrap and SPL require additional considerations for open loop recycling. The environmental burden of postconsumer steel scrap was taken from the most recent Worldsteel methodology report (Worldsteel Association 2011). In this document, the cutoff and end-of-life approaches are presented. The first approach considers postconsumer scrap “free” of any environmental burden from the upstream life cycles. This approach considers steel scrap as waste. In contrast, the second approach considers postconsumer steel scrap as a resource that substitutes primary steel production but with a lower environmental burden. We have applied the second approach to our modeling that is the approach supported by the metal industry (Atherton 2007).
SPL can be regenerated into hydrochloric acid and ferric oxide but this is not the case for the steelworks where SPL is being sourced from in this study. As such, if it is not used in the production of ferric chloride, it needs to be first neutralized with calcium oxide where the metals precipitate as sludge and then further treated through filtration, dewatering, and rendering with Portland cement to make it suitable for landfill disposal. Therefore, we have given ferric chloride a credit for the estimated avoided use of these materials according with the requirements for stabilization of pickle liquor presented in the literature (Stanczyk et al. 1982).
Solving allocation by physicochemical causation
In the chlor-alkali plant, electricity, salt, ancillary materials, and emissions to soil, water, and air are related to each of the outputs of the membrane cells through the electrochemical reaction of Equation (3):
Chlorine gas, hydrogen gas, and sodium hydroxide are produced in fixed proportions according to this reaction. Mass or molar relationships can be used to describe how these outputs are allocated to the inputs in the membrane cells. Using the most commonly applied mass allocation approach (Boustead 2005; Althaus et al. 2007), the key data are if 1000 kg of chlorine gas is produced, then 1126 kg of sodium hydroxide (100% w/w) and 28 kg of hydrogen gas are necessary coproducts. In reality, these mass relationships must be calibrated with the mass balance of the chlor-alkali facility to obtain the real allocation factors. For example, based on the above stoichiometry and using the molecular weights of the species involved, we will require 1.649 tonnes of sodium chloride per tonne of chlorine gas produced. In reality, however, this value is higher (EIPPCB 2013) due to the purge of brine from the chlor-alkali membrane plant circuit to control impurities. The allocation factors used here are presented in the Results and Discussion.
If molar relationships are used, the basic data is that 1 mol of chlorine gas is produced along with 2 mols of sodium hydroxide and 1 mol of hydrogen gas. When this approach is applied (Leimkuhler 2010), only 25% of the energy and material usage is ascribed to the oxidation of chloride ion to chlorine gas in the anode, whereas the other 75% is associated with the reduction of hydrogen ions to hydrogen gas and the release of hydroxide ions into the solution in the cathode, giving this latter compartment a bigger share of the environmental burden.
Another approach recommended in The International Reference Life Cycle Data System (ILCD) Handbook (ILCD 2010) is the use of an enthalpy basis for allocating the amount of energy used in the production of the outputs in the membrane cell. This approach makes sense in principle, but the Handbook is not entirely clear on which enthalpy is recommended. The first possible option is using enthalpy of formation, but this thermodynamic property is zero for elements such as chlorine and hydrogen gas. Using this approach to the outputs of the membrane cell will result in allocating the entire burden of electricity usage to sodium hydroxide that would seem a rather dubious outcome. Another possibility is applying enthalpy of combustion to the outputs, but in this case the entire burden will be ascribed to hydrogen gas because neither chlorine gas nor sodium hydroxide is flammable. These problems seem to be solved by using other thermodynamic property, exergy.
Exergy represents quantitatively the potential work embodied in fuels and nonfuel materials alike (Ayres et al. 1998). Calculating this property for the outputs of the membrane process we obtained 1743 kJ per tonne of chlorine, 2156 kJ for sodium hydroxide, and 3162 kJ for hydrogen. This form of allocation illustrates the potential for improvement in those chlor-alkali plants where hydrogen is not being used and also displays the relevance of exergy as a valuable indicator to measure efficiency in the chemical industry (Dincer 2002; Ayres et al. 2011). Allocation factors obtained for the application of exergy are presented in the Results and Discussion.
Solving allocation by economic relationships
In the case of allocation using economic relationships, a 3-year average annual price (Guinée et al. 2004) during the period 2008–2010 was used to calculate the associated allocation factors (presented in the Results and Discussion). For sodium hydroxide and chlorine gas leaving the electrolyzer, we used the same 3-year average market price as for sodium hydroxide at commercial concentration and liquefied chlorine respectively. For hydrogen gas, the natural gas market price in the Australian state of New South Wales (NSW) for the respective year was used as a proxy (ACCC 2009). In this case the gross energetic value of the total amount of hydrogen produced was calculated and then the price per giga joule (GJ) applied.
An additional consideration when economic allocation is used is the uncertainty in results stemming from variability in commodity prices (Pennington et al. 2010). To illustrate this point, the prices of sodium hydroxide and chlorine gas as taken from industry sources (Sydney Water, personal communication, August 17 2011) are presented as relative indices in Figure 2. The robustness of the results are tested in this article using deterministic and probabilistic approaches. The deterministic approach consists of calculating the economic allocation factors accordingly for the 2006–2010 period for which we had data. The probabilistic approach was undertaken using a 500 iteration Monte Carlo in GaBi analyst that is an in-built feature of GaBi 5 software (PE International 2012) assuming that prices fit a normal distribution. For interested readers, details of this tool are given in the GaBi database and modeling principles 2011 document (PE International 2011). This tool also requires the user to input the standard deviation (expressed in percentage) of the parameters under analysis. Using the price data gathered from our industry partners for the period 2006–2010, these values are 18.3% for sodium hydroxide, 14.9% for chlorine gas, and 2.23% for hydrogen gas. The output of this simulation is a probability distribution of the GWP results with details of the arithmetic median, standard deviation, and percentiles.
Figure 2. Australian relative retail price indices for chlorine gas and sodium hydroxide. Relative price index (y-axis; arbitrary units) shown for each chemical during the 2006-2010 period as a fraction of the baseline year (2006).
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