*3.4. Roles in Economic and Environmental Sustainability*

In energy-intensive plants that run on fossil fuels, it is essential to reduce greenhouse gas emissions. This can be achieved by introducing technologies for capture, utilisation, and storage of CO2 for such plants. In the first article on this topic, entitled "Techno-Economic and Environmental Evaluations of Decarbonized Fossil-Intensive Industrial Processes by Reactive Absorption and Adsorption CO2 Capture Systems", the authors Cormos et al. [134] analysed two technologies. The first was a well-known reactive gas-liquid absorption, where CO2 is absorbed in a solvent such as Methyl-DiEthanol-Amine (MDEA) [135]. Another technology was a newer reactive gas–solid adsorption system, in which CO2 is adsorbed onto solid calcium oxide, followed by thermal decomposition of the resulting calcium carbonate [136]. The authors conducted an analysis of techno-economic and environmental indicators for the integration of decarbonisation technologies into various processes, such as coal-based power generation, and steel and cement production. They compared several indicators, e.g., specific capital investment; production costs of electricity, steel, or cement; plant emissions; power consumption; power output; and avoided CO2 cost. The results showed that by integrating so-called decarbonisation technologies (more correctly decarbonisation emissions technologies), CO2 emissions were reduced by up to 90% compared to conventional plants without decarbonisation. However, the investment was higher, the efficiency lower, and the cost of produced electricity, steel, or cement higher. According to economic indicators, the use of pre-combustion capture technology is more favourable than post-combustion capture. Life Cycle Analysis showed that decarbonisation reduces the value of the Global Warming Potential, while other environmental indicators can increase, e.g., acidification, eutrophication, and toxicity. In the industrial processes studied in this paper, the technology of capturing CO2 by adsorption on solid calcium sorbent proved to be technologically and economically more favourable than the gas–liquid carbon capture system.

In the next article, Castor et al. [137] conducted "A Comparative Techno-Economic Analysis of Different Desalination Technologies in Off-Grid Islands". The authors compared the technical and economic properties of four desalination technologies to produce drinking water from seawater. These are multi-effect distillation, multi-stage flash, mechanical vapour compression, and reverse osmosis. These processes need a reliable energy source, and it is appropriate to integrate them with the energy production system, particularly on islands that are not connected to the power grid. The use of diesel generators is widespread on islands, but the use of renewable sources for water desalination, such as a combination of solar photovoltaic and reverse osmosis, is encouraged [138]. The authors used a system that combines a renewable source (photovoltaic) and a fossil source (diesel) while using a battery-based energy storage system. Using computer programs, the authors of this SI paper optimised the integrated energy–water system for a period of one year, taking into account hourly fluctuations in water and energy consumption. The cost of power, the cost of water, and the net present value of the costs of the various desalination technologies were calculated. The uncertainty in energy and water consumption was taken into account with a stochastic Monte Carlo simulation. The results show that diesel-based power production would dominate on smaller islands because of lower capital cost. At low fuel prices, distillation and flash desalination technologies would be suitable. If the fuel price is high, and a renewable source is preferred, the use of reverse osmosis could be preferred. This is especially true for large islands, where energy and water requirements are higher and high investment costs are offset by lower fuel consumption in reverse osmosis. Forecasts for 30 y ahead indicate that the cost of water produced by reverse osmosis would be the lowest of all of the technologies studied, while energy costs are expected to remain comparable for all technologies.

Aluminium production is an important energy consumer and emitter of CO2 emissions, although the energy efficiency of this process has improved significantly [139]. Gomilšek et al. [140], in their article entitled "Carbon Emissions Constrained Energy Planning for Aluminum Products", conducted planning and optimisation for various sources of electricity used in the production of aluminium products, such as slugs and evaporator panels. Fossil fuel, renewable, and nuclear energy sources were considered in their study. Electricity mixes should be derived from different sources that do not exceed the CO2 emission values required by the legal framework. The first technique for CO2 Constrained Energy Planning (CCEP) is the insight-based graphical targeting approach referred to as CEPA (Carbon Emission Pinch Analysis) and developed in 2007 by Tan and Foo [141]. CEPA uses a graphical approach based on the principles of traditional PA [142]. The numerical targeting approach with the cascade analysis methodology was originally developed for resource conservation networks [143] and had its roots in the Problem Table Algorithm, and the Heat Cascade developed for Maximum Heat Recovery networks [144]. The numerical targeting approach was further extended to determine the amount of low-carbon-emissions energy required to achieve the specified emission limits. In the case of the production of specific aluminium products, the authors found that about 20% of the energy should be replaced by sources with zero or low CO2 emissions to achieve the prescribed target emission. Optimal source mixes for power generation that would ensure emission target values at minimum cost were identified using an optimisation approach. In the studies of different scenarios, fossil fuel sources and nuclear energy were selected. The renewable sources were not beneficial due to the still higher price of renewable energy. The cost of power generated from an optimal mix of resources would be 26% higher than for the current power mix. However, the prescribed CO2 emission targets would be met. Finally, the authors summarised the advantages and disadvantages of the approaches used and recommended the development of combined methods that exploit the advantages of each approach.

Yang et al. [145], the authors of the article entitled "A Method for Analysing Energy-Related Carbon Emissions and the Structural Changes: A Case Study of China from 2005 to 2015", used Sankey diagrams to show the structure of energy consumption and carbon emissions resulting from the consumption of fossil fuels in China. Sankey diagrams were used to analyse China's energy consumption in 2005 [146]. However, calculating the components of the TRO indicator as proposed by the authors allowed trends to be identified, such as changes in total carbon emissions for individual sectors (T), relative growth in carbon emissions (R), and changes in the ratio of carbon emissions from a particular sector to total emissions (O). The visualisation of energy consumption and the resulting carbon emissions shows that coal is the predominant energy source and accounts for the largest share of emissions. Calculations of the individual components of the TRO index show that the share of coal in China s energy structure has decreased and natural gas has become an important energy source. The use of renewable resources, particularly wind energy, is developing. This paper concluded that the boom in the industry, the construction of infrastructure, and the rise in living standards are slowing the decline in coal consumption and the decarbonisation of the country. The methodology developed by the authors analyses the responsibility of the individual sectors for carbon emissions by visualisation and evaluates changes and trends using the TRO indicator. Thus, decision-makers can design more effective measures and solutions to reduce emissions.

Acrylic acid is an important chemical intermediate, used in particular by the polymer and textile industries to produce various end products. Premlall and Lokhat [147] report on simulation and design of acrylic acid production with a focus on its reactor system in an article entitled "Reducing Energy Requirements in the Production of Acrylic Acid: Simulation and Design of a Multitubular Reactor Train". The main objectives were more detailed design and optimisation of the reactor train and reduction of energy consumption. Two reaction steps were considered: the oxidation of propylene to acrolein and the oxidation of acrolein to acrylic acid. A plug-flow reactor model was used in Aspen Plus to simulate multitubular reactors with appropriate reaction kinetics for propylene oxidation [148] and for acrolein oxidation [149]. Side reactions produce CO2 and acetic acid, and it is important to select a catalyst with high selectivity. Bismuth molybdate and vanadium molybdate proved to be the most efficient catalysts for this application [150]. Authors of this SI paper determined the operating and design parameters of the reactor system, such as the number of tubes, the length of the reactor, the flow and temperature of the heat transfer fluid, the pressure drop, and the heat transfer area. To reduce energy consumption, the authors introduced an inert pre-heating zone into the first reactor. The heating medium used is molten salt, which absorbs the heat of the exothermic reaction and cools down again when the feed stream is preheated. Cold air is introduced into the outlet stream of the first

reactor, which lowers the inlet temperature in the second reactor and increases the heat absorption capacity in this reactor. The authors concluded that it is possible to reduce energy consumption by about 7 MW with the measures and optimisations they proposed.

A new idea based on the exergy concept was developed in the article "Thermodynamics-Based Process Sustainability Evaluation" by Varbanov et al. [151], who proposed the concepts of Exergy Profit and Footprint. All industrial and other human activities involve non-spontaneous processes and exergy is necessary to drive them. The authors developed the framework based on the concepts of exergy assets and liabilities for driving the processes. The formulated Exergy Profit criterion was demonstrated as an appropriate quantitative indicator of the sustainability contribution of the assessed processes. The concept of exergy is not as widespread as deserved because it is more abstract and often more difficult to understand, even for engineers, although it can be efficiently combined with energy and economic methods to comprehensively evaluate process systems [152]. The difficulty in its application can be seen from [153], who used exergy in their model but with a focus on the techno-economic optimisation of embedded multiple criteria, including environmental. Exergy assets are associated with the ability of process streams to extract useful work and drive the processes, including energy generation. Exergy liabilities represent the demands and deficits of exergy that have to be supplied to the processes. Exergy Profit and Footprint are calculated by balancing the assets and liabilities. The surplus of the assets over the liabilities results in the Exergy Profit whereas, in the case of deficit, an Exergy Footprint results. The sustainability contribution is higher for a higher value of the Exergy Profit. The concepts were illustrated with two examples from different problem domains. These were acetic acid waste recovery and reuse, in addition to the evaluation of municipal solid waste treatment options. This clearly demonstrated the applicability of the method to a wide range of systems.
