System description and general modelling approach

The PtF system under study is a site-specific integrated system as shown in Figure 4.1. It was specifically developed at the Research Centre ‘Forschungszentrum Jülich’ (North-Rhine Westphalia, Germany) in accordance with DECKER et al.

(2018), although adjusted for smaller scales in this study. It includes the following components: a) a biogas production plant using anaerobic digestion (AD), corresponding to a small-manure plant; b) a CO2 recovery unit, upgrading biogas to biomethane with a post-combustion unit for the flue gas; c) a CHP in which the biomethane is burnt; d) a wind turbine generator (WTG) and polymer electrolyte membrane (PEM) electrolyser for H2 supply, including a liquid organic hydrogen carrier (LOHC) storage facility for buffering; and e) a methanol synthesis plant.

Figure 4.1: Detailed Power-to-Fuel system under study showing the combination of a biogas plant (a) with gas separation (b) and combined heat and power plant (CHP) (c) and a wind turbine and electrolyser with storage facility (d), as well as a methanol plant (e).

Caption: own creation.

The overall concept is decentralised, so that the five subsystems at the location of an BGP can be integrated. Most components of the system meet the technology readiness level (TRL) of 9 and are readily available to be used in an operational environment. However, methanol production is not yet available on such a small scale. The membrane upgrading technology is applied with data from OESTER et al. (2018), a Suisse manufacturer. They merchandise plants on a similar scale as here, although they are not used on a commercial scale by small-manure plants in Germany yet. The TRL of the PEM electrolyser is assumed to be 8 as defined by SABA et al. (2018). PEM systems can be purchased from several manufacturers, although their current market penetration is limited in Germany. The main processes within the system of the standard case are described below, while major technical characteristics and associated process parameters are included in Table 4.1:

a) The biogas plant (BGP) represents a classic small-manure plant (75 kW). Inside its fermenter, a mix of manure and straw residues obtained from livestock farming is converted through AD into raw biogas and digestate. Both types of feedstock are considered residues; hence, their respective upstream production processes up to

b) Gas separation c)CHP

unit Electricity

grid District heating

network

e) Methanol plant

Electrolysis Renewable

power a)Biogas plant

CO₂ CH4

H₂

Manure Plant material

Heat Electricity

Raw biogas Digestate

Feedstock

Electricity

Storage

H₂

d)

the PtF facility gate are not of interest. The manure is directly transported from the stable to the facility through an automatic manure scraper into the preliminary storage tank, which is located underground and covered with concrete. The manure is pre-stored for a short period until it is pumped into the fermenter. The digestate obtained as a co-product from AD is then openly stored onsite and re-used as a fertiliser.

Table 4.1: Main process parameters characterising the innovative Power-to-Fuel system for the standard case (case 1) and associated data sources.

Parameter Value

Electricity CHP (kW) 75^a

Heat CHP (kW) 98^a

Engine output CHP (kW) 205^a

Plant electricity demand (%) 8^a

Plant heat demand (%) 35^a

Volume flow of raw biogas (m³/h) 39.39^b

Number of cows (providing manure for the 75 kW biogas plant) 126^c

CH4 losses during AD (%) 1.40^a

Methane slip during PSA (%) 1.50^d

Share of CO2 gain from post-combustion (%) 1.83^b

Electrolyser capacity (kW) 950^b

Wind turbine capacity (kW) 1040^b

FLH wind turbine 2000^b

Capacity of methanol plant (kWth,LHV) 138.38^b

FLH methanol plant 8500^a

a Rau (2019), ^b own calculation; approach shown in the remainder of this chapter, ^c calculated according to Rutzmoser et al. (2014), ^d Lohse (2019); WTG = wind turbine generator, PEM = Polymer electrolyte membrane, CHP = combined heat and power plant, LHV = Lower heating value.

b) An additional sub-system of gas separation is assumed that processes the biogas and provides CO2, as it is required for the methanol synthesis in this concept. Due to the already existing application of the membrane process at small-manure plants and its comparatively small space requirement, which would be preferable, this separation process is assumed in the following. A flue gas stream with 98.25% of CO2 and 1.75% CH4 is considered, assuming that biomethane from upgrading has a purity of 95% (LOHSE, 2019). Flue gas treatment through recuperative

post-combustion could possibly be carried out to achieve a pure stream of CO2. This process requires a minimum CH4 of 0.3vol% in the flue gas which is provided in this case.

c) The concentrated CH4, i.e. the biomethane, is subsequently burnt in the CHP unit. In industrial scale installations, the biomethane is injected into the gas grid in Germany. Nevertheless, feeding into the grid is not profitable for small-manure plants and there is not always a connection to the grid next to small-manure plants, which are often located in remote areas. This is why it was assumed that biomethane is used on site. Therefore, the biomethane is assumed to be used as a combustible in a CHP, which generates energy in the form of heat and electricity. According to SCHOLWIN et al. (2014, p. 12), every CHP can be fired with both NG and biogas from a technical point of view. This is an advantage for the system proposed, as it can be an expansion to already existing BGPs. The separated CO2 presents itself as a basic material for the added methanol synthesis. The available CO2 thus determines the capacity of the methanol plant.

d) A WTG is also implemented in the system to produce H2 by means of a PEM electrolyser. Its mode of operation was already described in 2.3. The required power for the electrolysis is supplied it. Since the WTG does not supply electricity for the entire operating time of the methanol plant, an LHOC storage facility is considered.

A tank provides a buffer facility whereby H2 can be stored after production. The capacities for the electrolyser and the WTG must be selected in such a way that continuous operation of the methanol plant is guaranteed. It is estimated that the wind turbine has a capacity of around 1 MW, producing electricity for water separation with a 950 kW electrolyser. It is also assumed that the H2 is produced in close proximity to the farm, hence no transport is involved.

e) Methanol is finally produced on a pilot scale in a plant with a nameplate capacity of 212 tonnes of methanol per year, assuming 8500 full load hours (FLH), corresponding to 138 kWth,LHV. It should be noted that the methanol synthesis runs at 80 bar and 250°C.

The developed model describes the PtF system underlining the assumption for the LCA and the cost analysis. The results and assumption have an impact on the LCA calculations and on the cost analysis, since calculated values act as operating conditions for the process design model of the system. In general, the goal of the simplified model is to quantify the amount of methanol that can be produced with a certain amount of biogas under certain environmental and economic conditions.

As the amount of biogas is unknown in this specific case, and varies slightly depending on digestion conditions, the known capacity of the CHP and the measured percentage composition of the biogas are used for the calculation. The model is based on primary data from measurements and simulations of a biogas plant in the East of Germany, as well as secondary data from the Ecoinvent 3.5 database and the literature. The model setup assures full load drive of the CHP.

Hence, if the capacity of the CHP is, e.g., 75 kWel, the PtF system runs such that the full capacity is utilised. As there are losses of CH4 through the biogas upgrading process, the initial CH4 stream and ultimately the initial biogas stream have to be greater. This can easily be achieved by using more feedstock, as long as the fermenter capacity allows it. As exact amounts of produced biogas were unknown, a model was developed to estimate the relevant gas flow and materials, whereby constant temperatures were assumed for simplification that are typical for these kind of BGPs as the logbook of the mentioned BGP indicates.