**4. Use Case Wet Pet Food**

In this section the production process of wet pet food is introduced and the basic condition for the case study are presented. Figure 3 shows the process flow diagram for wet pet food.

The frozen meat is first crushed and then processed in the grinder to a homogeneous mass. In the mixer, fresh meat and water are added. The mixing process requires thermal energy at a temperature level between 65 and 80 °C. The water-meat mixture is then emulsified and cooked in the steam tunnel and then cooled. Together with the gravy water, the mixture is filled into pouches and then sterilized in the retorts. The process concludes with the packaging of the pouches in boxes and cartons. Over 60% of the steam is used for the sterilisation process, while 6–8% are needed for the steam tunnels at 3 bar. Around 18% are required for the mixing, cleaning, hot water and other processes. The rest are losses and demands for pre-heating the fresh boiler water. Depending on the level of heat recovery and the set up of the production plant these values may differ.

**Figure 3.** Process flow diagram for wet pet food.

In the following, the process data of a wet pet food processing plant located in Spain is used within a simulation study to analyse different integration concepts for the presented technologies. Approximately 27,440 t of steam and 7800 MWh of electricity are required annually at this location. The steam is supplied at 9 bar and approximately 175 °C. The average steam demand is 3.1 t/h. However, the demand fluctuates between 1 t/h and 7 t/h. Higher peak demands up to 10 t/h occur rarely during the year. Figure 4 shows an example of the steam demand for four hours. Both, the minimum of 0 t/h and the maximum mass flow of 10 t/h as well as the strong fluctuations caused by the batch operation of the sterilization process are clearly visible. The irregularity in the steam demand places high requirements on the flexibility of steam production, which not every technological alternative can meet without restrictions.

In the reference case, the steam is produced by two conventional natural gas boilers. One acts as the leading boiler and the other one is a backup boiler. When the steam demand cannot be entirely satisfied by the leading boiler and its storage, the backup boiler supplies the rest. For the comparison of the different technologies the dimensions of power and steam accumulator has been optimised in a previous step.

**Figure 4.** Steam demand profile over four hours.

Table 3 shows the thermal and electrical energy demands, the corresponding energy costs and the *CO*2,*<sup>e</sup>* emission for the described reference case.


**Table 3.** Reference case.

For the calculation of the operating costs, the natural gas price varies between 0.027 e/kWh and 0.031 e/kWh. These natural gas prices were retrieved from the annual bill of the industrial site. For the case study, the prices are assumed to be fixed—biomass 0.028 e/kWh [54], biogas 0.0644 e/kWh and biomethane 0.095 e/kWh. For self-used electricity and sold electricity the Spanish stock market price [58] of 2017 in each time step is considered. The chosen values for emissions of natural gas are 241 g/kWh, of biomass 27 g/kWh [55], biogas 135 g/kWh [59], biomethane 146 g/kWh [59] and the emission for electricity are based on the grid electricity mix in each time step. This value varies between 90.13 g/kWh and 486.49 g/kWh [60]. For year 2017, the Spanish electricity mix is divided as follows—hydro 6.89%, wind 17.88%, solar photovoltaic 3.14%, other renewables 3.68%, nuclear 20.73%, combined cycle 13.84%, coal 16.81%, cogeneration 10.53%, others 3.08% and cross-border exchange 3.42% [61].

Table 4 summarises the investigated scenarios for the steam accumulator volumes, the size of the new technologies and the size of the backup boiler fired by natural gas or biogas.


