A concept to support the transformation from a linear to circular carbon economy: net zero emissions, resource efficiency and conservation through a coupling of the energy, chemical and waste management sectors

Coal and carbon-containing waste are valuable primary and secondary carbon carriers. In the current dominant linear economy, such carbon resources are generally combusted to produce electricity and heat and as a way to resolve a nation’s waste issue. Not only is this a wastage of precious carbon resources, which can be chemically utilized as raw materials for production of other value-added goods, it is also contrary to international efforts to reduce carbon emissions and increase resource efficiency and conservation. This article presents a concept to support the transformation from a linear ‘one-way cradle to grave manufacturing model’ toward a circular carbon economy. The development of new and sustainable value chains through the utilization of coal and waste as alternative raw materials for the chemical industry via a coupling of the energy, chemical and waste management sectors offers a viable and future-oriented perspective for closing the carbon cycle. Further benefits also include a lowering of the carbon footprint and increasing resource efficiency and conservation of primary carbon resources. In addition, technological innovations and developments that are necessary to support a successful sector coupling will be identified. To illustrate our concept, a case analysis of domestic coal and waste as alternative feedstock to imported crude oil for chemical production in Germany will be presented. Last but not least, challenges posed by path dependency along technological, institutional and human dimensions in the sociotechnical system for a successful transition toward a circular carbon economy will be discussed.


Introduction
Major global changes ranging from population growth, shifts in economic growth to breakthroughs in clean technology development and consumer technologies have led to a rapid increase in international demand for natural resources and raw materials. Confronted with challenges such as climate change, increasing natural resource depletion and a growing waste crisis, a predominant focus for decision-makers in numerous countries-in particular following the Paris Agreement-is the promotion of technological innovations that will pave the path for a low-carbon economy and the achievement of a circular economy. With its Energy Transition undertaking ('Energiewende'), Germany is taking a pioneering role in substituting conventional fossil energy resources with renewable energy for its power generation, heating and mobility sectors. The goal is to achieve greenhouse gas neutrality ('Treibhausgasneutralität') by 2050 [1]. To realize this ambitious target, other carbon-intensive industries such as the chemical and waste management sectors that are dependent on carbon feedstock for their production/operation are also increasingly under pressure to reduce their CO 2 emissions. At the same time, demands by end customers for sustainably produced products (e.g. PVC, ethanol and H 2 production from renewable and secondary carbon resources such as biomass, waste etc.) and progressively stricter regulations to reduce primary carbon resource consumption through increased utilization of secondary waste materials are also providing impetus for a transformation from a linear to a circular carbon economy.
Coal and carbon-containing waste (e.g. municipal waste) are valuable primary and secondary carbon carriers. In the current dominant linear economy, these carbon resources are combusted to produce electricity and heat and as a way to resolve a nation's waste issue. However, coal combustion and waste incineration are associated with considerable amounts of CO 2 and other emissions (e.g. organic and inorganic traces, fine particles). With the significant progress being made in renewable power generation, combustion of carbon resources is no longer in tune with the global drive toward CO 2 reduction. Moreover, as combustion essentially represents wastage of precious carbon resources, which can be chemically utilized as raw materials for production of other value-added goods, it is also contrary to international efforts to increase resource efficiency and conservation.
In this article, we present a concept to support the transformation from a linear 'one-way cradle to grave manufacturing model' [2] toward a circular carbon economy [3]. The development of new and sustainable value chains through the utilization of coal and waste as alternative raw materials for the chemical industry via a coupling of the energy, chemical and waste management sectors offers a viable and future-oriented perspective for closing the carbon cycle. Further benefits also include a lowering of the carbon footprint and increasing resource efficiency and conservation of primary carbon resources.
The article is structured as follows. First, trends and developments in the energy, chemical and waste management industries in Germany will be briefly reviewed. Second, the concept for coupling these sectors to support the transformation to a circular carbon economy will be introduced and the corresponding effect on CO 2 emissions will be presented. In addition, technological innovations and developments that are necessary to support a successful sector coupling of the energy, chemical and waste management sectors will be identified. To illustrate our concept, a case analysis of domestic coal and waste as alternative feedstock to imported crude oil for chemical production in Germany will also be shared. Finally, challenges posed by path dependency in the system for a successful transition in the coal and waste management sectors toward a circular carbon economy will be discussed.

Energy industry
Germany possesses a significant amount of domestic lignite (brown coal), with geological reserves of 72.7 billion tons, out of which 36.2 billion tons are deemed as economically minable [4]. To date, the country's energy transition has focused predominantly on achieving a transformation in its electricity system from fossil and nuclear to renewables. This will result in a stepwise reduction of its fossil power generation capacity, particularly for coal [3]. Coal mining, coal power generation and associated industries are key employers not only in German lignite regions situated in North Rhein-Westphalia, Saxony, Saxony-Anhalt and Brandenburg but also in neighboring coal regions in Poland and the Czech Republic. The changes brought about by Germany's Energy Transition as well as efforts to meet the EU climate and energy targets are already resulting in significant and challenging structural changes for the affected industries and population. After decades of coal combustion, a new and climate compatible utilization option is therefore urgently required to support a structured transition in these regions and to provide them with a sustainable long-term perspective.

Chemical industry
The chemical industry is a major industry sector in Germany. The sector generates immense revenues (185 billion Euros in 2016). It also accounts for ~447 000 jobs and investments of >7 billion Euros [5]. Since 1990, the German chemical industry was able to reduce their greenhouse gas emissions by almost 50% to <50 million tons of CO 2 equivalent [6]. However, concerns about feedstock prices, depleting oil resources, the concentration of oil and gas resources in politically unstable regions and demands by environmentally conscious end consumers are increasing pressure on the chemical industry to diversify their raw material basis. Moreover, criticisms are growing about the lack of consideration for 'carbon leakage' (i.e. greenhouse gas emissions that are associated with the extraction, processing and transport processes beyond Germany's national borders) in the upstream value chain of imported crude oil and natural gas-which accounts for ~85% of the carbon feedstock for the German chemical industry [7]. These considerations have motivated efforts by industry and political decision-makers to develop domestic and renewable carbon resources as alternative raw materials for its industry to address the trilemma of competitiveness, supply security and sustainability [8].

Waste management industry
In 2015, 402 million tons of waste were produced in Germany [9]. Annually, ~20 million tons of waste were combusted to produce electricity, process steam and/or heat [10]. This translates into ~15 million tons of CO 2 emissions into the atmosphere. A utilization of alternative feedstock to conventional (oil and natural gas) feedstock for chemical production could increase the overall value obtained from domestic carbon waste resources. In view of the large, untapped potential presented by plastic and other carbon-containing waste (e.g. municipal, industry, biomass waste and residue), this sector presents a huge opportunity for increasing resource efficiency through channeling domestic waste back into the value chain for multiple utilization cycles. In addition to promoting fossil resource conservation, this would also significantly reduce Germany's dependency on fossil imports and carbon leakage along international upstream value chains for its chemical production.
2 Sector coupling for transformation from a linear to a circular carbon economy

Coupling of the energy, chemical and waste management sectors
Olefins, especially ethylene and propylene, form the largest group of chemical intermediates (particularly for plastic production) with an annual production of ~10 million tons in Germany [5]. In Germany, olefins are mainly produced from imported crude oil [7].
With the global drive to reduce carbon footprints and transition to a circular economy, the potential of the energy, chemical and waste management sectors in contributing to achieving these objectives is significant. In this section, we present a concept to support the transformation from a linear to circular carbon economy and closing the carbon cycle through a coupling of these three sectors and replace imported crude oil with domestic carbon resources for olefin production (see Fig. 1).
The proposed concept encompasses four segments.
1) Waste processing for chemical recycling: Besides dumping, waste management globally still currently uses waste incineration. The latter represents a linear cradleto-grave model whereby carbon in the waste is fully released as CO 2 emissions (Fig. 2a). In Germany, under the current EU Emissions Trading System (ETS), CO 2 emissions from waste incineration plants receive ETS allowances (i.e. face no penalty payment). Nevertheless, this does not change the fact that the combustion of 100 units C (e.g. carbon atoms) of waste for electricity and heat will result in 100 C being released as CO 2 . The 20 million tons of waste, which is currently combusted in Germany, thus present an opportunity for binding carbon into chemical products via chemical utilization. This circular concept thus supports resource efficiency whereby valuable carbon resources are channeled back into the value chain for multiple utilization cycles. 2) Gasification of carbon resources, syngas processing and chemical synthesis as well as chemical products from domestic carbon resources: Gasification is the process of thermal conversion of carbon feedstock to obtain a synthesis gas (syngas) that consists mainly of H 2 and CO [see Equation (1) for a simplified equation of this process]. It is a key interface technology to convert any carbon feed material into syngas, which is required for any product synthesis [3]. Through synthesis, platform chemicals such as methanol (MeOH) can be produced from syngas. Subsequently [see Equation (2)], derivate such as olefins that form the basis raw material for the production of a wide range of chemical products can be synthesized via the methanol-to-olefins process [11]. Following their utilization, the spent products, i.e. waste can be brought back into the carbon cycle where-via  1 Transformation from a linear to circular carbon economy and closing the carbon cycle gasification and the subsequent synthesis-they are converted into new chemical products again, thus closing the carbon cycle and enabling the transformation to a circular carbon economy.
3) Availability of domestic lignite: In 2016, ~160 million tons of lignite was combusted in Germany to produce electricity and heat [12]. Similar to waste, 100% of the carbon in this primary carbon carrier is emitted into the atmosphere as CO 2 on combustion. However, the difference to waste combustion is that under the current ETS system, CO 2 emissions from fossil power plants face an ETS penalty (Fig. 2b).
Through the stepwise reduction of coal power generation in Germany, this opens up an opportunity for an alternative utilization for domestic lignite-as a partner for waste for chemical production-which has a lower carbon footprint. Three arguments on why lignite represents a potential partner for waste are: • From a technological perspective, ~50% carbon in the waste feedstock can be bonded in chemical products. Moreover, not all chemical products will re-enter the carbon cycle as waste materials following their utilization. To illustrate, consider the case of plastics in Germany. In 2015, while 10.1 million tons of plastics are consumed, only 5.9 million tons came back into the carbon system as waste [13]. This represents a gap that has to be compensated by feeding new primary carbon into the system for chemical production. • Carbon waste such as plastics includes many volatile compounds that go into the gas phase on heating during the gasification process. As gasification technology requires a certain amount of solid carbon to guarantee a stable process, the co-gasification of carbon waste with a feedstock having a higher fixed carbon content is therefore technologically required.  • There is considerable market competition for carbon waste as feedstock, e.g., by incinerators and as substitute fuel for electricity and heat production in power plants.

Waste
In view of the above considerations and the projected release of lignite from Germany's energy system, domestic lignite represents a potential and attractive partner to fulfill the role as a companion for carbon waste for chemical production. In the mid-term (i.e. 2025), a coupling of the energy with waste and chemical sectors in the form of conventional chemical utilization already supports the transition from a linear to circular carbon economy. As can be observed from the process chain calculations presented in Fig. 2c, even with the utilization of lignite as a co-feedstock for gasification, the majority of the carbon in the primary and secondary carbon carriers are bonded in the chemical products (104 C), resulting in reduced CO 2 emissions (96 C) compared to the combustion of waste and lignite as illustrated in Fig. 2a and b (200 C emitted CO 2 emissions altogether).
Utilization of waste and coal as alternative feedstock for chemical production will reduce the industry's demand for primary carbon feedstock such as crude oil and natural gas. Not only does this contribute to resource conservation, it also contributes to reducing carbon leakage associated with an international feedstock supply.

4) 'Green' H 2 from renewable electricity:
To close the carbon cycle, a coupling of the waste management and chemical sectors needs to be complemented with the energy sector not only for the coal input but also for the 'green' hydrogen. An integration of 'green' H 2 -produced via electrolysis powered by renewable energy-into the process enables closing the carbon cycle by avoiding carbon emissions in the form of CO 2 and other carbon components in purge gases etc. (i.e. the 'green' H 2 will bond with CO and CO 2 to produce additional syngas). Together with the interface technology gasification, this supports the achievement of net zero emissions, i.e. not just of CO 2 and other greenhouse gases but also emission of all other contaminants such as heavy metals, organic and inorganic traces and fine and ultrafine particulates.
In the long term (i.e. 2050+), an integration of 'green' H 2 is a probable scenario considering the projected surplus of renewable electricity resulting from Germany's Energiewende-via gasification to achieve net zero emissions. This projected surplus will represent a milestone in efforts to attain a closed carbon cycle whereby 100% of carbon in coal and waste feedstock can be bonded into the chemical products, thus resolving the problem of feedstock carbon losses (Fig. 2d). In addition, technological innovations and further developments could potentially support the mono-gasification of waste, thus eliminating the technological requirement for a solid/fixed carbondelivering partner. Furthermore, in focusing on specific waste segments that are challenging for combustion such as high calorific and problematic waste (e.g. high Cl containing, shredder light fractions, carbon and glass fiber plastics, composites etc.), chemical recycling could be a valuable partner for waste incinerators, thus addressing the market competition problem.

Required innovations and developments
The use of gasification to produce syngas from coal, and also from coal and waste, is a well-known tried-and-tested approach [3,14]. However, a successful transformation to a circular carbon economy would require further development of gasification technologies, which are currently available in the market as well as necessary accompanying technological innovations and developments along the entire process chain (see Fig. 3).
First, previous industrial experience with co-gasification of waste and coal for methanol production in Berrenrath (in the 1990s) and Schwarze Pumpe (till 2007) in Germany have shown that the feedstock preparation process (e.g. grinding, sieving, drying, compaction) is associated with significant effort and costs [14]. Utilization of coal and waste as feedstock with less treatment for the gasification process would have a considerable impact on lowering operational costs. This requires the development of an innovative 'lock-free' solid feeding system. Second, gasification technologies can generally be differentiated according to their gas-solid contact into fixed bed, fluidized bed and entrained flow [3]. Entrained-flow gasification technology is currently the technology having the highest market share with >50% of the cumulative syngas capacity worldwide [15]. The use of entrained-flow gasification is advantageous due to complete carbon conversion, high syngas yield and maximum available capacities of thermal input. However, the required particle sizes of <200 µm for entrained-flow gasification makes it unsuitable as waste cannot be easily pulverized. In contrast, fixed-bed and fluidized-bed technologies do not face such restrictions. However, both technologies do have disadvantages. With fixed-bed gasification, there are significant fractions of tar, oil and methane in the syngas, which reduces the syngas yield. Fluidized-bed gasification disadvantages are its association with incomplete carbon conversion, carbon-containing solid residues and considerable methane content in the syngas. To achieve a minimal CO 2 footprint and maximal resource efficiency in the co-utilization of domestic waste and coal for chemical production, the development of a next-generation gasification technology is necessary. This new technology would need to facilitate complete carbon transformation in the syngas (i.e. optimize cold gas efficiency and carbon retention in chemical products), support resource recovery (e.g. of metals before or after gasification) and use electricity as 'reaction partner' (in the short run through 'green' H 2 , and in the future direct use of 'green' electricity to support endothermic gasification reactions).
Third, water consumption is a key issue of concern for gasification technologies. In countries such as China where coal gasification is widely implemented for chemical production, water utilization and waste water treatment are high on the list of priority issues to resolve. Innovations to facilitate a closed water cycle and zero liquid discharge (ZLD) are thus urgently required. As defined by PROCESSNET-an initiative by the German Society for Chemical Engineering and Biotechnology (DECHEMA) and the Association of German Engineers-Society for Energy Process and Chemical Engineering (VDI-GVC)-under strict conditions, ZLD refers to a complete reduction of the water volume whereby water leaves the system only in the form of steam and solids are recycled or separated in dry form. Under restricted conditions, it refers to having no waste water leaving the system except for sludge, brine, aerosols or water by leaching [16]. In addition, development for a CO 2 -tolerant wash (i.e. CO 2 remains in the system after the selective removal of other gases such as H 2 S) is also necessary to ensure that all carbon streams (e.g. CO 2 emissions, carbon components in purge gases etc.) can be bonded in chemical products.
Finally, an integration of renewable generated 'green' H 2 directly into the syngas would avoid the need for watergas-shift to adjust the syngas composition and prevent CO 2 emissions from being released into the atmosphere. However, the widespread and easy availability of affordable 'green' H 2 is an obstacle to achieving zero CO 2 emissions. Hence, in addition to R&D focusing on expanding the production and lowering the cost of renewable generated H 2 , parallel efforts to develop innovative CO 2 -tolerant synthesis should also be intensified.

Key criterion for technology developmentminimal demand for renewable generated H 2
To achieve zero CO 2 emissions as hypothesized in the long-term perspective (i.e. Scenario 2050+) presented in Fig. 2d, all carbon waste streams (e.g. CO 2 emissions, carbon components in purge gases) must be recycled. A sufficient quantity of 'green' H 2 is needed to bond all 'waste' carbon atoms into chemical products. As mentioned above, the stable and permanent availability of 'green' H 2 represents a key obstacle to achieving zero emissions. A central criterion for the development and implementation of gasification technologies for sector coupling is therefore the minimal demand for 'green' H 2 .
In this section, we present a summary of results obtained from our case study analysis that focused on olefin production in Germany [17]. The objective is to illustrate the demand on renewable electricity to produce enough 'green' H 2 to achieve zero CO 2 emissions.
The reference scenario is based on carbon capture and utilization (CCU). This concept of utilizing CO 2 captured from large point sources (e.g. fossil power plants, steel and cement plants etc.) for chemical production has elicited increasing interest in the global community as a potential and attractive method to achieve a nation's CO 2 emission reduction targets. In this scenario, we assume that all carbon waste streams from the incineration of plastic waste in Germany are captured and bonded into chemical products through integration of 'green' H 2 . We also made the assumption that there is enough plastic waste available to support the production of 10 million t/a of olefins (Fig. 4a).
Our contrast scenario is based on syngas from waste and coal gasification. As mentioned in the previous section, commercialized fixed-bed, fluidized-bed and entrainedflow gasification technologies all face diverse shortcomings, which limit their suitability for the co-gasification of coal and waste. The disadvantages that make them unsuitable for sector coupling to close the carbon cycle are addressed by the development of a next-generation gasifier technology COORVED (CO 2 reduction by innovative gasifier design) [18,19]. In this design, the superior heat integration of a fixed-bed gasifier is combined with the moderate steam and oxygen consumption of a fluidizedbed gasifier and the gas quality of an entrained-flow gasifier. The carbon-containing ash agglomerates, which are formed in the jetting bed, are contacted with secondary gasification agent in the fixed-bed to ensure a complete carbon conversion and post-oxidation of the ash [3]. In the contrast scenario, the amount of renewable electricity required to produce sufficient 'green' H 2 to ensure zero CO 2 emissions for olefin production is determined using the COORVED gasification technology (Fig. 4b). Table 1 provides a description of where renewable energy is required (and produced in the case of waste incineration) in the production process, both for general operation and for producing 'green' H 2 for integration in the syngas (CO 2 and CO) and exhausted gases (the so-called diffuse CO 2 ) so as to bond all waste carbon into the chemical products. Figure 5 presents the model evaluation results for the quantity of renewable energy required to achieve zero CO 2 emissions for both CCU and the gasification scenarios. In the gasification scenario, we analyzed different combinations of waste and lignite. In our evaluations, we assume a 70% efficiency for the water electrolysis.
We observed that the electricity demand is significantly dominated by the supply of 'green' H 2 . A considerable amount of renewable generated 'green' H 2 is required to convert CO 2 in syngas into methanol in the CCU scenario. In contrast, the major proportion of 'green' H 2 is required for CO and CO 2 conversion from syngas to methanol in the case of gasification. Strikingly, compared to gasification, the total demand for renewable electricity in the CCU scenario is significantly higher. Even after deducting the power that is produced from the waste incineration process, the amount of renewable electricity that is required to produce 10 million t/a olefins via CCU would be higher than Germany's renewable generation of 188 TWh in 2016 [20]. In the case of gasification, the COORVED gasification technology requires considerably less 'green' H 2 to achieve   Direct power demand in process chain, e.g., for CO 2 compression e-H 2 for diffuse CO 2 'Green' hydrogen (e-H 2 ) for CO 2 conversion from exhaust gases into methanol e-H 2 for syngas CO 2 'Green' hydrogen (e-H 2 ) for CO 2 conversion from syngas into methanol e-H 2 for syngas CO 'Green' hydrogen (e-H 2 ) for CO conversion from syngas into methanol zero CO 2 emission compared to CCU. By increasing the plastic waste content for gasification, the hydrogen content in the syngas increases due to the higher H/C ratio, thus lowering the demand for 'green' hydrogen via electrolysis [17].
Results from process modeling suggest that in terms of 'green' H 2 demand, gasification is a more promising approach than CCU for olefin production without CO 2 emissions. Moreover, while additional measures are necessary to address other emissions from the incineration process (e.g. heavy metals, fine particulate) in the case of CCU, gasification technology has the advantage that it could achieve net zero emission for chemical production.
3 Challenges posed by path dependency in the sociotechnical system

Motivation for sector coupling
The proposed concept to support a transformation from a linear to a circular carbon economy is associated with various advantages. These include, besides closing the carbon cycle for chemical products, associated benefits such as: • providing a chemical storage for surplus renewable electricity, • feedstock diversification for petrochemical products which contributes to conserving primary carbon resources, reducing import dependency and carbon leakages along international supply chains, • reducing CO 2 emissions through chemical recycling of waste and chemical utilization of coal via sector coupling, • providing a higher value-added alternative to waste incineration and landfill for 'problematic' waste, which pose difficulties for combustion technologies (e.g. carbon/glass fiber reinforced polymer, light shredded fractions, mixed waste and residues) and • creating value domestically and providing an alternative perspective for Germany's domestic lignite mining areas to support a socially sustainable structural change in these regions.
However, while attractive in theory, a successful implementation of this concept to couple energy, chemical and waste management sectors for chemical production faces significant challenges posed by path dependency.

Path dependency and lock-in effects
The energy, chemical and waste management sectors exist in a large sociotechnical system. A sociotechnical system is made up of interrelated components connected in a complex network and infrastructure and includes components such as physical infrastructure (chemical plants, oil pipelines and waste incinerators), organizations (manufacturing industries, banks, R&D institutions), natural resources, informational elements (e.g. books, articles, news), legislative artifacts (e.g. laws, regulations, incentives) and human elements (e.g. social norms, perception, beliefs) [21][22][23]. Such systems are observed to exhibit path dependence and lock-in along multiple dimensions [24,25].
Along with technological and institutional dimensions, once investments have been made in particular products, technologies and infrastructures, increasing returns make it less costly to proceed on the existing path rather than to reverse or change it [26][27][28][29][30]. Researchers therefore emphasized how 'history matters', as decisions to embark along a particular developmental path tend to rely on and are constrained by earlier choices [29]. In Germany-one of the most industrialized nations in the world-the energy, chemical and waste management sectors are mature industrial branches where significant investments have already been made in specific technologies and in building up supporting infrastructures (e.g. coal mining industries, transport infrastructures and coal combustion technologies; waste infrastructure, international waste transport network and waste-to-energy technologies; international oil pipelines, cracker and reforming technologies). Such investments represent sunk costs whereby the continued adoption of a technology or process is often associated with growing benefits. Moreover, diverse complementary technical and institutional components would have developed over time, which further reinforce such increasing returns phenomenon. A combination of the above factors makes it costly to proceed off the existing path of oilbased chemical production, leading to considerable inertia in changing the existing status quo ('if it's not broken, why fixed it?'). Hence, the general preference is to invest in improving and further developing existing technologies and processes rather than radically changing the production value chains. This could eventually result in a society gradually locking itself into outcomes that are not necessarily superior to potential alternatives [26]. To support the transformation toward a circular carbon economy, regulatory frameworks and measures to support breaking out of diverse technological and institutional lock-ins in the energy, chemical and waste management sectors and promote sector coupling for chemical production are therefore necessary.
In addition, lock-in effects can also extend to the human dimension. Sociotechnical systems are not autonomous systems that are independent of human beings who, in modern societies, live in a technotope surrounded by technologies and material contexts shaping their perceptions, behavioral patterns and activities [21]. The deep embeddedness of technologies and infrastructures in our social environment could thus significantly shape how a nation's citizens view different carbon resources and their responses to associated developments. A lock-in or 'stickiness' in how coal and waste are viewed by individuals in a society could mean that prevailing perception-in providing a basis for judgment and decision-making-may discourage further analysis and lead to habitual responses and reactions that could subsequently hinder the adoption and societal support of new or innovative developments associated with such domestic resources [31,32]. This has significant implications for managing the transition of a nation's chemical production from a dependence on imported crude oil and natural gas to a reliance on domestic carbon resources as alternative feedstock. The following example from Germany illustrates the challenges posed by the social dimension in such transformation processes.

Perception and acceptance of domestic carbon resources in Germany
Although waste and coal are both significant sources of carbon in Germany, investigations into the perception and acceptance of waste as a domestic energy resource and as a valuable raw material alternative for the industries are limited [33,34]. In contrast, although Germany is the largest lignite producer in the world [35], coal is a highly controversial energy resource, which is scheduled for a phase-out from the electricity sector [8,36]. To illustrate, consider the following findings from various national and international surveys. In a special Eurobarometer study on knowledge and perception of energy technologies carried out in the European Union in 2006, German citizens expressed their opposition to the use of coal in their country. This aversion to coal was only surpassed by their opposition to nuclear energy [37]. Results from the 2011 special Eurobarometer on public awareness and acceptance of CO 2 capture and storage-implemented before the Fukushima nuclear incident-showed that the German population exhibited a lack of knowledge about the role of coal in Germany's electricity mix. Moreover, coal acceptance had also decreased in comparison to the 2006 study [38]. In contrast, a survey carried out shortly after the Fukushima accident focusing on acceptance for hard coal and lignite in Germany observed that the level of coal acceptance had tripled following the Japanese nuclear disaster [39]. In a 2012 national survey on acceptance for renewable energy, it was observed that in comparison to 2011, the acceptance for coal power plants in East German states has increased whereas the opposite is true in West German states. In another national survey in 2015 on the attitudes and assessment of German citizens regarding energy supply in Germany, it was observed that in spite of the critical discussions surrounding lignite, this domestic resource is still viewed as an important economic factor in Germany [40].
The utilization of domestic lignite as an alternative carbon feedstock to imported crude oil and natural gas for chemical production will mean an extension of lignite mining and utilization in the country. An intangible 'lock-in' of negative coal perception and public resistance toward coal-related developments will therefore pose another significant obstacle-in addition to the inertia in the existing system from barriers created by techno-institutional path dependence-for managing the transformation process. However, while the above surveys captured the general mood toward coal in the German society at the time of their implementation, the methodology utilized could not provide insights into underlying concerns or factors that are driving resistance or acceptance for coal. In addressing this gap, investigations found that young German adults commonly associated coal utilization with negative images of digging and mining, carbon emissions, filth and pollution. These negative associations are also observed to be resistant to change even in the aftermath of a significant energy catastrophe such as Fukushima. These research findings suggest that a strong association of coal with specific imageries and negative feelings is 'locked-in' in people's minds [32,41]. In addition, a nationwide survey in 2013 by the same researchers indicated that the majority of German citizens are also unaware of the potential of coal as a carbon feedstock for the chemical industry [42].
Taken together, the strong negative associations of coal with electricity generation and a lack of awareness of its potential as chemical feedstock in Germany could result in negative responses toward coal occurring unconsciously. Such intuitive reactions could have a deciding effect on people's receptiveness to proposed/planned developments, which are not associated with electricity production. Furthermore, research has demonstrated that multiple context factors, such as historical development, education, politics, media, economic relevance and impact as well as societal norm, not only shape but also reinforce how people view an energy source such that energy perception could remain relatively stable and resistant to change [31]. In the case of coal, this suggests that the current societal context and a climate of coal resistance could lead to a non-acceptance of coal by the German public no matter how it is used.

Conclusion
Coal and carbon-containing waste are valuable primary and secondary carbon carriers. As such, burning them to produce electricity and as a way to resolve a nation's waste issue are no longer in tune with the global drive toward greenhouse gas reduction, resource efficiency and conservation. Moreover, coal combustion and waste incineration are associated not only with significant greenhouse gas emissions but also with the release of multiple trace components and pollutants. With the increasing international focus on transiting toward a low-carbon and circular economy, this article presents a concept to support such a transformation for carbon-intensive industries, which, unlike the electricity sector, are dependent on carbon resources for their production.
The rapid development of renewable power generation in Germany, the increasing release of coal from the electricity sector and the predominant combustion of waste provide the opportunity for a sector coupling of the energy, chemical and waste management sectors to produce chemical products from domestic carbon resources. This opportunity is provided by gasification technology with the possibility of conversion of waste and coal together into synthesis gas for subsequent syntheses in the chemical industry to produce value-added goods. The utilization of domestic coal and waste resources as alternative feedstock for chemical production contributes to feedstock diversification for the chemical industry and reduces its dependency on imported primary carbon resources (i.e. crude oil and natural gas). Other benefits include reducing CO 2 emissions associated with waste incineration, the need for landfill space for waste (whose disposal is a challenge for most of the countries worldwide) and increased resource efficiency through binding carbon from waste in chemical products. With the projected increase in renewable generation capacity in Germany, such sector coupling also provides a chemical storage alternative for surplus renewable electricity while supporting the achievement of zero CO 2 emissions through an integration of renewable generated 'green' hydrogen in the production process. Importantly, following the massive reduction in coal power generation capacity as part of Germany's Energiewende (i.e. Energy Transition), it provides a new development perspective for domestic lignite mining areas, which is in line with the nation's climate objectives.
A successful transformation to a circular carbon economy would not only require further development of the interface technology gasification, accompanying innovations and developments along the entire process chain from the feed-in system, waste water and gas treatment to synthesis are also necessary. In addition, regulatory changes and measures are also urgently required to motivate carbon-intensive industries to abandon the current unnecessary carbon losses within the carbon cycle and break out of the inertia in the existing system to explore alternative and innovative cooperation with other sectors. Lastly, it is also essential to integrate a consideration of the human dimension in managing the transformation process. Neglecting intangible and qualitative aspects relating to citizens' knowledge, perception and acceptance while predominantly focusing on solving tangible and quantitative technical challenges could result in the development of technologies, which, although technically superior to other alternatives in facilitating a circular carbon economy, may face considerable resistance in the public realm.