**1. Introduction**

As part of the global energy transition and to avoid climate change, the Paris Agreement envisages a reduction of greenhouse gas emissions to 40% by 2040, starting from the base year 1990 [1]. There is a growing interest in companies to solve the question of their own climate neutrality in a transparent and sustainable way. In addition to their own motivation, customer demands for climate neutrality are becoming increasingly important from an economic point of view. Industry has a sector goal of reducing emissions by 49% to 51% (compared to 1990) by 2030 in Germany. To reach these goals industrial processing companies need to perform three steps. The first step is minimizing the energy demand by energy efficiency measures and secondly substituting fossil fuels by changing processes or implementing renewable energies. Compensating unavoidable emissions should only be the last step.

Process steam is widely used in the industry and is one of the main consumers of thermal energy. It is currently still often produced with fossil-fired steam boilers for economic reasons. In 2016, around 21.4% of Germany's total energy consumption was needed for processing heat. The share of renewable energies in this consumption was only 5.2% [2]. By 2050, the share of renewable energies in industry must rise to over 63% [3].

Bühler et al. [4] showed that an industrial integration of renewable energy based generation technologies is a complex problem due to many assumptions which have to be made as well as numerous statistical uncertainties. Therefore, different case studies from different industrial sectors and production system types are crucial to elucidate challenges and propose solutions for the energy transition. The paper does not consider solar thermal or biomass energy sources. Besides, it is focused on costs not emissions. For choosing the best energy supply for industrial sites top-down models often struggle to include technological explicitness [5]. This is especially important for batch processes with a fluctuating steam demand. For industrial sites, the most attractive renewable energy sources are are biomass, solar radiation (thermal or photovoltaics), ground heat and wind [6]. This indicates an electrification of the steam supply via heat pumps (HP) and electric boilers (EB) or a substitution of fossil fuels with renewable energy sources.

To reach a 100% renewable energy supply in New Zealand and California for milk production plants, Walmsley et al. [7] analysed geothermal steam and renewable electricity for the mechanical vapor recompression, a biomass boiler with a two-stage ammonia heat pump using the boiler flue gases and solar thermal energy. Also, the integration of solar thermal energy for a Danish milk powder factory was analysed [8]. For the state of California, 48 TWh per year for industrial heating demands below 260 °C can be covered by solar power technologies [9]. Solar thermal energy offers a good solution especially in sunny states like Spain, like Silva et al. [10] showed for saturated steam at 7 bar. Stark et al. [11] investigated a pharmaceutical production facility with several overlapping steam batch processes. The majority of the steam demand is supplied from a biomass CHP plant. By utilising a steam accumulator, the share of bio-steam can be increased and the supply stability is improved. Further, the design parameters and the influence on the turbine system of the plant were investigated by Stark et al. [12]. To process stability as well as the share of bio-steam which is used in the supply system can be improved. Pérez-Uresti et al. [13] presented a method for calculating levelised steam costs based on solar thermal energy, biomass or biogas. For medium pressure (MP) steam and a plant capacity of 30 kg/s the presented steam costs for biomass are 19.36 e/t, for solar thermal 30.65 e/t and biogas 25.73 e/t.

Peesel et al. [14] showed that the *CO*2,*e*-emissions can be reduced by up to 42% by the transition to a solid biomass-fuelled boiler system and up to 27% using a biomethane fuelled solid oxide fuel cell but the operating costs increase significantly. A bottom up methodology for assessing electrification options for industrial process was presented by Wiertzemena et al. [15]. The authors highlighted that the energy and carbon footprint consequences of electrification are hard to predict and therefore detailed simulation studies are required. For the year 2050, Johansson et al. [16] estimated the *CO*2,*e*-emissions reduction potential for the European petroleum refining industry including energy efficiency measures, fuel switching and carbon capture and storage. For a complex oil refinery four pathways for *CO*2,*e*-emissions reduction were analysed [17]. Energy efficiency measures followed by biomass gasification with carbon capture and storage is the most cost-effect pathway. Besides, industrial sites with an almost constant energy demand, part load characteristic may change the energy efficiency and cost calculation for steam systems. The part load performance of boilers and steam turbines is considered by Sun et al. [18] for analysing the steam costs by using a cumulative cost profile based on the marginal cost of steam.

However, a variety of technologies has to be investigated and different pathways should be investigated and implemented [6]. This is especially important as new developments can trigger significant cost reductions in certain technologies, like for example, optimized control [19] or load management. A system approach should be not restricted to certain technologies. Hybrid approaches are an option, for example, direct steam generation with solar collectors combined with biomass boilers [20].

In the food processing industry, the sterilisation process is the main energy consumer. For providing the steam often natural gas boilers are implemented. The short reaction times of the boilers help to manage the fluctuating steam demand due to batch sterilisation processes. A literature review and analysis of the present process revealed that sterilisation processes have been optimized in terms of temperature, time, material properties, heating medium and heat transfer coefficient [21]. To reduce high peak demands thermal process scheduling is possible [22]. Furthermore, a detailed analysis of the starting temperature and heat recovery potential has been carried out [23]. For batch processes, the time aspect and accumulators have to be considered as well. Especially for fluctuating renewable energy sources and fluctuating production energy demands [24]. For the technical requirements for the different steam utility systems the start up times are highly relevant especially for batch steam processes [25].

Although, alternative process steam generation technologies with renewable energies have already been investigated, there is a lack for the special requirements of batch processes. In addition, the volatile energy supply and energy demand require a dynamic simulation to represent the interaction of main supply technologies, steam accumulators and peak load boilers with sufficient accuracy. Furthermore, the cost and energy efficiency are highly depending on the steam's temperature, pressure and quality. This paper analyses the ratio of emissions savings and costs via a dynamic simulation for a total of seven technologies including specific investment and energy costs for a food processing plant with MP steam in Spain.

The purpose of this study is to highlight the *CO*2,*e*-emission reduction potentials and related economic consequences for changing the steam supply system to renewable energies. The seven different concepts include biomethane-fuelled solid oxide fuel cells (FC), biomass-fuelled boilers (BMB), biogas (BGB) or biomethane-fuelled boilers, micro gas turbines (MGT), EB, solar energy systems (SES) and HP are evaluated for their different implementation options in combination with load management measures, including a steam accumulator. In addition, the concepts are optimized in terms of the costs per saved *CO*2,*<sup>e</sup>* emissions per annum.

## **2. Description of Technologies**

In this section the technical functionality of seven different technologies for substituting the fossil fuel fired steam boilers are briefly described and an overview of the technical and economical data is given. Peculiarities for the integration into a steam system with fluctuating steam demand are explained and the models for the simulation study are described.
