High CO2 Emissions Research Articles

Research of the Impact of Hydrogen Metallurgy Technology on the Reduction of the Chinese Steel Industry’s Carbon Dioxide Emissions

The steel industry, which relies heavily on primary energy, is one of the industries with the highest CO2 emissions in China. It is urgent for the industry to identify ways to embark on the path to “green steel”. Hydrogen metallurgy technology uses hydrogen as a reducing agent, and its use is an important way to reduce CO2 emissions from long-term steelmaking and ensure the green and sustainable development of the steel industry. Previous research has demonstrated the feasibility and emission reduction effects of hydrogen metallurgy technology; however, further research is needed to dynamically analyze the overall impact of the large-scale development of hydrogen metallurgy technology on future CO2 emissions from the steel industry. This article selects the integrated MARKAL-EFOM system (TIMES) model as its analysis model, constructs a China steel industry hydrogen metallurgy model (TIMES-CSHM), and analyzes the resulting impact of hydrogen metallurgy technology on CO2 emissions. The results indicate that in the business-as-usual scenario (BAU scenario), applying hydrogen metallurgy technology in the period from 2020 to 2050 is expected to reduce emissions by 203 million tons, and make an average 39.85% contribution to reducing the steel industry’s CO2 emissions. In the carbon emission reduction scenario, applying hydrogen metallurgy technology in the period from 2020 to 2050 is expected to reduce emissions by 353 million tons, contributing an average of 41.32% to steel industry CO2 reduction. This study provides an assessment of how hydrogen metallurgy can reduce CO2 emissions in the steel industry, and also provides a reference for the development of hydrogen metallurgy technology.

How can there be an economic transition to a green ecosystem by adapting plastic-to-fuel technologies through renewable energy?

Plastics partially help define the way humanity lives today and plastic waste is an increasing phenomenon. It seems to be a serious concern that causes economic inefficiencies and environmental degradation that result in high CO2 emissions rates. To overcome this issue, regulations and policies on the production and behavior of plastics and energy should be reconsidered in the growing world economy. To this end, this study aims to foster economic pathways for decision-makers and answers the question of “How can there be an economic transition to a green eco-system by adapting plastic-to-fuel technologies through renewable energy?”. This strategy is introduced by an autoregressive distributed lag (ARDL) time-series methodology with the worldwide data from 1950 to 2022, considering energy production, economic growth, and CO2 emissions as independent variables; and plastic production as a dependent variable. The findings reveal the existence of a long-run cointegration relationship. An increase in renewable energy production (REP) and CO2 emissions causes a decrease in plastic production, while the opposite is valid for non-renewable energy production (NREP). The relationships between environmental degradation, energy production and economic growth are conducted through a global perspective. Therefore, this pioneering aspect of the article is expected to become a reference study for many studies in the future, in mitigating global warming by adapting plastic-to-fuel technologies. The paper shows how the production and use of renewable energy have critical importance in terms of the economic development levels of countries and therefore, can benefit countries (i.e., particularly developing and least developed countries) for environmentally friendly production and consumption patterns through a very important good: plastic.

Syngas optimization and tar reduction via multistage catalytic gasification: Effects of catalyst pore, catalyzer stage, and temperature profile

Biomass stands out as a promising availability source of sustainable carbon energy due to its low pollution profile and versatile applications. This study investigates the multistage catalytic gasification (MCG) of coconut shells, employing thermal and catalytic cracking processes in a two-step gasification approach. Local source catalysts, including dolomite, kaolin, and zeolite, were utilized as eco-friendly additives at temperatures ranging from 200 to 300 °C. Zeolite emerged as the most effective catalyst, producing the highest energy content at 6.21 MJ/kg. Conversely, kaolin resulted in higher CO2 emissions. A blend of zeolite and kaolin achieved the highest energy content at 5.43 MJ/kg. However, the combined use of catalysts showed negligible improvements compared to individual catalysts. The study demonstrates that multistage catalytic gasification enhances syngas production and reduces tar content during the gasification of coconut shells. Optimal pyrolysis temperatures were identified within the 500–600 °C range. In conclusion, the introduction of multiple stages in the catalytic cracking process ensures a stable breakdown of hydrocarbon chains, promoting the exposure of gasification products to lighter compounds. Multistage catalytic gasification enhances oxygen supply for oxidation, thereby improving syngas production while mitigating tar and ash content.

Intersection between tectonic faults and magmatic systems promotes swarms with large-magnitude earthquakes around the Tengchong volcanic field, southeastern Tibetan Plateau

Abstract Volcanic regions commonly host swarms comprising small to moderate-sized earthquakes while tectonic faults host mostly mainshock-aftershock sequences that can include very large earthquakes. In the southeastern Tibetan Plateau, large tectonic faults formed by the collision between the Indian and Eurasian plates intersect with the intraplate Tengchong volcanic field, and the seismic behavior of such an environment remains unclear. Here, we built a deep-learning-based high-precision earthquake catalog for the Tengchong volcanic field and found that (1) ~59% of the seismicity occurred as swarms but on faults aligned with the regional tectonic stress field; (2) all swarms contained fluid-diffusion-like migration fronts, with some occurring where high CO2 emissions have been detected; and (3) a year-long swarm, including two ML 5.2 earthquakes within two months, revealed complex fluid-fault interaction. Combined with the historical occurrences of M >6 earthquake swarms around the Tengchong volcanic field, our observations suggest potential increased likelihood of swarms with large-magnitude earthquakes where large tectonic faults and magmatic systems intersect.

Increasing risks of extreme salt intrusion events across European estuaries in a warming climate

Over the last decade, many estuaries worldwide have faced increased salt intrusion as a result of human activities and a changing climate. Despite its socio-economic importance, our current projections on the statistics of future salt intrusion are limited to case studies in certain regions. Here, we show that, compared to present-day conditions, river discharge in the summer months is projected to be reduced by 10–60% in 17 out of 22 investigated major European river basins at the end of the 21st century under the high CO2 emission scenario (Shared Socioeconomic Pathways, SSP 3-7.0). We find that the reduced future river discharge in the summer months, in turn, increases salt intrusion lengths by 10–30% in 9 representative European estuaries at low and mid latitudes. Our analysis further indicates that the European estuaries are projected to experience more than five times more frequent extreme salt intrusion events.

The role of mineral resources, sustainable finance, and innovation in promoting sustainable development under the IRA 2022

Under the Inflation Reduction Act (IRA) of 2022, promoting sustainable development has gained significant attention, particularly concerning mineral resources, sustainable finance, and innovation. This study investigates the interplay between these factors and their role in advancing sustainable development objectives. Through the utilization of asymmetrical relationship analysis, specifically the Nonlinear Autoregressive Distributed Lag (NARDL) approach, the study examines the potential impact of natural resources (NTR), sustainable finance (SUSFIN), and innovation (INNO) on environmental sustainability (ENV). The findings reveal intriguing dynamics. Positive shocks in natural resources (NTR) tend to contribute to increased CO2 emissions, while negative shocks lead to a reduction in CO2 emissions. Conversely, negative shocks in sustainable finance (SUSFIN) and innovation (INNO) are associated with higher CO2 emissions, whereas positive shocks are linked to emission reduction. These results underscore the importance of a balanced integration of natural resources, sustainable finance, and innovation to achieve a synergistic effect in promoting environmental sustainability. Based on these findings, a strategic amalgamation of these elements is recommended to maximize their positive impact on sustainable development. By effectively leveraging mineral resources, sustainable finance, and innovation within the framework of the Inflation Reduction Act, policymakers and stakeholders can contribute to a more sustainable future, aligning with the goals and objectives of the IRA.

The influence of the hydrometeorological factors on the CO2 fluxes from the oligotrophic bog surface.

The influence of the hydrometeorological factors on the CO2 fluxes from the oligotrophic bog surface.

Analysis of the Effect of Renewable Energy Consumption and Industrial Production on CO2 Emissions in Turkic Republics by Panel Data Analysis Method

This study aims to analyze the impact of energy consumption and industrial production on CO2 emissions in the Turkic Republics using the Panel Data method for the period 2000-2020. However, Turkmenistan was excluded from the analysis because the relevant data could not be accessed. Data from Kazakhstan, Azerbaijan, Kyrgyzstan, and Uzbekistan were analyzed. CO2 emission creates problems in terms of environment and sustainability and is a parameter that all countries and international organizations carefully monitor and try to reduce. Using panel data regression, this study examined whether renewable energy and industrial production and countries impact CO2 emissions. The findings revealed that as renewable energy consumption increases, CO2 emissions decrease. However, the effect of industrial production on CO2 emissions was not statistically significant. These results demonstrate that the industrial development of selected countries does not pose a CO2 emission problem. A comparison of countries showed that the high CO2 emission value for Kazakhstan was also reflected in the panel data regression findings, and among the four countries, the country effect was positive only for Kazakhstan. This study is noteworthy in revealing the effect of renewable energy consumption on CO2 emissions. To reduce CO2 emissions and gain a better understanding of the impact of renewable energy consumption on CO2 emissions, future studies should include both developing countries and OECD countries, and compare the results obtained from them.

A Review of the Energy Policies of the BRICS Countries: The Possibility of Adopting a Just Energy Transition for South Africa

This review focuses on the Just Energy Transition (JET) policies of the BRICS member countries with the aim of finding lessons and possibly adopting some of the key energy policy aspects utilised in other countries as a benchmark for the South African context. We consider the present stage of JET in South Africa while being cognisant of the energy source mix supporting the electricity sector and the lifespan and condition of its power plants. An analysis of the energy mix revealed that all nations are dependent on electricity for energy, which is produced predominantly from fossil-fuelled power plants with high GHG emissions (concentrating on high CO2 emissions). It was concluded that some of the lessons learnt from the BRICS countries might be incorporated into a South African energy plan for the transition, with options and guidance for the formulation of policies. This study will attract a responsible, environmentally conscious audience that has the commitment and drive to combat global warming and curb climate change.

Novel application of sustainable coal-derived char in cement soil stabilization

Cement soil stabilization is a commonly used method to improve in-situ soil properties catering to different geotechnical applications. However, cement manufacturing is typically associated with high CO2 emissions and energy consumption. Coal char is a sustainable and environmentally friendly material derived from the coal pyrolysis process. Traditionally used for combustion and gasification, recent research has revealed its potential to improve the engineering performance of cement-based construction and building materials. This study explores the innovative use of coal char in cement soil stabilization. Examining various cement contents (5%, 10%, and 20%) and char contents (10%, 20%, and 30%), the properties of char-cement stabilized soils, including mineralogy, density, water content, thermal conductivity, unconfined compressive strength (UCS), and mechanical properties under triaxial compression, are comprehensively investigated and compared with cement stabilized soil. It is found that char promotes both cement hydration and reaction between soil minerals and cement. The thermal conductivity and UCS of char-cement stabilized soil are 0–9% lower and 8–16% higher, respectively than that of cement stabilized soil. Under triaxial compression, the addition of char in stabilized soil leads to 23.7% increase in shear strength, 17.7% increase in cohesion, and 16.7% increase in the angle of internal friction. In conclusion, the introduction of coal char into traditional cement soil stabilization demonstrates a novel approach to achieving sustainability and enhancing engineering performance in relevant geotechnical applications.

Dispersed Cu (Ni, Co) in MN3 moiety on graphene as active site via electrolytic water towards electro-epoxidation of ethylene

Ethylene oxide (EO) is an important petrochemical product, widely used in printing and dyeing, automotive, medicine, and other fields. Direct epoxidation of ethylene by electrocatalysis is a simple and sustainable route to produce EO, which can avoid the disadvantage of traditional industrial production, such as high temperature and CO2 emissions. An efficient electrocatalyst could generate active oxygen intermediate during the electrolytic water process, which escape from the tremendous activation barrier of breaking molecular oxygen. In addition, ethylene and active oxygen are both required to be adsorbed or produce interaction on the same active site, which is more effective to directly transfer oxygen. Motivated by this strategy, we used first-principle to screen the possibility of 27 single transition metal atoms in MN3 moiety as active site starting from the Gibbs free energy change (ΔG) of *O + C2H4 → *+ C2H4O. Dispersed Cu, Ni, Co can absorb both active oxygen and ethylene, and exhibits the excellent catalytic performance through stability, selectivity, thermodynamics and kinetics evaluation. Electronic structure analysis indicates that the catalytic activity roots from the high electron transfer from the absorbed ethylene to the empty M–O anti-bonding, which weaken the M–O bonding and promote the oxygen transfer. Notably, for CuN3@graphene, the applied potential to produce active oxygen intermediate is 1.53 V vs RHE, and the kinetic energy barrier for subsequent oxygen transfer to form EO is 1.00 eV. This study verified the feasibility of single atomic electrocatalysts for EO production and offers valuable clues to design/discover active center to explore experimentally.

Solid Flow in an Experimental Oxygen Blast Furnace Model: Effects of Recycled Gas and Raceway

The oxygen blast furnace (OBF) process with top gas recycling is recognized as a promising ironmaking process, due to its high productivity and low CO2 emissions. The solid flow plays a crucial role in this process. Therefore, the solid flow in OBF was investigated using a cold experimental OBF model in this paper. The results indicate that the plug flow is the primary solid flow pattern in the upper and middle zones of the OBF. A slight convergence flow and a smaller deadman zone were observed at the bosh. The influence of recycled gas on solid flow was found to be quite limited. Additionally, the raceway size affects the burden structure and deadman zone, but the production rate had little impact. Both the raceway size and production rate have different effects on the repose angle of the deadman zone, and it varies by 14–18°. The findings of this study may have important implications for understanding the structure of solid flow in OBF. The results can be used to optimize the process parameters and equipment design to improve the efficiency of iron production while reducing environmental impact.

Sustainable utilisation of rice husk for cleaner energy: A circular economy between agricultural, energy and construction sectors

Global research focuses on cleaner and sustainable energy alternatives like biomass due to the limited availability of coal and high CO2 emissions associated with coal combustion. Rice husk-based electricity generation is hindered by the absence of cogeneration facilities in rice mills. Rice (Oryza sativa) cultivation is wide-spread and constructing several biomass plants at various locations is uneconomical. Hence, instead of using as biomass, rice husk is field burnt as a method of disposal. Construction of only few rice cogenerations facilitates and using already existing sugar mills and coal based thermal plants cogeneration facilities in a combined way is cost-effective and sustainable solution. Therefore, the present study examines the synergistic utilisation of sugar mills, coal-based power plants and only few newly proposed biomass power plants for effective combustion of rice husk. Maharashtra, one of the largest states in India is considered as the study area. Already existing 181 sugar mills, 18 thermal power plants are considered in 36 districts of Maharashtra state. Site suitability analysis was performed using a multi-criteria decision-making technique and ArcGIS. Furthermore, sensitivity analysis was conducted to assess the robustness of the suitability index values. From the analysis, it is found that nine districts require new biomass power plants. In remaining 27 districts use of already existing cogeneration facilities at sugar mills and coal-based power plants are found to be economical than new plants. In addition, 98.7 % of carbon emission was reduced when rice husk is used instead of coal for generating the equivalent quantity of power.

Low CO2 emission cement from alkali-activated gravel wash mud and quartzite dust

High CO2 emissions and rising energy costs result in an increasing demand for innovative, environmentally friendly and energy-efficient construction materials. Gravel wash mud (GWM) is a widely available industrial by-product from gravel mining which may be used as an aluminosilicate source in the production of cementitious materials. In this study, GWM from Eifel in the SW Germany was used for the synthesis of alkali-activated cement. A high NaOH leachable Si and Al content and fine grain-size makes this material particularly applicable for synthesis of alkali-activated cements and geopolymers. Silicate dust from quartzite mining in Saarland is other industrial by-product used to improve the physical properties of the cement. Both materials were thermally and mechanically processed prior to chemical activation. A single-compound activator of NaOH solution was used, leading to no addition of soluble silicates. The main advantages of the new cement are zero CO₂ emissions from raw materials during calcination, comparatively low energy demand due to short grinding time, low temperature of calcination and low demand of alkali activator/no need for soluble silicates and valorisation of GWM and quartzite dust from aggregate mining. Experiments show that calcined kaolinite clay acts as a very effective binder in the cement formulation with quartzite dust. Consequently, a very small quantity of metakaolinite in the cement mixture is necessary, which allows for a reduced quantity of alkali-activator and related production costs of the cement. Consequently, the alkali-activated cement can provide an environmentally-friendly and cost-effective material for special applications with a high heat resistance or resistance to aggressive environments.

Analysis of traffic conflicts at big intersection types in urban areas

In the past decades, planners developed intersections for the caroriented city. Above all, they should handle motor vehicle traffic efficiently. Urban areas currently have a high proportion of private cars and, as a result, congestion and high CO2 emissions. At the same time, the importance of and demand for sustainable mobility is increasing. Pedestrians and cyclists are particularly vulnerable as the weakest road users. The goal of transport policy is to achieve “Vision Zero,” i.e., no serious injuries or fatalities in road traffic (Federal Ministry of Digital Affairs and Transport, 2022). Many accidents occur primarily at intersections and junctions. Road traffic authorities in Germany generally use accident data from the accident atlas as the basis for evaluating the road safety of intersections. Planners can improve traffic safety by avoiding critical situations. To this end, an intensive study of the development of conflicts as a precursor to accidents can provide further insights. The research in this project shows that such data on traffic conflicts are not available to road authorities. The project will investigate which and how many conflicts between pedestrian, bicycle, and motor vehicle traffic frequently occur at individual intersection types, such as signalized intersections or traffic circles. The initial goal of the project is to develop a survey concept. In the future, planners should be able to use this to quickly identify risks to pedestrian and bicycle traffic at the types of intersections studied. With the help of vehicle sensor data (e.g. from emergency brake assistants), further findings on the frequency and course of conflicts are to be derived. In addition, a methodology would be developed that will enable traffic planners and municipalities to carry out risk management for intersections in the future in order to improve traffic safety and promote local mobility. From the results of the conflict analysis, we will derive recommendations for transport planning and policy. An action guide will then present the recommendations. The end of the project will be in summer 2024.

Comprehensive review: Effects of climate change and greenhouse gases emission relevance to environmental stress on horticultural crops and management

Global climate change exerts a significant impact on sustainable horticultural crop production and quality. Rising Global temperatures have compelled the agricultural community to adjust planting and harvesting schedules, often necessitating earlier crop cultivation. Notably, climate change introduces a suite of ominous factors, such as greenhouse gas emissions (CGHs), including elevated temperature, increased carbon dioxide (CO2) concentrations, nitrous oxide (N2O) and methane (CH4) ozone depletion (O3), and deforestation, all of which intensify environmental stresses on crops. Consequently, climate change stands poised to adversely affect crop yields and livestock production. Therefore, the primary objective of the review article is to furnish a comprehensive overview of the multifaceted factors influencing horticulture production, encompassing fruits, vegetables, and plantation crops with a particular emphasis on greenhouse gas emissions and environmental stressors such as high temperature, drought, salinity, and emission of CO2. Additionally, this review will explore the implementation of novel horticultural crop varieties and greenhouse technology that can contribute to mitigating the adverse impact of climate change on agricultural crops.

Comparative analysis of the effects of waste shell aggregates on the material properties of cement mortars

This study investigates the effects of different waste shells on the material characteristics and properties of cement-based composites. The accumulation of large quantities of waste shells poses significant environmental concerns, leading to soil pollution and odors. Furthermore, the construction industry contributes to global environmental issues through high energy consumption and CO2 emissions during aggregate production. Recycling different waste shells, including cockles, oysters, and murex, after cleaning them using an ultrasonic device, and incorporating them as partial substitutes for fine aggregates, can potentially tackle both environmental challenges. The physical and mechanical properties, including freeze–thaw resistance, of mortar specimens with different types and amounts of waste shell aggregates were measured, to examine the effect of each shell aggregate on the specimens. The microstructural characteristics of the specimens were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and micro-computed tomography (micro-CT) and correlated with the material properties. The results demonstrate that mortars incorporating waste shells, with appropriate treatments such as 30% replacement and washing of the particles, exhibit compressive strength up to 90% of that observed in the normal case. Therefore, these shells can be effectively used as substitutes for fine aggregates.

Feature selection for coal heating level estimation in thermal power plants

Several recently signed environmental agreements and protocols emphasize the global need to reduce GHG emissions, with a focus on limiting coal consumption due to high NOx and CO2 emissions. However, many countries, including those in the Western Balkans, rely heavily on coal for electricity generation. The outdated thermal power plant infrastructure in these regions poses a major challenge when it comes to meeting modern environmental standards while maintaining efficiency. This study is part of the more comprehensive research which aims to develop an expert system that utilizes existing measurements to estimate key parameters crucial for both energy production and pollution reduction. The focus is on Serbian thermal power plants, particularly plant Nikola Tesla unit B1. One of the critical parameters for optimizing thermal power plant control loops is the heating value of coal, which is challenging to measure in real time due to the coal?s varying chemical compositions and caloric values. This paper examines 74 different parameters measured in 59 instances to estimate the hating value of coal at unit B1. Through detailed analysis and feature selection methods, including linear regression, this research aims to identify the most informative parameters for estimating the heating value of coal, which will improve the control system that enables more efficient and environmentally friendly power generation in coal fired thermal power plants.

Comparative analysis of decision making factors of hybrid and emission free heat source users in single family houses

Aim/purpose – Energy is traditionally produced using fossil fuels as raw materials, which impacts the environment negatively. Due to the scarcity of fossil fuel supplies, rising prices of energy carriers, and global trends, consumers are turning to renewable energy sources (RES) for home heating. The aim of this study was to determine whether the choice of house heating system using different sources than fossil fuels is driven by any additional components of single-family house users’ purchasing decisions besides cost. Based on research results, the prospects for further transformation of thermal ener- gy in single-family housing in Poland towards RES were determined. Design/methodology/approach – The research on the inhabitants of single-family houses was conducted in December 2022 in Poland. The research method used was a survey using the CATI and CAWI techniques. 600 respondents filled out the question- naire. The results were statistically analyzed. A non-parametric Kruskal-Wallis test (non- -parametric ANOVA) and a post-hoc test were used. The distributions’ normality and the variances’ homogeneity were measured using Kolmogorov–Smirnov and Brownian– –Forsythe tests. Calculations were performed using Statistica software. Findings – The choice of heating energy source in single-family houses, taking CO2 emis- sions into account, is not dictated by income level. Instead, it is determined by the number of people in the household. More numerous households opt for a zero-emission heat source. Households using emission-based heating energy sources are significantly more driven by the cost of using heating appliances; their purchase price, reliability, and product quality are more important than those using zero-emission methods. The partial use of renewable energy source solutions promotes further investment in emission-free heat sources. Research implications/limitations – The analysis illustrates the state of thermal trans- formation in single-family houses during the energy transition process at the national level. A limitation of the research is that it samples only one EU country struggling with too high CO2 emissions compared to other countries. Originality/value/contribution – A unique approach used in this study is to address the variation in incentives for renewable energy purchasing decisions, considering CO2 emissions. This aspect, although noticeable to some extent, is not directly taken into account by users who do not have the tools to assess their CO2 emissions. However, the proposed research approach showed that the CO2 emission level of a heating system is a factor differentiating some aspects of the decision-making process of system users. In particular, it showed what elements of the decision are essential in households that have not yet taken any action towards energy transformation. Keywords: energy, decision-making process, heat source, hybrid heat sources, emis- sion-free heat sources, consumers’ characteristics. JEL Classification: Q56, G51, O13.

The Analysis of Next-Generation Refrigerants in Terms of Energy, Exergy, and LCCP Perspective

In this study, commonly used hydrofluorocarbon (HFC)-based refrigerants R404A and R410A, as well as hydrofluoroolefin (HFO)-based environmentally friendly next-generation refrigerant R1234yf with a low global warming potential (GWP), were analyzed in terms of energy, exergy, and life cycle climate performance (LCCP) in refrigeration and air conditioning systems. All three refrigerants were examined at four different evaporation temperatures (-30°C, -15°C, -5°C, 0°C) with a constant condenser temperature of 50°C using a simulation program. For different evaporation temperatures, the performance of the refrigerants was evaluated using the first and second laws of thermodynamics, and performance coefficients, exergy efficiency, and exergy destruction were calculated. Additionally, the amount of kgCO₂e equivalent was calculated using the LCCP method. In the study, it was found that the compressor energy consumption of R410A and R1234yf refrigerants was similar and approximately 7% lower than that of the R404A refrigerant. The highest coefficient of performance (COP) value was determined for R1234yf. It was observed that R1234yf refrigerant had the highest exergy efficiency starting from -15°C. The kgCO₂e equivalent emission values calculated using the LCCP method revealed that R404A had the highest CO₂ emissions, while R1234yf had the lowest. Furthermore, based on the simulation study and theoretical calculations, it was determined that R410A and R1234yf refrigerants could be considered as alternative choices to R404A refrigerants in systems where two refrigerants are used.