2.2.2. Inventory Data for Chips and Pellets

To model the life cycle of wood chips, the ecoinvent 3.4 dataset "Wood chips, wet, measured as dry mass {CH}|market for|APOS" was used. This dataset includes both wood chips from industrial activities and forest management and represents the average Swiss market (assumed to be a good proxy for Germany). In particular, wood chips from forest management represents an 85% share of the modeled Swiss wood chip market.

For wood pellets, the ecoinvent 3.4 dataset "wood pellet, measured as dry mass {RER}|market for wood pellet|APOS" was used.

For *Miscanthus* pellets, a similar dataset was not available in ecoinvent. Hence, the inventory data from [42] were used together with the best practices reported in [43]. An average dry yield value of 23.5 t *Miscanthus* (85% dry matter) per hectare was used to estimate the land requirements to provide enough fuel for the HBP plant for one year. The planting rate of 16,000 *Miscanthus* per ha was taken from [43]. As *Miscanthus* is a perennial crop, field preparation activities such as herbicide application, harrowing and plantation, occur only during the first year. The lifetime of the crop was assumed to be 18 years [43] and therefore 1/18 of the impact from field preparation activities was apportioned to one year of operation of the HBP plant. Once the *Miscanthus* is collected from the field, it is necessary to transport it to the pelleting plant. The transport distance to the pelleting plant was assumed to be 10 km by tractor [42]. For the chipping of *Miscanthus*, the energy consumption of the chipper and the amount of lubricating oil were retrieved from the ecoinvent 3.4 datasets "Wood chips, wet, measured as dry mass {CH}|wood chips production, hardwood, at sawmill|APOS". For the pelleting of *Miscanthus*, the amounts of electricity, heat, lubricating oil, and water were retrieved from the ecoinvent 3.4 dataset "Wood pellet, measured as dry mass {RER}|wood pellet production|APOS". The transportation of *Miscanthus* pellets to the HBP plant was assumed to occur by truck and with an average distance of 100 km [42].

## 2.2.3. Inventory Data for the Manufacturing of the Power Plant

The HBP manufacturing consists of three sub-processes: the manufacturing of the gasifier, the manufacturing of the SOFC stack and its balance of plant (BoP), and the manufacturing of the GCU. The data for the manufacturing of the gasifier is based on HBP project data and shown in Table 3.


**Table 3.** Materials of the gasifier including the primary gas treatment zone.

Concerning the SOFC stack, its production was modeled considering secondary data from scientific literature and, to a lower extent, from ecoinvent database. The literature data was retrieved from studies where the SOFC stacks had a similar power capacity as the HBP technology. The amount of electricity, nickel oxide, solvents, materials for the binder, carbon black, and chromium steel, as well as direct emissions (released during the production of the stack) to the air of carbon dioxide, methyl ethyl ketone, and benzyl alcohol were taken from [17] and adjusted proportionally to the power capacity (factor of 0.793 based on 199 kWe of HBP SOFC versus 250 kWe of SOFC in [17]).

The data for the manufacturing of the anode, cathode, electrolyte, and the required ceramic materials (Lanthanum Strontium Manganite (LSM) and Yittria Stabilised Zirconia (YSZ)) were retrieved from [44].

The other secondary data for the SOFC stack, which were not available in [17,44], were retrieved from the already existing inventory in ecoinvent 3.4 called "Fuel cell, stack solid oxide, 125kW electrical, future {CH}|production APOS" and multiplied times 1.59 to account for the different size (assumption of linear proportionality of materials to the size as before).

For the production of the SOFC's BoP, data for the inputs of steel and energy were retrieved from [17]. The other data were instead retrieved from ecoinvent 3.4 dataset "Fuel cell, solid oxide, 125 kW electrical, future {CH}|production|APOS", which was modified as well by multiplying times 1.59.

The materials for manufacturing the cage of the GCU were assumed to be similar to the ones of the cage of the external reformer of the SOFC provided in ecoinvent 3.4. The 96 filter candles which are present in the GCU system at the beginning of the operation were included within the manufacturing stage. These candles are made from calcium-magnesium-silicate high-temperature fiberglass. The processes "Calcium borates {GLO}|market for|APOS", "Magnesium {GLO}|market for|APOS" and "Silica sand {DE}|production|APOS" from ecoinvent 3.4 were used as a proxy for CaMgO4Si. It was further assumed that 1.1 kg of material input would generate 1 kg of filter candles. The mass of each candle was derived from the technical sheet of the candles [45].

#### 2.2.4. Inventory Data for Operation and Maintenance

The system operation includes all the material and energy inputs needed to operate the plant during one year of service (e.g., gas cleaning sorbents, water), waste outputs (e.g., ash which needs to be disposed of) and direct emissions to the environment (e.g., pollutant gas released to air).

The resulting direct emissions to air from the HBP are summarised in Table 4. Data for such emissions were only available for wood chips and wood pellets. The emissions from the operation with *Miscanthus* pellets were assumed to be the same as for wood pellets. Data on the ash formation (grate ash and fly ash) was retrieved from [14].

**Table 4.** Direct emissions (mg) to air per MJ of overall energy output (heat and electricity). OGC = organic gaseous compounds, TSP = total suspended particle, NOX = nitrogen oxides. Maximum values shown in the table were used in the Life Cycle Inventory.


The operation of the gasifier needs 2.36 kg of natural gas for start-up operations and about 80.0 t of tap water per year for gasification air humidification (based on simulations from project data). According to measurements performed downstream the primary gas treatment zone, i.e., at GCU inlet, the syngas composition during utilization of wood chips is as follows (in volume percentage): 15.4% CO, 10.6% CO2, 1.8% CH4, 8.3% H2, 21.8% H2O, 41.1% N2. During the multiple tests performed, such a composition showed to be stable. After the primary treatment unit, the syngas typically shows contaminant concentrations in the range of 30 ppm for sulfur and 20 ppm for chlorine (on wet basis) when wood chips are used as fuel. The tar concentration at the inlet of the GCU was lower than 2.0 g/Nm3 on a dry basis and the particulate matter contents (TSP) of about 200 g/Nm3 on a dry basis were determined.

For the operation of the GCU, about 1.2 t of zinc oxides per year are needed for H2S removal. The GCU also requires 1200 Nm<sup>3</sup> of Nitrogen per year for the cleaning of the filter elements. One year of operation of the GCU requires also 4800 kg of dolomite mixed with 900 kg of sodium bicarbonate (from ecoinvent 3.4, Soda ash, dense {GLO}|market for|APOS) as coating materials respectively for Cl-sorption. The GCU has been designed to feed the SOFC with a product gas containing less than 5 ppm of chlorine, less than 1 ppm of sulfur, and less than 100 ppm of particulate matter (TPS < 0.1 mg/Nm3, on wet basis). Since the composition of the syngas is expected to be stable (confirmed also by the first test runs), the uncertainty about the simulated electric power output of the SOFC is expected to be very low.

The maintenance stage includes all the components which are replaced during the lifetime of the HBP plant. The SOFC stack and the GCU have a shorter lifetime than the average lifetime of the HBP CHP plant. Since the SOFC stack currently investigated for the HBP technology has an estimated lifetime of 5 years, the production of 1/5 extra SOFC stack per year was added to the maintenance stage. The GCU used for the HBP technology has a lifetime of 10 years, therefore, the production of 1/10 extra GCU per year was added to the maintenance stage. All other maintenance inputs (steel components and deionized water consumption for start-ups) for the SOFC were retrieved from [17] and scaled for the capacity of the SOFC under investigation. For the filter candles, an average of 30% of candles is estimated to be replaced each year of operation of the HBP technology. Therefore, the production of 29 extra candles per year was included in the maintenance stage.

#### 2.2.5. Inventory Data for End-of-Life Disposal

The main material employed in the components of the HBP is steel and can be recycled at the end of the life of each component. Based on the amount of steel present in the components (and their replacements), it was assumed that about 1900 kg of steel are recycled per average year of operation. The model included the energy for pressing and crushing the steel crap (based on [46]), a recycling efficiency (referred to as RRE in Equation (1)) of 88% [47], and a transportation distance of 100 km from [46]. Such transportation was assumed to occur mainly by 16–32 t lorries Euro 3 [46].

A recycling process is a typical example of a multifunctional process fulfilling two functions i.e., the treatment of waste and the production of a recycled product. Based on our goal, the modeling approach (i.e., attributional), and the recommendations by ISO 14044:2006, mass allocation was applied. This selection is based on the fact that ISO 14044:2006 prioritizes allocating by a physical property for open-loop recycling over the economic value or number of uses (among ISO third level allocations i.e., by other relationships). Additionally, system expansion cannot be applied since we want to isolate the

first function (first use of the material which led to its treatment) from the second function (next use or cycle of the material). The impacts arising from transportation (ET), recycling (ERC), and the extraction and processing of the primary material (Ev) were therefore allocated by mass between this life cycle and the following one (see Equation (1) expressing the allocated impact to our functions). The resulting mass allocation factor (1/(1 + RRE)) was 53% (1/1.88). The second part of Equation (1) related to the virgin material takes into account the fact that the primary production was already accounted entirely in the manufacturing phase, and therefore the corresponding burdens (e.g., extraction of raw material) that belong to the following life cycle should be subtracted.

$$\mathbf{E}\_{\text{steel displacement}} = \text{RRE}(\mathbf{E}\_{\text{T}} + \mathbf{E}\_{\text{RC}}) \frac{1}{1 + \text{RRE}} - \frac{\text{RRE}}{1 + \text{RRE}} \,\mathrm{E\_{v}} \tag{1}$$

There are some precious metals (e.g., used as catalytic materials) used in the power plant that, depending on the recovery efficiency and initial concentration, might be economically convenient to recover, though e.g., hydrometallurgical treatment [48]. Nevertheless, such specific recovery processes were not modeled because of the unavailability of Life Cycle Inventory data. Materials other than the steel used in the power plant components consist of hazardous waste (24 kg) and inert waste (10 t per year in the chips scenario, and 20 t per year for the pellets scenarios). The treatment of the hazardous waste was modeled through the ecoinvent dataset "Hazardous waste, for underground deposit {DE}|treatment of hazardous waste, underground deposit|APOS". The inert waste consists mainly of materials for sorbents and was modeled through the ecoinvent 3.4 dataset "Inert waste, for final disposal {CH}|market for inert waste, for final disposal|APOS".

#### 2.2.6. Inventory Data for the Competing Technologies

For the comparative analysis, the ecoinvent 3.4 datasets "1 MJ Heat, district or industrial, other than natural gas {CH}|heat and power co-generation, wood chips, 2000 kW, state-of-the-art 2014|APOS" and "1 kWh Electricity, high voltage {CH}|heat and power co-generation, wood chips, 2000 kW, state-of-the-art 2014|APOS" were used for the ORC. This dataset represents a state of the art ORC co-generation plant equipped with an electrostatic precipitator for particulate emission reduction and includes the infrastructure. For the separate production of heat and electricity, the ecoinvent 3.4 datasets "1 MJ Heat, central or small-scale, natural gas {CH}|heat production, natural gas, at boiler condensing modulating < 100kW|APOS" and "1 kWh Electricity, medium voltage {DE}|market for|APOS" were used.

Following the description provided in ecoinvent 3.4 for the ORC ecoinvent dataset, the capacity of the ORC plant is 1000 kW thermal, and 200 kW electric (similar to the electric output of the HBP technology). This information was used to estimate the exergy allocation factor of 46% for heat (assumed district heating provided at 90 ◦C as for HBP) and 54% for electricity. Based on 2015–2017 average prices for Germany, the economic allocation shares for ORC would be 66% for heat and 34% for electricity. As the total power input (as wood chips) is 2000 kW, this ORC plant has an overall energy efficiency of 60%, i.e., 10% electrical efficiency plus 50% thermal efficiency.
