The Net Zero Lab is composed of different experts in areas of energy integration, modelling and simulation, life cycle analysis, furnaces operation and revamping, energy audit, and financial analysis. This team counts with the participation of practitioners and researchers from EPFL, HES SO, OIKEN and NOVELIS Sierre.

This detailed study aims to assess the sustainable utilization of renewable energy resources, especially biomass, in order to decarbonize heavy industries, such as secondary aluminium production. To this end, this study explores the feasibility of integrating biomass gasification to produce and supply fuel to high temperature furnaces in Novelis Sierre plant, whereas cascading the low-grade waste heat for district heating purposes and other cogeneration applications. The analysis of the additional sustainable biomass potential indicates the availability of sufficient resources for a project of this kind in nearby cantons including Valais, Vaud, Fribourg, and Bern. Unlocking only 10% of the additional sustainable potential of biomass for energy use in these cantons is sufficient to supply the needs of Novelis aluminium plant and Sierre city during the winter season. Sourcing biomass from various regions in Switzerland remains economically viable; yet, procurement of biomass resources in farther locations may significantly increase wood energy prices. The energy consumption in biomass transportation slightly varies for different moisture content scenarios, with pelletized wood showing the most significant reduction. However, in this study wood chips supply is evaluated due to the local availability and fewer processing steps. Different biomass energy conversion technologies for combined fuel, power and heat are analyzed to determine the most promising applications to use the limited valuable biomass resource. The biomass gasification system, which allows producing the synthetic natural gas (SNG) fed to Novelis aluminium plant, is compared to the conventional biomass boilers and cogeneration units. This analysis also encompasses the use of two different district heating networks, based either on the traditional water-based systems or new generation anergy distribution systems. Groundwater heat pumps (HP) are used to balance the space heating demand of the city. Eight biomass energy conversion scenarios are evaluated, namely:

1. SNG + water CAD: Biomass-to-SNG route with waste heat released to a water district heating network (CAD) without electricity generation.
2. SNG + cogeneration + water CAD: Similar to case 1, but with combined electricity and heat production in a Rankine cycle.
3. SNG + CO2 CAD: Similar to case 1, but the CAD consists of a CO2 district heating network, instead of a water-based CAD, without electricity generation.
4. SNG + cogeneration + CO2 CAD: Similar to case 3, but with combined electricity and heat production in a Rankine cycle.
5. Syngas for CHP engine + water CAD: Biomass-derived syngas is used directly to run an internal combustion engine that produces combined power and heat. Fossil natural gas is imported from the grid to meet energy demand of the Novelis furnaces.
6. Boiler for SN + water CAD: A biomass boiler is used to produce high temperature steam in a steam network (SN) that generates power in a Rankine cycle. A water CAD supplies the city of Sierre. Novelis furnaces are fed with imported fossil natural gas.
7. Only heating boiler: Biomass is used only in a heating boiler to produce heat distributed through a conventional water CAD. Electricity and fossil natural gas are imported.
8. OxySNG + sCO2 + CO2 CAD: SNG is produced via biomass gasification and consumed in Novelis oxycombustion furnaces. The biogenic CO2 separated is sequestrated to achieve negative emissions. The CAD consists of a CO2 district heating network. A supercritical CO2 power generation cycle is employed.
9. OxySNG + P2G + sCO2 + CO2 CAD: Similar to the case 8, but SNG production is boosted by reacting the biogenic CO2 with hydrogen from water electrolysis in a methanation system. The so-called power-to-gas approach also enables the storage of renewable electricity in the form of SNG, in view of a limited availability of biomass resource. Waste heat is cascaded to a CO2 CAD.

It is worth noticing that direct heat integration of the gasification unit with the aluminium plant remains challenging, in view of the harsh composition of the gasifier effluents, which may compromise the quality of the finished aluminium products. The technology footprint and the complexity of batch processing of solid aluminium also introduce other operational hurdles that must be carefully considered. On the other hand, indirect thermal integration between the aluminium and the gasification process has multiple benefits. As a source of local sustainable natural gas from renewable resources, the exploitation of the biomass enhances the sustainability and energy supply security of heavy industries, in view of the volatile natural gas and electricity market prices. In addition, the strategy of cascading waste heat from the biomass energy conversion and the aluminium production, starting from the highest temperature applications, through cogeneration, preheating of internal combustion air and gasifiers inputs, finalizing with the heat supply to a district heating system (CAD) at lower temperatures proved to be a suitable solution for meet the energy demands of the different industrial and societal actors, whereas producing net negative CO2 emissions. In effect, the biomass energy conversion routes for SNG production demand the largest biomass (>720 kWh/tAl) and electricity (100-300 kWh/tAl) imports, but just to the advantage of replacing fossil natural gas. Moreover, pre-combustion carbon capture units in the SNG plant and oxycombustion furnaces in Novelis aluminium plant facilitate the separation of biogenic CO2 emissions, which can be captured and sequestered permanently, entailing net negative CO2 emissions (up to -143 kgCO2/tAl). In comparison, the traditional scenarios of biomass conversion are responsible for more than +120 kgCO2/tAl fossil CO2 emitted. Also, thanks to a thorough waste heat recovery through the biomass conversion units and the aluminium furnace stacks using Rankine or transcritical CO2 power systems, this approach increases the share of power self-generation to 30%. The case 4 presents the minimum energy consumption (i.e. SNG + cogeneration + CO2 CAD), which can be explained by a more rational use of the biomass energy integrated to a waste heat steam network for combined heat and power production, linked to an anergy CO2 network that capitalizes on the low grade waste to deliver the space heating to the city. The total energy consumption of this configuration is 14% lower than the solution in which biomass is only used in a combustion boiler to supply heat. Direct biomass combustion (case7) holds the highest associated CO2 emissions, due to the additional fossil natural gas consumption necessary to fuel the furnaces of Novelis Sierre plant.

A particular scenario in which biomass input is drastically reduced (89%) is the case in which power-to-gas technology and seasonal SNG/CO2 storage is implemented (case 9). In fact, the so-called power-to-gas approach aims to capitalize on renewable electricity to boost the SNG production, thus the import of the woody biomass is limited to four colder months. The biogenic CO2 is stored in liquid form, so that it can be upgraded to fuel by capitalizing on the renewable electricity available during the remaining months. The opex and the indirect emissions associated to the large electricity import are the main challenges of this integrated route (P2G), as the indirect associated emissions may offset the positive impact of the avoided biogenic emissions. It was found that the maximum power capacity of the electrolyzer is 1220 kWhee/tAl, which in turn produces 933 kWhH2/tAl (HHV). The maximum methanator capacity is 650 kWhSNG/tAl. The gasifier is consequently undersized, compared to the previous cases 1-4 and 8, only requiring a maximum biomass input of 250 kWhDB/tAl during the operative months (less than 7 MW for a typical 27.8 t/h aluminium remelting plant). Hence, the maximum gasifier size is sharply reduced to less than one third of the minimum capacity for the other SNG production routes. The integration of an anergy network for district heating purposes is competitive vis-à-vis a high temperature water based network. Finally, an extended analysis presented in Annex, considering the integration of aluminium plant in an industrial cluster together with methanol and sustainable aviation fuels plants shows opportunities of industrial symbiosis for the processes decarbonization. Waste heat cascading combined with heat pumping solutions show a high degree of compatibility with the future scenarios of decarbonization technologies and transition pathways. Overall, the project reveals that biomass gasification holds significant potential for decarbonizing heavy industry applications. Expectedly, biomass gasification setups are footprint intensive and costlier than simpler biomass boilers. Moreover, the developing technology readiness levels of some energy systems aggravates the risk perception, despite the efficiency and environmental benefits that more advanced energy technologies may have in the overall energy and industrial sector. However, an economic feasibility study based solely on present market conditions may be misleading, in view of the upcoming energy transition policies and anticipated cost-effective deployments of those advanced energy systems, which will help in displacing fossil resources and, thus, increase the industrial sustainability. In fact, new technological setups will be critical in scenarios of more stringent carbon taxations, as they can offset atmospheric emissions associated to the typical biomass energy use (e.g. direct biomass combustion), while helping to alleviate the environmental burden of the hard-to-abate heavy industries.